arXiv subjects

Thirupathi Sivarani

Publications and source records attributed to Thirupathi Sivarani.

At least 19 recordsLinked to original sources

arXiv2026

Unveiling the Nature of C/2023 A3 (Tsuchinshan-ATLAS): A Multi-Technique Observational Approach

Comet C/2023 A3 (Tsuchinshan-ATLAS) is a non-periodic dynamically new Oort cloud comet that was discovered independently by Purple Mountain Observatory in China and Asteroid Terrestrial-impact Last Alert System (ATLAS) telescopes in South Africa. The comet passed perihelion at a distance of 0.39 AU on 27 September 2024. It was visible to the naked eye (the brightest since the comet C/1995 O1 (Hale-Bopp)) and was dubbed the great comet of 2024. In this work, we investigate the nature of this comet, which is moving in a hyperbolic orbit (e > 1), by analysing its composition using various observational techniques and tracing its orbital evolution through high-precision N-body simulations.

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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

A chemodynamical study of $r$-process-enhanced stars

The $r$-process enrichment in the Galaxy still remains elusive with regard to its nucleosynthesis conditions and the astrophysical sites where it occurs. As part of ongoing efforts to pinpoint the origin of chemically peculiar $r$-process-enhanced (RPE) stars, we concentrate in this study on the kinematics of RPE stars to investigate possible variations in the $r$-process enrichment among the Galactic components. We calculate the orbital parameters of a sample of 472 metal-poor RPE stars and associate them to the Galactic bulge, disk and halo populations using a physically motivated classification based on apocenter distance and maximum absolute vertical height of the orbit. We show that the Toomre diagram does not properly separate stars in the disk and halo components when they are on highly eccentric and/or retrograde orbits. The Galactic disk and halo share a similar fraction of RPE stars, in contrast to the earlier perception that the majority of RPE stars belong to the halo. We find that the stars most likely to be accreted belong to the halo. However, 3/4 of the stars lie in a mixed-zone. The inner disk, inner halo and outer halo stars exhibit similar abundance trends for the n-capture elements.

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arXiv2025

On the Origin of Neutron-capture Elements in r-I and r-II Stars: A Differential-abundance Analysis

We present a strictly line-by-line differential analysis of a moderately $r$-process-enhanced star ($r$-I: HD~107752) with respect to a strongly $r$-process-enhanced star ($r$-II: CS~31082-0001) to investigate the possible common origin of their heavy-element nucleosynthesis with high-precision abundances. This study employs ESO data archive high-resolution and high signal-to-noise spectra taken with the UVES (VLT) spectrograph. Considering only the lines in common in both spectra, we estimate differential abundances of 16 light/Fe-peak elements and 15 neutron-capture elements. Abundances of O, Al, Pr, Gd, Dy, Ho, Er, and detection of Tm in HD~107752 are presented for the first time. We found three distinct features in the differential-abundance pattern. Nearly equal abundances of light elements up to Zn are present for both the stars, indicating a common origin for these elements; in addition to no noticable odd-even differential pattern. In the case of neutron-capture elements, the $r$-I star exhibits mildly depleted light $r$-process elements and more depleted heavier $r$-process elements relative to $r$-II star. We also show that among $r$-I and $r$-II stars, the ratio of lighter-to-heavier $r$-process elements (e.g. [(Sr,Y,Zr)/Eu]) exhibits a decreasing trend with respect to the overall $r$-process enhancement, forming a continuous sequence from $r$-I and $r$-II stars. Finally, we discuss the necessity of multiple sites for the formation of $r$-I stars.

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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

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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arXiv2024

A chemodynamical analysis of bright metal-poor stars from the HESP-GOMPA survey -- Indications of a non-prevailing site for light r-process elements

