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T. Rauch

Publications and source records attributed to T. Rauch.

126 records · Page 7Linked to original sources
arXiv2003

A Possible New Transiting Planet

Recently, 59 low-luminosity object transits were reported from the Optical Gravitational Lensing Experiment (OGLE). Our follow-up low-resolution spectroscopy of 16 candidates provided two objects, OGLE-TR-3 and OGLE-TR-10, which have companions with radii compatible with those of gas-giant planets. Further high-resolution spectroscopy revealed a very low velocity variation (<500m/s) of the host star OGLE-TR-3 which may be caused by its unseen companion. An analysis of the radial velocity and light curve results in M<2.5 M_jup, R<1.6 R_jup, and an orbital separation of about 5 R_sol, which makes it the planet with the shortest period known. This allows to identify the low-luminosity companion of OGLE-TR-3 as a possible new gas-giant planet. If confirmed, this makes OGLE-TR-3 together with OGLE-TR-56 the first extrasolar planets detected via their transit light curves.

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arXiv2003

The rotational velocity of the sdOB primary of the eclipsing binary system LB 3459 (AA Dor)

We present an analysis of the rotational velocity of the primary of LB 3459 based on 107 new high-resolution and high-S/N ESO VLT UVES spectra. 105 of them cover a complete orbital period (0.26 d) of this binary system. We have determined an orbital period of P = 22600.702 +/- 0.005 sec, a radial velocity amplitude of A_1 = 39.19 +/- 0.05 km/sec, and T_0 = 2451917.152690 +/- 0.000005. From simulations of the He II 4686A line profile (based on NLTE model atmosphere calculations), we derive v_rot = 47 +/- 5 km/sec. We present an animation which shows the orbital movement of the binary system, its synthetic lightcurve, and compares the phase-dependent variation of the predicted with the observed He II 4686A line profile. The radius of the cool component is almost the same size like Jupiter but its mass is about 70 times higher than Jupiter's mass. Thus, from its present mass (M_2 = 0.066 M_sun), the secondary of LB 3459 lies formally within the brown-dwarf mass range (0.013 - 0.08 M_sun). It might be a former planet which has survived the previous common-envelope phase and even has gained mass.

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arXiv2002

The Enigma of AA Dor

AA Dor (LB 3459) is an eclipsing, close binary (P = 0.26d) consisting of a sdOB primary star and an unseen secondary with an extraordinary small mass. The secondary is possibly a former planet which may have survived a common-envelope phase and has even gained mass. In order to investigate on an existing discrepancy between the components' mass derived from NLTE spectral analysis and subsequent comparison to evolutionary tracks and masses derived from radial-velocity and the eclipse curves, we performed phase-resolved high-resolution and high-SN spectroscopy with the UVES attached to the ESO VLT. From the obtained spectra, we have determined AA Dor's orbital parameters (P = 22600.702 +/- 0.005 sec, A = 39.19 +/- 0.05 km/sec, T0 = 2451917.152690) and the rotational velocity (v = 47 +/- 5 km/sec) of the primary.

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arXiv2002

The Rotational Velocity of Helium-rich Pre-White Dwarfs

Previous investigations on hydrogen-rich white dwarfs generally yield only very small rotational velocities (v_rot sin i). We have analyzed line profiles in high-resolution optical spectra of eight hydrogen-deficient (pre-) white dwarfs and find deviations from the dominant Stark line broadening in five cases which, interpreted as an effect of stellar rotation, indicate projected rotational velocities of 40 - 70 km/sec. For the three least luminous stars upper limits of v_rot sin i = 15 - 25 km/sec could be derived only. The resulting velocities correlate with luminosity and mass. However, since the mass-loss rate is correlated to the luminosity of a star, the observed line profiles may be affected by a stellar wind as well. In the case of RX J2117.1+3412, this would solve discrepancies to results of pulsational modeling (v_rot sin i ~ 0).

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arXiv2002

Model Photospheres with Accelerated Lambda Iteration

We address the classical stellar-atmosphere problem and describe our method of numerical solution in detail. The problem consists of the solution of the radiation transfer equation under the constraints of hydrostatic, radiative and statistical equilibrium (non-LTE). We employ the Accelerated Lambda Iteration (ALI) technique, and use statistical methods to construct non-LTE metal-line-blanketed model-atmospheres.

