arXiv subjects

Athira Unni

Publications and source records attributed to Athira Unni.

At least 19 recordsLinked to original sources

arXiv2026

Validating the SCALES DRP: A Performance Study of the Medium-Resolution IFS Mode

We present a performance verification of the medium-resolution integral field spectrograph (IFS) mode of the SCALES Data Reduction Pipeline (SCALES-DRP) using data obtained during recent cryogenic cooldown testing. SCALES (Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy), scheduled for deployment on the Keck II Telescope in late 2026, will be the first facility-class high-contrast thermal-infrared IFS, operating over the 2-5 micrometer wavelength range. The SCALES-DRP, developed within the Keck Data Reduction Pipeline Framework and implemented in Python, converts raw detector reads into calibrated three-dimensional datacubes using rectification matrices derived from position-dependent monochromatic lenslet point-spread functions, and supports both optimal and chi-sqaure-based spectral extraction to maximize signal-to-noise and extraction fidelity. We present laboratory verification results for the medium-resolution IFS mode, which combines a 0.34x0.36 arcsec lenslet subarray with an image slicer to achieve spectral resolutions of R ~ 2,000 - 5,000, focusing on the performance of the rectification matrices and the robustness of the extraction algorithms. These results demonstrate the readiness of the SCALES-DRP for instrument commissioning and early science operations. The pipeline is publicly available on GitHub.

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arXiv2026

Pre-shipment optical characterization of the SCALES instrument

The Slicer Combined with an Array of Lenslets for Exoplanet Spectroscopy (SCALES) instrument is a 1-5 micron imager and 2-5 micron integral field spectrograph, currently being commissioned on the Keck II Telescope. SCALES is optimized for exoplanet high-contrast imaging and spectroscopic characterization, and will be sensitive to older, colder exoplanets than existing instrumentation. The 12.3" x 12.3" imaging channel is designed to replicate the capabilities of NIRC2, and the low (R~35-200, 2.2" x 2.2" FOV) and medium (R~2500-5000, 0.36" x 0.34" FOV) spectral resolution modes offer new capabilities compared to existing Keck instrumentation. We present preliminary optical performance results from laboratory testing and commissioning of SCALES.

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arXiv2026

An Observational Study of Systematics in Differential Transmission Spectroscopy Using HFOSC on the Himalayan Chandra Telescope

Ground-based low-resolution transmission spectroscopy requires photometric precision of a few hundred parts per million, making it sensitive to instrumental and atmospheric systematics. This work studies the systematic effects affecting differential transmission spectroscopy using the Hanle Faint Object Spectrograph Camera (HFOSC) on the 2-m Himalayan Chandra Telescope (HCT). The study was motivated by an additional flux drop observed in the white-light curve of HAT-P-1 b. HAT-P-1 b is an ideal target for differential spectrophotometry because it has a visual binary companion with similar brightness at a suitable separation, allowing the companion star to be used as a reference. To investigate the origin of this feature, we analyzed several observational parameters, including FWHM variations, spectral trace motion, centroid drift, and spectral shifts. We also observed WASP-33 b in slitless mode to test whether differential slit losses could explain the observed systematic. In addition, observations of WASP-12 b were used to derive a broadband optical transmission spectrum using common-mode correction. The additional flux drop is unlikely to be caused only by differential slit losses, since similar differential centroid and spectral shifts are present in both slit and slitless observations. The results suggest that the observed systematic may be related to field-dependent distortions and pointing-dependent instrumental flexure, although its exact cause is still unknown. Overall, this work highlights the importance of understanding and reducing observational systematics in ground-based exoplanet transmission spectroscopy, especially for measurements that require photometric precision of a few hundred parts per million.

