Outer Disk Research Articles

Extended far-ultraviolet emission in distant dwarf galaxies.

Blue compact dwarf (BCD) galaxies are low-luminosity (absolute K-band magnitude,MK > -21 mag)1, metal-poor (1/50 ≤ Z/Z⊙ ≤ 1/2,where Z is the metallicity in terms of the solar metallicityZ⊙)2, centrally concentrated3 galaxies with bright clumps of star formation4. Cosmological surface-brightness dimming5 and the small size of BCDs limit their detection at high redshifts, making their formation process difficult to observe. Observations of BCDs are needed at intermediate redshifts, where they are still young enough to show their formative stages, particularly in the outer regions where cosmic gas accretion should drive evolution. Here we report the observation of excess far-ultraviolet (FUV) emission in the outer regions of 11 BCDs in the GOODS South field at redshifts between 0.1 and 0.24, corresponding to look-back times of 1.3-2.8 billion years in standard cosmology. These observations were made by the Ultra-Violet Imaging Telescope6 on AstroSat7. For ten BCDs, the radial profiles of the intrinsic FUV emission, corrected for the instrument point spread function, have larger scale lengths than their optical counterparts observed with the Hubble Space Telescope. Such shallow FUV profiles suggest extended star formation in cosmically accreting disks. Clumpy structure in the FUV also suggests that the outer FUV disks are gravitationally unstable. Dynamical friction on the clumps drives them inwards at an average rate exceeding 106 solar masses per billion years.

Quasi-simultaneous observations of radio and X-ray variability in three radio-quiet Seyfert galaxies

ABSTRACT Radio variability in some radio-quiet (RQ) active galactic nuclei suggests emission from regions close to the central engine, possibly the outer accretion disc corona. If the origins of the radio and the X-ray emission are physically related, their emission may be temporarily correlated, possibly with some time delays. We present the results of quasi-simultaneous radio and X-ray monitoring of three RQ Seyfert galaxies, Mrk 110, Mrk 766, and NGC 4593, carried out with the Very Large Array at 8.5 GHz over a period of about 300 d, and with the Rossi X-ray Timing Explorer at 2–10 keV over a period of about 2000 d. The radio core variability is likely detected in the highest resolution (A configuration) observations of Mrk 110 and NGC 4593, with a fractional variability amplitude of 6.3 per cent and 9.5 per cent, respectively. A cross-correlation analysis suggests an apparently strong (Pearson r = −0.89) and highly significant correlation (p = 1 × 10−6) in Mrk 110, with the radio lagging the X-ray by 56 d. However, a further analysis of the r values distribution for physically unrelated long time delays reveals that this correlation is not significant. This occurs since the Pearson correlation assumes white noise, while both the X-ray and the radio light curves follow red noise, which dramatically increases the chance, by a factor of ∼103, to get extremely high r values in uncorrelated data sets. A significantly longer radio monitoring with a higher sampling rate, preferably with a high-resolution fixed radio array, is required in order to reliably detect a delay.

A 2-hr binary period for the black hole transient MAXI J0637-430

ABSTRACT We revisit various sets of published results from X-ray and optical studies of the Galactic black hole (BH) candidate MAXI J0637-430, which went into outburst in 2019. Combining the previously reported values of peak outburst luminosity, best-fitting radii of inner and outer accretion disc, viewing angle, exponential decay time-scale, and peak-to-peak separation of the He II λ4686 disc emission line, we improve the constraints on the system parameters. We estimate a heliocentric distance d ≈ (8.7 ± 2.3) kpc, a projected Galactocentric distance R ≈ (13.2 ± 1.8) kpc and a height |z| ≈ (3.1 ± 0.8) kpc from the Galactic plane. It is the currently known Milky Way BH candidate located farthest from the Galactic Centre. We infer a BH mass M1 ≈ (5.1 ± 1.6)M⊙, a spin parameter a* ≲ 0.25, a donor star mass M2 ≈ (0.25 ± 0.07)M⊙, a peak Eddington ratio λ ≈ 0.17 ± 0.11 and a binary period $P_{\rm orb} \approx 2.2^{+0.8}_{-0.6}$ hr. This is the shortest period measured or estimated so far for any Galactic BH X-ray binary. If the donor star is a main-sequence dwarf, such a period corresponds to the evolutionary stage where orbital shrinking is driven by gravitational radiation and the star has regained contact with its Roche lobe (low end of the period gap). The three Galactic BHs with the shortest period (≲3 hr) are also those with the highest vertical distance from the Galactic plane (≳2 kpc). This is probably because binaries with higher binding energies can survive faster natal kicks.

