Current Research

Hubble Space Telescope spectroscopy of the Balmer lines in Sirius B
High-resolution extreme ultraviolet spectroscopy of G191-B2B – II.
Heavy element abundances in DAO white dwarfs measured from FUSE data

Comparison of the effective temperatures, gravities and helium abundances of DAO white dwarfs from Balmer and Lyman line studies.

A search for binarity using FUSE observations of DAO white dwarfs
Heavy-element abundance patterns in hot DA white dwarfs
Praesepe and the seven white dwarfs
A near-IR spectroscopic search for very-low-mass cool companions to notable DA white dwarfs.
A near-IR spectrum of the DO white dwarf RE J0503-285

Photospheric phosphorus in the FUSE spectra of GD71 and two similar DA white dwarfs

Hubble Space Telescope spectroscopy of the Balmer lines in Sirius B

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High-resolution extreme ultraviolet spectroscopy of G191-B2B – II.

This is a continuation of our detailed analysis of the high-resolution ( R = 4000) spectroscopic observation of the DA white dwarf G191-B2B, obtained by the first successful flight of the Joint Astrophysical Plasmadynamic Experiment (J-PEX) normal incidence sounding rocket-borne telescope. We have compared the data obtained with theoretical predictions for both homogenised and stratified atmosphere structures. In our earlier study, we obtained an unusually high ionization fraction for the ionized He II present along the line of site to the star. A better fit was obtained in this study by assuming, as suggested by STIS results, that this He II resides in two separate components, one in the interstellar cloud.

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Heavy element abundances in DAO white dwarfs measured from FUSE data

DAO white dwarfs are a group of white dwarfs that are observed to have both hydrogen and helium lines in their optical spectra. We present heavy element abundance measurements for 16 DAO white dwarfs, determined from Far-Ultraviolet Spectroscopic Explorer (FUSE) spectra, which all show evidence of absorption by heavy elements. We compared these abundances with values of temperature, gravity and helium abundance measured separately from optical and FUSE data by Good et al. (2004). It was found that the carbon abundance measurements from optical data form a trend with the abundance measurements for DA white dwarfs of Barstow et al. (2003) increasing abundance with temperature, although no trend was found in the FUSE measurements. Silicon abundances were also found to increase with temperature, but no similar trends were found in the abundances of nitrogen, oxygen, iron or nickel, and no dependence on gravity or helium abundances was observed.

The abundances measured when the optical and FUSE- derived values of temperature, gravity and helium abundance were adopted showed no systematic differences, and satisfactory of absorption line fits were achieved almost equally for both, although the models derived from the FUSE measurements seemed better at reproducing the strength of the heavy element absorption lines.

Comparison of the effective temperatures, gravities and helium abundances of DAO white dwarfs from Balmer and Lyman line studies.

Observing the hydrogen Balmer absorption series is a well-established method of determining the surface gravity and effective temperature of white dwarfs. In situations where the Balmer lines cannot be used (e.g. for objects in close binary systems where the white dwarf cannot be spatially resolved) it is possible to use the hydrogen Lyman series instead, but it is important that the results they give should be consistent if we are to rely on either set of measurements. Previous studies by Barstow et al. (2001) and Barstow et al. (2003) have shown that this is the case for DA white dwarfs that are cooler than ~ 50 000 K, but at higher temperatures the Lyman lines systematically yield higher values. Analysis of optical and FUSE spectra of 16 such white dwarfs show that the data sets continue to diverge at even higher temperatures, but the discrepancies decrease at lower gravities. Three white dwarfs have T eff derived from the Lyman line analysis in excess of 120 000 K. However, the helium abundances show good agreement even when the temperatures differ considerably, apart from for the two DAO+dM binaries.

Systematic differences between the Lyman and Balmer gravity measurements were not found. The agreement is generally better at higher gravity (log g >7.7).

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A search for binarity using FUSE observations of DAO white dwarfs

Spectra recorded by Far Ultraviolet Spectroscopic Explorer (FUSE) are built up from a number of exposures. Changes in the position of photospheric heavy element absorption lines between exposures, with respect to the stationary interstellar medium lines, would indicate binarity. We report on the use of this technique to search for binarity in DAO white dwarfs. The technique was found to be successful in picking out all 5 known binaries from the sample of 16, and significant radial velocity shifts were found in one additional star, Ton320, which is also known to have an infrared excess. Three quarters of the relatively low mass white dwarfs in the sample displayed no evidence of binarity.

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Heavy-element abundance patterns in hot DA white dwarfs

We present a series of systematic abundance measurements for 25 hot DA white dwarfs in the temperature range ~20 000–110 000K, based on far-ultraviolet spectroscopy with the Space Telescope Imaging Spectrograph (STIS)/Goddard High Resolution Spectrograph (GHRS) on-board Hubble Space Telescope, IUE and FUSE . Using our latest heavy-element blanketed non-local thermodynamic equilibrium (non-LTE) stellar atmosphere calculations we have addressed the heavy-element abundance patterns, making completely objective measurements of abundance values and upper limits using a c 2 fitting technique to determine the uncertainties in the abundance measurements. These can be related to the formal upper limits in those stars where particular elements are not detected.

