IC 2395 and BH 47: Only One Open Cluster in the Vela Constellation
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A&A 409, 541–551 (2003) Astronomy DOI: 10.1051/0004-6361:20031121 & c ESO 2003 Astrophysics IC 2395 and BH 47: Only one open cluster in the Vela constellation J. J. Clari´a1, E. Lapasset1, A. E. Piatti2, and A. V. Ahumada1 1 Observatorio Astron´omico, Universidad Nacional de C´ordoba, Laprida 854, 5000 C´ordoba, Argentina e-mail: [email protected]; [email protected] 2 Instituto de Astronom´ıa y F´ısica del Espacio, CC 67, Suc. 28, 1428, Buenos Aires, Argentina e-mail: [email protected] Received 2 May 2003 / Accepted 18 July 2003 Abstract. We report UBV photoelectric photometry for 273 stars in the fields of the open clusters IC 2395 and BH 47. Our postulate is that, rather than two different clusters in this region of Vela, there is only one, which we call IC 2395 = BH 47. The cluster is centered at about α = 8h42m. 5, δ = −48◦06.8 (2000), its angular diameter being ∼19. The analysis of the photometric data yields 61 probable cluster members and 16 possible members. Photometric membership probabilities show good agreement with those obtained from a proper motion study for 21 stars in common. The reddening across the cluster appears to be uniform, the mean E(B − V) value being 0.09 ± 0.02. The true distance modulus is (V − MV )0 = 9.52 ± 0.10, corresponding to a distance from the Sun of (800 ± 40) pc and 48 pc below the Galactic plane. The cluster age, determined by fitting isochrones with core overshooting, turns out to be (6 ± 2) Myr. There is a strong likelihood that IC 2395 = BH 47 is physically connected to the VelaOB1C association. Key words. methods: observational – techniques: photometric – stars: fundamental parameters – open clusters and associations: individual: IC 2395, BH 47 1. Introduction On the other hand, BH 47 (C0841-479) was first recognized as an open cluster by van den Bergh & Hagen (1975), who es- The photometric data reported in this paper are part timated its angular diameter as 13 . Although BH 47, centered of an observing project which is still ongoing at the ◦ at α = 8h42m. 6, δ = −48 07 (2000.0), is considered to be a dif- Observatorio Astron´omico de la Universidad Nacional de ferent, somewhat smaller cluster than IC 2395 in the WEBDA C´ordoba (Argentina), consisting in obtaining photometric Open Cluster Database (Mermilliod 2000), there seems to ex- and/or spectroscopic data of southern open clusters not yet ob- ist some confusion in the literature about these two objects. served or with incomplete observations (see, e.g., Clari´a et al. In fact, Lyngå (1959) has observed photoelectrically in the 1998, 2003; Piatti & Clari´a 2002; Piatti et al. 1999, 2003). UBV system 20 stars in the field of BH 47, but calls the clus- UBV photoelectric photometry remains a valuable tool for ob- ter IC 2395. Photographic UBV photometry of about 30 stars taining the fundamental parameters of star clusters with an- was later obtained by Lyngå (1962) nearly in the same cluster gular diameters exceeding the usual size of CCD cameras. field. He derived a colour excess E(B − V) = 0.11, a distance Information on cluster membership, distance, interstellar red- of 960 pc and a nuclear age of about 100 Myr for what he refers dening and age are obtained through the analysis of the colour– to as IC 2395, even though he is in fact dealing with BH 47. magnitude (CM) and colour–colour (CC) UBV diagrams. In their search for carbon stars in open clusters, Jørgensen & IC 2395 (IAU designation C0839-480), also known as Westerlund (1988) obtained UBVRI photoelectric photometry Collinder 192, is a medium-sized open cluster situated in the of 14 stars located properly in the field of IC 2395. Since sev- Vela constellation at α = 8h41m. 1, δ = −48◦12 (2000.0), eral of these stars seem to fall on the fainter portion of the clus- with Galactic coordinates l = 266◦.57, b = −3◦.81, as con- ter’s CM published by Lyngå (1962), Jørgensen & Westerlund signed in the WEDBA Open Cluster Catalogue (Mermilliod suggested that there is only one cluster with an angular diam- 2000). It was described by Ruprecht (1966) as a Trumpler eter of about 12 in this region of Vela. More recently, Loktin (1930) class II-3p system – i.e., a detached cluster of fewer et al. (2001) published a second version of their Open Cluster than 50 members, with little central concentration and a Catalogue (Loktin & Matkin 1994) reviewing the fundamen- medium