R. D. Jeffries

A Spitzer View of Protoplanetary Disks in the γ Velorum Cluster

We present new Spitzer Space Telescope observations of stars in the young (~5 Myr)γ Velorum stellar cluster. Combining optical and 2MASS photometry, we have selected 579 stars as candidate members of the cluster. With the addition of the Spitzer mid-infrared data, we have identified five debris disks around A-type stars and five to six debris disks around solar-type stars, indicating that the strong radiation field in the cluster does not completely suppress the production of planetesimals in the disks of cluster members. However, we find some evidence that the frequency of circumstellar primordial disks is lower, and the infrared flux excesses are smaller than for disks around stellar populations with similar ages. This could be evidence for a relatively fast dissipation of circumstellar dust by the strong radiation field from the highest mass star(s) in the cluster. Another possibility is that γ Velorum stellar cluster is slightly older than reported ages and the low frequency of primordial disks reflects the fast disk dissipation observed at ~5 Myr.

The lithium depletion boundary in NGC 2547 as a test of pre-main-sequence evolutionary models

Intermediate resolution spectroscopy from the European Southern Observatory Very Large Telescope is analysed for 63 photometrically selected low-mass (0.08-0.30 M ○. ) candidates of the open cluster NGC 2547. We have confirmed membership for most of these stars using radial velocities, and found that lithium remains undepleted for cluster stars with I > 17.54 ± 0.14 and K s > 14.86 ± 0.12. From these results, several pre-main-sequence evolutionary models give almost model independent ages of 34-36 Myr, with a precision of 10 per cent. These ages are only slightly larger than the ages of 25-35(±5) Myr obtained using the same models to fit isochrones to higher mass stars descending towards the zero-age main-sequence, both in empirically calibrated and theoretical colour-magnitude diagrams. This agreement between age determinations in different mass ranges is an excellent test of the current generation of low-mass pre-main-sequence stellar models and lends confidence to ages determined with either method between 30 and 120 Myr.

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 discovery of a low-mass, pre-main-sequence stellar association around γ Velorum

We report the serendipitous discovery of a population of low-mass, pre-main-sequence (PMS) stars in the direction of the Wolf-Rayet/O-star binary system gamma(2) Vel and the Vela OB2 association. We argue that gamma(2) Vel and the low-mass stars are truly associated and approximately coeval, and that both are at distances between 360 and 490 pc, disagreeing at the 2 sigma level with the recent Hipparcos parallax of gamma(2) Vel, but consistent with older distance estimates. Our results clearly have implications for the physical parameters of the gamma(2) Vel system, but also offer an exciting opportunity to investigate the influence of high-mass stars on the mass function and circumstellar disc lifetimes of their lower mass PMS siblings.

A survey for low-mass spectroscopic binary stars in the young clusters around σ Orionis and λ Orionis

