arXiv subjects

T. Rauch

Publications and source records attributed to T. Rauch.

At least 109 records · Page 6Linked to original sources

arXiv2004

Fluorine in extremely hot post-AGB stars

We have discovered lines of highly ionized fluorine (Fv and Fvi) in the far-UV spectra of extremely hot (Teff=85,000--150,000 K) post-AGB stars. Our sample comprises H-rich central stars of planetary nebulae as well as H-deficient PG1159 stars. We performed non-LTE calculations and find strong F overabundances (up to 10**-4 by mass, i.e., 250 times solar) in a number of PG1159 stars, while F is essentially solar in the H-rich stars. Since PG1159 stars are believed to exhibit intershell matter of the preceding AGB phase on their surface, their chemical analyses allow for a direct insight into nucleosynthesis processes during the AGB phase. The high F abundances in PG1159 stars confirm the conclusion from abundance determinations in giants, that F is synthesized in AGB stars and that the F enrichment in the intershell must be very high.

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arXiv2004

HST and FUSE Spectroscopy of Hot Hydrogen-Rich Central Stars of Planetary Nebulae

High-resolution UV spectra, obtained with HST and FUSE, enable us to analyse hot hydrogen-rich central stars in detail. Up to now, optical hydrogen and helium lines have been used to derive temperature and surface gravity. Those lines, however, are rather insensitive; in particular, neutral helium lines have completely vanished in the hottest central stars. Therefore, we have concentrated on ionization balances of metals, which have a rich line spectrum in the UV, to establish a new temperature scale for our sample. Furthermore, we have determined abundances of light metals, which had been poorly known before. They show considerable variation from star to star. We present results of quantitative spectral analyses performed with non-LTE model atmospheres.

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arXiv2004

Iron Abundance in Hydrogen-Rich Central Stars of Planetary Nebulae

We report on an on-going analysis of high-resolution UV spectra of hot hydrogen-rich central stars of planetary nebulae (CSPN), obtained with the Hubble Space Telescope and FUSE. Since UV spectra of many CSPN are dominated by Fe and Ni lines, we intend to use them as temperature indicators to check the CSPN temperature scale we have derived earlier from CNO ionization balances. Furthermore, the observed line strengths of heavy metals show large variations between different objects suggesting a possible spread in abundances. We will determine abundances of iron group elements by quantitative spectral analyses with non-LTE model atmospheres.

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arXiv2004

FUSE spectroscopy of PG1159 Stars

PG1159 stars are hot hydrogen-deficient post-AGB stars with effective temperatures within a range from 75000 K up to 200000 K. These stars are probably the result of a late helium-shell flash that had occurred during their first descent from the AGB. The lack of hydrogen is caused by flash-induced envelope mixing and burning of H in deeper regions. Now the former intershell matter is seen on the surface of the stars. Hence the stellar atmospheres show metal abundances drastically different from the solar values. Our sample comprises ten PG1159 stars with effective temperatures between 85000 K and 170000 K. We present first results of our spectral analysis based on FUV spectra obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).

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arXiv2004

Insight into AGB and post-AGB stellar evolution with FUSE

FUSE spectroscopy has proved that extremely hot hydrogen-deficient post-AGB stars (PG1159 stars) display matter on their surface that usually remains hidden in the region between the H- and He-burning shells of the former AGB star. Hence, the spectral analysis of PG1159 stars allows to study directly the chemistry of this intershell region which is the outcome of complicated burning and mixing processes during AGB evolution. Detailed abundance determinations provide constraints for these processes which are still poorly understood. With FUSE we have discovered high neon and fluorine overabundances. There is also a significant iron deficiency, which may be caused by s-process neutron capture transforming iron into heavier elements.

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arXiv2004

On Possible Oxygen/Neon White Dwarfs: H1504+65 and the White Dwarf Donors in Ultracompact X-ray Binaries

We discuss the possibility to detect O/Ne white dwarfs by evidence for Ne overabundances. The hottest known WD, H1504+65, could be the only single WD for which we might be able to proof its O/Ne nature directly. Apart from this, strong Ne abundances are known or suspected only from binary systems, namely from a few novae, and from a handful of LMXBs. We try to verify the hypothesis that the latter might host strongly ablated O/Ne WD donors, by abundance analyses of the accretion disks in these systems. In any case, to conclude on O/Ne WDs just by strong Ne overabundances is problematic, because Ne enrichment also occurs by settling of this species into the core of C/O WDs.

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arXiv2004

Identification of a DO White Dwarf and a PG1159 Star in the ESO SN Ia Progenitor Survey (SPY)

We present high-resolution VLT spectra of a new DO white dwarf and a new PG1159 star, which we identified in the ESO SPY survey. The PG1159 star is a low-gravity, extremely hot (Teff=160,000 K, log g=6) star, having a C/He dominated atmosphere with considerable amounts of O and Ne (He=38%, C=54%, O=6%, Ne=2% by mass). It is located within the planetary nebula nuclei instability strip and pulsations have been discovered. The DO is a unique object. From HeI/HeII line strengths we found Teff about 60,000 K, however, the HeII lines are extraordinarily strong and cannot be fitted by any model.

