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

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

Monte Carlo simulations of post-common-envelope white dwarf + main sequence binaries: The effects of including recombination energy

Detached WD+MS PCEBs are perhaps the most suitable objects for testing predictions of close-compact binary-star evolution theories, in particular, CE evolution. The population of WD+MS PCEBs has been simulated by several authors in the past and compared with observations. However, most of those predictions did not take the possible contributions to the envelope ejection from additional sources of energy (mostly recombination energy) into account. Here we update existing binary population models of WD+MS PCEBs by assuming that a fraction of the recombination energy available within the envelope contributes to ejecting the envelope. We performed Monte Carlo simulations of 10^7 MS+MS binaries for 9 different models using standard assumptions for the initial primary mass function, binary separations, and initial-mass-ratio distribution and evolved these systems using the publicly available BSE code. Including a fraction of recombination energy leads to a clear prediction of a large number of long orbital period (>~10 days) systems mostly containing high-mass WDs. The fraction of systems with He-core WD primaries increases with the CE efficiency and the existence of very low-mass He WDs is only predicted for high values of the CE efficiency (>~0.5). All models predict on average longer orbital periods for PCEBs containing C/O-core WDs than for PCEBs containing He WDs. This effect increases with increasing values of both efficiencies. Longer periods after the CE phase are also predicted for systems containing more massive secondary stars. The initial-mass-ratio distribution affects the distribution of orbital periods, especially the distribution of secondary star masses. Our simulations, in combination with a large and homogeneous observational sample, can provide constraints on the values of the CE efficiencies, as well as on the initial-mass-ratio distribution for MS+MS binary stars.Comment: 11 pages, 10 figures, accepted for publication in A&

DA white dwarfs from the LSS-GAC survey DR1: the preliminary luminosity and mass functions and formation rate

Modern large-scale surveys have allowed the identification of large numbers of white dwarfs. However, these surveys are subject to complicated target selection algorithms, which make it almost impossible to quantify to what extent the observational biases affect the observed populations. The LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope) Spectroscopic Survey of the Galactic anti-center (LSS-GAC) follows a well-defined set of criteria for selecting targets for observations. This advantage over previous surveys has been fully exploited here to identify a small yet well-characterised magnitude-limited sample of hydrogen-rich (DA) white dwarfs. We derive preliminary LSS-GAC DA white dwarf luminosity and mass functions. The space density and average formation rate of DA white dwarfs we derive are 0.83+/-0.16 x 10^{-3} pc^{-3} and 5.42 +/- 0.08 x 10^{-13} pc^{-3} yr^{-1}, respectively. Additionally, using an existing Monte Carlo population synthesis code we simulate the population of single DA white dwarfs in the Galactic anti-center, under various assumptions. The synthetic populations are passed through the LSS-GAC selection criteria, taking into account all possible observational biases. This allows us to perform a meaningful comparison of the observed and simulated distributions. We find that the LSS-GAC set of criteria is highly efficient in selecting white dwarfs for spectroscopic observations (80-85 per cent) and that, overall, our simulations reproduce well the observed luminosity function. However, they fail at reproducing an excess of massive white dwarfs present in the observed mass function. A plausible explanation for this is that a sizable fraction of massive white dwarfs in the Galaxy are the product of white dwarf-white dwarf mergers.Comment: 23 pages, 14 figures and 5 tables. Accepted for publication by MNRA