arXiv subjects

E. M. Sion

Publications and source records attributed to E. M. Sion.

43 records · Page 3Linked to original sources

arXiv2004

Far Ultraviolet Observations of the Dwarf Novae SS Aur and RU Peg in Quiescence

We have analyzed the Far Ultraviolet Spectrocopic Explorer (FUSE) spectra of two U Gem-Type dwarf novae, SS Aur and RU Peg, observed 28 days and 60 days (respectively) after their last outburst. In both systems the FUSE spectra (905 - 1182 A) reveal evidence of the underlying accreting white dwarf exposed in the far UV. Our grid of theoretical models yielded a best-fitting photosphere to the FUSE spectra with Teff=31,000K for SS Aur and Teff=49,000K for RU Peg. This work provides two more dwarf nova systems with known white dwarf temperatures above the period gap where few are known. The absence of CIII (1175 A) absorption in SS Aur and the elevation of N above solar suggests the possibility that SS Aur represents an additional accreting white dwarf where the surface C/N ratio derives from CNO processing. For RU Peg, the modeling uncertainties prevent any reliable conclusions about the surface abundances and rotational velocity.

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arXiv2003

Modeling the Heating and Cooling of WZ Sagittae following the July 2001 Outburst

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.

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arXiv2003

Hubble Space Telescope Observations of UV Oscillations in WZ Sagittae During the Decline from Outburst

We present a time series analysis of Hubble Space Telescope observations of WZ Sge obtained in 2001 September, October, November and December as WZ Sge declined from its 2001 July superoutburst. Previous analysis of these data showed the temperature of the white dwarf decreased from ~29,000 K to ~18,000 K. In this study we binned the spectra over wavelength to yield ultraviolet light curves at each epoch that were then analyzed for the presence of the well-known 27.87 s and 28.96 s oscillations. We detect the 29 s periodicity at all four epochs, but the 28 s periodicity is absent. The origin of these oscillations has been debated since their discovery in the 1970s and competing hypotheses are based on either white dwarf non-radial g-mode pulsations or magnetically-channelled accretion onto a rotating white dwarf. By analogy with the ZZ Ceti stars, we argue that the non-radial g-mode pulsation model demands a strong dependence of pulse period on the white dwarf's temperature. However, these observations show the 29 s oscillation is independent of the white dwarf's temperature. Thus we reject the white dwarf non-radial g-mode pulsation hypothesis as the sole origin of the oscillations. It remains unclear if magnetically-funnelled accretion onto a rapidly rotating white dwarf (or belt on the white dwarf) is responsible for producing the oscillations. We also report the detection of a QPO with period ~18 s in the September light curve. The amplitudes of the 29 s oscillation and the QPO vary erratically on short timescales and are not correlated with the mean system brightness nor with each other.

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arXiv2003

HST/STIS Spectroscopy of the Intermediate Polar EX Hydrae

We present the results of our analysis of six orbits of HST/STIS time-tagged spectroscopy of the intermediate polar, EX Hydrae. The high time and wavelength resolution of the HST/STIS spectra provided an excellent opportunity to study the UV properties of EX Hya. Measurements of the continuum and emission line fluxes corroborate earlier studies that show that the emission line fluxes are modulated more strongly than the continuum flux and originate from the accretion curtains, while the continuum flux originates from the white dwarf photosphere. The measured K amplitude of the narrow emission line radial velocity curve is used to calculate a white dwarf mass of 0.91+/-0.05 M_sun. Synthetic white dwarf photosphere and accretion disk spectral models are used to further refine the white dwarf and accretion disk properties. Using the spectral models, it is determined that EX Hya has a white dwarf of mass 0.9 M_sun, T=23000 K, an accretion disk truncated at 2.5 R_WD, and is at a distance of 60 pc.

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arXiv2001

HST/STIS spectroscopy of the exposed white dwarf in the short-period dwarf nova EK TrA

We present high resolution Hubble Space Telescope ultraviolet spectroscopy of the dwarf nova EK TrA obtained in deep quiescence. The Space Telescope Imaging Spectrograph data reveal the broad Ly-alpha absorption profile typical of a moderately cool white dwarf, overlayed by numerous broad emission lines of He, C, N, and Si and by a number of narrow absorption lines, mainly of CI and SiII. Assuming a white dwarf mass in the range 0.3-1.4Msun we derive Teff=17500-23400K for the primary in EK TrA; Teff=18800K for a canonical mass of 0.6Msun. From the narrow photospheric absorption lines, we measure the white dwarf rotational velocity, v*sin i=200+-100kms. Even though the strong contamination of the photospheric white dwarf absorption spectrum by the emission lines prevents a detailed quantitative analysis of the chemical abundances of the atmosphere, the available data suggest slightly sub-solar abundances. The high time resolution of the STIS data allows us to associate the observed ultraviolet flickering with the emission lines, possibly originating in a hot optically thin corona above the cold accretion disk.

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arXiv2001

A Determination of the Local Density of White Dwarf Stars

The most recent version of the Catalog of Spectroscopically Identified White Dwarfs lists 2249 white dwarf stars. Among these stars are 118 white dwarfs that have either reliable trigonometric parallaxes or color-based distance moduli which place them at a distance within 20 pc of the Sun. Most of these nearby white dwarfs are isolated stars, but 35 (30 % of the sample) are in binary systems, including such well known systems as Sirius A/B, and Procyon A/B. There are also three double degenerate systems in this sample of the local white dwarf population. The sample of local white dwarfs is largely complete out to 13 pc and the local density of white dwarf stars is found to be (5.5 +/- 0.8) x 10^-3 pc^-3 with a corresponding mass density of (3.7 +/- 0.5) x10^-3 M pc^-3.

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arXiv1998

HST/GHRS observations of AM Herculis

We present phase-resolved spectroscopy of AM Herculis obtained with the HST/GHRS when the system was in a high state. The ultraviolet light curve shows a quasi-sinusoidal modulation, which can be explained by a hot spot on the rotating white dwarf. The broad Lalpha absorption expected for photospheric radiation of a moderately hot white dwarf is largely filled in with emission. The UV/FUV spectrum of AM Her in high state can be quantitatively understood by a two-component model consisting of the unheated white dwarf plus a blackbody-like radiating hot spot. A kinematic study of the strong UV emission lines using Doppler tomography is presented. The characteristics of the low ionization species lines and the SiIV doublet can be explained within the classical picture, as broad emission from the accretion stream and narrow emission from the heated hemisphere of the secondary. However, we find that the narrow emission of the NV doublet originates from material of low velocity dispersion located somewhere between L_1 and the centre of mass. The high signal-to-noise spectra contain a multitude of interstellar absorption lines but no metal absorption lines from the white dwarf photosphere.

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