ACV a2 Canum Venaticorum stars

- These are main-sequence stars with spectral types B8p-A7p and displaying strong magnetic fields. Spectra

Observing Variable show abnormally strong lines of Si, Sr, Cr, and rare earths whose intensities vary with rotation, magnetic field, and brightness changes (05-160d or more). The amplitudes of the light changes are usually in the range of 0".'01-0".'I in V. ACVO (subtype) - Rapidly oscillating a2 Canum Venaticorum variables. These are nonra-dially pulsating, rotating magnetic variables of Ap spectral type. Pulsation periods are in the range 6"'-12'" (0004-001), while amplitudes of light variation caused by the pulsation are about 0".'01 in V. The pulsational variations are superposed on those caused by rotation. GCVS

a2 Canum Venaticorum stars, also known as Ap1 and roAp2 are stars in which the surface is severely depleted of helium (He) with, at the same time, overabundance of iron (Fe), silicon (Si) and chromium (Cr) in spots. These stars have been known since the early days of spectral classification, when the phenomenon was first detected.

Of interest when attempting to unambiguously classify these stars, the discrimination made in the GCVS in regards to ACV and SXARI stars seems irrelevant; one classification is often assigned to a star for which the other one would be more appropriate.

These chemically peculiar (CP) stars, in general, are stars that possess spectral signatures of chemical peculiarities such as strongly enhanced spectral lines of iron and rare-earth elements. In this group there is a magnetic sequence, referring to stars that demonstrate a strong, global magnetic field. This does not mean that the HgMn stars, or metallic-line (Am) stars, etc. have no magnetic field at all; however, stars designated within the nonmagnetic sequence may exist without a magnetic field or exist with a significantly weaker global effect or even with a strong field of complicated structure, such that the measurable effect, averaged-out over the visible disk, is insignificant. Ap stars have global surface magnetic fields in the order of 0.3 to 30 kg,3 and their effective magnetic-field strength varies with rotation, a situation that led to an interpretation in terms of the oblique-rotator model in which the

Observation

Faint stars Small amplitudes Short periods CCD or PEP

'The "p" suffix indicates lhat the star is chemically peculiar, in which the spectra reveal chemical signatures such as Fe and rare-earth elements.

2The "ro" prefix indicates rapidly oscillating Ap stars.

'Thousands of times stronger than that of the Sun.

magnetic axis is oblique to the rotation axis.4 The time-scales of light variations seen in Ap stars range from minutes to decades.

Ap stars are intrinsically slow rotators but the hotter stars rotate faster than the cooler ones. The length of the rotation period can be derived by plotting their spotted surface variations that change as the star rotates. Most periods are of the order of one day to one week, with a tail towards longer periods. Other sources of variability, such as binary motion or pulsation, may be superposed, so careful analysis is required.

A source of further information regarding chemically peculiar stars in general, and specifically a1 Canum Venaticorum variables, is the Peculiar Newsletter, of the IAU Working Group on Ap and Related Stars. This is a specialized astronomical circular founded in Vienna, Institut d'Astronomie de l'Université de Lausanne, CH-1290 Chavannes-des-Bois, Switzerland <http:// obswww.unige.ch/~north/APN/Welcome.html>.

Of particular interest may be Issue No 24, October 17, 1995, which contains the Table of Ap and Am Stars in order of variable-star names. The table includes not only the Ap and Am stars quoted as variable in the 4th edition of the General Catalog of Variable Stars but also those for which a variable-star name has more recently been assigned (name-lists Nos. 67-72). Also, each star of the Catalog général des étoiles Ap et Am having received a variable-star name is listed.

Observation Key

Mixed stars Small amplitudes ^^ Long periods <3> CCD or PEP

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