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1.
For several novae, a bright X‐ray source with a spectrum resembling the class of Super Soft X‐ray Sources (SSS) has been observed a few weeks to months after outburst. Novae are powered by explosive nuclear burning on the surface of a white dwarf, and enough energy is produced to power a radiatively driven wind. Owing to the evolution of the opacity of the ejecta, the observable spectrum gradually shifts from optical to soft X‐rays (SSS phase). It has sometimes been assumed that at the beginning of the SSS phase no more mass loss occurs. However, high‐resolution X‐ray spectra of some novae have shown highly blue‐shifted absorption lines, indicating a significant expansion. In this paper, I show that all novae that have been observed with X‐ray gratings during their SSS phase show significant blue shifts. I argue that all models that attempt to explain the X‐ray bright SSS phase have to accommodate the continued expansion of the ejecta (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

2.
We present X‐ray spectral analysis of the super‐soft source CAL87 using ASCA, Chandra, XMM‐Newton observations. Early ASCA CCD spectrum reported a strong oxygen absorption edge, which is considered to originate in the an optically thick white‐dwarf atmosphere. On the other hand, contemporaneous grating observations by Chandra and XMM‐Newton indicate emission line dominated spectra, which obviously indicate the optically thin origin. Fitting all the available CCD (ASCA and XMM‐Newton) and grating spectra (XMM‐Newton and Chandra) simultaneously, we show that the CAL87 X‐ray energy spectrum is in fact composed of both an optically thick component with deep absorption edges and an optically thin component with numerous emission lines. The current result supports the standard SSS model that the primary source of X‐ray emission is nuclear burning in the white dwarf atmosphere, surrounded by a highly photoionised, optically thin corona (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

3.
The white dwarf in the eclipsing binary system V471 Tau is viewed through the atmosphere of the active K star prior to ingress and after egress. In the far UV the surface brightness of the hot white dwarf far outshines the K star emission. We can use this to probe the structure of the extended K star atmosphere along one line of sight, in absorption, on spatial scales of the radius of the white dwarf (10,000 km). The time series of HST/STIS spectra which show a hot (>250,000 K) extended (>1 K star radius) atmosphere around the K star. We see discrete structures in the velocity‐resolved spectra, on spatial scales of less than 100,000 km. The mean velocity is that expected of gas in co‐rotation with the K star, but the discrete velocity structures have excursions of up to 70 km/s from the mean. The mean temperature seems to increase with height above the K star photosphere. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

4.
The analyses of X-ray emission from classical novae during the outburst stage have shown that the soft X-ray emission below 1 keV, which is thought to originate from the photosphere of the white dwarf, is inconsistent with the simple blackbody model of emission. Thus, ROSAT Position Sensitive Proportional Counter (PSPC) archival data of the classical Nova Mus 1983 (GQ Mus) have been re-analysed in order to understand the spectral development in the X-ray wavelengths during the outburst stage. The X-ray spectra are fitted with the hot white dwarf (WD) atmosphere emission models developed for the remnants of classical novae near the Eddington luminosity. The post-outburst X-ray spectra of the remnant white dwarf are examined in the context of evolution on the Hertzsprung–Russell diagram using C–O enhanced atmosphere models. The data obtained in 1991 August (during the ROSAT All Sky Survey) indicate that the effective temperature is         . The 1992 February data show that the white dwarf had reached an effective temperature in the range         with an unabsorbed X-ray flux (i.e. ∼ bolometric flux) between     and     . We show that the H burning at the surface of the WD had most likely ceased at the time of the X-ray observations. Only the 1991 August data show evidence for ongoing H burning.  相似文献   

5.
The radiation spectra of supersoft X-ray sources based on a model for hydrogen burning on the white dwarf surface are investigated by solving hydrostatic equilibrium, radiative equilibrium, statistical equilibrium and radiative transfer self-consistently for various sets of mass, radius and luminosity. It is found that the radiation spectrum shows many bound–free emission/absorption features and greatly deviates from the blackbody spectrum at the effective temperature. By the effect of incoherent Compton scattering, the bound–free emission/absorption features do not appear in strong emission/absorption edges, as predicted by coherent models without Compton scattering, but appear as weak humps and relatively shallow absorption edges. The difference between the incoherent model and the coherent model is prominent for L 0.5 L Edd. A calculated spectrum is fitted to the ASCA observations of RX J0925.7−4758. It is found that the entire spectrum of RX J0925.7−4758 cannot be reproduced by model atmospheres with any parameter sets. This suggests that the observed spectrum consists of two or more components. In this case, the atmospheric component is explained by the emission from a white dwarf near the Chandrasekhar limit (∼1.4 M⊙) with L ∼0.2 L Edd at a distance of 16–20 kpc.  相似文献   

