Large-amplitude variables near the Galactic Centre

Abstract

We report here the results of a 4-yr K-band (2.2 μm) survey for large-amplitude variable stars in a 24 × 24 arcmin2 area centred on the Galactic Centre. A total of 409 likely long-period variables (LPVs) were detected, for which positions, amplitudes, average magnitudes and periods were obtained whenever possible. The surface density of LPVs is more than ten times greater than in the Sgr I Baade window at l = -1°.37, b = -2°.63. The limits of completeness arising from interstellar and circumstellar absorption are discussed. Most of the area suffers interstellar extinction of 20 < AV < 40 mag. The shorter-period LPVs are less luminous than the longer-period ones and may be slightly under-represented in the data. Extremely heavy extinction (AV > 40 mag), which affects the probability of detecting variables, occurs in less than 25 per cent of the area. Almost all of the LPVs are Miras or OH/IR stars, with periods ranging from 150 d to about 800 d. K-band counterparts have been found for 59 per cent of the 109 known OH sources in the field. The average period of the variables found is 427 d, while that of the OH/IR stars is 524 d. For comparison, the average period in the Sgr I window, which contains no known OH/IR stars, is 333 d and only two stars are detected with P > 600 d. The survey field also contains a number of long-period, large-amplitude variables that are not OH emitters.