* Astronomy

Blobrana · L

RE: T Pyxidis

Blobrana · L

T Pyxidis is a recurrent nova and nova remnant in the constellation Pyxis. Its usual apparent magnitude is 15.5, but there occurred eruptions with maximal apparent magnitude of about 7.0 in the years 1890, 1902, 1920, 1944 and 1966.
It is a binary star about 6000 light years from Earth.
Source

Blobrana · L

Title: The Nova Shell and Evolution of the Recurrent Nova T Pyxidis
Authors: Bradley E. Schaefer, Ashley Pagnotta, Michael M. Shara

T Pyxidis is the prototypical recurrent nova (RN) with a mysterious nova shell. We report new observations of the shell with HST. The knots in the shell are expanding with velocities 500-715 km/s, for a distance of 3500 pc. The fractional expansion of the knots is constant, and this implies no significant deceleration. Hence, the knots were ejected by an eruption close to the year 1866. Knots have turned on after 1995, and this demonstrates that the knots are powered by shocks from the collision of the 1866 ejecta with fast ejecta from later RN eruptions. The 1866 ejecta has a total mass of 10^-4.5 Msun, which with the low ejection velocity shows that the 1866 event was an ordinary nova eruption, not a RN eruption. The accretion rate before the ordinary nova event must have been low (around the 4x10^-11 Msun/yr expected for gravitational radiation alone) and the matter accumulated on the surface of the white dwarf for ~750,000 years. The current accretion rate (>10^-8 Msun/yr) is 1000X higher than expected for a system below the period gap, with the plausible reason being that the 1866 event started a continuing supersoft source that drives the accretion. A key fact about T Pyx is that its accretion rate has been secularly declining since before the 1890 eruption, with the current rate being only 3% of its earlier rate. The decline in the observed accretion rate shows that the supersoft source is not self-sustaining, and we calculate that the accretion in T Pyx will effectively stop in upcoming decades. With this, T Pyx will enter a state of hibernation, lasting for an estimated 2,600,000 years, before gravitational radiation brings the system into contact again. Thus, T Pyx has an evolutionary cycle going from an ordinary CV state, to its current RN state, to a future hibernation state, and then repeating this cycle.

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Blobrana · L

Title: The secrets of T Pyxidis II. A recurrent nova that will not become a SN Ia
Authors: P. Selvelli, A. Cassatella, R. Gilmozzi, R. Gonzalez-Riestra

By various methods, we obtained L_disk ~ 70 L_{\odot} and \dot{M} ~1.1 x 10^{-8} Solar masses 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$_{accr}= \dot{M}_{pre-OB} \cdot \Delta t, and its comparison with the critical ignition mass M_{ign}. We found M_{accr} and M_{ign} to be in perfect agreement (with a value close to 5 x 10^{-7}M_{\odot}) for M_1 ~1.37 M_{\odot}, which provides a confirmation of the thermonuclear runaway theory. The comparison of the observed parameters of the eruption phase, with the corresponding values in the grid of models by Yaron and collaborators, provides satisfactory agreement for values of M$_1$ close to 1.35 M_{\odot} and log \dot{M} between -8.0 and -7.0, but the observed value of the decay time t_3 is higher than expected. The long duration of the optically thick phase during the recorded outbursts of T Pyx, a spectroscopic behaviour typical of classical novae, and the persistence of P Cyg profiles, constrains the ejected mass M_{ign} to within 10^{-5} - 10^{-4} M_{\odot}. 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 this interpretation.

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