Slow expansion of the shell of the recurrent nova T Pyxidis and detection of a faint extended envelope

Abstract

Deep CCD images of the recurrent nova T Pyx have revealed a faint, extended Hα + [N ii] halo twice as large as the previously detected shell. An [O iii] image of T Pyx shows a smooth, small shell. Comparison of 1980 and 1985 images of the Hα + [N ii] shell show an expansion of less than 10%. If the bright, inner shell is due to the 1966 eruption, it should have expanded ~36% from 1980 to 1985 (assuming uniform shell expansion). We rule out the possibility of the T Pyx shell being associated with a planetary nebula-type ejection for two reasons: the shell mass is less than 10^(-4) M_⊙, and the shell expansion velocity is ~350 km s^(-1). This expansion velocity is much slower than the 850 km s^(-1) and 2000 km s^(-1) velocities reported by Catchpole (1969) during the 1966 outburst. If the 10" diameter shell is from the 1966 outburst, then the ejecta have given up most of their bulk kinetic energy by interaction with circumstellar matter or significant amounts of (now visible) low-velocity material were ejected during the last outburst, or both. The lack of strong [O i] λ6300 and [S ii] λλ6717,34 emission lines argues against much shock interaction at the present era, and, indirectly, for the 1944 identification of the 10" shell, while thermonuclear runaway nova models support the multiple-velocity idea. A point-spread function subtracted from an Hα + [N ii] image of T Pyx has revealed a 2" radius ring around the central star. This may be the ejecta from the 1966 eruption. The photoionized shell gas implies that the central star should be UV-bright. High resolution Hubble Space Telescope imaging observations of the next T Pyx eruption might yield early detection of light echoes.

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