EDP Sciences
Free Access
Issue
A&A
Volume 604, August 2017
Article Number A78
Number of page(s) 56
Section Stellar structure and evolution
DOI https://doi.org/10.1051/0004-6361/201629503
Published online 11 August 2017

© ESO, 2017

1. Introduction

In recent decades significant progress has been made regarding the understanding of star formation. However, the formation of massive stars, which is the only mode of star formation that is observable in external galaxies, remains a key unsolved problem (Zinnecker & Yorke 2007; Beuther et al. 2007; Tan et al. 2014). One of the main reasons is that observations of the earliest phases in the life of a massive star are scarce. This is explained by the expected short formation timescale (~104−105 yr) of massive stars and the severe extinction (AV ~ 10–100 mag) by which the surrounding gas and dust obscure their birth places from view. Additionally, massive stars are rare and, as a consequence, are located at relatively large distances.

Observational and theoretical evidence is accumulating that the formation process of massive stars is through disk accretion, similar to low-mass stars. This is the case despite the physical processes acting near hot, massive stars (e.g. radiation pressure and ionisation) that counteract the accretion flow onto the forming star (e.g. Wolfire & Cassinelli 1987; Keto & Wood 2006; Krumholz et al. 2009; Klassen et al. 2016). In the accretion scenario, one expects that soon after the gravitational collapse of the parental cloud the pre-stellar core is surrounded by an extended accretion disk (size ~ 1000 AU; e.g. Beltrán et al. 2004; Cesaroni et al. 2005; Sandell & Wright 2010; Beltrán & de Wit 2016). Apparently this is not a very efficient process, as most of these high-mass protostars seem to drive outflows (e.g. Zhang et al. 2001). Already before arriving on the main sequence, the young massive star is expected to produce a strong ultraviolet (UV) radiation field ionising its surroundings (Bernasconi & Maeder 1996). At this stage the object becomes detectable at radio and infrared wavelengths through the heated dust and recombination of hydrogen in an expanding hyper- or ultra-compact H ii (UCHii) region (e.g. Churchwell 2002). Unfortunately, only very little is known about the physical properties of the central (massive) stars at this stage of formation (e.g. Martín-Hernández et al. 2003).

Given the short formation time of massive stars, the accretion rate must be high (up to 10-3M yr-1; Hosokawa et al. 2010). At these accretion rates, a massive young stellar object (mYSO) is expected to bloat up to about 100 R, resulting in a relatively low effective temperature and modest UV luminosity. The accretion process is unlikely to be constant with time; the blob structure observed in Herbig Haro outflows indicates that strong variations occur in the mass in- and outflow rate of young intermediate-mass stars (e.g. Reipurth & Bally 2001; Ellerbroek et al. 2013b). Simulations of different accretion models applied to mYSOs show that the accretion rate increases as the mYSO grows in mass (Davies et al. 2011). Once the accretion rate diminishes, the so-called bloated pre-main-sequence (PMS) star contracts to the main sequence on the Kelvin-Helmholtz timescale. Recently, candidates for such bloated, massive PMS stars have been spectroscopically confirmed (Ochsendorf et al. 2011; Bik et al. 2012; Ellerbroek et al. 2013a; Chen et al. 2015).

The environment (multiplicity and clustering) and the large distances to massive (proto)stars make their circumstellar disks very difficult to resolve (Beltrán & de Wit 2016). In contrast, disks around intermediate-mass stars (2 < M < 8 M) have been characterised using (sub)millimeter as well as optical and near-infrared (NIR) spectroscopy (see Bik & Thi 2004; Wheelwright et al. 2010; Ellerbroek et al. 2011; Alcalá et al. 2014). The rotational structure of the disk provides information about the role of magnetic fields that can slow down the rotation rate below pure Keplerian. Super-Keplerian rotation of the disk could indicate that the inner disk contributes significantly to the gravitational potential of the system (Beltrán & de Wit 2016). In the mass range from 20–30 M circumstellar disks have been detected, but there is no consensus about their general properties.

M17, located in the Carina-Sagittarius spiral arm of the Galaxy is one of the best-studied giant H ii regions (L = 3.6 × 106L, Povich et al. 2007). Its distance has been accurately determined by measuring the parallax of the CH3OH maser source G15.03-0.68: kpc (Xu et al. 2011). The bright blister H ii region is embedded in a giant molecular cloud complex and divides the molecular cloud into two components: M17 South and M17 North, containing a total gas mass (molecular and atomic) of about 6 × 104M (Povich et al. 2009). The centre of the H ii region hosts the cluster NGC 6618 including 16 O stars and more than 100 B stars (Chini et al. 1980; Hoffmeister et al. 2008), providing the ionising power for the H ii region. NGC 6618 is a young cluster, whose age is estimated at about 1 Myr (Hanson et al. 1997, hereafter H97; Broos et al. 2007; Povich et al. 2009), while the surrounding molecular cloud hosts PMS stars. The photodissociation region to the southwest of NGC 6618 includes several candidate mYSOs: the hyper-compact H ii region M17-UC1 surrounded by a circumstellar disk (Nielbock et al. 2007), and IRS5, a young possibly quadruple system of which the primary star, IRS5A (B3–B7 V/III), is a high-mass protostellar object (~9 M, ~ 105 yr; Chen et al. 2015). H97 discovered a sample of high-mass (5–20 M) YSOs in the same area. Their spectral energy distributions (SEDs) display a NIR excess and some show CO bandhead and double-peaked Pa δ emission, likely due to the presence of a circumstellar disk. These objects are the subject of study in this paper.

We are now on the verge of being able to quantitatively test the predictions of massive star formation models. Several samples of mYSOs have been presented (e.g. Bik et al. 2006; Urquhart et al. 2011; Ochsendorf et al. 2016). However, so far the photospheric spectrum has only been detected for a handful of mYSOs. We obtained high-quality spectra with X-shooter on the ESO Very Large Telescope (VLT) of candidate mYSOs, first identified by H97, and a few other O and B stars in M17. The large wavelength coverage of the X-shooter spectra (and additional infrared photometry) allows for a detailed analysis of the SED, including an infrared excess produced by the dust component in the accretion disk. The observed double-peaked emission lines provide us the opportunity to study the dynamical structure of the gaseous component of the disk.

This paper is organised as follows. In Sect. 2 we present our target sample, observations, and data reduction procedure. In Sect. 3 we provide the spectral classification of the (pre-) main-sequence OB stars in our sample. After that (Sect. 4) we present their SEDs. To further constrain the properties of the mYSOs, we modelled the optical spectra via FASTWIND. These results are shown in Sect. 5, where we also place the PMS objects in the Hertzsprung-Russell diagram and compare their position with PMS tracks and isochrones. Additional evidence for the presence of circumstellar disks is obtained from NIR spectroscopy (Sect. 6). We discuss the age distribution of the massive PMS population in M17, the extinction towards this region, the presence of circumstellar disks, and the spin properties of the young massive stars in Sect. 7. We summarise our conclusions in Sect. 8.

2. VLT/X-shooter spectroscopy

We obtained optical to NIR (300–2500 nm) spectra of candidate mYSOs and some OB stars in M17. We used the X-shooter spectrograph mounted on UT2 of the VLT (Vernet et al. 2011). The spectra of B275 and B358 were obtained as part of the X-shooter science verification programme; B275 has been subject of study in Ochsendorf et al. (2011). For some objects multiple spectra were taken. A log of the observations is listed in Table 1.

Table 1

Journal of X-shooter observations.

thumbnail Fig. 1

Left: NIR colour composite image of M17 based on 2MASS data: J (blue), H (green), and K (red). Our X-shooter targets are labelled: in black we show the objects that have no indication of a disk, in green the objects that show NIR excess only longward of 3 μm in the photometry (points to a dusty disk), in blue the objects with emission lines in the X-shooter spectrum and NIR excess (identifying them as mYSOs), and in red the post-AGB star B358; B311 is associated with the bow shock M17-S1 (Povich et al. 2008). The central cluster NGC 6618 includes several dozen OB-type stars that ionise the surrounding region. At the western edge of the H ii region several massive YSOs were identified. Apparently, massive star formation is continuing in the irradiated molecular cloud. Right: mid-infrared colour composite image of M17 based on Spitzer and Herschel: IRAC1 – 3.6 μm (blue), IRAC4 – 8.0 μm (green), and PACS – 70 μm (red). Here the distribution of the heated dust (blue) becomes apparent along with the PAH emission within the H ii region (green).

