Transit distances and compositions of low-velocity exocomets in the β Pic system

Institut d’Astrophysique de Paris, CNRS, UMR 7095, Sorbonne Université, 98 bis boulevard Arago, 75014 Paris, France

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Received: 24 October 2025
Accepted: 21 January 2026

Abstract

Beta Pictoris is a young nearby A5V star, about 20 Myr old, that is embedded in a prominent debris disc. For the past 40 years, variable absorption features, produced by the gaseous tails of exocomets transiting the star, have been observed in the stellar spectrum. Yet, despite the large number of observations available, the origin and dynamical evolution of the exocomets remain poorly understood. Here we present new spectroscopic observations of β Pic obtained on April 29, 2025, with the Hubble Space Telescope and the High Accuracy Radial Velocity Planet Searcher. We report the detection of three strong exocomet signatures at low radial velocities (−7.5, +2.5, and +10 km/s) from a large set of lines from various species and excitation levels. We show that the three exocometary tails have different excitation states, indicating that they are located at different distances from the star. Using a detailed modelling of the excitation state of the transiting gas, which includes both radiative and collisional excitation, we derived the transit distance of the three exocometary gaseous tails to be 0.88 ± 0.08, 4.7 ± 0.3, and 1.52 ± 0.15 au. These values are much larger than previous estimates, which generally placed the transient features within 0.2 au. This reveals that gaseous tails produced by exocomets sublimating close to the star can expand and migrate over large distances while still remaining detectable in absorption spectroscopy. Our study provides a new method for measuring the transit distance of exocomets based on excitation modelling; this complements the acceleration method, which is only applicable to high-velocity objects.