What one research team calls "the most famous recurrent nova in the galaxy" might whittle itself to nothing in as little as 100,000 years.
T Pyxidis is a pairing of two stars in a dysfunctional relationship. They orbit each other so closely that one of them, a white dwarf, is able to suck matter off of its companion, a red dwarf. About every 20 years — six times since 1890 — the pressure of the stolen mass builds up until the white dwarf suffers a thermonuclear meltdown and erupts in a nova, spewing the accumulated matter off into space and briefly becoming some 2,500 times brighter.
Most astronomers believe that white dwarfs in such binary systems continue to prey off their gravitationally weaker companions until they fatten up to 1.4 solar masses, at which point they explode as supernovae.
But recent observations proffer a darker fate for T Pyx — instead of bulking up, its white dwarf appears to be losing mass. "The most famous recurrent nova in the galaxy…is dying," conclude Joseph Patterson (Columbia University) and 13 amateur co-authors in a new study. Their evidence suggests a "suicide pact" that, in about 100,000 years, will leave the white dwarf whittled itself away to nothing or its red dwarf companion consumed entirely.
The surprising result comes from a vast archive of T Pyx's brightness measurements made over 1½ decades by the globe-spanning Center for Backyard Astrophysics network of amateur observers, which Patterson coordinates. These observations stretch back to 1998, when the CBA team determined the system's orbital period by monitoring telltale fluctuations in brightness as the two stars circled one another.
Timing is Everything
T Pyx's most recent outburst, in 2011, gave the team a perfect opportunity to go one step further. In theory, if mass is lost, the gravitational force between the two stars is weakened, and they'll take longer to orbit each other. So by measuring the orbital period before and after the outburst, Patterson and his observers could determine how much mass had been lost.
The CBA team found that the period increased from 1.829507 hours to 1.829606 hours, an increase of 0.0054%. Although this may seem tiny, Patterson was still amazed: "So large a period change is very, very surprising," he writes — seven times larger than predicted. This corresponds to a mass loss of 1026 kg, about a Neptune's worth — five times greater than what the white dwarf gained in the decades before its outburst. "It seems unlikely," the article states, "that the white dwarf…will ever increase its mass at all, much less reach 1.4 solar masses."
Patterson speculates that T Pyx is so weird due to the vulnerable nature of its companion, the lowly red dwarf. In most known recurrent novae, the mass donor is a red giant, rather than a dwarf. "The outburst does not tremendously affect a nearby red giant," explains Patterson, "but it tremendously affects a low-mass red dwarf." The red dwarf is overwhelmed, its outer layers becoming bloated by the intense radiation. The blast then drives these wispy outer layers away, instead of replenishing the white dwarf. This might explain the enormous mass loss seen during the 2011 outburst.
However, not everyone believes T Pyx is doomed to simply fade away. "It just absolutely flies in the face of theoretical expectation," says Edward Sion, an astronomer at Villanova University who is no stranger to T Pyx and its peculiarities. In 2010, he led a study that concluded the system's white dwarf is steadily gaining mass and might be only 10 million years away from going supernova. "I'm not saying Patterson's group is wrong," he said during a phone interview, "but we have to be very cautious."
Sion warns that Patterson's calculated accretion rate falls into a grey area filled with theoretical contradictions. For example, Patterson assumes that T Pyx's white dwarf has 70% of the Sun's mass. But in that case it shouldn't be accreting matter fast enough to trigger outbursts every 22 years. And yet, Sion continues, Patterson's assumed accretion rate is too high — material is piling up on the surface of the white dwarf rapidly enough to be burning constantly, not waiting decades before erupting.
Debate Advances Science
Sion has in mind a different picture of T Pyx. In his view, the white dwarf has a strong magnetic field that clears the inner regions of the surrounding accretion disk and funnels this material to its magnetic poles, creating hot spots visible in ultraviolet light. He's currently analyzing spectra of T Pyx from the Hubble Space Telescope taken late last year, and he believes the white dwarf will turn out to be much more massive, closer to the critical 1.4 solar-mass limit. If true, this model would align much more closely with theory, which predicts that such novae have smaller outbursts and larger accretion rates than what Patterson's team has found.
That would be just fine with Patterson. When predictions prove to be wrong, he comments, "they stimulate people to make measurements — and everyone wins." No matter the outcome, CBA observers advanced the scientific debate by measuring T Pyx's mass loss. As Patterson notes, "It was lovely to see an important result coming directly from good ol' Kepler's laws applied to a huge swath of data from backyard telescopes."
Mark Zastrow, a graduate student in the astronomy program at Boston University, is also a science writer and photographer. Follow him (@MarkZastrow) on Twitter.