Astron. Astrophys. 336, 925-941 (1998)

7. The period distribution and number densities

The period distribution for the Galactic Center OH/IR stars studied here is shown in Fig. 12. The 1612 MHz maser emission from these stars was found in 6 VLA fields with a total area of 3000 arcmin². In our study of 102 fields, each of size 1 arcmin², we found 29 new LPVs with K amplitudes 0.5 magnitudes, consistent with them being Mira-like, large-amplitude LPVs. To get an estimate of the total number of such LPVs in an area equivalent to that searched for OH/IR stars, we multiply by the area ratio 3000/102. The resultant period distribution for all large amplitude LPVs near the Galactic Center is also shown in Fig. 12. It can be seen that the OH/IR stars are only a small part of the total LPV population.

Fig. 12. The period distributions for LPVs near the Galactic Center, in Baade's Window (Glass et al. 1995) and in the Bulge fields studied by Whitelock et al. (1991). In the top panel, the hatched area represents OH/IR stars while the unhatched area represents LPVs detected in this infrared monitoring program but not detected at 1612Mhz. The numbers have been scaled to the area of the 1612 MHz survey. In the bottom two panels, the unhatched areas represent probable foreground objects.

The OH/IR star population and the complete LPV population near the Galactic Center can now be compared with other populations of LPVs in the Galactic bulge. The population of stars in Baade's window studied by Glass et al. (1995) should be relatively complete as it includes LPVs found in optical and near-infrared (I) surveys as well as IRAS sources. This sample of stars, whose period distribution is also shown in Fig. 12, should be directly comparable to the complete sample of LPVs near the Galactic Center. On the other hand, the sample of bulge IRAS sources studied by Whitelock et al. (1991) should be comparable to the sample consisting of Galactic Center OH/IR stars only, both these samples representing stars in the superwind phase near the end of AGB evolution.

Comparison of the complete samples of LPVs in Baade's Window and at the Galactic Center clearly shows that the LPVs near the Galactic Center attain periods much longer (P up to 1100 days) than the LPVs in Baade's Window (P up to 700 days). This is consistent with the existence of AGB stars near the Galactic Center that are more massive than any existing in Baade's Window. A similar conclusion was derived above based on the luminosities of the longest period objects.

Comparing the Galactic Center OH/IR stars with the bulge IRAS sources, we see once again that the OH/IR stars extend to much longer periods than the bulge sources. As noted above, this can be interpreted as being due to a younger population of AGB stars near the Galactic Center. However, the higher metallicity near the Galactic Center must also contribute to the longer periods observed there.

At the short period end, there are clearly LPVs near the Galactic Center with periods down to 200 days. LPVs with even shorter periods may exist but we did not search for them because of the sparse temporal sampling our data. From Fig. 12 it is clear that there are also many LPVs in Baade's Window with periods down to 200 days. As noted above, the existence of Galactic Centre LPVs with periods near 300 days suggests that an old, metal-poor population exists there.

Glass et al. (1996) have reported preliminary results of a search for LPVs in a area near the Galactic Center. They found 250 LPVs with K amplitude greater than 0.6 magnitudes in this area, slightly more than the number of LPVs (180) we would predict in a area from our results. Glass et al. were not able to determine accurate periods for their LPVs, but estimated that about half had 400 days, while very few had 300 days. Our results also show that a large fraction of the LPVs have 400 days. However, unlike Glass et al., we find considerable numbers of LPVs with 300 days. It will be interesting to see if the large Glass et al. survey shows substantial numbers of shorter period (300 days) LPVs when completed.

© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998
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