Our Galaxy’s youngest disc

ABSTRACT We investigate the structure of our Galaxy’s young stellar disc by fitting the distribution functions (DFs) of a new family to 5D Gaia data for a sample of $47\, 000$ OB stars. Tests of the fitting procedure show that the young disc’s DF would be strongly constrained by Gaia data if the distribution of Galactic dust were accurately known. The DF that best fits the real data accurately predicts the kinematics of stars at their observed locations, but it predicts the spatial distribution of stars poorly, almost certainly on account of errors in the best-available dust map. We argue that dust models could be greatly improved by modifying the dust model until the spatial distribution of stars predicted by a DF agreed with the data. The surface density of OB stars is predicted to peak at $R\simeq 5.5\, \mathrm{kpc}$, slightly outside the reported peak in the surface density of molecular gas; we suggest that the latter radius may have been underestimated through the use of poor kinematic distances. The velocity distributions predicted by the best-fitting DF for stars with measured line-of-sight velocities v∥ reveal that the outer disc is disturbed at the level of $10\, \mathrm{km\, s}^{-1}$ in agreement with earlier studies, and that the measured values of v∥ have significant contributions from the orbital velocities of binaries. Hence the outer disc is colder than it is sometimes reported to be.

Gas temperature structure across transition disk cavities

Context. Most disks observed at high angular resolution show signs of substructures, such as rings, gaps, arcs, and cavities, in both the gas and the dust. To understand the physical mechanisms responsible for these structures, knowledge about the gas surface density is essential. This, in turn, requires information on the gas temperature. Aims. The aim of this work is to constrain the gas temperature as well as the gas surface densities inside and outside the millimeter-dust cavities of two transition disks: LkCa15 and HD 169142, which have dust cavities of 68 AU and 25 AU, respectively. Methods. We use some of the few existing ALMA observations of the J = 6-5 transition of 13CO together with archival J = 2−1 data of 12CO, 13CO, and C18O. The ratio of the 13CO J = 6−5 to the J = 2−1 transition is used to constrain the temperature and is compared with that found from peak brightness temperatures of optically thick lines. The spectra are used to resolve the innermost disk regions to a spatial resolution better than that of the beam of the observations. Furthermore, we use the thermochemical code DALI to model the temperature and density structure of a typical transition disk as well as the emitting regions of the CO isotopologs. Results. The 13CO J = 6−5 and J = 2−1 transitions peak inside the dust cavity in both disks, indicating that gas is present in the dust cavities. The kinematically derived radial profiles show that the gas is detected down to 10 and 5-10 AU, much farther in than the dust cavities in the LkCa15 and HD 169142 disks, respectively. For LkCa15, the steep increase toward the star in the 13CO J = 6−5 transition, in contrast to the J = 2−1 line, shows that the gas is too warm to be traced by the J = 2−1 line and that molecular excitation is important for analyzing the line emission. Quantitatively, the 6−5/2−1 line ratio constrains the gas temperature in the emitting layers inside the dust cavity to be up to 65 K, warmer than in the outer disk, which is at 20-30 K. For HD 169142, the lines are optically thick, complicating a line ratio analysis. In this case, the peak brightness temperature constrains the gas in the dust cavity of HD 169142 to be 170 K, whereas that in the outer disk is only 100 K. The data indicate a vertical structure in which the 13CO 6-5 line emits from a higher layer than the 2-1 line in both disks, consistent with exploratory thermochemical DALI models. Such models also show that a more luminous central star, a lower abundance of polycyclic aromatic hydrocarbons, and the absence of a dusty inner disk increase the temperature of the emitting layers and hence the line ratio in the gas cavity. The gas column density in the LkCa15 dust cavity drops by a factor of >2 compared to the outer disk, with an additional drop of an order of magnitude inside the gas cavity at 10 AU. In the case of HD 169142, the gas column density drops by a factor of 200–500 inside the gas cavity. Conclusions. The gas temperatures inside the dust cavities steeply increase toward the star and reach temperatures of up to 65 K (LkCa15) and 170 K (HD 169142) on scales of ~15–30 AU, whereas the temperature gradients of the emitting layers in the outer disks are shallow, with typical temperatures of 20-30 and 100 K, respectively. The deep drop in gas column density inside the HD 169142 gas cavity at <10 AU could be due to a massive companion of several MJ, whereas the broad dust-depleted gas region from 10 to 68 AU for LkCa15 may imply several lower mass planets. This work demonstrates that knowledge of the gas temperature is important for determining the gas surface density and thus whether planets, and if so what kinds of planets, are most likely to be carving the dust cavities.

