Herbig Ae/Be star

A Herbig Ae/Be star is a pre-main-sequence star – a young star of spectral types A or B. These stars are still embedded in gas-dust envelopes and are sometimes accompanied by circumstellar disks. Hydrogen and calcium emission lines are observed in their spectra. They are 2–8 Solar mass objects, still existing in the star formation stage and approaching the main sequence.

Description

In the Hertzsprung–Russell diagram, Herbig Ae/Be stars are located to the right of the main sequence. They are named after the American astronomer George Herbig, who first distinguished them from other stars in 1960.
The original Herbig criteria were:

Spectral type earlier than F0,
Balmer emission lines in the stellar spectrum,
Projected location within the boundaries of a dark interstellar cloud,
Illumination of a nearby bright reflection nebula.

There are now several known isolated Herbig Ae/Be stars. Thus the most reliable criteria now can be:

Spectral type earlier than F0,
Balmer emission lines in the stellar spectrum,
Infrared radiation excess due to circumstellar dust.

Sometimes Herbig Ae/Be stars show significant brightness variability. They are believed to be due to clumps in the circumstellar disk. In the lowest brightness stage the radiation from the star becomes bluer and linearly polarized.
Analogs of Herbig Ae/Be stars in the smaller mass range – F, G, K, M spectral type pre-main-sequence stars – are called T Tauri stars. More massive stars in pre-main-sequence stage are not observed, because they evolve very quickly: when they become visible, the hydrogen in the center is already burning and they are main-sequence objects.

Planets

Planets around Herbig Ae/Be stars include: