Two families of exocomets in the β Pictoris system

Abstract

The young planetary system surrounding the star β Pictoris harbours active minor bodies. These asteroids and comets produce a large amount of dust and gas through collisions and evaporation, as happened early in the history of our Solar System. Spectroscopic observations of β Pictoris reveal a high rate of transits of small evaporating bodies, that is, exocomets. Here we report an analysis of more than 1,000 archival spectra gathered between 2003 and 2011, which provides a sample of about 6,000 variable absorption signatures arising from exocomets transiting the disk of the parent star. Statistical analysis of the observed properties of these exocomets allows us to identify two populations with different physical properties. One family consists of exocomets producing shallow absorption lines, which can be attributed to old exhausted (that is, strongly depleted in volatiles) comets trapped in a mean motion resonance with a massive planet. Another family consists of exocomets producing deep absorption lines, which may be related to the recent fragmentation of one or a few parent bodies. Our results show that the evaporating bodies observed for decades in the β Pictoris system are analogous to the comets in our own Solar System.

Exocomet orbit fitting: accelerating coma absorption during transits of β Pictoris

“…At this stage a "stellar" reference spectrum near each line is computed; the method follows very closely that described by Kiefer et al (2014), which used the highest flux values minus an estimated noise level across all spectra to derive a spectrum free of absorption. An additional step added here is interpolation across the circumstellar line, which is always present but varies in width/velocity slightly, so that reference divided (or subtracted) spectra do not show spurious variations that might be interpreted as FEB activity near zero velocity.…”

“…In the time since, observations have continued, and in late 2017 all 2642 public HARPS spectra of β Pictoris were downloaded from the ESO archive for use in this study. These spectra have been processed by the HARPS Data Reduction Software, and have been shown by Kiefer et al (2014) to be of sufficient quality for work related to FEBs; the spectra have a stable wavelength calibration that far exceeds our needs here, and the relative flux calibration across the spectral lines of interest and over the years since 2004 is high enough that variations at the ∼1% level are easily detected (see for example their Extended Data Fig. 2).…”

“…While orbital characteristics have been derived based on the evaporation theory (Beust et al 1990;Kiefer et al 2014), these are model-dependent. That is, while the evaporation theory clearly reproduces the observations very well, it necessarily includes many assumptions for quantities that may vary from comet to comet, and it would be far preferable to derive the orbits in a less model-dependent way.…”

“…The relative lack of blue shifted events (Lagrange-Henri et al 1992;Crawford et al 1998) means that most comets are approaching pericentre, with values for the longitude of pericentre ̟ (measured from the line of sight to the star), being between -50 and 130 • (Beust et al 1990;Kiefer et al 2014). …”

The orbit of beta Pictoris b as a transiting planet

“…In particular, Beust & Morbidelli (1996) show that the 4:1 mean-motion resonance is a very efficient mechanism for producting the evaporating exocomets as soon as the eccentricity of the perturbing planet is higher than about 0.05. This last scenario is strengthened by the discovery of two families of exocomets, one of which presents the periastron angledistance relationship for bodies trapped in a mean motion resonance (Kiefer et al 2014).…”