arXiv subjects

Subhajit Kar

Publications and source records attributed to Subhajit Kar.

12 recordsLinked to original sources

arXiv2026

Mapping the Inner Milky Way with Infrared-Derived Distances to AGB Stars

The structure and evolution of the Milky Way (MW) can be traced with distance estimates to the evolved stellar populations in the inner galactic region. However, direct astrometric distances remain unavailable or highly uncertain for the majority of these sources due to instrumental limitations, large angular diameters, and complex variability. In this work, we develop a supervised machine-learning model to estimate statistical distances to oxygen-rich asymptotic giant branch (AGB) stars selected from the AKARI mid-infrared survey. We build an XGBoost regression model that maps multi-band IR photometry to distance using a training set of AGB stars with previously derived SED-based distances achieving a mean absolute percentage error (MAPE) of 6% on an independent test set. Distance estimates for over 36,000 AGB sources are obtained within a 36% total error margin, greatly expanding distance coverage (0.5-20 kpc) for dust-obscured AGB populations in the Galactic plane. We find good agreement with reported distances for Galactic Mira variables and independent period-luminosity relations. Utilizing the expanded distance set, we further investigate the spatial distribution of Mira variables in the Galactic bulge and disk. Longer-period Miras preferentially trace the bulge's barred morphology compared to their shorter-period counterparts that populate the disk. We also find roughly constant, but differing, relative scale heights for the bulge and disk, with the bulge vertical dispersion about 30% larger. These findings show that IR photometry-derived statistical distances can recover large-scale Galactic structures and establish long-period Mira variables as efficient tracers of stellar populations in heavily obscured regions of the MW.

astro-ph.GA↗

arXiv2026

Large Magellanic Cloud Globular Clusters in the Near-infrared. I. RR Lyrae in Reticulum

Reticulum is an old, metal-poor, and sparsely populated globular cluster in the outer regions of the Large Magellanic Cloud (LMC) and hosts a rich population of RR Lyrae stars. Being as close as possible to a single stellar population with negligible metallicity spread and low reddening, Reticulum is an ideal laboratory for testing stellar pulsation models and calibrating population II distance indicators. We present homogeneous multi-epoch near-infrared (NIR, JHKs) observations of RR Lyrae variables in Reticulum obtained with the Flamingos-2 imager on the 8.1-m Gemini South Telescope. Using NIR light-curve templates, we derive accurate intensity-averaged magnitudes and peak-to-peak amplitudes for 32 RR Lyrae stars, including 22 fundamental-mode (RRab), 4 first-overtone (RRc), and 6 mixed-mode (RRd) pulsators. The empirical JHKs period-luminosity (PL) relations of Reticulum RR Lyrae are very tight, exhibiting dispersions (~0.05 mag) comparable to those observed in Galactic globular clusters. The derived PL slopes are shallower than those reported for Galactic cluster variables. Adopting recent empirical and theoretical period-luminosity-metallicity (PLZ) calibrations based on Galactic globular clusters and pulsation models, we derive a true distance modulus of $\mu_0 = 18.472 \pm 0.035$ mag to Reticulum. This cluster distance is in excellent agreement with the precise geometric distance to the LMC and places Reticulum close to the LMC barycentric distance. The well-characterized RR Lyrae population and a precise distance make Reticulum a potential anchor for calibrating Population II distance ladder.

