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Galactic Archeology
Galactic Archeology
Collaborators: Prof. Bacham E. Reddy (IIA) and Prof. Emeritus David L. Lambert (W.J. McDonald Observatory and Uni. of Texas)
(Project duration: 2017 onwards)
This project aims to understand the formation and evolution histories of our home galaxy, the Milky Way, by studying the kinematic, chemical compositions and ages of individual stars as well as different stellar populations in it.
See Publications for details about our published works under this project!
Lithium enrichment in the Galaxy
Lithium enrichment in the Galaxy
Collaborators: Prof. Bacham E. Reddy (IIA) and Prof. Emeritus David L. Lambert (W.J. McDonald Observatory and Uni. of Texas)
(Project duration: 2018 onwards)
See Publications for details about our published works under this project!
Evolution of Lithium in low-mass stars
Evolution of Lithium in low-mass stars
Collaborators: Prof. Bacham E. Reddy (IIA) and Prof. Emeritus David L. Lambert (W.J. McDonald Observatory and Uni. of Texas)
(Project duration: 2018 onwards)
See Publications for details about our published works under this project!
Measurements of the Solar coronal magnetic field through low-frequency radio observations
Measurements of the Solar coronal magnetic field through low-frequency radio observations
(Work for this project was carried out at IIA Bangalore and The Gauribidanure Observatory under the supervision of Prof. R. Ramesh from 2016 to 2017)
The Sun's tenuous outer atmosphere, the corona, consists of highly rarefied ionised gases from which radio emission originates. Any inference about the physical nature of observed solar radio emission requires a knowledge of the generation and propagation of radio waves in the ionised gas through which the radiation passes. In the absence of a magnetic field, the propagation of radio waves in the medium depends upon the bending due to a change of refractive index, the absorptive attenuation in the "pass" regions (regions with positive refractive index) and the obstruction by "stop" regions (regions with negative refractive index) along its path. The presence of a magnetic field makes the medium birefringent and imposes distinct polarisation on the waves depending on the magnitude of the magnetic field and its direction with respect to the direction of wave propagation.
In the present work, we are concerned about the generation and propagation of radio radiation in the solar corona in the presence of any localised density enhancement or magnetic field. Here we have tried to devise a way to estimate the solar magnetic field associated with different transients or activity in the solar atmosphere by making low-frequency solar radio observations.
The first phase of this work involved understanding the propagation of radio-frequency waves through the quiet and active solar corona using the ray-tracing technique. This also involved measuring the effects of solar activities on observable quantities like Stokes I and Stokes V parameters, which are further used to measure the degree of circular polarisation (DCP). Some of the preliminary results from the simulations are shown below.
Ray trajectory corresponding to 30 MHz radio radiation for active corona with enhancement factor C =5 and width of the active region (streamer) σ(sigma) = 0.5 Ro . Again, rays are sent from xo = 9 Ro, y0 = Ya , and zo = Za , where Ya and Za are the impact parameters along y-axis (east-west) and z-axis (north-south). (a) When activity is present at θ = 90 and φ = 0 degrees, where θ and φ are the spherical polar coordinates of the active region. (b) When activity is present at θ = 90 and φ = 30 degrees, where θ and φ are the spherical polar coordinates of the active region. Out of the shown three circles, the innermost (yellow) represents the optical Sun, i.e. radius = Ro, and the outer two are of r = 1.5 Ro and r = 2.0 Ro.
Plots for the turning points (the closest point of approach) of rays shooted from different impact parameters Ya, with density enhancement present at θ = 90 and φ = 30, 60, and 90 degrees, along with the case of no enhancement (blue line). Out of the shown three circles in the figure, the innermost (dotted line) represents the optical Sun i.e. radius = Ro, and the middle is of radius,r = 1.5 Ro and the outer one is r = 2.0 Ro.
