arXiv subjects

Riccardo Spinelli

Publications and source records attributed to Riccardo Spinelli.

9 recordsLinked to original sources

arXiv2026

A Critical UV Legacy: A Hubble Roadmap for HWO Science Readiness

The Habitable Worlds Observatory (HWO) will provide the first opportunity to directly image and spectrally characterize terrestrial exoplanets in the habitable zones of nearby stars. Maximizing its scientific return requires a comprehensive understanding of the high-energy radiation environments of target stars, which shape planetary atmospheres and govern the production, destruction, and detectability of biosignatures. Ultraviolet (UV) radiation plays a particularly critical role in atmospheric chemistry. Far-ultraviolet (FUV) and near-ultraviolet (NUV) photons regulate key photochemical pathways, influence ozone stability, and drive the formation of prebiotic molecules. However, the majority of high-priority HWO target stars lack high-quality UV observations. Existing datasets are sparse, heterogeneous, or limited by calibration uncertainties, and no comparable UV observatory is expected for at least 5-10 years (with UVEX offering more limited spectral resolution, wavelength coverage, and sensitivity). The Hubble Space Telescope (HST) remains the only observatory capable of acquiring high-resolution FUV and NUV spectra for these targets over the next 10-15 years. We therefore advocate for a coordinated HST program to systematically obtain UV spectra of high-priority HWO targets, ideally in conjunction with X-ray observations. This effort is essential for enabling accurate target prioritization, constraining stellar radiation environments, and ensuring robust interpretation of future HWO observations.

astro-ph.IM↗

arXiv2025

HWO Target Stars and Systems: A Survey of Archival UV and X-ray Data

We assess archival high-energy data for key stars on the Habitable Worlds Observatory (HWO) Target Stars and Systems 2025 list, as stellar radiation is critical to shaping and interpreting planetary atmospheres. Using a sample of 98 nearby stars (HWO Tier 1 targets), we compile and evaluate X-ray and ultraviolet (UV) data from archival eROSITA, Chandra, XMM-Newton, ROSAT, EUVE, Swift, FUSE, IUE, GALEX, and HST. We examine spectral and temporal coverage, assess data quality, and identify major gaps. UV data are moderately available, with most coverage coming from near-UV spectra from IUE. Far fewer stars have far-UV spectra, especially from HST. In the X-ray regime, some stars have high-quality spectra, while others are limited to shallow detections or broad-band photometry. A small fraction of the sample has both X-ray and UV spectra of sufficient quality to support full spectral energy distribution modeling. Truly comprehensive coverage across X-ray, extreme-UV, and both UV bands remains extremely rare. Most datasets are single-epoch, limiting assessments of variability and flares - key factors in atmospheric photochemistry and escape. Moreover, the lack of simultaneous or contemporaneous observations across bands adds further uncertainty. Our findings underscore the need for new space-based missions and coordinated multiwavelength campaigns, ideally with overlapping coverage, to improve stellar characterization for HWO. As several key observatories age and face potential decommissioning, there is a narrow window of opportunity to secure these critical data. Investing in this effort now will directly support the science goals of HWO and enhance future studies of planetary habitability.

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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

Windows Into Other Worlds: Pitfalls in the physical interpretation of exoplanet atmospheric spectroscopy

Atmospheric spectroscopy provides a window into the properties of exoplanets. However, the physical interpretation of retrieved data and its implications for the internal properties of exoplanets remains nebulous. This letter addresses three misconceptions held by some atmospheric spectroscopists regarding the connection between observed chemical abundances and theory: (1) Whether atmospheric spectroscopy can provide the bulk atmospheric chemistry, (2) whether it can identify if a planet is cloudless, and (3) whether atmospheric evaporation arguments can be used to dismiss certain compositions inferred through spectroscopy. This letter concludes by exploring applications of remote sensing in the quest for the search for life outside of our solar system.

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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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arXiv2023

