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Sujan Sengupta

Publications and source records attributed to Sujan Sengupta.

36 records · Page 2Linked to original sources
arXiv2011

Probing the Physical Properties of Directly Imaged Gas Giant Exoplanets Through Polarization

It is becoming clear that the atmospheres of the young, self-luminous extrasolar giant planets imaged to date are dusty. Planets with dusty atmospheres may exhibit detectable amounts of linear polarization in the near-infrared, as has been observed from some field L dwarfs. The asymmetry required in the thermal radiation field to produce polarization may arise either from the rotation-induced oblateness or from surface inhomogeneities, such as partial cloudiness. While it is not possible at present to predict the extent to which atmospheric dynamics on a given planet may produce surface inhomogeneities substantial enough to produce net non-zero disk integrated polarization, the contribution of rotation-induced oblateness can be estimated. Using a self-consistent, spatially homogeneous atmospheric model and a multiple scattering polarization formalism for this class of exoplanets, we show that polarization on the order of 1% may arise due to the rotation-induced oblateness of the planets. The degree of polarization for cloudy planets should peak at the same wavelengths at which the planets are brightest in the near-infrared. The observed polarization may be even higher if surface inhomogeneities exist and play a significant role. Polarized radiation from self-luminous gas giant exoplanets, if detected, provides an additional tool to characterize these young planets and a new method to constrain their surface gravity and masses.

astro-ph.EP↗
arXiv2011

Theoretical Spectra of the Am CVn Binary System SDSS J0926+3624 : Effects of Irradiation onto the Donor Stars

Taking into account a range of parameters determined from the evolutionary models and available observational data, the detailed non-LTE spectra for the primary star and the irradiated donor star in the only known eclipsing AM CVn system to date SDSS J0926+3624 are constructed based on the TLUSTY stellar atmosphere code. The combined spectrum of the primary and the donor stars along with a multi-color blackbody spectrum of the accretion disk that reproduces a detailed numerical model is compared to the SDSS optical spectrum of the system. The photometric flux of the primary star inferred from eclipse observations is compared with the synthetic spectrum. The model fit of the two independent observations provides an upper limit on the distance of the system for different effective temperatures of the primary. In addition, an upper limit on the combined flux of the disk and the donor in the infrared region wherein the contribution of the primary is negligible is also determined. It is shown that the spectrum of a sufficiently cool donor can exhibit emission lines due to irradiation from a hot primary and the emission features should be detectable in the infrared even though the contribution of the flux from the disk dominates. Thus, it is pointed out that infrared observations of the system would provide important information on the thermal state of the donor as well as useful insight on the thermal properties of the primary star and the accretion disk.

astro-ph.HE↗
arXiv2010

Observed Polarization of Brown Dwarfs Suggests Low Surface Gravity

Light scattering by atmospheric dust particles is responsible for the polarization observed in some L dwarfs. Whether this polarization arises from an inhomogeneous distribution of dust across the disk or an oblate shape induced by rotation remains unclear. Here we argue that the latter case is plausible and, for many L dwarfs, the more likely one. Furthermore evolutionary models of mature field L dwarfs predict surface gravities ranging from about 200 to 2500 m/s^2 (corresponding to masses of about 15 to 70 times of Jupiter mass). Yet comparison of observed spectra to available synthetic spectra often does not permit more precise determination of the surface gravity of individual field L dwarfs, leading to important uncertainties in their properties. Since rotationally-induced non-sphericity, which gives rise to non-zero disk-integrated polarization, is more pronounced at lower gravities, polarization is a promising low gravity indicator. Here we combine a rigorous multiple scattering analysis with a self-consistent cloudy atmospheric model and observationally inferred rotational velocities and find that the observed optical polarization can be explained if the surface gravity of the polarized objects is about 300 m/s^2 or less, potentially providing a new method for constraining L dwarf masses.

