arXiv subjects

Naman S. Bajaj

Publications and source records attributed to Naman S. Bajaj.

8 recordsLinked to original sources

arXiv2026

JWST/MIRI Reveals the Evolution from Molecular to Atomic Disk Winds

The evolution and dispersal of protoplanetary disks--governed by accretion, magnetically launched jets and winds, and photoevaporative winds--fundamentally shape planetary systems. Determining how these mass-loss processes co-evolve is crucial for constraining planet formation pathways. We analyze archival JWST/MIRI/IFU data of 72 inclined (i>40deg) mostly ClassII disks to identify and characterize spatially resolved jets and winds, focusing on [NeII] and H2 lines. Extended emission in H2 S(1), S(3), S(5), S(7) and/or [NeII] is detected toward 66 disks, revealing diverse morphologies. We develop a framework to identify conical H2 winds and high-velocity [NeII] jets perpendicular to the disk, detecting them toward 46 and 40 disks, respectively. All sources with [NeII] jets exhibit a corresponding wind traced in either H2 (85%) or [OI], establishing a connection between jets and winds. The detection fractions of [NeII]-jets and H2-winds correlate positively with mass accretion rate, with no dependence on disk inclination or stellar mass. Conversely, marginally resolved low-velocity [NeII] winds are found preferentially toward lower accretors. Among sources with H2 winds, detection of hotter winds traced by S(7) and S(5) declines more rapidly with decreasing accretion rate than the colder S(1) component. Comparison with high-resolution [OI]6300\textÅ spectroscopy reveals [OI] LVC and extended H2 wind detections preferentially toward moderate-to-high accretors (>~10^{-8.5}~Msun/yr), whereas lower accretors exhibit only [OI] and [NeII] winds. Together, these results indicate that atomic jets and atomic+molecular winds, consistent with an MHD disk-wind origin, dominate during early, actively accreting disk phases, while at lower accretion rates, jets weaken and winds become predominantly atomic.

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arXiv2026

Chemical Divergence and Water Depletion: Gas Properties of Evolved Upper Scorpius Disks Revealed by JWST/MIRI

Tracing the chemical evolution of protoplanetary disks over time requires observations of disks at different ages. However, most JWST/MIRI surveys published to date have targeted younger ($\sim$1-3 Myr) rather than older systems. We present the results of a JWST/MIRI MRS survey of the inner regions of 10 protoplanetary disks (ages $\sim$2-6 Myr, spectral types M0-M4.5) in the Upper Scorpius region previously characterized by the ALMA AGE-PRO large program. Using MCMC slab modeling, we fit to a wide variety of detected molecules, including H$_2$O, CO, C$_2$H$_2$, $^{13}$CCH$_2$, HCN, HC$_3$N, CO$_2$, $^{13}$CO$_2$, C$_2$H$_6$, C$_4$H$_2$, and OH, as well as C$_6$H$_6$, CH$_3$, and H$_2$ visually. We classify each disk along two independent axes-a Water Classification based on H$_2$O line luminosity (Water-Rich, Water-Poor, or Water-Absent) and a Chemotype based on the dominant non-water chemistry (Organic-Rich, CO$_2$-Dominated, or Molecule-Absent)-and find an unexpectedly high diversity of distinct chemical compositions within our population. We leverage the heterogeneity of detected molecules in our sample to present new characteristic "diagnostic" wavelength regions for most species. We find that carbon-based molecules consistently exhibit markedly lower excitation temperatures ($\lesssim$300 K) compared to younger ($\sim$1-3 Myr) star-forming regions ($\sim$600-1000 K), hinting at relatively colder molecular reservoirs. We also determine that Upper Scorpius disks show systematically lower water luminosities by factors of 10-1000. In particular, disks with strong carbon-based molecular features but no observed H$_2$O defy expectations of an inner-disk dust cavity or a low ($\lesssim3$) $R_{\rm gas}/R_{\rm dust}$ ratio, instead suggesting that the presence of a strong outer-disk dust trap largely controls the chemical outcome of the terrestrial planet-forming region.

