arXiv subjects

E. M. Sion

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

At least 37 records · Page 2Linked to original sources

arXiv2011

FUSE Observational Evidence of the Boundary Layer of MY Lyrae in the High State

We carry out a spectral analysis of the archival FUSE spectrum of the VY Scl nova-like cataclysmic variable MV Lyrae obtained in the high state. We find that standard disk models fail to fit the flux in the shorter wavelengths of FUSE (< 950$A). An improved fit is obtained by including a modeling of the boundary layer at the inner edge of the disk. The result of the modeling shows that in the high state the disk has a moderate accretion rate of about 2.E09 solar mass per year, a low inclination, a boundary layer with a temperature of around 100,000K, and size 0.20Rwd, and the white dwarf is possibly heated up to a temperature of 50,000K or higher.

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arXiv2010

Multi-site Observations of Pulsation in the Accreting White Dwarf SDSS J161033.64-010223.3 (V386 Ser)

Nonradial pulsations in the primary white dwarfs of cataclysmic variables can now potentially allow us to explore the stellar interior of these accretors using stellar seismology. In this context, we conducted a multi-site campaign on the accreting pulsator SDSS J161033.64-010223.3 (V386 Ser) using seven observatories located around the world in May 2007 over a duration of 11 days. We report the best fit periodicities here, which were also previously observed in 2004, suggesting their underlying stability. Although we did not uncover a sufficient number of independent pulsation modes for a unique seismological fit, our campaign revealed that the dominant pulsation mode at 609s is an evenly spaced triplet. The even nature of the triplet is suggestive of rotational splitting, implying an enigmatic rotation period of about 4.8 days. There are two viable alternatives assuming the triplet is real: either the period of 4.8 days is representative of the rotation period of the entire star with implications for the angular momentum evolution of these systems, or it is perhaps an indication of differential rotation with a fast rotating exterior and slow rotation deeper in the star. Investigating the possibility that a changing period could mimic a triplet suggests that this scenario is improbable, but not impossible. Using time-series spectra acquired in May 2009, we determine the orbital period of SDSS J161033.64-010223.3 to be 83.8 +/- 2.9 min. Three of the observed photometric frequencies from our May 2007 campaign appear to be linear combinations of the 609s pulsation mode with the first harmonic of the orbital period at 41.5min. This is the first discovery of a linear combination between nonradial pulsation and orbital motion for a variable white dwarf.

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arXiv2009

A Far Ultraviolet Spectroscopic Explorer Survey of High Declination Dwarf Novae

We present an analysis of the FUSE spectra of eight high-declination dwarf novae obtained from a Cycle 7 FUSE survey. These DN systems have not been previously studied in the UV and little is known about their white dwarfs (WDs) or accretion disks. We carry out the spectral analysis of the FUSE data using synthetic spectra generated with the codes TLUSTY and SYNSPEC. For two faint objects (AQ Men, V433 Ara) we can only assess a lower limit for the WD temperature or mass accretion rate. NSV 10934 was caught in a quiescent state and its spectrum is consistent with a low mass accretion rate disk. For 5 objects (HP Nor, DT Aps, AM Cas, FO Per and ES Dra) we obtain WD temperatures between 34,000K and 40,000K and/or mass accretion rates consistent with intermediate to outburst states. These temperatures reflect the heating of the WD due to on-going accretion and are similar to the temperatures of other DNs observed on the rise to, and in decline from outburst. The WD Temperatures we obtain should therefore be considered as upper limits, and it is likely that during quiescence AM Cas, FO Per and ES Dra are near the average WD Teff for catalcysmic variables above the period gap (30,000K), similar to U Gem, SS Aur and RX And.

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arXiv2009

The White Dwarf in EM Cygni: Beyond The Veil

We present a spectral analysis of the FUSE spectra of EM Cygni, a Z Cam DN system. The FUSE spectrum, obtained in quiescence, consists of 4 individual exposures (orbits): two exposures, at orbital phases phi ~ 0.65 and phi ~ 0.90, have a lower flux; and two exposures, at orbital phases phi =0.15 and 0.45, have a relatively higher flux. The change of flux level as a function of the orbital phase is consistent with the stream material (flowing over and below the disk from the hot spot region to smaller radii) partially masking the white dwarf. We carry out a spectral analysis of the FUSE data, obtained at phase 0.45 (when the flux is maximual, using the codes TLUSTY and SYNSPEC. Using a single white dwarf spectral component, we obtain a white dwarf temperature of 40,000K, rotating at 100km/s. The white dwarf, or conceivably, the material overflowing the disk rim, shows suprasolar abundances of silicon, sulphur and possibly nitrogen. Using a white dwarf+disk composite model, we obtain that the white dwarf temperature could be even as high as 50,000K, contributing more than 90% of the FUV flux, and the disk contributing less than 10% must have a mass accretion rate reaching 1.E-10 Msun/yr.In both cases, however, we obtain that the white dwarf temperature is much higher than previously estimated.