6.
We present spectroscopy and photometry of GD 448, a detached white dwarf – M dwarf binary with a period of 2.47 h. We find that the Na  I  8200-Å feature is composed of narrow emission lines, owing to irradiation of the M dwarf by the white dwarf, within broad absorption lines that are essentially unaffected by heating. Combined with an improved spectroscopic orbit and gravitational redshift measurement from spectra of the Hα line, we are able to derive masses for the white dwarf and M dwarf directly (0.41 ± 0.01 and 0.096 ± 0.004 M⊙, respectively). We use a simple model of the Ca II emission lines to establish the radius of the M dwarf assuming the emission from its surface to be proportional to the incident flux per unit area from the white dwarf. The radius derived is 0.125 ± 0.020 R⊙. The M dwarf appears to be a normal main-sequence star in terms of its mass and radius, and is less than half the size of its Roche lobe. The thermal time-scale of the M dwarf is much longer than the cooling age of the white dwarf, so we conclude that the M dwarf was unaffected by the common-envelope phase. The anomalous width of the Hα emission from the M dwarf remains to be explained, but the strength of the line may be due to X-ray heating of the M dwarf owing to accretion on to the white dwarf from the M dwarf wind.  相似文献   

7.
We use a two-temperature hydrodynamical formulation to determine the temperature and density structures of the post-shock accretion flows in magnetic cataclysmic variables (mCVs) and calculate the corresponding X-ray spectra. The effects of two-temperature flows are significant for systems with a massive white dwarf and a strong white-dwarf magnetic field. Our calculations show that two-temperature flows predict harder keV spectra than one-temperature flows for the same white-dwarf mass and magnetic field. This result is insensitive to whether the electrons and ions have equal temperature at the shock, but depends on the electron–ion exchange rate, relative to the rate of radiative loss along the flow. White-dwarf masses obtained by fitting the X-ray spectra of mCVs using hydrodynamic models including the two-temperature effects will be lower than those obtained using single-temperature models. The bias is more severe for systems with a massive white dwarf.  相似文献   

8.
The purpose of this paper is to present new full evolutionary calculations for DA white dwarf stars with the major aim of providing a physically sound reference frame for exploring the pulsation properties of the resulting models in future communications. Here, white dwarf evolution is followed in a self-consistent way with the predictions of time-dependent element diffusion and nuclear burning. In addition, full account is taken of the evolutionary stages prior to white dwarf formation. In particular, we follow the evolution of a 3-M⊙ model from the zero-age main sequence (the adopted metallicity is   Z =0.02)  , all the way from the stages of hydrogen and helium burning in the core up to the thermally pulsing phase. After experiencing 11 thermal pulses, the model is forced to evolve towards its white dwarf configuration by invoking strong mass loss episodes. Further evolution is followed down to the domain of the ZZ Ceti stars on the white dwarf cooling branch.
Emphasis is placed on the evolution of the chemical abundance distribution caused by diffusion processes and the role played by hydrogen burning during the white dwarf evolution. We find that discontinuities in the abundance distribution at the start of the cooling branch are considerably smoothed out by diffusion processes by the time the ZZ Ceti domain is reached. Nuclear burning during the white dwarf stage does not represent a major source of energy, as expected for a progenitor star of initially high metallicity. We also find that thermal diffusion lessens even further the importance of nuclear burning.
Furthermore, the implications of our evolutionary models for the main quantities relevant for adiabatic pulsation analysis are discussed. Interestingly, the shape of the Ledoux term is markedly smoother compared with previous detailed studies of white dwarfs. This is translated into a different behaviour of the Brunt–Väisälä frequency.  相似文献   

9.
An analysis of the UV oscillations in WZ Sge is presented, in which we obtain the oscillation amplitude spectra. We find a strong 27.9-s oscillation in our Hubble Space Telescope ( HST ) UV and zeroth-order light curves as well as weaker oscillations at 28.4 s in the UV and 29.1 s in the zeroth order. We find that the main oscillation amplitude spectrum can be fitted with static white dwarf spectra of about 17 000 K, an accretion hotspot of only a few 100 K hotter than the underlying white dwarf temperature or a variety of cool (<14 500 K) white dwarf pulsation amplitude spectra. A pulsating white dwarf can also explain the very blue colour of oscillations of different periods previously found in the optical. Comparing our results with those of Welsh et al., we see that the amplitude spectra of the main oscillations in WZ Sge measured with different periods in data sets from different epochs are similar to each other. Our results raise questions about using the magnetically accreting rotating white dwarf model to explain the oscillations. We suggest that the pulsating white dwarf model is still a viable explanation for the oscillations in WZ Sge.  相似文献   

