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Register a new userThe origin of s-process isotope heterogeneity in the solar protoplanetary disk
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Rocky asteroids and planets display nucleosynthetic isotope variations that are attributed to the heterogeneous distribution of stardust from different stellar sources in the solar protoplanetary disk. Here we report new high precision palladium isotope data for six iron meteorite groups, which display smaller nucleosynthetic isotope variations than the more refractory neighbouring elements. Based on this observation we present a new model in which thermal destruction of interstellar medium dust results in an enrichment of s-process dominated stardust in regions closer to the Sun. We propose that stardust is depleted in volatile elements due to incomplete condensation of these elements into dust around asymptotic giant branch (AGB) stars. This led to the smaller nucleosynthetic variations for Pd reported here and the lack of such variations for more volatile elements. The smaller magnitude variations measured in heavier refractory elements suggest that material from high-metallicity AGB stars dominated stardust in the Solar System. These stars produce less heavy s-process elements compared to the bulk Solar System composition.
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Meteorites contain relict decay products of short-lived radionuclides that were present in the protoplanetary disk when asteroids and planets formed. Several studies reported a high abundance of 60Fe (t1/2=2.62+/-0.04 Myr) in chondrites (60Fe/56Fe~6*10-7), suggesting that planetary materials incorporated fresh products of stellar nucleosynthesis ejected by one or several massive stars that exploded in the vicinity of the newborn Sun. We measured 58Fe/54Fe and 60Ni/58Ni isotope ratios in whole rocks and constituents of differentiated achondrites (ureilites, aubrites, HEDs, and angrites), unequilibrated ordinary chondrites Semarkona (LL3.0) and NWA 5717 (ungrouped petrologic type 3.05), metal-rich carbonaceous chondrite Gujba (CBa), and several other meteorites (CV, EL H, LL chondrites; IIIAB, IVA, IVB iron meteorites). We derive from these measurements a much lower initial 60Fe/56Fe ratio of (11.5+/-2.6)*10-9 and conclude that 60Fe was homogeneously distributed among planetary bodies. This low ratio is consistent with derivation of 60Fe from galactic background (60Fe/56Fe=2.8*10-7 in the interstellar medium from gamma-ray observations) and can be reconciled with high 26Al/27Al=5*10-5 in chondrites if solar material was contaminated through winds by outer layers of one or several massive stars (e.g., a Wolf-Rayet star) rich in 26Al and poor in 60Fe. We present the first chronological application of the 60Fe-60Ni decay system to establish the time of core formation on Vesta at 3.7 (+2.5/-1.7) Myr after condensation of calcium-aluminum-rich inclusions (CAIs).
We study the origin of tail-like structures recently detected around the disk of SU Aurigae and several FU~Orionis-type stars. Dynamic protostellar disks featuring ejections of gaseous clumps and quiescent protoplanetary disks experiencing a close encounter with an intruder star were modeled using the numerical hydrodynamics code FEOSAD. Both the gas and dust dynamics were taken into account, including dust growth and mutual friction between the gas and dust components. Only plane-of-the-disk encounters were considered. Ejected clumps produce a unique type of tail that is characterized by a bow-shock shape. Such tails originate from the supersonic motion of ejected clumps through the dense envelope that often surrounds young gravitationally unstable protostellar disks. The ejected clumps either sit at the head of the tail-like structure or disperse if their mass is insufficient to withstand the head wind of the envelope. On the other hand, close encounters with quiescent protoplanetary disks produce three types of the tail-like structure; we define these as pre-collisional, post-collisional, and spiral tails. These tails can in principle be distinguished from one another by particular features of the gas and dust flow in and around them. We find that the brown-dwarf-mass intruders do not capture circumintruder disks during the encounter, while the subsolar-mass intruders can acquire appreciable circumintruder disks with elevated dust-to-gas ratios, which can ease their observational detection. However, this is true only for prograde collisions; the retrograde intruders fail to collect appreciable amounts of gas or dust from the disk of the target. The predicted mass of dust in the model tail-like structures is higher than what was inferred for similar structures in SU~Aur, FU~Ori, and Z~CMa, making their observational detection feasible. Abridged.
Measurements of the gas mass are necessary to determine the planet formation potential of protoplanetary disks. Observations of rare CO isotopologues are typically used to determine disk gas masses; however, if the line emission is optically thick this will result in an underestimated disk mass. With ALMA we have detected the rarest stable CO isotopologue, 13C17O, in a protoplanetary disk for the first time. We compare our observations with the existing detections of 12CO, 13CO, C18O and C17O in the HD163296 disk. Radiative transfer modelling using a previously benchmarked model, and assuming interstellar isotopic abundances, significantly underestimates the integrated intensity of the 13C17O J=3-2 line. Reconciliation between the observations and the model requires a global increase in CO gas mass by a factor of 3.5. This is a factor of 2-6 larger than previous gas mass estimates using C18O. We find that C18O emission is optically thick within the CO snow line, while the 13C17O emission is optically thin and is thus a robust tracer of the bulk disk CO gas mass.
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of a protoplanetary disk around the T Tauri star Sz~84 and analyses of the structures of the inner cavity in the central region of the dust disk. Sz~84s spectral energy distribution (SED) has been known to exhibit negligible infrared excess at $lambda lesssim$10~$mu$m due to the disks cavity structure. Analyses of the observed visibilities of dust continuum at 1.3~mm and the SED indicate that the size of the cavity in the disk of large (millimeter size) dust grains is 8~au in radius and that in the disk of small (sub-micron size) dust grains is 60~au in radius. Furthermore, from the SED analyses, we estimate that the upper limit mass of small dust grains at $r<$60~au is less than $sim$10$^{-3}$~$M_{rm earth}$, which is $lesssim$0.01~% of the total (small~$+$~large) dust mass at $r<$60~au. These results suggest that large dust grains are dominant at $r<$60~au, implying that dust grains efficiently grow with less efficient fragmentation in this region, potentially due to weak turbulence and/or stickier dust grains. The balance of grain growth and dust fragmentation is an important factor for determining the size of large dust grains in protoplanetary disks, and thus Sz~84 could serve as a good testbed for investigations of grain growth in such disks.
While it is generally accepted that the magnetic field and its non-ideal effects play important roles during the stellar formation, simple models of pure hydrodynamics and angular momentum conservation are still widely employed in the studies of disk assemblage in the framework of the so-called alpha-disk model due to their simplicity. There has only been a few efforts trying to bridge the gap between a collapsing prestellar core and a developed disk. The goal of the present work is to revisit the assemblage of the protoplanetary disk (PPD), by performing 3D MHD simulations with ambipolar diffusion and full radiative transfer. We follow the global evolution of the PPD from the prestellar core collapse for 100 kyr, with resolution of one AU. The formed disk is more realistic and is in agreement with recent observations of disks around class-0 young stellar objects. The mass flux arriving onto the disk and the radial mass accretion rate within the disk are measured and compared to analytical self-similar models. The surface mass flux is very centrally peaked, implying that most of the mass falling onto the star does not transit through the mid-plane of the disk. The disk mid-plane is almost dead to turbulence, whereas upper layers and the disk outer edge are very turbulent. The snow-line is significantly further away than in a passive disk. We developed a zoomed rerun technique to quickly obtain a reasonable disk that is highly stratified, weakly magnetized inside, and strongly magnetized outside. During the class-0 phase of PPD formation, the interaction between the disk and the infalling envelope is important and ought not be neglected. Accretion onto the star is found to mostly depend on dynamics of the collapsing envelope, rather than the detailed disk structure.
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