arXiv subjects

S. Dreizler

Publications and source records attributed to S. Dreizler.

At least 181 records · Page 10Linked to original sources

arXiv2007

Spectral types of planetary host star candidates from OGLE III

The Optical Gravitational Lensing Experiment project has recently provided the OGLE III list of low-luminosity object transits from campaigns #3 and #4, reporting 40 new objects exhibiting the low-amplitude photometric eclipses expected for exoplanets. Compared to previous OGLE targets, these OGLE III candidates have been more restrictively selected and may contain low-mass planets. We have secured follow-up low-resolution spectroscopy for 28 candidates out of this list (and one from the OGLE Carina fields) to obtain an independent characterization of the primary stars by spectral classification and thus better constrain the parameters of their companions. We fed the constraints from these results back into an improved light curve solution. Together with the radius ratios from the transit measurements, we derived the radii of the low-luminosity companions. This allows us to examine the possible sub-stellar nature of these objects. Sixteen of the companions can be clearly identified as low-mass stars orbiting a main sequence primary, while 10 more objects are likely to have red giant primaries and therefore also host a stellar companion; 3 possibly have a sub-stellar nature R< 0.15 R_sun). The planetary nature of these objects should therefore be confirmed by dynamical mass determinations.

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arXiv2006

Spectral analyses of 16 DAO white dwarfs from the Sloan Digital Sky Survey

We present a spectral analysis of 16 DAO white dwarfs from the Sloan Digital Sky Survey Data Release 4. With our NLTE H+He model grid, we derived photospheric parameters for these objects. We compare our new results to literature values and divide the DAOs into two distinct groups: post-AGB and EHB progenitors.

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arXiv2006

Observational constraints on the evolutionary connection between PG 1159 stars and DO white dwarfs

The Sloan Digital Sky Survey has provided spectra of a large number of new PG 1159 stars and DO white dwarfs. This increase in known hot H-deficient compact objects significantly improves the statistics and helps to investigate late stages of stellar evolution. We have finished our analyses of nine PG 1159 stars and 23 DO white dwarfs by means of detailed NLTE model atmospheres. From the optical SDSS spectra, effective temperatures, surface gravities, and element abundances are derived by using our new automated chi^2-fitting in order to place the observed objects in an evolutionary context. Especially the connection between PG 1159 stars and DO white dwarfs has been investigated.

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arXiv2006

Hot subdwarfs from the ESO Supernova Ia Progenitor Survey: II. Atmospheric parameters of subdwarf O stars

We address the origin and evolutionary status of hot subdwarf stars by studying the optical spectral properties of 58 subdwarf O (sdO) stars. Combining them with the results of our previously studied subdwarf B (sdB) stars, we aim at investigating possible evolutionary links. We analyze high-resolution ESO VLT UVES spectra from the ESO Supernova Ia Progenitor Survey (SPY). Effective temperatures, gravities, and helium abundances are determined simultaneously by fitting the profiles of H and He lines using dedicated synthetic spectra in NLTE. Evidence for cool companions to 8 sdOs as well as a binary consisting of two sdO stars is found. A correlation between He abundances and the presence of carbon and/or nitrogen lines emerges: below solar He abundance, no sdO shows C or N lines. In contrast, C and/or N lines are present in ALL sdOs with super- solar He abundance. We thus use the solar He abundance to divide our sample into He-deficient and He-enriched sdOs. While He-deficient sdOs are scattered in a wide range of the Teff-log(g)-diagram, most of the He-enriched sdOs cluster in a narrow region at Teff = 40,000 ... 50,000K and log(g)=5.5 ... 6.0. An evolu- tionary link between sdBs and sdOs appears plausible only for the He-deficient sdOs indicating that they are the likely successors to sdBs. The properties of He-enriched sdOs cannot be explained with canonical single star evolutionary models. Alternative scenarios (late hot flasher) as well as for binary evolution (white dwarf merger; post-RGB evolution) are tested. While we regard the post-RGB scenario as inappropriate, the white dwarf merger and the late hot flasher scenarios remain viable to explain the origin of He-enriched sdOs.

