The critical binary star separation for a planetary system origin of white dwarf pollution

The atmospheres of between one quarter and one half of observed single white dwarfs in the Milky Way contain heavy element pollution from planetary debris. The pollution observed in white dwarfs in binary star systems is, however, less clear, because companion star winds can generate a stream of matter which is accreted by the white dwarf. Here we (i) discuss the necessity or lack thereof of a major planet in order to pollute a white dwarf with orbiting minor planets in both single and binary systems, and (ii) determine the critical binary separation beyond which the accretion source is from a planetary system. We hence obtain user-friendly functions relating this distance to the masses and radii of both stars, the companion wind, and the accretion rate onto the white dwarf, for a wide variety of published accretion prescriptions. We find that for the majority of white dwarfs in known binaries, if pollution is detected, then that pollution should originate from planetary material.Comment: Accepted for publication in MNRA

Post-common-envelope binaries from the Sloan Digital Sky Survey

Close binaries containing a compact object make up a wide variety of objects. The evolution of all close binaries depends crucially on the rate at which angular momentum is extracted from the binary orbit. The two most important sources of angular momentum loss are the common envelope phase and magnetic braking. Both processes have been known for long but are still poorly understood, and significant progress will only be achieved if they can be calibrated using innovative observational input. Post-common-envelope binaries are probably among the best-suited class of objects to improve our understanding of close binary evolution, because (1) they are both numerous and well-understood in terms of their stellar components, and (2) they are not contaminated by the presence of an accretion disc. The Sloan Digital Sky Survey provides the possibility of dramatically improving the observational size of known post-common-envelope binaries, with already more than 1500 white dwarf-main sequence binaries having been identified. The major task is now to identify those systems that have undergone a common envelope and to measure their binary parameters. This new, large sample of well-studied post-common-envelope binaries will then provide the much-needed constraints for further development of binary evolution theory. Through my PhD I dedicated all my efforts towards identifying post-common-envelope binaries, obtaining orbital periods of these new systems, and determining their stellar parameters. For this purpose, I adopted the following strategies: (1) About 10% of the white dwarf-main sequence binaries in the Sloan Digital Sky Survey have more than one survey spectrum available. By measuring radial velocities from the Na I ll 8183.27,8194.81 absorption doublet and/or the Ha emission line in the different spectra from each object, I was able to identify radial velocity variable stars, which are prime candidates for being post-common-envelope binaries. This method resulted in the identification of 18 new post-common-envelope binaries among 130 white dwarf-main sequence binaries with multiple Sloan spectra. In addition, using a spectral decomposition/ model atmosphere analysis I determined the stellar parameters such as mass, radius, and temperature for the white dwarfs, and spectral types of the main sequence stars in these 130 white dwarf-main sequence binaries, along with the distances to the systems. I discussed also an apparent systematic issue with the spectral type-radius relation of the companion stars in those white dwarf-main sequence binaries. (2) Follow-up observations by our team have lead to the identification of 89 postcommon-envelope binaries from Sloan, which triples the number previously known. Intense radial velocity studies have lead to the determination of orbital periods for 42 of these systems, seven of them discussed in detail in this thesis. (3) I have developed a procedure based on c2 template fitting and signal-to-noise ratio constraints to identify white dwarf-main sequence binary candidates in the Sloan Digital Sky Survey Data Release 6 spectroscopic data base. This catalogue contains 1591 white dwarf-main sequence binaries identified in this way. Using a spectral decomposition/model atmosphere analysis, I have derived white dwarf temperatures, masses, companion star spectral types, and distances, and discussed the distributions of these parameters. In addition, I have analysed the selection effects of white dwarf-main sequence binaries in Sloan. This sample is an excellent data base for future follow-up observational studies of white dwarf-main sequence binaries

