Ram Kesh Yadav

We present a multiscale kinematic study of the hub–filament system in Mon R2 using James Clerk Maxwell Telescope (JCMT) ¹³ CO(3–2) and Atacama Large Millimeter/submillimeter Array (ALMA) C ¹⁸ O(1–0) observations to investigate how molecular gas converges toward the central hub and connects across cloud-to-core scales. The ¹³ CO data reveal several velocity-coherent filaments with diverse position angles converging toward the central region hosting massive stars and stellar clusters, where distinct velocity components become spatially superposed. Two dominant velocity regimes at ∼8.8–10.2 and ∼10.4–11.8 km s ⁻¹ are identified, with their velocity separation decreasing toward the central hub. The position–position–velocity (PPV) and longitude–velocity maps of the ¹³ CO emission exhibit a butterfly/X-shaped morphology, consistent with converging motions. A phenomenological rotating-and-infalling envelope model reproduces the longitude–velocity morphology, indicating that rotation and infall may contribute to the observed kinematics, although the model does not uniquely identify the underlying mechanism. At higher resolution, the ALMA C ¹⁸ O observations resolve the molecular gas into multiple velocity-coherent filaments spanning ∼8–13 km s ⁻¹ , with distinct kinematic domains at ∼9–10.5, 10.5–11.5, and 12–13 km s ⁻¹ converging toward the hub. The ALMA and JCMT position–velocity (PV) diagrams show consistent velocity structures across their respective spatial scales, with the ALMA-resolved features embedded within the broader molecular gas velocity field traced by JCMT, demonstrating a multiscale kinematic connection. These results support a hierarchical picture in which converging flows and filamentary accretion contribute to hub growth and ongoing star formation in Mon R2, while other dynamical processes may also contribute to the observed kinematics.

Context . Infrared-dark clouds (IRDCs) are ideal sites for investigating the initial conditions of massive star and cluster formation. The A Lei Of the Habitat and Assembly of Infrared Dark Clouds (ALOHA IRDCs), a James Clerk Maxwell Telescope Large Program, has mapped nearby IRDCs utilising the SCUBA-2 continuum camera at the highest achievable sensitivity. Complementary molecular line observations are needed to characterise the physical, kinematic, and chemical properties of the dense gas. Aims . We aim to determine the thermal, kinematic, and chemical properties of clumps identified in the ALOHA IRDCs, and to assess their evolutionary status and level of star-forming activity. Methods . We performed single-pointing K -band and W -band observations towards 56 clumps pf ALOHA IRDCs using the Effelsberg 100 m and Yebes 40 m telescopes, respectively. We derived NH ₃ kinetic temperatures using the hyperfine group ratio method and identified infall and shock signatures from HCO ⁺ , H ¹³ CO ⁺ , SiO, and HNCO profiles. Water masers and NH ₂ D emission were used as complementary tracers of chemical evolution and star formation. Results . The clumps exhibit kinetic temperatures of 15–29 K. We detect NH ₂ D emission towards 18 sources; the NH ₂ D centroid velocities are consistent with those of NH ₃ , indicating the two species trace the same dense gas component. More than half of the clumps display blue-asymmetric HCO ⁺ profiles, identifying them as infall candidates. Water masers with prominent velocity ranges and variability are detected in 22 sources. Broad SiO emission (≳20 km s ⁻¹ ) indicates strong shocks, while narrower emission (≲6 km s ⁻¹ ) likely traces large-scale interactions or low-velocity shocks. Conclusions . The widespread infall signatures, shock tracers, masers, and NH ₂ D emission suggest that relatively quiescent, chemically young material can coexist with dynamically active gas affected by early protostellar feedback, providing insight into the coupled physical and chemical evolution of massive IRDC clumps.

