4 NGC 1348

This cluster was never studied before. According to Lyngå (1987) it has a diameter of 5, and the spectral type of the brightest stars is A0. The region we covered is shown in Fig. 8. NGC 1348 is identified by a weak over-density of stars in a rich stellar field. Unfortunately, proper motions are not available for all the stars in the region of NGC 1348, so we must rely only on photometry to derive cluster members and cluster fundamental parameters.

4.1 Star counts

We performed star counts in concentric rings around the brightest star ($\char93$1, Tycho 3325-48) in the field, assumed as the cluster center. The number density profile is shown in Fig. 9. The profile decreases gently up to the limit of the covered region, and therefore we suspect that the Lyngå  estimate has to be taken as a lower limit of NGC 1348 diameter, which would be at least 10$^{\prime}$. Anyway, NGC 1348 appears as a real concentration.

4.2 Color-Magnitude Diagram

The CMDs for all the stars measured in the direction of NGC 1348 is shown in Fig. 10. In the left panel we plot all the stars in the V vs. (B-V) plane, where in the middle panel the same stars are plotted in the V vs. (V-I) plane. The CMDs are not easy to interpret, since most of the stars are just Galactic disk field stars. This is confirmed by the CMD in the right panel, where a simulation is presented of the Galactic disk component toward NGC 1348. The simulation has been performed using the TRILEGAL code (Girardi et al. 2002), as calibrated by Groenewegen et al. (2002). From this figure it is evident that NGC 1348 emerges as an overdensity of stars brighter than $V \approx 16{-}17$ above the mean stellar background.

4.3 Two-color Diagram and member selection

We follow the same method adopted above for NGC 133 to derive individual reddenings and membership to the cluster. The results are shown in Figs. 11 and 12.

In Fig. 11 we plot all the stars having UBV photometry in the two-color diagram. The solid line is an empirical ZAMS from Schimdt-Kaler (1982). There seems to be two populations. One having a mean reddening $E(B-V)=0.85\pm0.15$ mag, which lies close to a ZAMS shifted by E(B-V)=0.85 mag (dashed line), and another one with much larger reddening. We consider this latter population as the Galactic disk component, made of stars located at different distances, and with a different amount of absorption. To guide the eye we have drawn another ZAMS (dotted line) shifted by E(B-V)=1.5 mag. The same conclusion can be drawn by inspecting Fig. 12, where we show the reddening distribution. This has a clear peak at E(B-V)=0.7-0.9 mag, and several smaller peaks at larger values of the reddening.

We identify NGC 1348 with the group of stars having reddening $E(B-V)=0.85\pm0.15$ mag (about 20 stars).

Figure 8: A XDSS red map of the covered region in the field of NGC 1348. North is up, East on the left.

Figure 9: Star counts in the field of of NGC 1348 as a function of the radius.

Figure 10: CMDs of the stars in the region of NGC 1348. Left panel: all the stars in the V vs. (B-V) plane. Central panel: all the stars in the V vs. (V-I) plane. Right panel: a simulation of the Galactic disk component in the direction of NGC 1348.

Figure 11: Color-color diagram for all the stars in the field of NGC 1348 having UBV photometry. The solid line is the Schmidt-Kaler (1982) empirical ZAMS, whereas the dashed and dotted lines are the same ZAMS, but shifted by E(B-V)=0.85 and 1.5, respectively. The cross indicates the typical error bars.

Figure 12: Reddening distribution for the stars in the region of NGC 1348 having UBV photometry.

4.4 Hints for NGC 1348 distance and age

In Fig. 13 we plot the reddening corrected CMDs for the likely members stars above derived. In both diagrams we have over-imposed the empirical Schmidt-Kaler (1982) ZAMS, shifted by $(m-M)_{\rm o} = 11.5\pm0.5$ mag, which provides a nice fit to the stellar distribution. This implies that NGC 1348 is located $1.9\pm0.4$ kpc away from the Sun, where the uncertainty mirrors the difficulty of the fit due to the almost vertical structure of the MS.

From the location of the stars in the (B-V) vs. (U-B) plane, we infer that the stars spectral types ranges from B5 to A5 by deriving the absolute colors from the ZAMS at the same position of the stars. Moreover by inspecting Fig. 13, one can readily see that the brighter stars are actually leaving the MS, whereas the stars at $V_{\rm o} \approx 11.00$ - with spectral types in the range B8-A0 - are much probably still on the MS. Therefore we derive a lower limit of 50 Myrs for the age of NGC 1348 (Girardi et al. 2000).

Figure 13: Reddening corrected CMDs of the likely member stars in the region of NGC 1348.