Detailed observations of the M17 molecular cloud complex.
TL;DR: In this article, a detailed molecular line observations of CO, CS, HCN, H/sub 2/CO, NH/sub 3/, and some of their isotopes toward the giant molecular cloud complex near M17 are presented and discussed.
Abstract: Detailed molecular line observations of CO, CS, H/sub 2/CO, NH/sub 3/, and some of their isotopes toward the giant molecular cloud complex near M17 are presented and discussed. Extensive /sup 12/CO and /sup 13/CO observations are made of two separation clouds characterized by velocities of 20 and 23 km s/sup -1/. The cloud at 20 km s/sup -1/ is found to be approximately 15' x 30' (10 x 20 pc) in extent and to contain a mass in excess of 3 x 10/sup 4/ M/sub sun/. Within the 20 km s/sup -1/ cloud are three bright spots of /sup 12/CO emission, two of which are shown to be localized high-density fragments a few parsecs in size. Subsequent identification of infrared sources and H/sub 2/O maser sources within their boundaries is interpreted as an indication that star formation has recently taken place. Observations of millimeter-wave CO, CS, HCN, H/sub 2/CO, and SO and centimeter-wave NH/sub 3/ emission and H/sub 2/CO absorption are combined with model cloud calculations manner to determine the physical and chemical conditions in M17 SW, the largest of the CO bright spots. Integrated intensity ratios of /sup 13/CO to C/sup 18/O are found to be significantly the terrestrialmore » value. Line widths of /sup 12/CO and /sup 13/CO are found to increase near M17 SW. It is found that the entire cloud cannot be in uniform spherical collapse of any form. Collapse, however, could be in progress in a small region around M17 SW, but lack of sufficient angular resolution prohibits direct observational tests of this possibility. It is concluded that line widths of approx.3 km s/sup -1/ must be maintained by some mechanism other than systematic collapse or expansion motions. Comparison of the molecular line observations with optical, infrared, and radio continuum data is made. The 20 km s/sup -/1 molecular cloud, the M17 H II region, and the star cluster NGC 6618 are apparently related in a geometrical evolutionary sequence. Cloud evolution seems to have proceeded from the outer edge of a proto-molecular cloud inward.« less
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TL;DR: In this paper, a multi-wavelength spectroscopic survey of the ionizing cluster of M17 was presented, revealing a population of young stellar objects (YSOs) of high mass (M similar to 5-20 M).
Abstract: We present a multiwavelength spectroscopic survey that reveals the ionizing cluster of M17 and uncovers a population of young stellar objects (YSOs) of high mass (M similar to 5-20 M.). The masses of the stars have been determined fairly accurately through optical or near-infrared spectral classification. We find strong circumstantial evidence for disks around the massive YSOs in the following forms: near-infrared excess, optical veiling, CO band-head emission, and/or Pa delta emission. We find a direct correlation between those YSOs that show CO band-head emission at 2.3 mu m and these stars in our survey that show Pa delta emission; in three of the four Pa delta emission stars this line is double peaked (suggestive of a bipolar wind or a rotating disk or envelope). Our data suggest that circumstellar material, possibly in the form of a disk, is prevalent among very young objects of fairly high mass. Based on considerations of disk lifetimes in other young clusters, the M17 cluster appears to be very young, perhaps less than 1 Myr.We have also identified at least nine O stars and a couple of late-O/early-B stars, most behind more than 8 mag of visible extinction using either optical or near-infrared spectral types. Several stars have inferred masses in excess of 60 M., and they look to be very close to the predicted zero-age main sequence with an estimated age of about 1 Myr, consistent with the age of the massive YSOs in the cluster. We have used the O stars to determine the distance to M17, which assumes the stars to lie on the zero-age main sequence (1300(-200)(+400) pc). While we attempt to determine an initial mass function for the cluster, it is incomplete even at high masses because of regions of extremely high extinction (A(V) > 20) in the cluster. We have also used the M17 O stars to study the dust properties in the local cloud and the behavior of the diffuse interstellar bands (DIBs) along this sight line, over the extinction range of A(V) = 3-10. The DIBs over this extinction range show little change in spectral shape nor a significant increase in strength. We suggest the features are already saturated at small A(V), or the material local to M17, where the increased extinction is being traced, does not contain the carriers of the DIB feature.
248 citations
TL;DR: In this paper, the authors combine diffuse emission photometry from GLIMPSE and several other Galactic plane surveys covering near-IR through radio wavelengths to synthesize a global spectral energy distribution (SED) for the M17 complex.