We present a comprehensive analysis of the detailed chemical abundances for a sample of 11 metal-poor, very metal-poor and extremely metal-poor stars ([Fe/H] = -1.65 to [Fe/H] = -3.0) as part of the HESP-GOMPA (Galactic survey Of Metal Poor stArs) survey. The abundance determinations encompass a range of elements, including C, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, and Ba, with a subset of the brighter objects allowing for the measurement of additional key elements. Notably, the abundance analysis of a relatively bright highly r-process-enhanced (r-II) star (SDSS J0019+3141) exhibits a predominantly main r-process signature and variations in the lighter r-process elements. Moreover, successful measurements of thorium in this star facilitate stellar age determinations. We find a consistent odd-even nucleosynthesis pattern in these stars, aligning with expectations for their respective metallicity levels, thus implicating Type II supernovae as potential progenitors. From the interplay between the light and heavy r-process elements, we infer a diminishing relative production of light r-process elements with increasing Type II supernova contributions, challenging the notion that Type II supernovae are the primary source of these light r-process elements in the early Milky Way. A chemodynamical analysis based on Gaia astrometric data and our derived abundances indicates that all but one of our program stars are likely to be of accreted origin. Additionally, our examination of alpha-poor stars underscores the occurrence of an early accretion event from a satellite on a prograde orbit, similar to that of the Galactic disc.

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arXiv2023

The development of HISPEC for Keck and MODHIS for TMT: science cases and predicted sensitivities

HISPEC is a new, high-resolution near-infrared spectrograph being designed for the W.M. Keck II telescope. By offering single-shot, R=100,000 between 0.98 - 2.5 um, HISPEC will enable spectroscopy of transiting and non-transiting exoplanets in close orbits, direct high-contrast detection and spectroscopy of spatially separated substellar companions, and exoplanet dynamical mass and orbit measurements using precision radial velocity monitoring calibrated with a suite of state-of-the-art absolute and relative wavelength references. MODHIS is the counterpart to HISPEC for the Thirty Meter Telescope and is being developed in parallel with similar scientific goals. In this proceeding, we provide a brief overview of the current design of both instruments, and the requirements for the two spectrographs as guided by the scientific goals for each. We then outline the current science case for HISPEC and MODHIS, with focuses on the science enabled for exoplanet discovery and characterization. We also provide updated sensitivity curves for both instruments, in terms of both signal-to-noise ratio and predicted radial velocity precision.

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arXiv2023

Chemodynamic analysis of four $r$-process enhanced stars observed with GTC

Here, we delineate a comprehensive abundance analysis of four $r$-process enhanced metal-poor stars observed with HORuS spectrograph on a 10-m class telescope, GTC. The high signal-to-noise ratio at $R \approx 25000$ spectral resolution allowed us to detect 16 light and 20 neutron-capture elements along with Th in two stars. Four of our program stars show signatures of mixing in their atmosphere. Through detailed abundance analysis of four $r$-process enhanced stars together with already identified $r$-process-rich stars in literature, we probe the production sites of neutron-capture elements. The [Zr/Eu] ratio as a function of metallicity shows the evidence of multiple channels for the production of $r$-process. Thorium to first and second $r$-process peak elements ratios also support the similar non-universality of neutron-capture elements. An increased sample of $r$-process enhanced stars will enable us understand different formation channels of neutron capture elements. Using the kinematic analysis, we found the clues of accretion for two of our program stars.

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arXiv2023

Decoding the compositions of four bright $r$-process-enhanced stars

There has been a concerted effort in recent years to identify the astrophysical sites of the $r$-process that can operate early in the Galaxy. The discovery of many $r$-process-enhanced (RPE) stars (especially by the $R$-process Alliance collaboration) has significantly accelerated this effort. However, only limited data exist on the detailed elemental abundances covering the primary neutron-capture peaks. Subtle differences in the structure of the $r$-process pattern, such as the relative abundances of elements in the third peak, in particular, are expected to constrain the $r$-process sites further. Here, we present a detailed elemental-abundance analysis of four bright RPE stars selected from the HESP-GOMPA survey. Observations were carried out with the 10-m class telescope Gran Telescopio Canarias (GTC), Spain. The high spectral signal-to-noise ratios obtained allow us to derive abundances for 20 neutron-capture elements, including the third $r$-process peak element osmium (Os). We detect thorium (Th) in two stars, which we use to estimate their ages. We discuss the metallicity evolution of Mg, Sr, Ba, Eu, Os, and Th in $r$-II and $r$-I stars, based on a compilation of RPE stars from the literature. The strontium (Sr) abundance trend with respect to europium (Eu) suggests the need for an additional production site for Sr (similar to several earlier studies); this requirement could be milder for yttrium (Y) and zirconium (Zr). We also show that there could be some time delay between $r$-II and $r$-I star formation, based on the Mg/Th abundance ratios.

