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The initial mass distribution in the solar nebula is a critical input to planet formation models that seek to reproduce today’s Solar System 1 . Traditionally, constraints on the gas mass distribution are derived from observations of the dust emission from disks 2 , 3 , but this approach suffers from large uncertainties in dust opacity and gas-to-dust ratio 2 . On the other hand, previous observations of gas tracers only probe surface layers above the bulk mass reservoir 4 . Here we present the first partially spatially resolved observations of the 13 C 18 O J  = 3–2 line emission in the closest protoplanetary disk, TW Hydrae, a gas tracer that probes the bulk mass distribution. Combining it with the C 18 O J  = 3–2 emission and the previously detected HD J  = 1–0 flux, we directly constrain the mid-plane temperature and optical depths of gas and dust emission. We report a gas mass distribution with radius, R , of 13 − 5 + 8 × ( R / 20 .5 au ) − 0.9 − 0.3 + 0.4  g cm −2 in the expected formation zone of gas and ice giants (5–21 au). We find that the mass ratio of total gas to millimetre-sized dust is 140 in this region, suggesting that at least 2.4 M ⊕ of dust aggregates have grown to centimetre sizes (and perhaps much larger). The radial distribution of gas mass is consistent with a self-similar viscous disk profile but much flatter than the posterior extrapolation of mass distribution in our own and extrasolar planetary systems. ALMA observations of TW Hydrae in the 13C18O J = 3–2 molecular line probe the mid-plane of the circumstellar disk where giant planets are expected to form. With other lines, the gas mass distribution, temperature and the gas-to-dust ratio are determined.

The Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) is a NASA Astrophysics MIDEX-class mission concept, with the stated goal of Following water from galaxies, through protostellar systems, to Earth’s oceans. This paper details the protoplanetary disk science achievable with OASIS. OASIS’s suite of heterodyne receivers allow for simultaneous, high spectral resolution observations of water emission lines The Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) is a NASA Astrophysics MIDEX-class mission concept, with the stated goal of Following water from galaxies, through protostellar systems, to Earth’s oceans. This paper details the protoplanetary disk science achievable with OASIS. OASIS’s suite of heterodyne receivers allow for simultaneous, high spectral resolution observations of water emission lines HD in 100+ disks, allowing for the most accurate determination of total protoplanetary disk gas mass to date. When combined with the contemporaneous water observations, the HD detection will also allow us to trace the evolution of water vapor across evolutionary stages. These observations will enable OASIS to characterize the time development of the water distribution and the role water plays in the process of planetary system formation.

Core-accretion theory predicts that the formation of giant planets predominantly occurs at the dense mid-plane of the inner ∼50 AU of protoplanetary disks. However, due to observational limitation, this critical region remains to be the least charted area in protoplanetary disks. With its great sensitivity, ALMA recently started to image optically thin line emissions arisen from the mid-plane of the inner 50AU in nearby disks, which unlocks an exciting new path to directly constrain the physical properties of the giant planet formation zone through gas tracers. Here we present the first spatially resolved observations of the 13C18O J=3-2 line emission in the TW Hya disk. We show that this emission is optically thin even inside the CO mid-plane snowline. Combining it with the C18O J=3-2 images and the previously detected HD J=1-0 flux, we directly constrain the mid-plane temperature and optical depths of the CO gas and dust. We report a mid-plane CO snowline at 20.5 ± 1.3 AU, a mid-plane temperature distribution of 27+4−3×(R/20.5AU)-0.47+0.06−0.07 K, and a gas mass distribution of 13+8−5×(R/20.5AU)-0.9+0.4−0.3 g cm−2 between 5-20.5 AU in the TW Hya protoplanetary disk. We find a total gas/mm-sized dust mass ratio of 140 ± 40 in this region, suggesting that ∼2.4 earth mass of dust aggregates have grown to > cm sizes (and perhaps much larger).

