Yuri Aikawa

The physical and chemical conditions of protoplanetary disks shape the properties of nascent planetary systems. The line ratios and relative abundances of the hydrogen cyanide (HCN)–hydrogen isocyanide (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 ≈0.1–0.7 and 0.1–0.4, respectively. For all sources, measured HNC fluxes exceed predictions from models of full (nontransition) disks by ≈3–10×, while HCN fluxes appear typical. The relative brightness of HNC versus 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.

The snowlines of major volatiles in protoplanetary disks play a pivotal role in dust evolutions and volatile delivery to nascent planetary systems. In this paper, we report the Atacama Large Millimeter/submillimeter Array Band 1 (≈7.5 mm) observations of methanol (CH ₃ OH) emission lines in the disk around the FU-Ori type star V883 Ori, where accretion outburst heats the disk and the majority of ices have sublimated. We detect three CH ₃ OH emission lines at an angular resolution of ≈ 0 . ″ 2 . The stacked CH ₃ OH image exhibits a centrally-peaked morphology in contrast to the previous (sub-)mm observations that show a central depression. By fitting radially-resolved line profiles, we derive the radial intensity profile of the CH ₃ OH emission where we find a steep increase at ≲40 au. The column density of CH ₃ OH reaches at least ∼10 ¹⁹ –10 ²⁰ cm ⁻² at ∼10 au. This provides direct evidence that a significant amount of warm gaseous methanol is present in the innermost region of the disk where its emission has been suppressed in previous (sub-)mm observations due to the optically thick dust emission. The steep increase in the intensity profile may indicate that the CH ₃ OH snowline in the midplane is located at ∼30–55 au, or that the CH ₃ OH emission traces the temperature structure given that the emission is likely optically thick. Our results demonstrate the capability and significance of (sub-)cm observations in probing the innermost opaque region of disks, paving the way for the future observations with upcoming facilities.

Aims . To advance our understanding of dust properties in class 0/I young stellar objects, it is essential to resolve and characterize their structures at multiple wavelengths and investigate how both grain growth and environmental factors shape their spectral properties. Methods . We present 0.″5 angular resolution ALMA observations of the GSS 30 complex at 1.2–3.0 mm, collected as part of the FAUST large programme, with a linear resolution of 69 au. We analysed the dust continuum emission and performed model fitting to constrain the dust mass and the disk structure. Results . For IRS3, the spectral index increases radially from 2.0 at the centre to 2.5 at the disk edge. In contrast, it decreases from 2.0 to as low as 1.6–1.8 along the outflow direction. An asymmetric distribution shows α as low as 1.6 to the northeastern blueshifted lobe, possibly due to cold outer envelope dust obscuring warmer inner regions. Spectral energy distribution fitting indicates maximum dust grain sizes of tens of microns and a dust mass of 650–1510 M _⊕ . IRS1 is connected with an extended structure to the north-east. It may be an outflow–disk complex or a trailing structure associated with a circumbinary disk, although we cannot rule out the possibility that it could also be a gas streamer accreting onto IRS1. At the centre of IRS1, the spectral index is < 0.8, which is consistent with marginally optically thick free-free emission. IRS2 exhibits brightness variations over 420 seconds of observation, with a multi-epoch comparison suggesting a flare lasting tens of minutes, indicating magnetic activity in the central star. Conclusions . This study presents a detailed millimetre multi-wavelength analysis of the GSS 30 IRS3 and IRS1 systems, highlighting the crucial role of environmental factors – specifically dust obscuration and the connecting streamer structures – in shaping the observed morphology of these early-stage disks. We also find that the IRS2 exhibits millimetre variability, which may originate from magnetic flares at the surface of the protostar.

Ammonia (NH ₃ ) is one of the key volatiles that plays a central role in nitrogen chemistry and its evolution during the epoch of star and planet formation. We present subarcsecond ( ∼ 0 . ″ 5 ) resolution observations of NH ₃ molecular emission lines with the Karl G. Jansky Very Large Array (VLA) toward the Class 0 multiple system IRAS 16293−2422 including source A and source B as major components. This comprises the most comprehensive set of NH ₃ line observations in protostellar sources to date, which includes 17 inversion transitions with a wide range of upper state energies ( E _u ) spanning from ∼23 K to ∼1580 K. We detect spatially resolved emission of a number of transitions and find that the high- E _u (≳1000 K) lines show compact distributions in the vicinity of protostars while low- E _u (≲150 K) lines exhibit more extended emission. Utilizing a two-component model, we constrain the rotation temperature and NH ₃ column density for both source A and source B. The rotation temperature of the warmer component reaches ∼200–300 K, indicating that the high- E _u lines selectively trace the inner hot region. We suggest that this hot NH ₃ gas in source A originated from the local shock heating based on the comparison with the previous high-resolution ALMA observations, while that in source B could be explained by the mass accretion heating in the innermost hot region. We also briefly discuss the chemistry related to NH ₃ based on the abundance ratios relative to major icy molecules derived using literature values.

