Long Period Variables in the Magellanic Clouds: OGLE + 2 MASS + DENIS

Abstract

The 68 000 I-band light curves of variable stars detected by the  survey in the Large and Small Magellanic Clouds (MCs) are fitted by Fourier series, and also correlated with the  and 2 all-sky release databases and with lists of spectroscopically confirmed M-, S- and C-stars. Lightcurves and the results of the lightcurve fitting (periods and amplitudes) and  and 2 magnitudes are presented for 2277 M-, S-, C-stars in the MCs. The following aspects are discussed: the K-band period-luminosity relations for the spectroscopically confirmed AGB stars, period changes over a timespan of about 17 years in a subset of about 400 LPVs, and candidate obscured AGB stars. The use of a sample of spectroscopically confirmed variables shows specifically that almost all carbon stars are brighter than the tip of the RGB, and occupy sequences A+, B+, C and D. It is shown (for the LMC where there is a sufficient number of spectroscopically identified M-stars) that for sequences A+ ,B +, C the M-stars are on average fainter than the C-stars, as expected from an evolutionary point of view and previously observed in MC clusters. However, this is not so for sequence D, suggesting that the origin of the so-called Long Secondary Periods is not related to an evolutionary effect. The fraction of objects that has a period in sequence D is also independent of chemical type. Three stars are identified that have been classified as oxygen-rich in the 1970s and carbon-rich in 1990s. Possibly they underwent a thermal pulse in the last 20 years, and dredged-up enough carbon to switch spectral type. The observations over almost two decades seem to suggest that up to 10% of AGB variables changed pulsation mode over that time span. More robust estimates will come from the ongoing and future (microlensing) photometric surveys. A sample of 570 variable red objects ((J − K) > 2.0 or (I − K) > 4.0) is presented in which most stars are expected to be dust-obscured AGB stars. Estimates are presented for cut-off si n (J −K) which should be applied to minimise dust obscuration in K, and based on this, C- and O-star K-band PL-relations for large amplitude variables in the SMC and LMC are presented.