arXiv subjects

L. Ilsedore Cleeves

Publications and source records attributed to L. Ilsedore Cleeves.

At least 19 recordsLinked to original sources

arXiv2026

Laboratory Evidence that Methanol-Rich Ice Mantles Lower Methyl Formate Binding Energies

Complex organic molecules are widely detected in star-forming regions and are important precursors to prebiotic species. Because these molecules are thought to form primarily within icy grain mantles, their desorption is assumed to be regulated by the sublimation of the surrounding H$_2$O ice matrix. However, observations increasingly reveal gas-phase abundances of methyl formate (MF) at temperatures well below the sublimation temperature of water ice (T ~ 100K), challenging this view. To investigate how ice composition and phase influence MF desorption, we present laboratory temperature-programmed desorption measurements of MF from four astrophysically relevant substrates: amorphous and crystalline H$_2$O and CH$_3$OH. Desorption kinetics were analyzed using pre-exponential factors derived from transition-state theory with leading-edge analysis and non-negative least-squares inversion to recover binding-energy distributions. We find that MF binds more weakly to CH$_3$OH ice than to H$_2$O-rich substrates, with representative binding energies of ~6247K for amorphous H$_2$O, ~6213K for crystalline H$_2$O, and ~5469K for amorphous CH$_3$OH. MF deposited on crystalline CH$_3$OH exhibits coverage-independent leading edges consistent with multilayer or island desorption, yielding an effective binding energy of 5333K. Enhanced surface mobility and reduced trapping efficiency on methanol ice, combined with weaker MF binding, provide a natural explanation for the early appearance of gas-phase MF observed in star-forming environments, with characteristic desorption temperatures of ~85K on CH$_3$OH and ~95-100K on H$_2$O. Together, these effects support a two-step thermal desorption pathway in which MF associated with CH$_3$OH-rich ice can be released prior to bulk H$_2$O sublimation.

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arXiv2026

Mapping Molecular Ion Distributions in a Diverse Sample of Protoplanetary Disks

Observations of ionization-tracing molecules in protoplanetary disks suggest that their ionization environments are diverse due to varying degrees of cosmic ray modulation, stellar activity, and environmental differences that alter the impact of ionizing radiation. To better understand ionization chemistry in disks, we present the largest survey of resolved observations of molecular ions to date. Using new and archival ALMA observations we detect HCO+, H13CO+, N2H+, and DCO+ toward seven protoplanetary disks (AS 209, DM Tau, GM Aur, HD 163296, LkCa 15, MWC 480, and V4046 Sgr). We compare distributions of ions to continuum features and find no robust relationships between the two. The observations do, however, reveal distinctions between Herbigs and T-Tauris and hint at a dichotomy between two types of T-Tauri ionization environments. The Herbig sources exhibit unique centrally-peaked N2H+ J = 4-3 emission, which we speculate could be tracing either a highly ionized surface layer or a zone of CO destruction, possibly due to their strong UV fields. Three T-Tauri sources in our sample exhibit bright, radially coincident rings in all optically thin molecular ion lines. We speculate that these radially coincident ion rings are evidence of a global change in ionization, possibly at the edge of a T-Tauriosphere. While it is not yet clear what factors most strongly influence the distribution of ion emission, this survey more than doubles the number of resolved ion observations in disks and points to potential distinctions in ion emission morphology related to the central star and environment.