A Dynamical Model of the M101/NGC 5474 Encounter

We present the first dynamical simulation that recreates the major properties of the archetypal nearby spiral galaxy M101. Our model describes a grazing but relatively close (∼14 kpc) passage of the companion galaxy NGC 5474 through M101's outer disk approximately 200 Myr ago. The passage is retrograde for both disks, yielding a relatively strong gravitational response while suppressing the formation of long tidal tails. The simulation reproduces M101's overall lopsidedness, as well as the extended NE Plume and the sharp western edge of the galaxy’s disk. The post-starburst populations observed in M101's NE Plume are likely a result of star formation triggered at the point of contact where the galaxies collided. Over time, this material will mix azimuthally, leaving behind diffuse, kinematically coherent stellar streams in M101's outer disk. At late times after the encounter, the density profile of M101's disk shows a broken “upbending” profile similar to those seen in spiral galaxies in denser environments, further demonstrating the connection between interactions and long-term structural changes in galaxy disks.

Predicting HCN, HCO+, multi-transition CO, and dust emission of star-forming galaxies

The interstellar medium is a turbulent, multiphase, and multi-scale medium that follows scaling relations that link the surface density, volume density, and velocity dispersion with the cloud size. Galactic clouds range from below 1 pc to about 100 pc in size. Extragalactic clouds appear to follow the same range, although they are only now becoming observable in atomic and molecular lines. Analytical models of galactic gaseous disks need to take the multi-scale and multiphase nature of the interstellar medium into account. They can be described as clumpy star-forming accretion disks in vertical hydrostatic equilibrium, with the mid-plane pressure balancing the gravity of the gaseous and stellar disk. Interstellar medium turbulence is taken into account by applying Galactic scaling relations to the cold atomic and molecular gas phases. Turbulence is maintained through energy injection by supernovae. With the determination of the gas mass fraction at a given spatial scale, the equilibrium gas temperature between turbulent heating and line cooling, the molecular abundances, and the molecular line emission can be calculated. The resulting model radial profiles of infrared, HI, CO, HCN, and HCO+ emission are compared to THINGS, HERACLES, EMPIRE, SINGS, and GALEX observations of 17 local spiral galaxies. The model free parameters were constrained for each galactic radius independently. The Toomre parameter, which measures the stability against star formation (cloud collapse), exceeds unity in the inner disk of a significant number of galaxies. In two galaxies it also exceeds unity in the outer disk. Therefore, in spiral galaxies Qtot = 1 is not ubiquitous. The model gas velocity dispersion is consistent with the observed HI velocity dispersion where available. Within our model, HCN and HCO+ is already detectable in relatively low-density gas (∼1000 cm−3). We derive CO and HCN conversion factors and molecular gas depletion times. Both conversion factors are consistent with values found in the literature. Whereas in the massive galaxies the viscous timescale greatly exceeds the star-formation timescale, the viscous timescale is smaller than the star-formation timescale within R ∼ 2 Rd, the disk scale length, in the low-mass galaxies. We suggest that massive spiral galaxies undergo starvation in the absence of gas accretion from the halo, whereas in low-mass galaxies the fuel for star formation reaches R ∼ 2 Rd from outside via a thick gas disk component with a high radial infall velocity observable in the HI line.

Structure and function of ABCA4 and its role in the visual cycle and Stargardt macular degeneration.

