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R. Klement,
H.-W. Rix,
C. Flynn, B. Fuchs,
T. C. Beers,
C. Allende Prieto,
D. Bizyaev,
H. Brewington,
Y. S. Lee,
E. Malanushenko,
V. Malanushenko,
D. Oravetz,
K. Pan,
P. Re Fiorentin,
A. Simmons,
and S. Snedden
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ABSTRACT: We have detected stellar halo streams in the solar neighborhood using data from the seventh public data release of the Sloan Digital Sky Survey (SDSS), which includes the directed stellar program Sloan Extension For Galactic Understanding and Exploration (SEGUE). In order to derive distances to each star, we used the metallicity-dependent photometric parallax relation from Ivezić et al. We examine and quantify the accuracy of this relation by applying it to a set of globular and open clusters observed by the SDSS/SEGUE and comparing the resulting sequence to the fiducial cluster sequences obtained by An et al. Our final sample consists of 22,321 nearby (d ≤ 2 kpc), metal-poor ([Fe/H] ≤–0.5) main-sequence stars with six-dimensional estimates of position and space velocity . We characterize the orbits of these stars through suitable kinematic proxies for their "effective" integrals of motion, angular momentum, eccentricity, and orbital polar angle and compare the observed distribution to expectations from a smooth distribution in four [Fe/H] bins. The metallicities provide an additional dimension in parameter space that is well suited to distinguish tidal streams from those of dynamical origin. On this basis, we identify at least five significant "phase-space overdensities" of stars on very similar orbits in the solar neighborhood to which we can assign unambiguously peaked [Fe/H] distributions. Three of them have been identified previously, including the halo stream discovered by Helmi et al. at a significance level of σ = 12.0. In addition, we find at least two new genuine halo streams, judged by their kinematics and [Fe/H], at σ = 2.9 and 4.8, respectively. For one stream the stars even show coherence in the configuration space, matching a spatial overdensity of stars found by Juric et al. at (R, z) (9.5, 0.8) kpc. Our results demonstrate the practical power of our search method to detect substructure in the phase-space distribution of nearby stars without making a priori assumptions about the detailed form of the gravitational potential.
The Astrophysical Journal 05/2009; 698(1):865. · 6.02 Impact Factor
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R. Klement,
H. -W. Rix,
C. Flynn, B. Fuchs,
T. C. Beers,
C. Allende Prieto,
D. Bizyaev,
H. Brewington,
Y. S. Lee,
E. Malanushenko,
V. Malanushenko,
D. Oravetz,
K. Pan,
P. Re Fiorentin,
A. Simmons,
S. Snedden
[show abstract]
[hide abstract]
ABSTRACT: We have detected stellar halo streams in the solar neighborhood using data
from the 7th public data release of the Sloan Digital Sky Survey (SDSS), which
includes the directed stellar program SEGUE: Sloan Extension For Galactic
Understanding and Exploration. In order to derive distances to each star, we
used the metallicity-dependent photometric parallax relation from Ivezic et al.
(2008) for which we examine and quantify the accuracy. Our final sample
consists of 22,321 nearby (d < 2 kpc), metal-poor ([Fe/H] < -0.5) main-sequence
stars with 6D estimates of position and space velocity. We characterize the
orbits of these stars through suitable kinematic proxies for their "effective"
integrals of motion, angular momentum, eccentricity, and orbital polar angle
and compare the observed distribution to expectations from a smooth
distribution in four [Fe/H] bins. On this basis we identify at least five
significant "phase-space overdensities" of stars on very similar orbits in the
solar neighborhood to which we can assign unambiguously peaked [Fe/H]
distributions. Three of them have been identified previously, including the
halo stream discovered by Helmi et al. (1999) at a significance level of 12.0.
In addition, we find at least two new genuine halo streams, judged by their
kinematics and [Fe/H], at significance levels of 2.9 and 4.8, respectively. For
one stream the stars even show coherence in configuration space, matching a
spatial overdensity of stars found by Juric et al. (2008) at (R,z) \approx
(9.5,0.8) kpc. Our results demonstrate the practical power of our search method
to detect substructure in the phase-space distribution of nearby stars without
making a-priori assumptions about the detailed form of the gravitational
potential.
