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We investigate the impact of nightside cloud formation on the observable day/night contrast of tidally locked terrestrial planet atmospheres. We demonstrate that, in the case where the planetary dayside is only 10 s of Kelvin hotter than the planetary nightside, the presence of optically thick nightside clouds can lead to observations that mimic a planet without an atmosphere, despite the planet actually hosting a significant (10 bar) atmosphere. The scenario presented in this work requires a level of intrinsic atmospheric day/night temperature contrast such that the nightside can form clouds while the dayside is too hot for cloud formation to occur. This scenario is most likely for hotter terrestrials and terrestrials with low volatile inventories. We thus note that a substantial dayside/nightside temperature difference alone does not robustly indicate that a planet does not host an atmosphere, and additional observations and modeling are essential for characterization. We further discuss several avenues for future study to improve our understanding of the terrestrial planets and how best to characterize them with JWST.

FU Ori– and EX Lup–type objects present natural experiments for understanding a critical stage in the star and planet formation process. These objects offer insight into the diversity of molecules available to forming planetary systems due to a sudden increase in accretion, and central luminosity causes the disk and surrounding material to increase in temperature. This allows for volatiles to sublimate off of grains and exist in the gas phase for tens to hundreds of years after initial outburst. While this dynamic stage may be common for solar-type protostars, observations of the chemical impact of these bursts are rare. In this article, we present observations from the NOrthern Extended Millimeter Array of five young stellar objects (YSOs) that have undergone outbursts within the past 100 yr and catalog the volatile chemistry found within ∼1000 au of the YSO. Only one source clearly shows a line-rich spectra with >11 molecules detected, including complex organics and water, as is expected for a spectra signature for a postoutburst source. This source is V1057 Cyg, and we present it as the northern analog to the well-studied and molecule-rich FU Ori source, V883 Ori. Our conclusions on the chemical inventory of the other four sources in our sample are sensitivity limited, as V1057 Cyg contains the highest disk/envelope gas mass.

Stable isotopic ratios constitute powerful tools for unraveling the thermal and irradiation history of volatiles. In particular, we can use our knowledge of the isotopic fractionation processes active during the various stages of star, disk, and planet formation to infer the origins of different volatiles with measured isotopic patterns in our own solar system. Observations of planet-forming disks with the Atacama Large Millimeter/submillimeter Array (ALMA) now readily detect the heavier isotopologues of C, O, and N, while the isotopologue abundances and isotopic fractionation mechanisms of sulfur species are less well understood. Using ALMA observations of the SO and SO _2 isotopologues in the nearby, molecule-rich disk around the young star Oph-IRS 48 we present the first constraints on the combined ^32 S/ ^34 S and ^32 S/ ^33 S isotope ratios in a planet-forming disk. Given that these isotopologues likely originate in relatively warm gas (>50 K), like most other Oph-IRS 48 volatiles, SO is depleted in heavy sulfur, while SO _2 is enriched compared to solar system values. However, we cannot completely rule out a cooler gas reservoir, which would put the SO sulfur ratios more in line with comets and other solar system bodies. We also constrain the S ^18 O/SO ratio and find the limit to be consistent with solar system values given a temperature of 60 K. Together these observations show that we should not assume solar isotopic values for disk sulfur reservoirs, but additional observations are needed to determine the chemical origin of the abundant SO in this disk, inform on what isotopic fractionation mechanism(s) are at play, and aid in unraveling the history of the sulfur budget during the different stages of planet formation.

Observations of disks with the Atacama Large Millimeter/submillimeter Array (ALMA) allow us to map the chemical makeup of nearby protoplanetary disks with unprecedented spatial resolution and sensitivity. The typical outer Class II disk observed with ALMA is one with an elevated C/O ratio and a lack of oxygen-bearing complex organic molecules, but there are now some interesting exceptions: three transition disks around Herbig Ae stars all show oxygen-rich gas traced via the unique detections of the molecules SO and CH _3 OH. We present the first results of an ALMA line survey at ≈337–357 GHz of such disks and focus this paper on the first Herbig Ae disk to exhibit this chemical signature—HD 100546. In these data, we detect 19 different molecules including NO, SO _2 , and CH _3 OCHO (methyl formate). We also make the first tentative detections of ${{\rm{H}}}_{2}^{13}\mathrm{CO}$ and ^34 SO in protoplanetary disks. Multiple molecular species are detected in rings, which are, surprisingly, all peaking just beyond the underlying millimeter continuum ring at ≈200 au. This result demonstrates a clear connection between the large dust distribution and the chemistry in this flat disk. We discuss the physical and/or chemical origin of these substructures in relation to ongoing planet formation in the HD 100546 disk. We also investigate how similar and/or different this molecular makeup of this disk is to other chemically well-characterized Herbig Ae disks. The line-rich data we present motivate the need for more ALMA line surveys to probe the observable chemistry in Herbig Ae systems, which offer unique insight into the composition of disks ices, including complex organic molecules.

