arXiv subjects
Publications and source records attributed to B. T. Gaensicke.
We have taken near-infrared (JHK) spectroscopic data of a sample of pre-CVs, in order to examine them for signatures of nuclear evolution. Here we present preliminary results for the three systems BPM 71214, RR Cae, and RE 1016-053.
We present an analysis of HST STIS data of VW Hyi acquired ~ 14 days after a superoutburst. At the time of the observation the system appears to be going into outburst with the longest wavelengths increasing in flux by a factor of 5, while the shortest wavelengths increase by only a factor of 2. Using the distance of 65 pc, a system inclination angle of 60 degrees and a white dwarf mass of 0.86 solar mass, we carried out model fits involving a white dwarf by itself, an optically thisck accretion disk by itself, a composite model using an optically thick accretion disk and a white dwarf, a two-temperature white dwarf model with a cooler more slowly rotating photosphere and a hotter, rapidly rotating accretion belt and a composite model involving a white dwarf and a rapidly rotating cooler disk ring heated up to a ``low'' temperature of \~13-14,000K. This component of temperature stays fairly constant throughout the HST observations while the area of the disk ring increases by a factor of 12. We see evidence of a delay in the UV emission consistent with the outburst beginning outside of a disk truncation radius.
We report the detection of Zeeman-split Lalpha absorption pi and sigma+ lines in the far-ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph spectrum of the magnetic cataclysmic variable RX J1554.2+2721. Fitting the STIS data with magnetic white dwarf model spectra, we derive a field strength of B~144MG and an effective temperature of 17000K<Teff<23000K. This measurement makes RX J1554.2+2721 only the third cataclysmic variable containing a white dwarf with a field exceeding 100MG. Similar to the other high-field polar AR UMa, RX J1554.2+2721 is often found in a state of feeble mass transfer, which suggests that a considerable number of high-field polars may still remain undiscovered.
We explore the origin of FUSE and HST STIS far UV spectra of the dwarf nova, EY Cyg, during its quiescence using \emph{combined} high gravity photosphere and accretion disk models as well as model accretion belts. The best-fitting single temperature white dwarf model to the FUSE plus HST STIS spectrum of EY Cygni has T$_{eff} = 24,000$K, log $g = 9.0$, with an Si abundance of 0.1 x solar and C abundance of 0.2 x solar but the distance is only 301 pc. The best-fitting composite model consists of white dwarf with T$_{eff} = 22,000$K, log $g = 9$, plus an accretion belt with T$_{belt} = 36,000$K covering 27% of the white dwarf surface with V$_{belt} sin i = 2000$ km/s. The accretion belt contributes 63% of the FUV light and the cooler white dwarf latitudes contribute 37%. This fit yields a distance of 351 pc which is within 100 pc of our adopted distance of 450 pc. EY Cyg has very weak C {\sc iv} emission and very strong N {\sc v} emission, which is atypical of the majority of dwarf novae in quiescence. We also conducted a morphological study of the surroundings of EY Cyg using direct imaging in narrow nebular filters from ground-based telescopes. We report the possible detection of nebular material^M associated with EY Cygni. Possible origins of the apparently large N {\scv}/C {\sc iv} emission ratio are discussed in the context of nova explosions, contamination of the secondary star and accretion of nova abundance-enriched matter back to the white dwarf via the accretion disk or as a descendant of a precursor binary that survived thermal timescale mass transfer. The scenario involving pollution of the secondary by past novae may be supported by the possible presence of a nova remnant-like nebula around EY Cyg.
We present time-resolved optical spectroscopy and photometry, and far-ultraviolet spectroscopy of HS 0728+6738, a cataclysmic variable discovered in the Hamburg Quasar Survey. We show that the system is a new eclipsing member of the SW Sex class of CVs with an orbital period of 3.21 hours. We derive an orbital inclination of ~85 +- 4 degrees from the average eclipse profile, making HS 0728+6738 the highest inclination SW Sex star known. The optical and far-ultraviolet emission lines are not or only weakly occulted during the eclipse, indicating the presence of line-emission sites either far outside the Roche lobe of the primary or, more likely, above the orbital plane of the binary. The photometric light curves exhibit fast variability with a period of \~7 min, which might be related to the spin of the white dwarf.