04/2009;
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ABSTRACT: Recent 3D high-resolution simulations of the interstellar medium in a star forming galaxy like the Milky Way show that supernova
explosions are the main driver of the structure and evolution of the gas. Its physical state is largely controlled by turbulence
due to the high Reynolds numbers of the average flows. For a constant supernova rate a dynamical equilibrium is established
within 200Myr of simulation as a consequence of the setup of a galactic fountain. The resulting interstellar medium reveals
a typical density/pressure pattern, i.e. distribution of so-called gas phases, on scales of 500–700pc, with interstellar
bubbles being a common phenomenon just like the Local Bubble and the Loop I superbubble, which are assumed to be interacting.
However, modeling the Local Bubble is special, because it is driven by a moving group, passing through its volume, as it is
inferred from the analysis of Hipparcos data. Adetailed analysis reveals that between 14 and 19 supernovae have exploded
during the last 15 Myr. The age of the Local Bubble is derived from comparison with Hi and UV absorption line data to be 14.5±0.40.7Myr. We further predict the merging of the two bubbles in about 3Myr from now, when the interaction shell starts to fragment.
The Local Cloud and its companion Hi clouds are the consequence of a dynamical instability in the interaction shell between the Local and the Loop I bubble.
Space Science Reviews 02/2009; 143(1):263-276. · 3.61 Impact Factor
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ABSTRACT: We use a new method to trace backwards the star formation history of the Milky Way disk, using a sample of M dwarfs in the solar neighbourhood which is representative for the entire solar circle. M stars are used because they show H_alpha emission until a particular age which is a well calibrated function of their absolute magnitudes. This allows us to reconstruct the rate at which disk stars have been born over about half the disk's lifetime. Our star formation rate agrees well with those obtained by using other, independent, methods and seems to rule out a constant star formation rate. The principal result of this study is to show that a relation of the Schmidt-Kennicut type (which relates the star formation rate to the interstellar gas content of galaxy disks) has pertained in the Milky Way disk during the last 5 Gyr. The star formation rate we derive from the M dwarfs and the interstellar gas content of the disk can be inferred as a function of time from a model of the chemical enrichment of the disk, which is well constrained by the observations indicating that the metallicity of the Galactic disk has remained nearly constant over the timescales involved. We demonstrate that the star formation rate and gas surface densities over the last 5 Gyrs can be accurately described by a Schmidt-Kennicutt law with an index of Gamma = 1.45 (+0.22,-0.09). This is, within statistical uncertainties, the same value found for other galaxies. Comment: 7 pages, 5 figures, accepted by Astron. J
10/2008;
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ABSTRACT: We have analyzed the phase space distribution of a sample of about 900 non--kinematically selected low metallicity stars in the solar vicinity. The stars primarily represent the thick disk and halo populations of the Milky Way. We aim to identify overdensely populated regions in phase space, which we interpret as signatures of star streams passing close to the Sun. The search was conducted in a space constructed from the angular momenta and eccentricities of the stellar orbits. Besides recovering all well known star streams in the thick disk, we isolated four statistically significant phase space overdensities amongst halo stars. One of them is associated with a previously known halo star stream, but three of them are novel features, which we propose be also considered as genuine halo streams. Comment: 6 pages, 5 figures, accepted by Astron. Astrophys
09/2007;
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ABSTRACT: We present a new unbiased search and analysis of all B stars in the solar neighbourhood (within a volume of 400 pc diameter) using the Arivel data base to track down the remains of the OB associations, which hosted the supernovae responsible for the Local Bubble in the interstellar gas. We find after careful dereddening and by comparison with theoretical isochrones, that besides the Upper Scorpius the Upper Centaurus Lupus and Lower Centaurus Crux subgroups are the youngest stellar associations in the solar neighbourhood with ages of 20 to 30 Myr, in agreement with previous work. In search for the ``smoking gun'' of the origin of the Local Bubble, we have traced the paths of the associations back into the past and found that they entered the present bubble region 10 to 15 Myr ago. We argue that the Local Bubble began to form then and estimate that 14 to 20 supernovae have gone off since. It is shown that the implied energy input is sufficient to excavate a bubble of the presently observed size. Comment: 10 pages, 8 figures, accepted by Mon. Not. R. Astron. Soc
09/2006;
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08/2006: pages 34-40;
