We have mapped 63 regions forming high-mass stars in CS J = 5 → 4 using the CSO. The CS peak position was observed in C34S J = 5 → 4 toward 57 cores and in 13CS J = 5 → 4 toward the nine brightest cores. The sample is a subset of a sample originally selected toward water masers; the selection on maser sources should favor sources in an early stage of evolution. The cores are located in the first and second Galactic quadrants with an average distance of 5.3 ± 3.7 kpc and were well detected with a median peak signal-to-noise ratio in the integrated intensity of 40. The integrated intensity of CS J = 5 → 4 correlates very well with the dust continuum emission at 350 μm. For 57 sufficiently isolated cores, a well-defined angular size (FWHM) was determined. The core radius (RCS), aspect ratio [(a/b)obs], virial mass (Mvir), surface density (Σ), and the luminosity in the CS J = 5 → 4 line (L(CS54)) are calculated. The distributions of size, virial mass, surface density, and luminosity are all peaked with a few cores skewed toward much larger values than the mean. The median values, μ1/2, are as follows: μ1/2 (RCS) = 0.32 pc, μ1/2 ((a/b)obs) = 1.20, μ1/2 (Mvir) = 920 M⊙, μ1/2 (Σ) = 0.60 g cm-2, μ1/2 (L(CS54)) = 1.9 × 10-2 L⊙, and μ1/2 (Lbol/Mvir) = 165 (L/M)⊙. We find a weak correlation between C34S line width and size, consistent with Δv ~ R0.3. The line widths are much higher than would be predicted by the usual relations between line width and size determined from regions of lower mass. These regions are very turbulent. The derived virial mass agrees within a factor of 2-3 with mass estimates from dust emission at 350 μm after corrections for the density structure are accounted for. The resulting cumulative mass spectrum of cores above 1000 M⊙ can be approximated by a power law with a slope of about -0.9, steeper than that of clouds measured with tracers of lower density gas and close to that for the total masses of stars in OB associations. The median turbulent pressures are comparable to those in UCH II regions, and the pressures at small radii are similar to those in hypercompact H II regions (P/k ~ 1010 K cm-3). The filling factors for dense gas are substantial, and the median abundance of CS is about 10-9. The ratio of bolometric luminosity to virial mass is much higher than the value found for molecular clouds as a whole, and the correlation of luminosity with mass is tighter.

A CS J = 5 → 4 mapping survey toward high-mass star-forming cores associated with water masers

We have mapped 63 regions forming high-mass stars in CS J=5-4 using the CSO. The CS peak position was observed in C34S J=5-4 towards 57 cores and in 13CS J=5-4 towards the 9 brightest cores. The sample is a subset of a sample originally selected toward water masers; the selection on maser sources should favor sources in an early stage of evolution. The integrated intensity of CS J=5-4 correlates very well with the dust continuum emission at 350 microns. The distributions of size, virial mass, surface density, and luminosity are all peaked with a few cores skewed towards much larger values than the mean. We find a weak correlation between C34S linewidth and size, consistent with Dv ~ R^{0.3}. The linewidths are much higher than would be predicted by the usual relations between linewidth and size determined from regions of lower mass. These regions are very turbulent. The derived virial mass agrees within a factor of 2 to 3 with mass estimates from dust emission at 350 microns after corrections for the density structure are accounted for. The resulting cumulative mass spectrum of cores above 1000 solar masses can be approximated by a power law with a slope of about -0.9, steeper than that of clouds measured with tracers of lower density gas and close to that for the total masses of stars in OB associations. The median turbulent pressures are comparable to those in UCHII regions, and the pressures at small radii are similar to those in hypercompact-HII regions (P/k ~ 10^{10} K cm^{-3}). The filling factors for dense gas are substantial, and the median abundance of CS is about 10^{-9}. The ratio of bolometric luminosity to virial mass is much higher than the value found for molecular clouds as a whole, and the correlation of luminosity with mass is tighter. (Abridged).

https://arxiv.org/pdf/astro-ph/0308310.pdf

A CS J=5-4 Mapping Survey Towards High-mass Star Forming Cores Associated with Water Masers

