arXiv subjects

T. Nagel

Publications and source records attributed to T. Nagel.

At least 19 recordsLinked to original sources

arXiv2020

The EXOTIME project: Signals in the $ O-C $ diagrams of the rapidly pulsating subdwarfs DW Lyn, V1636 Ori, QQ Vir, and V541 Hya

We aim to investigate variations in the arrival time of coherent stellar pulsations due to the light-travel time effect to test for the presence of sub-stellar companions. Those companions are the key to one possible formation scenario of apparently single sub-dwarf B stars. We made use of an extensive set of ground-based observations of the four large amplitude p-mode pulsators DW Lyn, V1636 Ori, QQ Vir, and V541 Hya. Observations of the TESS space telescope are available on two of the targets. The timing method compares the phase of sinusoidal fits to the full multi-epoch light curves with phases from the fit of a number of subsets of the original time series. Observations of the TESS mission do not sample the pulsations well enough to be useful due to the (currently) fixed two-minute cadence. From the ground-based observations, we infer evolutionary parameters from the arrival times. The residual signals show many statistically significant periodic signals, but no clear evidence for changes in arrival time induced by sub-stellar companions. The signals can be explained partly by mode beating effects. We derive upper limits on companion masses set by the observational campaign.

astro-ph.SR↗

arXiv2018

GD358: three decades of observations for the in-depth asteroseismology of a DBV star

We report on the analysis of 34 years of photometric observations of the pulsating helium atmosphere white dwarf GD358. The complete data set includes archival data from 1982-2006, and 1195.2 hours of new observations from 2007- 2016. From this data set, we extract 15 frequencies representing g-mode pulsation modes, adding 4 modes to the 11 modes known previously. We present evidence that these 15 modes are ell = 1 modes, 13 of which belong to a consecutive sequence in radial overtone k. We perform a detailed asteroseismic analysis using models that include parameterized, complex carbon and oxygen core composition profiles to fit the periods. Recent spectroscopic analyses place GD358 near the red edge of the DBV instability strip, at 24,000 plus or minus 500 K and a log g of 7.8 plus or minus 0.08 dex. The surface gravity translates to a mass range of 0.455 to 0.540 solar masses. Our best fit model has a temperature of 23,650 K and a mass of 0.5706 solar masses. That is slightly more massive than suggested by most the recent spectroscopy. We find a pure helium layer mass of 10^-5.50, consistent with the result of previous studies and the outward diffusion of helium over time.

astro-ph.SR↗

arXiv2017

A finite-strain hyperviscoplastic model and undrained triaxial tests of peat

This paper presents a finite-strain hyperviscoplastic constitutive model within a thermodynamically consistent framework for peat which was categorised as a material with both rate-dependent and thermodynamic equilibrium hysteresis based on the data reported in the literature. The model was implemented numerically using implicit time integration and verified against analytical solutions under simplified conditions. Experimental studies on the undrained relaxation and loading-unloading-reloading behaviour of an undisturbed fibrous peat were carried out to define the thermodynamic equilibrium state during deviatoric loading as a prerequisite for further modelling, to fit particularly those model parameters related to solid matrix properties, and to validate the proposed model under undrained conditions. This validation performed by comparison to experimental results showed that the hyperviscoplastic model could simulate undrained triaxial compression tests carried out at five different strain rates with loading/unloading relaxation steps.

cs.CE↗

arXiv2016

The gaseous debris disk of the white dwarf SDSS J1228+1040. HST/COS search for far-ultraviolet signatures

