6.1. The Wh-band
The main aim of our research is the study of Cepheid light curves in nearby galaxies. This study requires a certain amount of observing telescope time, and owing to the pressure on the available observational facilities in sites with good sky and seeing conditions, we were forced to exploit as much as possible the relatively small telescopes. This reason, coupled with the need of the highest signal-to-noise ratio for obtaining accurate light curves, has implied our decision of observing with no filter. The advantage with respect to usual filters in the optical range, such as Johnson or Gunn systems, is that the number of collected photons is larger by about a factor from 4 to 6, which means that, for the same exposure time, an observation in Wh band with a 0.9 m telescope is equivalent, in terms of collected photons, to an observation in V band with a 2.1 m. On the other hand, the background sky tends to increase in the near infrared and this effect should be more evident when observing galaxies with a strong red background. This is not the case of IC 1613, however, because we have estimated a similar star/sky intensity ratio in V and Wh bands.
The observational data of IC 1613 Cepheids show that, for exposure times of half a hour with a 0.9 m telescope and a back-illuminated CCD, it is possible to get light curves which are as accurate as 0.03 mag for stars with mag, and to detect variable stars as faint as 23.
The photometric properties of the Wh band appear reasonably good. The effective wavelength for A-G spectral types is intermediate between that of Johnson V and R bands, and Wh measurements correlate well with V and V-R. The obvious defect is that the photometry depends on the instrument. For example the response of the system changes when using a front-illuminated CCD instead of a back-illuminated one, because the effective wavelength in the front-illuminated case is closer to that of R band. Therefore some care will be required when merging differential Wh observations of the same stellar field obtained with different instruments. Systematic effects related to star colours are expected, but they can be probably corrected for; this requires, however, one very deep exposure in V and R or one observation with a larger telescope to get the colors of the faintest stars. Star colors are important in any case for discussing the nature of variables, therefore the suggested strategy for future work in this field shall include at least one observation in a photometric system with an adequately large telescope for obtaining this information.
It is interesting to compare, at least qualitatively, our results with those of analogous surveys such as DIRECT (Kaluzny et al. 1999), which is the project dedicated to the observations of M31. We recall that the distance of IC 1613, , is very similar to that of M31, (Madore & Freedman 1991), but the stars of M31 suffer of local reddening from 0 to 0.25. DIRECT uses telescopes of 1.2-1.3 m, front- and back-illuminated CCD detectors and exposure time of 900 s, and the number of collected photons would be approximately similar to that obtained by us with the 0.9 m telescope, exposure of 1800 sec and same filter. For a given period, the M31 Cepheid V light curves appear less accurate than those of IC 1613 in Wh band. Another indication is the faintest Cepheid with short period: in M31 the limit is about 4 d and mag (Stanek et al. 1998); in IC 1613 we estimate . We note that it is not easy to compare observations obtained with different telescopes, and the difference between M31 and IC 1613 results could depend in part also on crowding problems in M31, average seeing conditions, etc. In principle, however, we expect a gain of about 1.5 mag due to the use of the Wh band, for the same telescope and conditions.
6.2. PL relation
It is possible to derive a PL relation for Cepheids using Wh measurements, and we should expect a similar slope to that obtained for V and R. The relation is shown in Fig. 15 for 22 fundamental mode Cepheids (dashed line). The slope is -2.690.26 and the zero-point is 22.960.16; these figures were obtained without considering V2396 (with d; but this exclusion is unessential) and we have reported them just for illustrative purposes. A detailed discussion will be made when the observations of all the fields of IC 1613 will be reduced; in particular, we will look for the possible bending of the relation at very short period, a feature which was observed in the Small Magellanic Cloud (Bauer et al. 1998). The zero-point of our relation is obviously instrumentation dependent, even if in principle one could use a transformation such as Eq. 2. However the results of our study could be applied to distance determinations in another way. When Cepheids have been identified and their periods have been determined, then one observation in V band is sufficient for constructing a standard PL relation for the galaxy. Freedman (1988a) has discussed and applied in detail this method, that is the `single-phase' PL relation. In Paper II we will show an example of this application.
© European Southern Observatory (ESO) 1999
Online publication: August 25, 1999