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arXiv2025

A new broadband atmospheric dispersion corrector for HROS-TMT

Atmospheric dispersion causes light from celestial objects with different wavelengths to refract at varying angles as it passes through Earth's atmosphere. This effect results in an elongated image at the focal plane of a telescope and diminishes fiber coupling efficiency into spectrographs. We propose an optical design that incorporates a Rotational Atmospheric Dispersion Corrector (RADC) to address the broadband dispersion encountered in the multi-object mode of the High-Resolution Optical Spectrograph (HROS) on the Thirty Meter Telescope (TMT). The RADC corrects the dispersion across the entire wavelength range (0.31-1 micron), using Amici prisms optimized for over 90% transmission efficiency and minimal angular deviation of the beam from the optical axis after dispersion correction. For enhanced accuracy, particularly in the blue region, we have, for the first time, implemented the Filippenko (1982) model in Zemax via a custom Dynamic-Link Library (DLL) file.

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arXiv2025

Addressing wavelength-correlated systematics in exoplanet transmission spectroscopy: a 2D Gaussian Process approach

Ground-based transmission spectroscopy is often dominated by systematics, which obstructs our ability to leverage the advantages of larger aperture sizes compared to space-based observations. These systematics could be time-correlated, uniform across all spectroscopic light curves, or wavelength-correlated, which could significantly affect the characterization of exoplanet atmospheres. Gaussian Processes were introduced in transmission spectroscopy by Gibson et al. (2012) to model correlated systematics in a non-parametric way. The technique uses auxiliary information about the observation and independently fits each spectroscopic light curve to provide robust atmospheric retrievals. However, this method assumes that the uncertainties in the transmission spectrum are uncorrelated in wavelength, which can cause discrepancies and degrade the precision of atmospheric retrievals. To address this limitation, we explore a 2D GP framework formulated by Fortune et al. (2024) to simultaneously model time- and wavelength-correlated systematics. We present its application to ground-based observations of TOI-4153b obtained using the 2-m Himalayan Chandra Telescope (HCT). As we move towards detecting smaller and cooler planets, developing new methods to address complex systematics becomes increasingly essential.

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arXiv2025

Atmospheric Dispersion Measurement at Hanle Site

Atmospheric dispersion introduces wavelength-dependent effects that significantly impact ground-based observations, particularly in slit- and fibre-fed spectroscopic studies. These effects reduce the signal entering the spectrograph and introduce systematic errors in radial velocity measurements. To address this challenge, atmospheric dispersion correctors are utilised. However, many existing designs of these correctors, which are based on theoretical models, often lack practical validation and consistency. The forthcoming National Large Optical Telescope (NLOT) will be installed at Hanle, a site known for its favourable astronomical sky conditions. Thus, the design of an effective dispersion corrector for the instruments on the NLOT, specifically one that compensates for the measured dispersion, is crucial. For the first time, we have directly measured atmospheric dispersion at the Hanle site using the Himalayan Faint Object Spectrograph mounted on the Himalayan Chandra Telescope. In this study, we present our methodology, the dispersion measurements obtained within the 400 to 700 nm wavelength range, and a comparison with modelled dispersion values.

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arXiv2025

A multi-object approach for studying exoplanet atmospheres using high-resolution spectrographs

Atmospheric characterization of exoplanets has traditionally relied on Low-Resolution Transmission Spectroscopy (LRTS), obtained from both space- and ground-based facilities, as well as on High-Resolution Transmission Spectroscopy (HRTS). Although HRTS can resolve individual spectral lines, it is subject to normalization degeneracies that limit the accurate retrieval of key atmospheric parameters such as pressure, abundance, and cloud opacity. A promising strategy to mitigate this issue is to combine ground-based HRTS with space-based LRTS. However, this approach depends on two separate datasets, thereby requiring two independent observations. In this study, we explore the feasibility of Multi-Object High-Resolution Transmission Spectroscopy (Mo-HRTS) as a means to constrain atmospheric parameters in retrievals using a single dataset. Through simulations based on existing spectrograph specifications for a well-studied target, we demonstrate that low-resolution broadband transmission spectra can be extracted from Mo-HRTS data.

