arXiv subjects

Sreeja S Kartha

Publications and source records attributed to Sreeja S Kartha.

16 recordsLinked to original sources

arXiv2026

A multi-epoch spectral and photometric study to understand the nature of the interesting Be star candidate HD 249179

HD 249179, a B5 star with unclear classification as either a classical Be star or a high-mass X-ray binary (HMXB) system, is investigated using the first multi-epoch spectral and photometric analysis over 5.5 years (2017-2023). Optical spectroscopy from LAMOST, BeSS, and the Himalayan Chandra Telescope show large H-alpha variability (-17.6 to -3.2 angstrom, in six days, returning to -31.3 angstrom, in 2022) and the presence of Paschen and O I lines, confirming active circumstellar disc dynamics. H-alpha double peak profiles show weak V/R ratio variations 0.92--1.28 indicating one-armed density waves. TESS observations presents hybrid beta Cephei/slowly pulsating B-type variability with primary frequencies of 3.5 c/d and 1.5-1.6 c/d, and an outburst-like brightening with phase-dependent frequency evolution. Simultaneous anti-correlation between optical brightness and H$α$ equivalent width suggest moderate disc inclination. Gaia DR3 CMD analysis rules out HD 249179 as a BeXRB but instead places it in the classical Be-star region. The BeppoSAX survey, despite previous X-ray cataloging, found no emission above the sensitivity limits. Our analysis shows that HD 249179 is most likely a classical Be star with variable circumstellar disc activity and hybrid pulsations, and that there is no strong evidence for an HMXB classification.

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arXiv2025

Non-Linear Solar EUV-Driven Sodium Release from the Lunar Surface: A Contrast to the Linear PSD Model

The correlation between solar Extreme Ultra-Violet (EUV) radiation above 8.8 eV and the release of sodium from the lunar surface via photon-stimulated desorption (PSD) is investigated. We use simultaneous measurements of EUV photon flux and Na optical spectral line flux ($F_{Na}$) from the lunar exosphere. Data were acquired with the high-resolution (R$\sim$72000) Echelle Spectrograph on the 2.34-m Vainu Bappu Telescope during the lunar first quarter (January-March 2024), observing $Na\,I$ D2 and D1 flux at altitudes below $\sim590\,km$ from the surface. Simultaneous EUV and FUV measurements were acquired from the GOES-R Series Extreme Ultraviolet Sensor (EUVS), while NUV data were obtained from the Total and Spectral Solar Irradiance Sensor-1 (TSIS-1) aboard the ISS. We correlated $F_{Na}$ with EUV photon flux from EUVS across six bands spanning $256-1405\,\text{Å}$ (48.5-8.8 eV) and NUV (2000-4000 $\text{Å}$) from TSIS-1. A non-linear rise in lunar exospheric sodium with increasing EUV and FUV fluxes was observed, contrasting with previous linear PSD models. The EUV radiation above 10 eV drives sodium release, with 256-304 $\text{Å}$ wavelengths as dominant contributors. Additionally, the NUV flux and $F_{Na}$ are positively correlated, indicating the role of sodium release. The zenith column density averages $3.3\times10^{9}$ atoms cm$^{-2}$, with Characteristic temperatures averaging at $\sim$6700K and scale heights of $\sim$1500 km. Elevated temperatures and sodium densities during solar activity suggest enhanced Na release during flares. These results emphasize the need for a revised PSD model above 8.8 eV and improved constraints on the PSD cross-section.

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arXiv2025

Probing the resolved K-S relation in nearby galaxies: Insights from UVIT and ALMA observations

This study examines the resolved Kennicutt Schmidt (rKS) relation, defined as the connection between the star formation rate surface density (Sigma SFR) and the molecular gas mass surface density (Sigma H2) in the high-density central regions of three nearby barred spiral galaxies hosting AGN: NGC 1365, NGC 1433, and NGC 1566. Utilising high-resolution archival data from AstroSat/UVIT for UV imaging and the Atacama Large Millimeter/submillimeter Array (ALMA) for CO(2-1) molecular gas mapping, we explore recent star formation and gas distribution with a spatial resolution of about 120 to 132 pc. Our findings reveal a sublinear rKS law, with slopes ranging from about 0.17 to 0.71. Notably, NGC 1566 exhibits a robust rKS relation consistent with previous studies, while NGC 1365 and NGC 1433 show weaker correlations. These differences are likely due to the smaller number of identified star-forming regions in these galaxies compared to NGC 1566, as well as the central molecular gas concentrations and varying star formation activity in their bars and nuclear regions. These results also support the idea that the rKS relation deviates from linearity in extreme environments, such as starburst galaxies and galactic centres. Additionally, we find a generally low median star formation efficiency (SFE) within the bars of these galaxies, suggesting that while bars may drive nuclear starbursts and contribute to bulge growth, they do not significantly increase SFE. Furthermore, a negative correlation between SFE and Sigma H2 is observed across the sample, both within and outside the bar regions, suggesting that higher Sigma H2 may lead to lower SFE in the central regions of these galaxies. Our findings highlight that Sigma H2 plays a primary role in shaping the observed trends in SFE, rather than the presence of a bar itself.

