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

Abstract

We present HST-COS FUV and -STIS optical observations towards the young accreting brown dwarf 2MASS-J08440915-7833457 (J0844) from the ULLYSES DDT Program. We analyse hot FUV lines such C IV, Si IV, and N V, as well as fluorescent emission from H2. 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 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 x 10-11 Msol/yr. The accretion rate found based on the empirical LCIV-Macc relationship is twoorders of magnitude higher, suggesting this relationship may not hold at the lowest masses. We find the H2 emission appears to originate within the co-rotation radius, pointing to either disc truncation well inside the co-rotation radius or additional sources of H2 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 disc conditions to the relatively unexplored lowest mass regime.