168 research outputs found
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 ΝΝ 8183.27, 8194.81 absorption doublet
and/or HÎą 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
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
Exsolution process in white dwarf stars
White dwarf (WD) stars are considered cosmic laboratories to study the
physics of dense plasma. Furthermore, the use of WD stars as cosmic clocks to
date stellar populations and main sequence companions demands an appropriate
understanding of the WD physics in order to provide precise ages for these
stars. We aim at studying exsolution in the interior of WD stars, a process in
which a crystallized ionic binary mixture separates into two solid solutions
with different fractions of the constituents. Depending on the parent solid
mixture composition, this process can release or absorb heat, thus leading to a
delay or a speed-up of WD cooling. Relying on accurate phase diagrams for
exsolution, we have modeled this process in hydrogen-rich WDs with both
carbon-oxygen and oxygen-neon core composition, with masses ranging from 0.53
to 1.29Msun and from 1.10 to 1.29Msun, respectively. Exsolution is a slow
process that takes place at low luminosities (log(L/Lsun)-2.75) and
effective temperatures (Teff18 000K) in WDs. We find that exsolution
begins at brighter luminosities in CO than in ONe WDs of the same mass. Massive
WDs undergo exsolution at brighter luminosities than their lower-mass
counterparts. The net effect of exsolution on the WD cooling times depends on
the stellar mass and the exact chemical profile. For standard core chemical
profiles and preferred assumptions regarding miscibility gap microphysics, the
cooling delay can be as large as ~0.35 Gyrs at L/Lsun ~ -5. We have neglected a
chemical redistribution possibly associated with this process, which could lead
to a further cooling delay. Exsolution has a marginal effect on the WD cooling
times and, accordingly, we find no WD branches on the Gaia color magnitude
diagram associated with it. However, exsolution in massive WDs can alter the
faint end of the WD luminosity function, thus impacting WD cosmochronology.Comment: Accepted for publication in Astronomy & Astrophysic
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&
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