Free Astronomy Magazine November-December 2016

SPACE CHRONICLES ing gas clouds surrounding the dy- ing stars. Astronomers speculated that these knots were actually jets ejected by disks of material around companion stars that were not visi- ble in the Hubble images. Most stars in our Milky Way galaxy are mem- bers of binary systems. But the de- tails of how these jets were pro- duced remained a mystery. “We want to identify the process that causes these amazing transfor- mations from a puffed-up red giant to a beautiful, glowing planetary nebula,” Sahai added. “These dra- matic changes occur over roughly 200 to 1,000 years, which is the blink of an eye in cosmic time.” Sahai's team used Hubble's Space Telescope Imaging Spectrograph (STIS) to conduct observations of V Hydrae and its surrounding region over an 11-year period, first from 2002 to 2004, and then from 2011 to 2013. Spectroscopy decodes light from an object, revealing informa- tion on its velocity, temperature, lo- cation, and motion. The data showed a string of mon- strous, superhot blobs, each with a temperature of more than 17,000 degrees Fahrenheit — almost twice as hot as the surface of the Sun. The researchers compiled a detailed map of the blobs' location, allowing them to trace the first behemoth clumps back to 1986. “The observa- tions show the blobs moving over time,” Sahai said. “The STIS data show blobs that have just been ejected, blobs that have moved a lit- tle farther away, and blobs that are even farther away.” STIS detected the giant structures as far away as 37 billion miles away from V Hydrae, more than eight times farther away than the Kuiper Belt of icy debris at the edge of our solar system is from the Sun. The blobs expand and cool as they move farther away, and are then not detectable in visible light. But observations taken at longer sub-millimeter wavelengths in 2004, by the Submillimeter Array in Ha- waii, revealed fuzzy, knotty struc- tures that may be blobs launched 400 years ago, the researchers said. Based on the observations, Sahai and his colleagues Mark Morris of the University of California, Los Angeles, and Samantha Scibelli of the State University of New York at Stony Brook developed a model of a com- panion star with an accretion disk to explain the ejection process. “This model provides the most plau- sible explanation because we know that the engines that produce jets are accretion disks,” Sahai explained. “Red giants don't have accretion disks, but many most likely have companion stars, which presumably have lower masses because they are evolving more slowly. The model we propose can help explain the pres- ence of bipolar planetary nebulae, the presence of knotty jet-like struc- tures in many of these objects, and even multipolar planetary nebulae. We think this model has very wide applicability.” A surprise from the STIS observation was that the disk does not fire the monster clumps in exactly the same direction every 8.5 years. The direc- tion flip-flops slightly from side- to-side to back-and-forth due to a possible wobble in the accretion disk. “This discovery was quite sur- prising, but it is very pleasing as well because it helped explain some other mysterious things that had been observed about this star by others,” Sahai said. Astronomers have noted that V Hy- drae is obscured every 17 years, as if something is blocking its light. Sahai and his colleagues suggest that due to the back-and-forth wob- ble of the jet direction, the blobs al- ternate between passing behind and in front of V Hydrae. When a blob passes in front of V Hydrae, it shields the red giant from view. “This accretion disk engine is very stable because it has been able to launch these structures for hundreds of years without falling apart,” Sahai added. “In many of these systems, the gravitational attraction can cause the companion to actually spi- ral into the core of the red giant star. Eventually, though, the orbit of V Hydrae's companion will continue to decay because it is losing energy in this frictional interaction. However, we do not know the ultimate fate of this companion.” The team hopes to use Hubble to conduct further observations of the V Hydrae system, including the most recent blob ejected in 2011. The astronomers also plan to use the Atacama Large Millimeter/submilli- meter Array (ALMA) in Chile to study blobs launched over the past few hundred years that are now too cool to be detected with Hubble. n