Disk Evolution Research Articles

Two sequences of spiral galaxies with different shapes of the metallicity gradients

We considered two sequences of spiral galaxies with different shapes of the radial gas-phase oxygen abundance distributions from the galaxies in the survey Mapping Nearby Galaxies at Apache Point Observatory (MaNGA): (1) Galaxies in which the gradient is well approximated by a single linear relation across the whole disc, that is, galaxies with an S (slope) gradients, (2) galaxies in which the metallicity in the inner region of the disc is at a nearly constant level and the gradient is negative at larger radii, that is, galaxies with level-slope (LS) gradients. We also selected galaxies with a nearly uniform oxygen abundance across the whole galaxy, that is, galaxies with level (L) gradients (or O/H uniform galaxies) with a high oxygen abundance that can be the final evolutionary stage of the two galaxy sequences described above. The radial nitrogen abundance distributions in galaxies with LS oxygen abundance distributions also show breaks at radii smaller than the O/H distribution breaks. The observed behaviour of the oxygen and nitrogen abundances with radius in these galaxies can be explained by the time delay between the nitrogen and oxygen enrichment together with the variation in the star formation history along the radius. These galaxies clearly show the effect of the inside-out disc evolution model, which predicts that the galactic centre evolves more rapidly than the regions at greater galactocentric distances. We find that the shape of the radial abundance distribution in a galaxy is not related to its macroscopic characteristics (rotation velocity, stellar mass, isophotal radius, and star formation rate) and is independent of its present-day environment. The correlations between the gradient slopes and macroscopic characteristics of galaxies are weak in the sense that the scatter of the points in each diagram is large. The galaxies with different abundance distributions (S, LS, or L) in our sample are located within the main sequence of the star-forming galaxies in the diagram of star formation rate–stellar mass. We also examined the properties of the Milky Way in the context of the considered galaxy samples.

JWST Imaging of Edge-on Protoplanetary Disks. I. Fully Vertically Mixed 10 μm Grains in the Outer Regions of a 1000 au Disk

Scattered light imaging of protoplanetary disks provides key insights on the geometry and dust properties in the disk surface. Here, we present James Webb Space Telescope (JWST) 2–21 μm images of a 1000 au radius edge-on protoplanetary disk surrounding an 0.4 M ⊙ young star in Taurus, Two Micron All Sky Survey (2MASS) J04202144 + 2813491. These observations represent the longest wavelengths at which a protoplanetary disk is spatially resolved in scattered light. We combine these observations with Hubble Space Telescope optical images and Atacama Large Millimeter/submillimeter Array continuum and CO mapping. We find that the changes in the scattered light disk morphology are remarkably small across a factor of 30 in wavelength, indicating that dust in the disk surface layers is characterized by an almost gray opacity law. Using radiative transfer models, we conclude that grains up to ≳10 μm in size are fully coupled to the gas in this system, whereas grains ≳100 μm are strongly settled toward the midplane. Further analyses of these observations, and similar ones of other edge-on disks, will provide strong empirical constraints on disk dynamics and evolution and grain growth models. In addition, the 7.7 and 12. μm JWST images reveal an X-shaped feature located above the warm molecular layer traced by CO line emission. The highest elevations at which this feature is detectable roughly match the maximal extent of the disk in visible wavelength scattered light as well as of an unusual kinematic signature in CO. We propose that these phenomena could be related to a disk wind entraining small dust grains.

Analytical solutions for the evolution of MHD wind-driven accretion discs

ABSTRACT We present new analytical solutions for the evolution of protoplanetary discs (PPDs) where magnetohydrodynamic (MHD) wind-driven processes dominate. Our study uses a 1D model which incorporates equations detailing angular momentum extraction by MHD winds and mass-loss rates. Our solutions demonstrate that the disc retains its initial state during the early phases; however, it rapidly evolves towards a self-similar state in the later stages of disc evolution. The total disc mass undergoes a continuous decline over time, with a particularly rapid reduction occurring beyond a certain critical time threshold. This gradual decrease in mass is influenced by the wind parameters and the initial surface density of the disc. In the MHD wind-dominated regime, we show that the disc’s lifespan correlates positively with the magnetic lever arm up to a certain threshold, irrespective of the initial disc size. PPDs with a larger magnetic lever arm are found to maintain significantly higher total disc mass over extended periods compared to their counterparts. The mass ejection-to-accretion ratio increases in efficient wind scenarios and is further amplified by a steeper initial surface density profile. Our analysis also reveals varied evolutionary trajectories in the plane of accretion rate and total disc mass, influenced by magnetic parameters and initial disc size. In scenarios with efficient MHD winds, discs with bigger sizes have extended operation time for mechanisms governing planet formation.

