Outer Disk Research Articles

The extreme super-eddington NLS1 RX J0134.2-4258 – II. A weak-line Seyfert linking to the weak-line quasar

ABSTRACT RX J0134.2-4258 is one of the most super-Eddington narrow-line Seyfert 1 (NLS1) galaxies, on which we conducted a monitoring campaign from radio to X-rays. In this paper, we present a detailed analysis of its optical/UV spectra and broad-band spectral energy distribution. Our study shows that the preferred black hole mass of RX J0134.2-4258 is MBH ∼ 2 × 107 M⊙, giving a mass accretion rate through the outer disc of $\dot{m}_{\rm out} \sim 20$ (assuming zero spin), compared to the observed luminosity ratio Lbol/LEdd ∼ 6. This reduction in radiative efficiency is expected for super-Eddington flows, as power can be lost via advection and/or disc winds. We find that the optical/UV lines of RX J0134.2-4258 resemble those from weak-like quasars (WLQs), as it has notably weak C iv and N v emission lines. It also has drastic X-ray variability, again similar to that recently observed in some other WLQs. However, WLQs have systematically higher masses (≳108 M⊙), and lower Eddington ratios ($\dot{m}_{\rm out} \sim 1$) than RX J0134.2-4258. We compare instead to the most extreme NLS1s, with similarly large $\dot{m}_{\rm out}$ but smaller masses. These show similarly large reductions in radiative efficiency but their UV lines are not similarly wind dominated. We suggest a new category of weak-line Seyfert galaxies to describe sources like RX J0134.2-4258, and interpret its (so far unique) properties in a model, where the lower disc temperature in the higher mass black holes leads to the UV-line-driving mechanism, which enhances the super-Eddington radiation-pressure-driven wind.

Analysis of the arm-like structure in the outer disk of PDS 70

Context. Observing dynamical interactions between planets and disks is key to understanding their formation and evolution. Two protoplanets have recently been discovered within the PDS 70 protoplanetary disk, along with an arm-like structure toward the northwest of the star. Aims. Our aim is to constrain the morphology and origin of this arm-like structure, and to assess whether it could trace a spiral density wave caused by the dynamical interaction between the planet PDS 70c and the disk. Methods. We analyzed polarized and angular differential imaging (PDI and ADI) data taken with VLT/SPHERE, spanning six years of observations. The PDI data sets were reduced using the irdap polarimetric data reduction pipeline, while the ADI data sets were processed using mustard, a novel algorithm based on an inverse problem approach to tackle the geometrical biases spoiling the images previously used for the analysis of this disk. Results. We confirm the presence of the arm-like structure in all PDI and ADI data sets, and extract its trace by identifying local radial maxima in azimuthal slices of the disk in each data set. We do not observe a southeast symmetric arm with respect to the disk minor axis, which seems to disfavor the previous hypothesis that the arm is the footprint of a double-ring structure. If the structure traces a spiral density wave following the motion of PDS 70c, we would expect 11°.28−0°.86+2°.20 rotation for the spiral in six years. However, we do not measure any significant movement of the structure. Conclusions. If the arm-like structure is a planet-driven spiral arm, the observed lack of rotation would suggest that the assumption of rigid-body rotation may be inappropriate for spirals induced by planets. We suggest that the arm-like structure may instead trace a vortex appearing as a one-armed spiral in scattered light due to projection effects. The vortex hypothesis accounts for both the lack of observed rotation and the presence of a nearby sub-millimeter continuum asymmetry detected with ALMA. Additional follow-up observations and dedicated hydrodynamical simulations could confirm this hypothesis.

