- Published on 05 October 2018
In section 1. Letters to the Editor
Radial and vertical dust transport inhibit refractory carbon depletion in protoplanetary disks
How to explain the lack of carbon in the inner solar system? Carbon is abundant in the Sun and in the interstellar medium but it is depleted by a factor of 100 (compared to e.g., Si) in carbonaceous chondrites and by a factor of 10,000 in the Earth. More than half of the carbon in the interstellar medium is in the form of refractory material that would not vaporize in normal conditions at the Earth's distance from the Sun. It had been proposed that carbon could be extracted by UV radiation on small grains in the atmosphere of protoplanetary disks. Klarmann, Ormel, and Dominik show that in fact, when including radial migration of grains, this mechanism cannot explain the observed depletion. This seems to imply the existence of widespread (in time and location) high temperature (over 1000K) regions in the inner solar system combined with the quick formation of a giant planet core to form a barrier to incoming carbon-rich grains.
- Published on 03 October 2018
In section 10. Planets and planetary systems
Gravitational fragmentation and formation of giant protoplanets on orbits of tens of au
The origin of systems of giant planets orbiting at tens of au from their parent star, such as the iconic HR 8799, remains a mystery. Here, the authors have used high resolution hydrodynamic simulations of protostellar disks to show that gravitationally unstable clumps formed in the outer parts of these disks can indeed eventually lead to the formation of such planets. They show that the clumps initially migrate very rapidly (on 1000 to 10,000 years timescales) towards the central star but that this migration may be halted by Roche lobe overflow at distances of tens of au. If at this point the clumps are warmer than 2000K, a second collapse may occur to lead to the formation of a gas giant protoplanet. While their subsequent evolution (and migration) over millions of years remain to be studied, this seems to be a promising route for the formation of these systems.
- Published on 28 September 2018
In section 3. Cosmology
Cosmic microwave background constraints in light of priors over reionization histories
Understanding how the gas between galaxies was transformed from the neutral state left over from the recombination epoch into the ionized state that we observe today is one of the most urgent open questions in physical cosmology. Such process occurred within one billion years of the Big Bang, and is known as cosmic reionization. It can be investigated with several different techniques. Among the most widely used is the study of the imprint left by the scattering of cosmic microwave background photons of free electrons along their path from the last scattering surface to us. Such an integral signal has been measured with exquisite precision by the Planck satellite, and interpreted as indicating an early start to reionization. However, that interpretation is not unique as it depends on the detailed reionization history which is currently not known precisely. To overcome this problem the authors use a non-parametric reconstruction approach to set tight limits on the extent of early reionization. This result might be in tension with the recent EDGES detection of a 21 cm line reionization signal, if the latter is to be confirmed.
- Published on 28 September 2018
In section 1: Letters to the Editor
Surface waves in protoplanetary disks induced by outbursts: Concentric rings in scattered light
This Letter presents a remarkably simple idea for treating the variations observed in circumstellar protoplanetary disks during outbursts of the central star. The authors propose that during an EXOr or FXOr event, the outer disk is sufficiently heated by irradiation that it is no longer static (it remains in global dynamical balance but expands and contracts to respond to the additional heating) and that this leaves a fingerprint in the line profiles. The response is wavelike in space because the time duration of the outburst is finite, the different parts of the circumstellar disk respond with different phases so it appears that a wave moves outward in the disk when t is actually only an excitation of a localized response. The time-delayed responses can also produce moving features in emission or absorption, and the light curves in different infrared bands will certainly show the effect.
- Published on 11 September 2018
In section 6. Interstellar and circumstellar matter
Anatomy of the massive star-forming region S106. The OI 63 micron line observed with GREAT/SOFIA as a versatile diagnostic tool for the evolution of massive stars
The star-forming region S106 has been an object of intense interest for decades as a model region for studying massive star formation. These new spectroscopic observations, performed with GREAT/SOFIA have superb spatial (3 arcsec stepping, about 500 AU) and velocity (about 0.04 km/s) resolutions. They are supplemented with IRAM mm and archival VLA cm and Herschel IR imaging to produce a comprehensive, virtually tomographic, picture of the region. Particularly lovely is the association of different parts of the [O I] profile with structures in the cm radio imaging. The way in which the [O I] precisely traces the ionised gas (from cm observations) in the low velocity interval of the line profiles. The paper highlights how high spectral resolution and multiple tracers provide the three-dimensional ionization, density, and velocity structure, even distinguishing between shock and radiative excitations. This paper serves as a model analysis for future observational programs on spatially resolved star forming regions.
- Published on 04 September 2018
In section 8. Stellar atmospheres
Spatially resolved spectroscopy across stellar surfaces. III. Photospheric FeI lines across HD189733A (K1 V)
This paper presents spatially resolved, high spectral-resolution spectroscopy with the ESO HARPS, of photospheric Fe I lines over the stellar surface of the cool planet-hosting K1 V star, HD 189733A. This is possible during an exoplanet transit as successive portions of the stellar surface become hidden, and differential spectroscopy between these various transit phases provides spectra of small surface segments temporarily hidden behind the planet. This provides spectral line profiles that are free from rotational broadening, and so their gradual change from the center towards the stellar limb reflects the fine structure in the stellar atmosphere. These observations are compared to 3D hydrodynamic models to explore fine structure and 3D line formation in the atmosphere of this star. This detailed understanding is important to enable future searches for Earth-analogue exoplanets around K-type stars, where the more tranquilsurface granulation and lower ensuing micro-variability may allow suchdetections.