NASA Logo, National Aeronautics and Space Administration

Light Echoes around a Mysterious Nova

  • By Koji Mukai
  • June 24, 2013
  • Comments Off on Light Echoes around a Mysterious Nova

“In 1890 T Pyxidis had appeared, brightened, and disappeared. When I first came to Harvard they were still telling how it was found again during a routine survey of plates taken in 1919, and how Miss Leavitt exclaimed: ‘That star hasn’t been seen for almost thirty years!’ – the first recurrent nova to be discovered.”

– Cecilia Payne-Gaposchkin in Fifty Years of Novae

Henrietta Swan Leavitt in the above quote is well known for discovering the period luminosity relationship of Cepheid type variable stars, which has since become an essential rung of the cosmic distance ladder. Cecilia Payne-Gaposchkin may be less widely known, but her contributions are arguably even more important than those of Leavitt’s (try typing “the most brilliant Ph.D. thesis ever written in astronomy” in to your favorite search engine). Years after her thesis work, Payne-Gaposchkin turned her attention to novae, and thanks to brilliant scientists like her, we know a lot about these explosions. Yet there is still much about novae that we don’t understand, and most nova researchers would agree that T Pyxidis (or T Pyx for short) is the most puzzling of them all, recurrent or otherwise.

April 15, 2011 outburst of T Pyxidis
Animation courtesy of Ernesto Guido and Giovanni Sostero

The most recent eruption of T Pyx was discovered on 2011 April 14, the first time since 1966/1967. Several groups, including the E-nova collaboration that I am a member of, have been observing T Pyx using modern instruments that weren’t available in 1967. These include the Very Large Array radio telescope, the Swift satellite (for X-rays), and the Hubble Space Telescope. One such campaign led by Dr. Jennifer (Jeno) Sokoloski of Columbia University has resulted in a paper published in Astrophysical Journal Letters, and a press release, issued in early June during the American Astronomical Society meeting in Indianapolis.

To appreciate the significance of this research, you should know a little bit of what a nova is, which starts with knowing what a white dwarf is. Our Sun will one day become a white dwarf, when it can no longer sustain nuclear fusion – it will eject its still hydrogen-rich outer layers, leaving behind a slowly cooling ember made up mostly of carbon and oxygen atoms. The white dwarf will have about 60% of the present-day mass of the Sun but only be as big as the Earth. For our Sun, that will be it – a white dwarf slowly cooling and becoming dimmer. While the majority of stars will become white dwarfs, sometimes that’s not end of the story.

A white dwarf in a close binary system can acquire some of its companion’s mass – accretion for short. Accretion onto a white dwarf is a powerful energy source, and that’s how binaries like T Pyx shine most of the time. But accretion also supplies fresh nuclear fuel to the formerly fuel-less white dwarf. When enough matter is accreted (and there are certain other conditions, too, but let’s not get too technical here), the accreted layer turns into a gigantic fusion bomb. The star becomes much brighter for a period of weeks, months, or sometimes years. The accreted matter, the mass equivalent of the Earth, Neptune, or even possibly Saturn, are expelled at high velocities. This phenomenon is called a nova.

Anatomy of a Debris Disk Around T Pyxidis
Credit: NASA, ESA, and A. Feild (STScI/AURA)

This is different from a supernova, a one time event in which the star is essentially destroyed. A nova, on the other hand, leaves the underlying white dwarf largely undisturbed. It can go back to accreting and, eventually, have another nova outburst. But it takes time to accumulate enough fresh fuel – the recurrence times of many thousands of years and even millions of years are expected. But there are a few binaries which have been seen to go nova multiple times within the last 150 years or so – these are called recurrent novae. We think a recurrent nova needs a massive white dwarf (higher gravity means higher pressure, and hence easier to ignite the accreted fuel), it must accrete at a furious rate, and even so the envelope is likely to be less massive when it explodes than in a typical nova, say only about an Earth mass or two.

If a nova ejects exactly all the mass accreted since the last outburst, no more, no less, then the mass of the white dwarf will be unchanged over time. There are indications that some novae eject underlying white dwarf material, in addition to the accreted envelope – if so, the white dwarf will lose mass. If the white dwarf in some nova systems can gain mass, then it may become a Type Ia supernova one day, which has become one of the most talked-about rungs in the cosmic distance ladder in recent years. If we want to figure out exactly how white dwarf mass changes over the accretion and nova cycles, you want to observe recurrent novae – these are the only systems where you have a chance of measuring accreted mass vs. ejected mass.

This is one of the motivations for studying T Pyx – these studies have led to several puzzles that we haven’t yet solved. First, we now know the basic characteristics of the underlying binary, such as the orbital period. It turns out to be the type of binary which normally implies a low accretion rate – how does it manage to accrete at a high enough rate to be a recurrent nova? The brightness of T Pyx during the accretion phase appears to have been fading somewhat over the last century or so, implying that the accretion rate is slowly dropping. During the last decade, several papers speculated that T Pyx will not have a nova outburst during our lifetime because of this lowered accretion rate. Yet it did, in 2011, and it appears to have ejected a lot of mass – maybe something like that of Saturn – contrary to theoretical expectations. So what is going on?

Flash of Light from Erupting Star Illuminates Debris Disk
Credit: NASA, ESA, A. Crotts, J. Sokoloski, and H. Uthas (Columbia University), and S. Lawrence (Hofstra University)

Jeno Sokoloski is the driving force behind the E-nova collaboration, and is deeply involved in the radio and X-ray observations of T Pyx. In addition, she and her Columbia colleagues Drs. Arlin Crotts and Helena Uthas had the brilliant idea to study the light echo using the Hubble Space Telescope (another group, led by Dr. Michale Shara of the American Museum of Natural History, had a similar idea and obtained HST observations with a somewhat different set-up). The idea is to study the matter clumps ejected during previous nova outbursts as they are illuminated by the temporary brightening of T Pyx itself. Just like bats using echolocation, or air traffic controllers using a radar, this allowed Jeno and her colleagues to figure out the geometry of the ejecta. Also, by comparing the time lags to the angular separation of the particular echo-producing clump of matter from the central binary, they were able to estimate the distance to T Pyx – a key parameter which is difficult to estimate (see ladder, distance). The answer is 4.8 ± 0.5 kiloparsecs (about 15,600 ± 1,600 light years). As for the geometry, the ejecta from the previous nova outbursts appear to be mostly confined on a plane – not spherically symmetric like you might naively assume for a nuclear explosion on the entire surface of a white dwarf.

So that’s the story behind this press release – the geometry of the ejecta is an important piece of the puzzle, we will all use the distance they have estimated, and the research method (nova light echo) is pretty cool. Still, this obviously is not the end of the story – T Pyx remains as enigmatic as ever, so research continues. The E-nova collaboration alone has one paper on radio observations of T Pyx in the refereeing process, and another on X-ray observations to be submitted soon. Stay tuned for more surprises ahead!


Comments are closed.

NASA Logo, National Aeronautics and Space Administration