Abstract:
We present the results of a single-dish survey toward 95 very low luminosity objects (VeLLOs) in optically thick
(HCN 1−0) and thin (N2H+ 1−0) lines performed for the purpose of understanding the physical processes of
inward motions in the envelopes of the VeLLOs and characterizing their true nature. The normalized velocity
differences (dVHCN) between the peak velocities of the two lines were derived for 41 VeLLOs detected in both
lines. The δV distribution of these VeLLOs is found to be significantly skewed to the blue, indicating the
dominance of infalling motions in their envelopes. The infall speeds were derived for 15 infall candidates by using
the HILL5 radiative transfer model. The speeds were in the range of 0.03−0.3 km s−1
, with a median value of
0.16 km s−1
, consistent with the gravitational freefall speeds from pressure-free envelopes. The mass infall rates
calculated from the infall speeds are mostly of the order of 10−6 Me yr−1
, with a median value of
(3.4 ± 1.5) × 10−6 Me yr−1
. These are found to be also consistent with the values predicted with the inside-out
collapse model and show a fairly good correlation with the internal luminosities of the VeLLOs. This again
indicates that the infall motions observed toward the VeLLOs are likely to be due to the gravitational infall motions
in their envelopes. Our study suggests that most of the VeLLOs are potentially faint protostars, while two of the
VeLLOs could possibly be proto−brown dwarf candidates.