Context. The unstable mass accretion process in young stellar objects (YSOs) often triggers observable outbursts. These episodic accretion events play a critical role in stellar mass assembly during the pre-main-sequence phase. In this paper, we present observations of an eruptive young star in the Rosette Nebula, identified by the Gaia Science Alerts system using Gaia time-series data. Aims. We aim to investigate the evolution of the brightness and mass accretion rate of V557 Mon throughout its outburst and subsequent decline. In addition, we trace the evolution of the inner accretion disk during the outburst by monitoring molecular emission features. Methods. We compiled multi-band photometric time-series from Gaia, ZTF, and several 1 m-class ground-based telescopes and obtained optical and near-infrared spectra at multiple epochs covering the outburst and fading phases. Stellar parameters were derived from quiescent colour, spectra and spectral energy distribution (SED) fitting. We also measured the mass accretion rate and fit models to molecular emission bands. Results. Since late 2024, V557 Mon has undergone a year-long outburst consistent with EXor variability. Based on quiescent photometry, V557 Mon has a spectral type of M1 with an extinction of A _V = 1.8 ± 0.3 mag, consistent with a 0.4–0.5 M _⊙ star at an age of 2 Myr. Our multi-epoch spectra and u -band photometry indicate a peak accretion rate of 6.3 × 10 ⁻⁷ M _⊙ yr ⁻¹ during the outburst, roughly 70 times higher than in quiescence. We report the detection of hot water vapour emission bands, together with TiO, VO, and CO emission features. Using ExoMol models, we measured that the inner-disk temperature changed from 3000 K to 2000 K during the fading phase of the outburst. Conclusions. We report a recent EXor outburst in a low-mass Class II YSO. Our observations reveal the transient formation of a hot molecular inner disk, traced by variable water vapour emission during the EXor event. A positive correlation is found between the molecular excitation temperature and the overall stellar brightness.

To investigate the role of magnetic fields toward the G35N and G35S subregions in the G35.20–0.74 star-forming complex, we utilized multiwavelength polarimetric observations from the SOFIA/HAWC+ at 154 μ m and Atacama Cosmology Telescope (ACT) at 220 GHz/1.3 mm. The ACT 220 GHz polarization data (resolution ∼1 ′ ) show an hourglass-shaped plane-of-sky magnetic field morphologies toward both the subregions, though with distinct symmetry axes. SOFIA/HAWC+ 154 μ m data (resolution ∼13 . ″ 6) confirm an hourglass morphology in G35N, whereas G35S displays a different magnetic field configuration compared to the ACT observations. An hourglass morphology identified at clump scales (∼a parsec) toward G35N is consistent with the previously reported B -field morphology at core scales (∼0.05 pc), supporting the scenario of a magnetically regulated collapse. Using the SOFIA/HAWC+ data, we estimate magnetic field strengths of ∼600 ± 200 μ G in G35N and ∼850 ± 310 μ G in G35S. Energy balance analysis suggests that gravity and magnetic fields contribute comparably in G35N, while in G35S the gas dynamics are dominated by magnetic field, followed by gravity and turbulence. The higher field strength in G35S likely results from compression by the expanding H ii region, highlighting the impact of stellar feedback. The derived magnetic field strengths and corresponding magnetic energies should be treated as upper limits due to unresolved beam-scale correlations and the limited fitting range of the polarization angle structure function. Overall, our results show that magnetic fields decisively regulate star formation, with G35N shaped by magnetically controlled collapse and G35S being strongly influenced by stellar feedback.

We present the SOFIA/HAWC+ 214 μm polarimetric observations toward the infrared dark cloud G351.77-0.53 (hereafter G351), complemented by existing multi-wavelength data sets. Infrared excess from the embedded sources indicate ongoing star formation activity in the cloud. The G351 cloud hosts two prominent star-forming clumps, i.e., c1 and c2. The plane-of-the-sky magnetic field lines from Planck observations are predominantly oriented perpendicular to the filament’s major axis. Magnetic field orientations from SOFIA/HAWC+ 214 μm observations reveal distinct hourglass-shaped field configuration toward c1, while the field lines remain perpendicular to the rest of the filament. Using the Davis-Chandrasekhar-Fermi method, we estimate a mean plane-of-the-sky magnetic field strength of ~147 ± 60 μG in the G351 filament, with values reaching ~0.8 mG toward c1. The mass-to-flux ratio analysis indicates that the filament is magnetically transcritical, where the gravitational and magnetic field energies are comparable. The hourglass-shaped magnetic field observed toward c1 could result from magnetically regulated gravitational collapse, the alignment of converging sub-filaments with the magnetic field, or a combination of both processes. The energy budget analysis further indicates that magnetic fields play an important role in governing the cloud’s gas dynamics, followed by contributions from turbulence and gravity.

