Infrared monitoring of OH/IR stars near the Galactic Center

We have monitored 102 fields of size 1′ × 1′ within ∼0.7◦ of the Galactic Center for 1200 days in the K band. Each field was centered on one of the OH/IR stars found by Lindqvist et al. (1992). Infrared colours J−K, H−K and K−L were also obtained. Periods have been determined for 80 known OH/IR stars, including 5 from Sjouwerman (1997), and a further 29 previously unknown long-period variables (LPVs) of large (> 0.5 mag.) K amplitude. The overall period distribution of the variables extends from ∼200 days to ∼1100 days and it is dominated by LPVs with P < 600 days. The existence of LPVs with P < 300 days indicates the presence of sub-solar metallicity, old, low mass stars near the Galactic Center. These old LPVs have luminosities equal to those of similar LPVs elsewhere in the Galaxy. However, the LPVs with P > 300 days near the Galactic Center have lower luminosities and higher wind expansion velocities vexp at a given period than similar stars in the Galactic bulge or solar vicinity, indicating that the metal abundance of Galactic Center stars is ∼2-4 times solar. Another consequence of the observed low luminosities of the Galactic Center LPVs is that the (Mbol, logP ) and (K, logP ) relations must be metallicity dependent, at least for metallicities of solar or above. The maximum periods and luminosities of Galactic Center LPVs exceed those of bulge LPVs suggesting that there are AGB stars near the Galactic Center which are more massive than any stars in the bulge. AGB masses up to ∼4 M are relatively common, with a few AGB stars having masses up to ∼7 M . There should be ∼1000 main-sequence precursors in the same volume for each of these massive AGB stars. The high expansion velocity (vexp > 18 km s−1) OH/IR stars are separated from the lower expansion velocity OH/IR stars in the (Mbol, logP ) diagram: this separation suggests that the stars with higher vexp have higher metallicity, as well as being younger. The overall results suggest that ongoing star formation and metal enrichment have occurred near the Galactic Center.