5 Field red clumps

An evident feature of fields CMD is, except in the case of NGC 643, the occurrence of a well-defined red clump. This locus of the CM diagram, containing core helium-burning stars of intermediate and small mass, has captured the attention of many researchers as an alternative way to constrain the distance of extragalactic objects. Paczynski & Stanek (1998) suggested the use of red clump mean absolute I magnitude (M_I) as a standard candle, since there was theoretical evidence that absolute luminosity of such core helium-burning stars depends only weakly on their age and metallicity (e.g. Seidel et al. 1987; Castellani et al. 1992).

The calibration of this method is based on the absolute I magnitude of clump stars in the solar neighbourhood with trigonometric parallaxes measured by the Hipparcos satellite. Using this method, Udalski et al. (1998) and Stanek et al. (1998) found distances for the LMC and SMC 15% lower than values derived from other standard candles (such as Cepheids or RR Lyrae). However, recent theoretical investigations (Girardi et al. 1998; Girardi & Salaris 2001) suggest that population effects can influence M_I up to 0.4 mag, so results based on the red clump method should be regarded with suspicion if these effects have been neglected. Unfortunately, population effects are not easy to estimate, because they depend on the assumptions of the history star formation rate (SFR) and on the age-metallicity relation (AMR).

To add data to such a relevant problem we made use of our sample to derive mean V magnitude and colour ( $<V_{{\rm RC}}>$ and $<(B-V)_{{\rm RC}}>$) for red clumps in the investigated field regions. For each Cloud, we have defined a colour-magnitude box containing red clump: 0.60 $\leq$ $(B-V) \leq$ 1.15 and 18.5 $\leq V \leq$ 20.0 for LMC, 0.45  $\leq (B-V) \leq$ 1.00 and 18.7 $\leq V \leq$ 20.2 for SMC (examples are given in Fig. 26). For each field we have then constructed two histograms, the first representing the number of stars contained in bins of 0.05 mag with varying magnitude, the second representing the number of stars contained in colour bins of 0.02 mag with varying colour. $<V_{{\rm RC}}>$ has been determined by fitting the histograms with a function of the form (Girardi & Salaris 2001):

$$% N(V) = a + bV + cV^2 + d~ \exp \bigg[-\frac{(V - <V_{{\rm RC}}>)^2} {2\sigma_{V}^{2}}\bigg]$$ (6)

where $\sigma _{V}$ gives an estimate of the sharpness of the magnitude distribution of clump stars. In a similar way we have determined $<(B-V)_{{\rm RC}}>$. An example of the fits is given in Fig. 27.

Figure 26: CMD boxes containing red clump stars for NGC 1902 LMC field (left panel) and for NGC 220 SMC field (right panel). Similar boxes was defined for the other fields of our sample.

Figure 27: Results of the fitting procedure on NGC 220 field clump histograms in magnitude (left panel) and colour (right panel). Similar analysis has been performed on all the other red clumps.

Table 7 gives the derived values, whereas Fig. 28 shows the location of the clumps in the V, (B-V) plane. In the LMC case, red clumps seems to concentrate in magnitude at $V \sim 19.25$ mag, except for the NGC 2155 clump, which appears noticeably brighter (V = 19.00 mag) and slightly bluer. SMC clumps present an analogous concentration around $V \sim 19.48$ mag. A most evident difference in mean locations is that SMC red clumps are bluer ($\sim$0.10-0.15 mag) than for the LMC, in good agreement with theoretical predictions about metallicity effects (see Girardi 1999).

From the tabulation by Girardi & Salaris (2001) one finds that, if the field of the two Clouds have similar histories (but different metallicities), SMC clumps at Z = 0.004 are expected to be more luminous than clumps in the LMC (Z = 0.008) by 0.11-0.14 mag. By relying on such a figure one would derive a difference in distance moduli between the two Clouds of the order of 0.35 mag. Along a similar route, one finds that neither reddening nor metallicity can account for the peculiar behaviour of the NGC 2155 field, and apparently we remain with the only solution that these stars are really closer to us.

5.1 Comparison between field and cluster red clumps

Finally, we have adopted the already quoted procedure to also derive mean magnitudes and colours of the cluster red clumps, as given in Table 8. Contrary to the field, in this case one is dealing with a "snapshot'' depicting the luminosity of He burning stars for a given (cluster) age. However, the comparison with the correspondent values for the field as given in Table 7 show few differences, in many cases possibly connected with the large dispersion and poor statistics of the observational data. The only relevant evidence is for NGC 643, whose clump appears about 0.5 mag brighter than the mean magnitude of the other clumps in the SMC field. This could be the effect of a younger age, but it could be also attributed to a distance effect, NGC 643 (located 4$^\circ$ far from SMC center) being closer to us than the SMC.

 

 

Table 7: Mean values and statistical errors for field red clump. $N^{{\rm RC}}_{{\rm star}}$ gives the total number of stars in the box.

Field	$N^{{\rm RC}}_{{\rm star}}$	$<V_{{\rm RC}}>$	$\sigma_{{\rm RC}}^{(V)}$	$<(B-V)_{{\rm RC}}>$	$\sigma_{{\rm RC}}^{(B-V)}$
LMC
KMHK264	1789	19.29	0.15	0.875	0.050
NGC 1902	1284	19.28	0.13	0.880	0.045
NGC 2155	542	19.00	0.14	0.860	0.041
NGC 1777	654	19.28	0.15	0.915	0.033
IC 2146	514	19.22	0.07	0.895	0.037
SMC
L45	2914	19.47	0.17	0.780	0.061
NGC 220	4362	19.48	0.14	0.725	0.057
NGC 458	445	19.42	0.09	0.770	0.039
L9	710	19.52	0.17	0.735	0.037
L13	1373	19.48	0.10	0.745	0.055
NGC 152	1545	19.45	0.20	0.750	0.037

Figure 28: Mean positions in the V, (B-V) plane of LMC (left panel) and SMC (right panel) studied red clumps.

 

 

Table 8: Mean magnitudes $<V_{{\rm RC,c}}>$ and colours $<(B-V)_{{\rm RC,c}}>$ of the cluster red clumps, with $\sigma _{{\rm RC,c}}^{(V)}$ and $\sigma _{{\rm RC,c}}^{(B-V)}$ giving estimates of the clump sharpness in magnitude and colour, respectively.

Cluster	$<V_{{\rm RC,c}}>$	$\sigma _{{\rm RC,c}}^{(V)}$	$<(B-V)_{{\rm RC,c}}>$	$\sigma _{{\rm RC,c}}^{(B-V)}$
LMC
NGC 2155	18.87	0.12	0.85	0.07
NGC 1777	19.32	0.21	0.83	0.10
IC 2146	19.15	0.16	0.90	0.06
SMC
L9	19.57	0.19	0.75	0.08
NGC 152	19.52	0.16	0.75	0.08
NGC 643	19.00	0.20	0.80	0.06