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arXiv2025

Connecting the dots: Tracing the evolutionary pathway of Polar Ring Galaxies in the cases of NGC 3718, NGC 2685, and NGC 4262

Polar Ring Galaxies (PRGs) are a unique class of galaxies characterised by a ring of gas and stars orbiting nearly orthogonal to the main body. This study delves into the evolutionary trajectory of PRGs using the exemplary trio of NGC 3718, NGC 2685, and NGC 4262. We investigate the distinct features of PRGs by analysing their ring and host components to reveal their unique characteristics through Spectral Energy Distribution (SED) fitting. Using CIGALE, we performed SED fitting to independently analyse the ring and host spatially resolved regions, marking the first decomposed SED analysis for PRGs, which examines stellar populations using high-resolution observations from AstroSat UVIT at a resolved scale. The UV-optical surface profiles provide an initial idea that distinct patterns in the galaxies, with differences in FUV and NUV, suggest three distinct stages of ring evolution in the selected galaxies. The study of resolved-scale stellar regions reveals that the ring regions are generally younger than their host galaxies, with the age disparity progressively decreasing along the evolutionary sequence from NGC 3718 to NGC 4262. Star formation rates (SFR) also exhibit a consistent pattern, with higher SFR in the ring of NGC 3718 compared to the others, and a progressive decrease through NGC 2685 and NGC 4262. Finally, the representation of the galaxies in the HI gas fraction versus the NUV- r plane supports the idea that they are in three different evolutionary stages of PRG evolution, with NGC 3718 in the initial stage, NGC 2685 in the intermediate stage, and NGC 4262 representing the final stage. NGC 3718, NGC 2685, and NGC 4262 represent different stages of this evolution, highlighting the dynamic nature of PRGs and emphasising the importance of studying their evolutionary processes to gain insights into galactic formation and evolution.

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arXiv2024

Deciphering the properties of UV upturn galaxies in the Virgo cluster

The UV upturn refers to the increase in UV flux at wavelengths shorter than 3000 Å observed in quiescent early-type galaxies (ETGs), which still remains a puzzle. In this study, we aim to identify ETGs showing the UV upturn phenomenon within the Virgo galaxy cluster. We utilized a color-color diagram to identify all potential possible UV upturn galaxies. The Spectral Energy Distributions (SED) of these galaxies were then analyzed using the CIGALE software; we confirmed the presence of UV upturn in galaxies within the Virgo cluster. We found that the SED fitting method is the best tool to visualize and confirm the UV upturn phenomenon in ETGs. Our findings reveal that the population distributions regarding stellar mass and star formation rate properties are similar between UV upturn and red sequence galaxies. We suggest that the UV contribution originates from old stellar populations and can be modeled effectively without a burst model. Moreover, by estimating the temperature of the stellar population responsible for the UV emission, we determined it to be 13,000 K to 18,000 K. These temperature estimates support the notion that the UV upturn likely arises from the contribution of low mass evolved stellar populations (extreme horizontal branch stars). Furthermore, the Mg2 index, a metallicity indicator, in the confirmed upturn galaxies shows higher strength and follows a similar trend to previous studies. This study sheds light on the nature of UV upturn galaxies within the Virgo cluster and provides evidence that low-mass evolved stellar populations are the possible mechanisms driving the UV upturn phenomenon.

