Search

We present an artificial neural network (ANN) model of photospheric lithium depletion in cool stars (⁠|$3000\lt T_{\rm eff}/{\rm K} \lt 6500$|⁠), producing estimates and probability distributions of age from |$^7$| Li 6708 Å equivalent width (LiEW) and effective temperature data inputs. The model is trained on the same sample of 6200 stars from 52 open clusters, observed in the Gaia -ESO spectroscopic survey, and used to calibrate the previously published analytical eagles model, with ages 2–6000 Myr and |$-0.3 \lt $| [Fe/H] |$\lt 0.2$|⁠. The additional flexibility of the ANN provides some improvements, including better modelling of the 'lithium dip' at ages |$\lt 50$|  Myr and |$T_{\rm eff}\sim 3500$|  K, and of the intrinsic dispersion in LiEW at all ages. Poor age discrimination is still an issue at ages >1 Gyr, confirming that additional modelling flexibility is not sufficient to fully represent the LiEW–age–T |$_{\text{eff}}$| relationship, and suggesting the involvement of further astrophysical parameters. Expansion to include such parameters–rotation, accretion, and surface gravity–is discussed, and the use of an ANN means these can be more easily included in future iterations, alongside more flexible functional forms for the LiEW dispersion. Our methods and ANN model are provided in an updated version 2.0 of the eagles software. [ABSTRACT FROM AUTHOR]

We present the first large-scale 3D kinematic study of |$\sim$| 2700 spectroscopically confirmed young stars (⁠|$\lt $| 20 Myr) in 18 star clusters and OB associations (hereafter groups) from the combination of Gaia astrometry and Gaia –ESO Survey spectroscopy. We measure 3D velocity dispersions for all groups, which range from 0.61 to 7.4 km s |$^{-1}$| (1D velocity dispersions of 0.35–4.3 km s |$^{-1}$| ). We find the majority of groups have anisotropic velocity dispersions, suggesting they are not dynamically relaxed. From the 3D velocity dispersions, measured radii, and estimates of total mass, we estimate the virial state and find that all systems are super-virial when only the stellar mass is considered, but that some systems are sub-virial when the mass of the molecular cloud is taken into account. We observe an approximately linear correlation between the 3D velocity dispersion and the group mass, which would imply that the virial state of groups scales as the square root of the group mass. However, we do not observe a strong correlation between virial state and group mass. In agreement with their virial state, we find that nearly all of the groups studied are in the process of expanding and that the expansion is anisotropic, implying that groups were not spherical prior to expansion. One group, Rho Oph, is found to be contracting and in a sub-virial state (when the mass of the surrounding molecular cloud is considered). This work provides a glimpse of the potential of the combination of Gaia and data from the next generation of spectroscopic surveys. [ABSTRACT FROM AUTHOR]

We present an empirical model of age-dependent photospheric lithium depletion, calibrated using a large homogeneously analysed sample of 6200 stars in 52 open clusters, with ages from 2 to 6000 Myr and −0.3 < [Fe/H] < 0.2, observed in the Gaia -ESO spectroscopic survey. The model is used to obtain age estimates and posterior age probability distributions from measurements of the Li  i 6708 Å equivalent width for individual (pre) main-sequence stars with 3000 < T eff/K < 6500, a domain where age determination from the HR diagram is either insensitive or highly model-dependent. In the best cases, precisions of 0.1 dex in log age are achievable; even higher precision can be obtained for coeval groups and associations where the individual age probabilities of their members can be combined. The method is validated on a sample of exoplanet-hosting young stars, finding agreement with claimed young ages for some, but not others. We obtain better than 10 per cent precision in age, and excellent agreement with published ages, for seven well-studied young moving groups. The derived ages for young clusters (<1 Gyr) in our sample are also in good agreement with their training ages, and consistent with several published model-insensitive lithium depletion boundary ages. For older clusters, there remain systematic age errors that could be as large as a factor of 2. There is no evidence to link these errors to any strong systematic metallicity dependence of (pre) main-sequence lithium depletion, at least in the range −0.29 < [Fe/H] < 0.18. Our methods and model are provided as software – 'Empirical AGes from Lithium Equivalent widthS' (eagles). [ABSTRACT FROM AUTHOR]

The kinematics of stars in OB associations can provide insights into their formation, dynamical evolution, and eventual fate. The low-mass stellar content of OB associations are sufficiently numerous as to provide a detailed sampling of their kinematic properties, however spectroscopy is required to confirm the youth of individual stars and to get 3D kinematics. In this paper, we present and analyse results from a large spectroscopic survey of Vela OB2 conducted using 2dF/HERMES on the AAT. This spectroscopy is used to confirm the youth of candidate young stars and determine radial velocities, which are combined with proper motions and parallaxes from Gaia to measure 3D positions and velocities. We identify multiple separate kinematic groups in the region, for which we measure velocity dispersions and infer their virial states. We measure expansion rates for all these groups and find strong evidence for anisotropic expansion in the Vela OB2 association of at least 11σ significance in all three dimensions, as well as some evidence for expansion in the γ Vel and P Puppis clusters. We trace back the motions of these groups into the past and find that the open cluster NGC 2547 is an interloper in the Vela OB2 region and actually formed >100 pc away from the association. We conclude that Vela OB2 must have formed with considerable spatial and kinematic substructure over a time-scale of ∼10 Myr, with clear temporal substructure within the association, but no clear evidence for an age gradient. [ABSTRACT FROM AUTHOR]

A growing disquiet has emerged in recent years that standard stellar models are at odds with observations of the colour–magnitude diagrams (CMDs) and lithium depletion patterns of pre-main sequence stars in clusters. In this work we select 1246 high probability K/M-type constituent members of five young open clusters (5–125 Myr) in the Gaia-ESO Survey to test a series of models that use standard input physics and others that incorporate surface magnetic fields or cool starspots. We find that: standard models provide systematically under-luminous isochrones for low-mass stars in the CMD and fail to predict Li-depletion of the right strength at the right colour; magnetic models provide better CMD fits with isochrones that are ∼1.5–2 times older, and provide better matches to Li depletion patterns. We investigate how rotation periods, most of which are determined here for the first time from Transiting Exoplanet Survey Satellite data, correlate with CMD position and Li. Among the K-stars in the older clusters we find the brightest and least Li-depleted are the fastest rotators, demonstrating the classic 'Li-rotation connection' for the first time at ∼35 Myr in NGC 2547, and finding some evidence that it exists in the early M-stars of NGC 2264 at |$\lt 10\,$| Myr. However, the wide dispersion in Li depletion observed in fully convective M-dwarfs in the γ Vel cluster at ∼20 Myr appears not to be correlated with rotation and is challenging to explain without a very large (>10 Myr) age spread. [ABSTRACT FROM AUTHOR]

