Abstract: The Southern African Large Telescope (SALT) has surveyed a sample of 300 chemically-peculiar hot subdwarfs with a view to explore evolutionary pathways and connections between a large variety of stars. Observations have been obtained for the entire sample at intermediate resolution to provide classifications and atmosphere parameters, and a smaller sample at high resolution to permit detailed chemical abundance analyses. The first has provided a substantial sample of extremely helium-rich sdO stars, but has also led to significant discoveries of hot white dwarfs, pre-white dwarfs, heavy-metal subdwarfs and extreme helium stars. This paper provides an overview of the current status of the project. PubDate: Thu, 25 May 2023 00:00:00 +020
Abstract: The origin of subdwarf B stars (sdBs) is unclear. Different formation channels are proposed including: close-binary evolution, helium-mixing on the red giant branch and the hot flasher scenario. Circumstantial evidence exists that sdBs can result from single star evolution in high metallicity populations. We investigate this possibility by using membership in the thin, thick disk and halo populations as a metallicity indicator. Radial velocity measurements were com-bined with Gaia proper motions to calculate space motions in the Galaxy. Galactic orbits and population memberships were derived. Preliminary results indicate more sdBs are members of the thin disk (86 sdBs) than of thick disk and halo (28 sdBs). Apparently single sdBs are found among all three populations. We look into the temperature distributions of the different types and discuss implication for formation scenarios. PubDate: Tue, 23 May 2023 00:00:00 +020
Abstract: Post-common Envelope Eclipsing Binary (PCEB) systems with a white or sub-dwarf primary and a low-mass secondary are ideal systems to study Eclipse Timing Variations (ETVs) due to their relatively small total masses which make it easy to detect gravitationally bound, yet unseen, additional bodies even within planetary limits on long-period orbits. With the addition of the recent discovery of two planetary-mass bodies in the sdB + dM binary Kepler-451 system, the number of circumbinary planets in PCEBs approaches 30 now. However, some of these systems turned out to be dynamically unstable with the suggested planetary parameters. As more observations accumulate over time, there have been significant changes in both the number of suggested planets and their parameters. These necessitate a look back to the ETV analyses of these systems with new and more precise data spanning longer baselines. Within this contribution, we review the systems hosting substellar bodies suggested from the light-time effect they are claimed to be causing. We list the general problems encountered during the ETV analyses and the interpretation of the results, and comment on their possible solutions. We cal-culate and present the potentials of supporting evidence from astrometry, radial velocity, and transit observations as well as tests through dynamical stability analyses and magnetic activity assumptions. PubDate: Tue, 23 May 2023 00:00:00 +020
Abstract: Hyper-velocity stars (HVS) are enigmatic objects because they are travelling so fast that they escape from the Galaxy. Among hot subdwarfs, only one such star is known, the He-sdO US 708. The Hyper-MUCHFUSS collaboration provided additional HVS candidates. Here we revisit the fastest candidates including US 708 by analysing optical spectra and spectral energy distributions using a new grid of tailored model atmospheres and report preliminary results. The sample is dominated by H-rich subdwarfs and their distribution in the Kiel diagram appears to be bimodal for the sdB stars but otherwise fits canonical evolutionary models well. Gaia’s proper motion measurements allowed a precise kinematic investigation to be made. It turns out that all previously proposed HVS candidates are actually bound to the Galaxy, except US 708. The original candidate sample turns out to belong to an extreme halo population. The scarcity of available observations of US 708 calls for space-based UV and IR photometry as well as high precision radial velocity measurements. PubDate: Tue, 23 May 2023 00:00:00 +020