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arXiv2022

Li distribution, kinematics and detailed abundance analysis among very metal-poor stars in the Galactic halo from the HESP-GOMPA survey

We present a study on the detailed elemental abundances of newly identified bright very metal-poor stars with the detection of lithium, initially observed as part of the SDSS/MARVELS pre-survey. These stars were selected for high-resolution spectroscopic follow-up as part of the HESP-GOMPA survey. In this work, we discuss the Li abundances detected for several stars in the survey, which include main-sequence stars, subgiants, and red giants. Different classes of stars are found to exhibit very similar distributions of Li, which point towards a common origin. We derive a scaling relation for the depletion of Li as a function of temperature for giants and main-sequence stars; the majority of the samples from the literature were found to fall within 1sigma (0.19 and 0.12 dex/K for giants and dwarfs respectively) of this relationship. We also report the existence of a slope of the Li abundances as a function of distances from the Galactic plane, indicating mixed stellar populations. Most Li-rich stars are found to be in or close to the galactic plane. Along with Li, we have derived detailed abundances for C, odd-Z, alpha-, Fe-peak, and neutron-capture elements for each star. We have also used astrometric parameters from Gaia-EDR3 to complement our study, and derived kinematics to differentiate between the motions of the stars; those formed in situ and accreted. The stellar population of the Spite plateau, including additional stars from the literature, is found to have significant contributions from stars formed in situ and through accretion. The orbits for the program stars have also been derived and studied for a period of 5 Gyr backward in time.

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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.

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arXiv2022

Optical observations and dust modelling of comet 156P/Russell-LINEAR

Comet 156P/Russell-LINEAR is a short period Jupiter family comet with an orbital period of 6.44 years. The results from spectroscopic, photometric, polarimetric observations and dust modelling studies are presented here. From the spectroscopic study, strong emissions from $CN (Δν= 0)$, $C_3 (λ4050$ Å), $C_2 (Δν= +1)$ and $C_2 (Δν= 0)$ can be observed during both the epochs of our observations. The Q($C_2$)/Q(CN) ratio classifies the comet as a typical comet. The imaging data reveals the presence of jets. The dust emission from the comet is observed to have a non-steady state outflow due to the presence of these strong jets which subside in later epochs, resulting in a steady state outflow. Polarimetric study at two different phase angles reveals the degree of polarization to be comparable to Jupiter family comets at similar phase angles. Localized variations in polarization values are observed in the coma. The dust modelling studies suggest the presence of high amount of silicate/low absorbing material and indicate the coma to be dominated by higher amount of large size grains with low porosity having power law size distribution index = 2.4. The observed activity and dust properties points to a similarity to another Jupiter family comet, 67P/Churyumov-Gerasimenko.

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arXiv2022

The TESS-Keck Survey. VIII. Confirmation of a Transiting Giant Planet on an Eccentric 261 day Orbit with the Automated Planet Finder Telescope

We report the discovery of TOI-2180 b, a 2.8 $M_{\rm J}$ giant planet orbiting a slightly evolved G5 host star. This planet transited only once in Cycle 2 of the primary Transiting Exoplanet Survey Satellite (TESS) mission. Citizen scientists identified the 24 hr single-transit event shortly after the data were released, allowing a Doppler monitoring campaign with the Automated Planet Finder telescope at Lick Observatory to begin promptly. The radial velocity observations refined the orbital period of TOI-2180 b to be 260.8$\pm$0.6 days, revealed an orbital eccentricity of 0.368$\pm$0.007, and discovered long-term acceleration from a more distant massive companion. We conducted ground-based photometry from 14 sites spread around the globe in an attempt to detect another transit. Although we did not make a clear transit detection, the nondetections improved the precision of the orbital period. We predict that TESS will likely detect another transit of TOI-2180 b in Sector 48 of its extended mission. We use giant planet structure models to retrieve the bulk heavy-element content of TOI-2180 b. When considered alongside other giant planets with orbital periods over 100 days, we find tentative evidence that the correlation between planet mass and metal enrichment relative to stellar is dependent on orbital properties. Single-transit discoveries like TOI-2180 b highlight the exciting potential of the TESS mission to find planets with long orbital periods and low irradiation fluxes despite the selection biases associated with the transit method.

