Journal
ASTRONOMY & ASTROPHYSICS
Volume 551, Issue -, Pages -Publisher
EDP SCIENCES S A
DOI: 10.1051/0004-6361/201219433
Keywords
stars: early-type; subdwarfs; stars: atmospheres
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Funding
- National Science Foundation [AST-9819835]
- Science and Technology Facilities Council [ST/H001921/1] Funding Source: researchfish
- STFC [ST/H001921/1] Funding Source: UKRI
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An MK (Morgan-Keenan)-like system of spectral classification for hot subdwarfs is presented. We find that a three-dimensional spectral type, consisting of a spectral class, a luminosity class, and a helium class, is necessary to classify the sdO (subdwarf O) and sdB (subdwarf B) stars. In addition, the helium-strong stars appear to form two parallel spectral sequences: one showing strong lines of CII, CIII, or CIV, and the other with these same lines weak or absent. We also give a preliminary calibration of the new spectral types in terms of effective temperature, surface gravity, and surface helium-to-hydrogen abundance ratio, and show the relation between the new spectral types and the natural groups defined by the Palomar-Green (PG) survey.
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