Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userUnlocking CO Depletion in Protoplanetary Disks I. The Warm Molecular Layer
211
0
0.0
(
0
)
Authors
Kamber R. Schwarz
Ask ChatGPT about the research
No Arabic abstract
CO is commonly used as a tracer of the total gas mass in both the interstellar medium and in protoplanetary disks. Recently there has been much debate about the utility of CO as a mass tracer in disks. Observations of CO in protoplanetary disks reveal a range of CO abundances, with measurements of low CO to dust mass ratios in numerous systems. One possibility is that carbon is removed from CO via chemistry. However, the full range of physical conditions conducive to this chemical reprocessing is not well understood. We perform a systematic survey of the time dependent chemistry in protoplanetary disks for 198 models with a range of physical conditions. We varying dust grain size distribution, temperature, comic ray and X-ray ionization rate, disk mass, and initial water abundance, detailing what physical conditions are necessary to activate the various CO depletion mechanisms in the warm molecular layer. We focus our analysis on the warm molecular layer in two regions: the outer disk (100 au) well outside the CO snowline and the inner disk (19 au) just inside the midplane CO snow line. After 1 Myr, we find that the majority of models have a CO abundance relative to H$_2$ less than $10^{-4}$ in the outer disk, while an abundance less than $10^{-5}$ requires the presence of cosmic rays. Inside the CO snow line, significant depletion of CO only occurs in models with a high cosmic ray rate. If cosmic rays are not present in young disks it is difficult to chemically remove carbon from CO. Additionally, removing water prior to CO depletion impedes the chemical processing of CO. Chemical processing alone cannot explain current observations of low CO abundances. Other mechanisms must also be involved.
rate research
Read More
CO is thought to be the main reservoir of volatile carbon in protoplanetary disks, and thus the primary initial source of carbon in the atmospheres of forming giant planets. However, recent observations of protoplanetary disks point towards low volatile carbon abundances in many systems, including at radii interior to the CO snowline. One potential explanation is that gas phase carbon is chemically reprocessed into less volatile species, which are frozen on dust grain surfaces as ice. This mechanism has the potential to change the primordial C/H ratio in the gas. However, current observations primarily probe the upper layers of the disk. It is not clear if the low volatile carbon abundances extend to the midplane, where planets form. We have run a grid of 198 chemical models, exploring how the chemical reprocessing of CO depends on disk mass, dust grain size distribution, temperature, cosmic ray and X-ray ionization rate, and initial water abundance. Building on our previous work focusing on the warm molecular layer, here we analyze the results for our grid of models in the disk midplane at 12 au. We find that either an ISM level cosmic ray ionization rate or the presence of UV photons due to a low dust surface density are needed to chemically reduce the midplane CO gas abundance by at least an order of magnitude within 1 Myr. In the majority of our models CO does not undergo substantial reprocessing by in situ chemistry and there is little change in the gas phase C/H and C/O ratios over the lifetime of the typical disk. However, in the small sub-set of disks where the disk midplane is subject to a source of ionization or photolysis, the gas phase C/O ratio increases by up to nearly 9 orders of magnitude due to conversion of CO into volatile hydrocarbons.
Based on the interstellar CO/H2 ratio, carbon monoxide-based censuses of protoplanetary disks in Lupus, sigma Orionis, and Chamaeleon I found no disks more massive than the minimum-mass solar nebula, which is inconsistent with the existence of exoplanets more massive than Jupiter. Observations and models are converging on the idea that ionization-driven chemistry depletes carbon monoxide in T-Tauri disks. Yet the extent of CO depletion depends on the incident flux of ionizing radiation, and some T-Tauri stars may have winds strong enough to shield their disks from cosmic rays. There is also a range of X-ray luminosities possible for a given stellar mass. Here we use a suite of chemical models, each with a different incident X-ray or cosmic-ray flux, to assess whether CO depletion is a typical outcome for T-Tauri disks. We find that CO dissociation in the outer disk is a robust result for realistic ionization rates, with abundance reductions between 70% and 99.99% over 2 Myr of evolution. Furthermore, after the initial dissociation epoch, the inner disk shows some recovery of the CO abundance from CO2 dissociation. In highly ionized disks, CO recovery in the inner disk combined with depletion in the outer disk creates a centrally peaked CO abundance distribution. The emitting area in rare CO isotopologues may be an indirect ionization indicator: in a cluster of disks with similar ages, those with the most compact CO isotopologue emission see the highest ionization rates.
The $sigma$ Orionis cluster is important for studying protoplanetary disk evolution, as its intermediate age ($sim$3-5 Myr) is comparable to the median disk lifetime. We use ALMA to conduct a high-sensitivity survey of dust and gas in 92 protoplanetary disks around $sigma$ Orionis members with $M_{ast}gtrsim0.1 M_{odot}$. Our observations cover the 1.33 mm continuum and several CO $J=2-1$ lines: out of 92 sources, we detect 37 in the mm continuum and six in $^{12}$CO, three in $^{13}$CO, and none in C$^{18}$O. Using the continuum emission to estimate dust mass, we find only 11 disks with $M_{rm dust}gtrsim10 M_{oplus}$, indicating that after only a few Myr of evolution most disks lack sufficient dust to form giant planet cores. Stacking the individually undetected continuum sources limits their average dust mass to 5$times$ lower than that of the faintest detected disk, supporting theoretical models that indicate rapid dissipation once disk clearing begins. Comparing the protoplanetary disk population in $sigma$ Orionis to those of other star-forming regions supports the steady decline in average dust mass and the steepening of the $M_{rm dust}$-$M_{ast}$ relation with age; studying these evolutionary trends can inform the relative importance of different disk processes during key eras of planet formation. External photoevaporation from the central O9 star is influencing disk evolution throughout the region: dust masses clearly decline with decreasing separation from the photoionizing source, and the handful of CO detections exist at projected separations $>1.5$ pc. Collectively, our findings indicate that giant planet formation is inherently rare and/or well underway by a few Myr of age.
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.
We present new Atacama Large Millimeter/submillimeter Array (ALMA) observations for three protoplanetary disks in Taurus at 2.9,mm and comparisons with previous 1.3,mm data both at an angular resolution of $sim0.1$ (15,au for the distance of Taurus). In the single-ring disk DS Tau, double-ring disk GO Tau, and multiple-ring disk DL Tau, the same rings are detected at both wavelengths, with radial locations spanning from 50 to 120,au. To quantify the dust emission morphology, the observed visibilities are modeled with a parametric prescription for the radial intensity profile. The disk outer radii, taken as 95% of the total flux encircled in the model intensity profiles, are consistent at both wavelengths for the three disks. Dust evolution models show that dust trapping in local pressure maxima in the outer disk could explain the observed patterns. Dust rings are mostly unresolved. The marginally resolved ring in DS Tau shows a tentatively narrower ring at the longer wavelength, an observational feature expected from efficient dust trapping. The spectral index ($alpha_{rm mm}$) increases outward and exhibits local minima that correspond to the peaks of dust rings, indicative of the changes in grain properties across the disks. The low optical depths ($tausim$0.1--0.2 at 2.9,mm and 0.2--0.4 at 1.3,mm) in the dust rings suggest that grains in the rings may have grown to millimeter sizes. The ubiquitous dust rings in protoplanetary disks modify the overall dynamics and evolution of dust grains, likely paving the way towards the new generation of planet formation.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
