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Astron. Astrophys. 353, 557-568 (2000)

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2. Observations

New high-resolution spectra were obtained for CS 22948-27 and CS 29497-34, in the blue region, using the 3.6m telescope at ESO, equipped with the Cassegrain Échelle Spectrograph (CASPEC). The log of observations is reported in Table 1. The wavelength range of the spectra is 3800-5000 Å, but they are only useful redward of 4000 Å because the flux was too faint at shorter wavelengths. A slit width of 250 µm was used, yielding a spectral resolving power (as verified from the measured widths of lines in the thorium comparison lamp) of R = 21000. For the previously obtained EMMI spectra the resolving power was R = 30000. The signal-to-noise ratio of the spectra has been estimated from the ADU level, as compared to stars with well-defined continua, at 4500 Å for the CASPEC data and at 6000 Å for the EMMI data; the resulting S/N values are reported in Table 1. Reductions were carried out using the packages developed by Spite (1990).


[TABLE]

Table 1. Log of the observations.


Because only [FORMULA] colours were previously available for these stars (a measured colour for CS 22948-27, and an approximate colour for CS 29497-34), and the temperature is not accurately determined from this colour alone, new photometry of these stars in Johnson-Cousins B, V, R and I bands was obtained. The colours reported in Table 2 were derived using CCD observations obtained at the 0.9m Dutch telescope at ESO. The estimated calibration errors derived from the transformation from instrumental magnitudes to those in the standard system, using Landolt (1983, 1992) standards, are also given in Table 2. Because only one set of observations was obtained for each star, we are unable to comment on photometric errors arising from photon statistics, but these are expected to be small.


[TABLE]

Table 2. Observed colours in Johnson-Cousins system (date of the observations: 09/09/1996)


We draw attention to the fact that the measured colours are quite unusual: [FORMULA] is larger than [FORMULA], which is not expected in normal red stars. This is due to the fact that the atmospheres of these very metal-poor and C,N-enhanced stars are strongly affected by the presence of CH, CN and C2 bands, producing huge gaps in the flux distribution of the stars. It is therefore mandatory to take these effects into account when deducing the temperature (Sec. 3.3). The reddening estimates toward these stars, obtained from the Burstein & Heiles (1982) maps, is minimal: [FORMULA] in the case of CS 22948-27, and [FORMULA] in the case of CS 29497-34, hence no adjustment to the measured colors has been made.

We have also obtained new high-resolution spectra with FEROS, at the 1.52m ESO telescope, for which radial velocities were measured in order to check for possible temporal variations (Table 1).

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999
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