3. Photometric observations in the visible spectrum
Table 1. Summary of Photometric Data for Horizontal Branch Star Candidates.
The new photometric observations were made by Kinman with the Mk III photometer on the Kitt Peak 1.3-m telescope (with chopping secondary) on the system as described by KSK. Additional observations were also made with the Kitt Peak 0.9-m telescope using a 512 512 CCD under control of the CCDPHOT program; details of this observing system are given by Kinman (1998). The photometry gives () on the Johnson system and a hybrid index from the Strömgren u filter and the Johnson B filter. The vs () diagram can be used to separate BHB from other stellar types as described by KSK. A vs () diagram using the most recent data is shown in Fig. 3 of Kinman (1998). There is a satisfactory separation of the BHB stars and RR Lyrae stars with () 0.00, but bluer than this the separation becomes rapidly more difficult. The vs () diagram gives a satisfactory way of distinguishing fainter BHB stars at high galactic latitudes because the risk of confusion with other types of early-type stars is not too severe and the integration times are smaller than for the Strömgren photometry; this is an important consideration for faint stars. In the solar neighbourhood, however, there is a wide variety of early-type stars and these diagrams can only be used to provide BHB candidates.
Some idea of the accuracy of the adopted photometric data can be appreciated from the plots of against () shown in Fig. 1. With the exception of the Post-AGB star (BD +32 2188), which has a lower gravity than the remaining stars, the BHB star candidates approximately follow the linear relationship:
which is shown by the dashed line in Fig. 1. None of the BHB stars depart from this relation by more than 0.01 mag in . This suggests that these quantities are not likely to be in error by more than one or two hundredths of a magnitude.
Hipparcos magnitudes (which are of high accuracy and on a very homogeneous system) are available for twenty one of the thirty stars given in Table 1. It was found that the difference () between the Hipparcos magnitude and the mean V magnitude 5 for our BHB star candidates could be expressed as the following linear function of :
The Hipparcos Catalogue (Vol. 1) (1997) gives values of for various in Table 1.3.5 and values of for different in Table 1.3.7. Thus the catalogue values of may be obtained for various . These agree well with our linear relation at a of 0.00 and 0.22 but are up to 0.01 magnitudes larger at intermediate . The catalogue are for "early type stars" and we have preferred our relation because it refers to the specific class of stars that we are studying. Our linear relation was therefore used to convert the Hipparcos magnitudes to V magnitudes and these are our adopted magnitudes. If no Hipparcos magnitude is available, the weighted mean V magnitude was adopted.
Significant systematic differences exist between values of the Strömgren -index made by different observers (Joner & Taylor 1997). Fortunately, many of the BHB candidates have been observed by Stetson (1991) and were therefore on one system. New observations of a selection of our candidates were made using BHB (and other stars of similar colour that were observed by Stetson) as standards so as to be on his system 6. These new values are given in the first line of Table 1, when the source K(n,m,o) is given. It should be noted that the large radial velocities of BHB stars can cause their H line to be shifted (in the case of HD 161817 by as much as 6 Å) from the rest wavelength. The FWHM of the narrow H filter is only 30 Å, so that small inaccuracies may be expected from this cause. As a check, synthetic indices were determined by measuring the "magnitudes" of the H-line through 30Å and 150Å bandpasses in our spectra (which do not include H) using the magband routine in the CTIO package of IRAF. It was found that these synthetic indices (on the photoelectric system) could be derived as a linear function of the difference between the broad and narrow H "magnitudes"; these synthetic indices are given in Column 5 of Table 15. In general, these synthetic agree well with the mean photoelectric values of taken from Hauck & Mermilliod (1998) and given in Table 1 and with our adopted values that are also given in Table 16. The rms difference between our synthetic and the adopted photoelectric values for the BHB stars is 0.009 if we omit HD 161817 for which the difference is 0.031.
Photometric data both from the far ultraviolet and from the infrared can also be used for the determination of the interstellar reddening and stellar parameters. These data are discussed in Sects 5.4 and 7.5 respectively.
© European Southern Observatory (ESO) 2000
Online publication: December 15, 2000