arXiv subjects

Doug Johnstone

Publications and source records attributed to Doug Johnstone.

At least 19 recordsLinked to original sources

arXiv2026

The FU Ori outburst of PR Ori B

We report the discovery of a nearby FU Ori-type outburst (FUor), PR Ori B, in the L1641 cluster of the Orion star-forming region. The high-amplitude variability was first identified in the NEOWISE (3-5 $\mu$m) photometry of the unresolved PR Ori binary system. Long-term, resolved optical photometric monitoring demonstrates that PR Ori B is the driver of a $\Delta G=5~$mag outburst, while PR Ori A has remained constant over the last 20 years. The near-IR spectrum of PR Ori B changes from a late K-type spectral type during quiescence to a viscously heated disk during outburst, including deep absorption in $^{12}$CO and H$_2$O bands. The optical spectrum also exhibits features that are commonly associated with FUors, including P Cygni profiles in Na I D lines and absorption in the Ca II infrared triplet. The luminosity of the outburst (L$_{acc}\sim$30--40 L$_\odot$) is similar to that commonly observed in FUors. The comparison of Spitzer/IRS and VLT/VISIR spectroscopy shows some evidence of silicate crystallisation during the outburst. PR Ori B is one of the closest and brightest FUors discovered over the last few years, only one magnitude fainter than the archetype of the class FU Ori. The proximity and brightness will allow for future high angular resolution observations to probe the physics of the inner disk and to evaluate changes in the disk due to the increased luminosity.

astro-ph.SR↗

arXiv2026

A Candidate Innermost Fifth Planet In the HR 8799 System Revealed By JWST NIRISS Aperture Masking Interferometry

We detect a candidate fifth planet in the HR 8799 system, directly imaged with the JWST/NIRISS Aperture Masking Interferometer (AMI). The detected source lies just above a $3\sigma$ contrast curve at a contrast of $\sim2\times10^{-4}$ in the F380M filter at a projected separation of $\sim150$ mas, corresponding to a few-to-several Jupiter mass planet at an orbital radius of $\sim 7$au. The separation of the candidate is compatible with absolute proper-motion constraints from Gaia and Hipparcos assuming it is bound, while its orbital position lies near a stable orbital solution of a fifth planet in a 3:1 mean motion resonance with planet e. This detection was made possible by a new JWST/NIRISS AMI data pipeline that reaches the photon noise limited potential of AMI by accounting for the optical and electronic systematics that limited sensitivity in prior analyses. Confirmation of this candidate would make HR 8799 the first directly imaged five-planet system and provide insight into the orbital dynamics and the dynamical evolution of planetary systems with widely-separated gas giants.

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arXiv2026

CCAT: The Prime-Cam Instrument for the Fred Young Submillimeter Telescope -- Overview and Status

Prime-Cam is a first-generation science instrument for the CCAT Observatory's six-meter aperture Fred Young Submillimeter Telescope (FYST), under construction at an elevation of 5600 m on Cerro Chajnantor in Chile's Atacama Desert. Prime-Cam will deliver over ten times greater mapping speed at submillimeter wavelengths than current facilities for unprecedented broadband and spectroscopic measurements in windows between 1.4 -- 0.3 mm (220 -- 850 GHz). When fully populated, Prime-Cam will field over 100,000 kinetic inductance detectors across seven independently optimized instrument modules. With Prime-Cam, the CCAT Collaboration will address a suite of science goals, from Big Bang cosmology, to galaxy evolution and star formation over cosmic time. Prime-Cam is scheduled for integration in FYST in late 2026, followed by a year of early science observations with the 280 and 350 GHz instrument modules. We discuss the design and in-lab testing of the 1.8-m diameter Prime-Cam receiver and 280 GHz instrument module, and give an update on deployment status and early science plans.

astro-ph.IM↗

arXiv2026

Atmospheric Reconnaissance of TRAPPIST-1 f with JWST NIRISS SOSS: No Evidence for the Transit Light Source Effect

