Browsing by Author "Roy, Nirupam"

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    Critical properties of spherically symmetric accretion in a fractal medium
    (2007-07-18) Roy, Nirupam; Ray, Arnab K.
    Spherically symmetric transonic accretion of a fractal medium has been studied in both the stationary and the dynamic regimes. The stationary transonic solution is greatly sensitive to infinitesimal deviations in the outer boundary condition, but the flow becomes transonic and stable, when its evolution is followed through time. The evolution towards transonicity is more pronounced for a fractal medium than what is it for a continuum. The dynamic approach also shows that there is a remarkable closeness between an equation of motion for a perturbation in the flow, and the metric of an analogue acoustic black hole. The stationary inflow solutions of a fractal medium are as much stable under the influence of linearised perturbations, as they are for the fluid continuum.
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    Multiwavelength investigation of the temperature of the cold neutral medium
    (2005-11-01) Roy, Nirupam; Chengalur, Jayaram; Srianand, R.
    We present measurements of the H i spin temperatures (Ts) of the Cold Neutral Medium (CNM) towards radio sources that are closely aligned with stars for which published H2 ortho-para temperatures (T01) are available from UV observations. Our sample consists of 18 radio sources close to 16 nearby stars. The transverse separation of the lines of sight of corresponding the UV and radio observations varies from 0.1 to 12.0 pc at the distance of the star. The ultraviolet (UV) measurements do not have velocity information, so we use the velocities of low ionization species (e.g Na i/K i/C i) observed towards these same stars to make a plausible identification of the CNM corresponding to the H2 absorption. We then find that T01 and Ts match within observational uncertainties for lines-of-sight with H2 column density above 1015.8 cm−2, but deviate from each other below this threshold. This is consistent with the expectation that in the CNM Ts tracks the kinetic temperature due to collisions and that T01 is driven towards the kinetic temperature by proton exchange reactions.