Free Access
Issue
A&A
Volume 614, June 2018
Article Number A116
Number of page(s) 12
Section Galactic structure, stellar clusters and populations
DOI https://doi.org/10.1051/0004-6361/201731720
Published online 22 June 2018

© ESO 2018

1 Introduction

In the last decade, the study and discovery of young and massive star clusters have received a vital boost, thanks to near-infrared all-sky and large-scale photometric surveys. These surveys have

revealed regions of the Milky Way obscured by intense and variable interstellar extinction, unveiling the massive stars contained in deeply embedded clusters. The catalogues of cluster candidates detected using the near-infrared surveys 2MASS (Skrutskie et al. 2006), GLIMPSE (Benjamin et al. 2003), UKIDSS-GPS (Lucas et al. 2008), and Vista-VVV (Minniti et al. 2010; Saito et al. 2012) have reported close to 2000 objects. These catalogues have helped to improve the census of massive clusters (objects with total mass MT >104 M), but it remains incomplete; according to Hanson & Popescu (2008) and Ivanov et al. (2010) more than 100 massive clusters are expected to be part of the Milky Way. Fewer than 20 massive clusters have been reported in our Galaxy.

Cluster candidate catalogues are mostly based on the detection of stellar over-densities and are biased towards certain kind of objects. Therecent results derived from the VVV survey include catalogues focus on very young populations (less than 5 Myr) whose main-sequence stars are still deeply embedded (Barbá et al. 2015), and less extinct objects characterized by a stellar over-density detected by eye (Borissova et al. 2011, 2014) or by automated detection procedures (Solin et al. 2014; Ivanov et al. 2017). The confirmation of candidates as true objects is possible after spectroscopic or astrometric follow-up. In the case of the Borissova et al. (2011) catalogue, the spectroscopic data has revealed clusters with Wolf−Rayet stars (Chené et al. 2013, 2015), OB-type dwarfs (Ramírez Alegría et al. 2014a, 2016), or young stellar objects (Borissova et al. 2016)as part of their stellar population.

The MAssive Stars in Galactic Obscured MAssive clusterS (MASGOMAS) is an initiative to look for massive clusters by identifying over-densities of OB-star candidates as young stellar cluster candidates in the 2MASS and other IR catalogues. In the initial stages of the project, using a set of near-infrared photometric cuts (magnitude KS < 12.0, colour (JKS) > 1.0 and the reddening-free parameter QIR between –0.2 and 0.2, see Comerón & Pasquali 2005; Negueruela & Schurch 2007, and Sect. 2), we discovered one massive cluster in the direction of the close end of the Galactic Bar (Masgomas-1; Ramírez Alegría et al. 2012) and one cluster with two cores of massive star formation (Masgomas-4; Ramírez Alegría et al. 2014b).

As photometric cuts select OB-type stellar candidates with some level of reddening (limiting to (JKS) > 1.0 to avoid foreground contamination), MASGOMAS clusters are young (less than 10 Myr) and not very embedded objects, compared for example with the candidates from the (Barbá et al. 2015) catalogue. The stellar population in the MASGOMAS clusters is principally found in the main sequence. It may be affected by differential reddening, but can be distinguished from the field stellar population by comparing the cluster and control photometric diagrams (colour–magnitude, colour–colour, and QIR–magnitude).

As we explore deeper using near-infrared data, crowding and chance alignment are factors to be considered in the search of cluster candidates. Kinematic information, complemented with near-infrared photometry and spectroscopic follow-up for the candidates’ most probable star members are useful tools for separating the entangled stellar populations of the aligned clusters and for identifying the asterism in the catalogues.

In this article we report the discovery of a massive star population around two massive stars: the Wolf−Rayet WR122-11(also known as WR1583-B73; Faherty et al. 2014) and the blue supergiant/LBV candidate IRAS 18576+0341 (Ueta et al. 2001; Pasquali & Comerón 2002). We present the first candidates derived with the MASGOMAS automatic searching software in Sect. 2, the near-infrared spectrophotometric data in Sect. 3, followed by the spectral classification and the analysis of the massive star populations confirmed by the spectroscopy (Sects. 4 and 5).

2 Candidate discovery

The MASGOMAS project focuses on the search of over-densities formed by OB-type star candidates and the characterization of these candidates using near-infrared photometry and spectroscopy. We select the OB-type candidates using three photometric criteria: KS magnitudes less than 12 mag (a limit adopted by the instrument used in our spectroscopic follow-up), red (JKS) > 1.0 colours, and a reddening-free parameter QIR between –0.2 and 0.2 (Ramírez Alegría et al. 2012). The adopted QIR parameter is (1)

using the Rieke extinction law (Rieke et al. 1989), with R = 3.09 (Rieke & Lebofsky 1985)1.

Limiting QIR to between –0.2 and 0.2, we clean the photometry from disc giant stars and focus our search on OB-type dwarfs (with QIR ~ 0). The main expected contaminants from our method are A and early F dwarfs, although photometric errors or extinction law variations may shift some late-type stars into this zone. The photometric diagrams for the massive cluster Masgomas-1 (Ramírez Alegría et al. 2012)and the double-core young cluster Masgomas-4 (Ramírez Alegría et al. 2014b) support the use of this law to describe the extinction in this Galactic direction. After this first stage, when we had detected cluster candidates as over-densities by eye, we developed an automatic searching software based on AUTOPOP (Garcia et al. 2009, 2011) and adapted to use 2MASS photometry.

