It is uncertain whether or not low-mass Population III stars ever existed. While limits on the number density of Population III stars with $M_{\ast} \approx 0.8~M_{\odot}$ have been derived using Sloan Digital Sky Survey (SDSS) data, little is known about the occurrence of Population III stars at lower masses. In the absence of reliable parallaxes, the spectra of metal-poor main sequence (MPMS) stars with $M_{\ast} \lesssim 0.8~M_{\odot}$ can easily be confused with cool white dwarfs. To resolve this ambiguity, we present a classifier that differentiates between MPMS stars and white dwarfs based on photometry and/or spectroscopy without the use of parallax information. We build and train our classifier using state-of-the-art theoretical spectra and evaluate it on existing SDSS-based classifications for objects with reliable Gaia DR2 parallaxes. We then apply our classifier to a large catalog of objects with SDSS photometry and spectroscopy to search for MPMS candidates. We discover several previously unknown candidate extremely metal-poor (EMP) stars and recover numerous confirmed EMP stars already in the literature. We conclude that archival SDSS spectroscopy has already been exhaustively searched for EMP stars. We predict that the lowest-mass primordial-composition stars will have redder optical-to-infrared colors than cool white dwarfs at constant effective temperature due to surface gravity-dependent collision-induced absorption from molecular hydrogen. We suggest that the application of our classifier to data produced by next-generation spectroscopic surveys will set stronger constraints on the number density of low-mass Population III stars in the Milky Way.
We present the results of our survey searching for new white dwarf pulsators for observations by the TESS space telescope. We collected photometric time-series data on 14 white dwarf variable-candidates at Konkoly Observatory, and found two new bright ZZ Ceti stars, namely EGGR 120 and WD 1310+583. We performed the Fourier-analysis of the datasets. In the case of EGGR 120, which was observed on one night only, we found one significant frequency at 1332 microHz with 2.3 mmag amplitude. We successfully observed WD 1310+583 on eight nights, and determined 17 significant frequencies by the whole d…
Searching for White Dwarf Candidates Formed Through Binary evolution in Star Clusters
School of Physics and Astronomy, Sun Yat-sen University（中山大学物理学院）；CSST Science Center for the Guangdong–Hong Kong–Macau Greater Bay Area（粤港澳大湾区宇宙线科学中心）；Department of Physics and Astronomy, University of British Columbia（不列颠哥伦比亚大学物理与天文学系）；Department of Physics, Xi’an Jiaotong–Liverpool University（西交利物浦大学物理系）；Shanghai Key Laboratory for Astrophysics, Shanghai Normal University（上海师范大学天体物理上海市重点实验室）；National Astronomical Observatories（国家天文台）
A sample of 4 thousands Gaia sources with apparent $G$-band magnitudes of $<17$ and trigonometric parallaxes of $>3.33$ milli-arcseconds were cross-matched to the Transiting Exoplanet Survey high level science products and the Full-Frame Images in order to extract the light curves of possible white dwarf variables. Most of the targets in the sample were observed in at least 27-day observing cycle with 30-minutes cadence. Based on Lomb-Scargle periodogram constructed from de-trended light curves, more than 600 sources have indication of periodic variability. This paper presents the early result…
The ground state of matter may be strange quark matter (SQM), not hadronic matter. A whole sequence of SQM objects, ranging from strange quark stars and strange quark dwarfs to strange quark planets, can stably exist according to this SQM hypothesis. A strange dwarf has a mass similar to that of a normal white dwarf but could harbor an extremely dense SQM core (with a density as large as $\sim \rm 4\times10^{14}\,g\,cm^{-3} $) at the center so that its radius can be correspondingly smaller. In this study, we try to search for strange dwarfs among the observed "white dwarfs" by considering thei…
The {\it Gaia} satellite recently released parallax measurements for $\sim$260,000 high-confidence white dwarf candidates, allowing for precise measurements of their physical parameters. By combining these parallaxes with Pan-STARRS and $u$-band photometry, we measured the effective temperature and stellar mass for all white dwarfs in the Northern Hemisphere within 100 parsecs of the Sun, and identified a sample of ZZ Ceti white dwarf candidates within the so-called instability strip. We acquired high-speed photometric observations for 90 candidates using the PESTO camera attached to the 1.6-m…