We report on the analysis of new X-ray data obtained with XMM-Newton and Chandra from two ROSAT-discovered X-ray dim isolated neutron stars. RX J0806.4-4123 was observed with XMM-Newton in April 2003, 2.5 years after the first observation. The EPIC-pn data confirm that this object is an X-ray pulsar with 11.371 s neutron star spin period. The X-ray spectrum is consistent with absorbed black-body emission with a temperature kT = 96 eV and NH = 4x10^19 cm^-2 without significant changes between the two observations. Four XMM-Newton observations of RX J0420.0-5022 between December 2002 and July 2003 did not confirm the 22.7 s pulsations originally indicated in ROSAT data, but clearly reveal a 3.453 s period. A fit to the X-ray spectrum using an absorbed black-body model yields kT = 45 eV, the lowest value found from the small group of XDINs and NH = 1.0x10^20 cm^-2. Including a broad absorption line improves the quality of the spectral fits considerably for both objects and may indicate the presence of absorption features similar to those reported from RBS1223, RX J1605.3+3249 and RX J0720.4-3125. For both targets we derive accurate X-ray positions from the Chandra data and present an optical counterpart candidate for RX J0420.0-5022 with B = 26.6+/-0.3 mag from VLT imaging.
Using the Hubble space Telescope Fine Guidance Sensor, we have measured the trigonometric parallax of the bright cataclysmic variable 1223 Sgr. The absolute parallax is pi(abs)= 1.96+-0.18mas, making V1223 Sgr the most distant CV with a well-determined trigonometric parallax. This distance, a Lutz-Kelker correction, and the previously measured extinction yield an absolute visual high-state magnitude Mv=4.0+-0.2. We outline a model, which is consistent with the observed spin-down of the white dwarf and provides for much of the UV/optical emission by reverberation of X-rays. From previous X-ray and UV/optical data, we derive an accretion luminosity Lacc=(2.6+-0.8)e34 erg/s, a white dwarf mass M_1=0.93+-0.12Msun, and an accretion rate dot M=(1.4+-0.3)e17 g/s.
We report the discovery of one unique cataclysmic variable drawn from the Hamburg Quasar Survey, HS2331+3905. Follow-up observations obtained over three years unveiled a very unusual picture. The large amplitude 3.5 h radial velocity variations obtained from our optical spectroscopy is not the orbital period of the system, as one would normally expect. Instead, extensive CCD photometry strongly suggests that HS2331+3905 is a short orbital period cataclysmic variable with Porb = 81.09 min, containing a cold white dwarf which appears to exhibit ZZ Ceti pulsations.
We report follow-up observations of a new white dwarf/red dwarf binary HS2237+8154, identified as a blue variable star from the Hamburg Quasar Survey. Ellipsoidal modulation observed in the R band as well as the Halpha radial velocity variations measured from time-resolved spectroscopy determine the orbital period to be Porb=178.10+-0.08min. The optical spectrum of HS2237+8154 is well described by a combination of a Teff=11500+-1500K white dwarf (assuming log g=8) and a dM3.5+-0.5 secondary star. The distance implied from the flux scaling factors of both stellar components is d=105+-25pc. Combining the constraints obtained from the radial velocity of the secondary and from the ellipsoidal modulation, we derive a binary inclination of i~50deg-70deg and stellar masses of Mwd=0.47-0.67Msun and Msec=0.2-0.4Msun. All observations imply that the secondary star must be nearly Roche-lobe filling. Consequently, HS2237+8154 may be either a pre-cataclysmic variable close to the start of mass transfer, or (considering its orbital period) a cataclysmic variable that terminated mass transfer and entered the period gap, or a hibernating nova.
