Asubdwarf, sometimes denoted by "sd", is a star withluminosity class VI under theYerkes spectral classification system. They are defined asstars withluminosity 1.5 to 2magnitudes lower than that of main-sequence stars of the samespectral type. On aHertzsprung–Russell diagram subdwarfs appear to lie below themain sequence.[a]
The term "subdwarf" was coined byGerard Kuiper in 1939, to refer to a series of stars with anomalous spectra that were previously labeled as "intermediatewhite dwarfs".[1](p 87)
SinceKuiper coined the term, the subdwarf type has been extended to lower-mass stars than were known at the time. Astronomers have also discovered an entirely different group of blue-white subdwarfs, making two distinct categories:
Like ordinarymain-sequence stars, cool subdwarfs (of spectral types G to M) produce their energy fromhydrogenfusion. The explanation of their underluminosity lies in their lowmetallicity: These stars are not enriched in elements heavier thanhelium. The lower metallicity decreases theopacity of their outer layers and decreases theradiation pressure, resulting in a smaller, hotter star for a given mass.[2] This lower opacity also allows them to emit a higher percentage ofultraviolet light for the samespectral type relative to aPopulation I star, a feature known as theultraviolet excess.[1](p 87–92)Usually members of the Milky Way'shalo, they frequently have high space velocities relative to theSun.[3]
Cool subdwarfs of spectral type L and T exist, for exampleULAS J131610.28+075553.0 with spectral type sdT6.5.[3]
Subclasses of cool subdwarfs are as following:[4][5]
The lowmetallicity of subdwarfs is coupled with their old age. The early universe had a low content of elements heavier than helium and formed stars andbrown dwarfs with lower metallicity. Only latersupernovae,planetary nebulae andneutron star mergers enriched the universe with heavier elements. The old subdwarfs belong therefore often to the older structures in our Milky Way, mainly thethick disk and thegalactic halo. Objects in the thick disk or the halo have a high space velocity compared to theSun, which belongs to the youngerthin disk. A highproper motion can be used to discover subdwarfs. Additionally the subdwarfs have spectral features that make them different from subdwarfs with solar metallicity. All subdwarfs share the suppression of the near-infrared spectrum, mainly theH-band and K-band. The low metallicity increase thecollision induced absorption ofhydrogen, causing this suppressed near-infrared spectrum. This is seen as blue infrared colors compared to brown dwarfs with solar metallicity. The low metallicity also change other absorption features, such as deeperCaH andTiO bands at 0.7 μm in L-subdwarfs, a weakerVO band at 0.8 μm in early L-subdwarfs and strongerFeH band at 0.99 μm for mid- to late L-subdwarfs.[7]2MASS J0532+8246 was discovered in 2003 as the first L-type subdwarf,[8] which was later re-classified as an extreme subdwarf.[7] The L-type subdwarfs have subtypes similar to M-type subdwarfs: The subtypes subdwarf (sd), extreme subdwarfs (esd) and ultra subdwarfs (usd), which are defined by their decreasingmetallicity, compared to solar metallicity, which is defined on alogarithmic scale:[7]
For T-type subdwarfs only a small sample of subdwarfs and extreme subdwarfs is known.[9]
2MASSI J0937347+293142 is the first object that was discovered in 2002 as a T-type subdwarf candidate[8] and in 2006 it was confirmed to have low metallicity.[10] The first two extreme subdwarfs of type T were discovered in 2020 by scientists and volunteers of theBackyard Worlds project. The first extreme subdwarfs of type T areWISEA 0414−5854 andWISEA 1810−1010.[9] Subdwarfs of type T and Y have lessmethane in their atmosphere, due to the lower concentration ofcarbon in these subdwarfs. This leads to a bluer W1-W2 (WISE) or ch1-ch2 (Spitzer) color, compared to objects with similar temperature, but with solar metallicity.[11] The color of T-types as a single classification criterion can be misleading. The closestdirectly 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.[12]
The first Y-type subdwarf candidate was discovered in 2021, the brown dwarfWISE 1534–1043, which shows a moderate redSpitzer 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, Maroccoet al. (2021) argue that the most likely explanation is a cold very low-metal brown dwarf, maybe the first subdwarf of type Y.[13]
Binaries can help to determine the age and mass of these subdwarfs. The subdwarfVVV 1256−62B (sdL3) was discovered as a companion to ahalowhite dwarf, allowing the age to be measured at 8.4 to 13.8 billion years. It has a mass of 84 to 87MJ, making VVV 1256−62B likely ared dwarf star.[14] The subdwarfWolf 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 44.9MJ, making it a brown dwarf.
Hot subdwarfs, of bluish spectral types O and B are an entirely different class of object than cool subdwarfs; they are also called"extremehorizontal-branch stars".Hot subdwarf stars represent a late stage in the evolution of some stars, caused when ared giant star loses its outerhydrogen layers before the core begins to fusehelium.
The reasons for their premature loss of their hydrogen envelope are unclear, but the interaction of stars in abinary star system is thought to be one of the main mechanisms. Single subdwarfs may be the result of a merger of twowhite dwarfs or gravitational influence from substellar companions. B-type subdwarfs, being more luminous than white dwarfs, are a significant component in the hot star population of old stellar systems, such asglobular clusters andelliptical galaxies.[15][16]
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The heavy metal subdwarfs are a type of hot subdwarf star with high concentrations ofheavy metals. The metals detected includegermanium,strontium,yttrium,zirconium andlead. Known heavy metal subdwarfs includeHE 2359-2844,LS IV-14 116, andHE 1256-2738.[17]
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