C. R. Devey

Low mass stars, brown dwarf candidates and the mass function of the young open cluster NGC 2547

We present a catalogue of R c I c Z photometry over an area of 0.855 square degrees, centred on the young open cluster NGC 2547. The survey is substantially complete to limits of R c = 21.5, I c = 19.5, Z = 19.5. We use the catalogue to define a sample of NGC 2547 candidates with model-dependent masses of about 0.05 -1.0 M ○. . After correcting for incompleteness and estimating contamination by foreground field dwarfs, we investigate the mass function of the cluster, its binary content, and search for evidence of mass segregation among the lower mass stars. There is ample evidence for mass segregation between high (>3 M ○. ) and lower mass stars, but over the range 0.1 < M < 0.7 M ○. , the data are consistent with no further mass segregation. By fitting King profiles we conclude that at least 60 per cent of the low-mass stellar population are contained within our survey. The cluster mass function is remarkably similar to the Pleiades for 0.075 < M < 0.7 M ○. . Because of its age (≃ 30 Myr), we demonstrate that this mass function is robust to a number of systematic uncertainties likely to affect older and younger clusters and is therefore one of the best available estimates for the initial mass function in young disc populations. For 0.05 < M < 0.075 M ○. there is some evidence for a deficit of brown dwarfs in NGC 2547 compared with other clusters. This deficit may extend to lower masses or may only be a dip, perhaps caused by an imperfect understanding of the mass-magnitude relationship at temperatures of around 2800 K. Incompleteness in both our survey and the luminosity functions from which we estimate contamination by foreground objects leave this question open. The binary fraction for systems with mass ratios greater than about 0.5 is 20-35 per cent for M dwarfs in NGC 2547, quite consistent with that found in the field and other young clusters. The full photometric catalogue and our lists of candidate cluster members are made available in electronic format.

The stellar association around Gamma Velorum and its relationship with Vela OB2

We present the results of a photometric BVI survey of 0.9 deg 2 around the Wolf-Rayet binary γ 2 Vel and its early-type common proper motion companion γ 1 Vel (together referred to as the y Vel system). Several hundred pre-main-sequence (PMS) stars are identified and the youth of a subset of these is spectroscopically confirmed by the presence of lithium in their atmospheres, Ha emission and high levels of X-ray activity. We show that the PMS stars are kinematically coherent and spatially concentrated around y Vel. The PMS stars have similar proper motions to y Vel, to main-sequence (MS) stars around y Vel and to early-type stars of the wider Vela OB2 association of which γ 2 Vel is the brightest member. The ratio of MS stars to low-mass (0.1-0.6M ⊙ ) PMS stars is consistent with a Kroupa mass function. MS fitting to stars around y Vel gives an association distance modulus of 7.76 ± 0.07 mag, which is consistent with a similarly determined distance for Vela OB2 and also with interferometric distances to γ 2 Vel. High-mass stellar models indicate an age of 3-4Myr for γ 2 Vel, but the low-mass PMS stars have ages of ≃10 Myr according to low-mass evolutionary models and 5-10 Myr by empirically placing them in an age sequence with other clusters based on colour-magnitude diagrams and lithium depletion. We conclude that the low-mass PMS stars form a genuine association with y Vel, and this is a subcluster within the larger Vela OB2 association. We speculate that γ 2 Vel formed after the bulk of the low-mass stars, expelling gas, terminating star formation and unbinding the association. The velocity dispersion of the PMS stars is too low for this star-forming event to have produced all the stars in the extended Vela OB2 association. Instead, star formation must have been initiated at several sites within a molecular cloud either sequentially or simultaneously after some triggering event.

The lithium depletion boundary and the age of NGC 2547

We present the results of a photometric and spectroscopic survey of cool M dwarf candidates in the young open cluster NGC 2547. Using the 2dF fibre spectrograph, we have searched for the luminosity at which lithium remains unburned in an attempt to constrain the cluster age. The lack of a population of individual lithium-rich objects towards the faint end of our sample places a very strong lower limit to the cluster age of 35 Myr. However, the detection of lithium in the averaged spectra of our faintest targets suggests that the lithium depletion boundary lies at 9.5 < MI < 10.0 and that the cluster age is <54 Myr. The age of NGC 2547 judged from fitting isochrones to low-mass pre-main-sequence stars in colour‐magnitude diagrams is 20‐35 Myr using the same evolutionary models. The sense and size of the discrepancy in age determined by these two techniques are similar to those found in another young cluster, IC 2391, and in the low-mass pre-main-sequence binary system, GJ 871.1AB. We suggest that the inclusion of rotation or dynamo-generated magnetic fields in the evolutionary models could reconcile the two age determinations, but only at the expense of increasing the cluster ages beyond that currently indicated by the lithium depletion. Alternatively, some mechanism is required that increases the rate of lithium depletion in young, very low-mass fully convective

Mass segregation in the young open cluster NGC 2547

We present a study of mass segregation of the young (20-35 Myr isochronal age) open cluster NGC 2547. We find good evidence that mass segregation exists in NGC 2547 down to 3 M O ., and weak evidence for mass segregationdown to 1 M O .. Theoretical models of an initially unsegregated model of NGC 2547 using the NBODY2 code show weaker mass segregation, implying that at least some of the observed mass segregation has a primordial origin. We also report the discovery of three possible escaped cluster members, which share the proper motion and colours of the cluster, but lie nearly a degree from the cluster centre.

On the nature of Collinder 121: insights from the low-mass pre-main sequence

We present a VI photometric catalogue towards the open cluster Cr 121. XMM‐Newton and ROSAT data are used to discover a low-mass pre-main sequence (PMS) along this sightline. de Zeeuw et al. have identified Cr 121 as a moving group, using Hipparcos data, at a distance of 592 pc; we reject the scenario that these low-mass PMS stars are associated with that association. By considering the higher mass main sequence stellar membership of the groups along this sightline, the density of low-mass PMS stars and their age spread we argue that the low-mass PMS stars are associated with a young, compact cluster at a distance of 1050 pc. This is consistent with Collinder’s original description of Cr 121, and we argue that this distant compact cluster should retain its original designation. The moving group detected by de Zeeuw et al. resembles a foreground association and we agree with Eggen that this should be called CMa OB2. This study demonstrates that although the de Zeeuw et al. census of OB associations is an invaluable resource for studying local star formation, it must be interpreted in the context of other data when considering structure over distances of the same order as the limits of the Hipparcos parallaxes.