Outer disc edge: properties of low-frequency aperiodic variability in ultracompact interacting binaries

ABSTRACT Flickering, and more specifically aperiodic broad-band variability, is an important phenomenon used in understanding the geometry and dynamics of accretion flows. Although the inner regions of accretion flows are known to generate variability on relatively fast time-scales, the broad-band variability generated in the outer regions has mostly remained elusive due to its long intrinsic variability time-scales. Ultracompact AM CVn systems are relatively small when compared to other accreting binaries and are well suited to search and characterize low-frequency variability. Here, we present the first low-frequency power spectral analysis of the ultracompact accreting white dwarf system SDSS J1908+3940. The analysis reveals a low-frequency break at ∼6.8 × 10−7 Hz in the time-averaged power spectrum as well as a second higher frequency component with characteristic frequency of ∼1.3 × 10−4 Hz. We associate both components with the viscous time-scales within the disc through empirical fits to the power spectrum as well as analytical fits using the fluctuating accretion disc model. Our results show that the low-frequency break can be associated with the outer disc regions of a geometrically thin accretion flow. The detection of the low-frequency break in SDSS J1908+3940 provides a precedent for further detection of similar features in other ultracompact accreting systems. More importantly, it provides a new observable that can help constrain simulations of accretion flows.

Efficient planet formation by pebble accretion in ALMA rings

ABSTRACT In the past decade, ALMA observations have revealed that a large fraction of protoplanetary discs contains rings in the dust continuum. These rings are the locations where pebbles accumulate, which is beneficial for planetesimal formation and subsequent planet assembly. We investigate the viability of planet formation inside ALMA rings in which pebbles are trapped by either a Gaussian-shaped pressure bump or by the strong dust backreaction. Planetesimals form at the mid-plane of the ring via streaming instability. By conducting N-body simulations, we study the growth of these planetesimals by collisional mergers and pebble accretion. Thanks to the high concentration of pebbles in the ring, the growth of planetesimals by pebble accretion becomes efficient as soon as they are born. We find that type-I planet migration plays a decisive role in the evolution of rings and planets. For discs where planets can migrate inward from the ring, a steady state is reached where the ring spawns ∼20 M⊕ planetary cores as long as rings are fed with materials from the outer disc. The ring acts as a long-lived planet factory and it can explain the ‘fine-tuned’ optical depths of the observed dust rings in the DSHARP large program. In contrast, in the absence of a planet removal mechanism (migration), a single massive planet will form and destroy the ring. A wide and massive planetesimals belt will be left at the location of the planet-forming ring. Planet formation in rings may explain the mature planetary systems observed inside debris discs.

Can ultralight dark matter explain the age–velocity dispersion relation of the Milky Way disc: A revised and improved treatment

ABSTRACT Ultralight axion-like particles ma ∼ 10−22 eV, or Fuzzy Dark Matter (FDM), behave comparably to cold dark matter (CDM) on cosmological scales and exhibit a kpc-size de Broglie wavelength capable of alleviating established (sub-)galactic-scale problems of CDM. Substructures inside an FDM halo incur gravitational potential perturbations, resulting in stellar heating sufficient to account for the Galactic disc thickening over a Hubble time, as first demonstrated by Church et al. We present a more sophisticated treatment that incorporates the full baryon and dark matter distributions of the Milky Way and adopts stellar disc kinematics inferred from recent Gaia, APOGEE, and LAMOST surveys. Ubiquitous density granulation and subhalo passages, respectively, drive inner disc thickening and flaring of the outer disc, resulting in an observationally consistent ‘U-shaped’ disc vertical velocity dispersion profile with the global minimum located near the solar radius. The observed age–velocity dispersion relation in the solar vicinity can be explained by the FDM-substructure-induced heating and places an exclusion bound ma ≳ 0.4 × 10−22 eV. We assess non-trivial uncertainties in the empirical core–halo relation, FDM subhalo mass function and tidal stripping, and stellar heating estimate. The mass range ma ≃ 0.5–0.7 × 10−22 eV favoured by the observed thick disc kinematics is in tension with several exclusion bounds inferred from dwarf density profiles, stellar streams, and Milky Way satellite populations, which could be significantly relaxed due to the aforesaid uncertainties. Additionally, strongly anisotropic heating could help explain the formation of ultra-thin disc galaxies.