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arXiv2002

Metal abundances in PG1159 stars from Chandra and FUSE spectroscopy

We investigate FUSE spectra of three PG1159 stars and do not find any evidence for iron lines. From a comparison with NLTE models we conclude a deficiency of 1-1.5 dex. We speculate that iron was transformed into heavier elements. A soft X-ray Chandra spectrum of the unique H- and He-deficient star H1504+65 is analyzed. We find high neon and magnesium abundances and confirm that H1504+65 is the bare core of either a C-O or a O-Ne-Mg white dwarf.

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arXiv2002

Spectral Types of Planetary Host Star Candidates: Two New Transiting Planets?

Recently, 46 low-luminosity object transits were reported from the Optical Gravitational Lensing Experiment. Our follow-up spectroscopy of the 16 most promising candidates provides a spectral classification of the primary. Together with the radius ratio from the transit measurements, we derived the radii of the low-luminosity companions. This allows to examine the possible sub-stellar nature of these objects. Fourteen of them can be clearly identified as low-mass stars. Two objects, OGLE-TR-03 and OGLE-TR-10 have companions with radii of 0.15 R_sun which is very similar to the radius of the transiting planet HD209458B. The planetary nature of these two objects should therefore be confirmed by dynamical mass determinations.

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arXiv2002

H1504+65 - The Naked Stellar C/O Core of a Former Red Giant Observed with FUSE and Chandra

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 spectra show a wealth of absorption lines from highly ionized oxygen, neon and magnesium and suggest 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 a O-Ne-Mg white dwarf.

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arXiv2002

Iron abundance in hot hydrogen-deficient central stars and white dwarfs from FUSE, HST, and IUE spectroscopy

We present a first systematic investigation of the iron abundance in very hot (Teff>50,000K) hydrogen-deficient post-AGB stars. Our sample comprises 16 PG1159 stars and four DO white dwarfs. We use recent FUSE observations as well as HST and IUE archival data to perform spectral analyses with line blanketed NLTE model atmospheres. Iron is not detected in any PG1159 star. In most cases this is compatible with a solar iron abundance due to limited quality of HST and IUE data, although the tendency to an iron underabundance may be recognized. However, the absence of iron lines in excellent FUSE spectra suggests an underabundance by at least 1 dex in two objects (K1-16 NGC 7094). A similar result has been reported recently in the [WC]-PG1159 transition object Abell 78 (Werner et al. 2002). We discuss dust fractionation and s-process neutron-captures as possible origins. We also announce the first identification of sulfur in PG1159 stars.

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arXiv2002

Temperature Scale and Iron Abundances of Very Hot Central Stars of Planetary Nebulae

The determination of effective temperatures of very hot central stars (Teff>70000K) by model atmosphere analyses of optical H and He line profiles is afflicted with considerable uncertainty, primarily due to the lack of neutral helium lines. Ionization balances of metals, accessible only with UV lines, allow more precise temperature estimates. The potential of iron lines is pointed out. At the same time iron and other metal abundances, hardly investigated until today, may be derived from UV spectra. We describe recent HST spectroscopy performed for this purpose. A search for iron lines in FUV spectra of the hottest H-deficient central stars (PG1159-type, Teff>100000K) taken with FUSE was unsuccessful. The derived deficiency is interpreted in terms of iron depletion due to n-capture nucleosynthesis in intershell matter, which is now exposed at the stellar surface as a consequence of a late He shell flash.

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arXiv2002

Spectral analysis of the sdO K 648, the exciting star of the planetary nebula Ps 1 in the globular cluster M 15 (NGC 7078)

We present a spectral analysis of the sdO central star K 648 based on medium-resolution optical and high-resolution UV spectra. The photospheric parameters are determined by means of state-of-the-art NLTE model atmosphere techniques. We found Teff = 39 +/- 2 kK and log g = 3.9 +/- 0.2. The helium (He/H=0.08) and oxygen (O/H=0.001) abundances are about solar while carbon is enriched by a factor of 2.5 (C/H=0.001). Nitrogen (N/H = 10**(-6), [N/H] = -2.0) appears at a sub-solar value. However, these metal abundances are much higher than the cluster's metallicity M 15: [Fe/H] = -2.25). The surface composition appears to be a mixture of the original hydrogen-rich material and products of helium burning (3 alpha process) which have been mixed up to the surface. The abundances of He, C, and N are consistent with the nebular abundance, while O is considerably more abundant in the photosphere than in the nebula. From a comparison of its position in the log Teff - log g plane with evolutionary calculations a mass of 0.57 (+0.02, -0.01) Msun and a luminosity of 3810 +/- 1200 Lsun are deduced. Our spectroscopic distance d = 11.1 (+2.4, -2.9) kpc is in agreement with the distance of M 15 as determined by Alves et al. (2000). From the GHRS spectra we measure a radial velocity of vrad = -130 km/sec.

