An asteroseismic view of the radius valley: stripped cores, not born rocky [EPA]

Various theoretical models treating the effect of stellar irradiation on planetary envelopes predict the presence of a radius valley: i.e. a bimodal distribution of planet radii, with super-Earths and sub-Neptune planets separated by a valley at around $\approx 2~R_\oplus$. Such a valley was observed recently, owing to an improvement in the precision of stellar, and therefore planetary radii. Here we investigate the presence, location and shape of such a valley using a small sample with highly accurate stellar parameters determined from asteroseismology, which includes 117 planets with a median uncertainty on the radius of 3.3%. We detect a clear bimodal distribution, with super-Earths ($\approx 1.5~R_\oplus$) and sub-Neptunes ($\approx 2.5~R_\oplus$) separated by a deficiency around $2~R_\oplus$. We furthermore characterize the slope of the valley as a power law $R \propto P^\gamma$ with $\gamma = {-0.09^{+0.02}_{-0.04}}$. A negative slope is consistent with models of photo-evaporation, but not with the late formation of rocky planets in a gas-poor environment, which would lead to a slope of opposite sign. The exact location of the gap further points to planet cores consisting of a significant fraction of rocky material.

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V. Eylen, C. Agentoft, M. Lundkvist, et. al.
Tue, 17 Oct 17

Comments: Submitted to MNRAS

Checking the Compatibility of the Cold Kuiper Belt with a Planetary Instability Migration Model [EPA]

The origin of the orbital structure of the cold component of the Kuiper belt is still a hot subject of investigation. Several features of the solar system suggest that the giant planets underwent a phase of global dynamical instability, but the actual dynamical evolution of the planets during the instability is still debated. To explain the structure of the cold Kuiper belt, Nesvorny (2015, AJ 150,68) argued for a soft instability, during which Neptune never achieved a very eccentric orbit. Here we investigate the possibility of a more violent instability, from an initially more compact fully resonant configuration of 5 giant planets. We show that the orbital structure of the cold Kuiper belt can be reproduced quite well provided that the cold population formed in situ, with an outer edge between 44 and 45 au and never had a large mass.

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R. Gomes, D. Nesvorny, A. Morbidelli, et. al.
Tue, 17 Oct 17

Comments: N/A

On the Origin of Banded Structure in Dusty Protoplanetary Discs: HL Tau and TW Hya [EPA]

Recent observations of HL Tau revealed remarkably detailed structure within the system’s circumstellar disc. A range of hypotheses have been proposed to explain the morphology, including, e.g., planet-disc interactions, condensation fronts, and secular gravitational instabilities. While embedded planets seem to be able to explain some of the major structure in the disc through interactions with gas and dust, the substructure, such as low-contrast rings and bands, are not so easily reproduced. Here, we show that dynamical interactions between three planets (only two of which are modelled) and an initial population of large planetesimals can potentially explain both the major and minor banded features within the system. In this context, the small grains, which are coupled to the gas and reveal the disc morphology, are produced by the collisional evolution of the newly-formed planetesimals, which are ubiquitous in the system and are decoupled from the gas.

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A. Boley
Tue, 17 Oct 17

Comments: To appear in ApJ

Double-diffusive erosion of the core of Jupiter [EPA]

We present Direct Numerical Simulations of the transport of heat and heavy elements across a double-diffusive interface or a double-diffusive staircase, in conditions that are close to those one may expect to find near the boundary between the heavy-element rich core and the hydrogen-helium envelope of giant planets such as Jupiter. We find that the non-dimensional ratio of the buoyancy flux associated with heavy element transport to the buoyancy flux associated with heat transport lies roughly between 0.5 and 1, which is much larger than previous estimates derived by analogy with geophysical double-diffusive convection. Using these results in combination with a core-erosion model proposed by Guillot et al. (2004), we find that the entire core of Jupiter would be eroded within less than 1Myr assuming that the core-envelope boundary is composed of a single interface. We also propose an alternative model that is more appropriate in the presence of a well-established double-diffusive staircase, and find that in this limit a large fraction of the core could be preserved. These findings are interesting in the context of Juno’s recent results, but call for further modeling efforts to better understand the process of core erosion from first principles.

