arXiv subjects

C. Swastik

Publications and source records attributed to C. Swastik.

12 recordsLinked to original sources

arXiv2026

A Quiet Host in an Active Planet-Forming Disk: Optical Spectroscopy of WISPIT 2

WISPIT 2 is a young pre-main-sequence star hosting a multi-ringed transition disk and two directly imaged protoplanets, including the accreting WISPIT 2b, making it the closest known analogue to PDS 70. We present the first optical spectrum of its central star, obtained with HFOSC on the 2-m Himalayan Chandra Telescope, and derive its atmospheric parameters, test its youth, and constrain its accretion state. We analyse low-resolution spectra with iSpec and validate the pipeline at HFOSC resolution against Gaia FGK Benchmark Stars and K-type pre-main-sequence templates. We measure T_eff = 4551 +/- 150 K, log g = 4.32 +/- 0.18, and a low-resolution, model-dependent global metallicity [M/H] = -0.17 +/- 0.16. The surface gravity and Li I equivalent width support the pre-main-sequence nature of the host. H-alpha remains in net absorption but is partially filled by weak emission at only 1.5-2.0 sigma, approximately 1.1 dex below the expected chromospheric-noise level and therefore consistent with chromospheric activity rather than detectable accretion. We place a 95% upper limit on the stellar accretion rate of 3.6 x 10^-11 solar masses per year, below even the lowest monitored value for PDS 70 and implying a host-to-planet accretion-rate ratio below approximately 18. Both known double-protoplanet hosts therefore show strongly suppressed or undetectable stellar accretion. Larger spectroscopic samples are needed to determine whether this is common in multi-protoplanet transition disks.

astro-ph.SR↗

arXiv2025

Imaging the LkCa 15 system in polarimetry and total intensity without self-subtraction artefacts

Studying young protoplanetary disks is essential for understanding planet formation, but traditional angular differential imaging can introduce self-subtraction artefacts that hinder interpretation of small-scale structures. We present high-resolution total- and polarized-intensity Ks-band images of the LkCa~15 system obtained with SPHERE using near-simultaneous reference-star differential imaging (star-hopping), yielding self-subtraction-free images beyond 0.1 arcsec. LkCa~15 hosts a ~160 au protoplanetary disk and has previously been reported to harbour candidate protoplanets at separations of 15--18 au. We analyse the disk morphology and dust properties and search for super-Jupiter planets beyond 20 au. We first model the near-infrared scattered-light images together with ALMA submillimetre continuum data using RADMC-3D and a two grain-size (micron and millimetre) compact olivine model. While this model broadly reproduces the disk geometry, it overpredicts the degree of forward scattering in the near-infrared. To investigate this discrepancy, we extract the scattering phase function S(theta) and polarized fraction P(theta) from the SPHERE data and compare them with aggregate-scattering models. The observed phase functions disfavour compact Mie spheres and are better matched by porous aggregates (CAHP). Recomputing the scattered-light models with porous CAHP grains in the disk surface layer significantly improves agreement with the observed Ks-band morphology and polarization, while retaining compact millimetre grains to reproduce the ALMA continuum. No new planetary companions are detected; we place upper mass limits of ~1.5 MJ beyond 200 au and ~3.6 MJ in the inner disk. Our results demonstrate that combining star-hopping imaging with phase-function diagnostics provides strong constraints on dust grain properties in protoplanetary disks.

astro-ph.EP↗

arXiv2025

Exoplanet Classification through Vision Transformers with Temporal Image Analysis

The classification of exoplanets has been a longstanding challenge in astronomy, requiring significant computational and observational resources. Traditional methods demand substantial effort, time, and cost, highlighting the need for advanced machine learning techniques to enhance classification efficiency. In this study, we propose a methodology that transforms raw light curve data from NASA's Kepler mission into Gramian Angular Fields (GAFs) and Recurrence Plots (RPs) using the Gramian Angular Difference Field and recurrence plot techniques. These transformed images serve as inputs to the Vision Transformer (ViT) model, leveraging its ability to capture intricate temporal dependencies. We assess the performance of the model through recall, precision, and F1 score metrics, using a 5-fold cross-validation approach to obtain a robust estimate of the model's performance and reduce evaluation bias. Our comparative analysis reveals that RPs outperform GAFs, with the ViT model achieving an 89.46$\%$ recall and an 85.09$\%$ precision rate, demonstrating its significant capability in accurately identifying exoplanetary transits. Despite using under-sampling techniques to address class imbalance, dataset size reduction remains a limitation. This study underscores the importance of further research into optimizing model architectures to enhance automation, performance, and generalization of the model.