The original AUTOPOP algorithm consists of two main routines: an automatic finding of stellar groups and an isochrone analysis over those groups. Using the photometric criteria to select OB-type candidates, we used the first routine to detect over-densities of OB-type candidates. This routine, based on a friends-of-friends method, considers two stars as part of a group if their separation is less than a search distance DS. To be considered a cluster candidate, the number of members in the group, Nmem, must be higher than a threshold Nmin. For the test version of the algorithm we arbitrarily set the values of DS = 60′′ and Nmin = 10. The first test was focused on abox region of 3° × 3° and centred around l = 38° and b = 0°. This search reported two candidates with an interesting number of member candidates: Masgomas-5 (Nmem = 15; also known as Juchert 3, Kronberger et al. 2006), and the new candidate selected for spectroscopic follow-up Masgomas-6 (Nmem = 12). The cluster candidate parameters derived from the automatic algorithm are given in Table 1.

In Fig. 1 we present Masgomas-6 in a false colour GLIMPSE mid-infrared image. In the mid-infrared it is easy to observe an extended nebulosity surrounding the candidate. The extension of this nebulosity is larger than the estimated radius for Masgomas-6, and as we demonstrate later, with the information derived from the observed spectra, the distribution of the stellar population goes beyond the first estimation of the cluster candidate radius.

After the discovery of the cluster candidate, we reviewed the literature and found two objects, inside an area of 4′ around the centre of Masgomas-6, that can suggest the presence of a population of massive stars: the Wolf−Rayet WR122-11 (Faherty et al. 2014) and the blue supergiant/LBV candidate IRAS 18576+0341 (Ueta et al. 2001; Pasquali & Comerón 2002). There are no previous reports of a cluster hosting these two massive objects.

Table 1

Summary of Masgomas objects detected during different project phases.

thumbnail Fig. 1

GLIMPSE false colour (blue = IRAC I1, green = IRAC I3, red = IRAC I4) image for Masgomas-6. The extension ofthe cluster candidate derived from the automatic search algorithm is shown (solid blue circle). The areas used for decontamination are also shown: cluster field (dashed white circle) and control field (dashed green circle). We markedthe stars spectrally classified as early-type (dwarf, giant, or supergiant) and the evolved massive stars (Wolf−Rayets) with small green boxes, the late-type giants with red circles, the star without classification (star 14) with a black square. The length of the orientation arrows is 1.0 arcmin.

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3 Observational data

After detecting Masgomas-6 as a cluster candidate using the 2MASS photometry, we completed a near-infrared spectroscopic follow-up for the brightest most probable stellar members with LIRIS (at the William Herschel Telescope, Roque de Los Muchachos Observatory, La Palma; Manchado et al. 2004). For the selection of spectroscopic candidates and the photometricanalysis of the candidate we used the photometry from the UKIDSS Galactic Plane Survey (GPS; Lucas et al. 2008).

3.1 Photometry

The UKIDSS GPS is a near-infrared public legacy survey, completed with the Wide Field Camera (WFCAM) on the United Kingdom Infrared Telescope (UKIRT). The survey covered the regions between 141 < l < 230, − 5 < b < 5; 15 < l < 107, − 5 < b < 5; and − 2 < l < 15, − 2 < b < 2. The survey includes observationsin the J, H, and K bandpasses; the K bandpass is observed in two different epochs. A detailed description of the UKIDSS GPS data and its attributes (or parameters, including photometric flags), available through the WFCAM Science Archive (WSA) webpage is given by Lucas et al. (2008).

The UKIDSS GPS survey includes two flags particularly helpful for describing data quality: mergedClass and ppErrBits. The first flag helps to discriminate elongated from rounded sources. Using only sources with mergedClass = −1 or − 2, we only include photometry from objects whose probability of being stars is larger than 70%2. For the ppErrBits flag, which quantifies how reliable the photometry is, 98% of the sources used in our analysis have the highest quality flag (i.e. ppErrBits < 256)3.

Our saturation limits were J = 13.50, H = 12.00, and K = 11.00 mag. These limits are slightly higher than suggested in the main GPS paper (H = 12.25 and K = 11.50 mag). We observed a good behaviour of the sources in these limits after inspection of the associated error and the ppErrBits flags. We replaced the individual saturated photometry with the corresponding 2MASS photometry, transformed to the UKIDSS bandpass system using the photometric transformation given by Lucas et al. (2008), and assuming a standard value of E(BV) = 3.

Taking advantage of the better spatial resolution and depth, compared with the 2MASS photometry, we selected OB-type candidates using UKIDSS GPS photometry and the same strategy of photometric criteria used to detect the cluster candidates. In Figs. 2 and 3 we show the colour–magnitude, free-reddening parameter–magnitude (QIRK), and colour–colour diagrams for Masgomas-6. The stars selected for spectroscopic follow-up are marked with red and green numbers, using the identification numbers from Table A.1. In the diagrams, we present the photometric information from the cluster and control fields shown in Fig. 1. In the QIRKS diagram we see a vertical sequence of stars centred around QIR = 0, which are not present in the control field. We interpret these object as candidates for the cluster OB-type population.

thumbnail Fig. 2

Colour–magnitude (left) and free-reddening parameter–magnitude QIR (right) diagrams for Masgomas-6. Black dots show the position of the most probable cluster members, and grey symbols show the position of the field stars. We include in the figure the spectroscopically confirmed massive stars with green open squares and the late-type giant stars with red open circles. The individual identification numbers (ID, Table A.1) are also shown above each coloured symbol. In the pseudocolour–magnitude diagram, we can see that most of the massive stars are found between QIR values of –0.2 and 0.2, as expected from our selection method.

