This page accompanied a short animation that demonstrates the cross-correlation technique used in high-resolution exoplanet spectroscopy. The example target is HIP 78530 b, a substellar companion observed with the CRIRES spectrograph.
The problem
The observed spectrum in the demonstration is very noisy, with a signal-to-noise ratio of about 2. Only one detector is shown at first. To the eye, the carbon monoxide (CO) absorption lines are almost impossible to see.
The method
A model spectrum of CO is slid across the data at different trial velocities. At each step, the match between model and observation is measured. This produces the cross-correlation function. When the trial velocity equals the true radial velocity of the companion, the many weak lines add up together and a clear peak emerges, even though no single line stands out.
The result
At the end of the animation, the final cross-correlation function is shown, built as a weighted average of three detectors. The peak sits at the systemic velocity of the companion. Its shape is closely tied to the profile of the CO lines in the real spectrum, so the width of the peak carries information about the object’s rotation. This is why the technique matters for spin studies such as those described in Spinning Worlds.