The DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material. https://prints:443 2026-07-31T18:32:43Z 2026-07-31T18:32:43Z Mishra, Wageesh Agarwal, Anjali Srivastava, N http://hdl.handle.net/2248/9012 2026-07-16T06:32:30Z 2026-07-01T00:00:00Z

Three-phase evolution of aspect ratio in fast and slow CMEs from the Sun to 1 au Mishra, Wageesh; Agarwal, Anjali; Srivastava, N Coronal mass ejections (CMEs) undergo significant geometric evolution as they propagate from the Sun to 1 au, influencing their radial size, expansion, and space weather impact. We investigate the evolution of CME aspect ratio (κ) and expansion dynamics for four fast and four slow Earth-directed CMEs. Using multipoint coronagraphic observations with the graduated cylindrical shell model and corrected in situ measurements of associated magnetic clouds at 1 au, we track the evolution of κ from the low-middle corona to interplanetary space. We find that κ does notremain constant but exhibits a systematic three-phase evolution: a rise phase in the low-middle corona ( 10–15 R), a saturation phase at intermediate heights, and then a decline phase in the interplanetary space. The ratio of radial expansion speed to leading-edge speed (Vexp/VLE) decreases substantially from the corona to 1 au, indicating a reduction in radial expansion efficiency during interplanetary propagation. The consistent evolution of κ and Vexp/VLE suggests a transition from magnetically dominated expansion in the corona to a regime increasingly controlled by the heliospheric environment. We note that fast CMEs show stronger early expansion and evolve into larger, more radially extended structures, whereas slow CMEs exhibit a more gradual rise and a steeper decline. These results demonstrate that CME geometry evolves significantly during propagation and highlight the need to incorporate aspect ratio evolution in models to improve predictions of CME size, arrival time, and geoeffectiveness. Open Access; This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

2026-07-01T00:00:00Z Mahavarkar, Prasanna Sriram, S Joshi, Bhagyashree Hasan, Amirul Remya, B. S Chandra, Sarvesh http://hdl.handle.net/2248/9011 2026-07-16T06:29:41Z 2023-12-01T00:00:00Z

Designing a sky-scanning Fabry–Perot interferometer system with a large size integrating sphere Mahavarkar, Prasanna; Sriram, S; Joshi, Bhagyashree; Hasan, Amirul; Remya, B. S; Chandra, Sarvesh Integrating spheres can be manufactured at sizes ranging from 1 mm to 3 m in diameter. It has been agreed widely by manufacturers that fitting a large-sized integrating sphere as per the customer requirement is a complex process. Thus, these spheres due to its large beam divergence from the exit port, have never been deployed for the calibration of aeronomy applications. In this article a optical system is described to integrate a large-sized integrating sphere with the Fabry–Perot etalon and thus devise a set-up to generate the concentric ring pattern and record it on the detector. The experimental set-up was verified by optical simulation using ZEMAX (OpticStudio) so as to validate the system’s performance in the laboratory. The important parameter finesse is calculated and its accuracy is found to be 80%. Open Access; This is an open access article under the CC BY license.

2023-12-01T00:00:00Z Deepak http://hdl.handle.net/2248/9010 2026-07-16T06:27:44Z 2022-04-01T00:00:00Z