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arXiv2004

Photoionization models of roundish galactic planetary nebulae in the thick disk

We present the result of photo-ionizing modelling of the three planetary nebulae (PNe) A 20, A 15 and MeWe 1-3. All three objects are roughly roundish, highly excited and have a high galactic $z$. The PNe displayed low densities in the shell, but relatively dense halos. A low metallicity and a relative high electron temperature were found. Comparisons with radio observations confirmed the obtained properties. The objects are very likely originating from thick disk stellar progenitors. The distances found investigating the PNe shells are somewhat lower than those derived spectroscopically with the central stars in the past.

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arXiv2004

AcDc - A new code for the NLTE spectral analysis of accretion discs: application to the helium CV AM CVn

We present a recently developed code for detailed NLTE calculations of accretion disc spectra of cataclysmic variables and compact X-ray binaries. Assuming a radial structure of a standard alpha-disc, the disc is divided into concentric rings. For each disc ring the solution of the radiation transfer equation and the structure equations, comprising the hydrostatic and radiative equilibrium, the population of the atomic levels as well as charge and particle conservation, is done self-consistently. Metal-line blanketing and irradiation by the central object are taken into account. As a first application, we show the influence of different disc parameters on the disc spectrum for the helium cataclysmic variable AM CVn.

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arXiv2004

A close binary nucleus in the most oxygen-poor planetary nebula PN G 135.9+55.9

We report FUSE and new deep optical spectroscopic observations of PN G 135.9+55.9, the most oxygen-poor planetary nebula located in the Galactic halo. These observations allow us to estimate the gravity of the central star by fitting the profile of the observed H I absorption lines with NLTE model atmospheres. Our best fit implies that the central star is still in a pre-white dwarf stage. We also find large variability of the radial velocities of the absorption component of the Balmer lines on a timescale of hours. This is direct evidence that the nucleus of PN G 135.9+55.9 is a close binary. The large semi-amplitude of the radial velocity variations and the probably short period suggest a massive compact companion, likely a white dwarf. Although our orbital solutions are very preliminary, they indicate that the total mass of the system probably exceeds the Chandrasekhar limit. If confirmed, this would make this binary the potential progenitor of a type Ia supernova.

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arXiv2004

Identification of a DO white dwarf and a PG1159 star in the ESO SN Ia progenitor survey (SPY)

We present high-resolution VLT spectra of a new helium-rich DO white dwarf and a new PG1159 star, which we identified in the ESO SPY survey. We performed NLTE model atmosphere analyses and found that the PG1159 star is a low-gravity, extremely hot (Teff=160,000K, logg=6) star, having a carbon-helium dominated atmosphere with considerable amounts of oxygen and neon (He=38%, C=54%, O=6%, Ne=2% by mass). It is located within the planetary nebula nuclei instability strip, hence, future searches for an associated PN as well as for stellar pulsations might be successful. The DO white dwarf is a unique object. From the relative strength of neutral and ionized helium lines we found Teff approx. 60,000K, however, the HeII lines are extraordinarily strong and cannot be fitted by any model. Similar problems were encountered with hot subdwarfs and white dwarfs showing signatures of a super-hot wind. The reason is unknown.

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arXiv2004

Identification of neon in FUSE and VLT spectra of extremely hot hydrogen-deficient (pre-) white dwarfs

One of the strongest absorption lines observed in far-ultraviolet FUSE spectra of many PG1159 stars remained unidentified up to now. We show that this line, located at 973.3A, stems from NeVII. We also present new optical high-resolution spectra of PG1159 stars, obtained with the ESO VLT, which cover the NeVII 3644A line and a newly identified NeVII multiplet in the 3850-3910A region. We compare the observed neon lines with NLTE models and conclude a substantial neon overabundance in a number of objects. Although a detailed analysis is still to be performed in order to compare quantitatively the abundances with evolutionary theory predictions, this corroborates the idea that the PG1159 stars and their immediate progenitors, the [WC]-type nuclei of planetary nebulae, display intershell matter of their precursor AGB stars. Possibly as the consequence of a late He-shell flash, H-deficient and (s-processed) Fe-depleted matter, that is strongly enriched by 3-alpha-processed elements (C, O, Ne), is dredged up to the surface. Hence, a detailed study of the element abundance patterns in these peculiar stars gives the unique possibility to probe mixing and nucleosynthesis processes in the precursor AGB stars.