On Internal and External Alignment of Dust Grains in Protostellar Environments

Multiwavelength observations toward protostars reveal complex properties of dust polarization, which are challenging to interpret. Here we study the physical processes inducing the alignment of the grain axis of the maximum inertia moment with the angular momentum ( J ; i.e., internal alignment) and of J with the magnetic field (i.e., external alignment) of very large grains (VLGs; of radius a > 10 μm) using the alignment framework based on radiative torques (RATs) and mechanical torques (METs). We derive analytical formulae for critical sizes of grain alignment, assuming grains aligned at low-J and high-J attractors by RATs (METs). For protostellar cores, we find that super-Barnett relaxation induces efficient internal alignment for VLGs with large iron inclusions, but inelastic relaxation is efficient for VLGs regardless of composition aligned at high-J attractors by RATs (METs). For external alignment, VLGs with iron inclusions aligned at high-J attractors have magnetic alignment by RATs (B-RAT) or METs (B-MET), enabling dust polarization as a reliable tracer of magnetic fields in dense regions. Still, grains at low-J attractors or without iron inclusions have alignment with J along the radiation direction (k-RAT) or gas flow (v-MET). For protostellar disks, we find that super-Barnett relaxation is efficient for grains with large iron inclusions in the outer disk thanks to spin-up by METs, but inelastic relaxation is inefficient. VLGs aligned at low-J attractors can have k-RAT (v-MET) alignment, but grains aligned at high-J attractors likely exhibit B-RAT (B-MET) alignment. We also find that grain alignment by METs is more important than that by RATs in protostellar disks.

Dynamical mass measurements of two protoplanetary discs

ABSTRACT ALMA observations of line emission from planet forming discs have demonstrated to be an excellent tool to probe the internal disc kinematics, often revealing subtle effects related to important dynamical processes occurring in them, such as turbulence, or the presence of planets, that can be inferred from pressure bumps perturbing the gas motion, or from the detection of the planetary wake. In particular, we have recently shown for the case of the massive disc in Elias 2-27 as how one can use such kind of observations to measure deviations from Keplerianity induced by the disc self-gravity, thus constraining the total disc mass with good accuracy and independently on mass conversion factors between the tracer used and the total mass. Here, we refine our methodology and extend it to two additional sources, GM Aur and IM Lup, for which archival line observations are available for both the 12CO and the 13CO line. For IM Lup, we are able to obtain a consistent disc mass of $M_{\rm disc}=0.1 \, \mathrm{M}_{\odot }$, implying a disc-star mass ratio of 0.1 (consistent with the observed spiral structure in the continuum emission) and a gas/dust ratio of ∼65 (consistent with standard assumptions), with a systematic uncertainty by a factor of ≃ 2 due to the different methods to extract the rotation curve. For GM Aur, the two lines we use provide slightly inconsistent rotation curves that cannot be attributed only to a difference in the height of the emitting layer, nor to a vertical temperature stratification. Our best-fitting disc mass measurement is $M_{\rm disc}=0.26 \, \mathrm{M}_{\odot }$, implying a disc-star mass ratio of ∼0.35 and a gas/dust ratio of ∼130. Given the complex kinematics in the outer disc of GM Aur and its interaction with the infalling cloud, the CO lines might not well trace the rotation curve and our results for this source should then be considered with some caution.