We present the first time-resolved spectroscopic study of the cataclysmic variable DW Cancri. We have determined an orbital period of 86.10 +- 0.05 min, which places the system very close to the observed minimum period of hydrogen-rich cataclysmic variables. This invalidates previous speculations of DW Cnc being either a permanent superhumper below the period minimum or a nova-like variable with an orbital period longer than 3 hours showing quasi-periodic oscillations. The Balmer and HeI lines have double-peaked profiles and exhibit an intense S-wave component moving with the orbital period. Remarkably, the Balmer and HeI radial velocity curves are modulated at two periods: 86.10 +- 0.05 min (orbital) and 38.58 +- 0.02 min. The same short period is found in the equivalent width variations of the single-peaked HeII 4686 line. We also present time-resolved photometry of the system which shows a highly-coherent variation at 38.51 min, consistent with the short spectroscopic period. The large number of similarities with the short-period intermediate polar V1025 Cen lead us to suggest that DW Cnc is another intermediate polar below the period gap, and we tentatively identify the photometric and spectroscopic 38-min signals with the white dwarf spin period. DW Cnc has never been observed to undergo an outburst, but it occasionally exhibits low states \~2 mag fainter than its typical brightness level of V~14.5, resembling the behaviour of the high mass-transfer VY Scl stars.
We report on the progress of the development of CVcat, an interactive catalogue on Cataclysmic Variables, which is the first application based on AstroCat, a general framework for the installation and maintenance of web-based interactive astronomical databases. Registered users can contribute directly to the catalogue content by adding new objects, object properties, literature references, and annotations. The scientific quality control of the catalogue is carried out by a distributed editorial team. Searches in CVcat can be performed by object name, classification, certain properties or property ranges, and coordinates. Search results can be retrieved in several output formats, including XML. Old database states can be restored in order to ensure the citability of the catalogue. Furthermore, CVcat is designed to serve as a repository for reduced data from publications. Future prospects include the integration of AstroCat-based catalogues in the international network of Virtual Observatories.
FUSE and HST/STIS spectra of the dwarf nova WZ Sge, obtained during and following the early superoutburst of July 2001 over a time span of 20 months, monitor changes in the components of the system during its different phases. The synthetic spectral fits to the data indicate a cooling in response to the outburst of about 12,000K, from about 28,000K down to about 16,000K. The cooling time scale tau (of the white dwarf temperature excess) is of the order of about 100 days in the early phase of the cooling period, and increases to about 850 days toward the end of the second year following the outburst. In the present work, we numerically model the accretional heating and subsequent cooling of the accreting white dwarf in WZ Sge. The best compressional heating model fit is obtained for a 1.2 Msun white dwarf accreting at a rate of 9.E-9 Msun/yr for 52 days. However, if one assumes a lower mass accretion rate or a lower white dwarf mass, then compressional heating alone cannot account for the observed temperature decline, and other sources of heating have to be included to increase the temperature of the model to the observed value. We quantitatively check the effect of boundary layer irradiation as such an additional source.
Using the HST Fine Guidance Sensor, we have measured a high precision astrometric parallax of the cataclysmic variable EX Hydrae, pi=15.50+-0.29mas. From the wavelength-integrated accretion-induced energy flux, we derive a quiescent accretion luminosity for EX Hya of Lacc = (2.6+-0.6)x10e32 erg. The quiescent accretion rate then is Mdot=(6.2\+-1.5)x10e-11 (M1/0.5Msun)^(-1.61})Msun/yr. The time-averaged accretion rate, which includes a small correction for the rare outbursts, is 6% higher. We discuss the system parameters of EX Hya and deduce M1=0.4-0.7Msun, M2=0.07-0.10Msun, and i=76.0deg-77.6deg, using recent radial velocity measurements of both components and restrictions imposed by other observational and theoretical constraints. We conclude that the secondary is undermassive, overluminous, and expanded over a ZAMS star of the same mass. Near the upper limit to M1, the accretion rate of the white dwarf coincides with that due to near-equilibrium angular momentum loss by gravitational radiation and angular momentum transfer from the orbit into the spin-up of the white dwarf. Near the lower mass limit, the correspondingly higher accretion rate requires that either an additional angular momentum loss process is acting besides gravitational radiation or that accretion occurs on a near-adiabatic time scale. The latter possibility would imply that EX Hya is in a transient phase of high mass transfer and the associated spin-up of the white dwarf.