astro-ph.EP↗

arXiv2024

The 10 pc Neighborhood of Habitable Zone Exoplanetary Systems: Threat Assessment from Stellar Encounters & Supernovae

The habitability of a planet is influenced by both its parent star and the properties of its local stellar neighborhood. Potential threats to habitability from the local stellar environment mainly arise from two factors: cataclysmic events such as powerful stellar explosions and orbital perturbations induced by close stellar encounters. Among the 4,500+ exoplanet-hosting stars, about 140+ are known to host planets in their habitable zones. In this study, we use \textit{Gaia DR3} data to investigate the 10~pc stellar neighborhood of the 84 habitable zone systems (HZS) closest to the Sun. We assess the possible risks that local stellar environment of these HZS pose to their habitability. In particular, we find that HD~165155 has a high stellar density around it, making it likely to experience at least one flyby encounter within a span of 5~Gyr. We also identified two high-mass stars ($M \geq 8 M_\odot$) as potential progenitors of supernovae, which could threaten the long-term survivability of habitable zone systems HD~48265 and TOI-1227. Further, to quantify the similarity between habitable zone stars and the Sun, as well as their respective 10~pc stellar environments, we employ various astrophysical parameters to define a Solar Similarity Index (SSI) and a Neighborhood Similarity Index (NSI). Our analysis suggests that HD~40307 exhibits the closest resemblance to the solar system, while HD~165155 shows the least resemblance.

astro-ph.SR↗

arXiv2024

Age analysis of extrasolar planets: Insight from stellar isochrone models

There is growing evidence from stellar kinematics and galactic chemical evolution (GCE) suggesting that giant planets (M$_{P}\geq$0.3$M_{J}$) are relatively young compared to the most commonly occurring population of small planets (M$_{P} <$0.3$M_{J}$). To further test the validity of these results, we analyzed the ages for a large number of 2336 exoplanet hosting stars determined using three different but well-established isochrone fitting models, namely, PARSEC, MIST, and Yonsei Yale (YY). As input parameters, we used Gaia DR3 parallaxes, magnitudes, and photometric temperature, as well as spectroscopically determined more accurate temperatures and metallicities from the Sweet Catalog. Our analysis suggests that $\sim$~50$\%$ to 70$\%$ of stars with planets are younger than the sun. We also find that, among the confirmed exoplanetary systems, stars hosting giant planets are even younger compared to small planet hosts. The median age of $\sim$~2.61 to 3.48~Gyr estimated for the giant planet-hosting stars (depending on the model input parameters) suggests that the later chemical enrichment of the galaxy by the iron-peak elements, largely produced from Type Ia supernovae, may have paved the way for the formation of gas giants. Furthermore, within the giant planet population itself, stars hosting hot Jupiters (orbital period $\le$10 days) are found to be younger compared to the stellar hosts of cool and warm Jupiters (orbital period $>$10 days), implying that hot Jupiters could be the youngest systems to emerge in the progression of planet formation.

astro-ph.EP↗

arXiv2024

PDS 70 unveiled by star-hopping: total intensity, polarimetry and mm-imaging modeled in concert