ABCA4 is a member of the superfamily of ATP-binding cassette (ABC) transporters that is preferentially localized along the rim region of rod and cone photoreceptor outer segment disc membranes. It uses the energy from ATP binding and hydrolysis to transport N-retinylidene-phosphatidylethanolamine (N-Ret-PE), the Schiff base adduct of retinal and phosphatidylethanolamine, from the lumen to the cytoplasmic leaflet of disc membranes. This ensures that all-trans-retinal and excess 11-cis-retinal are efficiently cleared from photoreceptor cells thereby preventing the accumulation of toxic retinoid compounds. Loss-of-function mutations in the gene encoding ABCA4 cause autosomal recessive Stargardt macular degeneration, also known as Stargardt disease (STGD1), and related autosomal recessive retinopathies characterized by impaired central vision and an accumulation of lipofuscin and bis-retinoid compounds. High resolution structures of ABCA4 in its substrate and nucleotide free state and containing bound N-Ret-PE or ATP have been determined by cryo-electron microscopy providing insight into the molecular architecture of ABCA4 and mechanisms underlying substrate recognition and conformational changes induced by ATP binding. The expression and functional characterization of a large number of disease-causing missense ABCA4 variants have been determined. These studies have shed light into the molecular mechanisms underlying Stargardt disease and a classification that reliably predicts the effect of a specific missense mutation on the severity of the disease. They also provide a framework for developing rational therapeutic treatments for ABCA4-associated diseases.

Shocks in Radiatively Driven Time-dependent, Relativistic Jets around Black Holes

We study time-dependent relativistic jets under the influence of the radiation field of the accretion disk. The accretion disk consists of an inner compact corona and an outer sub-Keplerian disk. The thermodynamics of the fluid is governed by a relativistic equation of state (EOS) for multispecies fluid that enables us to study the effect of composition on jet dynamics. Jets originate from the vicinity of the central black hole, where the effect of gravity is significant and traverses large distances where only special relativistic treatment is sufficient. So we have modified the flat metric to include the effect of gravity. In this modified relativistic framework we have developed a new total variation diminishing routine along with a multispecies EOS for the purpose. We show that the acceleration of jets crucially depends on flow composition. All the results presented are transonic in nature; starting from very low injection velocities, the jets can achieve high Lorentz factors. For sub-Eddington luminosities, lepton-dominated jets can be accelerated to Lorentz factors >50. The change in radiation field due to variation in the accretion disk dynamics will be propagated to the jet in a finite amount of time. Hence, any change in radiation field due to a change in disk configuration will affect the lower part of the jet before it affects the outer part. This can drive shock transition in the jet flow. Depending on the disk oscillation frequency, amplitude, and jet parameters, these shocks can collide with each other and may trigger shock cascades.

Mg/Fe] ratios in the solar neighbourhood: Stellar yields and chemical evolution scenarios

Context. The [Mg/Fe] abundance ratios are a fundamental fossil signature used to trace the chemical evolution of the disc and to divide it into low-α and high-α populations. Despite the huge observational and theoretical efforts, discrepancies between models and data are still present and several explanations have been put forward to explain the [α/Fe] bimodality. Aims. In this work we take advantage of a new AMBRE:HARPS dataset, which provides new more precise [Mg/Fe] estimations and reliable stellar ages for a subsample of stars, to study the [α/Fe] bimodality and the evolution of the solar neighbourhood. Methods. The data are compared with detailed chemical evolution models for the Milky Way, exploring the most used prescriptions for stellar yields and different formation scenarios for the Galactic disc (i.e. the delayed two-infall and the parallel models), including prescriptions for stellar radial migration. Results. We see that most of the stellar yield prescriptions struggle to reproduce the observed trend of the data and that semi-empirical yields describe best the [Mg/Fe] evolution in the thick and thin discs. In particular, most of the yields still predict a steeper decrease of the [Mg/Fe] ratio at high metallicity than shown by the data. The bulk of the data are well reproduced by the parallel and two-infall scenarios, but both scenarios have problems in explaining the most metal-rich and metal-poor tails of the low-α data. These tails can be explained in light of radial migration from the inner and outer disc regions, respectively. Conclusions. Despite the evidence of stellar migration, it is difficult to estimate the actual contribution of stars from other parts of the disc to the solar vicinity in the data we adopt. However, the comparison between data and models suggests that peculiar histories of star formation, such as that of the two-infall model, are still needed to reproduce the observed distribution of stars.

