Spectrum of a typical A star.
Among A-type peculiar stars are Ae stars, Am stars (which have strong and often variable absorption lines of certains metals and deficiences of others), and Ap stars. Also, two of the main kinds of pulsating variables, RR Lyrae stars and Delta Scuti stars, have surface temperatures in the A-star range.
An Ae star is an A star with strong emission lines, usually of hydrogen, superimposed on an otherwise normal spectrum and caused by a circumstellar shell of heated material. Some Ae stars have only recently formed and may be surrounded by visible nebulosity, in which case they are known as Herbig Ae/Be stars.
An Am star, also known as a metallic-line star, is a type of A star whose spectrum has strong and often variable absorption lines of some metals (hence the "m"), such as zinc, strontium, zirconium, and barium, more typical of an F star, and deficiencies of others, such as calcium and/or scandium. These abundance anomalies are due to some elements being pushed to the surface because they are better light-absorbers, while other elements sink to lower levels under gravity – an effect that requires slow stellar rotation.
Normal A stars spin quickly, but the vast majority of Am stars are known to be members of close binary systems, with orbital periods of less than 1,000 days, in which the two stars slow each other down by tidal action. Familiar examples include Sirius and Acubens (Alpha Cancri). An example of an Am star which is single is HD 8801. This star is also unusual for an Am star in that it pulsates, and it is unique among known stars in that it pulsates intrinsically with both gamma Doradus and Delta Scuti frequencies.
|Distribution of calcium on the surface of the Ap
star HR 3831.
An Ap star is an A star whose spectrum has unusually strong lines of some ionized metals and rare earth elements, pointing to a vast overabundance (103 to 106 solar values) of these elements in the star's surface layers. More generally, the term Ap star, or peculiar A star, has come to encompass a range of chemically anomalous stars roughly between spectral types B5 (see Bp star) and F5 (see Fp star). The elements in overabundance vary from one Ap star to another and may include manganese, mercury, silicon, chromium, strontium, europium, and others. Ap stars typically have surface temperatures of 8,000 to 15,000 K, strong magnetic fields, and low rotational rates – properties that help explain their observed chemical anomalies.
The separation of elements is enabled by the slow spin and the relatively high temperature, and hence lack of convection. Separation happens because each ion has its own photoabsorption characteristics. If a certain element absorbs photons (light particles) more easily, it will tend to be pushed to the surface and become overabundant. Otherwise it will sink under the force of gravity and appear depleted in the star's spectrum. The strength of the magnetic field also plays a part in determining which elements are overabundant as shown by the fact that manganese stars – similar to Ap stars but without a strong magnetic field – have anomalies of the same order of magnitude but often not for the same elements. Variations in the spectrum of many Ap stars, associated with magnetic variations, can be understood in terms of the oblique rotator model, in which the spin axis and magnetic axis of the star are out of alignment.