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thumbnail Fig. 3

Colour–colour diagram for Masgomas-6. Black dots show the position of the most probable cluster members, and grey symbols show the position of the field stars. Objects with spectroscopic data are shown: massive stars (in green) and late-type giants (in red). In the figure we included the projection, following the extinction law by Rieke et al. (1989) (with R = 3.09; Rieke & Lebofsky 1985), for an O giant (blue solid line; intrinsic colour by Martins & Plez 2006) and a G5 giant (red dashedline; intrinsic colour by Cox 2000). The stellar population located above the red-dashed line corresponds mostly to the disc giant population and is observed only in the grey-field stars. The massive star population, confirmed spectroscopically, is observed close to the blue solid line and below the giant sequence.

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thumbnail Fig. 4

Individual H band (left) and K band (right) spectra for massive stars. Labels indicate the object’s ID (H-band spectra) and spectral type (K band spectra). This set includes early-type stars with luminosity classes I, III, and V, and two Wolf−Rayets. The spectral features used for the spectral classifications are labelled in grey. The bottom red spectrum plotted over spectrum29 corresponds to WR62-2, a WN8-9h object (Chené et al. 2015) shown for visual comparison with the new Wolf−Rayet star WR122-16. The red supergiant spectrum is shown in Fig. 5.

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thumbnail Fig. 5

Individual H band (left) and K band (right) spectra for late-type stars. In this figure we include objects with luminosity class III or V, the red supergiant star 28, and the unclassified object 14. The spectral features used for the spectral classifications are labelled in grey. Labels in each spectrum also indicate the individual ID (H-band spectra) and spectral type (K band spectra).

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3.2 Spectroscopy

For the spectroscopic follow-up, we used the LIRIS instrument, a near-infrared spectrograph and imager mounted at the Cassegrain focus of the 4.2 m William Herschel Telescope (La Palma, Canary Island, Spain). The camera is equipped with a Hawaii 1024 × 1024 HgCdTe array detector, with a field of view of 4.2′× 4.2′ and a spatial scale of 0. ′′25 pixel−1. The instrument has multi-object (MOS) and long-slit (LS) spectroscopic modes and if the H and K pseudogrism are used, the resolution is R ~ 2500 (Fragoso-López et al. 2008). The near-infrared spectra were obtained on 2013 June 29 and 30, using both spectroscopic observing modes.

For the MOS we designed two masks, focused mainly on OB-type stellar candidates. Mask A contained12 stars and mask B 13 stars. We selected each mask candidate trying to have a K dispersion of less than 2.5 mag to avoid large differences in the integration times and the spectral signal-to-noise ratios (S/N).

We included in the masks mainly stars with reddening-free parameters QIR between − 0.2 and 0.2 (characteristic of OB-type stars, Ramírez Alegría et al. 2012) and K less than 13 mag. Some stars which do not fulfil the QIR criteria, but do not overlap with the OB-type candidate slits during mask design, were also included in both masks A and B (stars 2, 7, 11, 20, and 21). The QIR value for these five objects ranges between –0.27 and 0.27.

The mask design also considered the spectral range derived from the slit position. For slits located in the positive half of the detector (from the centre to the right), we obtain spectra from 1.55 to 1.85 μm in the H band and from 2.06 to 2.40 μm in the K band. These spectral ranges include the He I 1.70 μm, He I 2.11 μm, He II 2.19 μm, and He II 1.69 μm lines, which are required for early-type stellar spectral identification and classification.

Both masks were observed using the H and K pseudogrism. The seeing values during observations, measured from the telluric lines in the spectra, were between 0.75 and 1.40 arcsec. Individual slits vary between 8.5 and 10 arcsec long and are 0.8 arcsec wide.

For long-slit spectroscopy we selected bright and red (magnitude KS < 9 and colour (JKS) > 2.0) objects as red supergiant candidates. For these candidates, we did not expect a reddening-free parameter QIR ~ 0; therefore, for long-slit spectroscopy candidates we relaxed this restriction. We observed a total of seven stars (objects 24, 25, 26, 27, 28, 29, and 30) separated in four slits of 0.75 arcsec wide. For both long-slit and MOS observations, the resolution was λ∕Δ λ = 1500–1700.

To remove the sky contribution in the data reduction, we observed using an ABBA patern (the star is located in positions A and B in the slit, the positions then being sequentially changed). Flat-fielding, spectral tracing, sky subtraction, coaddition, and extraction were applied using IRAF4 for the long-slit spectra and LIRISDR5, a package developed specifically for LIRIS data, for the MOS spectra.