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arXiv2006

Probing unexplored territories with MUSE: a second generation instrument for the VLT

The Multi Unit Spectroscopic Explorer (MUSE) is a second-generation VLT panoramic integral-field spectrograph under preliminary design study. MUSE has a field of 1x1 arcmin**2 sampled at 0.2x0.2 arcsec**2 and is assisted by the VLT ground layer adaptive optics ESO facility using four laser guide stars. The simultaneous spectral range is 465-930 nm, at a resolution of R~3000. MUSE couples the discovery potential of a large imaging device to the measuring capabilities of a high-quality spectrograph, while taking advantage of the increased spatial resolution provided by adaptive optics. This makes MUSE a unique and tremendously powerful instrument for discovering and characterizing objects that lie beyond the reach of even the deepest imaging surveys. MUSE has also a high spatial resolution mode with 7.5x7.5 arcsec**2 field of view sampled at 25 milli-arcsec. In this mode MUSE should be able to obtain diffraction limited data-cubes in the 600-930 nm wavelength range. Although the MUSE design has been optimized for the study of galaxy formation and evolution, it has a wide range of possible applications; e.g. monitoring of outer planets atmosphere, environment of young stellar objects, super massive black holes and active nuclei in nearby galaxies or massive spectroscopic surveys of stellar fields in the Milky Way and nearby galaxies.

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arXiv2006

Spectral analyses of eighteen hot H-deficient (pre-) white dwarfs from the Sloan Digital Sky Survey Data Release 4

Context: The Sloan Digital Sky Survey Data Release 4 has provided spectra of several new PG 1159 stars and DO white dwarfs. This increase in known hot H-deficient compact objects significantly improves the statistics and helps to investigate late stages of stellar evolution. Aims: From the optical SDSS spectra, effective temperatures and surface gravities are derived in order to place the observed objects in an evolutionary context. Especially the connection between PG 1159 stars and DO white dwarfs shall be investigated. Method: Using our non-LTE model atmospheres and applying chi^2-fitting techniques, we determine stellar parameters and their errors. We derive total stellar masses for the DO white dwarfs using model evolutionary tracks. Results: We confirm three PG 1159 stars, with one showing ultra-high excitation ion features, and one sdO which we originally classified as a PG 1159 star. Additionally, we re-analysed the known PG 1159 star, PG 1424+535, with our new models. Furthermore, we present the first spectral analyses of thirteen DO white dwarfs, three of which show M-star features in their spectra, while two display ultra-high excitation ion features.

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arXiv2006

VLT spectroscopy and non-LTE modeling of the C/O-dominated accretion disks in two ultracompact X-ray binaries

We present new medium-resolution high-S/N optical spectra of the ultracompact low-mass X-ray binaries 4U0614+091 and 4U1626-67, taken with the ESO Very Large Telescope. They are pure emission line spectra and the lines are identified as due to C II-IV and O II-III Line identification is corroborated by first results from modeling the disk spectra with detailed non-LTE radiation transfer calculations. Hydrogen and helium lines are lacking in the observed spectra. Our models confirm the deficiency of H and He in the disks. The lack of neon lines suggests an Ne abundance of less than about 10 percent (by mass), however, this result is uncertain due to possible shortcomings in the model atom. These findings suggest that the donor stars are eroded cores of C/O white dwarfs with no excessive neon overabundance. This would contradict earlier claims of Ne enrichment concluded from X-ray observations of circumbinary material, which was explained by crystallization and fractionation of the white dwarf core.

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arXiv2006

SDSSJ212531.92-010745.9 - the first definite PG1159 close binary system

The archival spectrum of SDSSJ212531.92-010745.9 shows not only the typical signature of a PG1159 star, but also indicates the presence of a companion. Our aim was the proof of the binary nature ofthis object and the determination of its orbital period.We performed time-series photometry of SDSSJ212531.92-010745.9. We observed the object during 10 nights, spread over one month, with the Tuebingen 80cm and the Goettingen 50cm telescopes. We fitted the observed light curve with a sine and simulated the light curve of this system with the nightfall program. Furthermore, we compared the spectrum of SDSSJ212531.92-010745.9 with NLTE models, the results of which also constrain the light curve solution. An orbital period of 6.95616(33)h with an amplitude of 0.354(3)mag is derived from our observations. A pulsation period could not be detected. For the PG1159 star we found, as preliminary results from comparison with our NLTE models, Teff about 90000K, log g about 7.60, and the abundance ratio C/He = 0.05 by number fraction. For the companion we obtained with a mean radius of 0.4 +/- 0.1 Rsol, a mass of 0.4 +/- 0.1 Msol, and a temperature of 8200K on the irradiated side, good agreement between the observed light curve and the nightfall simulation, but we do not regard those values as final.

