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Home All issues Volume 526 (February 2011) A&A, 526 (2011) A41 Full HTML
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Issue
A&A
Volume 526, February 2011
Article Number A41
Number of page(s) 4
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201015612
Published online 20 December 2010

© ESO, 2010

1. Introduction

As part of a program with the InfraRed Spectrograph (IRS, Houck et al. 2004) onboard the Spitzer Space Telescope (SST, Werner et al. 2004) aimed at characterizing the circumstellar disks of a flux-limited sample of infrared-excess young stellar objects (YSOs) in the Serpens Molecular Cloud (αJ2000 = 18h29m49s, δJ2000 =  +01d14m48s), an interesting object of unknown nature was discovered (SSTc2dJ18282720+0044450, or #17 in Oliveira et al. 2010, hereafter OL17). The very bright, high ionization emission lines seen in the mid-IR spectrum of this object are not consistent with it being a YSO.

Two types of galactic objects show such high excitation lines: dusty planetary nebulae (PNe, Bernard-Salas et al. 2009; Stanghellini et al. 2007; Guiles et al. 2007) and supernova remnants (SNRs, Sandstrom et al. 2009; Ghavamian et al. 2009). SNRs typically show very broad emission lines, produced by the high velocity shock waves (Fesen et al. 1985; Fesen & Hurford 1996; Stupar et al. 2007). PNe, on the other hand, are characterized by narrow emission lines arising from the low velocity expanding outer shells (Balick & Frank 2002; Górny et al. 2009). Both classes of objects have been extensively studied by several authors, although just a few are so dusty that they were initially discovered only at mid-infrared wavelengths. To distinguish between these two possibilities, further spectroscopy on OL17 was needed.

In this research note, we present the original IRS spectrum (Sect. 2) as well as follow-up VLT/X-shooter spectra obtained as part of the instrument science verification phase (Sect. 3) and report on our identification of this object as a PN (Sect. 4).

2. Spitzer/IRS data

The IRS spectrum of OL17 (αJ2000 = 18h28m27.2s, δJ2000 =  +00d44m45s) is presented in Fig. 1. The data were obtained as part of the SST Cycle 3 program (GO3 30223, PI: Pontoppidan) in the low-resolution module (R = λ / Δλ = 50–100; short low [SL], 5.2–14.5 μm and long low [LL], 14.0–38.0 μm). See Oliveira et al. (2010) for further details on the program and the procedures for data reduction.

The spectrum is dominated by strong [O iv] emission1 at 25.89 μm, and is accompanied by other prominent high ionization lines: [Ar iii] (8.99 μm), [S iv] (10.51 μm), [Ar v] (13.07 μm), [Ne v] (14.32, 24.32 μm), [Ne iii] (15.56 μm), and [S iii] (18.71 μm, 33.48 μm). The equivalent width (EW) of these lines, calculated from Gaussian fits, as well as the line fluxes are given in Table 1. Furthermore, the spectrum shows clear evidence for polycyclic aromatic hydrocarbon (PAH) emission (most notably at 6.2, 7.7, 11.2, and 12.8 μm). This emission most likely comes from a shell of dusty material that surrounds, and is illuminated by, the central object.

thumbnail Fig. 1

IRS spectrum of OL17, a PN/SNR candidate found among the sample from Oliveira et al. (2010). The [O iv] line (25.89 μm) clearly dominates the spectrum.

Table 1

Emission lines identified in the IRS spectrum of OL17 (Fig. 1).

Because OL17 lies along the line of sight towards the Serpens Molecular Cloud, it is appreciably extincted due to foreground material (AV ~ 6.0 mag). It has weak detections in both the R-band and IRAC1 band (3.5 μm), but is very prominent in the MIPS1 band (24 μm). It has a 3–24 μm slope () of  ≈1.8. As shown in Fig. 1, the MIPS1 band is dominated by the very strong [O iv] emission line. This explains why this object appeared so red in the infrared color-magnitude diagrams, leading to the original misclassification of the source as an YSO. The optical R-band image taken with the Wide Field Camera (WFC) on the Isaac Newton Telescope (INT, Spezzi et al. 2010), the IRAC1 (Harvey et al. 2006) and MIPS1 (Harvey et al. 2007) images are shown in Fig. 2 (see Table 2 for the photometry in different bands).

thumbnail Fig. 2

Optical and infrared images showing the INT/WFC R-band (top-left), IRAC1 (top-right), IRAC4 (bottom-left) and MIPS1 (bottom-right). All three images display the same field of view and a length scale of 30″ is shown for reference.

