Title: Placing the spotted T Tauri star LkCa 4 on an HR diagram,
Abstract: Ages and masses of young stars are often estimated by comparing their
luminosities and effective temperatures to pre-main sequence stellar evolution
tracks, but magnetic fields and starspots complicate both the observations and
evolution. To understand their influence, we study the heavily-spotted
weak-lined T-Tauri star LkCa 4 by searching for spectral signatures of
radiation originating from the starspot or starspot groups. We introduce a new
methodology for constraining both the starspot filling factor and the spot
temperature by fitting two-temperature stellar atmosphere models constructed
from Phoenix synthetic spectra to a high-resolution near-IR IGRINS spectrum.
Clearly discernable spectral features arise from both a hot photospheric
component $T_{\mathrm{hot}} \sim4100$ K and to a cool component
$T_{\mathrm{cool}} \sim2700-3000$ K, which covers $\sim80\%$ of the visible
surface. This mix of hot and cool emission is supported by analyses of the
spectral energy distribution, rotational modulation of colors and of TiO band
strengths, and features in low-resolution optical/near-IR spectroscopy.
Although the revised effective temperature and luminosity make LkCa 4 appear
much younger and lower mass than previous estimates from unspotted stellar
evolution models, appropriate estimates will require the production and
adoption of spotted evolutionary models. Biases from starspots likely afflict
most fully convective young stars and contribute to uncertainties in ages and
age spreads of open clusters. In some spectral regions starspots act as a
featureless veiling continuum owing to high rotational broadening and heavy
line-blanketing in cool star spectra. Some evidence is also found for an
anti-correlation between the velocities of the warm and cool components.