CLASSICAL T TAURI-LIKE OUTFLOW ACTIVITY IN THE BROWN DWARF MASS REGIME*

摘要

Over the last number of years, spectroscopic studies have strongly supported the assertion that protostellar accretion and outflow activity persist to the lowest masses. Indeed, previous to this work, the existence of three brown dwarf (BD) outflows had been confirmed by us. In this paper, we present the results of our latest investigation of BD outflow activity and report on the discovery of two new outflows. Observations to date have concentrated on studying the forbidden emission line (FEL) regions of young BDs and in all cases data have been collected using the UV-Visual Echelle Spectrometer (UVES) on the ESO Very Large Telescope. Offsets in the FEL regions are recovered using spectro-astrometry. Here, ISO-Oph 32 is shown to drive a blueshifted outflow with a radial velocity of 10-20 km s–1 and spectro-astrometric analysis constrains the position angle of this outflow to 240° ± 7°. The BD candidate, ISO-ChaI 217 is found to have a bipolar outflow bright in several key forbidden lines (VRAD = –20 km s–1, +40 km s–1) and with a P.A. of 193°-209°. A striking feature of the ISO-ChaI 217 outflow is the strong asymmetry between the red- and blueshifted lobes. This asymmetry is revealed in the relative brightness of the two lobes (redshifted lobe is brighter), the factor of 2 difference in radial velocity (the redshifted lobe is faster) and the difference in the electron density (again higher in the red lobe). Such asymmetries are common in jets from low-mass protostars and the observation of a marked asymmetry at such a low mass (0.1 M ☉) supports the idea that BD outflow activity is scaled down from low-mass protostellar activity. Also note that although asymmetries are unexceptional, it is uncommon for the redshifted lobe to be the brightest as some obscuration by the accretion disk is assumed. This phenomenon has only been observed in one other source, the classical T Tauri (CTTS) star RW Aur. The physical mechanism responsible for the brightening of the redshifted lobe although as yet unknown must also now apply at BD masses to include the ISO-ChaI 217 outflow. In addition to presenting these new results, a comprehensive comparison is made between BD outflow activity and jets launched by CTTSs. In particular, the application of current methods for investigating the excitation conditions and mass loss rates () in CTT jets to BD spectra is explored. Where possible, the Bacciotti & Eisl?ffel technique is used to study the ionization fraction, electron temperature, and total density. For LS-RCrA 1, ISO-ChaI 217 and ISO-Oph 102 are measured to be in the range 10–10 to 10–9 M ☉ yr–1 using a method based on the luminosity of the [O I]λ6300 and [S II]λ6731 lines. Mass loss rates for our sample of BD outflows are found to be comparable to the mass accretion rates. Overall, as our results and discussion show, what is currently known about outflow activity in the BD mass regime points to strong similarities between BD outflows and CTT jets.