Definition
Ap and Bp stars are subclasses of chemically peculiar (CP) stars belonging to the spectral types A (Ap) and B (Bp). They exhibit abnormal surface abundances of certain chemical elements and possess strong, organized magnetic fields compared with normal A‑ and B‑type stars.
Overview
These stars are main‑sequence objects with effective temperatures ranging roughly from 7,500 K to 30,000 K. The “p” denotes “peculiar,” reflecting spectral lines that are atypically strong or weak due to over‑ or under‑abundances of elements such as silicon (Si), chromium (Cr), strontium (Sr), europium (Eu), and helium (He). The magnetic fields, often of the order of a few hundred gauss up to several tens of kilogauss, are stable and globally ordered, usually approximated by a dipole configuration inclined to the rotation axis. Because of the magnetic field, the surface abundance anomalies are non‑uniform, leading to rotational modulation of their spectral lines and broad‑band photometric variability, a phenomenon known as the “oblique rotator model.”
Etymology/Origin
The designation “Ap” and “Bp” originates from the Morgan–Keenan (MK) spectral classification system, where the letters “A” and “B” identify temperature classes, and the suffix “p” (for “peculiar”) indicates deviations from the normal spectral pattern. The terms were first employed in the early‑20th century to categorize a growing number of A‑ and B‑type stars whose spectra displayed unusual metallic lines (e.g., the work of W. W. Morgan and P. C. Keenan).
Characteristics
| Feature | Ap Stars | Bp Stars |
|---|---|---|
| Temperature range | ~7,500–10,000 K (late A) | ~10,000–30,000 K (early B) |
| Typical magnetic field strength | 0.3–30 kG | 0.5–30 kG |
| Common chemical anomalies | Overabundances of Si, Cr, Sr, Eu; underabundance of Ca | Overabundances of Si, He‑weak or He‑strong groups; sometimes rare‑earth enhancements |
| Rotational periods | Generally slow (days to months) compared with normal A stars | Similar range; some Bp stars rotate faster but still slower than typical B stars |
| Variability | Periodic photometric and spectroscopic variability due to surface spots (oblique rotator) | Same mechanisms; variability often detected in ultraviolet and optical bands |
| Spectral peculiarities | Strong, sharp metallic lines, sometimes Zeeman splitting observable | Strong Si II/III lines, He I variations, occasionally resolved Zeeman components |
| Incidence | Approximately 5–10 % of A‑type stars | Roughly 5 % of early‑B stars, though exact fraction depends on detection limits |
The magnetic fields are believed to be fossil remnants from the star‑forming cloud, rather than generated by contemporary dynamo action. Diffusion processes—radiative levitation of certain ions and gravitational settling of others—combined with the magnetic field, produce the observed surface abundance patterns.
Related Topics
- Chemically Peculiar Stars (CP stars) – broader category including Am, HgMn, and λ Bootis stars.
- Oblique Rotator Model – theoretical framework describing the geometry of magnetic fields and rotational modulation.
- Stellar Magnetism – studies of magnetic field generation, structure, and evolution in stars.
- Diffusion in Stellar Atmospheres – processes responsible for element segregation in radiative envelopes.
- Alpha² Canum Venaticorum variables – a class of photometrically variable Ap stars named after the prototype α CMi.
- Helium‑weak and Helium‑strong stars – subcategories of Bp stars distinguished by anomalous helium line strengths.