Guillem Anglada-Escudé, R. Paul Butler
Doppler spectroscopy has uncovered or confirmed all the known planets
orbiting nearby stars. Two different approaches are used to obtain precision
Doppler measurements at optical wavelengths. The first approach is the gas cell
method, which is based on the least-squares matching of the absorption spectrum
of Iodine over-imposed to the spectrum of the star. The second method relies on
the construction of a stabilized spectrograph calibrated in wavelength with an
externally fed calibration source. The most precise stabilized spectrometer in
operation is HARPS, operated by ESO in La Silla/Chile. In the case of HARPS,
the Doppler measurement is obtained using the so--called Cross-Correlation
Function technique (CCF). It consists of multiplying the stellar spectrum with
a binary mask and finding the minimum of such product as a function of the
stellar Doppler shift. Such mask is weighted to account for the different
depths of the stellar lines. It is known that CCF is suboptimal in exploiting
the Doppler information in the stellar spectrum. Here, we describe an algorithm
to obtain precision RV measurements based on least squares matching of each
observation to a high signal-to-noise ratio template. Such algorithm is
implemented in our software called HARPS-TERRA (Template Enhanced Radial
velocity Re-analysis Application). We show that, compared to CCF, template
matching provides a significant improvement in accuracy, specially when applied
to M dwarfs. We conclude that other stabilized spectrographs should use a
similar approach to achieve the sub \ms precision required to detect
potentially habitable worlds around nearby stars.
View original:
http://arxiv.org/abs/1202.2570
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