Exocometary physics: material release and tails

Abstract

Despite decades of observations, the physical processes governing mass loss from small bodies beyond our Solar System remain poorly constrained. These exocomets are often treated as analogs of Solar System comet, yet the stellar environments they inhabit spans a wide range in terms of luminosity, stellar winds, and evolutionary stage, leading to potentially very diverse physical behaviors. Within our Solar System, small bodies lose material through a range of mechanisms, including sublimation, desorption, impacts, and/or sputtering. Once released, the composition and dynamics of the ejecta are then altered by additional processes, such as dust sublimation, ionization, and radiation pressure. In extrasolar systems, these mechanisms unfold under vastly different radiative and plasma conditions, leading to a rich diversity of mass-loss pathways and observable signatures. This work reviews our understanding of the mechanisms driving mass loss from small bodies and the subsequent evolution of ejecta in diverse stellar environments. We compare the physical and chemical mechanisms that drive gas and dust production, and investigate how they scale with stellar luminosity, temperature, and activity. We then examine the processes that modify the composition of the ejecta (e.g., dust sublimation, dissociation, or ionisation) and its dynamics (e.g., radiation pressure or stellar winds). To illustrate how these processes vary across different stellar environments, we use four well-studied planetary systems as case studies: the Sun, $β$ Pictoris, AU Microscopii, and WD 1145+017. By exploring how cometary tails behave under such diverse conditions, this work provides a physical framework for interpreting exocometary activity and sheds light on why A-type stars, such as the famous $β$ Pictoris, are over-represented in the population of exocomet-hosting stars.

Figures (21)

• Figure 1: Spectral energy distribution used for our case study stars. The fluxes are all provided for distances of 1 au.
• Figure 2: Example of mass loss in Solar System objects. Upper left: Sodium tail of Mercury, observed through a narrowband filter centered on the Na D lines around 589 nm. (Credit: Andrea Alessandrini). Mercury's sodium tail is produced from a variety of processes, including photon and thermal desorption, meteroid impact, and ion sputtering. Upper right: HST/WFC3 captured multiple tails emerging from the asteroid Dimorphos after impact with the DART spacecraft (Credits: NASA, ESA, STScI, Jian-Yang Li (PSI); Joseph DePasquale). Lower left: Gas and dust tails of C/2020 F3 (NEOWISE), driven by the sublimation of volatiles (Credits: Kiss Péter, 2020). Lower right: Sodium tail of (3200) Phaethon Zhang_2023, produced from the thermal desorption of Na atoms at the surface of the asteroid.
• Figure 3: Location of the sublimation fronts of a variety of volatiles for 5 stars, spanning a wide range of effective temperatures. The values are from Table 1 in Meech2005, and scaled to each star assuming the sublimation distance of each molecule scales with $L_\star^{0.5} \propto T_{\rm eff}^2 R_{\star}$, where $L_\star$, $T_{\rm eff}^2$ and $R_{\star}$ are the stellar luminosity, effective temperature and radius, respectively.
• Figure 4: Same as Fig. \ref{['fig:sublimationlines MS']} but for three white dwarfs with different ages (known as "cooling ages" in the white dwarf community). Again, values were derived from Meech2005, scaling the sublimation distance with $L_\star^{0.5}$. The age-luminosity relationship of white dwarfs was taken from veras2022 (Eq. 9). The second row ($T_{\rm eff} = 16\,000$ K) corresponds to WD 1145+017.
• Figure 5: Comparison of the distribution models used by lecavelier_des_etangs_library_1999 and bodman_kic_2016. In this example, $s_p = 0.50$ µ m, $s_0 = 0.10$, µ m, $n = 4.2$, and $\alpha = 4$.