Abstract: We present a search for new variable hot subdwarfs from Gaia eDR3 using the Zwicky Transient Facility DR9 and the 2.1 m Otto Struve Telescope at McDonald Observatory. We report the discovery of 22 HW Vir binaries, 24 reflection effect binaries, and 14 ellipsoidally modulated systems. Candidate selection for our high-speed observations was based on object location on the Gaia color-magnitude diagram and on intrinsic variability, which we estimate from the Gaia G flux uncertainties. Notable discoveries include a candidate 72 min period sdB+WD binary and two short period HW Vir binaries with periods around 67 min and 79 min, which includes the shortest-period eclipsing sdB+dM/BD discovered to date. PubDate: Tue, 23 May 2023 00:00:00 +020
Abstract: Some Be stars exhibit a bright and extremely hard X-ray emission, whose origin is debated. In this context, it has been advocated that stripped helium-star sdO companions could play a significant role. Using high quality datasets, we examined (1) the binarity of these peculiar Be stars and (2) the X-ray emission of known Be+sdO systems. In this contribution, we present the result of these two studies and assess their impact on the proposed scenario. Certaines étoiles de type Be présentent une émission en rayons X brillante et très dure, d’origine débattue. Il a notamment été proposé que des compagnons sdO pouvaient jouent un rôle majeur dans la génération de cette émission. Grâce à des donnés inédites de grande qualité, nous avons étudié (1) la binarité de ces étoiles Be particulières et (2) l’émission en rayons X de systèmes Be+sdO. Cette contribution présente les résultats de ces deux recherches et leur impact sur le scénario proposé. PubDate: Tue, 23 May 2023 00:00:00 +020
Abstract: We present an update of the grid of detailed atmosphere models and homogeneous synthetic spectra for hot, high-gravity subdwarf stars. High-resolution spectra and synthetic photometry were calculated in the wavelength range 1,000 Å – 10,000 Å using Non-LTE extensively line-blanketed atmosphere structures. Nous prèsentons une mise à jour de la grille des modèles d’atmosphère détaillés et des spectres synthétiques homogénes pour les étoiles sous-naines chaudes de haute gravité. Les spectres haute résolution et la photométrie synthétique ont été calculés dans la gamme de longueurs d’onde 1.000 Å - 10.000 Å à l’aide de structures atmosphériques non-LTE entiérement recouvertes de lignes. PubDate: Tue, 23 May 2023 00:00:00 +020
Abstract: The main goal of this project is to investigate star-planet interactions during the Red Giant Branch (RGB) phase. In particular, we focus on what happens to planetary systems once their host stars leave the main sequence and engulf close-in planets, presumably, perturbing the dy-namical architecture of the system. In this context, we wonder if close-in planets may survive this evolutionary phase, evolving towards a new stable configuration, or on the contrary, if these planets are destroyed and the entire system collapses. Whatever the outcome, how will this interaction alter the star and affect its subsequent evolution' Here, we search for observational evidence of transiting planets orbiting hot subdwarfs (sdBs) stars, which are direct post-RGB objects that have lost most of their envelopes. To this end, we analyze light curves from the space missions Kepler/K2, TESS, and CHEOPS to search for shallow periodic transits of close-in, previously engulfed planets or their remnants, such as the rocky cores of giant gas planets. By determining the occurrence of planets or remnants around sdB stars, we will place strong constraints on the evolution of planetary systems after the RGB phase of their host stars. This summary describes the methods, tools, and strategies used to explore the available photometric data in the search for transiting planets and establish detection limits that allow us to build planet occurrence rates around these stars. PubDate: Tue, 23 May 2023 00:00:00 +020
Abstract: 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. PubDate: Tue, 23 May 2023 00:00:00 +020
Abstract: TIC 137608661 is a new sdBV+dM reflection-effect binary discovered by the TESS space mission with an orbital period of 7.21 hours. In addition to the detailed study of this system that was recently published (Silvotti et al., 2022), this article contains a further discussion on its inclination. Moreover, from the 3D space velocities of the binary obtained from the system’s radial velocity and from the Gaia DR3 proper motion, it is shown that TIC 137608661 belongs to the galactic thin disk population. PubDate: Tue, 23 May 2023 00:00:00 +020
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