1,396 research outputs found
A Hipparcos census of the nearby OB associations
A comprehensive census of the stellar content of the nearby OB associations
is presented, based on Hipparcos positions, proper motions, and parallaxes.
Moving groups are identified by combining de Bruijne's refurbished convergent
point method with the `Spaghetti method' of Hoogerwerf & Aguilar. Monte Carlo
simulations are used to estimate the expected number of interloper field stars.
Astrometric members are listed for 12 young stellar groups, out to a distance
of ~650 pc. These are the 3 subgroups Upper Scorpius, Upper Centaurus Lupus and
Lower Centaurus Crux of Sco OB2, as well as Vel OB2, Tr 10, Col 121, Per OB2,
alpha Persei (Per OB3), Cas-Tau, Lac OB1, Cep OB2, and a new group designated
as Cep OB6. The selection procedure corrects the list of previously known
astrometric and photometric B- and A-type members, and identifies many new
members, including a significant number of F stars, as well as evolved stars,
e.g., the Wolf-Rayet stars gamma^2 Vel (Vel OB2) and EZ CMa (Col 121), and the
classical Cepheid delta Cep in Cep OB6. In the nearest associations the
later-type members include T Tauri objects and other pre-main sequence stars.
Astrometric evidence for moving groups in the fields of R CrA, CMa OB1, Mon
OB1, Ori OB1, Cam OB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is
inconclusive, due to their large distance or unfavorable kinematics.
The mean distances of the well-established groups are systematically smaller
than previous estimates. The mean motions display a systematic pattern, which
is discussed in relation to the Gould Belt. Six of the 12 detected moving
groups do not appear in the classical list of nearby OB associations. The
number of unbound young stellar groups in the Solar neighbourhood may be
significantly larger than thought previously.Comment: 51 pages, 30 PostScript figures, 6 tables in PostScript format,
default LaTeX using psfig.sty; accepted for publication in the Astronomical
Journal, scheduled for January 1999 issue. Abbreviated abstrac
OB Associations
Since the previous (1990) edition of this meeting enormous progress in the
field of OB associations has been made. Data from X-ray satellites have greatly
advanced the study of the low-mass stellar content of associations, while
astrometric data from the Hipparcos satellite allow for a characterization of
the higher-mass content of associations with unprecedented accuracy. We review
recent work on the OB associations located within 1.5 kpc from the Sun, discuss
the Hipparcos results at length, and point out directions for future research.Comment: To appear in The Physics of Star Formation and Early Stellar
Evolution II, eds C.J. Lada & N. Kylafis (Kluwer Academic), 30 pages, 9
EPS-figures, LaTeX using crckapb.sty, epsfig.sty, amssymb.st
Triaxial orbit-based modelling of the Milky Way Nuclear Star Cluster
We construct triaxial dynamical models for the Milky Way nuclear star cluster
using Schwarzschild's orbit superposition technique. We fit the stellar
kinematic maps presented in Feldmeier et al. (2014). The models are used to
constrain the supermassive black hole mass M_BH, dynamical mass-to-light ratio
M/L, and the intrinsic shape of the cluster. Our best-fitting model has M_BH =
(3.0 +1.1 -1.3)x10^6 M_sun, M/L = (0.90 +0.76 -0.08) M_sun/L_{sun,4.5micron},
and a compression of the cluster along the line-of-sight. Our results are in
agreement with the direct measurement of the supermassive black hole mass using
the motion of stars on Keplerian orbits. The mass-to-light ratio is consistent
with stellar population studies of other galaxies in the mid-infrared. It is
possible that we underestimate M_BH and overestimate the cluster's triaxiality
due to observational effects. The spatially semi-resolved kinematic data and
extinction within the nuclear star cluster bias the observations to the near
side of the cluster, and may appear as a compression of the nuclear star
cluster along the line-of-sight. We derive a total dynamical mass for the Milky
Way nuclear star cluster of M_MWNSC = (2.1 +-0.7)x10^7 M_sun within a sphere
with radius r = 2 x r_eff = 8.4 pc. The best-fitting model is tangentially
anisotropic in the central r = 0.5-2 pc of the nuclear star cluster, but close
to isotropic at larger radii. Our triaxial models are able to recover complex
kinematic substructures in the velocity map.Comment: 14 pages, 10 figures. Accepted for publication in MNRA
Large scale kinematics and dynamical modelling of the Milky Way nuclear star cluster
Within the central 10pc of our Galaxy lies a dense nuclear star cluster
(NSC), and similar NSCs are found in most nearby galaxies. Studying the
structure and kinematics of NSCs reveals the history of mass accretion of
galaxy nuclei. Because the Milky Way (MW) NSC is at a distance of only 8kpc, we
can spatially resolve the MWNSC on sub-pc scales. This makes the MWNSC a
reference object for understanding the formation of all NSCs. We have used the
NIR long-slit spectrograph ISAAC (VLT) in a drift-scan to construct an
integral-field spectroscopic map of the central 9.5 x 8pc of our Galaxy. We use
this data set to extract stellar kinematics both of individual stars and from
the unresolved integrated light spectrum. We present a velocity and dispersion
map from the integrated light and model these kinematics using kinemetry and
axisymmetric Jeans models. We also measure CO bandhead strengths of 1,375
spectra from individual stars. We find kinematic complexity in the NSCs radial
velocity map including a misalignment of the kinematic position angle by 9
degree counterclockwise relative to the Galactic plane, and indications for a
rotating substructure perpendicular to the Galactic plane at a radius of 20" or