Magnetised winds in transition discs

Context. Protoplanetary discs are cold, dense, and weakly ionised environments that witness planetary formation. Among these discs, transition discs (TDs) are characterised by a wide cavity (up to tens of au) in the dust and gas distribution. Despite this lack of material, a considerable fraction of TDs are still strongly accreting onto their central star, possibly indicating that a mechanism is driving fast accretion in TD cavities. Aims. The presence of radially extended ‘dead zones’ in protoplanetary discs has recently revived interest in magnetised disc winds (MDWs), where accretion is driven by a large magnetic field extracting angular momentum from the disc. We propose that TDs could be subject to similar disc winds, and that these could naturally explain the fast-accreting and long-lived cavities inferred in TDs. Methods. We present the results of the first 2.5D global numerical simulations of TDs harbouring MDWs using the PLUTO code. We imposed a cavity in the gas distribution with various density contrasts, and considered a power-law distribution for the large-scale magnetic field strength. We assume the disc is weakly ionised and is therefore subject to ambipolar diffusion, as expected in this range of densities and temperatures. Results. We find that our simulated TDs always reach a steady state with an inner cavity and an outer ‘standard’ disc. These models also maintain an approximately constant accretion rate through the entire structure, reaching 10−7 M⊚ yr−1 for typical surface density values. The MDW launched from the cavity is more magnetised and has a significantly larger lever arm (up to 10) than the MDW launched from the outer disc. The material in the cavity is accreted at sonic velocities, and the cavity itself is rotating at 70% of the Keplerian velocity due to the efficient magnetic braking imposed by the MDW. Overall, our cavity matches the dynamical properties of an inner jet emitting disc (JED) and of magnetically arrested discs (MADs) in black-hole physics. Finally, we observe that the cavity is subject to recurring accretion bursts that may be driven by a magnetic Rayleigh-Taylor instability of the cavity edge. Conclusions. Some strongly accreting TDs could be the result of magnetised wind sculpting protoplanetary discs. Kinematic diagnostics of the disc or the wind (orbital velocity, wind speeds, accretion velocities) could disentangle classic photo-evaporation from MDW models.

Chemical evolution in planet-forming regions with growing grains

Context. Planets and their atmospheres are built from gas and solid material in protoplanetary disks. This solid material grows from smaller micron-sized grains to larger sizes in the disks during the process of planet formation. This solid growth may influence the efficiency of chemical reactions that take place on the surfaces of the grains and in turn affect the chemical evolution that the gas and solid material in the disk undergoes, with implications for the chemical composition of the planets. Aims. Our goal is to model the compositional evolution of volatile ices on grains of different sizes, assuming both time-dependent grain growth and several constant grain sizes. We also examine the dependence on the initial chemical composition. Methods. The custom Walsh chemical kinetics code was used to model the chemical evolution. This code was upgraded to account for the time-evolving sizes of solids. Chemical evolution was modelled locally at four different radii in a protoplanetary disk midplane (with associated midplane temperatures of 120, 57, 25, and 19.5 K) for up to 10 Myr. The evolution was modelled for five different constant grain sizes, and in one model, the grain size changed with time according to a grain-growth model appropriate for the disk midplane. Results. Local grain growth, with conservation of the total grain mass, and assuming spherical grains, acts to reduced the total grain-surface area that is available for ice-phase reactions. This reduces the efficiency of these reactions compared to a chemical scenario with a conventional grain-size choice of 0.1 μm. The chemical evolution modelled with grain growth leads to increased abundances of H2O ice. For carbon in the inner disk, grain growth causes CO gas to overtake CO2 ice as the dominant carrier, and in the outer disk, CH4 ice becomes the dominant carrier. Larger grain sizes cause less change the C/O ratio in the gas phase over time than when 0.1 μm sized grains are considered. Overall, a constant grain size adopted from a grain evolution model leads to an almost identical chemical evolution as a chemical evolution with evolving grain sizes. A constant grain size choice, albeit larger than 0.1 μm, may therefore be an appropriate simplification when modelling the impact of grain growth on chemical evolution.

Numerical research on the effect of brackets installation angle on the propeller wake field and propulsion performance in a four-screw ship

A series of full-scale self-propulsion simulations of a four-screw ship with different bracket installation angles using the Reynolds-averaged Navier–Stokes (RANS) method were conducted. The effects of the bracket installation angle on the propeller wake field and propulsion performance were investigated and studied. The result shows that the nominal wake low-speed region generated by the shedding vortex moved inward with the bracket angle increased. When the bracket angle turns inward 3°, the effect of shedding vortex almost disappeared in the outer propeller disc. The outer propeller shows a significant increase in thrust and torque, and the time-average load difference between inner and outer propeller drops to below 5%. The research conducted in this paper fills a gap in the study of full-scale four-screw ship appendages. The design method for determining the bracket installation angle by inflow direction is of practical significance to the design of four-screw ships.