Post-common-envelope binaries from the Sloan Digital Sky Survey

Close binaries containing a compact object make up a wide variety of objects. The evolution of all close binaries depends crucially on the rate at which angular momentum is extracted from the binary orbit. The two most important sources of angular momentum loss are the common envelope phase and magnetic braking. Both processes have been known for long but are still poorly understood, and significant progress will only be achieved if they can be calibrated using innovative observational input. Post-common-envelope binaries are probably among the best-suited class of objects to improve our understanding of close binary evolution, because (1) they are both numerous and well-understood in terms of their stellar components, and (2) they are not contaminated by the presence of an accretion disc. The Sloan Digital Sky Survey provides the possibility of dramatically improving the observational size of known post-common-envelope binaries, with already more than 1500 white dwarf-main sequence binaries having been identified. The major task is now to identify those systems that have undergone a common envelope and to measure their binary parameters. This new, large sample of well-studied post-common-envelope binaries will then provide the much-needed constraints for further development of binary evolution theory. Through my PhD I dedicated all my efforts towards identifying post-common-envelope binaries, obtaining orbital periods of these new systems, and determining their stellar parameters. For this purpose, I adopted the following strategies: (1) About 10% of the white dwarf-main sequence binaries in the Sloan Digital Sky Survey have more than one survey spectrum available. By measuring radial velocities from the Na I ll 8183.27,8194.81 absorption doublet and/or the Ha emission line in the different spectra from each object, I was able to identify radial velocity variable stars, which are prime candidates for being post-common-envelope binaries. This method resulted in the identification of 18 new post-common-envelope binaries among 130 white dwarf-main sequence binaries with multiple Sloan spectra. In addition, using a spectral decomposition/ model atmosphere analysis I determined the stellar parameters such as mass, radius, and temperature for the white dwarfs, and spectral types of the main sequence stars in these 130 white dwarf-main sequence binaries, along with the distances to the systems. I discussed also an apparent systematic issue with the spectral type-radius relation of the companion stars in those white dwarf-main sequence binaries. (2) Follow-up observations by our team have lead to the identification of 89 postcommon-envelope binaries from Sloan, which triples the number previously known. Intense radial velocity studies have lead to the determination of orbital periods for 42 of these systems, seven of them discussed in detail in this thesis. (3) I have developed a procedure based on c2 template fitting and signal-to-noise ratio constraints to identify white dwarf-main sequence binary candidates in the Sloan Digital Sky Survey Data Release 6 spectroscopic data base. This catalogue contains 1591 white dwarf-main sequence binaries identified in this way. Using a spectral decomposition/model atmosphere analysis, I have derived white dwarf temperatures, masses, companion star spectral types, and distances, and discussed the distributions of these parameters. In addition, I have analysed the selection effects of white dwarf-main sequence binaries in Sloan. This sample is an excellent data base for future follow-up observational studies of white dwarf-main sequence binaries.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

White dwarf-main sequence binaries from LAMOST: the DR5 catalogue

We present the data release (DR) 5 catalogue of white dwarf-main sequence (WDMS) binaries from the Large Area Multi-Object fiber Spectroscopic Telescope (LAMOST). The catalogue contains 876 WDMS binaries, of which 757 are additions to our previous LAMOST DR1 sample and 357 are systems that have not been published before. We also describe a LAMOST-dedicated survey that aims at obtaining spectra of photometrically-selected WDMS binaries from the Sloan Digital Sky Survey (SDSS) that are expected to contain cool white dwarfs and/or early type M dwarf companions. This is a population under-represented in previous SDSS WDMS binary catalogues. We determine the stellar parameters (white dwarf effective temperatures, surface gravities and masses, and M dwarf spectral types) of the LAMOST DR5 WDMS binaries and make use of the parameter distributions to analyse the properties of the sample. We find that, despite our efforts, systems containing cool white dwarfs remain under-represented. Moreover, we make use of LAMOST DR5 and SDSS DR14 (when available) spectra to measure the Na I λλ 8183.27, 8194.81 absorption doublet and/or Hα emission radial velocities of our systems. This allows identifying 128 binaries displaying significant radial velocity variations, 76 of which are new. Finally, we cross-match our catalogue with the Catalina Surveys and identify 57 systems displaying light curve variations. These include 16 eclipsing systems, two of which are new, and nine binaries that are new eclipsing candidates. We calculate periodograms from the photometric data and measure (estimate) the orbital periods of 30 (15) WDMS binaries

Post-common envelope binaries from SDSS - XVI. Long orbital period systems and the energy budget of CE evolution

