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

TL;DR

This study introduces an excitation-driven method to measure exocomet transit distances in the β Pictoris system by modelling radiative and collisional excitation in their gaseous tails. Using coordinated HST UV spectroscopy and HARPS optical data from 2025-04-29, the authors identify three low-velocity exocomet signatures and fit a four-component model (three LVCs plus the disc) to 1360 absorption lines across 340 transitions. They derive distinct transit distances—$d_{LVC1}=0.88 \pm 0.08$ au, $d_{LVC2}=4.7 \pm 0.3$ au, and $d_{LVC3}=1.52 \pm 0.15$ au—with corresponding excitation states and ionisation conditions, revealing that gaseous tails can originate very close to the star and migrate outward while remaining detectable. The results challenge prior low-velocity distance estimates and imply complex tail dynamics and ionisation structures, highlighting the utility of excitation modelling as a complementary tool to velocity-based methods for studying exocometary systems.

Abstract

$β$ 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 $β$ 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.

Figures (14)

• Figure 1: Spectroscopic observations of $\beta$ Pic obtained on April 29, 2025. The HARPS spectrum is not flux-calibrated and was simply renormalized to a flux level similar to that of the STIS data. Numerous saturated absorption lines are visible in the STIS and HARPS spectra, attributed to Si ii, C i, Al ii, Fe ii, Mg ii and Ca ii. The HST spectra are consistent with a stellar model from the PHOENIX library PHOENIX at $T_{\rm eff} = 8000$ K (see Sect. \ref{['Sect. Input stellar spectrum']}).
• Figure 2: Zoomed-in view of the STIS spectra of $\beta$ Pic of April 29, 2025, around two Fe ii lines. Top: View of the strong Fe ii line at 2756.5 $\angstrom$. Archival STIS spectra of $\beta$ Pic covering the same line are shown in transparent font. The solid black line represents an estimate of the stellar photospheric continuum, free of circumstellar absorption. Bottom: Same view of the shallower Fe ii line at 2578.7 $\angstrom$. This wavelength region is covered by two orders of the STIS echelle spectrograph. Since these two lines rise from excited states of Fe$^+$, they don't show any signature from the circumstellar disc (see Sect. \ref{['Sect. Closer look at the central components']}).
• Figure 3: Zoomed-in view of the STIS spectra of $\beta$ Pic obtained on April 29, 2025, around four Fe ii lines rising from different excitation levels. Errors bars were tabulated from the STIS data reduction pipeline. Red ticks indicate the velocities of the three main LVCs. The circumstellar disc (green tick, 0 km/s) is detected in the ground state of Fe ii. Note that the $\lambda \lambda$2260 and 2622 $\angstrom$ lines are covered by two spectral orders.
• Figure 4: Same as Fig. \ref{['Fig. Fe II lines']} for two Ni ii lines and two Si ii lines. The disc is detected in the metastable state of Ni ii at 8393 cm$^{-1}$, and in the ground state of Si ii.
• Figure 5: Comparison between the PHOENIX model and the observed photospheric spectrum of $\beta$ Pic (also shown on Fig. \ref{['Fig. Comparison 2025 and before']}), reconstructed from archival observations. Fluxes are provided as observed from Earth (19.3 pc). The spectral region displayed includes several strong Fe ii transitions at 1608, 1612 and 1618 $\angstrom$.