Search

We report on the first space-based observations of the bright, high-amplitude hybrid pulsator V585 Peg (Balloon 090100001). TESS observed V585 Peg for 28 d during Sector 56 from which we detect 14 g-mode pulsations, 20 p-mode pulsations, and 26 combination frequencies. These pulsations are unusual and interesting in a number of ways; V585 Peg is a rare hybrid pulsator where both regions have many (>10) pulsations, the p-mode pulsations have clear frequency multiplets and overtone spacings while the g-mode pulsations do not, its multiplets have shown changes in splittings between years, and it appears to have two stochastic pulsations while most are stable. From p-mode multiplets, we find an average rotation period of 6.91 ± 0.83 d in which the different ℓ and m indices indicate latitudinal differential rotation. Unresolved g-mode multiplets limit rotation periods to be longer then 9.3 d for dipole modes, indicating V585 Peg to also be a radially differential rotator. We detect one secure p-mode overtone of 950  |$\mu$| Hz and four less sure ones of 314, 410, 440, and 554  |$\mu$| Hz. Two frequencies show stochastic properties indicating mode lifetimes of 3.5 and 6.2 h, which would be the shortest yet discovered. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present an overview of eclipsing systems of the HW-Virginis (HW-Vir) type, based on space observations from the Transiting Exoplanet Survey Satellite (TESS) mission. We perform a detailed analysis of the properties of AA Doradus (AA Dor), which was monitored for almost a full year. This excellent time series data set permitted us to search for both stellar pulsations and eclipse timing variations. In addition, we used the high-precision trigonometric parallax from Gaia Early Data Release 3 to make an independent determination of the fundamental stellar parameters. No convincing pulsations were detected down to a limit of 76 parts per million; however, we detected one peak with false alarm probability of 0.2 per cent. 20 s cadences being collected during Year 3 should confirm or reject our detection. From eclipse timing measurements we were able to confirm that the orbital period is stable, with an upper limit to any period change of 5.75 × 10−13 s s−1. The apparent offset of the secondary eclipse is consistent with the predicted Rømer delay when the primary mass is that of a canonical extended horizontal branch star. Using parallax and a spectral energy distribution corroborates that the mass of the primary in AA Dor is canonical, and its radius and luminosity are consistent with an evolutionary state beyond core helium burning. The mass of the secondary is found to be at the limit of hydrogen burning. [ABSTRACT FROM AUTHOR]

EPIC 216747137 is a new HW Virginis system discovered by the Kepler spacecraft during its K 2 'second life'. Like the other HW Vir systems, EPIC 216747137 is a post-common-envelope eclipsing binary consisting of a hot subluminous star and a cool low-mass companion. The short orbital period of 3.87 h produces a strong reflection effect from the secondary (∼9 per cent in the R band). Together with AA Dor and V1828 Aql, EPIC 216747137 belongs to a small subgroup of HW Vir systems with a hot evolved sdOB primary. We find the following atmospheric parameters for the hot component: T eff = 40400 ± 1000 K, log  g  = 5.56 ± 0.06, and log(N (He)/ N (H)) = −2.59 ± 0.05. The sdOB rotational velocity v  sin   i  = 51 ± 10 km s−1 implies that the stellar rotation is slower than the orbital revolution and the system is not synchronized. When we combine photometric and spectroscopic results with the Gaia parallax, the best solution for the system corresponds to a primary with a mass of about 0.62 M⊙ close to, and likely beyond, the central helium exhaustion, while the cool M-dwarf companion has a mass of about 0.11 M⊙. [ABSTRACT FROM AUTHOR]

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 FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