The variation of percent maximum degree of circular polarization (DCP) with respect to frequency (calculated from 20 to 80 MHz in steps of 5 MHz) for different position of density enhancement in the presence of magnetic filed strength, B = 0.25, 0.50, 0.75, 1.00, 1.25 and 1.50 G. The density enhancement is added at θ = 90 o , (a) φ = 0 degree, (b) φ = 30 degrees, (c) φ = 45 degrees, and (a) φ = 60 degrees. The enhancement factor (C) = 5, and the width of the active region (streamer) is σ(sigma) = 0.5 Ro.
The variation of percent maximum degree of circular polarization (DCP) with respect to magnetic filed strength (calculated from 0.25 to 1.50 G in steps of 0.25 G) for different position of density enhancement at different frequencies (from 20 to 80 MHz in steps of 5 MHz). The density enhancement is added at θ = 90 degrees, (a) φ = 0 degree, (b) φ = 30 degrees, (c) φ = 45 degrees, and (a) φ = 60 degrees. The enhancement factor (C) = 5, and the width of the active region (streamer) is σ(sigma) = 0.5 Ro.
In conclusion, from the ray-tracing calculation, it is observed that for magnetic field strength B > 0.75 G, the per cent maximum degree of circular polarisation (DCP) is greater than unity in the frequency range 20-80MHz, and facilities at Gauribidanur Radio Observatory are capable of measuring DCP values unity or greater in this frequency range, so facilities at Gauribidanur Radio Observatory can be used to estimate the solar coronal magnetic field. Also, in the present case, we have considered the thermal radio emission, so not only for the active regions but for quiet solar corona, this method can be used to estimate the solar coronal magnetic field.
In the second phase of this project, we used the observational data from The Gauribidanure Observatory to find the observational counterpart of results from the theoretical ray-tracing simulation. We used the data from the April and May of 2016. The data is basically the stokes intensities of the quiet Sun observed in the frequency range from 35 to 80 MHz. The Sun was not always very quiet during these two months, so using data from other instruments like Gauribidanur RAdioheliograPH (GRAPH) and Gauribidanur LOw-frequency Solar Spectrograph (GLOSS), we identified the best quiet Sun days from these two months. For the present study, the days for which the Sun was found to be quiet are 6, 8, 21, 23, 26, 27, 28 and 29 of April 2016. (Note: The Sun was quiet for some other days also but due to some technical reasons, data was not available, so we have neglected all such days for which data was not available). Now to find the DCP of the Sun, a new method is developed in which the DCP value for the entire spectrum, i.e. for all the frequencies, are calculated simultaneously instead of going for a particular frequency because the latter method resulted in highly biased DCP values due to noise and was making it very difficult to predict the accuracy of the estimated value of DCP. In the presently used method, the dynamic plots of Stokes-I and Stokes-V were plotted and then from these plots, the integration time is decided (by considering the transition time), and then the dynamic spectrums are integrated to get the amplitude vs frequency spectrum for both Stokes-I and Stokes-V for quiet Sun. We then plotted the Sun's Stokes-V as a function of Stokes-I, and the slope of this curve gives the DCP value. But this DCP value is not the actual DCP value from the Sun as there will be contamination from the background and due to the instrument also. We used Cygnus as a calibrator to remove this polarization due to the instrument and the background polarization. For calibration, similar to the Sun's case Cygnus's Stokes-V as a function of Stokes-I is plotted, and then the slope of that curve is calculated, which represents the DCP due to the instrument and the background. Now for the actual DCP from the Sun, we subtracted the Cygnus's DCP from the previously calculated DCP for the Sun. This resulted in a final DCP of 2.48% in the observed frequency range from 35 to 80 MHz. Now using the results from the simulation, the corresponding values of the magnetic field at frequencies from 35 to 80 MHz are determined. The estimated coronal magnetic field strengths as a function of frequency and heliocentric distance are shown below.
The variation of the estimated magnetic field with respect to frequency (35 to 80 MHz in steps of 5 MHz). For theoretical predictions, we assumed an enhancement at θ = 90 degrees, and φ = 45 degrees with the enhancement factor (C) = 5, and the width of the active region (streamer) as σ(sigma) = 0.5Ro.
