Astron. Astrophys. 318, L13-L16 (1997)

3. Results and discussion

In this section we will briefly discuss the four sources in our field that are possible candidates for young deeply embedded (proto-)stars. All of the sources have a colour index ( - ) greater than 9, calculated as the difference between the observed magnitudes. Such large excess is probably due to the reprocessing of the stellar radiation by optically thick dust cocoons. In fact, for both unreddened main sequence and evolved stars (giant branch and asymptothic giant branch stars), the value of the colour index should be in the range 0-3. Reddening in the galactic plane cannot increase this value up to 9 even for very distant objects.

3.1. The H₂ O maser source

In Fig. 1 the spectrum of the H₂ O maser observed at Medicina is reported. The velocity of the two main components () are on the lowest side of the values expected for this galactic longitude. A "mean" velocity of could be obtained for objects at large distance (). In the Columbia ¹² CO survey (Cohen et al. 1986) and in the radio recombination line observed toward the nearby HII region G (Lockman 1989) the bulk velocity is . No H₂ O emission was detected at this velocity (in Fig. 1 only a small part of the spectrum is shown). In Table 1 the parameters of the observed H₂ O maser and of the closest ISO source together with its associated K-band and m source are reported.

Fig. 1. Spectrum of the H2 O maser observed at Medicina. The intensity scale is in Jansky.

Table 1. Parameters of the H₂ O maser, and of the associated infrared source

Due to the extremely large number of K-band sources in the field () and the relatively large astrometric error, a reliable association between 15µm sources and K-band sources is a major problem in these regions, which is presently being studied in a thorough statistical way. In this case we have considered all the K sources detected in our image within from the ISO source, finding only three, very faint candidates. The source given in Table 1 is the closest to the ISO source (), the other two are at a distance of and respectively. All have a magnitude greater than 14.4 in K, implying that, regardless of which of them is associated with the ISO source, the ( - ) colour index is in any case greater than 10. Note that even taking into account the flux calibration uncertainties of the 15 m and the K-band data, the resulting colour index must exceed 9.5 (which remains one of the highest measured).

Four other 15µm sources are detected close to but outside the error box of the H₂ O maser position. The closest is at , and the others are at , from the maser position. Although the possibility that the H₂ O maser may be associated with one of these sources cannot be completely ruled out, this seems unlikely, and in the following we will assume that the maser is associated with the closest 15µm source. Certainly higher resolution observations of the H₂ O maser are needed to settle this point.

Somewhat surprisingly, there is no IRAS point source closely related to this source. The error ellipse of the closest IRAS-PSC source (IRAS19104+1040) is shown in Fig. 2, and it is almost coincident with a 15µm source which is not associated to masers and does not show a large infrared excess ( - ). Given the large infrared excess of our source one would expect a steeply rising spectrum toward the longer wavelengths. Inspection of the HIRES IRAS maps shows indeed that there are two sources in a area, and that there is a peak at 25 m at the position of the ISO source associated with the water maser.

Fig. 2. Greyscale 15 m image. The small ellipse represents the IRAS-PSC error box, the circle marks the position of the H2 O maser, the diameter of the circle is approximately equal to the Medicina pointing error box. The epoch of the coordinates is 2000.

One important question to be addressed is if the maser is associated with a late type star or with a young stellar object. In fact, sources with large H₂ O spectra and associated with IRAS have been suggested to be late type stars (see e.g. IRAS in Engels et al.  1984, and IRAS16342-3814 in Likkel & Morris  1988). However, we believe that the ISO source is probably a YSO, even though only detection of OH maser or CO emission could settle this point. The extreme colour index observed for the infrared source can be explained only in terms of warm dust emission, most probably heavily extincted at K. From the fluxes reported in Table 1 it can be clearly seen that the spectral energy distribution of this source is steeply rising toward the long wavelengths, with a spectral index greater than 2, as expected for young stellar objects still embedded in optically thick cocoons (Shu et al. 1987).

No radio continuum emission was detected from this source at 20 cm in the VLA survey of Zoonematkermani et al. (1990). This implies that no HII region has formed or that, if present, is too faint or compact (and self-absorbed) to be detectable at 20 cm, thus confirming the young nature of the object.

3.2. Other Young Stellar Objects candidates

In the field observed at 15µm we found a surprisingly large number (210) of sources with a ( - ) colour index greater than 6, corrensponding to sources per square degree. Among these, three have a colour index greater than 9. None of these three sources are near the position of the HII region G , which was not observed with ISOCAM to avoid saturation. A remarkable difference with this bright radio HII region, is that none of the 15 m sources have a radio continuum counterpart in the 20 cm VLA observations of Zoonematkermani et al. (1990), which excludes the presence of an evolved HII region, leaving only the possibility of an associated UCHII region, optically thick at 20 cm.

In order to check that the large colour index observed is not due to a spurious association with a faint background K-band star, but due to an intrinsic infrared excess of the sources, we have searched around the position of the candidate 15µm sources for the presence of bright nearby K sources, but with no success. The main difference in the infrared morphology between these three sources and that associated with the maser is that all of them show faint extended emission (on arcminute scale) at 15µm. The nature of this extended emission is currently under investigation. All the three sources coincide with an IRAS point source, but only in one the IRAS colours are those of a typical UCHII regions (Wood & Churchwell 1989). None of these sources have yet been observed in the H₂ O maser line.

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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