astro-ph.SR↗

arXiv2025

Linear stability of an oceanic front at finite Rossby number

Submesoscale currents in the ocean's mixed layer (ML), consisting of fronts, eddies, and filaments, are characterized by order one Rossby (Ro) and Richardson (Ri) numbers. These currents play a crucial role in mediating vertical exchange between the surface and ocean interior and in facilitating cross-scale energy transfers. Despite a growing understanding of their generation mechanisms and energy pathways, two fundamental questions remain unresolved - how does a finite Ro modify the dynamics of ML instabilities, and what mechanisms are responsible for ML frontal arrest when Ro is order one. In this study, we address these questions through a linear stability analysis of a two-dimensional, geostrophically adjusted oceanic front based on the analytical model of Ou(1984), which allows systematic exploration across a range of Ro. In the low Ro, order one Ri regime, the most unstable mode is that of baroclinic instability, with the buoyancy flux serving as the primary source of perturbation kinetic energy. As Ro increases, the dominant instability becomes an inertia-critical layer type, characterized by a resonant interaction between a Rossby wave and an inertia-gravity wave. In the order one Ro regime, the shear production terms become comparable to the buoyancy flux term and even dominate in the region where the adjusted front is strongest. Our results suggest that shear production should be included in parameterizations of ML instabilities.minate in the region where the adjusted front is strongest. Our results suggest that shear production should be included in parameterizations of ML instabilities.

physics.ao-ph↗

arXiv2024

Classification of Wolf Rayet stars using Ensemble-based Machine Learning algorithms

We develop a robust Machine Learning classifier model utilizing the eXtreme-Gradient Boosting (XGB) algorithm for improved classification of Galactic Wolf-Rayet (WR) stars based on Infrared (IR) colors and positional attributes. For our study, we choose an extensive dataset of 6555 stellar objects (from 2MASS and AllWISE data releases) lying in the Milky Way (MW) with available photometric magnitudes of different types including WR stars. Our XGB classifier model can accurately (with an 86\% detection rate) identify a sufficient number of WR stars against a large sample of non-WR sources. The XGB model outperforms other ensemble classifier models such as the Random Forest. Also, using the XGB algorithm, we develop a WR sub-type classifier model that can differentiate the WR subtypes from the non-WR sources with a high model accuracy ($>60\%$). Further, we apply both XGB-based models to a selection of 6457 stellar objects with unknown object types, detecting 58 new WR star candidates and predicting sub-types for 10 of them. The identified WR sources are mainly located in the Local spiral arm of the MW and mostly lie in the solar neighborhood.

astro-ph.SR↗

arXiv2024

Spontaneous emission of internal waves by a radiative instability

The spontaneous emission of internal waves (IWs) from balanced mesoscale eddies has been previously proposed to provide a source of oceanic IW kinetic energy (KE). This study examines the mechanisms leading to the spontaneous emission of spiral-shaped IWs from an anticyclonic eddy with an order-one Rossby number, using a high-resolution numerical simulation of a flat-bottomed, wind-forced, reentrant channel flow configured to resemble the Antarctic Circumpolar Current. It is demonstrated that IWs are spontaneously generated as a result of a loss of balance process that is concentrated at the eddy edge, and then radiate radially outward. A 2D linear stability analysis of the eddy shows that the spontaneous emission arises from a radiative instability which involves an interaction between a vortex Rossby wave supported by the radial gradient of potential vorticity and an outgoing IWs. This particular instability occurs when the perturbation frequency is superinertial. This finding is supported by a KE analysis of the unstable modes and the numerical solution, where it is shown that the horizontal shear production provides the source of perturbation KE. Furthermore, the horizontal length scale and frequency of the most unstable mode from the stability analysis agree well with those of the spontaneously emitted IWs in the numerical solution.

physics.flu-dyn↗

arXiv2024

Detection of high-frequency pulsation in WR 135: investigation of stellar wind dynamics

We report the detection of high-frequency pulsations in WR\,135 from short cadence (10\,minutes) optical photometric and spectroscopic time series surveys. The harmonics up to $6^{th}$ order are detected from the integrated photometric flux variations while the comparatively weaker $8^{th}$ harmonic is detected from the strengths of the emission lines. We investigate the driving source of the stratified winds of WR\,135 using the radiative transfer modeling code, CMFGEN, and find the physical conditions that can explain the propagation of such pulsations. From our study, we find that the optically thick sub-sonic layers of the atmosphere are close to the Eddington limit and are launched by the Fe-opacity. The outer optically thin super-sonic winds ($\tau_{ross}=0.1-0.01$) are launched by the He\,$\textsc{ii}$ and C\,$\textsc{iv}$ opacities. The stratified winds above the sonic point undergo velocity perturbation that can lead to clumps. In the optically thin supersonic winds, dense clumps of smaller size ($f_{VFF}=0.27-0.3$, where $f_{VFF}$ is the volume filling factor) pulsate with higher-order harmonics. The larger clumps ($f_{VFF}=0.2$) oscillate with lower-order harmonics of the pulsation and affect the overall wind variability.

