177,352 research outputs found
White Dwarfs in the European Galactic Plane Surveys (EGAPS)
No full textContains fulltext :
36699.pdf ( ) (Open Access)The space density of white dwarfs is highly uncertain even nearby. This results from the fact that the known sample of white dwarfs is largely incomplete in part because most white dwarfs have been discovered as by-products in non-dedicated surveys. In order to obtain more accurate white dwarf space densities and scale heights we must build up a complete sample of white dwarfs. The European Galactic Plane Surveys (EGAPS) are the best database to search for white dwarfs as they will provide broad band (U, g', r', i') and narrow band (Halpha and HeI) measurements for one per cent of all the stars in the Galaxy. By looking at the Galactic Plane, where most stars are, we ensure that we are obtaining a complete sample. The space densities obtained from EGAPS can then be compared with those found in high latitude surveys such as the Sloan Digital Sky Survey (SDSS). The methods used to identify white dwarfs using the colours available in EGAPS are described and some preliminary results presented.European Workshop on White Dwarf
High spatial resolution galactic 3D extinction mapping with IPHAS
No full textWe present an algorithm (MEAD, for 'Mapping Extinction Against Distance') which will determine intrinsic (r' - i') colour, extinction, and distance for early-A to K4 stars extracted from the IPHAS r'/ i'/H alpha photometric data base. These data can be binned up to map extinction in three dimensions across the northern Galactic plane. The large size of the IPHAS data base (similar to 200 million unique objects), the accuracy of the digital photometry it contains and its faint limiting magnitude (r' similar to 20) allow extinction to be mapped with fine angular (similar to 10 arcmin) and distance (similar to 0.1 kpc) resolution to distances of up to 10 kpc, outside the solar circle. High reddening within the solar circle on occasion brings this range down to similar to 2 kpc. The resolution achieved, both in angle and depth, greatly exceeds that of previous empirical 3D extinction maps, enabling the structure of the Galactic Plane to be studied in increased detail. MEAD accounts for the effect of the survey magnitude limits, photometric errors, unresolved interstellar medium (ISM) substructure and binarity. The impact of metallicity variations, within the range typical of the Galactic disc is small. The accuracy and reliability of MEAD are tested through the use of simulated photometry created with Monte Carlo sampling techniques. The success of this algorithm is demonstrated on a selection of fields and the results are compared to the literature
Follow-up Observations of SPY White Dwarf + M-Dwarf Binaries
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34787.pdf (Publisher’s version ) (Open Access)We present the results of follow-up observations of white-dwarf + M-dwarf binaries identified using spectra obtained as part of the SPY survey. Spectra of the Halpha region were obtained with the SPIRAL spectrograph on the AAT telescope. Of the eleven stars observed, seven are binaries with periods in the range 2.8 hours to 7.7 days. We also show that one of our targets, WD 0137-349, has a brown dwarf companion.15th European Workshop on White Dwarf
HS 0146+1847 – a DAZB white dwarf of very unusual composition
No full textHS 0146+1847, originally identified as a white dwarf candidate in
proper motion surveys, was rediscovered as a candidate in the
Hamburg Quasar Survey. Spectra obtained for the SPY (ESO
Supernova Ia Progenitor) survey show strong Balmer and Ca II lines,
suggesting a classification as DAZ white dwarf. Contrary to the
objects known so far in this class, HS 0146+1847 has a helium-rich
atmosphere at = 11 500 K. This is confirmed by very weak He
lines, changing the classification to DAZB. Mg and Fe lines are
also detected. We discuss the physics of Balmer line broadening by
neutral helium, present a spectral analysis and note some
implications for the accretion/diffusion scenario of heavy elements
in cool white dwarfs
Recent Observational Progress in AM CVn Binaries
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36392.pdf ( ) (Open Access)We present the results of some recent research on AM CVn systems. We present: X-ray/UV observations made using XMM--Newton; the X-ray grating spectrum of RX J1914+24; preliminary results of a search for radio emission from AM CVn binaries, and discuss the strategy and first results of the RATS project, whose main aim is to discover AM CVn systems.European Workshop on White Dwarf
Binary White Dwarfs in the Supernova Ia Progenitor Survey
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35071.pdf (author's version ) (Open Access)We report results of a systematic radial velocity survey for double degenerate (DD) binaries as potential progenitors of type Ia supernovae: SPY (ESO Supernovae Ia Progenitor surveY). More than 1000 white dwarfs (WDs) and pre-white dwarfs were observed with the VLT. The aim of SPY is to perform a statistically significant test of the DD scenario. We give an overview of the project and follow-up results. The sample is used for two tests of the contemporary picture of close binary evolution leading to DDs. It is shown that the SPY observations of low mass He-core WDs (HeWDs) are inconsistent with the idea that they all reside in close binary systems. We carry out a model atmosphere analysis of WDs residing in double-lined binaries and estimate individual cooling ages. For a number of systems the younger (and hotter) component is the less massive one, as expected. However, for some systems the mass ratio is inverted
Hot subdwarfs from the ESO supernova Ia progenitor survey
No full textAims.
