arXiv subjects

S. R. Das

Publications and source records attributed to S. R. Das.

9 recordsLinked to original sources

arXiv2022

Systems of Left Translates and Oblique Duals on the Heisenberg Group

In this paper, we characterize the system of left translates $\{L_{(2k,l,m)}g:k,l,m\in\mathbb{Z}\}$, $g\in L^2(\mathbb{H})$, to be a frame sequence or a \emph{Riesz} sequence in terms of the twisted translates of the corresponding function $g^λ$. Here, $(\mathbb{H}$ denotes the Heisenberg group and $g^λ$ the inverse Fourier transform of $g$ with respect to the central variable. This type of characterization for a \emph{Riesz} sequence allows us to find some concrete examples. We also study the structure of the oblique dual of the system of left translates $\{L_{(2k,l,m)}g:k,l,m\in\mathbb{Z}\}$ on $(\mathbb{H}$. This result is also illustrated with an example.

math.FA↗

arXiv2021

Gabor System Based on the Unitary Dual of the Heisenberg Group

In this paper Gabor system of certain type based on the unitary dual of the Heisenberg group $\mathbb{H}^n$ is introduced and a sufficient condition is obtained for the Gabor system to be a Bessel sequence for $L^2(\mathbb{R}^*,\mathcal{B}_2;dκ)$ using the $Schrödinger$ representation of $\mathbb{H}^n$, where $\mathcal{B}_2$ denotes the class of Hilbert-Schmidt operators on $L^2(\mathbb{R}^n)$ and $dκ$ denotes the Haar measure on $\mathbb{R}^*$. Further a necessary and sufficient condition is provided for the Gabor system to be an orthonormal system, a Parseval frame sequence, a frame sequence and a Riesz sequence.

math.FA↗

arXiv2021

Twisted B-splines in the complex plane

In this paper, we introduce the new class of twisted $B$-splines and study some properties of these B-splines. We also investigate the system of twisted translates and the wavelets corresponding to these twisted $B$-splines.

math.FA↗

arXiv2020

Probing the physical conditions and star formation processes in the Galactic HII region S305

We present multi-scale and multi-wavelength observations of the Galactic HII region S305, which is excited by massive O8.5V and O9.5V stars. Infrared images reveal an extended sphere-like shell (extension ~7.5 pc; at T_d = 17.5-27 K) enclosing the S305 HII region (size ~5.5 pc; age ~1.7 Myr). The extended structure observed in the Herschel temperature map indicates that the molecular environment of S305 is heated by the massive O-type stars. Regularly spaced molecular condensations and dust clumps are investigated toward the edges of the infrared shell, where the PAH and H_2 emission is also observed. The molecular line data show a signature of an expanding shell of molecular gas in S305. GMRT 610 and 1280 MHz continuum maps reveal overdensities of the ionized emission distributed around two O-type stars, which are surrounded by the horseshoe envelope (extension ~2.3 pc). A molecular gas deficient region/cavity is identified toward the center of the horseshoe envelope, which is well traced with PAH, H_2, molecular, and dust emission. The edges of the infrared shell are found to be located in the front of the horseshoe envelope. All these outcomes provide the observational evidence of the feedback of O-type stars in S305. Moreover, non-thermal radio emission is detected in S305 with an average spectral index alpha ~-0.45. The variations in alpha, ranging from -1.1 to 1.3, are explained due to soft synchrotron emission and either optically-thicker thermal emission at high frequencies or a suppression of the low-frequency emission by the Razin-Tsytovich effect.

astro-ph.GA↗

arXiv2016

Radio and infrared study of the star forming region IRAS 20286+4105

A multi-wavelength investigation of the star forming complex IRAS 20286+4105, located in the Cygnus-X region, is presented here. Near-infrared K-band data is used to revisit the cluster / stellar group identified in previous studies. The radio continuum observations, at 610 and 1280 MHz show the presence of a HII region possibly powered by a star of spectral type B0 - B0.5. The cometary morphology of the ionized region is explained by invoking the bow-shock model where the likely association with a nearby supernova remnant is also explored. A compact radio knot with non-thermal spectral index is detected towards the centre of the cloud. Mid-infrared data from the Spitzer Legacy Survey of the Cygnus-X region show the presence of six Class I YSOs inside the cloud. Thermal dust emission in this complex is modelled using Herschel far-infrared data to generate dust temperature and column density maps. Herschel images also show the presence of two clumps in this region, the masses of which are estimated to be {\sim} 175 M{\sun} and 30 M{\sun}. The mass-radius relation and the surface density of the clumps do not qualify them as massive star forming sites. An overall picture of a runaway star ionizing the cloud and a triggered population of intermediate-mass, Class I sources located toward the cloud centre emerges from this multiwavelength study. Variation in the dust emissivity spectral index is shown to exist in this region and is seen to have an inverse relation with the dust temperature.