The variation of the estimated magnetic field with respect to the heliocentric distances. When the density enhancement is considered at θ = 90 degrees, and φ = 45 degrees with the enhancement factor (C) = 5, and the width of the active region (streamer) as σ(sigma) = 0.5Ro .
Experimental investigation of cylindrical vector beams
Experimental investigation of cylindrical vector beams
(The work for this project was carried out at the Indian Institute of Guwahati (IITG) under the supervision of Prof. Bosanta R Boruah (IITG) as part of my master's thesis from 2014 to 2015. This work also includes contributions from Dr. Ranjan Kalita and Dr. Md. Abdul Gaffar, both of whom were doctoral students at IITG at that time.)
Vector beams have interesting polarisation profiles in the pupil plane. Because of their characteristic polarisation, they give rise to important properties in focal volume, which also vary with the system's numerical aperture. The cylindrical vector beams (CVB) are a special class of vector beams. Their polarisation states exhibit cylindrical symmetry across their cross-section. This is different from traditional polarization types, such as linearly, elliptically, and circularly polarized beams, in which the polarization state is spatially homogeneous across the cross-section of the beam. The CVBs are of increasing recent interest in applications such as atom trapping, laser cooling, lithography, electron acceleration, optical probes, optical data storage and high-resolution microscopy, which may help break the optical limit. In astronomy and astrophysics, learning about vector beams is of utmost importance for exploring outer space.
In this project, the properties and generation methods of vector beams are investigated. Starting from the B. Richards and E. Wolf (1959) paper, where an integral representation of field at the focal point due to linearly polarised light (x-polarised) is given, work is extended for y-polarised, arbitrary polarised and circularly polarised lights. Field representation near the focus of an aplanatic system due to radially and azimuthally polarised light given by K. S. Youngworth and T. G. Brown (2000) is also used. On the basis of these field representations and by extensive simulation, a thorough investigation of the time-averaged electric and magnetic energy density in the focal plane is done for different numerical aperture of the system. And finally, using computer-generated holography (CGHy), liquid crystal spatial light modulator (LCSLM) and basic optical components, an experimental set-up is designed to generate these beams.
Intensity and polarization distributions of cylindrical vector beams (CVB). (a) Radially polarised CVB. (b) Azimuthally polarised CVB; (c) A generalized cylindrical vector beam from a linear superposition of a purely radially and azimuthally polarised CVB. The white colour indicates the highest intensity while black represents the lowest intensity. Arrows denote the polarization directions.
Diagram showing the experimental set-up used to perform the experiment. He-Ne Laser is a randomly polarized laser. L1 to L8 are lenses. M1 to M5 are plane reflective mirrors. ID is an iris diaphragm. P1 is a polarizer to make the polarization vertical. P is a prism, and BS is a beam splitter. HWP is a half-wave plate placed at an angle of 45 degrees with the vertical. CCD is a charged coupled device to record the beam, and finally, the PC is a computer used to store the data from the CCD and control a multiplex hologram on the spatial light modulator (SLM).
As shown above, the preliminary beams obtained from the setup are not spherical and have different kinds of aberrations and helicity. Here, we have shown the original beam along with the beam with helicity +1 and +2. Observed aberrations on the beam, like vertical coma (Z7), horizontal coma (Z8), vertical trefoil (Z9), and primary spherical (Z11), are also shown.
This figure shows the (a) aberrated beams, (b) corrected beams, (c) beams with helicity +1, and (d) beams with helicity +2.
An expanded view of the Beam 1 and Beam 2 are shown in the top panels of columns one and two, respectively. Sine modulation on Beam 1 and Cosine modulation on Beam 2 is shown in the bottom panels of column one and two, respectively. The third column's top panel show a combined Beam 1 and Beam 2, while the bottom panel show a radially polarized beam generated by combining modulated Beam 1 and Beam 2.
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