3. X-shooter data

X-shooter (D’Odorico et al. 2006) is a unique multi-wavelength (300–2480 nm) medium resolution (R = 4000−14   000) spectrograph on the Cassegrain focus of UT2 on the Very Large Telescope (VLT). It consists of 3 independent arms that give simultaneous spectra long-ward of the atmospheric cutoff in the UV (“UVB” arm), optical (“VIS” arm) and near-infrared (“NIR” arm). The data2 presented here were obtained as part of the X-shooter 2nd science verification phase (SV2), during September–October 2009, PI: Oliveira). The total amount of time awarded was 1 hr, during which integrations of 2760, 2560 and 2880s were obtained with the UVB, VIS and NIR arms, respectively. The observations were performed using the 1″ × 12″ slit in “nodding-on-slit” mode to perform adequate background subtraction in the near-infrared. The spectral resolving powers achieved in this setup were  ≈ 5100 in UVB and NIR and  ≈ 8800 in VIS. The raw science data consist of images containing highly curved, multi-order, two-dimensional spectra that need to be rectified, wavelength calibrated and merged using an input model configuration that describes the X-shooter spectral format. Data reduction was performed using an internal release of the X-shooter data calibration pipeline version 0.9.4 (Andrea Modigliani, priv. comm.), together with the “com4” reference calibration data.

Table 2

Optical R-band magnitude and Spitzer IRAC/MIPS fluxes of OL17 (see also Fig. 2).

thumbnail Fig. 3

X-shooter spectra of OL17. Panels show the one-dimensional spectra extracted from the UV/blue (top), the visible (middle), and the near-infrared (bottom) arms. Dotted lines mark the major emission lines tabulated in Table 3.

thumbnail Fig. 4

A zoom-in of the optical spectrum covering Hα and the [N ii] and [S ii] line doublets. The weakness of [N ii] and [S ii] relative to Hα indicate that this object is a PN and not a SNR (see Sect. 4 for details). The top panel shows the two-dimensional spectrum at corresponding wavelengths (white pixels indicate large, positive pixel values). The slit height measures 12″, while the spatial extent of the resolved Hα is about 6″.

The extracted one-dimensional spectra are shown in Fig. 3, and show many strong, narrow emission lines especially in the optical and NIR. These spectra were extracted using an aperture with a length of  ~5″ in order to include most of the extended emission seen along the slit (see top panel of Fig. 4 for the two-dimensional spectrum). Since OL17 is seen through a dense molecular cloud, it is very faint. These exposures therefore allowed the detection of the brightest emission lines, but not the continuum. Although the spectra were not flux calibrated, the analysis below makes use of line ratios involving lines that are closely spaced in wavelength, and not affected by strong telluric absorption bands.

The main lines identified, and their EWs are listed in Table 3. The EWs were calculated by fitting the lines and a possible continuum using a Gaussian profile and should be considered (3σ) lower limits, as the continuum was not detected at a sufficient signal-to-noise in these observations. It is interesting to note that the faintness of the continuum as inferred from the optical spectrum implies that the object detected in the R-band image shown in the top-left panel of Fig. 2 must consist almost entirely of Hα line emission.

Table 3

Major emission lines identified in the X-shooter spectra (Fig. 3).

4. Results

According to Fesen et al. (1985), the main quantitative criterion used to distinguish SNRs from PNe is the strength of the [S ii] (λλ6717, 6731 Å) emission lines relative to that of Hα. That is, [S ii]/Hα > 0.4 for typical SNRs (Matonick & Fesen 1997; Blair & Long 2004). Furthermore, SNRs typically have very strong [N ii] (λλ6548, 6583 Å) emission. Figure 4 shows a zoom of the X-shooter/VIS spectrum in the wavelength range covering Hα and the [N ii] and [S ii] doublets as labelled. It is clearly seen that the [N ii] and [S ii] lines are not detected, while Hα is very strong. Upper limits for the undetected lines were derived, yielding a flux ratio of [N ii]/Hα ~ [S ii]/Hα ≤ 0.06 (3σ). This value is well below the value of 0.4 quoted above as the lower threshold for SNRs.

Furthermore, SNRs have broad, high velocity (Δv ≥ 300 km s-1) emission lines, while PNe show much narrower lines. This criterion was used by Fesen & Milisavljevic (2010) to classify the Spitzer source J222557 + 601148, previously believed to be a young galactic SNR (Morris et al. 2006), as a PN. As it can be seen in Fig. 5 for a selection of lines in all 3 X-shooter arms, the typical FWHM for lines seen in these data are  <15–74 km s-1 (deconvolved for instrumental resolutions of FWHM ~ 59 in UVB/NIR and 34 km s-1 in VIS). The low velocity of the lines, combined with the low ratio of [S ii]/Hα ≤ 0.06 allow the conclusion that OL17 is a PN and not a SNR.

