arXiv subjects

P. Godon

Publications and source records attributed to P. Godon.

26 records · Page 2Linked to original sources

arXiv2008

A Far Ultraviolet Archival Study of Cataclysmic Variables: I. FUSE and HST/STIS Spectra of the Exposed White Dwarf in Dwarf Nova Systems

We present a synthetic spectral analysis of Far Ultraviolet Spectroscopic Explorer (FUSE) and Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) spectra of 5 dwarf novae above and below the period gap during quiescence. We use our synthetic spectral code, including options for the treatment of the hydrogen quasi-molecular satellite lines (for low temperature stellar atmospheres), NLTE approximation (for high temperature stellar atmospheres), and for one system (RU Peg) we model the interstellar medium (ISM) molecular and atomic hydrogen lines. In all the systems presented here the FUV flux continuum is due to the WD. These spectra also exhibit some broad emission lines. In this work we confirm some of the previous FUV analysis results but we also present new results. For 4 systems we combine the FUSE and STIS spectra to cover a larger wavelength range and to improve the spectral fit. This work is part of our broader HST archival research program, in which we aim to provide accurate system parameters for cataclysmic variables above and below the period gap by combining FUSE and HST FUV spectra.

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arXiv2008

Hubble Space Telescope STIS Spectroscopy of Long Period Dwarf Novae in Quiescence

We present the results of a synthetic spectral analysis of HST STIS spectra of five long period dwarf novae obtained during their quiescence to determine the properties of their white dwarfs which are little known for systems above the CV period gap. The five systems, TU Men, BD Pav, SS Aur, TT Crt, and V442 Cen were observed as part of an HST Snapshot project. The spectra are described and fitted with combinations of white dwarf photospheres and accretion disks. We provide evidence that the white dwarfs in all five systems are at least partially exposed. We discuss the evolutionary implications of our model fitting results and compare these dwarf novae to previously analyzed FUV spectra of other dwarf novae above the period gap. The dispersion in CV WD temperatures above the period gap is substantially greater than one finds below the period gap where there is a surprisingly narrow dispersion in temperatures around 15,000K. There appears to be a larger spread of surface temperatures in dwarf novae above the period than is seen below the gap.

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arXiv2007

A Far Ultraviolet Study of the Nova-like V794 Aquilae

We present a spectral analysis of the dereddened FUSE and HST/STIS spectra separately and combined together assuming E(B-V)=0.1 & 0.2. Overall, we find that the model fits are in much better agreement with the dereddened spectra when E(B-V) is large, as excess emission in the longer wavelengths render the slope of the observed spectra almost impossible to fit, unless E(B-V)=0.2 . The best fit accretion disk model is obtained for E(B-V)=0.2 . A single white dwarf model leads to a rather hot temperature (30,000K < Twd < 55,000K depending on the assumptions) but does not provide a fit as good as the accretion disk model. A combination of a white dwarf plus a disk does not lead to a better fit. The same best fit disk model is consistently obtained when fitting the FUSE and HST/STIS spectra individually and when combined together, implying therefore that the disk model is the best fit not only in the least chi2 sense, but also as a consistent solution across a large wavelength span of observation. This is not the case with the single white dwarf model fitting which leads to a different (and therefore inconsistent) temperature for each different spectrum FUSE, STIS and FUSE+STIS.

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arXiv2006

HST/STIS Spectroscopy and Modeling of the Long Term Cooling of WZ Sagittae following the July 2001 Outburst

We present the last HST/STIS E140M FUV spectrum (1150-1725A) of the dwarf nova (DN) WZ Sge, obtained in July 2004, 3 years following the early superoutburst of July 2001. Single white dwarf (WD) synthetic spectral fits (log{g}=8.5) to the data indicate that the WD has a temperature T~15,000K, about ~1500K above its quiescent temperature and it is still showing the effect of the outburst. Taking into account temperature estimates of the earlier phase of the cooling, we model the cooling curve of WZ Sge, over a period of 3 years, using a stellar evolution code including accretion and the effects of compressional heating. Assuming that compressional heating alone is the source of the energy released during the cooling phase, we find that (1) the mass of the white dwarf must be quite large (~1.0 Msun); and (2) the mass accretion rate must have a time-averaged (over 52 days of outburst) value of the order of 1.E-8 Msun/yr or larger. The outburst mass accretion rate derived from these compressional heating models is larger than the rates estimated from optical observations and from a FUV spectral fit by up to one order of magnitude. This implies that during the cooling phase the energy released by the WD is not due to compressional heating alone.

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arXiv2006

A Far Ultraviolet Study of the Hot White Dwarf in the Dwarf Nova WW Ceti

We present a synthetic spectral analysis of IUE archival and FUSE FUV spectra of the peculiar dwarf nova WW Ceti. During the quiescence of WW Ceti, a white dwarf with Twd=26,000K can account for the FUV flux and yields the proper distance. However, the best agreement with the observations is provided by a two-temperature white dwarf model with a cooler white dwarf at Twd=25,000K providing 75% of the FUV flux and a hotter region (accretion belt or optically thick disk ring) with T=40,000K contributing 25% of the flux for the proper distance. We find from the FUSE spectrum that the white dwarf is rotating with a projected rotational velocity V sin{i} = 600 km/s. Our temperature results provide an additional data point in the distribution of Twd versus orbital period above the CV period gap where few Twds are available.

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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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