0.8pc. We determine the mass of the NSC within r = 4.2pc to 1.4 x 10^7 Msun. We
also show that our kinematic data results in a significant underestimation of
the supermassive black hole (SMBH) mass. The kinematic substructure and
position angle misalignment may hint at distinct accretion events. This
indicates that the MWNSC grew at least partly by the mergers of massive star
clusters. Compared to other NSCs, the MWNSC is on the compact side of the r_eff
- M_NSC relation. The underestimation of the SMBH mass might be caused by the
kinematic misalignment and a stellar population gradient. But it is also
possible that there is a bias in SMBH mass measurements obtained with
integrated light.Comment: 20 pages, 19 Figures, Accepted for publication in A&
Kinematics of Metal-Poor Stars in the Galaxy. III. Formation of the Stellar Halo and Thick Disk as Revealed from a Large Sample of Non-Kinematically Selected Stars
(Abbreviated) We present a detailed analysis of the space motions of 1203
solar-neighborhood stars with metal abundances [Fe/H] <= -0.6, on the basis of
a recently revised and supplemented catalog of metal-poor stars selected
without kinematic bias (Beers et al. 2000). This sample, having available
proper motions, radial velocities, and distance estimates for stars with a wide
range of metal abundances, is by far the largest such catalog to be assembled
to date. Unlike essentially all previous kinematically selected catalogs, the
metal-poor stars in our sample exhibit a diverse distribution of orbital
eccentricities, e, with no apparent correlation between [Fe/H] and e. This
demonstrates, clearly and convincingly, that the evidence offered by Eggen,
Lynden-Bell, and Sandage (1962) for a rapid collapse of the Galaxy, an apparent
correlation between the orbital eccentricity of halo stars with metallicity, is
basically the result of their proper-motion selection bias. However, even in
our non-kinematically selected sample, we have identified a small concentration
of high-e stars at [Fe/H] = -1.7, which may originate, in part, from infalling
gas during the early formation of the Galaxy. The implications of our results
for the formation of the Galaxy are also discussed, in particular in the
context of the currently favored CDM theory of hierarchical galaxy formation.Comment: 51 pages, including 17 figures, to appear in AJ (June 2000), full
paper with all figures embedded available at
http://pluto.mtk.nao.ac.jp/people/chiba/preprint/halo5
KMOS view of the Galactic Centre - II. Metallicity distribution of late-type stars
Knowing the metallicity distribution of stars in the Galactic Centre has
important implications for the formation history of the Milky Way nuclear star
cluster. However, this distribution is not well known, and is currently based
on a small sample of fewer than 100 stars. We obtained near-infrared K-band
spectra of more than 700 late-type stars in the central 4 pc^2 of the Milky Way
nuclear star cluster with the integral-field spectrograph KMOS (VLT). We
analyse the medium-resolution spectra using a full-spectral fitting method
employing the G\"ottingen Spectral library of synthetic PHOENIX spectra. The
derived stellar metallicities range from metal-rich [M/H]>+0.3 dex to
metal-poor [M/H]<-1.0 dex, with a fraction of 5.2(^{+6.0}+{-3.1}) per cent
metal-poor ([M/H]<-0.5 dex) stars. The metal-poor stars are distributed over
the entire observed field. The origin of metal-poor stars remains unclear. They
could originate from infalling globular clusters. For the metal-rich stellar
population ([M/H]>0 dex) a globular cluster origin can be ruled out. As there
is only a very low fraction of metal-poor stars in the central 4 pc^2 of the
Galactic Centre, we believe that our data can discard a scenario in which the
Milky Way nuclear star cluster is purely formed from infalling globular
clusters.Comment: 18 pages, 9 Figures, accepted for publication in MNRA
KMOS view of the Galactic Centre I. Young stars are centrally concentrated
The Galactic centre hosts a crowded, dense nuclear star cluster with a
half-light radius of 4 pc. Most of the stars in the Galactic centre are cool
late-type stars, but there are also >100 hot early-type stars in the central
parsec of the Milky Way. These stars are only 3-8 Myr old. Our knowledge of the
number and distribution of early-type stars in the Galactic centre is
incomplete. Only a few spectroscopic observations have been made beyond a
projected distance of 0.5 pc of the Galactic centre. The distribution and
kinematics of early-type stars are essential to understand the formation and
growth of the nuclear star cluster. We cover the central >4pc^2 of the Galactic
centre using the integral-field spectrograph KMOS. We extracted more than 1,000
spectra from individual stars and identified early-type stars based on their
spectra. Our data set contains 114 bright early-type stars: 6 have narrow
emission lines, 23 are Wolf-Rayet stars, 9 stars have featureless spectra, and
76 are O/B type stars. Our wide-field spectroscopic data confirm that the
distribution of young stars is compact, with 90% of the young stars identified
within 0.5 pc of the nucleus. We identify 24 new O/B stars primarily at large
radii. We estimate photometric masses of the O/B stars and show that the total
mass in the young population is >12,000M_sun. The O/B stars all appear to be
bound to the Milky Way nuclear star cluster, while less than 30% belong to the
clockwise rotating disk. The central concentration of the early-type stars is a
strong argument that they have formed in situ. A large part of the young O/B
stars is not on the disk, which either means that the early-type stars did not
all form on the same disk or that the disk is dissolving rapidly. [abridged]Comment: 27 pages, 17 figures, matches journal version: Corrected typos,
corrected Notes in Table B.
- …