3 NGC 133

NGC 133 is identified by a group of 5 bright stars in a very rich stellar field (see Fig. 1). One of these stars is the binary system BD+6293 (ADS 423A,B), whose A component is of B3 spectral type. It is located very low in the Galactic plane, and therefore presumably suffers from severe field star contamination. This cluster was never studied before, but for a photographic UBV survey carried out by Jasevicius (1964, 1970). This study however does not report estimates of the cluster fundamental parameters, but the author suggests that there is no cluster in the direction of NGC 133.

Figure 2: Star counts in the field of of NGC 133 as a function of the radius. Lower panel: all the stars. Upper panel: only stars brighter than V=14.

3.1 Star counts

According to Lyngå (1987), NGC 133 has a diameter of 7 arcmin, so our study covers the entire cluster region. To infer an estimate of the radius, we derive the surface stellar density by performing star counts in concentric rings around the center of the covered area, and then dividing by their respective surfaces. The final density profile and the corresponding Poisson error bars are depicted in Fig. 2. In the lower panel we take into account all the measured stars, whereas in the upper panel we only consider stars brighter than V = 14. By inspecting the lower panel, one can readily see that the profile decreases smoothly, and the cluster does not appear as a clear concentration. This is probably due to the heavy contamination by faint Galactic field stars. In fact, after selecting only the brightest stars, the cluster emerges more clearly (see upper panel). We estimate a cluster radius of about 2-2.5 arcmin, somewhat lower than the value reported by Lyngå (1987).

3.2 Proper motions

Important information on the kinematics of the luminous stars in and around NGC 133 can be derived from the proper motions available in the Tycho-2 catalogue. The Tycho-2 proper motions are based on the comparison between contemporary mean positions derived from the recent Tycho observations on-board Hipparcos and early-epoch positions observed many decades ago (see Høg et al. 2000 and references therein). Due to the long time-baseline they have rather high precision and therefore directly indicate the long-term mean tangential motions of the stars. We have collected proper motion components for 16 stars in a field of $10\arcmin 0 \times 10\arcmin 0$ centered in NGC 133. They are shown in the vector point diagram in Fig. 3 together with the errors reported in the Tycho 2 catalog. From this sample we have extracted 8 stars, which seem to crowd in the vector point diagram. By assuming that these stars are likely members, we derive the common mean motion (which we shall assume as the cluster mean motion):

$$\mu_\alpha = -0.7\pm4.7$$

$$\mu_\delta = -0.4\pm3.6 .$$

The errors reported in the Tycho 2 catalog amount at more than 2.5 mas/yr, and therefore we conclude that these 8 stars most probably share a common tangential motion since their components deviate less that 1$\sigma$ from the derived mean motion. Therefore in the direction of the object NGC 133 a star cluster seems to be present.

 

Table 3: Photometry of likely member stars in the field of NGC 133 brighter than V=14.

ID	TYC 4019-	Name	$\alpha ({\rm J}2000.0)$	$\delta ({\rm J}2000.0)$	V	(B-V)	(U-B)	(V-I)	$\mu_\alpha \cos \delta$ [mas/yr]	$\mu_\delta$ [mas/yr]	E(B-V)
1	2122	HIP-2466	00:31:14.9	+63:21:16.9	9.501	0.339	-0.083	0.494	$0.0\pm2.8$	$-0.9\pm2.7$	0.498
4	1038	BD+6293	00:31:17.7	+63:20:33.0	10.837	0.404	0.332	0.456	$-3.1\pm3.9$	$-2.8\pm3.9$	0.499
6	1038	BD+6293	00:31:17.5	+63:20:38.8	11.572	0.426	0.405	0.530	$2.2\pm3.7$	$-1.5\pm3.7$	0.502
7	2326		00:31:50.0	+63:23:54.1	12.112	0.323	0.508	1.527	$-7.4\pm5.1$	$-1.2\pm5.3$	0.540
8			00:31:02.2	+63:21:23.9	12.444	0.568	0.493	0.968			0.548
10			00:31:24.7	+63:30:27.4	12.913	0.542	0.307	0.695			0.579
13			00:31:07.2	+63:20:45.8	13.076	0.727	0.547	1.022			0.707
15			00:31:49.5	+63:20:51.9	13.343	0.485	-0.005	0.782			0.614
20			00:31:17.6	+63:22:33.4	14.076	0.640	0.034	1.224			0.703

Figure 3: Vector point plot of Tycho-2 proper motion and proper motion errors for the stars in the field of NGC 133. Open symbols indicate likely non-members, filled symbols likely members.

