We present a comprehensive overview of a volume-complete sample of white dwarfs located within 40 pc of the Sun, a significant proportion of which were detected in Gaia Data Release 3 (DR3). Our DR3 ...sample contains 1076 spectroscopically confirmed white dwarfs, with just five candidates within the volume remaining unconfirmed (more than 99 per cent spectroscopic completeness). Additionally, 28 white dwarfs were not in our initial selection from Gaia DR3, most of which are in unresolved binaries. We use Gaia DR3 photometry and astrometry to determine a uniform set of white dwarf parameters, including mass, effective temperature, and cooling age. We assess the demographics of the 40 pc sample, specifically magnetic fields, binarity, space density and mass distributions.
We present the first volume-limited sample of spectroscopically confirmed hot subluminous stars out to 500 pc, defined using the accurate parallax measurements from the {\em Gaia} space mission data ...release 3 (DR3). The sample comprises a total of 397 members, with 305 (\(\sim 77\%\)) identified as hot subdwarf stars, including 83 newly discovered systems. Of these, we observe that 178 (\(\sim58\%\)) are hydrogen-rich sdBs, 65 are sdOBs (\(\sim 21\%\)), 32 are sdOs (\(\sim 11\%\)), and 30 are He-sdO/Bs (\(\sim 10\%\)). Among them, 48 (\(\sim 16\%\)) exhibit an infrared excess in their spectral energy distribution fits, suggesting a composite binary system. The hot subdwarf population is estimated to be 90\% complete, assuming that most missing systems are these composite binaries located within the main sequence (MS) in the \emph{Gaia} colour-magnitude diagram (CMD). The remaining sources in the sample include cataclysmic variables (CVs), blue horizontal branch stars (BHBs), hot white dwarfs (WDs), and MS stars. We derived the mid-plane density \(\rho_{0}\) and scale height \(\mathrm{h}_{z}\) for the non-composite hot subdwarf star population using a hyperbolic sechant profile (sech\(^2\)). The best-fit values are \(\rho_{0}\,=\,5.17\pm 0.33 \times10^{-7}\) stars/pc\(^{3}\) and \(\mathrm{h}_{z} = 281 \pm 62\) pc. When accounting for the composite-colour hot subdwarfs and their estimated completeness, the mid-plane density increases to \(\rho_{0}\,=\,6.15^ {+1.16}_{-0.53} \times10^{-7}\) stars/pc\(^{3}\). This corrected space density is an order of magnitude lower than predicted by population synthesis studies, supporting previous observational estimates.
HD49798 / RXJ0648.0-4418 is the only confirmed X-ray binary in which the mass donor is a hot subdwarf star of O spectral type and, most likely, it contains a massive white dwarf (1.28\(\pm\)0.05 ...M\(_{\rm SUN}\)) with a very fast spin period of 13.2 s. Here we report the results of new XMM-Newton pointings of this peculiar binary, carried out in 2018 and in 2020, together with a reanalysis of all the previous observations. The new data indicate that the compact object is still spinning-up at a steady rate of \((-2.17\pm0.01)\times10^{-15}\) s s\(^{-1}\), consistent with its interpretation in terms of a young contracting white dwarf. Comparison of observations obtained at similar orbital phases, far from the ecplise, shows evidence for long term variability of the hard (\(>\)0.5 keV) spectral component at a level of \(\sim\)(70\(\pm\)20)\%, suggesting the presence of time-dependent inhomogeneities in the weak stellar wind of the HD49798 subdwarf. To investigate better the soft spectral component that dominates the X-ray flux from this system, we computed a theoretical model for the thermal emission expected from an atmosphere with element abundances and surface gravity appropriate for this massive white dwarf. This model gives a best fit with effective temperature of T\(_{\rm eff}\)=2.25\(\times\)10\(^5\) K and an emitting area with radius of \(\sim\)1600 km, larger than that found with blackbody fits. This model also predicts a contribution of the pulsed emission from the white dwarf in the optical band significantly larger than previously thought and possibly relevant for optical variability studies of this system.
