ABSTRACT
Transiting Exoplanet Survey Satellite (TESS) observations show CD−28° 1974 to be a gravity(g)-mode-dominated hybrid pulsating subdwarf B (sdBV) star. It shows 13 secure periods that form an ...ℓ = 1 asymptotic sequence near the typical period spacing. Extraordinarily, these periods lie between 1500 and 3300 s, whereas typical $\ell = 1\, g$ modes in sdBV stars occur between 3300 and 10 000 s. This indicates a structure somewhat different from typical sdBV stars. CD−28° 1974 has a visually close F/G main-sequence companion 1.33 arcsec away, which may be a physical companion. Gaia proper motions indicate a comoving pair with the same distance. A reanalysis of Ultraviolet and Visual Echelle Spectrograph (UVES) spectra failed to detect any orbital motion and the light curve shows no reflection effect or ellipsoidal variability, making an unseen close companion unlikely. The implication is that CD−28° 1974 has become a hot subdwarf via single star or post-merger evolution.
Abstract
We analyse the full Kepler short cadence data set of the pulsating subdwarf B star KIC 11558725. KIC 11558725 is in an sdB+WD binary system with a period of 10.05 d and is known to be ...subsynchronously rotating. From the full data set, we detected 245 pulsation frequencies, mostly in the gravity (g-) mode region, but some in the pressure (p-) mode region as well. We are able to identify 142 of these pulsations as ℓ ≤ 2 modes and 27 as ℓ = 6 modes. Frequency splittings in the g- and p-mode regions indicate that KIC 11558725 is a solid-body rotator with a rotation period of ∼44 d. The ℓ = 6 multiplets do not show a constant splitting, with the splitting increasing over the course of the observations. Multiplet structure constrains the inclination of the pulsation axis to be greater than ∼80°. KIC 11558725 also displays mode trapping in two regions of its asymptotic sequence.
Pulsations and eclipse-time analysis of HW Vir Baran, A S; Østensen, R H; Telting, J H ...
Monthly notices of the Royal Astronomical Society,
12/2018, Letnik:
481, Številka:
2
Journal Article
We present results from the final 6 months of a survey to search for pulsations in white dwarfs (WDs) and hot subdwarf stars with the Kepler spacecraft. Spectroscopic observations are used to ...separate the objects into accurate classes, and we explore the physical parameters of the subdwarf B (sdB) stars and white dwarfs in the sample. From the Kepler photometry and our spectroscopic data, we find that the sample contains five new pulsators of the V1093 Her type, one AM CVn type cataclysmic variable and a number of other binary systems.
This completes the survey for compact pulsators with Kepler. No V361 Hya type of short-period pulsating sdB stars were found in this half, leaving us with a total of one single multiperiodic V361 Hya and 13 V1093 Her pulsators for the full survey. Except for the sdB pulsators, no other clearly pulsating hot subdwarfs or white dwarfs were found, although a few low-amplitude candidates still remain. The most interesting targets discovered in this survey will be observed throughout the remainder of the Kepler mission, providing the most long-term photometric data sets ever made on such compact, evolved stars. Asteroseismic investigations of these data sets will be invaluable in revealing the interior structure of these stars and will boost our understanding of their evolutionary history.
The subdwarf-B star KIC 2991276 was monitored with the Kepler spacecraft for nearly three years. Two pulsation modes with periods of 122 and 132 s are clearly detected in the Fourier spectrum, as ...well as a few weaker modes with periods ranging from 118 to 216 s. Unlike the other subdwarf-B pulsators with similar high-quality Kepler lightcurves, the modes in KIC 2991276 do not display long-term coherency. Rather, their pulsation amplitudes vary substantially in amplitude and phase on timescales of about a month, sometimes disappearing completely. Thus, while the pulsations are seen to have amplitudes of up to 1.4% in individual months, the amplitude spectrum of the full lightcurve shows a broad, messy peak with an amplitude of only 0.23%. Such stochastic oscillations are normal in the Sun and other cool stars with solar-like pulsations and have been suspected for V361-Hya pulsators, but thanks to the exceptional coverage of Kepler data, this is the first unambiguous case established for a hot subdwarf.
We report on extended photometry of two pulsating subdwarf B (sdB) stars in close binaries. For both cases, we use rotational splitting of the pulsation frequencies to show that the sdB component ...rotates much too slowly to be in synchronous rotation. We use a theory of tidal interaction in binary stars to place limits on the mass ratios that are independent of estimates based on the radial velocity curves. The companions have masses below 0.26 M⊙. The pulsation spectra show the signature of high-overtone g-mode pulsation. One star, KIC 11179657, has a clear sequence of g modes with equal period spacings as well as several periodicities that depart from that trend. KIC 02991403 shows a similar sequence, but has many more modes that do not fit the simple pattern.
We present results of a Transiting Exoplanet Survey Satellite (TESS) search for short-period pulsations in compact stellar objects observed in years 1 and 3 of the TESS mission, during which the ...southern ecliptic hemisphere was targeted. We describe the TESS data used and the details of the search method. For many of the targets, we use unpublished spectroscopic observations to classify the objects. From the TESS photometry, we clearly identify 43 short-period hot-subdwarf pulsators, including 32 sdB stars, eight sdOB stars, two sdO stars, and, significantly, one He-sdOB star, which is the first of this kind to show short-period pulsations. Eight stars show signals at both low and high frequencies, and are therefore “hybrid” pulsators. We report the list of prewhitened frequencies and we show the amplitude spectra calculated from the TESS data. We make an attempt to identify possible multiplets caused by stellar rotation, and we select four candidates with rotation periods between 1 and 12.9 days. The most interesting targets discovered in this survey should be observed throughout the remainder of the TESS mission and from the ground. Asteroseismic investigations of these data sets will be invaluable in revealing the interior structure of these stars and will boost our understanding of their evolutionary history. We find three additional new variable stars but their spectral and variability types remain to be constrained.
The Kepler spacecraft observed ~150 000 stars over the course of its four-year mission, of which 18 were discovered to be pulsating subdwarf B stars, including KIC 2697388. We analyse three years of ...Kepler spacecraft short-cadence data as well as 21 low-resolution spectra of the pulsating subdwarf B star KIC 2697388. Our spectra have a radial-velocity scatter of 9.5 km s super( -1), and while insufficient to completely rule out binarity, we rule out short-period, low-inclination orbits for KIC 2697388. From the short-cadence Kepler data, we detect 253 periodicities, most with periods from 1 to 2.5 h, which we associate with gravity-mode pulsations. Twenty-three periods were also detected in the short-period pressure-mode region. We applied standard seismic tools for mode identification, including asymptotic overtone period spacings and rotationally induced frequency multiplets. We classify 89 per cent of the periodicities with mode identifications; most of low degree (l less than or equal to 2), but 42 are identified as l greater than or equal to 3. Frequency multiplets provide a rotation period for the star of ~42 d. A unique feature is seen in KIC 2697388's data; in all l greater than or equal to 2 multiplets, the splittings decrease over the course of the observations. If the trend continues, l greater than or equal to 2 multiplets would become singlets within a decade.
PDF
