arXiv subjects

D. Koester

Publications and source records attributed to D. Koester.

At least 109 records · Page 6Linked to original sources

arXiv2008

Post Common Envelope Binaries from SDSS. V: Four eclipsing white dwarf main sequence binaries

We identify SDSS011009.09+132616.1, SDSS030308.35+005444.1, SDSS143547.87+373338.5 and SDSS154846.00+405728.8 as four eclipsing white dwarf plus main sequence (WDMS) binaries from the Sloan Digital Sky Survey, and report on follow-up observations of these systems. Orbital periods and ephemerides have been established from multi-season photometry. SDSS1435+3733, with Porb=3h has the shortest orbital period of all known eclipsing WDMS binaries. Time-resolved spectroscopic observations have been obtained and the radial velocities of the secondary stars in all four systems were measured. A spectral decomposition/fitting technique was then employed to determine the white dwarf effective temperatures and surface gravities, as well as the spectral types of the companion stars. We used a light curve modeling code to further constrain the masses and radii of the components in all systems. All three DA white dwarfs have masses of Mwd~0.4-0.6Msun, in line with the expectations from close binary evolution. The DC white dwarf in SDSS0303+0054 has a mass of Mwd>0.85Msun, making it unusually massive for a post-common envelope system. Our new additions raise the number of known eclipsing WDMS binaries to fourteen, and we find that the average white dwarf mass in this sample is =0.57+/-0.16Msun, only slightly lower than the average mass of single white dwarfs. The majority of all eclipsing WDMS binaries contain low-mass (<0.6Msun) secondary stars, and will eventually provide valuable observational input for the calibration of the mass-radius relations of low-mass main sequence stars and of white dwarfs.

astro-ph↗

arXiv2008

SDSS White Dwarf mass distribution at low effective temperatures

The DA white dwarfs in the Sloan Digital Sky Survey, as analyzed in the papers for Data Releases 1 and 4, show an increase in surface gravity towards lower effective temperatures below 11500 K. We study the various possible explanations of this effect, from a real increase of the masses to uncertainties or deficiencies of the atmospheric models. No definite answer is found but the tentative conclusion is that it is most likely the current description of convection in the framework of the mixing-length approximation, which leads to this effect.

astro-ph↗

arXiv2008

SDSSJ084539.17+225728.0: the first DBZ white dwarf with a metal-rich gaseous debris disc

We report the discovery of a third white dwarf hosting a gaseous debris disc, SDSSJ084539.17+225728.0. The typical double-peaked CaII 8498,8542,8662A emission lines can be modelled in terms of a Keplerian gas disc with a radial extent from ~0.5Rsun to ~1.0. The effective temperature of SDSS0845+2257, Teff=18600+-500K, is comparable to the two other white dwarfs with gaseous discs, SDSS1043+0855 and SDSS1228+1040, and hence substantially hotter than the bulk of white dwarfs where dusty debris discs were identified through the presence of infrared excess flux. This may suggest that the conditions to produce emission lines from debris discs in the optical wavelength range are only met for a relatively narrow range in Teff. The observed asymmetry in the line profiles indicates a substantial eccentricity in the disc. Two spectra obtained four years apart reveal a significant change in the shapes and equivalent widths of the line profiles, implying that the circumstellar disc evolves on relatively short time scales. In contrast to SDSS1043+0855 and SDSS1228+1040, SDSS0845+2257 has a helium-dominated atmosphere. We detect photospheric absorption lines of He, Ca, Mg, and Si in the Sloan Digital Sky Survey spectrum, and hence classify SDSS0845+2257 as DBZ white dwarf. The abundances for the three metals determined from model atmosphere fits are Ca/He~1.3x10^-7, Mg/He~6.0x10^-6, and Si/He~8.0x10^-6. From the non-detection of Halpha we derive H/He<3x10^-5, which implies that the hydrogen-to-metal abundance ratio of the circumstellar material is >~1000 times lower than in the Sun. This lends strong support to the hypothesis that the gaseous and dusty debris discs found around roughly a dozen white dwarfs originate from the disruption of rocky planetary material.

astro-ph↗

arXiv2008

MAD@VLT: Deep into the Madding Crowd of Omega Centauri

We present deep and accurate Near-Infrared (NIR) photometry of the Galactic Globular Cluster (GC) Omega Cen. Data were collected using the Multi-Conjugate Adaptive Optics Demonstrator (MAD) on VLT (ESO). The unprecedented quality of the images provided the opportunity to perform accurate photometry in the central crowded regions. Preliminary results indicate that the spread in age among the different stellar populations in Omega Cen is limited.