In just over three years of operation, JWST has observed all seven planets of the TRAPPIST-1 system. The two innermost planets were found to have little to no atmosphere, barring the presence of high-altitude aerosols. Here we present the first JWST transit spectra of the habitable-zone exoplanet TRAPPIST-1 f, which were obtained with NIRISS SOSS over the course of five transits. At least one stellar flare occurred in each visit, but unlike observations of closer-in TRAPPIST-1 planets, no evidence for contamination of the transit spectra from unocculted stellar surface heterogeneities was found. This non-detection does not guarantee the absence of unocculted heterogeneities in all future transit observations of this planet, and it could be explained by the transit chord of TRAPPIST-1 f having properties similar to the average, out-of-transit, visible stellar hemisphere at the time of observation. The transit spectra exhibit slopes ranging from -365 ppm/um down to 15 ppm/um, which we attribute to stellar variability, that is, flares and/or smaller-scale events. The visits least affected by flares rule out H2/He-dominated atmospheres with surface pressures higher than about 20 mbar at 95% confidence. For high-mean-molecular-mass atmospheres, the exact upper limits on surface pressures depend on the reduction pipeline and on the treatment of the residual slopes in the transit spectra.

astro-ph.EP↗

arXiv2026

Accretion Burst Crystallizes Silicates in a Planet-Forming Disk

Crystalline silicates form at high temperatures (> 900 K; Fabian et al. 2000; Hallenbeck et al. 1998). Their presence in comets (Hanner et al. 1994; Hayward et al. 2000; Wooden et al. 2002; Shinnaka et al. 2018) suggests that high-temperature dust processing occurred in the early Solar System and was subsequently transported outward to comet-forming regions. However, direct evidence for this crystallization and redistribution in Sun-like protostars has remained elusive. By comparing James Webb Space Telescope (JWST) mid-infrared spectra of the periodically bursting protostar EC 53 (Lee et al. 2020), we detect crystalline silicate (forsterite and enstatite) emission features that appear only during the burst. The emergence of these features indicates active crystal formation via thermal annealing in the hot inner disk during the accretion burst. We also detect a nested outflow-a collimated atomic jet enclosed by slower molecular outflows, consistent with magnetohydrodynamic (MHD) wind models (Pascucci et al. 2025). This configuration provides a mechanism for outward transport of freshly crystallized silicates (Giacalone et al. 2019). Our results provide the first direct observational evidence of in-situ silicate crystallization during episodic accretion bursts in a very young star still embedded in its dense envelope. Although we do not directly detect grains transported to the outer disk, the observed trends are consistent with outward redistribution, indicating that both dust processing and transport occur during the earliest and most dynamic stages of star formation.

astro-ph.EP↗

arXiv2026

JWST/NIRCam Imaging of Young Stellar Objects. IV. Detailed Imaging of the Protoplanetary Disk around TW Hya

As the nearest protoplanetary disk to Earth ($d = 60.14$ pc), TW Hya is one of the most studied protoplanetary disks and a critical benchmark for testing planet formation theories. We present high-contrast coronagraphic imaging of the TW Hya disk from JWST/NIRCam across four filters (F187N, F200W, F356W, and F444W). We detect the disk's scattered-light emission in F200W, F356W, and F444W. An elliptical fit to the disk image yields an average inclination of $i = 8.74^{+1.03}_{-0.94}$ degrees and a position angle of $\mathrm{PA} = 75.62^{+7.86}_{-6.56}$ degrees. We find tentative evidence for radial variations in these parameters, a trend consistent with a disk warp. Our companion search yields no new detections, placing the lowest mass limits yet on companions that might be responsible for carving out the dust gap. Assuming no local extinction and a system age of 10 Myr, the F444W data are sensitive to masses down to $\sim 0.4\,M_{\rm Jup}$ at separations of $1$ arcsec ($\sim 60$ AU). Accounting for local disk extinction analogous to the AS 209 system, our limits reach sub-Jupiter masses beyond $2$ arcsec. Furthermore, our analysis provides a detailed view of a previously detected feature in the outer disk at $\sim 120$ AU, confirming its morphology as a distinct bifurcation structure. This feature may indicate the presence of complex substructures arising from dynamical planet-disk interactions. These results demonstrate JWST's ability to characterize the architecture of protoplanetary disks and constrain the properties of forming worlds.