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2.1. Target sample

We selected our targets based on the study of H97 who obtained K-band spectra of the young massive star population of M17, complemented by optical and 1 μm spectroscopy for some of the moderately reddened objects. Several of the targets have also been studied by Hoffmeister et al. (2008) and some are included in the studies of Nielbock et al. (2001), Broos et al. (2007), and Povich et al. (2009). H97 reported that B111, B164, B253, and B311 do not present any NIR excess nor emission lines in their spectra and they are likely OB-star members of the central cluster NGC 6618. It is likely that all our targets belong to NGC 6618, at least the (massive) main-sequence stars.

Based on the NIR excess derived from their position in the NIR colour-colour diagram (CCD; see also Sect. 2.2) and shown in the SED (see also Sect. 4) and the apparent presence of CO bandhead and/or double-peaked emission lines, the candidate mYSOs according to H97 are B163, B243, B268, B275, B289, B331, and B337. We also observed B215 (IRS15): this mYSO candidate has been studied by Nielbock et al. (2001). The spectral types from H97 and Hoffmeister et al. (2008) are listed in Table 3. We obtained an X-shooter spectrum of B358, but according to Chen et al. (2013) and H97 the spectrum is that of an early- to mid-G supergiant. This object has not been detected in X-rays (Broos et al. 2007) and it is likely a background post-AGB star. From examining the X-shooter spectrum we support this conclusion.

Figure 1 (left panel) shows a colour composite image of M17 based on 2MASS J (blue), H (green), and K (red) observations (Skrutskie et al. 2006). The stars for which we collected X-shooter spectra are labelled. The area includes the bright nebular emission produced by the H ii region and the central cluster NGC 6618. The mYSO candidates are predominantly located near the excited rim of the surrounding molecular cloud. This observation suggests that those objects could be the product of a later phase in the star formation process. In the right panel a colour composite is shown based on images taken with Spitzer (GLIMPSE survey: IRAC1 at 3.6 μm; green: IRAC4 at 8.0 μm, Benjamin et al. 2003) and Herschel (red: PACS at 70 μm; Pilbratt et al. 2010; Poglitsch et al. 2010). This image provides insight into the (dusty) surroundings of the H ii region. The 3.6 μm image traces the illuminated edges of the cloud irradiated by the massive stars, whereas the 8 μm emission is usually associated with emission features produced by polycyclic aromatic hydrocarbons (PAHs) and/or warm (100–200 K) dust that emits at these wavelengths when heated by ultraviolet radiation (Povich et al. 2009).

Throughout this paper we have colour-coded the labels to present our results: the objects whose PMS nature is confirmed are plotted in blue, OB stars in grey (black in Fig. 1), objects for which we cannot confirm the PMS nature in green, and the post-AGB star in red.

2.2. Colour-colour and colour-magnitude diagram of X-shooter targets in M17

The (K, JK) colour-magnitude diagram is shown in Fig 2; the J, H, and K-band magnitudes are from the 2MASS database (Cutri et al. 2003). Similar diagrams were presented by H97 and Hoffmeister et al. (2008), but these authors did not use the recent distance estimate of 1.98 kpc (Xu et al. 2011). The zero-age main sequence (ZAMS) is indicated with a drawn line; the dashed line represents the reddening vector. The ZAMS properties for spectral types later than B9 were taken from Cox (2000) and for the earlier type stars from H97. We overplot the properties for the O dwarf stars from Martins & Plez (2006) for reference. We used the Cardelli et al. (1989) extinction law to plot the reddening lines. The candidate mYSOs have a large (JK) colour and a bright K-band magnitude.

thumbnail Fig. 2

NIR colour-magnitude diagram for our targets in M17. The objects for which we confirm their mYSO nature are denoted by blue circles, the stars with NIR excess only longward of 2.5 μm, but without emission lines, are indicated with green triangles, and the O and B main-sequence stars are indicated with grey squares. The red star corresponds to B358, which is most probably a background post-AGB star. The location of the zero-age main sequence is shown for a distance of 1.98 kpc with a solid black line (H97; Cox 2000). In grey we overplot the updated parameters of O V stars according to Martins & Plez (2006). The dashed line represents the reddening line for an O3 V star with RV = 3.1. Some of the mYSO candidates are located above the reddening line indicating the presence of a NIR excess.

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Figure 3 presents the NIR colour-colour diagram including the OB-type stars for which an X-shooter spectrum was obtained. In the colour-colour diagram a NIR excess becomes very apparent, and thus the possible presence of circumstellar material. The solid line represents the unreddened main sequence. The dashed lines indicate the reddening lines for an O3 dwarf and a M0 giant. Most of the mYSO candidates are located to the right of the reddening line of an O3 V star, which is consistent with the expectation that these objects host a circumstellar disk. B163, B289, and B331 are candidate mYSOs according to H97; however, they do not exhibit a NIR excess in the colour-colour diagram. Nevertheless, B331 and B163 are mYSO candidates according to our criteria: they present double-peaked emission lines, CO bandhead emission and/or infrared excess beyond the K band. We discuss the SED and possible presence of an infrared excess at mid-infrared wavelengths as well in Sect. 4.

thumbnail Fig. 3

NIR colour-colour diagram for our targets in M17. The colours and symbols are the same as in Fig. 2. The dashed lines represent the reddening lines for an O3 V and an M0 III star. For this we took the average Galactic value of RV = 3.1. In Sect. 4 we show that RV varies from 3.3 to 4.6. The solid line indicates the unreddened main sequence. Most confirmed mYSOs show evidence for the presence of a NIR excess.

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2.3. Reduction VLT/X-shooter spectra

The X-shooter spectra were obtained under good weather conditions with seeing ranging from 0.̋5 and 1′′ and clear sky. With the exception of the 2012 B289 spectrum and the 2009 B275 science verification spectrum, the spectrograph slit widths used were 1′′ (UVB, 300–590 nm), 0.̋9 (VIS, 550–1020 nm), and 0.̋4 (NIR, 1000–2480 nm), resulting in a spectral resolving power of 5100, 8800, and 11 300, respectively. The slit widths for the 2010 B275 observations were 1.̋6, 0.̋9, and 0.̋9 resulting in a resolving power of 3300, 8800, and 5600, respectively. For the 2012 B289 observations we used the 0.̋8, 0.̋7, and 0.̋4 slits corresponding to a resolving power of 6200, 11 000, and 11 300 for the UVB, VIS, and NIR arms, respectively. The spectra were taken in nodding mode and reduced using the X-shooter pipeline (Modigliani et al. 2010) version 2.7.1 running under the ESO Reflex environment (Freudling et al. 2013) version 2.8.4.

The flux calibration was obtained using spectrophotometric standards from the ESO database. We then scaled the NIR flux to match the absolutely calibrated VIS spectrum. The telluric correction was performed via the software tool molecfit v1.2.01 (Smette et al. 2015; Kausch et al. 2015). Parts of the spectra are shown in Figs. 4 to 6 and the full spectrum is available at the CDS. We used the xsh_scired_slit_nod recipe to reduce the data, meaning that the sky subtraction is performed by subtracting the two different nodding positions. Owing to the spatial variation of the nebular lines along the slit, residuals from the sky subtraction are present in some of the reduced spectra (e.g. [N ii] 6548, 6584 Å, [S ii] 6716, 6731 Å and [S iii] 9069, 9532 Å, and the H  lines).

3. Spectra and spectral classification

Traditionally, the blue spectral region (400–500 nm) is used to perform the spectral classification of OB-type stars. In the case of the M17 sources studied in this paper the blue region is severely affected by extinction, but is for most of our objects detected in the X-shooter spectrum (Fig. 4). Many targets show the full hydrogen Balmer, Paschen, and Brackett (up to Brγ) series; He i and He ii lines, the latter in the O-type stars; the Ca triplet lines; and diffuse interstellar bands. Figures 5 and 6 provide an overview of the Ca triplet region (820–880 nm) and the K band (2100–2400 nm, including Brγ and the CO bandheads) for all targets. With the exception of B163, B331, and B337, which are strongly affected by interstellar absorption in the blue (AV > 10), we classified the stars based on the strength of the H, He, and metal lines in the UVB spectrum. For the O stars we visually compared our spectra with the previously catalogued and published spectra by the Galactic O Star Spectroscopic Survey (GOSSS; Sota et al. 2011, 2014; Maíz Apellániz et al. 2016). For the B stars we used the criteria and spectra published by Gray & Corbally (2009).

Table 2

Summary of the properties of the sources observed with X-shooter.

We looked for the presence of [O i] 6300. This forbidden line is associated with bi-polar outflows/jets, disk winds, and the disk surface in Herbig Ae/Be stars. The [O i] line is thought to originate from the region where the UV radiation from the star impinges on the disk surface (Finkenzeller 1985; van der Plas et al. 2008; Mendigutía et al. 2011). We also paid special attention to the luminosity subclass Vz, which refers to objects with a substantially stronger He ii  4686 line. These objects are hypothesised to be the youngest optically observable O-type stars (the “z” stands for ZAMS; Sabín-Sanjulián et al. 2014; Arias et al. 2016). Given the young age of M17, one might expect such stars to be present.