The Atacama Large Millimeter/submillimeter Array (ALMA) can probe the molecular content of planet-forming disks with unprecedented sensitivity. These observations allow us to build up an inventory of the volatiles available for forming planets and comets. Herbig Ae transition disks are fruitful targets due to the thermal sublimation of complex organic molecules (COMs) and likely H _2 O-rich ices in these disks. The IRS 48 disk shows a particularly rich chemistry that can be directly linked to its asymmetric dust trap. Here, we present ALMA observations of the IRS 48 disk where we detect 16 different molecules and make the first robust detections of ${{\rm{H}}}_{2}^{13}\,\mathrm{CO}$ , ^34 SO, ^33 SO, and c-H _2 COCH _2 (ethylene oxide) in a protoplanetary disk. All of the molecular emissions, aside from CO, are co-located with the dust trap, and this includes newly detected simple molecules such as HCO ^+ , $\mathrm{HCN}$ , and CS. Interestingly, there are spatial offsets between different molecular families, including between the COMs and sulfur-bearing species, with the latter being more azimuthally extended and radially located further from the star. The abundances of the newly detected COMs relative to CH _3 OH are higher than the expected protostellar ratios, which implies some degree of chemical processing of the inherited ices during the disk lifetime. These data highlight IRS 48 as a unique astrochemical laboratory to unravel the full volatile reservoir at the epoch of planet and comet formation and the role of the disk in (re)setting chemical complexity.

Protoplanetary disks (PPDs) are characterized by different kinds of gas dynamics and chemistry, which are coupled via ionization, heating, and cooling processes, as well as advective and turbulent transport. However, directly coupling gas dynamics with time-dependent chemistry is prohibitively computationally expensive when using comprehensive chemical reaction networks. In this paper, we evaluate the utility of a species-based network reduction method in different disk environments to produce small chemical networks that reproduce the abundances of major species found in large gas-phase chemistry networks. We find that the method works very well in disk midplane and surface regions, where approximately 20–30 gas-phase species, connected by ∼50–60 gas-phase reactions, are sufficient to reproduce the targeted ionization fraction and chemical abundances. Most species of the reduced networks, including major carriers of oxygen, carbon and nitrogen, also have similar abundances in the reduced and complete network models. Our results may serve as an initial effort for future hydrodynamic/magnetohydrodynamic simulations of PPDs incorporating time-dependent chemistry in appropriate regions. Accurately modeling the abundances of major species at intermediate disk heights, however, will require a much more extended network incorporating gas-grain chemistry and is left for future studies. [ABSTRACT FROM AUTHOR]

Volatiles, especially CO, are important gas tracers of protoplanetary disks (PPDs). Freeze-out and sublimation processes determine their division between gas and solid phases, which affects both which disk regions can be traced by which volatiles, and the formation and composition of planets. Recently, multiple lines of evidence have suggested that CO is substantially depleted from the gas in the outer regions of PPDs, i.e., more depleted than would be expected from a simple balance between freeze-out and sublimation. In this paper, we show that the gas dynamics in the outer PPDs facilitates volatile depletion through turbulent diffusion. Using a simple 1D model that incorporates dust settling, turbulent diffusion of dust and volatiles, as well as volatile freeze-out/sublimation processes, we find that as long as turbulence in the cold midplane is sufficiently weak to allow a majority of the small grains to settle, CO in the warm surface layer can diffuse into the midplane region and deplete by freeze-out. The level of depletion sensitively depends on the level of disk turbulence. Based on recent disk simulations that suggest a layered turbulence profile with very weak midplane turbulence and strong turbulence at the disk surface, CO and other volatiles can be efficiently depleted by up to an order of magnitude over Myr timescales. [ABSTRACT FROM AUTHOR]