221 research outputs found
A retrospective cohort study of liver toxicity in hepatitis B and C HIV co-infected patients in the HIV Ontario Observational Database
See Attache
The Pattern Speed of the Galactic Bar
Most late-type stars in the solar neighborhood have velocities similar to the
local standard of rest (LSR), but there is a clearly separated secondary
component corresponding to a slower rotation and a mean outward motion.
Detailed simulations of the response of a stellar disk to a central bar show
that such a bi-modality is expected from outer-Lindblad resonant scattering.
When constraining the run of the rotation curve by the proper motion of Sgr A*
and the terminal gas velocities, the value observed for the rotation velocity
separating the two components results in a value of (53+/-3)km/s/kpc for the
pattern speed of the bar, only weakly dependent on the precise values for Ro
and bar angle phi.Comment: 5 pages LaTeX, 2 Figs, accepted for publication in ApJ Letter
The Effect of the Outer Lindblad Resonance of the Galactic Bar on the Local Stellar Velocity Distribution
Hydro-dynamical modeling of the inner Galaxy suggest that the radius of the
outer Lindblad resonance (OLR) of the Galactic bar lies in the vicinity of the
Sun. How does this resonance affect the distribution function in the outer
parts of a barred disk, and can we identify any effect of the resonance in the
velocity distribution f(v) actually observed in the solar neighborhood? To
answer these questions, detailed simulations of f(v) in the outer parts of an
exponential stellar disks with nearly flat rotation curves and a rotating
central bar have been performed. For a model resembling the old stellar disk,
the OLR causes a distinct feature in f(v) over a significant fraction of the
outer disk. For positions <2kpc outside the OLR radius and at bar angles of
\~10-70 degrees, f(v) inhibits a bi-modality between the low-velocity stars
moving like the local standard of rest (LSR) and a secondary mode of stars
predominantly moving outward and rotating more slowly than the LSR.
Such a bi-modality is indeed present in f(v) inferred from the Hipparcos data
for late-type stars in the solar neighborhood. If one interpretes this observed
bi-modality as induced by the OLR -- and there are hardly any viable
alternatives -- then one is forced to deduce that the OLR radius is slightly
smaller than Ro. Moreover, by a quantitative comparison of the observed with
the simulated distributions one finds that the pattern speed of the bar is
1.85+/-0.15 times the local circular frequency, where the error is dominated by
the uncertainty in bar angle and local circular speed.
Also other, less prominent but still significant, features in the observed
f(v) resemble properties of the simulated velocity distributions, in particular
a ripple caused by orbits trapped in the outer 1:1 resonance.Comment: 14 pages, 10 figures (Fig.2 in full resolution available upon
request), accepted for publication in A
The Galactic Kinematics of Mira Variables
The galactic kinematics of Mira variables derived from radial velocities,
Hipparcos proper motions and an infrared period-luminosity relation are
reviewed. Local Miras in the 145-200day period range show a large asymmetric
drift and a high net outward motion in the Galaxy. Interpretations of this
phenomenon are considered and (following Feast and Whitelock 2000) it is
suggested that they are outlying members of the bulge-bar population and
indicate that this bar extends beyond the solar circle.Comment: 7 pages, 2 figure, to be published in Mass-Losing Pulsating Stars and
their Circumstellar Matter, Y. Nakada & M. Honma (eds) Kluwer ASSL serie
Population analysis of open clusters: radii and mass segregation
Aims: Based on our well-determined sample of open clusters in the all-sky
catalogue ASCC-2.5 we derive new linear sizes of some 600 clusters, and
investigate the effect of mass segregation of stars in open clusters. Methods:
Using statistical methods, we study the distribution of linear sizes as a
function of spatial position and cluster age. We also examine statistically the
distribution of stars of different masses within clusters as a function of the
cluster age. Results: No significant dependence of the cluster size on location
in the Galaxy is detected for younger clusters (< 200 Myr), whereas older
clusters inside the solar orbit turned out to be, on average, smaller than
outside. Also, small old clusters are preferentially found close to the
Galactic plane, whereas larger ones more frequently live farther away from the
plane and at larger Galactocentric distances. For clusters with (V - M_V) <
10.5, a clear dependence of the apparent radius on age has been detected: the
cluster radii decrease by a factor of about 2 from an age of 10 Myr to an age
of 1 Gyr. A detailed analysis shows that this observed effect can be explained
by mass segregation and does not necessarily reflect a real decrease of cluster
radii. We found evidence for the latter for the majority of clusters older than
30 Myr. Among the youngest clusters (between 5 and 30 Myr), there are some
clusters with a significant grade of mass segregation, whereas some others show
no segregation at all. At a cluster age between 50 and 100 Myrs, the
distribution of stars of different masses becomes more regular over cluster
area. In older clusters the evolution of the massive stars is the most
