ﻻ يوجد ملخص باللغة العربية
Determining the structure of and the velocity field in prestellar cores is essential to understanding protostellar evolution.} {We have observed the dense prestellar cores L 1544 and L 183 in the $N = 1 to 0$ rotational transition of CN and thcn in order to test whether CN is depleted in the high--density nuclei of these cores.} {We have used the IRAM 30 m telescope to observe along the major and minor axes of these cores. We compare these observations with the 1 mm dust emission, which serves as a proxy for the hydrogen column density.}{We find that while CNjone is optically thick, the distribution of thcnjone intensity follows the dust emission well, implying that the CN abundance does not vary greatly with density. We derive an abundance ratio of $rm [CN]/[hh]=dix{-9}$ in L 183 and 1-3tdix{-9} in L 1544, which, in the case of L 183, is similar to previous estimates obtained by sampling lower--density regions of the core.}{We conclude that CN is not depleted towards the high--density peaks of these cores and thus behaves like the N-containing molecules nhp and hhh. CN is, to our knowledge, the first C--containing molecule to exhibit this characteristic.
We study the abundance of CCH in prestellar cores both because of its role in the chemistry and because it is a potential probe of the magnetic field. We also consider the non-LTE behaviour of the N=1-0 and N=2-1 transitions of CCH and improve curren
The CS molecule is known to be absorbed onto dust in the cold and dense conditions, causing it to get significantly depleted in the central region of cores. This study is aimed to investigate the depletion of the CS molecule using the optically thin
We investigate general aspects of molecular line formation under conditions which are typical of prestellar cores. Focusing on simple linear molecules, we study formation of their rotational lines by radiative transfer simulations. We present a therm
We studied the abundance of HCN, H13CN, and HN13C in a sample of prestellar cores, in order to search for species associated with high density gas. We used the IRAM 30m radiotelescope to observe along the major and the minor axes of L1498, L1521E, an
We report the detection of D2CO in a sample of starless dense cores, in which we previously measured the degree of CO depletion. The deuterium fractionation is found extremely high, [D2CO]/[H2CO] ~ 1-10 %, similar to that reported in low-mass protost