HD 21699, also known asHR 1063 andV396 Persei, is a star about 580light years from the Earth, in the constellationPerseus.[1] It is a 5thmagnitude star, so it will be faintly visible to thenaked eye of an observer far fromcity lights. This is a variable star, whose brightness varies slightly from 5.45 to 5.53 during its 2.4761 day rotation period.[2] It has a remarkabledipole magnetic field which is displaced from the star's center by 0.4 stellar radii, the poles of which appear close to each other on thestellar surface.[11] HD 21699 is a member of theAlpha Persei Cluster.[12]
In 1967, Robert Garrison noted that the U-Bcolor of HD 21699 is significantly bluer (more negative) than thespectral type assigned to it (B8 III) would suggest.[14] Such a discrepancy suggests that the star ishelium-weak.[15] The star's helium-weak nature was confirmed byWilliam Morganet al. in 1971.[3] HD 21699 also has an enhanced silicon abundance.[16]
John Winzer observed HD 21699 during 1971 - 1972 and discovered that it is a variable star. He found it varied by 0.03, 0.04 and 0.05 magnitudes in the visible, blue and ultravioletphotometric bands, respectively. Though he found that the brightness varied periodically, he was unable to unambiguously assign a period to it. It was the first helium-weak star to be found to vary in brightness periodically.[15] In 1974, HD 21699 was assigned thevariable star designation V396 Persei.[17] In 1985, John Percy established that the star's variability period is2.49246±0.00035 days.[7]
In 1980, Werner Weiss deduced that HD 21699 has a magnetic field, based on aheuristic relationship between photometric colors and a star's surface magnetic field.[18] In 1984, Douglas Brownet al. announced that a magnetic field with a strength of about onekilogauss had been detected from observations ofZeeman splitting ofspectral lines.[19] That same year, Brownet al. announced thatInternational Ultraviolet Explorer data showed evidence of astellar wind flowing from HD 21699, which was constrained to flow from the region of the star's magnetic poles. This "plume" of gas sweeps across the line of sight for an observer on the Earth, as the star rotates.[20]
In 2007, Yu. V. Glagolevskij and G. A. Chuntonov examined the extensive data which had been collected for HD 21699, and concluded that the star has a very peculiar magnetic field. In their model, the field is a dipole, but it is displaced by0.4±0.1 stellar radii from the star's center. If the dipole were centered within the star, one would expect that the surface magnetic poles would be separated by 180° along a great circle which contained both poles. However, because the dipole is displaced from the star's center, the poles are separated by only 55°. Furthermore, the two magnetic poles lie almost exactly on the star's equator. Their estimate for the field's strength is21.8±0.2 kilogauss at the poles.[11]
Hubrig, S.; Mathys, G. (1994), Balona, Luis A.; Henrichs, Huib F.; Le Contel, Jean Michel (eds.), "Mass loss, magnetic field and chemical inhomogeneities in the He-weak star HD 21699",Pulsation; rotation; and mass loss in early-type stars: proceedings of the 162nd symposium of the International Astronomical Union held in Antibes; Juan-les-Pins; France; October 5-8; 1993, International Astronomical Union. Symposium no. 162, vol. 162, Dordrecht: Kluwer Academic Publishers, p. 167,Bibcode:1994IAUS..162..167H.
Shore, Steven N.; et al. (January 1990), "The Discovery of Magnetically Controlled Circumstellar Matter in the Helium-weak Stars HD 5737 and HD 79158",Astrophysical Journal,348: 242,Bibcode:1990ApJ...348..242S,doi:10.1086/168233.
Mallama, A. D.; Molnar, M. R. (June 1974), "UV Observations of the Helium-Weak Star HD 21699.",Bulletin of the American Astronomical Society,6: 307,Bibcode:1974BAAS....6R.307M.