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arXiv2026

Molecular Similarity and Water Diversity in Coeval Binary Disks: JWST/MIRI Observations of Sz 65 and Sz 66

We present JWST/MIRI Medium Resolution Spectrometer spectra of the wide-separation (projected separation $= 980$ au) binary protoplanetary disks Sz 65 (K7; $0.68~M_{\odot}$) and Sz 66 (M3; $0.30~M_{\odot}$), reduced using the uniform pipeline of the JWST Disk Infrared Spectral Chemistry Survey. Both disks show rich molecular emission, including H$_2$O, CO$_2$, HCN, C$_2$H$_2$, and OH. The scaled spectra of the two disks exhibit remarkably similar H$_2$O, CO$_2$, and HCN line emission in the 13--18 $μ$m region, with the only notable difference being stronger C$_2$H$_2$ emission in the primary (Sz 65). Beyond 18 $μ$m, the difference in H$_2$O line emission between the two disks increases. Both the flux ratios and the slab-model-derived mass ratios of cold to hot H$_2$O ($\sim$200 K to $\sim$750 K) and warm to hot H$_2$O ($\sim$450 K to $\sim$750 K) are significantly higher in the secondary (Sz 66). Because binary stars share nearly the same age and metallicity, and as both disks appear compact in millimeter emission ($<30$ au), we suggest that the excess cold H$_2$O in the secondary is best explained by its unstructured dust disk, in contrast to the primary, which shows gaps at 6 and 20 au. The enhanced cold water in the secondary is consistent with efficient pebble drift across the water snow line and increased H$_2$O vapor from the sublimation of icy mantles. Our results demonstrate that wide-separation binaries can serve as powerful control samples for isolating the impact of individual disk properties on inner-disk chemistry and evolution.

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arXiv2025

Class I/II Jets with JWST: Mass loss rates, Asymmetries, and Binary induced Wigglings

We present JWST NIRSpec spectro-imaging observations of jets from four edge-on protoplanetary disks that exhibit clear signatures of MHD disk winds. Bipolar jets are detected and spatially resolved in over 30 shock-excited forbidden lines, multiple Paschen and Brackett series lines of atomic hydrogen, and the high-energy excitation line of atomic helium (1.083 um). This helium line is the brightest jet-tracer towards HH 30 and FS TauB, which also exhibit asymmetric intensity between their red- and blue-shifted lobes in all tracers, including the [Fe II] and [He I] lines. Extinction maps reveal no significant differences across the lobes, suggesting an asymmetric jet-launching mechanism rather than environmental effects. Diagnostic line ratios yield consistent shock speeds of 50-60 km/s, jet ionization fractions of 0.1-0.2, and pre-shock electron densities of 1000 /cm^3. Combined with pixel-by-pixel electron density maps and [Fe II] line luminosities, we estimate jet mass-loss rates using three independent methods, averaging around a few 10^(-9) solar masses/yr. We estimate the accretion rates for these sources as 10 times the jet mass loss rates and find them to match well with the independently derived accretion estimates of other Class II sources in the Taurus star-forming region. Owing to JWST's high precision, we also investigate jet wiggling and find Tau 042021 to showcase the perfect case of mirror-symmetric wiggling, which can only be explained by the motion of the jet source around a stellar companion. Modeling this wiggling suggests Tau 042021 to host 0.33 and 0.07 solar masses binary at the center with binary separation of 1.35 au and an orbital period of 2.5 years.

