Vol. 645
14. Catalogs and data

The ROSAT Raster survey in the north ecliptic pole field X-ray catalogue and optical identifications

by G. Hasinger, M. Freyberg, E. M. Hu, et al. 2020, A&A, 645, A95

Archival data are a gift that keeps on giving. The X-ray satellite ROSAT performed a raster scan of the North Ecliptic Pole (NEP) region in 1992. These data have now been analyzed in detail and put into context with the aid of multiwavelength surveys from the infrared to the ultraviolet. Hasinger and collaborators identified 766 high-quality X-ray sources, with an optical identification and redshift, out of 805 sources. Thanks to the combination of a large solid angle (40 deg2) and sufficient X-ray sensitivity (~5x10−14 erg cm−2 s−1, 0.5-2 keV), their survey was able to unveil a dominant population of absorbed active galactic nuclei (AGN2) with optically very faint infrared X-ray counterparts, which become predominant at high luminosity (log(LX/(erg s−1))>44) and high redshift (z>1.5).

Vol. 645
1. Letters

A "no-drift" runaway pile-up of pebbles in protoplanetary disks in which midplane turbulence increases with radius

by R. Hyodo, S. Ida, T. Guillot, 2020, A&A, 645, L9 alt

Streaming instability is considered to be a promising way to form planetesimals directly from pebbles in protoplanetary disks, but it requires specific conditions of elevated local solid-to-gas ratios and moderate drag couplings between gas and solids. Several mechanisms have been proposed to locally enhance the spatial density of solids, such as the presence of snow lines and the pile-up of material at pressure maxima produced in different ways. Hyodo et al. present a new mechanism that does not call for snow lines or pressure maxima, but instead relies on the effect of radially increasing mid-plane turbulence. When pebbles drifting from the outer disk enter a dead zone, they experience a decrease in vertical turbulence, reducing their vertical scale height and elevating their concentration in the disk mid-plane. Using both 1D diffusion-advection simulations and analytical arguments, the authors show that for sufficiently large pebble-to-gas mass flux ratios, the gas-pebble friction (back-reaction) causes a decrease in the solids' drift velocity and thus their progressive pile-up, termed "no drift runaway accumulation.” More studies, including additional physical processes, such as Kelvin–Helmholtz and vertical shear instabilities, are needed to gain further insight into the concept.

Vol. 645
7. Stellar structure and evolution

Optically thin circumstellar medium in the beta Lyr A system

by M. Broz, D. Mourard, J. Budaj, P. Harmanec, H. Schmitt, I. Tallon-Bosc, D. Bonneau, H. Bozic, D. Gies, M. Slechta 2021, A&A, 645, A51

The paper examines beta Lyr A, which is an archetype of a Roche-lobe overflow binary at a rapid mass transfer phase. Such a binary helps us to understand binary evolution. The authors used an extensive observational dataset, including light curves, optical interferometric data, and high-resolution spectroscopy, for example, and they devised a very complicated model with 38 free parameters. The model contains the primary, the Roche-filling secondary, the optically thick disk, its optically thin atmosphere, the jets, and the shell. The authors are able for their preferred model to account for all types of available observational data and present a “fine” structure of the system.

Vol. 645
6. Interstellar and circumstellar matter

C18O, 13CO, and 12CO abundances and excitation temperatures in the Orion B molecular cloud

by A. Roueff, M. Gerin, P. Gratier, et al. 2020, A&A, 645, A26

Quantitative inference of the H2 column densities from 3mm molecular emission: Case study towards Orion B

by P. Gratier, J. Pety, E. Bron, et al. 2020, A&A, 645, A27

Tracers of the ionization fraction in dense and translucent gas. I. Automated exploitation of massive astrochemical model grids

by E. Bron, E. Roueff, M. Gerin, et al. 2020, A&A, 645, A28 alt

The increase in bandwidth and sensitivity afforded by the latest generation of milliliter wave receivers has made it possible to map large areas of the sky using multiple GHz bandwidths that contain dozens of molecular lines. Extracting information from these complex data sets represents a challenge to observers, and it has motivated the development of new semi-automatic analysis techniques. In this volume, the ORION-B team presents three papers that illustrate some of the new techniques developed to characterize the Orion B molecular cloud using observations carried out with the IRAM 30m telescope.
In one of the contributions, Pierre Gratier and collaborators present the use of supervised machine learning methods (Random Forests) to train a predictor of the H$_2$ column density. These authors used a limited set of molecular lines in the 3mm-wavelength band as input and Herschel-based dust-derived column densities as the “ground truth” output. With their new technique, Gratier et al. estimate that they can predict the H$_2$ column density within a typical factor of 1.2 compared to the Herschel-based column density estimates.
In a separate contribution, Antoine Roueff and collaborators use the Cramer Rao bound technique to analyze and estimate the precision of physical parameters derived from the analysis of low-J transitions for the most common CO isotopologues. These authors propose a maximum likelihood estimator to infer the physical conditions from the 1-0 and 2-1 transitions of CO isotopologues, and from its use, they find local deviations of the mean fractional abundances across the cloud that likely result from the competition between selective photodissociation, chemical fractionation, and the depletion on grain surfaces.
In a third contribution, Emeric Bron and collaborators searched for the best observables of the gas ionization fraction using a grid of astrochemical models, and they estimated their predictive power by training a Random Forest model. From this work, the authors identify several observables that are better and more widely applicable than the commonly-used DCO$^+$/HCO$^+$ ratio. They also provide analytical fits to estimate the ionization fraction from the best observables and to derive the associated uncertainties.

Vol. 645
10. Planets and planetary systems

Atmospheric Rossiter-McLaughlin effect and transmission spectroscopy of WASP-121b with ESPRESSO

by F. Borsa, R. Allart, N. Casasayas-Barris, et al. 2021, A&A, 645, A24 alt

WASP-121b is one of the most studied ultra-hot Jupiters, and it is part of a class of planets that orbit hot stars at short distances, resulting in effective temperatures in excess of 2000 K. Most molecular species in these objects are thermally dissociated into their atomic constituents and their upper atmospheres are presumably subject to intense escape. Previous transit spectroscopy studies of WASP-121b have revealed several atmospheric features due to Fe I, Fe II, Na I, Cr I, V I, and Halpha. Here, Borsa et al. report on a detailed study of WASP-121b with the ESPRESSO/VLT instrument, both in its 1-UT and 4-UT modes, enabling remarkable new findings. The atmosphere is detected in two complementary ways. The atmospheric Rossiter-McLaughlin effect, involving a subtle change in the star velocity during transit associated to absorption by the planet's atmosphere, which is mostly in atomic Fe here, is clearly detected and demonstrates that the atmosphere is predominantly blue-shifted by several km/s due to day-to-night winds. Second, the transmission spectrum shows, either directly or through the cross-correlation technique, that absorption in no less than a dozen of atomic spectral lines, including new detections of Mg, K, Ca II, and, most notably, Li, featuring the first unambiguous detection of lithium in an exoplanet. The Halpha and Ca II absorptions are very prominent, extending close to or beyond the Roche limit, likely probing ongoing atmospheric escape. These observations highlight the outstanding capabilities of ESPRESSO for exoplanetary studies.