arXiv subjects

M. R. Burleigh

Publications and source records attributed to M. R. Burleigh.

At least 91 records · Page 5Linked to original sources

arXiv2005

A search for binarity using FUSE observations of DAO white dwarfs

We report on a search for evidence of binarity in Far-Ultraviolet Spectroscopic Explorer (FUSE) observations of DAO white dwarfs. Spectra recorded by FUSE are built up from a number of separate exposures. Observation of changes in the position of photospheric heavy element absorption lines between exposures, with respect to the stationary interstellar medium lines, would reveal radial velocity changes - evidence of the presence of a binary system. This technique is successful in picking out all the white dwarfs already known to be binaries, which comprise 5 out of the sample of 16, but significant radial velocity shifts were found for only one additional star, Ton 320. This object is also known to have an infrared excess. DAOs can be separated broadly into low or normal mass objects. Low mass white dwarfs can be formed as a result of binary evolution, but it has been suggested that the lower mass DAOs evolve as single stars from the extended horizontal branch, and we find no evidence of binarity for 8 out of the 12 white dwarfs with relatively low mass. The existence of higher mass DAOs can also be explained if they are within binary systems, but of the four higher mass stars in the sample studied, PG 1210+533 and LB 2 do not exhibit significant radial velocity shifts, although there were only two exposures for the former object and the latter has an infrared excess.

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arXiv2005

Heavy element abundances in DAO white dwarfs measured from FUSE data

We present heavy element abundance measurements for 16 DAO white dwarfs, determined from Far-Ultraviolet Spectroscopic Explorer {FUSE} spectra. Evidence of absorption by heavy elements was found in the spectra of all the objects. Measurements were made using models that adopted the temperatures, gravities and helium abundances determined from both optical and FUSE data by Good et al. (2004). Despite the different evolutionary paths that the types of DAO white dwarfs are thought to evolve through, their abundances were not found to vary significantly, apart from for the silicon abundances. Abundances measured when the FUSE derived values of temperature, gravity and helium abundance were adopted were, in general, a factor 1-10 higher than those determined when the optical measure of those parameters was used. Satisfactory fits to the absorption lines were achieved in approximately equal number. The models that used the FUSE determined parameters seemed better at reproducing the strength of the nitrogen and iron lines, while for oxygen, the optical parameters were better.

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arXiv2005

Hubble Space Telescope Spectroscopy of the Balmer lines in Sirius B

Sirius B is the nearest and brightest of all white dwarfs, but it is very difficult to observe at visible wavelengths due to the overwhelming scattered light contribution from Sirius A. However, from space we can take advantage of the superb spatial resolution of the Hubble Space Telescope to resolve the A and B components. Since the closest approach in 1993, the separation between the two stars has become increasingly favourable and we have recently been able to obtain a spectrum of the complete Balmer line series for Sirius B using HST?s Space Telescope Imaging Spectrograph (STIS). The quality of the STIS spectra greatly exceed that of previous ground-based spectra, and can be used to provide an important determination of the stellar temperature (Teff = 25193K) and gravity (log g = 8.556). In addition we have obtained a new, more accurate, gravitational red-shift of 80.42 +/- 4.83 km s-1 for Sirius B. Combining these results with the photometric data and the Hipparcos parallax we obtain new determinations of the stellar mass for comparison with the theoretical mass-radius relation. However, there are some disparities between the results obtained independently from log g and the gravitational redshift which may arise from flux losses in the narrow 50x0.2arcsec slit. Combining our measurements of Teff and log g with the Wood (1995) evolutionary mass-radius relation we get a best estimate for the white dwarf mass of 0.978 M. Within the overall uncertainties, this is in agreement with a mass of 1.02 M obtained by matching our new gravitational red-shift to the theoretical M/R relation.

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arXiv2004

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

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 (M<0.072Msun). Furthermore, any spatially unresolved late-type companion to RE J0457-280, RE J0623-374, RE J0723-274 or RE J2214-491 most likely has M<0.082Msun. These results imply that if weak accretion from a nearby late-type companion is the cause of the unusual photospheric composition observed in a number of these degenerates then the companions are of very low mass, beyond the detection thresholds of this study. Furthermore, these results do not contradict a previously noted deficit of very-low-mass stellar and brown dwarf companions to main sequence F,G,K and early-M type primaries (a<1000AU).

