16 research outputs found
The mass-radius relation of intermediate-mass planets outlined by hydrodynamic escape and thermal evolution
We employ planetary evolution modeling to reproduce the MR distribution of
the 198 so far detected planets with mass and radius measured to the <45% and
<15% level, respectively, and less massive than 108Me. We simultaneously
account for atmospheric escape, based on the results of hydrodynamic models,
and thermal evolution, based on planetary structure evolution models. Since
high-energy stellar radiation affects atmospheric evolution, we account for the
entire range of possible stellar rotation histories. To set the planetary
parameters at formation, we use analytical approximations based on formation
models. Finally, we build a grid of synthetic planets with parameters
reflecting those of the observed distribution. The predicted radius spread
reproduces well the observed MR distribution, except for two distinct groups of
outliers (~20% of the population). The first one consists of close-in
Saturn-mass planets with Jupiter-like radii for which we underpredict the
radius likely because it lacks additional heating similar to that responsible
for inflation in hot Jupiters. The second group consists of warm sub-Neptunes,
which should host massive primordial H-dominated atmospheres, but instead
present high densities indicative of small gaseous envelopes. This suggests
that their formation, internal structure, and evolution are different from that
of atmospheric evolution through the escape of H-dominated envelopes accreted
onto rocky cores. The observed characteristics of low-mass planets (<10-15Me)
strongly depend on the impact of atmospheric escape, and thus on the evolution
of the host star, while primordial parameters are less relevant. Instead, for
more massive planets, the parameters at formation play the dominant role in
shaping the final MR distribution.Comment: 14 pages + 8 pages Appendix, 8+4 Figures; Accepted for publication in
A&
Precise photoionisation treatment and hydrodynamic effects in atmospheric modelling of warm and hot Neptunes
Observational breakthroughs in the exoplanet field of the last decade
motivated the development of numerous theoretical models describing atmospheres
and mass loss, which is believed to be one of the main drivers of planetary
evolution. We aim to outline for which types of close-in planets in the
Neptune-mass range the accurate treatment of photoionisation effects is most
relevant concerning atmospheric escape and the parameters relevant for
interpreting observations. We developed the CHAIN (Cloudy e Hydro Ancora
INsieme) model combining 1D hydrodynamic upper atmosphere model with the
non-LTE photoionisation and radiative transfer code Cloudy accounting for
photochemistry, detailed atomic level populations, and chemical reactions for
all elements up to zinc. We apply CHAIN to model the upper atmospheres of a
range of Neptune-like planets with masses between 1 and 50 M,
varying also the orbital parameters. For the majority of warm and hot Neptunes,
we find slower and denser outflows, with lower ion fractions, compared to the
predictions of the hydrodynamic model alone. Furthermore, we find significantly
different temperature profiles between CHAIN and the hydrodynamic model alone,
though the peak values are similar for similar atmospheric compositions. The
mass-loss rates predicted by CHAIN are higher for hot, strongly irradiated
planets and lower for more moderate planets. All differences between the two
models are strongly correlated with the amount of high-energy irradiation.
Finally, we find that the hydrodynamic effects impact significantly ionisation
and heating. The impact of the precise photoionisation treatment provided by
Cloudy strongly depends on the system parameters. This suggests that some of
the simplifications typically employed in hydrodynamic modelling might lead to
systematic errors when studying planetary atmospheres, even at a
population-wide level.Comment: 23 pages, 16 figures; Accepted for publication in A&
Planetary evolution with atmospheric photoevaporation II: Fitting the slope of the radius valley by combining boil-off and XUV-driven escape
The Kepler satellite has revealed a gap between sub-Neptunes and super-Earths
that atmospheric escape models had predicted as an evaporation valley. We seek
to contrast results from a simple XUV-driven energy-limited (ELIM) escape model
against those from a direct hydrodynamic (HYDRO) model. Besides XUV-driven
escape, the latter also includes the boil-off regime. We couple the two models
to an internal structure model and follow the planets' temporal evolution over
Gyr. To see the population-wide imprint of the two models, we first employ a
rectangular grid in initial conditions. We then study the slope of the valley
also for initial conditions derived from the Kepler planets. For the
rectangular grid, we find that the power-law slope of the valley with respect
to orbital period is -0.18 and -0.11 in the ELIM and HYDRO model, respectively.