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arXiv2026

A JWST, ALMA and VLA survey of the Ophiuchus-A star-forming region: Unveiling hidden dust mass and connecting infrared outflows to their radio origins

We present an infrared, millimetre, and radio survey of 20 Class 0-III young stellar objects in the Ophiuchus A L1688 star-forming cluster, combining high-resolution (7-25 au) VLA and JWST observations with archival ALMA data. We implement physically motivated models to derive dust and ionised gas properties, spectral behaviour and their relative contributions through the millimetre-centimetre radio spectral energy distribution. Our models reveal circumstellar dust disks that are, on average, tens to hundreds of times more massive than millimetre-only estimates (subject to uncertainties arising from the choice of dust opacity) and contain millimetre-sized grains even at the Class 0 stage. Owing to the VLA's high resolution we are able to connect outflows to their origins, detecting protostellar jet emission on scales of 10s-1000s au. Our results represent a homogeneous characterisation of the dust and ionised gas properties in Ophiuchus and present a potential solution to the long-standing 'missing disk mass' problem. However, our understanding is still limited by resolution and sensitivity at frequencies <40 GHz. Future facilities like the SKA and ngVLA are needed to provide the necessary capabilities to fully spatially resolve this emission (<0.18") even in one of the closest star-forming regions.

astro-ph.GA↗

arXiv2026

EPISODE II: Variability in the CO and H$_2$O rovibrational absorption lines in a periodically variable protostar EC 53

We present two-epoch JWST NIRSpec and MIRI observations of the young protostar EC 53 (V371 Ser), a periodically variable source with well-characterized quiescent and burst phases. The spectra in both epochs show absorption in the CO overtone ($\sim$2.3 $\mu$m) and fundamental ($\sim$4.6 $\mu$m) bands and the H$_2$O stretching ($\sim$2.7 $\mu$m) and bending ($\sim$6.0 $\mu$m) modes. We also obtained high-resolution ($R\approx45{,}000$) IGRINS spectra during the burst to constrain the CO overtone line profiles. LTE slab modeling yields gas temperatures of $\sim$1800 K (CO overtone) and $\sim$1200 K (CO fundamental), consistent with the overtone tracing hotter gas at smaller radii. The H$_2$O stretching-mode absorption shows no compelling evidence for variability, and the current JWST CO overtone data do not provide a robust constraint on overtone variability. In contrast, the CO fundamental and H$_2$O bending-mode features weaken by a factor of $\sim$2 during the burst, which is most naturally explained by continuum changes rather than large variations in absorbing gas. To quantify continuum dilution, we introduce a ``relative veiling'' that treats the quiescent spectrum as an internal reference and measures the change in the continuum excess between the two epochs. This formalism yields burst-to-quiescent hot-continuum ratios of $2.9\pm0.2$ for the CO overtone and $1.71\pm0.11$ for the CO fundamental. Using a viscous-disk prescription, these imply representative accretion-rate ratios of $\sim$3.6 and $\sim$2.0, respectively. The differing ratios suggest that inner-disk regions traced at different temperatures, and thus radii, respond differently across the burst cycle, consistent with episodic mass buildup in the inner disk during quiescence followed by more efficient transport through the innermost disk onto the protostar during the burst.

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arXiv2026

BISTRO Survey: Gravity-Dominated and Magnetically Regulated Star Formation in M17 SW

We present high-resolution magnetic field maps of the M17 SW molecular cloud using JCMT 850 $\mu$m dust polarization at a scale of 14$''$. The magnetic field exhibits a distinct arc-like structure that encircles three dense clumps (C1, C2, and C3). By combining polarization data with ammonia line observations, the plane-of-sky magnetic field strength, measured using the Skalidis-Tassis method to minimize angle dispersion errors, ranges from 0.1 to 2.4 mG (mean: 0.54 mG). Energy budget analysis reveals a hierarchy dominated by gravity ($e_G \approx 10^{-7.8}$ erg cm$^{-3}$), which exceeds both magnetic ($e_B \approx 10^{-8.3}$ erg cm$^{-3}$) and turbulent ($e_k \approx 10^{-8.7}$ erg cm$^{-3}$) energies. Since all three energy densities lie within one order of magnitude, gravitational dominance acts primarily as the global driver, while the system remains in a state of near-equipartition. Structurally, the northeastern boundary shows magnetic field lines perpendicular to the shock front, consistent with compression from the adjacent HII region. Within the cloud, magnetic field lines generally align with gravity to assist collapse, but turn perpendicular to gravity within curved accretion bridges. This configuration provides support against radial collapse while guiding gas flow. Kinematic evidence suggests that these channels transport material from Clump C3 onto the massive Clump C2. Star formation in M17 SW is globally driven by gravity but locally regulated by the magnetic field structure.

astro-ph.GA↗

arXiv2026

Radio Continuum and Water Maser Monitoring of the Outburst in HOPS 373: Free-Free Emission Does Not Respond to the Outburst