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.
Methods.We analyse high-resolution
(), high-quality optical spectra of sdO stars obtained with the ESO
VLT UVES echelle spectrograph in the course of the ESO Supernova Ia Progenitor
Survey (SPY). Effective temperatures, surface gravities, and photospheric
helium abundances are determined simultaneously by fitting the profiles of
hydrogen and helium lines using dedicated synthetic spectra calculated from an
extensive grid of NLTE model atmospheres.
Results.
We find spectroscopic or photometric evidence for cool companions to
eight sdO stars, as well as a binary consisting of two sdO stars.
A clear correlation between helium abundances and the presence of
carbon and/or nitrogen lines is found: below solar helium abundance, no sdO star
shows C or N lines. In contrast, C and/or N lines are present in the
spectra of all sdO stars with supersolar helium abundance. We
thus use the solar helium abundance to divide our sample into
helium-deficient and helium-enriched sdO stars.
While helium-deficient sdO stars are scattered in a wide range of the
–log(g)-diagram, most of the
helium-enriched sdO stars cluster in a narrow region at
temperatures between 40 000 and 50 000 K and gravities between
and 6.0.
Conclusions.
An evolutionary link between sdB stars and sdO stars appears plausible only for
the helium-deficient sdO stars. They probably have evolved away from the extreme
horizontal branch; i.e., they are the likely successors to sdB stars.
In contrast, the atmospheric properties
of helium-enriched sdO stars cannot be explained with canonical
single-star evolutionary models. Alternative scenarios for
both single-star (late hot flasher) and binary evolution
(white-dwarf merger; post-RGB evolution) fail to
reproduce the observed properties of helium-enriched sdO stars in detail.
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
helium-enriched sdO stars
High-resolution extreme ultraviolet spectroscopy of G191-B2B: structure of the stellar photosphere and the surrounding interstellar medium
No full textWe have continued our detailed analysis of the high-resolution (R= 4000) spectroscopic observation of the DA white dwarf G191-B2B, obtained by the Joint Astrophysical Plasmadynamic Experiment (J-PEX) normal incidence sounding rocket-borne telescope, comparing the observed data with theoretical predictions for both homogeneous and stratified atmosphere structures. We find that the former models give the best agreement over the narrow waveband covered by J-PEX, in conflict with what is expected from previous studies of the lower resolution but broader wavelength coverage Extreme Ultraviolet Explorer spectra. We discuss the possible limitations of the atomic data and our understanding of the stellar atmospheres that might give rise to this inconsistency. In our earlier study, we obtained an unusually high ionization fraction for the ionized He ii present along the line of sight to the star. In the present paper, we obtain a better fit when we assume, as suggested by Space Telescope Imaging Spectrograph results, that this He ii resides in two separate components. When one of these is assigned to the local interstellar cloud, the implied He ionization fraction is consistent with measurements along other lines of sight. However, the resolving power and signal-to-noise available from the instrument configuration used in this first successful J-PEX flight are not sufficient to clearly identify and prove the existence of the two components
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Two new ZZ Ceti pulsators from the HS and HE surveys
No full textWe report the detection of nonradial g-mode oscillations in the DA white
dwarfs HS 1039+4112 () and HE 1429-0343 () from time-series
photometry made at the Loiano 1.5 m telescope.
The two stars were previously selected as probable pulsators based on
two-color photometry and spectral analysis respectively.
Following our temperature and surface gravity determinations, HS 1039+4112
(Teff K, \mbox{\log g} =8.2) is located near the red edge of the
ZZ Ceti instability strip, whereas HE 1429-0343 (Teff K,
\mbox{\log g}=7.8) falls in the middle of the strip.
Both stars show a multi-mode behavior with the main periods at about 850
and 970 s respectively, and relatively large amplitudes (~7% and ~2.5%)
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