Abstract: We combine diffuse emission photometry from GLIMPSE and several other Galactic plane surveys covering near-IR through radio wavelengths to synthesize a global spectral energy distribution (SED) for the M17 complex. By balancing the integrated flux in the SED with the total bolometric luminosity of all known O and early B stars in the ionizing cluster, we estimate a distance to M17 of 1.6 kpc. At this distance, the observed total flux in the SED corresponds to a luminosity of 2.4 ± 0.3 × 106 L☉. We find that the SED from the H II region peaks at shorter wavelengths and has a qualitatively different shape than the SED from the photodissociation region (PDR). We find that polycyclic aromatic hydrocarbons (PAHs) are destroyed over a short distance or edge at the boundary of the H II region. We demonstrate that this PAH destruction edge can be located easily using GLIMPSE band-ratio images and confirm this using Spitzer IRS spectra. We investigate the relative roles of extreme ultraviolet (EUV) and X-ray photons in the destruction of PAHs, concluding that X-rays are not an important PAH destruction mechanism in M17 or, by extension, in any other Galactic H II region. Our results support the hypothesis that PAHs are destroyed by EUV photons within H II regions. PAHs dominate the mid-IR emission in the neutral PDR beyond the ionized gas.
206 citations
Cites background from "Detailed observations of the M17 mo..."
...The M17 H ii region is erupting from the side of the giant molecular cloud M17 SW, and the southern bar marks the primary interface between the ionization edge ( I-front) of the H ii region and the molecular gas (Lada 1976; FCM84)....
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...M17 is also considered a prime candidate site for sequential or stimulated star formation (Elmegreen&Lada 1976)....
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TL;DR: In this paper, the authors used the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) with complementary IR surveys to identify candidate young stellar objects (YSOs) throughout a 15? 1? field that includes the M17 complex.
Abstract: M17 is one of the youngest and most massive nearby star-formation regions in the Galaxy It features a bright H II region erupting as a blister from the side of a giant molecular cloud (GMC) Combining photometry from the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) with complementary infrared (IR) surveys, we identify candidate young stellar objects (YSOs) throughout a 15 ? 1? field that includes the M17 complex The long sightline through the Galaxy behind M17 creates significant contamination in our YSO sample from unassociated sources with similar IR colors Removing contaminants, we produce a highly reliable catalog of 96 candidate YSOs with a high probability of association with the M17 complex We fit model spectral energy distributions to these sources and constrain their physical properties Extrapolating the mass function of 62 intermediate-mass YSOs (M > 3?M ?), we estimate that >1000 stars are in the process of forming in the extended outer regions of M17 The remaining 34 candidate YSOs are found in a 017?deg2 field containing the well-studied M17 H II region and photodissociation region (PDR), where bright diffuse mid-IR emission drastically reduces the sensitivity of the GLIMPSE point-source detections By inspecting IR survey images from IRAS and GLIMPSE, we find that M17 lies on the rim of a large shell structure ~05 in diameter (~20 pc at 21?kpc) We present maps of 12CO and 13CO (J = 2 ? 1) emission observed with the Heinrich Hertz Telescope The CO emission shows that the shell is a coherent, kinematic structure associated with M17, centered at v = 19?km s?1 The shell is an extended bubble outlining the PDR of a faint, diffuse H II region several Myr old We identify a group of candidate ionizing stars within the bubble YSOs in our catalog are concentrated around the bubble rim, providing evidence that massive star formation has been triggered by the expansion of the bubble The formation of the massive cluster ionizing the M17 H II region itself may have been similarly triggered We conclude that the star formation history in the extended environment of M17 has been punctuated by successive waves of massive star formation propagating through a GMC complex
139 citations
Cites background from "Detailed observations of the M17 mo..."
...The Sanders et al. (1986) CO J = 1–0 maps clearly show both the M17 molecular cloud and the complex identified by Elmegreen & Lada (1976), and they reveal an additional molecular cloud at v = 20 km s−1, centered at (l, b) = (15.9◦,−0.7◦)....
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...Elmegreen & Lada (1976) presented CO observations of a large molecular cloud complex associated with M17 at v = 20 km s−1 extending more than 1◦ to the Southwest, parallel to the Galactic midplane (Fig....
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...…Berkeley, CA 94720 7 Department of Astronomy, University of Virginia, P.O. Box 3818, Charlottesville, VA 22903-0818 8 Spitzer Fellow, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 ular cloud, a GMC at a velocity of 20 km s−1 with a mass >3 × 104 M⊙ (Lada 1976)....
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...The final subsample excludes 135 candidate YSOs (Table 3) that either are unassociated with M17 or are associated with the extended molecular cloud complex to the southwest (Elmegreen & Lada 1976; Sanders et al. 1986)....
