The present investigation considers the effect of small perturbations given in the Coriolis and centrifugal forces on the location and stability of the equilibrium points in the Robe’s circular restricted three-body problem with non-spherical primary bodies. The felicitous equations of motion of $m_3$ are obtainedby taking into account the shapes of primaries $m_1$ and $m_2$, the full buoyancy force of the fluid which is filled inside $m_1$ of density $\rho_1$, the forces due to the gravitational attraction of the fluid and $m_2$. We assume that the massive body $m_1$ is an oblate spheroid and the $m_2$ a finite straight segment, and they move under a mutual gravitational attraction described by the Newton’s universal law of gravitation. In the present problem, $m_3$ is moving in the fluid and the rotating reference frame is used, its motion is bound to be affected by the perturbed Coriolis and centrifugal forces. In this attempt these effects along with the effects caused by the oblateness and length parameters $A$ and $l$ respectively, on the location and stability of the equilibrium pointsare observed. A pair of collinear equilibrium points $L_1$ and $L_2$ and infinite number of non-collinear equilibrium points are obtained. The stability of all the equilibrium points depends on the coefficients of their corresponding characteristic polynomials that are obtained with the help of linear variational equations.

AstroSat is an astronomical observatory from India which is in a low earth orbit of 650 km altitude with a 6-degree inclination and has been functioning successfully in orbit for the last 5 years. The simultaneous observation by the payloads allows imaging of celestial sources in various wavebands that has resulted in scientific achievements which are published in various forum. Uniqueness of its orbit, and scientific objectives calls for a different type of scheduling approach and a hands-free automated tasking of payloads, on this satellite. Several software architectures involving a Centralized data-basecalled Mission Control And Proposals (MCAP) with the Scheduler, AstroSat Scheduler TeRminus for PAyLoads (ASTRAL), AstroSat Scheduler Software for OpeRaTions (ASSORT), Command Sequence Generation (CSG), AstroSat Schedule Viewer (ASV), Astroviewer and AstroSat Long Term Planner(ASPlanner) were developed and used for celestial source observations. This paper describes briefly the scheduling approaches, their goals, utilization, lessons learnt and improvements carried out over the last 5 years.

Over the last 5 years, UVIT has completed observations of more than 500 proposals with $\sim$800 unique pointings. In addition, regular planned monitoring observations have been made and from their analysis various key parameters related to in orbit performance of UVIT have been quantified. The sensitivities of the UV channels have remained steady indicating no effect of potential molecular contamination confirming the adequacy of all the protocols implemented for avoiding contamination. The quality of the PSF through the years confirms adequacy of thermal control measures. The early calibrations obtained during the Performance Verification (PV) phase have been further revised for more subtle effects. These include flat fields and detector distortions with greater precision. The operations of UVIT have also evolved through inorbit experience, e.g. tweaking of operational sequencing, protocol for recovery from bright object detection (BOD) shutdowns, parameters for BOD thresholds, etc. Finally, some effects of charged particle hits on electronics led to optimised strategy for regular resetting. The Near-UV channel was lost in one of suchoperations. All the above in-orbit experiences are presented here.

We present far ultraviolet (FUV) observations of globular cluster NGC 4147 using three FUV filters, BaF2 (F154W), sapphire (F169M), and silica (F172M) of Ultra-Violet Imaging Telescope (UVIT) onboard the AstroSat satellite. We confirmed the cluster membership of the UVIT observed sources usingproper motions from Gaia data release 2 (GAIA DR2). We identified 37 blue horizontal branch stars (BHBs), one blue straggler star (BSS) and 15 variable stars using UV-optical color magnitude diagrams (CMDs). We find that all the FUV bright BHBs are second generation population stars. Using UV-optical CMDs, weidentify two sub-populations, BHB1 and BHB2, among the UV-bright BHBs in the cluster with stars count ratio of 24:13 for BHB1 and BHB2. The effective temperatures ($T_{\rm eff}$) of BHB1 and BHB2 were derived using color-temperature relation of BaSTI-IAC zero-age horizontal branch (ZAHB). We found that BHB1 stars are more centrally concentrated than BHB2 stars. We also derive physical parameters of the detected FUV bright BSS by fitting younger age BaSTI-IAC isochrones on optical and UV-optical CMDs.

