Cool sub-dwarf

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Cool Subdwarfs are - like the hot sub-dwarf with the spectral type (sd) - classified . The (sd) stands as a prefix for Sub-Dwarf, which corresponds to the English term for Unterdwarf . They have a surface temperature of around 2000 to 6000  K - accordingly they are divided into classes (sd) M to (sd) G. If one compares sub - dwarfs in the Hertzsprung-Russell diagram with ordinary dwarf stars of a certain mass (like the sun), the cool sub-dwarfs are located to the left of the main row because of their hotter surface, and a little further up because they are lighter; however, they are not bright enough to match the brightness of more massive main sequence stars. In this way, cool sub-dwarfs form their own "main row" below the usual ones and are therefore misleadingly referred to as less luminous.

Cool sub-dwarfs are generally very old Population II stars , which mostly belong to the galactic halo of the Milky Way and have high speeds relative to the Sun.

construction

Section through a cool sub-dwarf subtype M, not to scale

Cool sub-dwarfs are poor in metal , i. H. Elements heavier than helium are rarer in them than usual, which leads to a slightly different star structure . Apart from that, cool sub-dwarfs are like main sequence stars and get their energy from burning hydrogen . The lower metal abundance reduces the opacity of the star's interior (it becomes more translucent) and thus the outward radiation pressure in the star is reduced . This in turn has the consequence that the star is smaller and hotter than an ordinary Population I main sequence star of equal mass . Because of their lower opacity, cool sub-dwarfs shine more strongly in the ultraviolet than main sequence stars of the same spectral class.

Substellar sub-dwarfs

A recent development is the discovery of cool sub-dwarfs, which are around brown dwarfs ( Halo transitional brown dwarfs might act). The ones discovered so far have spectral classes up to late L. These sub-dwarfs also have sub-categories (spectral class L):

  • sdL: subdwarf
  • esdL: extreme subdwarf
  • usdL: ultra-subdwarf

Examples

Individual evidence

  1. ↑ James B. Kaler: Stars and their spectra . Spectrum Academic Publisher, ISBN 3-86025-089-2 , 1994.
  2. ↑ a b Z.H. Zhang et al .: Primeval very low-mass stars and brown dwarfs - III. The halo transitional brown dwarfs . In: MNRAS . 2018. arxiv : 1805.08033 . doi : 10.1093 / mnras / sty1352 .
  3. ^ Adam J. Burgasser: The First Substellar Subdwarf? Discovery of a Metal-poor L Dwarf with Halo Kinematics . In: The Astrophysical Journal . 592, No. 2, 2003, pp. 1186-1192. arxiv : astro-ph / 0304174 . bibcode : 2003ApJ ... 592.1186B . doi : 10.1086 / 375813 .
  4. ↑ a b Z.H. Zhang et al .: Primeval very low-mass stars and brown dwarfs - I. Six new L subdwarfs, classification and atmospheric properties . In: MNRAS . 464, No. 3, 2017, pp. 3040-3059. arxiv : 1609.07181 . bibcode : 2017MNRAS.464.3040Z . doi : 10.1093 / mnras / stw2438 .
  5. ^ ZH Zhang et al. =: Primeval very low-mass stars and brown dwarfs - II. The most metal-poor substellar object . In: MNRAS . 468, No. 1, 2017, pp. 261–271. arxiv : 1702.02001 . bibcode : 2017MNRAS.468..261Z . doi : 10.1093 / mnras / stx350 .