Fr.: modèle CAK
The standard model of → radiation-driven winds in which the acceleration of → stellar wind is provided by the → absorption and → scattering of ultraviolet photons in ions of abundant elements (→ CNO, → iron peak) in the → Lyman continuum. The model was developed by Castor et al. (1975), who assumed that the forces due to the radiative lines and the pressure gradients are functions of local velocity gradient, and used a large number (~ 105) of lines which have a statistical distribution in line strengths. The model led to predictions of → mass loss rates (M_dot) and terminal velocities as a function of stellar properties and the line statistics parameters. With the modifications by Friend and Abbott (1986), Pauldrach et al. (1986), and Kudritzki et al. (1989), CAK multi-line theory gives good agreement with observationally derived values of mass loss rate and → terminal velocity (v∞). CAK wind solutions predict the terminal velocity to be proportional to the → escape velocity and the mass loss rate to depend strongly on the stellar → luminosity. Observations over the past decades have shown that these main wind parameters, M_dot and v∞, indeed behave as predicted by CAK. This basic agreement between observations and theory provides strong evidence that the winds from → massive stars are driven by → radiation pressure and this has favored the CAK formalism. See also → multiple scattering. See the review by J. Puls et al. 2008, Astron. Astrophys. Rev. 16, 209.
CAK, the initials of the researchers who developed the model: J.I. Castor, D.C. Abbott, and R.I. Klein(1975, Radiation-driven winds in Of stars, ApJ 195, 157); → model.
ruydâd-e K-T (#)
Fr.: événement K-T
H and K lines
xatthâ-ye H o K
Fr.: raies H et K
Two prominent → absorption lines, at 3968.5 Å and 3933.7 Å respectively, in the spectra of stars like the → Sun and cooler due to → singly ionized → calcium (Ca II). The strength of H and K lines can be an indication of considerable magnetic activity in the → chromosphere of these stars. The Ca II H and K lines are also common in some kinds of → eruptive variable stars. These lines are not seen in → hot stars, and start to become visible in → A-type stars.
H and K, letters of alphabet, conventionally chosen; → line.
Fr.: système JHK
1) Three → atmospheric windows in the
→ near infrared portion of the
→ electromagnetic spectrum at 1.25 (J), 1.65 (H), and
2.20 (K) μm.
Letters of alphabet, used conventionally; → system.
Fr.: couronne K
The inner part of the → solar corona which extends to about two solar radii. It is due to the → Thomson scattering of light from the → photosphere by the free electrons in the corona. The K corona exhibits a → linearly polarized continuous spectrum. The high speeds of the scattering electrons (on the average 10,000 km s-1 for a temperature of 2 million K) smear out the → Fraunhofer lines except the → H and K lines.
Fr.: correction K
A → color index correction applied to the photometric magnitudes and colors of a distant galaxy to compensate for the "reddening" of the galaxy due to → cosmological redshift. K correction is intended to derive the magnitudes in the → rest frame of the galaxy. Typically it is given as K(z) = az + bz2, where a and b depend on galaxy types. Conversely, one may deduce the redshift of a galaxy by its colors and a K-correction model.
The term K correction, probably stems from the K-term used by C. W. Wirtz (1918, Astron. Nachr. 206, 109), where K stands for Konstante, the German word for constant. The K-term was a constant offset in the redshift applied to diffuse nebulae in that epoch (source: A. L. Kinney, 1996, ApJ 467, 38); → correction.
setâre-ye K (#)
Fr.: étoile de type K
An orange-red star of → spectral type K with a surface temperature of about 3600-5000 K. The spectra of K stars are dominated by the H and K lines of calcium and lines of neutral iron and titanium, with molecular bands due to cyanogen (CN) and titanium dioxide (TiO). Examples are → Arcturus and → Aldebaran.
K the letter of alphabet; → star.
ruydâd-e K-T (#)
Fr.: événement K-T
Same as the → Cretaceous-Tertiary event.
Fr.: mission K2
A follow-up mission of the → Kepler satellite funded by → NASA. K2 provides an opportunity to continue Kepler's observations in the field of → exoplanets and expand its role into new astrophysical observations by assigning to Kepler new mission.
Fr.: système MKS
The system of → MKS units.