arXiv subjects

Boris Gaensicke

Publications and source records attributed to Boris Gaensicke.

31 records · Page 2Linked to original sources

arXiv2014

Variable Emission from a Gaseous Disc around a Metal-Polluted White Dwarf

We present the discovery of strongly variable emission lines from a gaseous disc around the DA white dwarf SDSS J1617+1620, a star previously found to have an infrared excess indicative of a dusty debris disc formed by the tidal disruption of a rocky planetary body. Time-series spectroscopy obtained during the period 2006-2014 has shown the appearance of strong double-peaked Ca II emission lines in 2008. The lines were weak, at best, during earlier observations, and monotonically faded through the remainder of our monitoring. Our observations represent unambiguous evidence for short-term variability in the debris environment of evolved planetary systems. Possible explanations for this extraordinary variability include the impact onto the dusty disc of either a single small rocky planetesimal, or of material from a highly eccentric debris tail. The increase in flux from the emission lines is sufficient that similar events could be detected in the broadband photometry of ongoing and future large-area time domain surveys.

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arXiv2014

A magnetic accretion switch in pre-cataclysmic binaries?

We have investigated the mass accretion rate implied by published surface abundances of Si and C in the white dwarf component of the 3.62 hr period pre-cataclysmic binary and planet host candidate QS Vir (DA+M2-4). Diffusion timescales for gravitational settling imply $\dot{M} \sim 10^{-16}M_\odot$ yr$^{-1}$ for the 1999 epoch of the observations, which is three orders of magnitude lower than measured from a 2006 {\it XMM-Newton} observation. This is the first time that large accretion rate variations have been seen in a detached pre-CV. A third body in a 14 yr eccentric orbit suggested in a recent eclipse timing study is too distant to perturb the central binary sufficiently to influence accretion. A hypothetical coronal mass ejection just prior to the {\it XMM-Newton} observation might explain the higher accretion rate, but the implied size and frequency of such events appear too great. We suggest accretion is most likely modulated by a magnetic cycle on the secondary acting as a wind "accretion switch", a mechanism that can be tested by X-ray and ultraviolet monitoring. If so, QS Vir and similar pre-CVs could provide powerful insights into hitherto inscrutable cataclysmic variable and M dwarf magnetospheres, and mass and angular momentum loss rates.

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arXiv2013

Building galaxies, stars, planets and the ingredients for life between the stars. A scientific proposal for a European Ultraviolet-Visible Observatory (EUVO)

The growth of luminous structures and the building blocks of life in the Universe began as primordial gas was processed in stars and mixed at galactic scales. The mechanisms responsible for this development are not well understood and have changed over the intervening 13 billion years. To follow the evolution of matter over cosmic time, it is necessary to study the strongest (resonance) transitions of the most abundant species in the Universe. Most of them are in the ultraviolet (UV; 950A-3000A) spectral range that is unobservable from the ground. A versatile space observatory with UV sensitivity a factor of 50-100 greater than existing facilities will revolutionize our understanding of the Universe. Habitable planets grow in protostellar discs under ultraviolet irradiation, a by-product of the star-disk interaction that drives the physical and chemical evolution of discs and young planetary systems. The electronic transitions of the most abundant molecules are pumped by the UV field, providing unique diagnostics of the planet-forming environment that cannot be accessed from the ground. Earth's atmosphere is in constant interaction with the interplanetary medium and the solar UV radiation field. A 50-100 times improvement in sensitivity would enable the observation of the key atmospheric ingredients of Earth-like exoplanets (carbon, oxygen, ozone), provide crucial input for models of biologically active worlds outside the solar system, and provide the phenomenological baseline to understand the Earth atmosphere in context. In this white paper, we outline the key science that such a facility would make possible and outline the instrumentation to be implemented.

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arXiv2012

Hot subluminous Stars: Highlights from the MUCHFUSS and Kepler missions

Research into hot subdwarf stars is progressing rapidly. We present recent important discoveries. First we review the knowledge about magnetic fields in hot subdwarfs and highlight the first detection of a highly-magnetic, helium-rich sdO star. We briefly summarize recent discoveries based on Kepler light curves and finally introduce the closest known sdB+WD binary discovered by the MUCHFUSS project and discuss its relevance as progenitor of a double-detonation type Ia supernova.

