Neue Artikel in Astronomy & Astrophysics Volume 689 (September 2024)
von Michael Johne ·
Zwischen dem 23.08.2024 und 30.08.2024 wurden in der Astronomy & Astrophysics Volume 689 (September 2024) 4 neue Artikel veröffentlicht.
Link: https://www.aanda.org/articles/aa/full_html/2024/09/aa50629-24/aa50629-24.html
Autoren: S. Howard, S. Müller and R. Helled
Kurztext: The phase separation between hydrogen and helium at high pressures and temperatures leads to the rainout of helium in the deep interiors of Jupiter and Saturn. This process, also known as “helium rain”, affects their long-term evolution. Modeling the evolution and internal structure of Jupiter and Saturn (and giant exoplanets) relies on the phase diagram of hydrogen and helium. In this work, we simulated the evolution of Jupiter and Saturn with helium rain by applying different phase diagrams of hydrogen and helium and we searched for models that reproduce the measured atmospheric helium abundance in the present day. We find that a consistency between Jupiter’s evolution and the Galileo measurement of its atmospheric helium abundance can only be achieved if a shift in temperature is applied to the existing phase diagrams (−1250 K, +350 K or −3850 K depending on the applied phase diagram). Next, we used the shifted phase diagrams to model Saturn’s evolution and we found consistent solutions for both planets. We confirm that de-mixing in Jupiter is modest, whereas in Saturn, the process of helium rain is significant. We find that Saturn has a large helium gradient and a helium ocean. Saturn’s atmospheric helium mass fraction is estimated to be between 0.13 and 0.16. We also investigated how the applied hydrogen-helium equation of state and the atmospheric model affect the planetary evolution, finding that the predicted cooling times can change by several hundred million years. Constraining the level of super-adiabaticity in the helium gradient formed in Jupiter and Saturn remains challenging and should be investigated in detail in future research. We conclude that further explorations of the immiscibility between hydrogen and helium are valuable as this knowledge directly affects the evolution and current structure of Jupiter and Saturn. Finally, we argue that measuring Saturn’s atmospheric helium content is crucial for constraining Saturn’s evolution as well as the hydrogen-helium phase diagram.
Link: https://www.aanda.org/articles/aa/full_html/2024/09/aa49897-24/aa49897-24.html
Autoren: S. Shibata, R. Helled
Kurztext: Context. The composition of giant planets’ atmospheres is an important tracer of their formation history. While many theoretical studies investigate the heavy-element accretion within a gaseous protoplanetary disk, the possibility of solid accretion after disk dissipation has not been explored.
Aims. Here, we focus on the case of a gas giant planet excited to an eccentric orbit and assess the likelihood of solid accretion after disk dissipation. We follow the orbital evolution of the surrounding solid materials and investigate the scattering and accretion of heavy elements in the remnant solid disks.
Methods. We perform N-body simulations of planetesimals and embryos around an eccentric giant planet. We consider various sizes and orbits for the eccentric planet and determine the fate of planetesimals and embryos.
Results. We find that the orbital evolution of solids, such as planetesimals and embryos, is regulated by weak encounters with the eccentric planet rather than strong close encounters. Even in the region where the Safronov number is smaller than unity, most solid materials fall onto the central star or are ejected from the planetary system. We also develop an analytical model of the solid accretion along the orbital evolution of a giant planet, where the accretion probability is obtained as a function of the planetary mass, radius, semi-major axis, eccentricity, inclination, and solid disk thickness.
Conclusions. Our model predicts that ~0.01–0.1 M⊕ of solids is accreted onto an eccentric planet orbiting in the outer disk (~10 au). The accreted heavy-element mass increases (decreases) with the eccentricity (inclination) of the planet. We also discuss the possibility of collisions of terrestrial planets and find that ~ 10% of the hot Jupiters formed via high-eccentric migration collide with a planet of 10 M⊕. However, we find that solid accretion and collisions with terrestrial planets are minor events for planets in the inner orbit, and a different accretion process is required to enrich eccentric giant planets with heavy elements.
