Disc truncation in embedded star clusters: Dynamical encounters versus face-on accretion [GA]


Observations indicate that the dispersal of protoplanetary discs in star clusters occurs on time scales of about 5 Myr. Several processes are thought to be responsible for this disc dispersal. Here we compare two of these processes: dynamical encounters and interaction with the interstellar medium, which includes face-on accretion and ram pressure stripping. We perform simulations of embedded star clusters with parameterisations for both processes to determine the environment in which either of these processes is dominant. We find that face-on accretion, including ram pressure stripping, is the dominant disc truncation process if the fraction of the total cluster mass in stars is $\lesssim 30\,\%$ regardless of the cluster mass and radius. Dynamical encounters require stellar densities $\gtrsim 10^4$ pc$^{-3}$ combined with a mass fraction in stars of $\approx 90\,\%$ to become the dominant process. Our results show that during the embedded phase of the cluster, the truncation of the discs is dominated by face-on accretion and dynamical encounters become dominant when the intra-cluster gas has been expelled. As a result of face-on accretion the protoplanetary discs become compact and their surface density increases. In contrast, dynamical encounters lead to discs that are less massive and remain larger.

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T. Wijnen, O. Pols, F. Pelupessy, et. al.
Fri, 23 Jun 17

Comments: Accepted for publication in A&A, 14 pages, 8 figures, 1 table

Gap and rings carved by vortices in protoplanetary dust [EPA]


Large-scale vortices in protoplanetary disks are thought to form and survive for long periods of time. Hence, they can significantly change the global disk evolution and particularly the distribution of the solid particles embedded in the gas, possibly explaining asymmetries and dust concentrations recently observed at sub-millimeter and millimeter wavelengths. We investigate the spatial distribution of dust grains using a simple model of protoplanetary disk hosted by a giant gaseous vortex. We explore the dependence of the results on grain size and deduce possible consequences and predictions for observations of the dust thermal emission at sub-millimeter and millimeter wavelengths. Global 2D simulations with a bi-fluid code are used to follow the evolution of a single population of solid particles aerodynamically coupled to the gas. Possible observational signatures of the dust thermal emission are obtained using simulators of ALMA and ngVLA observations. We find that a giant vortex not only captures dust grains with Stokes number St < 1 but can also affect the distribution of larger grains (with St ‘~’ 1) carving a gap associated to a ring composed of incompletely trapped particles. The results are presented for different particle size and associated to their possible signatures in disk observations. Gap clearing in the dust spatial distribution could be due to the interaction with a giant gaseous vortex and their associated spiral waves, without the gravitational assistance of a planet. Hence, strong dust concentrations at short sub-mm wavelengths associated with a gap and an irregular ring at longer mm and cm wavelengths could indicate the presence of an unseen gaseous vortex.

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P. Barge, L. Ricci, C. Carilli, et. al.
Fri, 23 Jun 17

Comments: 11 pages, 11 figures, accepted for publication in A&A

MOVES I. The evolving magnetic field of the planet-hosting star HD189733 [SSA]


HD189733 is an active K dwarf that is, with its transiting hot Jupiter, among the most studied exoplanetary systems. In this first paper of the Multiwavelength Observations of an eVaporating Exoplanet and its Star (MOVES) program, we present a 2-year monitoring of the large-scale magnetic field of HD189733. The magnetic maps are reconstructed for five epochs of observations, namely June-July 2013, August 2013, September 2013, September 2014, and July 2015, using Zeeman-Doppler Imaging. We show that the field evolves along the five epochs, with mean values of the total magnetic field of 36, 41, 42, 32 and 37 G, respectively. All epochs show a toroidally-dominated field. Using previously published data of Moutou et al. 2007 and Fares et al. 2010, we are able to study the evolution of the magnetic field over 9 years, one of the longest monitoring campaign for a given star. While the field evolved during the observed epochs, no polarity switch of the poles was observed. We calculate the stellar magnetic field value at the position of the planet using the Potential Field Source Surface extrapolation technique. We show that the planetary magnetic environment is not homogeneous over the orbit, and that it varies between observing epochs, due to the evolution of the stellar magnetic field. This result underlines the importance of contemporaneous multi-wavelength observations to characterise exoplanetary systems. Our reconstructed maps are a crucial input for the interpretation and modelling of our MOVES multi-wavelength observations.

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R. Fares, V. Bourrier, A. Vidotto, et. al.
Fri, 23 Jun 17

Comments: 14 pages, 6 figures, accepted for publication in MNRAS

Cloud formation in metal-rich atmospheres of hot super-Earths like 55 Cnc e and CoRot7b [EPA]


Clouds form in the atmospheres of planets where they can determine the observable spectra, the albedo and phase curves. Cloud properties are determined by the local thermodynamical and chemical conditions of an atmospheric gas. A retrieval of gas abundances requires a comprehension of the cloud formation mechanisms under varying chemical conditions. With the aim of studying cloud formation in metal rich atmospheres, we explore the possibility of clouds in evaporating exoplanets like CoRoT-7b and 55 Cnc e in comparison to a generic set of solar abundances and the metal-rich gas giant HD149026b. We assess the impact of metal-rich, non-solar element abundances on the gas-phase chemistry, and apply our kinetic, non-equilibrium cloud formation model to study cloud structures and their details. We provide an overview of global cloud properties in terms of material compositions, maximum particle formation rates, and average cloud particle sizes for various sets of rocky element abundances. Our results suggest that the conditions on 55 Cnc e and HD149026b should allow the formation of mineral clouds in their atmosphere. The high temperatures on some hot-rocky super-Earths (e.g. the day-side of Corot-7b) result in an ionised atmospheric gas and they prevent gas condensation, making cloud formation unlikely on its day-side.