Shape of the outer stellar warp in the Large Magellanic Cloud disk

Warps are vertical distortions of the stellar or gaseous disks of galaxies. One of the proposed scenarios for the formation of warps involves tidal interactions among galaxies. A recent study identified a stellar warp in the outer regions of the south-western (SW) disk of the Large Magellanic Cloud (LMC) and suggested that it might have originated due to the tidal interaction between the LMC and the Small Magellanic Cloud (SMC). Due to the limited spatial coverage of the data, the authors could not investigate the counterpart of this warp in the north-eastern (NE) region, which is essential to understanding the global shape, nature, and origin of the outer LMC warp. In this work, we study the structure of the LMC disk using data on red clump stars from the Gaia Early Data Release 3 (EDR3), which cover the entire Magellanic system. We detected a warp in the NE outer LMC disk which is deviated from the disk plane in the same direction as that of the SW outer warp, but with a lower amplitude. This suggests that the outer LMC disk has an asymmetric stellar warp, which is likely to be a U-shaped warp. Our result provides an observational constraint to the theoretical models of the Magellanic system aimed at improving the understanding the LMC-SMC interaction history.

Morphology and dynamical stability of self-gravitating vortices

Context. Theoretical and numerical studies have shown that large-scale vortices in protoplanetary discs can result from various hydro-dynamical instabilities. Once produced, such vortices can survive nearly unchanged over a large number of rotation periods, slowly migrating towards the star. Lopsided asymmetries recently observed at sub-millimetre and millimetre wavelengths in a number of transition discs could be explained by the emission of the solid particles trapped by vortices in the outer disc. However, at such a distance from the star, disc self-gravity (SG) may affect the vortex evolution and must be included in models. Aims. Our first goal is to identify how vortex morphology is affected by its own gravity. Next, we look for conditions that a self-gravitating disc must satisfy in order to permit vortex survival at long timescales. Finally, we characterise as well as possible the persistent self-gravitating vortices we have found in isothermal and non-isothermal discs. Methods. We performed 2D hydrodynamic simulations using the RoSSBi 3.0 code. The outline of our computations was limited to Euler’s equations assuming a non-homentropic and non-adiabatic flow for an ideal gas. A series of 45 runs were carried out starting from a Gaussian vortex-model; the evolution of vortices was followed during 300 orbits for various values of the vortex parameters and the Toomre parameter. Two simulations, with the highest resolution thus far for studies of vortices, were also run to better characterise the internal structure of the vortices and for the purpose of comparison with an isothermal case. Results. We find that SG tends to destabilise the injected vortices, but compact small-scale vortices seem to be more robust than large-scale oblong vortices. Vortex survival critically depends on the value of the disc’s Toomre parameter, but may also depend on the disc temperature at equilibrium. Disc SG must be small enough to avoid destruction in successive splitting and an approximate ‘stability’ criterion is deduced for vortices. The self-gravitating vortices that we found persist during hundreds of rotation periods and look like the quasi-steady vortices obtained in the non-self-gravitating case. A number of these self-gravitating vortices are eventually accompanied by a secondary vortex with a horseshoe motion. These vortices reach a new rotational equilibrium in their core, tend to contract in the radial direction, and spin faster. Conclusions. We propose an approximate ‘robustness criterion’, which states that, for a given morphology, a vortex appears stable provided that the disc’s Toomre parameter overcomes a fixed threshold. Global simulations with a high enough numerical resolution are required to avoid inappropriate decay and to follow the evolution of self-gravitating vortices in protoplanetary discs. Vortices reach a nearly steady-state more easily in non-isothermal discs than in isothermal discs.

