Free Astronomy Magazine May-June 2018

21 MAY-JUNE 2018 SPACE CHRONICLES ! S urface bright- ness image of the HR 4796A debris ring. [G. Schneider et al.] of very fine dust was likely created from collisions among developing infant planets near the star, evi- denced by a bright ring of dusty de- bris seen 7 billion miles from the star. The pressure of starlight from the star, which is 23 times more lu- minous than the Sun, then expelled the dust far into space. But the dy- namics don’t stop there. The puffy outer dust structure is like a donut- shaped inner tube that got hit by a truck. It is much more extended in one direction than in the other and so looks squashed on one side even after accounting for its inclined pro- jection on the sky. This may be due to the motion of the host star plow- ing through the interstellar medium, like the bow wave from a boat crossing a lake. Or it may be influ- enced by a tidal tug from the star’s red dwarf binary companion (HR 4796B), located at least 54 billion miles from the primary star. “The dust distribution is a telltale sign of how dynamically interactive the inner system containing the ring is,” said Glenn Schneider of the Univer- sity of Arizona, Tucson, who used Hubble’s Space Telescope Imaging Spectrograph (STIS) to probe and map the small dust particles in the outer reaches of the HR 4796A sys- tem, a survey that only Hubble’s sen- sitivity can accomplish. “We cannot treat exoplanetary debris systems as simply being in isolation. Environ- mental effects, such as interac- tions with the in- terstellar medium and forces due to stellar compan- ions, may have long-term impli- cations for the evolution of such systems. The gross asym- metries of the outer dust field are telling us there are a lot of forces in play (beyond just host- star radiation pressure) that are moving the mate- rial around. We have seen effects like this in a few other systems, but here’s a case where we see a bunch of things going on at once,” Schneider further explained. Though long hypothe- sized, the first evidence for a debris disk around any star was uncovered in 1983 with NASA’s Infrared Astro- nomical Satellite. Later photographs revealed an edge-on debris disk around the southern star Beta Pic- toris. In the late 1990s, Hubble’s second-generation instruments, which had the capability of block- ing out the glare of a central star, al- lowed many more disks to be photographed. Now, such debris rings are thought to be common around stars. About 40 such sys- tems have been imaged to date, largely by Hubble.