Photospheric acne at the bottom of the main-sequence: Doppler images of M4.5 - M9V stars

Barnes, John; Haswell, Carole; Jeffers, Sandra; Jones, Hugh; Pavlenko, Yakiv; Lohr, Marcus and Jenkins, James (2016). Photospheric acne at the bottom of the main-sequence: Doppler images of M4.5 - M9V stars. In: The 19th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun (Feiden, Gregory ed.).



Starspots are an important manifestation of stellar activity and yet their distribution patterns on the lowest mass stars is not well known. Time series spectra of fully convective M dwarfs taken in the red-optical with UVES reveal numerous line profile distortions which are interpreted as starspots. New Doppler images of HU Del (GJ 791.2A; M4.5V), BL Ceti (GJ 65A; M5.5V) and UV Ceti (GJ 65B; M6V) at two epochs separated by three nights are presented. We find that contrast ratios corresponding to photosphere-spot temperature differences of only 100-400 K are sufficient to model the time series spectra of M4.5V - M9V stars. Starspots are reconstructed at a range of phases and latitudes with mean spot filling factors of only a few per cent. The distribution and low-contrast of the spots/spot-groups that we recover are likely to be responsible for the low amplitude photometric variability seen in late-M dwarfs. The stability of the spot patterns in the two sets of timeseries observations enables us to measure the latitude dependent differential rotation, which we find to be consistent with zero

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