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2. Observations and reductions

 

For the purpose of surface imaging, very high-resolution (about 170 000 at 6173Å), high S/N spectra (about 200 - 300) were obtained with the high-resolution echelle spectrograph SOFIN (Tuominen 1992) at the 2.56 m Nordic Optical Telescope (NOT) at Roque de los Muchachos Observatory, La Palma, Spain. The high resolution optical camera was used. The entrance slit width of 32tex2html_wrap_inline1741m is adjusted to give a resolution element of about 2 pixels FWHM, corresponding to tex2html_wrap_inline1743 on the sky. The spectra were recorded with an Astromed-3200 CCD camera (Mackay 1986) equipped with an EEV P88200 UV-coated tex2html_wrap_inline1745 CCD with a pixel size of 22.5 tex2html_wrap_inline1747m and operating at the optimal working temperature of 150K. Three observing runs for tex2html_wrap_inline1749 Gem were conducted in late 1993, 1994 and 1995. Typical integration times were 30-40 minutes.

The reductions of the echelle spectra used the 3A-software package (Acquisition, Archiving and Analysis; Ilyin 1996).

The reductions involved the usual procedures of cosmic spike removal, bias subtraction, flatfielding, subtraction of scattered light, extraction of the curved echelle orders and wavelength calibration. The result is for each order the intensity normalized to the local continuum vs. heliocentric wavelength.

Two steps deserve a more detailed description. Flatfielding uses summed, merged flatfields, i.e. the slit height is so large that the orders of the flatfield spectra overlap; many of those images are summed in order to improve the S/N ratio in the flatfields. Fringing is usually of very low amplitude or absent. In some cases, however, an ordinary flatfield image is available, taken with the same parameters and immediately before or after the stellar exposure, which, after flatfielding and filtering to improve S/N and retain only the fringes, can be used to correct the corresponding stellar spectrum for fringing.

In order to obtain the highest possible accuracy in the wavelengths, a two-dimensional dispersion curve is determined from the Thorium-Argon comparison spectrum; this has the main advantage to use many comparison lines from many orders, as opposed to the sometimes very low number of comparison lines available in one individual echelle order. The determination proceeds as follows:

The accuracy of the RVs is determined by the following factors:

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The accuracy of the dispersion curve: typically, about 150 comparison lines are taken into account. The dispersion curve within the master order is described by a polynomial of 2nd degree; the rms deviation of the lines from the final fit is generally about 2 mÅ at 6165Å or 0.1tex2html_wrap1823. Since due to the large number of comparison lines used the error of the coefficients is much less than that, we conclude that the statistical error of the dispersion curve is negligible.
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The stability of the RV across the chip: any systematical error of the two-dimensional dispersion curve should show up as a systematical variation of the RV across the different orders. This was checked in the spectrum of the RV-standard tex2html_wrap_inline1775 Oph: the RV is stable across the chip to within 0.03tex2html_wrap1825. Also for tex2html_wrap_inline1779 Gem with its much broader lines due to fast rotation the RV is stable to better than 0.1tex2html_wrap1827.
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The accuracy of the wavelength zero point: Each spectral image is shifted geometrically with respect to the image containing the comparison spectrum. Strong atmospheric Otex2html_wrap_inline1783 and Htex2html_wrap_inline1785O lines are used to establish the wavelength zero point. The standard wavelengths are taken from the solar catalogue by Pierce & Breckinridge (1973). In the present spectra, three different orders are available, each containing enough atmospheric lines to establish independently the wavelength zero point. They prove that there is no significant systematic variation of the shift across the image and allow an error estimate for the shift. Typically, the shift is determined with an accuracy of 0.1 to 0.2 CCD pixels. Since in echelle spectra in a good first approximation the dispersion is proportional to the wavelength this uncertainty transforms to a systematic error of the RV from the whole image of about 0.1 to 0.2tex2html_wrap1829.
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The accuracy of the template's RV: The RV of tex2html_wrap_inline1789 Gem is measured by cross-correlating the wavelength region around 6175Å with the same region of a spectrum of tex2html_wrap_inline1791 Oph (K2III), taken with the same equipment and reduced in the same way. The spectrum has been filtered to mimic the rotational broadening in the tex2html_wrap_inline1793 Gem spectra. tex2html_wrap_inline1795 Oph is a radial velocity standard (see Astronomical Almanac 1995), a candidate for a list of primary standards; we can thus assume that both the constancy and the value of its RV are established to high accuracy. According to the Astronomical Almanac (1995) its RV = tex2html_wrap_inline1797tex2html_wrap1831.
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The accuracy of the cross-correlation: At present, this cannot be determined independently. Therefore, we adopt an error of another 0.1tex2html_wrap1833 from this procedure.

If we assume these error sources to be independent and add up the variances from the different contributions, the final RV-error is about 0.3tex2html_wrap1835. Note that although all the above error sources are also present in the template spectrum only the second to last source enters into the RV-error of tex2html_wrap_inline1805 Gem, since the literature value for the RV of tex2html_wrap_inline1807 Oph is used to determine the RV from the cross-correlation.

The RVs measured from the SOFIN spectra are given in Table 1 (click here).

 table248
Table 1:   The radial velocities (RVs) measured from the SOFIN spectra. The heliocentric Julian date is given for mid-exposure. A typical error of the individual RV is 0.3tex2html_wrap1837

 table258
Table 2:   The previously unpublished RVs measured by Eker. The measurement procedure and the interpretation is given in Eker (1986). He estimates the RV error to be 0.5tex2html_wrap1841


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