Other
Scientific paper
Aug 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...288..782g&link_type=abstract
Astronomy and Astrophysics 288, 782-790 (1994)
Other
17
Stars: Evolution Of, Stars: Agb, Stars: Variables: Others, Magellanic Clouds
Scientific paper
From observations of LPVs (long period variables) in the LMC we derive that the ratio of the number of C-rich LPVs to the total number of carbon stars is ~0.05 and that the ratio of the number of oxygen-rich LPVs to the total number of oxygen-rich AGB stars is between 0.05 and 0.10. The lifetime of the LPV phase in the LMC is only a few 10^4^yr, considerable below estimates for the Galaxy, where the duration of the LPV phase (about 2 10^5^yr) is similar to the total AGB lifetime. If the possible incompleteness of the surveys for LPVs is invoked to explain this discrepancy in the lifetimes then the ratio of small amplitude variables to large amplitude variables must be about 30, considerably larger than the ratio of Semi Regular to Mira variables in the Galaxy (1-3). We present a simple model to explain the observed properties of LPVs in the LMC. It is assumed that pulsation only occurs in an instability strip in the HR diagram. The instability strip is characterised by three parameters: the temperature at some reference luminosity, the width of the instability strip and its slope dT_eff_/dM_bol_. The first two are free parameters in the model. Based on observations we use dT_eff_/dM_bol_=275K/mag for M_bol_>-5 and 100 K/mag for M_bol_<-5. An additional complication is that the pulsation period depends rather sensitively on the effective temperature scale. The location of the AGB tracks in the HR diagram (the zero point of the effective temperature scale) is the third free parameter. Both a model with a Reimers mass loss law inside and outside the instability strip, and a model with the mass loss in the instability strip given by a scaled version of the Bloecker & Schoenberner (1993) mass loss law, fit the observational constraints equally well. We conclude that first harmonic pulsation can be excluded unless the canonical relation between (J-K) color and effective temperature (based on lunar occultation observations) gives temperatures which are too high by ~20%, much larger than the estimated uncertainty of ~8% or possible systematic effects (<~10%). Fundamental mode pulsation is therefore probably the dominant pulsation mode among LPVs in the LMC. A second conclusion is that for most stars the instability strip is not the final phase of AGB evolution. Based on our calculations for individual stars we find that AGB stars more massive than about 1.2Msun_ spend a considerable amount of time in the phase between the end of pulsation and the end of the AGB. We propose an alternative explanation for (some of) the non-variable OH/IR stars in the Galaxy.
de Jong Teije
Groenewegen Martin A. T.
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