Astronomers used NASA’s Hubble Space Telescope to observe the light emitted by the close double-star system T Pyxidis (T Pyx), a recurrent nova, during its latest outburst in April 2011. Contrary to some predictions, the astronomers were somewhat surprised to find that the ejecta from earlier outbursts stayed in the vicinity of the star and formed a disk of debris. The discovery suggests that material continues expanding outward along the system’s orbital plane, but it does not escape the system.
“We fully expected this to be a spherical shell,” said Arlin Crotts of Columbia University in New York City. “This observation shows that it is a disk, and it is populated with fast-moving ejecta from previous outbursts.”
A nova erupts when a white dwarf, the burned-out core of a Sun-like star, has siphoned enough hydrogen off a companion star to trigger a thermonuclear runaway. As hydrogen builds up on the surface of the white dwarf, it becomes hotter and denser until it detonates like a colossal hydrogen bomb, leading to a 10,000-fold increase in brightness in a little more than one day. Nova explosions are extremely powerful, equal to a blast of 1 million billion tons of dynamite. T Pyx erupts every 12 to 50 years.
Jennifer Sokolosk of Columbia University suggests that these data indicate that the companion star plays an important role in shaping how material is ejected, presumably along the system’s orbital plane, creating the pancake-shaped disk. The disk is tilted about 30° from face-on toward Earth.
Using Hubble’s Wide Field Camera 3, the team took advantage of the blast of light emitted by the erupting nova to trace the light’s path as it lit up the disk and material from previous ejecta. The disk is so vast, about a light-year across, that the nova’s light cannot illuminate all of the material at once. Instead, the light sweeps across the material sequentially illuminating parts of the disk, a phenomenon called a light echo. The light reveals which parts of the disk are nearer to Earth and which sections are farther away. By tracing the light, the team assembled a 3-D map of the structure around the nova.
“We’ve all seen how light from the fireworks shells during the grand finale will light up the smoke and soot from shells earlier in the show,” said Stephen Lawrence of Hofstra University in Hempstead, New York. “In an analogous way, we’re using light from T Pyx’s latest outburst and its propagation at the speed of light to dissect its fireworks displays from decades past.”
Although astronomers have witnessed light propagating through material surrounding other novae, this is the first time that the immediate environment around an erupting star has been studied in 3-D. Astronomers have studied light echoes from other novae, but those phenomena illuminated interstellar material around the stars.
The team also used the light echo to refine estimates of the nova’s distance from Earth. The new distance is 15,600 light-years from Earth. Previous estimates were between 6,500 and 16,000 light-years. T Pyx is located in the southern constellation Pyxis the Compass.
The team is continuing to analyze the Hubble data to develop a model of the outflow. T Pyx has a history of petulant outbursts. Besides the 2011 event, other previous known eruptions occurred in 1890, 1902, 1920, 1944, and 1966.