Cosmic rays were removed by comparing the multiple images per
filter before we constructed the image shown as Plate 1. However,
owing to the short exposure times, particle hits did not significantly
disturb any of the stellar images of interest to us. Consequently, we
applied the standard photometric and astrometric iraf tasks
tasks to each of the four available images individually. This provided
us with valuable information on consistency and repeatability. The
accuracy of the individual positions in the HST celestial reference
frame, as shown in Table 1, is estimated at 0.1 px (or 0.005 arcsec)
for the central star of NGC 650 (No.1), and double that value for
the companions (Nos. 2 and 3). The internal photometric errors are
very small at a few thousands of a magnitude, but several additional
factors need to be taken into account. In the first place, we had to
use a very small extraction aperture (of 3 px, or 0.14 arcsec
) to separate the components 2 and 3. In
contrast, WFPC2 photometric calibrations (Holtzman et al. 1995) are
based on 1 arcsec images. We estimate the
additional uncertainty introduced by this procedure, based on the PSF
of the sufficiently isolated component 1, at no more than 0.03 mag.
Some overlap of the PSF's occur for the stars 2 and 3, but the
contribution to our extraction aperture, for which we corrected, is
never more than 0.15 mag (i.e. to the I magnitude of No. 3).
Finally, we converted the natural WFPC2 magnitudes to standard
V and I values using the (2nd order)
colour-transformation scheme of Holtzman et al. 1995. This also adds a
small contribution to the error budget. We conclude that the
zero-points of the magnitudes shown in Table 2 are probably correct to
0.05 mag, while the colours are a little better.
Table 1. HST/WFPC2 observations of the three resolved stellar components
Table 2. Intrinsic stellar colours for the three resolved stellar components of NGC650 assuming = -0.47 for the central star corresponding to E = 0.16 and A = 0.5.
The photometry in Table 1 may be compared to values given in the literature. Cudworth (1973) gives a photovisual magnitude of 17.5 for the central star and 17.7 for the companion. Shaw & Kaler (1985) give a value of V = 15.87 0.36 for the combination of stars, which is too bright. But they are measuring with a 40 diaphragm which not only allows a large amount of nebular light to enter the photometer (which they had to correct for) but measures the light of other stars in the field as well. As can be seen in Plate 1, there are several bright stars in a 40 field of view. Napiwotzki & Schönberner (1995) give V = 17.7 0.3 by measuring the continuum of spectrum of the central star. This value is only marginally fainter than our measurement, which might be caused by a small tracking error during the exposure through the slit of their spectrograph.
© European Southern Observatory (ESO) 1998
Online publication: June 12, 1998