Accretion Properties of the Young Brown Dwarf 2MASS J08440915-7833457

Abstract We present Hubble Space Telescope Cosmic Origins Spectrograph far-ultraviolet (FUV) and Space Telescope Imaging Spectrometer (STIS) optical observations toward the young accreting brown dwarf 2MASS J08440915-7833457 (J0844) from the UV Legacy Library of Young Stars as Essential Standards Director’s Discretionary Time program. We analyse hot FUV lines, such C IV , Si IV , and N V , as well as fluorescent emission from H 2 . Despite evidence for accretion, the C IV line profiles are narrower than in typical classical T Tauri stars (CTTSs), resembling weak-lined T Tauri stars more closely. Additionally, the C IV integrated line flux does not follow the level expected of an accreting object in the magnetically saturated regime. However, comparing J0844 to appropriate low-mass analogs, J0844 does show excess C IV emission characteristic of accretion, suggesting the magnetic saturation level may need to be redefined for the lowest-mass objects. The C IV /Si IV emission-line ratio is found to be 20, which is higher than for most CTTSs, with a few exceptions (e.g., TW Hya). We fit the STIS optical spectrum to calculate an accretion rate, which we find to be 4.2 × 10 −11 M ⊙ yr −1 . The accretion rate found based on the empirical L CIV – <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">acc</mml:mi> </mml:mrow> </mml:msub> </mml:math> relationship is 2 orders of magnitude higher, suggesting this relationship may not hold at the lowest masses. We find the H 2 emission appears to originate within the corotation radius, pointing to either disk truncation well inside the corotation radius or additional sources of H 2 emission that we do not consider (e.g., from the accretion flow itself). These data provide an extension of our current understanding of accretion and inner-disk conditions to the relatively unexplored lowest-mass regime.