Properties of Dense Molecular Gas along the Major Axis of M82

Dense gas is important for galaxy evolution and star formation. Optically thin dense-gas tracers, such as isotopologues of HCN, HCO+, etc., are very helpful in diagnosing the excitation conditions of dense molecular gas. However, previous studies of optically thin dense-gas tracers mostly focused on the average properties of galaxies as a whole, due to limited sensitivity and angular resolution. M82, a nearby prototype starburst galaxy, offers a unique case for spatially resolved studies with single-dish telescopes. With the IRAM 30 m telescope, we observed the J = 1 → 0 transition of H13CN, HC15N, H13CO+, HN13C, H15NC, and SiO J = 2 → 1, HC3N J = 10 → 9, and H2CO J = 2 → 1 toward five positions along the major axis of M82. The intensity ratios of I(HCN)/I(H13CN) and I(HCO+)/I(H13CO+) show a significant spatial variation along the major axis, with lower values in the central region than those on the disk, indicating higher optical depths in the central region. The optical depths of HCO+ lines are found to be systematically higher than those of HCN lines at all positions. Furthermore, we find that the 14N/15N ratios have an increasing gradient from the center to the outer disk.

No Evidence of the Significant Grain Growth but Tentative Discovery of Disk Substructure in a Disk around the Class I Protostar L1489 IRS

For revealing the first step of planet formation, it is important to understand how and when dust grains become larger in a disk around a protostar. To investigate the grain growth, we analyze dust continuum emission toward a disk around the Class I protostar L1489 IRS at 0.9 and 1.3 mm wavelengths obtained by the Atacama Large Millimeter/submillimeter Array. The dust continuum emission extends to a disk radius (r) of r ∼ 300 au, and the spectral index (α) is derived to be α ∼ 3.6 at a radius of r ∼ 100–300 au, similar to the interstellar dust. Therefore, the grain growth does not occur significantly in the outer disk (r ∼ 100–300 au). Furthermore, we tentatively identify a ring-like substructure at r ∼ 90 au even though the spatial resolution and sensitivity are not enough to determine this structure. If this is the real ring structure, the ring position and small dust in the disk outer part are consistent with the idea of the growth front. These results suggest that the L1489 protostellar disk may be the beginning of planet formation.

Isotopic evidence for two chondrule generations in CR chondrites and their relationships to other carbonaceous chondrites

Among primitive meteorites, CR chondrites have peculiar isotopic compositions, the origin of which is uncertain and may have involved contributions from primordial molecular cloud material or the chondrites' formation and agglomeration late during the evolution of the protoplanetary disk. Here, we report a comprehensive textural and isotopic characterization of type I CR chondrules and provide new insights on their formation conditions. We find that two chondrule populations characterized by different sizes and oxygen isotopic compositions co-exist in CR chondrites. The typically larger, 16O-poor (Δ17O>-4‰) chondrules (type I-CR chondrules) appear to have formed late out of a CR reservoir already populated by typically smaller, 16O-rich (Δ17O<-4‰) chondrules (type I-CO chondrules). Before formation of type I-CR chondrules, the CR reservoir was likely dominated by CI-like dust, in line with the proximity of CR with CI chondrites for many isotopic ratios. The CR reservoir thus may have largely belonged to the continuum shown by other carbonaceous chondrites, although some isotopic ratios maintain some originality and suggest isotopic variation of CI-like dust in the outer disk. Combined with literature data, our data (i) demonstrates that recycling processes are responsible for the singular compositions of CR chondrites and their chondrules for isotopic systems with drastically different geochemical behaviors (O, Cr, Te) and (ii) support the homogeneous distribution of 26Al throughout the protoplanetary disk.