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arXiv2025

A multi-functional fiber positioning system for Extremely Large Telescopes

We present a conceptual design for a fiber positioning system for multi-object high-resolution spectroscopy, designed to be compatible with the upcoming large telescopes with a wide field of view. The design incorporates multiple Atmospheric Dispersion Correctors (ADCs) and tip-tilt mirrors that receive non-telecentric input from individual targets and direct it to the ADCs. Here, we introduce a mechanical design for the fiber positioner that accommodates the optics and operates in a curved focal plane with a Radius of Curvature (R) of 3m. This mechanical design provides four degrees of freedom to access the focal volume, enhancing targeting efficiency. The proposed design and an efficient target allocation algorithm ensure a targeting efficiency of approximately 80-100% for a primary observation session. We also present a methodology for target assignment, positioning, and quantification based on sequential and Monte Carlo (MC) algorithms. This method has been tested on realistic fields with varying target densities to validate its performance.

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arXiv2025

An Initial Assessment of the Hanle Echelle Spectrograph for Exoplanet Atmosphere Studies

Transmission spectroscopy is an effective technique for probing exoplanetary atmospheres. While most observations have relied on space facilities such as HST and JWST, ground-based high-resolution transmission spectroscopy (HRTS) has also provided valuable insights by resolving individual atomic features. In this work, we present an initial performance assessment and feasibility test of the Hanle Echelle Spectrograph (HESP) on the 2 m Himalayan Chandra Telescope (HCT) for HRTS. As a benchmark, we observed the hot Jupiter HD 209458b during a single transit at a resolution of R = 30,000. We developed a Python-based, semi-automated data reduction and analysis pipeline that includes corrections for telluric contamination and stellar radial velocity shifts. The final achieved signal-to-noise ratio and spectral stability allow us to probe for features at the 0.1% level. This work establishes a methodology and demonstrates the operational capability of the HESP-HCT for obtaining high-resolution transmission spectra.

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arXiv2025

SCALES-DRP : A Data Reduction Pipeline for an Upcoming Keck Thermal Infrared Spectrograph

We present the end-to-end data reduction pipeline for SCALES (Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy), the upcoming thermal-infrared, diffraction-limited imager, and low and medium-resolution integral field spectrograph (IFS) for the Keck II telescope. The pipeline constructs a ramp from a set of reads and performs optimal extraction and chi-square extraction to reconstruct the 3D IFS datacube. To perform spectral extraction, wavelength calibration, and sky subtraction, the pipeline utilizes rectification matrices produced using position-dependent lenslet point spread functions (PSFs) derived from calibration exposures. The extracted 3D data cubes provide intensity values along with their corresponding uncertainties for each spatial and spectral measurement. The SCALES pipeline is under active development, implemented in Python within the Keck data reduction framework, and is openly available on GitHub along with dedicated documentation.

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arXiv2025

Volcanic Satellites Tidally Venting Na, K, SO2 in Optical & Infrared Light

Recent infrared spectroscopy from the James Webb Space Telescope (JWST) has spurred analyses of common volcanic gases such as carbon dioxide (CO2), sulfur dioxide (SO2), alongside alkali metals sodium (Na I) and potassium (K I) surrounding the hot Saturn WASP-39 b. We report more than an order-of-magnitude of variability in the density of neutral Na, K, and SO2 between ground-based measurements and JWST, at distinct epochs, hinting at exogenic physical processes similar to those sourcing Io's extended atmosphere and torus. Tidally-heated volcanic satellite simulations sputtering gas into a cloud or toroid orbiting the planet, are able to reproduce the probed line-of-sight column density variations. The estimated SO2 flux is consistent with tidal gravitation predictions, with a Na/SO2 ratio far smaller than Io's. Although stable satellite orbits at this system are known to be < 15.3 hours, several high-resolution alkali Doppler shift observations are required to constrain a putative orbit. Due to the Roche limit interior to the planetary photosphere at ~ 8 hours, atmosphere-exosphere interactions are expected to be especially important at this system.

