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Solar Neighbourhood
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Title: The Solar Neighborhood XXXVIII. Results from the CTIO/SMARTS 0.9m: Trigonometric Parallaxes for 151 Nearby M Dwarf Systems
Author: Jennifer G. Winters, R. Andrew Sevrinsky, Wei-Chun Jao, Todd J. Henry, Adric R. Riedel, John P. Subasavage, John C. Lurie, Philip A. Ianna

We present 160 new trigonometric parallaxes for 151 M dwarf systems from the REsearch Consortium On Nearby Stars (RECONS) group's long-term astrometry/photometry program at the CTIO/SMARTS 0.9m telescope. Most systems (124 or 82%) are found to lie within 25 pc. The stars have 119 mas/yr < mu < 828 mas/yr and 3.85 < (V-K) < 8.47. Among these are 58 systems from the SuperCOSMOS-RECONS (SCR) search, discovered via our proper motion trawls of the SuperCOSMOS digitized archival photographic plates, while the remaining stars were suspected via photometric distance estimates to lie nearby. Sixteen are systems that are newly discovered via astrometric perturbations to be binaries, many of which are ideal for accurate mass determinations due to their proximity and orbital periods on the order of a decade. A variability analysis of the stars presented, two-thirds of which are new results, shows six of the stars to vary by more than 20 mmag. This effort brings the total number of parallaxes for M dwarf systems measured by RECONS to nearly 500 and increases by 26% the number of southern M dwarf systems with accurate trigonometric parallaxes placing them within 25 pc.

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Title: On the relevance of chaos for halo stars in the Solar Neighbourhood
Author: Nicolßs P. Maffione, Facundo A. Gˇmez, Pablo M. Cincotta, Claudia M. Giordano, Andrew P. Cooper, Brian W. O'Shea

We show that diffusion due to chaotic mixing in the Neighbourhood of the Sun may not be as relevant as previously suggested in erasing phase space signatures of past Galactic accretion events. For this purpose, we analyse Solar Neighbourhood-like volumes extracted from cosmological simulations that naturally account for chaotic orbital behaviour induced by the strongly triaxial and cuspy shape of the resulting dark matter haloes, among other factors. In the approximation of an analytical static triaxial model, our results show that a large fraction of stellar halo particles in such local volumes have chaos onset times (i.e., the timescale at which stars commonly associated with chaotic orbits will exhibit their chaotic behaviour) significantly larger than a Hubble time. Furthermore, particles that do present a chaotic behaviour within a Hubble time do not exhibit significant diffusion in phase space.

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Title: The Solar Neighbourhood XXXVI: The Long-Term Photometric Variability of Nearby Red Dwarfs in the VRI Optical Bands
Author: Altonio D. Hosey, Todd J. Henry, Wei-Chun Jao, Sergio B. Dieterich, Jennifer G. Winters, John C. Lurie, Adric R. Riedel, John P. Subasavage

We present an analysis of long-term photometric variability for nearby red dwarfs at optical wavelengths. The sample consists of 264 M dwarfs south of DEC = +30 with V-K = 3.96-9.16 and Mv~10-20 (spectral types M2V-M8V), most of which are within 25 pc. The stars have been observed in the VRI filters for ~4-14 years at the CTIO/SMARTS 0.9m telescope. Of the 238 red dwarfs within 25 pc, we find that only ~8% are photometrically variable by at least 20 mmag (~2%) in the VRI bands. We find that high variability at optical wavelengths over the long-term can be used to identify young stars. Overall, however, the fluxes of most red dwarfs at optical wavelengths are steady to a few percent over the long term. The low overall rate of photometric variability for red dwarfs is consistent with results found in previous work on similar stars on shorter timescales, with the body of work indicating that most red dwarfs are only mildly variable. We highlight 17 stars that show long-term changes in brightness, sometimes because of flaring activity or spots, and sometimes because of stellar cycles similar to our Sun's solar cycle. Remarkably, two targets show brightnesses that monotonically increase (G 169-029) or decrease (WT 460AB) by several percent over a decade. We also provide long-term variability measurements for seven M dwarfs within 25 pc that host exoplanets, none of which vary by more than 20 mmag. Both as a population, and for the specific red dwarfs with exoplanets observed here, photometric variability is therefore often not a concern for planetary environments, at least at the optical wavelengths where they emit much of their light.

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Title: Nemesis encounters of nearby Hipparcos stars
Authors: Igor Yu. Potemine

Very close encounters of stars might lead to significant perturbations of their Oort-type clouds, planetesimal belts and planetary systems. We have calculated encounter parameters of Hipparcos stars using HIP2, Pulkovo and CRVAD2 catalogues of radial velocities. It turns out that some stars have encounters within 0.1 pc from each other and might be on an essential collision course up to few thousands AU. We present here examples with accurate astrometric data and stable with respect to errors of radial velocities within ▒ 0.3 km/s. They include beta vs. gamma Virginis, 61 Cygni vs. chi^1 Orionis as well as close encounters involving eta Bootis, AB Doradus, 61 Ursae Majoris and others.

