Uniform infall toward the cometary H II region in the G34.26+0.15 complex?

Abstract

Gas accretion is a key process in star formation. However, gas infall detections in high-mass, star-forming regions with high spatial resolution observations are rare. Here, we report the detection of gas infall toward a cometary ultracompact H II region ("C") in the G34.26+0.15 complex. The observations were made with the IRAM 30 m, the James Clerk Maxwell Telescope 15 m telescope, and the Submillimeter Array (SMA). The hot core associated with "C" has a mass of ∼76 ± 11 M⊙ and a volume density of (1.1 ± 0.2) × 108 cm-3. The HCN (3-2) and HCO+ (1-0) lines observed by single dishes and the CN (2-1) lines observed by the SMA show redshifted absorption features, indicating gas infall. We found a linear relationship between the line width and optical depth of the CN (2-1) lines. Those transitions with larger optical depths and line widths have larger absorption areas. However, the infall velocities measured from different lines seem to be constant, indicating that the gas infall is uniform. We also investigated the evolution of gas infall in high-mass, star-forming regions. A tight relationship was found between the infall velocity and the total dust/gas mass. At stages prior to the hot core phase, the typical infall velocity and mass infall rate are ∼1 km s-1 and ∼10-4 M⊙ yr-1, respectively. While in more evolved regions, the infall velocity and mass infall rates can reach as high as several km s-1 and ∼10-3-10-2 M yr-1, respectively. Accelerated infall has been detected toward some hypercompact H II and ultracompact H II regions. However, the acceleration phenomenon is not seen in more evolved ultracompact H II regions (e.g., G34.26+0.15). © 2013. The American Astronomical Society. All rights reserved.