Ap star

Ap stars (sometimes also Bp / Ap stars ) are hot stars with a surface temperature on the order of 10,000  Kelvin ( spectral class  A and B) and a chemical composition that differs greatly from the majority of early stars. In their sharp-line spectra they show exceptionally strong lines of chromium , manganese and silicon or strontium, as well as the absorption lines of some rare earths, which are barely detectable in normal stars .

The name "Ap" is made up of the spectral class and the "p" from peculiar star (English peculiar for special ). The lines often vary periodically or quasi-periodically, which is why the Ap stars are also referred to as spectrum variables . Sometimes this star class is divided into Si stars, Mn stars, Cr stars, etc.

properties

Ap stars have a magnetic field with a magnetic flux density of a few kilo- Gauss . In addition, they are often pulsating variable stars with low amplitudes and radial speeds with periods between 5 and 21 minutes. They are known as roAp stars for rapidly oscillating Ap stars . roAp stars only occur in a temperature range from 6400 to 8400  Kelvin . They vibrate non-radially in high harmonics , the restoring force being the pressure in the star's atmosphere . Ap stars rotate much more slowly than the normal A and B stars.

The area of ​​the Hertzsprung-Russell diagram in which the roAps lie is also populated by non-variable Ap stars. There seem to be no differences in mass, chemical composition, age, or magnetic field strength between the two groups.

Another mechanism for brightness changes in Ap stars is suspected to be variable absorption by magnetospheric clouds and rotational variability by spots in the photosphere .

The periods of the spectral changes are not constant for most Ap stars, both increases and decreases in the rotational speed were measured. These period changes are associated with:

• Changes in radius due to the evolution of the stars from the main sequence
• the reduction of torque by a stellar wind with or without an interaction with the stellar magnetic field
• Precession of the axis of rotation
• a light transit time effect through a companion.

The spectra of the Ap stars vary with the duration of rotation, which is explained by the model of the crooked rotator. It says that the metals characteristic of these stars are predominantly enriched at the magnetic poles and become visible or invisible to the observer due to the rotation. The cause is likely to be in differential chemical diffusion , which selectively lets some chemical elements with a small cross-section sink, while chemical elements with a large cross-section accumulate in the atmosphere due to the radiation pressure . Some noticeable chemical abundances, such as the abundance of lithium, could also arise from spallation processes near the magnetic poles of the Ap or Bp stars.

With polarimetic measurements, the Stokes parameters can be derived from which conclusions can be drawn about the geometry of the magnetic fields. The Ap stars show more complex multipole magnetic fields than simple dipoles with only one north and one south pole.

Emergence

The precursors of Ap stars are pre-main sequence stars with considerably higher rotation speeds (typical rotation period one day), called Herbig-Ae / Be stars . Most of the original torque is likely dissipated by an interaction of the star's magnetic field with the ionized inner zone of the protoplanetary disk . In order to increase the rotation time to values ​​of up to a month, as observed with Ap stars, further torque would have to be reduced afterwards by a stellar wind with freezing magnetic field lines .

Examples

• Theta Aurigae
• HD 215441
• V364 Carinae
• SX Arietis

See also

• At the star
• Mercury Manganese Star
• Lambda Bootis star

Individual evidence

1. ↑ H. Scheffler, H. Elsässer: Physics of the sun and the stars . BI Wissenschaftsverlag, Heidelberg 1990, ISBN 3-411-14172-7 .
2. ↑ M. Schoeller, S. Correia, S. Hubrig, DW Kurtz: Multiplicity of rapidly oscillating Ap stars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1208.0480 .
3. ↑ J. Krticka, J. Janik, H. Markova, Z. Mikulasek, J. Zverko, M. Prvak, M. Skarka: Ultraviolet and visual flux and line variations of one of the least variable Bp stars HD 64740 . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1306.2458v1 .
4. ↑ Zdeněk Mikulášek, Jiří Krtička, Jan Janík, Miloslav Zejda, Gegory W. Henry, Ernst Paunzen, Jozef Žižňovský, Juraj Zverko: Ap stars with variable periods . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1310.6640v1 .
5. ↑ S. Joshi, E. Semenko, P. Martinez, M. Sachkov, YC Joshi, S. Seetha, NK Chakradhari, DL Mary, V. Girish, BN Ashoka: A spectroscopic analysis of the chemically peculiar star HD207561 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.4805v1 .
6. ↑ CP Folsom et al .: Orbital parameters, chemical composition, and magnetic field of the Ap binary HD 98088 . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1302.2699v1 .
7. ↑ A. Shavrina et al .: Abnormal lithium abundance in several Ap Bp stars . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1304.4175v1 .
8. ↑ J. Silvester, GA Wade, O. Kochukhov, S. Bagnulo, CP Folsom, D. Hanes: Stokes IQUV Magnetic Doppler Imaging of Ap stars I. ESPaDOnS and NARVAL Observations . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.5692 .
9. ↑ E. Alecian et al .: A high-resolution spectropolarimetric survey of Herbig Ae / Be stars II. Rotation . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.2911 .