arXiv subjects

Kevikumar A. Lad

Publications and source records attributed to Kevikumar A. Lad.

7 recordsLinked to original sources

arXiv2026

Design and development of Fabry-Perot based wavelength calibration system for PARAS-2 spectrograph

Precise wavelength calibration is essential for high-precision radial velocity (RV) spectrographs, necessitating a stable calibrator that provides a dense grid of uniformly spaced lines to accurately determine stellar line positions and monitor instrumental drifts. In this work, we present the development of a cost-effective Fabry-Perot (FP) etalon-based wavelength calibrator designed to overcome the limitations of conventional sources such as hollow cathode lamps (HCLs) and iodine cells. This FP calibrator, combined with a Xenon (Xe) arc lamp assembly, has been integrated with the PARAS-2 spectrograph on the PRL 2.5m telescope at Mount Abu Observatory. Operated under controlled temperature and pressure conditions, the system generates a dense, comb-like spectrum covering 62 echelle orders with more than 10,000 well-defined and stable spectral lines, enabling precise measurement of instrumental drift. Initial results show that the free spectral range (FSR) varies from 0.16 \AA~near 4000 \AA~to 0.49 \AA~ near 7000 \AA, with a value of 0.3 \AA~around the central wavelength of 5500 \AA~. The estimated finesse ranges from 9 near 4000 \AA~to 19 near 6900 \AA, with an approximate value of 17 at 5500 \AA. The temperature and pressure stability tests demonstrate RMS variations of $0.002 ^\circ\mathrm{C}$ and $5\times10^{-4}$ mbar, respectively. Based on these values, the theoretical stability of the FP wavelength calibrator is estimated to be within 10 cm/s, establishing it as a reliable alternative to Laser Frequency Combs (LFCs) for high-resolution spectroscopic calibration. We present an initial assessment of the RV stability of the FP calibrator, yielding 40-70 cm/s of relative drifts, which are up for further investigations. The observed excess over the theoretically estimated limit is likely attributable to instabilities arising from arc wandering in the xenon arc lamp.

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arXiv2026

Design and Development of Cassegrain Module for PARAS-2 Spectrograph (CAMPAS)

We present here the design and development of the CAMPAS, the Cassegrain Module for the PARAS-2 spectrograph. PARAS-2 is a high-resolution fiber-fed echelle spectrograph developed for the PRL 2.5m Telescope. The CAMPAS acts as a coupler between two optical systems, the PRL 2.5m telescope and the PARAS-2 spectrograph. It has primarily been developed for the precise injection of the light beam into the optical fibers of the PARAS-2 spectrograph. The CAMPAS consists of a focal reducer, beam-guiding optics, an atmospheric dispersion corrector, optical fiber mounts; a calibration unit incorporating calibration lamps and beam-guiding optics; and other auxiliary subsystems. It was developed in-house at PRL, Ahmedabad, between the years 2020-2022 and was installed at one of the two side ports of the PRL 2.5m telescope in March 2022. It is one of the first light instruments for the PRL 2.5m Telescope. The CAMPAS serves critical purposes of precise fiber feed, PSF estimation, and atmospheric dispersion correction.

