Chasing coronaij x-ray emission to the end of the pleiades main sequence … and beyond?

Abstract

We have used a 40-ks XMM-Newton observation to examine the X-ray emission of 33 proposed M-type members in. the core of the Pleiades. We reach a limiting X-ray luminosity of L_X ≈ × 10 ²⁷ erg s ^-1 in the field centre and detect 17 of 19 members with M_Ic < 10.5 (where M_Ic is absolute magnitude in the Cousins I band, which is centered on 790 nm), and 2 of 14 fainter members. A further possible detection of the brown dwarf candidate Roque 9, with log L_X ≈ 27.8 and X-ray-to-bolometric luminosity ratio of log( L _X/L _bol) ≈ -2.5 , may be due to a chance alignment with a background source. Although the X-ray luminosity decreases sharply with increasing M_Ic (later-types, lower-masses), all but 3 members show L_X consistent with near-saturated emission and we do not see convincing evidence for a decrease in activity level indicative of a decrease in dynamo efficiency at the spectral type at which stellar structure is expected to become fully-convective.