Exocomets: Comets around Another Star
- Title: Exocomets in the circumstellar gas disk of HD 172555
- Authors: F. Kiefer, A. Lecavelier des Etangs, J.-C. Augereau, A Vidal-Madjar, A.-M. Lagrange, H. Beust
Comets are icy bodies that are pulled from the outer solar system towards the Sun. Upon approach, comets begin to heat up and display a long tail. Sometimes, we can see these bright comets on their trips around the Sun.
But what about around other star systems? Are there comets there too, or is this just something unique to our solar system?
That’s what Kiefer et al. discusses in their paper. The authors focused on the star HD 172555, a young A7V-type star in the Beta Pictoris moving group, a group of stars that have a common motion through space. A feature of this star is that it has a circumstellar disk of material orbiting it, which is mostly in the gaseous phase. By studying this star’s spectrum, it was revealed that absorption activities were taking place in the Calcium II (Ca II) lines.
Many stars have strong calcium absorption lines. What is interesting about HD 172555 is that the absorptions are variable and have a small time scale of about one day. The authors compared this with the spectra of the star Beta Pictoris (β Pic) and found similar features, which were identified as Falling Evaporating Bodies (FEBs). Also called exocomets, these FEBs are small comet-like objects that evaporate when too close to the star. It is believed that what was observed around β Pic is the same phenomena that the authors observed around the star HD 172555.
These exocomets are believed to occur around the star’s radial velocity (the speed towards the radius of the star) near periastron (the point where an exocomet would be nearest to the star). Based on the detection of exocomets, it was determined that the orientation of the circumstellar disk around the HD 172555 system is close to edge-on.
In order to further understand the nature of these exocomets, the authors believe that additional study is needed.
Image: Plots of the spectra of HD 172555. The black lines are stable absorptions in the circumstellar disk, whereas the red lines are additional absorption features, taken at a later time, most likely due to a FEB.