Type II Cepheids Pulsating in the First Overtone from the OGLE Survey. (arXiv:1902.02352v1 [astro-ph.SR])

Type II Cepheids Pulsating in the First Overtone from the OGLE Survey. (arXiv:1902.02352v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Soszynski_I/0/1/0/all/0/1">I. Soszy&#x144;ski</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Smolec_R/0/1/0/all/0/1">R. Smolec</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Udalski_A/0/1/0/all/0/1">A. Udalski</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pietrukowicz_P/0/1/0/all/0/1">P. Pietrukowicz</a>

We report the discovery of the first type II Cepheids (BL Herculis stars)
pulsating solely in the first overtone. We found two such objects among tens of
millions of stars regularly observed by the OGLE survey in the Large Magellanic
Cloud. Our classification and the pulsation mode identification is based on the
position of these stars on the period-luminosity and color-magnitude diagrams
and on the light curve analysis. We discuss why single-mode first-overtone BL
Her pulsators must be very rare. For the two discovered stars we present
non-linear models that successfully reproduce their light variation. These
models indicate that both first-overtone pulsators should be more massive than
it is typically assumed for BL Her stars, i.e. their masses should be above
0.75 Solar masses. However, the higher mass requires higher luminosity to match
the observed periods of the stars, which is inconsistent with observations.

We report the discovery of the first type II Cepheids (BL Herculis stars)
pulsating solely in the first overtone. We found two such objects among tens of
millions of stars regularly observed by the OGLE survey in the Large Magellanic
Cloud. Our classification and the pulsation mode identification is based on the
position of these stars on the period-luminosity and color-magnitude diagrams
and on the light curve analysis. We discuss why single-mode first-overtone BL
Her pulsators must be very rare. For the two discovered stars we present
non-linear models that successfully reproduce their light variation. These
models indicate that both first-overtone pulsators should be more massive than
it is typically assumed for BL Her stars, i.e. their masses should be above
0.75 Solar masses. However, the higher mass requires higher luminosity to match
the observed periods of the stars, which is inconsistent with observations.

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