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Double-Overtone Cepheids in the Large Magellanic Cloud

100%

Dziembowski W. A. , Smolec R.

2009

tom Vol. 59, No. 1

19--31

One of the most interesting results from the OGLE-III study of the LMC Cepheids is the large number of objects that pulsate simultaneously in the first and second overtone (denoted 1O/2O). Double-mode Cepheids yield important constraint on stellar evolution models. We show that great majority of the LMC 1O/2O Cepheids have masses M=3.0±0.5 Msolar. According to current stellar evolution calculations these masses are lower than needed for the blue loop in the helium burning phase to reach the instability strip. On the other hand, we found most of these stars significantly overluminous if they are crossing the instability strip before helium ignition. A possible solution of this discrepancy is to allow for a large overshooting from the convective core in the main sequence phase. We also discuss origin of double-mode pulsation. At the short period range we find two types of resonances that are conducive to this form of pulsation. However, at longer periods, it has a different (non-resonant) origin.

Large Magellanic Cloud Cepheids in the ASAS Data

71%

Karczmarek P. , Dziembowski W. A. , Lenz P. , Pietrukowicz P. , Pojmański G.

tom Vol. 61, No. 4

303--318

A catalog of Cepheids in the Large Magellanic Cloud (LMC) from the ASAS project is presented. It contains data on 65 fundamental mode pulsators with periods longer than about 8 days. The period-luminosity (PL) relation in the V-band does not significantly differ from the relation determined from the OGLE-III data extended toward longer periods but shows much larger scatter. For objects with periods longer than 40 d there is an evidence for a shallower PL relation. The rates of long-term period variations significant at 3σ level are found only for seven objects. The rates for 25 objects determined with the 1σ significance are confronted with the values derived from stellar evolution models. The models from various sources yield discrepant predictions. Over the whole data range, a good agreement with measurements is found for certain models but not from the same source.