Following this classification scheme we are able to determine the spectral types with an accuracy of one subtype, unless stated otherwise in Sect. 3. We constrained the luminosity class of the stars using the surface gravity derived in Sect. 5. An overview of the spectral features described in this section is presented in Table 2 and the results of the spectral classification are listed in Table 3. In most cases the spectral classification agrees well with that provided in the literature by H97 and Hoffmeister et al. (2008).

thumbnail Fig. 4

Blue region traditionally used for OB-star classification (left) and red region (right) including Hα for the targets observed with X-shooter. The rest wavelengths of the fitted spectral lines for each star are indicated by the vertical dashed lines. The spectra include prominent diffuse interstellar bands (DIBs). The spectra have been clipped and smoothed to remove the residuals of the sky subtraction. B163, B331, and B337 are not included because they are severely extincted in this wavelength range.

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thumbnail Fig. 5

Calcium triplet region for each of the targets. The Ca ii triplet is clearly present in B337, B275, and B268; all targets exhibit Paschen absorption lines, some with a central emission component. The O i 8450 line is detected in B331, B243, and B275. The narrow features are either telluric or residuals from the subtraction of sky emission lines.

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thumbnail Fig. 6

K-band spectra of our X-shooter targets. The location of the Brγ line and CO bandhead emission are indicated. Brγ is in emission in B331, B243, B275 and B268. B163, B331, B243, B275 and B268 show CO bandhead emission. The narrow features in the spectrum are residuals of the sky subtraction procedure. The spectra were smoothed to reduce the noise in the continuum and clipped to remove residuals of the sky subtraction in Brγ.

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Individual sources

In this section we discuss the X-shooter spectra of the individual targets. In parentheses we list the updated spectral types corresponding to this work. The spectra are shown in full in Appendix A.

B111 (O4.5 V): this clearly is an early O-type spectrum; the He ii lines at 4200, 4541, and 4686 Å are strong, consistent with an O4.5 V type star. The spectrum does not show He i 4387 and He i 4471 is weak. The hydrogen Balmer (including Hα) and Paschen series lines are strongly in absorption. Using its NIR spectrum, H97 classified B111 as a kO3-O4 based on the He ii, N ii lines, and the lack of C iv. According to our K-band spectrum B111 is a kO5-O6 type star, based on the presence of He ii lines and weak, if any, C iv emission. The spectrum does not include any (nebular) emission lines.

B163 (kA5): owing to severe interstellar extinction, it is not possible to identify any photospheric features in the UVB arm of the X-shooter spectrum. Longward of ~8500 Å  the first Paschen line becomes detectable. The NIR spectrum also shows the Brackett series in absorption and no elements other than H are seen. Based on this spectrum we assign a mid-A spectral type. Two of the CO first overtone transitions are detected, indicating the presence of a circumstellar disk. The spectrum includes nebular emission lines in the hydrogen series, He i lines (e.g. 10 830 Å), and [N iii] 9069, 9532 Å. This points to an energetic ionising radiation field as expected given the location of B163 near the core of NGC 6618 (Fig. 1). According to our criteria B163 is a mYSO.

B164 (O6 Vz): the presence of He ii 4541, 4686, and the fact that these lines are stronger than He i 4387 and 4471 demonstrate that B164 is an O-type star. The ratio He ii 4200/He i (+ii) 4026 Å is near unity, which indicates a spectral type O6 V. Based on the ratio EW(He ii 4686)/EW(He ii 4542)> 1.10 we conclude that it belongs to the Vz class. From the K-band classification we conclude that B164 is a kO5-O6 type star. Based on the N iii and He i lines B164 was classified as kO7-O8 by H97. No emission lines are detected.

B215 (IRS15; B0-B1 V): no He ii lines are detected, which indicates that B215 must be of later spectral type than B0. The hydrogen lines of the Balmer, Paschen, and Brackett series are in absorption and the He i absorption lines are weak (e.g. 4922). The absence of Mg ii 4481 suggests that the spectral type is earlier than B1; therefore, we conclude that this star is of spectral type B0-B1. The K-band classification is consistent with kO9-B1. Nielbock et al. (2001) classified this object as an extreme class I source and Chini et al. (2006) pointed out that it is a ~26M star surrounded by an extensive remnant disk. The spectrum contains nebular emission lines in the hydrogen series, He i lines, and forbidden emission lines [N ii] 6548, 6584, [S ii] 6716, 6731, and [S iii] 9069, 9532. We also identify [O i] 6300 emission: a broad component (~100 km s-1) on top of a nebular component. By inspecting the 2D frames we conclude that the emission seen in Hα has a nebular origin. From the X-shooter spectrum alone we cannot conclude that this is a mYSO.

B243 (B8 V): the hydrogen lines of the Balmer series are in absorption. The strongest Balmer lines show a central emission component; Hα is strongly in emission. The Paschen series absorption lines are filled in with double-peaked emission lines. The Brackett series lines mainly exhibit a double-peaked emission component. Furthermore, the “auroral” O i 7774 and 8446 lines show prominent double-peaked emission. The Ca ii triplet is not present. The spectrum includes He i absorption lines (e.g. 4471, 5876, and 10 830); the ratio He i 4026/He i 4009 is around two. The He i 4471 and Mg ii 4481 line ratio is close to unity, which points towards spectral type B8. The JHK CCD indicates NIR excess, weak CO-bandhead emission is detected, and it presents [O i] 6300 emission. Therefore we conclude that B243 is a mYSO.

Table 3

Stellar properties derived from best-fit FASTWIND parameters and the SED fitting.

B253 (B3-B5): the spectrum displays strong and broad hydrogen absorption lines with a central (nebular) emission component. He i absorption lines are present (e.g. 4026, 4471, 4922, and 10 830 Å) with a central (nebular) emission component as well; the He i lines are most prominent in the blue part of the spectrum. No He ii lines are detected, nor is the C ii 4267 Å line; therefore, we conclude that B253 is a B3-B5 type star. The NIR CMD and CCD show no evidence of NIR excess. Its K-band spectrum is kB5 or later. Many forbidden emission lines are present (e.g. [O iii], [N ii], and [S ii]) which, together with the He i emission, indicate a high degree of ionisation of the surrounding H ii region.

B268 (B9-A0): the hydrogen Balmer and Paschen series are prominently in absorption and include a central nebular emission component. The Hα and Hβ profiles are filled in by a circumstellar emission component. The He i lines are very weakly present; the nebular emission component dominates the line profiles. The fact that He i 4471 and Mg ii 4481 have almost the same strength indicates that the spectral type is late B to early A. The Ca triplet lines show double-peaked emission and a red-shifted absorption component that reaches below the continuum and could indicate the presence of an accretion flow (Hartmann et al. 1994). We confirm the findings of H97 who classified B268 as a mYSO, based on the presence of CO bandhead emission and the Paδ line.

B275 (B7 III): Ochsendorf et al. (2011) performed a detailed spectral classification, resulting in spectral type B7 III. They concluded that B275 is a PMS star contracting towards the main sequence. The He i 4009 Å and C ii 4267 Å lines are very weak and when considering the Si ii 4128 Å to Mg ii 4481 Å ratio the spectral type becomes B6-B7. The O i 8446 Å and Ca triplet lines show pronounced double-peaked emission, as do several of the hydrogen lines on top of a photospheric absorption profile. Both first- and second-overtone CO emission are detected. The [O i] 6300 emission line is present. Together with double-peaked emission features and NIR excess, this points to the presence of a rotating circumstellar disk. B275 is a mYSO.

B289 (O9.7 V): the spectrum includes He ii 4686 and He ii 5411 absorption lines, but the presence of He ii 4200 is hard to confirm given the low signal-to-noise ratio. The He i (+ii) 4026 line is present, He i 4144 and 4387 are weak, and He i 4471 and 5876 are strong. In addition, it is possible to identify the C iii 4647/50/51 line complex with a similar strength as the He ii 4686 line, so that the spectral type is O9.7 V. From the K band we obtain a kO9-B1 spectral type. According to H97, B289 might have a NIR excess and be a late O star. The JHK CCD does not display evidence of NIR excess and there are no emission lines, therefore we cannot conclude that it is a mYSO. The 1.5 and 2.0 μm photometry by Nielbock et al. (2001) using MANIAC mounted at the ESO La Silla 2.2 m telescope shows IR excess. It is one of the sources surrounded by an IR-bright dusty disk as reported by Chini et al. (2005). The spectrum shows some weak nebular emission lines.

