Astron. Astrophys. 347, 565-571 (1999)

1. Introduction

Outflows from young stars are an important ingredient in the star formation process. Through their momentum input into the surrounding medium they may have a major impact on the evolution of their parental molecular clouds. They could support these clouds against further collapse, but in many cases more likely will even disperse them with time. It is therefore important to find out how many outflows are active in a certain cloud.

So far, most of the searches for outflows concentrated on single sources that were known to exhibit outflow signatures in their spectra (e.g., Mundt & Eislöffel 1998). Such searches among pre-selected sources are, however, highly biased in several ways. Therefore, surveys of whole star forming clouds are necessary for a better census of the outflow activity. Recently, several such surveys for outflows have been carried out, following two different approaches: some surveys have searched for molecular hydrogen emission from outflows in the near-infrared 1-0 S(1) line at 2.12 µm (NGC 1333: Hodapp & Ladd 1995; L1641N: Stanke et al. 1998). Other surveys have been done in the optical [SII ]6716,6730 lines or H (NGC 1333: Bally et al. 1996; the Oph dark cloud: Wilking et al. 1997, Gómez et al. 1998; L1641N: Reipurth et al. 1998).

While searches in the near-infrared are less susceptible to extinction, searches in the optical can cover larger areas more rapidly because of the vastly larger fields of view of optical wide field imagers as compared to near-infrared cameras. Moreover, even deeply embedded molecular H₂ outflows, like L1448 (Eiroa et al. 1994) or Cep E (Noriega-Crespo 1997) show optical [SII ] emission at some distance from the source, where the extinction is lower.

Serpens is a nearby (310 pc, DeLara et al. 1991) star forming region, which has received considerable attention, because a large number of young and very young sources have been found there. Especially the striking Serpens Reflection Nebula (SRN) has attracted attention, and the stellar content of its embedded cluster has been studied at optical (Gomez de Castro et al. 1988), near-infrared (Eiroa & Casali 1992, Giovannetti et al. 1998), and mm-wavelengths (Chini et al. 1997; Reipurth et al. 1993). Recently, it has also been mapped with mm-interferometry (Testi & Sargent 1998). Apart from the stellar sources, a poorly defined molecular CO outflow has been found in the region of the SRN (Bally & Lada 1983). Subsequent observations at higher spatial resolution (White et al. 1995) showed that this is not a single flow, but in fact a superposition of several CO outflows. Consequently, the region around the SRN has also been mapped in the near-infrared 1-0 S(1) line of molecular hydrogen at 2.12 µm (Eiroa et al. 1997; Herbst et al. 1997; Huard et al. 1997), and a H₂ jet from the Class I source CK 8, as well as a large number of other H₂ emission features were found.

The region covered by these near-infrared observations also includes two other interesting objects: firstly, the enigmatic Serpens triple radio source (Rodríguez et al. 1980). This source consists of a VLA radio continuum source, and a remarkable non-thermal bipolar radio jet with knots moving at high proper motions (Rodríguez et al. 1989, Curiel et al. 1993). Secondly, a strongly variable young stellar object has been found about one arcmin north of the radio source, which seems to have undergone an EXor outburst (Hodapp et al. 1996).

In a search on a red Schmidt plate and with follow-up CCD imaging in the H line Reipurth & Eiroa (1992) found four Herbig-Haro objects HH 106 to 109 in Serpens, all situated further away from the SRN than the sources discussed so far. All this evidence for ongoing star formation and outflow activity has prompted us to carry out a large-scale search for HH objects and flows from young stars in Serpens.

Our observations are described in Sect. 2. In Sect. 3 we present the HH objects found in our survey, while in Sect. 4 we discuss possible sources for these HH objects and identify several extended flows.

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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