A Kinematically Unbiased Search for Nearby Young Stars in the Northern Hemisphere Selected Using SuperWASP Rotation Periods

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 days in the SuperWASP All-Sky Survey and X-ray counterparts in the ROSAT All-Sky Survey were chosen to create a catalog 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 (RV) measurements were used to identify binarity. Twenty-six 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-alpha and v\sin i support their youthful status. Based on their youth, their radial velocities and estimates of their 3-dimensional kinematics, we find 11 objects that may be members of known MGs, 8 that do not appear associated with any young MG and a further 7 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

The low mass population of the Vela OB2 association from Gaia

The first Gaia Data Release presents an opportunity to characterise 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 (PMS) stars over the wider association area. Our method picks out the known clusters of young stars around Velorum and NGC-2547, but also identifies other over-densities 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

Searching for new young stars in the northern hemisphere: The Pisces Moving Group

Using the kinematically unbiased technique described in Binks, Jeffries & Maxted (2015), we present optical spectra for a further 122 rapidly-rotating (rotation periods <6 days), 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 km s−1 in each Galactic space velocity coordinate. The group members are approximately co-eval in the colour-magnitude diagram, with an age of 30–50 Myr, and have similar, though not identical, kinematics to the Octans-Near MG. stars: low-mass, stars: pre-main-sequenc

The inflated radii of M-dwarfs in the Pleiades

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 6 M/M⊙ 6 0.8), fastrotating 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 the evolutionary models of Dotter et al. (2008) and Baraffe et al. (2015). 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 starspots 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

Spectroscopic Evidence for Starspots on the Secondary Star of SS Cygni

This is the author accepted manuscript. The final version is available from American Astronomical Society via the DOI in this recordCataclysmic variables are interacting binary systems where a cool, rapidly rotating secondary star passes material to a white dwarf. If this mass loss is to continue, then there must be continuous angular momentum loss from the system. By analogy with the Sun and other cool stars, it has been assumed that magnetic braking is responsible, angular momentum being carried away by an ionized wind from the secondary star, threading a dynamo-generated magnetic field. We have discovered TiO absorption in the spectrum of SS Cyg, whose secondary star should be too hot to show such features. The most likely explanation of its presence is cool star spots caused by the strong (0.1 T) fields required by the magnetic braking theories

No evidence for intense, cold accretion on to YSOs from measurements of Li in T-Tauri stars

We have used medium-resolution spectra to search for evidence that proto-stellar objects accrete at high rates during their early 'assembly phase'. Models predict that depleted lithium and reduced luminosity in T-Tauri stars are key signatures of 'cold' high-rate accretion occurring early in a star's evolution. We found no evidence in 168 stars in NGC 2264 and the Orion nebula cluster for strong lithium depletion through analysis of veiling-corrected 6708Å lithium spectral line strengths. This suggests that 'cold' accretion at high rates (M = 5 × 10-4 M⊙ yr-1) occurs in the assembly phase of fewer than 0.5 per cent of 0.3 = M⊙ = 1.9M⊙ stars. We also find that the dispersion in the strength of the 6708Å lithium line might imply an age spread that is similar in magnitude to the apparent age spread implied by the luminosity dispersion seen in colour-magnitude diagrams. Evidence for weak lithium depletion (<10 per cent in equivalent width) that is correlated with luminosity is also apparent, but we are unable to determine whether age spreads or accretion at rates less than 5 × 10-4 M⊙ yr-1 are responsible. ©2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.DJS is funded by a UK Science and Technology Facilities Council (STFC) studentship. The authors wish to thank Isabelle Baraffe for providing cold accretion models and useful discussions. Spectra were extracted and calibrated using the AF2 pipeline developed by Richard Jackson. This research is based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group (ING) in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. This research has made use of archival data products from the Two-Micron All-Sky Survey (2MASS), which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration (NASA) and the National Science Foundation

A lithium depletion boundary age of 22 Myr for NGC 1960

We present a deep Cousins RI photometric survey of the open cluster NGC 1960, complete to R_C \simeq 22, I_C \simeq 21, that is used to select a sample of very low-mass cluster candidates. Gemini spectroscopy of a subset of these is used to confirm membership and locate the age-dependent "lithium depletion boundary" (LDB) --the luminosity at which lithium remains unburned in its low-mass stars. The LDB implies a cluster age of 22 +/-4 Myr and is quite insensitive to choice of evolutionary model. NGC 1960 is the youngest cluster for which a LDB age has been estimated and possesses a well populated upper main sequence and a rich low-mass pre-main sequence. The LDB age determined here agrees well with precise age estimates made for the same cluster based on isochrone fits to its high- and low-mass populations. The concordance between these three age estimation techniques, that rely on different facets of stellar astrophysics at very different masses, is an important step towards calibrating the absolute ages of young open clusters and lends confidence to ages determined using any one of them.Based on observations made with the INT operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina). CPB acknowledges receipt of a Science and Technology Facilities Council postgraduate studentship. SPL is supported by a RCUK fellowship

