Type II Cepheids Pulsating in the First Overtone from the OGLE Survey

Abstract

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 M ⊙ . However, the higher mass requires higher luminosity to match the observed periods of the stars, which is inconsistent with observations.

Evolutionary and pulsation properties of Type II Cepheids

“…# Topology of the instability strip-Theory and observations show that TIICs mainly pulsate in the fundamental mode, because they reach luminosities brighter than the transition point. The possible occurrence of fainter TIICs evolving along the blueward excursion showed by some HB models, and in turn the presence of first overtone pulsators (Soszyński et al 2019), cannot be excluded.…”

Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation

“…Their masses are similar to RR Lyrae but they cross the instability strip at larger luminosities and pulsate with longer periods (P > 1 d). Type II Cepheids are F-mode pulsators except for a few doublemode stars pulsating simultaneously in the F and 1O modes (Smolec et al 2018;Udalski et al 2018) and two recently discovered 1O-mode pulsators (Soszyński et al 2019). BL Her variables are a subclass of Type II Cepheids with P 4 d.…”

TESS Observations of Cepheid Stars: First Light Results

“…They are low mass stars (0.5-0.6 M ) evolving on the blue horizontal branch, and pulsating almost exclusively in the fundamental radial mode. The first double-mode pulsators among them in the bulge (Smolec et al 2018b;Udalski et al 2018) and two purely first overtone candidates in the LMC (Soszyński et al 2019) have been discovered only very recently. Theoretical calculations suggest overtone pulsation to be very rare among BLHs, in agreement with the observations.…”