Context. Most ground-based planet search direct imaging campaigns use angular differential imaging, which distorts the signal from extended sources like protoplanetary disks. In the case PDS 70, a young system with two planets found within the cavity of a protoplanetary disk, obtaining a reliable image of both planets and disk is essential to understanding planet-disk interactions. Aims. Our goals are to reveal the true intensity of the planets and disk without self-subtraction effects for the first time, search for new giant planets beyond separations of 0.1" and to study the morphology of the disk shaped by two massive planets. Methods. We present YJHK-band imaging, polarimetry, and spatially resolved spectroscopy of PDS 70 using near-simultaneous reference star differential imaging, also known as star-hopping. We created a radiative transfer model of the system to match the near-infrared imaging and polarimetric data, along with sub-millimeter imaging from ALMA. Furthermore, we extracted the spectra of the planets and the disk and compared them. Results. We find that the disk is quite flared with a scale height of ~15% at the outer edge of the disk at ~90 au, similar to some disks in the literature. The gap inside of ~50 au is estimated to have ~1% of the dust density of the outer disk. The Northeast outer disk arc seen in previous observations is likely the outer lip of the flared disk. Abundance ratios of grains estimated by the modeling indicate a shallow grain-size index > -2.7, instead of the canonical -3.5. There is both vertical and radial segregation of grains. Planet c is well separated from the disk and has a spectrum similar to planet b, clearly redder than the disk spectra. Planet c is possibly associated with the sudden flaring of the disk starting at ~50 au. No new planets > 5 Mj were found.

astro-ph.EP↗

arXiv2023

Protoplanetary disks in $K_s$-band total intensity and polarized light

Diverse protoplanetary disk morphology can result from planet-disk interaction, suggesting planetary presence. To date, most scattered light imaging campaigns have probed polarized light, which is only a fraction of the total light and not very sensitive to planets. To observe and characterize protoplanetary disk systems in the near-infrared in both polarized and total intensity light, we carried out an unprecedented study of scattering properties of disks, as well as of any planetary companions. Using SPHERE with star-hopping at the Very Large Telescope, we observed 29 disk hosts and their reference stars in $K_s$-band polarized light. We extracted disks in total intensity by adopting the data imputation concept with sequential non-negative matrix factorization (DI-sNMF). We obtained high-quality disk images in total intensity for 15 systems and in polarized light for 23. For well-recovered disks in polarized light and total intensity, we parameterized the polarization fraction phase functions using scaled beta distribution: the peak of polarization fraction tentatively correlates with the peak scattering angle, which could be reproduced using certain compact dust, yet more detailed modeling studies are needed. We investigated the empirical DI-sNMF detectability of disks using logistic regression: total intensity detectability of disks primarily depends on host star brightness. For disks with SPHERE data in $Y$-/$J$-/$H$-band, we summarized their polarized color at ~90 deg scattering angle: most of disks are blue in polarized $J-K_s$ color, and they are relatively redder as stellar luminosity increases, indicating larger scatterers. High-quality disk imagery in both total intensity and polarized light thus allows for disk characterization in polarization fraction, and reduces the confusion between disk and planetary signals.

astro-ph.EP↗

arXiv2023

Age distribution of exoplanet host stars: Chemical and Kinematics age proxies from GAIA DR3

The GAIA space mission is impacting astronomy in many significant ways by providing a uniform, homogeneous and precise data set for over 1 billion stars and other celestial objects in the Milky Way and beyond. Exoplanet science has greatly benefited from the unprecedented accuracy of stellar parameters obtained from GAIA. In this study, we combine photometric, astrometric, and spectroscopic data from the most recent Gaia DR3 to examine the kinematic and chemical age proxies for a large sample of 2611 exoplanets hosting stars whose parameters have been determined uniformly. Using spectroscopic data from the Radial Velocity Spectrometer (RVS) onboard GAIA, we show that stars hosting massive planets are metal-rich and $α$-poor in comparison to stars hosting small planets. The kinematic analysis of the sample reveals that the stellar systems with small planets and those with giant planets differ in key aspects of galactic space velocity and orbital parameters, which are indicative of age. We find that the galactic orbital parameters have a statistically significant difference of 0.06 kpc for $Z_{max}$ and 0.03 for eccentricity respectively. Furthermore, we estimated the stellar ages of the sample using the MIST-MESA isochrone models. The ages and its proxies for the planet-hosting stars indicate that the hosts of giant planetary systems are younger compared to the population of stars harboring small planets. These age trends are also consistent with the chemical evolution of the galaxy and the formation of giant planets from the core-accretion process.