Gaseous and dust debris disks around white dwarfs (WDs) are formed from tidally disrupted planetary bodies. This offers an opportunity to determine the composition of exoplanetary material by measuring element abundances in the accreting WD's atmosphere. A more direct way to do this is through spectral analysis of the disks themselves. Currently, the number of chemical elements detected through disk emission-lines is smaller than that of species detected through lines in the WD atmospheres. We assess the far-ultraviolet (FUV) spectrum of one well-studied object (SDSS J122859.93+104032.9) to search for disk signatures at wavelengths <1050 angstrom, where the broad absorption lines of the Lyman series effectively block the WD photospheric flux. We performed FUV observations (950-1240 angstrom) with the Hubble Space Telescope/Cosmic Origins Spectrograph and used archival optical spectra. We compared them with non-local thermodynamic equilibrium model spectra. In addition, we investigate the Ca II infrared triplet (IRT) line profiles to constrain disk geometry and composition. No disk emission-lines were detected in the FUV spectrum, indicating that the disk effective temperature is about 5000K. The long-time variability of the Ca II IRT was reproduced with a precessing disk model of bulk Earth-like composition, having a surface mass density of 0.3g/cm2 and an extension from 55 to 90 WD radii. The disk has a spiral shape that precesses with a period of approximately 37 years, confirming previous results.

astro-ph.SR↗

arXiv2014

Non-LTE spectral models for the gaseous debris-disk component of Ton 345

For a fraction of single white dwarfs with debris disks, an additional gaseous disk was discovered. Both dust and gas are thought to be created by the disruption of planetary bodies. The composition of the extrasolar planetary material can directly be analyzed in the gaseous disk component, and the disk dynamics might be accessible by investigating the temporal behavior of the Ca II infrared emission triplet, hallmark of the gas disk. We obtained new optical spectra for the first helium-dominated white dwarf for which a gas disk was discovered (Ton 345) and modeled the non-LTE spectra of viscous gas disks composed of carbon, oxygen, magnesium, silicon, sulfur, and calcium with chemical abundances typical for solar system asteroids. Iron and its possible line-blanketing effects on the model structure and spectral energy distribution was still neglected. A set of models with different radii, effective temperatures, and surface densities as well as chondritic and bulk-Earth abundances was computed and compared with the observed line profiles of the Ca II infrared triplet. Our models suggest that the Ca II emission stems from a rather narrow gas ring with a radial extent of R=0.44-0.94 Rsol, a uniform surface density Sigma=0.3 g/cm2, and an effective temperature of Teff=6000 K. The often assumed chemical mixtures derived from photospheric abundances in polluted white dwarfs - similar to a chondritic or bulk-Earth composition - produce unobserved emission lines in the model and therefore have to be altered. We do not detect any line-profile variability on timescales of hours, but we confirm the long-term trend over the past decade for the red-blue asymmetry of the double-peaked lines.

astro-ph.SR↗

arXiv2012

Observations and NLTE Modeling of the Gaseous Planetary Debris Disk around Ton 345

Debris disks around single white dwarfs are thought to be the remains of planetary bodies disrupted by tidal forces. Ongoing accretion of the hereby produced dust allows to detect the planetary material in the white dwarf photosphere and to conclude on its chemical composition. As an alternative, the composition can in principle be determined directly from the emission lines of the sometimes additionally observed gaseous component of the disks. To this aim, we perform spectral modeling with our non-LTE code for accretion disks. We have obtained new observations of Ton 345 in order to look for long- and short-term variations in the disk line-profiles. We find that the prominent red-violet asymmetry of the Ca II infrared triplet almost disappeared. Line-profile variations during one night are not seen without doubt.

astro-ph.SR↗

arXiv2012

Empirical Determination of Convection Parameters in White Dwarfs I : Whole Earth Telescope Observations of EC14012-1446

We report on analysis of 308.3 hrs of high speed photometry targeting the pulsating DA white dwarf EC14012-1446. The data were acquired with the Whole Earth Telescope (WET) during the 2008 international observing run XCOV26. The Fourier transform of the light curve contains 19 independent frequencies and numerous combination frequencies. The dominant peaks are 1633.907, 1887.404, and 2504.897 microHz. Our analysis of the combination amplitudes reveals that the parent frequencies are consistent with modes of spherical degree l=1. The combination amplitudes also provide m identifications for the largest amplitude parent frequencies. Our seismology analysis, which includes 2004--2007 archival data, confirms these identifications, provides constraints on additional frequencies, and finds an average period spacing of 41 s. Building on this foundation, we present nonlinear fits to high signal-to-noise light curves from the SOAR 4.1m, McDonald 2.1m, and KPNO 2m telescopes. The fits indicate a time-averaged convective response timescale of 99.4 +/- 17 s, a temperature exponent 85 +/- 6.2 and an inclination angle of 32.9 +/- 3.2 degrees. We present our current empirical map of the convective response timescale across the DA instability strip.

