arXiv subjects

Federico Biassoni

Publications and source records attributed to Federico Biassoni.

4 recordsLinked to original sources

arXiv2025

Why M-dwarf flares have limited impact on the atmospheric evaporation of sub-Neptunes and Earth-sized planets

M-type stars are prime targets for exoplanet searches within their habitable zones (HZs). These stars also exhibit significant magnetic flaring activity, particularly during their first billion years, which can potentially accelerate the evaporation of the hydrogen-helium envelopes of close-in planets. We employ the time-dependent photoionization hydrodynamics code ATES to investigate the impact of flares on atmospheric escape, focusing on an Earth-sized and a sub-Neptune-sized planet orbiting an early M-type star at distances of 0.01, 0.1, and 0.18-0.36 AU-the inner and outer edges of the HZ. Stellar flaring is modeled as a 1 Gyr-long high-activity phase followed by a 4 Gyr-long low-activity phase, each characterized by an appropriate flare frequency distribution. We find that flares have a modest impact-less than a factor of two-on the cumulative atmospheric mass loss, with the greatest absolute enhancement occurring when the planets are at their closest separation. However, the relative enhancement in mass loss between flaring and non-flaring cases is greater at larger orbital separations. This trend arises because, as stellar irradiation fluctuates between quiescent levels and peak flares, the proportion of time that a planet spends in the energy-limited versus recombination-limited mass loss regimes depends on its orbital separation. Additionally, we demonstrate the existence of a characteristic flare energy, intermediate between the minimum and maximum values, that maximizes the fractional contribution to flare-driven mass loss. Our results indicate that the flaring activity of M-dwarfs does not significantly affect the atmospheric retention of close-in planets, including those within the HZ. The potential occurrence of rare super-flares, which current observational campaigns may be biased against, does not alter our conclusions.

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arXiv2024

High-resolution transmission spectroscopy of the hot-Saturn HD 149026b

Advances in modern technologies enable the characterisation of exoplanetary atmospheres, most efficiently exploiting the transmission spectroscopy technique. We performed visible (VIS) and near infrared (nIR) high-resolution spectroscopic observations of one transit of HD 149026b, a close-in orbit sub Saturn exoplanet. We first analysed the radial velocity data, refining the value of the projected spin-orbit obliquity. Then we performed transmission spectroscopy, looking for absorption signals from the planetary atmosphere. We find no evidence for H$\alpha$, NaI D2 - D1, MgI and LiI in the VIS and metastable helium triplet HeI(2$^3$S) in the nIR using a line-by-line approach. The non-detection of HeI is also supported by theoretical simulations. With the use of the cross-correlation technique, we do not detect TiI, VI, CrI, FeI and VO in the visible, and CH$_4$, CO$_2$, H$_2$O, HCN, NH$_3$, VO in the nIR. Our non-detection of TiI in the planetary atmosphere is in contrast with a previous detection. We performed injection-retrieval tests, finding that our dataset is sensitive to our TiI model. The non-detection supports the TiI cold-trap theory, which is valid for planets with $T_{\rm eq} <$ 2200 K like HD 149026b. Even if we do not attribute it directly to the planet, we find a possibly significant TiI signal highly redshifted ($\simeq$+20 km s$^{-1}$) with respect to the planetary restframe. Redshifted signals are also found in the FeI and CrI maps. While we can exclude an eccentric orbit to cause it, we investigated the possibility of material accretion falling onto the star, possibly supported by the presence of strong LiI in the stellar spectrum, without finding conclusive results. The analysis of multiple transits datasets could shed more light on this target.

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arXiv2024

Evaporation of Close-in Sub-Neptunes by Cooling White Dwarfs

Motivated by the recent surge in interest concerning white dwarf (WD) planets, this work presents the first numerical exploration of WD-driven atmospheric escape, whereby the high-energy radiation from a hot/young WD can trigger the outflow of the hydrogen-helium envelope for close-in planets. As a pilot investigation, we focus on two specific cases: a gas giant and a sub-Neptune-sized planet, both orbiting a rapidly cooling WD with mass $M_\ast$ = 0.6 \msun\ and separation $a$ = 0.02 AU. In both cases, the ensuing mass outflow rates exceed $10^{14}$ g sec$^{-1}$ for WD temperatures greater than $T_{\rm WD} \simeq$ 50,000 K. At $T_{\rm WD} \simeq$ 18,000 K [/22,000 K], the sub-Neptune [/gas giant] mass outflow rate approaches $10^{12}$ g sec$^{-1}$, i.e., comparable to the strongest outflows expected from close-in planets around late main-sequence stars. Whereas the gas giant remains virtually unaffected from an evolutionary standpoint, atmospheric escape may have sizable effects for the sub-Neptune, depending on its dynamical history, e.g., assuming that the hydrogen-helium envelope makes up 1 [/4] per cent of the planet mass, the entire envelope would be evaporated away so long as the planet reaches 0.02 AU within the first 230 [/130] Myr of the WD formation. We discuss how these results can be generalized to eccentric orbits with effective semi-major axis $a'=a/(1-e^2)^{1/4}$, which receive the same orbit-averaged irradiation. Extended to a much broader parameter space, this approach can be exploited to model the expected demographics of WD planets as a function of their initial mass, composition and migration history, as well as their potential for habitability.

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arXiv2023

Self-Consistent Modeling of Metastable Helium Exoplanet Transits

Absorption of stellar X-ray and Extreme Ultraviolet radiation in the upper atmosphere of close-in exoplanets can give rise to hydrodynamic outflows, which may lead to the gradual shedding of their primordial, light element envelopes. Excess absorption by neutral helium atoms in the metastable state has recently emerged as a viable diagnostic of atmospheric escape. Here we present a public module to the 1D photo-ionization hydrodynamic code ATES, designed to calculate the HeI triplet transmission probability for a broad range of planetary parameters. By relaxing the isothermal outflow assumption, the code enables a self-consistent assessment of the HeI triplet absorption depth along with the atmospheric mass loss rate and the outflow temperature profile, which strongly affects the recombination rate of HeII into HeI triplet. We investigate how the transit signal can be expected to depend upon known system parameters, including host spectral type, orbital distance, as well as planet gravity. At variance with previous studies, which identified K-type stars as favorable hosts, we conclude that late M-dwarfs with Neptune-sized planets orbiting at ~0.05-0.1 AU can be expected to yield the strongest transit signal well in excess of 30% for near-cosmological He/H abundances. More generally, we show that the physics which regulates the population and depletion of the metastable state, combined with geometrical effects, can yield somewhat counter-intuitive results, such as a non-monotonic dependence of the transit depth on orbital distance. These are compounded by a strong degeneracy between the stellar EUV flux intensity and the atmospheric He/H abundance, both of which are highly uncertain. Compared against spectroscopy data our modelling suggests that either a large fraction of the targets have helium depleted envelopes, or, that the input stellar EUV spectra are systematically overestimated.

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