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arXiv2012

A Spitzer Space Telescope Study of the Debris Disks around four SDSS White Dwarfs

We present Spitzer Space Telescope data of four isolated white dwarfs that were previously known to harbor circumstellar gaseous disks. IRAC photometry shows a significant infrared excess in all of the systems, SDSS0738+1835, SDSS0845+2257, SDSS1043+0855 and SDSS1617+1620, indicative of a dusty extension to those disks. The 4.5-micron excesses seen in SDSS0738, SDSS0845, and SDSS1617 are 7.5, 5.7 and 4.5 times the white dwarf contribution, respectively. In contrast, in SDSS1043, the measured flux density at 4.5 microns is only 1.7 times the white dwarf contribution. We compare the measured IR excesses in the systems to models of geometrically thin, optically thick disks, and find that we are able to match the measured SEDs to within 3 sigma of the uncertainties, although disks with unfeasibly hot inner dust temperatures generally provide a better fit than those below the dust sublimation temperature. Possible explanations for the dearth of dust around SDSS1043+0855 are briefly discussed. Including our previous study of SDSS1228+1040, all five white dwarfs with gaseous debris disks have significant amounts of dust around them. It is evident that gas and dust can coexist around these relatively warm, relatively young white dwarfs.

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arXiv2010

The cool end of the DZ sequence in the SDSS

We report the discovery of cool DZ white dwarfs, which lie in the SDSS (u-g) vs. (g-r) two-color diagram across and below the main sequence. These stars represent the extension of the well-known DZ sequence towards cooler temperatures.

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arXiv2009

The HYPERMUCHFUSS Campaign -- an undiscovered high velocity population

We present an overview and a status report of HYPERMUCHFUSS (HYPER velocity or Massive Unseen Companions of Hot Faint Underluminious Stars Survey) aiming at the detection of a population of high velocity subluminous B stars and white dwarfs. The first class of targets consists of hot subdwarf binaries with massive compact companions, which are expected to show huge radial velocity variations. The second class is formed by the recently discovered hyper-velocity stars, which are moving so fast that the dynamical ejection by a supermassive black hole seems to be the only explanation for their origin. Until now only one old hyper-velocity star has been found, but we expect a larger population. We applied an efficient selection technique for hot subdwarfs and white dwarfs with high galactic restframe velocities from the \emph{SDSS} spectral data base, which serve as first epoch observations for our campaign with the ESO VLT and NTT in Chile, the 3.5 m telescope at DSAZ observatory (Calar Alto) in Spain and the WHT on La Palma. The survey is nearing completion and provides us with promising candidates which will be followed up to measure their RV-curves to uncover massive companions or prove their nature as HVS.

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arXiv2008

SDSS J162718.39+120435.0 - a dwarf nova in the period gap

SDSS J162718.39+120435.0 was suspected of being a dwarf nova from spectroscopic observations made available by the Sloan Digital Sky Survey. Photometry conducted during the 2008 outburst, the first ever outburst of this object detected and followed up in real time, shows that the outburst reached magnitude 14.6 at maximum, had an amplitude of 4.6 magnitudes and lasted for at least 18 days. Common superhumps were detected with an amplitude of up to 0.4 magnitudes, confirming it to be a member of the SU UMa family. Initially the superhump period, Psh, was 0.10993(7) days (2.638 hours), but it subsequently reduced to Psh = 0.10890(9) days (2.614 hours) later in the outburst. The period change corresponds to the end of the plateau period of the outburst. The orbital period, Porb, was estimated from the two Psh values as between 0.1026 d and 0.1016 d, which places SDSS1627 near the centre of the period gap in the orbital period distribution of cataclysmic variables.

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arXiv2004

Observational studies of Cataclysmic Variable evolution: Of samples, biases and surprises

I present brief status reports on three large observational projects that are designed to test our current understanding of the evolution of cataclysmic variables (CVs): The spectroscopic selection of new CVs in the Hamburg Quasar Survey, the search for pre-CVs based on Sloan colours and UK Schmidt/6dF multiobject spectroscopy, and the identification of CVs that descended from supersoft X-ray binaries using a HST/STIS far-ultraviolet spectroscopic survey.

