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Astron. Astrophys. 330, L41-L44 (1998)

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3. Observations and data analysis

We obtained BVRIc photometry of the field around Sakurai's Object using the HSHT 60 cm telescope of the Toronto Southern Observatory (UTSO) in Las Campanas, Chile in April 1996 and April 1997 and the Dutch 90 cm telescope at ESO/La Silla in March 1997. During the latter run a set of deep exposures were taken even if Sakurai's Object was overexposed, to obtain a better signal to noise ratio for faint stars. We only used stars within the field of a single CCD frame (3.5 by 3.5 arcmin) centered on Sakurai's Object, thereby minimizing the effect of inhomogeneities in the interstellar medium. It is well established that the interstellar has an irregular distribution resulting in a patchy pattern of extinction (e.g. Burton 1991 and references therein). The basic assumption of the extinction method requires that all stars used belong to the same reddening vs. distance relation. For a small field this is virtually garanteed and we have also found no indication of inhomogeneous reddening from star counts in the frame. The stars used are identified by numbers in Fig. 1, which can also serve as a finding chart. For the determination of color equations, extinction coefficients and instrumental calibration a set of standard stars (Landolt 1992) were taken throughout the whole night. A spectrum of one unusual star was obtained with the 2.5 m du Pont telescope of the Las Campanas Observatory, Chile in April 1997. For obtaining spectral type and luminosity class from the BVRIc photometry we used a newly developed method (a (V-I) vs. (V-R)-(R-I) diagram) which is similar in approach to the one described by Dean et al. (1978) and extensively investigated for the determination of extinction distances of planetary nebulae by Pollacco & Ramsay (1992). The (V-R)-(R-I) parameter is nearly identical to the extinction free parameter Q:= (V-R) - 1.72 (R-I). Defining the errors on the (V-R)-(R-I) axis by gaussian errors gives us


while that one for Q is given by


Assuming similar rms errors [FORMULA] on all bands this results in an error that is about 1/3rd samller than that for the Q parameter. The fact that the angle between main sequence and extinction direction is nearly identical at both diagrams lead us to the choice mentioned above. This method gives a quite good separation of dwarfs. This enables us to reliably classify the vast majority of stars in the field, most of which are late-type stars. We therefore can use the typical stellar population for establishing the reddening vs distance relation and do not have to rely on early-type stars none of which was found in the field. Details of the method will be presented in an upcoming paper.

[FIGURE] Fig. 1. R band image of the field around Sakurai's Object (1) identifying all stars used in the photometry. The star maked by an arrow was investigated spectroscopically.


Table 1. Photometry, extinction and adopted spectral types from CCD photometry:

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© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998