We report results of our analysis of the Kepler superaperture LC data of the open cluster NGC6791 to search for pulsating sdB stars. We checked all pixels and we found only three sdB stars to be ...pulsating, KIC2569576 (B3), KIC2438324 (B4) and KIC2437937 (B5). These stars were known to be pulsators before, though we extended data coverage detecting more frequencies and features in their amplitude spectra, i.e. new multiplets and more complete period spacing sequences that we used for identifying geometry of the pulsation modes. The multiplet splittings were also used to derive rotation periods. The remaining known sdBs do not show any pulsation-related light variation down to our detection thresholds. We analyzed already existing spectroscopic observations taken with the HECTOSPEC at the MMT telescope in Smithsonian Arizona and with the GMOS at the Gemini North telescope, and fitted atmospheric parameters using the Balmer lines. Four stars, B3-B6, show atmospheric parameters that are consistent with g-mode dominated sdBs. We detected hints of radial velocity variability in B3, B5, and B6, indicating these three stars may be in binaries.
The main science aim of the BlackGEM array is to detect optical counterparts
to gravitational wave mergers. Additionally, the array will perform a set of
synoptic surveys to detect Local Universe ...transients and short time-scale
variability in stars and binaries, as well as a six-filter all-sky survey down
to ~22nd mag. The BlackGEM Phase-I array consists of three optical wide-field
unit telescopes. Each unit uses an f/5.5 modified Dall-Kirkham (Harmer-Wynne)
design with a triplet corrector lens, and a 65cm primary mirror, coupled with a
110Mpix CCD detector, that provides an instantaneous field-of-view of
2.7~square degrees, sampled at 0.564\arcsec/pixel. The total field-of-view for
the array is 8.2 square degrees. Each telescope is equipped with a six-slot
filter wheel containing an optimised Sloan set (BG-u, BG-g, BG-r, BG-i, BG-z)
and a wider-band 440-720 nm (BG-q) filter. Each unit telescope is independent
from the others. Cloud-based data processing is done in real time, and includes
a transient-detection routine as well as a full-source optimal-photometry
module. BlackGEM has been installed at the ESO La Silla observatory as of
October 2019. After a prolonged COVID-19 hiatus, science operations started on
April 1, 2023 and will run for five years. Aside from its core scientific
program, BlackGEM will give rise to a multitude of additional science cases in
multi-colour time-domain astronomy, to the benefit of a variety of topics in
astrophysics, such as infant supernovae, luminous red novae, asteroseismology
of post-main-sequence objects, (ultracompact) binary stars, and the relation
between gravitational wave counterparts and other classes of transients
AIMS: With our low-resolution spectroscopic observing program for selected blue proper motion stars, we tried to find new white dwarfs (WDs) in the solar neighbourhood. METHODS: We used the LSPM ...catalogue with a lower proper motion limit of 150mas/yr and the UCAC2 for proper motions down to about 90mas/yr. The LSPM and UCAC2 photometry was combined with Two Micron All Sky Survey (2MASS) near-infrared (NIR) photometry. Targets selected according to their blue optical-to-NIR and NIR colours were observed mainly at Calar Alto. The spectra were classified by comparison with a large number of already known comparison objects, including WDs, simultaneously observed within our program. Gaia DR2 parallaxes and colours were used to confirm or reject spectroscopic WD candidates and to derive improved effective temperatures. RESULTS: We found ten new WDs at distances between 24.4pc and 79.8pc, including six hot DA WDs: GD 221 (DA2.0), HD 166435 B (DA2.2), GD 277 (DA2.2), 2MASS J19293865+1117523 (DA2.4), 2MASS J05280449+4105253 (DA3.6), and 2MASS J05005185-0930549 (DA4.2). The latter is rather bright (G~12.6) and with its Gaia DR2 parallax of ~14mas it appears overluminous by about 3mag compared to the WD sequence in the Gaia DR2 colour-magnitude diagram. It may be the closest extremely low mass (ELM) WD to the Sun. We further classified 2MASS J07035743+2534184 as DB4.1. With its distance of 25.6pc it is the second nearest known representative of its class. With GD 28 (DA6.1), LP 740-47 (DA7.5), and LSPM J1919+4527 (DC10.3) three additional cool WDs were found. Gaia DR2 parallaxes showed us that four of our candidates but also two previously supposed WDs (WD 1004+665 and LSPM J1445+2527) are in fact distant Galactic halo stars with high tangential velocities. Among our rejected WD candidates, we identified a bright (G=13.4mag) G-type carbon dwarf, LSPM J0937+2803, at a distance of 272pc.