The presence or absence of heavy elements in the hot DA white dwarfs largely reflects what would be expected if radiative levitation is the supporting mechanism, although as reported by past authors the measured abundances do not match the predicted values very well. Almost all stars hotter than ~50 000 K contain heavy elements, with a narrow spread of abundances similar to those measured in G191-B2B. However, there is an unexplained dichotomy at lower temperatures with some stars having apparently pure H envelopes and others having detectable quantities of heavy elements, in which the heavy elements are often stratified and lying in the outermost layers of the envelope.

The abundance of Si decreases with temperature, the N abundance pattern splits into two groups at lower temperatures and below ~50 000 K the Fe and Ni abundances sharply decline to zero. When detected, the Fe and Ni abundances maintain an approximately constant ratio, close to the cosmic value of ~20. The strongest influence on abundance for the hottest white dwarfs appears to be gravity.

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Praesepe and the seven white dwarfs

White dwarfs are noticeably absent from open clusters, but we have discovered two new white dwarfs in the Praesepe cluster on our preliminary survey. They are WD0837+218 and WD0837+185. These are likely to be the degenerates of stars with masses between 2.6 and 8 solar masses and 2.4 and 3.5 solar masses respectively. Their cooling ages are approx. 280 and 500 million years respectively.

Open clusters provide useful material for the study of initial mass-final mass relationships, as members of such clusters share a common age, metal content and distance. Five white dwarfs have already been found in this particular cluster.

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A near-IR spectroscopic search for very-low-mass cool companions to notable DA white dwarfs.

There are many known examples of white dwarfs in binary systems with early-mid M dwarf companions, but few with late-M and cooler companions. We have undertaken a detailed near-IR spectroscopic analysis of eight notable white dwarfs, predominantly of southern declination. In each case the spectrum failed to reveal compelling evidence for the presence of a spatially unresolved, cool, late-type companion. Therefore, we have placed an approximate limit on the spectral-type of a putative companion to each degenerate. From these limits we conclude that if GD659, GD50, GD71 or WD2359-434 possesses an unresolved companion then most probably it is substellar in nature (less than 0.72 solar masses). Also, any late type-companion to RE J0457-280, RE J0623-374, RE J0723-274 or RE J2214-491 most likely has a mass less than 0.82 solar masses. If the unusual photospheric composition observed in these white dwarfs is due to weak accretion from a nearby late-type companion, the companion is very low-mass and cannot be detected by this study. These results do not contradict the previously noted deficit of very-low-mass stellar and brown dwarf companions to main sequence, F, G, K and early M-type primaries.

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A near-IR spectrum of the DO white dwarf RE J0503-285

The intriguing DO white dwarf RE J0503-285 has discrepancies between the observed photospheric abundances and model predictions. It is relatively metal-rich, and Nickel has been detected in the photosphere despite the absence of iron. Given that the cosmic abundance ratio of iron to nickel is 18:1, this is unusual. This report uses a near-IR spectroscopic analysis of RE J0503-285 to test the theory that these discrepancies may be due to accretion from a spatially-unresolved, cool, late-type companion. The spectrum failed to reveal evidence of such a companion, but assuming that one such does exist we can place an approximate limit on its spectral type and mass (later than M8, M<0.085M solar ). This result rules out ongoing interaction between the white dwarf and a close companion with M>0.085M solar as responsible for the discrepancies between the observed photospheric abundances and model predictions.

Near-IR spectroscopy (solid grey line) and 2MASS JHK photometry (filled circles) of the white dwarf RE J0503-285. A He+C+H non-LTE synthetic white dwarf spectrum of appropriate effective temperature, surface gravity and normalisation (solid black line) and a hybrid white dwarf + late-type dwarf model representing our estimated limit on the spectral type of a putative spatially unresolved companion (dotted black line, WD+M8) are overplotted. We have labelled the more prominent white dwarf HeII lines and telluric water vapour features present in this dataset.

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Photospheric phosphorus in the FUSE spectra of GD71 and two similar DA white dwarfs

Using FUSE data we detected traces of phosphorus in the atmospheres of GD71 and two similar DA white dwarfs. This is the first detection of a trace metal in the photosphere of the spectrophotometric standard star GD71. These three objects represent the coolest DA white dwarfs in which photospheric phosphorus has been observed. We used a grid of homogeneous non-local thermodynamic equilibrium synthetic spectra to measure abundances of [P/H]= and in GD71, RE J1918+595 and RE J0605-482 respectively. Phosphorus at this level has no significant impact on the overall energy distribution of GD71.

Although accretion of wind material from a cool companion can lead to heavy element abundances in white dwarf stars, it seems unlikely that this is the source of the phosphorus observed in these stars, as no evidence for radial velocity variations has been found for GD71 (Maxted et al., 2000). However, equilibrium radiative levitation calculations indicate that phosphorus should be supported in the photospheres of typical DA white dwarfs to T eff ~ 32 500 K, and our measured values are consistent with these theoretical values. It is also possible that gravitational settling could be the cause, or both together, their effects possibly modified by weak mass loss.

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