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arXiv2008

Far Ultraviolet Spectroscopic Explorer Spectroscopy of the Nova-like BB Doradus

We present an analysis of the Far Ultraviolet Spectroscopic Explorer ({\it{FUSE}}) spectra of the little-known southern nova-like BB Doradus. The spectrum was obtained as part of our Cycle 8 {\it FUSE} survey of high declination nova-like stars. The FUSE spectrum of BB Dor, observed in a high state, is modeled with an accretion disk with a very low inclination (possibly lower than 10deg). Assuming an average WD mass of 0.8 solar leads to a mass accretion rate of 1.E-9 Solar mass/year and a distance of the order of 650 pc, consistent with the extremely low galactic reddening in its direction. The spectrum presents some broad and deep silicon and sulfur absorption lines, indicating that these elements are over-abundant by 3 and 20 times solar, respectively.

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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

The First Direct Spectroscopic Detection of a White Dwarf Primary in an AM CVn System

We report the results of a synthetic spectral analysis of Hubble STIS spectra of the AM CVn-type cataclysmic variable CP Eri obtained when the system was in quiescence. The FUV spectrum is best fitted by a helium-dominated, hybrid composition (DBAZ) white dwarf with Teff=17,000K +/- 1000K, log g=8, He abundance ~1000 times solar, H abundance ~0.1 times solar, metallicity Z ~ 0.05 times solar, V sin i = 400 km/s. This is the first directly detected primary white dwarf in any AM CVn and the surface abundance and rotation rate for the white dwarf primary are the first to be reported for AM CVn systems. The model-predicted distance is ~ 1000 pc. The spectral fits using pure He photospheres or He-rich accretion disks were significantly less successful. Based upon the analysis of our FUV spectra, CP Eri appears to contain a hybrid composition DBAZ white dwarf with a metallicity which sets it apart from the other two AM Cvn stars which have been observed in quiescence and are metal-poor. The implications of this analysis for evolutionary channels leading to AM CVn systems are discussed.

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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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arXiv2005

The accreting white dwarfs in BW Scl, BC UMa and SW UMa

We have observed the short-period dwarf novae BW Scl, BC UMa and SW UMa using the Hubble Space Telescope/Space Telescope Imaging Spectrograph. In all three systems, the white dwarf is the dominant source of far-ultraviolet flux, even though in BC UMa and SW UMa an additional continuum component contributes ~10% and ~20% of the 1400A flux, respectively. Fitting the data with detailed white dwarf model spectra, we determine the effective temperatures to be 14800+-900K (BW Scl), 15200+-1000K (BC UMa), and 13900+-900K (SW UMa). The additional continuum component in BC UMa and SW UMa is equally well described by either a blackbody or a power law, which could be associated with emission from the hot spot or from an optically thin accretion disk (or an optically thin layer on top of a colder optically thick disk), respectively. Modelling the narrow metal lines detected in the STIS spectra results in sub-solar abundances of carbon, oxygen and silicon for all three systems, and also suggests substantial supra-solar abundances of aluminium. The narrow absorption line profiles imply low white dwarf rotation rates, v sin(i)<=300km/s for the three white dwarfs. SW UMa is the only system that shows significant short-term variability in the far-ultraviolet range, which is primarily associated with the observed emission lines.

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arXiv2004

Far-ultraviolet Spectroscopy of Magnetic Cataclysmic Variables

We have obtained HST/STIS data for a total of eleven polars as part of a program aimed to compile a homogeneous database of high-quality far-ultraviolet (FUV) spectra for a large number of cataclysmic variables (CVs). Of the eleven polars, eight were found in a state of low accretion activity (V347Pav, VVPup, V834Cen, BLHyi, MRSer, STLMi, RXJ1554.2+2721 and V895Cen) and three in a state of high activity (CDInd, ANUMa and UWPic). The STIS spectra of the low-state polars unambiguously reveal the photospheric emission of their white dwarf (WD) primaries. We have used pure hydrogen WD models to fit the FUV spectra of the low-state systems (except RX J1554.2+2721, which is a high-field polar) in order to measure the WD effective temperatures. In all cases, the fits could be improved by adding a second component, which is presumably due to residual accretion onto the magnetic pole of the WD. The WD temperatures obtained range from 10800K to 14200K for log g = 8.0. Comparing the WD temperatures of polars to those of non-magnetic CVs, we find that at any given orbital period the WDs in polars are colder than those in non-magnetic CVs. The temperatures of polars below the period gap are consistent with gravitational radiation as the only active angular momentum loss mechanism. The differences in WD effective temperatures between polars and non-magnetic CVs are significantly larger above the period gap, suggesting that magnetic braking in polars might be reduced by the strong field of the primary.