We present a spectroscopic survey of 248 white dwarf candidates within 40 pc
of the Sun; of these 244 are in the southern hemisphere. Observations were
performed mostly with the Very Large Telescope ...(X-Shooter) and Southern
Astrophysical Research Telescope. Almost all candidates were selected from
$\textit{Gaia}$ Data Release 3 (DR3). We find a total of 246 confirmed white
dwarfs, 209 of which had no previously published spectra, and two main-sequence
star contaminants. Of these, 100 white dwarfs display hydrogen Balmer lines, 69
have featureless spectra, and two show only neutral helium lines. Additionally,
14 white dwarfs display traces of carbon, while 37 have traces of other
elements that are heavier than helium. We observe 36 magnetic white dwarfs
through the detection of Zeeman splitting of their hydrogen Balmer or metal
spectral lines. High spectroscopic completeness (> 97 per cent) has now been
reached, such that we have 1058 confirmed $\textit{Gaia}$ DR3 white dwarfs out
of 1083 candidates within 40 pc of the Sun at all declinations.
We present simultaneous, multi-colour optical light curves of the companion star to the black-widow pulsar PSR J2051-0827, obtained approximately 10 years apart using ULTRACAM and HiPERCAM, ...respectively. The ULTRACAM light curves confirm the previously reported asymmetry in which the leading hemisphere of the companion star appears to be brighter than the trailing hemisphere. The HiPERCAM light curves, however, do not show this asymmetry, demonstrating that whatever mechanism is responsible for it varies on timescales of a decade or less. We fit the symmetrical HiPERCAM light curves with a direct-heating model to derive the system parameters, finding an orbital inclination of \(55.9^{+4.8}_{-4.1}\) degrees, in good agreement with radio-eclipse constraints. We find that approximately half of the pulsar's spin-down energy is converted to optical luminosity, resulting in temperatures ranging from approximately \(5150^{+190}_{-190}\) K on the day side to \(2750^{+130}_{-150}\) K on the night side of the companion star. The companion star is close to filling its Roche lobe (\(f_{\rm RL} =0.88^{+0.02}_{-0.02}\)) and has a mass of \(0.039^{+0.010}_{-0.011}\) M\(_{\odot}\), giving a mean density of \(20.24^{+0.59}_{-0.44}\) g cm\(^{-3}\) and an apsidal motion constant in the range \(0.0036 < k_2 < 0.0047\). The companion mass and mean density values are consistent with those of brown dwarfs, but the apsidal motion constant implies a significantly more centrally-condensed internal structure than is typical for such objects.
We perform ground based photometric observations of 22 DA white dwarf stars, 10 already known ZZ Cetis and 12 candidates with atmospheric parameters inside the classical instability strip. We report ...on the discovery of four new variable DA white dwarf stars. Two objects are near the middle of the instability strip, SDSS J082804.63+094956.6 and SDSS J094929.09+101918.8, and two red edge pulsators, GD 195 and L495-82. In addition, we classified four objects as possible variables, since evidence of variability was detected in the light curve, but the S/N was not sufficient to establish a definite detection. Follow-up observations were performed for 10 know ZZ Ceti stars to verify period stability and search for new periodicities. For each confirmed variable, we perform a detailed asteroseismological fit and compare the structural parameters obtained from the best fit models with those obtained from spectroscopy and photometry from Gaia. Finally we present a study of the asteroseismological properties of a sample of 91 ZZ Ceti stars.
HiPERCAM is a portable, quintuple-beam optical imager that saw first light on the 10.4-m Gran Telescopio Canarias (GTC) in 2018. The instrument uses re-imaging optics and 4 dichroic beamsplitters to ...record \(u_s g_s r_s i_s z_s\) (\(320-1060\) nm) images simultaneously on its five CCD cameras, each of 3.1 arcmin (diagonal) field of view. The detectors in HiPERCAM are frame-transfer devices cooled thermo-electrically to 183 K, thereby allowing both long-exposure, deep imaging of faint targets, as well as high-speed (over 1000 windowed frames per second) imaging of rapidly varying targets. A comparison-star pick-off system in the telescope focal plane increases the effective field of view to 6.7 arcmin for differential photometry. Combining HiPERCAM with the world's largest optical telescope enables the detection of astronomical sources to \(g_s \sim 23\) in 1 s and \(g_s \sim 28\) in 1 h. In this paper we describe the scientific motivation behind HiPERCAM, present its design, report on its measured performance, and outline some planned enhancements.
