arXiv subjects

Shubhendra Nath Das

Publications and source records attributed to Shubhendra Nath Das.

3 recordsLinked to original sources

arXiv2026

TOI-6884b: A low-mass brown dwarf transiting a slightly evolved star

We report the discovery of a low-mass transiting brown dwarf orbiting TOI-6884 (TIC~156514476, $T_{\rm mag}=11.4$) from NASA's \textit{Transiting Exoplanet Survey Satellite} (\textit{TESS}) mission. The \textit{TESS} light curves initially suggested an orbital period of $\sim$14.42~days; however, our high-precision ground-based radial velocity measurements and multi-epoch time-series photometry reveal this to be a harmonic alias. We determine the true orbital period to be $4.808264^{+0.000015}_{-0.000014}$~days and confirm the substellar nature of the companion. TOI-6884b has a mass of $26.32^{+0.98}_{-0.93}\,M_{\mathrm{J}}$, a radius of $0.927^{+0.51}_{-0.52}\,R_{\mathrm{J}}$, and resides on a nearly circular orbit ($e=0.067^{+0.010}_{-0.012}$). Its host star is a late F-type slightly evolved star with $M_\star = 1.410^{+0.075}_{-0.069}\,M_\odot$,\msun, $R_\star = 1.840^{+0.072}_{-0.073}\,R_\odot$, $\log{g} = 4.057^{+0.045}_{-0.039}$, $[{\rm Fe/H}] = 0.094^{+0.073}_{-0.068}$~dex, and $T_{\rm eff}=6330^{+180}_{-160}$,\mathrm{K}$. TOI-6884b is a key addition to the small population of well-characterized transiting brown dwarfs orbiting host stars that have evolved off the main sequence. The detection of such systems will contribute to our understanding of the dynamical histories and structural evolution of short-period substellar companions around evolved stars.

astro-ph.EP↗

arXiv2026

Direct Optical Evidence of Late-Stage Infall in AB Aurigae: A Stagnant [O I] Reservoir and a Crushed Magnetosphere

Massive planet-carved cavities in transition disks should theoretically throttle inward gas transport, challenging our understanding of how central stars maintain vigorous accretion. To investigate how macro-scale late-stage infall traverses these gaps, we present multi-epoch, extreme-resolution (R ~ 107,000) PARAS-2 optical spectroscopy of the benchmark Herbig Ae system AB Aurigae. By resolving the kinematics of H-alpha, He I 5876, [O I] 6300, 6363, and Na I D, we map the innermost accretion environment. We find that the [O I] emission is centered near the stellar rest velocity with symmetric broadening of ~ 35 km/s. Restricted to T <= 3800 K, this profile traces a stagnant, gravitationally bound Keplerian gas reservoir at ~ 1 au. Therefore, it provides strong optical evidence that late-stage infall accumulates in an inner gas reservoir and subsequently feeds the innermost dust cavity. From this reservoir, gas is transported inward and crashes onto the star, driving a highly active accretion rate of dM/dt ~ 4 x 10^-7 M_sun/yr. The associated ram pressure crushes the stellar magnetosphere to R_mag ~ 1.2 R_star, which explains the restricted He I free-fall velocities and the highly variable inner wind. We also isolate a stable, slow H-alpha wind component, likely tracing an extended photoevaporative disk wind.

astro-ph.SR↗

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 Å~near 4000 Å~to 0.49 Å~ near 7000 Å, with a value of 0.3 Å~around the central wavelength of 5500 Å~. The estimated finesse ranges from 9 near 4000 Å~to 19 near 6900 Å, with an approximate value of 17 at 5500 Å. 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.

astro-ph.IM↗