The structure of our Galaxy has been studied from ultraviolet (UV) star counts obtained with the Ultra-Violet Imaging Telescope (UVIT) on board the AstroSat satellite, in Far-UV (FUV) and Near-UV (NUV) bands. The F154W (BaF$_2$) and N263M (NUVB4) filters were used in the FUV and NUV bands,respectively. The point sources are separated from the extra-galactic sources of UVIT observations using infrared (IR) color cut method. The observed UVIT star counts match well with the simulations obtained from the Besanc¸on model of stellar population synthesis towards several Galactic directions. We alsoestimated the scale length and scale height of the thick disc and the scale height of the thin disc using the space density function and the exponential density law for the stars of intermediate galactic latitudes. The scale length of the thick disc ranges from 3.11 to 5.40 kpc whereas the scale height ranges from $530 \pm 32$ pc to $630 \pm 29$ pc. The scale height of the thin disc comes out to be in the range of $230 \pm 20$ pc to $330 \pm 11$ pc.

Ultra Violet Imaging Telescope (UVIT) is one of the 5 instruments on AstroSat satellite, which was launched on September 28, 2015. UVIT was designed to make images with a resolution of <1:8$''$, simultaneously in two ultraviolet channels: Far Ultraviolet (130–180 nm) and Near Ultraviolet (200–300 nm). Images are also made in visible region (320–550 nm) for tracking drifts in pointing. The shortest wavelengths to be observed with UVIT can be heavily absorbed by mono-molecular deposits/contamination on the optical surfaces.Keeping contamination under control in UVIT was a major challenge and it required a variety of actions: (i) strict control of the payload materials and process, (ii) mechanical configuration, (iii) baking of all the parts to release all the adsorbed molecules etc., (iv) assembly in ultra cleanrooms, (v) pre-inspection and auditing of all the areas, in which UVIT was placed, for any potential for contamination, (vi) continuous purging, with ultrapure nitrogen gas, till a few days before the launch, etc. In order to minimise any possible cross contaminationsfrom the other payloads/satellite, the doors of UVIT were opened 2 months after the launch. The high performance in the orbit and high stability of the sensitivity over 4 years in the orbit shows that the contamination was negligible. This paper presents the processes and protocols followed during the integration and testingphase to minimise the contamination in order to prevent any performance degradation.

AstroSat is India’s first mission dedicated to space-based Astronomy and carries a complement of instruments sensitive over a wide spectral region covering visible, ultraviolet, soft X-ray and hard X-ray bands. The spacecraft platform provides the required stability, pointing, power, thermal control, and interface requirements. Among the critical functions of the Attitude and Orbit Control System (AOCS) were, manoeuvre the spacecraft from one target source to another in an optimal way while avoiding the sun, compensate for the rotational disturbance torque created by the scanning of the SSM payload on the other payloads, primarily the UVIT payload, provide for calibration of all the payloads, which involved manoeuvring through 2-dimensional target patterns. All the above novel control functions, along with the routine house-keeping operations, were designed, developed, and tested extensively on ground, beforedeclaring them as flight-worthy. The payload performance and the resulting science are a testimony to the excellent performance of the on-board Control System.

The Cadmium–Zinc–Telluride Imager (CZTI) onboard AstroSat is designed for hard X-ray imaging and spectroscopy in the energy range of 20–100 keV. The CZT detectors are of 5-mm thickness and hence have good efficiency for Compton interactions beyond 100 keV. The polarisation analysis using CZTIrelies on such Compton events and have been verified experimentally. The same Compton events can also be used to extend the spectroscopy up to 380 keV. Further, it has been observed that about 20% pixels of the CZTI detector plane have low gain, and they are excluded from the primary spectroscopy. If these pixels are included, then the spectroscopic capability of CZTI can be extended up to 500 keV and further up to 700 keV with a better gain calibration in the future. Here we explore the possibility of using the Compton events as well as the low gain pixels to extend the spectroscopic energy range of CZTI for ON-axis bright X-ray sources. We demonstrate this technique using Crab observations and explore its sensitivity.