Multiple kinematical populations in Vela OB2 from Gaia DR1 data

Context. Recent results using radial-velocity measurements from the Gaia-ESO Survey have led to the discovery of multiple kinematic populations across the Vela OB2 association. We present here a proper-motion study of the same region. Aims. Our aim is to test whether or not the radial-velocity populations have a counterpart in proper-motion space, and if so, how the two sets of kinematical data complement each other. Methods. This work is based on parallaxes and proper motions from the TGAS catalogue, as part of Gaia DR1. Results. Two distinct proper-motion populations are found dispersed across ~ 5 degrees (or ~ 30 pc at their likely distances). Their detailed correspondence to the radial-velocity populations could not be tested because of the paucity of common objects. However, compelling indications are found that one of the new proper-motion populations consists mostly of members of the young cluster NGC 2547, and the other is related to the 2 Vel cluster. Constraints on the age of the two populations, both of which appear to be only 1

A survey for low mass spectroscopic binary stars in the young clusters around sigma Orionis and lambda Orionis

We have obtained multi-epoch, high-resolution spectroscopy of 218 candidate low-mass stars and brown dwarfs in the young clusters around sigma Ori and lambda Ori. We find that 196 targets are cluster members based on their radial velocity, the equivalent width of their NaI 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 line width and shape. The sample covers the magnitude range Ic=14-18.9 (mass =~ 0.55-0.03 Msun), but all of the binary stars are brighter than Ic=16.6 (mass =~ 0.12Msun) and 10 are brighter than Ic=15.5 (mass =~ 0.23Msun). 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% confidence. The spectroscopic binary fraction for stars more massive than about 0.1Msun (Ic < 16.9) is f_bright=0.095(+0.012)(-0.028). The 90% confidence upper limit to the spectroscopic binary fraction for very low mass (VLM) stars (mass < 0.1Msun) and brown dwarfs (BDs) is f_faint < 7.5%. The hypothesis that f_bright and f_faint are equal can be rejected with 90% confidence. We conclude that we have found strong evidence for a change in the fraction of spectroscopic binaries among young VLM stars and brown dwarfs 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 semi-major axes, or both. (Abridged

Pre-main-sequence isochrones - II. Revising star and planet formation time-scales

archiveprefix: arXiv primaryclass: astro-ph.SR keywords: techniques: photometric, stars: evolution, stars: formation, stars: fundamental parameters, Hertzsprung-Russell and colour-magnitude diagrams, stars: pre-main-sequence adsurl: http://adsabs.harvard.edu/abs/2013MNRAS.434..806B adsnote: Provided by the SAO/NASA Astrophysics Data SystemWe have derived ages for 13 young (<30 Myr) star-forming regions and find that they are up to a factor of 2 older than the ages typically adopted in the literature. This result has wide-ranging implications, including that circumstellar discs survive longer (≃ 10–12 Myr) and that the average Class I lifetime is greater (≃1 Myr) than currently believed. For each star-forming region, we derived two ages from colour–magnitude diagrams. First, we fitted models of the evolution between the zero-age main sequence and terminal-age main sequence to derive a homogeneous set of main-sequence ages, distances and reddenings with statistically meaningful uncertainties. Our second age for each star-forming region was derived by fitting pre-main-sequence stars to new semi-empirical model isochrones. For the first time (for a set of clusters younger than 50 Myr), we find broad agreement between these two ages, and since these are derived from two distinct mass regimes that rely on different aspects of stellar physics, it gives us confidence in the new age scale. This agreement is largely due to our adoption of empirical colour–Teff relations and bolometric corrections for pre-main-sequence stars cooler than 4000 K. The revised ages for the star-forming regions in our sample are: ∼2 Myr for NGC 6611 (Eagle Nebula; M 16), IC 5146 (Cocoon Nebula), NGC 6530 (Lagoon Nebula; M 8) and NGC 2244 (Rosette Nebula); ∼6 Myr for σ Ori, Cep OB3b and IC 348; ≃10 Myr for λ Ori (Collinder 69); ≃11 Myr for NGC 2169; ≃12 Myr for NGC 2362; ≃13 Myr for NGC 7160; ≃14 Myr for χ Per (NGC 884); and ≃20 Myr for NGC 1960 (M 36).CPMB is funded by a UK Science and Technology Facilities Council (STFC) studentship. SPL is supported by an RCUK fellowship. The authors would like to thank Charles D. H. Williams for maintaining the Xgrid facilities at the University of Exeter which were used to reduce the photometric data presented in this study. The authors thank Amelia Bayo for bringing to our attention the important work on the λ Ori region published in Bayo et al. (2011) and Bayo et al. (2012) which we overlooked in our original submission. The inclusion of these works does not change the results or conclusions of the paper. The authors also thank the referee for useful comments and constructive suggestions that have greatly improved this work. This research has made use of data obtained at the Isaac Newton Telescope which is operated on the island of La Palma by the Isaac Newton Group (ING) in the Spanish Observatorio del Roque de los Muchachos of the Institutio de Astrofisica de Canarias. This research has also made use of archival data products from the Two-Micron All-Sky Survey (2MASS), which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration (NASA) and the National Science Foundation