Accuracy of radial-velocity measurements for early-type stars - I. Spectral-type mismatch in A-type synthetic spectra
Astron. Astrophys. Suppl. Ser.
Accuracy of radial-velocity measurements for early-type stars
W. Verschueren? 0 1
M. David 0 1
R.E.M. Gri n?? 0 1
0 Send o print requests to: W. Verschueren
1 Astrophysics Research Group, University of Antwerp (RUCA) , Groenenborgerlaan 171, B-2020 Antwerpen , Belgium
Measuring accurate radial velocities of rotating early-type (O-B-A) stars using cross-correlation techniques is hampered by the fact that object-template spectrum mismatch causes systematic errors that do not cancel out su ciently in these spectra. This series of papers aims at quantifying those mismatch errors, understanding their astrophysical origin, and developing strategies to avoid them maximally. In this rst paper, we employ synthetic spectra to study mismatch between A-type main-sequence stars caused solely by di erences in Te and log g. We show that this spectral-type mismatch varies greatly throughout the spectrum, though with some degree of systematic dependence on rotational velocity and, to a lesser degree, on temperature. We propose a scheme for selecting spectral regions that should provide, for main-sequence A-type stars, accuracies better than 1 km s−1 for v sin i 150 km s−1 and between 1−2 km s−1 for v sin i up to 300 km s−1. The scheme includes su cient spectral information to keep random errors conveniently small, but excludes all wavelength sub-intervals which produce systematic errors much larger than the above mentioned accuracy. Our predictions con rm the success of the methodology of Fekel (1985, 1999). We conclude that the proposed scheme needs further testing on a broad sample of real A-type spectra to see under what conditions of stellar individuality it may break down.
methods; numerical | techniques; radial velocities | techniques; spectroscopic | stars; early-type | stars; kinematics
1. Introduction
Techniques to determine radial velocities (RVs) of stars by
measuring the Doppler shifts of their spectral lines have
continuously improved throughout the last half-century.
Cross-correlation techniques that use optical templates
(e.g. Baranne et al. 1979)
or numerical ones
(e.g. Scarfe
et al. 1990; Latham 1992; Baranne et al. 1996)
are
particulary e cient in that they combine all spectral information
and therefore optimize the measuring precision (random
errors). There is, however, no guarantee that the
accuracy (systematic errors) is optimized as well in this way.
Cross-correlations of spectra give rise to systematic errors
through \spectrum mismatch" between object and
template spectrum. More speci cally, such errors arise from
asymmetrical di erences, when (for example) the
components of a blended line do not have the same relative
strength in both spectra. In fact, spectrum mismatch is
all but inevitable except in the singular case when both
spectra arise from the same non-variable star (and
provided that neither the observations nor the data reduction
process introduced signi cant di erential error).
Spectrum mismatch is caused by di erences in
atmospheric parameters (Te , log g, abundances), rotational
velocity, atmospheric velocity elds (e.g. convection, wind,
pulsation), and all peculiarities which individual stars or
speci c groups of stars may exhibit (e.g. magnetic- eld
structure). If the template is a synthetic spectrum, the
amount of spectrum mismatch will obviously depend on
the degree of sophistication included in the model, on the
accuracy of the line data, and on the amount of
information one has about the object; even the best
presentday spectrum synthesis does not simulate all the ne
details of an observed spectrum. Incidentally, if RVs are
determined not by cross-correlating spectra but by
measuring the centroids of individual features, spectrum
mismatch causes small wavelength o sets in those measured
centroids.
For most categories of late-type (F-G-K) stars, radial
velocities are routinely obtained nowadays with precisions
and accuracies of the order of 0:1 − 1 km s−1. That suc- types, some lines are formed in atmospheric velocity elds
cess derives from intrinsic characteristics of late-type spec- as large as several km s−1
(e.g. Ebbets 1979)
. It will
theretra, namely a high density of lines, small line widths, and fore be appreciated that RV measurements of early-type
lines which generally bear a family resemblance to one an- stars are prone to systematic errors that can amount to
other over a wide range of spectral types. Thus, one can several km s−1. Furthermore, they cannot be anchored to
achieve small random errors from spectra with relatively the late-type absolute zero-point in a simple way. There
low signal-to-noise ratio (S=N ) and with a small wave- has therefore been little progress in de ning a system
length coverage. Furthermore, systematic mismatch errors of early-type RV standard stars
(see Latham & Stefanik
between those spectra are likely to be much smaller than 1992)
; the lack of early-type stars with RVs known
ac1 km s−1 becau (...truncated)