Properties and Gravitational Collapse of the Core in G19.61 – 0.23

We present the results of an observational study toward the core in G19.61 - 0.23 with the Submillimeter Array. The continuum at 900 Delta *mm revealed that the core has a mass of 15 M. The rotational temperature and the column density of CH3CN are 552 K and 3.4 X 1016 cm-2, respectively. No UC H II region or infrared source was found at the peak position of the core. Inverse P Cygni profiles were clearly exhibited in the 13CO J = 3-2 and CN N = 3-2 lines, strongly indicating that material infall is taking place in this compact hot core. The mass-infall rate estimated from the 13CO J = 3-2 line is 6.1 X 10-3 M yr-1, which agrees with the value calculated from a free-fall model. The mass-infall rate, together with the difference of the infall velocities estimated from the 13CO J = 3-2 and CN J = 3-2 lines as well as the spatial distribution of their absorption show that the inflow motion originates from gravitational potential and is consistent with the inside-out model. The prior maser observations indicated outflow activities in this core. The signatures of both inflow and outflow activities suggest that there may be an embedded early high-mass star forming via gas accretion. After comparing the emissions of CO and its isotopes with the other widely used inflow motion tracers, we find that the CO lines may also be useful to trace inflow motion in a small scale when the continuum background is bright enough.