Transit distances and composition of low-velocity exocomets in the $\beta$ Pic system

Abstract

$\beta$ Pictoris is a young nearby A5V star, about 20 Myr old, embedded in a prominent debris disc. For the past 40 years, variable absorption features have been observed in the stellar spectrum, produced by the gaseous tails of exocomets transiting the star. 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 $\beta$ Pic, obtained on April 29, 2025, with the Hubble Space Telescope and the HARPS spectrograph. We report the detection of three strong exocomet signatures at low radial velocities (-7.5, +2.5 and +10 km/s), in 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 derive the transit distance of the three exocometary gaseous tails to be $0.88\pm 0.08$, $4.7\pm 0.3$, and $1.52\pm 0.15$ 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 to measure the transit distance of exocomets, based on excitation modelling, complementing the acceleration method only applicable for high-velocity objects.

arXiv commentAccepted for publication in Astronomy and Astrophysics

References in corpus (31)

• emcee: The MCMC Hammer
• A new extensive library of PHOENIX stellar atmospheres and synthetic spectra
• A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
• A giant planet imaged in the disk of the young star Beta Pictoris
• The chemical make-up of the Sun: A 2020 vision
• Molecular Gas Clumps from the Destruction of Icy Bodies in the $\beta$ Pictoris Debris Disk
• STIS Coronagraphic Observations of Beta Pictoris
• Direct confirmation of the radial-velocity planet $\beta$ Pic c
• Dynamical traceback age of the $\beta$ Pictoris moving group
• The Spatial Structure of the beta Pictoris Gas Disk
• High Resolution HST-STIS Spectra of CI and CO in the Beta Pictoris Circumstellar Disk
• Exocomets in the circumstellar gas disk of HD 172555
• Transiting exocomets detected in broadband light by TESS in the $\beta$ Pictoris system
• Widespread atomic gas emission reveals the rotation of the beta Pictoris disk
• The Inner Disk Structure, Disk-Planet Interactions, and Temporal Evolution in the Beta Pictoris System: A Two-Epoch HST/STIS Coronagraphic Study
• Atomic data from the Iron Project. LXIV. Radiative transition rates and collision strengths for Ca II
• Exocomets size distribution in the $\beta$ Pictoris planetary system
• Exposing the Gas Braking Mechanism of the beta Pictoris Disk
• Herschel detects oxygen in the beta Pictoris debris disk
• New exocomets of $\beta$ Pic
• First Detection of Hydrogen in the β Pictoris Gas Disk
• UV Spectroscopy of Star-Grazing Comets within the 49 Ceti Debris Disk
• Exocomet Orbit Fitting: Accelerating Coma Absorption During Transits of $\beta$ Pictoris
• VLT imaging of the β Pictoris gas disk
• Collision strengths and transition probabilities for Co II infrared forbidden lines
• Detection of Nitrogen gas in the $\beta$ Pictoris circumstellar disk
• Curves of growth for transiting exocomets: Application to Fe II lines in the Beta Pictoris system
• Population of excited levels of Fe+, Ni+ and Cr+ in exocomets gaseous tails
• Abundances of refractory ions in Beta Pictoris exocomets
• Exocomets of $\beta$ Pictoris I: Exocomet destruction, sodium and disk line variability in 17 years of HARPS observations
• A nomenclature for individual exocomets