Combining the individual spectra, we discarded cosmic rays and hot pixels that might mimic spectral lines. For wavelength calibration, argon and xenon lamps were observed, both lamps (continuum-subtracted) being used to calibrate the K band spectra and the argon lamp only for the H-band spectra. The telluric subtraction was carried out using MOLECFIT (Kausch et al. 2015; Smette et al. 2015), a software that corrects the atmospheric absorption lines by fitting a synthetic transmission spectrum. The software models the synthetic spectrum by estimating the column density of atmospheric molecules (water, carbon dioxide, methane, nitrous oxide, and ozone) from the physical parameters during the observing night (external and internal temperature, relativity humidity, observatory coordinates and altitude, wind speed, and atmospheric pressure).

To quantify the spectra quality, we measured the S/N per resolution element using the IRAF task SLOT. We used as spectrum continuum the spectral ranges 1.611−1.641 μm, 1.650−1.675 μm, 2.060−2.105 μm, and 2.190−2.247 μm for early-type stars, and 1.675−1.711 μm and 2.075−2.2080 μm for late-type stars. Most of our spectra have S/N greater than 70 (necessary to detect weak absorption features for R ~ 1000 or larger, Hanson et al. 1996), and the final values are given in Table A.1.

4 Spectral classification

We classify our spectra following the same procedure as for Masgomas-1 and Masgomas-4 (Ramírez Alegría et al. 2012, 2014b). Using spectral libraries with similar resolution to LIRIS spectra, we identify spectral lines and compare theshapes and depths. For early-type stars, we used the Hanson et al. (1996), Meyer et al. (1998), Hanson et al. (1998), and Hanson et al. (2005) atlases (this last one, degraded to our spectral resolution). For later spectral types, we used Wallace & Hinkle (1997) and the IRTF spectral library (Rayner et al. 2003; Cushing et al. 2004).

The near-infrared early-type spectra show mainly the hydrogen Brackett series and helium (He I and He II) lines. The extension of the Brackett series is useful to set the upper limit (earliest spectral type) for the classification. The later the spectral type, the more extended the Brackett series to the blue part of the spectra. For example, the H I (4–12) line at 1.64 μm is barely noticed for stars earlier than O5. The shape of the Brackett series helps to distinguish between luminosity class I (very narrow and deep lines) and III−V (broader and shallower lines).

The separation between luminosity classes III and V may be more challenging, but the shape and presence of He I and He II lines can help in the classification, for spectra with SN ~ 70 or higher. The presence of He I and He II lines are also good indicators of the spectral type. For dwarf stars the presence of He II at 1.69 μm and/or 2.19 μm lines indicates a spectral type not later than O7−8. In the case of the He I 1.70 μm, 2.06 μm, and 2.11 μm, the detection in the spectra sets a lower limit for the spectral type at B2−3. At our resolution and S/N, this set of lines allows a spectral classification of two subtypes, a similar uncertainty to that reported by Hanson et al. (2010) and Negueruela et al. (2010), with comparable datasets. For some of our stars, the luminosity classification is not possible, and we prefer to discard them from the analysis.

For late-type stars, the luminosity classification is based on the detection and equivalent width of CO(ν = 3–6) 1.62 μm, CO(ν = 0–2) 2.29 μm, and CO(ν = 1–3) 2.32 μm. All spectra show the CO band at 1.62 μm, discarding the dwarf luminosity class, and only the spectra whose equivalent width between 2.294 and 2.304 μm univocally indicates a supergiant luminosity class from the relation by Davies et al. (2007) were classified as red supergiants.

Most of our spectra show the features from OB-type stars, but luminosity classes range from dwarfs to supergiants. The presence and shape of the Brackett series and the He I lines in absorption indicate that the earliest dwarf stars have a spectral type of O9. In the group of dwarfs objects, we classify stars 10, 22, and 23 (classified as O9 V), and stars 11, 18, & 19 (classified as B3 V). For the first group the He I lines at 1.70 and 2.11 μm and the Brackett series are clear in both H and K bands. The He I lines are clear, but not as deep as expected for the Brackett series in luminosity class III or I stars. The spectra are similar to the O9 V HD 93521 spectrum (standard star, observed by our group using LIRIS). The spectra from the second group, classified as B3 V, have a clear Brackett series extending until the Brγ H I (4–15) line. The He I 2.11 μm is weak and fit the B3 V HR 5191 spectrum from Meyer et al. (1998).

Stars 1, 3, and 13 are classified as early-type giants (O8−9 III). The spectral features have similar depth and shape to those observed in stars HD 37043 (O9 III; Hanson et al. 1996), HD 36861 (O8 III; Hanson et al. 1996), and HR 1899 (O9 III; Meyer et al. 1998). For star 1, the He I 2.06−2.11 μm lines are similar to those observed in the O9 III star HD 37043 (Hanson et al. 1996). For star 3, we observe the depth of He I 1.70 μm at a similar level to that of giant stars and the Brackett series shallower than that observed in O8−9 III stars (Meyer et al. 1998; Hanson et al. 2005). In the case of star 13 the classification is mostly based on its broadand shallow Brackett series, observing a H I (4–7) peculiar emission line, plus its He I 1.70 μm line, which is too deep to be considered a dwarf star.

