Accretion through circumstellar disks plays an important role in star formation and in establishing the properties of the regions in which planets form and migrate. The mechanisms by which protostellar and protoplanetary disks accrete onto low-mass stars are not clear; angular momentum transport by magnetic fields is thought to be involved, but the low-ionization conditions in major regions of protoplanetary disks lead to a variety of complex nonideal magnetohydrodynamic effects whose implications are not fully understood. Accretion in pre-main-sequence stars of masses ≲1M⊙ (and in at least some 2–3-M⊙ systems) is generally funneled by the stellar magnetic field, which disrupts the disk at scales typically of order a few stellar radii. Matter moving at near free-fall velocities shocks at the stellar surface; the resulting accretion luminosities from the dissipation of kinetic energy indicate that mass addition during the T Tauri phase over the typical disk lifetime ∼3 Myr is modest in terms of stellar evolution, but is comparable to total disk reservoirs as estimated from millimeter-wave dust emission (∼10−2 M⊙). Pre-main-sequence accretion is not steady, encompassing timescales ranging from approximately hours to a century, with longer-timescale variations tending to be the largest. Accretion during the protostellar phase—while the protostellar envelope is still falling onto the disk—is much less well understood, mostly because the properties of the central obscured protostar are difficult to estimate. Kinematic measurements of protostellar masses with new interfometric facilities should improve estimates of accretion rates during the earliest phases of star formation.

Context. With the Herschel Space Observatory, lines of simple molecules (C + , O, and high-J lines of CO, J up 14) have been observed in the atmosphere of protoplanetary disks. When combined with ground-based data on [C i], all principle forms of carbon can be studied. These data allow us to test model predictions for the main carbon-bearing species and verify the presence of a warm surface layer. The absence of neutral carbon [C i], which is predicted by models to be strong, can then be interpreted together with ionized carbon [C ii] and carbon monoxide. Aims. We study the gas temperature, excitation, and chemical abundance of the simple carbon-bearing species C, C + , and CO, as well as O by the method of chemical-physical modeling. Using the models, we explore the sensitivity of the lines to the entering parameters and constrain the region from which the line radiation emerges. Methods. Numerical models of the radiative transfer in the lines and dust are used together with a chemical network simulation and a calculation of the gas energetics to obtain the gas temperature. We present our new model, which is based on our previous models but includes several improvements that we report in detail, together with the results of benchmark tests. Results. A model of the disk around the Herbig Be star HD 100546 is able to reproduce the CO ladder together with the atomic finestructure lines of [O i] and either [C i] or [C ii]. We find that the high-J lines of CO can only be reproduced by a warm atmosphere with T gas T dust . The low-J lines of CO, observable from the ground, are dominated by the outer disk with a radius of several 100 AU, while the high-J CO observable with Herschel-PACS are dominated from regions within some tens of AU. The spectral profiles of high-J lines of CO are predicted to be broader than those of the low-J lines. We study the effect of several parameters including the size of the disk, the gas mass of the disk, the PAH abundance and distribution, and the amount of carbon in the gas phase. Conclusions. The main conclusions of our work are (i) only a warm atmosphere with T gas T dust can reproduce the CO ladder. (ii) The CO ladder together with [O i] and the upper limit to [C i] can be reproduced by models with a high gas/dust ratio and a low abundance of volatile carbon. These models however produce too small amounts of [C ii]. Models with a low gas/dust ratio and more volatile carbon also reproduce CO and [O i], are in closer agreement with observations of [C ii], but overproduce [C i]. Owing to the uncertain origin of the [C ii] emission, we prefer the high gas/dust ratio models, indicating a low abundance of volatile carbon.

Abstract:The statistics of discovered exoplanets suggest that planets form efficiently. However, there are fundamental unsolved problems, such as excessive inward drift of particles in protoplanetary disks during planet formation. Recent theories invoke dust traps to overcome this problem. We report the detection of a dust trap in the disk around the star Oph IRS 48 using observations from the Atacama Large Millimeter/submillimeter Array (ALMA). The 0.44-millimeter-wavelength continuum map shows high-contrast crescent-shaped emission on one side of the star originating from millimeter-sized grains, whereas both the mid-infrared image (micrometer-sized dust) and the gas traced by the carbon monoxide 6-5 rotational line suggest rings centered on the star. The difference in distribution of big grains versus small grains/gas can be modeled with a vortex-shaped dust trap triggered by a companion.Main Text: While the ubiquity of planets is confirmed almost daily by detections of new exoplanets (1), the exact formation mechanism of planetary systems in disks of gas and dust around young stars remains a long-standing problem in astrophysics (2). In the standard coreaccretion picture, dust grains must grow from submicron sizes to ~10 M Earth rocky cores within the ~10 Myr lifetime of the circumstellar disk. However, this growth process is stymied by what is usually called the "radial drift and fragmentation barrier": Particles of intermediate size (~1 m at 1 AU, or ~1 mm at 50 AU from the star) acquire high drift velocities toward the star with respect to the gas (3,4). This leads to two major problems for further growth (5): First, highvelocity collisions between particles with different drift velocities cause fragmentation. Second, even if particles avoid this fragmentation, they will rapidly drift inward and thus be lost into the star before they have time to grow to planetesimal size. This radial drift barrier is one of the most persistent issues in planet formation theories. A possible solution is dust trapping in so-called pressure bumps: local pressure maxima where the dust piles up. One example of such a pressure bump is an anticyclonic vortex which can trap dust particles in the azimuthal direction (6-10).Using the Atacama Large Millimeter/submillimeter Array (ALMA), we report a highly asymmetric concentration of millimeter-sized dust grains on one side of the disk of the star Oph IRS 48 in the 0.44 millimeter (685 GHz) continuum emission (Fig. 1). We argue that this can be understood in the framework of dust trapping in a large anticyclonic vortex in the disk.The young A-type star Oph IRS 48 (distance ~ 120 pc, 1 pc =3.1·1013 km) has a well studied disk with a large inner cavity (deficit of dust in the inner disk region), a so-called transition disk. Mid-infrared imaging at 18.7 μm reveals a disk ring in the small dust grain (size ~50 μm) emission at an inclination of ~50°, peaking at 55 AU radius (1 AU = 1.5·10 8 km = distance from Earth to the Sun) or 0.46 arcseconds from the star (11). Spatially resolved obs...