Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...460..359h&link_type=abstract
Astrophysical Journal v.460, p.359
Astronomy and Astrophysics
Astronomy
71
Ism: General, Stars: Formation, Ism: H Ii Regions, Ism: Individual Alphanumeric: G9.62+0.19, Ism: Kinematics And Dynamics, Ism: Molecules, Radio Continuum: Ism
Scientific paper
We present high-resolution observations of CH3CN (J = 6-5), C18O (J = 1-0), and 2.7 mm continuum toward a region of massive star formation in the 09.62+0.19 H II region complex. Our observations reveal three embedded centers of massive star formation located along the major axis of an elongated molecular cloud core. Two of those (components D and F) are observed close to the maximum of C18O column density, whereas component E is located at the edge of the molecular core. Based on the C18O observations, we derive cloud core dimensions of 0.6 × 0.3 pc, a total mass of 1000 Msun, and a mean hydrogen density of 2 × 105 cm-3 for an assumed temperature of 30 K. The cloud core has a systematic velocity gradient of 9 km s-1 pc-1 along its major axis. The embedded centers of massive star formation are all detected in the 2.7 mm continuum, and all three are bright, compact sources of CH3CN (J = 6-5) emission.
Component D is an ultracompact (UC) H II region with an electron density of 1.8 × 105 cm-3, an electron temperature of 8000 K, a diameter ≤ 0.014 pc, and an ionizing star of type B0.5 zero-age main sequence (ZAMS). For component E, the power-law spectrum of index 1.1 between 2 cm and 2.7 mm, together with the small angular diameter, suggests that the continuum emission is produced either by a partially ionized stellar wind or by an UC H II region of electron density 8.0 ×x 105 cm-3 and a diameter ≤ 0.0025 pc, ionized by a star of type B1 ZAMS, plus some excess dust emission. Our C180 and 2.7 mm continuum data indicate that the mass associated with component is in the range 122-160 Msun. For component F, continuum emission is only detected at 2.7 mm. It is likely that most or all of this emission is produced by warm circumstellar dust. If so, then we estimate a total mass associated with component F of 55-160 Msun, depending on the temperature of the gas and dust, which is not well determined for this object. Broad line wings in the C18O line are detected close to the position of component F; we interpret this is as being due to a bipolar molecular flow. Thus, component F is probably a pre-main-sequence star which may still be in a rapid accretion phase, and we postulate that it is the youngest of the three centers of massive star formation in the 09.62+0.19 cloud core. In fact, it may be one of the youngest massive stars yet identified.
Each of the continuum components D, E, and F are enclosed in very compact, dense, warm molecular cocoons. Analysis of our CH3CN and C180 data indicates gas temperatures in the range 50 K (component D) to possibly as high as 300 K (component F), but more likely 100 K (components E and F). The optical depths in the lower K-transitions of CH3CN (J = 6-5) are greater than unity toward all three sources. From our large velocity gradient analysis, we obtain methyl cyanide column densities in the range 5 × 1015 cm-2 (component D) to 1 >c 1017 cm-2 (component F); the methyl cyanide abundance is clearly enhanced at least in component F and probably also in components D and E. Hydrogen densities are in excess of 107 in components E and F.
The systematic increase in density, column density, and temperature in components D, E, and F, respectively, together with properties of the continuum emission at the three positions, leads us to suggest that components D-F probably represent an evolutionary sequence, with component D being the oldest and component F being the youngest among the three centers of massive star formation.
Cesaroni Riccardo
Churchwell Edward
Hofner Peter
Kurtz Stan
Walmsley Charles Malcolm
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