astro-ph.SR↗
arXiv2009

Multiple Scattering Polarization of Substellar-mass Objects : T-dwarfs

While there have been multiple observational programs aimed at detecting linear polarization of optical radiation emitted by ultracool dwarfs, there has been comparatively less rigorous theoretical analysis of the problem. The general expectation has been that the atmospheres of those substellar-mass objects with condensate clouds would give rise to linear polarization due to scattering. Because of rotation-induced non-sphericity, there is expected to be incomplete cancellation of disk-integrated net polarization and thus a finite polarization. For cloudless objects, however, only molecular Rayleigh scattering will contribute to any net polarization and this limit has not been well studied. Hence in this paper we present a detailed multiple scattering analysis of the polarization expected from those T-dwarfs whose spectra show absence of condensates. For this, we develop and solve the full radiative transfer equations for linearly polarized radiation. Only atomic and molecular Rayleigh scattering are considered to be the source of polarization. We compute the local polarization at different angular directions in a plane-parallel atmospheres calculated for the range of effective temperatures of T dwarfs and then average over the whole surface of the object. The effects of gravity and limb darkening as well as rotation induced non-sphericity are included. It is found that the amount of polarization decreases with the increase in effective temperature. It is also found that significant polarization at any local point in the atmosphere arises only in the optical (B-band). However, the disk integrated polarization--even in the B-band--is negligible. Hence we conclude that, unlike the case for cloudy L dwarfs, polarization of cloudless T-dwarfs by atomic and molecular scattering may not be detectable.

astro-ph.SR↗
arXiv2008

Cloudy Atmosphere of the Extra-solar Planet HD189733b : A Possible Explanation of the Detected B-band Polarization

The peak amplitude of linear polarization detected recently from an extrasolar hot giant planet HD 189733b, is a few times of $10^{-4}$, more than an order of magnitude higher than all theoretical predictions. Rayleigh scattering off $H_2$ and $He$ may although give rise to a planet-star flux ratio of the order of $10^{-4}$ in the blue band, it cannot account for the high polarization unless the planet has an unusually extended atmosphere. Therefore, it is suggested that the high polarization should be attributed to the presence of a thin cloud of sub-micron size dust grains in the upper visible atmosphere which supports the observational finding of an almost feature-less transmission spectrum in the optical with no indication of the expected alkaline absorption features. It is found that the polarimetry observation allows for a small eccentricity of the orbit that is predicted from the time delay of the secondary eclipse of the planet. The estimated longitude of the ascending node is $16^o\pm6$ which interestingly coincides with the observationally inferred location of the peak hemisphere-integrated brightness.

astro-ph↗
arXiv2005

Polarization of Starlight by Unresolved and Oblate Extra-solar Planet in Elliptical Orbit

We calculate the degree of linear polarization of radiation from stars having planets that may not be resolved spatially. We assume single scattering by water and silicate particulates in the planetary atmosphere. The dilution of the reflected polarized radiation of the planet by the unpolarized stellar radiation and the effect of oblateness of the planet as well as its elliptical orbit are included. We employ a chemical equilibrium model to estimate the number density of water and silicate condensates and calculate the degree of linear polarization at R band of starlight as a function of (1) mean size of condensates, (2) planetary oblateness, (3) inclination angle, (4) phase angle, (5) orbital eccentricity e and (6) the epoch of periastron passage. We show that the polarization profile alters significantly at all inclination angles when elliptical orbit is considered and the degree of polarization peaks at the epoch of periastron passage. We predict that detectable amount of linear polarization may arise if the planetary atmosphere is optically thin, the mean size of the condensates is not greater than a few microns and the oblateness of the planet is as high as that of Jupiter.

astro-ph↗
arXiv2005

Polarization of L Dwarfs by Dust Scattering

The degree of polarization in L dwarfs of spectral types L0 to L6 resulting from dust scattering in a rotation-induced oblate photosphere is calculated. Assuming that forsterite is the main condensate, the atmospheric dust distribution is derived for different spectral types based on a chemical equilibrium model. The degree of polarization at optical is then calculated using a single scattering model. The expected linear polarization at optical is found to peak at around spectral type L1. For a fixed rotational velocity, the degree of polarization decreases from hotter to cooler objects. However, with the increase in mean grain size, the degree of linear polarization reduces significantly. We fit the recently observed linear polarimetric data of L dwarfs and find that single dust scattering model coupled with the chemical equilibrium models of condensates is consistent with the observational results.

