Blazhko effect in the Galactic bulge fundamental mode RR Lyrae stars – II. Modulation shapes, amplitudes, and periods

Abstract

The number of stars observed by the Optical Gravitational Lensing Experiment (OGLE) project in the Galactic bulge offers an invaluable chance to study RR Lyrae stars in a statistical manner. We used data of 3141 fundamental-mode RR Lyrae stars showing the Blazhko effect observed in OGLE-IV to investigate a possible connection between modulation amplitudes and periods, light curve and pulsation characteristics. We found that there is no simple monotonic correlation between any combination of two parameters concerning the Blazhko and pulsation amplitudes, periods and the shape of the light curves. There are only systematic limits. There is a bottom limit of the modulation period with respect to the pulsation period. We also found that the possible range of modulation amplitudes decreases with increasing pulsation period which could point towards that the Blazhko effect is suppressed in cooler, larger, more luminous and less metal abundant bulge RR Lyrae stars. Our investigation revealed that the distribution of the modulation periods can be described with two populations of stars with the mean modulation periods of 48 and 186 days. There is a certain region with a low density of the modulated stars, which we call the Blazhko valley, in the pulsation period-modulation period plane. Based on the similarity of the modulation envelopes, basically every star can be assigned to one of six morphological classes. Double modulation was found in 25 per cent of the studied stars. Only 6.3 per cent of modulated stars belongs to the Oosterhoff group II.

On the Use of Field RR Lyrae as Galactic Probes. I. The Oosterhoff Dichotomy Based on Fundamental Variables*

“…The relative fractions for RRc variables need to be cautiously treated, since they are roughly two dozen. In passing, we also note that the current findings support recent results by Skarka et al (2020) based on a large sample (more than 3,300) of Galactic Bulge and cluster Blazhko RRLs.…”

“…Indeed, the modulations change from ∼0.7 mag, close to the fundamental blue edge, to ∼0.05 mag, close to the fundamental red edge (see Fig. 10 in Skarka et al 2020, andalso Jurcsik et al 2011;Benkő et al 2014;Braga et al 2016). The RRc variables are also affected by the Blazhko phenomenon, but the fraction is significantly smaller (∼6%, Netzel et al 2018) and the amplitude modulation is, at most, of the order of ∼0.2 mag.…”

First results on RR Lyrae stars with the TESS space telescope: untangling the connections between mode content, colors and distances

“…We were able to determine or estimate the Blazhko periods in five stars. The shortest one is SU Hyi, with a period of P BL = 5.55 d, which is among the shortest values ever recorded (Skarka et al 2020). We investigate the light curve of SU Hyi in more detail in Sect.…”

“…The Blazhko effect comes in many shapes and forms. Recenty, Skarka et al (2020) separated the modulated OGLE stars into six morphological groups. Our TESS light curves are unfortunately too short for similar exercises, but we can still examine how the amplitudes and phases vary over time.…”

RR Lyrae and Type II Cepheid Variables in Globular Clusters: Optical and Infrared Properties

“…The Blazhko effect results in (multi-)periodic modulations of the light curves on a timescale that is much longer than the primary pulsation period. The Blazhko effect is not well understood, despite a number of investigations that are even based on unprecedentedly high-precision photometry from Kepler and TESS [70][71][72][73][74]. The space-based photometry has also revealed several low-amplitude additional modes and dynamical effects in the pulsation, including period doubling of the fundamental mode [72,74].…”