Research Highlights
ARTICLES | IIA

We present a detailed study of the interacting triple active galactic nuclear system, NGC 7733-34, focusing on stellar kinematics, ionised gas characteristics, and star formation within the central region and stellar bars of both galaxies. We performed a comprehensive analysis using archival data from MUSE, HST/ACS, and DECaLS, complemented with observations from UVIT and IRSF. We identified a disc-like bulge in both NGC 7733 and NGC 7734 through a 2D decomposition. A central nuclear structure, with a semi-major axis of ∼1.113 kpc, was detected in NGC 7733 via a photometric and kinematic analysis, confirmed by the strong anticorrelation between V/σ and h3, indicative of circular orbits in the centre. NGC 7734 lacks a distinct nuclear structure. The presence of a disc-like bulge results in an anti-correlation between V/σ and h3, along with diffuse light. However, it does show higher central velocity dispersion, possibly attributed to an interaction with a smaller clump, which is likely a fourth galaxy within the system. Both galaxies demonstrate ongoing star formation, evidenced by FUV and Hα observations. NGC 7734 shows recent star formation along its bar, while NGC 7733 experiences bar quenching. The star formation rate (SFR) analysis of NGC 7734 reveals that the bar region’s SFR dominates the galaxy’s overall SFR. Conversely, in NGC 7733, the lack of star formation along the bar and the presence of a Seyfert 2 active galactic nucleus (AGN) at the galaxy centre suggest the possibility of link among both. However, this would not affect the galaxy’s overall star formation. Our findings provide valuable insights into the stellar and gas kinematics, star formation processes, and AGN feedback mechanisms in interacting galaxies hosting stellar bars.

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Context. We study the galaxies hosting ultra-strong Mg II (USMgII) absorbers at small impact parameters of ∼2″ (5‑20 kpc) spanning a redshift range of 0.4 ≤ z ≤ 1.7 using deep high-resolution images from the Hyper Suprime-Cam Subaru Strategic Survey and spectra from the Sloan Digital Sky Survey (SDSS). Aims. Our aim is to explore the physical origin of the USMgII absorbers and characterize the associated galaxies. Methods. We performed a galaxy spectral energy distribution (SED) fitting using optical and near-IR multiband data to identify potential absorber host galaxies. Further, we searched for the [O II] nebular emission line from absorber galaxies in the SDSS fiber spectra. Results. From a total of 418 USMgII absorbers with W2796 ≥ 3 Å along 412 quasar sight lines, we detected 50 galaxies based on [O II] λλ3727, 3729 nebular emission detected at the ≥2σ level. Utilizing the [O II] emission from the stacked spectrum and employing the best-fit galaxy SED template, we further identified 86 galaxies, leading to a total of 136 bona fide USMgII galaxies. With a prerequisite of having a minimum of four HSC passbands available, we found a detection rate of ∼38% at an average impact parameter of 11.4 kpc. We find that galaxies hosting USMgII systems are typically star-forming main sequence galaxies, with 21% exhibiting a starburst nature. The non-zero [O II] emission along the "clear" sight lines, with no stellar counterpart, indicates that the USMgII absorbers may likely emanate from the unseen faint galaxies near the quasar. The USMgII absorbers preferentially align along the major and minor axes of the galaxy, which suggests that they originate in the disk or large-scale wind. We show that the distribution of W2796 as a function of the impact parameter indicates a discernible radial dependence for the "disk" and "wind" subsets, with the observed large scatter in W2796 potentially attributed to large-scale outflows. The quasar sight line hosting USMgII systems show a factor of three higher galaxy surface density at impact parameters of ≲50 kpc, highlighting the multiple pathways that give rise to USMgII absorption.