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R. Moll, P. Garaud, C. Mankovich, et. al.
Tue, 17 Oct 17

Comments: Accepted for publication in ApJ

High-Resolution Spectroscopic Detection of TiO and Stratosphere in the Day-side of WASP-33b [EPA]

We report high-resolution spectroscopic detection of TiO molecular signature in the day-side spectra of WASP-33 b, the second hottest known hot Jupiter. We used High-Dispersion Spectrograph (HDS; R $\sim$ 165,000) in the wavelength range of 0.62 — 0.88 $\mu$m with the Subaru telescope to obtain the day-side spectra of WASP-33 b. We suppress and correct the systematic effects of the instrument, the telluric and stellar lines by using SYSREM algorithm after the selection of good orders based on Barnard star and other M-type stars. We detect a 4.8-$\sigma$ signal at an orbital velocity of $K_{p}$= +237.5 $^{+13.0}{-5.0}$ km s$^{-1}$ and systemic velocity $V{sys}$= -1.5 $^{+4.0} {-10.5}$ km s$^{-1}$, which agree with the derived values from the previous analysis of primary transit. Our detection with the temperature inversion model implies the existence of stratosphere in its atmosphere, however, we were unable to constrain the volume-mixing ratio of the detected TiO. We also measure the stellar radial velocity and use it to obtain a more stringent constraint on the orbital velocity, $K{p} = 239.0^{+2.0}_{-1.0}$ km s$^{-1}$. Our results demonstrate that high-dispersion spectroscopy is a powerful tool to characterize the atmosphere of an exoplanet, even in the optical wavelength range, and show a promising potential in using and developing similar techniques with high-dispersion spectrograph on current 10m-class and future extremely large telescopes.

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S. Nugroho, H. Kawahara, K. Masuda, et. al.
Tue, 17 Oct 17

Comments: Accepted for publication in the Astronomical Journal, 20 pages, 18 figures, 2 tables

Capture of exocomets and the erosion of the Oort cloud due to stellar encounters in the Galaxy [EPA]

The Oort cloud (OC) probably formed more than 4$\,$Gyr ago and has been moving with the Sun in the Galaxy since, exposed to external influences, most prominently to the Galactic tide and passing field stars. Theories suggest that other stars might posses exocomets distributed similarly to our OC. We study the erosion of the OC and the possibility for capturing exocomets during the encounters with such field stars. We carry out simulations of flybys, where both stars are surrounded by a cloud of comets. We measure how many exocomets are transferred to the OC, how many OC’s comets are lost, and how this depends on the other star’s mass, velocity and impact parameter. Exocomets are transferred to the OC only during relatively slow ($\lesssim0.5\,$km$\,$s$^{-1}$) and close ($\lesssim10^5\,$AU) flybys and these are expected to be extremely rare. Assuming that all passing stars are surrounded by a cloud of exocomets, we derive that the fraction of exocomets in the OC has been about $10^{-5}$–$10^{-4}$. Finally we simulate the OC for the whole lifetime of the Sun, taking into account the encounters and the tidal effects. The OC has lost 25–65% of its mass, mainly due to stellar encounters, and at most 10% (and usually much less) of its mass can be captured. However, exocomets are often lost shortly after the encounter that delivers them, due to the Galactic tide and consecutive encounters.

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J. Hanse, L. Jilkova, S. Zwart, et. al.
Tue, 17 Oct 17

Comments: Accepted for publication in MNRAS. 16 pages, 10 figures

Exoplanet phase curves at large phase angles. Diagnostics for extended hazy atmospheres [EPA]

At optical wavelengths, Titan’s brightness for large Sun-Titan-observer phase angles significantly exceeds its dayside brightness. The brightening that occurs near back-illumination is due to moderately large haze particles in the moon’s extended atmosphere that forward-scatter the incident sunlight. Motivated by this phenomenon, here we investigate the forward scattering from currently known exoplanets, its diagnostics possibilities, the observational requirements to resolve it, and potential implications. An analytical expression is derived for the amount of starlight forward-scattered by an exponential atmosphere that takes into account the finite angular size of the star. We use this expression to tentatively estimate how prevalent this phenomenon may be. Based on numerical calculations that consider exoplanet visibility, we identify numerous planets with predicted out-of-transit forward scattering signals of up to tens of parts-per-million provided that aerosols of $>$1 micron size form over an extended vertical region near the optical radius level. We propose that the interpretation of available optical phase curves should be revised to constrain the strength of this phenomenon that might provide insight into aerosol scale heights and particle sizes. For the relatively general atmospheres considered here, forward scattering reduces the transmission-only transit depth by typically less than the equivalent to a scale height. For short-period exoplanets the finite angular size of the star severely affects the amount of radiation scattered towards the observer at mid-transit.

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A. Munoz and J. Cabrera
Tue, 17 Oct 17

Comments: 20 pages; In press, MNRAS. this https URL