Dense Gas and Star Formation: Characteristics of Cloud Cores Associated with Water Masers

Abstract

We have observed 150 regions of massive star formation, selected originally by the presence of an H 2 O maser, in the J = 5→4, 3→2, and 2→1 transitions of CS, and 49 regions in the same transitions of C 34 S. Over 90% of the 150 regions were detected in the J = 2→1 and 3→2 transitions of CS and 75% were detected in the J=5→4 transition. We have combined the data with the J = 7→6 data from our original survey (Plume et al. 1992) to determine the density by analyzing the excitation of the rotational levels. Using Large Velocity Gradient (LVG) models, we have determined densities and column densities for 71 of these regions. The gas densities are very high ( log(n) = 5.9), but much less than the critical density of the J=7→6 line. Small maps of 25 of the sources in the J = 5→4 line yield a mean diameter of 1.0 pc. Several estimates of the mass of dense gas were made for the sources for which we had sufficient information. The mean virial mass is 3800 M ⊙ . The mean ratio of bolometric luminosity to virial mass (L/M ) is 190, about 50 times higher than estimates using CO emission, suggesting that star formation is much more efficient in the dense gas probed in this study. The gas depletion time for the dense gas is roughly 1.3 × 10 7 yr, comparable to the timescale for gas dispersal around open clusters and OB associations. We find no statistically significant linewidth-size or density-size relationships in our data. Instead, both linewidth and density are larger for a given size than would be predicted by the usual relationships. We find that the linewidth increases with density, the opposite of what would be predicted by the usual arguments. We estimate that the luminosity of our Galaxy (excluding the inner 400 pc) in the CS J = 5→4 transition is 15 to 23 L ⊙ , considerably less than the luminosity in this line within the central 100 pc of NGC 253 and M82. In addition, the ratio of far-infrared luminosity to CS luminosity is higher in M82 than in any cloud in our sample. Table 2 CS Source List CS J = 2-1 CS J = 3-2 CS J = 5-4 Source Flag T R * ∫T R * dV V LSR FWHM T R * ∫T R * dV V LSR FWHM T R * ∫T R * dV V LSR FWHM Name K K km s -1 km s -1 km s -1 K K km s -1 km s -1 km s -1 K K km s -1 km s -1 km s -1Table 2 CS Source List (Continued)4 Flag CS J = 2-1 CS J = 3-2 CS J = 5-4 Source T R * ∫T R * dV V LSR FWHM T R * ∫T R * dV V LSR FWHM T R * ∫T R * dV V LSR FWHM Name K K km s -1 km s -1 km s -1 K K km s -1 km s -1 km s -1 K K km s -1 km s -1 km s -1Table 2 CS Source List (Continued)6 CS J = 2-1 CS J = 3-2 CS J = 5-4 Source Flag T R * ∫T R * dV V LSR FWHM T R * ∫T R * dV V LSR FWHM T R * ∫T R * dV V LSR FWHM Name K K km s -1 km s -1 km s -1 K K km s -1 km s -1 km s -1 K K km s -1 km s -1 km s -1

The earliest phases of high-mass star formation: a 3 square degree millimeter continuum mapping of Cygnus X

“…The presence of an envelope around the IRAS sources is confirmed by their association with high-density gas, and the existence of hot gas by the detection of a hot core and water or methanol masers (e.g. Bronfman et al 1996;Plume et al 1997). In this way, studies by e.g., Molinari et al (2000) and Sridharan et al (2002) have identified massive young stellar objects harboring highluminosity infrared protostars (see also Mueller et al 2002;Faúndez et al 2004).…”

The Physical Conditions for Massive Star Formation: Dust Continuum Maps and Modeling

“…The objects in this study were selected from the sample of Plume, Jaffe, & Evans (1992) and Plume et al (1997) of massive star-forming cores associated with water masers. Table 1 lists the sources and their observed properties.…”

The Dynamical Structure and Evolution of Giant Molecular Clouds

“…This result does not apply to unbound clumps in GMCs, which can have lower column densities [5], nor to the OB star-forming clumps studied by Plume et al [87], which haveN H22 ∼ 60.…”

“…Thus, the dynamically regulated rate appears to be within the range observed in these clumps assuming that the star formation has been occurring for the past 1-3 Myr. The OB star-forming clumps observed by Plume et al [87] are an order of magnitude more massive than these, and it is not known if their properties are also consistent with dynamically regulated star formation.…”