Astron. Astrophys. 320, 594-604 (1997)

1. Introduction

A reference working model for the cradle of newly born luminous stars has been around for many years. Since the star itself is not visible, all the attention is focused on the cradle itself, i.e.: UC HII regions, hot molecular cores, cool dust envelopes responsible for the FIR emission, hot dust cocoons bright in the near IR, bipolar outflows, H₂ jets, masers, etc. However, many of these features usually occur on widely different scales. Consequently, the correlation among them may often be confused, and misleading conclusions can be reached, because of the lack of matching (high) resolutions.

Different and complementary approaches have been used to search for the earliest phases of massive stars. Several pieces of evidence suggest that H₂ O masers are one of the best indicators for selecting the target fields and that the study of their association with near IR sources, UC HII regions and hot molecular cores are the necessary follow-ups to unveal the earliest YSOs. In fact, even though originally H₂ O masers were discovered in diffuse HII regions, it has now become clear from surveys of IRAS selected sources (Palla et al. 1993) that H₂ O masers not associated with diffuse HII regions may be the majority, up to 80%, suggesting that the maser emission occurs in an earlier phase, much before the onset of an (UC) HII region (Codella et al 1994, Codella & Felli 1995). Also, on simple energetic arguments (Elitzur et al. 1989), H₂ O masers must be very close to the stellar source of energy (from 10¹⁴ to 10¹⁶ cm). Consequently, VLA maser positions with a precision better than offer the most accurate locations to be searched.

The new scenario that emerges from these studies is that in the very early stages bounded ionized winds or UC HII regions maybe present around the star and close to the H₂ O maser, but so small and optically thick that they are undetectable in the radio continuum. These early stages are usually highly obscured also at K band and maybe observable in the near IR thanks to the emission of hot dust near the star, or in molecular lines sensitive to high densities (Cesaroni et al. 1994). As the ionized region expands, the HII region becomes detectable, the H₂ O maser disappears and the K band dust emission strongly decreases (Testi et al. 1994; Hunter et al. 1995; Tofani et al. 1995; Felli et al. 1996).

In the present paper we test these ideas on the S 235 A - B star forming complex. S 235 is the most prominent of a group of optical nebulosities which lie toward the anticenter of our Galaxy. The area around S 235 contains traces of both advanced evolutionary stages (S 235 itself) and less evolved stages such as three small optical nebulosities: S 235 A and S 235 B about south of S 235 and apart from each other, and S 235 C, further to the south. S 235 is connected to an extended molecular cloud at -20 km s^-1, while S 235 A, S 235 B and S 235 C are embedded in a smaller molecular cloud at -17 km s^-1, elongated in the north-south direction and with a maximum extension of . The region has been mapped in several molecular lines, but with a resolution of the same order as the mutual separation between S 235 A and S 235 B, so that it is difficult to make precise association of molecular peaks with either of the two nebulosities (Evans & Blair 1981; Ho et al. 1981; Sandell et al. 1983; Lafon et al. 1983; Stutzki et al. 1984; Nakano & Yoshida, 1986 [hereafter NY]; Wilking et al. 1989; Snell et al. 1990; Plume et al. 1992). S 235 A and S 235 B are also strong near IR sources, known as IRS3 and IRS4 (Evans & Blair 1981; Evans et al. 1981). Both are associated with H emission (Krassner et al. 1982), but only S 235 A has corresponding radio emission (Israel & Felli 1978), which suggests a classical HII region nature. S 235 B, despite many attempts, remains undetected in the radio continuum. Its nature is still a puzzle, expecially considering that it has strong Br and Br emission (Krassner et al. 1982; Thompson et al. 1983). A highly self-absorbed UC HII region or a stellar wind have been put forward as possible explanations, but are somehow in contradiction with its bright aspect in H , which would exclude a strong local obscuration. S 235 C is also detected in the radio continuum, has a partial shell morphology and is associated with a HH object.

In between S 235 A and S 235 B a highly variable H₂ O maser has been detected (see e.g. Persi et al. 1994, Tofani et al. 1995). No radio continuum emission from a point source () down to 0.3 mJy at 8.4 GHz has been found in a region in radius around the maser (Tofani et al. 1995). Three possible sources of excitation of the maser have been considered: 1) the S 235 A HII region, 2) the S 235 B YSO and 3) a so far undetected YSO closer to the maser (Tofani et al. 1995). While the first one is improbable based on simple morphological arguments (basically the large distance of the maser from the outer boundary of the HII region), the remaining two (as well as the true nature of S 235 B) demand more observations. We present here arcsec resolution near IR images, high resolution molecular observations and an extended patrol of the H₂ O maser emission which can improve our understanding of this star forming region and possibly reveal the true location of a new YSO in a very early phase.

A distance of 1.8 kpc will be assumed, following NY, for the molecular cloud and the stellar cluster .

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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