Vol. 639
6. Interstellar and circumstellar matter

Gap, shadows, spirals, and streamers: SPHERE observations of binary-disk interactions in GG Tauri A

by M. Keppler, A. Penzlin, M. Benisty, et al. 2020, A&A, 639, A62 alt

A large fraction of stars are found in multiple systems. More than 4000 detections of extrasolar planets around single stars raise the question as to if and how planets in binary systems may form. Since protoplanetary disks are the birthplaces of planets, thus dictating the initial conditions, their characterization is crucial to understanding the planet formation process. Unlike single star systems, numerical simulations predict that the gravitational impact of the central binary will severely perturb its surrounding disk, leading to the formation of a gap through which material in the form of streamers will feed the central stars. New high-resolution scattered-light observations of GG Tau A, one of the most massive and largest circumbinary protoplanetary disks, reveal binary-disk interactions in unprecedented detail. The disk shows evidence of a large gap, several shadowed regions, spiral structures, as well as streamer-like filaments connecting the outer disk and the central binary. These filaments are possibly generated by periodic perturbations from the binary, tearing off material from the inner edge of the outer disk once per orbit as predicted by numerical simulations.

Vol. 639
7. Stellar structure and evolution

Observational appearance of rapidly rotating neutron stars. X-ray bursts, cooling tail method, and radius determination

by V. F. Suleimanov, J. Poutanen, K. Werner, 2020, A&A, 639, A33

There are several ongoing efforts to measure the neutron star equation of state. Various methods have been suggested to measure the neutron star mass and radius, including the cooling tail during "type I bursts", which occur when sufficient accreting matter deposits onto the neutron star surface, thus igniting the thermonuclear explosions. A specific spectral pattern is predicted due to the cooling neutron star surface, which can predict the neutron star radius with good accuracy. In this work, Suleimanov and colleagues compute the emission pattern for a rapidly rotating oblate (i.e., non-spherical) neutron star. This effect turns out to be important and overestimated by 3–3.5 km the neutron star radius (~30%).

Vol. 639
6. Interstellar and circumstellar matter

Molecular globules in the Veil bubble of Orion IRAM 30 m 12CO, 13CO, and C18O (2–1) expanded maps of Orion A

by J. R. Goicoechea, C. H. M. Pabst, S. Kabanovic, et al. 2020, A&A, 639, A1

Expanding bubbles in Orion A: [C II] observations of M 42, M 43, and NGC 1977

by C. H. M. Pabst, J. R. Goicoechea, D. Teyssier, et al. 2020, A&A, 639, A2

Strong winds and ultraviolet (UV) radiation from O-type stars disrupt and ionize their molecular core birthplaces, sweeping up material into parsec-sized shells. The "Veil bubble" in Orion represents a nearby example of such a shell, where the complexities of stellar feedback can be studied in detail. The Veil lies in front of the well-known M42 HII region, and it is driven by the Trapezium-cluster stars. In such a harsh environment dominated by shocks and UV radiation, little molecular emission is expected to be found. As Goicoechea et al. show, however, some pockets of molecular gas have managed to survive in the Veil, and they have been detected with the IRAM 30 m telescope as small CO globules. These globules have small masses and are moderately dense, and they seem to be confined by the shell's external pressure and are likely supported by magnetic fields. They are either transient objects that formed by hydrodynamic instabilities or pre-existing over-dense structures of the original molecular cloud. In a companion paper, Pabst et al. analyze [CII]158 microns observations of the Veil and other nearby bubbles in Orion, which were carried out with the upGREAT instrument on board SOFIA. The Veil bubble is found to have an expansion time of 0.2 Myr and to be driven by the mechanical energy input from the wind of the O-type star θ1 Ori C, the most massive star in the Trapezium. Whereas the nearby bubbles associated with M43 and NGC 1977 are caused by the thermal expansion of the gas ionized by their central later-type less massive stars.

Vol. 638
2. Astrophysical processes

Fermi Large Area Telescope observations of the fast-dimming Crab Nebula in 60–600 MeV

by Paul K. H. Yeung and Dieter Horns 2020, A&A, 638, A147

The supernova that exploded in 1054 gave rise to the following well-known nebula since the manufacturing of large telescopes: the Crab Nebula, which is the first object in the Messier’s list of nebulae. The Crab Nebula is also one of the brightest objects in the X-ray sky; it consists of a pulsar at its center and a powered wind nebula. The Crab is a very stable object in X-rays and it has been used by astronomers to calibrate instruments since the dawn of X-ray astronomy. At very high energies (60-600 MeV), the Crab is known to produce bright gamma-ray flares at a rate of ~1 per year. In this paper, Yeung and Horns report the discovery of fast dimming in the 60-600 MeV flux from the Crab, observed by the Fermi Large Area Telescope. Typically, the Crab weakens by a factor of ~5 in a timescale of a few days and remains faint for several weeks. This fast variability lends support to models which ascribe to the inner knot of the Crab Nebula (a spot at ~0.3 arcsec from the pulsar itself and visible in Hubble Space telescope images) from which very high energy emission is observed from the Crab, arising as synchrotron emission.

