Signatures of Infall in Regions of Low Mass Star Formation

Details Signatures of Infall in Regions of Low Mass Star Formation Signatures of Infall in Regions of Low Mass Star Formation

Jan 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt.........6m&link_type=abstract

Thesis (PHD). HARVARD UNIVERSITY , Source DAI-B 59/01, p. 259, Jul 1998, 232 pages.

Physics

18

Interstellar Medium

Scientific paper

We used mm-wavelength lines to probe the dense gas kinematics around nearby low mass young stellar objects (YSOs). We used optically thin lines to study the underlying core rest velocity, and optically thick lines to study the foreground absorption velocity field. We surveyed 47 nearby low mass YSOs to determine the frequency of infall-like and expansion-like spectral signatures. We used δ v = (vthick - vthin) / Δ vthin, to characterize the spectral line asymmetries. The distribution of δ v is skewed toward negative (blue-shifted) velocities. This excess is much more significant for Class 0 than for Class I sources, indicating that we detect infall motions toward Class 0 and not toward Class I sources. We present maps of 12 YSOs from the survey having strong H2CO line asymmetries. The H2CO self-absorption features are concentrated toward the YSOs in all sources, having predominant infall-like line asymmetries over spatial scales of 0.01--0.04 pc, where the mean gas density is ~105 cm-3. We derive typical infall speeds of 0.01 -- 0.1 km s-1 over the cores, indicating mass accretion rates of (0.1 - 10) × 10-6 Modot yr-1. We find strong velocity gradients, up to 6 km s-1 pc-1, close to the core half maximum contours, suggestive of core motions through an inter-core medium. We present results of spherically symmetric radiative transfer calculations with a variety of cloud models. We find that the single most important parameter that determines the line profile shapes is the inward speed in the outer layers of the cores, which are predominantly responsible for the spectral line self-absorption. Our main conclusions are Infall motions are detected toward 40-50% of Class 0 sources, and < 10% of Class I sources. Most sources with infall motions have line widths dominated by non-thermal, rather than thermal motions Infall motions are extended over spatial scales of 0.01--0.04 pc. Infall motions are distinguishable from bipolar outflows by analyzing the spatial distribution of the line asymmetries. Dense star forming cores are apparently moving in an inter core medium, consistent with them being gravitationally bound objects. The observed strong line asymmetries require high infall speeds at large distances from the YSOs, which are inconsistent with pure inside-out collapse models.

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