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arXiv2026

The ALMA View of the Edge-on Gomez's Hamburger System: A Highly-Dynamic, Asymmetric Protoplanetary Disk Reveals the Earliest Phases of Giant Planet Formation

Chemical tracers provide some of the strongest observational signatures of ongoing planet formation and localized dynamical perturbations in protoplanetary disks. In particular, sulfur-bearing molecules are predicted to be enhanced in regions of shock heating, ice sublimation, and gravitational instability. Here, we present high-angular-resolution ($\approx$0.$^{\prime\prime}$2) Atacama Large Millimeter/submillimeter Array observations of $^{12}$CO J=3-2, $^{13}$CO J=3-2, CS J=7-6, and SO J$_{\rm N}$=8$_8$-7$_7$ toward the large, edge-on Gomez's Hamburger ('GoHam'; IRAS 18059-3211) disk. We detect a narrow, one-sided arc of SO emission that peaks near a previously-identified gas over-density, suggesting localized heating around an early-stage giant protoplanet or disk fragment. The edge-on geometry of GoHam enables us to place this chemical signature in the broader context of the disk gas and dust structure. To do so, we map the vertical distribution of molecular gas relative to millimeter- and (sub)-micron-sized dust, identify a pronounced north-south continuum asymmetry, and detect non-Keplerian $^{12}$CO and $^{13}$CO emission indicative of a disk wind. We also derive a dynamical stellar mass of 2.2 $\pm$ 0.5 M$_{\odot}$ and a revised dust-extinction-map-based distance of 139 $\pm$ 24 pc, which places GoHam in the outskirts of the Scorpius-Centaurus association. Together, these observations reveal a highly dynamic disk in which localized sulfur chemistry may trace one of the earliest observable stages of wide-separation giant planet formation.

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arXiv2026

SMA Observations Reveal Abundant HNC Chemistry in Transition Disks

The physical and chemical conditions of protoplanetary disks shape the properties of nascent planetary systems. The line ratios and relative abundances of the HCN-HNC isomer pair are well-suited for tracing these gas conditions, since isomer chemistry is linked to the underlying temperature, elemental abundances, and irradiation environment of the emitting gas. While HCN emits bright lines regularly observed in disks, the fainter HNC lines are targeted significantly less often, precluding our ability to calibrate the HNC-to-HCN ratio as a tracer of disk properties. Here, we present new Submillimeter Array observations of five transition disks around the T Tauri stars GM Aur, J1604, LkCa 15, GG Tau, and V4046 Sgr, covering the J=3-2 and J=4-3 lines of HCN and HNC. We detected at least one line of both HCN and HNC in each source and measured disk-integrated HNC-to-HCN flux and column density ratios of $\approx$0.1-0.7 and 0.1-0.4, respectively. For all sources, measured HNC fluxes exceed predictions from models of full (non-transition) disks by $\approx$3-10x, while HCN fluxes appear typical. The relative brightness of HNC vs. HCN in transition disks strongly suggests a link between the presence of a cavity and efficient HNC production. We corroborate this trend using chemical models of the DM Tau transition disk, showing HNC production and destruction are connected to the radiation environment. In our sample, disk-integrated HNC-to-HCN column density ratio shows no trend with disk gas temperature but positively correlates with disk mass due to increased HNC abundance in the larger reservoirs of cooler gas in more massive disks.

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arXiv2026

Angular Momentum of Planet-Forming Disks: Implications for Infall Driven Misalignments

Context. A significant fraction (>30%) of planet-forming disks and planetary are misaligned with respect to the rotational axis of their host stars, yet the dominant mechanism responsible for these misalignments remains unclear. Aims. We aim to observationally constrain the angular momentum of Class II protoplanetary disks and assess whether late-stage infall of material can bring sufficient angular momentum to tilt them. Methods. We first computed the angular momenta of 15 disks with surface density profiles inferred from dynamical modeling of high angular resolution ALMA observations. Based on this sample, we derived a relation linking disk angular momentum to stellar mass, disk mass, and the radius enclosing 90% of the 13CO flux and used it to estimate angular momenta of 18 more disks. We then compared disk values with theoretical predictions for late-stage accretion from clouds and observed streamers. Results. Angular momentum for most disks is lower than what theoretical models predict for late infall. This is also in qualitative agreement with comparison with streamer observations, however, characterization of mass of reservoirs feeding the streamers is needed to confirm this picture. Conclusions. Interactions with nearby clouds, resulting in late-stage infall of material onto Class II disks, can potentially explain the observed misalignments within disks and planetary systems.