Accretion-driven outbursts in young stellar objects remain poorly understood, largely limited by a statistically small sample of closely followed-up events. This underscores the importance of a thorough exploration of each outbursting object. We studied a peculiar outbursting system, Gaia24ccy, which exhibited two Δ g ∼ 3.8 mag outbursts in 2019 and 2024. The system consists of two unresolved, nearly identical, and rapidly rotating young stars: Gaia24ccy A (1.1419 days) and Gaia24ccy B (1.7898 days). Periodogram analyses just before the onset of the outbursts suggest Gaia24ccy B to be the outbursting component. Unlike any previously known EXor sources, the two outburst profiles show very similar evolution: both rose at the same rate for the first 15 days, followed by multiple “subbursts” on timescales of 10−20 days. The 2019 outburst lasted 145–255 days, while the 2024 outburst persisted for 367 days. We infer the unstable region to lie at r _trigger ≃ 0.019–0.047 au (∼5–12.3 R _⋆ ). The accreted mass per event, M _acc ∼ 10 ⁻⁵ M _⊙ , can be provided by a compact inner-disk reservoir. The photometric rise and decay timescales, together with the mid-infrared (MIR) color evolution, favor a thermal–viscous trigger in a hot inner disk, while the presence of rich emission-line spectra indicates concurrent magnetospheric compression—together forming a hybrid picture. Finally, we explain the reddening of the MIR color observed during the outburst as a consequence of the competing emission from the viscous disk and the photosphere.

We present a detailed analysis of the long-term photometric and spectroscopic evolution of V1180 Cas over a decade, aiming to identify the dominant mechanisms behind its variability. We combine multiband light curves from 1999 to 2025 with over 30 epochs of optical to near-infrared (NIR) spectroscopy (0.5–2.5 μ m), analyzing variability patterns, color behavior, and emission line diagnostics. We investigate the temporal evolution of accretion and outflow indicators and their correlation with photometric states. The light curve reveals a transition from sporadic early dimming events to a quasi-periodic pattern since 2018, with 11 major dips showing asymmetry and stochastic substructure. Color–magnitude diagrams show classic UXor-like blueing during deep minima, while NIR and mid-infrared color changes indicate thermal evolution of the disk. Spectroscopic analysis reveals persistent hydrogen, Ca II , He I , and forbidden line emission. Accretion diagnostics track photometric variability, and forbidden lines often intensify during dips, implying a physical link between extinction and outflows. Estimated accretion rates range from ∼10 ⁻⁸ to 10 ⁻⁷ M _⊙ yr ⁻¹ ; the outflow rate and density diagnostics are consistent with disk winds and shock-excited jets. V1180 Cas demonstrates dual-mode variability driven by both variable circumstellar extinction and episodic accretion events. The hybrid UXor/EXor behavior, combined with evolving disk signatures and persistent outflows, suggests a young stellar object undergoing coupled accretion–extinction–outflow evolution. Continued monitoring will be essential to fully resolve the physical processes shaping its variability.

We report the discovery of 45 compact hub-filament systems (HFSs; median size ∼2.4 pc) in infrared-dark clouds (IRDCs) in the W33 complex, located at the junction of the Scutum and Norma spiral arms. Using Spitzer 8 and 24 µ m, and unWISE 12 μ m images, HFSs are identified as regions where three or more filaments converge onto a central hub, appearing as absorption features toward IRDCs. In each IRDC, HFSs mainly lie at the intersections of elongated substructures, associated with groups of protostars and lacking radio continuum emission. Minimum Spanning Tree (MST) analysis shows that protostars are closely associated with the HFSs, with protostellar core separations of ≤0.7 pc, indicating strong clustering within fragmented structures. The HFSs form two main groupings spanning 10–15 pc, with member separations of 1–3.3 pc. Around 65% are tightly clustered (5 cm ⁻³ ), whereas the core spacing is smaller than the Jeans length, suggesting that thermal fragmentation may influence core formation but alone cannot explain the larger-scale protostellar distribution. All these findings together support a picture in which fragments of clouds/filaments form clumps hosting compact HFSs that facilitate efficient and clustered star formation, often yielding massive stars.