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Register a new userThe RMS Survey: A Galaxy-wide Sample of Massive Young Stellar Objects
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Authors
J. S. Urquhart
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Here we describe the Red MSX Source (RMS) survey which is the largest, systematic, galaxy-wide search for massive young stellar objects (MYSOs) yet undertaken. Mid-IR bright point sources from the MSX satellite survey have been followed-up with ground-based radio, millimetre, and infrared observations to identify the contaminating sources and characterise the MYSOs and UCHII regions. With the initial classification now complete the distribution of sources in the galaxy will be discussed, as well as some programmes being developed to exploit our sample.
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Near-infrared H- and K-band spectra are presented for 247 objects, selected from the Red MSX Source (RMS) survey as potential young stellar objects (YSOs). 195 (~80%) of the targets are YSOs, of which 131 are massive YSOs (L_BOL > 5x10^3 L_solar), M > 8M_solar. This is the largest spectroscopic study of massive YSOs to date, providing a valuable resource for the study of massive star formation. In this paper we present our exploratory analysis of the data. The YSOs observed have a wide range of embeddedness (2.7 < A_V < 114), demonstrating that this study covers minimally obscured objects right through to very red, dusty sources. Almost all YSOs show some evidence for emission lines, though there is a wide variety of observed properties. The most commonly detected lines are Brgamma, H_2, fluorescent FeII, CO bandhead, [FeII] and HeI 2-1 2^1S-2^1P, in order of frequency of occurrence. In total, ~40% of the YSOs display either fluorescent FeII 1.6878um or CO bandhead emission (or both), indicative of a circumstellar disc; however, no correlation of the strength of these lines with bolometric luminosity was found. We also find that ~60% of the sources exhibit [FeII] or H_2 emission, indicating the presence of an outflow. Three quarters of all sources have Brgamma in emission. A good correlation with bolometric luminosity was observed for both the Brgamma and H_2 emission line strengths, covering 1 L_solar< L_BOL < 3.5x10^5 L_solar. This suggests that the emission mechanism for these lines is the same for low-, intermediate-, and high-mass YSOs, i.e. high-mass YSOs appear to resemble scaled-
We present the results of ammonia observations towards 66 massive star forming regions identified by the Red MSX source survey. We have used the Green Bank Telescope and the K-band focal plane array to map the ammonia NH3 (1,1) and (2,2) inversion emission at a resolution of 30 arcsec in 8 arcmin regions towards the positions of embedded massive star formation. We have identified a total of 115 distinct clumps, approximately two-thirds of which are associated with an embedded massive young stellar object or compact HII region, while the others are classified as quiescent. There is a strong spatial correlation between the peak NH3 emission and the presence of embedded objects. We derive the spatial distribution of the kinetic gas temperatures, line widths, and NH$_3$ column densities from these maps, and by combining these data with dust emission maps we estimate clump masses, H$_2$ column densities and ammonia abundances. The clumps have typical masses of ~1000 Msun and radii ~0.5 pc, line widths of ~2 km/s and kinetic temperatures of ~16-20 K. We find no significant difference between the sizes and masses of the star forming and quiescent subsamples; however, the distribution maps reveal the presence of temperature and line width gradients peaking towards the centre for the star forming clumps while the quiescent clumps show relatively uniform temperatures and line widths throughout. Virial analysis suggests that the vast majority of clumps are gravitationally bound and are likely to be in a state of global free fall in the absence of strong magnetic fields. The similarities between the properties of the two subsamples suggest that the quiescent clumps are also likely to form massive stars in the future, and therefore provide a excellent opportunity to study the initial conditions of massive pre-stellar and protostellar clumps.
We present the results from NH$_{3}$ mapping observations towards 34 regions identified by the Red MSX Source (RMS) survey. We have used the Australia Telescope Compact Array to map ammonia (1,1) and (2,2) inversion emission spectra at a resolution of 10 with velocity channel resolution of 0.4$,$km$,$s$^{-1}$ towards the positions of embedded massive star formation. Complementary data have been used from the ATLASGAL and GLIMPSE Legacy Surveys in order to improve the understanding of the regions and to estimate physical parameters for the environments. The fields have typical masses of ~1000$,$M$odot$, radii of ~0.15$,$pc and distances of ~3.5$,$kpc. Luminosities range between ~10$^{3}$ to ~10$^{6}$$,$L$odot$ and kinetic temperatures between 10 and 40$,$K. We classify each field into one of two subsets in order to construct an evolutionary system for massive star formation in these regions based on the morphology and relative positions of the NH$_{3}$ emission, RMS sources and ATLASGAL thermal dust emission. Differences in morphology between NH$_{3}$ emission and ATLASGAL clumps are shown to correspond to evolutionary stages of ongoing massive star formation in these regions. The study has been further refined by including the positions of known methanol and water masers in the regions to gain insight into possible protostellar regions and triggered star formation.
The purpose of this research is to study the connection of global properties of eight young stellar clusters projected in the Vista Variables in the Via Lactea (VVV) ESO Large Public Survey disk area and their young stellar object population. The analysis in based on the combination of spectroscopic parallax-based reddening and distance determinations with main sequence and pre-main sequence ishochrone fitting to determine the basic parameters (reddening, age, distance) of the sample clusters. The lower mass limit estimations show that all clusters are low or intermediate mass (between 110 and 1800 Mo), the slope Gamma of the obtained present-day mass functions of the clusters is close to the Kroupa initial mass function. On the other hand, the young stellar objects in the surrounding clusters fields are classified by low resolution spectra, spectral energy distribution fit with theoretical predictions, and variability, taking advantage of multi-epoch VVV observations. All spectroscopically confirmed young stellar objects (except one) are found to be massive (more than 8 Mo). Using VVV and GLIMPSE color-color cuts we have selected a large number of new young stellar object candidates, which are checked for variability and 57% are found to show at least low-amplitude variations. In few cases it was possible to distinguish between YSO and AGB classification on the basis of the light curves.
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