astro-ph.SR↗

arXiv2007

Polarimetric studies of comet Halley

In the present work, the non-spherical grain characteristics of comet Halley are analysed using the T-matrix method at $λ= 0.365, 0.485$ and $0.684μm$ respectively. In order to analyse the polarisation data of comet Halley, the dust size distribution function derived by Das et al. (2004) for comet Halley is used in the present work. The size range of the grains is taken to be 0.01$μm \le$ s $\le 3μm$. Using the T-matrix method, the best fit values of complex refractive indices $(n, k)$ and aspect ratio ($E$) are determined at three different wavelengths $ 0.365, 0.485$ and $0.684μm$ and the corresponding values are given by $(1.380, 0.043, 0.962), (1.378, 0.049, 0.962)$ and $(1.377, 0.058,0.962)$ respectively. After comparing the above result with Mie theory result, it is found that prolate grains give the best fit to the observed polarisation data of comet Halley. Also the negative polarisation behaviour of comet Halley is discussed thereafter.

astro-ph↗

arXiv2000

Supergravity couplings to Noncommutative Branes, Open Wilson Lines and Generalised Star Products

Noncommutative gauge theories can be constructed from ordinary $U(\infty)$ gauge theories in lower dimensions. Using this construction we identify the operators on noncommutative D-branes which couple to linearized supergravity backgrounds, from a knowledge of such couplings to lower dimensional D-branes with no $B$ field. These operators belong to a class of gauge invariant observables involving open Wilson lines. Assuming a DBI form of the coupling we show, to second order in the gauge potential but to all orders of the noncommutativity parameter, that our proposal agrees with the operator obtained in terms of ordinary gauge fields by considering brane actions in backgrounds and then using the Seiberg-Witten map to rewrite this in terms of noncommutative gauge fields. Our result clarify why a certain {\it commutative} but {\it non-associative} ``generalized star product'' appears both in the expansion of the open Wilson line, as well as in string amplitude computations of open string - closed string couplings. We outline how our procedure can be used to obtain operators in the noncommutative theory which are holographically dual to supergravity modes.

hep-th↗

arXiv2000

Magnetic Moments of Branes and Giant Gravitons

We study the magnetic analogue of Myers' Dielectric Effect and, in some cases, relate it to the blowing up of particles into branes, first investigated by Greevy, Susskind and Toumbas. We show that $D0$ branes or gravitons in M theory, moving in a magnetic four-form field strength background expand into a non-commutative two sphere. Both examples of constant magnetic field and non-constant fields in curved backgrounds generated by branes are considered. We find, in all cases, another solution, consisting of a two-brane wrapping a classical two-sphere, which has all the quantum numbers of the $D0$ branes. Motivated by this, we investigate the blowing up of gravitons into branes in backgrounds different from $AdS_m \times S^n$. We find the phenomenon is quite general. In many cases with less or even no supersymmetry we find a brane configuration which has the same quantum numbers and the same energy as a massless particle in supergravity.

hep-th↗

arXiv1999

Supergravity with Self-dual B fields and Instantons in Noncommutative Gauge Theory

We study Type IIB supergravity in the presence of (euclidean) D3 branes and nonzero self-dual B-fields. We point out that the Einstein frame metric is identical to the full geometry for D3 branes without B fields turned on. Furthermore, in a decoupling limit in which the theory is conjectured to be dual to noncommutative Yang-Mills theory, the entire Einstein metric remains intact, and in particular, is asymptotically flat. We construct D-instanton solutions in this geometry. We show that in the decoupling limit the D-instanton action agrees with the action of the corresponding instanton in the noncommutative Yang-Mills theory and is expressed in terms of the open string coupling. Some other aspects of this correspondence, which have unusual features because the underlying metric is asymptotically flat, are explored.

hep-th↗