Article

Comparing radial velocities of atmospheric lines with radiosonde measurements

Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.23). 10/2011; 420(4). DOI: 10.1111/j.1365-2966.2011.20015.x
Source: arXiv

ABSTRACT The precision of radial velocity (RV) measurements depends on the precision
attained on the wavelength calibration. One of the available options is using
atmospheric lines as a natural, freely available wavelength reference. Figueira
et al. (2010) measured the RV of O2 lines using HARPS and showed that the
scatter was only of ~10 m/s over a timescale of 6 yr. Using a simple but
physically motivated empirical model, they demonstrated a precision of 2 m/s,
roughly twice the average photon noise contribution. In this paper we take
advantage of a unique opportunity to confirm the sensitivity of the telluric
absorption lines RV to different atmospheric and observing conditions: by means
of contemporaneous in-situ wind measurements by radiosondes.
The RV model fitting yielded similar results to that of Figueira et al.
(2010), with lower wind magnitude values and varied wind direction. The probes
confirmed the average low wind magnitude and suggested that the average wind
direction is a function of time as well. The two approaches deliver the same
results in what concerns wind magnitude and agree on wind direction when
fitting is done in segments of a couple of hours. Statistical tests show that
the model provides a good description of the data on all timescales, being
always preferable to not fitting any atmospheric variation. The smaller the
timescale on which the fitting can be performed (down to a couple of hours),
the better the description of the real physical parameters. We conclude then
that the two methods deliver compatible results, down to better than 5 m/s and
less than twice the estimated photon noise contribution on O2 lines RV
measurement. However, we cannot rule out that parameters alpha and gamma
(dependence on airmass and zero-point, respectively) have a dependence on time
or exhibit some cross-talk with other parameters (abridged).

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