Chemodynamic studies of the galaxy Deepak Based on data from large spectroscopic surveys like the GALactic Archaeology with HERMES (GALAH) and the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST), along with astrometric and photometric data from the Gaia survey, we have addressed three major questions in the field of the chemical and dynamical evolution of the Galaxy. The first of these three questions is understanding the evolution of lithium (Li) in the Galaxy. For this, we used the spectroscopic data from the GALAH survey along with astrometric and photometric data from the Gaia survey. We found that Li has significantly increased in the Galactic disc compared to the observed value in the metal-poor halo stars and the theoretically predicted primordial value from the Standard Big Bang Nucleosynthesis models. Search for an explanation for such large-scale Li enrichment throughout the Galactic disc led us to study the evolution of Li in low-mass stars (the second problem we have addressed). According to the standard evolutionary models, Li gets depleted as stars ascend on the red-giant branch (RGB). However, about a per cent of giants have been found to have a thousandfold higher Li than the value predicted by standard models. These rare giants with high Li, well known as the Li-rich giants, have puzzled astronomers for over four decades since their first discovery by Wallerstein and Sneden (1982). To understand the evolution of Li in low-mass stars, we used data from the GALAH and Gaia surveys. From the second data release of the GALAH survey, we discovered 335 new Li-rich giants (with A(Li) ≥ 1.8 dex), of which 20 are super Li-rich with A(Li) ≥ 3.2 dex. We further discovered that almost all of these Li-rich giants are core He-burning (CHeB) giant stars, and Li enrichment in red giants is likely associated with the He-core flash. We also found that the Li-rich giants do not belong to any particular dynamic group in the Galaxy and are found throughout the Galaxy. However, they are more prevalent among giants of the Galactic thin disc than the thick disc and halo. We further found that chemically Li-rich and normal giants are similar except for Li abundance. We also found that the probability of becoming a Li-rich giant is approximately independent of a star’s mass, although the majority of the Li-rich giants are found to be low mass (M ≤ 2 M⊙). The frequency of occurrence of Lienriched giants among normal giants is about one per cent and slightly dependent on metallicity. Li-enriched and normal giants are also found to have a similar projected rotational velocity, suggesting that Li-enrichment in giants is not linked to scenarios such as mergers and tidal interaction between binary stars. To find more clues about the origin of Li enrichment in giants, we studied the correlation between giants’ Li abundance and asteroseismic parameters. Data for the CHeB giants suggest a decrease in A(Li) with an increase in the gravity mode period spacing ∆Π1, which is known to increase with time at the start of the CHeB phase suggesting the enriched Li in He-core burning giants decreases as stars evolve. Based on asteroseismic data also, we found no evidence of Li enrichment at the luminosity bump. In conclusion, these studies have helped to uncover the almost four-decade-old mystery of the origin of Li-rich giant stars, and now we know that all the low-mass giants experience Li enrichment during the CHeB phase to a varying degree. Lastly, to uncover the formation and evolution history of the Galaxy, we studied the chemical and kinematic properties of various pro-grade (like the Splash) and retrograde (like Gaia-Enceladus/Sausage (GE/S), Thamnos and Sequoia) substructures in the Galactic halo along with the Galactic thin and thick disc. We also studied the age distributions of all these Galactic components. We found that the star formation in the Splash, which is the major in situ component of the halo and has a median [Fe/H] of −0.75±0.24 dex, peaked about 13 Gyr ago. On the other hand, the star formation in the GE/S, which is the halo’s largest accreted component and has a median [Fe/H] of −1.31 ± 0.23 dex, peaked about 1.5 Gyr later than the Splash. The Galactic thin and thick discs are found to have peak ages of about 5.0 and 11.5 Gyr, respectively. The GE/S and Sequoia are also found to have a distinct chemical evolution from the Splash, whose chemical composition is found to be similar to the metal-poor stars of the thick disc. In conclusion, our studies support the idea of galaxy formation by hierarchical clustering in a Lambda cold dark matter universe. © Indian Institute of Astrophysics; Thesis Supervisor Prof. Bacham E. Reddy

2022-04-01T00:00:00Z Goswami, P. P Goswami, A http://hdl.handle.net/2248/9009 2026-07-16T06:26:05Z 2026-07-01T00:00:00Z

Three extremely metal-poor stars: Discovery of a new CEMP-no star Goswami, P. P; Goswami, A We have conducted detailed high-resolution spectroscopic studies on three extremely metal-poor (EMP) stars HE 0401−0138, HE 1153−0518, and HE 1246−1344. For the stars HE 0401−0138 and HE 1246−1344, we have estimated the abundances of C, Na, Mg, Ca, Sc, Ti, Cr, Mn, Co, Ni, Sr, and Ba along with the upper limits for Li, O, La, Ce, Pr, Nd, Sm, and Eu. For HE 1153−0518, abundances of seven light elements from C through Ni and two heavy elements, Y and Ba, have been derived, together with upper limits for Li, O, and La. Based on their observed abundance patterns, HE 0401−0138 and HE 1246−1344 are classified as normal EMP stars, whereas HE 1153−0518 is identified as a newly discovered CEMP-no star. HE 1153−0518 shows strong carbon enhancement with a high absolute carbon abundance, extreme sodium enrichment, very low neutron-capture element abundances, and a very low carbon isotopic ratio (12C/13C = 2.0). Its spectral energy distribution shows clear infrared excess, indicating the presence of circumstellar dust. The abundance pattern of HE 1153−0518 suggests enrichment by early nucleosynthetic processes, such as faint core-collapse supernovae with mixing and fallback, while the possibility of binary interaction and subsequent internal mixing cannot be ruled out. The discovery and detailed study of HE 1153−0518 add an important object to the small population of highA(C) CEMP-no stars and provide valuable constraints on early chemical enrichment pathways and the nature of the first generations of stars. Open Access; This is an Open Access article distributed under the terms of the Creative Commons Attribution License which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

2026-07-01T00:00:00Z