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arXiv2024

JWST captures a sudden stellar outburst and inner disk wall destruction

We present JWST/MIRI observations of T~Cha, a highly variable ($ΔV \sim$3-5\,mag) accreting Sun-like star surrounded by a disk with a large ($\sim 15$\,au) dust gap. We find that the JWST mid-infrared spectrum is signiticantly different from the {\it Spitzer} spectrum obtained 17 years before, where the emission at short wavelengths ($5-10 μm$) has decreased by $\sim 2/3$ while at longer wavelengths ($15-25 μm$) it increased by up to a factor of $\sim 3$. This 'seesaw' behavior is contemporary with a fairly constant higher optical emission captured by the All Sky Automated Survey. By analyzing and modelling both SEDs, we propose that JWST caught the star during an outburst that destructed the asymmetric inner disk wall responsible for the high optical variability and lower $15-25$\,micron\ emission during the {\it Spitzer} time. The dust mass lost during this outburst is estimated to be comparable ($\sim 1/5$) to the upper limit of the total micron-sized dust mass in the inner disk of T~Cha now. Monitoring this system during possible future outbursts and more observations of its quiescent state will reveal if the inner disk can be replenished or will continue to be depleted and vanish.

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arXiv2024

JWST/NIRSpec Reveals the Nested Morphology of Disk Winds from Young Stars

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

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arXiv2024

Modeling JWST MIRI-MRS Observations of T Cha: Mid-IR Noble Gas Emission Tracing a Dense Disk Wind

[Ne II] 12.81 $μ\mathrm{m}$ emission is a well-used tracer of protoplanetary disk winds due to its blueshifted line profile. MIRI-MRS recently observed T Cha, detecting this line along with lines of [Ne III], [Ar II] and [Ar III], with the [Ne II] and [Ne III] lines found to be extended while the [Ar II] was not. In this complementary work, we use these lines to address long-debated questions about protoplanetary disk winds regarding their mass-loss rate, the origin of their ionization, and the role of magnetically-driven winds as opposed to photoevaporation. To this end, we perform photoionization radiative transfer on simple hydrodynamic wind models to map the line emission. We compare the integrated model luminosities to those observed with MIRI-MRS to identify which models most closely reproduce the data and produce synthetic images from these to understand what information is captured by measurements of the line extents. Along with the low degree of ionization implied by the line ratios, the relative compactness of [Ar II] compared to [Ne II] is particularly constraining. This requires Ne II production by hard X-rays and Ar II production by soft X-rays (and/or EUV) in an extended ($\gtrsim 10$ au) wind that is shielded from soft X-rays - necessitating a dense wind with material launched on scales down to ~1 au. Such conditions could be produced by photoevaporation, whereas an extended MHD wind producing equal shielding would likely underpredict the line fluxes. However, a tenuous inner MHD wind may still contribute to shielding the extended wind. This picture is consistent with constraints from spectrally-resolved line profiles.

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arXiv2024

JWST MIRI/MRS Observations of T Cha: Discovery of a Spatially Resolved Disk Wind

Understanding when and how circumstellar disks disperse is crucial to constrain planet formation and migration. Thermal winds powered by high-energy stellar photons have long been theorized to drive disk dispersal. However, evidence for these winds is currently based only on small (~3-6 km/s) blue-shifts in [Ne II] 12.81 um lines, which does not exclude MHD winds. We report JWST MIRI MRS spectro-imaging of T Cha, a disk with a large dust gap (~30 au in radius) and blue-shifted [Ne II] emission. We detect four forbidden noble gas lines, [Ar II], [Ar III], [Ne II], and [Ne III], of which [Ar III] is the first detection in any protoplanetary disk. We use line flux ratios to constrain the energy of the ionizing photons and find that Argon is ionized by EUV whereas Neon is most likely ionized by X-rays. After performing continuum and Point Spread Function (PSF) subtraction on the IFU cube, we discover a spatial extension in the [Ne II] emission off the disk continuum emission. This is the first spatially resolved [Ne II] disk wind emission. The mostly ionic spectrum of T Cha, in combination with the extended [Ne II] emission, points to an evolved stage for any inner MHD wind and is consistent with the existence of an outer thermal wind ionized and driven by high-energy stellar photons. This work acts as a pathfinder for future observations aiming at investigating disk dispersal using JWST.

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