A search for variable subdwarf B stars in TESS full frame images – I. Variable objects in the southern ecliptic hemisphere

ABSTRACT We report the results of our search for pulsating subdwarf B stars in full frame images, sampled at 30 min cadence and collected during Year 1 of the TESS mission. Year 1 covers most of the southern ecliptic hemisphere. The sample of objects we checked for pulsations was selected from a subdwarf B stars data base available to public. Only two positive detections have been achieved, however, as a by-product of our search we found 1807 variable objects, most of them not classified, hence their specific variability class cannot be confirmed at this stage. Our preliminary discoveries include: 2 new subdwarf B (sdB) pulsators, 26 variables with known sdB spectra, 83 non-classified pulsating stars, 83 eclipsing binaries (detached and semidetached), a mix of 1535 pulsators and non-eclipsing binaries, two novae, and 77 variables with known (non-sdB) spectral classification. Among eclipsing binaries we identified two known HW Vir systems and four new candidates. The amplitude spectra of the two sdB pulsators are not rich in modes, but we derive estimates of the modal degree for one of them. In addition, we selected five sdBV candidates for mode identification among 83 pulsators and describe our results based on this preliminary analysis. Further progress will require spectral classification of the newly discovered variable stars, which hopefully include more subdwarf B stars.

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.

Hot subdwarf B stars with neutron star components

Context. Subdwarf B stars (sdBs) play a crucial role in stellar evolution, asteroseismology, and far-UV radiation of early-type galaxies, and have been intensively studied with observation and theory. It has theoretically been predicted that sdBs with neutron star (NS) companions exist in the Galaxy, but none have been discovered yet. This remains a puzzle in this field. In a previous study (hereafter Paper I), we have studied the formation channels of sdB+NS binaries from main-sequence (MS) stars plus NS binaries by establishing a model grid, but it is still unclear how these binaries consisting of MS stars and NS binaries came to be in the first place. Aims. We systematically study the formation of sdB+NS binaries from their original zero-age main-sequence progenitors. We bridge the gap left by our previous study in this way. We obtain the statistical population properties of sdB+NS binaries and provide some guidance for observational efforts. Methods. We first used Hurley’s rapid binary evolution code BSE to evolve 107 primordial binaries to the point where the companions of NS+MS, NS+Hertzsprung gap star, and NS+Giant Branch star binaries have just filled their Roche lobes. Next, we injected these binaries into the model grid we developed in Paper I to obtain the properties of the sdB+NS populations. We adopted two prescriptions of NS natal kicks: the classical Maxwellian distribution with a dispersion of σ = 265 km s−1, and a linear formula that assumes that the kick velocity is associated with the ratio of ejected to remnant mass. Different values of αCE, where αCE is the common-envelope ejection efficiency, were chosen to examine the effect of common-envelope evolution on the results. Results. In the Galaxy, the birthrate of sdB+NS binaries is about 10−4 yr−1 and there are ∼7000 − 21 000 such binaries. This contributes 0.3−0.5% of all sdB binaries in the most favorable case. Most Galactic sdB+NS binaries (≳60%) arise from the channel of stable mass transfer. The value of αCE has little effect on the results, but when we use the linear formula prescription of NS natal kick, the number and birthrate doubles in comparison to the results we obtained with the Maxwellian distribution. The orbital periods of sdB+NS binaries from different formation channels differ significantly, as expected. This results in two peaks in the radial velocity (RV) semi-amplitude distribution: 100 − 150 km s−1 for stable mass transfer, and 400 − 600 km s−1 for common-envelope ejection. However, the two sdB+NS binary populations exhibit similar delay-time distributions, which both peak at about 0.2 Gyr. This indicates that Galactic sdB+NS binaries are born in very young populations, probably in the Galactic disk. The sdB+NS binaries produced from the common-envelope ejection channel are potential sources of strong gravitational wave radiation (GWR), and about ∼100 − 300 could be detected by the Laser Interferometer Space Antenna (LISA) with a signal-to-noise ratio of 1. Conclusions. Most sdB+NS binaries are located in the Galactic disk with small RV semi-amplitudes. SdB+NS binaries with large RV semi-amplitudes are expected to be strong GWR sources, some of which could be detected by LISA in the future.

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.

Mode identification in three pulsating hot subdwarfs observed with TESS satellite

ABSTRACT We report on the detection of pulsations of three pulsating subdwarf B stars observed by the Transiting Exoplanet Survey Satellite (TESS) satellite and our results of mode identification in these stars based on an asymptotic period relation. SB 459 (TIC 067584818), SB 815 (TIC 169285097), and PG 0342 + 026 (TIC 457168745) have been monitored during single sectors resulting in 27 d coverage. These data sets allowed for detecting, in each star, a few tens of frequencies that we interpreted as stellar oscillations. We found no multiplets, though we partially constrained mode geometry by means of period spacing, which recently became a key tool in analyses of pulsating subdwarf B stars. Standard routine that we have used allowed us to select candidates for trapped modes that surely bear signatures of non-uniform chemical profile inside the stars. We have also done statistical analysis using collected spectroscopic and asteroseismic data of previously known subdwarf B stars along with our three stars. Making use of high precision trigonometric parallaxes from the Gaia mission and spectral energy distributions we converted atmospheric parameters to stellar ones. Radii, masses, and luminosities are close to their canonical values for extreme horizontal branch stars. In particular, the stellar masses are close to the canonical one of 0.47 M⊙ for all three stars but uncertainties on the mass are large. The results of the analyses presented here will provide important constrains for asteroseismic modelling.

Asteroseismology of tidally distorted sdB stars

ABSTRACT Most subdwarf B stars are located in post-common envelope binaries. Many are in short-period systems subject to tidal influence, and many show pulsations useful for asteroseismic inference. In combination, one must quantify when and how tidal distortion affects the normal modes. We present a method for computing tidal distortion and associated frequency shifts. Validation is by application to polytropes and comparison with previous work. For typical sdB stars, a tidal distortion to the radius of between $0.2\,$ and $2\,$ per cent is generated for orbital periods of 0.1 d. Application to numerical helium core-burning stars identifies the period and mass-ratio domain where tidal frequency shifts become significant and quantifies those shifts in terms of binary properties and pulsation modes. Tidal shifts disrupt the symmetric form of rotationally split multiplets by introducing an asymmetric offset to modes. Tides do not affect the total spread of a rotationally split mode unless the stars are rotating sufficiently slowly that the rotational splitting is smaller than the tidal splitting.

K2 observations of the sdBV + dM/bd binaries PHL 457 and EQ Psc

ABSTRACT 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 Teff 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.