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arXiv2026

First Detection of HC5N in a Class II Disk around TW Hya

Over the last decade of ALMA's operation the molecular inventory of protoplanetary disks has expanded rapidly, revealing a diverse set of nitrogen-bearing organics and carbon-chain molecules that trace both prebiotic chemistry and fundamental disk physics. Despite this progress, detections of larger species such as cyanopolyynes have remained limited, leaving larger carbon-chain chemistry in Class II disks largely unconstrained. Here, we report the first detection of HC5N toward the TW Hya protoplanetary disk, representing the largest cyanopolyyne identified to date in a Class II system. We derive a HC5N column density for two rotational transitions J = 41-40 and J = 37-36, N_T ~ 10^12 cm^-2 for assumed T_rot = 20-50 K and optically thin emission in LTE. We compare HC5N and HC3N formation mechanisms and analyze the HC3N/HC5N ratio. We use a chemical model to estimate the expected abundance and emitting layer of HC5N in a TW Hya-like disk. Although HC5N emission is spatially unresolved, measured column densities suggest an origin in the warm molecular layer where CN-based pathways are active. This detection extends the known carbon-chain chemistry in Class II disks and demonstrates that long cyanopolyynes can form and persist in planet-forming environments.

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arXiv2026

JWST-DECO: The Impact of Accretion on Mid-Infrared Observable Water in Planet-forming Disks

The inner few au of a protoplanetary disk hosts the majority of observed exoplanets and is the primary planet-forming zone of the disk. The mid-IR spectra of disks, with its rich forest of water lines, provides key insights into the composition of forming planets. One of the strongest trends seen with data from Spitzer and now JWST is a correlation between the increase in water line flux and accretion luminosity of a system. We set out to reproduce and understand this trend by adding an accretion module to the thermo-chemical code DALI, and explore how viscous accretion heating and the addition of accretion luminosity impacts the 2D temperature structure and the observable water reservoir. We reproduce the trend that the observed water mass increases with accretion rate, with hot, warm, and cool water being more to less strongly correlated, respectively. Our model suggests that these trends are due to an increased emitting area with accretion rate, with some of the cool and warm population becoming hidden underneath an optically thick dust surface and being constrained to a smaller disk volume. This trend is driven by the accretion-related increase in central luminosity, while viscous heating centralized to the midplane has no impact on observed water mass.

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arXiv2026

Characterizing the Extended Molecular Hydrogen Winds in Protoplanetary Disks from the JWST Disk Infrared Spectroscopic Chemistry Survey

We present a comprehensive analysis of extended H$_2$ emission from 34 protoplanetary disks observed with the JWST Disk Infrared Spectroscopic Chemistry Survey (JDISCS), supplemented by archival data. We investigated the morphology, kinematics, excitation conditions, and mass dynamics of H$_2$. Extended emission from pure rotational H$_2$ lines is found to be common, with 16 sources exhibiting clear signatures of disk winds. These include monopolar and bipolar structures in inclined disks and ring-like or bubble-like morphologies in face-on systems features indicative of wide-angle disk winds. Our analysis shows that the H$_2$ is consistent with slow {(4.2$^{+6.7}_{-3.0}$ km s$^{-1}$)} MHD driven winds. For ten disks, we model the wind morphology and find a median half-opening angle of $45\arcdeg^{+5}_{-4}$ and a characteristic power-law index of $α\sim$ 1.6. Excitation analysis yields a median gas temperature of 624 $\pm$ 130 K and a column density of $\log(N_{\mathrm{tot}}\,[\mathrm{cm}^{-2}]) = 18.6 \pm 0.6$. The median wind mass-loss rate, ${\rm log_{10}}(\dot{\rm M}_{\rm wind}^{\rm tot}) = -9_{-0.4}^{+0.8}\,{\rm M_\odot\,yr^{-1}}$, implies that, if molecular winds are the dominant mechanism responsible for disk dispersal, a typical disk with a mass of $2-3\,M_{\rm Jup}$ would dissipate on a $\sim$2-3 Myr timescale, consistent with observed disk lifetimes. The $\dot{\rm M}_{\mathrm{\rm wind}}^{\rm tot}$ span a relatively narrow range ($\sim$2 dex) and do not correlate strongly with accretion rates onto the star, suggesting that the mass loss rate and the accretion rates are probing different timescales. Our findings demonstrate that spatially extended warm H$_2$ emission is a widespread and reliable tracer of molecular disk winds in protoplanetary systems.