Spectroscopy from the final internal data release of the Gaia –ESO Survey (GES) has been combined with Gaia EDR3 to assign membership probabilities to targets observed towards 63 Galactic open clusters and 7 globular clusters. The membership probabilities are based chiefly on maximum likelihood modelling of the 3D kinematics of the targets, separating them into cluster and field populations. From 43 211 observed targets, 13 985 are identified as highly probable cluster members (P > 0.9), with an average membership probability of 0.993. The addition of GES radial velocities successfully drives down the fraction of false positives and we achieve better levels of discrimination in most clusters over the use of astrometric data alone, especially those at larger distances. Since the membership selection is almost purely kinematic, the union of this catalogue with GES and Gaia is ideal for investigating the photometric and chemical properties of clusters as a function of stellar mass, age, and Galactic position. [ABSTRACT FROM AUTHOR]

Astrometry and photometry from Gaia and spectroscopic data from the Gaia -ESO Survey (GES) are used to identify the lithium depletion boundary (LDB) in the young cluster NGC 2232. A specialized spectral line analysis procedure was used to recover the signature of undepleted lithium in very low luminosity cluster members. An age of 38 ± 3 Myr is inferred by comparing the LDB location in absolute colour−magnitude diagrams (CMDs) with the predictions of standard models. This is more than twice the age derived from fitting isochrones to low-mass stars in the CMD with the same models. Much closer agreement between LDB and CMD ages is obtained from models that incorporate magnetically suppressed convection or flux-blocking by dark, magnetic starspots. The best agreement is found at ages of 45−50 Myr for models with high levels of magnetic activity and starspot coverage fractions >50 per cent, although a uniformly high spot coverage does not match the CMD well across the full luminosity range considered. [ABSTRACT FROM AUTHOR]

New fibre spectroscopy and radial velocities from the WIYN telescope are used to measure photospheric lithium in 242 high-probability, zero-age main-sequence F- to K-type members of the rich cluster M35. Combining these with published rotation periods, the connection between lithium depletion and rotation is studied in unprecedented detail. At T eff < 5500 K there is a strong relationship between faster rotation and less Li depletion, although with a dispersion larger than measurement uncertainties. Components of photometrically identified binary systems follow the same relationship. A correlation is also established between faster rotation rate (or smaller Rossby number), decreased Li depletion and larger stellar radius at a given T eff. These results support models where star-spots and interior magnetic fields lead to inflated radii and reduced Li depletion during the pre-main-sequence (PMS) phase for the fastest rotators. However, the data are also consistent with the idea that all stars suffered lower levels of Li depletion than predicted by standard PMS models, perhaps because of deficiencies in those models or because saturated levels of magnetic activity suppress Li depletion equally in PMS stars of similar T eff regardless of rotation rate, and that slower rotators subsequently experience more mixing and post-PMS Li depletion. [ABSTRACT FROM AUTHOR]

The Gaia -ESO Survey (GES) observed many open clusters as part of its programme to spectroscopically characterize the various Milky Way populations. GES spectroscopy and Gaia astrometry from its second data release are used here to assign membership probabilities to targets towards 32 open clusters with ages from 1 to 3800 Myr, based on maximum likelihood modelling of the 3D kinematics of the cluster and field populations. From a parent catalogue of 14 398 individual targets, 5032 stars with uniformly determined 3D velocities, T eff, log  g , and chemistry are assigned cluster membership with probability >0.9, and with an average probability of 0.991. The robustness of the membership probabilities is demonstrated using independent membership criteria (lithium and parallax) in two of the youngest clusters. The addition of radial velocities improves membership discrimination over proper motion selection alone, especially in more distant clusters. The kinematically selected nature of the membership lists, independent of photometry and chemistry, makes the catalogue a valuable resource for testing stellar evolutionary models and investigating the time evolution of various parameters. [ABSTRACT FROM AUTHOR]

The kinematics of low-mass stars in nearby OB associations can provide clues about their origins and evolution. Combining the precise positions, proper motions, and parallaxes given in the second Gaia Data Release with radial-velocity measurements obtained with the Hermes spectrograph at the Anglo-Australian Telescope, we have an opportunity to study in detail the kinematics of low-mass stars belonging to the nearby γ Vel cluster and the Vela OB2 association it is projected against. The presence of lithium is used to confirm the youth of our targets. We separate our sample into the cluster and association populations based on the Gaia-ESO Survey membership probabilities their parallaxes, and kinematics. We find strong evidence for expansion in the OB association population with at least 4σ significance along all three axes, though the expansion is notably anisotropic. We discuss these results in the context of cluster and association dispersal theories. [ABSTRACT FROM AUTHOR]

The combination of precise radial velocities from multi-object spectroscopy and highly accurate proper motions from Gaia DR2 opens up the possibility for detailed 3D kinematic studies of young star-forming regions and clusters. Here, we perform such an analysis by combining Gaia -ESO Survey spectroscopy with Gaia astrometry for ∼900 members of the Lagoon Nebula cluster, NGC 6530. We measure the 3D velocity dispersion of the region to be |$5.35^{+0.39}_{-0.34}$|  km s−1, which is large enough to suggest the region is gravitationally unbound. The velocity ellipsoid is anisotropic, implying that the region is not sufficiently dynamically evolved to achieve isotropy, though the central part of NGC 6530 does exhibit velocity isotropy that suggests sufficient mixing has occurred in this denser part. We find strong evidence that the stellar population is expanding, though this is preferentially occurring in the declination direction and there is very little evidence for expansion in the right ascension direction. This argues against a simple radial expansion pattern, as predicted by models of residual gas expulsion. We discuss these findings in the context of cluster formation, evolution, and disruption theories. [ABSTRACT FROM AUTHOR]

Rotation periods from Kepler K2 are combined with projected rotation velocities from the WIYN 3.5 m telescope to determine projected radii for fast-rotating, low-mass (⁠|$0.15\, \le \, M/\mathrm{ M}_{\odot }\, \le 0.6$|⁠) members of the Praesepe cluster. A maximum likelihood analysis that accounts for observational uncertainties, binarity, and censored data yields marginal evidence for radius inflation – the average radius of these stars is 6 ± 4 per cent larger at a given luminosity than predicted by commonly used evolutionary models. This overradius is smaller (at 2σ confidence) than was found for similar stars in the younger Pleiades using a similar analysis; any decline appears due to changes occurring in higher mass (⁠|${\gt }0.25 \, \mathrm{M}_{\odot }$|⁠) stars. Models incorporating magnetic inhibition of convection predict an overradius, but do not reproduce this mass dependence unless superequipartition surface magnetic fields are present at lower masses. Models incorporating flux blocking by starspots can explain the mass dependence but there is no evidence that spot coverage diminishes between the Pleiades and Praesepe samples to accompany the decline in overradius. The fastest rotating stars in both Praesepe and the Pleiades are significantly smaller than the slowest rotators for which a projected radius can be measured. This may be a selection effect caused by more efficient angular momentum loss in larger stars leading to their progressive exclusion from the analysed samples. Our analyses assume random spin-axis orientations; any alignment in Praesepe, as suggested by Kovacs, is strongly disfavoured by the broad distribution of projected radii. [ABSTRACT FROM AUTHOR]

