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Subsections

5 Discussion


5.1 Distribution in the Galaxy

Several attempts to establish a distribution of methanol masers in the Galaxy have been done. It was shown, that the masers are concentrated mostly in the inner Galaxy (see e.g. van der Walt et al. 1995). MacLeod & Gaylard (1992) reported an asymmetry in the distribution of 6.7 GHz masers in the inner Galaxy: the number of masers, detected in the first quadrant is about 20% lower than the number of masers, detected in the fourth quadrant. MacLeod & Gaylard (1992) and Schutte et al. (1993) concluded that the probability to detect a 6.7 GHz maser toward an IRAS source with colours typical for ultracompact HII regions in the Carina arm is lower than in the inner arms. They suggested that it may be due to the combination of sensitivity limitations and the Galactic metallicity gradient. However, Gaylard & MacLeod (1993) made a deeper search at 6.7 GHz in which they detected several 6.7 GHz sources in the Carina arm and concluded that the low detection rate in the Carina arm reported by Macleod & Gaylard (1992) and Schutte et al. (1993) is due to limited sensitivity of their surveys.

It is of interest to study the detection rate of methanol masers in the whole Galaxy. As a first step, we made a histogram of the detection rate versus galactic longitude (Fig. 2). The detection rate was calculated as the ratio of the number of detections to the total number of reported observations per galactic longitude bin. The histogram shows that the probability to detect the 6.7 GHz methanol masers is higher in the inner Galaxy. It does not change very much if one uses only IRAS sources selected by Wood & Churchwell colour criteria.

  
\begin{figure}
{
\includegraphics [width=8.8cm]{fig2.eps}
}\end{figure} Figure 2: 6.7 GHz methanol maser detection rate versus galactic longitude

The distribution of 6.7 GHz masers on the Galactic plane is shown in Fig. 3. This diagram is based on our data and the results of Menten et al. (1991a); Schutte et al. (1993); MacLeod & Gaylard (1992); Caswell et al. (1995a) and van der Walt et al. (1995). In total, 270 sources were taken into consideration. Only near kinematic distances were used in cases of ambiguity. No kinematic distances were calculated in the zone, restricted by two dashed lines, i.e., closer than 10$^{\circ}$ to the center-anticenter line, where kinematic distances are highly unreliable. The cross point of the dashed lines represents the position of the Sun; the Galactic center is located at the (0,0) position. The arc in the left part of the diagram is the Sagittarius arm.

 
\begin{figure}
{
\includegraphics [width=8.8cm]{fig3.eps}
}\end{figure} Figure 3: The distribution of 6.7 GHz masers on the Galactic plane


5.2 Velocity dispersion

The 6.7 GHz methanol masers typically show a significant velocity offset from the radial velocity of host molecular clouds. Figure 4 shows the histogram of the absolute difference between the radial velocity of the strongest methanol feature and the radial velocity of thermal CS lines (in some sources it was CO) for 157 methanol masers taken from this survey and surveys by Menten (1991a); Shutte et al. (1993); van der Walt et al. (1995), and Caswell et al. (1995a) and for which the cloud velocity was available.

  
\begin{figure}
{
\includegraphics [width=8.8cm]{fig4.eps}
}\end{figure} Figure 4: Modulo of velocity difference between the 6.7 GHz peak velocity and CS velocity

The average difference is 5.5 $\pm$ 0.7  km s-1, which is a factor of two to three larger than the thermal velocity dispersion, even in hot molecular clouds. In some sources this difference is larger than 10 km s-1. The velocity shift can be seen in the spectra in Fig. 1 where the CS velocity is shown by dashed lines. The methanol spectra themselves show a large velocity dispersion, the difference between extreme spectral features is typically 5 to 15 km s-1, with 25 to 30 km s-1 in some sources. Figure 5 shows a histogram of the velocity range for 157 6.7 GHz methanol masers taken from all available surveys (this paper, Menten 1991a; Shutte et al. 1993; van der Walt et al. 1995, and Caswell et al. 1995a).

  
\begin{figure}
{
\includegraphics [width=8.8cm]{fig5.eps}
}\vspace{-3mm}\end{figure} Figure 5: The distribution of 6.7 GHz methanol masers velocity range

Is is evident that the velocity range less than 2 km s-1 is rare, the mean velocity range is 8.0 $\pm$ 0.5 km s-1, and exceeds velocity dispersion of host molecular clouds. In this respect the 6.7 GHz methanol masers are similar to H2O masers with high velocity spectral features, although to a smaller scale. One possible explanation of the large velocity dispersion in the spectra of 6.7 GHz methanol masers could be related to the excitation mechanism. A large velocity gradient can provide easier escape of photons, a condition required in some maser models to maintain the population inversion.

Another possible explanation of the large velocity dispersion is that the maser condensations are gravitationally bound to massive stars, and are circling around them with Keplerian velocities. If the mass of the star is $30\ M_\odot$ (O-star), then the Keplerian velocity of 5 km s-1 corresponds to a distance from the star of 1080 A.U. The observed linear separation between 6.7 GHz maser spots is of this order of magnitude (Norris et al. 1993). Moreover, in many cases the maser spots lay along straight lines or arcs, and it was suggested (Norris et al. 1993) that they originate in circumstellar disks. The large range of methanol maser velocity features is consistent with their circumstellar origin. The massive O-stars associated with 6.7 GHz methanol masers could be very young, newly born stars embedded in a dense dust molecular core. The star itself is invisible because of a large extinction in the core, but could ionize an ultracompact HII region, which can be detected as a continuum radio source. The association of 6.7 GHz methanol masers with ultracompact HII regions established in this study (see Sect. 5.3) corroborates this conclusion.


