Earth and Planetary Astrophysics (astro-ph.EP)

Abstract: Forming planets around young, fast-rotating solar-like stars are exposed to an intense X-ray/extreme ultraviolet radiation field and strongly magnetized stellar winds, as a consequence of the high magnetic activity of these stars. Under these conditions, Earth-like exoplanets may experience a rapid loss of their primordial hydrogen atmospheres, resulting in atmosphere-less rocky obstacles for the stellar winds. The interaction of stellar winds with those planets leads to the formation of potentially observable structures due to the formation of large-scale magnetic field and density disturbances in the vicinity of these planets, such as bow shocks, induced magnetospheres and comet-like tails. In this work, we study the interaction between the stellar winds of active, fast-rotating solar-like stars in the superfast-magnetosonic regime with Earth-like, unmagnetized, tenuous atmosphere, planetary obstacles through numerical 3D simulations using the PLUTO magnetohydrodynamical code. The properties of AB Doradus, a nearby young star with a small rotation period (0.51 days) and a strong flaring activity, have been used to parameterize this early wind state. Bow shock and induced magnetosphere formation are characterized through the alfv\'enic Mach number MA of the wind, for different stellar wind configurations. Large bow shocks, up to an extension of ~7.0 planetary radii are found for low-MA winds. The general increase of density, temperature and magnetic field in these large-scale structures formed around planets may result in potentially detectable spectral signatures.

Abstract: Protoplanetary disks emit radiation across a broad range of wavelengths, requiring a multiwavelength approach to fully understand their physical mechanisms and how they form planets. Observations at sub-millimeter to centimeter wavelengths can provide insights into the thermal emission from dust, free-free emission from ionized gas, and possible gyro-synchrotron emission from the stellar magnetosphere. This Letter focuses on CX Tau, a ${\sim}0.4\,M_\odot$ star with an extended gas emission and a compact and apparently structureless dust disk, with an average millimeter flux when compared to Class II sources in Taurus. We present Karl G. Jansky Very Large Array (VLA) observations in 4 bands (between 9.0 mm and 6.0 cm) and combine them with archival data from the Atacama Large Millimeter/submillimeter Array (ALMA), the Submillimeter Array (SMA) and the Plateau de Bure Interferometer (PdBI). Such a multiwavelength approach allows to separate the dust continuum from other emissions. After isolating the dust thermal emission, we derived an upper limit of the dust disk extent at 1.3 cm which is consistent with theoretical predictions of a radial drift-dominated disk. Centimeter data show a peculiar behavior: deep observations at 6.0 cm did not detect the source, while at 1.3 cm the flux density is anomalously higher than adjacent bands. Intraband spectral indices suggest a dominant contribution from free-free emission, whereas gyro-synchrotron emission is excluded. To explain these observations, we propose strong variability of the free-free emission with timescales shorter than a month. Another possible interpretation is the presence of anomalous microwave emission from spinning dust grains.

Abstract: Transmission spectroscopy supports the presence of uncharacterised, light-scattering and -absorbing aerosols in the atmospheres of many exoplanets. The complexity of factors influencing the formation, 3-D transport, radiative impact, and removal of aerosols makes it challenging to match theoretical models to the existing data. Our study simplifies these factors to focus on the interaction between planetary general circulation and haze distribution at the planetary limb. We use an intermediate complexity general circulation model, ExoPlaSim, to simulate idealised organic haze particles as radiatively active tracers in the atmospheres of tidally locked terrestrial planets for 32 rotation rates. We find three distinct 3-D spatial haze distributions, corresponding to three circulation regimes, each with a different haze profile at the limb. All regimes display significant terminator asymmetry. In our parameter space, super-Earth-sized planets with rotation periods greater than 13 days have the lowest haze optical depths at the terminator, supporting the choice of slower rotators as observing targets.

Abstract: The orbits of some warm Jupiters are highly inclined (20$^\circ$-50$^\circ$) to those of their exterior companions. Comparable misalignments are inferred between the outer and inner portions of some transition discs. These large inclinations may originate from planet-planet and planet-disc secular resonances that sweep across interplanetary space as parent discs disperse. The maximum factor by which a seed mutual inclination can be amplified is of order the square root of the angular momentum ratio of the resonant pair. We identify those giant planet systems (e.g. Kepler-448 and Kepler-693) which may have crossed a secular resonance, and estimate the required planet masses and semimajor axes in transition discs needed to warp their innermost portions (e.g. in CQ Tau). Passage through an inclination secular resonance could also explain the hypothesized large mutual inclinations in apsidally-orthogonal warm Jupiter systems (e.g. HD 147018).