Schematic representation of the Herzprung Russel diagram with the Sun compared to a young and bright B star.
Stars form with an initial mass distribution represented by the Initial Mass Function or IMF.
The most common parametrization of the IMF is the Sapeter IMF, which has a powe-law slope.
Thus most of the stars in the Galaxy are low-mass stars, with few giants.
A proof of the “dynamo” model is represented by the close link between stellar rotation and X-ray emission: stars that rotate quickly have higher levels of X-ray emission than stars that rotate slowly.
This relationship is linear for slowly rotating stars (at normal velocities for stars on the main sequence).
X-ray activity is also related to age: this evidence can be explained if the loss of angular momentum is due to mass loss over the life of the star.
The X-ray behaviour of ultrafast rotating stars is peculiar with respect to the trends observed in low-mass stars:
Possible reasons:
Close binaries have enhanced X-ray emission.
Young stars are known to have copious X-ray activity, well in excess of the one predicted from the dynamo model which explains main sequence stars.
Multiwavelength observations over the last two decades have shown magnetic activity in young pre-main sequence (pre-MS) stars or T Tauri stars (TTs) is greatly enhanced compared to the main sequence Sun.
Why?
Young stars show no correlation between rotational period and X-ray luminosity.
This is in direct contrast to the trend in main-sequence stars, and contradicts the commonly-held perception that high X-ray emission in young stars is a result of faster rotational velocities.
Young stars show no correlation between disk presence and X-ray luminosity.
“Box plots” encompass distributed data. Box centers indicate median values.
Young stars show correlation between bolometric and X-ray luminosity.
There is a correlation between bolometric and luminosity and X-ray luminosity, but this is likely due to the larger surface area of stars with greater bolometric luminosity.
2. Absorption and scattering processes – Part I
3. Absorption and scattering processes – Part II
4. Emission processes – Part I
5. Emission processes – Part II
6. Instruments for X-ray and γ-ray Astrophysics – Part I
7. Instruments for X-ray and γ-ray Astrophysics – Part II
8. X-rays from the solar system
9. X-rays from low-mass and PMS stars
12. Evolution of Shell-type Supernova remnants
13. X-ray binaries
14. X-ray emission in normal galaxies
15. Active Galactic Nuclei – part I
16. Active Galactic Nuclei – Part II
17. Active Galactic Nuclei – Part III
18. Clusters of Galaxies – Part I
Feigelson et al. 2003. “X-rays in the Orion Nebula cluster: constraints on the origins of magnetic activity in pre-main-sequence stars.”
Feigelson & Montmerle, 1999. “High-energy processes in young stellar objects.”