Unraveling the Origins and Development of the Galactic Disk through Metal-Poor Stars

The Milky Way is a spiral galaxy comprising three main components: the Bulge, the Disk, and the Halo. Of particular interest is the Galactic disk, which holds a significant portion of the baryonic matter angular momentum and harbors at least two primary stellar populations: the thin and thick disks. Understanding the formation and evolution of the Galactic disk is crucial for comprehending the origins and development of our Galaxy. Stellar archaeology offers a means to probe the disk’s evolution by listening to the cosmological narratives of its oldest and most pristine stars, specifically the metal-poor stars. In this study, we employed accurate photometric metallicity estimates and Gaia Early Data Release 3 astrometry to curate a pure sample of the oldest Galactic stars. This proceeding presents a summary of our primary findings.

On the evolution of a twisted thin accretion disc in eccentric inclined supermassive binary black holes

ABSTRACT We propose a model of a twisted accretion disc around a Kerr black hole interacting with a secondary black hole of a smaller mass on an inclined eccentric orbit. We use parameters of the system, which may be appropriate for the so-called precessing massive model of OJ 287. We calculate expressions for torque exerted on the disc by the secondary and a contribution of the secondary to the apsidal precession of disc elements by a double averaging procedure over the periods of the secondary and the disc elements. These expressions are used at all scales of interest, including the ones inside the binary orbit. We calculate numerically the evolution of the disc tilt and twist assuming a flat initial configuration. We consider the disc aspect ratio h/r = 10−3, a rather large viscosity parameter α = 0.1 and several values of the primary rotational parameter, χ. We find that, after a few periods of Lense–Thirring precession of the orbit, the disc relaxes to a quasi-stationary configuration in the precessing frame with a non-trivial distribution of the disc inclination angle, β, over the radial scale. We propose an analytic model for this configuration. We show that the presence of the twisted disc leads to multiple crossings of the disc by the secondary per one orbital period, with time periods between the crossings being different from the flat disc model. Our results should be taken into account in the modelling of OJ 287. They can also be applied to similar sources.

Snapshots of an Evolving Solar Nebula Recorded in Nucleosynthetic Sr and Ba Signatures of Early Condensates

The discovery of extreme strontium isotope anomalies (μ 84Sr) in refractory leachates from Allende fine-grained calcium-aluminum-rich inclusions (CAIs) is at odds with long-standing predictions regarding the homogenization of presolar components in the CAI-forming region. Elucidating the stellar source(s) of these phases and the mechanisms for their preservation holds potential significance in understanding the dynamics and evolution of the protoplanetary disk. Here we present barium isotope data for the same set of leachates previously analyzed for μ 84Sr. Our results show fairly homogeneous Ba isotope anomalies across leachates (∼100–200 ppm variability), in contrast to the observed μ 84Sr variations (up to ∼8%). Secondary phases extracted in earlier leaching steps (L1 and L3) reveal trends in μ 137Ba and μ 138Ba akin to that of mainstream SiC and a second nucleosynthetic component. We show that SiC X grains from Type II supernovae are good end-member candidates for explaining the intra-leachate spread in L1 and L3 μ 13x Ba. Notably, neither s-variability nor X-variability appears to contribute to trends in the barium isotope anomalies of the most refractory components (L4 and L5). We propose that the contrast in isotope anomaly systematics between the labile and refractory leachates could reflect a shift in the nucleosynthetic signatures of reservoirs sampled by these components. These observations are consistent with extreme 84Sr p-excesses manifesting only in L4 and L5 leachates. Finally, the decoupled Sr and Ba isotope anomalies point to a nucleosynthetic source that significantly overproduces strontium relative to barium, such as electron-capture supernovae or the collapse of rotating massive stars.