astro-ph↗
arXiv2003

Effects of Dust Scattering in Expanding Spherical Nebulae

The mean intensity of planetary nebulae with an expanding atmosphere is modeled by considering dusty and dust-free atmospheres. The bulk matter density is determined from the adopted velocity field through the equation of continuity. The gas is assumed to consist of hydrogen and helium and the gas-to-dust mass ratio is taken to be $3\times10^{-4}$. The Rayleigh phase function is employed for atomic scattering while the full Mie theory of scattering is incorporated for determining the dust scattering and absorption cross-section as well as the phase function for the angular distribution of photons after scattering. It is shown that in a dust free atmosphere, the mean intensity increases with the increase in the expansion velocity that makes the medium diluted. The mean intensity profile changes significantly when dust scattering is incorporated. The increase in forward scattering of photons by the dust particles yields into an increase in the mean intensity as compared to that without dust. The mean intensity increases as the particle size is increased. Thus it is shown that both the expansion of the medium and the presence of dust play important role in determining the mean intensity of a planetary nebulae.

astro-ph↗
arXiv2003

Explaining the Observed Polarization from Brown Dwarfs by Single Dust Scattering

Recent observation of linear optical polarization from brown dwarfs confirms the dust hypothesis in the atmospheres of brown dwarfs with effective temperature higher than 1400 K. The observed polarization could arise due to dust scattering in the rotation induced oblate photosphere or due to the scattering by non-spherical grains in the spherical atmosphere or by the anisotropic distribution of dust clouds. Assuming single scattering by spherical grains in a slightly oblate photosphere consistent with the projected rotational velocity, the observed optical linear polarization is modeled by taking grains of different sizes located at different pressure height and of different number density. Minimum possible oblateness of the object due to rotation is considered in order to constrain the grain size. It is shown that the observed polarization from the L-dwarfs 2MASSW J0036+1821 and DENIS-P J0255-4700 can well be explained by several sets of dust parameters and with the minimum possible oblateness. Models for the observed polarization constrain the maximum size of grains. It is emphasized that future observation of polarization at the blue region will further constrain the grain size.

astro-ph↗
arXiv2002

Anisotropic Sunyaev-Zel'dovich Effect

The spectral distortion in the Cosmic Microwave Background Radiation (CMBR) caused from the inverse Compton scattering of low energy CMB photons by thermally distributed hot electrons in massive clusters of galaxies - better known as the thermal Sunyaev-Zel'dovich effect is usually estimated by the analytical or semi-analytical solutions of Kompaneets equation. These solutions are based on the assumption that the scattering is isotropic, i.e., the intensity of the CMBR remains angle-independent after scattering. In this letter, I present the solution of the full radiative transfer equations by incorporating anisotropic inverse Compton scattering of CMBR photons by relativistic electrons and show that the thermal Sunyaev-Zel'dovich effect is strongly angle-dependent. The spectral distortion in the CMBR alters significantly in different angular directions. The amount of distortion increases several times when the angle between the axis of symmetry and the photon ray path is increased. It is pointed out that for anisotropic scattering the Compton y parameter is angle dependent. Hence it is important to consider the direction of the emergent radiation with respect to the axis of symmetry in the electron rest frame while estimating any cosmological quantity from the Sunyaev-Zel'dovich effect. ~

astro-ph↗
arXiv2001

Probing Dust in the Atmosphere of Brown Dwarfs Through Polarization

Theoretical analysis and observational evidences indicate that a brown dwarf with effective temperature greater than 1400 K would have dust cloud in its atmosphere. In this letter, we show that dust scattering should yield polarized continuum radiation from the relatively warm brown dwarfs and the polarized flux profile could be a potential diagnosis tool for the optical and the physical properties of dust grains. The degree of polarization due to multiple scattering will be more in the optical region if the particle size is small while significant polarization should be detected in the infra-red region if the particle size is large. It is pointed out that the departure from sphericity in the shape of the object due to rapid rotation and due to tidal effect by the companion in a binary system ensures the disc integrated polarization to be non-zero.

astro-ph↗
arXiv2000

Joule Heating in Neutron Stars under Strong Gravitation

Considering Joule heating caused by the dissipation of the magnetic field in the neutron star crust to be an efficient mechanism in maintaining a relatively high surface temperature in very old neutron stars, the role of general relativity is investigated. It is found that, although the effect of space-time curvature produced by the intense gravitational field of the star slows down the decay rate of the magnetic field, modification of the initial magnetic field configuration and the initial field strength by the space-time curvature results in increasing the rate of Joule heating. Hence the space-time curvature supports Joule heating in maintaining a relatively high surface temperature which is consistent with the bservational detection.