We have obtained multi-epoch, high-resolution spectroscopy of 218 candidate low-mass stars and brown dwarfs (BDs) in the young clusters around a Ori and \ Ori. We find that 196 targets are cluster members based on their radial velocity, the equivalent width of their Na Ι 8200 lines and the spectral type from their TiO band strength. We have identified 11 new binary stars among the cluster members based on their variable radial velocity and an additional binary from the variation in its linewidth and shape. Of these, six are double-lined spectroscopic binaries (SB2) where the components of the binary are of comparable brightness. The others are single-lined binaries (SB 1) in which the companion is faint or the spectra of the stars are blended. There are three narrow-lined SB 1 binaries in our sample for which the companion is more than 2.5 mag fainter than the primary. This suggests that the mass ratio distribution for the spectroscopic binaries in our sample is broad but that there may be a peak in the distribution near q = 1. The sample covers the magnitude range I C = 14-18.9 (mass ≈0.55-0.03 M ⊙ ), but all of the binary stars are brighter than I C = 16.6 (mass ≈30.12 M ⊙ ) and 10 are brighter than I C = 15.5 (mass ≈0.23 M ⊙ ). There is a significant lack of spectroscopic binaries in our sample at faint magnitudes even when we account for the decrease in sensitivity with increasing magnitude. We can reject the hypothesis that the fraction of spectroscopic binaries is a uniform function of I C magnitude with more than 99 per cent confidence. The spectroscopic binary fraction for stars more massive than about 0.1 M ⊙ (I C < 16.9) is f bright = 0.095 +0.012 -0.028 . The 90 per cent confidence upper limit to the spectroscopic binary fraction for very low-mass (VLM) stars (mass <0.1 M ⊙ ) and BDs is f faint < 7.5 per cent. The hypothesis that f bright and /faint are equal can be rejected with 90 per cent confidence. The average detection probability for our survey is 50 per cent or more for binaries with separations up to 0.28 au for stars with I C < 16.9 and 0.033 au for the fainter stars in our sample. We conclude that we have found strong evidence for a change in the fraction of spectroscopic binaries among young VLM stars and BDs when compared to more massive stars in the same star-forming region. This implies a difference in the total binary fraction between VLM stars and BDs compared to more massive stars or a difference in the distribution of semimajor axes, or both.

Contamination and exclusion in the σ Orionis young group

We present radial velocities for 38 low-mass candidate members of the σ Orionis young group. We have measured their radial velocities by cross-correlation of high-resolution (R ≈ 6000) AF2/Wide Field Fibre Optical Spectrograph (WYFFOS) spectra of the gravity-sensitive Na I doublet at 8183, 8195 A. The total sample contained 117 objects, of which 54 have sufficient signal-to-noise ratio to detect Na I at an equivalent width of 3 A; however, we only detect Na I in 38 of these. This implies that very low-mass members of this young group display weaker Na I absorption than similarly aged objects in the Upper Scorpius OB association. We develop a technique to assess membership using radial velocities with a range of uncertainties that does not bias the selection when large uncertainties are present. The resulting membership probabilities are used to assess the issue of exclusion in photometric selections, and we find that very few members are likely to be excluded by such techniques. We also assess the level of contamination in the expected pre-main-sequence region of colour‐magnitude space brighter than I = 17. We find that contamination by non-members in the expected pre-main-sequence region of the colour‐magnitude diagram is small. We conclude that although radial velocity alone is insufficient to confirm membership, high signal-to-noise ratio observations of the Na I doublet provide the opportunity to use the strength of Na I absorption in concert with radial velocities to asses membership down to the lowest masses, where lithium absorption no longer distinguishes youth.

Membership, metallicity and lithium abundances for solar-type stars in NGC 6633

We present spectroscopic observations of candidate F, G and K type stars in NGC 6633, an open cluster with a similar age to the Hyades. From the radial velocities and metal-line equivalent widths we identify 10 new cluster members including one short period binary system. Combining this survey with that of Jeffries (1997), we identify a total of 30 solar-type members. We have used the F and early G stars to spectroscopically estimate [Fe/H]= 0.096 ± 0.081 for NGC 6633. When compared with iron abundances in other clusters, determined in a strictly comparable way, we can say with more precision that NGC 6633 has (0.074±0.041)dex less iron than the Pleiades and (0.206±0.040)dex less iron than the Hyades. A photometric estimate of the overall metallicity from the locus of cluster members in the B-V, V-Ic plane, yields [M/H]= 0.04 ± 0.10. A new estimate, based upon isochrones that are empirically tuned to fit the Pleiades, gives a distance modulus to NGC 6633 that is 2.41 ± 0.09 larger than the Pleiades. Lithium abundances have been estimated for the NGC 6633 members and compared with consistently determined Li abundances in other clusters. Several mid F stars in NGC 6633 show strong Li depletion at approximately the same effective temperature that this phenomenon is seen in the Hyades. At cooler temperatures the Li abundance patterns in several open clusters with similar ages (NGC 6633, Hyades, Praesepe and Coma Berenices) are remarkably similar, despite their differing [Fe/H]. There is however evidence that the late G and K stars of NGC 6633 have depleted less Li than their Hyades counterparts. This qualitatively agrees with models for pre-main sequence Li depletion that feature only convective mixing, but these models cannot simultaneously explain why these stars have in turn depleted Li by more than 1 dex compared with their ZAMS counterparts in the Pleiades. Two explanations are put forward. The first is that elemental abundance ratios, particularly [O/Fe], may have non-solar values in NGC 6633 and would have to be higher than in either the Hyades or Pleiades. The second is that additional non-convective mixing, driven by angular momentum loss, causes additional photospheric Li depletion during the first few hundred Myr of main sequence evolution.