Optical Spectroscopy of 1A 0535+262 Before, During, and After the 2020 Giant X-ray Outburst

We present the findings from our study of the Be/X-ray binary 1A 0535+262/HD 245770 during the giant X-ray outburst in October 2020. We utilized the 1.2-m telescope at Mount Abu Infrared observatory for optical observations of the Be companion star. The outburst reached a peak X-ray flux of approximately 11 Crab in the 15-50 keV range, marking the highest ever recorded X-ray outburst from the pulsar. We conducted optical observations in the 6000-7200 Å range before, during, and after the X-ray outburst, aiming to examine the evolution of the circumstellar disc of the Be star from February 2020 to February 2022. Our optical spectra displayed prominent emission lines at 6563 Å (H I), 6678 Å (He I), and 7065 Å (He I). Notably, the Hα line exhibited significant variability in the spectra. Prior to and during the outburst, the line profiles appeared single-peaked, and asymmetric with broad red and blue wings, respectively. However, post-outburst observations revealed a double-peaked profile with asymmetry in the blue wing. Our pre-outburst observations confirmed a larger Be circumstellar disc that diminished in size as the outburst progressed. Additionally, the observed variations in the Hα line profile and parameters indicate the presence of a highly misaligned, precessing, and warped Be disc.

Evolution of the Magnetic Field in High- and Low-β Disks with Initially Toroidal Fields

Abstract We present the results from a pair of high-resolution, long-timescale (∼105 GM/c 3), global, three-dimensional magnetohydrodynamical accretion disk simulations with differing initial magnetic plasma β in order to study the effects of the initial toroidal field strength on the production of a large-scale poloidal field. We initialize our disks in approximate equilibrium with purely toroidal magnetic fields of strength β 0 = 5 and 200. We also perform a limited resolution study. We find that simulations of differing field strengths diverge early in their evolution and remain distinct over the time studied, indicating that the initial magnetic conditions leave a persistent imprint in our simulations. Neither simulation enters the magnetically arrested disk regime. Both simulations are able to produce poloidal fields from initially toroidal fields, with the β 0 = 5 simulation evolving clear signs of a large-scale poloidal field. We make a cautionary note that computational artifacts in the form of large-scale vortices may be introduced in the combination of initially weak field and disk-internal mesh refinement boundaries, as evidenced by the production of an m = 1 mode overdensity in the weak field simulation. Our results demonstrate that the initial toroidal field strength plays a vital role in the simulated disk evolution for the models studied.

Evolution of eccentric stellar discs around supermassive black holes: the complex disc disruption dynamics and the milliparsec stars

ABSTRACT We study the 10 Myr evolution of parsec-scale stellar discs with initial masses of Mdisc = 1.0–$7.5 \times 10^4\, \mathrm{M}_\odot$ and eccentricities einit = 0.1–0.9 around supermassive black holes (SMBHs). Our disc models are embedded in a spherical background potential and have top-heavy single and binary star initial mass functions (IMF) with slopes of 0.25–1.7. The systems are evolved with the N-body code BIFROST, including post-Newtonian (PN) equations of motion and simplified stellar evolution. All discs are unstable and evolve on Myr time-scales towards similar eccentricity distributions peaking at e⋆ ∼ 0.3–0.4. Models with high einit also develop a very eccentric (e⋆ ≳ 0.9) stellar population. For higher disc masses Mdisc ≳ 3 × 104 M⊙, the disc disruption dynamics is more complex than the standard secular eccentric disc instability with opposite precession directions at different disc radii – a precession direction instability. We present an analytical model describing this behaviour. A milliparsec population of N ∼ 10–100 stars forms around the SMBH in all models. For low einit, stars migrate inward while for einit ≳ 0.6 stars are captured by the Hills mechanism. Without PN, after 6 Myr, the captured stars have a sub-thermal eccentricity distribution. We show that including PN effects prevents this thermalization by suppressing resonant relaxation effects and cannot be ignored. The number of tidally disrupted stars is similar or larger than the number of milliparsec stars. None of the simulated models can simultaneously reproduce the kinematic and stellar population properties of the Milky Way centre clockwise disc and the S-cluster.