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arXiv2004

An HST STIS Observation of VW Hydri at the Exact FUV Onset of an Outburst

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.

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arXiv2004

Dwarf Novae with Newly Determined Parallaxes: Model Analyses of VY Aquari, RU Pegasi, and T Leonis

Using newly determined parallaxes for dwarf novae, we have derived outburst accretion rates for VY Aqr, RU Peg and T Leo and for T Leo during quiescence. The two short-period dwarf novae, VY Aqr and T Leo, show good agreement with optically thick steady-state accretion disks in outburst, whereas RU Peg shows a significant departure from a steady-state disk. We have determined that the white dwarf in T Leo has T$_{eff} = 16,000 \pm 1000$ K, a value consistent with long term compressional heating when gravitational wave emission drives mass transfer. The white dwarf in T Leo has a temperature in the same narrow range as other WZ Sge-like dwarf novae.

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arXiv2004

Synthetic Spectral Analysis of the Hot Component in the S-Type Symbiotic Variable EG Andromeda

We have applied grids of NLTE high gravity model atmospheres and optically thick accretion disk models for the first time to archival IUE and FUSE spectra of the S-type symbiotic variable EG And taken at superior spectroscopic conjunction when Rayleigh scattering should be minimal and the hot component is viewed in front of the red giant. For EG And's widely accepted, published hot component mass, orbital inclination and distance from the Hipparcos parallax, we find that hot, high gravity, NLTE photosphere model fits to the IUE spectra yield distances from the best-fitting models which agree with the Hipparcos parallax distance but at temperatures substantially lower than the modified Zanstra temperatures. NLTE fits to an archival FUSE spectrum taken at the same orbital phase as the IUE spectra yield the same temperature as the IUE temperature (50,000K). However, for the same hot component mass, inclination and parallax-derived distance, accretion disk models at moderately high inclinations, $\sim 60-75^{\circ}$ with accretion rates $\dot{M} = 1\times 10^{-8}$ to $1\times 10^{-9} M_{\sun}/yr$ for white dwarf masses $M_{wd} = 0.4 M_{\sun}$ yield distances grossly smaller than the distance from the Hipparcos parallax. Therefore, we rule out an accretion disk as the dominant source of the FUV flux. Our findings support a hot bare white dwarf as the dominant source of FUV flux.

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arXiv2004

Composite Accretion Disk and White Dwarf Photosphere Analyses of the FUSE and HST Observations of EY Cygni

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.

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arXiv2004

Surface Hydrogen Modeling of Super Soft X-ray Sources: Are They Supernova Ia Progenitors?

Nova explosions occur on the white dwarf (WD) component of a Cataclysmic Variable stellar system which is accreting matter lost by a companion. A Type Ia supernova explosion is thought to result when a WD, in a similar binary configuration, grows in mass to the Chandrasekhar Limit. Here, we present calculations of accretion of Solar matter, at a variety of mass accretion rates, onto hot ($2.3 \times 10^{5}$K), luminous (30L$_\odot$), massive (1.25M$_\odot$, 1.35M$_\odot$) Carbon-Oxygen WDs. In contrast to our nova simulations where the WD has a low initial luminosity and a thermonuclear runaway (TNR) occurs and ejects material, these simulations do not eject material (or only a small fraction of the accreted material) and the WD grows in mass. A hydrogen TNR does not occur because hydrogen fuses to helium in the surface layers, and we call this process Surface Hydrogen Burning (SHB). As the helium layer grows in mass, it gradually fuses either to carbon and oxygen or to more massive nuclei depending on the WD mass and mass accretion rate. If such a WD were to explode in a SN Ia event, therefore, it would show neither hydrogen nor helium in its spectrum as is observed. Moreover, the luminosities and effective temperatures of our simulations agree with the observations of some of the Super Soft X-ray Binary Sources and, therefore, our results strengthen previous speculation that some of them (CAL 83 and CAL 87 for example) are probably progenitors of SN Ia explosions. Finally, we have achieved SHB for values of the mass accretion rate that almost span the observed values of the Cataclysmic Variables.

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