A. Rebassa-Mansergas

New white dwarf stars in the Sloan Digital Sky Survey Data Release 10

We report the discovery of 9 089 new spectroscopically confirmed white dwarfs and subdwarfs in the Sloan Digital Sky Survey Data Release 10. We obtain Teff, log g and mass for hydrogen atmosphere white dwarf stars (DAs) and helium atmosphere white dwarf stars (DBs), and estimate the calcium/helium abundances for the white dwarf stars with metallic lines (DZs) and carbon/helium for carbon dominated spectra DQs. We found 1 central star of a planetary nebula, 2 new oxygen spectra on helium atmosphere white dwarfs, 71 DQs, 42 hot DO/PG1159s, 171 white dwarf+main sequence star binaries, 206 magnetic DAHs, 327 continuum dominated DCs, 397 metal polluted white dwarfs, 450 helium dominated white dwarfs, 647 subdwarfs and 6888 new hydrogen dominated white dwarf stars.

Monte Carlo simulations of post-common-envelope white dwarf + main sequence binaries: comparison with the SDSS DR7 observed sample

Context. Detached white dwarf + main sequence (WD+MS) systems represent the simplest population of post-common envelope binaries (PCEBs). Since the ensemble properties of this population carries important information about the characteristics of the common-envelope (CE) phase, it deserves close scrutiny. However, most population synthesis studies do not fully consider the effects of the observational selection biases of the samples used to compare with the theoretical simulations. Aims. Here we present the results of a set of detailed Monte Carlo simulations of the population of WD+MS binaries in the Sloan Digital Sky Survey (SDSS) Data Release 7. Methods. We used up-to-date stellar evolutionary models, a complete treatment of the Roche lobe overflow episode, and a full implementation of the orbital evolution of the binary systems. Moreover, in our treatment we took the selection criteria and all the known observational biases into account. Results. Our population synthesis study allowed us to make a meaningful comparison with the available observational data. In particular, we examined the CE efficiency, the possible contribution of internal energy, and the initial mass ratio distribution (IMRD) of the binary systems. We find that our simulations correctly reproduce the properties of the observed distribution of WD+MS PCEBs. In particular, we find that once the observational biases are carefully considered, the distribution of orbital periods and of masses of the WD and MS stars can be correctly reproduced for several choices of the free parameters and different IMRDs, although models in which a moderate fraction (≤10%) of the internal energy is used to eject the CE and in which a low value of CE efficiency is used (≤0.3) seem to fit the observational data better. We also find that systems with He-core WDs are over-represented in the observed sample, because of selection effects. Conclusions. Although our study represents an important step forward in modeling the population of WD+MS PCEBs, the still scarce observational data preclude deriving a precise value of the several free parameters used to compute the CE phase without ambiguity or ascertaining which the correct IMRD might be.

The SDSS spectroscopic catalogue of white dwarf-main-sequence binaries: new identifications from DR 9–12

We present an updated version of the spectroscopic catalogue of white dwarf-main-sequence (WDMS) binaries from the Sloan Digital Sky Survey (SDSS). We identify 938 WDMS binaries within the data releases (DR) 9–12 of SDSS plus 40 objects from DR 1–8 that we missed in our previous works, 646 of which are new. The total number of spectroscopic SDSS WDMS binaries increases to 3294. This is by far the largest and most homogeneous sample of compact binaries currently available. We use a decomposition/fitting routine to derive the stellar parameters of all systems identified here (white dwarf effective temperatures, surface gravities and masses, and secondary star spectral types). The analysis of the corresponding stellar parameter distributions shows that the SDSS WDMS binary population is seriously affected by selection effects. We also measure the Nau2009I λλ 8183.27, 8194.81 absorption doublet and H α emission radial velocities (RV) from all SDSS WDMS binary spectra identified in this work. 98 objects are found to display RV variations, 62 of which are new. The RV data are sufficient enough to estimate the orbital periods of three close binaries.