B311 (O8.5 Vz): the He i (+ii) 4026 and He i 4471 lines are almost equally strong and sharp. He iλλ 4144, 4387, and He iiλλ 4541, 4686 also show up in the spectrum. The C iii 4068/69/70 complex is in absorption together with Si iv 4089 and 4116. We classify this star as O8.5 Vz because the ratio EW(He ii 4686)/EW(He i 4471) > 1.10. From the NIR spectrum, H97 classified it as later than O9-B2 because it has He i in absorption and lacks N iii and He ii. Nielbock et al. (2001) and Chini et al. (2005) detected an IR-bright dusty disk in the N and Q bands. However, Povich et al. (2008) resolved a bow shock associated with it, which is responsible for the NIR emission. B311 is a main-sequence star according to our criteria. The result of the K-band classification agrees well with the visual spectral type (kO9-B1). Nebular emission is weakly present.

B331 (late-B): the UVB part of the spectrum is not detected due to the severe interstellar extinction towards this source. The red and NIR spectrum is dominated by strong emission lines. In the case of the hydrogen series lines these are superposed on a broad and shallow photospheric profile. According to Hoffmeister et al. (2008), B331 is a B2 V-type star based on its visual spectrum. No helium lines are detected in the NIR part of the spectrum, indicating that B331 is a late-B or early-A-type star. It exhibits Brγ, O i, CO bandhead emission, and several double-peaked emission lines, indicating the presence of a rotating circumstellar disk. The SED includes a NIR excess, making it a bonafide mYSO. The spectrum contains some weak nebular emission lines.

B337 (late-B): this object is deeply embedded so that it is not possible to detect the blue part of the spectrum. It does show the Paschen series in absorption with a narrow central nebular emission component. The Ca triplet lines show pronounced double-peaked emission and a blue absorption component remnant of the sky subtraction. The SED presented by H97 is consistent with a B5 V type star when correcting for the distance. No He i lines can be identified in the X-shooter spectrum pointing towards a late-B or early-A-type star. The NIR CCD indicates a NIR excess and the Ca ii triplet lines are in emission and double peaked; we do not detect CO bandhead emission. B337 is a mYSO.

4. Spectral energy distribution

We constructed the SED of the star by dereddening the X-shooter spectrum and the available photometric data extending into the mid-infrared wavelength range. We fit the slope of the SED in the photospheric domain (400–820 nm) to Castelli & Kurucz models (Kurucz 1993; Castelli & Kurucz 2004). As an initial guess, we took the associated Teff and log  g from the Castelli & Kurucz2 model corresponding to the spectral type reported in Sect. 3. We then cross-checked our choice of Teff and log  g with the results obtained in Sect. 5 and performed the necessary iterations. The final values of Teff and log  g, corresponding to the best fit Castelli & Kurucz models used, are shown in Fig. 7. We thereby constrain the visual extinction AV and the total to selective extinction RV = AV/E(BV) towards the objects, adopting the parametrisation of Cardelli et al. (1989). From comparing these SEDs to observed spectra a NIR excess should become apparent (Fig. 7).

The flux calibrated X-shooter spectra and the photometric data points are shown by the black lines and squares, respectively. The photometric data in the UBVRI bands were taken from Chini et al. (1980) who used the 123 cm telescope on Calar Alto and the 50 cm telescope on the Gamsberg in south-west Africa. These measurements have an accuracy of ±0.5 mag. We highlight that the apertures used for this study are so large that the measurements might be contaminated by close neighbours. For B215 and B331 we obtained the V-band magnitude from Hoffmeister et al. (2008), and for B111 from the AAVSO Photometric all sky survey (APASS)3. The JHK magnitudes were taken from the 2MASS survey (Skrutskie et al. 2006), which provides photometry for our objects with an accuracy better than 0.3 mag with the exception of B215 whose JHK photometry we obtained from Chini & Wargau (1998). The mid-IR magnitudes were measured with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope (Fazio et al. 2004) as part of the Spitzer Legacy Science Program GLIMPSE (Benjamin et al. 2003); the photometric accuracy achieved for our targets is better than 0.2 mag. The N and Q-band magnitudes (10.5 and 20 μm) were obtained (when available) from Nielbock et al. (2001) who observed M17 with the ground-based infrared camera MANIAC. The aperture sizes were 2′′ for B337, 3′′ for B331 and B275, 5′′ for B311, 7′′ for B215, and 10′′ for B289; the accuracy in their measurements varies from 10 to 30%. The dereddened spectra and photometry are shown in blue. The dashed grey line represents the Castelli & Kurucz model corresponding to the spectral type of the star. The temperature and log g corresponding to the best Castelli & Kurucz model are indicated in the bottom right corner; these values are not identical to the temperature and log g of the star, but correspond to the adopted Castelli & Kurucz model. The resulting extinction and stellar parameters are labelled as well.

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SEDs of our X-shooter targets. The dashed line shows the Castelli & Kurucz model, the blue line the dereddened X-shooter spectrum, and the solid black line the flux calibrated spectrum. The squares and diamonds give the observed and dereddened magnitudes, respectively. In the top part of the plots we indicate the spectral type of the star and the parameters of the corresponding Castelli & Kurucz model. In the bottom part of the plots we list the extinction parameters, and the radius and luminosity of the star resulting from our SED fit. The spectra of B163, B331, and B337 were clipped in order to show only the part of the spectrum used for the analysis.

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4.1. Extinction parameters

In order to obtain independent values of the total extinction AV and the total to selective extinction RV, we implemented a χ2 fitting algorithm. We first dereddened the X-shooter spectrum varying RV from 2 to 5.5 in steps of 0.1 and then fitted the slope of the Castelli & Kurucz model to that of the dereddened X-shooter spectrum in the photospheric domain to determine the total V-band extinction, AV. This allowed us to constrain RV for all sources, except for B163. For this star we lack spectral coverage at λ< 850 nm. As the infrared excess starts at 1000 nm, a spectral range that is too limited is available for constraining RV.

The obtained values for RV range from 3.3 to 4.7, and AV varies from ~6 to ~14 mag (Table 3). H97 observe a similar range, while Hoffmeister et al. (2008) found RV = 3.9 ± 0.2 for their sample. The latter authors argue that the extinction to M17 is best described by a contribution of foreground extinction (AV = 2 mag with RV = 3.1, the average Galactic value) plus an additional contribution produced by local ISM dust. We derive RV and AV for each individual sight line. The AV values that we obtained agree within the errors with those calculated by Povich et al. (2009) from the total hydrogen column density N(H).

The X-shooter spectra include strong diffuse interstellar bands (DIBs). Hanson et al. (1997) reported that their strength did not vary with AV (or E(BV)) as seen in other Galactic sightlines. Massive star forming regions are known to exhibit anomalous extinction properties (Dahlstrom et al. 2013; Oka et al. 2013; Ellerbroek et al. 2013a). We will report the DIB behaviour towards sight lines in M17 in a separate paper.

4.2. Stellar radius

To estimate the radius of the stars we scaled the observed flux to the flux produced by the stellar surface given by the Castelli & Kurucz model, using the distance to M17 (d = 1.98 kpc). The difference between the measured flux Fλ and the flux from the model FKur can be corrected for by multiplying by a factor (R/d)2, where R is the stellar radius and d is the distance to the Sun. We calculated the luminosity log L/L using the V-band magnitude from Chini et al. (1980), the distance to M17, the AV values from our fit (Sect. 4.1), and the bolometric correction corresponding to the spectral type from Cox (2000).

To assess the uncertainties in the parameters, we calculate the probability P that its χ2 value differs from the best-fit χ2 due to random fluctuations: P = 1−Γ(χ2/ 2/ 2), where Γ is the incomplete gamma function and ν the number of degrees of freedom. We normalise the χ2 such that the reduced χ2 corresponding to the best-fitting model has a value of unity and we select all models with P ≥ 0.32 as acceptable fits representing the 68% confidence interval. The finite exploration of the parameter space may result in an underestimation of the confidence interval near the borders of P(χ2) = 0.32. To avoid an underestimation of the errors we select as boundaries of the 68% confidence interval the first combination of parameters that do not satisfy P(χ2) ≥ 0.32 (see Tramper et al. 2014; Ramírez-Agudelo et al. 2017). As in the case of B275 (Ochsendorf et al. 2011), the obtained stellar radius of the mYSOs does not correspond to the value expected for a main-sequence star. This corroborates our earlier finding that these mYSOs indeed are massive PMS stars that are still contracting towards the main sequence. In Sect. 5 we show that this is consistent with the log g values independently measured from the broadening of the diagnostic lines.

