Uncertainties in (E)UV model atmosphere fluxes

During the comparison of synthetic spectra calculated with two NLTE model atmosphere codes, namely TMAP and TLUSTY, we encounter systematic differences in the EUV fluxes due to the treatment of level dissolution by pressure ionization. Systematic differences may occur due to a code-specific cutoff frequency of the H I Lyman bound-free opacity. This is the case for TMAP and TLUSTY. Both codes predict the same flux level at wavelengths lower than about 1500 A for stars with effective temperatures below about 30000K only, if the same cutoff frequency is chosen. In the case of Sirius B, we demonstrate an uncertainty in modeling the EUV flux reliably in order to challenge theoreticians to improve the theory of level dissolution. The theory of level dissolution in high-density plasmas, which is available for hydrogen only should be generalized to all species. Especially, the cutoff frequencies for the bound-free opacities should be defined in order to make predictions of UV fluxes more reliable.Comment: 3 pages, 5 figure

Irradiated atmospheres of accreting magnetic white dwarfs with an application to the polar AM Herculis

We present a pilot study of atmospheres of accreting magnetic white dwarfs irradiated by intense fluxes at ultraviolet to infrared wavelengths. The model uses a standard LTE stellar atmosphere code which is expanded by introducing an angle-dependent external radiation source. The present results are obtained for an external source with the spectral shape of a 10 000 K blackbody and a freely adjustable spectral flux. The model provides an explanation for the observed largely filled-up Lyman lines in the prototype polar AM Herculis during its high states. It also confirms the hypotheses (i) that irradiation by cyclotron radiation and other radiation sources is the principle cause for the large heated polar caps surrounding the accretion spots on white dwarfs in polars and (ii) that much of the reprocessed light appears in the far ultraviolet and not in the soft X-ray regime as suggested in the original simple theories. We also briefly discuss the role played by hard X-rays in heating the polar cap

Zeeman tomography of magnetic white dwarfs. IV, The complex field structure of the polars EF Eridani, BL Hydri and CP Tucanae

Context. The magnetic fields of the accreting white dwarfs in magnetic cataclysmic variables (mCVs) determine the accretion geometries, the emission properties, and the secular evolution of these objects. Aims. We determine the structure of the surface magnetic fields of the white dwarf primaries in magnetic CVs using Zeeman tomography. Methods. Our study is based on orbital-phase resolved optical flux and circular polarization spectra of the polars EF Eri, BL Hyi, and CP Tuc obtained with FORS1 at the ESO VLT. An evolutionary algorithm is used to synthesize best fits to these spectra from an extensive database of pre-computed Zeeman spectra. The general approach has been described in previous papers of this series. Results. The results achieved with simple geometries as centered or offset dipoles are not satisfactory. Significantly improved fits are obtained for multipole expansions that are truncated at degree lmax = 3 or 5 and include all tesseral and sectoral components with 0 ≤ m ≤ l. The most frequent field strengths of 13, 18, and 10MG for EF Eri, BL Hyi, and CP Tuc, and the ranges of field strength covered are similar for the dipole and multipole models, but only the latter provide access to accreting matter at the right locations on the white dwarf. The results suggest that the field geometries of the white dwarfs in short-period mCVs are quite complex, with strong contributions from multipoles higher than the dipole in spite of a typical age of the white dwarfs in CVs in excess of 1 Gyr. Conclusions. It is feasible to derive the surface field structure of an accreting white dwarf from phase-resolved low-state circular spectropolarimetry of sufficiently high signal-to-noise ratio. The fact that independent information is available on the strength and direction of the field in the accretion spot from high-state observations helps in unraveling the global field structure

Tunable UV laser photolysis of organometallics with product detection by laser mass spectroscopy: Trimethylaluminum.

We use tunable UV laser light in the region 200–320 nm, produced by frequency doubling the output of a dye laser, for the decomposition of organometallic compounds. This method has been applied to TMA, trimethylaluminum Al(CH3)3. Only the TMA monomer absorbs UV light for λ >220 nm. TMA decomposes by one-photon absorption mainly into two channels: aluminum atoms Al plus organic fragments, and aluminummonomethyl AlCH3 molecules plus organic fragments. The ratio [Al]/[AlCH3] is wavelength dependent. We present a mechanism to explain the photolysis of trimethyl compounds of group III elements (Al, Ga, In)

Spectroscopy of Seven Cataclysmic Variables with Periods Above Five Hours

We present spectroscopy of seven cataclysmic variable stars with orbital periods P(orb) greater than 5 hours, all but one of which are known to be dwarf novae. Using radial velocity measurements we improve on previous orbital period determinations, or derive periods for the first time. The stars and their periods are TT Crt, 0.2683522(5) d; EZ Del, 0.2234(5) d; LL Lyr, 0.249069(4) d; UY Pup, 0.479269(7) d; RY Ser, 0.3009(4) d; CH UMa, 0.3431843(6) d; and SDSS J081321+452809, 0.2890(4) d. For each of the systems we detect the spectrum of the secondary star, estimate its spectral type, and derive a distance based on the surface brightness and Roche lobe constraints. In five systems we also measure the radial velocity curve of the secondary star, estimate orbital inclinations, and where possible estimate distances based on the MV(max) vs.P(orb) relation found by Warner. In concordance with previous studies, we find that all the secondary stars have, to varying degrees, cooler spectral types than would be expected if they were on the main sequence at the measured orbital period.Comment: 25 pages, 2 figures, accepted for Publications of the Astronomical Society of the Pacifi