range in the brightness of the stars. tal parameters of 423 clusters. Their updated distances are now Send offprint requests to: J. J. Clari´a, based on a Hyades distance modulus of 3.27, this value being e-mail: [email protected] 0.06 mag smaller than that found from Hipparcos parallaxes, Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20031121 542 J. J. Clari´a et al.: The open cluster IC 2395 about 12−15 standard stars taken from the lists of Cousins (1973, 1974), Landolt (1973) and Graham (1982) were ob- served each night to insure accurate transformations to the stan- dard system. The dispersions in the individual measures for the observed stars yield the internal mean errors of a single UBV observation. These errors are σV ≤ 0.015 mag, σB−V ≤ 0.010 mag and σU−B ≤ 0.016 mag for V < 12 mag, increas- ing to 0.019, 0.023 and 0.031 for V > 12 mag, respectively. A comparison of the observed mean values with the published ones for the standard stars yields the external mean errors of a single observation, an indication of how closely the standard system has been reproduced. The averaged external mean er- rors are smaller than 0.015 mag for the V magnitude and the (B − V) and (U − B) colours. All these errors are nearly similar to those in previous papers (e.g., Clari´a et al. 1991). No evi- dence of systematic differences either in the V magnitude or in the colours among the stars measured in LCO and CTIO was detected. Consequently, mean UBV values were computed for the observed stars. Table 1 presents the results of our photometry. The first column gives a running star number. Columns 2 and 3 re- fer to linear coordinates X and Y in pixels for the positions Fig. 1. Identification chart for stars observed in the fields of the open of stars in Fig. 1. The figure is 1000 × 1000 pixels and the clusters IC 2395 and BH 47. The positions and approximate sizes zero point corresponds to the upper left corner. The scale of IC 2395 (the largest circle) and BH 47 (smaller circle) are indi- − on the figure is 3.1 pixel 1. Only star numbers correspond- cated. The positions of the observed stars can be determined from = . the linear coordinates X and Y listed in Table 1. The zero point of ing to the observed stars brighter than V 10 0 are indi- these coordinates corresponds to the upper left corner and the size cated in Fig. 1. Columns 4 and 5 list the equatorial coordi- of the figure is 1000 × 1000 pixels. Individual equatorial coordinates nates Right Ascension and Declination for the epoch 2000.0, Right Ascension and Declination (2000.0) for all the observed stars while Cols. 6−9 give in succession the V magnitude, the are given in Table 1. Only observed stars brighter than V = 10.0 are B − V and U − B colours and the number of individual identified. Image taken from the Digitized Sky Survey, produced at UBV photoelectric observations for each star made on dif- the Space Telescope Science Institute under US Government grant ferent nights. Column 10 provides the HD or CD numbers NAG W-2166. (when available) as well as star numbers from Lyngå (1962) and Jørgensen & Westerlund (1988) preceded by L and JW, namely (m − M)V = 3.33 ± 0.01 (Perryman et al. 1998). Using respectively. Column 11 lists the MK spectral types taken the original data from Lyngå (1959, 1962), Loktin et al. (2001) from Houck (1978) or Garrison et al. (1977), when avail- derived the following parameters for IC 2395: d = 705 pc, able, and the last column gives an indication whether the star E(B − V) = 0.066 and 16 Myr. is believed to be a probable member (M), a possible mem- In the present study we report UBV photoelectric obser- ber (PM), or a non-member (NM) of the cluster IC 2395 = vations of 273 stars brighter than V = 15.0, distributed in an BH 47 (see Sect. 5.1). Only a portion of Table 1 is shown here area of about 50 × 50 centered at about α = 8h42m. 2 (2000), for guidance regarding its form and content, its whole con- δ = −48◦09.3, which includes both IC 2395 and BH 47 (Fig. 1). tent being available in electronic form at the CDS via anony- We will use the UBV data to search for any evidence that the mous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via two groups of stars belong to the same cluster. We will also http://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/409/541 use the data to determine the basic parameters of this group of stars. 3. Comparison with other UBV data Eighteen stars measured in the present study have previously 2.