14 new eclipsing white dwarf plus main-sequence binaries from the SDSS and Catalina surveys

We report on the search for new eclipsing white dwarf plus main-sequence (WDMS) binaries in the light curves of the Catalina surveys. We use a colour-selected list of almost 2000 candidate WDMS systems from the Sloan Digital Sky Survey, specifically designed to identify WDMS systems with cool white dwarfs and/or early M-type main-sequence stars. We identify a total of 17 eclipsing systems, 14 of which are new discoveries. We also find three candidate eclipsing systems, two main-sequence eclipsing binaries and 22 non-eclipsing close binaries. Our newly discovered systems generally have optical fluxes dominated by the main-sequence components, which have earlier spectral types than the majority of previously discovered eclipsing systems. We find a large number of ellipsoidally variable binaries with similar periods, near 4 h, and spectral types M2–3, which are very close to Roche lobe filling. We also find that the fraction of eclipsing systems is lower than found in previous studies and likely reflects a lower close binary fraction among WDMS binaries with early M-type main-sequence stars due to their enhanced angular momentum loss compared to fully convective late M-type stars, hence causing them to become cataclysmic variables quicker and disappear from the WDMS sample. Our systems bring the total number of known detached, eclipsing WDMS binaries to 71.

The evolution of stellar metallicity gradients of the Milky Way disk from LSS-GAC main sequence turn-off stars: a two-phase disk formation history?

Accurate measurements of stellar metallicity gradients in the radial and vertical directions of the disk and their temporal variations provide important constraints on the formation and evolution of the Milky Way disk. We use 297 042 main sequence turn-off stars selected from the LAMOST Spectroscopic Survey of the Galactic Anti-center (LSS-GAC) to determine the radial and vertical gradients of stellar metallicity, ∆[Fe/H]/∆ R and ∆[Fe/H]/∆| Z | of the Milky Way disk in the direction of the anti-center. We determine ages of those turn-off stars by isochrone fitting and measure the temporal variations of metallicity gradients. We have carried out a detailed analysis of the selection effects resulting from the selection, observation and data reduction of LSS-GAC targets and the potential biases of a magnitude limited sample on the determinations of metallicity gradients. Our results show that the gradients, both in the radial and vertical directions, exhibit significant spatial and temporal variations. The radial gradients yielded by stars with the oldest ages ( ≿ 11 Gyr) are essentially zero at all heights from the disk midplane, while those given by younger stars are always negative. The vertical gradients deduced from stars with the oldest ages ( ≿ 11 Gyr) are negative and only show very weak variations with Galactocentric distance in the disk plane, R , while those yielded by younger stars show strong variations with R . After being essentially flat at the earliest epochs of disk formation, the radial gradients steepen as age decreases, reaching a maximum (steepest) at age 7–8 Gyr, and then they flatten again. Similar temporal trends are also found for the vertical gradients. We infer that the assembly of the Milky Way disk may have experienced at least two distinct phases. The earlier phase is probably related to a slow, pressure-supported collapse of gas, when the gas settles down to the disk mainly in the vertical direction. In the later phase, there are significant radial flows of gas in the disk, and the rate of gas inflow near the solar neighborhood reaches a maximum around a lookback time of 7–8 Gyr. The transition between the two phases occurs around a lookback time between 8 and 11 Gyr. The two phases may be responsible for the formation of the Milky Way’s thick and thin disks, respectively. Also, as a consequence, we recommend that stellar age is a natural, physical criterion to distinguish stars from the thin and thick disks. From an epoch earlier than 11 Gyr to one between 8 and 11 Gyr, there is an abrupt, significant change in magnitude of both the radial and vertical metallicity gradients, suggesting that stellar radial migration is unlikely to play an important role in the formation of the thick disk.

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

Context. Detached white dwarf + main sequence (WD+MS) post-common-envelope binaries (PCEBs) are perhaps the most suitable objects for testing predictions of close-compact binary-star evolution theories, in particular, common-envelope (CE) evolution. Consequently, the population of WD+MS PCEBs has been simulated by several authors in the past and the predictions have been compared with the observations. However, most of those theoretical predictions did not take into account the possible contributions to the envelope ejection from additional sources of energy (mostly recombination energy) stored in the envelope. nAims. Here we update existing binary population models of WD+MS PCEBs by assuming that in addition to a fraction aCE of the orbital energy, a fraction arec of the recombination energy available within the envelope contributes to ejecting the envelope. nMethods. We performed Monte Carlo simulations of 107 MS+MS binaries for 9 different combinations of aCE and arec using standard assumptions for the initial primary mass function, binary separations, and initial-mass-ratio distribution and evolved these systems using the publicly available binary star evolution (BSE) code. nResults. Including a fraction of the 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 (MWD ? 0.5 M?) increases with the CE efficiency and the existence of very low-mass He WDs (?0.3 M?) is only predicted for high values of the CE efficiency, i.e. aCE ? 0.5. All models predict on average longer orbital periods for PCEBs containing C/O-core WDs (MWD ? 0.5 M?) than for PCEBs containing He WDs. This effect increases with increasing values of both efficiencies, i.e., aCE and arec. 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. nConclusions. Our simulations, in combination with a large and homogeneous observational sample, can provide constraints on the values of aCE and arec, as well as on the initial-mass-ratio distribution for MS+MS binary stars.