We present VLA C-band (5~cm) continuum, K-band (1.3~cm) continuum, and water maser (22.235 GHz) monitoring of the protostar HOPS-373. We additionally present the contemporaneous monitoring for 95 sources within the 5~cm field of view for over two years during the peak of the HOPS-373 outburst and an additional epoch in 2026. HOPS-373 is a binary Class 0 protostar located in the Orion star forming region that was found to have a $\sim$4$\times$ luminosity burst from the JCMT Transient Survey and NEOWISE monitoring. We do not find evidence for a change in the free-free emission traced by VLA 5~cm continuum during the peak of its outburst or during the decline. Moreover, the 1.3~cm continuum does not show significant variability between the NE and SW components of the HOPS-373 binary. The water maser emission is highly variable toward HOPS-373, multiple velocity components are detected at different (or the same) times and the maser spots are located close to the 1.3~cm continuum source of HOPS-373-SW. There is tentative evidence for the water maser spots to be propagating away from the source, but there is not a robust connection between the outburst and the observed maser activity. The lack of correlation between outburst and free-free emission from HOPS-373 indicates that the free-free emission may not directly respond to increases in the accretion rate and subsequently the outflow rate. The lack of a link could be due to the outflow mostly being neutral, or there may be offsets in the timescale for the free-free response.

astro-ph.SR↗

arXiv2026

The Days Drag On on WASP-121 b: Interpreting its NIRISS Spectroscopic Phase Curve with General Circulation Models

Ultra-hot Jupiters present extreme atmospheric phenomena not found in the Solar System. These planets' daysides experience strong temperature inversions, molecular species (including H2) dissociate, and magnetism disrupts their atmospheric circulation. On their nightsides H2 can recombine and clouds may form. Spectroscopic phase curves let us measure these spatially inhomogeneous conditions, which can then be interpreted with three-dimensional (3-D) models. In this work we compare the JWST/NIRISS spectroscopic phase curve of the ultra-hot Jupiter WASP-121 b to state-of-the-art 3-D models with varying modeling assumptions, including the aforementioned physical phenomena. We demonstrate the importance of accurately accounting for the planet's radius in comparison between data and models, as it changes the implied overall planetary emission. We find that the 3-D models predict planet emission $\sim$12% higher than observed, contributing to a continued tension between measured and predicted hot Jupiter albedos. We identify multiple pieces of evidence that confirm a strong source of drag operating in this planet's atmosphere. In addition, the nightside emission spectrum is devoid of strong absorption features, which may be best explained by nightside clouds. One feature of the dataset that is not matched by the 3-D models is a trend of increasing eastward phase offset with decreasing wavelength, for wavelengths shorter than $\sim$1.4 \textmu m. This result is not consistent with reflection from dayside clouds, nor can it be explained by removing atmospheric opacity sources. Our analysis highlights the complexities in generating 3-D models and interpreting observations of ultra-hot Jupiters in the JWST era.

astro-ph.EP↗

arXiv2026

The long-term outburst(s) of GPSV16: from an intermediate to a FUor classification

FU Ori outbursts are thought to play a key role in stellar mass assembly and in the chemistry of protoplanetary disks during the early formation of stars. However, uncertainties remain regarding the universality of these events and the physical mechanism driving the high-amplitude variability. In this work, we present an analysis of optical, near- and mid-IR photometry (ZTF, UKIDSS GPS, NEOWISE) and near-IR spectra (IRTF, Gemini) of the eruptive variable Class I YSO GPSV16. The YSO, associated with the HII region G71.52$-$00.38 ($d=3.61$~kpc), showed two outbursts, one with $\Delta K_{\rm s}=2.2$~mag (2005-2012) and a second starting in 2016 with $\Delta K_{\rm s}=5.6$~mag and accretion luminosity of $\sim$130 L$_{\odot}$. The outbursts displayed distinct spectroscopic characteristics: the first showed emission lines associated with a hot inner disk surface, whereas the second showed absorption lines arising from the cooler upper layers of a viscously heated disk. These features likely arose due to the different accretion rates reached during each outburst. The second outburst showed a two-stage mid-IR rise, requiring $\approx8.4$ years to reach peak brightness. The mid-IR rise also started 8 years before the onset of the optical outburst. The wavelength-dependent light curve points to an instability that is triggered at larger distances within the accretion disk and propagates inward. Assuming a propagation time of 8 years for the accretion wave, we estimate that the second outburst started at a distance of $r\sim0.4$~AU. These results show how long-term, multi-wavelength photometric monitoring can help identify the disk instabilities that trigger eruptions in YSOs.

