English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

We have combined 2MASS and POSS II data in a search for nearby ultracool(later than M6.5) dwarfs with K_s<12. Spectroscopic follow-up observationsidentify 53 M7 to M9.5 dwarfs and seven L dwarfs. The observed space density is0.0045 +- 0.0008 M8-M9.5 dwarfs per cubic parsec, without accounting forbiases, consistent with a mass function that is smooth across thestellar/substellar limit. We show the observed frequency of H alpha emissionpeaks at ~100% for M7 dwarfs and then decreases for cooler dwarfs. In absoluteterms, however, as measured by the ratio of H alpha to bolometric luminosity,none of the ultracool M dwarfs can be considered very active compared toearlier M dwarfs, and we show that the decrease that begins at spectral type M6continues to the latest L dwarfs. We find that flaring is common among thecoolest M dwarfs and estimate the frequency of flares at 7% or higher. We showthat the kinematics of relatively active (EW_{H alpha}>6 Angstroms) ultracool Mdwarfs are consistent with an ordinary old disk stellar population, while thekinematics of inactive ultracool M dwarfs are more typical of a 0.5 Gyr oldpopulation. The early L dwarfs in the sample have kinematics consistent withold ages, suggesting that the hydrogen burning limit is near spectral typesL2-L4. We use the available data on M and L dwarfs to show that chromosphericactivity drops with decreasing mass and temperature, and that at a given (M8 orlater) spectral type, the younger field (brown) dwarfs are less active thanmany of the older, more massive field stellar dwarfs. Thus, contrary to thewell-known stellar age-activity relationship, low activity in field ultracooldwarfs can be an indication of comparative youth and substellar mass.Comment: 39 pages including tables and figures, to appear in August 2000 A.

New Neighbors from 2MASS: Activity and Kinematics at the Bottom of the Main Sequence