Hills Observatory: 1 January 2013 to 26 May 2020
Variable stars with nonuniform surface brightness and/or ellipsoidal shapes, whose
variability is caused by axial rotation with respect to the observer. The nonuniformity
of surface brightness distributions may be caused by the presence of spots or by some
thermal or chemical inhomogeneity of the atmosphere caused by a magnetic field
whose axis is not coincident with the rotation axis.
These stars are subdivided into the following types:
ACV Alpha2 Canum Venaticorum variables. These are main-sequence stars with
spectral types B8p-A7p and displaying strong magnetic fields. Spectra show
abnormally strong lines of Si, Sr, Cr, and rare earths whose intensities vary with
rotation. They exhibit magnetic field and brightness changes (periods of 0.5-160
days or more). The amplitudes of the brightness changes are usually withine
0.01-0.1 mag in V.
ACVO Rapidly oscillating Alpha2 CVn variables. These are nonradially pulsating,
rotating magnetic variables of Ap spectral type (DO Eri). Pulsation periods are in
the range of 6-12 mmag (0.004-0.01 days), while amplitudes of light variation
caused by the pulsation are about 0.01 mag in V. The pulsational variations are
superposed on those caused by rotation.
BY BY Draconis-type variables, which are emission-line dwarfs of dKe-dMe spectral
type showing quasiperiodic light changes with periods from a fraction of a day to
120 days and amplitudes from several hundredths to 0.5 mag in V. The light
variability is caused by axial rotation of a star with a variable degree of
nonuniformity of the surface brightness (spots) and chromospheric activity.
Some of these stars also show flares similar to those of UV Cet stars, and in
those cases they also belong to the latter type and are simultaneously considered
ELL Rotating ellipsoidal variables (b Per, Alpha Vir). These are close binary systems
with ellipsoidal components, which change combined brightnesses with periods
equal to those of orbital motion because of changes in emitting areas toward an
observer, but showing no eclipses. Light amplitudes do not exceed 0.1 mag in V.
FKCOM FK Comae Berenices-type variables. These are rapidly rotating giants with
nonuniform surface brightnesses, which have G-K spectral types with broad H
and K Ca II emission and sometimes Halpha. They may also be spectroscopic
binary systems. Periods of light variation (up to several days) are equal to
rotational periods, and amplitudes are several tenths of a magnitude. It is not
excluded that these objects are the product of further evolution of EW (W UMa)
close binary systems (see below).
PSR Optically variable pulsars (CM Tau), which are rapidly rotating neutron stars with
strong magnetic fields, radiating in the radio, optical, and X-ray regions. Pulsars
emit narrow beams of radiation, and periods of their light changes coincide with
rotational periods (from 0.004 to 4 s), while amplitudes of the light pulses reach
SXARI SX Arietis-type variables. These are main-sequence B0p-B9p stars with
variable-intensity He I and Si III lines and magnetic fields. They are sometimes
called helium variables. Periods of light and magnetic field changes (about 1 day)
coincide with rotational periods, while amplitudes are approximately 0.1 mag in V.
These stars are high-temperature analogs of the ACV variables.