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arXiv2026

CORINOS V: Radiative transfer effects in protostellar ice observations

Recent observations of protostars with the James Webb Space Telescope have revealed unprecedented chemical complexity from their ice absorption features. However, these spectra are likely influenced by radiative transfer effects, and there is little understanding of how this impacts our ability to identify, quantify, and interpret the observed ice features. We have developed a new modeling framework to investigate the radiative transfer through icy protostellar envelopes, and apply this to the IRAS 15398-3359 protostar observed by the JWST CORINOS program. The modeled H$_2$O and CO column densities are similar to previous empirical studies, but we require a high CO$_2$/H$_2$O ratio of 76% to match the optical depth of the 15 $μ$m band. We use our modeled continuum to calculate a 6-10 $μ$m optical depth spectrum, and see considerable differences compared to a simple polynomial continuum model, underscoring the challenges with quantifying trace ice species in this range. For this source, we find that the observed absorption predominantly originates along the viewing line of sight between 1000 - 2000 au, peaking at the transition from the outflow cavity to the envelope; the spectra are largely insensitive to absorption from ices in the outer envelope, which extends out to 20,000 au. Lastly, we show that depending on how the line of sight intersects the cavity, the apparent CO$_2$/H$_2$O and CO/H$_2$O column density ratios can be underestimated compared to the underlying ice abundance ratios. Together this provides important context for interpreting the ice constraints derived from JWST observations of protostars.

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arXiv2026

Protoplanetary disk cavities with JWST-MIRI: a dichotomy in molecular emission

The evolution of planet-forming regions in protoplanetary disks is of fundamental importance to understanding planet formation. Disks with a central deficit in dust emission, a "cavity", have long attracted interest as potential evidence for advanced disk clearing by protoplanets and/or winds. Before JWST, infrared spectra showed that these disks typically lack the strong molecular emission observed in full disks. In this work, we combine a sample of 12 disks with millimeter cavities of a range of sizes ($\sim2$-70 au) and different levels of millimeter and infrared continuum deficits. We analyze their molecular spectra as observed with MIRI on JWST, homogeneously reduced with the new JDISCS pipeline. This analysis demonstrates a stark dichotomy in molecular emission where "molecule-rich" (MR) cavities follow global trends between water, CO, and OH luminosity and accretion luminosity as in full disks, while "molecule-poor" (MP) cavities are significantly sub-luminous in all molecules except sometimes OH. Disk cavities generally show sub-luminous organic emission, higher OH/H$_2$O ratios, and suggest a lower water column density. The sub-thermal excitation of CO and water vibrational lines suggests a decreased gas density in the emitting layer in all cavities, supporting model expectations for C$_2$H$_2$ photodissociation. We discover a bifurcation in infrared index (lower in MR cavities) suggesting that the molecular dichotomy is linked to residual $μ$m-size dust within millimeter disk cavities. Put together, these results suggest a feedback process between dust depletion, gas density decrease, and molecule dissociation. Disk cavities may have a common evolutionary sequence where MR switch into MP over time.

