sdBV Stars Research Articles

Kepler, K2, and TESS observations: ensemble and comparative asteroseismology

The original Kepler mission detected 18 pulsating subdwarf B (sdBV) stars, K2 observed 161 of our proposed targets with 41 sdB stars found to show p- or g-mode pulsations, and TESS has observed about 1,000 of our proposed targets. All these data should provide ∼300 sdBV stars, from which asteroseismology will provide a host of measurables. Combined with temperatures and gravities (and radii and masses from GAIA parallaxes!), we have a powerful set of observations with which to compare models. Here we review our seismology progress with sdBV stars.

Pulsating subdwarf B stars observed with K2 during Campaign 7 and an examination of seismic group properties

ABSTRACT We report the discovery of four new pulsating subdwarf B (sdBV) stars from Campaign 7 of the Kepler spacecraft’s K2 mission. EPIC 215776487, EPIC 217280630, EPIC 218366972, and EPIC 218717602 are all gravity (g)-mode pulsators and we also detect two pressure (p)-mode pulsations in EPIC 218717602. We detect asymptotic $\ell \, =\, 1$ sequences in all four stars, allowing us to identify nearly all of the g modes. We detect evenly spaced frequency multiplets in EPIC 218717602 from which we determine a rotation period near 7 d. Spectroscopic observations determine that EPIC 218366972 is in a 5.92 d binary with most likely a white dwarf companion of canonical mass while the others have no detected companions. As we detect no multiplets in EPIC 218366972, it is added to the growing list of subsynchronously rotating stars. With 40 Kepler-detected sdBV stars and a growing number of Transiting Exoplanet Survey Satellite (TESS) publications, we update an examination of the group properties to provide direction for models. We notice a correlation between effective temperature and period of maximum pulsation amplitude, at least for g-mode pulsations, and update the previously observed effective temperature–rotation period relation.

TESS observations of pulsating subdwarf B stars: extraordinarily short-period gravity modes in CD−28° 1974

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.

K2 observations of the pulsating subdwarf B stars UY Sex and V1405 Ori

ABSTRACT We processed and analysed K2 observations of the pulsating subdwarf-B (sdBV) stars UY Sex and V1405 Ori. We detect 97 p-mode pulsations in UY Sex while we discover V1405 Ori to be a rare rich hybrid pulsator with over 100 p-mode pulsations and 19 g-mode pulsations. We detect frequency multiplets, which we use to identify pulsation modes as well as determine rotation periods. For UY Sex, we find a rotation period of the envelope of 24.6 ± 3.5 d and for V1405 Ori, we find a rotation period of 0.555 ± 0.029 d for the p modes and a marginal detection of 4.2 ± 0.4 d for the g modes. We discover that V1405 Ori is unique among sdBV stars observed to date. It is a rich hybrid pulsator, allowing us to simultaneously probe the envelope and interior; its frequency multiplets indicate V1405 Ori to be rotating differentially with the core rotating more slowly than the envelope, and it is also in a short-period binary (0.398 d) with an envelope that is nearly but not quite tidally locked. For both stars, we have obtained spectroscopic follow-up observations and examine combining them with Gaia parallaxes and archival photometry to determine fundamental properties. Our derived masses are inconsistent with spectroscopy and previous determinations and indicate problems with the methodology.

Analysis of putative exoplanetary signatures found in light curves of two sdBV stars observed by Kepler

Context. We investigate the validity of the claim that invokes two extreme exoplanetary system candidates around the pulsating B-type subdwarfs KIC 10001893 and KIC 5807616 from the primary Kepler field. Aims. Our goal was to find characteristics and the source of weak signals that are observed in these subdwarf light curves. Methods. To achieve this, we analyzed short- and long-cadence Kepler data of the two stars by means of a Fourier transform and compared the results to Fourier transforms of simulated light curves to which we added exoplanetary signals. The long-cadence data of KIC 10001893 were extracted from CCD images of a nearby star, KIC 10001898, using a point spread function reduction technique. Results. It appears that the amplitudes of the Fourier transform signals that were found in the low-frequency region depend on the methods that are used to extract and prepare Kepler data. We demonstrate that using a comparison star for space telescope data can significantly reduce artifacts. Our simulations also show that a weak signal of constant amplitude and frequency, added to a stellar light curve, conserves its frequency in Fourier transform amplitude spectra to within 0.03 μHz. Conclusions. Based on our simulations, we conclude that the two low-frequency Fourier transform signals found in KIC 5807616 are likely the combined frequencies of the lower amplitude pulsating modes of the star. In the case of KIC 10001893, the signal amplitudes that are visible in the light curve depend on the data set and reduction methods. The strongest signal decreases significantly in amplitude when KIC 10001898 is used as a comparison star. Finally, we recommend that the signal detection threshold is increased to 5σ (or higher) for a Fourier transform analysis of Kepler data in low-frequency regions.

