Exocomets of β Pictoris

I. Exocomet destruction, sodium absorption and disk line variability in 17 years of HARPS observations

Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 118, 221 00 Lund, Sweden

^★ Corresponding author.

Received: 21 July 2024
Accepted: 26 May 2025

Abstract

Context. The young β Pictoris system has been monitored with high-resolution optical spectrographs for decades. These observations have revealed strongly variable stochastic absorption in the Ca II H&K lines attributed to infalling cometary bodies.

Aims. Since 2003, over 9000 HARPS observations of β Pictoris have been taken, and many of these have not yet been used for exocomet studies. We aim to search these spectra for new exocomet phenomenology enabled by the long-time coverage and large volume of this dataset.

Methods. We systematically carried out telluric correction of the HARPS spectra using molecfit, compared multiyear observations at the wavelengths of the Ca II and Na I lines, and used a Bayesian fitting algorithm to extract exocomet line parameters. We explored the usage of an unbiased reference spectrum with which to calibrate the continuum and investigate Keplerian orbital solutions to observed exocomet acceleration.

Results. We find a general absence of exocometary sodium line absorption, with only two instances of clear (~2% deep) exocometary sodium out of 198 nights of observation, as well as a weaker (~1%) feature that persists over 13 nights in 2004. We find that these events occur during times of exceptionally deep Ca II absorption, at the same redshift, implying that strongly Ca II-evaporating exocomets also exhibit detectable levels of Na I, in spite of the vast majority of Na I being rapidly photoionized in close proximity to the star. We find long-lived CaII absorption in 2017 and 2018 that persists on a timescale of a year, which may be difficult to explain with the classical exocomet model. Lastly, we investigated two strongly accelerating, blueshifted exocomet features observed in 2019 that show strong and sudden departures from Keplerian motion, suggesting rapid changes to the dynamics of the exocomet cloud. We hypothesize that this is caused by the destruction of the comet nuclei shortly after their periastron passages.