The first Gaia Data Release presents an opportunity to characterize the low-mass population of OB associations, providing larger statistical samples and better constraints on the formation and evolution of clusters and associations. Using previously known low-mass members in a small region of Vela OB2, we have designed selection criteria that combine Gaia and 2MASS photometry, independently of any astrometric information, to identify low-mass pre-main-sequence stars over the wider association area. Our method picks out the known clusters of young stars around γ2 Velorum and NGC-2547, but also identifies other overdensities that may represent previously unknown clusters. There are clear differences in the spatial distributions of the low-mass and the high-mass OB populations, suggesting either that the structure and dynamics of these populations has evolved separately or that the initial mass function can vary considerably on small scales within a single association. [ABSTRACT FROM AUTHOR]

Rotation periods obtained with the Kepler satellite have been combined with precise measurements of projected rotation velocity from the WIYN 3.5-m telescope to determine the distribution of projected radii for several hundred low-mass (0.1 ≤ M /M⊙ ≤ 0.8), fast-rotating members of the Pleiades cluster. A maximum likelihood modelling technique, that takes account of observational uncertainties, selection effects and censored data, and considers the effects of differential rotation and unresolved binarity, has been used to find that the average radius of these stars is 14 ± 2 per cent larger at a given luminosity than predicted by current evolutionary models of Dotter et al. and Baraffe et al. The same models are a reasonable match to the interferometric radii of older, magnetically inactive field M dwarfs, suggesting that the over-radius may be associated with the young, magnetically active nature of the Pleiades objects. No evidence is found for any change in this over-radius above and below the boundary marking the transition to full convection. Published evolutionary models that incorporate either the effects of magnetic inhibition of convection or the blocking of flux by dark star-spots do not individually explain the radius inflation, but a combination of the two effects might. The distribution of projected radii is consistent with the adopted hypothesis of a random spatial orientation of spin axes; strong alignments of the spin vectors into cones with an opening semi-angle <30° can be ruled out. Any plausible but weaker alignment would increase the inferred over-radius. [ABSTRACT FROM AUTHOR]

Using the kinematically unbiased technique described in Binks, Jeffries & Maxted (2015), we present optical spectra for a further 122 rapidly rotating (rotation periods <6 d), X-ray active FGK stars, selected from the SuperWASP survey. We identify 17 new examples of young, probably single stars with ages of <200 Myr and provide additional evidence for a new Northern hemisphere kinematic association: the Pisces moving group (MG). The group consists of 14 lithium-rich G- and K-type stars that have a dispersion of only ∼3 kms-1 in each Galactic space velocity coordinate. The group members are approximately coeval in the colour–magnitude diagram, with an age of 30-50 Myr, and have similar, though not identical, kinematics to the Octans-Near MG. [ABSTRACT FROM AUTHOR]

We present a family of self-consistent axisymmetric rotating globular cluster models which are fitted to spectroscopic data for NGC 362, NGC 1851, NGC 2808, NGC 4372, NGC 5927 and NGC 6752 to provide constraints on their physical and kinematic properties, including their rotation signals. They are constructed by flattening Modified Plummer profiles, which have the same asymptotic behaviour as classical Plummer models, but can provide better fits to young clusters due to a slower turnover in the density profile. The models are in dynamical equilibrium as they depend solely on the action variables. We employ a fully Bayesian scheme to investigate the uncertainty in our model parameters (including mass-tolight ratios and inclination angles) and evaluate the Bayesian evidence ratio for rotating to non-rotating models. We find convincing levels of rotation only in NGC 2808. In the other clusters, there is just a hint of rotation (in particular, NGC 4372 and NGC 5927), as the data quality does not allow us to draw strong conclusions. Where rotation is present, we find that it is confined to the central regions, within radii of R ≤ 2rh. As part of this work, we have developed a novel q-Gaussian basis expansion of the line-of-sight velocity distributions, from which general models can be constructed via interpolation on the basis coefficients. [ABSTRACT FROM AUTHOR]

We present a search for debris discs amongstM-dwarf members of nearby, young (5–150 Myr) moving groups (MGs) using infrared (IR) photometry, primarily from theWide Infrared Survey Explorer (WISE). A catalogue of 100 MG M dwarfs that have suitable WISE data is compiled, and 19 of these are found to have significant IR excess emission at 22 μm. Our search is likely to be complete for discs where the ratio of flux from the disc to flux from the star fd/f∗ > 10-3. The spectral energy distributions are supplemented with Two-Micron All-Sky Survey (2MASS) photometry and data at longer wavelengths, and fitted with simple disc models to characterize the IR excesses. There is a bimodal distribution – 12 targets have W1 − W4 > 3, corresponding to fd/f∗ > 0.02, and are likely to be gas-rich, primordial discs. The remaining seven targets have W1 − W4 < 1 (fd/f∗ ≲ 10-3) and include three objects with previously known or suspected debris discs and four new debris disc candidates that are all members of the Beta Pic MG. All of the IR excesses are identified in stars that are likely members of MGs with age <30 Myr. The detected debris disc frequency falls from 13 ± 5 per cent to <7 per cent (at 95 per cent confidence) for objects younger or older than 30 Myr, respectively. This provides evidence for the evolution of debris discs on this time-scale and does not support models where the maximum of debris disc emission occurs much later in lower mass stars. [ABSTRACT FROM AUTHOR]

We show that non-magnetic models for the evolution of pre-main-sequence (PMS) stars cannot simultaneously describe the colour-magnitude diagram (CMD) and the pattern of lithium depletion seen in the cluster of young, low-mass stars surrounding γ 2 Velorum. The age of 7.5 ± 1 Myr inferred from the CMD is much younger than that implied by the strong Li depletion seen in the cluster M-dwarfs, and the Li depletion occurs at much redder colours than predicted. The epoch at which a star of a given mass depletes its Li and the surface temperature of that star are both dependent on its radius. We demonstrate that if the low-mass stars have radii ~10 per cent larger at a given mass and age, then both the CMD and the Li-depletion pattern of the Gamma Velorum cluster are explained at a common age of ≃18-21 Myr. This radius inflation could be produced by some combination of magnetic suppression of convection and extensive cool starspots. Models that incorporate radius inflation suggest that PMS stars, similar to those in the Gamma Velorum cluster, in the range 0.2 < M/M⊙ < 0.7, are at least a factor of 2 older and ~7 per cent cooler than previously thought and that their masses are much larger (by >30 per cent) than inferred from conventional, non-magnetic models in the Hertzsprung-Russell diagram. Systematic changes of this size may be of great importance in understanding the evolution of young stars, disc lifetimes and the formation of planetary systems. [ABSTRACT FROM AUTHOR]