astro-ph.SR↗

arXiv2011

Non-LTE models for the gaseous metal component of circumstellar discs around white dwarfs

Gaseous metal discs around single white dwarfs have been discovered recently. They are thought to develop from disrupted planetary bodies. Spectroscopic analyses will allow us to study the composition of extrasolar planetary material. We investigate in detail the first object for which a gas disc was discovered (SDSS J122859.93+104032.9). Therefor we perform non-LTE modelling of viscous gas discs by computing the detailed vertical structure and line spectra. The models are composed of carbon, oxygen, magnesium, silicon, calcium, and hydrogen with chemical abundances typical for Solar System asteroids. Line asymmetries are modelled by assuming spiral-arm and eccentric disc structures as suggested by hydrodynamical simulations. The observed infrared Ca II emission triplet can be modelled with a hydrogen-deficient metal gas disc located inside of the tidal disruption radius, with an effective temperature of about 6000 K and a surface mass density of 0.3 g/cm^2. The inner radius is well constrained at about 0.64 Solar radii. The line profile asymmetry can be reproduced by either a spiral-arm structure or an eccentric disc, the latter being favoured by its time variability behaviour. Such structures, reaching from 0.64 to 1.5 Solar radii, contain a mass of about 3 to 6*10^21 g, the latter equivalent to the mass of a 135-km diameter Solar System asteroid.

astro-ph.SR↗

arXiv2011

Asymmetric Line Profiles in Spectra of Gaseous Metal Disks Around Single White Dwarfs

Metal-rich debris disks were discovered around several single DAZ and DBZ white dwarfs, which may stem from disruption of smaller rocky planetesimals. In some cases, the material in addition forms a gaseous disk. For SDSS J122859.93+104032.9, the double peaked infrared Ca II triplet at about 8500 angstrom exhibits a strong red/violet asymmetry. With the Tuebingen non-LTE Accretion Disk code AcDc we calculated the spectrum and vertical structure of the disk, assuming the chemical mixture of the disk's material being similar to a chondrite-like asteroid, the most prominent type in our own solar system. By modifying the code to simulate different non axis-symmetrical disk geometries, the first preliminary results are in good agreement with the observed asymmetric line profile.

astro-ph.EP↗

arXiv2010

Asymmetric Line Profiles in Spectra of Gaseous Metal Disks Around Single White Dwarfs

Around several single DAZ and DBZ white dwarfs metal-rich disks have been observed, which are mostly believed to originate from disruption of smaller rocky planetesimals. In some cases the material does not (only) form a dusty but gaseous disk. In the case of SDSS J122859.93+104032.9 the double peaked infrared Ca II triplet at about 8500 angstrom, one of only two emission features of the spectra, exhibits a strong red/violet asymmetry. Assuming a composition similar to a chondrite-like asteroid, being the most prominent type in our own solar system, we calculated the spectrum and vertical structure of the disk using the Tuebingen NLTE accretion disk code "AcDc". Modified to simulate different non axis-symmetrical disk geometries, the first preliminary results are in good agreement with the observed asymmetric line profile.

astro-ph.SR↗

arXiv2010

Pulsational Mapping of Calcium Across the Surface of a White Dwarf

We constrain the distribution of calcium across the surface of the white dwarf star G29-38 by combining time series spectroscopy from Gemini-North with global time series photometry from the Whole Earth Telescope. G29-38 is actively accreting metals from a known debris disk. Since the metals sink significantly faster than they mix across the surface, any inhomogeneity in the accretion process will appear as an inhomogeneity of the metals on the surface of the star. We measure the flux amplitudes and the calcium equivalent width amplitudes for two large pulsations excited on G29-38 in 2008. The ratio of these amplitudes best fits a model for polar accretion of calcium and rules out equatorial accretion.

astro-ph.SR↗

arXiv2009

Towards a dynamical mass of a PG 1159 star: radial velocities and spectral analysis of SDSS J212531-010745