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ABSTRACT: We analysed the fine structure of the phase space distribution function of nearby subdwarfs using data extracted from various catalogues. Applying a new search strategy based on Dekker's theory of galactic orbits, we found four overdensely populated regions in phase space. Three of them were correlated with previously known star streams: the Hyades-Pleiades and Hercules streams in the thin disk of the Milky Way and the Arcturus stream in the thick disk. In addition we find evidence for another stream in the thick disk, which resembles closely the Arcturus stream and probably has the same extragalactic origin. Comment: 6 pages, 5 figures, 2 tables, accepted by Astron. Astrophys
12/2005;
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ABSTRACT: We present an analysis of the space motions of 742 subdwarf stars based on the sample of Carney et al. (1994, CLLA). Hipparcos parallaxes, TYC2+HIP proper motions and Tycho2 proper motions were combined with radial velocities and metallicities from CLLA. The kinematical behavior is discussed in particular in relation to their metallicities. The majority of these sample stars have metal abundances of [Fe/H] >-1 and represent the thick disk population. The halo component, with [Fe/H] <-1.6, is characterized by a low mean rotation velocity and a radially elongated velocity ellipsoid. In the intermediate metallicity range (-1.6 < [Fe/H] <-1), we find a significant number of subdwarfs with disklike kinematics. We interpret this population of stars as a metal-weak thick disk population. Comment: 6 pages, 7 figures, accepted by Astronomy & Astrophysics
01/2005;
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Astronomische Nachrichten Supplement. 07/2004; 325:97.
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ABSTRACT: In this review we concentrate on the dynamical evolution of the Milky Way as monitored in the solar neighbourhood. The relevant data sets are presented and discussed in detail. In the second part we review various mechanisms, which drive the dynamical evolution.
10/2000;
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Astrophysics and Space Science 06/1999; 265(1):247-248. · 1.69 Impact Factor
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ABSTRACT: The statistical test described by Wielen et al.(1994) is used to derive new zero-points of ground-based Cepheid period-luminosity (PL) and period-luminosity-colour (PLC) relations. Eleven relations are compared with the Hipparcos data. Our results argue for a typical increase of the adopted distance scale by about 8% +- 8%. Our zero-point for the PL relation of Caldwell & Laney (1991) is in agreement with that of Feast & Catchpole (1997).
01/1999;
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ABSTRACT: A sample of subdwarfs with accurate space velocities and standarized metallicities is presented. This was constructed by combining Hipparcos parallaxes ond proper motions with radial velocities and metallicities from Carney et al. (1994; CLLA). The accurate Hipparcos parallaxes lead to an - upward - correction factor of 11% of the photometric distance scale of CLLA. The kinematical behaviour of the subdwarfs is discussed in particular in relation to their metallicities. Most of the stars turn out to be thick disk stars, but the sample contains also many genuine halo stars. While the extreme metal poor halo does not rotate, a population of subdwarfs with metallicities in the range -1.6 < [Fe/H] < -1.0 dex appears to rotate around the galactic center with a mean rotation speed of about 100 km/s. Comment: Latex, 4 pages, 6 figures. To appear in the proceedings of the `Galaxy Evolution' conference held in Meudon, September, 21 - 25, 1998
11/1998;
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ABSTRACT: Based on the recent discoveries of free floating brown dwarfs we derive estimates of the local mass density of this population of objects. Mass density estimates from various surveys span the range 0.03 to 0.005 M_Sun/pc^3. These estimates are compared with the local mass densities of the other constituents of the galactic disk and, in particular, with the dynamically determined total local mass density. We argue that brown dwarfs might indeed contribute significantly to the local mass budget, but that a local mass density as high as 0.03 M_Sun/pc^3 as suggested by Ruiz et al. (1997) is rather unlikely.
11/1998;
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ABSTRACT: We have searched the Forth Catalogue of Nearby Stars for halo stars and identified 15 subdwarfs and a high velocity white dwarf in the solar neighbourhood. This search was motivated mainly by the recent determinations of MACHO masses of about 0.5 M_sun, which are typical for halo dwarfs. The local mass density of the stars is 1.5 10^{-4} M_sun pc^{-3}, which is only 3% of the current estimate of the local mass density of the MACHO population. We compare the local density of subdwarfs with constraints set by HST observations of distant red dwarfs. Using models of the stellar halo with density laws that fall off like r^{-\alpha}, \alpha = 3.5 to 4, we find that the HST constraints can only be matched, if we assume that the stellar halo is flattened with an axial ratio of about 0.6. The non-detection of the analogues of MACHOs in the solar neighbourhood allows to set an upper limit to the luminosity of MACHOs of M_B > 21 magnitudes.
