Shudong Zhou

Evidence for protostellar collapse in B335

We have observed five rotational transitions of H2CO and CS toward the Bok globule, B335, with high spatial and spectral resolution. The characteristic shape of the observed profiles provides direct, kinematic evidence of collapse. In addition, we have modeled line profiles of collapsing dense cores with density and velocity structures taken from the theory of Shu and coworkers. Using the age of collapse as the only free parameter, we found that the strengths and profiles of the observed lines can be well fitted by the theoretical model. Our best-fit model gives an age of 1.5 x 10 exp 5 yr, corresponding to an infall radius of 0.04 pc and a total mass of 0.4 solar mass for the central star and disk. Outside the infall radius, there is a static envelope with a r exp -2 density distribution, an average temperature of 13 K, and a turbulent velocity (1/e width) of 0.12 km/s. The CS abundance is 3.6 x 10 exp -9 with about 30 percent uncertainty.

New Protostellar Collapse Candidates: An HCO+ Survey of the Class 0 Sources

We have observed 23 Class 0 sources in the HCO+ J = 4− 3 and 3 − 2 lines. Nine sources, including three sources previously suggested to be collapsing, have the correct (blue) spectral line asymmetry for infall in both lines. The H13CO+ J = 4 − 3 and 3 − 2 lines were also observed to find the velocity of the ambient gas. While most of the sources with the line asymmetry expected for infall are collapse candidates, further evidence is needed to establish that infall is taking place. The HCO+ spectra are not conclusive because bipolar outflows produce strong emission, which can confuse the issue.

Testing star formation theories - VLA observations of H2CO in the BOK globule B335

The H2CO 6-cm line was observed with the VLA toward B335. The line is seen in absorption against the cosmic background radiation in a ring-shaped region centered on the infrared source. The data were analyzed with both LVG and microturbulent radiative transfer models. The two models agree qualitatively but differ quantitatively. The microturbulent model, which is physically more appropriate for this source, yields a best-fit power-law density profile of the form n proportional to 1/r exp alpha with alpha = 1.5-2.0, a temperature of 13-15 K, and an H2CO abundance of 2 x 10 to the -9th. Comparison of observations with theoretical models for low-mass star formation in the subcritical and superumbral regime (Shu et al., 1987) shows good agreement. The density estimates are considerably lower than previous density estimates from dust emission. By considering the effects of temperature and density gradients, it is possible to produce a better ageement between the estimates from dust and H2CO. 38 refs.

A C(18)O survey of dense cores in the Taurus molecular cloud: Signatures of evolution and protostellar collapse

We have mapped 11 dense cores in the Taurus molecular cloud in the C(18)O J = 2 goes to 1 line at a linear resolution of 0.02 pc. The core masses derived from C(18)O range from 0.06 to 5 solar mass. Five of them have embedded infrared sources, and six do not. Dense cores without infrared sources show multiple emission peaks. In contrast, dense cores with infrared sources have a single peak and smaller sizes. The cores with infrared sources have line widths that are 2-3 times the value expected from correlations found in previous surveys. This enhancement may be accounted for by models of gravitational collapse. The data are consistent with the idea that dense cores evolve first toward smaller sizes and smaller line width along the line width-size relation, and then toward larger line width and constant or smaller sizes as an infrared source becomes observable. A good collapse candidate, L1527, is identified based on the shapes of C(18)O and H2CO lines.

New VLA Observations of NH 3 in S140

The S140 molecular cloud was observed in the NH 3 (1, 1) and (2, 2) lines with 5″ resolution. Compared with the previous VLA observations, the new observations have 3 times better flux sensitivity, 4 times better spatial resolution, and 2.5 times better velocity resolution. These advantages allowed the detection of 80%-85% of the single-dish flux in the two NH 3 lines. The NH 3 emission region consists of a long filament perpendicular to the optical bright rim and an arclike structure along the edge of the bright rim. Within 15″ of IRS 1, there is an absence of NH 3 emission which is probably caused by a true decrease in NH 3 column density, not by missing short spacings or changes in the partition function due to a temperature increase

CO observations of L1582

In this paper, the bright-rimmed cloud L1582 is mapped in (C-12)O and (C-13)O, and the embedded NH3 core is mapped in the CS(3 - 2) line. The optical bright rim coincides with the cloud edge seen in CO. Both the kinetic temperature and column density maps show elongated features along the bright rim, and the NH3 core is located right near the edge of the cloud inside a column density plateau region corresponding to an optical extinction patch. The density enhancement in the CS core is about a factor of three over the ambient cloud and the core mass is estimated at eight solar. The dust luminosity can be supplied by radiation from the Gamma Ori OB association. The most plausible heating source for the cloud gas is the photoelectric ejection of energetic electrons from grains induced by the far-UV flux from the Gamma Ori OB association. 25 references.