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Estimating kinetic temperature from H I 21 cm absorption studies: correction for the turbulence broadening

Neutral hydrogen 21 cm transition is a useful tracer of the neutral interstellar medium. However, inferring physical condition from the observed 21 cm absorption and/or emission spectra is often not straightforward. One complication in estimating the temperature of the atomic gas is that the line width may have significant contribution from non-thermal broadening. We propose a formalism here to separate the thermal and non-thermal broadening using a self-consistent model of turbulence broadening of the HI 21 cm absorption components. Applying this novel method, we have estimated the spin and the kinetic temperature of diffuse Galactic neutral hydrogen, and found that a large fraction of gas has temperature in the unstable range. The turbulence is found to be subsonic or transonic in nature, and the clouds seem to have a bimodal size distribution. Assuming that the turbulence is magnetohydrodynamic in nature, the estimated magnetic field strength is of μG order, and is found to be uncorrelated with the HI number density.