Cadmium–Zinc–Telluride Imager (CZTI) onboard AstroSat has been a prolific Gamma-Ray Burst (GRB) monitor. While the 2-pixel Compton scattered events (100–300 keV) are used to extract sensitive spectroscopic information, the inclusion of the low-gain pixels ($\sim$20% of the detector plane) aftercareful calibration extends the energy range of Compton energy spectra to 600 keV. The new feature also allows single-pixel spectroscopy of the GRBs to the sub-MeV range which is otherwise limited to 150 keV. We also introduced a new noise rejection algorithm in the analysis (‘Compton noise’). These new additionsnot only enhances the spectroscopic sensitivity of CZTI, but the sub-MeV spectroscopy will also allow proper characterization of the GRBs not detected by Fermi. This article describes the methodology of single, Compton event and veto spectroscopy in 100–900 keV combined for the GRBs detected in the first year of operation. CZTI in last five years has detected $\sim$20 bright GRBs. The new methodologies, when applied on the spectral analysis for this large sample of GRBs, has the potential to improve the results significantly and help in better understanding the prompt emission mechanism.

The results presented in this paper are obtained from low-latitude ionospheric total electron content (TEC) variation during the chosen geomagnetic storm events happening during the solar cycle 24.We include the four intense geomagnetic storms that occurred on 26 September 2011, 15 July 2012, 19 February 2014 and 20 December 2015, depending upon the availability of TEC data. For this, we have used the TEC data from low-latitude station Varanasi (geographic latitude 25$^{\circ}$, 16$'$N, geographic longitude 82$^{\circ}$, 59$'$E and geomagnetic latitude 16$^{\circ}$, 24$'$N) and an equatorial station Bengaluru (geographic latitude 13$^{\circ}$, 02$'$N, geographic longitude 77$^{\circ}$, 34$'$E and geomagnetic latitude 04$^{\circ}$, 68$'$N). The storm-induced TEC changes at chosen stations have been discussed in terms of local time, storm wind effect, neutral wind, composition changes and variation in the dawn–dusk component of the interplanetary electric field (IEF $E_y$).

In this work, we study the synthetic explosions of a massive star. We take a 100 $M_{\odot}$ zero-age main-sequence (ZAMS) star and evolve it until the onset of core-collapse using $\mathtt{MESA}$. Then, the resulting star model is exploded using the publicly available stellar explosion code, $\mathtt{STELLA}$. The outputs of $\mathtt{STELLA}$ calculations provide the bolometric light curve and photospheric velocity evolution along with other physical properties of the underlying supernova. In this paper, the effects of having a large Hydrogen-envelope on the supernova light curve have been explored.We also explore the effects of the presence of different amounts of nickel mass and the variation of the explosion energy of the supernovae from such heavy progenitors, on thebolometric light curves and photospheric velocities.