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arXiv2004

Rotational period of WD1953-011 - a magnetic white dwarf with a star spot

WD1953-011 is an isolated, cool (7920 +/- 200K, Bergeron, Legget & Ruiz, 2001) magnetic white dwarf (MWD) with a low average field strength (~70kG, Maxted et al. 2000) and a higher than average mass (~0.74 M_sun, Bergeron et al. 2001). Spectroscopic observations taken by Maxted et al. (2000) showed variations of equivalent width in the Balmer lines, unusual in a low field white dwarf. Here we present V band photometry of WD1953-011 taken at 7 epochs over a total of 22 months. All of the datasets show a sinusoidal variation of approximately 2% peak-to-peak amplitude. We propose that these variations are due to a star spot on the MWD, analogous to a sunspot, which is affecting the temperature at the surface, and therefore its photometric magnitude. The variations have a best-fit period over the entire 22 months of 1.4418 days, which we interpret as the rotational period of the WD.

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arXiv2004

Praesepe and the seven white dwarfs

We report the discovery, from our preliminary survey of the Praesepe open cluster, of two new spectroscopically confirmed white dwarf candidate members. We derive the effective temperatures and surface gravities of WD0837+218 and WD0837+185 (LB5959) to be 17845 +555 -565K and log g = 8.48 +0.07 -0.08 and 14170 +1380 -1590K and log g =8.46 +0.15 -0.16 respectively. Using theoretical evolutionary tracks we estimate the masses and cooling ages of these white dwarfs to be 0.92+/-0.05Msun and 280 +40 -30Myrs and 0.90+/-0.10Msun and 500 +170 -100Myrs respectively. Adopting reasonable values for the cluster age we infer the progenitors of WD0837+218 and WD0837+185 had masses of 2.6 < M < Mcrit Msun and 2.4 < M < 3.5Msun respectively, where Mcrit is the maximum mass of a white dwarf progenitor. We briefly discuss these findings in the context of the observed deficit of white dwarfs in open clusters and the initial mass-final-mass relationship.

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arXiv2003

Photometric variability of the unique magnetic white dwarf GD356

GD356 is a magnetic white dwarf (B = 13MG) that uniquely displays weak resolved Zeeman triplets of Halpha and Hbeta in emission. As such, GD356 may be the only known white dwarf with some kind of chromosphere, although accretion from the interstellar medium or more exotic mechanisms cannot be ruled out. Here, we report the detection of low amplitude (+/-~0.2%) near-sinusoidal photometric (V-band) variability in GD356, with a period of 0.0803 days (~115 minutes). We interpret this as the rotation period of the star. We model the variability with a dark spot (by analogy with star spots) covering 10% of the stellar surface. It seems likely that this spot is also the site of the Zeeman emission, requiring the presence of a temperature inversion. We show that the spot is never totally visible or obscured, and that both polar and equatorial spots produce good fits to the data when viewed at high and low inclination respectively.

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arXiv2003

A comparison of DA white dwarf temperatures and gravities from FUSE Lyman line and ground-based Balmer line observations

Observation of the strengths and profiles of the hydrogen Balmer absorption series is an established technique for determining the effective temperature and surface gravity of hot H-rich white dwarf stars. In principle, the Lyman series lines should be equally useful but, lying in the far-UV, are only accessible from space. Nevertheless, there are situations (for example, where the optical white dwarf spectrum is highly contaminated by the presence of a companion) in which use of the Lyman series may be essential. Therefore, it is important to establish whether or not the Lyman lines provide an equally valid means of measurement. We have already made a first attempt to study this problem, comparing Lyman line measurements from a variety of far-UV instruments with ground-based Balmer line studies. Within the measurement uncertainties we found the results from each line series to be broadly in agreement. However, we noted a number of potential systematic effects that could bias either measurement. With the availability of the FUSE data archive and observations from our own Guest Observer programmes, we now have an opportunity of examining the use of the Lyman series in more detail from observations of 16 DA white dwarfs. The new results partially reproduce the earlier study, showing that Balmer and Lyman line determined temperatures are in good agreement up to \~50000K. However, above this value there is an increasing systematic difference between the Lyman and Balmer line results, the former yielding the higher temperature.

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arXiv2003

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 ~20000-110000K, based on far-UV spectroscopy with STIS/GHRS on HST, IUE and FUSE. Using our latest heavy element blanketed 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 2 fitting technique to determine the uncertainties in the abundance measurements, which can be related to the formal upper limits in those stars where particular elements are not detected. We find that 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 the measured abundances do not match the predicted values very well, as reported by other authors in the past. Almost all stars hotter than ~50000K contain heavy elements. For most of these the spread in element abundances is quite narrow and similar to the abundances measured in G191-B2B. However, there is an unexplained dichotomy at lower temperatures with some stars having apparently pure H envelopes and others detectable quantities of heavy elements. The heavy elements present in these cooler stars are often stratified, lying in the outermost layers of the envelope. A few strong temperature/evolutionary effects are seen in the abundance measurements. There is a decreasing Si abundance with temperature, the N abundance pattern splits into two groups at lower temperature and there is a sharp decline in Fe and Ni abundance to zero, below ~50000K. When detected, the Fe and Ni abundances maintain an approximately constant ratio, close to the cosmic value ~20.

