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(abridged) We present an analysis of high resolution optical spectra for a sample of very young, mid- to late M, low-mass stellar and substellar objects in Upper Sco and Taurus. Effective temperatures (T_eff) and surface gravities are derived from a multi-feature spectral analysis using TiO, NaI and KI, through comparison with the latest synthetic spectra. In combination, they allow us to determine T_eff to within 50K and gravity to within 0.25 dex. Our high-resolution analysis does not require extinction estimates. Moroever, it yields T_eff and gravities independent of theoretical evolutionary models. We find that our gravities for most of the sample agree remarkably well with the isochrone predictions for the likely cluster ages. However, discrepancies appear in our coolest targets: these appear to have significantly lower gravity (by up to 0.75 dex) than our hotter objects, even though our entire sample covers a relatively narrow range in T_eff (about 300K). This drop in gravity is also implied by inter-comparisons of the data alone, without recourse to synthetic spectra. We consider, and argue against, dust opacity, cool stellar spots or metallicity differences leading to the observed spectral effects; a real decline in gravity is strongly indicated. Such gravity variations are contrary to the predictions of the evolutionary tracks, causing improbably low ages to be inferred from the tracks for our coolest targets. We venture that these results may arise from evolutionary model uncertainties related to accretion, deuterium-burning and/or convection effects. Finally, when combined with photometry and distances, our technique for deriving gravities and temperatures provides a way of obtaining masses and radii for substellar objects independent of evolutionary models, as presented in Paper II., 74 pages, incl. 11 figures, accepted ApJ

We analyse high-resolution optical spectra of a sample of low-mass, very young, mid-to late M stellar and substellar objects in Upper Scorpius and Taurus. Precise effective temperatures (± 50K) and surface gravities (± 0.25 dex) are derived from amulti-feature spectral analysis using TiO, NaI and KI, through comparison with the latest synthetic spectra. Our high-resolution analysis is independent of extinction estimates. Combining our Teff and gravities with distance and photometry information then allows us to derive extinctions, masses, radii and luminosities. We emphasize that our analysis is independent of theoretical evolutionary models. As such, it offers a new way of deriving fundamental parameters for low-mass stellar and substellar objects, and of testing the model evolutionary tracks. We find discrepancies between our results and the theoretical tracks for the coolest, lowest mass targets. Our gravities for these objects are significantly (∼ 0.75 dex) lower than for hotter ones. This is at odds with the predictions of the evolutionary models, and causes improbably low ages to be derived from the theoretical tracks for the coolest objects. Similarly, our radii and luminosities for the coolest, lowest mass objects are much larger than predicted by the evolutionary models. These results lead us to suggest that significant improvements in the evolutionary models may be necessary for very cool, ultra-low mass objects, perhaps in the treatment of convection and/or initial conditions. We also find two of our coolest targets to be planetary mass objects (“planemos”), with mass ∼ 7 MJ. Thus, some late-M PMS objects may be much less massive than current evolutionary models indicate. Equivalently, young planemos may be more luminous than expected. Combined with the detection of planemos by other groups, our results imply that such objects may not be too rare.

Young stars surrounded by disks with very low mass accretion rates are likely in the final stages of inner disk evolution and therefore particularly interesting to study. We present ultraviolet (UV) observations of the ~5-9 Myr old stars RECX-1 and RECX-11, obtained with the Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph on the Hubble Space Telescope, as well as optical and near-infrared spectroscopic observations. The two stars have similar levels of near-UV emission, although spectroscopic evidence indicates that RECX-11 is accreting and RECX-1 is not. The line profiles of Hα and He I λ10830 in RECX-11 show both broad and narrow redshifted absorption components that vary with time, revealing the complexity of the accretion flows. We show that accretion indicators commonly used to measure mass accretion rates, e.g., U-band excess luminosity or the Ca II triplet line luminosity, are unreliable for low accretors, at least in the middle K spectral range. Using RECX-1 as a template for the intrinsic level of photospheric and chromospheric emission, we determine an upper limit of 3 × 10^(–10) M_☉ yr–1 for RECX-11. At this low accretion rate, recent photoevaporation models predict that an inner hole should have developed in the disk. However, the spectral energy distribution of RECX-11 shows fluxes comparable to the median of Taurus in the near-infrared, indicating that substantial dust remains. Fluorescent H2 emission lines formed in the innermost disk are observed in RECX-11, showing that gas is present in the inner disk, along with the dust.