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Title: The Solar Neighbourhood XXVIII: The Multiplicity Fraction of Nearby Stars from 5 to 70 AU and the Brown Dwarf Desert Around M Dwarfs
Authors: Sergio B. Dieterich, Todd J. Henry, David A. Golimowski, John E. Krist, Angelle M. Tanner

We report on our analysis of HST/NICMOS snapshot high resolution images of 255 stars in 201 systems within ~10 parsecs of the Sun. Photometry was obtained through filters F110W, F180M, F207M, and F222M using NICMOS Camera 2. These filters were selected to permit clear identification of cool brown dwarfs through methane contrast imaging. With a plate scale of 76 mas/pixel, NICMOS can easily resolve binaries with sub-arcsecond separations in the 19".5x19".5 field of view. We previously reported five companions to nearby M and L dwarfs from this search. No new companions were discovered during the second phase of data analysis presented here, confirming that stellar/substellar binaries are rare. We establish magnitude and separation limits for which companions can be ruled out for each star in the sample, and then perform a comprehensive sensitivity and completeness analysis for the subsample of 138 M dwarfs in 126 systems. We calculate a multiplicity fraction of 0.0{-0.0}^{+3.5}% for L companions to M dwarfs in the separation range of 5 to 70 AU, and 2.3_{-0.7}^{+5.0}% for L and T companions to M dwarfs in the separation range of 10 to 70 AU. We also discuss trends in the colour-magnitude diagrams using various colour combinations and present astrometry for 19 multiple systems in our sample. Considering these results and results from several other studies, we argue that the so-called "brown dwarf desert" extends to binary systems with low mass primaries and is largely independent of primary mass, mass ratio, and separations. While focusing on companion properties, we discuss how the qualitative agreement between observed companion mass functions and initial mass functions suggests that the paucity of brown dwarfs in either population may be due to a common cause and not due to binary formation mechanisms.

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Title: Dark gas in the Solar neighbourhood from extinction data
Authors: D. Paradis, K. Dobashi, T. Shimoikura, A. Kawamura, T. Onishi, Y. Fukui, J.-P. Bernard

When modelling infrared or gamma-ray data as a linear combination of observed gas tracers, an excess of emission has been pointed out with respect to expectations from known neutral and atomic gas as traced by HI and CO measurements respectively. This excess could result from an additional gas component. This gas, called "dark-gas" (DG) has been observed in our Galaxy, as well as in the Magellanic Clouds. In this paper, we investigate for the first time the correlation between visible extinction (Av) data and the gas tracers on large scales in the solar neighbourhood. Our work focuses on the solar neighbourhood (|b|>10\degr), as well as the inner and outer Galaxy and on four individual regions: Taurus, Orion, Cepheus-Polaris and Aquila-Ophiuchus. Thanks to the recent production of an all-sky Av map, we first perform the correlation between Av and HI and CO emission over the most diffuse regions, to derive the optimal (Av/NH)^(ref) ratio. We then iterate the analysis over the entire regions to estimate the CO-to-H2 conversion factor as well as the DG mass fraction. The average extinction to gas column density ratio in the solar neighbourhood is found to be (Av/NH)^(ref)=6.53 10^(-22) mag. cm^2, with significant differences between the inner and outer Galaxy. We derive an average XCO value of 1.67 10^(20) H2 cm^(-2)/(K km s^(-1)). In the solar neighbourhood, the gas mass in the dark component is found to be 19% relative to that in the atomic component and 164% relative to the one traced by CO. These results are compatible with the recent analysis using Planck data within the uncertainties of our measurements. We estimate the fraction of dark gas to the total molecular gas to be 0.62 in the solar neighbourhood. The HI-to-H2 and H2-to-CO transitions appear for Av \simeq0.2 and Av\simeq1.5, respectively, in agreement with theoretical models of dark-H2 gas.

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Posts: 131433
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Nearby White Dwarfs
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Title: Know Your Neighbourhood: A Detailed Model Atmosphere Analysis of Nearby White Dwarfs
Authors: Noemi Giammichele, Pierre Bergeron, Patrick Dufour

We present improved atmospheric parameters of nearby white dwarfs lying within 20 pc of the Sun. The aim of the current study is to obtain the best statistical model of the least-biased sample of the white dwarf population. A homogeneous analysis of the local population is performed combining detailed spectroscopic and photometric analyses based on improved model atmosphere calculations for various spectral types including DA, DB, DC, DQ, and DZ stars. The spectroscopic technique is applied to all stars in our sample for which optical spectra are available. Photometric energy distributions, when available, are also combined to trigonometric parallax measurements to derive effective temperatures, stellar radii, as well as atmospheric compositions. A revised catalogue of white dwarfs in the solar neighbourhood is presented. We provide, for the first time, a comprehensive analysis of the mass distribution and the chemical distribution of white dwarf stars in a volume-limited sample.