The electron density (ne) can be derived from the ratio between the [S iii] lines at 33.48 and 18.71 μm in the IRS spectrum, using the method of Herter et al. (1982). The ratio of [S iii]λ33.48/18.71 ~ 1.9 yields an electron density of  ~1–100 cm-3 (Rubin 1989), where the large range is due to the fact that we are in the regime where the line ratio is least sensitive to the density. However, we should note that the line at 33.47 μm appears to be twice as broad as that at 18.71 μm (possibly due to a data artifact). If we conservatively estimate that  ~50% of the flux in this line is indeed due to contamination, the [Siii] 33.47/18.71 ratio would yield a density of order 1000 cm-3, more typical of PNe. In any case, we note that unlike the intensity ratios of [Nii]/Ha and [S ii]/Hα, ne is a rather poor diagnostic for discriminating between SNRs and PNe as both span a similarly large range in density (Sabbadin et al. 1977; Riesgo & López 2005).

thumbnail Fig. 5

A sample of the narrow emission lines present in the X-shooter spectra: [O iii] (left) at 5007 Å, Hα (middle) and H i (Paβ, right) at 12 818 Å.

While the distance to this PN cannot be determined without knowing its intrinsic size, its angular extent of  ≈ 6″ (determined from the extent of Hα in the two-dimensional spectrum) and the large extinction it experiences in the bluer parts of the spectrum suggest that this PN must lie at a distance of at least several kpc, i.e., firmly behind the Serpens Molecular Cloud (distance of  ~415 pc, Dzib et al. 2010).

We summarize as follows:

  • 1.

    The Spitzer/IRS spectrum of SSTc2dJ18282720+0044450 from Oliveira et al. (2010) shows strong high ionization emission lines, compatible with both Planetary Nebulae and Supernova Remnants, one of only a handful such objects identified in the mid-IR to date.

  • 2.

    High quality VLT/X-shooter spectra, obtained during the science verification run, show a wealth of narrow emission lines in the UV/blue, optical and near-IR arm data, illustrating the enormous potential of X-shooter as a diagnostic tool for optically faint sources discovered at other wavelengths.

  • 3.

    The optical line ratios are compatible with PNe and not with SNRs: the low values measured for the line ratio [S ii]/Hα, one of the main diagnostic criteria for distinguishing between the two types of objects, allows a straightforward classification of this object as a PN.

  • 4.

    The PN interpretation is confirmed by the relative narrow (Δv ~ 8–74 km s-1) width of the emission lines seen throughout the spectra.

  • 5.

    This previously unknown PN now has public, high-quality spectroscopic data covering a very large bandwidth ranging from 0.3 to 35 μm. We encourage the use of these data3 by the community wishing to further study objects of this class.

1

In principle, the [O iv] may suffer from contamination by [Fe ii] at the resolution of the LL module, but this is ruled out in this case following our conclusion that this object is a PN and given that [Fe ii] is relatively weak in such objects.

2

Program ID 60.A-9416(A).

3

The data are publicly available and can be accessed through the data archives at ESO and the Spitzer Science Center: http://archive.eso.org/eso/eso_archive_main.htmlhttp://ssc.spitzer.caltech.edu/spitzerdataarchives/

Acknowledgments

Astrochemistry at Leiden is supported by a Spinoza grant from the Netherlands Organization for Scientific Research (NWO) and by the Netherlands Research School for Astronomy (NOVA) grants. The authors would like to express their gratitude to Bram Venemans and Brent Groves for help with the X-Shooter data reduction and electron densities, respectively.

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All Tables

Table 1

Emission lines identified in the IRS spectrum of OL17 (Fig. 1).

In the text
Table 2

Optical R-band magnitude and Spitzer IRAC/MIPS fluxes of OL17 (see also Fig. 2).

In the text
Table 3

Major emission lines identified in the X-shooter spectra (Fig. 3).

In the text

All Figures

thumbnail Fig. 1

IRS spectrum of OL17, a PN/SNR candidate found among the sample from Oliveira et al. (2010). The [O iv] line (25.89 μm) clearly dominates the spectrum.
In the text
thumbnail Fig. 2

Optical and infrared images showing the INT/WFC R-band (top-left), IRAC1 (top-right), IRAC4 (bottom-left) and MIPS1 (bottom-right). All three images display the same field of view and a length scale of 30″ is shown for reference.
In the text
thumbnail Fig. 3

X-shooter spectra of OL17. Panels show the one-dimensional spectra extracted from the UV/blue (top), the visible (middle), and the near-infrared (bottom) arms. Dotted lines mark the major emission lines tabulated in Table 3.
In the text
thumbnail Fig. 4

A zoom-in of the optical spectrum covering Hα and the [N ii] and [S ii] line doublets. The weakness of [N ii] and [S ii] relative to Hα indicate that this object is a PN and not a SNR (see Sect. 4 for details). The top panel shows the two-dimensional spectrum at corresponding wavelengths (white pixels indicate large, positive pixel values). The slit height measures 12″, while the spatial extent of the resolved Hα is about 6″.
In the text
thumbnail Fig. 5

A sample of the narrow emission lines present in the X-shooter spectra: [O iii] (left) at 5007 Å, Hα (middle) and H i (Paβ, right) at 12 818 Å.
In the text

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