IIA ARVIND K DATTATREYUnveiling dynamics and variability in open clusters: insights from a comprehensive analysis of six galactic clusters
We present a kinematic and dynamical analysis of six Galactic open clusters – NGC 2204, NGC 2660, NGC 2262, Czernik 32, Pismis 18, and NGC 2437, using Gaia DR3. We used Bayesian and Gaussian Mixture Model (GMM) methods to identify cluster members, but chose GMM because it is more appropriate for low-massstars. Estimated distancesrange from 1.76 to 4.20 kpc and ages from 0.199 to 1.95 Gyr, confirming their intermediate-age nature. King model fits indicate compact morphologies, with core radii of 1–10 arcmin and cluster radii of 5–24 arcmin. We identify 13 blue straggler stars and 3 yellow straggler stars members, whose central concentrations suggest origins via masstransfer orstellar collisions. The massfunction slopes(0.96–1.19) are flatter than the Salpeter value, which indicates that these clusters have undergone dynamical mass segregation. Orbit integration within a Galactic potential indicates nearly circular orbits (eccentricities 0.02–0.10), vertical excursions within ±132 pc, and guiding radii near the solar circle, suggesting disc confinement. These clusters likely formed in the thin disc and are shaped by Galactic tidal perturbations, facilitating the rapid loss of low-mass members. Additionally, 12 variable stars were found across 4 clusters using Transiting Exoplanet Survey Satellite (TESS) light curves, including γ Doradus and SPB pulsators, eclipsing binaries, and a yellow straggler candidate. Periods were derived via Lomb–Scargle analysis. Two eclipsing binaries (TIC 94229743 and TIC 318170024) were modelled using PHOEBE, yielding mass ratios of 1.37 and 2.16, respectively. Our findings demonstrate that integrating orbital dynamics and variable star studies presents valuable insights into the evolutionary pathways of open clusters
IIA SHANTIKUMAR, N. SDependence of water vapor radiative forcing on aerosol loading: Analyses over the densely populated and highly polluted Indo-Gangetic Plain region
This study analyzes the aerosol and precipitable water vapor (PWV) properties at six sites in the Indo-Gangetic Plains (IGP), a densely populated and highly polluted region. The main objective is to explore how the columnar PWV is related to the attenuation of shortwave solar radiation (SWR), as well as the combined role of aerosol properties and PWV on radiative forcing based on AERONET data and model (SBDART) simulations. The analysis revealed high aerosol optical depth (AOD) values (0.4–0.6) throughout the year in all the sites, associated with increased PWV (4–5 cm) during the summer monsoon. Comprehensive investigation shows that changes in PWV levels also affect aerosols’ size distribution, optical properties and radiation balance in a similar way - but in different magnitudes - between the examined sites. The water vapor radiative effect (WVRE) is highly dependent on aerosol presence, with its magnitude for both surface (− 130 to − 140 Wm− 2) and atmospheric forcing becoming higher under clean atmospheres (without aerosols). Aerosol presence is also considered in the computations of the WVRE. In that case, the WVRE becomes more pronounced at the top of the atmosphere (TOA) (30 to 35 Wm− 2) but exhibits a lower forcing impact on the surface (about − 45 Wm− 2) and within the atmosphere (70–80 Wm− 2), suggesting important aerosol-PWV interrelations. The atmospheric heating rate due to PWV is more than double (3.5–4.5 K Day− 1) that of aerosols (1–1.9 K Day− 1), highlighting its essential role in radiative effects and climate implications over the IGP region. The radiative impacts of PWV and aerosols are further examined as a function of the single scattering albedo, solar zenith angle, and absorbing AOD at the different sites, revealing dependence on both astronomical and atmospheric variables related to aerosol absorption, thus unravelling the combined role of aerosols and PWV in climate implications.