5. Modelling the photospheric spectrum

To further constrain the temperature, luminosity, projected rotational velocity, and surface gravity of the stars we used an automatic fitting algorithm developed by Mokiem et al. (2005) and Ramírez-Agudelo et al. (2017), which compares the H i, He i, He ii, and N absorption lines with model profiles produced by the non-LTE stellar atmosphere code FASTWIND (Puls et al. 2005; Rivero González et al. 2012). For our PMS stars we did not fit the He and N abundances but fixed them to be consistent with the solar values. This is in agreement with what we would expect for such a young stellar population. This method applies the genetic fitting algorithm PIKAIA (Charbonneau 1995), which allows us to explore the parameter space in an extensive way (Mokiem et al. 2005; Tramper et al. 2014). FASTWIND calculates non-LTE line-blanketed stellar atmospheres and accounts for a spherically symmetric stellar wind. This code can be used to examine the dependence of H, He i, and He ii photospheric lines on Teff and log g. Its application to B stars has been successfully tested by Lefever et al. (2010).

Inputs to the fit are the absolute V-band magnitude (MV) and the radial velocity (RV) of the star. We calculated MV using the apparent magnitude reported by Chini et al. (1980) and the extinction coefficient AV found in Sect. 4 adopting a distance of 1.98 kpc. We measured the RV following the procedure described by Sana et al. (2013) where we simultaneously adjust the spectral lines for a given object, taking into account variations in the signal-to-noise ratio. The RV values vary from –11 to 20 kms-1 and are listed in Table 3; for B243, B268, and B289 we list the RV values obtained from the 2013, 2013, and 2012 observations, respectively. We calculated the RV dispersion using the standard deviation of a Gaussian distribution (σ1D) and we find the strikingly low value of ~5 km s-1. This is in contrast with the expectation that the massive star population in M17 includes many (close) binaries (e.g. Kouwenhoven et al. 2007; Sana et al. 2012; Dunstall et al. 2015). This points to a lack of short period binaries or a low binary fraction. A quantitative investigation of the low σ1D is presented in Sana et al. (2017).

Several atmospheric parameters are obtained from the fitting procedure. The effective temperature (Teff) can be constrained using the relative strength of the H i, He i, and He ii lines. The surface gravity (log g) is obtained from fitting the wings of the Stark-broadened H i lines. This parameter allows us to constrain the luminosity class (see Mokiem et al. 2005). The projected rotational velocity (vsini) is a natural outcome of the fitting procedure as the models are convolved with a rotational profile to reproduce the observed spectrum. The mass-loss rate is mainly determined by fitting the Hα line. We only obtain a reliable measurement for B111 (log  = −6.00 ± 0.1) and B164 (log  = −6.35 ± 3.65), the two hottest stars in our sample. The parameter describing the rate of acceleration of the outflow (β) cannot be constrained by our data and was therefore fixed to the theoretical value predicted by Muijres et al. (2012), i.e. β = 0.8 for main-sequence stars. Given the low mass-loss rates, the value adopted for β does not affect the model results. Another parameter in the fitting procedure is the microturbulent velocity; we allowed this parameter to vary from 5 to 50 kms-1, but due to the lack of Si lines in our spectra this parameter remains poorly constrained. This does not affect the determination of the other stellar parameters.

The bolometric luminosity (log L/L) is obtained by applying the bolometric correction to the absolute magnitude used as input. The luminosity and effective temperature are used to calculate the radius of the star R. Using log g we can calculate the spectroscopic mass of the star Mspec = gR2/G, where G is the gravitational constant. Given the uncertainty in log g it is very difficult to constrain Mspec as is evident from Table 3.

To calculate the errors in the parameters we follow the procedure described by Tramper et al. (2014) and Ramírez-Agudelo et al. (2017). The best-fitting model is selected based on the χ2 value in the same way as described in Sect. 4.2. The normalisation of the χ2 in this approach is only valid if the best fit is a good representation of the data, which we checked visually for each case (see Appendix B). We select all models with P(χ2) ≥ 0.05 as acceptable fits, representing the 95% confidence interval to the fitted parameters. The results from the fitting procedure are listed in Table 3; the temperatures obtained with FASTWIND are consistent with the spectral types derived in Sect. 3. The best-fitting model and other acceptable fits (5% significance level or higher models) are shown in Appendix B. The lines used for each fit are shown in Figs. B.1 to B.5; we give the same weight to all the lines in the fitting procedure.

It is important to point out that the confidence intervals cited in this paper represent the validity of the models and the errors on the fit. They do not include the contribution from systematic errors due to the model assumptions, continuum placement biases, etc. For a detailed analysis of the systematic errors and their impact on the parameter determination, see Ramírez-Agudelo et al. (2017). As can be seen in Table 3 the error bars differ significantly from star to star. Large error bars are obtained in two cases: first, when the parameter space is poorly explored near the border of the confidence interval and, therefore, the first model that does not satisfy P(χ2) = 0.05 is considerably outside the confidence interval; and, second, when the parameter is poorly constrained because of, for example, a low signal-to-noise ratio.

For B275 a good fit of the spectrum could not be obtained while leaving all parameters free. To mitigate this, we first constrained vsin i using a fit to only the helium lines. The obtained range of valid values was then used in a fit including the full set of diagnostic lines. Hβ was not included in this fit because of the presence of a very strong 4880 Å DIB blending the red wing of the line. This approach results in an acceptable fit to the spectrum, which is consistent with the results presented in Ochsendorf et al. (2011); the red wings of He i 6678 and 5875 are not well represented. The morphology of these lines and other He i lines could be an indication that this star is in a binary system; follow-up observations of this source are required to confirm or reject this hypothesis.

5.1. Comparison of the radius estimated by two methods

The radius of the star is estimated in two different ways following the procedures described in the previous sections. In Fig. 8 we show R obtained by fitting the SEDs to Castelli & Kurucz models versus that obtained via the genetic algorithm fitting. The diagonal line represents the one-to-one correlation and each symbol corresponds to one of our targets. B163, B331, and B337 were left out because for these stars it was not possible to identify any absorption lines, and therefore we did not include them in the GA fitting. The conclusion is that the two methods yield radii that are consistent within the errors.

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Comparison of the values for the stellar radius R obtained via SED fitting and GA analysis. The symbols and error bars correspond to the results obtained in Sects. 4 and 5; the dashed line represents the one-to-one correlation. The blue dots show the location of the targets for which we found evidence of a gaseous disk; the green triangles denote the stars that show IR excess longward of 2.5 μm and no emission lines in their spectra. The grey squares show the objects without disks. The values are consistent within the errors.

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5.2. Hertzsprung-Russell diagram

Figure 9 shows the theoretical Hertzsprung-Russell diagram (HRD) based on the values of Teff and L obtained in the previous sections. We plotted the PMS tracks from Hosokawa & Omukai (2009) with the ZAMS mass labelled and open symbols indicating lifetimes. We also present the birth line for accretion rates of 10-3, 10-4, and 10-5M yr-1. Assuming that the accretion is constant and that the Hosokawa tracks provide a correct description of the PMS evolution we conclude that 80% of our sample must have experienced an on average high accretion rate (10-3–10-4Myr-1) and the remaining stars an accretion rate of at least 10-5Myr-1.

All our confirmed PMS stars (blue circles) are located far away from the ZAMS and their positions can be compared with PMS tracks. The O stars (B111, B164, and B311; grey squares) and the stars without disk signatures (B289 and B215; green triangles) in the X-shooter spectra are located at or near the ZAMS. The position of the B-type star B253 is consistent with it being a PMS star, but its spectrum shows no signatures for the presence of a circumstellar disk.

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Hertzsprung-Russell diagram for the observed objects in M17. The blue dots show the location of the targets for which we found evidence of a gaseous disk; the green triangles denote the stars that show IR excess longward of 2.5 μm and no emission lines in their spectra. The grey squares show the objects without disk signatures. The stars for which the parameters are derived using FASTWIND are shown with filled symbols. The open symbols represent the stars for which log  L/L and Teff were derived from their spectral type. The ZAMS is represented by the thick black line and the solid lines correspond to PMS tracks from Hosokawa & Omukai (2009) with the ZAMS mass labelled and the lifetimes indicated as open symbols. The dotted, dashed, and dashed-dotted lines are the birth lines for accretion rates of 10-3, 10-4, and 10-5Myr-1, respectively. The grey lines indicate radii of 1, 10, and 30 R.

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6. Evidence of the presence of circumstellar disks

In this section we present an overview of the emission line features thought to be produced by the circumstellar disks. We discuss the nature of the disks of the PMS objects identified in the previous sections and measure the disk rotational velocity, Vdisk, using several hydrogen lines. In some of the objects we identify CO bandhead emission. The infrared excess observed in the SEDs (Sect. 4) provides information on the dust component of the disk.

