Spectroscopic and photometric characterization of cool and evolved stars

 The cool and evolved stars are in post-main-sequence evolutionary phases with low-to intermediate-mass (0.8–8 Msun) in the Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) phase. Precise estimation of fundamental parameters is very important to characterize the stellar populations in the galactic and extragalactic environments. Also, the atmospheres of AGB stars are strongly affected by the radial pulsation, and therefore, the time-dependent spectroscopy provides the understanding of ongoing physical processes in its atmosphere. I shall talk about the precise estimation of fundamental parameters from dominant spectral features of K–M giants. For this, we construct a new near-infrared (1.50–2.45 μm) spectral library of 72 K–M giants using TIRSPEC instrument on 2-m HCT. Using TIRSPEC and IRTF spectral library, the behaviour of spectral features with fundamental parameters are investigated. Additionally, we derive new empirical correlations between fundamental parameters and 12CO-bands strength and study their relative effectiveness for the estimation of parameters. Furthermore, we explore various indices for an optimal correlation of stellar parameters estimation of cool giants. The presentation also focuses on the characterization of a red optical transient, which was found as an O-rich (C/O<1) Mira variable, and a poorly known OH/IR star from long-term optical/near-infrared photometric and spectroscopic observations. The variability period and wavelength-dependent variability amplitudes are estimated from best-fit light curves. We implement the radiative transfer code to construct the best-fit SED from multi-wavelength data. The time-dependent spectroscopy allows to probe the atmosphere of those pulsating variables and reveals spectral variability in unprecedented detail.