Virtually all close compact binary stars are formed through common-envelope (CE) evolution. It is generally accepted that during this crucial evolutionary phase a fraction of the orbital energy is used to expel the envelope. However, it is unclear whether additional sources of energy, such as the recombination energy of the envelope, play an important role. Here we report the discovery of the second and third longest orbital period post-common envelope binaries (PCEBs) containing white dwarf (WD) primaries, i.e. SDSSJ121130.94-024954.4 (Porb = 7.818 +- 0.002 days) and SDSSJ222108.45+002927.7 (Porb = 9.588 +- 0.002 days), reconstruct their evolutionary history, and discuss the implications for the energy budget of CE evolution. We find that, despite their long orbital periods, the evolution of both systems can still be understood without incorporating recombination energy, although at least small contributions of this additional energy seem to be likely. If recombination energy significantly contributes to the ejection of the envelope, more PCEBs with relatively long orbital periods (Porb >~ 1-3 day) harboring massive WDs (Mwd >~ 0.8 Msun) should exist.Comment: Accepted for publication in MNRAS. 8 pages, 6 figures and 4 table

Gaia 0007-1605: an old triple system with an inner brown dwarf-white dwarf binary and an outer white dwarf companion

We identify Gaia 0007–1605 A,C as the first inner brown dwarf–white dwarf binary of a hierarchical triple system in which the outer component is another white dwarf (Gaia 0007–1605 B). From optical/near-infrared spectroscopy obtained at the Very Large Telescope with the X-Shooter instrument and/or from Gaia photometry plus spectral energy distribution fitting, we determine the effective temperatures and masses of the two white dwarfs (12,018 ± 68 K and 0.54 ± 0.01 M¿ for Gaia 0007–1605 A and 4445 ± 116 K and 0.56 ± 0.05 M¿ for Gaia 0007–1605 B) and the effective temperature of the brown dwarf (1850 ± 50 K; corresponding to spectral type L3 ± 1). By analyzing the available TESS light curves of Gaia 0007–1605 A,C we detect a signal at 1.0446 ± 0.0015 days with an amplitude of 6.25 ppt, which we interpret as the orbital period modulated from irradiation effects of the white dwarf on the brown dwarf's surface. This drives us to speculate that the inner binary evolved through a common-envelope phase in the past. Using the outer white dwarf as a cosmochronometer and analyzing the kinematic properties of the system, we conclude that the triple system is about 10 Gyr old.Postprint (published version

The mass function of hydrogen-rich white dwarfs: robust observational evidence for a distinctive high-mass excess near 1Msun

The mass function of hydrogen-rich atmosphere white dwarfs has been frequently found to reveal a distinctive high-mass excess near 1Msun. However, a significant excess of massive white dwarfs has not been detected in the mass function of the largest white dwarf catalogue to date from the Sloan Digital Sky Survey. Hence, whether a high-mass excess exists or not has remained an open question. In this work we build the mass function of the latest catalogue of data release 10 SDSS hydrogen-rich white dwarfs, including the cool and faint population (i.e. effective temperatures 6,000 <~ Teff <~ 12,000 K, equivalent to 12 mag <~ Mbol <~ 13 mag). We show that the high-mass excess is clearly present in our mass function, and that it disappears only if the hottest (brightest) white dwarfs (those with Teff >~ 12,000 K, Mbol <~ 12 mag) are considered. This naturally explains why previous SDSS mass functions failed at detecting a significant excess of high-mass white dwarfs. Thus, our results provide additional and robust observational evidence for the existence of a distinctive high-mass excess near 1Msun. We investigate possible origins of this feature and argue that the most plausible scenario that may lead to an observed excess of massive white dwarfs is the merger of the degenerate core of a giant star with a main sequence or a white dwarf companion during or shortly after a common envelope event.Comment: Accepted for publication by MNRA

The star formation history of Gaia white dwarf population through its colour-magnitude diagram

White dwarfs are the most common stellar remnants. Furthermore, as being old objects, their study could shed new light on different questions related to the history, formation and evolution of the Galaxy. Despite these objects have been broadly studied from a theoretical point of view, the observational data has been limited to a poor statistical sample, due to the intrinsic low luminosity of white dwarfs. However, thanks to the recent Gaia EDR3, for the first time, a significant sample of the White dwarf population of our Galaxy, containing around 13,000 objects up to 100 pc from the Sun, has been obtained. Such data, in particular its color-magnitude diagram, provides the ideal scenario for extracting the maximum information. This communication reports a work-in-progress of a widely applied technique for recovering the star formation history of galaxies through its color-magnitude diagram applied, in this case, for first time to the local White dwarf population.Peer ReviewedPostprint (published version