def physical_separation(d, theta):
    """
    Returns separation between two objects in the plane of the sky in AU given distance and parallax
    Distance and parallax must be astropy unit objects.
    Args:
        d (float): distance
        theta (float): parallax
    Return:
        separation in AU
    Written by: Logan Pearce, 2017
    """
    from astropy import units as u
    d = d.to(u.pc)
    theta = theta.to(u.arcsec)
    a = d * theta
    return a.to(u.au, equivalencies=u.dimensionless_angles())
