The ¹³CO-rich atmosphere of a young accreting super-Jupiter

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Isotope abundance ratios play an important role in astronomy and planetary sciences, providing insights in the origin and evolution of the Solar System, interstellar chemistry, and stellar nucleosynthesis. In contrast to deuterium/hydrogen ratios, carbon isotope ratios are found to be roughly constant (~89) in the Solar System, but do vary on galactic scales with ¹²C/¹³C~68 in the current local interstellar medium. In molecular clouds and protoplanetary disks, ¹²CO/¹³CO isotopologue ratios can be altered by ice and gas partitioning, low-temperature isotopic ion exchange reactions, and isotope-selective photodissociation. Here we report on the detection of ¹³CO in the atmosphere of the young, accreting giant planet TYC 8998-760-1 b at a statistical significance of >6σ. Marginalizing over the planet’s atmospheric temperature structure, chemical composition, and spectral calibration uncertainties, suggests a ¹²CO/¹³CO ratio of ~31, a significant enrichment in ¹³C with respect to the terrestrial standard and the local interstellar value. Since the current location of TYC 8998 b at >160 au is far beyond the CO snowline, we postulate that it accreted a significant fraction of its carbon from ices enriched in ¹³C through fractionation. Future isotopologue measurements in exoplanet atmospheres can provide unique constraints on where, when and how planets are formed.

A search for He I airglow emission from the hot Jupiter τ Boo b

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Context. It has been suggested that the helium absorption line at 10830 Å that originates from the metastable triplet state is an excellent probe for the extended atmospheres of hot Jupiters and their hydrodynamic escape processes. It has recently been detected in the transmission spectra of a handful of planets. The isotropic reemission will lead to helium airglow that may be observable at other orbital phases.

Aims: We investigate the detectability of He I emission at 10830 Å in the atmospheres of exoplanets using high-resolution spectroscopy. This would provide insights into the properties of the upper atmospheres of close-in gas giants.

Methods: We estimated the expected strength of He I emission in hot Jupiters based on their transmission signal. We searched for the He I 10830 Å emission feature in τ Boo b in three nights of high-resolution spectra taken by CARMENES at the 3.5m Calar Alto telescope. The spectra from each night were corrected for telluric absorption, sky emission lines, and stellar features, and were shifted to the planetary rest frame to search for the emission.

Results: The He I emission is not detected in τ Boo b at a 5σ contrast limit of 4 × 10^-4 for emission line widths of >20 km/s. This is about a factor 8 above the expected emission level (assuming a typical He I transit absorption of 1% for hot Jupiters). This suggests that targeting the He I emission with well-designed observations using upcoming instruments such as VLT/CRIRES+ and E-ELT/HIRES is possible.

Planet Formation in Highly Inclined Binary Systems

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Stars are commonly formed in binary systems, which provide a natural laboratory for studying planet formation in extreme conditions. We numerically study the late stage of terrestrial planet formation, i.e., from embryos to full planets, in binary systems of various orbital configurations. We identify an orbital alignment effect; namely, although an inclined binary generally misaligns the planetary orbits with respect to the spin axis of the primary host star (i.e., causing large obliquity), it could align the planetary orbits with respect to the binary orbit. Such an orbital alignment effect is caused by the combination of orbital differential precession and self-damping, and it is mostly significant in cases of intermediate binary separations, i.e., 40–200 au for planet formation around 1 au from the primary stars. In such intermediate separation binaries, somewhat contrary to intuition, the binary companion can aid planet growth by having increased the rate of collisions, forming significantly more massive but fewer planets. On the other two ends, the companion is either too close, and thus plays a violently disruptive role, or too wide to have a significant effect on planet formation. Future observations that can discover more planet-bearing binary star systems and constrain their masses and 3D orbital motions will test our numerical findings.

Animation of a typical simulation with a binary separation of 80 au and inclination of 60 deg. The orbits of planetary embryos (denoted with the cyan spheres) were initially aligned with respect to the spin of the primary but finally aligned with respect to the binary orbit (not shown). The green arrows denote the normal lines of the primary spin plane (L_spin) and of the binary orbital plane (L_binary) respectively.