Indian Institute of Astrophysics

The ultraviolet (UV) spectral domain occupies a unique position in stellar astrophysics, serving as the bridge between the thermal continuum of photospheres and the high-energy, non-thermal processes of stellar coronae and winds. This article provides a review of stellar physics in the UV, addressing both the theoretical framework and observational applications across the Hertzsprung-Russell diagram. We explicitly structure our discussion around key scientific questions, demonstrating that accurate spectral synthesis in this regime demands Non-LTE radiative transfer codes, which in turn rely on precise atomic collision and recombination rates. We highlight how a critical scarcity of modern laboratory astrophysics data limits these models, particularly for complex ions. Moving to observational diagnostics, we review how UV spectroscopy constrains diffusion and radiatively driven winds in hot subluminous stars, and traces shock dynamics and abundance patterns in Planetary Nebulae and Supernova Remnants. In the context of star clusters, we illustrate how UV sensitivity to light-element variations (C, N, O) allows us to disentangle multiple stellar populations that appear degenerate in optical bands. We conclude that future progress depends on facilities capable of high-resolution spectroscopy, time-domain monitoring, and polarimetry to recover these diagnostic tracers and resolve the physics of stellar feedback.

As an update on the initial findings of DESI, the new results provide the first hint of potential deviations from a cosmological constant (omega= -1), which, if confirmed with significance > (2-4)sigma, would challenge the validity of Lambda within the ACDM model. We explore the Generalized Emergent Dark Energy (GEDE) model using recent BAO measurements from DESI DR2, Type Ia supernova compilations, and CMB distance priors. Employing nested sampling, we constrain the parameter Delta, which characterizes deviations from ACDM. Our analysis shows that with CMB+DESI DR2 alone, GEDE tends to prefer positive values of Delta. However, when different SNe Ia calibrations are included, the model favors negative values of Delta, corresponding to an earlier injection of dark energy. The Marginalized constraints on !(z) further shows that GEDE sharply emerges but then asymptotes to omega = -1 without crossing it. At z similar to 1 data, GEDE provides a better fit than ACDM, while at z less than or similar to 0.5 the data favor omega > -1, bringing the model deviate from ACDM. Bayesian model comparison shows weak support for GEDE with CMB+DESI DR2 (ln BF = 1.96), moderate with PP (ln BF = 2.65), weak-to-moderate with Union3 (lnBF = 2.34), and weak with DES-SN5Y (ln BF = 1.44). Overall, GEDE is consistent with current data and mildly favored when SNe Ia are included, making it a viable extension of Lambda CDM that merits further investigation with future high precision measurements.

We present new constraints on an interacting dark matter-dark energy scenario motivated by string compactification, where a scalar field adiabatically tracks the minimum of an effective potential sourced by dark matter density. In this study, we focus on the Chameleon dark energy model and numerically solve the Klein-Gordon equation using a shooting algorithm to determine precise initial conditions such that the field rests at effective potential minima today. We perform a comprehensive Markov Chain Monte Carlo (MCMC) analysis using a combination of datasets, including Planck, BAO (SDSS and DESI DR2), Pantheon+, and SH0ES. Our analysis shows a mild preference for a higher nonzero dark sector coupling, compared to earlier works on similar models, for two particular combinations of datasets: (i) Planck + DESI DR2 BAO +.Pantheon+ and (ii) Planck + SDSS BAO + Pantheon+ + SH0ES. Notably, the inclusion of DESI DR2 and SH0ES data increases the inferred interaction strength to beta similar to 0.3 (68% C.L.) and yields weak and positive evidence in favor of the model over Lambda CDM, with Delta chi min2=-4.75,-6.41 and Delta AIC= -0.75, -2.41, respectively. This model remains consistent with a phantom crossing at redshift z similar to 0.5, in agreement with the trend indicated by DESI observations. However, due to the settlement of the scalar field at the minima of the effective potential at the present epoch, the effective dark energy equation of state asymptotically approaches weff -> -1. leading to only weak evidence in favor of this model when analyzed using the DESI DR2 dataset.

Observational data play a pivotal role in identifying cosmological models that are both theoretically consistent and empirically viable. In this work, we investigate the level of preference for dynamical dark energy over a cosmological constant using current late-time observational datasets, including cosmic chronometers (CC), baryon acoustic oscillations from Dark Energy Spectroscopic Instrument (DESI) DR2, and different Type Ia supernova catalogs (Pantheon ^+ , DES-Dovekie, Union3). We analyze various dynamical dark-energy models, including ω CDM, o ω CDM, ω _0 ω _a CDM, logarithmic, exponential, Jassal–Bagla–Padmanabhan (JBP), Barboza–Alcaniz (BA), and GEDE. In most cases, the oΛCDM and o ω CDM models favor an open Universe. For the o ω CDM, the inclusion of DES-Dovekie or Union3 data together with CC and DESI DR2 favors a nearly flat geometry. Using the CC + DESI DR2 dataset, the preference for dynamical dark energy lies between the 1 σ and 2 σ level. When different supernova catalogs (DES-Dovekie or Union3) are included, the deviation from ΛCDM in the ω CDM, ω _0 ω _a CDM, logarithmic, JBP, BA, and GEDE models increases to the 2 σ –2.74 σ level, while the Pantheon ^+ sample yields deviations below the 2 σ level. We find consistent evidence for ω _0 > −1 and ω _a < 0 across all dark-energy models, indicating a preference for dynamical dark energy characterized by a Quintom-B-type scenario. The ΛCDM paradigm has long served as the standard framework of modern cosmology; however, recent DESI DR2 results have exposed emerging tensions with the cosmological constant Λ, hinting at possible new physics in the dark-energy sector. Even so, the currently available data are still not strong enough to definitively rule out the ΛCDM model.

Context. The Sun’s magnetic field exhibits the 11 year solar cycle as well as shorter periodicities, popularly known as the quasi-biennial oscillations (QBOs) and Rieger-type periods. Although several theories have been proposed to explain the origin of QBOs and Rieger-type periods, no single theory has had widespread acceptance. Aims. We explore whether the Babcock–Leighton dynamo can produce Rieger-type periodicity and QBOs and investigate their underlying physical mechanisms. Methods. We used the observationally guided 3D kinematic Babcock–Leighton dynamo model, which has emerged as a successful model for reproducing many characteristic features of the solar cycle. We used Morlet wavelet and global wavelet power spectrum techniques to analyze the data obtained from the model. Results. In our model, we report QBOs and Rieger-type periods for the first time. Further, we investigated the individual Babcock–Leighton parameters (fluctuations in flux, latitude, time delay, and tilt scatter) role in the occurrence of QBOs and Rieger-type periods. We find that while fluctuations in the individual parameters of the Babcock–Leighton process can produce QBOs and Rieger-type periodicity, their occurrence probability is enhanced when considering combined fluctuations of all parameters in the Babcock–Leighton process. Finally, we find that with the increase in dynamo supercriticality, the model tends to suppress the generation of Rieger-type periodicity. Thus, this result supports earlier studies that suggest the solar dynamo is not highly supercritical. Conclusions. The Babcock–Leighton dynamo model successfully reproduces QBOs and Rieger-type periodicities that are observed in various solar activity data.