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arXiv2025

Doppler Shifted Transient Sodium Detection by KECK/HIRES

We carried out the first high-resolution transit observations of the exoplanet WASP-49 Ab with Keck/HIRES. Upon custom wavelength calibration we achieve a Doppler RV precision of $<$ 60 ${\rm m\,s}^{-1}$. This is an improvement in RV stability of roughly 240 ${\rm m\,s}^{-1}$ with respect to the instrument standard. We report an average sodium flux residual of $Δ\mathcal{F}_{NaD}/ \mathcal{F}_{\star} (λ) \sim$ 3.2 $\pm$ 0.4 $\%$ (8.0 $σ$) comparable to previous studies. Interestingly, an average Doppler shift of -6.2 $\pm$ 0.5 ${\rm km\,s}^{-1}$ (12.4 $σ$) is identified offset from the exoplanet rest frame. The velocity residuals \textit{in time} trace a blueshift (v$_{Γ, ingress} \sim$ -10.3 $\pm$ 1.9 ${\rm km\,s}^{-1}$) to redshift (v$_{Γ, egress} \sim$ +4.1 $\pm$ 1.5 ${\rm km\,s}^{-1}$) suggesting the origin of the observed sodium is unlikely from the atmosphere of the planet. The average Na light curves indicate a depth of $Δ\mathcal{F}_{NaD} /\mathcal{F}_{\star} (t) \sim$ 0.47 $\pm$ 0.04 % (11.7 $σ$) enduring $\lesssim$ 90 minutes with a half-max duration of $\sim$ 40.1 minutes. Frequent high-resolution spectroscopic observations will be able to characterize the periodicity of the observed Doppler shifts. Considering the origin of the transient sodium gas is of unknown geometry, a co-orbiting natural satellite may be a likely source.

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arXiv2024

Low-resolution Transit Spectroscopy of Three Hot Jupiters Using the 2m Himalayan Chandra Telescope

Here, we present the low-resolution transmission spectroscopy of three giant planets using the Himalayan Faint Object Spectrograph Camera (HFOSC) on the 2m Himalayan Chandra Telescope (HCT) in Hanle, India. It is the first application of transmission spectroscopy with HCT. This study presents results from a single transit, each for three planets: HAT-P-1b, KELT- 18b and WASP-127b. The selection of suitable reference stars assisted in accurately tracking slit losses for the long cadence observations that are needed to achieve the required Signal to Noise Ratio (SNR). We employ the Common Mode Correction (CMC) technique, utilizing a white light transit curve to minimize time dependent systematic errors. The observed spectra for WASP-127b and HAT-P-1b agree with previous low-resolution transit spectroscopic observations using other observing facilities. We confirm the presence of Rayleigh scattering in the atmosphere of WASP-127b. In addition, we provide the first low-resolution transmission spectrum for KELT-18b. Modeling the exoplanet atmosphere with HFOSC and available IR observations from HST and SPITZER for WASP-127b and HAT-P-1b shows that HFOSC can be an alternative optical instrument to use in conjunction with IR observations to constrain the atmospheric parameters better.

