HD 187474

Lore & Background

In 1958, Horace Babcock announced that HD 187474 has a magnetic field, which he estimated at 1867 gauss. He found the star remarkable because it was the only A-type star he had observed that seemed to have a magnetic field that did not vary in time. This perceived consistency later turned out to be the result of Babcock's observations covering only a small portion of the star's unexpectedly long variability period. The star's variability was apparently discovered by Babcock sometime prior to 1976, the year it received the variable star designation V3961 Sagittarii, but the result was never published by him.

HD 187474 is a single-lined spectroscopic binary, discovered by Sylvia Burd at Palomar Observatory. The finding was communicated to S. Leeman by Babcock, and Leeman published it in 1964 along with orbital elements derived by Burd. The initial estimate of the orbital period was 700 days. Several groups have attempted to determine the strength and geometry of the star's magnetic field. In 1987, Pierre Didelon derived a surface field strength of 1 kilogauss from Zeeman splitting observations. In 2000, John Landstreet and Gautier Mathys found that the variation of the measured magnetic field as the star rotated was far from sinusoidal, and obtained an acceptable fit with a model containing colinear dipole, quadrupole and octupole terms. Two years later, Stefano Bagnulo et al. modeled the field as a dipole inclined 80° with respect to the rotation axis plus a nonlinear quadrupole term. The next year, V. R. Khalack et al. modeled the field as a set of virtual magnetic charges with zero total charge. In 2005, Yu. V. Glagolevskij modeled the field as a dipole displaced from the star's center and inclined relative to the rotation axis by 24°.

Reader's Guide

HD 187474 holds significance as an extreme outlier among Alpha2 Canum Venaticorum variables due to its rotation period of 2,345 days, which is more than an order of magnitude longer than typical for the class. This unusual slowness initially misled Horace Babcock into believing the star's magnetic field was constant, as his observations covered only a small fraction of its rotation cycle. The star's magnetic field has been the subject of multiple modeling efforts, each employing different assumptions—dipole plus multipole terms, inclined dipole with nonlinear quadrupole, virtual magnetic charges, and a displaced dipole—reflecting the complexity of the field geometry. As a single-lined spectroscopic binary with an orbital period initially estimated at 700 days, the star also offers a rare opportunity to study magnetic field evolution in a binary system. The discovery of its variability by Babcock, though never published, and the subsequent assignment of the variable star designation V3961 Sagittarii in 1976, underscore the star's role in advancing understanding of magnetic fields in chemically peculiar A-type stars. The diversity of magnetic field models proposed between 1987 and 2005 highlights ongoing uncertainty about the field's true configuration.

Did You Know?

• Horace Babcock initially thought the star's magnetic field did not vary, but this was because his observations covered only a small part of its long rotation cycle.
• Multiple models of the star's magnetic field have been proposed, including dipole plus quadrupole and octupole terms, an inclined dipole with nonlinear quadrupole, virtual magnetic charges, and a displaced dipole.