Like ordinary
main-sequence stars, cool subdwarfs (of spectral types G to M) produce their energy from
hydrogen fusion. The explanation of their underluminosity lies in their low
metallicity: These stars are not enriched in elements heavier than
helium. The lower metallicity decreases the
opacity of their outer layers and decreases the
radiation pressure, resulting in a smaller, hotter star for a given mass. 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. •
SSSPM J1549-3544 == Hot (blue) subdwarfs ==