astro-ph.SR↗

arXiv2026

Magnetic field alignment with dense cores in the transition between cloud and core scales

In a magnetically-dominated model of star formation, we expect to see alignments between the magnetic field orientation of star-forming dense cores and the cloud-scale magnetic field. Pandhi et al. (2023) showed instead, however, that the orientation of cores and their angular momentum vectors appear random with respect to the larger-scale magnetic field, implying that magnetic fields may play a diminished role in core formation and evolution. Here, we use higher-resolution dust polarization data from the B-Fields In Star-forming Region Observations (BISTRO) survey on the James Clerk Maxwell Telescope (JCMT) to investigate the change in the magnetic field orientation from cloud scales to core scales, and reassess any correlations between core-scale magnetic fields, core orientations and core velocity gradients. We produce a catalog of 79 cores over 14 star-forming regions with averaged core-scale magnetic field orientations. We find that the core-scale magnetic field is more disordered compared to the cloud-scale field, as measured by an increased standard deviation in the magnetic field vector orientations. Alignment between the core-scale and cloud-scale field varies greatly between regions. Our results are consistent with random alignments between the core-scale magnetic field, core orientation, and core velocity gradient, in agreement with the results by Pandhi et al. (2023) for the cloud-scale field. We conclude that there is a clear change in the magnetic field in the transition from cloud- to core-scales. Our results suggest that the magnetic field may not play a dominant role in the evolution of dense cores on core scales.

astro-ph.GA↗

arXiv2026

EPISODE IV: Ice Inventory in the Envelope of EC 53

We present the 1.6$-$28 $\mu$m spectra of the young protostar EC 53, obtained with JWST NIRSpec IFU and MIRI MRS during the quiescent and burst phases of its periodic brightness variations. To isolate ice absorption features, we modeled and removed the mid-infrared silicate dust absorption using a dedicated continuum-fitting procedure. In the optical depth spectrum, we first fit the broad H$_2$O ice features and then decomposed the major ice components, including NH$_3$, CO$_2$, CH$_3$OH, CO, and CH$_4$, by matching laboratory profiles for both pure and H$_2$O-mixed ices. The 4.62 $\mu$m and 6.85 $\mu$m bands are attributed to OCN$^-$ and NH$_4^+$ ions, respectively. Minor or tentative contributions from complex species (HCOOH, H$_2$CO, CH$_3$COOH, CH$_3$CHO, CH$_3$CH$_2$OH, and NH$_2$CHO) are also considered to our global ice analysis. The silicate-corrected spectra reveal no measurable change in any ice absorption band between the two phases, indicating that moderate and short-period accretion bursts in EC~53 do not significantly alter the physical or chemical state of the ices within its envelope. The derived abundances of these major species relative to H$_2$O significantly exceed the values typically observed toward other embedded protostars. Finally, we place the ice inventory of EC~53 in the context of other protostellar systems observed with JWST, highlighting that its chemically rich, thermally quiescent ice reservoir provides a benchmark for studying ice evolution under episodic accretion.

astro-ph.SR↗

arXiv2026

Direct evidence for magnetohydrodynamic disk winds driving rotating outflows in protostar HOPS 358

Angular momentum removal is a fundamental requirement for star and planet formation, yet the mechanisms driving this process remain debated. Magnetohydrodynamic disk winds, launched along magnetic field lines from extended disk regions, offer a promising solution, particularly in regions where magnetorotational turbulence is weak. Here we present high-resolution Atacama Large Millimeter/submillimeter Array observations of the Class 0 protostar HOPS 358, revealing a rotating, nested outflow structure traced by H2CO, SO, and CH3OH emission. The outflow preserves the disk's rotational sense and is aligned with the disk axis, providing direct observational evidence for a magnetically launched disk wind. From the measured kinematics, we derive a dimensionless magnetic lever arm of approximately 2.3 and constrain the wind-launching region to radii of 10-18 astronomical units within the planet-forming zone. These results demonstrate that magnetohydrodynamic disk winds operate during the deeply embedded phase, efficiently extracting angular momentum while shaping disk evolution and establishing initial conditions for planet formation.

