1 research outputs found
Probing the evolutionary status of starless cores through N2H+ and N2D+ observations
We have undertaken a survey of N2H+ and N2D+ towards 31 low-mass starless
cores using the IRAM 30m telescope. Our main objective has been to determine
the abundance ratio of N2D+ and N2H+ towards the nuclei of these cores and thus
to obtain estimates of the degree of deuterium enrichment, a symptom of
advanced chemical evolution according to current models. We find that the
N(N2D+)/N(N2H+) ratio is larger in more "centrally concentrated cores" with
larger peak H2 and N2H+ column density than the sample mean. The deuterium
enrichment in starless cores is presently ascribed to depletion of CO in the
high density (> 3*10^4 cm-3) core nucleus. To substantiate this picture, we
compare our results with observations in dust emission at 1.2 mm and in two
transitions of C18O. We find a good correlation between deuterium fractionation
and N(C18O)/N(H2) for the nuclei of 14 starless cores. We, thus, identified a
set of properties that characterize the most evolved, or "pre-stellar",
starless cores. These are: higher N2H+ and N2D+ column densities, higher
N(N2D+)/N(N2H+), more pronounced CO depletion, broader N2H+ lines with infall
asymmetry, higher central H2 column densities and a more compact density
profile than in the average core. We conclude that this combination of
properties gives a reliable indication of the evolutionary state of the core.
Seven cores in our sample (L1521F, OphD, L429, L694, L183, L1544 and TMC2) show
the majority of these features and thus are believed to be closer to forming a
protostar than are the other members of our sample. Finally, we note that the
subsample of Taurus cores behaves more homogeneously than the total sample, an
indication that the external environment could play an important role in the
core evolution.Comment: 42 pages, 17 figures, 12 tables, published in ApJ. Figures 8 to 15
available in gif format; full resolution version can be downloaded at
http://cfa-www.harvard.edu/~acrapsi/publications/ Replaced with final version
as published on the journa