astro-ph.SR↗

arXiv2022

Carbon abundance of stars in the LAMOST-Kepler field

The correlation between host star iron abundance and the exoplanet occurrence rate is well-established and arrived at in several studies. Similar correlations may be present for the most abundant elements, such as carbon and oxygen, which also control the dust chemistry of the protoplanetary disk. In this paper, using a large number of stars in the Kepler field observed by the LAMOST survey, it has been possible to estimate the planet occurrence rate with respect to the host star carbon abundance. Carbon abundances are derived using synthetic spectra fit of the CH G-band region in the LAMOST spectra. The carbon abundance trend with metallicity is consistent with the previous studies and follows the Galactic chemical evolution (GCE). Similar to [Fe/H], we find that the [C/H] values are higher among giant planet hosts. The trend between [C/Fe] and [Fe/H] in planet hosts and single stars is similar; however, there is a preference for giant planets around host stars with a sub-solar [C/Fe] ratio and higher [Fe/H]. Higher metallicity and sub-solar [C/Fe] values are found among younger stars as a result of GCE. Hence, based on the current sample, it is difficult to interpret the results as a consequence of GCE or due to planet formation.

astro-ph.EP↗

arXiv2022

Galactic chemical evolution of exoplanet host stars: Are high-mass planetary systems young?

The imprints of stellar nucleosynthesis and chemical evolution of the galaxy can be seen in different stellar populations, with older generation stars showing higher $α$-element abundances while the later generations becoming enriched with iron-peak elements. The evolutionary connections and chemical characteristics of circumstellar disks, stars, and their planetary companions can be inferred by studying the interdependence of planetary and host star properties. Numerous studies in the past have confirmed that high-mass giant planets are commonly found around metal-rich stars, while the stellar hosts of low-mass planets have a wide range of metallicity. In this work, we analyzed the detailed chemical abundances for a sample of $>900$ exoplanet hosting stars drawn from different radial velocity and transit surveys. We correlate the stellar abundance trends for $α$ and iron-peak elements with the planets' mass. We find the planet mass-abundance correlation to be primarily negative for $α$-elements and marginally positive or zero for the iron-peak elements, indicating that stars hosting giant planets are relatively younger. This is further validated by the age of the host stars obtained from isochrone fitting. The later enrichment of protoplanetary material with iron and iron-peak elements is also consistent with the formation of the giant planets via the core accretion process. A higher metal fraction in the protoplanetary disk is conducive to rapid core growth, thus providing a plausible route for the formation of giant planets. This study, therefore, indicates the observed trends in stellar abundances and planet mass are most likely a natural consequence of Galactic chemical evolution.

astro-ph.EP↗

arXiv2021

Host star metallicity of directly imaged wide-orbit planets: implications for planet formation

Directly imaged planets are self-luminous companions of pre-main sequence and young main sequence stars. They reside in wider orbits ($\sim10\mathrm{s}-1000\mathrm{s}$~AU) and generally are more massive compared to the close-in ($\lesssim 10$~AU) planets. Determining the host star properties of these outstretched planetary systems is important to understand and discern various planet formation and evolution scenarios. We present the stellar parameters and metallicity ([Fe/H]) for a subsample of 18 stars known to host planets discovered by the direct imaging technique. We retrieved the high-resolution spectra for these stars from public archives and used the synthetic spectral fitting technique and Bayesian analysis to determine the stellar properties in a uniform and consistent way. For eight sources, the metallicities are reported for the first time, while the results are consistent with the previous estimates for the other sources. Our analysis shows that metallicities of stars hosting directly imaged planets are close to solar with a mean [Fe/H] = $-0.04\pm0.27$~dex. The large scatter in metallicity suggests that a metal-rich environment may not be necessary to form massive planets at large orbital distances. We also find that the planet mass-host star metallicity relation for the directly imaged massive planets in wide-orbits is very similar to that found for the well studied population of short period ($\lesssim 1$~yr) super-Jupiters and brown-dwarfs around main-sequence stars.

astro-ph.EP↗

arXiv2020

Bouncing Universe with exotic radiation

Scenario bouncing can give the cosmology singularity problem a possible way out. Finding a solution for the universe's bouncing model requires proper estimation of the state equation. We present two such state equations that give us the solution for a bounce. One such case is the exotic radiation, where we assumed that exotic radiation dominated the universe during the bounce occurrence. We also considered another case where quintom matter scenario existed previously, and the newly proposed exotic radiation scenario coexisted. In these two cases, we have shown that all the necessary conditions for the bounce are fulfilled. Such new ways certainly increase support and flexibility for the bouncing universe model.

gr-qc↗