Role of redox conditions and thermal metamorphism in the preservation of Cr isotopic anomalies in components of non-carbonaceous chondrites

Mass-independent isotopic anomalies in meteorites are probes to the dynamics and evolution of the protoplanetary disc and the reservoirs from which planetary bodies accreted. Variable Cr nucleosynthetic compositions between meteorite groups are observed but the cause is not well understood. Investigations on presolar carriers in unequilibrated chondrites are thus required to establish these relationships. For that purpose, we analysed the Cr isotope compositions of leachates (stepwise dissolution solutions) of one ordinary chondrite and two enstatite chondrites of EL and EH subgroups previously measured for Nd and Sm isotopes. The leachates of the ordinary chondrite display large variations of their Cr isotope compositions, whereas the leachates of the enstatite chondrites show very limited variations of their nucleosynthetic compositions, confirming other studies. The Cr isotope compositions of leachates of unequilibrated chondrites are significantly modified by thermal processing and aqueous alteration under various conditions. The leachates of the enstatite chondrites are relatively homogeneous for Cr isotope composition, suggesting that presolar carriers of Cr are not stable under very reduced conditions and are likely oxidised phases. After careful examination of the role of parent body processing, we conclude that there is one predominant presolar carrier enriched in 54Cr that produced most of the nucleosynthetic variations observed in leachates. This carrier is likely depleted in the carbonaceous reservoir relative to the non-carbonaceous reservoir. The drastically different isotope compositions of Cr and heavy refractory lithophile elements such as Nd of leachates of chondrites reflect relative preservation biases of their respective presolar carriers, namely a still poorly characterised presolar oxide and silicon carbide (SiC), respectively.

Protostellar discs fed by dense collapsing gravomagneto sheetlets

ABSTRACT Stars form from the gravitational collapse of turbulent, magnetized molecular cloud cores. Our non-ideal MHD simulations reveal that the intrinsically anisotropic magnetic resistance to gravity during the core collapse naturally generates dense gravomagneto sheetlets within inner protostellar envelopes – disrupted versions of classical sheet-like pseudo-discs. They are embedded in a magnetically dominant background, where less dense materials flow along the local magnetic field lines and accumulate in the dense sheetlets. The sheetlets, which feed the disc predominantly through its upper and lower surfaces, are the primary channels for mass and angular momentum transfer from the envelope to the disc. The protostellar disc inherits a small fraction (up to 10 per cent) of the magnetic flux from the envelope, resulting in a disc-averaged net vertical field strength of 1–10 mG and a somewhat stronger toroidal field, potentially detectable through ALMA Zeeman observations. The inherited magnetic field from the envelope plays a dominant role in disc angular momentum evolution, enabling the formation of gravitationally stable discs in cases where the disc field is relatively well-coupled to the gas. Its influence remains significant even in marginally gravitationally unstable discs formed in the more magnetically diffusive cases, removing angular momentum at a rate comparable to or greater than that caused by spiral arms. The magnetically driven disc evolution is consistent with the apparent scarcity of prominent spirals capable of driving rapid accretion in deeply embedded protostellar discs. The dense gravomagneto sheetlets observed in our simulations may correspond to the ‘accretion streamers’ increasingly detected around protostars.

Insights into the first and second hydrostatic core stages from numerical simulations

The theory of how low mass stars form from the collapse of a dense molecular cloud core has been well-established for decades. Thanks to significant progress in computing and numerical modelling, more physical models have been developed and a wider parameter space explored to understand the early stages of star formation more fully. In this review, I describe the expected physical properties of the first and second core stages and how the inclusion of different physics affects those predicted characteristics. I provide an overview of chemical models and synthetic observations, looking towards the positive identification of the first core in nature, which remains elusive. However, there are a few likely candidate first cores, which are listed, and I briefly discuss the recent progress in characterising the youngest protostellar sources. Chemistry will be instrumental in the firm identification of the first core so we require robust theoretical predictions of the chemical evolution of protostellar cores, especially of the first and second core outflows. Looking ahead, simulations can shed light on how the protostellar collapse phase shapes the evolution of the protostellar disc. Simulations of dust evolution during protostellar core collapse show there is significant enhancement in grain size and abundance towards the centre of the core. Chemical models show that the warm, dense conditions of the first core drive chemical evolution. There is a wide scope for further study of the role that the first and second core stages play in determining the structure and composition of the protostellar disc and envelope and, of course, the eventual influence on the formation of planets.

