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

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2. Observations and results

We have observed 53 IRAS sources listed by MCBB in sample A with a declination greater than [FORMULA]. In addition, we have selected 3 X-ray sources from the list of Caillault et al. (1995) which have probably galactic stellar counterparts. Table 1 gives the sources of sample A with their identification number, the IRAS name, the equatorial (1950) coordinates, and the observed rms (1 [FORMULA])in the [FORMULA] O line. The three sources from Caillault et al. (1995) are labelled by their MBM number. We have also chosen 35 sources from sample B with the same characteristics. Their properties are listed in Table 2. Thus, our final lists contain a total of 91 sources.

The observations were made with the 100-m antenna during several runs in August and September 1995 and September 1996. At the frequency of the water maser line (616 [FORMULA] 523 ; 22235.07985 MHz) the HPBW is [FORMULA]. The system temperature ranged from 70 K to 140 K depending on the weather conditions. The intensity scale of the spectra was calibrated on the continuum sources NGC7027 and 3C286, using the values given by Baars et al. (1977) and Ott et al. (1994). The calibration accounted for the dependence of the gain on the elevation. The flux density uncertainties are of the order of 20%. The pointing accuracy was better than [FORMULA], corresponding to a point source sensitivity uncertainty of about 15%. The observations were obtained in total power mode using position switching, with an integration time of about 20 minutes on source for those listed in Table 1 and of 3 minutes for the sources in Table 2. Spectral information was obtained with a 1024-channel three-level autocorrelation spectrometer, providing 0.330 km s-1 sampling and a total velocity coverage of [FORMULA] 160 km s-1 across a 25 MHz bandpass. The average detection level (3 [FORMULA]) is 0.18 Jy for sample A and 0.51 Jy for sample B.

We did not observe maser emission features in any of our targets. The overall negative result of the survey is not totally unexpected for a variety of reasons. The first one is that the IRAS sources identified by MCBB are very weak. Studies of water masers associated with low-mass stars have shown that the frequency of occurrence decreases with the infrared luminosity of a source and that the emission is highly variable, sometime reaching two orders of magnitude in a period of several months (Persi et al. (1994 ), Codella et al. (1996 ), Wilking et al. (1994 ) and Claussen et al. (1996 )). Perhaps more importantly, there is still no direct evidence that the faint IRAS sources at high-latitudes are indeed associated with molecular gas and/or with YSOs. In fact, the translucent high-latitude clouds are often identified with the molecular component of the IRAS 100 [FORMULA] m cirrus and it is very likely that the majority of the IRAS sources observed by us are infrared cirrus clumps associated with atomic and not molecular hydrogen. In support of the latter possibility, we mention the case of the source MCBB77. In the course of the first run, we had tentatively detected a weak emission feature which motivated us to search for the presence of molecular gas in the region and to confirm the association of the IRAS source with a new star forming site. We have observed several molecular transitions using the 100-m MPIfR and the 30-m Pico Veleta antennas, including 12 CO ([FORMULA]), [FORMULA] (1,1), [FORMULA] (2,2), but did not detect emission down to about 0.1 K in CO and 0.07 K in ammonia. Similarly, continuum observations at 1.3 millimeter did not show the presence of cold dust in the region. Subsequent observations at 22 GHz did not confirm the occurrence of water maser emission. These negative results indicate that the presence of a faint IRAS source is not necessarily an indication of ongoing star formation.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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