Pulsations and eclipse-time analysis of HW Vir

We analysed recent K2 data of the short-period eclipsing binary system HW Vir, which consists of a hot subdwarf-B type primary with an M-dwarf companion. We determined the mid-times of eclipses, calculated O–C diagrams, and an average shift of the secondary minimum. Our results show that the orbital period is stable within the errors over the course of the 70 days of observations. Interestingly, the offset from mid-orbital phase between the primary and the secondary eclipses is found to be 1.62 s. If the shift is explained solely by light-travel time, the mass of the sdB primary must be 0.26 M⊙, which is too low for the star to be core-helium burning. However, we argue that this result is unlikely to be correct and that a number of effects caused by the relative sizes of the stars conspire to reduce the effective light-travel time measurement. After removing the flux variation caused by the orbit, we calculated the amplitude spectrum to search for pulsations. The spectrum clearly shows periodic signal from close to the orbital frequency up to 4600 µHz, with the majority of peaks found below 2600 µHz. The amplitudes are below 0.1 part-per-thousand, too low to be detected with ground-based photometry. Thus, the high-precision data from the Kepler spacecraft has revealed that the primary of the HW Vir system is a pulsating sdBV star. We argue that the pulsation spectrum of the primary in HW Vir differs from that in other sdB stars due to its relatively fast rotation that is (nearly) phase-locked with the orbit.

Problems with exoplanets around sdBV and sdO stars from primary Kepler field

AbstractThe existence of exoplanets around evolved objects is one of the most interesting subjects from the viewpoint of planetary system evolution and its fate. What happens to the exoplanets engulfed in the host star envelope during red giant branch (RGB) phase? Can planets survive this evolutionary stage of the host star? Here, we are showing that at least some of the exoplanetary candidates recently found around a couple of sdBV stars, KIC 5807616 and KIC 10001893, might not be exoplanets after all. One “exoplanetary signal” visible in the light curve FT of KIC 10001893 can be just a frequency combination of stellar pulsation modes, while others are likely artifacts. Similarly, low frequency signals found in KIC 5807616 light curve FT, are beating frequencies of stellar oscillations, rather than resulting from the exoplanetary radiation. We also analyzed frequency and amplitude changes of the signal around 0.256 c/d (∼3.9 day) visible in the light curve FT of the KIC 10449976 sdO star. Our simulations show that it is difficult to reproduce the observed signal frequency variations by the weather changes in the exoplanet atmosphere.

A review of seismic observations of Kepler and K2-Observed sdBV stars

Abstract This paper reviews recent seismic findings from Kepler and K2 data. Using three years of short cadence Kepler (K1) data, it is possible to examine time evolution of pulsations in an unprecedented way. While K2 observations are shorter, only three months, they are important as they are finding more sdBV stars than K1 did. Most importantly, K2 is discovering more p−mode pulsators with coverage not possible to get from the ground.

Super-Nyquist asteroseismology with the Kepler Space Telescope

Barycentric corrections made to the timing of Kepler observations, necessitated by variations in light arrival time at the satellite, break the regular time-sampling of the data -- the time stamps are periodically modulated. A consequence is that Nyquist aliases are split into multiplets that can be identified by their shape. Real pulsation frequencies are distinguishable from these aliases and their frequencies are completely recoverable, even in the super-Nyquist regime, that is, when the sampling interval is longer than half the pulsation period. We provide an analytical derivation of the phenomenon, alongside demonstrations with simulated and real Kepler data for \delta Sct, roAp, and sdBV stars. For Kepler data sets spanning more than one Kepler orbital period (372.5 d), there are no Nyquist ambiguities on the determination of pulsation frequencies, which are the fundamental data of asteroseismology.

Pulsations driven by theɛ-mechanism in post-merger remnants: First results

Helium-rich subdwarfs are a rare subclass of hot subdwarf stars which constitute a small and inhomogeneous group showing varying degrees of helium enrichment. Only one star, LS IV °14 116 has been found to show multiperiodic luminosity variations. The variability of LS IV °14 116 has been explained as the consequence of nonradial g-mode oscillations, whose excitation is difficult to understand within the frame of the standard κ-mechanism driving pulsations in sdBV stars. In a recent study, we have proposed that the pulsations of LS IV °14 116 might be driven through the ɛ-mechanism acting in unstable He-burning zones in the interior of the star, that appear before the quiescent He-burning phase. One of the few accepted scenarios for the formation of He-rich subdwarfs is the merger of two He-core white dwarfs. As part of this project, we present a study of the ɛ-mechanism in post-merger remnants, and discuss the results in the light of the pulsations exhibited by LS IV °14 116.