HD 4539 (alias PG 0044 + 097 or EPIC 220641886) is a bright (V  = 10.2) long-period V1093 Her-type subdwarf B (sdB) pulsating star that was observed by the Kepler spacecraft in its secondary (K 2) mission. We use the K 2 light curve (78.7 d) to extract 169 pulsation frequencies, 124 with a robust detection. Most of these frequencies are found in the low-frequency region typical of gravity (g-)modes, but some higher frequencies corresponding to pressure (p-)modes are also detected. Therefore HD 4539 is a hybrid pulsator and both the deep and surface layers of the star can potentially be probed through asteroseismology. The lack of any frequency splitting in its amplitude spectrum suggests that HD 4539 has a rotation period longer than the K 2 run and/or that it is seen pole-on. From asymptotic period spacing we see many high-degree modes, up to l  = 12, in the spectrum of HD 4539, with amplitudes as low as a few ppm. A large fraction of these modes can be identified and for ∼29 per cent of them we obtain a unique and robust identification corresponding to l ≤ 8. Our study includes also a new determination of the atmospheric parameters of the star. From low-resolution spectroscopy we obtain T eff  = 22 800 ± 160 K, log  g  = 5.20 ± 0.02, and log(N (He)/ N (H))  = −2.34 ± 0.05. By fitting the SED we obtain T eff  = 23 470 |$^{+650}_{-210}$|  K, R ⋆  = 0.26 ± 0.01 R⊙, and M ⋆  = 0.40 ± 0.08 M⊙. Moreover, from 11 high-resolution spectra we see the radial velocity variations caused by the stellar pulsations, with amplitudes of ≈150 m s−1 for the main modes, and we can exclude the presence of a companion with a minimum mass higher than a few Jupiter masses for orbital periods below ∼300 d. [ABSTRACT FROM AUTHOR]

We present an analysis of two pulsating subdwarf B stars PHL 457 and EQ Psc observed during the K2 mission. The K2 light curves of both stars show variation consistent with irradiation of a cooler companion by the hot subdwarf. They also show higher frequency oscillations consistent with pulsation. Using new spectroscopic data, we measured the radial velocity, effective temperature, surface gravity, and helium abundance of both hot subdwarfs as a function of orbital phase. We confirm the previously published spectroscopic orbit of PHL 457, and present the first spectroscopic orbit of EQ Psc. The orbital periods are 0.313 and 0.801 d, respectively. For EQ Psc, we find a strong correlation between T eff and orbital phase, due to contribution of light from the irradiated companion. We calculated amplitude spectra, identified significant pulsation frequencies, and searched for multiplets and asymptotic period spacings. By means of multiplets and period spacing, we identified the degrees of several pulsation modes in each star. The g-mode multiplets indicate subsynchronous core rotation with periods of 4.6 d (PHL 457) and 9.4 d (EQ Psc). We made spectral energy disctribution (SED) fits of PHL 457 and EQ Psc using available broad-band photometry and Gaia data. While the SED of PHL 457 shows no evidence of a cool companion, the SED for EQ Psc clearly shows an infrared (IR) access consistent with a secondary with a temperature of about 6800 K and a radius of 0.23 R⊙. This is the first detection of an IR access in any sdB + dM binary. [ABSTRACT FROM AUTHOR]

This paper presents spectroscopic and seismic analyses of two rare hybrid pulsating subdwarf B (sdB) stars, PG 0048+091 and PG 1315−123, which are dominated by pressure (p)-mode pulsations. These are only the second and third sdB pulsators with detected frequency multiplets in both the p- and g (gravity)-mode regions. This allows an examination of the radial rotation profiles and p- and g-mode overtone spacings. Frequency multiplets indicate that PG 1315−123 rotates uniformly, as a solid body, while PG 0048+091 is rotating faster in the outer envelope. g- and p-mode overtone spacings for PG 1315−123 agree with previous findings and model predictions at 237 and ∼1000 s while those for PG 0048+091 are significantly shorter at 207 and <250 s, respectively. The cause of PG 0048+091's discrepancies is likely related to core structural differences which could be related to progenitor mass. Spectroscopic findings show that both stars lie on the hot end of the instability region and are most likely in long-period binaries with main-sequence companions. Their high temperatures challenge driving theory, which produces g-mode pulsations only at cooler temperatures. Another challenge is the appearance of pulsations with both driven and stochastic properties. Such a variety of pulsation properties makes these stars very interesting and important for understanding hot horizontal-branch stars. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We report the discovery of three pulsating subdwarf B stars in binary systems observed with the Kepler space telescope during Campaign 5 of K2. EPIC 211696659 (SDSS J083603.98+155216.4) is a g-mode pulsator with a white dwarf companion and a binary period of 3.16 d. EPICs 211823779 (SDSS J082003.35+173914.2) and 211938328 (LB 378) are both p-mode pulsators with main-sequence F companions. The orbit of EPIC 211938328 is long (635 ± 146 d) while we cannot constrain that of EPIC 211823779. The p modes are near the Nyquist frequency and so we investigate ways to discriminate super- from sub-Nyquist frequencies. We search for rotationally induced frequency multiplets and all three stars appear to be slow rotators with EPIC 211696659 subsynchronous to its orbit. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

