Hunting exocomets. The study: “Exocomets: A spectroscopic survey” of I. Rebollido (Universidad Autónoma de Madrid) recently appeared on Astronomy & Astrophysics

The Solar System is populated by a variety of objects: gaseous giants, rocky planets, dwarf planets, moons, asteroids, comets, and dust. To date, more than 4000 planets orbiting around other stars were identified, and the presence of planets seems to be ubiquitous in stars of our Galaxy. Are the other class of objects that we observe in the Solar System common in the other stars such as planets?

 

For instance, astronomers have identified several stars with a debris disk similar to the Kuiper Belt of the Solar System. Besides, in a few stars with the effective temperature hotter than 10000 degrees (stars with spectral class O, B, and A), absorbing features due to hot circumstellar gas were observed. In a few other cases, non periodic absorbing features due to comets transiting along the line of sight were observed. In these stars, these features are due to the absorption of the stellar radiation by the material in the exocomets tails. The most famous of these stars is β Pictoris. In this star, spectroscopic observations have allowed astronomers to identify a few hundreds absorbing features due to exocomets. These events can be sorted into two classes: events due to exocomets with a small abundances of volatile elements, and events due to fragmentation of solid bodies orbiting around the star.

 

In total, there are 20 known stars showing absorbing features due to exocomets. Anyway, it is reasonable to expect that exocomets must be as common as exoplanets. In order to understand how common exocomets are in stars of our Galaxy, the team led by the astronomer I. Rebollido (Dpto. Física Teórica, Universidad Autónoma de Madrid) analyzed spectroscopic observations of 117 main sequence stars with a spectral class between B8 (effective temperature of 11500 degrees) and G8 (5500 degrees), searching for absorption features in the spectra due to exocomets. The observations for this survey (the most extensive dedicated to the search of exocomets so far) were acquired using spectrographs of several observatories: HERMES mounted on the Mercator Telescope, FIES on the Nordic Optical Telescope, FEROS and MPEG/ESO 2.2 on the La Silla Observatory. Among the observed 117 stars, about 50% (60 stars) show absorption features due to intervening gas: In 30 cases the gas is associated with the interstellar medium, and thus not associated with the star; in the other 30 stars, all with an effective temperature larger than 7000 degrees (OB and A stars), the absorption events are due to hot circumstellar gas. Among these latter stars, in 16 cases also events similar to β Pictoris events, i.e. due to exocomets transits, were observed. The absorption features due to exocomets are more common in stars with an edge-on debris disk, where geometrical effects result in larger chances to observe the transits. The results of this study are described in the paper: “Exocomets: A spectroscopic survey“, recently appeared on Astronomy & Astrophysics. Among the coauthors, the astronomer J. Maldonado of INAF – Astronomical Observatory of Palermo.

 

The figure (click here to visualize the entire image) shows a map of the sky with the positions of the stars studied in this paper. These stars are marked with different symbols: “Young Disks”: young stars with disks; “NIR excesses”: stars with infrared emission from circumstellar material; “Shell stars”: stars with absorption features due to circumstellar gas; “λ Boo stars”: stars with peculiar chemical abundances; “previous FEBs”: stars known to host exocomets; “Debris disks”: stars with debris disks; “Prev. detected gas”: stars known to have absorption features due to circumstellar gas.

 

Mario Giuseppe Guarcello  ( follow mguarce)