astro-ph↗

arXiv2008

Post common envelope binaries from SDSS. II: Identification of 9 close binaries with VLT/FORS2

The Sloan Digital Sky Survey turned out to be very efficient in identifying white dwarf/main sequence binaries (WDMS). The population of WDMS systems consists of wide systems whose stellar components evolve like single stars and - more interesting in the context of close binary evolution - PCEBs. We pursue a large-scale follow-up survey to identify and characterise the PCEBs among the WDMS binaries that have been found with SDSS. We use a two-step strategy with the identification of PCEBs among WDMS in the first and orbital period determinations in the second phase. Here we present first results of our VLT/FORS2 pilot-study that has the target of identifying the PCEBs among the fainter (g>~18.5) SDSS-WDMS. From published SDSS catalogues we selected 26 WDMS targets to be observed with VLT/FORS2. Using a spectral decomposition/fitting technique we determined the white dwarf effective temperatures and surface gravities, masses, and secondary star spectral types for all WDMS in our sample. We used the NaI 8183.27,8194.81 doublet to measure radial velocity variations of our targets and performed additional follow-up spectroscopy using Magellan-Clay/LDSS3 of two systems showing significant radial velocity variations. Among the 26 WDMS systems we find 9 strong PCEB candidates showing clear (>=3sigma) radial velocity variations and we estimate the fraction of PCEBs among SDSS WDMS to be ~35%. We find indications for a dependence of the relative number of PCEBs among SDSS-WDMS on the spectral type of the secondary star. These results are subject to small number statistics and need to be confirmed by additional observations. For two of the identified PCEB candidates, SDSSJ1047+0523 and SDSSJ1414-0132, we measured the orbital periods to be 9.17hrs and 17.48hrs respectively.

astro-ph↗

arXiv2008

The Absolute Flux Distribution of LDS749B

Observations from the Space Telescope Imaging Spectrograph define the flux of the DBQ4 star LDS749B from 0.12-1.0 μm with an uncertainty of ~1% relative to the three pure hydrogen WD primary HST standards. With T_{eff}=13575 K, log g=8.05, and a trace of carbon at <1x10^{-6} of solar, a He model atmosphere fits the measured STIS fluxes within the observational noise, except in a few spectral lines with uncertain physics of the line broadening theory. Upper limit to the atmospheric hydrogen and oxygen fractions by number are 1x10^{-7} and 7x10^{-10}, respectively. The excellent agreement of the model flux distribution with the observations lends confidence to the accuracy of the modeled IR fluxes beyond the limits of the STIS spectrophotometry. The estimated precision of ~1% in the predicted IR absolute fluxes at 30 μm should be better than the model predictions for Vega and should be comparable to the absolute accuracy of the three primary WD models.

astro-ph↗

arXiv2007

On the radial distribution of white dwarfs in the Galactic globular cluster Omega Cen

We present deep and accurate photometry (F435W, F625W, F658N) of the Galactic Globular Cluster Omega Cen collected with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). We identified ~ 6,500 white dwarf (WD)candidates and compared their radial distribution with that of Main Sequence (MS) stars. We found a mild evidence that young WDs (0.1 < t < 0.6 Gyr) are less centrally concentrated when compared to MS stars in the magnitude range 25 < F435W < 26.5.

astro-ph↗

arXiv2007

On the white dwarf cooling sequence of the globular cluster Omega Centauri

We present deep and precise photometry (F435, F625W, F658N) of Omega Cen collected with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). We have identified ~ 6,500 white dwarf (WD) candidates, and the ratio of WD and Main Sequence (MS) star counts is found to be at least a factor of two larger than the ratio of CO-core WD cooling and MS lifetimes. This discrepancy is not explained by the possible occurrence of a He-enhanced stellar population, since the MS lifetime changes by only 15% when changing from a canonical (Y=0.25) to a He-enhanced composition (Y=0.42). The presence of some He-core WDs seems able to explain the observed star counts. The fraction of He WDs required ranges from 10% to 80% depending on their mean mass and it is at least five times larger than for field WDs. The comparison in the Color Magnitude Diagram between theory and observations also supports the presence of He WDs. Empirical evidence indicates that He WDs have been detected in stellar systems hosting a large sample of extreme horizontal branch stars, thus suggesting that a fraction of red giants might avoid the He-core flash.