We observe several double-peaked emission lines and/or CO bandhead emission in the visual to NIR spectrum in six of our targets: B163, B243, B268, B275, B331, and B337. A selection of double-peaked lines along the X-shooter wavelength range is shown in Figs. 10 to 14. The fact that we see double-peaked emission is indicative of a rotating circumstellar disk. For all of these objects we observe an infrared excess in their SEDs (Sect. 4). The atypical morphologies observed in the Balmer and Brackett series of B243 and B268 might be an indication of active accretion; nevertheless, further higher resolution observations are needed to confirm this scenario.

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Double-peaked emission lines in M17-B243. These profiles indicate the presence of a circumstellar rotating disk. The X-shooter spectrum is shown with the solid black lines and the two Gaussian functions fitted to the profiles are shown in red. The lines are plotted in the heliocentric frame and the dashed grey line represents the radial velocity of the star. Each line is labelled on the left side of the plots and the peak-to-peak separation is shown on the right side of the profiles.

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thumbnail Fig. 11

Same as Fig. 10 for M17-B268.

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thumbnail Fig. 12

Same as Fig. 10 for M17-B275.

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thumbnail Fig. 13

Same as Fig. 10 for M17-B311.

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thumbnail Fig. 14

Same as Fig. 10 for M17-B337.

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6.1. Velocity structure of the gaseous disks

To measure the characteristic projected rotational velocity properties of the gaseous component of the circumstellar disks, we selected a sample of double-peaked emission lines for each of the sources and fitted two Gaussian functions to measure the peak-to-peak separation. The projected rotational velocity measured in this way corresponds to half of the peak-to-peak separation. The Gaussian functions fitted to the lines are shown with the solid red lines in Figs. 10 to 14 and the results from these measurements are listed in Table 4. The Balmer, Paschen, and Brackett series and Ca ii triplet lines are plotted in each of the columns (from left to right) and lines are labelled on the left side of the plot. The measured peak-to-peak separation is indicated to the right.

Table 4

Projected disk rotational velocity measured from half the peak-to-peak separation of the lines originating in the circumstellar disk.

Figure 15 shows the disk projected velocities measured from each of the hydrogen lines against log λflu, where flu is the oscillator strength. We use log λflu as a measure of the relative strength of a given line within a line series. The oscillator strengths for the hydrogen lines were obtained from Goldwire (1968). For B163 and B337 we do not have sufficient velocity measurements to draw any conclusions. For four out of the six gaseous disks detected (B243, B268, B275, and B331) we see a clear trend of the projected disk rotational velocity from the hydrogen recombination lines with line strength. This suggests a structured velocity profile of the gaseous disks, which is in qualitative agreement with the prediction that the high excitation lines form in the inner region (dense and hot) of the disk while the low excitation lines form over a larger area (and on average more slowly rotating part) of the disk.

thumbnail Fig. 15

Projected disk rotational velocity, Vdisk, against oscillator strength for double-peaked lines along the wavelength coverage of X-shooter for the stars indicated in the upper right corner. The Paschen series with dashed-dotted lines (black) and the Brackett series with dotted lines (blue).

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B268 and B275 also exhibit Ca ii triplet and O i double-peaked emission lines. The velocities measured from these lines are higher than those of the hydrogen lines. This indicates that these lines are formed closer to the star than the hydrogen lines.

Assuming that the disks have a Keplerian velocity structure and adopting the mass of the central object, it is possible to roughly calculate the distance from the star at which the lines are formed. This results in a range from a few hundred to one thousand R (tens to a few hundreds R).

6.2. CO bandhead emission

CO overtone emission is produced in high-density (1010–1011 cm-3) and high-temperature (2500–5000 K) environments (Charbonneau 1995). CO is easily dissociated and therefore must be shielded from the strong UV radiation coming from the star. These conditions are expected in the inner regions of (accretion) disks, which makes the CO bandheads an ideal tool to trace the disk structure around mYSOs (e.g. Bik et al. 2006; Ilee et al. 2013). The shape of the CO lines can be modelled by a circumstellar disk in Keplerian rotation (Bik & Thi 2004; Blum et al. 2004; Davies et al. 2010; Wheelwright et al. 2010; Ilee et al. 2013). The blue shoulder in the bandheads is a measure of the inclination of the disk; an extended blue shoulder indicates a high inclination angle (i.e. near edge-on view).

We detect CO-bandhead emission in B163, B243, B268, B275, and B331 (see Fig. 6). Although CO-bandhead emission is rare, it is also seen in some B[e] supergiants (Lamers et al. 1998). However, these evolved B[e] stars show numerous forbidden emission lines in their optical spectra, while we only observe the well-known [O i] 6300 in some of the sources. Hence, the observed spectra support our hypothesis that the observed disks are related with the accretion process in contrast to a possible origin such as in (more evolved) B[e] stars (see Bik & Thi 2004). Modelling of the bandheads and the double-peaked hydrogen emission lines will be the subject of a forthcoming paper.

7. Discussion

7.1. Age distribution of the massive PMS population in M17

Age estimates of M17 show that its stellar population is not older than 1 Myr. There is no evidence for a supernova explosion in that area (Broos et al. 2007), which is consistent with such a young age. We estimated the age of the main-sequence stars by comparing their position in the HRD with Milky Way evolutionary tracks from Brott et al. (2011). We compare the position of our stars with evolutionary tracks that account for the effects of rotation considering a flat distribution of spin rates starting at the measured vsini. To do so, we used the Bayesian tool BONNSAI4 from Schneider et al. (2014). Within the uncertainties of the models, the ages obtained for B111, B164, and B311 are 1.60, 0.98, and 0.82 Myr, in fair agreement with the age estimates for the region (Table 5). As a way of testing our classification of the PMS stars, we estimated their ages using main-sequence tracks through BONNSAI. We obtained main-sequence ages ranging from 10 to 90 Myr, which would be inconsistent with M17 being a young region. Assuming that our classified PMS stars are part of M17, we concluded that these stars cannot be post-, but must be PMS objects.

We estimated the age of the PMS stars by comparing their effective temperatures and luminosities with PMS tracks (Hosokawa & Omukai 2009). For these stars we find an age span from tens of thousands of years to a few hundred thousand years.

If we compare the position of the (presumable) main-sequence stars B215, B253, and B289 to isochrones, we obtain age estimates of ~9, ~50, and ~4 Myr, respectively. B253 does not present emission lines nor IR excess in its SED, and B215 and B289 do not present emission lines in the X-shooter spectrum but have IR excess longward of 2.3 μm. If these sources are PMS stars this suggests that their circumstellar disks have already (at least partially) disappeared (see also Sect. 7.2).

Table 5

Age of our targets estimated by comparing their position in the HRD with PMS tracks from Hosokawa & Omukai (2009, third column) and main-sequence tracks from Brott et al. (2011, fourth column).

Given the size of our sample and the uncertainties in the models we cannot draw a firm conclusion about the age of M17 nor about the possible presence of two distinct populations in this region (Povich et al. 2009). Nevertheless we observe a trend of age with luminosity: the less luminous objects are further away from the ZAMS than their more luminous counterparts. This is in line with the idea (McKee & Tan 2003; Zinnecker & Yorke 2007; Davies et al. 2011) that more massive stars contract faster and therefore spend less time on the PMS tracks than lower mass stars.

7.2. Extinction towards the PMS stars in M17

Table 3 lists the extinction properties of our targets. We find that the extinction in the V band varies from ~5 to ~14 mag and that the total to selective extinction is 3.3 <RV< 5. The two sources with the highest extinction are situated near or in the irradiated molecular cloud (see Fig. 1), but there is no general trend with location in the H  ii region. The sources with higher AV tend to have larger values of RV, although the correlation is weak. The overall conclusion is that the extinction is quite patchy with substantial variation on a spatial scale of 50 arcsec (corresponding to a geometrical scale of 0.5 pc at the distance of M17), similar to the findings of H97. A dust disk local to the star, for example, may dominate the line-of-sight extinction towards the sources (see Ellerbroek et al. 2013a). Studies of individual sources therefore should not rely on average properties of the region, but should be based on a detailed investigation of the extinction in the line of sight.

7.3. Presence of circumstellar disks around massive PMS stars

We detect signatures of circumstellar disks in six of our sources: B163, B243, B268, B275, B331, and B337. The full sample includes two O-type stars, B111 and B164, which are not classified as potential YSO sources by H97, but as main-sequence objects, and do not reveal disk signatures. Two stars, B215 (IRS15) and B289, do not show evidence of gaseous disk material in their spectral lines but feature excess infrared continuum emission that is indicative of dust in the circumstellar environment. We thus find that 60% of our massive YSO candidates show clear evidence for circumstellar disks in their spectrum and another 20% likely also feature disks based on NIR excess emission.