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arXiv2024

The Interaction Jigsaw: Investigating star formation in interacting galaxies

Interaction between galaxies play a pivotal role in their evolution. Ongoing star formation in spiral galaxies can be affected by these processes. We select a sample of interacting galaxies in field environments at various interaction stages and are nearly face-on and chose galaxy pairs NGC 2207/IC 2163, NGC 4017/4016 (ARP 305) and NGC 7753/7752 (ARP 86). We use the UltraViolet Imaging Telescope (UVIT) onboard AstroSat to characterize the star-forming regions in the galaxy with a superior resolution of ~1.4". We identified and characterized star-forming regions in the UVIT images of the sample and correlated them with the neutral hydrogen (HI) distribution. We detected localized regions of enhancement in star formation surface density and distortions in the sample of galaxies. We found this consistent with the distribution of HI in the galaxy. These are possible evidence of past and ongoing interactions affecting the star formation properties in the galaxies. We then conducted a study to understand whether the observed enhancements hold true for a wider sample of interacting galaxies. We observe a moderate enhancement in the star formation rate (SFR) with the interaction class, with the maximum of 1.8 being in the merger class of galaxies. We studied the SFR enhancement for the main galaxies in our sample as a function of pair mass ratio and pair separation. We observe a strong anti-correlation between the SFR enhancement and pair mass ratio and no linear correlation between the enhancement and pair separation. This suggests that the enhancement in interaction-induced star formation may be more strongly influenced by the pair mass ratios, rather than the pair separation. We also infer that the pair separation can possibly act as a limiting parameter for the SFR enhancement.

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arXiv2024

Study of the Balmer decrements for Galactic classical Be stars using the Himalayan Chandra Telescope of India

In a recent study, Banerjee et al. (2021) produced an atlas of all major emission lines found in a large sample of 115 Galactic field Be stars using the 2-m Himalayan Chandra Telescope (HCT) facility located at Ladakh, India. This paper presents our further exploration of these stars to estimate the electron density in their discs. Our study using Balmer decrement values indicate that their discs are generally optically thick in nature with electron density (n_e) in their circumstellar envelopes (CEs) being in excess of 10^13 cm^-3 for around 65% of the stars. For another 19% stars, the average n_e in their discs probably range between 10^12 cm^-3 and 10^13 cm^-3. We noticed that the nature of the Hα and H\b{eta} line profiles might not influence the observed Balmer decrement values (i.e. D_34 and D_54) of the sample of stars. Interestingly, we also found that around 50% of the Be stars displaying D_34 greater than 2.7 are of earlier spectral types, i.e. within B0 -B3.

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arXiv2024

Beyond the Rings: Polar Ring Galaxy NGC 4262 and its Globular Cluster System

In the context of the hierarchical model of galaxy evolution, polar ring galaxies (PRGs) are considered the intermediate phase between ongoing mergers and quiescent galaxies. This study explores the globular cluster system (GCS) and its properties in the nearest PRG, NGC4262, serving as a pilot investigation to study GCS in nearby PRGs. We utilize wide and deep field observations of the CFHT as part of the NGVS to investigate the GCS of NGC4262. We presented the first optical image of NGC4262 with an optically faint ring component. The photometric analysis of the GCS displays a distinct color bimodality. We estimate the total number of GCs for NGC4262 to be 266$\pm$16 GCs with a specific frequency of 4.2$\pm$0.8 and a specific mass of 0.23$\pm$0.01, which is relatively high compared to other galaxies of similar mass and environmental conditions. The spatial and azimuthal distributions of subpopulations reveal strong evidence of previous interactions within the host galaxy. The color distribution of GCS in NGC4262 shows a gradient of -0.05$\pm$0.01 within 5.5\arcmin, supporting the notion of past interactions and evolutionary transitions. PRG NGC4262 conforms to the overall trend of the GCS mass with respect to the halo mass. Furthermore, our investigation of the global scaling relations between GCS and host galaxy parameters provides further support for the hypothesis that PRGs are an intermediate phase connecting ongoing mergers and quiescent galaxies.

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arXiv2024

Disentangling the association of PAH molecules with star formation : Insights from JWST and UVIT

Polycyclic Aromatic Hydrocarbons (PAHs) are ubiquitous complex molecules in the interstellar medium and are used as an indirect indicator of star-formation. On the other hand the ultraviolet (UV) emission from the young massive stars directly traces the star formation activity in a galaxy. The James Webb Space Telescope (JWST), along with the UltraViolet Imaging Telescope (UVIT), opened up a new window of opportunity to make a better understanding of the properties of the PAH molecules associated with the star-forming regions. In this study, we investigate how the resolved scale properties of PAH molecules in nearby galaxies are affected by star-formation. We analyze the PAH features observed at 3.3, 7.7, and 11.3 μm using F335M, F770W, and F1130W images obtained from JWST. Additionally, we utilize UVIT images to assess the star formation associated with these PAH emitting regions. Our study focuses on three galaxies, namely NGC 628, NGC 1365, and NGC 7496, selected based on the availability of both JWST and UVIT images. Based on the resolved scale study on the PAH bright regions using JWST and UVIT images, we found that the fraction of ionized PAH molecules is high in the star-forming regions with high ΣSFR. We observed that emission from smaller PAH molecules is more in the star-forming regions with higher ΣSFR. Our study suggests that the PAH molecules excited by the photons from SF regions with higher ΣSFR are dominantly smaller and ionized molecules. UV photons from the star-forming regions could be the reason for a higher fraction of the ionized PAHs. We suggest that the effect of high temperature in the star-forming regions and the formation of smaller PAH molecules in the star-forming regions might also be resulting in the higher fraction of emission in the F335MPAH band.