Type Ia Supernovae (SNe Ia) are securely understood to come from the thermonuclear explosion of a white dwarf as a result of binary interaction, but the nature of that binary interaction and the secondary object is uncertain. Recently, a double white dwarf model known as the dynamically driven double-degenerate double-detonation (D6) model has become a promising explanation for these events. One realization of this scenario predicts that the companion may survive the explosion and reside within the remnant as a fast moving ($V_{peculiar} >1000$ km s$^{-1}$), overluminous ($L > 0.1 L_\odot$) wh…
Our knowledge of the magnetism in white dwarfs is based on an observational dataset that is biased in favour of stars with very strong magnetic fields. Most of the field measurements available in the literature have a relatively low sensitivity, while current instruments allow us to detect magnetic fields of white dwarfs with sub-kG precision. With the aim of obtaining a more complete view of the incidence of magnetic fields in degenerate stars, we have started a long-term campaign of high-precision spectropolarimetric observations of white dwarfs. Here we report the results obtained so far wi…
We report on multi-band photometric observations of four binary millisecond pulsars with the Gran Telescopio Canarias. The observations led to detection of binary companions to PSRs J1630+3734, J1741+1351 and J2042+0246 in the Sloan g', r' and i' bands. Their magnitudes in the r' band are $\approx$24.4, 24.4 and 24.0, respectively. We also set a 3$σ$ upper limit on the brightness of the PSR J0557+1550 companion in the r' band of $\approx$25.6 mag. Combining the optical data with the radio timing measurements and white dwarf cooling models, we show that the detected companions are cool low-mass…
We present spectropolarimetric observations and modelling of 12 DQ white dwarfs. Modelling is based on the method presented in Berdyugina et al. (2005). We use the model to fit the C_2 absorption bands to get atmospheric parameters in different configurations, including stellar spots and stratified atmospheres, searching for the best possible fit. We still have problem to solve before we can give temperature estimates based on the Swan bands alone.
Searching for GEMS: Discovery of the Nearby Post-Common-Envelope Binary System TIC-460388167
University of Wyoming（怀俄明大学）；NASA Goddard Space Flight Center（美国国家航空航天局戈达德太空飞行中心）；Carnegie Institution for Science（卡内基科学机构）；The University of Arizona（亚利桑那大学）；The University of Texas at Austin（得克萨斯大学奥斯汀分校）；The Pennsylvania State University（宾夕法尼亚州立大学）
White dwarfs with metal lines in their spectra act as signposts for post-main sequence planetary systems. Searching the Sloan Digital Sky Survey (SDSS) data release 12, we have identified 231 cool (<9000 K) DZ white dwarfs with strong metal absorption, extending the DZ cooling sequence to both higher metal abundances, lower temperatures, and hence longer cooler ages. Of these 231 systems, 104 are previously unknown white dwarfs. Compared with previous work, our spectral fitting uses improved model atmospheres with updated line profiles and line-lists, which we use to derive effective temperatu…
We report on the detection of the two shortest period non-interacting white dwarf binary systems. These systems, SDSS J143633.29+501026.8 and SDSS J105353.89+520031.0, were identified by searching for radial velocity variations in the individual exposures that make up the published spectra from the Sloan Digital Sky Survey. We followed up these systems with time series spectroscopy to measure the period and mass ratios of these systems. Although we only place a lower bound on the companion masses, we argue that they must also be white dwarf stars. With periods of approximately 1 hour, we estim…
The sample of white dwarfs included in the local 20 pc volume documents, fairly accurately, the total production of white dwarfs over roughly 10 Gyr of stellar evolution in this part of the Milky Way Galaxy. In this sample, we have been systematically searching for magnetic white dwarfs. Here we report the discovery of six new magnetic white dwarfs, with a field strength from a few MG to about 200MG. Two of these stars show H lines that are split and polarised by the magnetic field. One star shows extremely weak spectral lines in intensity, to which highly polarised narrow features correspond.…