astro-ph.SR↗

arXiv2024

Investigation of [KSF2015] 1381-19L, a WC9-type star in the high extinction Galactic region

We report a multi-wavelength study of the Wolf Rayet (WR) star: [KSF2015] 1381-19L, which is located in the solar metallicity region (Z=0.014) of the Milky Way Galaxy, strongly obscured by the interstellar dust. We perform a detailed characterization of the stellar atmosphere by fitting the spectral emission lines observed in the Optical and Near-InfraRed (NIR) bands, using CMFGEN. The best-fitted spectroscopic model indicates a highly luminous ($10^{5.89}L_{\odot}$) star with a larger radius ($15\,R_{\odot}$) and effective temperature, wind terminal velocity, and chemical composition similar to that of Galactic WC9-dusty (WC9d)-type stars. The atmospheric ionization structure shows coexisting ionization states of different elements, simultaneously affecting the opacity and thermal electron balance. Fitting of the spectral energy data (SED) reveals high interstellar optical extinction ($A_{V}=$ 8.87) while the IR extinction is found to be comparatively lower ($A_{K_{s}}=$ 0.98). We do not detect any excess emission at near-IR wavelengths due to dust. Upon comparison of our results with the GENEVA single star evolutionary models (Z=0.014), we identify the best possible progenitors ( a rotating star of $67\,M_{\odot}$ and a non-rotating star of $90\,M_{\odot}$).

astro-ph.SR↗

arXiv2023

Morpho-kinematic and photoionization models of the multipolar structures in planetary nebula NGC 6572

We have studied the planetary nebula (PN) NGC 6572 through 3D morpho-kinematic and photoionization modelling. The 3D morphology is reconstructed from the Hubble Space Telescope images in different narrow band filters and position-velocity spectra. The PN have a multipolar morphology consisting of highly collimated outflows. The nebular image show signatures of multiple lobes within a spiral-ring-like structure. The multipolar structure is modelled with two bipolar shells (axes ratios 5.5:1 and 3:1), having closed and opened lobes, respectively. A toroidal structure (radius:height = 1:3) surrounds the shells at the waist. The toroidal axis aligns with the major axes of the bipolar shells. Our study reveals the nebula to have a history of collimated polar outflow perpendicular to a higher density equatorial wind with the outflow seemingly have episodes of changing direction of ejection. We construct a photoionization model of NGC 6572 using the deep optical spectra obtained at the 2 m Himalayan Chandra Telescope. For the photoionization model, we configure the input shell geometry in form of a highly bipolar nebular shell with reference to the 3D morphology. Our photoionization model satisfactorily reproduces the observables. We estimate the nebular elemental abundances, and important characteristic parameters of the central star (e.g., effective temperature, luminosity, gravity, mass, etc.) and the nebula (e.g., hydrogen density profiles, radii, etc.). We compare the resolved H$β$, [O III], and [N II] profiles in the 4.2 m William Herschel Telescope with that from the photoionization model and find a good characteristic match.