ALMA Images the Eccentric HD 53143 Debris Disk

We present ALMA 1.3 mm observations of the HD 53143 debris disk—the first infrared or millimeter image produced of this ∼1 Gyr old solar analog. Previous HST STIS coronagraphic imaging did not detect flux along the minor axis of the disk, which could suggest a face-on geometry with two clumps of dust. These ALMA observations reveal a disk with a strikingly different structure. In order to fit models to the millimeter visibilities and constrain the uncertainties on the disk parameters, we adopt a Markov Chain Monte Carlo approach. This is the most eccentric debris disk observed to date with a forced eccentricity of 0.21 ± 0.02, nearly twice that of the Fomalhaut debris disk, and also displays an apocenter glow. Although this eccentric model fits the outer debris disk well, significant interior residuals remain, which may suggest a possible edge-on inner disk, which remains unresolved in these observations. Combined with the observed structure difference between HST and ALMA, these results suggest a potential previous scattering event or dynamical instability in this system. We also note that the stellar flux changes considerably over the course of our observations, suggesting flaring at millimeter wavelengths. Using simultaneous TESS observations, we determine the stellar rotation period to be 9.6 ± 0.1 days.

Reliability and limitations of inferring birth radii in the Milky Way disc

ABSTRACT Recovering the birth radii of observed stars in the Milky Way is one of the ultimate goals of Galactic Archaeology. One method to infer the birth radius and the evolution of the interstellar medium (ISM) metallicity assumes a linear relation between the ISM metallicity with radius at any given look-back time. Here, we test the reliability of this assumption by using four zoom-in cosmological hydrodynamic simulations from the NIHAO-UHD project. We find that one can infer precise birth radii only when the stellar disc starts to form, which for our modelled galaxies happens ∼10 Gyr ago, in agreement with recent estimates for the Milky Way. With a current day measurement of ISM metallicity gradient of −0.05 dex and a dispersion of 0.03 dex, the intrinsic uncertainty in inferring Rbirth is ∼0.6 kpc. At later times, the linear correlation between the ISM metallicity and radius increases, as stellar motions become more ordered and the azimuthal variations of the ISM metallicity start to drop. The formation of a central bar and perturbations from mergers can increase this uncertainty in the inner and outer disc, respectively.

Unveiling the outer dust disc of TW Hya with deep ALMA observations

ABSTRACT The radial extent of millimetre dust in protoplanetary discs is often far smaller than that of their gas, mostly due to processes such as dust growth and radial drift. However, it has been suggested that current millimetre continuum observations of discs do not trace their full extent due to limited sensitivity. In this letter, we present deep (19 $\mu$Jy beam−1) moderate resolution (0.37 arcsec) ALMA observations at 1 mm of the nearest protoplanetary disc, TW Hya. Using the visibility analysis tool frank, we reveal a structured millimetre intensity distribution out to 100 au, well beyond previous estimates of 60–70 au. Our analysis suggests the presence of a new millimetre continuum gap at 82 au, coincident with similar features seen in optical/near-infrared scattered light and millimetre molecular line observations. Examination of the fit residuals confirms the presence of the previously reported au-scale continuum excess at 52 au (P.A. = 242.5°). Our results demonstrate the utility of combining deep, moderate-resolution observations with super-resolution analysis techniques to probe the faintest regions of protoplanetary discs.

The peculiar spectral evolution of the new X-ray transient MAXI J0637–430

ABSTRACT We studied the transient Galactic black hole candidate MAXI J0637−430 with data from Insight-HXMT, Swift, and XMM–Newton. The broad-band X-ray observations from Insight-HXMT help us constrain the power-law component. MAXI J0637–430 is located at unusually high Galactic latitude; if it belongs to the Galactic thick disc, we suggest a most likely distance ≲7 kpc. Compared with other black hole transients, MAXI J0637–430 is also unusual for other reasons: a fast transition to the thermal dominant state at the start of the outburst; a low peak temperature and luminosity (we estimate them at ≈0.7 keV and ≲0.1 times Eddington, respectively); a short decline time-scale; a low soft-to-hard transition luminosity (≲0.01 times Eddington). We argue that such properties are consistent with a small binary separation, short binary period (P ∼ 2 h), and low-mass donor star (M2 ∼ 0.2 M⊙). Moreover, spectral modelling shows that a single disc blackbody component is not a good fit to the thermal emission. Soft spectral residuals, and deviations from the standard $L_{\rm disc} \propto T_{\rm in}^4$ relation, suggest the need for a second thermal component. We propose and discuss various scenarios for such component, in addition to those presented in previous studies of this source. For example, a gap in the accretion disc between a hotter inner ring near the innermost stable orbit, and a cooler outer disc. Another possibility is that the second thermal component is the thermal plasma emission from an ionized outflow.