Figure 4: CMDs of the stars in the region of NGC 133. Panel a) all the stars in the V vs. (B-V) plane. Panel b) all the stars in the V vs. (V-I) plane. Panel c) the CMD by Jasevicius (1964). Panel d) A simulation of the Galactic disk component in the direction of NGC 133.

3.3 Color-Magnitude Diagrams

The Color-Magnitude Diagrams (CMDs) for all the stars measured in the direction of NGC 133 are shown in Fig. 4. In panel a) we plot all the stars in the V vs. (B-V) plane, where in panel b) the same stars are plotted in the V vs. (V-I) plane. For comparison, the photographic CMD by Jasevicius (1964) is presented in panel c). The comparison - in the sense (this study-Jasevicius) - yields:

$$\Delta V = 0.07\pm0.06$$

$$\Delta (B-V) = 0.11\pm0.05$$

$$\Delta (U-B) = 0.13\pm0.09$$

for 29 common stars.

Clearly, the present study supersedes that of Jasevicius (1964). The CMDs are not easy to interpret, since most of the stars are just Galactic disk field stars. This is corroborated by the CMD in panel d), where a simulation is presented of the Galactic disk component toward NGC 133. The simulation has been performed using the TRILEGAL code (Girardi et al. 2002), as calibrated by Groenewegen et al. (2002). Also from this figure is evident that NGC 133 is a small group of stars brighter than $V \approx 14$above the mean stellar background.

Figure 5: Color-color diagram for all the stars in the field of NGC 133 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)=0.55 and 1.4, respectively. The cross indicates the typical error bars.

Figure 6: Reddening distribution for the stars in the region of NGC 133 having UBV photometry.

Figure 7: Reddening corrected CMDs of the likely member stars in the region of NGC 133.

3.4 Two-color diagram and members selection

We derive cluster membership by grouping stars according to their mean reddening. Individual reddenings have been computing by means of the usual reddening free parameter Q, following the procedure outlined in detail in Patat & Carraro (2001). The results are shown in Figs. 5 and 6.

In Fig. 5 we plot all the stars having UBV photometry in the two-color diagram. The solid line is an empirical ZAMS from Schmidt-Kaler (1982). There seem to be two pupulations. One having a mean reddening $E(B-V)=0.60\pm0.10$ mag, which crowds close to a ZAMS shifted by E(B-V)=0.60 mag (dashed line), and another one with a much larger reddening. We consider this latter population as the Galactic disk component, made of stars placed 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. 6, where we show the reddening distribution. This has a clear peak at $E(B-V) \approx 1.1$ mag, but at the same time exhibits an hint for a secondary peak at E(B-V)=0.5-0.7 mag. However, the statistical significance of this secondary peak is quite low, and has to be taken as no more than an indication. An additional confirmation derives from Schlegel et al. (1998) extinction maps, which in the direction of NGC 133 give A_V = 1.426 mag. By assuming a standard value of the total to selective absorption ratio R_V = 3.1, the reddening toward NGC 133 becomes $E(B-V) \approx 0.50$ mag, in close agreement with our results.

In conclusion, we would like to argue that the population of stars having $E(B-V)=0.60\pm0.10$ mag (about 20 stars) identify the open cluster NGC 133.

Now, we need to compare these findings with the proper motion data, to check for consistency. Out of 16 stars which have Tycho 2 proper motions, we were able to secure photometry only for 5 (see Table 3). In this table, we list likely members derived from the analysis of the two-color diagram brighter than $V \approx 14.0$. The 4 stars with proper motion compatible with the mean turn out to be also photometric members (see the last entry in the table reporting the reddening), whereas the star $\char93 2$ (Tycho 4019-744), which has $\mu_{\alpha}=19.6$ mas/yr, $\mu_{\delta}=-6.9$ mas/yrs, turns out to have E(B-V)=0.902, which makes it both a photometric and an astrometric non-member. This result makes us confindent when using photometrically selected cluster members.

It is however worth noting that these results have to be confirmed by extending proper motion measurements to dimmer magnitudes, and by providing radial velocities of the brightest stars.

3.5 Hints for NGC 133 distance and age

In Fig. 7 we plot the reddening corrected CMDs for the likely members stars. In both diagram we have over-imposed the empirical Schmidt-Kaler (1982) ZAMS, shifted by $(m-M)_{\rm o} =9.0~\pm~0.3$ mag, which provides a nice fit of the stars distribution. This implies that NGC 133 is located $630\pm150$ pc 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 B0 to A5 by deriving the absolute colors from the ZAMS at the same position of the stars. This result agrees with the B3spectral type reported for ADS 423A ($\char93 4$ in the present numbering). If the stars having B0 spectral type are still along the MS, we derive an upper limit of 10 Myrs for the age of NGC 133 (Girardi et al. 2000).