Stars 2,5, 9, and 15 show spectral features observed in supergiants and giants stars. For these stars we could only determine the spectral type with a luminosity class between I and III. K band spectrum of star 2 does not show distinctly spectral lines, but in the H-band spectrum we observe a strong He I 1.70 μm absorption line, more profound than the Brackett series. This characteristic is not seen in dwarf stars. The He I line is deeper than observed in the O9 I HD30614 spectrum (Hanson et al. 1998), but the Brackett series is too shallow to fit a supergiant luminosity class. Spectra of stars 5, 9, and 15 also present the He I 1.70−2.11 μm line in absorption, deeper than the line observed in star 2, and the Brackett series is observed until H I (4–15). The shape and depth of the He I is compatible with O9 I stars (Hanson et al. 2005), but the depth of the Brackett series is in agreement with a giant luminosity class. The shape of the lines does not allow a luminosity class to be determine for these objects.

Star 4 is the only one whose spectrum was observed with multi-object mask and was classified as an early supergiant. Its H-band spectrum does not show the Brackett series, but we observe the He II 1.69 μm line, deeper than the He I 1.70 and 2.11 μm lines. The He II/He I 1.69/1.70 ratio indicates an O5−6 I spectral type (e.g. Cyg OB2 8C; Hanson et al. 2005). The C IV 2.08 μm in emission also supports an O5−6 I spectral type for this object.

The spectrum of number 16 shows a narrow Brackett series both in H and K band, which is characteristic of A-supergiants. We classify this object as A0 I, after comparing it with star A0 I HR 3975.

Star 20 shows broad emission lines and corresponds to the Wolf−Rayet WR122-11. The broad He I 1.70, 2.06, 2.11, and He II 1.57, 1.69, 2.03, 2.16, 2.19, 2.35 lines in emission dominate both H and K band spectra, and the Brackett series appears weakly in emission. Our K band spectrum is similar to the WR134 spectrum from Figer et al. (1997), and we confirm the WN6 classification for this object given by Faherty et al. (2014).

The long-slit spectra include very bright stars, RSG candidates, and bright OB-type candidates which could not be included in the mask. From this group of seven spectra we classify four as massive evolved objects (stars 25, 27, 28, and 29).

Star 25 shows a deeper CO band at 1.62 μm than do stars 24 and 26. The Al II, Ca I, and Mg I lines fit a supergiant (we used star M1 I-HD 14404 from IRTF library as an example) and a giant luminosity class (e.g. star M5 III HD 175865). For this star, the equivalent width of the 12CO (2,0) band in the region between 2.294 and 2.304 μm is EW25 = 23.29 Å. In the relation given by Davies et al. (2007), this star was classified as mid M giant or early M supergiant. Thus, we could not determine the luminosity class for this star and we discarded it from the analysis.

The spectrum of star 27 (the blue supergiant/LBV candidate IRAS 18576+0341, Ueta et al. 2001; Pasquali & Comerón 2002) shows the Brackett series with a central narrow absorption profile mixed with a broader emission profile. The depth of the absorption line component is similar to that observed in B8 supergiants (for example, HR 1713), but the emission component could affect the natural depth of the series. We also observed in emission two Na I lines at 2.206 and 2.209 μm, and one Mg II at 2.137 μm.

When we compare our spectrum with the one published by Clark et al. (2009), we observe weaker emission features, and the previously stronger emission Brackett γ H I (4–7) line now mixed with an absorption central feature (not seen in the spectra presented by Clark et al. 2003, 2009). This mixed profile in the Brackett γ line is also observed in the H-band Brackett lines. We classify this star as B8 I, in agreement with Clark et al. (2003) and because of the emission contaminating the photometry, we preferred to discard this star from the distance and extinction estimations, assuming the distance of 10 kpc estimated by Ueta et al. (2001).

Star 28 has clear and deep CO bands in both the H and K band spectra. The depth of both CO bands implies a supergiant luminosity class. Comparison with IRTF spectra indicates that star 28 is later than M5 I. The equivalent width of the 12CO (2,0) band is EW28 = 34.66 Å in the region between 2.294 and 2.304 μm. The equivalent width and the shape of the spectrum point to a spectral type of M5 I for this star.

The spectrum of star 29 has a broad Brackett series in emission. The He I lines (at 1.70 μm, 2.11 μm, and 2.16 μm) and N III lines (at 2.11 and 2.25 μm) are evident in emission. The He II lines can be observed in absorption clearly at 1.70 and 2.19 μm (the latter present a P Cygni profile). The spectrum is similar to a WN object (O4−6If+/WN9 VVVCL 73-2 by Chené et al. 2013, WN8-9h WR 62-2 by Chené et al. 2015, or F2 and F7 by Martins et al. 2008), without a narrow C IV emission line detected in our spectrum. We classify star 29 as WN8-9h and name it WR122-16, following the nomenclature by the “Galactic Wolf Rayet Catalogue” (version 1.19, P. Crowther, priv. comm.).

The contaminants in this group are characterized by CO bands in both the H and K band spectra, but the depth and shape of the CO bands and the observed metallic lines eliminate them as supergiant objects. The shape of the CO band at 1.619 μm of star 24 resembles a late M giant spectrum (e.g. the M5 III HD 175865 from IRTF library). In the case of star 26, we only have K band spectrum. The shape and depth of its CO band and the Ca I lines indicate a late M giant type (the same as star 24). The spectrum of star 30 also shows 12CO Δ ν = 3 and 12CO Δν = 2 bands, but with depths similar to a giant luminosity class. We assigned the spectral type K2 III to this star.

