Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userUnlocking CO Depletion in Protoplanetary Disks II. Primordial C/H Predictions Inside the CO Snowline
175
0
0.0
(
0
)
Authors
Kamber R. Schwarz
Ask ChatGPT about the research
No Arabic abstract
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.
rate research
Read More
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.
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.
Snowlines are key ingredients for planet formation. Providing observational constraints on the locations of the major snowlines is therefore crucial for fully connecting planet compositions to their formation mechanism. Unfortunately, the most important snowline, that of water, is very difficult to observe directly in protoplanetary disks due to its close proximity to the central star. Based on chemical considerations, HCO$^+$ is predicted to be a good chemical tracer of the water snowline, because it is particularly abundant in dense clouds when water is frozen out. This work maps the optically thin isotopologue H$^{13}$CO$^+$ ($J=3-2$) toward the envelope of the low-mass protostar NGC1333-IRAS2A (observed with NOEMA at ~0.9 resolution), where the snowline is at larger distance from the star than in disks. The H$^{13}$CO$^+$ emission peaks ~2 northeast of the continuum peak, whereas the previously observed H$_2^{18}$O shows compact emission on source. Quantitative modeling shows that a decrease in H$^{13}$CO$^+$ abundance by at least a factor of six is needed in the inner ~360 AU to reproduce the observed emission profile. Chemical modeling predicts indeed a steep increase in HCO$^+$ just outside the water snowline; the 50% decrease in gaseous H$_2$O at the snowline is not enough to allow HCO$^+$ to be abundant. This places the water snowline at 225 AU, further away from the star than expected based on the 1D envelope temperature structure for NGC1333-IRAS2A. In contrast, DCO$^+$ observations show that the CO snowline is at the expected location, making an outburst scenario unlikely. The spatial anticorrelation of the H$^{13}$CO$^+$ and H$_2^{18}$O emission provide a proof of concept that H$^{13}$CO$^+$ can be used as a tracer of the water snowline.
[Abridged] Planet formation is expected to be enhanced around snowlines in protoplanetary disks, in particular around the water snowline. However, the close proximity of the water snowline to the host star and water in the Earths atmosphere makes a direct detection of the water snowline in protoplanetary disks challenging. Following earlier work on protostellar envelopes, the aim of this research is to investigate the validity of HCO$^+$ and H$^{13}$CO$^+$, as tracers of the water snowline in protoplanetary disks, as HCO$^+$ is destroyed by gas-phase water. Two small chemical networks are used to predict the HCO$^+$ abundance in a typical Herbig Ae disk. Subsequently, the corresponding emission profiles are modelled for H$^{13}$CO$^+$ and HCO$^+$ $J=2-1$, which provides the best balance between brightness and optical depth effects of the continuum emission. The HCO$^+$ abundance jumps by two orders of magnitude just outside the water snowline at 4.5 AU. We find that the emission of H$^{13}$CO$^+$ and HCO$^+$ is ring-shaped due to three effects: destruction of HCO$^+$ by gas-phase water, continuum optical depth, and molecular excitation effects. The presence of gas-phase water causes an additional drop of only $sim$13% and 24% in the center of the disk, for H$^{13}$CO$^+$ and HCO$^+$, respectively. For the much more luminous outbursting source V883Ori, our models predict that the effect of dust and excitation are not limiting if the snowline is located outside $sim$40 AU. Our analysis of ALMA observations of HCO$^+$ $J=3-2$ is consistent with the water snowline located around 100 AU. The HCO$^+$ abundance drops steeply around the water snowline, but dust and excitation can conceal the drop in HCO$^+$ emission due to the water snowline. Therefore, locating the water snowline with HCO$^+$ in Herbig disks is very difficult, but it is possible for outbursting sources like V883Ori.
We present spatially resolved ALMA images of CO J=3-2 emission from the protoplanetary disk around HD100546. We model the spatially-resolved kinematic structure of the CO emission. Assuming a velocity profile which prescribes a flat or flared emitting surface in Keplerian rotation, we uncover significant residuals with a peak of $approx7delta v$, where $delta v = 0.21$ km s$^{-1}$ is the width of a spectral resolution element. The residuals reveal the possible presence of a severely warped and twisted inner disk extending to at most 100au. Adapting the model to include a misaligned inner gas disk with (i) an inclination almost edge-on to the line of sight, and (ii) a position angle almost orthogonal to that of the outer disk reduces the residuals to $< 3delta v$. However, these findings are contrasted by recent VLT/SPHERE, MagAO/GPI, and VLTI/PIONIER observations of HD100546 that show no evidence of a severely misaligned inner dust disk down to spatial scales of $sim 1$au. An alternative explanation for the observed kinematics are fast radial flows mediated by (proto)planets. Inclusion of a radial velocity component at close to free-fall speeds and inwards of $approx 50$au results in residuals of $approx 4 delta v$. Hence, the model including a radial velocity component only does not reproduce the data as well as that including a twisted and misaligned inner gas disk. Molecular emission data at a higher spatial resolution (of order 10au) are required to further constrain the kinematics within $lesssim 100$au. HD100546 joins several other protoplanetary disks for which high spectral resolution molecular emission shows that the gas velocity structure cannot be described by a purely Keplerian velocity profile with a universal inclination and position angle. Regardless of the process, the most likely cause is the presence of an unseen planetary companion. (Abridged)
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