Binary systems of a hot subdwarf B (sdB) star + a white dwarf (WD) with orbital periods less than 2-3 hours can come into contact due to gravitational waves and transfer mass from the sdB star to the WD before the sdB star ceases nuclear burning and contracts to become a WD. Motivated by the growing class of observed systems in this category, we study the phases of mass transfer in these systems. We find that because the residual outer hydrogen envelope accounts for a large fraction of an sdB star's radius, sdB stars can spend a significant amount of time ($\sim$10s of Myr) transferring this s…
Magnetic fields have been detected in several hundred white dwarfs, with strengths ranging from a few kG to several hundred MG. Only a few of the known fields have a mean magnetic field modulus below about 1 MG. We are searching for new examples of magnetic white dwarfs with very weak fields, and trying to model the few known examples. Our search is intended to be sensitive enough to detect fields at the few kG level. We have been surveying bright white dwarfs for very weak fields using spectropolarimeters at the Canada-France-Hawaii telescope, the William Herschel telescope, the European Sout…
An increasing number of white dwarf stars show atmospheric chemical composition polluted by heavy elements accreted from debris disk material. The existence of such debris disks strongly suggests the presence of one or more planet(s) whose gravitational interaction with rocky planetesimals is responsible for their disruption by tidal effect. The ZZ Ceti pulsator and polluted DAZ white dwarf GD 133 is a good candidate for searching for such a potential planet. We started in 2011 a photometric follow-up of its pulsations. As a result of this work in progress, we used the data gathered from 2011 …
L-type and T-type dwarfs span the boundaries between main-sequence stars, brown dwarfs, and planetary-mass objects. For these reasons, L and T dwarfs are the perfect laboratories for exploring the relationship between planet formation and moon formation, and evidence suggests they may be swarming with close-in rocky satellites, though none have been found to date. The discovery of satellites orbiting L or T dwarfs will have transformative implications for the nature of planets, moons and even life in the Universe. These transiting satellites will be prime targets for characterization with NASA…
We provide a general analysis on the properties of emitting material of some rapidly evolving and luminous transients discovered recently with the Pan-STARRS1 Medium Deep Survey. It is found that these transients are probably produced by a low-mass non-relativistic outflow that is continuously powered by a newly born, rapidly spinning, and highly magnetized neutron star. Such a system could originate from an accretion-induced collapse of a white dwarf or a merger of a neutron star-neutron star binary. Therefore, observations to these transients would be helpful for constraining white dwarf and…
Aims. We use the Kepler data accumulated on the pulsating DB white dwarf KIC 08626021 to explore in detail the stability of its oscillation modes, searching in particular for evidences of nonlinear behaviors. Methods. We analyse nearly two years of uninterrupted short cadence data, concentrating in particular on identified triplets due to stellar rotation that show intriguing behaviors during the course of the observations. Results. We find clear signatures of nonlinear effects attributed to resonant mode coupling mechanisms. We find that a triplet at 4310 μHz and this doublet at 3681 μHz (mos…
The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims at finding hot subdwarf stars with massive compact companions (massive white dwarfs M>1.0 Msun, neutron stars or stellar mass black holes). The existence of such systems is predicted by binary evolution theory and some candidate systems have been found. We classified about 1400 hot subdwarf stars from the Sloan Digital Sky Survey (SDSS) by colour selection and visual inspection of their spectra. Stars with high velocities have been reobserved and individual SDSS spectra have been analysed. In total…
We present our results on the continuation of our survey searching for new ZZ Ceti stars, inspired by the recently launched TESS space mission. The seven targets were bright DA-type white dwarfs located close to the empirical ZZ Ceti instability strip. We successfully identified one new pulsator candidate, namely PM J22299+3024, derived detection limits for possible pulsations of four objects for the first time, and determined new detection limits for two targets.