5 Analysis

From the 30 spectra, we estimated the individual distances, extinctions and radial velocities (RVs). Using together the photometrically derived distances and the spectroscopically derived RVs, we characterized the massive star population of the cluster candidate Masgomas-6, discovering the presence in the candidate area of two different groups of massive stars, separated by 4 kpc.

5.1 Individual distances

To estimate the individual distances for the stars with spectral classification we compared the apparent magnitude with the intrinsic magnitude corresponding to the spectral type, assuming the Rieke et al. (1989) extinction law, with R = 3.09 (Rieke & Lebofsky 1985). This selection of extinction law was the same used for the discovery of the cluster candidate. The intrinsic magnitudes and colours for O-type stars, for all luminosity classes, are from Martins et al. (2005). For stars later than O9.5 V, we used the intrinsic magnitudes and colours from Cox (2000). The spectral type uncertainty dominates the distance errors, and we estimated it by deriving the individual distance for the same star assuming ± 2 spectral subtypes, except for the blue supergiant (BSG) stars 2, 4, 5, and 9.

For these objects intrinsic magnitudes by Martins et al. (2005) have practically no variation (meaning extinction and distance estimates almost constant in the ± 2 spectral subtypes range). Using the apparent magnitudes from the BSG-population in Arches (Martins et al. 2008) and Mercer 30 (de la Fuente et al. 2016), we estimated a dispersion of ~0.6 mag in K band per 2 spectral subtypes. We used this dispersion in magnitude to estimate the uncertainties in extinction and distance estimates for the BSG in our sample.

For the Wolf−Rayet objects, we used the absolute magnitude calibration by Rosslowe & Crowther (2015). From a sample of 126 Wolf−Rayets with known distances, the authors derive near-infrared JHKS absolute magnitudes for nitrogen, carbon, and oxygen-type WR.

In Table A.1 we present the individual extinction and distance determinations for the spectroscopically observed stars. We distinguished two groups of distances for the massive stars: one nearby group (located between 4 and 5 kpc) and one distant group (with distances greater than 6.5 kpc). Because the populations lie in the same line of sight, decontaminating the colour–magnitude diagram using statistical field-star decontamination is not possible, and deriving physical parameters for the clusters or associations (such as total mass by IMF integration or age determination via isochrone fitting) from the cleaned photometric diagrams is also not possible. Using only the decontaminated colour–magnitude and colour–colour diagrams (Figs. 2 and 3), it is not possible to distinguish two or more stellar populations at different distances based on differences in the extinction law (across the line of sight or a product of differential reddening in any of the clusters or associations).

5.2 Radial velocities

We used theIRAF task RVIDLINES to measure radial velocities from the observed spectra. The task compares the wavelength shift in spectral absorption lines relative to specified rest wavelengths (Table 2 includes the rest wavelength used for both early- and late-type objects). After measuring a series of wavelength shifts, the task estimates their average and the radial velocity. This task is more suitable for determining radial velocity for spectra with low S/N or with a few lines compared with a task based on template cross-correlation (such as FXCOR). The line centring includes both a standard centring algorithm (similar to the IDENTIFY task) or a Gaussian fitting and a deblending centring algorithm. Using the task RVCORRECT from the same IRAF RV package, we corrected the radial velocities to the local standard of rest, considering the observation date and time for each spectrum.

Because of the small number of lines identified in each spectrum, the errors of the LSR radial velocities are large and strongly dominated by the error derived from the initial RVIDLINES estimates. We also included quadratically the error associated with the RVCORRECT task, but they are a smaller contributionto the total error. The values of the corrected radial velocity and the associated error for each star are given in Table A.1. We do not report radial velocities for stars 14 (without spectral classification), 20 (WN6), 27 (B8 I), and 29 (WN8-9h). In the first case, we were not able to observe spectral features in the stellar spectrum, and for stars 20, 27, and 29 we expected a wind-dominated line profile.

Table 2

Spectral lines wavelengths.

5.3 Masgomas-6a and Masgomas-6b

Using together the individual distances, extinctions, and radial velocity estimates, we can improve the identification of the massive star populations at different distances. In Fig. 6 we see that for distant objects (i.e. d > 6 kpc, with the exception of star 16) the mean radial velocity is below 50 km s−1 (no distant stars with large radial velocity), and for close objects the radial velocities range between 50 and 180 km s−1. Compared with the radial velocity measures for masers by Reid et al. (2014), we see that in the Galactic direction l = 37–38°, the expected radial velocity for distant stars is also less than 50 km s−1, and they are almost exclusively members of the Perseus arm. In the same galactic longitude but for nearby stars, we find a population mixed with the Scutum−Centaurus and the Sagittarius arms.

Late-type stars (Table A.1; stars 6, 7, 8, 12, 17, 21, 24, 25, 26, and 30) present radial velocities in the whole range as expected for stars closer than 4 kpc. These objects should belong to the close part of the Sagittarius arm and also to the tangent section of the Scutum−Centaurus (including the base of this arm and the near end of the Galactic bar). Due to our uncertainties in the distance determination, it is not possible to separate the population because of the close arms.

