5 The open cluster vdB-Hagen 99

During a blue-red photographic survey of the southern Milky Way van den Bergh & Hagen (1975) provided a list of 262 known or suspected open clusters. Among them, 64 groups were newly recognized. One of this is the scarcely populated and loose open cluster vdB-Hagen 99, which lies in the outskirts of the Carina complex.

5.1 Previous investigation

vdB-Hagen 99 was studied by Landolt et al. (1990), who emphasize the importance of this cluster due to the probable membership of four known or suspected variables. They obtained multicolor broad-band UBVRI photoelectric photometry for 48 stars, and intermediate- and narrow-band photometry for 56 stars up to V=12. Moreover they obtained spectra for 21 stars in the region of the cluster and additional photometry for 11 fainter stars, with $13 \leq\ V \leq\ 16$. The main results of their investigation are that vdB-Hagen 99 is a sparse open cluster with at least 24 candidate members. The real existence of the cluster is argued on the basis of the narrow sequences the bright stars form in different color-color diagrams. Although dominated by variable extinction, vdB-Hagen 99 has a low mean reddening E(B-V)=0.05. Moreover it is 10⁸ yrs old and at a distance of about half a kpc. Finally it contains 8 photometric variables.

Figure 12: CMDs for all stars in the region of vdB-Hagen 99. The dashed line indicates the limiting magnitude reached by Landolt et al. (1990).

 

 

Table 7: Photometry of the stars in the field of the open cluster vdB-Hagen 99 in common with Landolt et al. (1990). The name of the stars follow Landolt et al. (1990). The suffix CP refers to the present study, whereas LPLM indicates Landolt et al. (1990) photometry.

ID	Name	$V_{\rm CP}$	$(B-V)_{\rm CP}$	$(U-B)_{\rm CP}$	$V_{\rm LPLM}$	$(B-V)_{\rm LPLM}$	$(U-B)_{\rm LPLM}$
2	CPD -582451	10.037	0.086	0.047	10.078	0.114	0.093
4	CPD -582452	10.062	0.990	0.788	10.101	1.068	0.803
6	CPD -582447	10.765	0.553	0.127	10.782	0.648	0.175
7	CPD -582442	11.056	0.283	0.024	11.077	0.333	0.071
8	CPD -582440	10.244	0.094	0.012	10.236	0.127	0.085
11	VV Car	11.748	1.747	1.319	11.775	1.784	1.356

5.2 The present work

We obtained CCD UBVRI photometry for 900 stars in the region shown in Fig. 11, up to V=20. Our survey supersedes the previous one, whose limiting magnitude was about V=12.

The measured stars are shown in Fig. 12, in the planes V-(B-V), V-(V-I) and V-(V-R). These CMDs resemble those of NGC 3114 (see Fig. 2) and Collinder 228 (see Fig. 7), with a MS extending from V=10 up to V=20 and with some evidence of the RG branch of the field stars population. The similarity is not surprising, since all the clusters are projected toward the Carina spiral arm.

We have 6 stars in common with Landolt et al. (1990), which are listed in Table 7. The mean differences turn out to be:

$$V_{\rm CP} - V_{\rm LPLM} = -0.023\pm0.016$$

$$(B-V)_{\rm CP} - (B-V)_{\rm LPLM} = -0.053\pm0.025$$

$$(U-B)_{\rm CP} -(U-B)_{\rm LPLM} = -0.044\pm 0.017 ,$$

where CP indicates our photometry, whereas LPLM stands for Landolt et al. (1990). Taking into account the different techniques used in extracting the photometry, the agreement is very good both for magnitude and colors.

Figure 13: vdB-Hagen 99 stars brighter than V = 17 in the (B-I)-(B-V) plane.

Figure 14: Two color diagram for the stars in the field of vdB-Hagen 99 brighter than V=17. The arrow indicates the reddening vector. The solid line is the empirical un-reddened ZAMS from Schmidt-Kaler (1982), while the dashed and dashed-dotted lines are the same ZAMS, but shifted by E(B-V)=0.10 and E(B-V)=0.50, respectively.