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arXiv2024

Redshifted Sodium Transient near Exoplanet Transit

Neutral sodium (Na I) is an alkali metal with a favorable absorption cross section such that tenuous gases are easily illuminated at select transiting exoplanet systems. We examine both the time-averaged and time-series alkali spectral flux individually, over 4 nights at a hot Saturn system on a $\sim$ 2.8 day orbit about a Sun-like star WASP-49 A. Very Large Telescope/ESPRESSO observations are analyzed, providing new constraints. We recover the previously confirmed residual sodium flux uniquely when averaged, whereas night-to-night Na I varies by more than an order of magnitude. On HARPS/3.6-m Epoch II, we report a Doppler redshift at $v_{ Γ, \mathrm{NaD}} =$ +9.7 $\pm$ 1.6 km/s with respect to the planet's rest frame. Upon examining the lightcurves, we confirm night-to-night variability, on the order of $\sim$ 1-4 % in NaD rarely coinciding with exoplanet transit, not readily explained by stellar activity, starspots, tellurics, or the interstellar medium. Coincident with the $\sim$+10 km/s Doppler redshift, we detect a transient sodium absorption event dF$_{\mathrm{NaD}}$/F$_{\star}$ = 3.6 $\pm$ 1 % at a relative difference of $ΔF_{\mathrm{NaD}} (t) \sim$ 4.4 $\pm$ 1 %, enduring $Δt_{\mathrm{NaD}} \gtrsim$ 40 minutes. Since exoplanetary alkali signatures are blueshifted due to the natural vector of radiation pressure, estimated here at roughly $\sim$ -5.7 km/s, the radial velocity is rather at +15.4 km/s, far larger than any known exoplanet system. Given that the redshift magnitude v$_Γ$ is in between the Roche limit and dynamically stable satellite orbits, the transient sodium may be a putative indication of a natural satellite orbiting WASP-49 A b.

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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.

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arXiv2024

Optical spectroscopy of comets using Hanle Echelle Spectrograph (HESP)

Observing the vibrational/rotational lines in a comet's optical spectrum requires high-resolution spectroscopy, as they are otherwise seen as a blended feature. To achieve this, we have obtained medium and high-resolution (R ($λ/Δλ$) = 30000 and 60000) spectra of several comets, including C/2015 V2 (Johnson), 46P/Wirtanen, 41P/Tuttle-Giacobini-Kresák and 38P/Stephan-Oterma, using the Hanle Echelle Spectrograph (HESP) mounted on the 2-m Himalayan Chandra Telescope (HCT) in India. The spectra effectively cover the wavelength range 3700 - 10,000 Å, allowing us to probe the various vibrational bands and band sequences to identify the rotational lines in the cometary molecular emission. We were also able to separate the cometary Oxygen lines from the telluric lines and analyse the green-to-red (G/R) forbidden oxygen [OI] ratios in a few comets. For comets C/2015 V2, 46P, and 41P, the computed G/R ratios, 0.04$\pm$0.01, 0.04$\pm$0.01, and 0.08$\pm$0.02 respectively, point to H$_2$O being a major source of Oxygen emissions. Notably, in the second fibre pointing at a location 1000 km away from the photocenter of comet 46P, the G/R ratio reduced by more than half the value observed in the first fibre, indicating the effects of quenching within the inner coma. We also measured the NH$_2$ ortho-to-para ratio of comet 46P to be about 3.41$\pm$0.05 and derived an ammonia ratio of 1.21$\pm$0.03 corresponding to a spin temperature of $\sim$26 K. With these, we present the results of the study of four comets from different cometary reservoirs using medium and high-resolution optical spectroscopy, emphasising the capabilities of the instrument for future cometary studies.

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arXiv2023

The Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy (SCALES): driving science cases and expected outcomes

The Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy (SCALES) is a $2-5~μ$m, high-contrast integral field spectrograph (IFS) currently being built for Keck Observatory. With both low ($R\lesssim250$) and medium ($R\sim3500-7000$) spectral resolution IFS modes, SCALES will detect and characterize significantly colder exoplanets than those accessible with near-infrared ($\sim1-2~μ$m) high-contrast spectrographs. This will lead to new progress in exoplanet atmospheric studies, including detailed characterization of benchmark systems that will advance the state of the art of atmospheric modeling. SCALES' unique modes, while designed specifically for direct exoplanet characterization, will enable a broader range of novel (exo)planetary observations as well as galactic and extragalactic studies. Here we present the science cases that drive the design of SCALES. We describe an end-to-end instrument simulator that we use to track requirements, and show simulations of expected science yields for each driving science case. We conclude with a discussion of preparations for early science when the instrument sees first light in $\sim2025$.

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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.

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