Amplitude variations in pulsating sdB stars

Frequency analyses over two or more seasons are presented for several pulsating sdB stars. These results indicate that amplitude variations in sdBV stars might be common and occur on timescales from less than a day to years. It is not clear whether such variations are a result of real amplitude modulation or of the fact that very close frequencies have not all been resolved. In either case, it would appear to be necessary to carry out reasonably extensive studies before one can claim to have resolved “all” frequencies, and to exhibit care before selecting frequencies for ‘ $\dot{P}$ ’ studies.

RAT0455+1305: another pulsating hybrid sdB star

RAT0455+1305 was discovered during the Rapid Temporal Survey which aims in finding any variability on timescales of a few minutes to several hours. The star was found to be another sdBV star with one high amplitude mode and relatively long period. These features along with estimation of T_eff and log g makes this star very similar to Balloon 090100001. Encouraged by prominent results obtained for the latter star we have decided to perform white light photometry on RAT0455+1305. In 2009 we used the 1.5m telescope located in San Pedro Martir Observatory in Mexico. Fourier analysis confirmed the dominant mode found in the discovery data, uncovered another peak close to the dominant one, and three peaks in the low frequency region. This shows that RAT0455+1305 is another hybrid sdBV star pulsating in both p- and g-modes.

The planet-hosting subdwarf B star V 391 Pegasi is a hybrid pulsator

Context. A noticeable fraction of subdwarf B stars shows either short-period ( p -mode) or long-period ( g -mode) luminosity variations, with two objects so far known to exhibit hybrid behaviour, i.e. showing both types of modes at the same time. The pulsating subdwarf B star V 391 Pegasi (or HS 2201+2610), which is close to the two known hybrid pulsators in the log g – T eff plane, has recently been discovered to host a planetary companion. Aims. In order to learn more about the planetary companion and its possible influence on the evolution of its host star (subdwarf B star formation is still not well understood), an accurate characterisation of the host star is required. As part of an ongoing effort to significantly improve the asteroseismic characterisation of the host star, we investigate the low-frequency behaviour of HS 2201+2610. Methods. We obtained rapid high signal-to-noise photometric CCD ( B -filter) and PMT (clear-filter) data at 2 m-class telescopes and carried out a careful frequency analysis of the light curves. Results. In addition to the previously known short-period luminosity variations in the range 342 s–367 s, we find a long-period variation with a period of 54 mn and an amplitude of 0.15 per cent. This can most plausibly be identified with a g -mode pulsation, so that HS 2201+2610 is a new addition to the short list of hybrid sdB pulsators. Conclusions. Along with the previously known pulsating subdwarf B stars HS 0702+6043 and Balloon 090100001 showing hybrid behaviour, the new hybrid HS 2201+2610 is the third member of this class. This important property of HS 2201+2610 can lead to a better characterisation of this planet-hosting star, helping the characterisation of its planetary companion as well. Current pulsation models cannot yet reproduce hybrid sdBV stars particularly well and improved pulsation models for this object have to include the hybrid behaviour.

Observations of the pulsating subdwarf B star Feige 48: Constraints on evolution and companions

Since pulsating subdwarf B (sdBV or EC14026) stars were first discovered, observational efforts have tried to realize their potential for constraining the interior physics of extreme horizontal branch stars. Difficulties encountered along the way include uncertain mode identifications and a lack of stable pulsation mode properties. Here we report on Feige 48, an sdBV star for which follow-up observations have been obtained spanning more than four years. These observations show some stable pulsation modes. We resolve the temporal spectrum into five stable pulsation periods in the range 340–380 s with amplitudes less than 1 per cent, and two additional periods that appear in one data set each. The three largest amplitude periodicities are nearly equally spaced, and we explore the consequences of identifying them as a rotationally split l= 1 triplet by consulting a representative stellar model. The general stability of the pulsation amplitudes and phases allows us to use the pulsation phases to constrain the time-scale of evolution for this sdBV star. Additionally, we are able to place interesting limits on any stellar or planetary companion to Feige 48.