Young star clusters consisting of massive stars are the ideal sites to study the star formation processes and influence of massive stars on the subsequent star formation. NGC 1893 is a young star cluster associated with the Hii region Sh2-236. It contains about five ‘O’-type stars and several early ‘B’-type stars. It is located at a moderate distance of $\sim$3.25 kpc and has a reddening, $E(B - V)\sim 0.4$ mag. To characterize the young low-mass stellar population in the central portion of the cluster, we carried out deep $VI$ band observations of the region using the $4{\rm K} \times 4{\rm K}$ CCD IMAGER mounted on the 3.6-m Devasthal Optical Telescope. Our analysis shows that the present data are deep enough to detect stars below $V \sim 24$ mag. Wefound optical counterparts of $\sim$220 candidates, including young stars and unclassified cluster members from Caramazza et al. (2008). We estimated the membership probabilities of the Gaia sources (mostly bright starswith $G$ < 19 mag) located within the cluster radius using the Gaia EDR3. Toward the fainter end, we used the optical color-magnitude diagram (CMD) to select the cluster members from a sample of young stars. The locations of young stars on the CMD show that a majority of them are low-mass stars with age <10 Myr. The unclassified candidates and X-ray sources from Caramazza et al. (2012) are also found to be young low-mass stars. In total, we identified $\sim$425 young stars with age <10 Myr, and 110 of these are new. Most of these stars appear as kinematic members of the cluster. By examining the CMD for the stars in the cluster region, we suggest that the cluster has insignificant contamination due to field stars in the pre-main-sequence zone ofthe CMD. The slope of the mass function in the mass range $0.2 \leq M/M_{\odot} \leq 2.5$ is found to be $\Gamma=-1.43\pm 0.15$, consistent with those of other star-forming complexes. The spatial distribution of the young stars as a function of mass suggests that toward the cluster center, most of the stars are massive.

The 4m international liquid mirror telescope (ILMT) installation activities have recently been completed at the Devasthal observatory (Uttarakhand, India). The ILMT will perform continuous observation of a narrow strip of the sky ($\sim$27$'$) passing over the zenith in the SDSS $g'$, $r'$ and $i'$ bands. Incombination with a highly efficient 4k$\times$4k CCD camera and an optical corrector, the images will be secured at the prime focus of the telescope using the time delayed integration technique. The ILMT will reach $\sim$22.5mag ($g'$-band) in a single scan and this limiting magnitude can be further improved by co-adding the nightly images. The uniqueness of the one-day cadence and deeper imaging with the ILMT will make it possible to discover and study various galactic and extra-galactic sources, specially variable ones. Here, we present the latest updates of the ILMT facility and discuss the preparation for the first light, which is expected during early 2022. We also briefly explain different steps involved in the ILMT data reduction pipeline.

Optical follow-up observations of optical afterglows of gamma-ray bursts are crucial to probe the geometry of outflows, emission mechanisms, energetics and burst environments. We performed the follow-up observations of GRB 210205A and ZTF21aaeyldq (AT2021any) using the 3.6m Devasthal opticaltelescope (DOT) around one day after the burst to deeper limits due to the longitudinal advantage of the place. This paper presents our analysis of the two objects using data from other collaborative facilities, i.e., 2.2m Calar Alto Astronomical Observatory (CAHA) and other archival data. Our analysis suggests that GRB 210205A is a potential dark burst once compared with the X-ray afterglow data. Also, comparing results with other known and well-studied dark GRBs samples indicate that the reason for the optical darkness of GRB210205A could either be intrinsic faintness or a high redshift event. Based on our analysis, we also found that ZTF21aaeyldq is the third known orphan afterglow with a measured redshift except for ZTF20aajnksq (AT2020blt) and ZTF19abvizsw (AT2019pim). The multiwavelength afterglow modeling of ZTF21aaeyldq using the afterglowpy package demands a forward shock model for an ISM-like ambient medium with a rather wider jet opening angle. We determine circumburst density of $n_0 =0.87$ cm$^{-3}$, kinetic energy $E_k=3.80 \times 10^{52}$ erg and the afterglow modeling also indicates that ZTF21aaeyldq is observed on-axis ($\theta_{\rm obs}$ < $\theta_{\rm core}$) and a gamma-ray counterpart was missed by GRBs satellites. Our results emphasize that the 3.6m DOT has a unique capability for deep follow-up observations of similar and other new transients for deeper observations as a part of time-domain astronomy in the future.

