Measuring precise radial velocities and cross-correlation function line-profile variations using a Skew Normal density. (arXiv:1811.12718v1 [astro-ph.EP])

Measuring precise radial velocities and cross-correlation function line-profile variations using a Skew Normal density. (arXiv:1811.12718v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Simola_U/0/1/0/all/0/1">Umberto Simola</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dumusque_X/0/1/0/all/0/1">Xavier Dumusque</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cisewski_Kehe_J/0/1/0/all/0/1">Jessi Cisewski-Kehe</a>

Stellar activity is one of the primary limitations to the detection of
low-mass exoplanets using the radial-velocity (RV) technique. We propose to
estimate the variations in shape of the CCF by fitting a Skew Normal (SN)
density which, unlike the commonly employed Normal density, includes a skewness
parameter to capture the asymmetry of the CCF induced by stellar activity and
the convective blueshift. The performances of the proposed method are compared
to the commonly employed Normal density using both simulations and real
observations, with different levels of activity and signal-to-noise ratio. When
considering real observations, the correlation between the RV and the asymmetry
of the CCF and between the RV and the width of the CCF are stronger when using
the parameters estimated with the SN density rather than the ones obtained with
the commonly employed Normal density. Using the proposed SN approach, the
uncertainties estimated on the RV defined as the median of the SN are on
average 10% smaller than the uncertainties calculated on the mean of the
Normal. The uncertainties estimated on the asymmetry parameter of the SN are on
average 15% smaller than the uncertainties measured on the Bisector Inverse
Slope Span (BIS SPAN), which is the commonly used parameter to evaluate the
asymmetry of the CCF. We also propose a new model to account for stellar
activity when fitting a planetary signal to RV data. Based on simple
simulations, we were able to demonstrate that this new model improves the
planetary detection limits by 12% compared to the model commonly used to
account for stellar activity. The SN density is a better model than the Normal
density for characterizing the CCF since the correlations used to probe stellar
activity are stronger and the uncertainties of the RV estimate and the
asymmetry of the CCF are both smaller.

Stellar activity is one of the primary limitations to the detection of
low-mass exoplanets using the radial-velocity (RV) technique. We propose to
estimate the variations in shape of the CCF by fitting a Skew Normal (SN)
density which, unlike the commonly employed Normal density, includes a skewness
parameter to capture the asymmetry of the CCF induced by stellar activity and
the convective blueshift. The performances of the proposed method are compared
to the commonly employed Normal density using both simulations and real
observations, with different levels of activity and signal-to-noise ratio. When
considering real observations, the correlation between the RV and the asymmetry
of the CCF and between the RV and the width of the CCF are stronger when using
the parameters estimated with the SN density rather than the ones obtained with
the commonly employed Normal density. Using the proposed SN approach, the
uncertainties estimated on the RV defined as the median of the SN are on
average 10% smaller than the uncertainties calculated on the mean of the
Normal. The uncertainties estimated on the asymmetry parameter of the SN are on
average 15% smaller than the uncertainties measured on the Bisector Inverse
Slope Span (BIS SPAN), which is the commonly used parameter to evaluate the
asymmetry of the CCF. We also propose a new model to account for stellar
activity when fitting a planetary signal to RV data. Based on simple
simulations, we were able to demonstrate that this new model improves the
planetary detection limits by 12% compared to the model commonly used to
account for stellar activity. The SN density is a better model than the Normal
density for characterizing the CCF since the correlations used to probe stellar
activity are stronger and the uncertainties of the RV estimate and the
asymmetry of the CCF are both smaller.

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