def virial_mass(FWHM, vel_disp):
    """
    Calculates the Vyrial Theorem based Dynamical mass 
    FWHM is the deconvolved size of the source in pc
    vel_disp is the velocity dispersion in km/s
    http://adsabs.harvard.edu/abs/2018arXiv180402083L
    Leroy et al 2018
    """
    M = 892. * FWHM * (vel_disp**2.)
    return M