Multiwavelength observations of MAXI J1820+070 during its outburst decay and subsequent mini-outburst

ABSTRACT We present results from quasi-simultaneous multiwavelength observations of the Galactic black hole X-ray transient MAXI J1820+070 during the decay of the 2018 outburst and its entire subsequent mini-outburst in March 2019. We fit the X-ray spectra with phenomenological and Comptonizaton models and discuss the X-ray spectral evolution in comparison with the multiwavelength behaviour of the system. The system showed a rebrightening in UV/Optical/NIR bands 7 d after the soft-to-hard transition during the main outburst decay while it was fading in X-rays and radio. In contrast, the mini-outburst occurred 165 d after the hard state transition of the initial outburst decay and was detected in all wavelengths. For both events, the measured time-scales are consistent with those observed in other black hole systems. Contemporaneous hard X-ray/soft γ-ray observations indicate a non-thermal electron energy distribution at the beginning of the UV/Optical/NIR rebrightening, whereas a thermal distribution can fit the data during the hard mini-outburst activity. The broad-band spectral energy distributions until the rebrightening are consistent with the irradiated outer accretion disc model. However, both the SEDs produced for the peak of rebrightening and close to the peak of mini-outburst provided good fits only with an additional power law component in the UV/Optical/NIR frequency ranges, which is often interpreted with a jet origin.

Dispersal of protoplanetary discs: how stellar properties and the local environment determine the pathway of evolution

ABSTRACT We study the evolution and final dispersal of protoplanetary discs that evolve under the action of internal and external photoevaporation, and different degrees of viscous transport. We identify five distinct dispersal pathways, which are (i) very long-lived discs ($\gt 20\,$ Myr), (ii) inside-out dispersal where internal photoevaporation dominates and opens inner holes, (iii) outside-in dispersal where external photoevaporation dominates through disc truncation, and two intermediate regimes characterized by lingering material in the inner disc with the outer disc dispersed predominantly by either internal or external photoevaporation. We determine how the lifetime, relative impact of internal and external winds, and clearing pathway vary over a wide, plausible, parameter space of stellar/disc/radiation properties. There are a number of implications, for example, in high UV environments because the outer disc lifetime is shorter than the time-scale for clearing the inner disc, we do not expect transition discs to be common, which appears to be reflected in the location of transition disc populations towards the Orion Nebular Cluster. Irrespective of environment, we find that ongoing star formation is required to reproduce observed disc fractions as a function of stellar cluster age. This work demonstrates the importance of including both internal and external winds for understanding protoplanetary disc evolution.

High thermal stability of nanocrystalline FeNi2CoMo0.2V0.5 high-entropy alloy by twin boundary and sluggish diffusion

A nanocrystalline (NC) face-centered cubic FeNi2CoMo0.2V0.5 high-entropy alloy was produced by high pressure torsion (HPT). The evolutions of microhardness and microstructure of the NC alloy during subsequent isochronal annealing were investigated systematically by electron back-scattering diffraction (EBSD) and high-resolution transmission electron microscopy (HRTEM). It was found that nano-grains and deformation nano-twin lamella were obtained at outer disk edge after HPT process with a hardness plateau of 450 HV. Isochronal annealing below 600 °C induced an evident hardening without precipitation effect, due to the annihilation of mobile dislocations and sustained deformation twin barriers. Evident recrystallization and grain growth of the NC FeNi2CoMo0.2V0.5 high-entropy alloy occurred during isochronal annealing at temperatures higher than 600 °C. The activation energies of recrystallization and grain growth of the NC FeNi2CoMo0.2V0.5 high-entropy alloy were calculated to be 350 kJ/mol and 272 kJ/mol, respectively, corresponding to a slow defects recovery process and a swift GB migration process. The high thermal stability of the NC FeNi2CoMo0.2V0.5 high-entropy was mainly caused by kinetic sluggish diffusion effect and deformation twin boundaries with thermodynamic low boundary energy, which retarded the movements of dislocations and grain boundary.