prominent effect we observe.Comment: 14 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
Constraining the fundamental parameters of the O-type binary CPD-41degr7733
Using a set of high-resolution spectra, we studied the physical and orbital
properties of the O-type binary CPD-41 7733, located in the core of \ngc. We
report the unambiguous detection of the secondary spectral signature and we
derive the first SB2 orbital solution of the system. The period is 5.6815 +/-
0.0015 d and the orbit has no significant eccentricity. CPD-41 7733 probably
consists of stars of spectral types O8.5 and B3. As for other objects in the
cluster, we observe discrepant luminosity classifications while using
spectroscopic or brightness criteria. Still, the present analysis suggests that
both components display physical parameters close to those of typical O8.5 and
B3 dwarfs. We also analyze the X-ray light curves and spectra obtained during
six 30 ks XMM-Newton pointings spread over the 5.7 d period. We find no
significant variability between the different pointings, nor within the
individual observations. The CPD-41 7733 X-ray spectrum is well reproduced by a
three-temperature thermal mekal model with temperatures of 0.3, 0.8 and 2.4
keV. No X-ray overluminosity, resulting e.g. from a possible wind interaction,
is observed. The emission of CPD-41 7733 is thus very representative of typical
O-type star X-ray emission.Comment: Accepted by ApJ, 15 pages, 9 figure
Towards absolute scales of radii and masses of open clusters
Aims: In this paper we derive tidal radii and masses of open clusters in the
nearest kiloparsecs around the Sun. Methods: For each cluster, the mass is
estimated from tidal radii determined from a fitting of three-parametric King's
profiles to the observed integrated density distribution. Different samples of
members are investigated. Results: For 236 open clusters, all contained in the
catalogue ASCC-2.5, we obtain core and tidal radii, as well as tidal masses.
The distributions of the core and tidal radii peak at about 1.5 pc and 7 - 10
pc, respectively. A typical relative error of the core radius lies between 15%
and 50%, whereas, for the majority of clusters, the tidal radius was determined
with a relative accuracy better than 20%. Most of the clusters have tidal
masses between 50 and 1000 , and for about half of the clusters, the
masses were obtained with a relative error better than 50%.Comment: 11 pages, 7 figures, accepted for publication in Astronomy &
Astrophysic
Variability of Be Stars in Southern Open Clusters
We recently discovered a large number of highly active Be stars in the open
cluster NGC 3766, making it an excellent location to study the formation
mechanism of Be star disks. To explore whether similar disk appearances and/or
disappearances are common among the Be stars in other open clusters, we present
here multiple epochs of H-alpha spectroscopy for 296 stars in eight open
clusters. We identify 12 new transient Be stars and confirm 17 additional Be
stars with relatively stable disks. By comparing the H-alpha equivalent widths
to the photometric y - H-alpha colors, we present a method to estimate the
strength of the H-alpha emission when spectroscopy is not available. For a
subset of 128 stars in four open clusters, we also use blue optical
spectroscopy and available Stromgren photometry to measure their projected
rotational velocities, effective temperatures, and polar surface gravities. We
combine our Be star detections from these four clusters to investigate physical
differences between the transient Be stars, stable Be stars, and normal B-type
stars with no line emission. Both types of Be stars are faster rotating
populations than normal B-type stars, and we find no significant physical
differences between the transient and stable Be stars in our sample.Comment: Accepted to ApJ; small corrections to Table 5 and associated figure
Spectroscopy of New High Proper Motion Stars in the Northern Sky. I. New Nearby Stars, New High Velocity Stars, and an Enhanced Classification Scheme for M Dwarfs
We define an enhanced spectral classification scheme for M dwarf stars, and
use it to derive spectral classification of 104 northern stars with proper
motions larger than 0.5"/yr which we discovered in a survey of high proper
motion stars at low galactic latitudes. The final tally is as follows: 54 M
dwarfs, 25 sdK and sdM subdwarfs, 14 esdK and esdM extreme subdwarfs, and 11 DA
and DC white dwarfs. Among the most interesting cases, we find one star to be
the coolest subdwarf ever reported (LSR2036+5059, with spectral type sdM7.5), a
new M9.0 dwarf only about 6pc distant (LSR1835+3259), and a new M6.5 dwarf only
7pc from the Sun (LSR2124+4003). Spectroscopic distances suggests that 27 of
the M dwarfs, 3 of the white dwarfs, and one of the subdwarfs (LSR2036+5059)
are within 25pc of the Sun, making them excellent candidates for inclusion in
the solar neighborhood census. Estimated sky-projected velocities suggest that
most of our subdwarfs and extreme subdwarfs have halo kinematics. We find that
several white dwarfs and non metal-poor M dwarfs also have kinematics
consistent with the halo, and we briefly discuss their possible origin.Comment: 53 pages, 18 figures, 5 tables, accepted by The Astronomical Journal;
this updated version includes radial velocities for the stars and an expanded
discussio
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