10.
Using a parametrized function for the mass loss at the base of the post-shock region, we have constructed a formulation for magnetically confined accretion flows which avoids singularities, such as the infinity in density, at the base associated with all previous formulations. With the further inclusion of a term allowing for the heat input into the base from the accreting white dwarf, we are also able to obtain the hydrodynamic variables to match the conditions in the stellar atmosphere. (We do not, however, carry out a mutually consistent analysis for the match.) Changes to the emitted X-ray spectra are negligible unless the thickness of mass leakage region at the base approaches or exceeds one per cent of the height of the post-shock region. In this case the predicted spectra from higher-mass white dwarfs will be harder, and fits to X-ray data will predict lower white dwarf masses than previous formulations.  相似文献   

11.
We investigate the evolution of cooling helium atmosphere white dwarfs using a full evolutionary code, specifically developed to follow the effects of element diffusion and gravitational settling on white dwarf cooling. The major difference between this work and previous work is that we use more recent opacity data from the OPAL project. Since, in general, these opacities are higher than those available 10 years ago, at a given effective temperature, convection zones go deeper than in models with older opacity data. Thus convective dredge-up of observationally detectable carbon in helium atmosphere white dwarfs can occur for thicker helium layers than found by Pelletier et al. We find that the range of observed C to He ratios in different DQ white dwarfs of similar effective temperature is well explained by a range of initial helium layer mass between 10−3 and 10−2 M⊙, in good agreement with stellar evolution theory, assuming a typical white dwarf mass of 0.6 M⊙. We also predict that oxygen will be present in DQ white dwarf atmospheres in detectable amounts if the helium layer mass is near the lower limit compatible with stellar evolution theory. Determination of the oxygen abundance has the potential of providing information on the profile of oxygen in the core and hence on the important 12C(α,γ)16O reaction rate.  相似文献   

12.
Ultra‐cool dwarf variability studies have matured into a multi‐wavelength, multi‐method probe of ultra‐cool atmospheres. They have the unique potential to address the question of heterogeneity on the ultra‐cool dwarf surface. The constraints on the models that we can gain though time‐sensitive observations are however hampered by the weak signal detected so far, and the limitations of current atmospheric models, otherwise quite successful, to predict dynamical, or even static 2‐D atmosphere characteristics. Here I review the situation of the ultra‐cool dwarf variability studies: possible sources of variability; methods, their limitations and their results; tentative interpretation and prospects. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

13.
We describe fits to the Ginga LAC and ASCA GIS and SIS X-ray spectra of the intermediate polar V1223 Sgr using a multi-temperature emission model and including reflection from the white dwarf surface. Shock temperatures of and were obtained for the Ginga LAC, ASCA GIS and SIS instruments, respectively, giving a mean value of The data reveal significant amounts of reflection as well as a heavy metal underabundance by a factor of 2.
Multiple absorption components are required to successfully model the observed X-ray spectra at low energies, suggesting the presence of complex intrinsic absorption. All data sets reveal a low-energy spectral hardening at the minimum in the spin cycle caused by an increase in the amount of absorption at this phase. The requirement of such complex absorption is becoming increasingly common in the good quality X-ray spectra from magnetic cataclysmic variables.  相似文献   

14.
Recent observations of nearby star forming regions have offered evidence that young brown dwarfs undergo a period of mass accretion analogous to the T Tauri phase observed in young stars. Brown dwarf analogs to stellar protostars, however, have yet to be definitively observed. These young, accreting objects would shed light on the nature of the dominant brown dwarf formation process, as well as provide ideal laboratories to investigate the dependence of the accretion mechanism on protostellar mass. Recent near infrared surveys have identified candidate proto‐brown dwarfs and characterized low mass protostars in nearby star forming regions. These techniques allow near infrared spectra to diagnose the effective temperature, accretion luminosity, magnetic field strength and rotation velocity of young low mass stars across the stellar/substellar boundary. The lowest mass proto‐brown dwarfs (M < 40 MJup), however, will prove challenging to observe given current near IR observational capabilities. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

15.
We present a detailed calculation of model atmospheres for DA white dwarfs. Our atmosphere code solves the atmosphere structure in local thermodynamic equilibrium with a standard partial linearization technique, which takes into account the energy transfer by radiation and convection. This code incorporates recent improved and extended data base of collision-induced absorption by molecular hydrogen. We analyse the thermodynamic structure and emergent flux of atmospheres in the range 2500 T eff60 000 K and 6.5log  g 9.0. Bolometric correction and colour indices are provided for a subsample of the model grid. Comparison of the colours is made with published observational material and results of other recent model calculations.
Motivated by the increasing interest in helium-core white dwarfs, we analyse the photometric characteristics of these stars during their cooling, using evolutionary models recently available. Effective temperatures, surface gravities, masses and ages have been determined for some helium-core white dwarf candidates, and their possible binary nature is briefly discussed.  相似文献   

