A subdwarf, sometimes denoted by "sd", is a star with luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type. On a Hertzsprung–Russell diagram subdwarfs appear to lie below the main sequence.
The term "subdwarf" was coined by Gerard Kuiper in 1939, to refer to a series of stars with anomalous spectra that were previously labeled as "intermediate white dwarfs". This lower opacity also allows them to emit a higher percentage of ultraviolet light for the same spectral type relative to a Population I star, a feature known as ultraviolet excess.
Usually members of the Milky Way's halo, they frequently have high space velocities relative to the Sun.
Cool subdwarfs of spectral type L and T exist, such as ULAS J131610.28+075553.0 with spectral type sdT6.5.
; cool subdwarf: Examples: Kapteyn's Star (sdM1), GJ 1062 (sdM2.5)
; extreme subdwarf: Example: APMPM J0559-2903 (esdM7)
; ultrasubdwarf: Example: LSPM J0822+1700 (usdM7.5) 2MASS J0532+8246 was discovered in 2003 as the first L-type subdwarf, which was later re-classified as an extreme subdwarf.
2MASSI J0937347+293142 is the first object that was discovered in 2002 as a T-type subdwarf candidate The first two extreme subdwarfs of type T were discovered in 2020 by scientists and volunteers of the Backyard Worlds project. The first extreme subdwarfs of type T are WISEA 0414−5854 and WISEA 1810−1010. The color of T-types as a single classification criterion can be misleading. The closest directly imaged exoplanet, COCONUTS-2b, was first classified as a subdwarf of type T due to its color, while not showing a high tangential velocity. Only in 2021 it was identified as an exoplanet.
The first Y-type subdwarf candidate was discovered in 2021, the brown dwarf WISE 1534–1043, which shows a moderate red Spitzer Space Telescope color (ch1-ch2 = 0.925±0.039 mag). The very red color between J and ch2 (J-ch2 > 8.03 mag) and the absolute brightness would suggest a much redder ch1-ch2 color of about 2.4 to 3 mag. Due to the agreement with new subdwarf models, together with the high tangential velocity of 200 km/s, Kirkpatrick, Marocco et al. (2021) argue that the most likely explanation is a cold very low-metal brown dwarf, maybe the first subdwarf of type Y.
Binaries can help to determine the age and mass of these subdwarfs. The subdwarf VVV 1256−62B (sdL3) was discovered as a companion to a halo white dwarf, allowing the age to be measured at 8.4 to 13.8 billion years. It has a mass of 84 to 87 , making VVV 1256−62B likely a red dwarf star. The subdwarf Wolf 1130C (sdT8) is the companion of an old subdwarf-white dwarf binary, which is estimated to be older than 10 billion years. It has a mass of 44.9 , making it a brown dwarf.
Examples of cool subdwarfs
- Kapteyn's Star
- Groombridge 1830
- Mu Cassiopeiae
- 2MASS J05325346+8246465, a possible halo brown dwarf and the first substellar subdwarf.
Heavy metal subdwarfs
The heavy metal subdwarfs are a type of hot subdwarf star with high concentrations of heavy metals. The metals detected include germanium, strontium, yttrium, zirconium and lead. Known heavy metal subdwarfs include HE 2359-2844, LS IV-14 116, and HE 1256-2738.