17 Comae Berenices

Lore & Background

17 Comae Berenices is a multiple star system located about 240 light-years away in the northern constellation Coma Berenices. The brightest component, 17 Com A, is visible to the naked eye at magnitude 5.2. It has a magnitude 6.6 companion, 17 Com B, separated by an angular distance at a position angle of 251° as of 2018, and a fainter 14th-magnitude star near 17 Com B.

The system's double nature was first recorded by F. G. W. Struve in 1836. The two main stars share a common proper motion, suggesting they may be physically associated. Component B is itself a binary system, though only its brighter member is detectable in the spectrum; the Washington Double Star Catalogue lists the fainter member as component C, with a magnitude of 13.7. The stars have been proposed as members of the Coma Star Cluster, but this remains uncertain.

17 Com A was identified as chemically peculiar by A. J. Cannon before 1918. In 1932, W. W. Morgan noted that its spectral lines varied in strength and appearance, and detected europium lines. H. W. Babcock and T. G. Cowling measured the Zeeman effect in 1953, confirming the star has a magnetic field. E. P. J. van den Heuvel observed a blue excess in 1967, suggesting it may be a blue straggler. G. W. Preston and colleagues found in 1969 that both the star's luminosity and magnetic field vary on a timescale of about five days.

The primary is a magnetic chemically peculiar Ap star, classified as A0p or A0 SrCrEu, indicating enhanced strontium, chromium, and europium. Its atmosphere also shows elevated silicon and a significant helium deficiency. It carries the variable star designation AI Com and is classified as an Alpha2 Canum Venaticorum variable, with a suspected Delta Scuti variable nature. It is also considered a suspected blue straggler. Estimated to be 101 million years old, the star has more than double the Sun's mass and twice its radius, and radiates 43 times the Sun's luminosity from a photosphere at roughly 10,000 K. Its projected rotational velocity is 20 km/s.

The co-moving companion, component B, is a single-lined spectroscopic binary with an orbital period of 68.3 days and an eccentricity of 0.3. The visible star in this pair is a strong Am star, with a classification of kA2hA9VmF0—meaning its calcium K-lines match an A0 star, its hydrogen lines an A9 star, and its metallic lines an F0 star.

Reader's Guide

17 Comae Berenices holds significance as a well-studied magnetic chemically peculiar star, one of the early objects in which the Zeeman effect was used to demonstrate the presence of a stellar magnetic field. Its variability in luminosity and magnetic field strength over a roughly five-day cycle, documented by Preston and associates, contributed to the understanding of rotational modulation in Ap stars. The system's possible membership in the Coma Star Cluster, though disputed, ties it to broader questions of stellar association and evolution. The primary's classification as a suspected blue straggler and Delta Scuti variable adds further interest, as such stars challenge standard models of stellar evolution. The binary nature of component B, with its 68.3-day orbital period and Am star characteristics, provides a contrasting example of chemical peculiarity in a close binary system. Together, the components of 17 Comae Berenices offer a rich case study in stellar magnetism, chemical abundance anomalies, and multiple-star dynamics.

Did You Know?

• 17 Com A has the variable star designation AI Com.
• The magnetic field of 17 Com A was demonstrated via the Zeeman effect in 1953.