Search

G23.33-0.30 is a 600 M⊙ infrared dark molecular filament that exhibits large NH3 velocity dispersions (σ ∼ 8 km s−1) and bright, narrow NH3(3, 3) line emission. We have probed G23.33-0.30 at the < 0.1 pc scale and confirmed that the narrow NH3(3, 3) line is emitted by four rare NH3(3, 3) masers, which are excited by a large-scale shock impacting the filament. G23.33-0.30 also displays a velocity gradient along its length, a velocity discontinuity across its width, shock-tracing SiO(5–4) emission extended throughout the filament, and broad turbulent line widths in NH3(1, 1) through (6, 6), CS(5–4), and SiO(5–4), as well as an increased NH3 rotational temperature (Trot) and velocity dispersion (σ) associated with the shocked, blueshifted component. The correlations among Trot, σ, and VLSR imply that the shock is accelerating, heating, and adding turbulent energy to the filament gas. Given G23.33-0.30's location within the giant molecular cloud G23.0-0.4, we speculate that the shock and NH3(3, 3) masers originated from the supernova remnant (SNR) W41, which exhibits additional evidence of an interaction with G23.0-0.4. We have also detected the 1.3 mm dust continuum emission from at least three embedded molecular cores associated with G23.33-0.30. Although the cores have moderate gas masses (M = 7–10 M⊙), their large virial parameters (α = 4–9) suggest that they will not collapse to form stars. The turbulent line widths of the (α > 1) cores may indicate negative feedback due to the SNR shock. [ABSTRACT FROM AUTHOR]

Using molecular line data from the Millimetre Astronomy Legacy Team 90 GHz Survey, we have searched the optically thick HCO+ (1–0) line for the “blue asymmetry” spectroscopic signature of infall motion in a large sample of high-mass, dense molecular clumps observed to be at different evolutionary stages of star cluster formation according to their mid-infrared appearance. To quantify the degree of the line asymmetry, we measure the asymmetry parameter , the fraction of the integrated intensity that lies to the blueshifted side of the systemic velocity determined from the optically thin tracer N2H+ (1–0). For a sample of 1093 sources, both the mean and median of A are positive ( and 0.065 ± 0.009, respectively) with high statistical significance, and a majority of sources (a fraction of 0.607 ± 0.015 of the sample) show positive values of A, indicating a preponderance of blue asymmetric profiles over red asymmetric profiles. Two other measures, the local slope of the line at the systemic velocity and the δv parameter of Mardones et al. (1997), also show an overall blue asymmetry for the sample, but with smaller statistical significance. This blue asymmetry indicates that these high-mass clumps are predominantly undergoing gravitational collapse. The blue asymmetry is larger () for the earliest evolutionary stages (quiescent, protostellar, and compact H ii region) than for the later H ii region () and photodissociation region () classifications. [ABSTRACT FROM AUTHOR]

How high-mass star clusters form, especially young massive clusters (YMCs: age <100 Myr; mass >104 ), remains an open problem, largely because they are so rare that examples of their cold, dense, molecular progenitors continue to be elusive. The molecular cloud G337.342−0.119, the “Pebble,” is a candidate cold progenitor. Although G337.342−0.119 was originally identified as four separate ATLASGAL clumps, the similarities in their molecular line velocities and line widths in the MALT90 data set demonstrate that these four clumps are in fact one single, coherent cloud. This cloud is unique in the MALT90 survey for its combination of both cold temperatures (Tdust ∼ 14 K) and large line widths (ΔV ∼ 10 km s−1). The near/far kinematic distance ambiguity is difficult to resolve for G337.342−0.119. At the near kinematic distance (4.7 kpc), the mass is 5000 and the size is 7 × 2 pc. At the far kinematic distance (11 kpc), the mass is 27,000 and the size is 15 × 4 pc. The unusually large line widths of G337.342−0.119 are difficult to reconcile with a gravitationally bound system in equilibrium. If our current understanding of the Galaxy’s Long Bar is approximately correct, G337.342−0.119 cannot be located at its end. Rather, it is associated with a large star-forming complex that contains multiple clumps with large line widths. If G337.342−0.119 is a prototypical cold progenitor for a high-mass cluster, its properties may indicate that the onset of high-mass star cluster formation is dominated by extreme turbulence. [ABSTRACT FROM AUTHOR]