Astronomical Spectroscopy - Physics - University of Cincinnati

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that it is often better to simply measure the centers of individual spectral lines. Broad-lined
objects, such as Wolf-Rayet stars, also require some thought and care as to how to best
obtain radial velocities.
The velocity dispersion of a galaxy is measured using a similar method in order to obtain
the width of the cross-correlation function. The intrinsic line widths of the radial velocity
standards must be removed in quadrature. Thus sharp-lined radial velocity standards work
better than those with wide lines.
Precision Radial Velocities. The desire to detect exoplanets has changed the meaning
of “precision” radial velocities from the 1-2 km per second regime to the 1-100 meters per
second regime. Several methods have been developed to achieve this. The traditional method
of wavelength calibration does not achieve the needed precision as the comparison arcs and
the starlight are not collimated in exactly the same method. Early work by Campbell &
Walker (1979) achieved a precision of 10 m s −1 using a hydrogen fluoride cell inserted into
the beam that provided an evenly spaced absorption spectrum. More modern techniques
achieve 1 m s −1 precision. One example is the High-Accuracy Radial Velocity Planetary
Searcher (HARPS) deployed on on ESO’s 3.6-m telescope on La Silla. It uses a conventional
ThAr discharge lamp for wavelength reference. The ThAr source and the star’s light each
enter its own fiber, which then feeds a bench-mounted, extremely stable spectrograph. Both
the ThAr reference source and the star are observed simultaneously. (See Pepe et al. 2004.)
The recent emphasis on planet detection among late-type (cool) stars has driven some
of this work into the NIR, where ammonia gas cells provide a stable reference (Valdivielso
et al. 2010, Bean et al. 2010).
Laboratory Wavelengths. The traditional source of wavelengths for astronomers has
been Moore (1972), a reprint of her original 1945 table. More up-to-date line lists can be
found on the Web, allowing one to search the National Institute of Standards and Technology
Atomic Spectra Database. The official site is http://www.nist.gov/physlab/data/asd.cfm,
but a very useful search interface can be found at http://www.pa.uky.edu/∼peter/atomic/.
One must make sure that one is using the “air” wavelengths rather than “vacuum” wavelengths
when observing from the ground.
One problem is that many lines in stellar (and reference sources!) are actually blends of
lines. So, tables of “effective wavelengths” can be found for stellar lines for stars of different
spectral types. These are generally scattered throughout the literature; see, for example,
Conti et al. (1977) for a line list for O-type stars.