The discovery of lithium-rich giants contradicts expectations from canonical stellar evolution. Here we report on the serendipitous discovery of 20 Li-rich giants observed during the Gaia-ESO Survey, which includes the first nine Li-rich giant stars known towards the CoRoT fields. Most of our Li-rich giants have near-solar metallicities and stellar parameters consistent with being before the luminosity bump. This is difficult to reconcile with deep mixing models proposed to explain lithium enrichment, because these models can only operate at later evolutionary stages: at or past the luminosity bump. In an effort to shed light on the Li-rich phenomenon, we highlight recent evidence of the tidal destruction of close-in hot Jupiters at the sub-giant phase.We note that when coupled with models of planet accretion, the observed destruction of hot Jupiters actually predicts the existence of Li-rich giant stars, and suggests that Li-rich stars should be found early on the giant branch and occur more frequently with increasing metallicity. A comprehensive review of all known Li-rich giant stars reveals that this scenario is consistent with the data. However, more evolved or metal-poor stars are less likely to host close-in giant planets, implying that their Li-rich origin requires an alternative explanation, likely related to mixing scenarios rather than external phenomena. [ABSTRACT FROM AUTHOR]

The young open cluster NGC3293 is included in the observing program of the Gaia-ESO survey (GES). The radial velocity values provided have been used to assign cluster membership probabilities by means of a single-variable parametric analysis. These membership probabilities are compared to the results of the photometric membership assignment of NGC3293, based on UBVRI photometry. The agreement of the photometric and kinematic member samples amounts to 65 per cent, and could increase to 70 per cent as suggested by the analysis of the differences between both samples. A number of photometric PMS candidate members of spectral type F are found, which are confirmed by the results from VPHAS photometry and SED fitting for the stars in common with VPHAS and GES data sets. Excesses at midand near-infrared wavelengths, and signs of Hα emission, are investigated for them. Marginal presence of Hα emission or infilling is detected for the candidate members. Several of them exhibit moderate signs of U excess and weak excesses at mid-IR wavelengths. We suggest that these features originate from accretion discs in their last stages of evolution. [ABSTRACT FROM AUTHOR]

Optical spectroscopic observations are reported for 24 and 23, nearby, proper-motion-selected M-dwarf candidate members of the Beta Pictoris and AB Doradus moving groups (BPMG and ABDMG). Using kinematic criteria, the presence of both Hα emission and high X-rayto- bolometric luminosity, and position in absolute colour-magnitude diagrams, 10 and 6 of these candidates are confirmed as likely members of the BPMG and ABDMG, respectively. Equivalent widths or upper limits for the Li I 6708 Å line are reported and the lithium depletion boundary (LDB) age of the BPMG is revisited. Whilst non-magnetic evolutionary models still yield an estimated age of 21±4Myr, models that incorporate magnetic inhibition of convection imply an older age of 24 ± 4 Myr. A similar systematic increase would be inferred if the stars were 25 per cent covered by dark magnetic starspots. Since young, convective M-dwarfs are magnetically active and do have starspots, we suggest that the original LDB age estimate is a lower limit. The LDB age of the ABDMG is still poorly constrained - non-magnetic evolutionary models suggest an age in the range 35-150 Myr, which could be significantly tightened by new measurements for existing candidate members. [ABSTRACT FROM AUTHOR]

We present a kinematically-unbiased search to identify young, nearby low-mass members of kinematic moving groups (MGs). Objects with both rotation periods shorter than 5 d in the SuperWASP All-Sky Survey and X-ray counterparts in the ROSAT All-Sky Survey were chosen to create a catalogue of several thousand rapidly rotating, X-ray active FGK stars. These objects are expected to be either young single stars or tidally locked spectroscopic binaries. We obtained optical spectra for a sub-sample of 146 stars to determine their ages and kinematics, and in some cases repeat radial velocity measurements were used to identify binarity. 26 stars are found to have lithium abundances consistent with an age of ≤200 Myr, and show no evidence for binarity and in most cases measurements of H α and v sin i support their youthful status. Based on their youth, their radial velocities and estimates of their three-dimensional kinematics, we find 11 objects that may be members of known MGs, eight that do not appear associated with any young MG and a further seven that are close to the kinematics of the recently proposed 'Octans-Near' MG, and which may be the first members of this MG found in the Northern hemisphere. The initial search mechanism was ~18 per cent efficient at identifying likely-single stars younger than 200 Myr, of which 80 per cent were early-K spectral types. [ABSTRACT FROM AUTHOR]

In a coeval group of low-mass stars, the luminosity of the sharp transition between stars that retain their initial lithium and those at slightly higher masses in which Li has been depleted by nuclear reactions, the lithium depletion boundary (LDB), has been advanced as an almost model-independent means of establishing an age scale for young stars. Here, we construct polytropic models of contracting pre-main sequence stars (PMS) that have cool, magnetic star-spots blocking a fraction β of their photospheric flux. Star-spots slow the descent along Hayashi tracks, leading to lower core temperatures and less Li destruction at a given mass and age. The age, τLDB, determined from the luminosity of the LDB, LLDB, is increased by a factor of (1 − β)−E compared to that inferred from unspotted models, where E ≃ 1 + dlog τLDB/dlog LLDB and has a value ∼0.5 at ages <80 Myr, decreasing to ∼0.3 for older stars. Spotted stars have virtually the same relationship between K-band bolometric correction and colour as unspotted stars, so this relationship applies equally to ages inferred from the absolute K magnitude of the LDB. Low-mass PMS stars do have star-spots, but the appropriate value of β is highly uncertain with a probable range of 0.1 < β < 0.4. For the smaller β values, our result suggests a modest systematic increase in LDB ages that is comparable with the maximum levels of theoretical uncertainty previously claimed for the technique. The largest β values would however increase LDB ages by 20–30 per cent and demand a re-evaluation of other age estimation techniques calibrated using LDB ages. [ABSTRACT FROM AUTHOR]

We present an isochrone server for semi-empirical pre-main-sequence model isochrones in the following systems: Johnson–Cousins, Sloan Digital Sky Survey, Two-Micron All-Sky Survey, Isaac Newton Telescope (INT) Wide-Field Camera and INT Photometric Hα Survey (IPHAS)/UV-Excess Survey (UVEX). The server can be accessed via the Cluster Collaboration webpage http://www.astro.ex.ac.uk/people/timn/isochrones/. To achieve this, we have used the observed colours of member stars in young clusters with well-established age, distance and reddening to create fiducial loci in the colour–magnitude diagram. These empirical sequences have been used to quantify the discrepancy between the models and data arising from uncertainties in both the interior and atmospheric models, resulting in tables of semi-empirical bolometric corrections (BCs) in the various photometric systems. The model isochrones made available through the server are based on existing stellar interior models coupled with our newly derived semi-empirical BCs. As part of this analysis, we also present new cluster parameters for both the Pleiades and Praesepe, yielding ages of $135^{+20}_{-11}$ and $665^{+14}_{-7}\,\rm {Myr}$ as well as distances of 132 ± 2 and 184 ± 2 pc, respectively (statistical uncertainty only). [ABSTRACT FROM AUTHOR]

