Herbig-Ae / Be star

Herbig Ae / Be stars (after their first descriptor, George H. Herbig ) are certain young stars that are less than 10 million years old.

Their mass is in the range of 2 to about 10  solar masses , and they are often found in areas with increased star formation . The temperature inside is not yet high enough to allow stable hydrogen burning ; instead, the energy emitted comes from its contraction. Thus the Herbig Ae / Be stars in the Hertzsprung-Russell diagram are still on the way to the main sequence .

definition

Since this class of stars was first described in 1960, the criteria for classifying objects have undergone various changes. Today, slightly different working definitions are used for categorization as Herbig-Ae / Be stars. These include in particular the following characteristic features:

• a spectral class of A or B, the range sometimes being expanded to include the immediately adjacent spectral types up to F2 and O9;
• strong infrared excess due to dust in a circumstellar disk , from matter on the star accreted (the e in the name comes from the word excess);
• the Balmer lines in issue.

development

Herbig-Ae / Be stars arise from T-Tauri stars , pre-main sequence stars with a spectral type of F3 and later. Most T-Tauri stars have less than 2 solar masses, but some more massive stars develop into Herbig-Ae / Be stars in the course of further contraction. (Stars with more than 8 solar masses do not show a pre-main sequence stage, as they develop so quickly that the hydrogen burn will already ignite if they can not be observed in a molecular cloud due to high extinction .)

Later, Herbig Ae / Be stars evolve into A and B stars on the main sequence.

variability

The luminosity of Herbig Ae / Be stars is variable on time scales from seconds to hundreds of days (i.e. years). Some stars are pulsating variables of the Delta Scuti or Gamma Doradus type . In addition, there are flares and a modulation of the light curve with the duration of the star's rotation. This suggests a global magnetic field , which is unexpected in the case of radiative energy transport in the photosphere . These magnetic fields have been proven by spectropolarimetric measurements .

Another source of variability appears to be changes in absorption in the circumstellar disks of the UX Orionis stars . It is assumed that condensation has already formed in the protoplanetary disks, which lead to increased absorption in the direction of the earth when passing in front of the disk.

Accretion discs

Herbig Ae / Be stars are divided into two groups according to their spectral course in the infrared :

• Group I sources show a spectral energy distribution that can be explained by blackbody radiation from a single temperature and a stronger infrared excess.
• Group II sources, on the other hand, can be described by a power law .

It is believed that Group I sources evolve into Group II sources when dust particles coagulate and the dust disc flattens out due to collisions. In addition, the radiation from the young star could inflate the inner part of the disk and thereby change the spectral energy distribution towards Group II sources. With the group II accretion discs , the inner part of the disc appears to be expanded and shields the outer part from the intense UV radiation . Therefore, larger dust grains can condense in these stars, which are not destroyed by photodissociation within a very short time .

In most Herbig-Ae / Be stars, the accretion disk dissolves within a period of 3 million years, the higher the mass of the star, the faster this process occurs. This is a consequence of the rise in temperature and luminosity, as well as the accelerated development with higher mass. The relatively short period of time is sufficient for exoplanets to form , some of which have already been detected around the early stars. The accretion rate over a period of one million years is around 10 ^−4.5 solar masses per year. During this time, most of the star's mass is accreted.

Dust disks have often been found around the successors of the Herbig Ae / Be stars, main sequence stars between 10 and 20 million years old. The detection in the far infrared was initially surprising, since dust is removed from the star systems within a short time by the radiation pressure . Hence there needs to be a continuous source of dust recurrence. Today it is believed that the dust is created in numerous collisions of asteroids . The corresponding disks are therefore called debris disks .

Examples

• HD 100546
• MWC 480

See also

• UX Orionis star
• Be star

Individual evidence

1. ^ MA Pogodin et al .: Measuring the mass accretion rates of Herbig Ae / Be stars with X-shooter . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.3732v1 .
2. ^ CP Folsom et al .: Chemical abundances of magnetic and non-magnetic Herbig Ae / Be stars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1202.1845v1 .
3. ↑ SM Rucinski et al .: Photometric variability of the Herbig Ae star HD 37806 . In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1008.4599 .
4. ^ Jose A. Caballero: The occultation events of the Herbig Ae / Be star V1247 Ori . In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1002.4092 .
5. ↑ KM Maaskant et al .: Identifying gaps in flaring Herbig Ae / Be disks using spatially resolved mid-infrared imaging. Are all group I disks transitional? In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1305.3138v1 .
6. ^ Jose A. Caballero: On the interplay between flaring and shadowing in disks around Herbig Ae / Be stars. In: Astrophysics. Solar and Stellar Astrophysics . 2009, arxiv : 0907.2102 .
7. ^ B. Acke et al .: Parameters of Herbig Ae / Be and Vega-type stars . In: Astrophysics. Solar and Stellar Astrophysics . 2008, arxiv : 0811.3557 .
8. ↑ I. Mendigutıa, A. Mora, B. Montesinos, C. Eiroa, G. Meeus, B. Merın, and RD Oudmaijer: Accretion-related properties of Herbig Ae / Be stars Comparison with T Tauris . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.4734v1 .
9. ↑ G. Meeus, B. Montesino, I. Mendigutıa, I. Kamp, WF Thi, C. Eiroa, CA Grady, G. Mathews, G. Sandell, C. Martin-Zaıdi, S. Brittain, WRF Dent, C. Howard, F. Menard, C. Pinte, A. Roberge, B. Vandenbussche and JP Williams: AGASPS observations of Herbig Ae / Be stars with PACS / Herschel? The atomic and molecular content of their protoplanetary discs . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.3413v1 .