Anomalous double-mode RR Lyrae stars in the Magellanic Clouds

Abstract

We report the discovery of a new subclass of double-mode RR Lyrae stars in the Large and Small Magellanic Clouds. The sample of 22 pulsating stars have been extracted from the latest edition of the OGLE collection of RR Lyrae variables in the Magellanic System. The stars pulsating simultaneously in the fundamental (F) and first-overtone (1O) modes have distinctly different properties than regular double-mode RR Lyrae variables (RRd stars). The P 1O /P F period ratios of our anomalous RRd stars are within a range 0.725-0.738, while "classical" double-mode RR Lyrae variables have period ratios in the range 0.742-0.748. In contrast to the typical RRd stars, in the majority of the anomalous pulsators the F-mode amplitudes are higher than the 1O-mode amplitudes. The light curves associated with the F-mode in the anomalous RRd stars show different morphology than the light curves of, both, regular RRd stars and single-mode RRab stars. Most of the anomalous double-mode stars show long-term modulations of the amplitudes (Blazhko-like effect). Translating the period ratios into the abundance parameter, Z, we find for our stars Z ∈ (0.002, 0.005) -an order of magnitude higher values than typical for RR Lyrae stars. The mass range of the RRd stars inferred from the W I vs. P F diagram is (0.55 − 0.75) M ⊙ . These parameters cannot be accounted for with single star evolution assuming a Reimers-like mass loss. Much greater mass loss caused by interaction with other stars is postulated. We blame the peculiar pulsation properties of our stars to the parametric resonance instability of the 1O-mode to excitation of the F-and 2O-modes as with the inferred parameters of the stars 2ω 1O ≈ ω F + ω 2O .

Highlights on eclipsing binary variables from Araucaria Project

“…is consistent with theoretical expectations for a short period Cepheid and interferometric measurements for δ Cep [5]. Recent discovery of the first eclipsing binary consisting of two classical Cepheid variables OGLE-LMC-CEP-1718 [3] is an important step towards better understanding of binary pulsators. A detached system of 413-day eccentric (e = 0.276) orbit consists of two overtone pulsators, with pulsation periods of 1.96 and 2.48 days.…”

“…The accuracy of previous efforts to establish a dynamical Cepheid mass from single-lined non-eclipsing binaries was typically about 15 − 30%, which was not good enough to resolve the mass discrepancy problem [2]. The discovery of an eclipsing binary system OGLE-LMC-CEP0227 [3] consisting of a classical Cepheid pulsating with a period of 3.8 days together with a stable red giant in a 310-day orbit allowed to determine the mass of the pulsator to 4.165 M with a precision of 1% [4]. This determination agrees with its pulsation mass, providing strong evidence that pulsation theory correctly and precisely predicts the masses of classical Cepheids.…”

A new survey of cool supergiants in the Magellanic Clouds

“…-HV 1956 ([M2002] 58738): this star is a well-known longperiod Cepheid variable (Butler 1976;Eggen 1977), with a very long period of ∼210 days (Soszyñski et al 2010). This star has been classified as M0-1 I by (Prevot et al 1983), G2 Ib by Wallerstein (1984), and K2 I by Massey (2002).…”

The K2 RR Lyrae Survey

“…Besides the half-integer frequencies due to period-doubling [12], extra frequencies have been found around the expected location of the second radial overtone frequency [13], around P X /P 1 = 0.61 [14], where P 1 and P X are the periods of first radial overtone and a presumably nonradial mode which seems to be ubiquitous in RRc and RRd stars [15]. In addition, anomalous first overtone / fundamental mode (P 1 /P 0 ) period ratios were uncovered [16], [17], [18], [19].…”