Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...502..296o&link_type=abstract
Astrophysical Journal v.502, p.296
Astronomy and Astrophysics
Astronomy
112
Ism: Clouds, Ism: Molecules, Stars: Formation, Stars: Pre-Main-Sequence, Surveys
Scientific paper
This paper discusses star formation in the 40 C18O molecular cloud cores in Taurus by studying relations between the cores and young stellar objects (YSOs). These C18O cores constitute a complete sample of dense gas in Taurus as described in detail by Onishi and coworkers. The YSOs treated in the present work fall into the following four categories: (1) starless H13CO+ compact cores, (2) cold IRAS sources, (3) warm IRAS sources, and (4) T Tauri stars without far-infrared emission. Starless H13CO+ compact cores are assumed to be protostellar condensations that are in a phase just prior to star formation, although they are not yet considered to be YSOs in the usual sense. In the present study, we shall call the objects 1 and 2 cold objects and objects 3 and 4 warm objects. They constitute the most complete sample of YSOs, including compact dense molecular condensations, at present. It is found that C18O cores with cold objects have clearly higher molecular column densities than the rest. These C18O cores also tend to be massive and large in size. If we assume that the cold objects represent the protostellar condensations or protostars in the main accretion phase, the results imply that the actual process of star formation, i.e., collapse and significant mass accretion, takes place when a column density of the molecular cloud core becomes greater than a certain value. This threshold value for star formation is ~8.0 x 1021 cm-2 in average column density. On the other hand, the C18O cores without cold objects are characterized by a typical column density of 5.5 x 1021 cm-2, significantly smaller than the threshold value. The fact that all the C18O cores whose average column density is higher than 8.0 x 1021 cm-2 are associated with cold objects suggests that star formation takes place immediately and without exception when the column density satisfies the above value. Such C18O cores with cold objects typically have more than one YSO. The number of YSOs in a C18O core increases with core mass. The C18O core mass divided by the number of YSOs does not depend strongly on the C18O core mass and is nearly a constant mass of 11 M&sun;, i.e., the number of YSOs within a C18O core is proportional to the core mass. This suggests that formation of a protostellar condensation requires ~11 M&sun; molecular gas of n(H)2 ~ 104 cm-3 indicating a rather uniform star formation efficiency (Mstar/Mgas) of ~6% (the mass of a YSO is taken to be 0.7 M&sun;) at a density of ~104 cm-3. The mean projected separation of the cold objects in a C18O core is estimated to be ~0.3 pc. This separation is roughly in a range explicable by existing theoretical studies of gravitational instability, while the core morphology is not yet exactly taken into account in these studies. The C18O data suggest that the amount of the surrounding gas decreases on a timescale of, ~5 x 105 yr or larger. Dissipation of surrounding gas occurs not only in the vicinity of a star but also in the scale of C18O cores, >~0.1 pc. This dissipation cannot be explained simply by an outflow or mass accretion onto a central star.
Fukui Yasuo
Kawamura Akiko
Mizuno Akira
Ogawa Hideo
Onishi Toshikazu
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