A polytropic model is used to investigate the effects of dark photospheric spots on the evolution and radii of magnetically active, low-mass (M < 0.5 M⊙), pre-main-sequence (PMS) stars. Spots slow the contraction along Hayashi tracks and inflate the radii of PMS stars by a factor of (1 − β)−N compared to unspotted stars of the same luminosity, where β is the equivalent covering fraction of dark starspots and N ≃ 0.45 ± 0.05. This is a much stronger inflation than predicted by Spruit & Weiss for main-sequence stars with the same β, where N ∼ 0.2–0.3. These models have been compared to radii determined for very magnetically active K- and M-dwarfs in the young Pleiades and NGC 2516 clusters, and the radii of tidally locked, low-mass eclipsing binary components. The binary components and zero-age main-sequence K-dwarfs have radii inflated by ∼10 per cent compared to an empirical radius–luminosity relation that is defined by magnetically inactive field dwarfs with interferometrically measured radii; low-mass M-type PMS stars, that are still on their Hayashi tracks, are inflated by up to ∼40 per cent. If this were attributable to starspots alone, we estimate that an effective spot coverage of 0.35 < β < 0.51 is required. Alternatively, global inhibition of convective flux transport by dynamo-generated fields may play a role. However, we find greater consistency with the starspot models when comparing the loci of active young stars and inactive field stars in colour–magnitude diagrams, particularly for the highly inflated PMS stars, where the large, uniform temperature reduction required in globally inhibited convection models would cause the stars to be much redder than observed. [ABSTRACT FROM AUTHOR]

We present a model that predicts the light-curve amplitude distribution for an ensemble of low-mass magnetically active stars, under the assumptions that stellar spin axes are randomly orientated and that cool starspots have a characteristic scalelength and are randomly distributed across the stellar surfaces. The model is compared with observational data for highly magnetically active M-dwarfs in the young cluster NGC 2516. We find that the best-fitting starspot scalelength is not constrained by these data alone, but requires assumptions about the overall starspot-filling factor and starspot temperature. Assuming a spot coverage fraction of 0.4 ± 0.1 and a starspot to unspotted photosphere temperature ratio of 0.7 ± 0.05, as suggested by the inflated radii of these stars compared to evolutionary model predictions and by TiO band measurements on other active cool stars of earlier spectral type, the best-fitting starspot angular scalelength is 3.5+ 2− 1 degrees, or a linear scalelength of ∼25 000 km. This linear scalelength is similar to large sunspot groups, but two to five times smaller than the starspots recently deduced on an active G-dwarf using eclipse mapping by a transiting exoplanet. However, the best-fitting spot scalelength in the NGC 2516 M-dwarfs increases with the assumed spot temperature ratio and with the inverse square root of the assumed spot-filling factor. Hence, the light-curve amplitude distribution might equally well be described by these larger spot scalelengths if the spot-filling factors are <0.1 or the spot temperature ratio is >0.9. [ABSTRACT FROM AUTHOR]

ABSTRACT We present a critical assessment of commonly used pre-main-sequence isochrones by comparing their predictions to a set of well-calibrated colour-magnitude diagrams of the Pleiades in the wavelength range 0.4- [ABSTRACT FROM AUTHOR]

ABSTRACT We present far-red, intermediate-resolution spectroscopy of 572 photometrically selected, low-mass stars (0.2 < M/M⊙ < 0.7) in the young open cluster NGC 2516, using the FLAMES spectrograph at the Very Large Telescope. Precise radial velocities confirm membership for 210 stars that have published rotation periods from spot-modulated light curves and for another 144 stars in which periodic modulation could not be found. The two subsamples are compared and no significant differences are found between their positions in colour-magnitude diagrams, the distribution of their projected equatorial velocities or their levels of chromospheric activity. We rule out differing observational sensitivity as an explanation and conclude that otherwise similar objects, with equally high levels of chromospheric activity, do not exhibit spot-induced light-curve modulation because their significant spot coverage is highly axisymmetric. We propose that the spot coverage consists of large numbers of small, dark spots with diameters of about 2°. This explains why about half of cluster members do not exhibit rotationally modulated light curves and why the light-curve amplitudes of those that do have mean values of only 0.01-0.02 mag. [ABSTRACT FROM AUTHOR]

ABSTRACT We present an analysis of Spitzer IRAC (3.6-8 μm) and MIPS (24 μm) imaging of members of the 16 Myr old open cluster NGC 1960 (M36). Models of terrestrial planet formation indicate that rocky planets are likely to achieve their final masses at around 10-30 Myr, and thus this cluster is at an interesting epoch for planet formation. We find 21 B-F5 type stars and 14 F6-K9 type stars which have 24 μm excess emission, and thus determine that >30 per cent of B-F5 type stars and >23 per cent of F6-K9 type stars in this cluster have 24 μm excess emission. These excess frequencies are similar to those observed in other clusters of similar age. Three early-type stars have excesses at near-infrared wavelengths. Analysis of their spectral energy distributions confirms that these are true debris discs and not remnant primordial or transitional discs. None of the 61 Sun-like stars has confirmed near-infrared excess, and we can place a limit on the frequency of 8 μm excess emission around Sun-like stars of <7 per cent. All of the detected excesses are consistent with emission from debris discs and are not primordial. [ABSTRACT FROM AUTHOR]

ABSTRACT The wide luminosity dispersion seen for stars at a given effective temperature in the Hertzsprung-Russell diagrams of young clusters and star-forming regions is often interpreted as due to significant (∼10 Myr) spreads in stellar contraction age. In the scenario where most stars are born with circumstellar discs, and that disc signatures decay monotonically (on average) over time-scales of only a few Myr, any such age spread should lead to clear differences in the age distributions of stars with and without discs. We have investigated large samples of stars in the Orion Nebula Cluster (ONC) using three methods to diagnose disc presence from infrared measurements. We find no significant difference in the mean ages or age distributions of stars with and without discs, consistent with expectations for a coeval population. Using a simple quantitative model, we show that any real age spread must be smaller than the median disc lifetime. For a lognormal age distribution, there is an upper limit of <0.14 dex (at 99 per cent confidence) to any real age dispersion, compared to the ≃0.4 dex implied by the Hertzsprung-Russell diagram. If the mean age of the ONC is 2.5 Myr, this would mean at least 95 per cent of its low-mass stellar population have ages between 1.3-4.8 Myr. We suggest that the observed luminosity dispersion is caused by a combination of observational uncertainties and physical mechanisms that disorder the conventional relationship between luminosity and age for pre-main-sequence stars. This means that individual stellar ages from the Hertzsprung-Russell diagram are unreliable and cannot be used to directly infer a star formation history. Irrespective of what causes the wide luminosity dispersion, the finding that any real age dispersion is less than the median disc lifetime argues strongly against star formation scenarios for the ONC lasting longer than a few Myr. [ABSTRACT FROM AUTHOR]