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TL;DR: In this article, the authors derived kinematic distances to a large sample of massive cold dust clumps from their measured line velocities, which were then used to estimate masses and sizes of ATLASGAL sources.
Abstract: Context. The formation of high mass stars and clusters occurs in giant molecular clouds. Objects in evolved stages of massive star formation such as protostars, hot molecular cores, and ultracompact HII regions have been studied in more detail than earlier, colder objects. Further progress thus requires the analysis of the time before massive protostellar objects can be probed by their infrared emission. With this in mind, the APEX Telescope Large Area Survey of the whole inner Galactic plane at 870 μ m (ATLASGAL) has been carried out to provide a global view of cold dust and star formation at submillimetre wavelengths.Aims. We derive kinematic distances to a large sample of massive cold dust clumps from their measured line velocities. We estimate masses and sizes of ATLASGAL sources, for which the kinematic distance ambiguity is resolved.Methods. The ATLASGAL sample is divided into groups of sources, which are located close together, mostly within a radius of 2 pc, and have velocities in a similar range with a median velocity dispersion of ~1 km s-1 . We use NH3 , N2 H+ , and CS velocities to calculate near and far kinematic distances to those groups.Results. We obtain 296 groups of ATLASGAL sources in the first quadrant and 393 groups in the fourth quadrant, which are coherent in space and velocity. We analyse HI self-absorption and HI absorption to resolve the kinematic distance ambiguity to 689 complexes of submm clumps. They are associated with 12 CO emission probing large-scale structure and 13 CO (1–0) line as well as the 870 μ m dust continuum on a smaller scale. We obtain a scale height of ~28 ± 2 pc and displacement below the Galactic midplane of ~−7 ± 1 pc. Within distances from 2 to 18 kpc ATLASGAL clumps have a broad range of gas masses with a median of 1050 M ⊙ as well as a wide distribution of radii with a median of 0.4 pc. Their distribution in galactocentric radii is correlated with spiral arms.Conclusions. Using a statistically significant ATLASGAL sample we derive a power-law exponent of −2.2 ± 0.1 of the clump mass function. This is consistent with the slope derived for clusters and with that of the stellar initial mass function. Examining the power-law index for different galactocentric distances and various source samples shows that it is independent of environment and evolutionary phase. Fitting the mass-size relationship by a power law gives a slope of 1.76 ± 0.01 for cold sources such as IRDCs and warm clumps associated with HII regions.
125 citations
TL;DR: In this paper, the authors presented a survey of OH maser emission towards a sample of high mass protostellar objects made using the Nancay and GBT telescopes, and the results of this survey are compared with previous H 2 O and class II CH 3 maser observations of the same objects.
Abstract: Context. Masers are important tracers of the early evolution of young high mass stars, but the relationship between different types of maser and the evolutionary state of the exciting source remains unclear. Aims. To determine whether OH masers are common towards candidate high mass protostellar objects. Methods. We present a survey of OH maser emission towards a sample of high mass protostellar objects made using the Nancay and GBT telescopes. Results. OH maser emission was detected towards 63 objects with 36 new detections. There are 56 star-forming regions and 7 OH/IR candidates. Nearly half of the detected sources have OH flux densities ≤1 Jy. There is no evidence that sources with OH masers have a different range of luminosities from the non-maser sources. The results of this survey are compared with previous H 2 O and class II CH 3 OH maser observations of the same objects. Some of the detected sources are only associated with OH masers and some sources are only associated with the 1720 MHz OH maser line. The velocity range of the maser emission suggests that the water maser sources may be divided into two groups. The detection rates and velocity range of the OH and Class II CH 3 OH masers support the idea that there is a spatial association of the OH and Class II CH 3 OH masers. The sources span a wide range in R, the ratio of the methanol maser peak flux to OH 1665 MHz maser peak flux, however there are only a few sources with intermediate values of R, 8 < R < 32, which has characterised previous samples. The majority of the sources are either methanol-favoured or OH-favoured. Sources which have masers of any species, OH, water or methanol, have redder [100 μm-12 μm] IRAS colours than those without masers. However, there is no evidence for different maser species tracing different stages in the evolution of these young high mass sources. Conclusions. The detection of OH masers towards 26% of a sample of 217 sources should remove any doubt about the existence of OH maser emission towards these objects or this early evolutionary stage. Previous observations which have shown that the OH maser emission from similar sources traces the circumstellar disks around the objects. This combined with the sensitivity of the OH emission to the magnetic field, make the newly detected sources interesting candidates for future follow-up at high angular resolution.
82 citations