5.3 6.7 GHz masers and ultracompact HII regions

As noted in the introduction, our observing sample contained 326 IRAS sources which we selected with the colour criteria for ultracompact HII regions by Wood and Churchwell (1989). For this sample the methanol maser detection rate was 11 per cent. Of the total of 326 IRAS sources 76 coincided with ultracompact HII regions detected as compact continuum radio sources at 5 GHz by Becker et al. (1994) in a deep survey with the VLA. In 19 of them 6.7 GHz methanol masers were detected. The corresponding detection rate for the compact radio sources is 25 per cent which is considerably higher than the mean rate 11 per cent for the whole IRAS sample. Therefore the compact thermal continuum radio sources are much better candidates for detection of 6.7 GHz methanol masers than IRAS sources selected solely with Wood and Churchwell criteria.


5.4 The comparison of 6.7 GHz and 44 GHz masers

It was noted by Slysh et al. (1994c) that 6.7 GHz and 44 GHz masers co-exist in many star forming regions, although they belong to different classes of methanol masers and require different physical conditions for the inversion to take place. It was further noted that there is no one-to-one correspondence in their spectra, and there is a tendency of anticorrelation of flux densities between 6.7 GHz and 44 GHz masers, which may appear since external radiation, necessary to invert the 50-61A+ transition, quenches the 44 GHz masers (Cragg et al. 1992). The anticorrelation can be assessed in Fig. 6 which was constructed using an extended set of sources. In Fig. 6 we show a histogram of the $\log$ flux density ratio between 6.7 GHz and 44 GHz masers in the same sources. The distribution extends from 0.01 to 200 with a dip at -0.2. Sources predominantly emitting at 44 GHz ($\log$ ratio less than -0.2) belong to Class I while those with ratio greater than -0.2 are Class II masers. So the ratio between 6.7 GHz and 44 GHz is a good parameter for the classification of methanol masers.

  
\begin{figure}
{
\includegraphics [width=8.8cm]{fig6.eps}
}\vspace{-3mm}\end{figure} Figure 6: The flux density ratio between 6.7 GHz and 44 GHz masers in the same sources


5.5 Far Infrared Pump

Following Kemball et al. (1988), we found a correlation between 6.7 GHz methanol flux density and 60 $\mu$m flux density of the associated IRAS sources (Fig. 7). The sample in the diagram contains all the known methanol masers which have IRAS identification. With the exception of seven masers, the 6.7 GHz flux is lower than one fourth of 60 $\mu$m flux. This result is similar to that of Kemball et al. (1988) for 12 GHz methanol masers, but is based on a larger sample of sources and confirms that there is enough far-infrared quanta to pump the 6.7 GHz methanol masers. Van der Walt et al. (1995) reached a similar conclusion based partly on the same observational results. A result of Ellingsen et al. (1996) survey, that shows that there can be a methanol maser without any FIR emission, is in contrast with this conclusion, but it may be due to confusion, as the survey has been done in the direction of the Galactic Center. An unbiased survey of a less dense portion of the galactic plane would help to clarify this point.

  
\begin{figure}
{
\includegraphics [width=8.8cm]{fig7.eps}
}\vspace{-3mm}\end{figure} Figure 7: Maser flux as a function of 60 $\mu$m flux of associated IRAS objects


5.6 Variability

It is well established, that the 6.7 GHz masers are variable (see, e.g., Caswell et al. 1995b). The study of maser variability, which requires more systematic and long term observations, was beyond the scope of this work; however, a comparison of our spectra with those published in the literature shows variations larger than the estimated errors for some sources. In those cases when the spectral resolution is different, as in van der Walt et al. (1995), or when the observed positions differ by a sgnificant fraction of the telescope beamwidths, such as in the case of 02455+6034 and 18324-0820, the variability of the source cannot be asserted. MacLeod & Gaylard (1992); Gaylard & MacLeod (1993); Schutte et al. (1993); van der Walt et al. (1995) made their surveys with the 26-m Hartebeesthoek radio telescope whose beamwidth is 7 arcmin, close to the Medicina value, and their observed positions are either the same as ours or the difference is not larger than 20-30 arcsec. Therefore when the spectra, obtained by these authors, differ from ours (in 174.20-0.08, 17589-2312, 18310-0825, 18379-0546, 78.12+3.63) these differences most likely may be attributed to the source variability. This conclusion is particularly true in the case of 174.20-0.08 where the number of the spectral components and the relative intensities are different. In the case of 17599-2148 we have a non-detection while it was observed by Schutte et al. (1993) with $S_{\nu}=14.6$ Jy well over our sensitivity limit. Other examples of possible variability are our detection of weak masers towards 18064-2008, 18236-1241, and 19220+1432, where no 6.7 GHz masers were found by van der Walt et al. (1995). Thus, our results, like those by Caswell et al. (1995b), show that the 6.7 GHz masers are often variable.



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