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arXiv2023

DES J024008.08-551047.5:A new member of the polar ring galaxy family

During the visual observations of optical imaging data obtained from the DECaLS, a serendipitous discovery emerged, revealing the presence of a ringed galaxy, DES J024008.08-551047.5 (DJ0240). We performed one dimensional isophotal and two-dimensional GALFIT analysis to confirm the orthogonal nature of the ring galaxy and identify distinct components within the host galaxy. We discovered the galaxy DJ0240 as a potential PRG candidate with a ring component positioned almost perpendicular to the host galaxy. The position angles of the ring and host components have been determined to be 80 and 10 degrees, respectively, indicating that they are nearly orthogonal to each other. We observed that the ring component extends three times more than the host galaxy and shows a distinct colour separation, being bluer than the host. The estimated g - r colour values of host and ring components are 0.86+/-0.02 and 0.59+/-0.10 mag, respectively. The colour value of the ring component is similar to typical spiral galaxies. The host galaxy`s colour and the presence of a bulge and disk components indicate the possibility of the host galaxy being a lenticular type. Based on the comparison of photometric properties between the PRGs and other ring-type galaxies (RTGs), our findings reveal a subtle, yet noticeable, colour difference between the host and ring components. We observed that both host and ring components of DJ0240 align more closely with PRGs than with RTGs. Furthermore, we compared the sersic index values of the ring component (nring) of galaxy DJ0240 with a selected sample of PRGs and Hoag-type galaxies. The results showed DJ0240 had a remarkably low nring value of 0.13, supporting the galaxy`s classification as a PRG. Hence, we suggest that the ring galaxy DJ0240 is a highly promising candidate for inclusion in the family of PRGs.

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arXiv2023

What drives the wheels of evolution in NGC 1512? A UVIT study

Environmental and secular processes play a pivotal role in the evolution of galaxies. These can be due to external processes such as interactions or internal processes due to the action of bar, bulge and spiral structures. Ongoing star formation in spiral galaxies can be affected by these processes. Studying the star formation in the galaxy can give insights into the evolution of the galaxy. The ongoing interaction between barred-spiral galaxy NGC 1512 and its satellite NGC 1510 offers an opportunity to investigate how galactic interactions and the presence of a galactic bar influence the evolution of NGC 1512. We aim to understand the recent star formation activity in the galaxy pair and thus gain insight into the evolution of NGC 1512. The UltraViolet Imaging Telescope (UVIT) onboard AstroSat enables us to study the star-forming regions in the galaxy with a spatial resolution of ~85 pc in the galaxy rest frame. We identified and studied 175 star-forming regions in UVIT FUV image of NGC 1512 and correlated with the neutral hydrogen (HI) distribution. We detected localized regions of star formation enhancement and distortions in the galactic disk. This is consistent with HI distribution in the galaxy. This is evidence of past and ongoing interactions affecting the star formation properties of the galaxy. We studied the properties of the inner ring. We find that the regions of the inner ring show maximum star formation rate density (log(SFRDmean[Msolaryr-1kpc-2]) ~ -1.7) near the major axis of the bar, hinting at a possible crowding effect in these regions. The region of the bar in the galaxy is also depleted of UV emission. This absence suggests that the galactic bar played an active role in the redistribution of gas and quenching of star formation inside identified bar region. Hence, we suggest that both the secular and environmental factors might influence the evolution of NGC 1512.

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arXiv2023

Estimation of Stellar Parameters and Mass Accretion Rate of Classical T Tauri Stars from LAMOST DR6

Classical T Tauri stars are low-mass pre-main sequence stars with an active circumstellar environment. In this work we present the identification and study of 260 Classical T Tauri stars using LAMOST Data Release 6, among which 104 stars are newly identified. We distinguish Classical T Tauri stars from Giants and main-sequence dwarfs based on the log g values and the presence of H (alpha) emission line and infrared excess that arises from the circumstellar accretion disk. We estimated the mass and age of 210 stars using the Gaia color-magnitude diagram. The age is from 0.1 to 20 Myr, where 90% of the stars have age below 10 Myr and the mass ranges between 0.11 to 1.9 M(solar). From the measured H(alpha) equivalent widths, we homogeneously estimated the mass accretion rates for 172 stars, with most values ranging from 10^-7 to 10^-10 M(solar) yr^-1. The mass accretion rates are found to follow a power law distribution with the mass of the star, having a relation of the form Macc proportional to M(star)^1.43 +/- 0.26, in agreement with previous studies.

