THE ASTROPHYSICAL JOURNAL, 459:278-287, 1996 March 1

       OPENING A NEW WINDOW ON Ap STAR ATMOSPHERES: A T-tau RELATION FOR
              HR 3831 FROM ITS LIMB-DARKENED PULSATION AMPLITUDES

                              JAYMIE M. MATTHEWS
    Department of Geophysics and Astronomy, University of British Columbia,
             Vancouver, BC V6T 1Z4, Canada; matthews@astro.ubc.ca

                               WILLIAM H. WEHLAU
            Department of Astronomy, University of Western Ontario,
                        London, Ontario N6A 3K7, Canada

                                   JOHN RICE
           Department of Physics and Astronomy, Brandon University,
          Brandon, Manitoba R7A 6A9, Canada; rice@mickey.brandonu.ca

                                      AND

                              GORDON A. H. WALKER
    Department of Geophysics and Astronomy, University of British Columbia,
              Vancouver, BC V6T 1Z4, Canada; walker@astro.ubc.ca


                                   ABSTRACT

        We demonstrate a new approach for probing the atmospheres of
     selected chemically peculiar magnetic stars.  Using the Cerro Tololo
     Inter-American Observatory (CTIO) 1.0 and 1.5 m telescopes
     simultaneously on two nights in 1991, we obtained optical and infrared
     high-speed photometry of the rapidly oscillating Ap (roAp) star HR
     3831 (=HD 83368), which pulsates with a period of ~11.8 minutes.
     Oscillation amplitudes (or upper limits) in each of eight bandpasses
     were measured by Fourier analysis, revealing the very steep decline in
     amplitude with increasing wavelength characteristic of roAp stars.
     Matthews and coworkers have shown that this can be explained by the
     wavelength dependence of limb darkening and its filtering effect on
     the integrated amplitude of an (l,m) = (1,0) nonradial pulsation mode.
     Since the eigenfrequency spectrum of HR 3831 is dominated by just such
     a dipole mode, it is possible to infer limb-darkening coefficients
     beta(lambda) for its atmosphere from our amplitude measurements.
     These coefficients are used to derive a source function through the
     Eddington-Barber relation, which leads to a relation for temperature
     versus optical depth in the stellar photosphere.
        Our results show that the T-tau(5000) curve is much steeper for HR
     3831 than for the Sun (at least at the rotational phase when the
     star's magnetic pole dominates the visible hemisphere).  We also
     calculate the gradient dT/dtau as a function of wavelength to compare
     with the curve expected for H- continuous opacity.  This type of
     diagnostic may become a useful tool for checking the predictions of
     diffusion theory.

     Subject headings: stars: atmospheres -- stars: individual (HR 3831)
                       -- stars: oscillations -- stars: peculiar