Alert Notice 815: T Pyx monitoring requested for upcoming HST observations

February 28, 2023

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Further to AAVSO Alert Notice 784, Dr. Patrick Godon (Villanova University) requests AAVSO observers' assistance in monitoring the recurrent nova T Pyx, beginning immediately, in support of upcoming HST ultraviolet and optical spectroscopic observations scheduled for March 20-21, 2023. These observations are the first of two sets planned this year for T Pyx.

Dr. Godon writes that monitoring T Pyx and knowing its status is essential to be sure that it has not gone "...into outburst during (or before) the HST observations. When in outburst...T Pyx can reach V=6 or so! Such a flux would destroy the detectors of HST and grill some of its hardware. AAVSO observations will also reveal whether other unusual source fluctuations occur.

"T Pyx is a recurrent nova, and as such it has erupted six times since 1890 (it erupted in 1890, 1902, 1920, 1944, 1967, and 2011). The binary is made of a white dwarf which accretes matter from its companion star. While white dwarfs are made mostly of carbon and oxygen, the matter from the other star is made mostly of hydrogen. The hydrogen-rich material forms an accretion disk around the white dwarf and this material moves inwards onto the white dwarf. The hydrogen accumulates into a layer onto the white dwarf surface and due to the high gravity of the white dwarf that layer is being compressed, its temperature and density increase as more material is transferred from the companion star onto the white dwarf. When the pressure and temperature is high enough at the base of the hydrogen layer (after usually thousands of years), it undergoes fusion (into helium) and the thermonuclear reaction propagates all over the surface of the white dwarf (thermonuclear runaway) to consume all the hydrogen. That explosion is the nova which ejects material (forming a shell, or at least some clumps). T Pyxidis underwent six such nova explosions since 1890: the recurrence time for the explosions  of T Pyx is 20 to 40 years (rather than 1000s of years), which makes T Pyx a fast recurrent novae. T Pyx's last eruption was in 2011 and it is not expected to go into another outburst for another 30 years or more. T Pyx now has come back to a "quiescent" state, where it has completely cooled down from its eruption and the ejected material is not interacting as strongly as in the early phase following the explosion. 

"It is therefore a good time to observe T Pyx with HST, to obtain an ultraviolet and an optical spectra of the white dwarf with its accretion disk. The accretion disk is the dominant source in the UV and optical, and the spectra can tell us how much matter is being accreted onto the white dwarf. Since the white dwarf is also losing mass during its eruptions, one can find out whether the mass of the white dwarf is increasing with time and whether it can reach the Chandrasekhar limit for a supernova explosion."

Beginning immediately, please observe T Pyx in V one to two times per night; V is preferred to other filters or unfiltered, and B observations would also be valuable. Visual observations are welcome. It is particularly important to have positive V observations available to the HST team from the nights of March 16/17 through 20/21, and essential for them to have a positive V observation from March 19/20, as they will make their decision about permitting the observation 24 hours before the scheduled observation time. Please continue nightly snapshots until April 7.

Please submit observations as soon after making them as possible to the AAVSO International Database. If T Pyx appears to be going into outburst, report observations immediately and email AAVSO ( T Pyx is around magnitude 15.5 V at minimum, and can brighten to magnitude 6 in outburst. Observations brighter than 14.0-14.5 should be treated as a possible outburst beginning.

Coordinates (2000.0):  R.A. 09 04 41.51   Dec.  -32 22 47.6   (from VSX page for T Pyx)

Charts with comparison stars for T Pyx may be created using the AAVSO Variable Star Plotter (VSP).

Please submit observations to the AAVSO International Database using the name T PYX.

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen using material provided by Dr. Godon.


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