astro-ph↗

arXiv2007

The pre-cataclysmic variable, LTT560

System parameters of the object LTT560 are determined in order to clarify its nature and evolutionary status. We apply time-series photometry to reveal orbital modulations of the light curve, time-series spectroscopy to measure radial velocities of features from both the primary and the secondary star, and flux-calibrated spectroscopy to derive temperatures of both components. We find that LTT560 is composed of a low temperature T~7500K DA white dwarf as the primary and an M5.5+-1 main-sequence star as the secondary component. The current orbital period is Porb=3.54(07)h. We derive a mass ratio Msec/Mwd = 0.36(03) and estimate the distance to d=25-40pc. Long-term variation of the orbital light curve and an additional H-alpha emission component on the white dwarf indicate activity in the system, probably in the form of flaring and/or accretion events.

astro-ph↗

arXiv2007

The Chemical Composition of an Extrasolar Minor Planet

We report the relative abundances of 17 elements in the atmosphere of the white dwarf star GD 362, material that, very probably, was contained previously in a large asteroid or asteroids with composition similar to the Earth/Moon system. The asteroid may have once been part of a larger parent body not unlike one of the terrestrial planets of our solar system.

astro-ph↗

arXiv2007

Post Common Envelope Binaries from SDSS. I: 101 white dwarf main sequence binaries with multiple SDSS spectroscopy

We present a detailed analysis of 101 white dwarf-main sequence binaries (WDMS) from the Sloan Digital Sky Survey (SDSS) for which multiple SDSS spectra are available. We detect significant radial velocity variations in 18 WDMS, identifying them as post common envelope binaries (PCEBs) or strong PCEB candidates. Strict upper limits to the orbital periods are calculated, ranging from 0.43 to 7880 d. Given the sparse temporal sampling and relatively low spectral resolution of the SDSS spectra, our results imply a PCEB fraction of >=15% among the WDMS in the SDSS data base. Using a spectral decomposition/fitting technique we determined the white dwarf effective temperatures and surface gravities, masses, and secondary star spectral types for all WDMS in our sample. Two independent distance estimates are obtained from the flux scaling factors between the WDMS spectra, and the white dwarf models and main sequence star templates, respectively. Approximately one third of the systems in our sample show a significant discrepancy between the two distance estimates. In the majority of discrepant cases, the distance estimate based on the secondary star is too large. A possible explanation for this behaviour is that the secondary star spectral types that we determined from the SDSS spectra are systematically too early by 1-2 spectral classes. This behaviour could be explained by stellar activity, if covering a significant fraction of the star by cool dark spots will raise the temperature of the inter-spot regions. Finally, we discuss the selection effects of the WDMS sample provided by the SDSS project.

astro-ph↗

arXiv2007

High-Resolution Spectra Observed for SPY. II. DB and DBA Stars

We present a detailed spectroscopic analysis of the stars with helium-dominated spectra in the ESO Supernova Ia Progenitor Survey (SPY). Atmospheric parameters, masses, and abundances of trace hydrogen are determined and discussed in the context of spectral evolution of white dwarfs. Our final sample contains 71 objects, of which 6 are new detections and 14 are reclassified from DB to DBA because of the presence of H lines. 55% of the DB sample show hydrogen and are thus DBA, a significantly higher fraction than found before. The large incidence of DBA, and the derived total hydrogen masses are compatible with the scenario that DBs ``reappear'' around 30000 K from the DB gap by mixing and diluting a thin hydrogen layer of the order of E-15 Msun. This hydrogen mass is then during the evolution continuously increased by interstellar accretion. There are indications that the accretion rate increases smoothly with age or decreasing temperature, a trend which continuous even below the current low temperature limit (Dufour 2006). A remaining mystery is the low accretion rate of H compared to that of Ca observed in the DBZA, but a stellar wind extending down to the lowest temperatures with decreasing strength might be part of the solution.

