Chapter IV: Constraining Gas Masses of CO-Depleted Disks with Mul-
5.4 Discussion
Midplane
Composition CH3OH CO CH3 HCN
CH4
H2CO
COH2O H2CO CH3OH
N2
HCN Surface (z/h=5)
Composition Models:
CO CO2
% C
CO H2O CO2
O O2
NO CH3OH Start
% O
N2
N
% NNO
CO2
CO CO
CO2
H2O O2
N2
CO CO2
% C
CO H2O CO2
O O2
NO CO2 Start
% O
N2
N
% NNO
C(gr) H2O N N2
CO CO2
% C
CO H2O CO2
O O2
NO C(gr) Start
% O
N2
N
% NNO
Figure 5.4: Major carbon, oxygen, and nitrogen species at five scale heights above the midplane in the disk surface layers (top row) and at the disk mid- plane (bottom row) at a radius of 1 au in our models. Results are shown at 106 years for three different models, from left to right: all of initial carbon starting in CH3OH ice, CO2 ice, and amorphous carbon grains. Bar colors indicate the distribution of each element by number of atoms across chemical species. All chemical species comprising more than 1% of the total budget for each element are labeled. Bars filled in solid colors represent species in the gas phase whereas hatch patterns represent ices.
Because of the oxygen content of CH3OH and CO2, the presence of H2O-poor gas beyond the H2O snowline has little effect on the main carbon products in the surface layers. For the amorphous carbon grain case however, the surface products change substantially (Fig. 5.5). The lack of H2O in the gas limits the amount of oxygen available to produce CO. Instead, carbon is distributed over various nitrogen-bearing and hydrocarbon species, many containing large carbon chains.
Figure 5.5: Comparions of major carbon, oxygen, and nitrogen species for dis- tinct gas-phase C/O ratios. Plotted are two scenarios, first the initial model where carbon starts in the form of amorphous carbon grains and the inner disk gas composition is dominated by radial drift of solids and the subsequent va- porization of their volatile-rich icy surfaces (A) and then an additional carbon grain model that assumes that the inner disk gas composition is dominated by accretion of H2O-poor gas from the cold outer disk where the bulk of the wa- ter is frozen on grain surfaces (B). Results are shown at 106 years for the disk surface at five scale heights above the midplane at a radius of 1 au. Bar colors indicate the distribution of each element by number of atoms across chemical species. All chemical species comprising more than 1% of the total budget for each element are labeled. The label “Other” represents C6H2, C10H2, HC9N ice, NC6N, C8H2, C7H2, HC3N, C4H2, HC5N, and HNC at levels of 1–4%.
Bars filled in solid colors represent species in the gas phase whereas hatch patterns represent ices.
the the outer disk gas of TW Hya (Walsh et al. 2016). Further exploration of carbon species including CO, CO2, CH4, and CH3OH in protoplanetary disks will be completed by JWST.
Our results suggest that abundant reservoirs that trace the original carbon carriers from the outer disk are only preserved up to 2–3 scale heights above the midplane. Further, chemical abundances in the higher surface layers ap- pear very similar regardless of the initial carbon carrier because over short timescales the final compositions converge to the same dominant species. In these high layers, which are above the dust optical surface at mid-infrared wavelengths where key lines of these species will be observed, the original carriers are rapidly destroyed by photochemistry. Given the optical depth
limitations, we expect the observed compositions in the infrared to be largely uniform across many protoplanetary disks. If this is not the case, it may mean that photochemistry is much less efficient than modeled here or that dynam- ical processes are constantly resupplying material from the outer disk and/or midplane. Current observational constraints do not rule out disk compositions that are relatively uniform across different sources. Local thermal equilibrium (LTE) models fit to Spitzer and Keck-NIRSPEC survey data find that the best-fit column density ratios of CO2, HCN, and C2H2 relative to H2O range from 10−4–10−2within 1σand the reported scatter in CO/H2O is only a factor of ∼5–10 (Salyk et al. 2011).
Photochemistry may vary for different stellar environments. In fact, Herbig disks, which have stronger UV fields relative to T-Tauri sources, have been observed in CO but these same disks lack detections of HCN, C2H2, and CO2 emission seen in many T-Tauri disks (Pontoppidan et al. 2010; Salyk et al. 2011). Sources with lower UV fluxes may reprocess carbon carriers more slowly, preserving the more diverse chemical compositions such as those seen closer to the midplane in our models. However, lower UV fluxes may also result in colder disks, where increased settling causes increased penetration of UV photons into the disk, moving the photoactive layer closer to the midplane.
Fully understanding this process would require iterative modeling of the verti- cal disk structure and radiation environment, particularly for the dense inner disk regions.
The rate of vertical motion of material from the midplane to the observable surface layers depends on the means of angular momentum transport in the disk. Given the rapid photochemistry converting the initial carbon carriers into CO at the disk surface at timescales <1 yr, resupply from the midplane would need to be extremely efficient in order to maintain a diversity of carbon species in the photon-dominated surface layers. The diffusion timescale for a single scale height at 1 au is about 15 yrs based on the formula for the diffusion coefficient of gas from Dullemond and Dominik (2004) and turbulence characterized by a standard α value of 0.01. Uncertainty in the diffusion coefficient by orders of magnitude could largely vary this timescale estimate.
If vertical mixing of materials occurs more slowly than the photochemistry, it would result in the surface composition spreading throughout the entire column.
Although infrared observations are limited to the disk surface due to the opti- cally thick dust emission at these wavelengths, longer wavelength observations may probe deeper. Observatories such as the future Square Kilometer Ar- ray (SKA) with low frequencies of 50–350 MHz and mid-range frequencies of 350 MHz to 15.3 GHz, corresponding to cm to m wavelengths, may detect signatures from the midplane for low-frequency emitters including CH3OH.
This would provide crucial information about midplane CH3OH abundances that could help distinguish the carbon carrier scenarios.