AAVSO Alert Notice 784 gives preliminary announcement of an observing campaign on the recurrent nova T Pyx in support of HST observations to be carried out in 2022-2023. Please see the notice for details and observing instructions.
There are threads for this campaign under the following forums:
Please subscribe to these threads if you are participating in the campaign so you can be updated. Join in the discussion or ask questions there!
Many thanks, and Good observing,
Elizabeth O. Waagen, AAVSO HQ
Thank you Elizabeth for putting a preliminary Alert Notice about T Pyxidis!
And thanks to all the AAVSO observers who have been observing cataclysmic variables for countless (and sleepless) nights providing extremely valuable data for many decades! I have been working on cataclysmic variables for more than 3 decades now, and I have often used the AAVSO data (light curve generator) to check many of the cataclysmic variables I have worked on (especially in the last 20 years; before that I was working on Theory rather than Observations).
T Pyxidis 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 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. For this reason T Pyx will be observed with HST in the coming HST Cycle 30, starting anywhere between October 1st 2022 and September 30th 2023.
The "windows of opportunity" for the HST observations have not yet been fixed and it might take weeks to months before I am informed.
While it seems that the recurrence time of T Pyx nova explosions is increasing (from 12 years in 1902 to 44 years in 2011), and that T Pyx might not erupt for another 30 years or so, we cannot rule out an unexpected behavior from the system.
However, if T Pyxidis goes into another one of its (recurrent) nova eruptions just as HST is looking at it, this will basically "fry" the detector and likely damage the hardware of the HST instruments/detectors ("STIS" for optical spectroscopy and "COS" for the UV spectroscopy). So in order to ensure that T Pyx is not erupting during that time, T Pyx needs to be monitored in the weeks and days before the HST observations. For HST safe target offset procedure to work properly, continuous photometric monitoring is required for at least 7 consecutive days leading to the HST observations (and within the 24 hrs of the target being observed by HST). The HST observations will likely be split into two, such that there will be two HST observations separated by (likely) days-to-a-month (one observation for the STIS optical, and one for the COS ultraviolet). Each HST observations will need photometric monitoring.
I am making a similar post on the same thread in the other forum.