astro-ph.SR↗

arXiv2026

Time-variable Scattered Light in Herbig Disks Observed with Subaru/SCExAO

Using the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument, we present near-infrared K-band polarimetric imaging of nine Herbig stars selected from a volume-limited sample within 200 pc. We detect the disks around MWC 480, HD 163296, and HD 143006 for the first time with SCExAO, and compare these observations with previous VLT/SPHERE datasets to identify surface-brightness variability. In MWC 480, we resolve two azimuthal brightness dips near the disk minor axis and find evidence that one of them shifted between 2021 and 2022. In HD 163296, we identify an apparent linear azimuthal motion of a localized peak in polarized intensity along the outer ring over a 15-month baseline. The rapid motion of these features relative to the local Keplerian velocity suggests that the observed variability is driven by changing illumination rather than physical material motion. Due to uncertainties in the underlying scattering background, however, we cannot determine the precise physical origin of the variability. No significant disk variability is detected in HD 143006 over a 10-month baseline. We also report the first detection of a protoplanetary disk using the fast-PDI mode on SCExAO, illustrating both the promise and current limitations of this observing mode. Finally, we report non-detections toward HD 144432, HD 56895, PDS 76, HIP 80425, HD 148352, and HIP 81474. All non-detections with Meeus classifications belong to Group II systems and are likely self-shadowed. For these six systems, we measure the system-integrated polarization fraction and angle of linear polarization, providing quantitative constraints on their unresolved circumstellar environments.

astro-ph.EP↗

arXiv2026

The Outflow of the B335 Protostar II: After the Outburst

The B335 protostar has recently undergone a major, still ongoing, outburst detected in scattered light from its outflow cavity, offering a rare opportunity to study its impact on a protostellar jet. We use JWST/NIRCam photometry of background stars behind B335 from 2.7 to 4.4 $\mu$m to map extinction and H$_2$O ice absorption, showing that the outflow has carved a cavity in the molecular core. We measure proper motions of shock fronts emerging from the protostar of 131--227 km s$^{-1}$. The kinematic age of the most prominent shock front, 3E, corresponds to the early phase of the current outburst. JWST/NIRSpec IFU data show that the youngest shock, 2E, exhibits ionic lines but no molecular emission. Shock 3E shows strong CO emission together with H$_2$ and [\ion{Fe}{2}], whereas older shocks show weaker CO and are dominated by H$_2$ and [\ion{Fe}{2}]. The feature 0E, closest to the protostar, appears to be a stationary shock. CO-line-removed spectra near the protostar show that the unsaturated absorption features of $^{13}$CO$_2$, OCN$^-$, and OCS increase strongly toward the source. The ice properties are otherwise similar to those along lower-extinction sight lines. In the central bipolar reflection nebula, CO gas is seen in scattered emission from the immediate protostellar surroundings, but a few arcsec farther out, absorption by cooler CO gas in the outflow cavity is detected.

astro-ph.SR↗

arXiv2026

The compositions of the HR 8799 planets reflect accretion of both solids and metal-enriched gas

With four giant planets ($m\sim5-10~M_{\rm Jup}$, $T_\rm{eff}\sim900-1200$ K) orbiting between 15-70 au, HR 8799 provides an unparalleled testbed for studying giant planet formation and probing compositional trends across the protoplanetary disk. We present new JWST/NIRSpec IFU observations ($2.85-5.3~\mu$m, $R\approx2700$) that now include the spectrum of HR 8799 b, and higher S/N spectra for HR 8799 c, d, and e compared to that in Ruffio & Xuan et al. We detect CO, CH$_4$, H$_2$O, H$_2$S, CO$_2$, and for planet b, NH$_3$. We combine the NIRSpec spectra with $1-5 \mu$m photometry to perform atmospheric retrievals that account for disequilibrium chemistry and clouds, and allow C/H, O/H, N/H, and S/H to scale independently. While the four planets are similarly enriched in carbon and oxygen, with C/H and O/H between $3-5\times$ stellar, we observe a tentative trend of increasing S/H - a tracer of refractory solids - from $2-5 \times$ stellar with increasing orbital distance. From HR 8799 b's NH$_3$ abundance, we estimate $\rm N/H=21.2^{+16.2}_{-8.8}\times$ stellar, suggesting the outer planet accreted significant amounts of N-rich gas. Overall, the elemental abundance patterns we observe are consistent with a picture where planet b formed between the CO snowline and the more-distant N$_2$ snowline, while the inner planets accreted $3 \times$ stellar CO-enriched disk gas within the CO snowline. The excess volatile mass from pebble drift and evaporation implies an integrated pebble flux of $750 \pm 200~M_{\oplus}$. The increase in the planets' S/H with orbital distance implies more solid accretion further out, which is quantitatively compatible with expectations from both pebble and planetesimal accretion ($2 \times$ Minimum Mass Solar Nebula) paradigms.

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