Diversity of dynamical phenomena in type II Cepheids of the OGLE collection

Abstract

We have analysed the photometry of 924 type II Cepheids from the Galactic bulge observed by the Optical Gravitational Lensing Experiment. Our goal was to detect and analyse various dynamical phenomena, including, e.g. multi-mode pulsation, perioddoubled pulsation, period-k pulsation, or quasi-periodic modulation of pulsation. First examples of double-mode BL Her-type stars, pulsating simultaneously in the fundamental and first overtone modes are reported. Two double-mode pulsators are not isolated cases however, but form an extension of recently identified class of long-period RRd stars. Period doubling effect was detected in three BL Her-type stars: two were identified previously and one is a new discovery. Through identification of numerous period-doubled W Vir stars we show that the appearance of the period doubling effect, and hence the transition towards RV Tau class, is a smooth process that starts at pulsation periods slightly above 15 d. Interchange of the deep and shallow pulsation cycles is identified in tens of RV Tau variables. One RV Tau star is a strong candidate for period-4 pulsation. Quasi-periodic modulation of pulsation in all sub-classes of type II Cepheids is reported for the first time. Modulation was detected in 16 BL Her, 9 W Vir and in 7 RV Tau stars. In the last case, modulation is the most likely scenario, but two other scenarios, excitation of non-radial modes, or period-4 pulsation, can also account for the observed pulsation pattern. Irregular changes of pulsation amplitude and period on various time-scales are common in all subgroups of type II Cepheids.

TESS Observations of Cepheid Stars: First Light Results

“…Chaotic behavior was confirmed both from the ground (Buchler et al 1996;Kolláth et al 1998) and via Kepler data (Bódi et al 2016;Plachy et al 2018). Period doubling was also detected both with ground-and space-based observations in each subclass of Type II Cepheids Plachy et al 2017;Smolec et al 2018b;Jurkovic et al 2020). We note that light curves of Type II Cepheids can also be affected by extrinsic variations, usually connected to binarity.…”

“…The decade-long monitoring led to many discoveries, including low-amplitude additional modes and modulation (e.g. Moskalik et al 2006;Smolec et al 2018b;Süveges & Anderson 2018). Multi-epoch space photometry is also available for a large number of Cepheids with the Gaia Data Release 2 (DR2, Rimoldini et al 2019).…”

“…So far the BRITE satellites observed the largest number of Cepheids, and detected phenomena ranging from low-amplitude modulation cycles to extra modes and jitter in them (Smolec et al 2017(Smolec et al , 2018a. Some Type II Cepheids (T2CEP) are known to exhibit non-linear dynamical phenomena (Smolec 2016;Smolec et al 2018b). These are believed to be the results of destabilization of the fundamental mode by non-linear coupling to another pulsation mode.…”

OGLE-GAL-ACEP-091: The First Known Multi-mode Anomalous Cepheid

“…For years, all type II Cepheids were considered to be pure fundamental-mode pulsators, until Smolec et al (2018) identified the first two cases of beat type II Cepheids in the OGLE Collection of Variable Stars (OCVS). Currently, a total of five objects belonging to this class are known in the Galaxy (Udalski et al 2018;Soszyński et al 2020).…”

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

“…The first two double-mode type II Cepheids pulsating simultaneously in the fundamental and first-overtone modes were discovered by Smolec et al (2018), who identified them among nearly one thousand type II Cepheids detected by the OGLE survey in the Galactic bulge (Soszyński et al 2017b). These two unique objects have (fundamental-mode) periods equal to 1.04 d and 1.18 d, so they belong to the shortest-period subclass of type II Cepheids -BL Herculis stars -just above the conventional borderline (1 d) between RR Lyr stars and type II Cepheids.…”