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arXiv2002

PG1115+166 - a long period DA+DB binary

We present spectra of the DAB white dwarf PG1115+166. Radial velocity measurements of the Balmer lines and the HeI6678 line show that this is a binary white dwarf with a period of 30.09d in which the Balmer lines move in anti-phase to the HeI line, i.e., PG1115+166 is a DA+DB binary. The minimum masses of the stars are M_DA=0.43+-0.15 solar masses and M_DB=0.52+-0.12 solar masses. The separation of the stars is about 45 solar radii which is much smaller than a typical AGB progenitor of a white dwarf, implying that there has been at least one common envelope phase in this binary. Indeed, it is possible that this binary may have suffered up to three mass transfer episodes - two associated with the red giant phase prior to the formation of each white dwarf and a third associated with the ``born-again'' red giant phase of the DB white dwarf. PG1115+166 has the longest orbital period of any post common envelope white dwarf - white dwarf binary found to date. Published models for the formation of white dwarf - white dwarf binaries do not predict any white dwarfs with the combination of long orbital period and high mass found in PG1115+166. We conclude that PG1115+166 is a key object for testing models of binary star evolution and it may also be a key object for our understanding of the formation of DB white dwarfs. We outline the observational tests which can be applied to scenarios for the formation of PG1115+166 and apply them to the simplest case of a single common envelope phase. This suggests that some part of the internal energy stored in the envelope of the AGB star, e.g., as ionized hydrogen, may have contributed to the ejection of the common envelope, but there are several unanswered questions concerning this simple scenario.

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arXiv2002

Imaging planets around nearby white dwarfs

We suggest that Jovian planets will survive the late stages of stellar evolution, and that white dwarfs will retain planetary systems in wide orbits (>5AU). Utilising evolutionary models for Jovian planets, we show that infra-red imaging with 8m class telescopes of suitable nearby white dwarfs should allow us to resolve and detect companions >3Mjup. Detection of massive planetary companions to nearby white dwarfs would prove that such objects can survive the final stages of stellar evolution, place constraints on the frequency of main sequence stars with planetary systems dynamically similar to our own and allow direct spectroscopic investigation of their composition and structure.

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arXiv2001

The low mass white dwarf companion to Beta Crateris

We present FUSE H Lyman series spectroscopy of the hot white dwarf companion to the 4th magnitude A1III star Beta Crateris, which shows that is has an unusually low mass, 0.43Msun, and has almost certainly evolved through binary interaction. This system could be a long-sought remnant of Algol-type evolution, although radial velocity measurements appear to show that the pair are not close. Instead, micro-variations in Beta Crt's proper motion as measured by Hipparcos suggests that the period could be as high as 10 years. However, a low mass white dwarf in a system with a period >3 years is difficult to explain by conventional models for binary evolution. We speculate on alternative models for the evolution of this system which involve an eccentric binary or multiple components.

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arXiv2001

A ROSAT WFC observation of SW UMa: the EUV behaviour of dwarf novae in superoutburst explained

During re-processing and analysis of the entire ROSAT Wide Field Camera (WFC) pointed observations database, we discovered a serendipitous, off-axis detection of the cataclysmic variable SW UMa at the onset of its 1997 October superoutburst. Although long outbursts in this SU UMa-type system are known to occur every ~450 days, none had ever been previously observed in the extreme ultra-violet (EUV) by ROSAT. The WFC observations began just ~13 hr after the optical rise was detected. With a peak count rate of 4.5 count/s in the S1 filter, SW UMa was temporarily the third brightest object in the sky in this waveband. Over the next ~19 hr the measured EUV flux dropped to <2 count/s while the optical brightness remained essentially static at V~11. Similar behaviour has also been recently reported in the EUV light curve of the related SU UMa-type binary OY Car during superoutburst (Mauche and Raymond 2000). In contrast, U Gem-type dwarf novae show no such early EUV dip during normal outbursts. Therefore, this feature may be common in superoutbursts of SU UMa-like systems. We expand on ideas first put forward by Osaki (1994, 1995) and Mauche and Raymond (2000) and offer an explanation for this behaviour by examining the interplay between the thermal and tidal instabilities which affect the accretion disks in these systems.