Testing external photoevaporation in the σ-Orionis cluster with spectroscopy and disk mass measurements (Corrigendum)

{The evolution of protoplanetary disks is regulated by an interplay of several processes, either internal to the system or related to the environment. As most of the stars and planets, including our own Solar System, have formed in massive stellar clusters that contain OB-type stars, studying the effects of UV radiation on disk evolution is of paramount importance.} % aims heading (mandatory) {Here we test the impact of external photoevaporation on the evolution of disks in the mid-age ($sim$3--5 Myr) $sigma$-Orionis cluster by conducting the first combined large-scale UV to IR spectroscopic and mm-continuum survey of this region.} % methods heading (mandatory) {We study a sample of 50 targets located at increasing distances from the central, massive OB system $sigma$-Ori. We combine new spectra obtained with VLT/X-Shooter, used to measure mass accretion rates and stellar masses, with new and previously published ALMA measurements of disk dust and gas fluxes and masses.} % results heading (mandatory) {We confirm the previously found decrease of $M_{rm dust}$ in the inner $sim$0.5 pc of the cluster, and discover an additional dependence with the stellar host mass. The disks around the more massive stars ($ge$ 0.4msun) and located in the inner part ($<$ 0.5 pc) of the cluster have $M_{rm dust}$ about an order of magnitude lower than the more distant ones, in contrast to the rather flat $M_{rm dust}$ distribution as a function of the projected separation from $sigma$-Ori, for the disks around the lowest-mass stars in the sample. About half of the sample is located in the region of the $dot{M}_{rm acc}$ vs $M_{rm disk}$ expected by models of external photoevaporation, i.e. showing shorter disk lifetimes than expected for their ages. The shorter disk lifetimes is observed for all targets with projected separation from $sigma$-Ori $<$ 0.5 pc, proving that the presence of a massive stellar system affects disk evolution. } % conclusions heading (optional), leave it empty if necessary {External photoevaporation is a viable mechanism to explain the observed shorter disk lifetimes and lower $M_{rm dust}$ in the inner $sim$0.5 pc of the $sigma$-Orionis cluster, where the effects of this process are more pronounced. Follow-up observations of the low stellar mass targets are crucial to constrain disk dispersion time scales in the cluster and to confirm the dependence of the external photoevaporation process with stellar host mass. This work confirms that the effects of external photoevaporation are significant down to at least impinging radiation as low as $sim 10^{4}$ G$_0$. }

A Primordial Origin for the Gas-rich Debris Disks around Intermediate-mass Stars

While most debris disks consist of dust with little or no gas, a fraction have significant amounts of gas detected via emission lines of CO, ionized carbon, and/or atomic oxygen. Almost all such gaseous debris disks known are around A-type stars with ages up to 50 Myr. We show, using semianalytic disk evolution modeling, that this can be understood if the gaseous debris disks are remnant protoplanetary disks that have become depleted of small grains compared to the interstellar medium. Photoelectric heating by the A stars’ far-UV (FUV) radiation is then inefficient, while the stars’ extreme-UV (EUV) and X-ray emissions are weak owing to a lack of surface convective zones capable of driving magnetic activity. In this picture, it is relatively difficult for stars outside the range of spectral types from A through early B to have such long-lived gas disks. Less-massive stars have stronger magnetic activity in the chromosphere, transition region, and corona with resulting EUV and X-ray emission, while more-massive stars have photospheres hot enough to produce strong EUV radiation. In both cases, primordial disk gas is likely to photoevaporate well before 50 Myr. These results come from 0D disk evolution models where we incorporate internal accretion stresses, MHD winds, and photoevaporation by EUV and X-ray photons with luminosities that are functions of the stellar mass and age. A key issue this work leaves open is how some disks become depleted in small dust so that FUV photoevaporation slows. Candidates include the grains’ growth, settling, radial drift, radiation force, and incorporation into planetary systems.

