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

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3. Discussion

Evidence for star formation in high-latitude clouds has been found in half a dozen regions and about twelve T Tauri and Herbig Ae/Be stars and YSOs candidates have been identified (see MCBB). However, these clouds are all Lynds clouds and therefore not primarily translucent. Martín & Kun (1996) have identified spectroscopically four new T Tauri stars among 100 H [FORMULA] candidates discovered by Kun (1992). Since these stars are associated with the dark cloud L134, the authors conclude that no translucent cloud has yet proved to form low-mass stars. In our study, we have observed two isolated T Tauri stars, LkH [FORMULA] 264 and St 202, and one source in MBM12, probably the best studied high-latitude cloud. None of these sources shows maser emission.

What can we say then about the star formation efficiency of high-latitude clouds? MCBB estimate an upper limit of a few tenths of one percent assuming that all the candidates in their sample are indeed young stars of one solar mass. This value is lower than the typical efficiency of few percent determined in low-mass SFRs. On the other hand, in the two dense cores studied by Reach et al. (1995) the derived star formation rate is larger than that in the solar neighbourhood, suggesting that such clouds could be a significant birth site for low-mass stars. It is worth noting that the distribution in the colour-colour plot of the sources associated with high-latitude clouds (see Fig. 1) is indistinguishable from that of the IRAS sources found in the dense molecular cores of nearby star forming regions (see Fig. 3 of Codella & Palla 1995). These sources are the best examples of protostellar candidates: the average FIR luminosity (1 to 10 [FORMULA]) is higher than that of the present sample ([FORMULA] 1 [FORMULA]) and the maser detection rate is correspondingly higher ([FORMULA] 10%). Our negative results indicate that, even accounting for the high degree of variability, water maser emission is a rare phenomenon in high-latitude clouds and that the efficiency of star formation in these regions could be much lower than the suggested value of a few tenths of a percent. This is consistent with the fact that most high-latitude clouds do not appear to be gravitationally bound and in the process of forming stars.

[FIGURE] Fig. 1. [25-12] vs. [60-12] colour distribution of the 192 IRAS point sources associated with translucent clouds from MCBB (see text). The 91 sources observed at 22.2 GHz are shown as filled circles. The boxes delimit the colours of T Tauri stars (Emerson 1987) and ultracompact (UC) HII regions (Wood & Churchwell 1989).
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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998