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OATS-DAUT SEMINAR
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Speaker: Pierluigi Selvelli (INAF-OATS, I)

Title: The secrets of T Pyx. A recurrent nova that will not become a SN Ia

Date: Wednesday, February 4th, 2009
Time: 11:00
Venue: Villa Bazzoni

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Abstract:
Recurrent novae represent a convenient laboratory to compare the predictions of the thermonuclear runaway theory with the observations. From the observed mass accretion rate (dM/dt) and the observed duration of the inter-outburst interval, one can obtain a direct estimate of the total mass accreted (M_accr) between two successive outbursts. This quantity can be compared with both the (theoretical) critical ignition mass M_ign and the mass of the ejected shell M_ej. Observations of RNe in quiescence and outburst can also be used to determine the secular balance between the total accreted mass M_accr and that ejected in the explosive phase M_ej, and therefore to investigate the possible role of RNe as progenitors of SN Ia. By various methods, for T Pyx, we obtained L_disk ~ 70 L_sun and dM/dt ~ 1.1 10^-8 M_sun yr^-1. These values were about twice as high in the pre-1966-outburst epoch. This allowed the first direct estimate of the total mass accreted before outburst, M_accr=(dM/dt)_pre-OB &Delta t, and its comparison with the critical ignition mass M_ign. We found M_accr and M_ign to be in perfect agreement (with a value close to 5 &sdot 10^-7M_sun) for M₁ ~ 1.37 M_sun, which provides a confirmation of the thermonuclear runaway theory. The comparison of the observed parameters of the eruption phase, with the corresponding values in the theoretical grids provides satisfactory agreement for values of M₁ close to 1.35 M_sun and log(dM/dt) between -8.0 and -7.0, but the observed value of the decay time t₃ is higher than expected. The long duration of the optically thick phase during the recorded outbursts of T Pyx, a spectroscopic behavior typical of classical novae, and the persistence of P Cyg profiles, constrains the ejected mass M_ign to within 10^-5 - 10^-4 M_sun. Therefore, T Pyx ejects far more material than it has accreted, and the mass of the white dwarf will not increase to the Chandrasekhar limit as generally believed in recurrent novae. A detailed study based on the UV data excludes the possibility that T Pyx belongs to the class of the supersoft X-ray sources, as has been postulated. XMM-NEWTON observations have revealed a weak, hard source and confirmed our interpretation..
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contact: Pierluigi Selvelli (OATS)