The magnetic field and helium variation of the helium-strong star HD 184927.

Details The magnetic field and helium variation of the helium-strong star HD 184927. The magnetic field and helium variation of the helium-strong star HD 184927.

Apr 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997a%26a...320..172w&link_type=abstract

Astronomy and Astrophysics, v.320, p.172-176

Astronomy and Astrophysics

Astrophysics

14

Polarization, Stars: Chemically Peculiar, Stars: Individual: Hd 184927, Stars: Magnetic Fields

Scientific paper

We have obtained 29 longitudinal magnetic field measurements of the helium-strong star HD 184927 using the University of Western Ontario photoelectric polarimeter and the Special Astrophysical Observatory Zeeman Analyzer. These measurements, obtained over ~5500-days, confirm the presence of a variable line-of-sight magnetic field. A period search of these data reveals several acceptable rotational periods, the most significant of which is 9.52961d +/-0.00731d. This period is the only one consistent with those determined previously from analysis of photometric, equivalent width and magnetic variations (Bond & Levato 1976PASP...88..905B; Levato & Malaroda 1979PASP...91..789L). When phased with this period, the longitudinal field data describe a smooth sinusoidal curve with extrema of -0.7kG and +1.8kG. When we phase the helium equivalent width measurements and u-band photometry of Bond & Levato (1976PASP...88..905B) with our adopted period, the maximum of each of these quantities occurs at the same phase as the longitudinal field maximum. This indicates that an enhanced patch of helium may exist in the photosphere of HD 184927 near the positive magnetic pole. There is no evidence for such an enhancement near the negative pole. This suggests that significant differences may exist in the magnetic field of HD 184927 at the positive and negative magnetic poles, consistent with the presence of an important magnetic quadrupole component. By synthesizing C II λ6582.9 at a single phase, we have attempted, with moderate success, to define the magnetic field geometry. Using the line profile morphology, the observed longitudinal field variation, an the position of HD 184927 on the logT_eff_-logg diagram (which implies a radius R=6.6+/-0.8Rsun_), we find the projected rotational velocity v_e_sin i=14.5+/-2.5km/s, the inclination of the rotational axis to the line-of-sight i=25+/-5°, and the obliquity of the magnetic symmetry axis β=78+/-3°. Possible magnetic configurations range from nearly dipolar, to those which contain quadrupole and octupole components comparable in polar strength to the dipole component.

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