We present an analysis of the rotation of young stars in the associations Cepheus OB3b, NGC 2264, 2362 and the Orion Nebula Cluster (ONC). We discover a correlation between rotation rate and position in a colour-magnitude diagram (CMD) such that stars which lie above an empirically determined median pre-main sequence rotate more rapidly than stars which lie below this sequence. The same correlation is seen, with a high degree of statistical significance, in each association studied here. If position within the CMD is interpreted as being due to genuine age spreads within a cluster, then the stars above the median pre-main sequence would be the youngest stars. This would in turn imply that the most rapidly rotating stars in an association are the youngest, and hence those with the largest moments of inertia and highest likelihood of ongoing accretion. Such a result does not fit naturally into the existing picture of angular momentum evolution in young stars, where the stars are braked effectively by their accretion discs until the disc disperses. Instead, we argue that, for a given association of young stars, position within the CMD is not primarily a function of age, but of accretion history. We show that this hypothesis could explain the correlation we observe between rotation rate and position within the CMD. [ABSTRACT FROM AUTHOR]

We present Spitzer IRAC and MIPS 24-μm imaging of members of the 27 ± 5 Myr old open cluster IC 4665. Models for the assembly of terrestrial planets through planetesimal collisions and mergers predict episodic dust debris discs at this epoch. We determine that per cent of the solar-type (F5-K5) cluster members have excess emission at 24 μm indicative of these debris discs, the highest frequency of the clusters studied with Spitzer to date. The majority of these discs have intermediate levels of excess (), and no source is found to have extreme levels of excess indicative of a recent transient event as opposed to steady-state collisional evolution. We find no evidence of a link between multiplicity and 24-μm excess in this cluster sample. Only the early-type star TYC424-473-1 () has significant near-infrared excess from 4.5 μm as measured with IRAC. Two solar-type targets have low-significance 8-μm excess but no significant 24-μm excess. All other targets show no evidence for near-infrared excess which could indicate the presence of an optically thick primordial disc, demonstrating that the observed 24-μm excess arises from a debris disc. [ABSTRACT FROM AUTHOR]

At fast rotation rates, the coronal activity of G- and K-type stars has been observed to 'saturate' and then decline again at even faster rotation rates - a phenomenon dubbed 'supersaturation'. In this paper, we investigate coronal activity in fast-rotating M-dwarfs using deep XMM-Newton observations of 97 low-mass stars of known rotation period in the young open cluster NGC 2547 and combine these with published X-ray surveys of low-mass field and cluster stars of known rotation period. Like G- and K-dwarfs, we find that M-dwarfs exhibit increasing coronal activity with decreasing Rossby number , the ratio of period to convective turnover time, and that activity saturates at for . However, supersaturation is not convincingly displayed by M-dwarfs, despite the presence of many objects in our sample with , where supersaturation is observed to occur in higher mass stars. Instead, it appears that a short rotation period is the primary predictor of supersaturation; d for K-dwarfs and perhaps d for M-dwarfs. These observations favour the 'centrifugal stripping' model for supersaturation, where coronal structures are forced open or become radiatively unstable as the Keplerian corotation radius moves inside the X-ray-emitting coronal volume. [ABSTRACT FROM AUTHOR]

We report radial velocities (RVs), projected equatorial velocities and Ca ii triplet (CaT) chromospheric activity indices for 237 late-K to mid-M candidate members of the young open cluster NGC 2516. These stars have published rotation periods between 0.1 and 15 d. Intermediate-resolution spectra were obtained using the Giraffe spectrograph at the Very Large Telescope. Membership was confirmed on the basis of their RVs for 210 targets. For these stars, we see a marked increase in the fraction of rapidly rotators as we move to cooler spectral types. About 20 per cent of M0–M1 stars have km s−1, increasing to 90 per cent of M4 stars. Activity indices derived from the first two lines of the CaT (8498 and 8542 Å) show differing dependencies on the rotation period and mass for stars lying above and below the fully convective boundary. Higher mass stars, of spectral type K3–M2.5, show chromospheric activity which increases with a decreasing Rossby number (the ratio of period to convective turnover time), saturating for Rossby numbers 0.1. For cooler stars, which are probably fully convective and almost all of which have Rossby numbers 0.1, there is a clear decrease in chromospheric activity as increases, amounting to a fall of about a factor of 2–3 between spectral types M2.5 and M4. This decrease in activity levels at low Rossby numbers is not seen in X-ray observations of the coronae of cluster M dwarfs or of active field M dwarfs. There is no evidence for supersaturation of chromospheric activity for stars of any spectral type at Rossby numbers 0.01. We suggest that the fall in the limiting level of chromospheric emission beyond spectral type M3 in NGC 2516 is, like the simultaneous increase in rotation rates in field stars, associated with a change in the global magnetic topology as stars approach the fully convective boundary and not due to any decrease in dynamo-generated magnetic flux. [ABSTRACT FROM AUTHOR]

We present a photometric study of I-band variability in the young association Cepheus OB3b. The study is sensitive to periodic variability on time-scales of less than a day, to more than 20 d. After rejection of contaminating objects using and narrow-band Hα photometry, we find 475 objects with measured rotation periods, which are very likely pre-main-sequence members of the Cep OB3b star-forming region. We revise the distance and age to Cep OB3b, putting it on the self-consistent age and distance ladder of Mayne & Naylor. This yields a distance modulus of 8.8 ± 0.2 mag, corresponding to a distance of 580 ± 60 pc, and an age of 4–5 Myr. The rotation period distribution confirms the general picture of rotational evolution in young stars, exhibiting both the correlation between accretion (determined in this case through narrow-band Hα photometry) and rotation expected from disc locking, and the dependence of rotation upon mass that is seen in other star-forming regions. However, this mass dependence is much weaker in our data than found in other studies. Comparison to the similarly aged NGC 2362 shows that the low-mass stars in Cep OB3b are rotating much more slowly. This points to a possible link between star-forming environment and rotation properties. Such a link would call into question models of stellar angular momentum evolution, which assume that the rotational period distributions of young clusters and associations can be assembled into an evolutionary sequence, thus ignoring environmental effects. [ABSTRACT FROM AUTHOR]

We investigate to what extent the spin axes of stars in young open clusters are aligned. Assuming that the spin vectors lie uniformly within a conical section, with an opening half-angle between (perfectly aligned) and (completely random), we describe a Monte Carlo modelling technique that returns a probability density for this opening angle given a set of measured values, where i is the unknown inclination angle between a stellar spin vector and the line of sight. Using simulations we demonstrate that although azimuthal information is lost, it is easily possible to discriminate between strongly aligned spin axes and a random distribution, providing that the mean spin-axis inclination lies outside the range . We apply the technique to G- and K-type stars in the young Pleiades and Alpha Per clusters. The values are derived using rotation periods and projected equatorial velocities, combined with radii estimated from the cluster distances and a surface brightness/colour relationship. For both clusters we find no evidence for spin-axis alignment: is the most probable model and with 90 per cent confidence. Assuming a random spin-axis alignment, we redetermine the distances to both clusters, obtaining for the Pleiades and for Alpha Per. If the assumption of random spin-axis alignment is discarded however, whilst the distance estimate remains unchanged, it has an additional per cent uncertainty. [ABSTRACT FROM AUTHOR]