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arXiv2005

Exciting new features in the frequency spectrum of the EC14026 star HS0702+6043 - Simultaneous g-modes and p-modes in a sdB pulsator

The discovery of a long-period g-mode oscillation in the previously known short-period p-mode sdB pulsator HS0702+6043 makes this star an extraordinary object, unique as a member of the family of sdB pulsators, and one of the very few known pulsating stars overall exhibiting excited modes along both the acoustic and gravity branches of the nonradial pulsation spectrum. Because p-modes and g-modes probe different regions of a pulsating star, HS0702+6043 holds a tremendous potential for asteroseismological investigations. We present preliminary results from the first extended campaign on this object.

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arXiv2005

HS0702+6043: A star showing both short-period p-mode and long-period g-mode oscillations

[Context.] The hot subdwarf B star HS0702+6043 is known as a large-amplitude, short-period p-mode pulsator of the EC14026 type. Its atmospheric parameters place it at the common boundary between the empirical instability regions of the EC14026 variables and the typically cooler long-period g-mode pulsators of the PG1716 kind. [Aims.] We analyse and interpret the photometric variability of HS0702+6043 in order to explore its asteroseismological potential. [Methods.] We report on rapid wide band CCD photometric observations to follow up on and confirm the serendipitous discovery of multiperiodic long-period luminosity variations with typical time scales of ~1h in HS0702+6043, in addition to the two previously known pulsations at 363s and 383s. In particular, we isolate a relatively low-amplitude (~4 mmag), long-period (3538+-130s) light variation. [Results.] We argue that the most likely origin for this luminosity variation is the presence of an excited g-mode pulsation. If confirmed, HS0702+6043 would constitute a rare addition to the very select class of pulsating stars showing simultaneously parts of their pressure and gravity mode pulsation spectra. The asteroseismological potential of such stars is immense, and HS0702+6043 thus becomes a target of choice for future investigations. While our discovery appears consistent with the location of HS0702+6043 at the common boundary between the two families of pulsating sdB stars, it does challenge theory's current description of stability and driving mechanisms in pulsating B subdwarfs.

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arXiv2005

On the most metal-poor PN and its binary central star

PN G135.9+55.9 is the most metal-poor PN known in our Galaxy. The central star resides in a short-period binary system with a compact component, probably a white dwarf. We describe new observations, which allowed us to determine the orbital period. The lower limit for the combined mass of both stars is close to the Chandrasekhar limit for white dwarfs, making this binary a possible progenitor of a supernova type Ia. The binary system must have recently emerged from a common envelope phase.

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arXiv2005

Spectral analyses of DO white dwarfs and PG1159 stars from the Sloan Digital Sky Survey

We present a model atmosphere analysis of ten new DO white dwarfs and five new PG 1159 stars discovered in the Sloan Digital Sky Survey DR1, DR2 and DR3. This is a significant increase in the number of known DOs and PG 1159 stars. DO white dwarfs are situated on the white dwarf cooling sequence from the upper hot end Teff ~ 120 000 K down to the DB gap (Teff ~ 45 000 K). PG 1159 stars on the other hand feature effective temperatures which exceed Teff = 65 000 K with an upper limit of Teff = 200 000 K and are the proposed precursors of DO white dwarfs. Improved statistics are necessary to investigate the evolutionary link between these two types of stars. From optical SDSS spectra effective temperatures, surface gravities and element abundances are determined by means of non-LTE model atmospheres.

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arXiv2005

Light and heavy metal abundances in hot central stars of planetary nebulae

We present new results from our spectral analyses of very hot central stars achieved since the last IAU Symposium on planetary nebulae held in Canberra 2001. The analyses are mainly based on UV and far-UV spectroscopy performed with the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer but also on ground-based observations performed at the Very Large Telescope and other observatories. We report on temperature, gravity, and abundance determinations for the CNO elements of hydrogen-rich central stars. In many hydrogen-deficient central stars (spectral type PG1159) we discovered particular neon and fluorine lines, which are observed for the very first time in any astrophysical object. Their analysis strongly confirms the idea that these stars exhibit intershell matter as a consequence of a late helium-shell flash.

