EDP Sciences

Vol. 603
In section 10. Planets and planetary systems

Spectral and atmospheric characterization of 51 Eridani b using VLT/SPHERE

by M. Samland, P. Mollière, M. Bonnefoy, et al. A&A 603, A57


alt

51 Eridani b is a highly interesting planet discovered recently through direct imaging. It is young (20 Myr), nearby (30 pc), has a mass around ten times that of Jupiter, and an orbital distance of about 14 au. Most importantly, its angular separation makes it an excellent target for direct spectroscopic characterization. Samland et al. use SPHERE at the VLT to obtain a precise spectrum of the planet from 0.95 to 1.7 microns. The planet is outside of the color-color relations for brown dwarfs. Models indicate that it is cloudy and has a very high metallicity ([Fe/H]=1.0 +/- 0.1), significantly above that of its parent star. This latter implies that the atmosphere of 51 Eri b is as metal-rich as that of Saturn. Given the trend between increasing planetary mass and decreasing atmospheric metallicity in the solar system, this is a somewhat surprising result, but definitely a very significant one.

Vol. 603
In section 8. Stellar atmospheres

The full spectral radiative properties of Proxima Centauri

by I. Ribas, M. D. Gregg, T. S. Boyajian, and E. Bolmont A&A 603, A58


alt

The discovery of Proxima Centauri b, a terrestrial temperate planet, presents the opportunity of studying a nearby, potentially habitable world. Understanding the radiation environment of the planet is a prerequisite for modeling its habitability. In this paper, the authors derive top-of-atmosphere fluxes on the planet from the X-ray to the mid-IR spectral domains. They also also aim at constraining the fundamental properties of the star, namely its mass, radius, effective temperature, and luminosity. The top-of-atmosphere average XUV irradiance on Proxima b is 0.293 W m^-2, nearly 60 times higher than Earth, and the total irradiance is 877 ± 44 W m^-2 , or 64 ± 3% of the solar constant but with a significantly redder spectrum. The fundamental properties of Proxima Centuri are M = 0.120±0.003 solar mass, R = 0.146±0.007 solar radius, Teff = 2980±80K, L = 0.00151 ± 0.00008 solar luminosity. The analysis also reveals a 20% excess in the 3–30 μm range that is best interpreted as arising from warm dust in the system.

Vol. 603
In section 10. Planets and planetary systems

Observational evidence for two distinct giant planet populations

by N.C. Santos, V. Adibekyan, P. Figueira, et al. A&A 603, A30


alt

Giant planets are thought to form in circumstellar disks, while brown dwarfs and stars are thought to form directly by gravitational collapse and fragmentation of a gas cloud. We do not know how efficient these mechanisms are, whether they overlap in mass and whether other mechanisms also exist. Santos et al. provide observational evidence for the existence of two distinct populations of planets/brown dwarfs: objects with masses smaller than approximately four Jupiter masses show a strong preference for metal-rich stars. At larger masses, between 4 and 20 Mjup, this preference disappears, indicating that these brown dwarfs/massive giant planets formed with a different mechanism. Although the mechanisms themselves remain to be identified, statistical studies of the population of exoplanets are beginning to shed light on their formation.

Vol. 603
In section 6. Interstellar and circumstellar matter

Fragmentation and disk formation in high-mass star formation: The ALMA view of G351.77-0.54 at 0.06'' resolution

by H. Beuther, A. J. Walsh, K. G. Johnston, et al. A&A 603, A10


alt

Understanding the fragmentation of high-mass gas clumps and the subsequent formation and evolution of accretion disks around young high-mass protostars remains an unsolved question in high-mass star formation research. In this paper, the authors report ALMA observationsof the massive hot core region G351.77-0.54 with baselines up to 1.5 km, leading to at an unprecedented spatial resolution of 0.06 arcsec (130 AU) at a source distance of 2.2 kpc. Within the inner few 1000 AU, G351.77 fragments into at least four cores (see figure). The kinematics of the central structure (#1) reveal contributions from a rotating disk, an infalling envelope and potentially also an outflow, whereas the spectral profile toward source #2 can be attributed to infall, with a rate of upto 10ˆ-4 - 10ˆ-3 solar mass per year. A stability analysis of the rotating central structure suggests that it is axisymmetrically stable.However, asymmetric instabilities such as spiral arms may still occur on smaller, so far unresolved, spatial scales.

Vol. 602
In section 6. Interstellar and circumstellar matter

SPHERE/ZIMPOL observations of the symbiotic system R Aquarii. I. Imaging of the stellar binary and the innermost jet clouds

by H.M. Schmid, A. Bazzon, J. Milli, et al. A&A 602, A53


The symbiotic binary system R Aqr consists of a Mira variable, a hot companion with a spectacular jet outflow, and an extended emission line nebula. The authors observed R Aqr in the Halpha and other filters with the visual camera subsystem ZIMPOL, which is part of the new extreme adaptive optics (AO) instrument SPHERE at the VLT. They analyzed the high resolution ≈ 25 mas images from SPHERE-ZIMPOL and determined the position, size, geometric structure, and line fluxes of the jet source and the clouds in the innermost region 2′′ ( 400 AU) f R Aqr. They resolved for the first time the R Aqr binary and found that the jet source is 46 ± 1 mas west (position angle −85.5◦ ± 1.0◦) of the Mira star. The central jet source is the strongest Hα component. Within 0.5′′ (100 AU) in the SW of the central source, they observed a string of bright clouds arranged in a zig-zag pattern, and further out, at 1′′ − 2′′, fainter and more extended bubbles. In the N and NE they found a bright, very elongated filamentary structure between 0.2′′ − 0.7′′ and faint perpendicular “wisps” further out. Cloud densities are very high, implying a short recombination timescale of a year or less.

Vol. 602
In section 7. Stellar structure and evolution

Kepler sheds new and unprecedented light on the variability of a blue supergiant: Gravity waves in the O9.5Iab star HD188209

by C. Aerts, S. Símon-Díaz, S. Bloemen, et al. A&A 602, A32


Space missions such as CoRoT and Kepler have had a tremendous impact on asteroseismological studies of low mass stars. Higher-mass stars (> 8 M_sun) and their evolved descendants have not been as amenable to similar scrutiny primarily because their oscillation periods are longer, leading to multi-periodic beating patterns with periods of several years, thus rendering observations unfeasible. For massive O and B supergiants, the situation is further complicated by the presence of other phenomena such as strong stellar winds. Although the saturation limit for the Kepler CCDs is ~11.5 mag., Aerts et al. were able to observe a 5.5 mag. O supergiant, HD188209, using a strategic placement of masks, thereby enabling a study of the target in scattered light over a time period of about four years. Extensive ground-based spectroscopy was also acquired. By combining these unique datasets, the authors were able to infer the presence of convectively driven internal gravity waves, making HD188209 the first case of a massive supergiant for which such variability has been observed.