arXiv subjects

Himadri Sekhar Das

Publications and source records attributed to Himadri Sekhar Das.

5 recordsLinked to original sources

arXiv2026

Distance Estimates to Five Bok Globules Using Gaia DR3 Parallaxes and Near-Infrared Photometry: Validation with 3D Dust Maps

Accurate distances to small, opaque Bok globules are difficult to obtain due to their compact sizes and lack of embedded standard candles. We estimate distances to five globules -- CB4, CB24, CB56, CB60, and CB188 -- using a near-infrared (NIR) extinction technique applied to 2MASS photometry, combined with stellar distances from \textit{Gaia} DR3 parallaxes. Extinction-distance profiles reveal distinct rises marking each cloud, yielding median distances of 766~pc (CB4), 354~pc (CB24), 483~pc (CB56), 1143~pc (CB60), and 926~pc (CB188). Weighted-mean values agree within $8.0\%$, supporting the internal consistency of the method and interquartile-range-based uncertainties span 3-9\% across the sample. Independent validation with Bayestar19 3D dust maps supports the CB4 and CB188 results. These refined distances provide an improved basis for future determinations of cloud masses, densities, and star-formation efficiencies, including the first robust measurement for CB56 and a revised upward distance for CB188.

astro-ph.GA↗

arXiv2019

Photometry, spectroscopy, and polarimetry of distant comet C/2014 A4 (SONEAR)

We analyze the dust environment of the distant comet C/2014 A4 (SONEAR), with a perihelion distance near 4.1~au, using comprehensive observations obtained by different methods. We present an analysis of spectroscopy, photometry, and polarimetry of comet C/2014 A4 (SONEAR), which were performed on November 5~--~7, 2015, when its heliocentric distance was 4.2~au and phase angle was 4.7$^\circ$. Long-slit spectra and photometric and linear polarimetric images were obtained using the focal reducer SCORPIO-2 attached to the prime focus of the 6-m telescope BTA (SAO RAS, Russia). We simulated the behavior of color and polarization in the coma presenting the cometary dust as a set of polydisperse polyshapes rough spheroids. No emissions were detected in the 3800~--~7200~$Å$ wavelength range. The continuum showed a reddening effect with the normalized gradient of reflectivity 21.6$\pm$0.2\% per 1000~$Å$ within the 4650~--~6200~$Å$ wavelength region. The fan-like structure in the sunward hemisphere was detected. The radial profiles of surface brightness differ for r-sdss and g-sdss filters, indicating predominance of submicron and micron-sized particles in cometary coma. The dust color (g--r) varies from 0.75$ \pm $0.05$^m$ to 0.45$ \pm $0.06$^m$ along the tail. For aperture radius near 20~000~km, the dust productions in various filters were estimated as $Afρ$~=~680$\pm$18~cm (r-sdss) and 887$ \pm $16~cm (g-sdss). The polarization map showed spatial variations of polarization over the coma from about --3\% near the nucleus to --8\% at cometocentric distance about 150~000~km. Our simulations show that the dust particles were dominated (or covered) by ice and tholin-like organics. Spatial changes in the color and polarization can be explained by particle fragmentation.

astro-ph.EP↗

arXiv2018

Dependence of light scattering properties on porosity, size and composition of dust aggregates

In this work, we study the light scattering properties of dust aggregates ($0.7μm \lesssim R_c \lesssim 2.0μm$) having a wide range of porosity ($\mathcal{P}$ = 0.59 to 0.98). The simulations are executed using the Superposition T-matrix code with BCCA, BA, BAM1 and BAM2 clusters of varying porosity. We investigate the nature and dependencies of the different scattering parameters on porosity, size and composition of the aggregated particles for wavelengths 0.45 $μ$m and 0.65 $μ$m. We find that the scattering parameters are strongly correlated with the porosity of the aggregated structures. Our results indicate that, when the porosity of the aggregates decreases, keeping characteristic radius of the aggregates ($R_c$) same for all structures, there is an enhancement in the negative polarization branch (NPB) which is accompanied by a substantial increase in the anisotropies present in the material. Also at the exact backscattering region, the anisotropies are found to be linearly correlated with the porosity of the aggregated structure. The computational study reveals that, for low absorbing materials ($k \le 0.1$), the negative polarization minimum ($P_{min}$) is strongly correlated with the associated anisotropies. Finally, we put forward a qualitative comparison between our computationally obtained results and some selected data from the Amsterdam Light Scattering Database for both low and high absorbing materials. The experimental results also suggest that an increase in the NPB is always accompanied by an enhancement in the anisotropy at the backscattering region.

astro-ph.EP↗

arXiv2017

Correlation among extinction efficiency and other parameters in an aggregate dust model

We study the extinction properties of highly porous BCCA dust aggregates in a wide range of complex refractive indices ($1.4 \le n \le 2.0$, $0.001 \le k \le 1.0$) and wavelength ($0.11μm \le λ\le 3.4μm$). An attempt has been made for the first time to investigate the correlation among extinction efficiency ($Q_{ext}$), the composition of dust aggregates ($n, k$), the wavelength of radiation ($λ$) and size parameter of the monomers ($x$). If $k$ is fixed at any value between 0.001 and 1.0, $Q_{ext}$ increases with increase of $n$ from 1.4 to 2.0. $Q_{ext}$ and $n$ are correlated via \emph{linear} regression when the cluster size is small whereas the correlation is \emph{quadratic} at moderate and higher sizes of the cluster. This feature is observed at all wavelengths (UV to optical to infrared). We also find that the variation of $Q_{ext}$ with $n$ is very small when $λ$ is high. When $n$ is fixed at any value between 1.4 and 2.0, it is observed that $Q_{ext}$ and $k$ are correlated via polynomial regression equation (of degree 1, 2, 3 or 4), where the degree of the equation depends on the cluster size, $n$ and $λ$. The correlation is linear for small size and quadratic/cubic/quartic for moderate and higher sizes. We have also found that $Q_{ext}$ and $x$ are correlated via a polynomial regression (of degree 3,4 or 5) for all values of $n$. The degree of regression is found to be $n$ and $k$-dependent. The set of relations obtained from our work can be used to model interstellar extinction for dust aggregates in a wide range of wavelengths and complex refractive indices.

astro-ph.EP↗

arXiv2015

Study of grain alignment efficiency and a distance estimate for small globule CB4

We study the polarization efficiency (defined as the ratio of polarization to extinction) of stars in the background of the small, nearly spherical and isolated Bok globule CB4 to understand the grain alignment process. A decrease in polarization efficiency with an increase in visual extinction is noticed. This suggests that the observed polarization in lines of sight which intercept a Bok globule tends to show dominance of dust grains in the outer layers of the globule. This finding is consistent with the results obtained for other clouds in the past. We determined the distance to the cloud CB4 using near-infrared photometry (2MASS JHKS colors) of moderately obscured stars located at the periphery of the cloud. From the extinction-distance plot, the distance to this cloud is estimated to be ($459 \pm 85$) pc.

astro-ph.SR↗