Vol. 638
7. Stellar structure and evolution

An in-depth reanalysis of the alleged type Ia supernova progenitor Henize 2-428

by N. Reindl, V. Schaffenroth, M. M. Miller Bertolami, S. Geier, N. L. Finch, M. A. Barstow, S. L. Casewell, and S. Taubenberger 2020, A&A, 638, A93

The planetary nebula Hen 2-428 was reported in 2015 to have a double degenerate core with an orbital period of 4.2 hours and, more excitingly, a combined mass of 1.76Msun, which is above the Chandrasekhar mass limit. The period and the mass imply that the system should merge in 700 million years and trigger in a type Ia supernova event. Reindl et al. most recently carried out an in-depth reanalysis of the alleged type Ia progenitor. They show that blends of He II λ 5412 Angstrom with DIBs resulted in an overestimation of the previously reported mass. The new total mass of the double degenerates is 1.08 Msun, a measurement that no longer exceeds the Chandrasekhar mass limit.

Vol. 638
4. Extragalatic astronomy

The GOGREEN Survey: A deep stellar mass function of cluster galaxies at 1.0 < z < 1.4 and the complex nature of satellite quenching

by R. F. J. van der Burg, G. Rudnick, M. L. Balogh, et al. 2020, A&A, 638, A112

The origin of the bimodality of galaxies that separates star-forming from passive galaxies remains incompletely understood. In particular, it is not clear whether the mechanisms at play in the present-day Universe can also explain how star-formation quenched in distant galaxies. To address this question, van den Burg et al. realized the most precise measurement of the stellar mass function of galaxies in the dense environment of 11 galaxy clusters at z=1 to 1.4 using photometric measurements from the blue to the 4.5 microns bands. Surprisingly, what they find is in stark contrast to what several studies concluded from the study of nearby galaxies. These studies refer to the following two mechanisms that act separately and independently in the local Universe: a mass-independent environmental quenching and an environment-independent mass quenching. When they compare the stellar mass functions of star-forming and passive galaxies in the clusters and in the field at z=1-1.4, the authors find no signature of purely mass-independent environmental quenching. Instead galaxies in clusters appear to quench through the same processes as those in the field, simply they do so at an earlier time. They conclude that galaxies that are destined to become part of z=1-1.4 clusters start their formation “early” with respect to galaxies in the field, but they quench via a similar physical process.

Vol. 638
1. Letters

Evidence for a rapid decrease of Pluto’s atmospheric pressure revealed by a stellar occultation in 2019

by K. Arimatsu, G. L. Hashimoto, M. Kagitani, et al. 2020, A&A, 638, L5 alt

Pluto's atmosphere originates from sublimation-condensation equilibria of surface volatiles, primarily N2. Pluto's strongly elliptical orbit and high obliquity lead to marked seasonal variability of the local insolation. Regular monitoring of stellar occultations over the last 30 years has revealed that although Pluto has been receding from the Sun since 1989, its atmospheric pressure has increased by a factor of ~3 from 1988-2016. Based on climate models, this is best understood to be due to the evolution of insolation over Sputnik Planitia, the largest reservoir of N2 ice on Pluto, and models have predicted that the pressure should progressively decline after 2015, which is a consequence of the subsolar point moving to higher northern latitudes. Using a single-chord stellar occultation observed from Hawaii, Arimatsu et al. determine that Pluto's atmospheric pressure in 2019 was approximately 21% lower than it was in 2016. Although significant at 2.4 sigma only, this result! suggests that Pluto's long-expected atmospheric decline has started, and at a pace faster than anticipated by models, possibly pointing to additional regions of nitrogen ice deposition. Continuing the monitoring of Pluto's atmosphere is needed to confirm the trend and improve our understanding of Pluto's climate and its atmospheric sublimation-condensation cycles.

Vol. 638
10. Planets and planetary systems

Revised mass-radius relationships for water-rich rocky planets more irradiated than the runaway greenhouse limit

by M. Turbet, E. Bolmont, D. Ehrenreich, P. Gratier, J. Leconte, F. Selsis, N. Hara, and C. Lovis 2020, A&A, 638, A41 alt

With seven rocky planets on temperate orbits, of which three are considered to be in the habitable zone, the TRAPPIST-1 system is one of the best characterized and fascinating planetary systems. Based on knowledge of their masses, radii, and densities, previous studies inferred that most of the seven planets are likely enriched in volatiles (e.g., water), up to tens of percent of the planetary masses. However, the three innermost planets (TRAPPIST-1b, c, and d) receive more irradiation from the star than the runaway greenhouse limit, implying that all of their putative water reservoir could actually form a steam atmosphere, and it has been shown in a recent study by Turbet et al. (2019) that this situation leads to strong atmospheric expansion. Here the authors apply these concepts to propose revised mass-radius relationships for this class of planets, providing, in particular, empirical formulas for the thickness of the H2O steam atmosphere as a function of planet core gravity and the radius, water content, and irradiation. For a given water-to-rock mass ratio, these new mass-radius relationships lead to planet bulk densities that are much lower than if water is assumed to be in its condensed form, implying that the latter assumption considerably overestimates the planets' bulk water endowment. The application to the TRAPPIST-1 system implies that when assuming a terrestrial core composition, TRAPPIST-1b, c, and d can accommodate a water mass fraction of 0.1-2 %, at most, and perhaps even much less.

Vol. 638
1. Letters

Unexpected late-time temperature increase observed in the two neutron star crust-cooling sources XTE J1701-462 and EXO 0748-676

by A. S. Parikh, R. Wijnands, J. Homan, et al., 2020, A&A, 638, L2

During an X-ray outburst, a transient neutron star accretes matter which then heats the crust. After the outburst has ended, the neutron star cools, providing invaluable insights into the properties of the crust. Without further sources of heat, the compact object is expected to continue to cool. This simple physical picture is challenged by the observations presented in this Letter, in which a late time rise in temperature has been observed in two transient systems years after the outburst ended. As the authors have stated, this rise in temperature is unexplained. Further studies should lead to a deeper understanding of the dense matter physics.