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arXiv2021

Activity of the first interstellar comet 2I/Borisov around perihelion: Results from Indian observatories

Comet 2I/Borisov is the first true interstellar comet discovered. Here we present results from observational programs at two Indian observatories, 2 m Himalayan Chandra Telescope at the Indian Astronomical Observatory, Hanle (HCT) and 1.2 m telescope at the Mount Abu Infrared Observatory (MIRO). Two epochs of imaging and spectroscopy were carried out at the HCT and three epochs of imaging at MIRO. We found CN to be the dominant molecular emission on both epochs, 31/11/2019 and 22/12/2019, at distances of r$_H$ = 2.013 and 2.031 AU respectively. The comet was inferred to be relatively depleted in Carbon bearing molecules on the basis of low $C_2$ and $C_3$ abundances. We find the production rate ratio, Q($C_2$)/Q(CN) = 0.54 $\pm$ 0.18, pre-perihelion and Q($C_2$)/Q(CN) = 0.34 $\pm$ 0.12 post-perihelion. This classifies the comet as being moderately depleted in carbon chain molecules. Using the results from spectroscopic observations, we believe the comet to have a chemically heterogeneous surface having variation in abundance of carbon chain molecules. From imaging observations we infer a dust-to-gas ratio similar to carbon chain depleted comets of the Solar system. We also compute the nucleus size to be in the range $0.18\leq r \leq 3.1$ Km. Our observations show that 2I/Borisov's behaviour is analogous to that of the Solar system comets.

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arXiv2020

Abundance Analysis of New $r$-Process-Enhanced Stars from the HESP-GOMPA Survey

We present a study on the detailed chemical abundances of five new relatively bright $r$-process-enhanced stars that were initially observed as part of the SDSS/MARVELS pre-survey. These stars were selected, on the basis of their metallicities and carbon abundances, among a total of 60 stars, for high-resolution spectroscopic follow-up as part of the HESP-GOMPA survey (Hanle Echelle SPectrograph -- Galactic survey Of Metal Poor stArs). Here we discuss the three new $r$-I and two new $r$-II stars found in this survey. We have carried out a detailed abundance analysis for each of these stars, at a resolving power of $R \sim 30,000$, and compare our results to the existing literature. We could measure three of the first $r$-process-peak elements (Sr, Y and Zr) in all five stars, while Ba, Ce, Nd, Sm, Eu, and Dy could be detected among the second $r$-process-peak elements. Thorium could also be detected in one of the targets, which is found to be an actinide-boost star. We have carried out a comparative study among the sub-populations of the $r$-process-enhanced stars and other stars of the Milky Way halo population to constrain the origin of this class of objects. These bright $r$-process-enhanced stars provide an excellent opportunity to study the nucleosynthesis history of this population in great detail, and shed light on their chemical-enrichment histories.

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arXiv2019

High-resolution Infrared Spectrograph for Exoplanet Characterization with the Keck and Thirty Meter Telescopes

HISPEC (High-resolution Infrared Spectrograph for Exoplanet Characterization) is a proposed diffraction-limited spectrograph for the W.M. Keck Observatory, and a pathfinder for the MODHIS facility project (Multi-Object Diffraction-limited High-resolution Infrared Spectrograph) on the Thirty Meter Telescope. HISPEC/MODHIS builds on diffraction-limited spectrograph designs which rely on adaptively corrected single-mode fiber feeds. Seeing-limited high-resolution spectrographs, by virtue of the conservation of beam etendue, grow in volume following a D^3 power law (D is the telescope diameter), and are subject to daunting challenges associated with their large size. Diffraction-limited spectrographs fed by single mode fibers are decoupled from the telescope input, and are orders of magnitude more compact and have intrinsically stable line spread functions. Their efficiency is directly proportional to the performance of the adaptive optics (AO) system. AO technologies have matured rapidly over the past two decades and are baselined for future extremely large telescopes. HISPEC/MODHIS will take R>100,000 spectra of a few objects in a 10" field-of-view sampled at the diffraction limit (~10-50 mas), simultaneously from 0.95 to 2.4 microns (y-K). The scientific scope ranges from exoplanet infrared precision radial velocities, spectroscopy of transiting, close-in, and directly imaged exoplanets (atmospheric composition and dynamics, RM effect, spin measurements, Doppler imaging), brown dwarf characterization, stellar physics/chemistry, proto-planetary disk kinematics/composition, Solar system, extragalactic science, and cosmology. HISPEC/MODHIS features a compact, cost-effective design optimized to fully exploit the existing Keck-AO and future TMT-NFIRAOS infrastructures and boost the scientific reach of Keck Observatory and TMT soon after first light.

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