The survey of planetary nebulae in Andromeda (M31)

Context. The Andromeda (M31) galaxy presents evidence of recent substantial mass accretion. This is unlike what has happened in the Milky Way, which has experienced a rather quiescent evolution. Aims. We use oxygen and argon abundances for planetary nebulae (PNe) with low internal extinction (progenitor ages of > 4.5 Gyr) and high extinction (progenitor ages < 2.5 Gyr), as well as those of the HII regions, to constrain the chemical enrichment and star formation efficiency in the thin and thicker discs of M31. Methods. The argon element is produced in larger fractions by Type Ia supernovae compared to oxygen. We find that the mean log(O/Ar) values of PNe as a function of their argon abundances, 12 + log(Ar/H), trace the interstellar medium (ISM) conditions at the time of birth of the M31 disc PN progenitors. Thus, the chemical enrichment and star formation efficiency information encoded in the [α/Fe] versus [Fe/H] distribution of stars is also imprinted in the oxygen-to-argon abundance ratio log(O/Ar) versus argon abundance for the nebular emissions of the different stellar evolution phases. We propose using the log(O/Ar) versus (12 + log(Ar/H)) distribution of PNe with different ages to constrain the star formation histories of the parent stellar populations in the thin and thicker M31 discs. Results. For the inner M31 disc (RGC < 14 kpc), the chemical evolution model that reproduces the mean log(O/Ar) values as a function of argon abundance for the high- and low-extinction PNe requires a second infall of metal-poorer gas during a gas-rich (wet) satellite merger. This wet merger triggered the burst of star formation seen by the PHAT survey in the M31 disc, ∼3 Gyr ago. A strong starburst is ongoing in the intermediate radial range (14 ≤ RGC ≤ 18 kpc). In the outer M31 disc (RGC > 18 kpc), the log(O/Ar) versus argon abundance distribution of the younger high-extinction PNe indicates that they too were formed in a burst, though mostly from the metal-poorer gas. Present-day HII regions show a range of oxygen-to-argon ratios, which is indicative of spatial variations and consistent with a present-day rainfall of metal-poorer gas onto the disc with different degrees of mixing with the previously enriched ISM. Conclusions. We implement the use of the log(O/Ar) versus argon abundance distribution for emission nebulae as a complement to the [α/Fe] versus [Fe/H] diagram for stars, and use it to constrain the star formation efficiency in the thin and thicker discs of M31. Diagrams for M31 PNe in different age ranges reveal that a secondary infall of gas affected the chemical evolution of the M31 thin disc. In M31, the thin disc is younger and less radially extended, formed stars at a higher star formation efficiency, and had a faster chemical enrichment timescale than the more extended thicker disc. Both the thin and thicker discs in M31 reach similar high argon abundances (12 + log(Ar/H)) ≃ 6.7. The chemical and structural properties of the thin and thicker discs in M31 are thus remarkably different from those determined for the Milky Way thin and thick discs.

Milky Way's Eccentric Constituents with Gaia, APOGEE, and GALAH

We report the results of an unsupervised decomposition of the local stellar halo in the chemodynamical space spanned by the abundance measurements from APOGEE DR17 and GALAH DR3. In our Gaussian mixture model, only four independent components dominate the halo in the solar neighborhood, three previously known, Aurora, Splash, and Gaia-Sausage/Enceladus (GS/E), and one new, Eos. Only one of these four is of accreted origin, namely, the GS/E, thus supporting the earlier claims that the GS/E is the main progenitor of the Galactic stellar halo. We show that Aurora is entirely consistent with the chemical properties of the so-called Heracles merger. In our analysis in which no predefined chemical selection cuts are applied, Aurora spans a wide range of [Al/Fe] with a metallicity correlation indicative of a fast chemical enrichment in a massive galaxy, the young Milky Way. The new halo component dubbed Eos is classified as in situ given its high mean [Al/Fe]. Eos shows strong evolution as a function of [Fe/H], where it changes from being the closest to GS/E at its lowest [Fe/H] to being indistinguishable from the Galactic low-α population at its highest [Fe/H]. We surmise that at least some of the outer thin disk of the Galaxy started its evolution in the gas polluted by the GS/E, and Eos is evidence of this process.