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Posts: 131433
Date:
RE: Solar neighbourhood
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Title: Fluorine abundances in dwarf stars of the solar neighbourhood
Authors: A. Recio-Blanco, P. de Laverny, C. Worley, N. C. Santos, C. Melo, G. Israelian

In spite of many observational efforts aiming to characterise the chemical evolution of our Galaxy, not much is known about the origin of fluorine (F). Models suggest that the F found in the Galaxy might have been produced mainly in three different ways, namely, Type II supernovae, asymptotic giant branch nucleosynthesis, or in the core of Wolf-Rayet stars. Only a few observational measurements of F abundances are available in the literature and mostly for objects whose characteristics might hamper an accurate determination of fluorine abundance (e.g.,complex mixing and nucleosynthesis processes, external/internal contamination). We derive the F abundances for a set of nine cool main-sequence dwarfs in the solar neighbourhood, based on an unblended line of the HF molecule at 2.3 microns. In addition, we study the s-process elements of five of these stars. We acquire data using the high-resolution IR-spectrograph CRIRES and gather FEROS data from the European Southern Observatory archive. Several of the analysed stars seem to be slightly fluorine enhanced with respect to the Sun, although no correlation is found between the F abundance and the iron content. In addition, the most fluorine enriched stars are also yttrium and zirconium enriched, which suggests that AGB fluorine nucleosynthesis is the dominant source of fluorine production for the observed stars. Nevertheless, the correlation between [F/Fe] and the s-elements is rather weak and possibly masked by the uncertainties in the F abundance measurements. Finally, we compare our derived F abundances to previous measurements of alpha-element and iron-peak element abundances. Type II core collapse Supernovae do not appear to be the main site of F production for our targets, as no correlation seems to exist between the [F/Fe] and the [alpha/Fe] ratios.

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Posts: 131433
Date:
The Solar Neighbourhood
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Title: The Solar Neighbourhood. XXVI. AP Col: The Closest (8.4 pc) Pre-Main-Sequence Star
Authors: Adric R. Riedel, Simon J. Murphy, Todd J. Henry, Carl Melis, Wei-Chun Jao, John P. Subasavage

We present the results of a multi-technique investigation of the M4.5Ve flare star AP Col, which we discover to be the nearest pre-main-sequence star. These include astrometric data from the CTIO 0.9m, from which we derive a proper motion of 342.0▒0.5 mas yr^-1, a trigonometric parallax of 119.21▒0.98 mas (8.39▒0.07 pc), and photometry and photometric variability at optical wavelengths. We also provide spectroscopic data, including radial velocity (22.4▒0.3 km s^-1), lithium Equivalent Width (EW) (0.28▒0.02 A), H-alpha EW (-6.0 to -35 A), vsini (11▒1 km s^-1), and gravity indicators from the Siding Spring 2.3-m WiFeS, Lick 3-m Hamilton echelle, and Keck-I HIRES echelle spectrographs. The combined observations demonstrate that AP Col is the closer of only two known systems within 10 pc of the Sun younger than 100 Myr. Given its space motion and apparent age of 12-50 Myr, AP Col is likely a member of the recently proposed ~40 Myr old Argus/IC 2391 association.

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Posts: 131433
Date:
RE: Solar neighbourhood
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Title: The Dark Matter Density in the Solar Neighbourhood reconsidered
Authors: W. de Boer, M. Weber (Karlsruhe Institute of Technology (KIT), Karlsruhe)

Both the gas flaring and the dip in the rotation curve, which was recently reconfirmed with precise measurements using the VERA VLBI array in Japan, suggest doughnut-like substructure in the dark matter (DM) halo. A global fit to all available data shows that the data are indeed best described by an NFW DM profile complemented by two doughnut-like DM substructures with radii of 4.2 and 12.4 kpc, which coincide with the local dust ring and the Monocerus ring of stars, respectively. Both regions have been suggested as regions with tidal streams from "shredded" satellites. If real, the radial extensions of these nearby ringlike structures enhance the local dark matter density by a factor of four to about 1.3 ▒0.3 GeV/cm│.
It is shown that i) this higher DM density is perfectly consistent with the local gravitational potential determining the surface density and the local matter density (Oort limit), ii) previous determinations of the surface density were biased by the assumption of a smoothly varying DM halo and iii) the s-shaped gas flaring is explained. Such a possible enhancement of the local DM density is of great interest for direct DM searches and would change the directional dependence for indirect DM searches.

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