Starspot Distributions on Fully Convective M Dwarfs: Implications for Radial Velocity Planet Searches

Abstract

Since M4.5 -M9 dwarfs exhibit equatorial rotation velocities of order 10 kms −1 on average, radial velocity surveys targeting this stellar population will likely need to find methods to effectively remove starspot jitter. We present the first high resolution Doppler images of the M4.5 dwarf, GJ 791.2A, and the M9 dwarf, LP 944-20. The time series spectra of both objects reveal numerous line profile distortions over the rotation period of each star which we interpret as starspots. The transient distortions are modelled with spot/photosphere contrast ratios that correspond to model atmosphere temperature differences of T phot -T spot = 300 K and 200 K. GJ 791.2A is a fully convective star with v sin i = 35.1 kms −1 . Although we find more starspot structure at high latitudes, we reconstruct spots at a range of phases and latitudes with a mean spot filling of ∼ 3%. LP 944-20 is one of the brightest known late-M dwarfs, with spectral type M9V and v sin i = 30.8 kms −1 . Its spectral time series exhibits two dominant transient line distortions that are reconstructed as high latitude spots, while a mean spot filling factor of only 1.5% is found. The occurrence of low-contrast spots at predominantly high latitudes, which we see in both targets here, is in general likely to be responsible for the low amplitude photometric variability seen in late-M dwarfs. For GJ 791.2A, the radial velocities induced by the starspot features yield an r.m.s. velocity variability of 138 ms −1 , which can be reduced by a factor of 1.9 using our reconstructed surface brightness distributions.

Modelling the RV jitter of early-M dwarfs using tomographic imaging

“…Although relying on a simple assumption, this method gives promising results, and allows us to get a better insight on the origin of the activity RV jitter of early M dwarfs. This study of slowly-rotating early-M dwarfs complements the analysis already done for late M dwarfs (e.g., Barnes et al 2011Barnes et al , 2014Barnes et al , 2015.…”

ACCESS I. AN OPTICAL TRANSMISSION SPECTRUM OF GJ 1214b REVEALS A HETEROGENEOUS STELLAR PHOTOSPHERE

“…Mid-to late-M dwarfs have also been found to have abundant polar spots from Doppler imaging. Barnes et al (2015) found the M4.5 dwarf GJ791.2A to have a mean spot coverage of 3.2% and a maximum spot coverage of 82.3% during its rotation, assuming spots are 300K cooler than the photosphere. On the Sun, spots are accompanied by bright plage, which include faculae with temperature contrasts of 300-500K with their surroundings (Topka et al 1997).…”

“…The spectra S o and S u are generated using the values for Fe H o [ ] and Fe H u [ ] (the metallicities of the occulted and unocculted spectra, respectively) and linearly interpolating between the closest spectra in the PHOENIX model grid. Following the results of Doppler imaging studies of Mdwarfs, we adopt = F 0.032, which is the mean spot filling factor Barnes et al (2015) found for the M4.5dwarf GJ791.2A over its rotation period. We fix the metallicity of the occulted spectrum to the best-fit value for the metallicity of GJ1214, ) using the mean and standard deviation of the residuals found by fitting the joint data set to the exoplanetʼs model transmission Figure 6.…”

“…Results from MCMC Fits to CPAT-absorber Models for Cloudy Atmospheres Barnes et al (2015). The free parameters in the model are the temperature contrast DT , which determines T u relative to T o , and R R p s o (…”

New Low-mass Eclipsing Binary Systems in Praesepe Discovered by K2

“…Such a scenario has been suggested to explain observations of the late-type EB H II 2407 in the Pleiades . Indeed, there is observational evidence (Barnes et al 2015) and theoretical motivation (Schuessler & Solanki 1992;Granzer et al 2000) for polar spots on rapidly rotating, fully convective stars.…”