CDROM/APJ/V459/P278 Ap Star Atmospheres (Matthews+ 1996) ================================================================================ Opening a New Window on Ap Star Atmospheres: A T-tau Relation for HR 3831 From Its Limb-Darkened Pulsation Amplitudes Jaymie M. Matthews, William H. Wehlau, John Rice, and Gordon A. H. Walker <1996, ApJ, 459, 278> =1996ApJ...459..278M ================================================================================ 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. Keywords: stars: atmospheres -- stars: individual (HR 3831) -- stars: oscillations -- stars: peculiar File Summary: -------------------------------------------------------------------------------- File Name Lrecl Records Explanations -------------------------------------------------------------------------------- hr3831b.dat 17 151 Differential photometry, b bandpass hr3831v.dat 17 152 Differential photometry, v bandpass hr3831y.dat 17 152 Differential photometry, y bandpass hr3831r.dat 17 144 Differential photometry, R bandpass hr3831i.dat 17 141 Differential photometry, I bandpass hr3831j.dat 17 91 Differential photometry, J bandpass hr3831h.dat 17 92 Differential photometry, H bandpass hr3831k.dat 17 92 Differential photometry, K bandpass -------------------------------------------------------------------------------- Byte-by-byte Description of file: hr3831b.dat hr3831v.dat hr3831y.dat hr3831r.dat hr3831i.dat hr3831j.dat hr3831h.dat hr3831k.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 9 D9.5 d HJD Julian Date - 2448000 10-17 F8.4 mag dMag Differential magnitude normalized to the mean -------------------------------------------------------------------------------- ================================================================================ (End) Lee Brotzman [ADS] 22-Apr-96