A Millimeter-multiwavelength Continuum Study of VLA 1623 West

VLA 1623 West is an ambiguous source that has been described as a shocked cloudlet as well as a protostellar disk. We use deep ALMA 1.3 and 0.87 mm observations to constrain its shape and structure to determine its origins better. We use a series of geometric models to fit the uv visibilities at both wavelengths with GALARIO. Although the real visibilities show structures similar to what has been identified as gaps and rings in protoplanetary disks, we find that a modified flat-topped Gaussian model at high inclination provides the best fit to the observations. This fit agrees well with expectations for an optically thick, highly inclined disk. Nevertheless, we find that the geometric models consistently yield positive residuals at the four corners of the disk at both wavelengths. We interpret these residuals as evidence that the disk is flared in the millimeter dust. We use a simple toy model for an edge-on flared disk and find that the residuals best match a disk with flaring that is mainly restricted to the outer disk at R ≳ 30 au. Thus, VLA 1623W may represent a young protostellar disk where the large dust grains have not yet had enough time to settle into the midplane. This result may have implications for how disk evolution and vertical dust settling impact the initial conditions leading to planet formation.

Inside-out planet formation – VII. Astrochemical models of protoplanetary discs and implications for planetary compositions

ABSTRACT Inside-out planet formation (IOPF) proposes that the abundant systems of close-in Super-Earths and Mini-Neptunes form in situ at the pressure maximum associated with the dead zone inner boundary (DZIB). We present a model of physical and chemical evolution of protoplanetary disc midplanes that follows gas advection, radial drift of pebbles, and gas-grain chemistry to predict abundances from ∼300 au down to the DZIB near 0.2 au. We consider typical disc properties relevant for IOPF, i.e. accretion rates $10^{-9}\lt \dot{m}/ (\mathrm{ M}_\odot \:{\rm {yr}}^{-1})\lt 10^{-8}$ and viscosity parameter α = 10−4, and evolve for fiducial duration of 105 yr. For outer, cool disc regions, we find that C and up to $90{{\ \rm per\ cent}}$ of O nuclei start locked in CO and $\rm O_2$ ice, which keeps abundances of $\rm CO_2$ and $\rm H_2O$ one order of magnitude lower. Radial drift of icy pebbles is influential, with gas-phase abundances of volatiles enhanced up to two orders of magnitude at icelines, while the outer disc becomes depleted of dust. Discs with decreasing accretion rates gradually cool, which draws in icelines closer to the star. At ≲ 1 au, advective models yield water-rich gas with C/O ratios ≲ 0.1, which may be inherited by atmospheres of planets forming here via IOPF. For planetary interiors built by pebble accretion, IOPF predicts volatile-poor compositions. However, advectively enhanced volatile mass fractions of $\sim 10{{\ \rm per\ cent}}$ can occur at the water iceline.

The disturbed outer Milky Way disc

ABSTRACT The outer parts of the Milky Way’s disc are significantly out of equilibrium. Using only distances and proper motions of stars from Gaia’s Early Data Release 3, in the range |b| < 10°, 130° < ℓ < 230°, we show that for stars in the disc between around 10 and $14\, \mathrm{kpc}$ from the Galactic centre, vertical velocity is strongly dependent on the angular momentum, azimuth, and position above or below the Galactic plane. We further show how this behaviour translates into a bimodality in the velocity distribution of stars in the outer Milky Way disc. We use an N-body model of an impulse-like interaction of the Milky Way disc with a perturber similar to the Sagittarius dwarf to demonstrate that this mechanism can generate a similar disturbance. It has already been shown that this interaction can produce a phase spiral similar to that seen in the Solar neighbourhood. We argue that the details of this substructure in the outer galaxy will be highly sensitive to the timing of the perturbation or the gravitational potential of the Galaxy, and therefore may be key to disentangling the history and structure of the Milky Way.

Turbulence in outer protoplanetary discs: MRI or VSI?