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arXiv2005

The MultiSite Spectroscopic Telescope campaign: 2m spectroscopy of the V361 Hya variable PG1605+072

We present results and analysis for the 2m spectroscopic part of the MultiSite Spectroscopic Telescope (MSST) campaign undertaken in May/June 2002. The goal of the project was to observe the pulsating subdwarf B star PG1605+072 simultaneously in velocity and photometry and to resolve as many of the >50 known modes as possible, which will allow a detailed asteroseismological analysis. We have obtained over 150 hours of spectroscopy, leading to an unprecedented noise level of only 207m/s. We report here the detection of 20 frequencies in velocity, with two more likely just below our detection threshold. In particular, we detect 6 linear combinations, making PG1605+072 only the second star known to show such frequencies in velocity. We investigate the phases of these combinations and their parent modes and find relationships between them that cannot be easily understood based on current theory. These observations, when combined with our simultaneous photometry, should allow asteroseismology of this most complicated of sdB pulsators.

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arXiv2005

The D/H Ratio in the Interstellar Medium toward the White Dwarf PG0038+199

We determine the D/H ratio in the interstellar medium toward the DO white dwarf PG0038+199 using spectra from the Far Ultraviolet Spectroscopic Explorer (FUSE), with ground-based support from Keck HIRES. We employ curve of growth, apparent optical depth and profile fitting techniques to measure column densities and limits of many other species (H2, NaI, CI, CII, CIII, NI, NII, OI, SiII, PII, SIII, ArI and FeII) which allow us to determine related ratios such as D/O, D/N and the H2 fraction. Our efforts are concentrated on measuring gas-phase D/H, which is key to understanding Galactic chemical evolution and comparing it to predictions from Big Bang nucleosynthesis. We find column densities log N(HI) = 20.41+-0.08, log N(DI)=15.75+-0.08 and log N(H2) = 19.33+-0.04, yielding a molecular hydrogen fraction of 0.14+-0.02 (2 sigma errors), with an excitation temperature of 143+-5K. The high HI column density implies that PG0038+199 lies outside of the Local Bubble; we estimate its distance to be 297 (+164,-104)pc (1 sigma). D/[HI+2H2] toward PG0038+199 is 1.91(+0.52,-0.42) e-5 (2 sigma). There is no evidence of component structure on the scale of Delta v > 8 km/s based on NaI, but there is marginal evidence for structure on smaller scales. The D/H value is high compared to the majority of recent D/H measurements, but consistent with the values for two other measurements at similar distances. D/O is in agreement with other distant measurements. The scatter in D/H values beyond ~100pc remains a challenge for Galactic chemical evolution.

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arXiv2005

The Spectral Variability of Pulsating Stars: PG1159-035

With 10m class telescopes as well as with time-tagging detectors on board of HST and FUSE, the analysis of time-resolved spectra for pulsating white dwarfs becomes feasible. We present simulated time-resolved spectra for the hot pulsating white dwarf PG1159-035 and compare these models with observational data of the 516s mode based on HST-STIS spectroscopy. A determination of the pulsation mode by the spectral variability of PG1159-035 seems to be impossible for the moment.

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arXiv2005

Spectral Analyses of DO White Dwarfs and PG1159 Stars from the Sloan Digital Sky Survey

SDSS (DR1 and DR2) has recently proposed 7 new DO white dwarfs as well as 6 new PG1159 stars. This is a significant increase in the known number of DOs and PG1159 stars. Our spectral analyses provide stellar parameters which can then be used to derive constraints for the evolution of H-deficient white dwarfs. A comprehensive understanding of these objects is still severely hampered by low-number statistics.

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arXiv2005

NLTE Analysis of the Ultra-short Period White-dwarf Binary RX J0806.3+1527

RX J0806.3+1527 is suspected to be a double-degenerate white dwarf binary. We present first results of our NLTE analysis of its optical spectrum. The VLT/FORS1 data show a composite spectrum consisting of a blue continuum and superimposed emission lines of the HeII Pickering series and, possibly, the H Balmer series. Our models are based on hot white dwarf atmospheres and include illumination effects onto the secondary star. The physical parameters and chemical abundances derived from the comparison of the observed spectrum with a grid of model atmospheres provide constraints on the true nature of this enigmatic binary and on the models proposed so far.

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