Volume 470, Number 2, August I 2007
|Page(s)||639 - 652|
|Section||Interstellar and circumstellar matter|
|Published online||25 May 2007|
INAF, Istituto di Radioastronomia, CNR, Via Gobetti 101, 40129 Bologna, Ital e-mail: email@example.com
2 Steward Observatory, The University of Arizona, Tucson, AZ 85721, USA
3 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street MS42, Cambridge, MA 02138, USA
4 INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
5 Max-Planck-Insitut fuer Radioastronomie, Auf dem Huegel 69, 53121 Bonn, Germany
Accepted: 11 May 2007
Aims.We have observed several emission lines of two Nitrogen-bearing (C2H5CN and C2H3CN) and two Oxygen-bearing (CH3OCH3 and HCOOCH3) molecules towards a sample of well-known hot molecular cores (HMCs) in order to check whether the chemical differentiation seen in the Orion-HMC and W3(H2O) between O- and N-bearing molecules is a general property of HMCs.
Methods.With the IRAM-30m telescope we have observed 12 HMCs in 21 bands, centered at frequencies from 86 250 to 258 280 MHz.
Results.In six sources, we have detected a number of transitions sufficient to derive their main physical properties. The rotational temperatures obtained from C2H5CN, C2H3CN and CH3OCH3 range from ~100 to ~150 K in these HMCs. The total column densities of these molecules are of the order of ~1015-1017cm-2. Single Gaussian fits performed to unblended lines show a marginal difference in the line peak velocities of the C2H5CN and CH3OCH3 lines, indicating a possible spatial separation between the region traced by the two molecules. On the other hand, neither the linewidths nor the rotational temperatures and column densities confirm such a result. The average molecular abundances of C2H5CN, C2H3CN and CH3OCH3 are in the range ~10-9-10-10, comparable to those seen in the Orion hot core. In other HMCs Bisschop et al. 2007 found comparable values for C2H5CN but values ~2.5 times larger for CH3OCH3. By comparing the abundance ratio of the pair C2H5CN/C2H3CN with the predictions of theoretical models, we derive that the age of our cores ranges between 3.7 and 5.9 104 yr.
Conclusions.The abundances of C2H5CN and C2H3CN are strongly correlated, as expected from theory which predicts that C2H3CN is formed through gas phase reactions involving C2H5CN . A correlation is also found between the abundances of C2H5CN and CH3OCH3, and C2H3CN and CH3OCH3. In all tracers the fractional abundances increase with the H2 column density while they are not correlated with the gas temperature. On average, the chemical and physical differentiation between O- and N-bearing molecules seen in Orion and W3(H2O) is not revealed by our observations. We believe that this is partly due to the poor angular resolution of our data, which allows us to derive only average values over the sources of the discussed parameters.
Key words: stars: formation / radio lines: ISM / ISM: molecules
© ESO, 2007
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