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Astron. Astrophys. 340, 402-414 (1998)

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7. NGC 2451

NGC 2451 is located in Puppis at [FORMULA] ([FORMULA]). During the last 30 years a debate has arisen about its existence and physical parameters. Williams (1967a,b) measured three-colour photometry for 213 stars in a 1 square degree field centered on HD 63032 and determined a distance of [FORMULA] pc to the cluster. Eggen (1983) however re-investigated this region and found a wide range of distance moduli between [FORMULA] 6.0 and 9.0 with no prominent concentration. Lyngå & Wramdemark (1984) argued on the basis of Eggen's photometry that there is a clustering of stars brighter than 9th magnitude at a distance of [FORMULA] pc. In a later investigation Eggen (1986) found a concentration of stars of spectral types between B8 and A0 at distances less than 250 pc, too.

Röser & Bastian (1994) investigated a [FORMULA] by [FORMULA] area centered on HD 63032. On the basis of proper motions from the PPM Catalogue they argued for a star cluster in this region, which they called Puppis Moving Group. They derived a distance of [FORMULA] pc to this cluster, in accordance with Lyngå & Wramdemark (1984) and Eggen (1986). Furthermore, they found evidence for a second cluster at a distance of approximately 400 pc. Platais et al. (1996), using proper motions and CCD BV photometry confirmed the reality of the first cluster, which they called NGC 2451 A. They also found evidence for a second cluster, NGC 2451 B, at a distance of about 400 pc.

The Hipparcos data offer the opportunity to re-investigate the nature of NGC 2451. In a first step we selected all stars in a [FORMULA] by [FORMULA] area centered on [FORMULA] = -38:O4 from the Hipparcos Catalogue. Their proper motions are shown in Fig. 9, where some stars with high proper motions have already been omitted. Two conspicuous concentrations appear, located at approximately [FORMULA] mas/yr and [FORMULA] mas/yr. The first group is the cluster NGC 2451 A, while the second concentration is mainly due to the background of Milky Way stars. We will first discuss NGC 2451 A.

[FIGURE] Fig. 9. Proper motions of all Hipparcos stars in the direction of NGC 2451.

7.1. NGC 2451 A

Fig. 10a shows the proper motions of the possible members in greater detail. It is possible that all stars share a common proper motion. Looking at the spatial distribution of the stars (Fig. 10b) one notices a clear concentration of the hypothetical members. This is a strong argument in favour of a star cluster. A cluster of 100 [FORMULA] would have a tidal radius of roughly 6 pc. At a distance of 220 pc this corresponds to an angular diameter of [FORMULA]. Hence, with the exception of the star at the right edge (HIP 36653), all stars may be bound to the same cluster. We note, that HIP 36653 has also the most discrepant proper motion in Fig. 10a, so that its membership is doubtful. Fig. 10c shows the parallaxes of the probable cluster members. It is possible that all stars are at the same distance. Excluding HIP 36653, we obtain a mean parallax of [FORMULA] mas for the cluster. This is in perfect agreement with the photometric distance of [FORMULA] pc (corresponding to a parallax of [FORMULA] mas) that Platais et al. determined for NGC 2451 A.

[FIGURE] Fig. 10a-d. Proper motions (a ), spatial distribution (b ), parallaxes (c ) and colours and magnitudes (d ) of the suspected members of NGC 2451 A. The star shown with an open circle is HIP 36653.

Fig. 10d finally shows a colour-magnitude diagram of NGC 2451 A. Although there is some scatter, the possible members form a well-defined main-sequence. NGC 2451 A therefore beyond doubt is a true star cluster. We have listed the possible members in Table 6. Based on their magnitudes, colours and parallaxes, two additional Hipparcos stars may be members of NGC 2451 A: HIP 37514 and HIP 37915. Their proper motions differ significantly from the rest of the cluster stars, but this may be due to binary effects. There are some hints that HIP 37514 is a binary, since it has a G-type solution in the Hipparcos catalogue. In addition, the radial velocity of HIP 37915 is very close to that of HIP 37297. Furthermore, both stars are located in the central part of the cluster, adding further evidence for their membership.


Table 6. Members and possible members of NGC 2451 A
Columns 1 - 10: Same as Table 1. Columns 11 and 12 are taken from the WEB Catalogue (Duflot et al. 1995) of radial velocities. Column 13 gives our decision upon membership

The radial velocities seem to exclude two stars as possible cluster members: Star 7 (HIP 37666) has a very discrepant velocity, and the velocities of stars 1 (37297) and 9 (37752) differ also significantly from each other. However, we note that HIP 37666 is a double star and its radial velocity is based on only three measurements, which weakens the discrepancy of its radial velocity.

7.2. NGC 2451 B

Röser & Bastian (1994) found evidence for a second cluster among the stars in a one-square-degree field centered on the K4 giant HD 63032. Based on its distance modulus they found that the star itself could be a member of this cluster.

In contrast to NGC 2451 A it is difficult to identify a second cluster in the proper motion diagram of Fig. 9. If it exists, its proper motion must be either un-distinguishable from the bulk of Milky Way stars or it contains only a few Hipparcos stars. In order to examine the reality of NGC 2451 B, we proceeded in the following way: We first searched the list of Williams (1967a) for stars which are close to the lower of the two main-sequences in Fig. 5 of Röser & Bastian (1994) and are therefore possible cluster members. These stars were then identified in the Hipparcos and ACT catalogues. Fig. 11 shows the proper motions of the stars found.

[FIGURE] Fig. 11. Proper motions of stars from the list of Williams that may be members of NGC 2451 B. The solid line shows the expected proper motions of stars in the direction of NGC 2451 and with distances between 200 and 1000 pc, assuming that the space motions consist of solar motion and galactic rotation only.

There is a clear concentration of proper motions around [FORMULA] mas/yr, [FORMULA] mas/yr. Assuming a Milky Way model with the 1985 IAU recommended values of galactic rotation (A = -14 km/sec/kpc, B = -12 km/sec/kpc, [FORMULA] = 8.5 kpc, [FORMULA] = 220 km/sec) and assuming a local solar motion of [FORMULA] km/sec, we predict proper motions between [FORMULA] mas/yr (for [FORMULA] pc) and [FORMULA] mas/yr (for [FORMULA] pc) for the NGC 2451 field. The concentration in the proper motions is somewhat offset from this line and probably not due to the Milky Way background. It may be a hint of a star cluster.

To check the cluster hypothesis further, we next searched the ACT Catalogue for stars with proper motions within [FORMULA] to [FORMULA] mas/yr, [FORMULA] mas/yr. Fig. 12 shows the positions on the sky of the stars found. The possible cluster members show a concentration near [FORMULA], [FORMULA] -37:O8, which supports the cluster hypothesis. NGC 2451 B may therefore be a true star cluster.

[FIGURE] Fig. 12. Positions of all ACT stars with proper motions close to [FORMULA] mas/yr, [FORMULA] mas/yr.

We assume that all stars from Williams list with proper motions within [FORMULA] to [FORMULA] mas/yr, [FORMULA] mas/yr are possible cluster members and have collected them in Table 7. It is unclear, whether HD 63032 belongs to this cluster. Its proper motion ([FORMULA] mas/yr, [FORMULA] mas/yr) differs clearly from the mean of the ACT stars. In addition, as a cluster member, it would be about five magnitudes brighter than the brightest main-sequence star.


Table 7. Possible members of NGC 2451 B
Columns 1, 3 and 4 are taken from Williams (1967a). Columns 5 to 10 are taken from the ACT Catalogue, except for HD 63032 where they are taken from Hipparcos

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Online publication: November 9, 1998