We have used fibre spectroscopy to establish cluster membership and examine pre-main-sequence (PMS) lithium depletion for low-mass stars (spectral types F to M) in the sparse young cluster IC 4665. We present a filtered candidate list of 40 stars that should contain 75 per cent of single cluster members with in the central square degree of the cluster. Whilst F- and G-type stars in IC 4665 have depleted little or no lithium, the K- and early M-type stars have depleted more Li than expected when compared with similar stars in other clusters of known age. An empirical age estimate based on Li depletion among the late-type stars of IC 4665 would suggest that it is older than 100 Myr. This disagrees entirely with ages determined either from the nuclear turn-off, from isochronal matches to low-mass stars or from the re-appearance of lithium previously found in much lower mass stars (the ‘lithium depletion boundary’). We suggest that other parameters besides age, perhaps composition or rotation, are very influential in determining the degree of PMS Li depletion in stars with . Further work is required to identify and assess the effects of these additional parameters, particularly to probe conditions at the interface between the subphotospheric convection zone and developing radiative core. Until then, PMS Li depletion in F- to early M-type stars cannot be confidently used as a precise age indicator in young clusters, kinematic groups or individual field stars. [ABSTRACT FROM AUTHOR]

Using a novel technique, which combines previously determined rotation periods with new spectroscopic determinations of projected rotation velocity, we have determined radii for fast-rotating, low-mass M-dwarfs in the young, solar-metallicity open cluster, NGC 2516. The mean radii are larger than model predictions at a given absolute I or K magnitude and also larger than the measured radii of magnetically inactive M-dwarfs; the difference increases from a few per cent to 50 per cent for the lowest luminosity stars in our sample. We show that a simple two-temperature starspot model is broadly capable of explaining these observations, but requires spot coverage fractions of about 50 per cent in rapidly rotating M-dwarfs. [ABSTRACT FROM AUTHOR]

We present the results of a photometric BVI survey of 0.9 deg2 around the Wolf–Rayet binary Vel and its early-type common proper motion companion Vel (together referred to as the γ 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, Hα emission and high levels of X-ray activity. We show that the PMS stars are kinematically coherent and spatially concentrated around γ Vel. The PMS stars have similar proper motions to γ Vel, to main-sequence (MS) stars around γ Vel and to early-type stars of the wider Vela OB2 association of which Vel is the brightest member. The ratio of MS stars to low-mass PMS stars is consistent with a Kroupa mass function. MS fitting to stars around γ Vel gives an association distance modulus of , which is consistent with a similarly determined distance for Vela OB2 and also with interferometric distances to Vel. High-mass stellar models indicate an age of 3–4 Myr for 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 γ Vel, and this is a subcluster within the larger Vela OB2 association. We speculate that 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. [ABSTRACT FROM AUTHOR]

NGC 6611 is the massive young cluster (2–3 Myr) that ionizes the Eagle Nebula. We present very deep photometric observations of the central region of NGC 6611 obtained with the Hubble Space Telescope and the following filters: ACS/WFC F775W and F850LP and NIC2 F110W and F160W, loosely equivalent to ground-based IZJH filters. This survey reaches down to . We construct the initial mass function (IMF) from well into the brown dwarf regime (down to ). We have detected 30–35 brown dwarf candidates in this sample. The low-mass IMF is combined with a higher-mass IMF constructed from the ground-based catalogue from Oliveira et al. We compare the final IMF with those of well-studied star-forming regions: we find that the IMF of NGC 6611 more closely resembles that of the low-mass star-forming region in Taurus than that of the more massive Orion Nebula Cluster. We conclude that there seems to be no severe environmental effect in the IMF due to the proximity of the massive stars in NGC 6611. [ABSTRACT FROM AUTHOR]

We have obtained multi-epoch, high-resolution spectroscopy of 218 candidate low-mass stars and brown dwarfs (BDs) in the young clusters around σ Ori and λ Ori. We find that 196 targets are cluster members based on their radial velocity, the equivalent width of their Na i 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 (SB1) in which the companion is faint or the spectra of the stars are blended. There are three narrow-lined SB1 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 . The sample covers the magnitude range (mass ), but all of the binary stars are brighter than (mass ) and 10 are brighter than (mass ). 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 IC magnitude with more than 99 per cent confidence. The spectroscopic binary fraction for stars more massive than about is . The 90 per cent confidence upper limit to the spectroscopic binary fraction for very low-mass (VLM) stars (mass ) and BDs is per cent. The hypothesis that fbright and ffaint 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 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. [ABSTRACT FROM AUTHOR]

The radii of young pre-main-sequence (PMS) stars in the Orion Nebula Cluster (ONC) have been estimated using their rotation periods and projected equatorial velocities. Stars at a given effective temperature have a spread in their geometrically estimated projected radii, that is larger than can be accounted for with a coeval model, observational uncertainties and randomly oriented rotation axes. It is shown that the required dispersion in radius (a factor of 2–3 full width at half-maximum) can be modelled in terms of a spread in stellar ages larger than the median age of the cluster, although the detailed star formation history cannot be uniquely determined using present data. This technique is relatively free from systematic uncertainties (binarity, extinction, variability, distance) that have hampered previous studies of the ONC star formation history using the conventional Hertzsprung–Russell diagram. However, the current ONC rotational data are biased against low-luminosity objects, so the deduced dispersions in radius and inferred age are probably underestimates. In particular, the ages of a tail of PMS stars that appear to be ≥10Myr old in the Hertzsprung–Russell diagram cannot be verified with present data. If projected equatorial velocities were measured for these objects it could easily be checked whether their radii are correspondingly smaller than the bulk of the ONC population. [ABSTRACT FROM AUTHOR]

The distance to the Orion Nebula cluster (ONC) is estimated using the rotational properties of its low-mass pre-main-sequence (PMS) stars. Rotation periods, projected equatorial velocities and distance-dependent radius estimates are used to form an observational sin i distribution (where i is the axial inclination), which is modelled to obtain the distance estimate. A distance of is found from a sample of 74 PMS stars with spectral types between G6 and M2, but this falls to when PMS stars with accretion discs are excluded on the basis of their near-infrared excess. Since the radii of accreting stars are more uncertain and probably systematically underestimated, then this closer distance is preferred. The quoted uncertainties include statistical errors and uncertainties due to a number of systematic effects including binarity and inclination bias. This method is geometric and independent of stellar evolution models, though does rely on the assumption of random axial orientations and the Cohen & Kuhi effective temperature scale for PMS stars. The new distance is consistent with, although lower and more precise, than most previous ONC distance estimates. A closer ONC distance implies smaller luminosities and an increased age based on the positions of PMS stars in the Hertzsprung–Russell diagram. [ABSTRACT FROM AUTHOR]