The ultraviolet habitable zone of exoplanets

The dozens of rocky exoplanets discovered in the Circumstellar Habitable Zone (CHZ) currently represent the most suitable places to host life as we know it outside the Solar System. However, the presumed presence of liquid water on the CHZ planets does not guarantee suitable environments for the emergence of life. According to experimental studies, the building blocks of life are most likely produced photochemically in presence of a minimum ultraviolet (UV) flux. On the other hand, high UV flux can be life-threatening, leading to atmospheric erosion and damaging biomolecules essential to life. These arguments raise questions about the actual habitability of CHZ planets around stars other than Solar-type ones, with different UV to bolometric luminosity ratios. By combining the "principle of mediocricy" and recent experimental studies, we define UV boundary conditions (UV-habitable Zone, UHZ) within which life can possibly emerge and evolve. We investigate whether exoplanets discovered in CHZs do indeed experience such conditions. By analysing Swift-UV/Optical Telescope data, we measure the near ultraviolet (NUV) luminosities of 17 stars harbouring 23 planets in their CHZ. We derive an empirical relation between NUV luminosity and stellar effective temperature. We find that eighteen of the CHZ exoplanets actually orbit outside the UHZ, i.e., the NUV luminosity of their M-dwarf hosts is decisively too low to trigger abiogenesis - through cyanosulfidic chemistry - on them. Only stars with effective temperature >3900 K illuminate their CHZ planets with enough NUV radiation to trigger abiogenesis. Alternatively, colder stars would require a high-energy flaring activity.

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arXiv2022

Irradiation-driven escape of primordial planetary atmospheres II. Evaporation efficiency of sub-Neptunes through hot Jupiters

Making use of the publicly available 1D photoionization hydrodynamics code ATES we set out to investigate the combined effects of planetary gravitational potential energy ($ϕ_p\equiv GM_p/R_p$) and stellar X-ray and Extreme Ultraviolet (XUV) irradiation ($F_{\rm XUV}$) on the evaporation efficiency ($η$) of moderately-to-highly irradiated gaseous planets, from sub-Neptunes through hot Jupiters. We show that the (known) existence of a threshold potential above which energy-limited escape (i.e., $η\simeq 1$) is unattainable can be inferred analytically. For $\log ϕ_p\gtrsim \log ϕ_p^{\rm thr}\approx [12.9-13.2]$ (in cgs units), most of the energy absorption occurs where the average kinetic energy acquired by the ions through photo-electron collisions is insufficient for escape. This causes the evaporation efficiency to plummet with increasing $ϕ_p$,. Whether or not planets with $ϕ_p\lesssim ϕ_p^{\rm thr}$ exhibit energy-limited outflows is regulated primarily by the stellar irradiation level. Specifically, for low-gravity planets, above $F_{\rm XUV}\simeq 10^{4-5}$ erg cm$^{-2}$s$^{-1}$ Ly$α$ losses overtake adiabatic and advective cooling and the evaporation efficiency of low-gravity planets drops below the energy-limited approximation, albeit remaining largely independent of $ϕ_p$Further, we show that whereas $η$ increases as $F_{\rm XUV}$ increases for planets above $ϕ^{\rm thr}_p$, the opposite is true for low-gravity planets. This behavior can be understood by examining the relative fractional contributions of advective and radiative losses as a function of atmospheric temperature. This novel framework enables a reliable, physically motivated prediction of the expected evaporation efficiency for a given planetary system; an analytical approximation of the best-fitting $η$ is given in the appendix.

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arXiv2021

Irradiation-driven escape of primordial planetary atmospheres I. The ATES photoionization hydrodynamics code

Intense X-ray and ultraviolet stellar irradiation can heat and inflate the atmospheres of closely orbiting exoplanets, driving mass outflows that may be significant enough to evaporate a sizable fraction of the planet atmosphere over the system lifetime. The recent surge in the number of known exoplanets, together with the imminent deployment of new ground and space-based facilities for exoplanet discovery and characterization, requires a prompt and efficient assessment of the most promising targets for intensive spectroscopic follow-ups. To this purpose, we developed ATES (ATmospheric EScape); a new hydrodynamics code that is specifically designed to compute the temperature, density, velocity and ionization fraction profiles of highly irradiated planetary atmospheres, along with the current, steady-state mass loss rate. ATES solves the one-dimensional Euler, mass and energy conservation equations in radial coordinates through a finite-volume scheme. The hydrodynamics module is paired with a photoionization equilibrium solver that includes cooling via bremsstrahlung, recombination and collisional excitation/ionization for the case of a primordial atmosphere entirely composed of atomic hydrogen and helium, whilst also accounting for advection of the different ion species. Compared against the results of 14 moderately-to-highly irradiated planets simulated with The PLUTO-CLOUDY Interface (TPCI), ATES yields remarkably good agreement at a significantly smaller fraction of the computational time. A convergence study shows that ATES recovers stable, steady-state hydrodynamic solutions for systems with $\log(-ϕ_p) \lesssim 12.9 + 0.17\log F_{\rm XUV}$. Incidentally, atmospheres of systems above this threshold are generally thought to be undergoing Jeans escape. The code, which also features a user-friendly graphic interface, is available publicly as an online repository.

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