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arXiv2022

Emission line star catalogues post-Gaia DR3: A validation of Gaia DR3 data using LAMOST OBA emission catalogue

Gaia DR3 and further releases have the potential to identify and categorise new emission-line stars in the Galaxy. We perform a comprehensive validation of astrophysical parameters from Gaia DR3 with the spectroscopically estimated emission-line star parameters from LAMOST OBA emission catalogue. We compare different astrophysical parameters provided by Gaia DR3 with those estimated using LAMOST spectra. By using a larger sample of emission-line stars, we perform a global polynomial and piece-wise linear fit to update the empirical relation to convert Gaia DR3 pseudo-equivalent width to observed equivalent width, after removing the weak emitters from the analysis. We find that the emission-line source classifications given by DR3 is in reasonable agreement with the classification from LAMOST OBA emission catalogue. The astrophysical parameters estimated by esphs module from Gaia DR3 provides a better estimate when compared to gspphot and gspspec. A second-degree polynomial relation is provided along with piece-wise linear fit parameters for the equivalent width conversion. We notice that the LAMOST stars with weak Hα emission are not identified to be in emission from BP/RP spectra. This suggests that emission-line sources identified by Gaia DR3 is incomplete. In addition, Gaia DR3 provides valuable information about the binary and variable nature of a sample of emission-line stars.

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arXiv2022

Decoding the X-ray flare from MAXI J0709-159 using optical spectroscopy and multi-epoch photometry

We present a follow-up study on the recent detection of two X-ray flaring events by MAXI/GSC observations in soft and hard X-rays from MAXI J0709-159 in the direction of HD 54786 (LY CMa), on 2022 January 25. The X-ray luminosity during the flare was around 10^(37) erg/s (MAXI), which got reduced to 10^(32) erg/s (NuSTAR) after the flare. We took low-resolution spectra of HD 54786 from HCT and VBT facilities in India, on 2022 February 1 and 2. In addition to H-alpha emission, we found emission lines of He I in the optical spectrum of this star. By comparing our spectrum of the object with those from literature we found that He I lines show variability. Using photometric study we estimate that the star is having effective temperature of 20000 K. Although HD 54786 is reported as a supergiant in previous studies, our analysis favours it to be evolving off the main sequence in the Color-Magnitude Diagram. We could not detect any infrared excess, ruling out the possibility of IR emission from a dusty circumstellar disc. Our present study suggests that HD 54786 is a Be/X-ray binary system with a compact object companion, possibly a neutron star.

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arXiv2021

Identification of emission-line stars in transition phase from pre-main sequence to main sequence

Pre-main sequence (PMS) stars evolve into main sequence (MS) phase over a period of time. Interestingly, we found a scarcity of studies in existing literature that examines and attempts to better understand the stars in PMS to MS transition phase. The purpose of the present study is to detect such rare stars, which we named as 'Transition Phase' (TP) candidates - stars evolving from the PMS to the MS phase. We identified 98 TP candidates using photometric analysis of a sample of 2167 classical Be (CBe) and 225 Herbig Ae/Be (HAeBe) stars. This identification is done by analyzing the near- and mid-infrared excess and their location in the optical color-magnitude diagram. The age and mass of 58 of these TP candidates are determined to be between 0.1-5 Myr and 2-10.5 M$_\odot$, respectively. The TP candidates are found to possess rotational velocity and color excess values in between CBe and HAeBe stars, which is reconfirmed by generating a set of synthetic samples using the machine learning approach.

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arXiv2020

A statistical search for Star-Planet Interaction in the UltraViolet using GALEX

Most (~82%) of the over 4000 confirmed exoplanets known today orbit very close to their host stars, within 0.5 au. Planets at such small orbital distances can result in significant interactions with their host stars, which can induce increased activity levels in them. In this work, we have searched for statistical evidence for Star-Planet Interactions (SPI) in the ultraviolet (UV) using the largest sample of 1355 GALEX detected host stars with confirmed exoplanets and making use of the improved host star parameters from Gaia DR2. From our analysis, we do not find any significant correlation between the UV activity of the host stars and their planetary properties. We further compared the UV properties of planet host stars to that of chromospherically active stars from the RAVE survey. Our results indicate that the enhancement in chromospheric activity of host stars due to star-planet interactions may not be significant enough to reflect in their near and far UV broad band flux.

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