ABSTRACT The outer protoplanetary discs (PPDs) can be subject to the magnetorotational instability (MRI) and the vertical shear instability (VSI). While both processes can drive turbulence in the disc, existing numerical simulations have studied them separately. In this paper, we conduct global 3D non-ideal magnetohydrodynamic (MHD) simulations for outer PPDs, with ambipolar diffusion and instantaneous cooling, and hence conductive to both instabilities. Given the range of ambipolar Elsässer numbers (Am) explored, it is found that the VSI turbulence dominates over the MRI when ambipolar diffusion is strong (Am = 0.1); the VSI and MRI can co-exist for Am = 1; and the VSI is overwhelmed by the MRI when ambipolar diffusion is weak (Am = 10). Angular momentum transport process is primarily driven by MHD winds, while viscous accretion due to MRI and/or VSI turbulence makes a moderate contribution in most cases. Spontaneous magnetic flux concentration and formation of annular substructures remain robust in strong ambipolar diffusion-dominated discs (Am ≤ 1) with the presence of the VSI. Ambipolar diffusion is the major contributor to the magnetic flux concentration phenomenon rather than advection.

A correlation between H α trough depth and inclination in quiescent X-ray transients: evidence for a low-mass black hole in GRO J0422+32

ABSTRACT We present a new method to derive binary inclinations in quiescent black hole (BH) X-ray transients (XRTs), based on the depth of the trough (T) from double-peaked H α emission profiles arising in accretion discs. We find that the inclination angle (i) is linearly correlated with T in phase-averaged spectra with sufficient orbital coverage (≳50 per cent) and spectral resolution, following i(deg) = 93.5 × T + 23.7. The correlation is caused by a combination of line opacity and local broadening, where a leading (excess broadening) component scales with the deprojected velocity of the outer disc. Interestingly, such scaling allows to estimate the fundamental ratio M1/Porb by simply resolving the intrinsic width of the double-peak profile. We apply the T–i correlation to derive binary inclinations for GRO J0422+32 and Swift J1357−0933, two BH XRTs where strong flickering activity has hindered determining their values through ellipsoidal fits to photometric light curves. Remarkably, the inclination derived for GRO J0422+32 (i = 55.6 ± 4.1○) implies a BH mass of $2.7^{+0.7}_{-0.5}$ M⊙ thus placing it within the gap that separates BHs from neutron stars. This result proves that low-mass BHs exist in nature and strongly suggests that the so-called ‘mass gap’ is mainly produced by low number statistics and possibly observational biases. On the other hand, we find that Swift J1357−0933 contains a $10.9^{+1.7}_{-1.6}$ M⊙ BH, seen nearly edge on ($i=87.4^{+2.6}_{-5.6}$ deg). Such extreme inclination, however, should be treated with caution since it relies on extrapolating the T–i correlation beyond i ≳ 75○, where it has not yet been tested.

Cluster environment quenches the star formation of low-mass satellite galaxies from the inside-out

ABSTRACT Environment plays a critical role in the star formation history of galaxies. Tidal and hydrodynamical stripping, prominent in cluster environment, can remove the peripheral gas of galaxies and star formation may thus be environmentally suppressed from the outside-in. We revisit the environmental dependence of the radial gradient of specific star formation rate (sSFR) profile. We probe the radial gradient by using the archival spectral indices D4000n and HδA measured from SDSS fibre spectra, to indicate central sSFR and the total sSFR from fitting the spectral energy distribution. Despite the low spatial resolution, the wealth of SDSS data allows to disentangle the dependences on stellar mass, sSFR, and environment. We find that low-mass satellite galaxies in the mass range $9\lt \mathrm{log}\, \mathcal {M}_{\star }/\mathcal {M}_{\odot }\lt 9.8$ on average quench in more inside-out pattern compared to isolated galaxies matched in mass, sSFR, and fibre coverage. This environmental effect is particularly strong for galaxies below the star formation main sequence, and peaks for those in the core of massive clusters where the phase-space diagram reveals clear links between the inside-out quenching and orbital properties. Our results suggest that both tidal and hydrodynamical interactions in cluster environment suppress the star formation of satellites mainly from the inside-out. As accreted gas of low angular momentum from hot gas haloes is an important source for replenishing central gas reservoir, we discuss how gas stripping in clusters may lead to starvation and cause inside-out quenching when the outer star-forming discs are not significantly affected.