Issue A&A Volume 433, Number 2, April II 2005 Page(s) 535 - 552 Section Interstellar and circumstellar matter DOI http://dx.doi.org/10.1051/0004-6361:20041914

A&A 433, 535-552 (2005)
DOI: 10.1051/0004-6361:20041914

Chemical evolution in the environment of intermediate mass young stellar objects

NGC 7129 - FIRS 2 and LkH 234

A. Fuente¹, J. R. Rizzo^{1, 2}, P. Caselli³, R. Bachiller¹ and C. Henkel<sup>4

1</sup>  Observatorio Astronómico Nacional (IGN), Campus Universitario, Apdo. 112, 28803 Alcalá de Henares (Madrid), Spain
    e-mail: a.fuente@oan.es
²  Departamento de Física, Universidad Europea de Madrid, Urb. El Bosque, 28670 Villaviciosa de Odón, Spain
³  Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁴  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

(Received 27 August 2004 / Accepted 15 November 2004)

Abstract
We have carried out a molecular survey of the Class 0 IM protostar NGC 7129 - FIRS 2 (hereafter FIRS 2) and the Herbig Be star LkH 234 with the aim of studying the chemical evolution of the envelopes of intermediate-mass (IM) young stellar objects (YSOs). The two objects have similar luminosities (~500 ) and are located in the same molecular cloud which minimizes the chemical differences due to different stellar masses or initial cloud conditions. Moreover, since they are located at the same distance, we have the same spatial resolution in both objects. A total of 17 molecular species (including rare isotopes) have been observed in both objects and the structure of their envelopes and outflows has been determined with unprecedent detail.

Our results show that the protostellar envelopes are dispersed and warmed up during the evolution of the YSO into a pre-main sequence star. In fact, the envelope mass decreases by a factor >5 from FIRS 2 to LkH 234, while the kinetic temperature increases from ~13 K to 28 K. On the other hand, there is no molecular outflow associated with LkH 234. The molecular outflow seems to stop before the star becomes visible.

These physical changes strongly affect the chemistry of their envelopes. The N₂H⁺ and NH₃ abundances seem to be quite similar in the two objects. However, the H¹³CO⁺ abundance is a factor of ~3 lower in the densest part of FIRS 2 than in LkH 234, very likely because of depletion. In contrast, the SiO abundance is larger by a factor of ~100 in FIRS 2 than in LkH 234. CS presents complex behavior since its emission arises in different envelope components (outflow, cold envelope, hot core) and could also suffer from depletion. The CH₃OH and H₂CO column densities are very similar in FIRS 2 and LkH 234 which implies that the beam-averaged abundances are a factor >5 larger in LkH 234 than in FIRS 2. The same is found for the PDR tracers CN and HCN which have similar column densities in both objects. Finally, complex behavior is found for the deuterated compounds. While the DCO⁺/H¹³CO⁺ ratio decreases by a factor of ~4 from FIRS 2 to LkH 234, the D₂CO/H₂CO ratios is within a factor 1.5 in both objects. The detection of a warm CH₃CN component with T_k >63 K shows the existence of a hot core in FIRS 2. Thus far, only a handful of hot cores have been detected in low and intermediate mass stars.

Based on our results in FIRS 2 and LkH 234, we propose some abundance ratios that can be used as chemical clocks for the envelopes of IM YSOs. The SiO/CS, CN/N₂H⁺, HCN/N₂H⁺, DCO⁺/HCO⁺ and D₂CO/DCO⁺ ratios are good diagnostics of the protostellar evolutionary stage.

Key words: stars: formation -- stars: pre-main sequence -- stars: individual: LkH 234 -- ISM: abundances -- ISM: clouds -- ISM: individual objects: NGC 7129

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