Vol. 640
6. Interstellar and circumstellar matter

First detection of 13CH in the interstellar medium

The CH radical has long been recognized as a reliable proxy of molecular hydrogen in diffuse and translucent clouds. Its rare isotopolog $^{13}$CH, however, had been alluded to as a detection in the interstellar medium. In this paper, Jacob et.al present the detection of $^{13}$CH in absorption toward Sgr B2(M) and other galactic star-forming regions using the upGREAT receiver on board SOFIA. Combining these new data with previous observations of the main CH isotopolog toward the same sight lines, the authors have determined an improved estimate for the 12C^13 ratio as a function of the galactocentric distance.

Vol. 640
10. Planets and planetary systems

Masses for the seven planets in K2-32 and K2-233. Four diverse planets in resonant chain and the first young rocky worlds

K2-32 and K2-233 are two planetary systems which host four and three planets, respectively. They were detected by the K2 mission transiting their host star. In this study, the authors measured their masses and bulk densities for the first time by using a long-term radial velocity follow-up with the HARPS instrument. K2-32 displays a similar architecture as our Solar System but in a scaled-down version, with a small rocky world in the inner regions, followed by a transitional sub-Saturn planet and two mini-Neptunes in the external region. All of them are in a compact resonant configuration inside half of Mercury’s orbit, which makes them key targets for atmospheric studies. With regard to K2-233, it is a young 600 Myr old star. Lillo-Box et al. confirmed in this study the rocky nature of the two inner planets in the system, thus representing the first instances of rocky worlds with measured masses around a young star. The two planets K2-233 b and c show masses and radii corresponding to a telluric composition. The results of this study allow for further investigations on the atmosphere of lower-mass planets, the early evolution of multi-planetary systems with rocky worlds, and the planet-planet interactions in compact multi-planetary systems in near resonant chains. The transiting nature of all of these planets and the precise bulk densities established in this work put them at the forefront of future atmospheric studies.

Vol. 640
1. Letters

Near-infrared emission lines trace the state-independent accretion disc wind of the black hole transient MAXI J1820+070

Blue-shifted absorption lines in the X-ray spectra of low mass X-ray binaries (LMXBs) provide the signature of hot winds from their accretion disk, carrying a sizable amount of material away. Cold winds also exist and are observed with the same characteristics in the optical spectra. Hot winds are preferentially observed during the "soft" state of LMXBs, when a disk is present and extends close to the compact object, dominating the X-ray spectrum. Cold winds are instead observed during the "hard" state when the standard accretion disk is pushed further out. Cold winds are revealed by blue-shifted lines in the optical spectra, mainly Hydrogen Balmer lines and He I (5976A). Here, Sánchez-Sierras and T. Muñoz-Darias monitored the evolution of the outburst from the black hole transient MAXI J1820+070 of which seven observations were taken with the X-shooter instrument at the ESO-VLT. These observations encompass the hard-soft-hard outburst evolution. Blue-shifted optical lines were previously observed in the "hard" state of MAXI J1820+070. The near-infrared (NIR) arm of X-shooter turned out to be particularly important on this topic: Blue-shifted Hydrogen Pashen and Brackett lines were observed during soft and hard states. The line shift is similar to what has been observed for optical lines. This result shows that NIR lines can be used to trace the cold wind across the entire LMXB outburst evolution and that cold winds are also present in the "soft" state.

Vol. 639
1. Letters

Broad-line type Ic SN 2020bvc. Signatures of an off-axis gamma-ray burst afterglow

Gamma-ray bursts (GRBs) are the brightest explosions in the Universe. There are two types of GRBs, and the long-duration ones are the outcome of the death of massive stars. A narrow, collimated jet pointing toward the Earth marks the GRB. At the same time, the star explodes giving rise to a supernova. GRBs that explode in the local Universe are almost always associated with broad-line, type Ic supernovae (i.e., those that were stripped more during the evolutionary path which brought them to explode). The supernova emission is isotropic and the GRB emission is collimated: We do expect that most of the type Ic supernovae lack a GRB, being that the Earth was missed by the jet. However, if we are just outside the GRB narrow jet, it is possible to observe an "orphan" GRB, that is to say a relativistic explosion without a high-energy signature. In this letter, Izzo et al. provide strong evidence for an off-axis GRB or choked jet (i.e., a jet which fails to break out from the star). This evidence comes from delayed X-ray emission - GRBs have a decaying X-ray afterglow and supernovae rarely have X-ray emission - and from the very high expansion velocity which is 20% of the speed of light, which were revealed from an early optical spectrum that was taken 1.5 d after the explosion. The off-axis viewing angle is ~25 degrees. The GRB barely missed the Earth and the authors were able to detect the X-ray emission thanks to its closeness of just 120 Mpc.

Vol. 639
6. Interstellar and circumstellar matter

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

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

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

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

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

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

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

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.