IIA SANTANU MONDALDistinct Fe K Line Complexes in MAXI J1744−294 Revealed by XRISM High-resolution Spectroscopy
The newly discovered Galactic transient MAXI J1744−294 went into its first X-ray outburst in 2025. We study the spectral properties of this source in the 2–10 keV energy band during this outburst using X-ray data from the XRISM satellite for both of its Resolve and Xtend instruments, taken on 2025 March 3. High-resolution spectroscopy has revealed, for the first time, complex iron line features in this source, corresponding to distinct components of Fe XXV emission and Fe XXVI absorption lines. Such a detailed structure has not been reported in other low-mass X-ray binaries to date, prior to the XRISM era. Our analysis shows that the line complexes arise from two highly ionized plasmas with an ionization rate ∼103 erg cm s −1 with distinct turbulent velocities—one broad (vturb ≈ 2513 km s −1) from hot gas at the inner accretion disk and one narrow (vturb ≈ 153 km s−1) scattered by nearby photoionized gas. These results offer new insight into the reprocessing of continuum in stratified media, either in the accretion disk or winds, or both, for X-ray binaries in the soft state. The data are well described by models with spin, mass of the black hole, and accretion disk inclination 0.63─0.70, 7.9 ± 2.2 M⊙, and 19°─24°. The fitted spectral model parameters suggest that the source is in the soft spectral state. The source is situated in a crowded field near the Galactic center, resulting in a large hydrogen column density.
IIA VIPIN KUMAR SHARMADoes DESI DR2 Challenge ΛCDM Paradigm?
Although the debate about the systematic errors of DESI DR1 is still open, recent DESI DR2 is consistent with DESI DR1 and further strengthens the results of DESI DR1. In our analysis, both the LRG1 point at zeff = 0.510 and the LRG3+ELG1 point at zeff = 0.934 are in tension with the ΛCDM-anchored value of Ωm inferred from Planck and the Type Ia supernovae compilations Pantheon+, Union3, and DES-SN5YR. For luminous red galaxy 1 (LRG1) the tensions are 2.42σ, 1.91σ, 2.19σ, and 2.99σ, respectively; for LRG3+emission line galaxy 1 (ELG1) they are 2.60σ, 2.24σ, 2.51σ, and 2.96σ, respectively. From low to high redshift bins, DESI DR2 shows improved consistency relative to DESI DR1: the Ωm tension decreases from 2.20σ to 1.84σ. However, DESI DR2 alone does not provide decisive evidence against the ΛCDM model, and the apparent signal is largely driven by specific tracers, LRG1 and LRG2. In the ω0ωaCDM analysis, including all tracers yields a posterior mean with ω0 > −1, which aligns with scenarios of dynamical dark energy as a potential explanation and suggests that the DESI DR2 challenges the ΛCDM paradigm. While removing LRG1 and/or LRG2 fully restores ΛCDM concordance (i.e., ω0 → −1), we also find ω0(LRG1)>ω0(LRG2) , indicating LRG1 drives the apparent dynamical dark energy trend more strongly. Model selection using the natural log Bayes factor lnBF≡ln(ZΛCDM/Zω0ωaCDM) shows weak evidence for ΛCDM when LRG1, LRG2, or both are removed, and it is inconclusive for the full sample; thus, the data do not require the extra ωa freedom, and the apparent ω0 > −1 preference should be interpreted cautiously as a manifestation of the ω0─ωa degeneracy under limited per tracer information.
