Udem, Th.; Wilken, T.; Steinmetz, T.; Rebolo, R.; Probst, R. A.; Pasquini, L.; Manescau, A.; Hänsch, T. W.; Holzwarth, R.; Molaro, P.; González Hernández, J. I.; Lo Curto, G.; Monai, S.; Esposito, M.
Bibliographical reference
Astronomy and Astrophysics, Volume 560, id.A61, 9 pp.
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12
2013
Journal
Citations
47
Refereed citations
43
Description
Context. The solar spectrum is a primary reference for the study of
physical processes in stars and their variation during activity cycles.
High resolution spectra of the Sun are easily obtained from spatially
selected regions of the solar disk, while those taken over the
integrated disk are more problematic. However, a proxy can be obtained
by using solar light reflected by small bodies of the solar system. Aims: In November 2010 an experiment with a prototype of a laser
frequency comb (LFC) calibration system was performed with the HARPS
spectrograph of the 3.6m ESO telescope at La Silla during which high
signal-to-noise spectra of the Moon were obtained. We exploit those
Echelle spectra to study a portion of the optical integrated solar
spectrum and in particular to determine the solar photospheric line
positions. Methods: The DAOSPEC program is used to measure solar
line positions through Gaussian fitting in an automatic way. The solar
spectra are calibrated both with an LFC and a Th-Ar. Results: We
first apply the LFC solar spectrum to characterize the CCDs of the HARPS
spectrograph. The comparison of the LFC and Th-Ar calibrated spectra
reveals S-type distortions on each order along the whole spectral range
with an amplitude of ±40 m s-1 . This confirms the
pattern found in the first LFC experiment on a single order and extends
the detection of the distortions to the whole analyzed region revealing
that the precise shape varies with wavelength. A new data reduction is
implemented to deal with CCD pixel inequalities to obtain a wavelength
corrected solar spectrum. By using this spectrum we provide a new LFC
calibrated solar atlas with 400 line positions in the range of 476-530,
and 175 lines in the 534-585 nm range corresponding to the LFC
bandwidth. The new LFC atlas is consistent on average with that based on
FTS solar spectra, but it improves the accuracy of individual lines by a
significant factor reaching a mean value of ≈10 m s-1 .
Conclusions: The LFC-based solar line wavelengths are essentially
free of major instrumental effects and provide a reference for absolute
solar line positions at the date of Nov. 2010, i.e. an epoch of low
solar activity. We suggest that future LFC observations could be used to
trace small radial velocity changes of the whole solar photospheric
spectrum in connection with the solar cycle and for direct comparison
with the predicted line positions of 3D radiative hydrodynamical models
of the solar photosphere. The LFC calibrated solar atlas can be also
used to verify the accuracy of ground or space spectrographs by means of
the solar spectrum.
Observations based on the ESO 3.6 m telescope at La Silla, Chile.Full
Table 3 and a solar spectrum are only available at the CDS via anonymous
ftp to http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/560/A61
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Rebolo López