16.
We have examined the evolution of merged low-mass double white dwarfs that become luminous helium stars. We have approximated the merging process by the rapid accretion of matter, consisting mostly of helium, on to a carbon–oxygen (CO) white dwarf. After a certain mass is accumulated, a helium shell flash occurs, the radius and luminosity increase and the star becomes a yellow giant. Mass accretion is stopped artificially when the total mass reaches a pre-determined value. When the mass above the helium-burning shell becomes small enough, the star evolves blueward almost horizontally in the Hertzsprung–Russell diagram. The theoretical models for the merger of a 0.6-M⊙ CO white dwarf with a 0.3-M⊙ He white dwarf agree very well with the observed locations of extreme helium stars in the  log  T eff–log  g   diagram, with their observed rates of blueward evolution, and with luminosities and masses obtained from their pulsations. Together with predicted merger rates for  CO+He  white dwarf pairs, the evolutionary time-scales are roughly consistent with the observed numbers of extreme helium stars. Predicted surface carbon and oxygen abundances can be consistent with the observed values if carbon and oxygen produced in the helium shell during a previous asymptotic giant branch phase are assumed to exist in the helium zone of the initial CO white dwarfs. These results establish the  CO+He  white dwarf merger as the best, if not only, viable model for the creation of extreme helium stars and, by association, the majority of R Coronae Borealis stars.  相似文献   

17.
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18.
We apply our technique for indirect imaging of the accretion stream to the polar HU Aquarii, using eclipse profiles observed when the system was in a high accretion state. The accretion stream is relatively luminous, contributing as much as the accretion region on the white dwarf, or more, to the overall system brightness. We model the eclipse profiles using a model stream consisting of a ballistic trajectory from the L1 point followed by a magnetically channelled trajectory that follows a dipole field line out of the orbital plane. We perform model fits using two geometries: a stream that accretes on to both footpoints of the field line, and a stream that accretes only on to the footpoint of the field line above the orbital plane. The stream images indicate that the distribution of emission along the stream is not a simple function of the radial distance from the white dwarf. The stream is redirected by the magnetic field of the white dwarf at a distance 1.0–1.3×1010 cm from the white dwarf; this implies a mass transfer rate in the range 8–76×1016 g s−1. The absorption dips in the light curve indicate that the magnetically entrained part of the stream moves from 42° to 48° from the line of centres over the three nights of observation. This is in close agreement with the results of the one-footpoint models, suggesting that this is the more appropriate geometry for these data. The stream images show that, in almost all sections of the stream, the flux peaks in B and is successively fainter in U , V and R .  相似文献   

19.
Equilibrium models of differentially rotating nascent neutron stars are constructed, which represent the result of the accretion-induced collapse of rapidly rotating white dwarfs. The models are built in a two-step procedure: (1) a rapidly rotating pre-collapse white dwarf model is constructed; (2) a stationary axisymmetric neutron star having the same total mass and angular momentum distribution as the white dwarf is constructed. The resulting collapsed objects consist of a high-density central core of size roughly 20 km, surrounded by a massive accretion torus extending over 1000 km from the rotation axis. The ratio of the rotational kinetic energy to the gravitational potential energy of these neutron stars ranges from 0.13 to 0.26, suggesting that some of these objects may have a non-axisymmetric dynamical instability that could emit a significant amount of gravitational radiation.  相似文献   

20.
Nova V5116 Sgr 2005 No. 2, discovered on 2005 July 4, was observed with XMM‐Newton in March 2007, 20 months after the optical outburst. The X‐ray spectrum showed that the nova had evolved to a pure supersoft X‐ray source, indicative of residual H‐burning on top of the white dwarf. The X‐ray light‐curve shows abrupt decreases and increases of the flux by a factor 8 with a periodicity of 2.97 h, consistent with the possible orbital period of the system. The EPIC spectra are well fit with an ONe white dwarf atmosphere model, with the same temperature both in the low and the high flux periods. This rules out an intrinsic variation of the X‐ray source as the origin of the flux changes, and points to a possible partial eclipse as the origin of the variable light curve. The RGS high resolution spectra support this scenario showing a number of emission features in the low flux state, which either disappear or change into absorption features in the high flux state. A new XMM‐Newton observation in March 2009 shows the SSS had turned off and V51 16 Sgr had evolved into a weaker and harder X‐ray source (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

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