The evolutionary scenarios which are commonly accepted for PG 1159 stars are mainly based on numerical simulations, which have to be tested and calibrated with real objects with known stellar parameters. One of the most crucial but also quite uncertain parameters is the stellar mass. PG 1159 stars have masses between 0.5 and 0.8 M_sun, as derived from asteroseismic and spectroscopic determinations. Such mass determinations are, however, themselves model-dependent. Moreover, asteroseismically and spectroscopically determined masses deviate systematically for PG 1159 stars by up to 10%. SDSS J212531.92-010745.9 is the first known PG 1159 star in a close binary with a late-main-sequence companion allowing a dynamical mass determination. We have obtained 14 Calar Alto spectra of SDSS J212531.92-010745.9 covering the full orbital phase range. A radial velocity curve was extracted for both components. With co-added phase-corrected spectra the spectral analysis of the PG 1159 component was refined. The irradiation of the companion by the PG 1159 star is modelled with PHOENIX, yielding constraints on radii, effective temperature and separation of the system's components. The light curve of SDSS J212531.92-010745.9, obtained during three seasons of photometry with the Goettingen 50 cm and Tuebingen 80 cm telescopes, was modelled with both the NIGHTFALL and PHOEBE programs.

astro-ph.SR↗

arXiv2009

Accretion Disk Winds in AM CVn Binaries - a Monte Carlo Approach

AM CVn systems are interacting binaries similar to cataclysmic variables (CVs), but more compact with orbital periods of less than 80 minutes. The primary is a white dwarf, whereas the nature of the secondary is not completely clear, yet. Abundances and composition of the outer layer of the secondary can be found by analysis of the accretion disk (presented by Nagel et al. these proceedings). Spectra from high-state AM CVn systems do not only show typical signatures of accretion disks, but also P Cygni line profiles, a sign of outflow being present in the system. Here we present the first quantitative spectral analysis of an accretion-disk wind in AM CVn systems. Emergent wind spectra are modeled with our 3-D Monte Carlo radiative transfer code WOMPAT. We show that P Cygni profiles can be reproduced with our wind models.

astro-ph.SR↗

arXiv2008

2006 Whole Earth Telescope Observations of GD358: A New Look at the Prototype DBV

We report on the analysis of 436.1 hrs of nearly continuous high-speed photometry on the pulsating DB white dwarf GD358 acquired with the Whole Earth Telescope (WET) during the 2006 international observing run, designated XCOV25. The Fourier transform (FT) of the light curve contains power between 1000 to 4000 microHz, with the dominant peak at 1234 microHz. We find 27 independent frequencies distributed in 10 modes, as well as numerous combination frequencies. Our discussion focuses on a new asteroseismological analysis of GD358, incorporating the 2006 data set and drawing on 24 years of archival observations. Our results reveal that, while the general frequency locations of the identified modes are consistent throughout the years, the multiplet structure is complex and cannot be interpreted simply as l=1 modes in the limit of slow rotation. The high k multiplets exhibit significant variability in structure, amplitude and frequency. Any identification of the m components for the high k multiplets is highly suspect. The k=9 and 8 modes typically do show triplet structure more consistent with theoretical expectations. The frequencies and amplitudes exhibit some variability, but much less than the high k modes. Analysis of the k=9 and 8 multiplet splittings from 1990 to 2008 reveal a long-term change in multiplet splittings coinciding with the 1996 "sforzando" event, where GD358 dramatically altered its pulsation characteristics on a timescale of hours. We explore potential implications, including the possible connections between convection and/or magnetic fields and pulsations. We suggest future investigations, including theoretical investigations of the relationship between magnetic fields, pulsation, growth rates, and convection.

astro-ph↗

arXiv2008

Spectral modeling of gaseous metal disks around DAZ white dwarfs

We report on our attempt for the first non-LTE modeling of gaseous metal disks around single DAZ white dwarfs recently discovered by Gaensicke et al. and thought to originate from a disrupted asteroid. We assume a Keplerian rotating viscous disk ring composed of calcium and hydrogen and compute the detailed vertical structure and emergent spectrum. We find that the observed infrared CaII emission triplet can be modeled with a hydrogen-deficient gas ring located at R=1.2 R_sun, inside of the tidal disruption radius, with Teff about 6000 K and a low surface mass density of about 0.3 g/cm**2. A disk having this density and reaching from the central white dwarf out to R=1.2 R_sun would have a total mass of 7 10**21 g, corresponding to an asteroid with about 160 km diameter.