KIC 10001893 is a V1093 Her type pulsating subdwarf-B (sdB) star, which was observed extensively by the Kepler spacecraft. Itwas a part of the survey searching for compact pulsators in the Kepler field of view. An amplitude spectrum of the star demonstrates a rich content of g modes between 102 and 496 μHz as well as a few p modes above 2000 μHz. In total, we found 110 frequencies. The g-mode region contains 104 frequencies, while the p-mode region contains just 6, altogether indicating the hybrid nature of KIC 10001893. The main goal of our analysis was to identify the detected modes and to find some features, which will significantly help modelling the star. We found no multiplets that point at a pole-on orientation; however, we defined modal degrees and relative radial orders using asymptotic period spacing. Eventually, we assigned 32 dipole l = 1 and 18 quadrupole l = 2 modes. The most remarkable feature we found is trapped modes, which are clearly seen in a reduced-period diagram. It is the second time that three trapped modes are detected in one pulsating sdB star. Since the more trapped modes we find, the better sounding of the stellar interior we can do, this feature provides important constraints on the physical structure of the star. [ABSTRACT FROM AUTHOR]

We apply asteroseismic tools to the newly discovered subdwarf B (sdB) pulsator EPIC 203948264, observed with K2, the two-gyro mission of the Kepler space telescope. A time series analysis of the 83-d Campaign 2 (C2) short-cadence data set has revealed a g-mode pulsation spectrum with 22 independent pulsation periods between 0.5 and 2.8 h. Most of the pulsations fit the asymptotic period sequences for ℓ = 1 or 2, with average period spacings of 261.3 ± 1.1 and 151.18 ± 0.37 s, respectively. The pulsation amplitudes are below 0.77 ppt and vary over time. We include updated spectroscopic parameters, including atmospheric abundances and radial velocities, which give no indication for binarity in this star. We detect one possible low-amplitude multiplet, which corresponds to a rotation period of 46 d or longer. EPIC 203948264 appears as another slowly rotating sdB star. [ABSTRACT FROM AUTHOR]

In preparation for the upcoming all-sky data releases of the Gaia mission we compiled a catalog of known hot subdwarf stars and candidates drawn from the literature and yet unpublished databases. The catalog contains 5613 unique sources and provides multiband photometry from the ultraviolet to the far infrared, ground based proper motions, classifications based on spectroscopy and colors, published atmospheric parameters, radial velocities and light curve variability information. Using several different techniques we removed outliers and misclassified objects. By matching this catalog with astrometric and photometric data from the Gaia mission, we will develop selection criteria to construct a homogeneous, magnitude-limited all-sky catalog of hot subdwarf stars based on Gaia data. [ABSTRACT FROM AUTHOR]

UVO 0825+15 is a hot bright helium-rich subdwarf which lies in K2 Field 5 and in a sample of intermediate helium-rich subdwarfs observed the Subaru High Dispersion Spectrograph. The K2 light curve shows low-amplitude variations, whilst the Subaru spectrum shows Pb IV absorption lines, indicative of a very high lead overabundance. UVO 0825+15 also has a high proper motion with kinematics typical for a thick disc star. Analyses of ultraviolet and intermediate dispersion optical spectra rule out a short-period binary companion and provide fundamental atmospheric parameters of Teff = 38 900 ± 270 K, log g/cm s-2 = 5.97 ± 0.11, log nHe/nH = -0.57 ± 0.01, EB - V ≈ 0.03, and angular radius θ = 1.062 ± 0.006 × 10-11 radians (formal errors). The high-resolution spectrum shows that carbon is >2 dex subsolar, iron is approximately solar, and all other elements heavier than argon are at least 2-4 dex overabundant, including germanium, yttrium and lead. Approximately 150 lines in the blueoptical spectrum remain unidentified. The chemical structure of the photosphere is presumed to be determined by radiatively dominated diffusion. The K2 light curve shows a dominant period around 10.8 h, with a variable amplitude, its first harmonic, and another period at 13.3 h. The preferred explanation ismultiperiodic non-radial oscillation due to gmodes with very high radial order, although this presents difficulties for pulsation theory. Alternative explanations fail for lack of radial-velocity evidence. UVO 0825+15 represents the fourth member of a group of hot subdwarfs having helium-enriched photospheres and 3-4 dex overabundances of trans-iron elements and is the first lead-rich subdwarf to show evidence of pulsations. [ABSTRACT FROM AUTHOR]