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

The mass function of hydrogen-rich atmosphere white dwarfs has been frequently found to reveal a distinctive high-mass excess near 1 M-circle dot. 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 (SDSS). 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 ten SDSS hydrogen-rich white dwarfs, including the cool and faint population (i.e. effective temperatures 6000 less than or similar to T-eff less than or similar to 12 000 K, equivalent to 12 mag less than or similar to M-bol less than or similar to 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 T-eff greater than or similar to 12 000 K, M-bol less than or similar to 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 1 M-circle dot. 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.

Detached cataclysmic variables are crossing the orbital period gap

A central hypothesis in the theory of cataclysmic variable (CV) evolution is the need to explain the observed lack of accreting systems in the ≃2–3 h orbital period range, known as the period gap. The standard model, disrupted magnetic braking (DMB), reproduces the gap by postulating that CVs transform into inconspicuous detached white dwarf (WD) plus main sequence systems, which no longer resemble CVs. However, observational evidence for this standard model is currently indirect and thus this scenario has attracted some criticism throughout the last decades. Here, we perform a simple but exceptionally strong test of the existence of detached CVs (dCVs). If the theory is correct, dCVs should produce a peak in the orbital period distribution of detached close binaries consisting of a WD and an M4–M6 secondary star. We measured six new periods which brings the sample of such binaries with known periods below 10 h to 52 systems. An increase of systems in the ≃2–3 h orbital period range is observed. Comparing this result with binary population models, we find that the observed peak cannot be reproduced by post-common envelope binaries (PCEBs) alone and that the existence of dCVs is needed to reproduce the observations. Also, the WD mass distribution in the gap shows evidence of two populations in this period range, i.e. PCEBs and more massive dCVs, which is not observed at longer periods. We therefore conclude that CVs are indeed crossing the gap as detached systems, which provides strong support for the DMB theory.

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 wellcharacterised 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±0:16×10 3 pc 3 and 5:42 ± 0:08 × 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.

White dwarf-main sequence binaries from LAMOST: the DR1 catalogue

Context. White dwarf-main sequence (WDMS) binaries are used to study several different important open problems in modern astrophysics. nAims. The Sloan Digital Sky Survey (SDSS) identified the largest catalogue of WDMS binaries currently known. However, this sample is seriously affected by selection effects and the population of systems containing cool white dwarfs and early-type companions is under-represented.Here we search for WDMS binaries within the spectroscopic data release 1 of the LAMOST (Large sky Area Multi-Object fiber Spectroscopic Telescope) survey. LAMOST and SDSS follow different target selection algorithms. Hence, LAMOST WDMS binaries may be drawn from a different parent population and thus help in overcoming the selection effects incorporated by SDSS on the current observed population. nMethods. We develop a fast and efficient routine based on the wavelet transform to identify LAMOST WDMS binaries containing a DA white dwarf and a M dwarf companion, and apply a decomposition/fitting routine to their LAMOST spectra to estimate their distances and measure their stellar parameters, namely the white dwarf effective temperatures, surface gravities and masses, and the secondary star spectral types. nResults. We identify 121 LAMOST WDMS binaries, 80 of which are new discoveries, and estimate the sample to be sim90 per cent complete. The LAMOST and SDSS WDMS binaries are found to be statistically different. However, this result is not due to the different target selection criteria of both surveys, but likely a simple consequence of the different observing conditions. Thus, the LAMOST population is found at considerably shorter distances (sim50-450 pc) and is dominated by systems containing early-type companions and hot white dwarfs. (abridged)