3 Spectroscopy

Since the radial velocity of the A component was reported as variable in three references - data in literature varies from -32 to +10.5 km s^-1 - we also took several CCD spectra with the coudé echelle spectrograph ECHELEC attached to the 1.52 m ESO telescope. The CCD chip RCA #13 (known to be affected by few defect pixel rows) was used. The average linear dispersion was 3.8 Å mm^-1, the slit width was in the range 1.4$^{\prime\prime}$ to 2.5$^{\prime\prime}$, equivalent to a resolving power of 23000 to 32000. The useful wavelength range was about 300 Å, and various central wavelengths were chosen. The reduction of the spectra was performed with the Munich Image Data Analysis System (MIDAS). For the determination of radial velocities and equivalent widths, the spectral lines HeI 4471, NII 4481, NIII 4511, HeII 4541, SiIV 4631, CIII 4647, SiIV 4654, HeII 4686, HeI 4713 and HeI 4922 were fitted with single Gaussian profiles, using the Newton-Raphson method to optimize the central wavelength, the full width at half-maximum and the amplitude of the lines. For the line HeI 4471, which is a blend of two components, a weighted average 4471.508 was used as the reference table wavelength.

  

Table 6: Heliocentric radial velocities from ECHELEC spectra (in km s^-1) for HD 164492 (ADS 10991 A)

  

Table 7: Equivalent widths for HD 164492 (ADS 10991 A) in Å derived from ECHELEC spectra

^a standard deviation relative to averaged value.

Figure 2 shows two typical merged ECHELEC-spectra of HD 164492 (A). The most prominent lines are indicated, and features, which are due to the CCD defects, are marked as "CCD". Table 6 represents the radial velocities, and in Table 7 the measured equivalent widths are given. The most accurate radial velocities result from HeII 4686 (its average value is +2.8 $\pm$ 0.4 km s^-1), which is the strongest helium line in the spectra.

Our results do not confirm any radial velocity variability of the main component A of HD 164492 (A). It should be stressed that in many cases low-dispersion spectra of OB stars yield imaginary radial velocity variations, which cannot be confirmed by modern measurements using high dispersion spectra (see e.g. Mayer et al. 1994).

  

Figure 2: Typical merged ECHELEC spectra of HD 164492 (component A) taken at hel. JD 2448459.7366 (above) and 2448763.8435 (below). Some prominent lines are indicated, features caused by a CCD defect are marked as <CCD>