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-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 ≤ 2), but 42 are identified as ℓ ≥ 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 ≥ 2 multiplets, the splittings decrease over the course of the observations. If the trend continues, ℓ ≥ 2 multiplets would become singlets within a decade. [ABSTRACT FROM AUTHOR]

We present a spectroscopic survey of known and candidate γ Doradus stars. The high-resolution, high signal-to-noise spectra of 52 objects were collected by five different spectrographs. The spectral classification, atmospheric parameters (Teff, logg, ξ), vsini and chemical composition of the stars were derived. The stellar spectral and luminosity classes were found between G0-A7 and IV-V, respectively. The initial values for Teff and logg were determined from the photometric indices and spectral energy distribution. Those parameters were improved by the analysis of hydrogen lines. The final values of Teff, logg and ξ were derived from the iron lines analysis. The Teff values were found between 6000K and 7900K, while logg values range from 3.8 to 4.5 dex. Chemical abundances and vsini values were derived by the spectrum synthesis method. The vsini values were found between 5 and 240 km s-1. The chemical abundance pattern of γ Doradus stars were compared with the pattern of non-pulsating stars. It turned out that there is no significant difference in abundance patterns between these two groups. Additionally, the relations between the atmospheric parameters and the pulsation quantities were checked. A strong correlation between the vsini and the pulsation periods of γ Doradus variables was obtained. The accurate positions of the analysed stars in the Hertzsprung-Russell diagram have been shown. Most of our objects are located inside or close to the blue edge of the theoretical instability strip of γ Doradus. [ABSTRACT FROM AUTHOR]

We report a new subdwarf B pulsator, PG 1142-037, discovered during the first full-length campaign of K2, the two-gyro mission of the Kepler space telescope. 14 periodicities have been detected between 0.9 and 2.5 hr with amplitudes below 0.35 parts-per-thousand. We have been able to associate all of the pulsations with low-degree, l ≤ 2 modes. Follow-up spectroscopy of PG 1142 has revealed it to be in a binary with a period of 0.54 d. Phase-folding the K2 photometry reveals a two-component variation including both Doppler boosting and ellipsoidal deformation. Perhaps the most surprising and interesting result is the detection of an ellipsoidal, tidally distorted variable with no indication of rotationally induced pulsation multiplets. This indicates that the rotation period is longer than 45 d, even though the binary period is near 13 h. [ABSTRACT FROM AUTHOR]

We present our analysis of Kepler data of a pulsating subdwarf B star, KIC8302197. We used Q5-17 data and applied a Fourier technique to extract 30 significant pulsation modes. We searched for multiplets and period-spacing sequences to perform a mode identification and to derive a rotation period. To our surprise, KIC 8302197 does not show any multiplets. We explain the lack of multiplets by either a very slow rotation (longer than ~1000 days) or a unique (pole-on) orientation of the pulsation axis. Our mode identification relied solely on period spacing. We were successful in identifying modal degrees of most of the detected modes. An analysis of the period stability did not show any evidence of a companion to the host star. In addition to photometric data, several spectroscopic observations were collected. Our twelve radial-velocity measurements constrain a possible orbital radial-velocity amplitude to be smaller than about 10 km s-1. Furthermore, based on color indices we constrained a possible companion to be an M or later type main sequence, a compact or a substellar object. We found that the atmospheric parameters (Teff = 27 450 ± 200 K, log g = 5.438 ± 0.033 dex, and log(nHe/nH) = -2.56 ± 0.07 dex) of KIC 8302197 are consistent with other slow pulsating subdwarf B stars. From the optical spectra we derived C, N, O, Si and Fe abundances, and set an upper limit for the S abundance. [ABSTRACT FROM AUTHOR]