We have used RCIC CCD photometry from the Isaac Newton telescope and intermediate-resolution spectroscopy from the Gemini North telescope to identify and characterize low-mass pre-main-sequence stars in the young open cluster NGC 2169. Isochrone fitting to the high- and low-mass populations yields an intrinsic distance modulus of 10.13+0.06−0.09 mag and a model-dependent age of 9 ± 2 Myr. Compared to the nearby, kinematically defined groups of a similar age, NGC 2169 has a large low-mass population which potentially offers a more precise statistical investigation of several aspects of star formation and early stellar evolution. By modelling the distribution of low-mass stars in the IC versus RC− IC diagram, we find that any age spread among cluster members has a Gaussian full width at half-maximum (FWHM) ≤ 2.5 Myr. A young age and a small age spread (<10 Myr) are supported by the lack of significant lithium depletion in the vast majority of cluster members. There is no clear evidence for accretion or warm circumstellar dust in the low-mass members of NGC 2169, bolstering the idea that strong accretion has ceased and inner discs have dispersed in almost all low-mass stars by ages of 10 Myr. [ABSTRACT FROM AUTHOR]

We have selected pre-main-sequence (PMS) stars in 12 groups of notional ages ranging from 1 to 35 Myr, using heterogeneous membership criteria. Using these members we have constructed empirical isochrones in V, V− I colour–magnitude diagrams. This allows us to identify clearly the gap between the radiative main sequence and the convective PMS (the R–C gap). We follow the evolution of this gap with age and show that it can be a useful age indicator for groups less than ≃15 Myr old. We also observe a reduction in absolute spreads about the sequences with age. Finally, the empirical isochrones allow us to place the groups in order of age, independently of theory. The youngest groups can be collated into three sets of similar ages. The youngest set is the ONC, NGC 6530 and IC 5146 (nominally 1 Myr); next Cep OB3b, NGC 2362, λ Ori and NGC 2264 (nominally 3 Myr); and finally σ Ori and IC 348 (nominally 4–5 Myr). This suggests Cep OB3b is younger than previously thought, and IC 348 older. For IC 348 the stellar rotation rate distribution and fraction of stars with discs imply a younger age than we derive. We suggest this is because of the absence of O-stars in this cluster, whose winds and/or ionizing radiation may be an important factor in the removal of discs in other clusters. [ABSTRACT FROM AUTHOR]

We present a maximum-likelihood method for fitting two-dimensional model distributions to stellar data in colour–magnitude space. This allows one to include (for example) binary stars in an isochronal population. The method also allows one to derive formal uncertainties for fitted parameters, and assess the likelihood that a good fit has been found. We use the method to derive an age of and a true distance modulus of from the V versus V− I diagram of NGC 2547 (the uncertainties are 67 per cent confidence limits, and the parameters are insensitive to the assumed binary fraction). These values are consistent with those previously determined from low-mass isochronal fitting, and are the first measurements to have statistically meaningful uncertainties. The age is also consistent with the lithium depletion age of NGC 2547, and the Hipparcos distance to the cluster is consistent with our value. The method appears to be quite general and could be applied to any N-dimensional data set, with uncertainties in each dimension. However, it is particularly useful when the data are sparse, in the sense that both the typical uncertainties for a data point and the size of structure in the function being fitted are small compared with the typical distance between data points. In this case binning the data will lose resolution, whilst the method presented here preserves it. Software implementing the methods described in this paper is available from . [ABSTRACT FROM AUTHOR]

We have used precise radial velocity measurements for a large number of candidate low-mass stars and brown dwarfs to show that the young σ Ori ‘cluster’ consists of two spatially superimposed components which are kinematically separated by 7 km s−1 in radial velocity, and which have different mean ages. We examine the relationship of these two kinematic groups to other populations in the Orion OB1 association, and briefly discuss the consequence of mixed age samples for ongoing investigations of the formation and evolution of low-mass objects in this much-observed region. [ABSTRACT FROM AUTHOR]

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 . 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. [ABSTRACT FROM AUTHOR]

We report on XMM–Newton observations of the young open cluster NGC 2547 which allow us to characterize coronal activity in solar-type stars, and stars of lower mass, at an age of 30 Myr. X-ray emission is seen from stars at all spectral types, peaking among G stars at luminosities (0.3–3 keV) of and declining to among M stars with masses . Coronal spectra show evidence for multi-temperature differential emission measures and low coronal metal abundances of . The G- and K-type stars of NGC 2547 follow the same relationship between X-ray activity and Rossby number established in older clusters and field stars, although most of the solar-type stars in NGC 2547 exhibit saturated or even supersaturated X-ray activity levels. The median levels of Lx and in the solar-type stars of NGC 2547 are very similar to those in T-Tauri stars of the Orion Nebula cluster (ONC), but an order of magnitude higher than in the older Pleiades. The spread in X-ray activity levels among solar-type stars in NGC 2547 is much smaller than in older or younger clusters. Coronal temperatures increase with and surface X-ray flux. The most active solar-type stars in NGC 2547 have coronal temperatures intermediate between those in the ONC and the most active older zero-age main-sequence (ZAMS) stars. We show that simple scaling arguments predict higher coronal temperature in coronally saturated stars with lower gravities. A number of candidate flares were identified among the low-mass members and a flaring rate [for total flare energies ] of one every was found for solar-type stars, which is similar to rates found in the ONC and Pleiades. Comparison with ROSAT High Resolution Imager (HRI) data taken 7 yr earlier reveals that only 10–15 per cent of solar-type stars or stars with exhibit X-ray variability by more than a factor of 2. This is comparable with clusters of similar age but less than in both older and younger clusters. The similar median levels of X-ray activity and rate of occurrence for large flares in NGC 2547 and the ONC demonstrate that the X-ray radiation environment around young solar-type stars remains relatively constant over their first 30 Myr. [ABSTRACT FROM AUTHOR]

High-resolution spectroscopy is used to determine the detailed chemical abundances of a group of eight F- and G-type stars in the young open cluster Blanco 1. An average [Fe/H] of (internal error) ± 0.04 (external error) is found, considerably lower than a previous spectroscopic estimate for this cluster. The difference is due mainly to our adoption of significantly cooler temperatures that are consistent with both photometric and spectroscopic constraints. Blanco 1 exhibits subsolar [Ni/Fe] , [Si/Fe] , [Mg/Fe] and [Ca/Fe] ; ratios that are not observed among nearby field stars. The material from which Blanco 1 formed may not have been well mixed with interstellar matter in the galactic disc, which tallies with its current location about 240 pc below the galactic plane. A simultaneous deficit of Ni and alpha elements with respect to Fe is hard to reconcile with most published models of yields from supernovae of types Ia and II. The revised abundances for Blanco 1 indicate that overall radiative opacities in its stars, and hence convective zone properties at a given mass, are similar to those in the Pleiades at approximately the same age. This can explain a previous observation that the Li-depletion patterns of G- and K-type stars in the two clusters are indistinguishable. The lower overall metallicity of Blanco 1 now makes it less attractive as a target for discovering transiting, short-period exoplanets. [ABSTRACT FROM AUTHOR]