We present a sample of 30 well observed Post Common Envelope Binaries (PCEBs). Deriving the cooling age of the white dwarfs, we show that the PCEB population is dominated by young systems. Having calculated the orbital evolution of the systems under the assumption of two different prescriptions for the angular momentum loss, we find that most of the systems have not yet completed a significant fraction of their PCEB life time. We therefore predict the existence of a large population of old PCEBs containing cold white dwarfs (T_eff<15000 K). Our calculations show that nearly half of the PCEBs in our sample will evolve into a semi-detached configuration and start mass transfer in less than the Hubble-time. These systems are thus representative for progenitors of the current CV population. Only one of them (V471 Tau) will evolve into a long-period (Porb>4h) CV, and a significant fraction of the systems will start mass transfer in the period gap. Having estimated the distances of the PCEBs in the sample, we derive a space density of ~6-30x1e-6 [pc-3], depending on the assumed angular momentum loss prescription. Taking into account the evolutionary time scales we compute a lower limit for the CV space density, predicted by the currently known PCEB population of ~10 x 1e-5 [pc-3]. Finally, we discuss possible observational selection effects and conclude that the observed PCEB population is probably highly incomplete.
We report the identification of the cataclysmic variable 1RXSJ062518.2+733433 as an intermediate polar. The orbital period of the system is determined to be 283.0+-2 min from the radial velocity variation of Halpha, measured in an extensive set of time-resolved spectroscopy. Differential optical photometry obtained over a base line of three weeks reveals the presence of coherent variability with a period of 19.788+-0.002 min, which we suggest to be the white dwarf spin period. The power spectrum of our photometry also contains a strong signal near the spectroscopically determined orbital period. The emission lines in 1RXSJ062518.2+733433 display a complex multicomponent structure. In the trailed spectrogram of HeI 6678 we detect a narrow component with a radial velocity semi-amplitude of ~140kms, consistent with a possible origin on the irradiated face of the secondary. The absence of eclipses gives an upper limit on the binary inclination of i<=60deg.
CVcat is a database that contains published data on cataclysmic variables and related objects. Unlike in the existing online sources, the users are allowed to add data to the catalogue. The concept of an ``open catalogue'' approach is reviewed together with the experience from one year of public usage of CVcat. New concepts to be included in the upcoming AstroCat framework and the next CVcat implementation are presented. CVcat can be found at http://www.cvcat.org.
We have developed a new method to derive the magnetic field distribution on the surfaces of rotating magnetic white dwarfs from phase-resolved flux and circular polarization spectra. An optimization code based on an evolutionary strategy is used to fit synthetic Zeeman spectra for a variety of model geometries described in the framework of a truncated multipole expansion. We demonstrate that the code allows the reconstruction of relatively complex fields using noise-added synthetic input spectra. As a first application, we analyze flux and circular polarization spectra of the polar EF Eri in a low state of accretion taken with FORS1 at the ESO VLT.
We report on the first far-UV observations of the asynchronous polar BY Cam made by the Far-Ultraviolet Spectroscopic Explorer (FUSE). The source is known to exhibit the most extreme NV/CIV emission resonance line ratio observed among polars. The FUSE observations reveal a OVI resonance line weaker than in the prototype of polars, AM Her, with the absence of a detectable narrow component. The OVI broad line is detected with an equivalent width of the same order as in AM Her, the blueward doublet component is clearly present but the redward component is strongly affected by H2 absorption. The presence of a strong NIII line and weak CIII lines also confirms the peculiar CNO line flux. We compare the resonance CNO line intensities with the predictions of the CLOUDY plasma code coupled to a geometrical model of the accretion column. Varying the temperature and/or intensity of the ionising spectrum is unable to reproduce the observed broad line ratios. A solution is obtained by significantly altering the element abundances with a strong depletion of C, overabundance of N and a weak underabundance of O. This confirms previous suggestions of non-solar abundances which may result from redistribution in the accreted material following nova outbursts and/or the secondary nuclear evolution. A very significant H2 absorption is observed in front of the source, a possible indication for either the existence of a dense interstellar cloud or of circumstellar material.