A refinement of the rotation period of the roap star hr 3831 and the discovery of a significant phase lag between the times of pulsation and magnetic extrema and the time of mean-light extremum

Abstract

HR 3831 is a rapidly oscillating Ap star with a pulsational period of 11.67 min. The amplitude of this pulsation ranges from O to about 5 mmag as a function of the rotational aspect of the star. We use 268 hr of new high-speed photometric observa­ tions obtained in 1990 and 1991 plus 238 hr of published high-speed photometry from 1980, 1981, 1985 and 1986 to show that the period of pulsation amplitude modulation is equal to the period of mean-light variation. We use this to show that an interpretation of the pulsation-amplitude modulation as rotationally perturbed m­ modes requires that Cnt≤2 X 10-5 at the 3σ a confidence level, making that interpretation highly unlikely. We also show that the time of pulsation maximum pre­ cedes the time of mean-light extremum by 0.177 ± 0.007 d (0.062 ± 0.002 rotation periods), a 25σ a difference. The time of magnetic extremum coincides with the time of pulsation-amplitude maximum but differs from mean-light extremum by 0.158±0.032 d, a 5σ difference. This argues that HR3831 is an oblique pulsator with the magnetic axis and pulsation axis aligned, rather than a spotted pulsator with amplitude modulation caused by surface inhomogeneities. lt also indicates a deviation from cylindrical symmetry which makes the interpretation of the pulsation-amplitude modulation in terms of the oblique-pulsator model more complex than has yet been considered. By shifting the pulsation-amplitude variations into phase with the mean­ light variations and normalizing both data sets, we refine the rotation frequency for HR3831 to be vrot=0.3506333±0.0000006 d-1=4058.256±0.007 nHz, or Prot=2.851982±0.000005 d.