astro-ph.SR↗

arXiv2022

Energy exchanges between a two-dimensional front and internal wave modes

Fronts and near-inertial waves are energetic motions in the upper ocean that can interact and provide a route for kinetic energy (KE) dissipation of balanced oceanic flows. A quasilinear model is developed to study the KE exchanges between a two-dimensional geostrophically-balanced front undergoing strain-induced semigeostrophic frontogenesis and internal wave (IW) vertical modes. The quasilinear model is solved numerically for variable imposed strain magnitudes, initial IW vertical modes, and for both minimum frequency (near-inertial, NI) and high-frequency IWs. The front-IW KE exchanges are quantified separately during two frontogenetic stages -- an exponential sharpening stage that is characterized by a low Rossby number and is driven by the imposed geostrophic strain, followed by a superexponential sharpening stage that is characterized by an order-one Rossby number and is driven by the convergence of the ageostrophic secondary circulation. It is demonstrated that high-frequency IWs quickly escape the frontal zone and are very efficient at extracting KE from the imposed geostrophic strain field through the deformation shear production (DSP) mechanism. Part of the extracted KE is then converted to wave potential energy. Minimum frequency IWs remain locked to the frontal zone and therefore exchange energy with the ageostrophic frontal circulation. During the exponential stage, IWs extract KE from the geostrophic strain through DSP and transfer it to the frontal secondary circulation via the ageostrophic shear production (AGSP) mechanism. During the superexponential stage a newly identified mechanism, convergence production (CP), which is directly linked to the convergent secondary circulation, plays an important role in the NIW KE budget.

physics.ao-ph↗

arXiv2019

An inverse technique for reconstructing ocean's density stratification from surface data

In this article, we propose an inverse technique that accurately reconstructs the ocean's density stratification profile simply from free surface elevation data. Satellite observations suggest that ocean surface contains the signature of internal tides, which are internal gravity waves generated by the barotropic tides. Since internal tides contain the information of ocean's density stratification, the latter can in principle be reconstructed from the free surface signature. We consider a simple theoretical model that approximates a continuously stratified ocean as discrete layers of constant buoyancy frequency; this facilitates the derivation of a closed-form dispersion relation. First, we numerically simulate internal tide generation for toy ocean scenarios and subsequently perform Space-Time Fourier Transform (STFT) of the free surface, which yields internal tide spectra with wavenumbers corresponding to the tidal frequency. The density profile is reconstructed by substituting these wavenumbers into the dispersion relation. Finally, we consider a more realistic situation with rotation, bottom topography, shear and density profiles representative of the Strait of Gibraltar. Density reconstruction in the presence and absence of shear are respectively found to be $90.2\%$ and $94.2\%$ accurate.

physics.flu-dyn↗

arXiv2018

Ocean bathymetry reconstruction from surface data using hydraulics theory

Here we propose a technique that successfully reconstructs ocean bathymetry from the free surface velocity and elevation data. This technique is based on the principles of open-channel hydraulics, according to which a sub-critical flow over a seamount creates a free surface dip. The proposed method recognizes that such free surface dip contains the signature of the bottom topography, hence inverts the free surface to reconstruct the topography accurately. We applied our inversion technique on re-analysis data, and reconstructed the Mediterranean and the Red sea bathymetries of $1/12\degree$ resolution with approximately $90$\% accuracy.

physics.flu-dyn↗

arXiv2018

Selection of axial dipole from a seed magnetic field in rapidly rotating dynamo models

In this study, we investigate preferences of dipolar magnetic structure from a seed magnetic field in the rapidly rotating spherical shell dynamo models. In this study, we set up a realistic model to show the effect of the Lorentz force in the polarity selection. The important results that has come out from our study is that the magnetic field acts on the flow much before the saturation. Our study suggests that the growth of the magnetic field is not a kinematic effect as one might think off, rather a dynamic effect. This dynamic effect grows as the field generated with time and finally brings the saturation to the dynamo action. Previous studies show that Lorentz force effect the flow when Elsasser number more or less 1 and the studies were focused on the saturation by looking at the time-averaged quantities. However, in this study, we show a clear effect of the Lorentz force even at Elsasser number of $0.3-0.4$. To show the effect of the Lorentz force, we did two different simulations, one is a nonlinear model and another is kinematic model and shows that how a magnetic field can change the flow structure and by doing that the generated field changes, while this kind of behavior is not observed in kinematic dynamo models. This study shows a scale dependent behaviour of the kinetic helicity at two different spectral range.

physics.geo-ph↗