arXiv subjects

S. Starrfield

Publications and source records attributed to S. Starrfield.

95 records · Page 6Linked to original sources

arXiv2002

The X-ray emission from Nova V382 Velorum: II. The super-soft component observed with BeppoSAX

Nova Velorum 1999 (V382 Vel) was observed by BeppoSAX 6 months after optical maximum and was detected as a bright X-ray supersoft source, with a count rate 3.454+-0.002 cts/s in the LECS. It was the softest and most luminous supersoft source observed with this instrument. The flux in the 0.1-0.7 keV range was not constant during the observation. It dropped by a factor of 2 in less than 1.5 hour and then was faint for at least 15 minutes, without significant spectral changes. The observed spectrum is not well fit with atmospheric models of a hot, hydrogen burning white dwarf. This is due mainly to a supersoft excess in the range 0.1-0.2 keV, but the fit can be significantly improved at higher energy if at least one emission feature is superimposed. We suggest that a ``pseudocontinuum'' was detected, consisting of emission lines in the supersoft X-ray range superimposed on the thermal continuum of a white dwarf atmosphere. As a result, an accurate determination of the effective temperature and gravity of the white dwarf at this post-outburst stage is not possible.

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arXiv2001

The X-ray emission from Nova V382 Velorum: I. The hard component observed with BeppoSAX

We present BeppoSAX observations of Nova Velorum 1999 (V382 Vel), done in a broad X-ray band covering 0.1-300 keV only 15 days after the discovery and again after 6 months. The nova was detected at day 15 with the BeppoSAX instruments in the energy range 1.8-10 keV and we attribute the emission to shocks in the ejecta. The plasma temperature was kT~6 keV and the unabsorbed flux was F(x)~4.3 x 10(-11) erg/cm**2/s. The nebular material was affected by high intrinsic absorption of the ejecta. 6 months after after the outburst, the intrinsic absorption did not play a role, the nova had turned into a bright supersoft source, and the hot nebular component previously detected had cooled to a plasma temperature kT<=1 keV. No emission was detected in either observation above 20 keV.

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arXiv2000

Non-LTE Modeling of Nova Cygni 1992

We present a grid of nova models that have an extremely large number of species treated in non-LTE, and apply it to the analysis of an extensive time series of ultraviolet spectroscopic data for Nova Cygni 1992. We use ultraviolet colors to derive the time development of the effective temperature of the expanding atmosphere during the fireball phase and the first ten days of the optically thick wind phase. We find that the nova has a pure optically thick wind spectrum until about 10 days after the explosion. During this interval, we find that synthetic spectra based on our derived temperature sequence agree very well with the observed spectra. We find that a sequence of hydrogen deficient models provides an equally good fit providing the model effective temperature is shifted upwards by ~1000 K. We find that high resolution UV spectra of the optically thick wind phase are fit moderately well by the models. We find that a high resolution spectrum of the fireball phase is better fit by a model with a steep density gradient, similar to that of a supernova, than by a nova model.

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arXiv1997

Detailed NLTE Model Atmospheres for Novae during Outburst: I. New Theoretical Results

We present new detailed NLTE calculations for model atmospheres of novae during outburst. This fully self-consistent NLTE treatment for a number of model atoms includes 3922 NLTE levels and 47061 NLTE primary transitions. We discuss the implication of departures from LTE for the strengths of the lines in nova spectra. The new results show that our large set of NLTE lines constitute the majority of the total line blanketing opacity in nova atmospheres. Although we include LTE background lines, their effect are small on the model structures and on the synthetic spectra. We demonstrate that the assumption of LTE leads to incorrect synthetic spectra and that NLTE calculations are required for reliably modeling nova spectra. In addition, we show that detailed NLTE treatment for a number of ionization stages of iron changes the results of previous calculations and improve the fit to observed nova spectra.

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arXiv1997

Abundance analysis of the slow nova PW Vulpeculae 1984

We determine the elemental abundances for the ejecta of the slow nova PW Vul 1984. Our technique uses a minimization of the emission line fits of a photoionization model to available ultraviolet, optical and infrared spectra. We find the following abundances (by number) with respect to solar: He/H = 1.0 $\pm$ 0.4, C/H = 7.0 $^{+7}_{-4}$, N/H = 85 $^{+59}_{-41}$ and O/H = 6 $^{+7}_{-2}$. In addition, there is weak evidence for solar Ne and Mg and twice solar Fe. Previous studies (Saizar et al. 1991 and Andreä et al. 1991, 1994) of PW Vul have yielded considerable differences in their derived elemental abundances for the ejecta. Our abundances fall in between the previous studies. To explain the discrepant abundances, we analyze in detail the data and methods used to obtain the previous results. The abundances of Saizar et al. (1991) are significantly smaller then our values because of the lower electron temperature used by Saizar et al. in deriving elemental abundances from ion abundances. Andreä et al. (1991) used an ionization correction method to obtain their abundances and verified their results with a photoionization model (Andreä et al. 1994). Our analysis of their data shows that the absolute fluxes of the optical emission lines used by Andreä are underestimated by 15% leading to a factor of 2 increase in their derived abundances. We also find the photoionization model used by Andreä et al. (1994) predicts 2 times more carbon than the photoionization code we used even when fitting the same data with similar model parameters.

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