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arXiv2004

Observations and 3D photoionisation modelling of the Wolf-Rayet planetary nebula NGC 1501

Deep optical spectra of the high excitation planetary nebula NGC 1501 and its W04 central star are presented. A recombination line abundance analysis of the central star's emission-line spectrum yields He:C:O mass fractions of 0.36:0.48:0.16, similar to those of PG1159 stars. A detailed empirical analysis of the nebular collisionally excited line (CEL) and optical recombination line (ORL) spectrum is presented, together with fully three-dimensional photoionisation modelling of the nebula. We found very large ORL-CEL abundance discrepancy factors (ADFs) for O^2+ (32) and Ne^2+ (33). The mean value of 5100K for the T_e derived from HeI recombination lines ratios is 6000K lower than the value of 11100K implied by the [OIII] line ratio. A three-dimensional photoionisation model of NGC 1501 was constructed using the photoionisation code MOCASSIN, based on our new spectroscopic data and using the three-dimensional electron density distribution determined from long-slit echellograms of the nebula by Ragazzoni et al. (2001). The initial models showed higher degrees of ionisation of heavy elements than indicated by observations. We investigated the importance of the missing low-temperature dielectronic recombination rates for third-row elements and have estimated upper limits to their rate coefficients. Our model heavily under-predicts the optical recombination line emission. We conclude that the presence of a hydrogen-deficient, metal-rich component is necessary to explain the observed ORL spectrum of this object. The existence of such knots could also provide a softening of the radiation field, via the removal of ionising photons by absorption in the knots, thereby helping to alleviate the over-ionisation of the heavy elements in our models.

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arXiv2004

Chandra and FUSE spectroscopy of the hot bare stellar core H1504+65

H1504+65 is an extremely hot hydrogen-deficient white dwarf with an effective temperature close to 200,000 K. We present new FUV and soft X-ray spectra obtained with FUSE and Chandra, which confirm that H1504+65 has an atmosphere primarily composed of carbon and oxygen. The Chandra LETG spectrum (60-160 Angstroem) shows a wealth of photospheric absorption lines from highly ionized oxygen, neon, and - for the first time identified in this star - magnesium and suggests relatively high Ne and Mg abundances. This corroborates an earlier suggestion that H1504+65 represents a naked C/O stellar core or even the C/O envelope of an O-Ne-Mg white dwarf.

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arXiv2004

A Dor - An Eclipsing Subdwarf - Brown Dwarf Binary

AA Dor is an eclipsing, close, post common-envelope binary (PCEB) consisting of a sdOB primary star and an unseen cool companion with an extraordinary low mass. The evolution of this system appears enigmatic - results of a recent spectral analysis of the primary has shown that the secondary is formally a brown dwarf. We present results of a spectral analysis which is based on phase-resolved high-resolution and high-SN spectroscopy.

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arXiv2003

Modeling of Oxygen-Neon Dominated Accretion Disks in Ultracompact X-ray Binaries: 4U1626-67

We report on first results of computing synthetic spectra from H/He-poor accretion disks in ultracompact LMXBs. We aim at the determination of the chemical composition of the very low-mass donor star, which is the core of a former C/O white dwarf. The abundance analysis allows to draw conclusions on gravitational settling in WDs which is an important process affecting cooling times and pulsational g-mode periods.

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arXiv2003

NLTE Spectral Analysis of Central Stars of PNe Interacting with the ISM

The analysis of Planetary Nebulae (PNe) provides a tool to investigate the properties of their exciting central stars (CSPN) at the moment of the PN ejection as well as on the properties of the ambient interstellar medium (ISM). The spectral analysis of the CSPN is a prerequisite to calculate the ionizing flux which is a crucial input for reliable PN modeling. In the framework of a systematic study of PNe interacting with the ISM, we present preliminary results of ongoing NLTE spectral analyses of ten of their CS based on new optical medium-resolution spectra.

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arXiv2003

Three-dimensional photoionisation modelling of the hydrogen-deficient knots in the planetary nebula Abell 30

We have constructed a photoionization model, using the 3-D Monte Carlo code MOCASSIN for one of the hydrogen-deficient knots (J3) of the born-again planetary nebula Abell 30. The model consists of spherical knots, comprising a cold, dense, hydrogen-deficient core with very high metal abundances. The inner core is surrounded by a less dense hydrogen-deficient and metal-enriched gas envelope, with less extreme abundances. The envelope of the knot might have been formed by the mixing of the knot material with the surrounding nebular gas. This bi-chemistry, bi-density model did not produce enough heating to match the fluxes of the collisionally excited emission lines (CELs) and of the optical recombination lines (ORLs) observed in the spectrum of the knot. We therefore included heating by photoelectric emission from dust grains in the thermal equilibrium calculations, and found that dust-to-gas ratios of 0.077 and 0.107 by mass for the central core and the envelope of the knot, respectively, are sufficient to fit the spectrum. Surprisingly, photoelectric emission from grains is the dominant source of heating in the hot envelope of the knot, while heating by photoionization of helium and heavy elements dominates in the cold core. The chemical abundances inferred from our modelling indicate, in agreement with the empirical analysis of Wesson et al. (2003), that the C/O ratio in the two regions of the knot is less than unity, contrary to theoretical predictions for born-again nebulae.

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