IIA ADITYA, KEvolution of Low Surface Brightness Ultrathin Galaxies: The Role of Dark Matter Halo and Bar Formation on Disk Thickness
We investigate how stellar disks sustain their ultrathin structure throughout their evolution. We follow the evolution of ultrathin stellar disks with varying dark matter (DM) halo concentration (c) using collisionless N-body simulations with AREPO. We test models embedded in steep (c = 12), shallow (c = 2), and intermediate (c = 6) DM concentrations. Our models match the observed structural properties of the stellar disk in the low surface brightness (LSB) ultrathin galaxy FGC 2366, specifically its surface brightness, disk scalelength, and vertical thinness (hz/RD = 0.1), while excluding gas, allowing us to isolate the effects of DM. The internal disk heating mechanism driven by bars is suppressed in the LSB ultrathin stellar disks regardless of the DM concentration. The ratio of disk thickness (hz) to scalelength (RD) remains constant at ≤0.1 throughout their evolution. To clearly establish that the LSB nature of stellar disks is the key to preventing disk thickening, we construct the initial conditions by increasing the stellar mass fraction from fs ∼ 0.01 to 0.02 and 0.04, respectively, while keeping the total mass equal to 1011M⊙ and hz/RD ≤ 0.1 unchanged. We find that models with a higher stellar mass fraction embedded in a shallow DM potential (c = 2) form bars and undergo significant disk thickening (hz/RD ≫ 0.1) concurrent with the bar growth. We conclude that if the LSB disks are thin to begin with, they remain so throughout their evolution in isolation, regardless of the concentration of the DM halo.
IIA NITESH K. DUBEYIsentropic processes for axisymmetric black holes
We demonstrate that the isentropic absorption of a classical charged test particle is classically forbidden for all (3 þ 1)-dimensional stationary, nonextremal, axisymmetric black holes in any diffeomorphism invariant theory of gravity. This result is derived purely from the near-horizon geometry and thermodynamic properties of the black hole spacetime, independent of the specific gravitational theory. We further consider the Kerr-Newman black hole in general relativity and analyse, using the quantum tunneling approach, the conditions under which isentropic absorption may be allowed. Broader implications for the second law and extremality bounds are discussed.
IIA DIPANKAR BANERJEEThe Effect of Poloidal Magnetic Field and Helicity Injection in a Breakout Coronal Mass Ejection
Coronal mass ejections (CMEs), as crucial drivers of space weather, necessitate a comprehensive understanding of their initiation and evolution in the solar corona in order to better predict their propagation. Solar Cycle 24 exhibited lower sunspot numbers compared to Solar Cycle 23, along with a decrease in the heliospheric magnetic pressure. Consequently, a higher frequency of weak CMEs was observed during Solar Cycle 24. Forecasting CMEs is vital, and various methods, primarily involving the study of the global magnetic parameters using data sets like Space-weather Helioseismic and Magnetic Imager Active Region Patches, have been employed in earlier works. In this study, we perform numerical simulations of CMEs within a magnetohydrodynamics framework using Message Passing Interface–Adaptive Mesh Refinement Versatile Advection Code in 2.5D. By employing the breakout model for CME initiation, we introduce a multipolar magnetic field configuration within a background bipolar magnetic field, inducing shear to trigger the CME eruption. Our investigation focuses on understanding the impact of the background global magnetic field on CME eruptions. Furthermore, we analyze the evolution of various global magnetic parameters in distinct scenarios (failed eruption, single eruption, and multiple eruptions) resulting from varying amounts of helicity injection in the form of shear at the base of the magnetic arcade system. Our findings reveal that an increase in the strength of the background poloidal magnetic field constrains CME eruptions. Furthermore, we establish that the growth rate of absolute net current helicity is the crucial factor that determines the likelihood of CME eruptions.
IIA SRIRAM, SDMD-based Multi-Object Spectrograph (D-MOS): AIV and first light results
A Digital Micromirror Device (DMD)-based Multi-Object Spectrograph (D-MOS) with an integrated imager has been developed. The optical performance of the MOS is evaluated through comprehensive laboratory calibration and on-sky observations using the 1.3-meter J.C. Bhattacharya (JCB) Telescope at the Vainu Bappu Observatory (VBO). The system is designed to assess the viability of using a DMD as a programmable slit mechanism for future ultraviolet-optical space missions. A complete imager-cum-spectrograph assembly was constructed using off-the-shelf optical components and configured for operation in the optical band, employing a DLP9500 DMD with a 1920×1080 micromirror array. Calibration experiments established the DMD-to-detector coordinate mapping and validated the strategies for object selection and slit placement. On-sky tests in crowded stellar fields confirmed successful slit targeting, precise object alignment, and multiplexed spectral acquisition. The spectrograph achieved a peak efficiency of 32%, a spectral resolving power of R∼1000 at 6000Å, a multiplexing capability of up to 46 slits (extendable to 85), and a contrast ratio of ∼ 6000. These results demonstrate the robustness and effectiveness of the DMD MOS system under real observational conditions and raise its TRL level for use in next-generation spectroscopic space missions.