Link: https://www.aanda.org/articles/aa/full_html/2024/09/aa46570-23/aa46570-23.html
Autoren: L. Mignon, X. Delfosse, X. Bonfils, N. Meunier, N. Astudillo-Defru, G. Gaisne, T. Forveille, F. Bouchy, G. Lo Curto, S. Udry, D. Segransan, N. Unger, C. Lovis, N. C. Santos, M. Mayor
Kurztext: Context. The census of planets around M dwarfs in the solar neighbourhood meets two challenges: detecting the best targets for the future characterisation of planets with ELTs, and studying the statistics of planet occurrence that are crucial to formation scenarios. The radial velocity (RV) method remains the most appropriate for such a census as it is sensitive to the widest ranges of masses and periods. HARPS, mounted on the 3.6 m telescope at La Silla Observatory (ESO, Chile), has been obtaining velocity measurements since 2003, and can therefore be used to analyse a very large and homogeneous dataset.
Aims. We performed a homogeneous analysis of the RV time series of 200 M dwarfs observed with HARPS from 2003 to 2019 (gathering more than 15 000 spectra), with the aim of understanding detectable signals such as stellar and planetary companions and activity signals.
Methods. The RVs were computed with a template matching method before carrying out the time series analysis. First, we focused on the systematic analysis of the presence of a dominant long-term pattern in the RV time series (linear or quadratic trend and sine function). Then, we analysed higher-frequency perdiodic signals using periodograms of the residual time series and Keplerian function fitting.
Results. We found long-term variability in 57 RV time series (28.5%). This led to the revision of the parameters of the massive planet (GJ 9482 b), as well as the detection of four substellar and stellar companions (around GJ 3307, GJ 4001, GJ 4254, and GJ 9588), for which we characterised inclinations and masses by combining RV and astrometry. The periodic analysis allowed us to recover 97% of the planetary systems already published in this sample, but also to propose three new planetary candidates orbiting GJ 300 (7.3 M⊕), GJ 654(5 M⊕), and GJ 739 (39 M⊕), which require additional measurements before they can be confirmed.
Link: https://www.aanda.org/articles/aa/full_html/2024/09/aa50107-24/aa50107-24.html
Autoren: H. Diamond-Lowe, G. W. King, A. Youngblood, A. Brown, W. S. Howard, J. G. Winters, D. J. Wilson, K. France, J. M. Mendonça, L. A. Buchhave, L. Corrales, L. Kreidberg, A. A. Medina, J. L. Bean, Z. K. Berta-Thompson, T. M. Evans-Soma, C. Froning, G. M. Duvvuri, E. M.-R. Kempton, Y. Miguel, J. S. Pineda, C. Schneider
Kurztext: The high-energy radiative output, from the X-ray to the ultraviolet, of exoplanet host stars drives photochemical reactions and mass loss in the upper regions of planetary atmospheres. In order to place constraints on the atmospheric properties of the three closest terrestrial exoplanets transiting M dwarfs, we observe the high-energy spectra of the host stars LTT 1445A and GJ 486 in the X-ray with XMM-Newton and Chandra and in the ultraviolet with HST/COS and STIS. We combine these observations with estimates of extreme-ultraviolet flux, reconstructions of the Lyα lines, and stellar models at optical and infrared wavelengths to produce panchromatic spectra from 1 Å to 20 µm for each star. While LTT 1445Ab, LTT 1445Ac, and GJ 486b do not possess primordial hydrogen-dominated atmospheres, we calculate that they are able to retain pure CO2 atmospheres if starting with 10, 15, and 50% of Earth’s total CO2 budget, respectively, in the presence of their host stars’ stellar wind. We use age-activity relationships to place lower limits of 2.2 and 6.6 Gyr on the ages of the host stars LTT 1445A and GJ 486. Despite both LTT 1445A and GJ 486 appearing inactive at optical wavelengths, we detect flares at ultraviolet and X-ray wavelengths for both stars. In particular, GJ 486 exhibits two far-ultraviolet flares with absolute energies of 1029.5 and 1030.1 erg (equivalent durations of 4357 ± 96 and 19 724 ± 169 s) occurring 3 h apart. Based on the timing of the observations, we suggest that these high-energy flares are related and indicative of heightened flaring activity that lasts for a period of days, but our interpretations are limited by sparse time-sampling. Consistent high-energy monitoring is needed to determine the duration and extent of high-energy activity on individual M dwarfs and the population as a whole.
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