We report VLA HI Zeeman observations toward the W49A star-forming region and the SNR W49B. Line of sight magnetic fields (Blos) of 60 to 300 muG at 25" resolution were detected toward W49A at velocities of ~4 and ~7 km/s. The Blos values measured toward W49A show a significant increase in field strength with higher resolution especially for the ~4 km/s HI gas. The HI gas in the velocity range -5 to 25 km/s toward W49A shows good agreement both kinematically and spatially with molecular emission intrinsically associated with W49A. Based on comparisons with molecular data toward W49A, we suggest that the 4 km/s HI gas is directly associated with the northern part of the HII region ring, while the 7 km/s HI gas seems to originate in a lower density halo surrounding W49A. We estimate that the W49A North core is significantly subvirial, and that the total kinetic +magnetic energies amount to less than 1/3 of the total W49A North gravitational energy. These magnetic field results suggest that W49A North is unstable to overall gravitational collapse in agreement with evidence that the halo is collapsing onto the W49A North ring of HII regions. 
The majority of the HI column density toward W49B comes from Sagittarius Arm clouds along the line of sight at ~40 km/s and ~60 km/s. No significant magnetic fields were detected toward W49B. Comparison of the spectral distribution of HI gas toward W49A and W49B suggests that evidence placing W49B 3 kpc closer to the sun (i.e. at 8 kpc) than W49A is quite weak.

VLA HI Zeeman Observations Toward the W49 Complex

We report Very Large Array (VLA) H I Zeeman observations toward the W49A star-forming region and the supernova remnant W49B. Line-of-sight magnetic fields (Blos) of 60-300 μG at 25'' resolution were detected toward W49A at velocities of ~4 and ~7 km s-1. The Blos values measured toward W49A show a significant increase in field strength with higher resolution, especially for the ~4 km s-1 H I component. The H I gas in the velocity range -5-25 km s-1 toward W49A shows good agreement both kinematically and spatially with molecular emission intrinsically associated with W49A. On the basis of comparisons with molecular data toward W49A, we suggest that the ~4 km s-1 H I component is directly associated with the northern part of the H II region ring, while the ~7 km s-1 H I component seems to originate in a lower density halo surrounding W49A. We estimate that the W49A North core is significantly subvirial (2/ ~ 0.2) and that the total kinetic+magnetic energies amount to less than of the total W49A North gravitational energy. These magnetic field results suggest that W49A North is unstable to overall gravitational collapse, in agreement with evidence that the halo is collapsing onto the W49A North ring of H II regions. The majority of the H I column density toward W49B comes from Sagittarius arm clouds along the line of sight at ~40 and ~60 km s-1. No significant magnetic fields were detected toward W49B. Comparison of the spectral distribution of H I gas toward W49A and W49B suggests that evidence placing W49B 3 kpc closer to the Sun (i.e., at 8 kpc) than W49A is quite weak. Although we cannot place W49B at the same distance as W49A, we find the morphology of a ~5 km s-1 H I component toward the southern edge of W49B suggestive of an interaction.

Very Large Array H I Zeeman Observations toward the W49 Complex

We present subarcsecond resolution observations of the star-forming region W49N made with the Berkeley-Illinois-Maryland Association array at 1.4 mm wavelength in the continuum and CH3CN J = 12-11 lines. The continuum image, at a resolution of 018 (2000 AU), shows many of the previously identified ultracompact H II regions and at least one new source whose steep spectrum to short wavelengths indicates dust emission. This compact dust concentration also shows the strongest CH3CN emission in the region. Additional peaks of CH3CN emission likely mark hot cores produced by other deeply embedded young massive stars that may be precursors to O-type stars like those that power the ultracompact H II regions. The number ratio of hot cores to ultracompact H II regions found in W49N provides a measure of the relative timescales of these early evolutionary phases. Although the samples are small, the source counts suggest that the hot core lifetime is most likely shorter than, but comparable to, the ultracompact H II region lifetime.

Hot Cores in W49N and the Timescale for Hot Core Evolution

Observations of the W49A North star-forming region in the J = 2-1 line of CS have been made with the BIMA array with an angular resolution of 46 × 38; complementary observations in the J = 2-1 line of C18O and J = 3-2 line of C34S were made with the IRAM 30 m telescope, with angular resolutions of 12'' and 17'' respectively. The molecular complex is elongated along a diagonal from northeast to southwest. The position-velocity diagrams for CS convolved to 12'' resemble those of C18O and the J = 5-4 line of C34S (from Serabyn et al.). There is a change in velocity along the major axis of the molecular complex. Most spectra toward W49A North show two main velocity components, the relative intensities of which vary with position along the major axis of the molecular complex. At the higher resolution of ~4'', absorption of the continuum emission at 3 mm by CS is clearly observed. The apparent velocity gradient is seen to be part of a C-shaped distribution in the CS position-velocity image. Furthermore, as is found in the J = 1-0 HCO+ data by Welch et al., the optically thick CS J = 2-1 line exhibits an inverse P Cygni profile toward H II region G. These two characteristics of the CS data lend support to the global collapse model (Welch et al.). The high-resolution CS profiles in the central region all show varying degrees of absorption. At lower resolution the surrounding CS emission fills in much of the absorption and the inverse P Cygni shape disappears.

David E. Upham

Papers

Measurements of the J=2-1 Lines of CS and C18O toward the Star-forming Region W49A North