Simulation of Stellar Objects in SDSS Color Space

We present a simulation of the spatial, luminosity and spectral distributions of four types of stellar objects. We simulate: (1) Galactic stars, based on a Galactic structure model, a stellar population synthesis model, stellar isochrones, and stellar spectral libraries; (2) white dwarfs, based on model atmospheres, the observed luminosity function, mass distribution, and Galactic distribution of white dwarfs; (3) quasars, based on their observed luminosity function and its evolution, and models of emission and absorption spectra of quasars; and (4) compact emission line galaxies, based on the observed distribution of their spectral properties and sizes. The results are presented in the color system of the Sloan Digital Sky Survey (SDSS), with realistic photometric error and Galactic extinction. The simulated colors of stars and quasars are compared with observations in the SDSS system and show good agreement. The stellar simulation can be used as a tool to analyze star counts and constrain models of Galactic structure, as well as to identify stars with unusual colors. The simulation can also be used to establish the quasar target selection algorithm for the SDSS.Comment: 52 pages, 22 figures, to appear in AJ; simulated catalogs available at http://www.astro.princeton.edu/~fan/sdss_simu.htm

S4N: A Spectroscopic Survey of Stars in the Solar Neighborhood

[ABRIDGED] We report the results of a high-resolution spectroscopic survey of all the stars more luminous than Mv = 6.5 mag within 14.5 pc from the Sun. We derive stellar parameters and perform a preliminary abundance and kinematic analysis of the F-G-K stars in the sample. The inferred metallicity ([Fe/H]) distribution is centered at about -0.1 dex, and shows a standard deviation of 0.2 dex. We identify a number of metal-rich K-type stars which appear to be very old, confirming the claims for the existence of such stars in the solar neighborhood. With atmospheric effective temperatures and gravities derived independently of the spectra, we find that our classical LTE model-atmosphere analysis of metal-rich (and mainly K-type) stars provides discrepant abundances from neutral and ionized lines of several metals. Based on transitions of majority species, we discuss abundances of 16 chemical elements. In agreement with earlier studies we find that the abundance ratios to iron of Si, Sc, Ti, Co, and Zn become smaller as the iron abundance increases until approaching the solar values, but the trends reverse for higher iron abundances. At any given metallicity, stars with a `low' galactic rotational velocity tend to have high abundances of Mg, Si, Ca, Sc, Ti, Co, Zn, and Eu, but low abundances of Ba, Ce, and Nd. The Sun appears deficient by roughly 0.1 dex in O, Si, Ca, Sc, Ti, Y, Ce, Nd, and Eu, compared to its immediate neighbors with similar iron abundances.Comment: 24 pages, 19 figures, to appear in A&A; data can be accessed from http://hebe.as.utexas.edu/s4n/ or http://www.astro.uu.se/~s4n

The stellar and sub-stellar IMF of simple and composite populations

The current knowledge on the stellar IMF is documented. It appears to become top-heavy when the star-formation rate density surpasses about 0.1Msun/(yr pc^3) on a pc scale and it may become increasingly bottom-heavy with increasing metallicity and in increasingly massive early-type galaxies. It declines quite steeply below about 0.07Msun with brown dwarfs (BDs) and very low mass stars having their own IMF. The most massive star of mass mmax formed in an embedded cluster with stellar mass Mecl correlates strongly with Mecl being a result of gravitation-driven but resource-limited growth and fragmentation induced starvation. There is no convincing evidence whatsoever that massive stars do form in isolation. Various methods of discretising a stellar population are introduced: optimal sampling leads to a mass distribution that perfectly represents the exact form of the desired IMF and the mmax-to-Mecl relation, while random sampling results in statistical variations of the shape of the IMF. The observed mmax-to-Mecl correlation and the small spread of IMF power-law indices together suggest that optimally sampling the IMF may be the more realistic description of star formation than random sampling from a universal IMF with a constant upper mass limit. Composite populations on galaxy scales, which are formed from many pc scale star formation events, need to be described by the integrated galactic IMF. This IGIMF varies systematically from top-light to top-heavy in dependence of galaxy type and star formation rate, with dramatic implications for theories of galaxy formation and evolution.Comment: 167 pages, 37 figures, 3 tables, published in Stellar Systems and Galactic Structure, Vol.5, Springer. This revised version is consistent with the published version and includes additional references and minor additions to the text as well as a recomputed Table 1. ISBN 978-90-481-8817-