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arXiv2025

CORINOS IV: Quantifying Baseline-Fitting Uncertainties in SO$_2$ Ice Measurements with JWST/MIRI

Sulfur dioxide (SO$_2$) ice has been tentatively detected in protostellar envelopes, but its reliability as a solid-state sulfur reservoir remains unclear. We present new measurements of SO$_2$ ice from 6.8-8.5 $μ$m toward four Class 0 protostars observed with JWST's Mid-Infrared (MIRI) Medium Resolution Spectrometer, as part of the COMs ORigin Investigated by the Next-generation Observatory in Space (CORINOS) program. The sample spans a luminosity range from 1 $L_\odot$ (B335, IRAS 15398-3359) to 10 $L_\odot$ (L483, Ser-emb~7). To assess continuum placement uncertainty in absorption spectra, we apply randomized polynomial fits over the restricted region. We fit laboratory spectra from the Leiden Ice Database for Astrochemistry (LIDA) using the open-source Python library Omnifit. We detect the 7.7 $μ$m CH$_4$ band in all sources and find its column density robust to baseline choice, providing a reference for evaluating the weaker SO$_2$ feature on its blue shoulder and quantifying baseline-related uncertainty. Three SO$_2$ laboratory ices were tested: pure SO$_2$ ice yields 0.3-1.2% of volatile sulfur may be locked in SO$_2$ ice (lower and upper limits); CH$_3$OH:SO$_2$ ice gives 0.02-0.18%, but with lower quality fitting. The best-fitting H$_2$O:SO$_2$ ice yields 0.2-0.9%, which we consider the most realistic. These ranges define plausible bounds on SO$_2$ ice abundances in our sample. We find evidence for SO$_2$ in Ser-emb 7, L483, and IRAS 15398-3359, but emphasize the noisy spectrum of B335 prevents a definitive detection. Comparing SO$_2$ ice abundances across the different environments, we assess how conditions influence role of SO$_2$ as a potential sulfur reservoir and implications for the longstanding ``missing sulfur'' problem.

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arXiv2025

A tale of three tails: A misaligned streamer and mysterious structures around [BHB2007]1

Recent discoveries of streamer-like structures around protostellar sources challenge the traditional picture of isolated, axisymmetric star formation. Here, we present new ALMA observations of [BHB2007]1, a flat-spectrum source connected to at least three such elongated structures. Two of these features are symmetrically located to the north and south of the disk, with velocities aligned with the disk on their respective sides. However, their unbound kinematics and curved morphology make it difficult to determine their origin. Possible explanations include outflows, interactions with the nearby BHB2 system, and hyperbolic infall, but none fully account for all observed properties. In contrast, a newly identified collimated structure to the west shows clear evidence of gravitationally bound infall. Estimates of its mass, mass infall rate, and angular momentum suggest that this infalling streamer would roughly double the mass budget available to form planets and tilt the disk by a few tens of degrees. Furthermore, its misalignment with the midplane of the disk and the lack of diffuse envelope emission indicate that the streamer may have formed due to gravitational capture of cloud material unrelated to the source's natal core. Together, these findings support a more dynamic picture of star formation, one where environmental interactions continue to shape conditions for building planetary systems.

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arXiv2025

A Submillimeter Survey of CS Excitation in Protoplanetary Disks: Evidence of X-ray-Driven Sulfur Chemistry