For the initial conditions derived from the Kepler planets, the results are
similar (-0.16 and -0.10). While the slope found with the ELIM model is steeper
than observed, the one of the HYDRO model is in excellent agreement with
observations. The reason for the shallower slope is caused by the two regimes
in which the ELIM model fails: First, puffy planets at low stellar irradiation.
For them, boil-off dominates mass loss. However, boil-off is absent in the ELIM
model, thus it underestimates escape relative to HYDRO. Second, massive compact
planets at high XUV irradiation. For them, the ELIM approximation overestimates
escape relative to the HYDRO case because of cooling by thermal conduction,
neglected in the ELIM model. The two effects act together in concert to yield
in the HYDRO model a shallower slope of the valley that agrees very well with
observations. We conclude that an escape model that includes boil-off and a
more realistic treatment of cooling mechanisms can reproduce one of the most
important constraints, the valley slope.Comment: 20 pages, 11 figures, accepted to A&
The Transiting Multi-planet System HD15337: Two Nearly Equal-mass Planets Straddling the Radius Gap
We report the discovery of a super-Earth and a sub-Neptune transiting the star HD 15337 (TOI-402, TIC 120896927), a bright (V = 9) K1 dwarf observed by the Transiting Exoplanet Survey Satellite (TESS) in Sectors 3 and 4. We combine the TESS photometry with archival High Accuracy Radial velocity Planet Searcher spectra to confirm the planetary nature of the transit signals and derive the masses of the two transiting planets. With an orbital period of 4.8 days, a mass of and a radius of 1.64 ± 0.06 R â, HD 15337 b joins the growing group of short-period super-Earths known to have a rocky terrestrial composition. The sub-Neptune HD 15337 c has an orbital period of 17.2 days, a mass of , and a radius of 2.39 ± 0.12 R â, suggesting that the planet might be surrounded by a thick atmospheric envelope. The two planets have similar masses and lie on opposite sides of the radius gap, and are thus an excellent testbed for planet formation and evolution theories. Assuming that HD 15337 c hosts a hydrogen-dominated envelope, we employ a recently developed planet atmospheric evolution algorithm in a Bayesian framework to estimate the history of the high-energy (extreme ultraviolet and X-ray) emission of the host star. We find that at an age of 150 Myr, the star possessed on average between 3.7 and 127 times the high-energy luminosity of the current Sun
The Multiplanet System TOI-421: A Warm Neptune and a Super Puffy Mini-Neptune Transiting a G9 V Star in a Visual Binary
We report the discovery of a warm Neptune and a hot sub-Neptune transiting TOI-421 (BD-14 1137, TIC 94986319), a bright (V = 9.9) G9 dwarf star in a visual binary system observed by the Transiting Exoplanet Survey Satellite (TESS) space mission in Sectors 5 and 6. We performed ground-based follow-up observationsâcomprised of Las Cumbres Observatory Global Telescope transit photometry, NIRC2 adaptive optics imaging, and FIbre-fed EchellĂ© Spectrograph, CORALIE, High Accuracy Radial velocity Planet Searcher, High Resolution Ăchelle Spectrometer, and Planet Finder Spectrograph high-precision Doppler measurementsâand confirmed the planetary nature of the 16 day transiting candidate announced by the TESS team. We discovered an additional radial velocity signal with a period of five days induced by the presence of a second planet in the system, which we also found to transit its host star. We found that the inner mini-Neptune, TOI-421 b, has an orbital period of P_b = 5.19672 ± 0.00049 days, a mass of M_b = 7.17 ± 0.66 Mâ, and a radius of R_b = 2.68^(+0.19)_(-0.18) Râ, whereas the outer warm Neptune, TOI-421 c, has a period of Pc = 16.06819 ± 0.00035 days, a mass of M_c = 16.42^(+1.06)_(-1.04) Mâ, a radius of R_c = 5.09^(+0.16)_(-0.15) Râ and a density of Ï_c = 0.685^(+0.080)_(-0.072) g cmâ»Âł. With its characteristics, the outer planet (Ï_c = 0.685^(+0.080)_(-0.072) g cmâ»Âł) is placed in the intriguing class of the super-puffy mini-Neptunes. TOI-421 b and TOI-421 c are found to be well-suited for atmospheric characterization. Our atmospheric simulations predict significant Lyα transit absorption, due to strong hydrogen escape in both planets, as well as the presence of detectable CH4 in the atmosphere of TOI-421 c if equilibrium chemistry is assumed