Kavli Affiliate: Sara Seager
| First 5 Authors: H. N. Smitha, Alexander I. Shapiro, Veronika Witzke, Nadiia M. Kostogryz, Yvonne C. Unruh
| Summary:
Accurate calculations of starspot spectra are essential for multiple
applications in astronomy. The current standard is to represent starspot
spectra by spectra of stars that are cooler than the quiet star regions. This
implies approximating a starspot as a non-magnetic 1D structure in
radiative-convective equilibrium, parametrizing convective energy transport by
mixing length theory. It is the inhibition of convection by the starspot
magnetic field that is emulated by using a lower spot temperature relative to
the quiet stellar regions. Here, we take a different approach avoiding the
approximate treatment of convection and instead self-consistently accounting
for the interaction between matter, radiation, and the magnetic field. We
simulate spots on G2V, K0V, M0V stars with the 3D radiative
magnetohydrodynamics code MURaM and calculate spectra ($R approx 500$ from
250~nm to 6000~nm) using ray-by-ray radiative transfer with the MPS-ATLAS code.
We find that the 1D models fail to return accurate umbral and penumbral spectra
on K0V and M0V stars where convective and radiative transfer of energy is
simultaneously important over a broad range of atmospheric heights rendering
mixing length theory inaccurate. However, 1D models work well for G2V stars,
where both radiation and convection significantly contribute to energy transfer
only in a narrow region near the stellar surface. Quantitatively, the 1D
approximation leads to errors longward of 500 nm of about 50% for both umbral
and penumbral flux contrast relative to quiet star regions on M0V stars, and
less than 2% (for umbrae) and 10% (for penumbrae) for G2V stars.
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