IIA MRIDWEEKA SINGHA tale of three cataclysmic variables with distinct superhumps
Superhumps are among the most commonly observed variable features in the light curves of cataclysmic variables (CVs). To study the superhump behaviour of CVs, we present Transiting Exoplanet Survey Satellite (TESS) observations of three CVs: CRTS J110014.7+131552, SDSS J093537.46+161950.8, and [PK2008] HalphaJ130559. Among them, a super-outburst has been observed in CRTS J110014.7+131552, which is associated with the precursor outburst, where prominent superhumps have been observed during maximum of the outburst with a mean period of 0.06786(1) d. We observed variations in the superhump period, along with changes in the shape of the light curve profile and the amplitude of the superhumps during different phases of the outburst, indicating disc-radius variation as well as periodically variable dissipation at the accretion stream’s bright spot. The data on SDSS J093537.46+161950.8 reveal previously unknown variations modulated with periods of 0.06584(2) d and 2.36(2) d, related to the positive superhump and the disc-precession periods, respectively, which can reasonably be interpreted as a result of the prograde precession of an eccentric accretion disc. Despite its short orbital period, the lack of outburst activity, its stable long-term brightness, discovery spectrum, and absolute magnitude suggest that the object might not be an SU UMa type dwarf nova. Instead, it could belong to the group of highmass-transfer CVs below the period gap: either a rare class of nova-like variables or a high-luminosity intermediate polar, a subclass of magnetic CVs. For [PK2008] HalphaJ130559, a new average orbital period of 0.15092(1) d has been identified. Additionally, this system displays previously undetected average periods of 0.14517(3) d and 3.83(1) d, which could be provisionally identified as negative superhump and disc-precession periods, respectively. If the identified simultaneous signals do indeed reflect negative superhump and disc-precession period variations, then their origin might be associated with the retrograde precession of a tilted disc and its interaction with the secondary stream.
IIA SRIRAM, SDigital micromirror device characterization in optical band for astronomical multi-object spectrograph
The Digital Micromirror Device (DMD), a micro-electro-mechanical system (MEMS) consisting of individually controllable micromirrors, has emerged as a versatile tool for astronomical instrumentation, particularly in multi-object spectroscopy (MOS). Unlike traditional slit masks or fiber-based systems, DMDs offer dynamic reconfigurability, enabling efficient light modulation and enhanced spectral acquisition. Their adaptability has led to widespread adoption in ground-based spectrographs (e.g., RITMOS, BATMAN, SAMOS, IRMOS) and feasibility studies for space missions (e.g., EUCLID, CASTOR, SUMO, SIRMOS). DMDs have demonstrated robustness in space qualification tests, including radiation exposure, thermal cycling, and mechanical stress, making them viable for space-based applications. Recent advancements, such as UV-transparent windows and enhanced coatings, further expand their potential for ultraviolet astronomy. In India, the success of AstroSat’s Ultra Violet Imaging Telescope (UVIT) has motivated the development of the next-generation INdian Spectroscopic and Imaging Space Telescope (INSIST), which includes a DMD-based MOS for UV/optical observations. To advance its Technology Readiness Level (TRL), we evaluated the Texas Instruments DLP9500 DMD (1920 × 1080 micromirrors, 10 µm pitch) in the optical band, assessing key parameters such as diffraction efficiency, reflectivity, contrast, micromirror repeatability, and Point Spread Function (PSF) alignment. This study establishes a foundation for future UV-optimized DMD applications in INSIST and other astronomical missions.