The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims at finding sdBs with compact companions like supermassive white dwarfs (M>1.0 Msun), neutron stars or black holes. The existence of such systems is predicted by binary evolution theory and recent discoveries indicate that they are likely to exist in our Galaxy. A determination of the orbital parameters is sufficient to put a lower limit on the companion mass by calculating the binary mass function. If this lower limit exceeds the Chandrasekhar mass and no sign of a companion is visible in the spect…
A large fraction of white dwarfs (WDs) may host planets in their habitable zones. These planets may provide our best chance to detect bio-markers on a transiting exoplanet, thanks to the diminished contrast ratio between the Earth-sized WD and its Earth-sized planets. The JWST is capable of obtaining the first spectroscopic measurements of such planets, yet there are no known planets around WDs. Here we propose to take advantage of the unique capability of the Kepler spacecraft in the 2-Wheels mode to perform a transit survey that is capable of identifying the first planets in the habitable zo…
The direct detection of a stellar system that explodes as a Type Ia supernova (SN Ia) has not yet been successful. Various indirect methods have been used to investigate SN Ia progenitor systems but none have produced conclusive results. A prediction of single-degenerate models is that H- (or He-) rich material from the envelope of the companion star should be swept up by the SN ejecta in the explosion. Seven SNe Ia have been analysed to date looking for signs of H-rich material in their late-time spectra and none were detected. We present results from new late-time spectra of 11 SNe Ia obtain…
A Calibrated Bayesian Search for Potential Chemical Technosignatures in Polluted White Dwarf
Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University（北京师范大学前沿天文与天体物理研究所）；School of Physics and Astronomy, Beijing Normal University（北京师范大学物理与天文学院）；College of Computer and Information Engineering, Dezhou University（德州学院计算机与信息工程学院）
Discovery of a compact hierarchical triple main-sequence star system while searching for binary stars with compact objects
Fukui Prefectural University（福井县立大学）；Subaru Telescope Okayama Branch, National Astronomical Observatory of Japan（日本国立天文台 Subaru 望远镜冈山分部）；Nishi-Harima Astronomical Observatory, Center for Astronomy, University of Hyogo（兵库大学天文学中心西播磨天文台）；Department of Earth and Planetary Sciences, Institute of Science Tokyo（东京科学大学地球行星科学系）；Department of Earth and Space Science, Graduate School of Science, Osaka University（大阪大学理学研究科地球宇宙科学系）；Astrobiology Center（天体生物学中心）
Searching for compact objects (black holes, neutron stars, or white dwarfs) in the Milky Way is essential for understanding the stellar evolution history, the physics of compact objects, and the structure of our Galaxy. Compact objects in binaries with a luminous stellar companion are perfect targets for optical observations. Candidate compact objects can be achieved by monitoring the radial velocities of the companion star. However, most of the spectroscopic telescopes usually obtain stellar spectra at a relatively low efficiency, which makes a sky survey for millions of stars practically imp…
A TESS View of Post-Eruption Variability in the Novae V1405 Cas,V1716 Sco, and V1674 Her
Universidad Nacional de Hurlingham (UNAHUR)（乌尔希林国立大学）；Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)（国家科学研究委员会）；Advanced Technologies Research Institute, Faculty of Materials Science and Technology in Trnava, Slovak University of Technology in Bratislava（布拉迪斯拉发斯洛伐克技术大学特伦材料科学与技术学院先进技术研究所）；INAF-Osservatorio Astronomico di Padova（帕多瓦天文台）；Department of Astronomy, University of Wisconsin（威斯康星大学天文学系）
AM CVn systems are ultra-compact, helium-rich, accreting binaries with degenerate or semi-degenerate donors. We report the discovery of five new eclipsing AM CVn systems with orbital periods of 61.5, 55.5, 53.3, 37.4, and 35.4 minutes. These systems were discovered by searching for deep eclipses in the Zwicky Transient Facility (ZTF) lightcurves of white dwarfs selected using Gaia parallaxes. We obtained phase-resolved spectroscopy to confirm that all systems are AM CVn binaries, and we obtained high-speed photometry to confirm the eclipse and characterize the systems. The spectra of two long-…
Low-cost mass-produced sensors and optics have recently made it feasible to build telescope arrays which observe the entire accessible sky simultaneously. In this article we discuss the scientific motivation for these telescopes, including exoplanets, stellar variability and extragalactic transients. To provide a concrete example we detail the goals and expectations for the Evryscope, an under-construction 780 MPix telescope which covers 8,660 square degrees in each two-minute exposure; each night, 18,400 square degrees will be continuously observed for an average of approximately 6 hours. Des…
We present results of a Hubble Space Telescope far-ultraviolet (FUV) survey searching for white dwarf (WD) companions to blue straggler stars (BSSs) in open cluster NGC 188. The majority of NGC 188 BSSs (15 of 21) are single-lined binaries with properties suggestive of mass-transfer formation via Roche lobe overflow, specifically through an asymptotic giant branch star transferring mass to a main sequence secondary, yielding a BSS binary with a WD companion. In NGC 188, a BSS formed by this mechanism within the past 400 Myr will have a WD companion hot and luminous enough to be directly detect…