In the present work, recent characterization results of the 4K$\times$4K CCD imager (a first light instrument of the 3.6m devasthal optical telescope; DOT) and photometric calibrations are discussed along with measurements of the extinction coefficients and sky brightness values at the location of the 3.6m DOTsite basedon the imaging data taken between 2016 and 2021. For the 4K$\times$4K CCD imager, all given combinations of gains (1, 2, 3, 5 and 10 e$^-$/ADU) and readout noise values for the three readout speeds (100 kHz, 500 kHz and 1 MHz) are verified using the sky flats and bias frames taken during early 2021; measured values resemble well with the theoretical ones. Using color–color and color–magnitude transformation equations, color coefficients ($\alpha$) and zero-points ($\beta$) are determined to constrain and examine their long-term consistencies and any possible evolution based on UBVRI observations of several Landolt standard fields observed during 2016–2021. Our present analysis exhibits consistency among estimated a values within the 1$\sigma$ and does not show any noticeable trend with time. We also found that the photometric errors and limiting magnitudes computed using the data taken using the CCD imager follow the simulated ones published earlier. The average extinction coefficients, their seasonal variations and zenith night-sky brightness values for the moon-less nights for all ten Bessell and SDSS filters are also estimated and found comparable to those reported for other good astronomical sites.

In this paper, we have provided a theoretical analysis on enhanced terahertz (THz) generation by beating two amplitude-modulated laser beams in the collisional plasma under the effect of static electric and magnetic fields. Two amplitude-modulated laser beams of slightly different frequencies ($\omega_1$, $\omega_2$) with wavenumbers ($k_1$, $k_2$) are propagating along the same direction under the effect of these fields. The coupling between the various nonlinear terms like the nonlinear velocity and electron density in the collisional plasma results in the THz generation. The electric field, magnetic field, and propagation direction of lasers are mutually perpendicular to each other. The applied static magnetic and electric fields assist to enhance the nonlinear current density and normalized THz amplitude. The normalized THz amplitude showsnotable enhancement with the increase of applied static electric and magnetic fields. This scheme can be employed to generate and detect the THz radiations for making interesting astronomical observations.

The present study investigates the combined effects of the oblateness and straight segment on the positions and linear stability of the equilibrium points in the restricted $2 + 2$ body problem. The present model holds fourteen equilibrium points, out of which six are collinear with the centers of the primaries and the rest are non-collinear. It has been observed that the positions of all the equilibrium points are subsequently affected by the oblateness and length of the primary bodies. The linear stability of the equilibrium points is also presented by slightly perturbing the position of the equilibrium points. It is observed that for aconsidered set of parameters, all the fourteen equilibrium points are unstable. An application of the present model is also studied, for which the position and stability of the equilibrium points are investigated for the Earth-22 Kalliope-dual satellite system. It has been observed that for this system, all the equilibrium points are unstable except for two non-collinear equilibrium points that are found to be stable.

Gamma-ray bursts are traditionally classified as short and long bursts based on their $T_{90}$ value (the time interval during which an instrument observes 5% to 95% of gamma-ray/hard X-ray fluence). However, $T_{90}$ is dependent on the detector sensitivity and the energy range in which the instrument operates. As a result, different instruments provide different values of $T_{90}$ for a burst. GRB 210217A is detected with different duration by Swift and Fermi. It is classified as a long/soft GRB by Swift-BAT with a $T_{90}$ value of 3.76 s. On the other hand, the sub-threshold detection by Fermi-GBM classified GRB 210217A as a short/hard burst with a duration of 1.024 s. We present the multi-wavelength analysis of GRB 210217A (lying in the overlapping regime of long and short GRBs) to identify its actual class using multi-wavelength data. We utilized the $T_{90}$-hardness ratio, $T_{90}-E_p$ and $T_{90}-t_{\rm mvts}$ distributions of the GRBs to find the probability of GRB 210217A being a short GRB. Further, we estimated the photometric redshift of the burst by fitting the joint XRT/UVOT SED and placed the burst in the Amati plane. We found that GRB 210217A is an ambiguous burst showing properties of both short and long class of GRBs.