5.3 Reddening

To have an idea of the cluster mean reddening we selected all the stars brighter than V=17, and use their position in (B-I)-(B-V) plane, following the method described in Sect. 3.3. The least squares fit yields $E(B-V)=0.10\pm 0.08$. Again, the large uncertainty is due to the scatter of the stars in this plane, and indicates the presence of stars with different reddening, as already argued by Landolt et al. (1990).

Candidate members can be searched for by considering the color color diagram in Fig. 14, where filled circles represent stars having $E(B-V)=0.04\pm 0.03$, whereas open circles indicate stars having larger reddening. Tentatively, we suggest the possibility that two distinct populations are actually present: a group of eight bright stars which have the same low reddening (filled circles), and all the other stars which have a larger reddening with a significant scatter.

We argue that the brighter stars are candidate members of vdB-Hagen 99, whereas all the other stars having larger value of E(B-V) (see Fig. 14) are probably field stars.

 

 

Table 8: List of the new candidate members of vdB-Hagen 99 obtained in the present study.

ID	X	Y	V	(B-V)	(U-B)	(V-R)	(R-I)
3	182.75	235.10	10.244	0.044	0.212	0.026	0.027
8	106.44	341.37	11.165	-0.037	-0.027	-0.005	0.005
10	-7.76	-166.28	11.399	0.007	0.160	0.008	0.080
13	242.58	-76.50	12.229	0.014	0.126	0.000	0.081

5.4 Distance and age

In order to test this hypothesis, we construct the reddening corrected CMDs in the V₀-(B-V)₀ and V₀-(U-B)₀ planes for all the stars for which we could obtain a reddening estimate (see Fig. 15). Filled symbols indicate cluster candidate members, whereas open symbols indicated background stars.

As suggested above, two distinct populations are readily visible.

Most of the stars we measured are located beyond vdB-Hagen 99, at the distance of the Carina spiral arm (2.5-3.0 kpc). They are indicated with open symbols, and fitted with an empirical ZAMS shifted by (m-M)₀=12.20.

With filled triangles we indicate Landolt et al. (1990) candidate members, 20 stars in total. They define a tight sequence along the empirical ZAMS (solid line) shifted by (m-M)₀=8.30. Noticeably, all the empty triangles, which identify stars that have been suggested by Landolt et al. (1990) not be cluster members, actually lie close to the field stars sequence.

The stars indicated with filled circles are probable cluster members observed by us. Four of them - namely CPD -582451, CPD -582440, CPD -582442 and CPD -582447 - are in common with Landolt et al. (1990). The other 4 are probable new candidate members, and their properties are summarized in Table 8. The remaining two common stars (CPD -582452 and VV Car) are red stars of GK spectral type belonging to the field.

This way we increased the number of cluster members, suggesting that they are at least 28. Finally, the stars redder than vdB-Hagen 99 members are probably interlopers stars, located between us and the cluster.

As for the age, most of the stars lie close to the ZAMS, with the exception of the brightest ones. This is an indication that the cluster is young, as already claimed by Landolt et al. (1990). To have an idea of the cluster age, we over-imposed in Fig. 15 a solar metallicity isochrone (dotted line) from Girardi et al. (2000) for the age of $1.25 \times 10^{8}$ yrs, which nicely fits the evolved stars.

In conclusion, these results confirm that vdB-Hagen 99 is a young cluster projected toward the Carina spiral arm, at a distance of about 500 pc from the Sun. The mean reddening of cluster members turns out to be $E(B-V)=0.04\,\pm\, 0.03$, in agreement with the findings of Landolt et al. (1990).

Figure 15: Reddening corrected CMD for the stars in the field of vdB-Hagen 99. Filled symbols indicate cluster members, whereas open circles are field stars. Triangles refer to Landolt et al. (1990) photometry, whilst circles indicate stars whose photometry has been presented in this study. Finally, the dotted line is an isochrone for the age of $1.25 \times 10^{8}$ yrs, whereas the other two lines are the same empirical ZAMS, shifted by ( m-M)=12.20 (dashed line) and (m-M)=8.30 (solid line). See text for additional details.