In the case of nearby early-type stars, objects 10, 22, 23 (O9 V stars), and 13 (O9 IIIe) present distances and radial velocities compatible with a single cluster or association. The mean distance (3.9 kpc) and radial velocity (96 ± 6 km s−1) place this population as part of the close intersection of the Sagittarius arm. We refer to this group of massive stars as Masgomas-6a and it may include the two Wolf−Rayets, the WN6 WR122-11 (star 20 at 4.8 kpc) and WN8-9h WR122-16 (star 29 at 4.4 kpc), as part of its stellar population. Stars 11 and 18 (both B3 V) have a similar distance (mean distance of 4.4 kpc), but their larger radial velocities would place them in the Scutum−Centaurus arm. Star 19 (B3 V) exhibits a very low radial velocity, and because it is the limit of the massive star classification we prefer not to consider it as part of Masgomas-6a.

The group of distant stars includes the blue (star 4), yellow (star 16), and red (star 28) supergiants, plus the O-giants (stars 1 and 3). All objects in this group (except for star 16-A0I) show a radial velocity below 50 km s−1 and distance estimate greater than 6.0. We include the LBV candidate in this group (star 27, classified by us as B8 I) based on the distance estimate by Ueta et al. (2001). We do not estimate either radial velocity or extinction for this object, but the previously estimated distance of 10 kpc for the LBV agrees with its membership to the distant group (hereafter Masgomas-6b). The individual distances for Masgomas-6b indicates a single distance of 9.6 ± 0.4 kpc, mean extinction of 2.5 mag, and radial velocity of 64 ± 6 km s−1.

Assuming that both Masgomas-6a and Masgomas-6b populations from a stellar cluster or association can be described using a Kroupa initial mass function (IMF, Kroupa 2001), we fitted it to the observed massive stars and integrated this function to derive a lower limit for the total mass of each cluster/association. For Masgomas-6a, we have six objects with masses between 15 and 40 M, limits set by the O9 V star (Martins et al. 2005) and the WN8-9h Wolf−Rayet (Ekström et al. 2012).

For a crude estimate of the total mass (lower limit) of Masgomas-6a, we fitted the Kroupa IMF to the middle point in the mass range (i.e. 27.5 M), with the uncertainty derived from the spectral type determination of two subtypes (i.e. mass between 18.8 and 42.5 M). After integrating the IMF from log (M) = −1.0 to 1.6 dex, we derived for Masgomas-6a a total mass (lower limit) of (9.0 − 1.3) × 103 M. Following the same procedure, we found for Masgomas-6b a lower limit for its total mass of (10.5 − 1.5) × 103 M. For both objects, the presence of evolved massive stars (the WN8-9h Wolf−Rayet in Masgomas-6a and the LBV candidate in Masgomas-6b) sets an age limit of 5 Myr, similar to the age estimated by Chené et al. 2013 for VVV CL009.

thumbnail Fig. 6

Individual distance and radial velocity estimates for stars with spectroscopic observations. Objects are identified by the ID number from Table A.1, and massive stars are shown as green circles and late-type giants as red triangles. The vertical lines mark the distance estimates for the Wolf−Rayet stars (red line, objects 20 and 29) and the LBV (blue line, object 27). Only stars with assigned luminosity classes are included in the figure.

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6 Conclusions

We present the spectroscopic confirmation of a massive star population around the first cluster candidate derived from an automatic search software based on AUTOPOP (Garcia et al. 2009, 2011), and adapted to use 2MASS photometry. The spectroscopic observations of 30 stars from the candidate Masgomas-6 revealed two populations of massive stars in the same line of sight: Masgomas-6a and Masgomas-6b.

The stellar population of both groups appears mixed in the colour–magnitude and colour–colour diagrams, and we could not observe differences in the extinction law to help a photometric distinction between the two populations. Using the spectroscopic data, particularly the individual distance, extinction and radial velocity estimates, we could distinguish both groups and estimate their distances (~3.9 and ~9.6 kpc) and radial velocities (~96 and ~64 km s−1) for Masgomas-6a and Masgomas-6b, assuming that each of them forms a physical entity.

Masgomas-6a is part of the Scutum−Centaurus arm, close to the base of the arm and the near end of the Galactic bar. This region of intense star formation contains six red supergiant clusters: RSGC1 (Figer et al. 2006), RSGC2 (Davies et al. 2007), RSGC3 (Clark et al. 2009), Alicante 8 (Negueruela et al. 2010), Alicante 7 (Negueruela et al. 2011), and Alicante 10 (González-Fernández & Negueruela 2012), plus Masgomas-1, the first massive stellar cluster found by our group (Ramírez Alegría et al. 2012). The Galactic longitude of Masgomas-6a would place it on the outer edge of the arm, but still very close by distance and direction to this very active star-forming region. With Masgomas-6a we also reported the discovery of a new Wolf−Rayet object, WR122-16, and associated the previously known WR122-11 as part of their massive population.

The discovery of Masgomas-6b and its evolved-massive population confirms that the LBV candidate IRAS 18576+0341 is not in isolation. The seven evolved massive objects are part of the Perseus arm and have a common radial velocity. The extension and shape of both massive groups need to be confirmed with an extended spectroscopic study of the region.