astro-ph↗
arXiv2000

Line Formation in the Atmosphere of Brown Dwarf Gliese 229B: $CH_4$ at 2.3 micron

We investigate the formation of methane line at 2.3 ${\rm μm}$ in Brown Dwarf Gliese 229B. Two sets of model parameters with (a) $T_{\rm eff}=940 $K and $\log (g) =5.0$, (b) $T_{\rm eff}=1030 $K and $ \log(g)=5.5$ are adopted both of which provide excellent fit for the synthetic continuum spectra with the observed flux at a wide range of wavelengths. In the absence of observational data for individual molecular lines, we set the additional parameters that are needed in order to model the individual lines by fitting the calculated flux with the observed flux at the continuum. A significant difference in the amount of flux at the core of the line is found with the two different models although the flux at the ontinuum remains the same. Hence, we show that if spectroscopic observation at $2.3{\rm μm}$ with a resolution as high as $R \simeq 200,000$ is possible then a much better constraint on the surface gravity and on the metallicity of the object could be obtained by fitting the theoretical model of individual molecular line with the observed data.

astro-ph↗
arXiv1998

Evolution of Crustal Magnetic Fields in Isolated Neutron Stars : Combined Effects of Cooling and Curvature of Space-time

The ohmic decay of magnetic fields confined within the crust of neutron stars is considered by incorporating both the effect of neutron star cooling and the effect of space-time curvature produced by the intense gravitational field of the star. For this purpose a stationary and static gravitational field has been considered with the standard as well as the accelerated cooling models of neutron stars. It is shown that general relativistic effect reduces the magnetic field decay rate substantially. At the late stage of evolution when the field decay is mainly determined by the impurity-electron scattering, the effect of space-time curvature suppresses the role of the impurity content significantly and reduces the decay rate by more than an order of magnitude. Even with a high impurity content the decay rate is too low to be of observational interest if the accelerated cooling model along with the effect of space-time curvature is taken into account. It is, therefore, pointed out that if a decrease in the magnetic field strength by more than two orders of magnitude from its initial value is detected by observation then the existence of quark in the core of the neutron star would possibly be ruled out.

astro-ph↗
arXiv1997

Charged particle trajectories in a toroidal magnetic and rotation-induced electric field around a black hole

Trajectories of charged particle in combined poloidal, toroidal magnetic field and rotation-induced unipolar electric field superposed in Schwarzschild background geometry have been investigated extensively in the context of accreting black holes. The main purpose of the paper is to obtain a reasonably well insight on the effect of spacetime curvature to the electromagnetic field surrounding black holes. The coupled equations of motion have been solved numerically and the results have been compared with that for flat spacetime. It is found that the toroidal magnetic field dominates the induced electric field in determining the motion of charged particles in curved spacetime. The combined electromagnetic field repels a charged particle from the vicinity of a compact massive object and deconfines the particle from its orbit. In the absence of toroidal magnetic field the particle is trapped in a closed orbit. The major role of gravitation is to reduce the radius of gyration significantly while the electric field provides an additional force perpendicular to the circular orbit. Although the effect of inertial frame dragging and the effect of magnetospheric plasma have been neglected, the results provide a reasonably well qualitative picture of the important role played by gravitation in modifying the electromagnetic field near accreting black holes and hence the results have potentially important implications on the dynamics of the fluid and the radiation spectrum associated with accreting black holes.

gr-qc↗
arXiv1997

General Relativistic Effects on the Ohmic Decay of Crustal Magnetic Fields in Neutron Stars

The ohmic decay of magnetic fields confined within the crust of neutron stars is considered by incorporating the effect of space-time curvature produced by the intense gravitational field of the star. It is shown that general relativistic effect reduces the magnetic field decay rate substaintially and specially at the late time of the evolution the decay rate decreases by several orders of magnitude when compared with the case without the relativistic effect.

astro-ph↗
arXiv1995

TOWARDS A PHYSICAL INTERPRETATION OF THE ANGULAR MOMENTUM PARAMETER ASSOCIATED WITH THE KERR METRIC

The role of the parameter `$a$' associated with the Kerr metric which represents the angular momentum per unit mass of a rotating massive object has been converted into determining the rotationally induced quadrapole electric field outside a rotating massive object with external dipole magnetic field. A comparison of the result with that of the Newtonian case implies that the parameter `$a$' represents the angular momentum per unit mass of a rotating hollow object.

gr-qc↗