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arXiv2026

TOI-7154b: A Close-in Massive Brown Dwarf in an Eccentric Orbit

We report here the discovery and characterization of a high-mass transiting brown dwarf in a close-in orbit around its host star, TOI-7154. Initially, the host star was identified as an exoplanetary candidate from the TESS photometry data. Later, with the mass measurements from the RV follow-up using the PARAS-2 and TRES spectrographs, the companion is found to be sub-stellar in nature. TOI-7154, is a G-type main-sequence metal-rich star metallicity $\mathrm{[Fe/H]} = 0.154^{+0.077}_{-0.075}\,\text{dex}$, effective temperature $T_{\mathrm{eff}} = 5564^{+100}_{-110}\,\text{K}$, mass $M_\star = 0.939^{+0.047}_{-0.043}\,M_{\odot}$, radius $R_\star = 0.949^{+0.032}_{-0.030}\,R_{\odot}$, and surface gravity $\log g = 4.456^{+0.036}_{-0.036}$. With the joint analysis of the TESS photometry and the PARAS-2 and TRES radial velocities we found that TOI-7154b orbits its host star in $P = 8.860073\pm 0.000029\,\text{d}$, eccentric ($e = 0.2482 \pm 0.0024$) orbit and its radius is smaller than that of Jupiter ($R_{b} = 0.827^{+0.040}_{-0.037}\,R_{\mathrm{J}}$). With a mass near the hydrogen-burning boundary ($M_{b} = 71.7^{+2.4}_{-2.2}\,M_{\mathrm{J}}$) which separates brown dwarfs from very low-mass stars, TOI-7154b occupies a critical position in the regime for probing the transition between sub-stellar and stellar objects. The system is very old, with its age estimated to be $7.2^{+3.9}_{-3.6}\,\text{Gyr}$ by MIST isochrones, while Galactic kinematics indicate an age of $\sim4-5\,\text{Gyr}$. {Our tidal evolution simulations indicate a stellar dissipation factor of $Q_\star'\lesssim10^6$. Since the presence of any companion is currently ruled out by observations, the presence of eccentricity in this old system is, therefore, indicative of it having stellar-like fragmentation origins.

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arXiv2026

Development of ProtoPol: a medium resolution echelle spectro-polarimeter for PRL telescopes, Mt Abu, India -- Part I : the design, development and laboratory characterization

ProtoPol is a medium-resolution echelle spectro-polarimeter developed for Physical Research Laboratory (PRL) 1.2m and 2.5m telescopes, Mt. Abu, India. Though initially conceived to evaluate the development methodology of the echelle spectro-polarimeter, it was subsequently elevated to the level of a full-fledged back-end instrument for PRL telescopes. ProtoPol is developed on the traditional concept of using a half-wave plate with Wollaston prism to achieve the separation of two mutually orthogonal polarized beams. These separated beams are modulated and directed into an echelle spectrometer which is employs an echelle grating and two plane reflection gratings as the cross-dispersers. Therefore, the cross-dispersed spectra for two orthogonal polarized beams are recorded in multiple orders on a CCD detector. ProtoPol is designed to operate in the visible and near IR spectral range, 4000 - 9600 angstrom, with a spectral resolution ($\delta$$\lambda$) around 0.4-0.75 angstrom. The uniqueness of ProtoPol lies in its design which has entirely been developed with commercially available off-the-shelf optical and opto-mechanical components. This feature makes ProtoPol a noteworthy development as it offers a cost-effective way to develop spectro-polarimeters with such resolutions for small-aperture (2-3m) telescopes around the world, in a much shorter development period. ProtoPol has been successfully developed and commissioned on PRL 1.2m and 2.5m telescopes since December 2023, and a variety of observations have been carried out for instrument characterization, performance verification, and scientific purposes. This is the first of the two-part research articles series, wherein we present the design and development methodology of ProtoPol, along with its laboratory characterization and performance.

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arXiv2025

TOI-6038 A b: A dense sub-Saturn in the transition regime between the Neptunian ridge and savanna

We present the discovery and characterization of a sub-Saturn exoplanet, TOI-6038~A~b, using the PARAS-2 spectrograph. The planet orbits a bright ($m_V=9.9$), metal-rich late F-type star, TOI-6038~A, with $T_{\rm{eff}}=6110\pm100~\mathrm{K}$, $\log{g}=4.118^{+0.015}_{-0.025}$, and $[{\rm{Fe/H}}]=0.124^{+0.079}_{-0.077}$ dex. The system also contains a wide-orbit binary companion, TOI-6038~B, an early K-type star at a projected separation of $\approx3217$ AU. We combined radial velocity data from PARAS-2 with photometric data from the Transiting Exoplanet Survey Satellite (TESS) for joint modeling. TOI-6038~A~b has a mass of $78.5^{+9.5}_{-9.9}~M_\oplus$ and a radius of $6.41^{+0.20}_{-0.16}~R_\oplus$, orbiting in a circular orbit with a period of $5.8267311^{+0.0000074}_{-0.0000068}$ days. Internal structure modeling suggests that $\approx74\%$ of the planet's mass is composed of dense materials, such as rock and iron, forming a core, while the remaining mass consists of a low-density H/He envelope. TOI-6038~A~b lies at the transition regime between the recently identified Neptunian ridge and savanna. Having a density of $ρ_{\rm{P}}=1.62^{+0.23}_{-0.24}\rm~g\,cm^{-3}$, TOI-6038~A~b is compatible with the population of dense ridge planets ($ρ_{\rm{P}}\simeq$ 1.5-2.0 $\rm~g\,cm^{-3}$), which have been proposed to have reached their close-in locations through high-eccentricity tidal migration (HEM). First-order estimates suggest that the secular perturbations induced by TOI-6038~B may be insufficient to drive the HEM of TOI-6038~A~b. Therefore, it is not clear whether HEM driven by a still undetected companion, or early disk-driven migration, brought TOI-6038~A~b to its present-day close-in orbit. Its bright host star makes TOI-6038~A~b a prime target for atmospheric escape and orbital architecture observations, which will help us to better understand its overall evolution.

