Herbig Ae/Be star
Herbig-Ae/Be are young stars with an age of less than 10 million years, and typically an early spectral type of A or B. They show a strong infrared excess due to dust formation in a circumstellar disk is accreted from the matter on the star. Their mass is in the range of 2 to about 10 solar masses and they are often found in areas with enhanced star formation. The temperature in the interior is not yet high enough to allow a stable hydrogen burning, thus the Herbig located in the Hertzsprung -Russell diagram still on the way to the main sequence. The radiated energy is obtained even from the contraction of the star.
Definition
Was first described for this class of stars in 1960 by George H. Herbig, the criteria for the classification of objects have undergone various transformations. There will now slightly different working definitions for the categorization of stars as Herbig application. For the classification of this class are used in particular the following characteristics:
- They have a spectral class of A or B, the area is sometimes also extended to the immediately adjacent spectral types to F2 and O9,
- They show the Balmer lines in emission, and
- They have an infrared excess due to dust from a circumstellar disk.
The Herbig evolve to A- and B- stars on the main sequence. Pre-main sequence with a spectral type of F3 and later are called T Tauri stars. Most T Tauri stars have masses of less than 2 solar masses. Some high-mass T Tauri stars evolve but Herbig-Ae/Be-Sternen the course of further contraction. Stars with masses greater than 8 solar masses show no Vorhauptreihenstadium because they evolve so fast that the hydrogen burning already ignites at a time in which they are unobservable in a molecular cloud due to high extinction.
- Group I sources show a spectral energy distribution that can be explained by a blackbody radiation at a single temperature.
- Group II sources, however, can be rather described by a power law.
It is believed that the Group I sources evolve to group II sources when dust particles coagulate and by collisions, the dust disk becomes flatter. In addition, the radiation of the young star may exaggerate the inner part of the disk, thereby changing the spectral energy distribution in the direction of group II sources.
Variability
The luminosity of Herbig-Ae/Be-Sternen is variable on time scales of seconds to hundreds of days. Some stars are pulsating variables of type Delta Scuti and gamma Doradus. Besides occur flares and a modulation of the light curve with the rotation period of the star. This suggests a global magnetic field at the Herbig-Ae/Be-Sternen, which is unexpected in a radiative energy transport in the photosphere. These magnetic fields have been proved by spectropolarimetric measurements in some Herbig-Ae/Be-Sternen. Another source of variability appear to be changes in absorption in the circumstellar disks in the UX Orionis stars. It is believed that already have formed in the proto plans Aryan discs condensations that lead the pre-transition before the disc to an increased absorption in the direction of the earth.
Accretion disks
The accretion disks of Herbig-Ae/Be-Sternen be divided into two groups on the basis of the infrared excess, wherein the group - I- slices show a greater excess. In group II, the inner discs of the disc appears to be expanded and shields the outer portion with respect to the intense UV radiation. Therefore condense large dust grains, which are not destroyed within a short time by photodissociation in these stars. The accretion disk dissolves in most Herbig-Ae/Be-Sternen within a period of 3 million years ago, and this process goes faster the higher the mass of the star. This is a consequence of the increase in the temperature and the brightness as well as to the accelerated development of higher mass. This brief period of time sufficient for the formation of exoplanets, some of which have already been demonstrated by the early stars. The accretion rate is in the period of one million years at approximately 10 to 4.5 solar masses per year. In this period, most of the mass of the star is accreted. To the successors of the Herbig, main-sequence stars with ages 10 to 20 million years ago, dust disks have been found frequently. The proof in the far infrared was zunäscht surprising since the radiation pressure dust away from the star systems within a short time. Therefore, there must be a continuous source for the Neueinstehung dust. It is believed today that the dust is produced in numerous collisions of asteroids. The corresponding slices are called debris disks.