T Pyxidis is the only recurrent nova surrounded by knots of material ejected
in previous outbursts. Following the eruption that began on 2011 April 14.29,
we obtained seven epochs (from 4 to 383 days after eruption) of Hubble Space
Telescope narrowband Ha images of T Pyx . The flash of radiation from the nova
event had no effect on the ejecta until at least 55 days after the eruption
began. Photoionization of hydrogen located north and south of the central star
was seen 132 days after the beginning of the eruption. That hydrogen recombined
in the following 51 days, allowing us to determine a hydrogen atom density of
at least 7e5 cm^-3 - at least an order of magnitude denser than the previously
detected, unresolved [NII] knots surrounding T Pyx. Material to the northwest
and southeast was photoionized between 132 and 183 days after the eruption
began. 99 days later that hydrogen had recombined. Both then (282 days after
outburst) and 101 days later, we detected almost no trace of hydrogen emission
around T Pyx. There is a large reservoir of previously unseen, cold diffuse
hydrogen overlapping the previously detected, [NII] - emitting knots of T Pyx
ejecta. The mass of this newly detected hydrogen is probably an order of
magnitude larger than that of the [NII] knots. We also determine that there is
no significant reservoir of undetected ejecta from the outer boundaries of the
previously detected ejecta out to about twice that distance, near the plane of
the sky. The lack of distant ejecta is consistent with the Schaefer et al
(2010) scenario for T Pyx, in which the star underwent its first eruption
within five years of 1866 after many millennia of quiescence, followed by the
six observed recurrent nova eruptions since 1890. This lack of distant ejecta
is not consistent with scenarios in which T Pyx has been erupting continuously
as a recurrent nova for many centuries or millennia.Comment: 27 pages, 10 figures, submitted to the Astrophysical Journa