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 (∼2.3 μ m) and fundamental (∼4.6 μ m) bands and the H ₂ O stretching (∼2.7 μ m) and bending (∼6.0 μ m) modes. We also obtained high-resolution ( R ≈ 45,000) IGRINS spectra during the burst to constrain the CO overtone line profiles. Local thermodynamic equilibrium (LTE) slab modeling yields gas temperatures of ∼1800 K (CO overtone) and ∼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 ∼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 ± 0.2 for the CO overtone and 1.71 ± 0.11 for the CO fundamental. Using a viscous-disk prescription, these imply representative accretion-rate ratios of ∼3.6 and ∼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.

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 HC ₅ N toward the TW Hya protoplanetary disk, representing the largest cyanopolyyne identified to date in a Class II system. We derive a HC ₅ N 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 HC ₅ N and HC ₃ N formation mechanisms and analyze the HC ₃ N/HC ₅ N ratio. We use a chemical model to estimate the expected abundance and emitting layer of HC ₅ N in a TW Hya–like disk. Although HC ₅ N emission is spatially unresolved and 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.

We investigate how misalignment between the core angular momentum and the large-scale magnetic field affects protostellar outflows, and whether a single protostellar system can drive multiple outflow components. We perform three-dimensional nonideal magnetohydrodynamic simulations of magnetized rotating cores, focusing on the formation of a protostar, a circumstellar disk, and magnetically driven outflows. The initial angle between the core angular-momentum vector and the magnetic field is systematically varied from 0 ^∘ to 90 ^∘ . All models launch a classical magnetocentrifugal disk wind (DW) roughly along the local disk normal. For large misalignment, the system also develops a spiralflow (SF) component that propagates parallel to the disk plane. In a representative model with a 60° misalignment, the outflow transitions from a DW-dominated to an SF-dominated phase, with the SF becoming more massive and more extended than the DW, and the two components intermittently coexisting. Across the model suite, the maximum mass and size ratios of SF to DW, as well as the relative lifetimes of the two components, increase for misalignment angles ≳60°. We propose that secondary, misaligned outflows (or their fossil remnants) observed in some protostellar systems can be interpreted as the SF component, while the main bipolar outflow traces the DW from the same misaligned system.

The sulfur chemistry in protoplanetary disks directly affects the composition and potential habitability of nascent planets, but its volatile inventory remains highly uncertain. Here, we present deep Atacama Large Millimeter/submillimeter Array observations of hydrogen sulfide (H ₂ S) along with SO and SO ₂ in the disk around HD 163296 at an angular resolution of ≈ 0 . ″ 3 (or ≈30 au). We detect unresolved, compact emission of H ₂ S and SO (and tentatively SO ₂ ) at the disk center with a broad line width of ∼40 km s ⁻¹ , suggesting that the emission is originating from the innermost regions. By fitting line profiles with a geometrically thin Keplerian-rotating disk model, we constrain the emitting radii and gas temperatures of these molecules to be ≈3–5 au and ≳90–120 K, respectively, consistent with sublimation of sulfur-bearing molecules along with water ice in the inner warm region. While the higher or comparable column density of H ₂ S with respect to SO and SO ₂ indicates that H ₂ S is an important volatile sulfur reservoir in the disk, the limited constraints mean that we cannot rule out significantly depleted volatile sulfur as also commonly inferred in other planet-forming disks. Further observations are needed to better constrain the disk’s sulfur inventory, unravel how sulfur compounds are reprocessed in disks, and shed light on the nature of less-volatile species, such as salts and sulfide minerals, which may occupy a significant portion of the sulfur budget.

We present the 1.6–28 μ m spectra of the young protostar EC 53, obtained with the James Webb Space Telescope (JWST) Near Infrared Spectograph Integral Field Unit and MIRI Medium Resolution Spectroscopy 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 μ m and 6.85 μ 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 for 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.

We present deep, high-resolution (∼100 mas) Karl G. Jansky Very Large Array Ka -band (9.1 mm) observations of the disk around MWC 480 and infer dust properties through a combined analysis with archival Atacama Large Millimeter/submillimeter Array data at 0.87, 1.17, 1.33, and 3.0 mm. The prominent dust ring at 95 au (B95) is detected at 9.1 mm for the first time, while the faint outer ring at 160 au is not revealed. Through nonparametric visibility modeling, we identified two new annular features: a plateau within 20–50 au across all wavelengths, and a shoulder exterior to the B95 ring at 0.87, 1.17, and 1.33 mm, consistent with signatures of planet–disk interaction. We find that the width of the B95 ring remains constant across wavelengths, suggesting that fragmentation dominates over radial diffusion or that unresolved substructure is present within the ring. Resolved spectral modeling yields two families of dust solutions that reproduce the observations equally well: compact grains or highly porous (90%) grains, with carbonaceous components dominated by refractory organics or amorphous carbon, respectively. The inferred maximum grain sizes peak at the locations of the two rings and reach centimeter within the B95 ring. The total dust masses are 86 0 − 78 + 95 M ⊕ / 150 0 − 330 + 440 M ⊕ (large/small-grain solution in inner disk) and 23 0 − 13 + 14 M ⊕ for the two dust mixtures. The B95 ring alone contains 10 0 − 5 + 5 M ⊕ and 4 3 − 2 + 2 M ⊕ , respectively, sufficient to assemble the cores of giant planets. Finally, we highlight the power of broadband, multiwavelength observations in placing better constraints on dust composition and porosity in protoplanetary disks.