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Astron. Astrophys. 335, 243-247 (1998)

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1. Introduction

In the first half of this decade, André et al. (1993) recognized a new class of cool, low-mass protostellar objects, which they called Class 0 sources and which are the youngest objects known, with ages of the order of [FORMULA] years. Observationally, Class 0 objects (also often referred to as submillimetre protostars) are characterized by having strong submillimetre luminosity, no (or very weak) near-mid IR emission, and a spectral energy distribution (SED) which can be fitted by a single-temperature black body (André et al., 1993). The submillimetre luminosity is a relative measure of the total circumstellar mass, which is higher than the central stellar mass in the Class 0 sources (e.g. André 1997). Virtually all of them drive collimated molecular outflows (Bachiller 1996) and, in some cases, shock excited H2 emission (e.g. Davis et al. 1994) or HH objects (Eiroa et al. 1994) have been detected. All the Class 0 sources known to date are low-luminosity objects and, consequently, low-mass protostars (see the list compiled by Bachiller 1996). There is, however, no reason why more massive class 0 objects, the protostellar progenitors of Herbig AeBe (HAeBe) stars, should not also be detected. In fact, intermediate-mass stars have a pre-main sequence evolution with times spanning several 105 years (e.g. Palla & Stahler 1990, 1993).

NGC 7129 is a reflection nebula immersed in a very active and complex molecular cloud (see e.g. Hartigan & Lada 1985, Miranda et al. 1993, and references therein). The distance to the region has been estimated to be 1 kpc (Racine 1968), which is the value usually assumed for NGC 7129. Shevchenko & Yakubov (1989) have estimated a distance of 1250 [FORMULA] 50 pc. In this paper, we will assume a value of 1 kpc for our calculations, since the small difference of both distance estimates does not qualitatively affect the main conclusions. The nebula is illuminated by several HAeBe stars, the most prominent being LkH [FORMULA] 234, which form part of a PMS stellar cluster. Bechis et al (1978) and Harvey et al. (1984) detected far-infrared extended emission and two point-like sources. One of these is associated with the star LkH [FORMULA] 234 and the second, called FIRS 2, lacks any optical or near-infrared counterpart. FIRS 2 coincides with a 13 CO column density peak (Bechis et al. 1978), with a high-density NH3 cloudlet (Güsten & Marcaide 1986), and is close to an H2 O maser (Rodriguez et al. 1980). In addition, a bipolar CO outflow is also associated with FIRS 2 (Edwards & Snell 1983).

In this work we present (sub)millimetre observations of FIRS 2 and IRAS maps of the whole NGC 7129 region. The main result of the analysis of these data is that FIRS 2 is an intermediate-luminosity object with properties similar to those of the low-mass Class 0 sources. This suggests that we have found a very young object, FIRS2, which is most likely to be an intermediate-mass counterpart of the Class 0 protostars.

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© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998