Exocomet orbital distribution around β Pictoris

Abstract

The ~23 Myr young star β Pictoris (β Pic) is a laboratory for planet formation studies because of its observed debris disk, its directly imaged super-Jovian planets β Pic b and c, and the evidence of extrasolar comets that regularly transit in front of the star. The most recent evidence of exocometary transits around β Pic came from stellar photometric time series obtained with the TESS space mission. Previous analyses of these transits constrained the orbital distribution of the underlying exocomet population to a range between about 0.03 and 1.3 AU assuming a fixed transit impact parameter. We examine the distribution of the observed transit durations (Δt) to infer the orbital surface density distribution (δ) of the underlying exocomet sample. The effect of the geometric transit probability for circular orbits was properly taken into account, but we assumed that the radius of the transiting comets and their possible clouds of evaporating material are much smaller than the stellar radius. We show that a narrow belt of exocomets around β Pic, in which the transit impact parameters are randomized but the orbital semimajor axes are equal, results in a pile-up of long transit durations. This is in contrast to observations, which reveal a pile-up of short transit durations (Δt ≈ 0.1 d) and a tail of only a few transits with Δt > 0.4 d. A flat density distribution of exocomets between about 0.03 and 2.5 AU results in a better match between the resulting Δt distribution and the observations, but the slope of the predicted Δt histogram is not sufficiently steep. An even better match to the observations can be produced with a δ ∝ aβ power law. Our modeling reveals a best fit between the observed and predicted Δt distribution for β = −0.15−0.10+0.05. A more reasonable scenario in which the exocometary trajectories are modeled as hyperbolic orbits can also reproduce the observed Δt distribution to some extent. Future studies might reproduce the observed Δt distribution with a full exploration of the four-dimensional parameter space of highly eccentric orbits, and they might need to relax our assumption that the transiting objects are smaller than the stellar disk. The number of observed exocometary transits around β Pic is currently too small to validate the previously reported distinction of two distinct exocomet families, but this might be possible with future TESS observations of this star. Our results nevertheless imply that cometary material exists on highly eccentric orbits with a more extended range of semimajor axes than suggested by previous spectroscopic observations.