SIMBAD references

2005ApJ...626..498M - Astrophys. J., 626, 498-522 (2005/June-2)

The T Tauri phase down to nearly planetary masses: echelle spectra of 82 very low mass stars and brown dwarfs.


Abstract (from CDS):

Using the largest high-resolution spectroscopic sample to date of young, very low mass stars and brown dwarfs, we investigate disk accretion in objects ranging from just above the hydrogen-burning limit all the way to nearly planetary masses. Our 82 targets span spectral types from M5 to M9.5, or masses from 0.15 Mdown to about 15 jupiters. They are confirmed members of the ρ Ophiuchus, Taurus, Chamaeleon I, IC 348, R Coronae Australis, Upper Scorpius, and TW Hydrae star-forming regions and young clusters, with ages from <1 to ∼10 Myr. The sample contains 41 brown dwarfs (spectral types ≥M6.5). We have previously presented high-resolution optical spectra for roughly half the sample; the rest are new. This is a close to complete survey of all confirmed brown dwarfs known so far in the regions examined, except in ρ Oph and IC 348 (where we are limited by a combination of extinction and distance). We find that (1) classical T Tauri-like disk accretion persists in the substellar domain down to nearly the deuterium-burning limit; (2) while an Hα 10% width ≳200 km/s is our prime accretion diagnostic (following our previous work), permitted emission lines of Ca II, O I, and He I are also good accretion indicators, just as in classical T Tauri stars (we caution against a blind use of Hα width alone, since inclination and rotation effects on the line are especially important at the low accretion rates in very low mass objects); (3) the Ca II λ8662 line flux is an excellent quantitative measure of the accretion rate in very low mass stars and brown dwarfs (as in higher mass classical T Tauri Stars), correlating remarkably well with the M{dot} obtained from veiling and Hα modeling; (4) the accretion rate diminishes rapidly with mass–our measurements support previous suggestions that M{dot}∝M2*(albeit with considerable scatter) and extend this correlation to the entire range of substellar masses; (5) the fraction of very low mass stellar and substellar accretors decreases substantially with age, as in higher mass stars; (6) at any given age, the fraction of very low mass stellar and substellar accretors is comparable to the accretor fraction in higher mass stars; and (7) a number of our sources with infrared excesses arising from dusty disks do not evince measurable accretion signatures, with the incidence of such a mismatch increasing with age: this implies that disks in the low-mass regime can persist beyond the main accretion phase and parallels the transition from the classical to post-T Tauri stage in more massive stars. These strong similarities at young ages, between higher mass stars on the one hand and low-mass bodies close to and below the hydrogen-burning limit on the other, are consistent with a common formation mechanism in the two mass regimes.

Abstract Copyright:

Journal keyword(s): Stars: Circumstellar Matter - Stars: Planetary Systems - Stars: Formation - Stars: Low-Mass, Brown Dwarfs - Stars: Pre-Main-Sequence - Techniques: Spectroscopic

CDS comments: Table 1: USco DENIS HHMMSS (= USDEN HHMMSS in the text) are DENIS-P HHMMSS.s+DDMMSS

Simbad objects: 128

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