We analyse 2.75 yr of Kepler spacecraft observations of the pulsating subdwarf B star KIC 10670103. These 1.4 million measurements have an impressive duty cycle of 93.8 per cent, a frequency resolution of 0.017 μHz, and a 5σ detection limit of 0.1 parts-per-thousand (ppt). We detect 278 periodicities, making KIC 10670103 the richest pulsating subdwarf B star to date. Frequencies range from 23 to 673 μHz (0.4 and 11.8 h), with amplitudes from the detection limit up to 14 ppt. Follow-up spectroscopic data were obtained from which it was determined that KIC 10670103 does not show significant radial velocity variations. Updated atmospheric model fits determined Teff = 21 485 ± 540 K, log g = 5.14 ± 0.05, and log N(He)/N(H) = −2.60 ± 0.04. We identify pulsation modes using asymptotic period spacings and frequency multiplets. The frequency multiplets indicate a spin period of 88 ± 8 d. Of the 278 periodicities detected in KIC 10670103, 163 (59 per cent) have been associated with low-degree (ℓ ≤ 2) pulsation modes, providing tight constraints for model fitting. While the data are exquisite, amplitudes (and some frequencies) are not stable over the course of the observations, requiring tools which are non-standard for compact pulsators such as sliding Fourier transforms and Lorentzian fitting. Using the 163 identified pulsation modes, it is possible to make detailed examinations of the pulsation structure; including where the pulsation power is concentrated in radial order, over what frequency range mode trapping is inefficient, and how power switches between multiplet members. [ABSTRACT FROM AUTHOR]

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 KIC2991276 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. [ABSTRACT FROM AUTHOR]

ABSTRACT We present our analyses of 15 months of Kepler data on KIC 10139564. We detected 57 periodicities with a variety of properties not previously observed all together in one pulsating subdwarf B (sdB) star. Ten of the periodicities were found in the low-frequency region, and we associate them with nonradial g modes. The other periodicities were found in the high-frequency region, which are likely p modes. We discovered that most of the periodicities are components of multiplets with a common spacing. Assuming that multiplets are caused by rotation, we derive a rotation period of 25.6 ± 1.8 d. The multiplets also allow us to identify the pulsations to an unprecedented extent for this class of pulsator. We also detect l ≥ 2 multiplets, which are sensitive to the pulsation inclination and can constrain limb darkening via geometric cancellation factors. While most periodicities are stable, we detected several regions that show complex patterns. Detailed analyses showed that these regions are complicated by several factors. Two are combination frequencies that originate in the super-Nyquist region and were found to be reflected below the Nyquist frequency. The Fourier peaks are clear in the super-Nyquist region, but the orbital motion of Kepler smears the Nyquist frequency in the barycentric reference frame and this effect is passed on to the sub-Nyquist reflections. Others are likely multiplets but unstable in amplitudes and/or frequencies. The density of periodicities also makes KIC 10139564 challenging to explain using published models. This menagerie of properties should provide tight constraints on structural models, making this sdB star the most promising for applying asteroseismology. To support our photometric analysis, we have obtained spectroscopic radial-velocity measurements of KIC 10139564 using low-resolution spectra in the Balmer-line region. We did not find any radial-velocity variation. We used our high signal-to-noise average spectrum to improve the atmospheric parameters of the sdB star, deriving Teff = 31 859 K and log g = 5.673 dex. [ABSTRACT FROM AUTHOR]

ABSTRACT We have used 605 days of photometric data from the Kepler spacecraft to study KIC 6614501, a close binary system with an orbital period of 0.157 497 47(25) days (3.779 939 h), that consists of a low-mass subdwarf B (sdB) star and a white dwarf (WD). As seen in many other similar systems, the gravitational field of the WD produces an ellipsoidal deformation of the sdB which appears in the light curve as a modulation at two times the orbital frequency. The ellipsoidal deformation of the sdB implies that the system has a maximum inclination of ∼40°, with i ≈ 20° being the most likely. The orbital radial velocity (RV) of the sdB star is high enough to produce a Doppler beaming effect with an amplitude of 432 ± 5 ppm, clearly visible in the folded light curve. The photometric amplitude that we obtain, K1 = 85.8 km s−1, is ∼12 per cent less than the spectroscopic RV amplitude of 97.2 ± 2.0 km s−1. The discrepancy is due to the photometric contamination from a close object at about 5 arcsec north-west of KIC 6614501, which is difficult to remove. The atmospheric parameters of the sdB star, Teff = 23 700 ± 500 K and log g = 5.70 ± 0.10, imply that it is a rare object below the extreme horizontal branch (EHB), similar to HD 188112. The comparison with different evolutionary tracks suggests a mass between ∼0.18 and ∼0.25 M⊙, too low to sustain core helium burning. If the mass was close to 0.18-0.19 M⊙, the star could be already on the final He-core WD cooling track. A higher mass, up to ∼0.25 M⊙, would be compatible with a He-core WD progenitor undergoing a cooling phase in a H-shell flash loop. A third possibility, with a mass between ∼0.32 and ∼0.40 M⊙, cannot be excluded and would imply that the sdB is a 'normal' (but with an unusually low mass) EHB star burning He in its core. In all these different scenarios, the system is expected to merge in less than 3.1 Gyr due to gravitational wave radiation. [ABSTRACT FROM AUTHOR]

