On the pulsation modes and masses of RGB OSARGs

Abstract

OSARG (OGLE Small Amplitude Red Giants) variables are RGB or AGB stars that show multi-periodic light variations with periods of about 10-100 days. Comparing linear nonadiabatic pulsation periods and period ratios with observed ones, we determined pulsation modes and masses of the RGB OSARG variables in the LMC. We found that pulsations of OSARGs involve radial 1st to 3rd overtones, p4 of l = 1, and p2 of l = 2 modes. The range of mass isfound to be 0.9-1.4Mfor RGB OSARGs and their mass-luminosity relation is logL/L� = 0.79M/M� + 2.2. 2. MODELS We have obtained linear nonadiabatic pulsation periods for envelope models along the evolutionary tracks calculated with the MESA code (3) for several initial masses and adopting a mixing-length of 1.5 pressure scale height. We have adopted the chemical composition (X,Z) = (0.71,0.01) for the LMC and used OPAL opacity tables (4). 3. MODE IDENTIFICATION Since the pulsation period itself depends on radius and mass, the period ratios are useful to identify pulsation modes, while pulsation periods are used to determine the appropriate luminosity (or mass) ranges. The left panel of Fig. 1 shows the period versus period-ratio diagram (also called Petersen diagram) for radial pulsations of 1.1Mmodels with luminosity (logL/L� ) between 2.7 and 3.35, and the observed values for the RGB OSARGs. Period ratios of about 0.5 correspond to b3/b1, and ratios of about 0.7 correspond to b2/b1 and b3/b2. From the figure we conclude that radial 1st, 2nd, and 3rd overtones in the range of 3.0 < logL/L� < 3.15 correspond to b1, b2, and b3, respectively. To explain period ratios larger than about 0.9, it is necessary to consider non-radial pulsations. The presence of such high period ratios indicates that each ridge in the P-L plane might consist of more than one mode. The middle and right panels of Fig. 1 show period ratios obtained between l = 1 and radial modes and l = 2 and radial modes respectively, for 1.1MRGB models within the luminosity range 3.0 < logL/L� < 3.15. These figures indicate a period ratio of about 0.9 that can be explained by the