Photoevaporation obfuscates the distinction between wind and viscous angular momentum transport in protoplanetary discs

ABSTRACT How protoplanetary discs evolve remains an unanswered question. Competing theories of viscosity and magnetohydrodynamic disc winds have been put forward as the drivers of angular momentum transport in protoplanetary discs. These two models predict distinct differences in the disc mass, radius, and accretion rates over time, that could be used to distinguish them. However that expectation is built on models that do not include another important process – photoevaporation, both internally by the host star and externally by neighbouring stars. In this work we produce numerical models of protoplanetary discs including viscosity, magnetohydrodynamic disc winds, and internal and external photoevaporation. We find that even weak levels of external photoevaporation can significantly affect the evolution of protoplanetary discs, influencing the observable features such as disc radii, that might otherwise distinguish between viscous and wind driven discs. Including internal photoevaporation further suppresses differences in evolution between viscous and wind driven discs. This makes it much more difficult than previously anticipated, to use observations of nearby star forming regions to determine whether discs are viscous or wind driven. Interestingly we find that evolved protoplanetary discs in intermediate FUV environments may be the best cases for differentiating whether they evolve through viscosity or magnetohydrodynamic disc winds. Ultimately this work demonstrates the importance of understanding what are the key evolutionary processes and including as many of those as possible when exploring the evolution of protoplanetary discs.

The effect of Jupiter on the CAI storage problem

Context. Meteorites preserve an imprint of conditions in the early Solar System. By studying the distribution of calcium-aluminium-rich inclusions (CAIs) that are embedded within meteorites, we can learn about the dynamical history of the protoplanetary disk from which our Solar System formed. A long-standing problem concerning CAIs is the CAI storage problem. CAIs are thought to have formed at high temperatures near the Sun, but they are primarily found in carbonaceous chondrites, which formed much further out, beyond the orbit of Jupiter. Additionally, radial drift of CAI particles should have removed them from the solar protoplanetary disk several million years before the parent bodies of meteorites in which they are encountered would have accreted. Aims. We revisit a previously suggested solution to the CAI storage problem by Desch, Kalyaan, and Alexander which proposed that CAIs were mixed radially outward through the disk and subsequently got trapped in a pressure maximum created by Jupiter’s growing core opening a planet gap. Our aim is to investigate whether their solution still works when we take into account the infall phase during which the disk builds up from the collapse of a molecular cloud core. Methods. We build a 1D numerical code in Python using the DISKLAB package to simulate the evolution of the solar protoplanetary disk, starting with a collapsing molecular cloud. CAIs are created in the model by thermal processing of solar nebula composition dust, and subsequently transported through the disk by turbulent diffusion, radial drift and advection by the gas. Results. We find that outward transport of CAIs during the infall phase is very efficient, possibly mixing them all the way into the far outer disk. Subsequent inward radial drift collects CAIs in the pressure maximum beyond Jupiter’s orbit while draining the inner disk, roughly reproducing parts of the result by Desch et al. By introducing CAI formation so early, abundances out to 100 AU remain significant, possibly not consistent with some meteoritic data. It is possible to create a disk that does not expand as far out and also does not push CAIs as far out by using a very slowly rotating cloud.

Millimeter emission in photoevaporating disks is determined by early substructures

Context. Photoevaporation and dust-trapping are individually considered to be important mechanisms in the evolution and morphology of protoplanetary disks. However, it is not yet clear what kind of observational features are expected when both processes operate simultaneously. Aims. We studied how the presence (or absence) of early substructures, such as the gaps caused by planets, affects the evolution of the dust distribution and flux in the millimeter continuum of disks that are undergoing photoevaporative dispersal. We also tested if the predicted properties resemble those observed in the population of transition disks. Methods. We used the numerical code Dustpy to simulate disk evolution considering gas accretion, dust growth, dust-trapping at substructures, and mass loss due to X-ray and EUV (XEUV) photoevaporation and dust entrainment. Then, we compared how the dust mass and millimeter flux evolve for different disk models. Results. We find that, during photoevaporative dispersal, disks with primordial substructures retain more dust and are brighter in the millimeter continuum than disks without early substructures, regardless of the photoevaporative cavity size. Once the photoevaporative cavity opens, the estimated fluxes for the disk models that are initially structured are comparable to those found in the bright transition disk population (Fmm > 30 mJy), while the disk models that are initially smooth have fluxes comparable to the transition disks from the faint population (Fmm < 30 mJy), suggesting a link between each model and population. Conclusions. Our models indicate that the efficiency of the dust trapping determines the millimeter flux of the disk, while the gas loss due to photoevaporation controls the formation and expansion of a cavity, decoupling the mechanisms responsible for each feature. In consequence, even a planet with a mass comparable to Saturn could trap enough dust to reproduce the millimeter emission of a bright transition disk, while its cavity size is independently driven by photoevaporative dispersal.

