Discovery of the spectroscopic binary nature of the Cepheids X Puppis and XX Sagittarii

We present the analysis of photometric and spectroscopic data of two bright Galactic Cepheids, X Puppis and XX Sagittarii. Based on the available data in the literature as well as own observations spanning 75 years, we conclude that both Cepheids belong to spectroscopic binary systems. However, the data are not sufficient to determine the orbital periods nor other elements for the orbit. This discovery corroborates the statement on the high frequency of occurrence of binaries among the classical Cepheids, a fact to be taken into account when calibrating the period-luminosity relationship for Cepheids. The photometric data revealed that the pulsation period of X Pup is continuously increasing with Delta P=0.007559 d/century likely caused by stellar evolution. The pulsation period of XX Sgr turned out to be very stable in the last ~100 years.Comment: 6 pages, 5 figure

Tidally Induced Oscillations and Orbital Decay in Compact Triple-Star Systems

We investigate the nature of tidal effects in compact triple-star systems. The hierarchical structure of a triple system produces tidal forcing at high frequencies unobtainable in binary systems, allowing for the tidal excitation of high frequency p-modes in the stellar components. The tidal forcing exists even for circular, aligned, and synchronized systems. We calculate the magnitude and frequencies of three-body tidal forcing on the central primary star for circular and coplanar orbits, and we estimate the amplitude of the tidally excited oscillation modes. We also calculate the secular orbital changes induced by the tidally excited modes, and show that they can cause significant orbital decay. During certain phases of stellar evolution, the tidal dissipation may be greatly enhanced by resonance locking. We then compare our theory to observations of HD 181068, which is a hierarchical triply eclipsing star system in the Kepler field of view. The observed oscillation frequencies in HD 181068 can be naturally explained by three-body tidal effects. We then compare the observed oscillation amplitudes and phases in HD 181068 to our predictions, finding mostly good agreement. Finally, we discuss the past and future evolution of compact triple systems like HD 181068.Comment: 19 pages, 8 figures, published in MNRA

Discovery of the spectroscopic binary nature of three bright southern Cepheids

We present an analysis of spectroscopic radial velocity and photometric data of three bright Galactic Cepheids: LR Trianguli Australis (LR TrA), RZ Velorum (RZ Vel), and BG Velorum (BG Vel). Based on new radial velocity data, these Cepheids have been found to be members of spectroscopic binary systems. The ratio of the peak-to-peak radial velocity amplitude to photometric amplitude indicates the presence of a companion for LR TrA and BG Vel. IUE spectra indicate that the companions of RZ Vel and BG Vel cannot be hot stars. The analysis of all available photometric data revealed that the pulsation period of RZ Vel and BG Vel varies monotonically, due to stellar evolution. Moreover, the longest period Cepheid in this sample, RZ Vel, shows period fluctuations superimposed on the monotonic period increase. The light-time effect interpretation of the observed pattern needs long-term photometric monitoring of this Cepheid. The pulsation period of LR TrA has remained constant since the discovery of its brightness variation. Using statistical data, it is also shown that a large number of spectroscopic binaries still remain to be discovered among bright classical Cepheids.Comment: 9 pages, 14 figure

Variability of M giant stars based on Kepler photometry: general characteristics

M giants are among the longest period pulsating stars which is why their studies were traditionally restricted to analyses of low-precision visual observations, and, more recently, accurate ground-based data. Here we present an overview of M giant variability on a wide range of time-scales (hours to years), based on the analysis of 13 quarters of Kepler long-cadence observations (one point per every 29.4 min), with a total time-span of over 1000 d. About two-thirds of the sample stars have been selected from the All Sky Automated Survey (ASAS)-North survey of the Kepler field, with the rest supplemented from a randomly chosen M giant control sample. We first describe the correction of the light curves from different quarters, which was found to be essential. We use Fourier analysis to calculate multiple frequencies for all stars in the sample. Over 50 stars show a relatively strong signal with a period equal to the Kepler-year and a characteristic phase dependence across the whole field of view. We interpret this as a so far unidentified systematic effect in the Kepler data. We discuss the presence of regular patterns in the distribution of multiple periodicities and amplitudes. In the period-amplitude plane we find that it is possible to distinguish between solar-like oscillations and larger amplitude pulsations which are characteristic for Mira/semiregular stars. This may indicate the region of the transition between two types of oscillations as we move upward along the giant branch

HD181068 : A TRIPLY ECLIPSING SYSTEM WITH INTRINSICALLY VARIABLE RED GIANT COMPONENT

We present the analysis of HD 181068 which is one of the first triply eclipsing triple system discovered. Using Kepler photometry, ground based spectroscopic and interferometric measurements, we determined the stellar and orbital parameters of the system. We show that the oscillations observed in the red giant component of the system are tidally forced oscillations, while one of the most surprising results is that it does not show solar-like oscillations