astro-ph↗

arXiv2007

The pulsation modes of the pre-white dwarf PG 1159-035

PG 1159-035, a pre-white dwarf with T_eff=140,000 K, is the prototype of both two classes: the PG1159 spectroscopic class and the DOV pulsating class. Previous studies of PG 1159-035 photometric data obtained with the Whole Earth Telescope (WET) showed a rich frequency spectrum allowing the identification of 122 pulsation modes. In this work, we used all available WET photometric data from 1983, 1985, 1989, 1993 and 2002 to identify the pulsation periods and identified 76 additional pulsation modes, increasing to 198 the number of known pulsation modes in PG 1159-035, the largest number of modes detected in any star besides the Sun. From the period spacing we estimated a mass M = 0.59 +/- 0.02 solar masses for PG 1159-035, with the uncertainty dominated by the models, not the observation. Deviations in the regular period spacing suggest that some of the pulsation modes are trapped, even though the star is a pre-white dwarf and the gravitational settling is ongoing. The position of the transition zone that causes the mode trapping was calculated at r_c = 0.83 +/- 0.05 stellar radius. From the multiplet splitting, we calculated the rotational period P_rot = 1.3920 +/- 0.0008 days and an upper limit for the magnetic field, B < 2000 G. The total power of the pulsation modes at the stellar surface changed less than 30% for l=1 modes and less than 50% for l=2 modes. We find no evidence of linear combinations between the 198 pulsation mode frequencies. PG 1159-035 models have not significative convection zones, supporting the hypothesis that nonlinearity arises in the convection zones in cooler pulsating white dwarf stars.

astro-ph↗

arXiv2007

A new period determination for the close PG1159 binary SDSSJ212531.92-010745.9

Methods to measure masses of PG1159 stars in order to test evolutionary scenarios are currently based on spectroscopic masses or asteroseismological mass determinations. One recently discovered PG1159 star in a close binary system may finally allow the first dynamical mass determination, which has so far been analysed on the basis of one SDSS spectrum and photometric monitoring. In order to be able to phase future radial velocity measurements of the system SDSSJ212531.92-010745.9, we follow up on the photometric monitoring of this system to provide a solid observational basis for an improved orbital ephemeris determination. New white-light time series of the brightness variation of SDSSJ212531.92-010745.9 with the Tuebingen 80cm and Goettingen 50cm telescopes extend the monitoring into a second season (2006), tripling the length of overall observational data available, and significantly increasing the time base covered. We give the ephemeris for the orbital motion of the system, based on a sine fit which now results in a period of 6.95573(5)h, and discuss the associated new amplitude determination in the context of the phased light curve variation profile. The accuracy of the ephemeris has been improved by more than one order of magnitude compared to that previously published for 2005 alone.

astro-ph↗

arXiv2007

Synthetic NLTE accretion disc spectra for the dwarf nova SS Cyg during an outburst cycle

Dwarf nova outbursts result from enhanced mass transport through the accretion disc of a cataclysmic variable system. We assess the question of whether these outbursts are caused by an enhanced mass transfer from the late-type main sequence star onto the white dwarf (so-called mass transfer instability model, MTI) or by a thermal instability in the accretion disc (disc instability model, DIM). We compute non-LTE models and spectra of accretion discs in quiescence and outburst and construct spectral time sequences for discs over a complete outburst cycle. We then compare our spectra to published optical spectroscopy of the dwarf nova SS Cygni. In particular, we investigate the hydrogen and helium line profiles that are turning from emission into absorption during the rise to outburst. The evolution of the hydrogen and helium line profiles during the rise to outburst and decline clearly favour the disc-instability model. Our spectral model sequences allow us to distinguish inside-out and outside-in moving heating waves in the disc of SS Cygni, which can be related to symmetric and asymmetric outburst light curves, respectively.

astro-ph↗