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arXiv2001

Effects of CSPN Models on PNe Shell Modeling

Modeling planetary nebulae (PNe) shells using different ionizing spectra for a hot central star (CSPN) of evolved PNe, we found that especially a blackbody model leads to wrong nebular diagnostics and abundances.

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arXiv2001

Identification of the 4486,4504A emission lines in O-type spectra

Inspired by an appeal to the community from Walborn (these proceedings) we decided to solve the long-standing problem concerning the nature of the 4486,4504A emission lines, which are frequently observed in O-type spectra and which are particularly prominent in supergiants. We claim that these lines emerge from sulfur, namely by de-excitation of a highly excited S IV doublet state. We prove this by an exploratory NLTE calculation with a detailed model atom.

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arXiv2000

The iron abundance in hot central stars of planetary nebulae derived from IUE spectra

We present the first attempt to determine the iron abundance in hot central stars of planetary nebulae. We perform an analysis with fully metal-line blanketed NLTE model atmospheres for a sample of ten stars (T_eff >= 70.000 K) for which high-resolution UV spectra are available from the IUE archive. In all cases lines of Fe VI or Fe VII can be identified. As a general trend, the iron abundance appears to be subsolar by 0.5-1 dex, however, the S/N of the IUE spectra is not sufficient to exclude a solar abundance in any specific case. Improved spectroscopy by either FUSE or HST is necessary to verify the possibility of a general iron deficiency in central stars. The suspected deficiency may be the result of gravitational settling in the case of three high-gravity objects. For the other stars with low gravity and high luminosity dust fractionation during the previous AGB phase is a conceivable origin.

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arXiv1999

HST/STIS ultraviolet spectroscopy of the supersoft X-ray source RX J0439.8-6809

We present ultraviolet observations of the supersoft X-ray source RXJ0439.8-6809 obtained with the Hubble Space Telescope Imaging Spectrograph. The ultraviolet spectrum is a very blue continuum overlayed with interstellar absorption lines. The observed broad Lyman alpha absorption line is consistent with an interstellar column density of neutral hydrogen Nh=(4.0+-1.0)e20 cm-2. The light curve obtained from the time-tagged dataset puts a 3sigma upper limit of 0.04mag on the ultraviolet variability of RXJ0439.8-6809 on time scales between 10s and 35min. The long-term X-ray light curve obtained from our three-year ROSAT HRI monitoring of RXJ0439.8-6809 shows the source with a constant count rate, and implies that the temperature did not change more than a few 1000K. If RXJ0439.8-6809 is a massive extremely hot pre-white dwarf on the horizontal shell-burning track, opposed to the alternative possibility of a very compact double-degenerate supersoft X-ray binary, its constant temperature and luminosity are a challenge to stellar evolution theory. Interestingly, RXJ0439-6809 is found close to the theoretical carbon-burning main-sequence.

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arXiv1996

Temperature determination of the cool DO white dwarf HD149499B from EUVE observations

We present a LTE model atmosphere analysis of the medium and long wavelength spectrum of the cool DO white dwarf HD149499B. This observation in the spectral range 230-700 A supplements a previous analysis of an ORFEUS spectrum between 912 and 1170 A which yielded the atmospheric parameters Teff=49500+-500 K and log g = 7.97+-0.08 and a hydrogen-to-helium number ratio of 0.22+-0.11. The EUVE data are in full agreement with the ORFEUS result and allow a more precise determination of the effective temperature (Teff=49500+-200 K) and the interstellar hydrogen column density (NH=7.E18cm^-2). None of the features in the EUVE spectrum could be identified with any additional absorber besides helium. Exploratory calculations with fully metal blanketed LTE model atmospheres show that the metal abundances predicted by current diffusion theory are clearly at variance with the observed spectrum.

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