The recently discovered subdwarf B (sdB) pulsator KIC11 558 725 is one of the 16 pulsating sdB stars detected in the Kepler field. It features a rich g-mode frequency spectrum, with a few low-amplitude p-modes at short periods. This makes it a promising target for a seismic study aiming to constrain the internal structure of this star, and of sdB stars in general. We have obtained ground-based spectroscopic radial-velocity measurements of KIC 11 558 725 based on low-resolution spectra in the Balmer-line region, spanning the 2010 and 2011 observing seasons. From these data we have discovered that KIC11 558 725 is a binary with period P = 10.05 d, and that the radial-velocity amplitude of the sdB star is 58 km s-1. Consequently the companion of the sdB star has a minimum mass of 0.63 M☉, and is therefore most likely an unseen white dwarf. We analyse the near-continuous 2010-2011 Kepler light curve to reveal the orbital Doppler-beaming effect, giving rise to light variations at the 238 ppm level, which is consistent with the observed spectroscopic orbital radial-velocity amplitude of the subdwarf. We use the strongest 70 pulsation frequencies in the Kepler light curve of the subdwarf as clocks to derive a third consistent measurement of the orbital radial-velocity amplitude, from the orbital light-travel delay. The orbital radius asdB sin i = 11.5 R☉ gives rise to a light-travel time delay of 53.6 s, which causes aliasing and lowers the amplitudes of the shortest pulsation frequencies, unless the effect is corrected for. We use our high signal-to-noise average spectra to study the atmospheric parameters of the sdB star, deriving Teff = 27 910K and log g = 5.41 dex, and find that carbon, nitrogen and oxygen are underabundant relative to the solar mixture. Furthermore, we analyse the Kepler light curve for its pulsational content and extract more than 160 significant frequencies. We investigate the pulsation frequencies for expected period spacings and rotational splittings. We find period-spacing sequences of spherical-harmonic degrees ɭ = 1 and ɭ = 2, and we associate a large fraction of the g-modes in KIC 11 558 725 with these sequences. From frequency splittings we conclude that the subdwarf is rotating subsynchronously with respect to the orbit. [ABSTRACT FROM AUTHOR]

BSTRACT [ABSTRACT FROM AUTHOR]

BSTRACT [ABSTRACT FROM AUTHOR]

We present our discovery of a new hybrid pulsating subdwarf B (sdB) star, J08069+1527. The effective temperature and surface gravity of 28 500 ± 400 K and 5.37 ± 0.04 dex, respectively, place this object inside the instability strip and also among other pulsating hot subdwarfs of a hybrid nature, right next to another fascinating star: Balloon 090100001. From this proximity, we anticipated this star could pulsate in both high- and low-frequency modes. Indeed, our analysis of photometric data confirmed our prediction. We detected two peaks in the high-frequency region and two other peaks at low frequencies. In addition, the amplitude of the dominant mode is very high and comparable to the dominant peaks in other hybrid sdB stars. Since this star is bright, we performed time-series low-resolution spectroscopy. Despite a low signal-to-noise ratio, we were able to detect the main peak from these data. All our results strongly indicate that J08069+1527 is a high-amplitude pulsating hot sdB star of hybrid nature. By analogy to the other pulsating sdB star, we judge that the dominant mode we detected here has radial nature. Future stellar modelling should provide us with quite good constrains as p and g modes presented in this star are driven in different parts of its interior. [ABSTRACT FROM AUTHOR]