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The study examined sensitivity analysis of aerosol optical and radiative properties due to different versions of SKYRAD.pack module (i.e. versions 4.2 and 5.0) along with stability and performance of sky radiometer instruments (POM-01), operating at Hanle, Leh and Merak, located at high-altitude background sites in the most climate sensitive Hindu Kush Himalayan region. The study utilized long-term aerosol measurements during 2008–2024 for examining the stability and performance of the instruments. As a part of sensitivity analysis, coarse-mode aerosol optical depth (AOD) was found to be higher at version 4.2, while fine-mode AOD showed higher at version 5.0, but interestingly the variation of total AOD was found to be insignificant. Further, single scattering albedo (SSA) at version 5.0 was overestimated from 4.2 version. Among the parameters, aerosol asymmetry parameter (AS) showed significantly larger difference between the two versions with overestimation at 4.2 version. Such large differences of AS may be attributed to variations in aerosol radiative forcing parameters. Further, variation of ±2% calibration constants (F0I) in the sensitivity analysis showed significant variation in the retrieval parameters. Aerosol volume size distribution at three sites showed dominantly trimodal pattern at version 4.2, while version 5.0 showed dominance of bi-modal distribution, which may be attributed from significant variation of AS between the two versions. These findings highlighted the importance of performing calibration procedures frequently to ensure the quality controlled data at background sites in particular, and sensitivity analysis for aerosol retrieval parameters in different versions of the SKYRAD.pack software tool.

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We present a study of correlations between high Li abundances and strong chromospheric He i λ10830 absorption-line strengths in Kepler field giant stars. Our sample includes 84 giants with detectable solar-like oscillations in their light curves, and their Li abundances come from the literature or are measured here using LAMOST medium-resolution spectra. Evolutionary phases are determined through asteroseismic analysis, with mixed-mode period spacing (ΔP) used to infer the time evolution of red clump (RC) giants. Near-IR observations of the He i λ10830 line were obtained with the high-resolution Habitable-zone Planet Finder spectrograph on the Hobby–Eberly Telescope. We find high Li abundances and strong He i lines exclusively among RC giants, with their absence in red giant branch stars suggesting a shared origin linked to the He flash. Additionally, a steady decline in He i strength with decreasing Li abundance among RC giants indicates a correlation between these properties. Older, Li-normal RC giants are He weak, while most younger, super-Li-rich giants are He strong, suggesting temporal evolution of both phenomena. We hypothesize that the core He flash and subsequent subflashes may enhance Li abundances in RC giant photospheres and trigger heightened chromospheric activity, leading to stronger He i λ10830 lines in younger RCs. Over time, following He flash, chromospheric activity diminishes, resulting in weaker He i lines in older, Li-normal RCs.

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We present a comprehensive photometric and spectroscopic study of the Type IIP supernova (SN) 2018is. The V band luminosity and the expansion velocity at 50 days post-explosion are −15.1 ± 0.2 mag (corrected for AV = 1.34 mag) and 1400 km s−1, classifying it as a low-luminosity SN II. The recombination phase in the V band is shorter, lasting around 110 days, and exhibits a steeper decline (1.0 mag per 100 days) compared to most other low-luminosity SNe II. Additionally, the optical and near-infrared spectra display hydrogen emission lines that are strikingly narrow, even for this class. The Fe II and Sc II line velocities are at the lower end of the typical range for low-luminosity SNe II. Semi-analytical modelling of the bolometric light curve suggests an ejecta mass of ∼8 M⊙, corresponding to a pre-supernova mass of ∼9.5 M⊙, and an explosion energy of ∼0.40 × 1051 erg. Hydrodynamical modelling further indicates that the progenitor had a zero-age main sequence mass of 9 M⊙, coupled with a low explosion energy of 0.19 × 1051 erg. The nebular spectrum reveals weak [O I] λλ6300,6364 lines, consistent with a moderate-mass progenitor, while features typical of Fe core-collapse events, such as He I, [C I], and Fe I, are indiscernible. However, the redder colours and low ratio of Ni to Fe abundance do not support an electron-capture scenario either. As a low-luminosity SN II with an atypically steep decline during the photospheric phase and remarkably narrow emission lines, SN 2018is contributes to the diversity observed within this population.

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The growth of a large-scale magnetic field in the Sun and stars is usually possible when the dynamo number (D) is above a critical value Dc. As the star ages, its rotation rate and thus D decrease. Hence, the question is how far the solar dynamo is from the critical dynamo transition. To answer this question, we have performed a set of simulations using Babcock Leighton type dynamo models at different values of dynamo supercriticality and analyzed various features of magnetic cycle. By comparing the recovery rates of the dynamo from the Maunder minimum and statistics (numbers and durations) of the grand minima and maxima with that of observations and we show that the solar dynamo is only about two times critical and thus not highly supercritical. The observed correlation between the polar field proxy and the following cycle amplitudes and Gnevyshev–Ohl rule are also compatible with this conclusion.

