Cooler Dwarf Stars

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Science  04 Aug 2000:
Vol. 289, Issue 5480, pp. 697
DOI: 10.1126/science.289.5480.697d

The coolest stars with just enough mass to fuse hydrogen are the M-dwarfs. Two new classes of brown dwarfs have been added to the end of this stellar sequence. The L-dwarfs are slightly cooler and less massive than M-dwarfs, and they cannot sustain hydrogen fusion; T-dwarfs are even cooler and less massive than L-dwarfs. Hundreds of T-dwarfs and tens of L-dwarfs have been discovered in the solar neighborhood during the last 4 years.

Kirkpatrick et al. describe photometric and spectroscopic work on 67 newly identified L-dwarfs, revealing that L-dwarfs have temperatures between 1300 to 2000 Kelvin and that the atomic lithium abundance declines in the cooler L-dwarfs, which suggests that lithium is being converted to a Li-rich molecule. Burgasser et al. concentrate on one T-dwarf (2MASSW J1237392+652615) and note a hydrogen alpha emission that persisted during three nights of observations. This emission indicates that the T-dwarf may have either a magnetic field that creates stellar flares in its chromosphere or an active accretion disk (along with a close binary companion) that emits Ha. Thus, the L-dwarfs are more abundant than previously suspected, and the lowly T-dwarfs may be more active than expected. — LR

Astron. J.120, 447 (2000); Astron. J. 120, 473 (2000).

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