We study the effect of heating–cooling imbalance on slow magnetohydrodynamic waves in solar coronal loops with time-varying background temperature in the presence of thermal conduction, opticallythin radiation and heating. The MHD equations governing the plasma motion are solved numerically to examine the effects of heating–cooling imbalance on slow waves in the presence of thermal conduction and radiation. It is found that the amplitude of perturbed velocity decreases in the case of increasing background temperature, whereas the perturbed velocity amplitude increases in the case of decaying background temperature. The heating–cooling imbalance influences the damping of slow waves. Damping of waves is stronger for characteristic time $\tau=1000$ s than the damping for $\tau=3000$ s in both time-varying background temperature plasmas.

The advanced gravitational wave (GW) detector network has started routine detection of signals from merging compact binaries. Data indicate that in a fair fraction of these sources, at least one component was a neutron star, bringing with it the possibility of electromagnetic (EM) radiation. So far, a confirmed link between EM and GW radiation has been established for only one source, GW170817. Joint analysis ofbroadband multi-wavelength data and the GW signal have yielded rich information spanning fields as varied as jet physics, cosmology and nucleosynthesis. Here, we discuss the importance of such joint observations, as well as current and near-future efforts to discover and study more EM counterparts to GW sources.

In this paper, we present multi-band photometric observations and analysis of the host galaxies for a sample of five interesting gamma-ray bursts (GRBs) observed using the 3.6mDevasthal optical telescope (DOT) and the back-end instruments. The host galaxy observations of GRBs provide unique opportunities to estimatethe stellar mass, ages, star-formation rates and other vital properties of the burst environments and hence, progenitors. We performed a detailed spectral energy distribution (SED) modeling of the five host galaxies using an advanced tool called $\mathtt{Prospector}$, a stellar population synthesis model. Furthermore, we comparedthe results with a larger sample of well-studied host galaxies of GRBs, supernovae and normal star-forming galaxies. Our SED modeling suggests that GRB 130603B, GRB 140102A, GRB 190829A and GRB 200826A have massive host galaxies with high star-formation rates (SFRs). On the other hand, a supernovae-connectedGRB 030329 has a rare low-mass galaxy with a low star-formation rate.We also find that GRB 190829A has the highest (in our sample) amount of visual dust extinction and gas in its local environment of the host, suggesting that the observed very high-energy emission from this burst might have a unique local environment. Broadly,the five GRBs in our sample satisfy the typical correlations between host galaxies parameters and these physical parameters are more common to normal star-forming galaxies at the high-redshift Universe. Our results also demonstrate the capabilities of 3.6m DOT and the back-end instruments for the deeper photometric studies ofthe host galaxies of energetic transients, such as GRBs, supernovae and other transients in the long run.

In this work, we employ two publicly available analysis tools to study four hydrogen (H)–stripped core–collapse supernovae (CCSNe), namely, SN 2009jf, iPTF13bvn, SN 2015ap and SN 2016bau. We use the modular open-source fitter for transients (${\texttt{MOSFiT}}$) to model the multi-band light curves. ${\texttt{MOSFiT}}$ analyses show ejecta masses ($\log M_{ej}$) of $0.80^{+0.18}_{−0.13}$ $M_{\odot}$, $0.15^{+0.13}_{−0.09}$ $M_{\odot}$, $0.19^{+0.03}_{−0.03}$ $M_{\odot}$ and $0.19^{−0.01}_{+0.02}$ $M_{\odot}$ for SN 2009jf, iPTF13vn, SN 2015ap and SN 2016au, respectively. Later, modules for experiments in stellar astrophysics(MESA), is used to construct models of stars from pre-main sequence up to core collapse, which serve as the possible progenitors of these H-stripped CCSNe. Based on literature, we model a 12 $M_{\odot}$ ZAMS star as the possible progenitor for iPTF13vn, SN 2015ap and SN 2016bau, while a 20 $M_{\odot}$ ZAMS star is modeled as the possible progenitor for SN 2009jf. Glimpses of stellar engineering and physical properties of models at various stages of their lifetime have been presented to demonstrate the usefulness of these analysis threads to understand the observed properties of several classes of transients in detail.

