Context. Fundamental parameters and physical processes leading to the formation of white dwarfs (WDs) may be constrained and refined by discovering WDs in open clusters (OCs). Cluster membership can be utilized to establish the precise distances, luminosities, ages, and progenitor masses of such WDs. Aims. We compile a list of probable WDs that are OC members in order to facilitate WD studies that are impractical or difficult to conduct for Galactic field WDs. Methods. We use recent catalogs of WDs and OCs that are based on the second data release of the Gaia satellite mission (GDR2) to identify WDs that are OC members. This crossmatch is facilitated by the astrometric and photometric data contained in GDR2 and the derived catalogs. Assuming that most of the WD members are of the DA type, we estimate the WD masses, cooling ages, and progenitor masses. Results. We have detected several new likely WD members and reassessed the membership of the literature WDs that had been previously associated with the studied OCs. Several of the recovered WDs fall into the recently reported discontinuity in the initialfinal mass relation (IFMR) around M i ∼ 2.0M , which allows for tighter constrains on the IFMR in this regime.
Context. We investigate the occurrence of stellar bow shocks around high-mass X-ray binaries (HMXBs) in the Galaxy. Aims. We seek to conduct a survey of HMXBs in the mid-infrared to search for the presence of bow shocks around these objects. Methods. Telescopes operating in the mid-infrared, such as the Spitzer Space Telescope or Wide-field Infrared Survey Explorer (WISE), are potent tools for searching for the stellar bow shocks. We used the available archival data from these telescopes to search for bow shock candidates around the confirmed and candidate HMXBs in the Galaxy. Results. We detected extended mid-infrared structures around several surveyed confirmed and candidate HMXBs. Two of these structures, associated with Vela X-1 and 4U 1907+09, are genuine bow shocks that have been studied previously. However, there are no new unambiguous bow shocks among the rest of the objects. The paucity of bow shocks around HMXBs suggests that the majority of these systems still reside within hot, low-density bubbles around their parent star clusters or associations. This also implies that the dynamical ejection of massive binaries is apparently less efficient than the ejections caused by the supernova explosions inside a binary.
Since the discovery of the spectral peculiarities of their prototype α2 Canum Venaticorum in 1897, the so-called ACV variables, which are comprised of several groups of chemically peculiar stars of the upper main sequence, have been the target of numerous photometric and spectroscopic studies. Especially for the brighter ACV variables, continuous observations over about a century are available, which are important to study long-term effects such as period changes or magnetic cycles in these objects. The present work presents an analysis of 165 Ap/CP2 and He-weak/CP4 stars using light curves obtained by the Solar Mass Ejection Imager (SMEI) between the years 2003 and 2011. These data fill an important gap in observations for bright ACV variables between the Hipparcos and TESS satellite missions. Using specifically tailored data treatment and period search approaches, we find variability in the accuracy limit of the employed data in 84 objects. The derived periods are in excellent agreement with the literature; for one star, the here presented solution represents the first published period. We discuss the apparently constant stars and the corresponding level of non-variability. From an investigation of our target star sample in the Hertzsprung-Russell diagram, we deduce ages between 100 Myr and 1 Gyr for the majority of our sample stars. Our results support that the variable CP2/4 stars are in a more advanced evolutionary state and that He and Si peculiarities, preferentially found in the hotter, and thus more massive, CP stars, produce larger spots or spots of higher contrast.
Chemically peculiar (CP) stars of the upper main sequence are characterized by specific anomalies in the photospheric abundances of some chemical elements. The group of CP2 stars, which encompasses classical Ap and Bp stars, exhibits strictly periodic light, spectral, and spectropolarimetric variations that can be adequately explained by the model of a rigidly rotating star with persistent surface structures and a stable global magnetic field. Using observations from the Kepler K2 mission, we find that the B9pSi star HD 174356 displays a light curve variable in both amplitude and shape, which is not expected in a CP2 star. Employing archival and new photometric and spectroscopic observations, we carry out a detailed abundance analysis of HD 174356 and discuss its photometric and astrophysical properties in detail. We employ phenomenological modelling to decompose the light curve and the observed radial velocity variability. Our abundance analysis confirms that HD 174356 is a silicon-type CP2 star. No magnetic field stronger than 110 G was found. The star’s light curve can be interpreted as the sum of two independent strictly periodic signals with $P_1=4{_{.}^{\rm d}}043\, 55(5)$ and $P_2=2{_{.}^{\rm d}}111\, 69(3)$. The periods have remained stable over 17 yr of observations. In all spectra, HD 174356 appears to be single-lined. From the simulation of the variability characteristics and investigation of stars in the close angular vicinity, we put forth the hypothesis that the peculiar light variability of HD 174356 arises in a single star and is caused by rotational modulation due to surface abundance patches (P1) and g-mode pulsation (P2).
We combine Strömgren-Crawford uvbyβ photometry with Gaia-DR2 parallaxes for a sample of 880 supergiants of O-B9 spectral types to explore the potential of these data in providing physical parameters of such stars. We obtained reddening corrections and photometric absolute magnitudes via the existing uvbyβ calibrations. We calculated Gaia-based absolute magnitudes applying individual photometric reddening corrections for each star. The uvbyβ photometry is particularly useful in this case, since it provides color excess independent of spectral classification, thus overcoming possible MK classification inconsistencies. After evaluating and correcting the effect of luminosity class mis-classifications on the photometric absolute magnitudes, we found a reasonable agreement between the photometric and Gaia-based absolute magnitudes for this sample. At this point, the colormagnitude M V versus (b − y) 0 diagrams based on photometry and Gaia show slight discrepancies, which have to be addressed in the light of the next Gaia data releases. The derived precise reddening corrections should further help to refine the physical parameters of these stars and address their connection to OB-associations and regions of star formation.
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