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arXiv2000

Resolving Sirius-like binaries with the Hubble Space Telescope

We present initial results from a Hubble Space Telescope ultraviolet imaging survey of stars known to have hot white-dwarf companions which are unresolved from the ground. The hot companions, discovered through their EUV or UV emission, are hidden by the overwhelming brightnesses of the primary stars at visible wavelengths. Out of 17 targets observed, we have resolved eight of them with the Wide Field Planetary Camera~2, using various ultraviolet filters. Most of the implied orbital periods for the resolved systems are hundreds to thousands of years, but in at least three cases (56 Persei, zeta Cygni, and REJ1925-566) it shouldbe possible to detect the orbital motions within the next few years, and they may eventually yield new dynamically determined masses for the white-dwarf components. The 56 Persei and 14 Aurigae systems are found to be quadruple and quintuple, respectively, including the known optical components as well as the newly resolved white-dwarf companions. The mild barium star zeta Cygni, known to have an 18 year spectroscopic period, is marginally resolved. All of these new resolved Sirius-type binaries will be useful in determining gravitational redshifts and masses of the white-dwarf components.

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arXiv2000

FUSE Observations of the hottest DA white dwarfs

We report early FUSE observations of the very hot DA white dwarfs PG1342+444 and REJ0558-371. Detection of photospheric absorption lines allows us to estimate the abundances of C, O, Si, P, S and Fe, the first measurements reported for DA stars at such high temperatures. Values of Teff and log g determined for PG1342+444 from the Lyman line series disagree with the results of the standard Balmer line analysis, an issue that requires further investigation.

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arXiv2000

Heavy element abundances in DA white dwarfs

We present a series of systematic abundance measurements for a group of hot DA white dwarfs in the temperature range 20,000-75,000K, based on far-UV spectroscopy with STIS on HST, IUE and FUSE. Using our latest heavy element blanketed non-LTE stellar atmosphere calculations we have addressed the heavy element abundance patterns for the hottest stars for the first time, showing that they are similar to objects like G191-B2B. The abundances observed in the cooler (<50,000K) white dwarfs are something of a mystery. Some of the patterns (e.g. REJ1032) can be explained by self-consistent levitation-diffusion calculations but there is then a serious difficulty in understanding the appearance of the apparently pure H atmospheres. We also report the detection of photospheric HeII in the atmosphere of WD2218+706.

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arXiv2000

An Early-Time Infrared and Optical Study of the Type Ia Supernova 1998bu in M96

We present first-season infrared (IR) and optical photometry and spectroscopy of the Type Ia Supernova 1998bu in M96. We also report optical polarimetry of this event. SN1998bu is one of the closest type Ia Supernovae of modern times and the distance of its host galaxy is well-determined. We find that SN1998bu is both photometrically and spectroscopically normal. However, the extinction to this event is unusually high, with Av=1.0 +/-0.11. We find that SN1998bu peaked at an intrinsic Mv=-19.37 +/-0.23. Adopting a distance modulus of 30.25 (Tanvir et al. 1999) and using Phillips et al.'s (1999) relations for the Hubble constant we obtain Ho=70.4 +/-4.3 km/s/Mpc. Combination of our IR photometry with those of Jha et al. (1999) provides one of the most complete early-phase IR light curves for a SN Ia published so far. In particular, SN 1998bu is the first normal SN Ia for which good pre-maximum (in the B band) IR coverage has been obtained. It reveals that the J, H and K light curves peak about 5 days earlier than the flux in the B-band curve.

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arXiv2000

The Schweizer-Middleditch star revisited

We have re-observed and re-analysed the optical spectrum of the Schweizer-Middleditch star, a hot subdwarf which lies along almost the same line-of-sight as the centre of the historic SN1006 supernova remnant (SNR). Although this object is itself unlikely to be the remnant of the star which exploded in 1006AD, Wellstein et al. (1999) have demonstrated that it could be the remnant of the donor star in a pre-supernova Type Ia interacting binary, if it possesses an unusually low mass. We show that, if it had a mass of 0.1-0.2 Msun, the SM star would lie at the same distance (~800pc) as the SNR as estimated by Willingale et al. (1995). However, most distance estimates to SN1006 are much larger than this, and there are other convincing arguments to suggest that the SM star lies behind this SNR. Assuming the canonical subdwarf mass of 0.5 Msun, we constrain the distance of the SM star as 1050 pc<d<2100 pc. This places the upper limit on the distance of SN1006 at 2.1 kpc.

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