The Age-Metallicity Relation in the Thin Disk of the Galaxy

HST trigonometric distances, photometric metallicities, isochronic ages from the second revised version of the Geneva--Copenhagen survey, and uniform spectroscopic Fe and Mg abundances from our master catalog are used to construct and analyze the age--metallicity and age-relative Mg abundance relations for stars of the thin disk. The influences of selection effects are discussed in detail. It is demonstrated that the radial migration of stars does not lead to appreciable distortions in the age dependence of the metallicity. During the first several billion years of the formation of the thin disk, the interstellar material in this disk was, on average, fairly rich in heavy elements ( ~-0.2) and poorly mixed. However, the metallicity dispersion continuously decreased with age, from \sigma_{[Fe/H]}~0.22 to ~0.13. All this time, the mean relative abundance of Mg was somewhat higher than the solar value (~0.1). Roughly four to five billion years ago, the mean metallicity began to systematically increase, while retaining the same dispersion; the mean relative Mg abundance began to decrease immediately following this. The number of stars in this subsystem increased sharply at the same time. These properties suggest that the star-formation rate was low in the initial stage of formation of the thin disk, but abruptly increased about four to five billion years ago.Comment: 16 page, 7 figures, accepted 2011, Astron. Rep., v.55, No.8, p.667-68

Effect of hygroscopic growth on the aerosol light-scattering coefficient: A review of measurements, techniques and error sources

Knowledge of the scattering enhancement factor, f(RH), is important for an accurate description of direct aerosol radiative forcing. This factor is defined as the ratio between the scattering coefficient at enhanced relative humidity, RH, to a reference (dry) scattering coefficient. Here, we review the different experimental designs used to measure the scattering coefficient at dry and humidified conditions as well as the procedures followed to analyze the measurements. Several empirical parameterizations for the relationship between f(RH) and RH have been proposed in the literature. These parameterizations have been reviewed and tested using experimental data representative of different hygroscopic growth behavior and a new parameterization is presented. The potential sources of error in f(RH) are discussed. A Monte Carlo method is used to investigate the overall measurement uncertainty, which is found to be around 20 e40% for moderately hygroscopic aerosols. The main factors contributing to this uncertainty are the uncertainty in RH measurement, the dry reference state and the nephelometer uncertainty. A literature survey of nephelometry-based f(RH) measurements is presented as a function of aerosol type. In general, the highest f(RH) values were measured in clean marine environments, with pollution having a major influence on f(RH). Dust aerosol tended to have the lowest reported hygroscopicity of any of the aerosol types studied. Major open questions and suggestions for future research priorities are outlined.This work was supported by the Andalusia Regional Government through projects P10-RNM-6299 and P12-RNM-2409; by the Spanish Ministry of Economy and Competitiveness and FEDER through project CGL2013_45410-R; and by European Union’s Horizon 2020 research and innovation programme under grant agreement No 654109, ACTRIS-2. G. Titos was partially funded by Programa del Plan Propio de Investigación “Contrato Puente” of the University of Granada. We thank the Stockholm International Meteorological Institute (IMI) for travel support of G. Titos