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arXiv2024

Discovery and characterization of a dense sub-Saturn TOI-6651b

We report the discovery and characterization of a transiting sub-Saturn exoplanet TOI-6651b using PARAS-2 spectroscopic observations. The host, TOI-6651 ($m_{V}\approx 10.2$), is a sub-giant, metal-rich G-type star with $[{\rm Fe/H}] = 0.225^{+0.044}_{-0.045}$, $T_{\rm eff} = 5940\pm110\ \mathrm{K}$, and $\log{g} = 4.087^{+0.035}_{-0.032}$. Joint fitting of the radial velocities from PARAS-2 spectrograph and transit photometric data from Transiting Exoplanet Survey Satellite (TESS) reveals a planetary mass of $61.0^{+7.6}_{-7.9}\ M_\oplus$ and radius of $5.09^{+0.27}_{-0.26}\ R_\oplus$, in a $5.056973^{+0.000016}_{-0.000018}$ day orbit with an eccentricity of $0.091^{+0.096}_{-0.062}$. TOI-6651b has a bulk density of $2.52^{+0.52}_{-0.44}\ \mathrm{g\ cm^{-3}}$, positioning it among the select few known dense sub-Saturns and making it notably the densest detected with TESS. TOI-6651b is consistent with the positive correlation between planet mass and the host star's metallicity. We find that a considerable portion $\approx$ 87% of the planet's mass consists of dense materials such as rock and iron in the core, while the remaining mass comprises a low-density envelope of H/He. TOI-6651b lies at the edge of the Neptunian desert, which will be crucial for understanding the factors shaping the desert boundaries. The existence of TOI-6651b challenges conventional planet formation theories and could be a result of merging events or significant atmospheric mass loss through tidal heating, highlighting the complex interplay of dynamical processes and atmospheric evolution in the formation of massive dense sub-Saturns.

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arXiv2024

The PRL 2.5m Telescope and its First Light Instruments: FOC & PARAS-2

We present here the information on the design and performance of the recently commissioned 2.5-meter telescope at the PRL Mount Abu Observatory, located at Gurushikhar, Mount Abu, India. The telescope has been successfully installed at the site, and the Site Acceptance Test (SAT) was completed in October 2022. It is a highly advanced telescope in India, featuring the Ritchey-Chr$\acute{e}$tien optical configuration with primary mirror active optics, tip-tilt on side-port, and wave front correction sensors. Along with the telescope, its two first light instruments {namely Faint Object Camera (FOC) and PARAS-2} were also integrated and attached with it in the June 2022. {FOC is a} camera that uses a 4096 X 4112 pixels detector SDSS type filters with enhanced transmission and known as u', g', r', i', z'. It has a limiting magnitude of 21 mag in 10 minutes exposure in the r'-band. The other first light instrument PARAS-2 is a state-of-the-art high-resolution fiber-fed spectrograph operates in 380-690 nm wave-band, aimed to unveil the super-Earth like worlds. The spectrograph works at a resolution of $\sim$107,000, making it the highest-resolution spectrograph in Asia to date, which is under {ultra}-stable temperature and pressure environment, at 22.5 $\pm$ 0.001 $^{\circ}$C and 0.005 $\pm$ 0.0005 mbar, respectively. Initial calibration tests of the spectrograph using a Uranium Argon Hollow Cathode Lamp (UAr HCL) have yielded intrinsic instrumental RV stability down to 30 cm s$^{-1}$.

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