We present the discovery of non-radial pulsations in five hot subdwarf B (sdB) stars based on 27 d of nearly continuous time series photometry using the Kepler spacecraft. We find that every sdB star cooler than K that Kepler has observed (seven so far) is a long-period pulsator of the V1093 Her (PG 1716) class or a hybrid star with both short and long periods. The apparently non-binary long-period and hybrid pulsators are described here. The V1093 Her periods range from 1 to 4.5 h and are associated with -mode pulsations. Three stars also exhibit short periods indicative of -modes with periods of 2-5 min and in addition, these stars exhibit periodicities between both classes from 15 to 45 min. We detect the coolest and longest-period V1093 Her-type pulsator to date, KIC010670103 ( K, h) as well as a suspected hybrid pulsator, KIC002697388, which is extremely cool ( K) and for the first time hybrid pulsators which have larger -mode amplitudes than -mode ones. All of these pulsators are quite rich with many frequencies and we are able to apply asymptotic relationships to associate periodicities with modes for KIC010670103. Kepler data are particularly well suited for these studies as they are long duration, extremely high duty cycle observations with well-behaved noise properties. [ABSTRACT FROM AUTHOR]

We present results from the first two quarters of a survey to search for pulsations in compact stellar objects with the Kepler spacecraft. The survey sample and the various methods applied in its compilation are described, and spectroscopic observations are presented to separate the objects into accurate classes. From the Kepler photometry we clearly identify nine compact pulsators and a number of interesting binary stars. Of the pulsators, one shows the strong, rapid pulsations typical of a V361 Hya-type sdB variable (sdBV); seven show long-period pulsation characteristics of V1093 Her-type sdBVs; and one shows low-amplitude pulsations with both short and long periods. We derive effective temperatures and surface gravities for all the subdwarf B stars in the sample and demonstrate that below the boundary region where hybrid sdB pulsators are found, all our targets are pulsating. For the stars hotter than this boundary temperature a low fraction of strong pulsators (10 per cent) is confirmed. Interestingly, the short-period pulsator also shows a low-amplitude mode in the long-period region, and several of the V1093 Her pulsators show low-amplitude modes in the short-period region, indicating that hybrid behaviour may be common in these stars, also outside the boundary temperature region where hybrid pulsators have hitherto been found. [ABSTRACT FROM AUTHOR]

2M1938+4603 (KIC 9472174) displays a spectacular light curve dominated by a strong reflection effect and rather shallow, grazing eclipses. The orbital period is 0.126 d, the second longest period yet found for an eclipsing sdB+dM, but still close to the minimum 0.1-d period among such systems. The phase-folded Kepler light curve was used to detrend the orbital effects from the data set. The amplitude spectrum of the residual light curve reveals a rich collection of pulsation peaks spanning frequencies from ∼50 to 4500 μHz. The presence of a complex pulsation spectrum in both the p- and g-mode regions has never before been reported in a compact pulsator. Eclipsing sdB+dM stars are very rare, with only seven systems known and only one with a pulsating primary. Pulsating stars in eclipsing binaries are especially important since they permit masses derived from seismological model fits to be cross-checked with orbital mass constraints. We present a first analysis of this star based on the Kepler 9.7-d commissioning light curve and extensive ground-based photometry and spectroscopy that allow us to set useful bounds on the system parameters. We derive a radial-velocity amplitude , inclination angle , and find that the masses of the components are and . [ABSTRACT FROM AUTHOR]

Using the large photometric and spectroscopic data sets of the ν Eridani multisite campaign given in our two recent papers (Aerts et al. and Handler et al.), we present an extended frequency analysis and a photometric mode identification. For the extended frequency analysis, we used an improved radial velocity time series, the second-moment time series and the line profiles themselves. In the radial velocity time series, we can now detect an additional pulsation frequency that was previously only found in photometric time series. We also report several new candidate pulsation frequencies. For seven frequencies, the photometric mode identification indicates that they belong to a radial mode and six dipole modes, and for three frequencies the degree ℓ could not be unambiguously determined. We also placed ν Eri in the Hertzsprung–Russell diagram by determining using Geneva plus Strömgren photometric calibrations, spectral energy distribution fitting, by non-local thermodynamic equilibrium hydrogen, helium and silicon line profile fitting, and by determining using the Hipparcos parallax and an Hβ calibration. [ABSTRACT FROM AUTHOR]