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

Abstract

Comets are a remarkable feature in our night sky, visible on their passage through the inner Solar system as the Sun's energy sublimates ices and liberates surface material, generating beautiful comae, dust, and ion tails. Comets are also thought to orbit other stars, and are the most promising interpretation of sporadic absorption features (i.e. transits) seen in spectra of stars such as β Pictoris and 49 Ceti. These "exocomets" are thought to form and evolve in the same way as in the Solar system, and as in the Solar system we may gain insight into their origins by deriving their orbits. In the case of β Pictoris, orbits have been estimated indirectly, using the radial velocity of the absorption features coupled with a physical evaporation model to estimate the stellocentric distance at transit d tr . Here, we note that the inferred d tr imply that some absorption signatures should accelerate over several hours, and show that this acceleration is indeed seen in HARPS spectra. This new constraint means that orbital characteristics can be obtained directly, and the pericentre distance and longitude constrained when parabolic orbits are assumed. The results from fitting orbits to 12 accelerating features, and a handful of non-accelerating ones, are in broad agreement with previous estimates based on an evaporation model, thereby providing some validation of the exocomet hypothesis. A prediction of the evaporation model, that coma absorption is deeper for more distant transits, is also seen here.

Transiting exocomets detected in broadband light by TESS in the β Pictoris system

“…Similar detections have also been made towards other stars, for example HD 172555 (Kiefer et al 2014a). Models based on these detections have included cometary comae and have placed the exocomets at distances of a few to several tens of stellar radii, which was confirmed by their observed acceleration (Beust et al 1990;Kennedy 2018). Subsequent work theoretically predicted that occultations of the stellar light by these exocomets could be detected in broadband photometric time series.…”

“…This work recovered the events discovered by Rappaport et al (2018), found one more around HD 182952, and argued that the host stars are likely young (∼100Myr old). For completeness, we note that the deep and irregular dimming events seen towards KIC 8462852 have also been interpreted as having a cometary origin (Boyajian et al 2016;Wyatt et al 2018;Kiefer et al 2017), although the shape and depth of these events are very different to those discovered by Rappaport et al (2018) and Kennedy (2018).…”

“…Subsequent work theoretically predicted that occultations of the stellar light by these exocomets could be detected in broadband photometric time series. Lecavelier Des and Lecavelier Des Etangs (1999) estimated that the depths of such signals would be of the order of tenths of a percent in flux and suggested the detection of an exocomet passage in front of β Pictoris in observations from 1981 (Lecavelier Des Etangs et al 1995, 1997Lamers et al 1997).…”

Exocomets from a Solar System Perspective

“…Rapidly varying absorption features (on the order of hours) are observed in sequential radial velocity spectra and are interpreted as being due to accelerating exocomets. The measured acceleration constrains the distance at which the exocomets pass in front of the star which is found to be within a few tens of stellar radii (Kennedy 2018). These observations are consistent with analytical estimates by who estimated that low-velocity Ca ii absorption features correspond to distances of ∼ 15 to 30 stellar radii (R * ) whereas high-velocity features correspond to distances of 5 to 8 R * .…”

Vertical evolution of exocometary gas: I. How vertical diffusion shortens the CO lifetime

“…In addition, in a few systems gas absorption lines have been shown to be variable and Doppler shifted, indicating that the hot gas originates from sublimating exocomets on highly eccentric orbits (e.g. Kennedy 2018). In this paper, we focus on the study of cold gas found at tens of au.…”