Magnetic support, wind-driven accretion, coronal heating, and fast outflows in a thin magnetically arrested disc

ABSTRACT Accretion discs properties should deviate from standard theory when magnetic pressure exceeds the thermal pressure. To quantify these deviations, we present a systematic study of the dynamical properties of magnetically arrested discs (MADs), the most magnetized type of accretion disc. Using an artificial cooling function to regulate the gas temperature, we study MADs of three different thermal thicknesses, hth/r = 0.3, 0.1, and 0.03. We find that the radial structure of the disc is never mostly supported by the magnetic field. In fact, thin MADs are very near Keplerian. However, as discs gets colder, they become more magnetized and the largest deviations from standard theory appear in our thinnest disc with hth/r = 0.03. In this case, the disc is much more extended vertically and much less dense than in standard theory because of vertical support from the turbulent magnetic pressure and wind-driven angular momentum transport that enhances the inflow speed. The thin disc also dissipates a lot of thermal energy outside of z/r = ±0.03 and a significant fraction of this dissipation happens in mildly relativistic winds. The enhanced dissipation in low-density regions could possibly feed coronae in X-ray binaries (XRBs) and active galactic nuclei (AGNs). Wind-driven accretion will also impact the dynamical evolution of accretion discs and could provide a mechanism to explain the rapid evolution of changing-look AGN and the secular evolution of XRBs. Finally, our MAD winds have terminal velocities and mass-loss rates in good agreement with the properties of ultrafast outflows observed in AGN.

Photoevaporation and Close Encounters: How the Environment around Cygnus OB2 Affects the Evolution of Protoplanetary Disks

In our Galaxy, star formation occurs in a variety of environments, with a large fraction of stars formed in clusters hosting massive stars. OB stars have an important feedback on the evolution of protoplanetary disks orbiting around nearby young stars and likely on the process of planet formation occurring in them. The nearby massive association Cyg OB2 is an outstanding laboratory to study this feedback. It is the closest massive association to our Sun and hosts hundreds of massive stars and thousands of low-mass members, both with and without disks. In this paper, we analyze the spatial variation of the disk fraction (i.e., the fraction of cluster members bearing a disk) in Cyg OB2 and study its correlation with the local values of far-ultraviolet (FUV) and extreme-ultraviolet (EUV) radiation fields and the local stellar surface density. We present definitive evidence that disks are more rapidly dissipated in the regions of the association characterized by intense local UV fields and large stellar density. In particular, the FUV radiation dominates disk dissipation timescales in the proximity (i.e., within 0.5 pc) of the O stars. In the rest of the association, EUV photons potentially induce a significant mass loss from the irradiated disks across the entire association, but the efficiency of this process is reduced at increasing distances from the massive stars owing to absorption by the intervening intracluster material. We find that disk dissipation due to close stellar encounters is negligible in Cyg OB2 and likely to have affected 1% or fewer of the stellar population. Disk dissipation is instead dominated by photoevaporation. We also compare our results to what has been found in other young clusters with different massive populations, concluding that massive associations like Cyg OB2 are potentially hostile to protoplanetary disks but that the environments where disks can safely evolve in planetary systems are likely quite common in our Galaxy.

Twenty-five Years of Accretion onto the Classical T Tauri Star TW Hya

Accretion plays a central role in the physics that governs the evolution and dispersal of protoplanetary disks. The primary goal of this paper is to analyze the stability over time of the mass accretion rate onto TW Hya, the nearest accreting solar-mass young star. We measure veiling across the optical spectrum in 1169 archival high-resolution spectra of TW Hya, obtained from 1998–2022. The veiling is then converted to accretion rate using 26 flux-calibrated spectra that cover the Balmer jump. The accretion rate measured from the excess continuum has an average of 2.51 × 10−9 M ⊙ yr−1 and a Gaussian distribution with an FWHM of 0.22 dex. This accretion rate may be underestimated by a factor of up to 1.5 because of uncertainty in the bolometric correction and another factor of 1.7 because of excluding the fraction of accretion energy that escapes in lines, especially Lyα. The accretion luminosities are well correlated with He line luminosities but poorly correlated with Hα and Hβ luminosity. The accretion rate is always flickering over hours but on longer timescales has been stable over 25 years. This level of variability is consistent with previous measurements for most, but not all, accreting young stars.