Cosmic dawn (CD) and the epoch of reionization (EoR) are the most important parts of cosmic history during which the first luminous structures emerged. These first objects heated and ionized the neutral atomic hydrogen in the intergalactic medium. The redshifted 21-cm radiation from the atomic hydrogen providesan excellent direct probe to study the evolution of neutral hydrogen (Hi) and thus reveal nature of the first luminous objects, their evolution and role in this last phase transition of the Universe and formation and evolution of the structures thereafter. Direct mapping of the Hi density during the CD–EoR is rather difficult with thecurrent and forthcoming instruments due to stronger foreground and other observational contamination. The first detection of this redshifted Hi signal is expected to be done through statistical estimators. Given the upmost importance of the detection and analysis of the redshifted 21-cm signal, physics of CD–EoR is considered asone of the objective of the upcoming SKA-low telescope. This paper summarizes the collective effort of Indianastronomers to understand the origin of the redshifted 21-cm signal, sources of first ionizing photons, their propagation through the IGM, various cosmological effects on the expected 21-cm signal, various statistical measures of the signal like power spectrum, bispectrum, etc. A collective effort on detection of such signalby developing e stimators of the statistical measures with rigorous assessment of their expected uncertainties, various challenges like that of the large foreground emission and calibration issues are also discussed. Various versions of the detection methods discussed here have also been used in practice with the Giant MeterwaveRadio Telescope with successful assessment of the foreground contamination and upper limits on the matter density in reionization and post-reionization era. The collective efforts compiled here has been a large part of the global effort to prepare proper observational technique, analysis procedure for the first light of the CD–EoR through the SKA-low.

Understanding the explosion mechanism of type Ia supernova is among the most challenging issues in astrophysics. Accretion of matter on a carbon–oxygen (CO) white dwarf (WD) from a companion star is one of the most important keys in this regard. Our aim is to study the effects of WD composition on variousparameters during the accretion of helium-rich matter at a slow rate. We have used the computer simulation code Modules for Experiments in Stellar Astrophysics (MESA) to understand the variations in the properties,such as specific heat ($C_P$) and degeneracy parameter ($\eta$). The profile of specific heat shows a discontinuity and that of the degeneracy parameter shows a dip near the ignition region. As expected, the size of WD decreasesand g increases during the accretion. However, a red-giant-like expansion is observed after the rapid ignition towards the end. Our study explains the reason behind the delay in onset of helium ignition due to the differencein carbon abundance in a CO-WD. We found that WDs of the lower abundance of carbon, accrete slightly longer before the onset of helium ignition.

Mid-infrared emission spectra, obtained from ISO archive, of thirteen astrophysical objects as well as computed spectra of 27 polycyclic aromatic hydrocarbon (PAH) molecules are studied. All the objects show strong aromatic infrared band (AIB) features with variations that correlate with object type. Based on AIB peakpositions, the features for IRC $+$10216, Monoceros R2, and IC 5117 and PN-SwSt 1 are classified as type `A’, `B’ or `C’ for the first time. The AIBs at 6.2, 7.7 and 11.2 $\mu$m are used to obtain band intensity ratios for 6.2/7.7 and 11.2/6.2, which respectively indicate PAH size as number of carbon atoms and the ionization conditionsof the medium. The smaller value of 6.2/7.7 points towards the presence of large PAH molecules, while higher value of 11.2/6.2 ratio relates to harsh conditions around the object. In general, for star-forming regions, the 6.2/7.7 band ratio obtained is >1 and the 11.2/6.2 ratio is >2, while for late type carbon stars, these valuesare <1 and <2. This indicates that small/medium-sized ionized PAHs are likely in star-forming regions and large PAHs in evolved stars. For each of the 27 plain PAH molecules, the integrated intensity in these bands is obtained from the computed infrared spectra and the band ratios are calculated. The ratio 6.2/7.7 in severalcomputed medium and large sized PAH cations is in the range of observed ratio in most objects, but some molecules show large variations in band ratios, indicating that PAHs possible in interstellar medium could be more complex and with irregular structures.