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G. Mahapatra, C. Helling and Y. Miguel
Fri, 23 Jun 17

Comments: 19 pages, accepted for publication in MNRAS

The Detectability of Radio Auroral Emission from Proxima B [EPA]


Magnetically active stars possess stellar winds whose interaction with planetary magnetic fields produces radio auroral emission. We examine the detectability of radio auroral emission from Proxima b, the closest known exosolar planet orbiting our nearest neighboring star, Proxima Centauri. Using the Radiometric Bode’s Law, we estimate the radio flux produced by the interaction of Proxima Centauri’s stellar wind and Proxima b’s magnetosphere for different planetary magnetic field strengths. For plausible planetary masses, Proxima b produces 6-83 mJy of auroral radio flux at frequencies of 0.3-0.8 MHz for planetary magnetic field strengths of 1-3 B$_{\oplus}$. According to recent MHD models that vary the orbital parameters of the system, this emission is expected to be highly variable. This variability is due to large fluctuations in the size of Proxima b’s magnetosphere as it crosses the equatorial streamer regions of the dense stellar wind and high dynamic pressure. Using the MHD model of Garraffo et al. 2016 for the variation of the magnetosphere radius during the orbit, we estimate that the observed radio flux can vary nearly by an order of magnitude over the 11.2 day period of Proxima b. The detailed amplitude variation depends on the stellar wind, orbital, and planetary magnetic field parameters. We discuss observing strategies for proposed future space-based observatories to reach frequencies below the ionospheric cut off ($\sim 10$ MHz) as would be required to detect the signal we investigate.

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B. Burkhart and A. Loeb
Thu, 22 Jun 17

Comments: Submitted to ApJL, comments welcome

Escape and fractionation of volatiles and noble gases from Mars-sized planetary embryos and growing protoplanets [EPA]


Planetary embryos form protoplanets via mutual collisions, which can lead to the development of magma oceans. During their solidification, large amounts of the mantles’ volatile contents may be outgassed. The resulting H$_2$O/CO$_2$ dominated steam atmospheres may be lost efficiently via hydrodynamic escape due to the low gravity and the high stellar EUV luminosities. Protoplanets forming later from such degassed building blocks could therefore be drier than previously expected. We model the outgassing and subsequent hydrodynamic escape of steam atmospheres from such embryos. The efficient outflow of H drags along heavier species (O, CO$_2$, noble gases). The full range of possible EUV evolution tracks of a solar-mass star is taken into account to investigate the escape from Mars-sized embryos at different orbital distances. The envelopes are typically lost within a few to a few tens of Myr. Furthermore, we study the influence on protoplanetary evolution, exemplified by Venus. We investigate different early evolution scenarios and constrain realistic cases by comparing modeled noble gas isotope ratios with observations. Starting from solar values, consistent isotope ratios (Ne, Ar) can be found for different solar EUV histories, as well as assumptions about the initial atmosphere (either pure steam or a mixture with accreted H). Our results generally favor an early accretion scenario with a small amount of accreted H and a low-activity Sun, because in other cases too much CO$_2$ is lost during evolution, which is inconsistent with Venus’ present atmosphere. Important issues are likely the time at which the initial steam atmosphere is outgassed and/or the amount of CO$_2$ which may still be delivered at later evolutionary stages. A late accretion scenario can only reproduce present isotope ratios for a highly active young Sun, but then very massive steam atmospheres would be required.

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P. Odert, H. Lammer, N. Erkaev, et. al.
Thu, 22 Jun 17

Comments: 53 pages, 7 figures, 3 tables, submitted to Icarus

EPIC 228735255b – An eccentric 6.57 day transiting hot Jupiter in Virgo [EPA]


We present the discovery of EPIC 228735255b, a P= 6.57 days Jupiter-mass (M$P$=1.019$\pm$0.070 M${Jup}$) planet transiting a V=12.5 (G5-spectral type) star in an eccentric orbit (e=$0.120^{+0.056}{-0.046}$) detected using a combination of K2 photometry and ground-based observations. With a radius of 1.095$\pm$0.018R${Jup}$ the planet has a bulk density of 0.726$\pm$0.062$\rho_{Jup}$. The host star has a [Fe/H] of 0.12$\pm$0.045, and from the K2 light curve we find a rotation period for the star of 16.3$\pm$0.1 days. This discovery is the 9th hot Jupiter from K2 and highlights K2’s ability to detect transiting giant planets at periods slightly longer than traditional, ground-based surveys. This planet is slightly inflated, but much less than others with similar incident fluxes. These are of interest for investigating the inflation mechanism of hot Jupiters.

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H. Giles, D. Bayliss, N. Espinoza, et. al.
Thu, 22 Jun 17

Comments: Submitted to MNRAS, 11 pages, 10 figures