MSST observations of the pulsating sdB star PG 1605+072

We present the first results from the MultiSite Spectroscopic Telescope (MSST) observations of the sdBV star PG 1605+072. Pulsating sdB stars (also known as EC 14026 stars) offer the chance to gain new insights into the formation and evolution of extreme Horizontal Branch stars using the tools of asteroseismology. PG 1605+072 is an outstanding object in its class, with the richest frequency spectrum, the longest periods, and the largest variations. The MSST campaign took place in May/June 2002 immediately following the Whole Earth Telescope Xcov22 run, which observed PG 1605+072 as an alternate target. We will first give an overview of the project and its feasibility, after which we will present the massive data set, made up of 399 h of photometry and 151 h of spectroscopy. The overall aims of the project are to examine light/velocity amplitude ratios and phase differences, changes in equivalent width/line index, and λ-dependence of photometric amplitudes, and to use these properties for mode identification.

Multisite spectroscopic and photometric observations of the pulsating sdB star PG 1605+072

AbstractWe present the first results from the MultiSite Spectroscopic Telescope (MSST) observations of the sdBV star PG 1605+072. Pulsating sdB stars (V361 Hya stars) offer the chance to gain new insights into the formation and evolution of extreme Horizontal Branch stars using the tools of asteroseismology. PG 1605+072 is an outstanding object in its class, with the richest frequency spectrum, the longest periods, and the largest variations. The MSST campaign took place in 2002 May/June and we present here the massive data set, made up of 399 hr of photometry and 151 hr of spectroscopy. The overall aims of the project are to examine light/velocity amplitude ratios and phase differences, changes in equivalent width/line index, and λ-dependence of photometric amplitudes, and to use these properties for mode identification.

Colour and radial velocity variations in pulsating subluminous B stars

The utility of pulsations for the investigation of the structure and evolution of subdwarf B stars is considerable. However, the small number of detected modes generally limits the potential for a traditional seismological analysis such as that carried out for the Sun. Therefore, it is crucial to acquire additional primary data to characterise the stellar oscillations more completely. We review recent studies of radial velocity amplitudes and introduce new multi-colour photometry of small-amplitude sdBV stars. We also discuss a set of models for radial velocity and colour variations and demonstrate how these may be used to infer the spherical and azimuthal degrees of observed pulsations.

HS0702+6043 – A new large amplitude sdB variable at the cool end of the instability region

We report on time series photometry of hot sdB stars from the Hamburg Schmidt survey carried out at the Calar Alto 1.23 m and 2.2 m telescopes. Among 14 spectroscopically selected candidates we detected oscillations in the sdB star HS0702+6043 with a period of 363 s and an amplitude of 29 mmag which is among the largest amplitude observed in sdBV stars. A second period of 382 s with a significantly smaller amplitude (3.8 mmag) might be present. Our NLTE model atmosphere analysis of the time-averaged optical spectra, obtained at the Calar Alto 3.5 m telescope, indicates that HS0702+6043 has K and . This places the star at the cool end of the theoretical sdBV instability strip (EC 14026 stars). Among the other thirteen sdB stars we confirmed the variablity of the previously discovered sdBV HS0815+4243 and identified twelve stars to be stable, for which we give upper limits for light variations.

PG 1325+101 and PG 2303+019: Two new large amplitude subdwarf B pulsators

We report the detection of short period oscillations in the sdB stars PG 1325+101 (B = 13.8) and PG 2303+019 (alias HS 2303+0152, B = 16.0) from time-series photometry made at the Nordic Optical Telescope of a sample of 21 candidates. Both stars are multi-mode pulsators with at least three distinct periods in the range 100-140 s, and relatively large amplitudes up to 2.6 and 1.6% respectively. Moreover PG 1325+101 shows the shortest pulsation period ever registered among sdBV stars, 68.9 s, which corresponds to the first harmonic of the main signal. Following previous temperature and gravity determinations for PG 1325+101 (T e f f = 34500 K, log g = 6.1) and our NLTE model atmosphere analysis for PG 2303+019 (T e f f = 35 200 K, log g = 5.7), both stars are well inside the theoretical sdBV instability strip.

First results from the multisite time-series campaign on the sdBV star HS2201+2610

In this article we present the first results of about 95 hours of time-series photometry on the low gravity sdB pulsating star HS 2201+2610, obtained from a multi-site time-series campaign in September-October 2000. The temporal spectrum of HS 2201+2610 shows 3 close frequencies at 2860.9, 2824.1 and 2880.8 μHz with amplitudes of about 1 %, 0.4 % and 0.1 % respectively. Due to the small number of excited modes, a mode identification is possible in principle but not easy; no clear signatures of rotational splitting are seen.