Radially extended disk winds could be the key to unlocking how protoplanetary disks accrete and how planets form and migrate. A distinctive characteristic is their nested morphology of velocity and chemistry. Here we report James Webb Space Telescope near-infrared spectrograph spectro-imaging of four young stars with edge-on disks, three of which have already dispersed their natal envelopes. For each source, a fast collimated jet traced by [Fe ii ] is nested inside a hollow cavity within wider lower-velocity H 2 . In one case, a hollow structure is also seen in CO ro-vibrational ( v  = 1 → 0) emission but with a wider opening angle than the H 2 , and both of those are nested inside an Atacama Large Millimeter Array CO ( J  = 2 → 1) cone with an even wider opening angle. This nested morphology, even for sources with no envelope, strongly supports theoretical predictions for wind-driven accretion and underscores the need for theoretical work to assess the role of winds in the formation and evolution of planetary systems. JWST observations of outflows from four young stars reveal in each case a molecular wind with a central cavity surrounding a fast jet. These results point to disk winds driving accretion, with implications for planet formation and evolution.

Today, with the wealth of data provided by the Atacama Large Millimeter/ submillimeter Array (ALMA), we are beginning to characterizing the chemistry associated with the early stages of planet formation. Planets are born within disks of gas, primarily in molecular form, and dust. ALMA enables us to, for the first time, resolve these disks down to the radii of giant planet formation, and in some instances even into the zone where Earth-like planets are born. In this dissertation I explore one of the major results from ALMA regarding the disposition of the primary carriers of carbon and nitrogen within protoplanetary disks. The state of carbon and nitrogen has important implications for the composition of planets. Knowing the abundance of gas phase species in the disk provides the starting composition for the atmospheres of gaseous giant planets while the composition of ices influence the composition of solid bodies, such as terrestrial planets. Using both models and observations, this dissertation explores the evolution of volatile molecules in protoplanetary disks. Using chemical models, I have shown that volatile nitrogen in protoplanetary disks is likely found mainly in the form of molecular nitrogen, a molecule which remains in the gas phase throughout much of the disk (Chapter 2). The rest of this dissertation focuses on the chemistry of carbon, as the main carbon carriers are more readily accessible to observational characterization. My analysis of CO isotopologue emission in the protoplanetary disk TW Hydrae, in conjunction with emission from the molecular hydrogen isotopologue HD, reveals that CO gas, the primary carrier of volatile carbon, is under-abundant relative to the total gas mass throughout the disk (Chapter 3). I thus demonstrate that it is CO, and not the total gas, which is missing in this one system. To explore the potential cause of this depletion I then ran a large grid of chemical models for disks with a wide range of physical conditions in order to analyze how effective chemical reactions are at removing volatile molecules from the gas. I found that in both the upper layers of the disk (Chapter 4) and in the midplane (Chapter 5), an ISM level cosmic ray ionization rate, one unattenuated by disk winds, is needed to reduce the CO gas abundance by greater than an order of magnitude during the typical disk lifetime. In the absence of cosmic rays, chemical processes involving ultraviolet or X-ray photons can also reprocess CO on timescales of several million years, though not to the extent seen in the high cosmic ray rate models. I conclude that chemistry is unlikely to be the only cause of volatile depletion, given that many young, 1-3 million year old, protoplanetary disks have measured CO abundances one to two orders of magnitude below expectations. Other processes, such as vertical mixing of the gas and grain growth, must also contribute. The results of my chemical modeling suggest that, under certain circumstances, gas giants which form after a million years of chemical evolution may accrete envelopes under-abundant in volatile elements such as carbon, nitrogen, and oxygen. To conclude, a summary of the findings and future directions are discussed in Chapter 6.

The Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) is a NASA Astrophysics MIDEX-class mission concept, with the stated goal of following water from galaxies, through protostellar systems, to Earth's oceans. This paper details the protoplanetary disk science achievable with OASIS. OASIS's suite of heterodyne receivers allow for simultaneous, high spectral resolution observations of water emission lines spanning a large range of physical conditions within protoplanetary disks. These observations will allow us to map the spatial distribution of water vapor in disks across evolutionary stages and assess the importance of water, particularly the location of the midplane water snowline, to planet formation. OASIS will also detect the H2 isotopologue HD in 100+ disks, allowing for the most accurate determination of total protoplanetary disk gas mass to date. When combined with the contemporaneous water observations, the HD detection will also allow us to trace the evolution of water vapor across evolutionary stages. These observations will enable OASIS to characterize the time development of the water distribution and the role water plays in the process of planetary system formation.