09/1997;
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ABSTRACT: We have derived the galactic space velocities of 130 RR Lyrae stars, using proper motions measured by Hipparcos. The distances of the stars are taken from a photometric calibration which is confirmed by Hipparcos parallaxes. We present the mean velocities and the velocity dispersions of the RR Lyrae stars as a function of the metallicity [Fe/H]. The data do not allow to decide safely whether the kinematical properties of halo and disk objects show a smooth transition or whether halo and disk stars belong to two distinct populations. The difference in the mean motions of metal-poor and metal-rich RR Lyrae stars is in good agreement with a circular velocity of about 220 km/s at the Sun. Adopting such a circular velocity, the system of metal-poor RR Lyrae stars shows no significant net rotation in our Galaxy. The metalrich RR Lyrae stars move like typical disk stars. Key words: RR Lyrae stars; galactic kinematics; galactic dynamics; galactic evolution; galactic halo; galactic disk; Hippar...
08/1997;
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ABSTRACT: We have derived the galactic space velocities of 130 RR Lyrae stars, using proper motions measured by Hipparcos. The distances of the stars are taken from a photometric calibration which is confirmed by Hipparcos parallaxes. We present the mean velocities and the velocity dispersions of the RR Lyrae stars as a function of the metallicity [Fe/H]. The data do not allow to decide safely whether the kinematical properties of halo and disk objects show a smooth transition or whether halo and disk stars belong to two distinct populations. The difference in the mean motions of metal-poor and metal-rich RR Lyrae stars is in good agreement with a circular velocity of about 220 km/s at the Sun. Adopting such a circular velocity, the system of metal-poor RR Lyrae stars shows no significant net rotation in our Galaxy. The metalrich RR Lyrae stars move like typical disk stars. Key words: RR Lyrae stars; galactic kinematics; galactic dynamics; galactic evolution; galactic halo; galactic disk; Hippar...
08/1997;
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ABSTRACT: Hipparcos parallaxes and proper motions were combined with radial velocities and metallicities from Carney et al. (1994). As a result a sample of 237 high-velocity stars was obtained having highly accurate space velocities and uniformly determined metallicities. Hipparcos parallaxes show: (a) the Carney et al. (1994) sample is still contaminated by subgiants and giants. (b) the photometric distances in Carney et al. (1994) were underestimated by about 16 per cent. The kinematical properties of these subdwarfs are discussed as a function of their metallicities. From an enlarged sample of subdwarfs, 93 halo subdwarfs could be selected indicating that a possible rotation of the galactic halo is improbable. Key words: space astrometry; subdwarfs; photometric distances; kinematics. 1. INTRODUCTION Hipparcos parallaxes and proper motions (ESA, 1997) became available for a predetermined sample of 731 high-velocity stars proposed in 1981 for observation by the Hipparcos satellite. Since then ...
08/1997;
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[show abstract]
[hide abstract]
ABSTRACT: Hipparcos parallaxes and proper motions were combined with radial velocities and metallicities from Carney et al. (1994). As a result a sample of 237 high-velocity stars was obtained having highly accurate space velocities and uniformly determined metallicities. Hipparcos parallaxes show: (a) the Carney et al. (1994) sample is still contaminated by subgiants and giants. (b) the photometric distances in Carney et al. (1994) were underestimated by about 16 per cent. The kinematical properties of these subdwarfs are discussed as a function of their metallicities. From an enlarged sample of subdwarfs, 93 halo subdwarfs could be selected indicating that a possible rotation of the galactic halo is improbable. Key words: space astrometry; subdwarfs; photometric distances; kinematics. 1. INTRODUCTION Hipparcos parallaxes and proper motions (ESA, 1997) became available for a predetermined sample of 731 high-velocity stars proposed in 1981 for observation by the Hipparcos satellite. Since then ...
08/1997;
