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Register a new userLight Curve Patterns and Seismology of a White Dwarf with Complex Pulsation
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Zs\\'ofia Bogn\\'ar
Publication date
2009
fields
Physics
and research's language is
English
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The ZZ Ceti star KUV 02464+3239 was observed over a whole season at the mountain station of Konkoly Observatory. A rigorous frequency analysis revealed 6 certain periods between 619 and 1250 seconds, with no shorter period modes present. We use the observed periods, published effective temperature and surface gravity, along with the model grid code of Bischoff-Kim, Montgomery and Winget (2008) to perform a seismological analysis. We find acceptable model fits with masses between 0.60 and 0.70 M_Sun. The hydrogen layer mass of the acceptable models are almost always between 10^-4 and 10^-6 M_*. In addition to our seismological results, we also show our analysis of individual light curve segments. Considering the non-sinusoidal shape of the light curve and the Fourier spectra of segments showing large amplitude variations, the importance of non-linear effects in the pulsation is clearly seen.
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We obtained the absolute magnitudes, distances, and white dwarf (WD) masses of 32 recent galactic novae based on the time-stretching method for nova light curves. A large part of the light/color curves of two classical novae often overlap each other if we properly squeeze/stretch their timescales. Then, a target nova brightness is related to the other template nova brightness by $(M_V[t])_{rm template} = (M_V[t/f_{rm s}] - 2.5 log f_{rm s})_{rm target}$, where $t$ is the time, $M_V[t]$ is the absolute $V$ magnitude, and $f_{rm s}$ is their timescaling ratio. Moreover, when these two time-stretched light curves, $(t/f_{rm s})$-$(M_V-2.5 log f_{rm s})$, overlap each other, $(t/f_{rm s})$-$(B-V)_0$ do too, where $(B-V)_0$ is the intrinsic $B-V$ color. Thus, the two nova tracks overlap each other in the $(B-V)_0$-$(M_V-2.5 log f_{rm s})$ diagram. Inversely using these properties, we obtain/confirm the distance and reddening by comparing each nova light/color curves with the well calibrated template novae. We classify the 32 novae into two types, LV Vul and V1500 Cyg types, in the time-stretched $(B-V)_0$-$(M_V-2.5 log f_{rm s})$ color-magnitude diagram. The WD mass is obtained by direct comparison of the model $V$ light curves with the observation. Thus, we obtain a uniform set of 32 galactic classical novae that provides the distances and WD masses from a single method. Many novae broadly follow the universal decline law and the present method can be applied to them, while some novae largely deviate from the universal decline law and so the method cannot be directly applied to them. We discuss such examples.
V959 Mon is one of the gamma-ray detected novae. It was optically discovered about 50 days after the gamma-ray detection due to proximity to the Sun. The nova speed class is unknown because of lack of the earliest half of optical light curve and short supersoft X-ray phase due to eclipse by the disk rim. Using the universal decline law and time-stretching method, we analyzed the data of V959 Mon and obtained nova parameters. We estimated the distance modulus in the V band to be (m-M)_V=13.15pm0.3 for the reddening of E(B-V)=0.38pm0.01 by directly comparing with the similar type of novae, LV Vul, V1668 Cyg, IV Cep, and V1065 Cen. The distance to V959 Mon is 2.5pm0.5 kpc. If we assume that the early phase light curve of V959 Mon is the same as that of time-stretched light curves of LV Vul, our model light curve fitting suggests that the white dwarf (WD) mass is 0.9-1.15 M_sun, being consistent with a neon nova identification. At the time of gamma-ray detection the photosphere of nova envelope extends to 5-8 R_sun (about two or three times the binary separation) and the wind mass-loss rate is (3-4)times 10^{-5} M_sun yr^{-1}. The period of hard X-ray emission is consistent with the time of appearance of the companion star from the nova envelope. The short supersoft X-ray turnoff time is consistent with the epoch when the WD photosphere shrank to behind the elevating disk rim, that occurs 500 days before nuclear burning turned off.
The classical nova V2491 Cyg was once suggested to be a recurrent nova. We have broadly reproduced the light curve of V2491 Cyg by a nova outburst model on a cold $1.36~M_odot$ white dwarf (WD), which strongly suggests that V2491 Cyg is a classical nova outbursting on a cold very massive WD rather than a recurrent nova outbursting on a warmer WD like the recurrent nova RS Oph. In a long-term evolution of a cataclysmic binary, an accreting WD has been settled down to a thermal equilibrium state with the balance of gravitational energy release and neutrino loss. The central temperature of the WD is uniquely determined by the energy balance. The WD is hot (cold) for a high (low) mass-accretion rate. We present the central temperatures, ignition masses, ignition radii, and recurrence periods for various WD masses and mass-accretion rates. In a classical nova, which corresponds to a low mass-accretion rate, the WD is cool and strongly degenerated and the ignition mass is large, which result in a strong nova outburst. In a recurrent nova, the WD is relatively warmer because of a high mass accretion rate and the outburst is relatively weaker. The gravitational energy release substantially contributes to the luminosity during the recurrent nova outbursts. We compare physical properties between classical novae and recurrent novae and discuss the essential differences between them.
We present some of the results of a survey aimed at exploring the asteroseismological potential of the newly-discovered carbon-atmosphere white dwarfs. We show that, in certains regions of parameter space, carbon-atmosphere white dwarfs may drive low-order gravity modes. We demonstrate that our theoretical results are consistent with the recent exciting discovery of luminosity variations in SDSS J1426+5752 and some null results obtained by a team of scientists at McDonald Observatory. We also present follow-up photometric observations carried out by ourselves at the Mount Bigelow 1.6-m telescope using the new Mont4K camera. The results of follow-up spectroscopic observations at the MMT are also briefly reported, including the surprising discovery that SDSS J1426+5752 is not only a pulsating star but that it is also a magnetic white dwarf with a surface field near 1.2 MG. The discovery of $g$-mode pulsations in SDSS J1426+5752 is quite significant in itself as it opens a fourth asteroseismological window, after the GW Vir, V777 Her, and ZZ Ceti families, through which one may study white dwarfs.
Ultra-massive DA WD stars are expected to harbor ONe cores resulting from the progenitor evolution through the Super-AGB phase. As evolution proceeds during the WD cooling phase, a crystallization process resulting from Coulomb interactions in very dense plasmas is expected to occur, leading to the formation of a highly crystallized core. Pulsating ultra-massive WDs offer a unique opportunity to infer and test the occurrence of crystallization in WD interiors as well as physical processes related with dense plasmas. We aim to assess the adiabatic pulsation properties of ultra-massive DA WD with ONe cores. We studied the pulsation properties of ultra-massive DA WD stars with ONe cores. We employed a new set of ultra-massive WD evolutionary sequences of models with stellar masses in the range 1.10 $leq M_{star}/M_{sun} leq$ 1.29 computed by taking into account the complete evolution of the progenitor stars and the WD stage. When crystallization set on in our models, we took into account latent heat release and also the expected changes in the core chemical composition that are due to phase separation according to a phase diagram suitable for O and Ne plasmas. We computed nonradial pulsation g-modes of our sequences of models at the ZZ Ceti phase by taking into account a solid core. We explored the impact of crystallization on their pulsation properties, in particular, the structure of the period spectrum and the distribution of the period spacings. We find that it would be possible, in principle, to discern whether a WD has a nucleus made of CO or a nucleus of ONe by studying the spacing between periods. The features found in the period-spacing diagrams could be used as a seismological tool to discern the core composition of ultra-massive ZZ Ceti stars, something that should be complemented with detailed asteroseismic analysis using the individual observed periods.
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