Radially extended disk winds could be the key to unlocking how protoplanetary disks accrete and how planets form and migrate. A distinctive characteristic is their nested morphology of velocity and chemistry. Here we report JWST/NIRSpec spectro-imaging of four young stars with edge-on disks in the Taurus star-forming region that demonstrate the ubiquity of this structure. In each source, a fast collimated jet traced by [Fe II] is nested inside a hollow cavity within wider lower-velocity H2 and, in one case, also CO ro-vibrational (v=1-0) emission. Furthermore, in one of our sources, ALMA CO(2-1) emission, paired with our NIRSpec images, reveals the nested wind structure extends further outward. This nested wind morphology strongly supports theoretical predictions for wind-driven accretion and underscores the need for theoretical work to assess the role of winds in the formation and evolution of planetary systems

Recent observations show that the CO gas abundance, relative to H\\(_2\\), in many 1-10 Myr old protoplanetary disks may be heavily depleted, by a factor of 10-100 compared to the canonical interstellar medium value of 10\\(^{-4}\\). When and how this depletion happens can significantly affect compositions of planetesimals and atmospheres of giant planets. It is therefore important to constrain if the depletion occurs already at the earliest protostellar disk stage. Here we present spatially resolved observations of C\\(^{18}\\)O, C\\(^{17}\\)O, and \\(^{13}\\)C\\(^{18}\\)O \\(J\\)=2-1 lines in three protostellar disks. We show that the C\\(^{18}\\)O line emits from both the disk and the inner envelope, while C\\(^{17}\\)O and \\(^{13}\\)C\\(^{18}\\)O lines are consistent with a disk origin. The line ratios indicate that both C\\(^{18}\\)O and C\\(^{17}\\)O lines are optically thick in the disk region, and only \\(^{13}\\)C\\(^{18}\\)O line is optically thin. The line profiles of the \\(^{13}\\)C\\(^{18}\\)O emissions are best reproduced by Keplerian gaseous disks at similar sizes as their mm-continuum emissions, suggesting small radial separations between the gas and mm-sized grains in these disks, in contrast to the large separation commonly seen in protoplanetary disks. Assuming a gas-to-dust ratio of 100, we find that the CO gas abundances in these protostellar disks are consistent with the ISM abundance within a factor of 2, nearly one order of magnitude higher than the average value of 1-10 Myr old disks. These results suggest that there is a fast, \\(\\sim\\)1 Myr, evolution of the abundance of CO gas from the protostellar disk stage to the protoplanetary disk stage.

MWC349A, which had remained an ordinary member of the MWC catalog for a few decades, is now known as: (1) the brightest stellar source of radio continuum; (2) the only known high-gain natural maser in hydrogen recombination lines; and (3) the only strictly proven natural high-gain laser (in IR hydrogen recombination lines). These phenomena seem to occur in the circumstellar disk seen almost edge-on. They help us understand the structure and kinematics of the disk. The evolutionary status of MWC 349A is still debated: a young HAeBe star with a pre-planetary disk or an old B[e] star or even a protoplanetary nebula? We discuss new observational data obtained at the Maria Mitchell Observatory and elsewhere which may cast light on this issue.

The inner disk of the young star PDS 70 may be a site of rocky planet formation, with two giant planets detected further out. Solids in the inner disk may inform us about the origin of this inner disk water and nature of the dust in the rocky planet-forming regions. We aim to constrain the chemical composition, lattice structure, and grain sizes of small silicate grains in the inner disk of PDS 70, observed both in JWST/MIRI MRS and Spitzer IRS. We use a dust fitting model, called DuCK, based on a two-layer disk model. We use Gaussian Random Field and Distribution of Hollow Spheres models to obtain two sets of dust opacities. The third set of opacities is obtained from aerosol spectroscopy. We use stoichiometric amorphous silicates, forsterite, and enstatite in our analysis. We also used iron-rich and magnesium-rich amorphous silicate and fayalite dust species to study the iron content. The Gaussian Random Field opacity agrees well with the observed spectrum. In both MIRI and Spitzer spectra, amorphous silicates are the dominant dust species. Crystalline silicates are dominated by iron-poor olivine. We do not find strong evidence for enstatite. Moreover, the MIRI spectrum indicates larger grain sizes than the Spitzer spectrum, indicating a time-variable small grain reservoir. The inner PDS 70 disk is dominated by a variable reservoir of optically thin warm amorphous silicates. We suggest that the small grains detected in the inner PDS 70 disk are likely transported inward from the outer disk as a result of filtration and fragmentation at the ice line. In addition, the variation between MIRI and Spitzer data can be explained by the grain growth over 15 years and a dynamical inner disk where opacity changes occur resulting from the highly variable hot innermost dust reservoir.