Very Low Mass Stars and Brown Dwarfs

In the very coolest main-sequence stars, energy transport is dominated by convection throughout all of their interiors. In such fully convective stars, a solar-like aQ-dynamo, thought to be localized in a transition layer between radiative and convective zones, cannot operate since there is no radiative core. The disappearance of radiative cores takes place at ^0.35MQ (corresponding to spectral type ~M3). Stars below this mass limit are referred to as very-low mass (VLM) stars. Similar to the case of the pre-main sequence stars (see Sect. 10.4), alternative field generating mechanisms, such as an a2-dynamo or a turbulent dynamo, may be at work in VLM stars, but unfortunately, theory makes no quantitative and testable predictions on their efficiency.

Proceeding toward even lower masses on the main-sequence yet another critical point is reached near 0.07-0.08M0. Objects below this limit are not able to sustain stable hydrogen burning because their interior temperature is lower than the threshold (~6106 K) required to initiate hydrogen burning through the protonproton chain. These "substellar" objects are called brown dwarfs. Lacking an internal energy source, brown dwarfs cool down and become fainter with age.

Establishing, if a given object is substellar or not, is quite cumbersome in practice, because none of the crucial physical parameters, like mass, effective temperature, and age, are directly observable. The discovery of the coolest stars and brown dwarfs within the last years, mainly by infrared sky surveys, has required the extension of the historical spectral type sequence (OBAFGKM) at the cool end, where the spectral classes "L" and "T" have been introduced. The spectral type/effective temperature calibrations depend on several atmospheric parameters, e.g., gravity, such that the conversion is different for young brown dwarfs on the premain sequence and for evolved brown dwarfs on the main sequence. Therefore, late-M or early L objects can be stars or brown dwarfs depending on their age. Objects in the transition region are termed brown dwarf candidates. Only if spectroscopic methods have confirmed their substellar nature, they are considered bona-fide brown dwarfs.

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