Hunting for rapid rotators in the field...
Rapidly rotating giant stars are a natural place to look for the chemical signature of planet accretion. These stars' unusually large rotation speeds suggests the possibility of angular momentum augmentation. Previously, I combed through over a thousand spectra of K giant stars looking for rapid rotators. (Click on the figure below to see how rotation changes the widths of stellar lines.) These spectra were originally taken in preparation for NASA's Space Interferometry Mission. Out of nearly 1300 stars, I found only 28 rapid rotators. This catalog was published in 2011.
... and in open clusters.I am currently hunting for rapid rotators among the red giant stars of open clusters like the NGC 2660, seen below. I have collected hundreds of spectra using the duPont and Magellan Clay telescopes at Las Campanas Observatory.
Chemical signatures of planet accretion
The question I spend most of my time working on is this:
Do any rapid rotators show unusual abundance patterns that would implicate planet accretion as the source of the enhanced rotation?
The most obvious abundance signature would be the presence of an element that get destroyed during stellar evolution and are severely depleted in red giant stars' atmospheres. Lithium is the best example. A Jupiter-mass planet (with a normal Li abundance) can have more total Li atoms than the atmosphere of a red giant star. And, red giant stars atmospheres can comprise 80% of the stars total mass. Unfortunately, it is still not clear whether planet accretion is a unique explanation for both rotation and enriched lithium in red giant stars. I invite you to check out the paper on the most Li-rich star I have studied so far.
My collaborators and I recently finished a study of the lithium abundances in a sample of slowly rotating and rapidly rotating red giant stars. We found that the rapidly rotating stars tend to have higher abundances of lithium, which is consistent with the idea that engulfing planets can spin up a red giant (making it a rapid rotator) and enrich its surface lithium abundances. However, both the normal red giants and the rapid rotators show a very large range of lithium abundances, which makes it difficult to pinpoint which stars have lithium abundances consistent with normal stellar evolution and which ones do not. You can read the results of our paper here .
I am currently working on trying to identify other chemical abundance patterns one might expect to see in red giant stars that have accreted planets. The biggest difficulty is that the planets in our solar system that are the most chemically distinct from the Sun (e.g., Mercury or Earth) are also the least massive. An accreted terrestrial planet would leave behind a distinct chemical fingerprint, but would that fingerprint be too small to detect? A Jupiter mass planet would leave behind a large fingerprint, but its fingerprint would be nearly identical to the star's! (Aside from a few depleted elements, like Li.)