astro-ph↗

arXiv2007

Six New ZZ Ceti Stars from the SPY and the HQS Surveys

We report on the discovery of six new ZZ Ceti stars. They were selected as candidates based on preparatory photometric observations of objects from the Hamburg Quasar Survey (HQS), and based on the spectra of the Supernova Ia Progenitor Survey (SPY). Time-series photometry of 19 candidate stars was carried out at the Nordic Optical Telescope (NOT) at Roque de Los Muchachos Observatory, Spain. The new variables are relatively bright, 15.4<B<16.6. Among them is WD1150-153, which is the third ZZ Ceti star that shows photospheric CaII in its spectrum.

astro-ph↗

arXiv2007

XMM-Newton observations of EF Eridani: the textbook example of low-accretion rate polars

Archival X-ray observations of EF Eridani obtained in a low state revealed distinct X-ray detections at a luminosity L_X ~ 2 10^{29} erg/s, three orders of magnitude below its high state value. The plasma temperature was found to be as low as $kT \loa 2$ keV, a factor 10 below the high state. The X-ray/UV/IR spectral energy distribution suggests faint residual accretion rather than coronal emission as being responsible for the low-state X-ray emission. EF Eri thus showed a clear transition from being shock-dominated in the high state to be cyclotron-dominated in the low state. From the optical/UV spectral energy distribution we re-determine the photospheric temperature of the white dwarf to \~10000K. Contrary to earlier claims, WD model atmospheres produce sufficient UV flux to reproduce the published GALEX flux and orbital modulation.

astro-ph↗

arXiv2007

The Lowest Mass White Dwarf

Extremely low mass white dwarfs are very rare objects likely formed in compact binary systems. We present MMT optical spectroscopy of 42 low mass white dwarf candidates serendipitously discovered in a survey for hypervelocity B-type stars. One of these objects, SDSS J0917+46, has Teff= 11,288 \pm 72 K and log g = 5.48 \pm 0.03; with an estimated mass of 0.17 M_sun, it is the lowest gravity/mass white dwarf currently known. However, 40 of the low mass candidates are normal DA white dwarfs with apparently inaccurate SDSS g magnitudes. We revisit the identification of low mass white dwarf candidates previously found in the SDSS, and conclude that four objects have M < 0.2 M_sun. None of these white dwarfs show excess emission from a binary companion, and radial velocity searches will be necessary to constrain the nature of the unseen companions.

astro-ph↗

arXiv2007

Velocity Fields at the Surface of ZZ Ceti Stars

We demonstrate that the peculiar line profiles observed in DA white dwarfs in the temperature range of the ZZ Ceti variables can be explained by the surface velocity fields associated with the pulsations.

astro-ph↗

arXiv2007

White Dwarf Mass Distribution in the SDSS

We determined masses for the 7167 DA and 507 DB white dwarf stars classified as single and non-magnetic in data release four of the Sloan Digital Sky Survey (SDSS). We obtained revised Teff and log g determinations for the most massive stars by fitting the SDSS optical spectra with a synthetic spectra grid derived from model atmospheres extending to log g=10.0. We also calculate radii from evolutionary models and create volume-corrected mass distributions for our DA and DB samples. The mean mass for the DA stars brighter than g=19 and hotter than Teff=12000K is M(DA)= 0.593+/-0.016M(Sun). For the 150 DBs brighter than g=19 and hotter than Teff=16000K, we find M(DB)=0.711+/-0.009 M(Sun). It appears the mean mass for DB white dwarf stars may be significantly larger than that for DAs. We also report the highest mass white dwarf stars ever found, up to 1.33 M(Sun).

astro-ph↗

arXiv2006

Towards a pure ZZ Ceti instability strip

We have observed again two stars inside the ZZ Ceti instability strip that were previously classified as not-observed-to-vary (NOV) by Mukadam et al. (2004) and found them to be low-amplitude variables. Some evidence points to a pure ZZ Ceti instability strip; other evidence contests it. The two stars previously classified as NOV have Sloan Digital Sky Survey (SDSS) spectroscopic effective temperatures that place them inside the ZZ Ceti instability strip, and they were "contaminating" the strip as constant stars, which could indicate that the instability strip was no longer a simple evolutionary stage. A pure instability strip indicates that pulsation is a normal phase which all DAs must go through. We used effective temperatures derived from SDSS optical spectra by comparing them with model atmospheres to look for pulsators through time-resolved photometry and stars previously classified as NOV. Our new results indicate, but do not prove, a pure instability strip, because there are still other NOV stars that need to be observed again. Additionally, we have discovered five other ZZ Ceti stars based on their effective temperatures.

astro-ph↗