docs: def escape_velocity(M,R):
      """ 
      escape velocity.
      
      Parameters
      ----------
      M : float
          Mass in solar masses.
      R : float
          Radius in solar radiu.
  
      Returns
      -------
      v_escape
          in km/s.
          
      """
  
      ve = np.sqrt(2.*grav_const*M*msun_g/(R*rsun_cm))
      ve = ve*1.e-5
      return ve