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We model the kinematics of the Small Magellanic Cloud (SMC) by analyzing the proper motions (PMs) from Gaia DR3 of nine different stellar populations, including young main-sequence (MS) stars (<2 Gyr), red giant branch stars, red clump stars, red giants with line-of-sight velocities, and three groups of star clusters. This analysis is carried out using a robust Markov Chain Monte Carlo method, to derive up to seven kinematic parameters. We trace the evolution from a nonrotating flattened elliptical system, as mapped by the old population, to a rotating highly stretched disk structure, as denoted by the young MS stars and clusters (<400 Myr). We estimate that the inclination i (∼58°–82°) decreases and the position angle Θ (∼180°–240°) increases with age. We estimate an asymptotic velocity of ∼49–89 km s‑1 with a scale radius of ∼6–9 kpc for the young MS populations, with velocity dispersion of ∼11 km s‑1, suggesting a rotation-supported disk structure. Our models estimate a line-of-sight extension of ∼30 kpc, in agreement with observations. We identify four regions of the SMC showing anomalies in the residual PM: the East Anomaly, the Southeast Anomaly (SEA), the South Anomaly, and the West Anomaly. The SEA appears like an infalling feature and is identified for the first time. The tidal imprints observed in the residual PM of the SMC suggest that its evolution is considerably shaped by the recent interaction with the Large Magellanic Cloud.

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We present a detailed analysis of an extragalactic slow classical nova in M31 exhibiting multiple peaks in its light curve. Spectroscopic and photometric observations were used to investigate the underlying physical processes. Shock-induced heating events resulting in the expansion and contraction of the photosphere are likely responsible for the observed multiple peaks. Deviation of the observed spectrum at the peak from the models also suggests the presence of shocks. The successive peaks occurring at increasing intervals could be due to the series of internal shocks generated near or within the photosphere. Spectral modeling suggests a low-mass white dwarf (WD) accreting slowly from a companion star. The ejecta mass, estimated from spectral analysis, is ~10−4 M⊙, which is typical for a slow nova. We estimate the binary, by comparing the archival Hubble Space Telescope data and eruption properties with stellar and nova models, to comprise a 0.65 M⊙ primary WD and a K iii cool evolved secondary star.

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The γ-ray detection from an astrophysical object indicates the presence of an extreme environment where highenergy radiation is produced. With the continuous monitoring of the γ-ray sky by the Fermi Large Area Telescope (LAT) leading to deeper sensitivity, high-energy γ-ray emission has now been detected from a diverse class of jetted active galactic nuclei (AGNs). Here, we present the results of a multiwavelength study of the radio source DA 362, which was reported to be a blazar candidate of uncertain type. However, it was recently identified as a bona fide compact symmetric object (CSO) based on its subkiloparsec, bipolar radio morphology, and lack of radio variability. This makes DA 362 only the fourth γ-ray-emitting object of this enigmatic class of radio-loud AGNs. Using five very-long-baseline interferometry observations covering 1996–2018, we found the jet separation velocity to be subluminal (vapp ~ 0.2c), thus supporting its CSO nature. Its Fermi-LAT observations revealed a γray flaring activity, a phenomenon never detected from the other three γ-ray-detected CSOs. This object is bright in the near-infrared band but extremely faint in the optical-UV filters, hinting at possible obscuration. Swift X-Ray Telescope observation of DA 362 reveals an extremely hard X-ray spectrum, though a strong claim cannot be made due to large uncertainties. We conclude that deeper observations are needed to probe the broadband properties of this enigmatic object and to understand the origin of high-energy γ-ray emission

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A fine abundance analysis of a recently discovered hydrogen-deficient carbon (HdC) star, A980, is presented. Based on the observed high-resolution optical spectrum, we ascertain that A980 is a cool extreme helium (EHe) star and not an HdC star. Singly ionized germanium Ge II lines are identified in A980’s optical spectrum. These are the first-ever detections of germanium lines in an EHe star's observed spectrum and provide the first measurements of germanium abundance in an EHe star. The overabundance of germanium in A980’s atmosphere provides us with evidence for the synthesis of germanium in EHe stars. Among the known cool EHe stars, A980 exhibits a maximum enhancement of the s-process elements based on a significant number of transitions. The measured elemental abundances reveal signs of H-burning, He-burning, and specifically the nucleosyntheses of the key elements Ge, Sr, Y, Zr, and Ba. The nucleosyntheses of these key elements are discussed in light of asymptotic giant branch evolution and the expectation from the accretion of an He white dwarf by a C–O white dwarf or by a neutron star

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