The Gaia-ESO Survey::the Galactic Thick to Thin Disc transition

(Abridged) We have used the atmospheric parameters, [alpha/Fe] abundances and radial velocities, determined from the Gaia-ESO Survey GIRAFFE spectra of FGK-type stars (iDR1), to provide a chemo-kinematical characterisation of the disc stellar populations. We focuss on a subsample of 1016 stars with high quality parameters, covering the volume |Z|<4.5kpc and R in the range 2-13kpc. We have identified a thin to thick disc separation in the [alpha/Fe] vs [M/H] plane, thanks to the presence of a low-density region in the number density distribution. The thick disc stars seem to lie in progressively thinner layers above the Galactic plane, as metallicity increases and [alpha/Fe] decreases. The thin disc population presents a constant value of the mean distance to the plane at all metallicities. Our data confirm the already known correlations between V_phi and [M/H] for the two discs. For the thick disc sequence, a study of the possible contamination by thin disc stars suggests a gradient up to 64km/s/dex. The distributions of V_phi, V_Z, and orbital parameters are analysed for the chemically separated samples. Concerning the gradients with galactocentric radius, we find for the thin disc a flat behaviour of V_phi, a [M/H] gradient of -0.058dex/kpc and a small positive [alpha/Fe] gradient. For the thick disc, flat gradients in [M/H] and [alpha/Fe] are derived. Our chemo-kinematical analysis suggests a picture in which the thick disc seems to have experienced a settling process, during which its rotation increased progressively, and, possibly, the V_phi dispersion decreased. At [M/H]-0.25dex and [alpha/Fe]0.1dex, the mean characteristics of the thick disc in distance to the Galactic plane, V_phi, V_phi dispersion and eccentricity agree with those of the thin disc stars, suggesting a possible connection between these populations at a certain epoch of the disc evolution.Comment: Accepted for publication in Astronomy and Astrophysic

Discovery of the benchmark metal poor T8 dwarf BD+01 2920B

We have searched the WISE first data release for widely separated (<10,000AU) late T dwarf companions to Hipparcos and Gliese stars. We have discovered a new binary system containing a K-band suppressed T8p dwarf WISEP J1423+0116 and the mildly metal poor ([Fe/H]=-0.38+-0.06) primary BD+01 2920 (Hip 70319), a G1 dwarf at a distance of 17.2pc. This new benchmark has Teff=680+-55K and a mass of 20-50 Mjup. Its spectral properties are well modelled except for known discrepancies in the Y and K bands. Based on the well determined metallicity of its companion, the properties of BD+01 2920B imply that the currently known T dwarfs are dominated by young low-mass objects. We also present an accurate proper motion for the T8.5 dwarf WISEP J075003.84+272544.8.Comment: MNRAS, accepted 2012 January 1

Dynamics of Disks and Warps

This chapter reviews theoretical work on the stellar dynamics of galaxy disks. All the known collective global instabilities are identified, and their mechanisms described in terms of local wave mechanics. A detailed discussion of warps and other bending waves is also given. The structure of bars in galaxies, and their effect on galaxy evolution, is now reasonably well understood, but there is still no convincing explanation for their origin and frequency. Spiral patterns have long presented a special challenge, and ideas and recent developments are reviewed. Other topics include scattering of disk stars and the survival of thin disks.Comment: Chapter accepted to appear in Planets, Stars and Stellar Systems, vol 5, ed G. Gilmore. 32 pages, 17 figures. Includes minor corrections made in proofs. Uses emulateapj.st

Prognostic model to predict postoperative acute kidney injury in patients undergoing major gastrointestinal surgery based on a national prospective observational cohort study.

Background: Acute illness, existing co-morbidities and surgical stress response can all contribute to postoperative acute kidney injury (AKI) in patients undergoing major gastrointestinal surgery. The aim of this study was prospectively to develop a pragmatic prognostic model to stratify patients according to risk of developing AKI after major gastrointestinal surgery. Methods: This prospective multicentre cohort study included consecutive adults undergoing elective or emergency gastrointestinal resection, liver resection or stoma reversal in 2-week blocks over a continuous 3-month period. The primary outcome was the rate of AKI within 7 days of surgery. Bootstrap stability was used to select clinically plausible risk factors into the model. Internal model validation was carried out by bootstrap validation. Results: A total of 4544 patients were included across 173 centres in the UK and Ireland. The overall rate of AKI was 14·2 per cent (646 of 4544) and the 30-day mortality rate was 1·8 per cent (84 of 4544). Stage 1 AKI was significantly associated with 30-day mortality (unadjusted odds ratio 7·61, 95 per cent c.i. 4·49 to 12·90; P < 0·001), with increasing odds of death with each AKI stage. Six variables were selected for inclusion in the prognostic model: age, sex, ASA grade, preoperative estimated glomerular filtration rate, planned open surgery and preoperative use of either an angiotensin-converting enzyme inhibitor or an angiotensin receptor blocker. Internal validation demonstrated good model discrimination (c-statistic 0·65). Discussion: Following major gastrointestinal surgery, AKI occurred in one in seven patients. This preoperative prognostic model identified patients at high risk of postoperative AKI. Validation in an independent data set is required to ensure generalizability