The sulfur chemistry in protoplanetary disks influences the properties of nascent planets, including potential habitability. Although the inventory of sulfur molecules in disks has gradually increased over the last decade, CS is still the most commonly-observed sulfur-bearing species and it is expected to be the dominant gas-phase sulfur carrier beyond the water snowline. Despite this, few dedicated multi-line observations exist, and thus the typical disk CS chemistry is not well constrained. Moreover, it is unclear how that chemistry - and in turn, the bulk volatile sulfur reservoir - varies with stellar and disk properties. Here, we present the largest survey of CS to date, combining both new and archival observations from ALMA, SMA, and NOEMA of 12 planet-forming disks, covering a range of stellar spectral types and dust morphologies. Using these data, we derived disk-integrated CS gas excitation conditions in each source. Overall, CS chemistry appears similar across our sample with rotational temperatures of ${\approx}$10-40 K and column densities between 10$^{12}$-10$^{13}$ cm$^{-2}$. CS column densities do not show strong trends with most source properties, which broadly suggests that CS chemistry is not highly sensitive to disk structure or stellar characteristics. We do, however, identify a positive correlation between stellar X-ray luminosity and CS column density, which indicates that the dominant CS formation pathway is likely via ion-neutral reactions in the upper disk layers, where X-ray-enhanced S$^+$ and C$^+$ drive abundant CS production. Thus, using CS as a tracer of gas-phase sulfur abundance requires a nuanced approach that accounts for its emitting region and dependence on X-ray luminosity.

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arXiv2025

Investigating the Sulfur Mystery in Protoplanetary Disks Through Chemical Modeling

Sulfur is a critical element to life on Earth, and with detections of sulfur-bearing molecules in exoplanets and comets, questions arise as to how sulfur is incorporated into planets in the first place. In order to understand sulfur's journey from molecular clouds to planets, we need to understand the molecular forms that sulfur takes in protoplanetary disks, where the rotational emission from sulfur-bearing molecules in the gas phase indicates a very low abundance. To address this question, we have updated the 2D time-dependent disk chemical modeling framework of Fogel et al. (2011) to incorporate several new sulfur species and hundreds of new sulfur reactions from the literature. Specifically, we investigate the main molecular forms that sulfur takes in a disk orbiting a solar mass young T Tauri star. We explore the effects of different volatile (reactive) sulfur abundances, C/O ratios, initial sulfur molecular forms, and cosmic-ray ionization rates. We find that a high C/O ratio can explain both the prevalence of CS observed in disks and the lack of SO detections, consistent with previous results. Additionally, initial sulfur form greatly affects the ice abundances in the lower layers of the disk, which has implications for comet formation and future observations with JWST.

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arXiv2025

The Far-Infrared Enhanced Survey Spectrometer (FIRESS) for PRIMA: Science Drivers

We present the science drivers for the Far-Infrared Enhanced Survey Spectrometer (FIRESS), one of two science instrument on the PRobe Infrared Mission for Astrophysics (PRIMA). FIRESS is designed to meet science objectives in the areas of the origins of planetary atmospheres, the co-evolution of galaxies and supermassive black holes, and the buildup of heavy elements in the Universe. In addition to these drivers, FIRESS is envisioned as a versatile far-infrared spectrometer, capable of addressing science questions in most areas of astrophysics and planetary astronomy as part of a dominant General Observer (GO) program with 2/3 of the current science cases using FIRESS. We summarize how the instrument design choices and parameters enable the main science drivers as well as a broad and vibrant GO program.

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arXiv2025

Cosmic cascades: How disk substructure regulates the flow of water to inner planetary systems

The influx of icy pebbles to the inner regions of protoplanetary disks constitutes a fundamental ingredient in most planet formation theories. The observational determination of the magnitude of this pebble flux and its dependence on disk substructure (disk gaps as pebble traps) would be a significant step forward. In this work we analyze a sample of 21 T Tauri disks (with ages $\approx 0.5{-}2\mathrm{~Myr}$) using JWST/MIRI spectra homogeneously reduced with the JDISCS pipeline and high-angular-resolution ALMA continuum data. We find that the 1500/6000 K water line flux ratio measured with JWST - a tracer of cold water vapor and pebble drift near the snowline - correlates with the radial location of the innermost dust gap in ALMA continuum observations (ranging from 8.7 to 69 au), confirming predictions from recent models that study connections between the inner and outer disk reservoirs. We develop a population synthesis exploration of pebble drift in gapped disks and find a good match to the observed trend for early and relatively effective gaps, while scenarios where pebble drift happens quickly, gaps are very leaky, or where gaps form late are disfavored on a population level. Inferred snowline pebble mass fluxes (ranging between $10^{-6}$ and $10^{-3}~M_\oplus/\mathrm{yr}$ depending on gap position) are comparable to fluxes used in pebble accretion studies and those proposed for the inner Solar System, while system-to-system variations suggest differences in the emerging planetary system architectures and water budgets.