In Fig. 9 we labelled the sources with a gaseous disk detectable in the X-shooter spectrum with a blue dot, those with only NIR excess in the SED with a green triangle, and the O and B stars with a grey square. The stars with gaseous disks are located further away from the ZAMS than the other sources. However, we cannot link this in a straightforward way to an evolutionary effect. Having noted this, and taking into account that our sources with non-detectable disks are younger than 105 yr, we can conclude that massive stars up to ~20 M retain disks up to less than 105 yr upon arrival on the ZAMS. We identify the presence of [O i] 6330 emission in three sources (B215, B243, and B275). In Herbig Ae/Be this line is mostly formed in stars with a flaring disk (van der Plas et al. 2008). The spectral profiles of some lines in B268 and B243 might be an indication that this star is actively accreting. For the other sources, as we do not find strong evidence of infall or the presence of jets, we do not know whether these sources are actively accreting or that the observed disks are structures remnant of the formation process.

7.4. Spin properties of the massive PMS stars in M17

The projected rotational velocity (vsini) distribution for 216 O stars in 30 Dor has been published by Ramírez-Agudelo et al. (2013). Their distribution shows a two-component structure consisting of a peak at 80 kms-1 with 80% of the stars having 0 <vsini< 300kms-1 and a high velocity tail (containing 20% of the stars) extending up to 600 kms-1. Dufton et al. (2013) studied 300 stars spanning spectral types from O9.5 to B3 in 30 Dor; they find a bimodal distribution with 25% of the stars with 0 <vsini< 100kms-1 and a high velocity tail between 200 <vsini< 350kms-1. They estimated vsini using a Fourier transform method, which allows them to separate the rotational broadening from other broadening mechanisms. Penny & Gies (2009) measured vsini for 97 OB stars in the Milky Way. These authors find that 80% of their sample rotate slower than 200 kms-1 and that the remaining 20% has 200 <vsini< 400kms-1. Table 3 shows the vsini measured for the stars in our sample. We find that around 30% of our sample is rotating relatively fast (around 200 kms-1).

We show the cumulative vsini distribution functions of these works together with our results in Fig. 16. To quantitatively compare these distributions we performed a Kuiper test, which allows us to test the null hypothesis that two observed distributions are drawn from the same parent distribution. The significance level of the Kuiper statistic, pK, is a percentage that indicates the similarity of the compared distributions. Small values of pK show that our cumulative distribution is significantly different from that it is compared to. The pK values obtained from comparing our distribution with those of Penny & Gies (2009), Ramírez-Agudelo et al. (2013), and Dufton et al. (2013) are pK = 13%, 99%, and 18%, respectively. As these values are not lower than 10%, they do not allow us to reject the null hypothesis.

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Comparison of the cumulative distribution of projected rotational velocities of our sample (purple, 9 stars); Dufton et al. (2013; blue, 251 stars); Ramírez-Agudelo et al. (2013; green, 216 stars); and Penny & Gies (2009; red, 106 stars). The dashed line indicates the distribution of projected rotational velocities of our stars upon arrival at the ZAMS assuming rigid rotation (analogous to homologous contraction) and angular momentum conservation.

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Upon arrival on the ZAMS our sample will span a roughly similar range of masses as the B-star sample in 30 Dor (Dufton et al. 2013). Therefore, it is most appropriate to compare our findings to this sample. According to the models by Brott et al. (2011) and Ekström et al. (2012) the vsini distribution of stars in our mass range will not change significantly during the first few Myr of evolution. Dufton et al. (2013) point out that macroturbulent motions only need to be taken into account in cases of relatively slow spinning stars (vsini < 80 km s-1). Dufton et al. (2013) discuss the possibility of their presumably single star sample to be polluted by (relatively rapidly spinning) post-interaction binaries. We estimated the vsini that our PMS stars will have upon arrival on the ZAMS using the ZAMS radii corresponding to the end of the PMS tracks from Hosokawa & Omukai (2009). We applied angular momentum conservation and assumed that the stars are rigidly rotating, which is analogous to homologous contraction. When comparing this vsini distribution (dashed line in Fig. 16) with the B-star sample in 30 Dor we find that pk = 87%. Assuming that our stars represent a progenitor population of the LMC B-star sample studied by Dufton et al. (2013), and neglecting metallicity effects and pollution from post-interaction systems in the B stars in 30 Dor sample, we find that the contraction of the PMS stars during the main sequence is consistent with being homologous. Of course, it is premature to firmly state this given the small sample size and the caveats mentioned above.

8. Conclusions

We performed VLT/X-shooter observations of young OB stars in M17. We classified and modelled the photospheric spectra via FASTWIND to derive their stellar parameters. We identified the presence of gaseous and dusty disks in some of them based on the emission lines in the spectrum and on the IR excess observed in the SED. We confirm the PMS nature of six objects in this region and conclude that they are on their way to becoming B main-sequence stars with masses ranging from 6 to 20 M. This constitutes a unique sample of PMS stars that allows us to test theoretical star formation models. Our findings can be summarised as follows:

  • We confirm the PMS nature of six of the mYSO candidatespresented by H97. We conclude that most of our objects must haveexperienced, on average, high accretion rates.

  • The age of the PMS objects was obtained by comparing their position in the HRD with PMS tracks of Hosokawa & Omukai (2009), whereas the ages of the OB stars were estimated using the tool BONNSAI (Schneider et al. 2014) and comparing them to tracks of Brott et al. (2011). For the O stars we obtained ages younger than 2 Myr. The PMS stars have estimated lifetimes of a few hundred thousand years. Given the uncertainty in the age and the fact that we have only a few stars, we cannot conclude that the PMS population corresponds to a second generation of star formation in M17 but found no indication in favour either.

  • We measured the visual and total to selective extinction towards our objects by fitting Castelli & Kurucz models corresponding to the spectral types. We confirm that the extinction towards M17 is highly variable, as usually observed in star forming regions. We point out that a dust disk local to the star may dominate the line-of-sight extinction towards the sources. Based on the NIR excess (>2 μm) observed in the SEDs, we found dusty disks in eight of our targets.

  • We found evidence of gaseous circumstellar disks in six of our targets, all of which also include a dust component, via double-peaked emission lines. We measured the projected rotational velocity of the disks from each of the double-peaked lines and found a structured velocity profile among the hydrogen recombination lines for four out of the six disks. For two out of the six disks we were able to identify Ca ii triplet and O i double-peaked emission lines. The velocities measured from these lines are larger than from those measured from hydrogen suggesting that they are formed closer to the star than the hydrogen lines.

  • We measured the projected rotational velocities, vsini, of our stars. About 30% of our sample rotate at around 200 kms-1 or faster. The PMS objects are expected to contract and therefore have spun up upon arrival on the main sequence. Assuming homologous contraction and the absence of processes causing angular momentum loss in their remaining PMS evolution, the vsini distribution of our sample appears to be consistent with that of the B stars in 30 Dor once they ignite hydrogen. We note that the contraction in PMS stars is not well understood. Two of the objects will have a vsini > 300km s-1 upon arrival on the main sequence.

With this unique, although still small, sample we show the potential for constraining models of early stellar evolution. A larger sample is needed in order to robustly assess the validity of different theories.


4

The BONNSAI web-service is available at www.astro.uni-bonn.de/stars/bonnsai

Acknowledgments

We thank the anonymous referee for carefully reading the manuscript and many helpful and insightful comments and suggestions. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. This research made use of Astropy, a community-developed core Python package for Astronomy (Astropy Collaboration, 2013). MCRT is funded by the Nederlandse Onderzoekschool Voor Astronomie (NOVA).

References

Appendix A: Full X-shooter spectra

thumbnail Fig. A.1

Normalised VLT/X-shooter spectra of the objects studied in this paper (Table 1), combining the UVB, VIS, and NIR arms. Line identifications are indicated above the spectrum. We point out the presence of telluric absorption features centred at 690, 720, 820, 920, 1400, 2010 and 2060 nm. The attenuated region between the H and the K band (1830–1980 nm) has been omitted. The blue part of the spectrum for B163, B331, and B337 is not visible due to severe interstellar extinction.

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Appendix B: FASTWIND fitting results

In this appendix we show in detail the model fits for our targets and the spectral lines used in each of the cases.

thumbnail Fig. B.1

Observed spectra (black dots), the best-fitting model (red), and other acceptable fits (5% significance level or higher; green) for B111 and B164. The vertical dashed lines indicate the spectral range used for the fit and the horizontal black line gives the position of the continuum.

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thumbnail Fig. B.2

Same as Fig. B.1 for B215 and B243.

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thumbnail Fig. B.3

Same as Fig. B.1 for B253 and B268.

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thumbnail Fig. B.4

Same as Fig. B.1 for B275 and B289.

