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Astron. Astrophys. 327, 755-757 (1997)

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1. Introduction

Since their discovery by Magnani et al. (1985, hereafter MBM), molecular clouds at high latitudes ([FORMULA]) have received considerable attention. The majority of high-latitude clouds are characterized by low average volume densities ([FORMULA] 100 cm-3), masses (in the range 10 to 100 [FORMULA]), and have visual extinctions between 1 and 5 magnitudes (van Dishoeck et al. 1991). A catalog of all the presently known clouds has been compiled by Magnani et al. (1996). From their analysis, Magnani et al. (1996) conclude that the majority of the high-latitude clouds are local objects (the mean distance of the sample is 150 pc), possibly located at the near edge of the Local Bubble. Their ensemble should contribute to 10% to 20% of the total molecular gas budget of the local ISM.

Multitransition observations of several molecular species indicate the existence of small cores with densities in excess of [FORMULA] cm-3 (Turner et al. 1992; Reach et al. 1995). These conditions are very similar to those observed in dark clouds at low galactic latitudes which are well known sites of low-mass star formation. The question has naturally raised whether the translucent molecular clouds share this property with standard dark clouds.

There have been two recent attempts to answer this question. In the first one, Magnani et al. (1995, hereafter MCBB) have compiled a list of all possible star forming sites based on data from the IRAS Faint Source Survey (Moshir et al. 1989). By using far-infrared colours typical of pre-main-sequence stars and young stellar objects (YSOs), they identified 127 candidate sources not associated with regions of known molecular clouds (hereafter referred to as sample A), plus another 65 sources with less reliability (sample B). MCBB estimate that the star formation efficiency in high-latitude clouds would be at most of order of [FORMULA] 1%, assuming that all the faint IRAS sources were genuine young stars of solar mass. In a second paper, Caillault et al. (1995) have used X-ray emission as a diagnostic of the nature of the sources embedded in some high-latitude clouds. A search through the Einstein IPC X-ray database yielded negative results, with the exception of one star located however in a Lynds dark cloud (L1457, MBM12-1). The result was considered not too surprising, and Caillault et al. (1995) concluded that the translucent clouds have yet to reveal any evidence of star formation.

Guided by the MCBB paper, we have set out an experiment aimed at detecting 22.2 GHz water maser emission in a sample of their sources. The presence of [FORMULA] O maser emission reveals dense gas (with densities in excess of [FORMULA] cm-3) in star forming regions. It is well known that [FORMULA] O masers are associated with objects in the earliest evolutionary phases independent of their luminosity/mass. Several surveys towards HII regions (Codella et al. 1994; Codella & Palla 1995), IRAS sources in the inner and outer Galaxy (Wouterloot et al. 1993, 1995), CO outflow sources (Tofani et al. 1995) and low-mass stars (Wilking et al. 1994) have greatly expanded our knowledge of the occurrence of the maser phenomenon. The detection of water masers can therefore be considered a secure identifying criterion even in the case of the infrared sources embedded within high-latitude clouds.

Previous searches for water masers at high-latitudes have yielded only upper limits to the [FORMULA] O flux. In a survey of 1409 IRAS sources selected using the Emerson (1987) colour criteria for YSOs, Palumbo et al. (1994) and Codella et al.(1995) have observed 80 sources with [FORMULA] with an average rms of 2 Jy. Maser emission was found only in two sources within known star forming regions (Orion and HH 7-11).

It is well known that the luminosity of the maser source ([FORMULA]) scales with the far-infrared (FIR) luminosity of the associated IRAS source, and the typical ratio of the luminosities is of order [FORMULA] - [FORMULA] (e.g. Palagi et al. 1993). Since the average luminosity of the IRAS sources in the MCBB sample is [FORMULA] 0.1 [FORMULA], assuming a distance of 150 pc, the expected [FORMULA] O luminosity is about [FORMULA] [FORMULA], or a peak flux of [FORMULA] 0.3 Jy for isotropic emission and a linewidth of 1 km s-1. However, the flux could be quite smaller than the isotropic value and this calls for high sensitivity observations. In this paper, we report on the results of such a survey performed with the MPIfR Effelsberg (Bonn, Germany) 100-m radiotelescope.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998