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arXiv2025

The Multiband Imaging Survey for High-Alpha PlanetS (MISHAPS) I: Preliminary Constraints on the Occurrence Rate of Hot Jupiters in 47 Tucanae

The first generation of transiting planet searches in globular clusters yielded no detections, and in hindsight, only placed occurrence rate limits slightly higher than the measured occurrence rate in the higher-metallicity Galactic thick disk. To improve these limits, we present the first results of a new wide field search for transiting hot Jupiters in the globular cluster 47~Tucanae. We have observed 47~Tuc as part of the Multiband Imaging Survey for High-Alpha Planets (MISHAPS). Using 24 partial and full nights of observations taken with the Dark Energy Camera on the 4-m Blanco telescope at CTIO, we perform a search on 19,930 stars in the outer regions of the cluster. Though we find no clear planet detections, by combining our result with the upper limit enabled by Gilliland et al.'s 2000 Hubble search for planets around an independent sample of 34,091 stars in the inner cluster, we place the strongest limit to date on hot Jupiters with periods of $0.8 \leq P \leq 8.3$ days and $0.5~R_{\rm Jup} \leq R_{\rm P} \leq 2.0~R_{\rm Jup}$ of $f_{\rm HJ} < 0.11\%$, a factor of ${\sim}$4 below the occurrence rate in the \textit{Kepler} field. Our search found 35 transiting planet candidates, though we are ultimately able to rule out each without follow-up observations. We also found 4 eclipsing binaries, including 3 previously-uncataloged detached eclipsing binary stars.

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arXiv2025

A Multi-line Analysis of the Distribution and Excitation of CS and H$_2$CS in the HD 163296 Disk

The abundance and distribution of sulfur-bearing molecules in protoplanetary disks directly influences the composition and potential habitability of nascent planets in addition to providing powerful probes of the physical gas conditions in the disks themselves. Here, we present new and archival ALMA and SMA observations of CS and H$_2$CS, and their C$^{34}$S and H$_2$C$^{34}$S isotopologues, at high-angular resolution (${\approx}$0."2-0."4; 20-40 au) in the HD 163296 disk, which reveal a central cavity and multi-ringed emission structure. These observations comprise the most comprehensive, multi-line CS data in a planet-forming disk to date, spanning a wide range of excitation conditions from E$_{\rm{u}}$=7.1 K to 129.3 K, and include new detections of C$^{34}$S, H$_2$CS, and H$_2$C$^{34}$S in this system. Using these data, we derive spatially-resolved rotational temperature and column density profiles for all species. We find a column density ratio N(H$_2$CS)/N(CS) $\approx$ 0.5, which is comparable to that of the similar MWC 480 disk and suggests that organic sulfur compounds may constitute a large fraction of the volatile sulfur reservoir in disks around Herbig stars generally. We derive $^{32}$S/$^{34}$S ratios of ${\approx}$5 (CS/C$^{34}$S) and ${\approx}$2 (H$_2$CS/H$_2$C$^{34}$S) based on disk-averaged and spatially-resolved analyses. Both values are consistent across these two pairs of optically-thin molecules and are well-below the expected ISM ratio of ${\approx}$22, suggesting significant sulfur fractionation. We also constrain the CS emitting layer ($z/r\lesssim 0.1$) using the vertical separations of the disk surfaces in the channel maps and based on the known 2D gas structure of the HD 163296 disk combined with our excitation analysis.

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