Planet-Eating Stars
Tuesday, January 22, 2008
Science Update - A look at CfA discoveries from recent journals

Approximately 250 extra-solar planets have been discovered in the past decade. Of these, about a third seem to orbit very close to their parent star, within one astronomical unit (1 AU is the average distance of the earth from the sun). Stars swell in size as they age -- the sun, for example, will likely grow to a size of one AU in another eight billion years or so. This raises the dramatic question: what happens to these closely orbiting planets when the star ages? The answer to this question will help astronomers accurately estimate the frequency of planets even around older stars in which they may have been digested, as well as address how the star itself might be affected by the process.

SAO astronomers Allesandro Massarotti, Dave Latham, and Robert Stefanik, together with a colleague, observed 761 nearby evolved stars whose distances are well known -- all are closer than 326 light-years. They used the spectrometer on the recently closed 1.5 m Wyeth Reflector at the Oak Ridge Observatory in Harvard, MA, and complemented that data with results from the 1.5 m Tillinghast Reflector and from the MMT, both on Mt. Hopkins, AZ. Their technique was to look for evidence for enhanced stellar rotation, since the ingestion of a planet should speed up a star's rotation. The method is complicated because stars naturally rotate, often have binary companions whose orbital motions blur the effect of excess rotation, and have atmospheric turbulence that can be
mistaken for rotation in the data. The team was able to model the results and apply statistical and comparative techniques to sort out all these effects successfully.

In the end, these scientists discovered or improved the orbital parameters on seventy-five binary systems that were in their sample, calculated improved stellar temperatures and other parameters for the others, and ended up with three isolated, old stars that convincingly appear to be have excess rotation (about nine kilometers per second), very likely due to the ingestion of a giant planet. The new results support the idea that aging stars can -- and have -- swallowed their closely orbiting planets, as well as help refine our understanding of how a star's rotation is affected by aging.