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Register a new userKIC5950759: a high-amplitude $delta$ Sct star with amplitude and frequency modulation near the terminal age main sequence
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Amongst the intermediate mass pulsating stars known as $delta$ Sct stars is a subset of high-amplitude and predominantly radial-mode pulsators known as high-amplitude $delta$ Sct (HADS) stars. From more than 2000 $delta$ Sct stars observed by the Kepler space mission, only two HADS stars were detected. We investigate the more perplexing of these two HADS stars, KIC5950759. We study its variability using ground- and space-based photometry, determine its atmospheric parameters from spectroscopy and perform asteroseismic modelling to constrain its mass and evolutionary stage. From spectroscopy, we find that KIC5950759 is a metal-poor star, which is in agreement with the inferred metallicity needed to reproduce its pulsation mode frequencies from non-adiabatic pulsation models. Furthermore, we combine ground-based WASP and Kepler space photometry, and measure a linear change in period of order $dot{P}/P simeq 10^{-6}$ yr$^{-1}$ for both the fundamental and first overtone radial modes across a time base of several years, which is at least two orders of magnitude larger than predicted by evolution models, and is the largest measured period change in a $delta$ Sct star to date. Our analysis indicates that KIC5950759 is a metal-poor HADS star near the short-lived contraction phase and the terminal-age main sequence, with its sub-solar metallicity making it a candidate SX Phe star. KIC5950759 is a unique object amongst the thousands of known $delta$ Sct stars and warrants further study to ascertain why its pulsation modes are evolving remarkably faster than predicted by stellar evolution.
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The detection of small-amplitude nonradial modes in high-amplitude Delta Sct (HADS) variables has been very elusive until at least five of them were detected in the light curve of V974 Oph obtained from ground-based observations. The combination of radial and nonradial modes has a high asteroseismic potential, thanks to the strong constraints we can put in the modelling. The continuous monitoring of ASAS 192647-0030.0=CoRoT 101155310 (P=0.1258 d, V=13.4) ensured from space by the CoRoT (Convection, Rotation and planetary Transits) mission constitutes a unique opportunity to exploit such potential. The 22270 CoRoT measurements were performed in the chromatic mode. They span 152 d and cover 1208 consecutive cycles. After the correction for one jump and the long-term drift, the level of the noise turned out to be 29 micromag. The phase shifts and amplitude ratios of the coloured CoRoT data, the HARPS spectra, and the period-luminosity relation were used to determine a self-consistent physical model. In turn, it allowed us to model the oscillation spectrum, also giving feedback on the internal structure of the star. In addition to the fundamental radial mode f1=7.949 c/d with harmonics up to 10f1, we detected 12 independent terms. Linear combinations were also found and the light curve was solved by means of 61 frequencies (smallest amplitude 0.10 mmag). The newest result is the detection of a periodic modulation of the f1 mode (triplets at +/-0.193 c/d centred on f1 and 2f1), discussed as a rotational effect or as an extension of the Blazhko effect to HADS stars. The physical model suggests that CoRoT 101155310 is an evolved star, with a slight subsolar metallic abundance, close to the terminal age main sequence. All the 12 additional terms are identified with mixed modes in the predicted overstable region.
People cannot witness the stellar evolution process of a single star obviously in most cases because of its extremely secular time-scale, except for some special time nodes in it (such as the supernova explosion). But in some specific evolutionary phases, we have the chances to witness such process gradually on human times-scales. When a star evolved leaving from the main sequence, the hydrogen nuclei fusion in its core is gradually transferring into the shell. In the Hertzsprung-Russell diagram, its evolutionary phase falls into the Hertzsprung gap, which is one of the most rapidly evolving phases in the life of a star. Here we report a discovery of a rapidly evolving high-amplitude $delta$ Scuti star KIC6382916 (J19480292+4146558) which is crossing the Hertzsprung gap. According to the analysis of the archival data, we find three independent pulsation modes of it, whose amplitudes and frequencies are variating distinctly in 4 years. The period variation rates of the three pulsation modes are one or two orders larger than the best seismic model constructed by the standard evolution theory, which indicates the current theory cannot precisely describe the evolution process in this rapidly evolving phase and needs further upgrades. Moreover, the newly introduced Interaction Diagram can help us to find the interactions between the three independent pulsation modes and their harmonics/combinations, which opens a new window to the future asteroseismology.
We have investigated the nature of the variability of CHS7797, an unusual periodic variable in the Orion Nebula Cluster. An extensive I-band photometric data set of CHS7797 was compiled between 2004-2010 using various telescopes. Further optical data have been collected in R and z bands. In addition, simultaneous observations of the ONC region including CHS7797 were performed in the I, J, Ks and IRAC [3.6] and [4.5] bands over a time interval of about 40d. CHS7797 shows an unusual large-amplitude variation of about 1.7 mag in the R, I, and z bands with a period 17.786. The amplitude of the brightness modulation decreases only slightly at longer wavelengths. The star is faint during 2/3 of the period and the shape of the phased light-curves for seven different observing seasons shows minor changes and small-amplitude variations. Interestingly, there are no significant colour-flux correlations for wavelengths smaller than 2microns, while the object becomes redder when fainter at longer wavelengths. CHS7797 has a spectral type of M6 and an estimated mass between 0.04-0.1Msun. The analysis of the data suggests that the periodic variability of CHS7797 is most probably caused by an orbital motion. Variability as a result of rotational brightness modulation by spots is excluded by the lack of any color-brightness correlation in the optical. The latter indicates that CHS7797 is most probably occulted by circumstellar matter in which grains have grown from typical 0.1 microns to 1-2 micron sizes. We discuss two possible scenarios in which CHS7797 is periodically eclipsed by structures in a disc, namely that CHS7797 is a single object with a circumstellar disc, or that CHS7797 is a binary system, similar to KH15D, in which an inclined circumbinary disc is responsible of the variability. Possible reasons for the typical 0.3mag variations in I-band at a given phase are discussed.
We report on a multi-site photometric campaign on the high-amplitude $delta$ Scuti star V2367 Cyg in order to determine the pulsation modes. We also used high-dispersion spectroscopy to estimate the stellar parameters and projected rotational velocity. Time series multicolour photometry was obtained during a 98-d interval from five different sites. These data were used together with model atmospheres and non-adiabatic pulsation models to identify the spherical harmonic degree of the three independent frequencies of highest amplitude as well as the first two harmonics of the dominant mode. This was accomplished by matching the observed relative light amplitudes and phases in different wavebands with those computed by the models. In general, our results support the assumed mode identifications in a previous analysis of Kepler data.
We present a simulation showing that super-Nyquist frequencies may have periodic amplitude and frequency modulations, even if actually stable, in time series sampled like the Kepler data. These modulations are caused by the barycentric time correction, which destroys the evenly spaced time measurements, making the Nyquist frequency variable over the spacecraft orbit around the Sun. These modulations can easily be identified in pulsating stars from Keplers photometric data.
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