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thumbnail Fig. B.5

Same as Fig. B.1 for B311.

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All Tables

Table 1

Journal of X-shooter observations.

Table 2

Summary of the properties of the sources observed with X-shooter.

Table 3

Stellar properties derived from best-fit FASTWIND parameters and the SED fitting.

Table 4

Projected disk rotational velocity measured from half the peak-to-peak separation of the lines originating in the circumstellar disk.

Table 5

Age of our targets estimated by comparing their position in the HRD with PMS tracks from Hosokawa & Omukai (2009, third column) and main-sequence tracks from Brott et al. (2011, fourth column).

All Figures

thumbnail Fig. 1

Left: NIR colour composite image of M17 based on 2MASS data: J (blue), H (green), and K (red). Our X-shooter targets are labelled: in black we show the objects that have no indication of a disk, in green the objects that show NIR excess only longward of 3 μm in the photometry (points to a dusty disk), in blue the objects with emission lines in the X-shooter spectrum and NIR excess (identifying them as mYSOs), and in red the post-AGB star B358; B311 is associated with the bow shock M17-S1 (Povich et al. 2008). The central cluster NGC 6618 includes several dozen OB-type stars that ionise the surrounding region. At the western edge of the H ii region several massive YSOs were identified. Apparently, massive star formation is continuing in the irradiated molecular cloud. Right: mid-infrared colour composite image of M17 based on Spitzer and Herschel: IRAC1 – 3.6 μm (blue), IRAC4 – 8.0 μm (green), and PACS – 70 μm (red). Here the distribution of the heated dust (blue) becomes apparent along with the PAH emission within the H ii region (green).

Open with DEXTER
In the text
thumbnail Fig. 2

NIR colour-magnitude diagram for our targets in M17. The objects for which we confirm their mYSO nature are denoted by blue circles, the stars with NIR excess only longward of 2.5 μm, but without emission lines, are indicated with green triangles, and the O and B main-sequence stars are indicated with grey squares. The red star corresponds to B358, which is most probably a background post-AGB star. The location of the zero-age main sequence is shown for a distance of 1.98 kpc with a solid black line (H97; Cox 2000). In grey we overplot the updated parameters of O V stars according to Martins & Plez (2006). The dashed line represents the reddening line for an O3 V star with RV = 3.1. Some of the mYSO candidates are located above the reddening line indicating the presence of a NIR excess.

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In the text
thumbnail Fig. 3

NIR colour-colour diagram for our targets in M17. The colours and symbols are the same as in Fig. 2. The dashed lines represent the reddening lines for an O3 V and an M0 III star. For this we took the average Galactic value of RV = 3.1. In Sect. 4 we show that RV varies from 3.3 to 4.6. The solid line indicates the unreddened main sequence. Most confirmed mYSOs show evidence for the presence of a NIR excess.

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In the text
thumbnail Fig. 4

Blue region traditionally used for OB-star classification (left) and red region (right) including Hα for the targets observed with X-shooter. The rest wavelengths of the fitted spectral lines for each star are indicated by the vertical dashed lines. The spectra include prominent diffuse interstellar bands (DIBs). The spectra have been clipped and smoothed to remove the residuals of the sky subtraction. B163, B331, and B337 are not included because they are severely extincted in this wavelength range.

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In the text
thumbnail Fig. 5

Calcium triplet region for each of the targets. The Ca ii triplet is clearly present in B337, B275, and B268; all targets exhibit Paschen absorption lines, some with a central emission component. The O i 8450 line is detected in B331, B243, and B275. The narrow features are either telluric or residuals from the subtraction of sky emission lines.

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In the text
thumbnail Fig. 6

K-band spectra of our X-shooter targets. The location of the Brγ line and CO bandhead emission are indicated. Brγ is in emission in B331, B243, B275 and B268. B163, B331, B243, B275 and B268 show CO bandhead emission. The narrow features in the spectrum are residuals of the sky subtraction procedure. The spectra were smoothed to reduce the noise in the continuum and clipped to remove residuals of the sky subtraction in Brγ.

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In the text
thumbnail Fig. 7

SEDs of our X-shooter targets. The dashed line shows the Castelli & Kurucz model, the blue line the dereddened X-shooter spectrum, and the solid black line the flux calibrated spectrum. The squares and diamonds give the observed and dereddened magnitudes, respectively. In the top part of the plots we indicate the spectral type of the star and the parameters of the corresponding Castelli & Kurucz model. In the bottom part of the plots we list the extinction parameters, and the radius and luminosity of the star resulting from our SED fit. The spectra of B163, B331, and B337 were clipped in order to show only the part of the spectrum used for the analysis.

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In the text
thumbnail Fig. 8

Comparison of the values for the stellar radius R obtained via SED fitting and GA analysis. The symbols and error bars correspond to the results obtained in Sects. 4 and 5; the dashed line represents the one-to-one correlation. The blue dots show the location of the targets for which we found evidence of a gaseous disk; the green triangles denote the stars that show IR excess longward of 2.5 μm and no emission lines in their spectra. The grey squares show the objects without disks. The values are consistent within the errors.

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In the text
thumbnail Fig. 9

Hertzsprung-Russell diagram for the observed objects in M17. The blue dots show the location of the targets for which we found evidence of a gaseous disk; the green triangles denote the stars that show IR excess longward of 2.5 μm and no emission lines in their spectra. The grey squares show the objects without disk signatures. The stars for which the parameters are derived using FASTWIND are shown with filled symbols. The open symbols represent the stars for which log  L/L and Teff were derived from their spectral type. The ZAMS is represented by the thick black line and the solid lines correspond to PMS tracks from Hosokawa & Omukai (2009) with the ZAMS mass labelled and the lifetimes indicated as open symbols. The dotted, dashed, and dashed-dotted lines are the birth lines for accretion rates of 10-3, 10-4, and 10-5Myr-1, respectively. The grey lines indicate radii of 1, 10, and 30 R.

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In the text
thumbnail Fig. 10

Double-peaked emission lines in M17-B243. These profiles indicate the presence of a circumstellar rotating disk. The X-shooter spectrum is shown with the solid black lines and the two Gaussian functions fitted to the profiles are shown in red. The lines are plotted in the heliocentric frame and the dashed grey line represents the radial velocity of the star. Each line is labelled on the left side of the plots and the peak-to-peak separation is shown on the right side of the profiles.

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In the text
thumbnail Fig. 11

Same as Fig. 10 for M17-B268.

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In the text
thumbnail Fig. 12

Same as Fig. 10 for M17-B275.

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In the text
thumbnail Fig. 13

Same as Fig. 10 for M17-B311.

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In the text
thumbnail Fig. 14

Same as Fig. 10 for M17-B337.

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In the text
thumbnail Fig. 15

Projected disk rotational velocity, Vdisk, against oscillator strength for double-peaked lines along the wavelength coverage of X-shooter for the stars indicated in the upper right corner. The Paschen series with dashed-dotted lines (black) and the Brackett series with dotted lines (blue).

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In the text
thumbnail Fig. 16

Comparison of the cumulative distribution of projected rotational velocities of our sample (purple, 9 stars); Dufton et al. (2013; blue, 251 stars); Ramírez-Agudelo et al. (2013; green, 216 stars); and Penny & Gies (2009; red, 106 stars). The dashed line indicates the distribution of projected rotational velocities of our stars upon arrival at the ZAMS assuming rigid rotation (analogous to homologous contraction) and angular momentum conservation.

Open with DEXTER
In the text
thumbnail Fig. A.1

Normalised VLT/X-shooter spectra of the objects studied in this paper (Table 1), combining the UVB, VIS, and NIR arms. Line identifications are indicated above the spectrum. We point out the presence of telluric absorption features centred at 690, 720, 820, 920, 1400, 2010 and 2060 nm. The attenuated region between the H and the K band (1830–1980 nm) has been omitted. The blue part of the spectrum for B163, B331, and B337 is not visible due to severe interstellar extinction.

Open with DEXTER
In the text
thumbnail Fig. B.1

Observed spectra (black dots), the best-fitting model (red), and other acceptable fits (5% significance level or higher; green) for B111 and B164. The vertical dashed lines indicate the spectral range used for the fit and the horizontal black line gives the position of the continuum.

Open with DEXTER
In the text
thumbnail Fig. B.2

Same as Fig. B.1 for B215 and B243.

Open with DEXTER
In the text
thumbnail Fig. B.3

Same as Fig. B.1 for B253 and B268.

Open with DEXTER
In the text
thumbnail Fig. B.4

Same as Fig. B.1 for B275 and B289.

Open with DEXTER
In the text
thumbnail Fig. B.5

Same as Fig. B.1 for B311.

Open with DEXTER
In the text

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