Circumstellar discs in the young σ Orionis cluster

We present new K- and L � -band imaging observations for members of the young (3‐5 Myr) σ Orionis cluster, obtained at the United Kingdom Infrared Telescope (UKIRT) with the UKIRT 1‐5 micron Imager Spectrometer (UIST). We determine (K − L � ) colour excesses with respect to the photospheres, finding evidence for warm circumstellar dust around 27 out of 83 cluster members that have masses between 0.04 and 1.0 M� . This indicates a circumstellar disc frequency of at least (33 ± 6) per cent for this cluster, consistent with previous determinations from smaller samples and also consistent with the 3-Myr disc half-life suggested by Haisch et al. There is marginal evidence that the disc frequency declines towards lower masses, but the data are also consistent with no mass dependence at all. There is no evidence for spatial segregation of objects with and without circumstellar discs.

The low-mass initial mass function in the young cluster NGC 6611

A B ST R A C T NGC 6611 isthe m assiveyoung cluster(2 3M yr)thationisesthe EagleNebula.W e presentverydeep photom etricobservationsofthecentralregionofNGC 6611obtained with the Hubble Space Telescope and the followinglters:ACS/W FC F775W and F850LP and NIC2 F110W and F160W ,loosely equivalent to ground-based IZJH �lters.ThissurveyreachesdowntoI � 26m ag.W econstructtheInitialM assFunction (IM F) from �1.5Mwellinto the brown dwarfregim e (down to �0.02M � ).W e have detected 30 35 brown dwarfcandidates in this sam ple.The low-m ass IM F is com bined with a higher-m ass IM F constructed from the groundbased catalogue from Oliveira etal.(2005).W ecom parethenalIM F with thoseofwellstudied star form ingregions:wend thattheIM F ofNGC 6611m oreclosely resem blesthatofthe low-m assstarform ing region in Taurusthan thatofthe m orem assiveOrion Nebula Cluster(ONC).W econcludethatthereseem sto beno severeenvironm entaleectin theIM F dueto theproxim ity ofthem assivestarsin NGC 6611. K ey w ords: stars:late-type { stars:low-m ass,brown dwarfs { stars:lum inosity function,m ass function { stars:pre-m ain-sequence { open cluster and associations: individual:NGC 6611.

Chandra X-Ray Observations of the Young Open Cluster NGC 2516

We present a comprehensive study of the Chandra X-ray observations of the young open cluster NGC 2516. We have analyzed eight individual Chandra observations. We have combined the data to achieve the greatest sensitivity, reaching down to log fX = -14.56 (ergs s-1 cm -2), or log LX = 28.69 (ergs s-1) at the distance of NGC 2516. Out of 284 X-ray sources detected, 155 are identified with photometric cluster members, with very little ambiguity. We have studied the X-ray luminosity functions for the various spectral types, correcting for nonmember contamination those for later type stars. We find strong X-ray emission from a couple of B stars that cannot be due to companions. Among A stars, chemically peculiar stars show a detection fraction much larger than normal A stars. The luminosity functions decrease monotonically from F to M stars, and those for K stars are significantly lower than similar stars in the Pleiades, a cluster only slightly younger than NGC 2516. We discuss possible causes of this discrepancy, including differences in age and stellar rotation.