6 Clusters and non-stellar objects

6.1 Hodge open clusters

Little progress has been made in this area since the original suggestion of Baade (1963) that the number of clusters in IC1613 is very low. Hodge (1978) identified by visual inspection 43 cluster candidates. Except for a number of cluster stars (usually 4-5 up to V=23 mag) there is no other information about them.

We identified twelve Hodge clusters in our field of observation. Growth curves aperture photometry was applied to eight of them in order to determine the total U (except for C20), B and V magnitudes (Table 7). Local sky background reaches up to diaphragms of about 7 arcsec ($\approx$ 24pc). This is not much above the diameters given by Hodge ($\approx$ 17pc). The only exception is C40, noted by Hodge as "loose", with a diameter of 10 arcsec ($\approx$ 34pc). We have UBV photometry for the brightest stars of each cluster. The integrated magnitudes and colors of the brightest cluster members differ slightly from the total aperture magnitudes and colors.

The (U-B) vs. (B-V) diagram is presented in Fig.  16. The observed distribution of clusters in the color-color (U-B, B-V) diagram can be related to the SWB classification which was originally based on the Gunn photometric system (Bica et al. 1996). It is basically an age sequence. Six clusters from our sample fall into the SWB0 type zone (Bica et al. 1992) for very young clusters associated with the H II regions and ages between 0 and 10Myr. Only C12, noted by Hodge as "red giants", falls into the SWB1 type zone of clusters around 10Myr. The integrated colors of the clusters belonging to this group may reach very red B-V colors owing to a red supergiant phase.

  

Figure 16: (U-B) vs. (B-V) diagram for Hodge clusters. The border line between SWB0 and SWB1 zones is shown. Two new cluster candidates are marked by filled circles

Two new cluster candidates are identified using the method described above. The magnitudes and colors for them are given in Table 7. The new cluster candidates also fall into the SWB0 type zone (Bica et al. 1992) for very young clusters (marked in Fig. 16 by filled circles).

6.2 Non-stellar objects

There is a remarkable lack of globular clusters in IC1613 (Hodge 1978; Freedman 1988a). This fact was a prompt to carry out a new search for globulars. All objects with $\chi^2\gt 1.5$ were derived from our ALLSTAR photometry lists in BV. Six most probable nonstellar objects were selected after a careful "object by object" inspection of the resultant profiles and a thorough check for unresolved close neighbours. Following Crampton et al. (1985) the full widths of the half-maximum (FWHM) and the quarter maximum (FWQM) were measured for these objects as well as for a representative sample of 30 randomly scattered and well resolved stars. A plot of FWHM vs. FWQM (in pixels) for the V frame is presented in Fig. 17. It shows a clear separation between the sample stars (filled circles) and the non-stellar objects (open circles). We then made an additional growth curves aperture photometry for non-stellar candidates (Table 8). All objects are relatively bright but red. Taking into account the very low reddening towards IC1613 we conclude that they can not be globulars which confirms the lack of globular clusters in this galaxy. According to Fabry Pérot images (Rosado 1998) these are no peculiar velocities or emissions around these nonstellar objects. We can only speculate that these objects are perhaps stars embedded into shells of dust but most probably they are distant galaxies visible through the disk of IC1613.

  

Figure 17: The FWHM vs. FWQM plot for nonstellar objects (open circles) and sample stars (filled diamonds)