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arXiv2003

WZ Sge: FUSE Spectroscopy of the 2001 Outburst

The short-period cataclysmic variable WZ Sge underwent its first dwarf nova outburst in 23 years in 2001 July. Here we describe 905-1187 Ang. spectra obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) during the outburst and within the first few months after the outburst. The first spectrum, obtained about 7 days after the outburst began and 6.4 days past optical maximum, is dominated by a broad (50-60 Ang. EW) feature due to O VI and can be characterized in terms of emission from a highly inclined disk modulated by the effects of a highly ionized wind. WZ Sge is fainter in later observations (starting about 45 days after optical maximum) and increasingly "red" in the far ultraviolet. The spectral shape increasingly resembles that expected from a metal-enriched white dwarf photosphere, although it seems likely that some disk contribution remains at least through the second FUSE observation, when the visible light curve showed the system to be undergoing a rebrightening event. The absorption lines observed on 29 September and in early November, when the visible light curve was in a fairly steady decline toward quiescence, are narrow, with widths (FWHM) of order 0.7-1.3 Ang. (200-400 km/s). The most natural explanation for these spectra is that the continuum arises from a relatively massive, slowly rotating white dwarf, and that the lines are created by a combination of metals in the atmosphere and of absorbing material along the line of sight to the white dwarf. If the white dwarf has log{g}= 8.5, then the white dwarf cooled from ~25,000 K on 29 September to 23,000 K in early November.

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arXiv2003

Far-UV FUSE spectra of peculiar magnetic cataclysmic variables

We present far-UV spectra of the three magnetic cataclysmic variables (MCVs) BY Cam, V1309 Ori and AE Aqr obtained with the FUSE satellite. These MCVs have revealed strongly unusual NV and CIV UV resonance lines. The FUSE spectra exhibit broad OVI lines as well as a strong NIII line at 991A, while the CIII 1175A line is nearly absent, supporting non-solar CNO abundances of the accreting matter in these sources. The spectrum of BY Cam shows molecular H2 lines which might be of circumstellar nature. The flaring activity of AE Aqr is also observed in the far-UV range. The radial velocities of the broad OVI components in AE Aqr are orbitally modulated and would indicate an emission region close to the magnetosphere.

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arXiv2001

The Intriguing New Cataclysmic Variable KUV03580+0614

Photometric and spectroscopic observations of the candidate cataclysmic variable KUV03580+0614, independently found from the Hamburg survey as HS0357+0614, reveal this to be a cataclysmic variable with an orbital period of 3.4 hrs. CVs with periods in this range are usually novalikes belonging to the SW Sex or Intermediate Polar categories. The variability, spectroscopic characteristics and the lack of strong X-rays indicate a system similar to V795 Her, LS Peg and AH Men.

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arXiv1997

Anomalous cooling of the massive white dwarf in U Geminorum following a narrow dwarf nova outburst

We obtained Hubble GHRS medium resolution (G160M grating) phase-resolved spectroscopic observations of the prototype dwarf nova U Geminorum during dwarf nova quiescence, 13 days and 61 days following the end of a narrow outburst. The spectral wavelength ranges were centered upon three different line regions: N V (1238Å, 1242Å), Si III (1300Å) and He II (1640Å). All of the quiescent spectra at both epochs are dominated by absorption lines and show no emission features. The Si III and He II absorption line velocities versus orbital phase trace the orbital motion of the white dwarf but the N~V absorption velocities appear to deviate from the white dwarf motion. We confirm our previously reported low white dwarf rotational velocity, V sin i= 100 km/s. We obtain a white dwarf orbital velocity semi-amplitude K1=107 km/s. Using the gamma-velocity of Wade (1981) we obtain an Einstein redshift of 80.4 km/s and hence a carbon core white dwarf mass of ~1.1 Msun. We report the first subsolar chemical abundances of C and Si for U Gem with C down by 0.05 with respect to the Sun, almost certainly a result of C depletion due to thermonuclear processing. This C-depletion is discussed within the framework of a weak TNR, contamination of the secondary during the common envelope phase, and mixing of C-depleted white dwarf gas with C-depleted matter deposited during a dwarf nova event. Remarkably the Teff of the white dwarf 13 days after outburst is only 32,000K, anomalously cooler than previous early post-outburst measurements. Extensive cooling during an extraordinarily long (210 days) quiescence followed by accretion onto an out-of-equilibrium cooled degenerate could explain the lower Teff.

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