To determine whether Masgomas-6a and Masgomas-6b are clusters or associations is beyond the scope of our available data. The accuracy of our RV measures does not allow us to study the internal dispersion on each population. A more in-depth kinematic study with more extensive and precise measurements of individual radial velocities is required to understand whether the massive stars are members of a cluster or an OB-association. Future data should allow extending the study to the less massive population associated with any of the objects reported in this work.

Appendix

Table A.1

Spectroscopically observed stars.

Acknowledgements

We are very thankful to Amparo Marco and Ignacio Negueruela for very fruitful discussions and help, to Karla Peña Ramírez for her collaboration during the text edition, and to the anonymous referee for the comments on the article. S. R. A. acknowledges the support of the FONDECYT Iniciación project No. 11171025 and the CONICYT + PAI “Concurso Nacional Inserción de Capital Humano Avanzado en la Academia 2017” project PAI 79170089. J.B. acknowledges support by the Ministry for the Economy, Development, and Tourism’s Programa Iniciativa Científica Milenio through grant IC120009, awarded to The Millennium Institute of Astrophysics (MAS). A. H. and K. R. acknowledge support by the Spanish MINECO under projects AYA 2015-68012-C2-1 and SEV 2015-0548. M. G. acknowledges support from grants FIS2012-39162-C06-01, ESP2013-47809-C3-1-R and ESP2015-65597-C4-1-R. Based on observations made with the William Herschel Telescope (WHT). The WHT is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This research has made use of NASA’s Astrophysics Data System. We estimated asymmetric errors using the Java applet by Barlow (2004), available at http://www.slac.stanford.edu/~barlow/statistics.html.

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1

The reddening-free parameter, is defined as , and the numerical value of depends on the extinction law.

4

IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.

All Tables

Table 1

Summary of Masgomas objects detected during different project phases.

Table 2

Spectral lines wavelengths.

Table A.1

Spectroscopically observed stars.

All Figures

thumbnail Fig. 1

GLIMPSE false colour (blue = IRAC I1, green = IRAC I3, red = IRAC I4) image for Masgomas-6. The extension ofthe cluster candidate derived from the automatic search algorithm is shown (solid blue circle). The areas used for decontamination are also shown: cluster field (dashed white circle) and control field (dashed green circle). We markedthe stars spectrally classified as early-type (dwarf, giant, or supergiant) and the evolved massive stars (Wolf−Rayets) with small green boxes, the late-type giants with red circles, the star without classification (star 14) with a black square. The length of the orientation arrows is 1.0 arcmin.

Open with DEXTER
In the text
thumbnail Fig. 2

Colour–magnitude (left) and free-reddening parameter–magnitude QIR (right) diagrams for Masgomas-6. Black dots show the position of the most probable cluster members, and grey symbols show the position of the field stars. We include in the figure the spectroscopically confirmed massive stars with green open squares and the late-type giant stars with red open circles. The individual identification numbers (ID, Table A.1) are also shown above each coloured symbol. In the pseudocolour–magnitude diagram, we can see that most of the massive stars are found between QIR values of –0.2 and 0.2, as expected from our selection method.

Open with DEXTER
In the text
thumbnail Fig. 3

Colour–colour diagram for Masgomas-6. Black dots show the position of the most probable cluster members, and grey symbols show the position of the field stars. Objects with spectroscopic data are shown: massive stars (in green) and late-type giants (in red). In the figure we included the projection, following the extinction law by Rieke et al. (1989) (with R = 3.09; Rieke & Lebofsky 1985), for an O giant (blue solid line; intrinsic colour by Martins & Plez 2006) and a G5 giant (red dashedline; intrinsic colour by Cox 2000). The stellar population located above the red-dashed line corresponds mostly to the disc giant population and is observed only in the grey-field stars. The massive star population, confirmed spectroscopically, is observed close to the blue solid line and below the giant sequence.

Open with DEXTER
In the text
thumbnail Fig. 4

Individual H band (left) and K band (right) spectra for massive stars. Labels indicate the object’s ID (H-band spectra) and spectral type (K band spectra). This set includes early-type stars with luminosity classes I, III, and V, and two Wolf−Rayets. The spectral features used for the spectral classifications are labelled in grey. The bottom red spectrum plotted over spectrum29 corresponds to WR62-2, a WN8-9h object (Chené et al. 2015) shown for visual comparison with the new Wolf−Rayet star WR122-16. The red supergiant spectrum is shown in Fig. 5.

Open with DEXTER
In the text
thumbnail Fig. 5

Individual H band (left) and K band (right) spectra for late-type stars. In this figure we include objects with luminosity class III or V, the red supergiant star 28, and the unclassified object 14. The spectral features used for the spectral classifications are labelled in grey. Labels in each spectrum also indicate the individual ID (H-band spectra) and spectral type (K band spectra).

Open with DEXTER
In the text
thumbnail Fig. 6

Individual distance and radial velocity estimates for stars with spectroscopic observations. Objects are identified by the ID number from Table A.1, and massive stars are shown as green circles and late-type giants as red triangles. The vertical lines mark the distance estimates for the Wolf−Rayet stars (red line, objects 20 and 29) and the LBV (blue line, object 27). Only stars with assigned luminosity classes are included in the figure.

Open with DEXTER
In the text

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