261 research outputs found
Chemical abundances in seven red giants of NGC 2360 and NGC 2447
Chemical abundances of about fifteen elements from oxygen to europium are
measured in seven red giants of the two open clusters NGC 2360 and NGC 2447.
The effective temperatures of the giants are determined spectroscopically by
taking advantage of their known masses (~2 Mo in NGC 2360 and ~3 Mo in NGC
2447) and bolometric magnitudes.
The average iron abundances we obtain for the two clusters are [Fe/H]=0.07
for NGC 2360 and [Fe/H]=0.03 for NGC 2447.
Evolutionary stellar model calculations are performed in the mass range 1-4
Mo in order to analyze the surface Na and O abundances predicted after the
first dredge-up. The sodium abundance shows a well defined correlation with
stellar mass in the 2-3 Mo range. The agreement between our Na abundance
determinations in NGC 2360 and our model predictions at 2 Mo is very good. In
contrast, the overabundance in one of the three stars in NGC 2447 exceeds that
predicted at 3 Mo by ~0.08 dex, which is significant compared to the
observational error bars. The effects of core overshooting, convection
prescription, metallicity and nuclear reaction rates on the Na surface
predictions of our models are investigated.
An oxygen deficiency relative to iron by 0.2 dex is measured in our stars, in
disagreement with our model predictions. Assuming that the Sun is 0.1-0.3 dex
enriched in oxygen relative to neighbor stars could explain the discrepancy.Comment: 11 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
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
Clinical impact of double protease inhibitor boosting with Lopinavir/Ritonavir and Amprenavir as part of salvage antiretroviral therapy
Purpose: Double protease inhibitor (PI) boosting is being explored as a new strategy in salvage antiretroviral (ARV) therapy. However, if a negative drug interaction leads to decreased drug levels of either or both PIs, double PI boosting could lead to decreased virologic response. A negative drug interaction has been described between amprenavir (APV) and lopinavir/ritonavir (LPV/r). This observational cohort study assessed the virologic impact of the addition of APV to a salvage ARV regimen, which also contains LPV/r, compared to a regimen containing LPV/r alone. Method: Patients initiated on a salvage ARV regimen that included LPV/r obtained from the expanded access program in Toronto, Canada, were evaluated. APV (600-1,200 mg bid) was added at the discretion of the treating physician. Results: Using multivariate Cox proportional hazards models, we found that the addition of APV to a LPV/r-containing salvage regimen was not significantly associated with time to virologic suppression (< 50 copies/mL; adjusted hazard ratio [HR] = 0.75, p = .12) or with time to virologic rebound (adjusted HR = 1.46, p = .34). Those patients who received higher doses of APV had an increased chance of virologic suppression (p = .03). In a subset of 27 patients, the median LPV Ctrough was significantly lower in patients receiving APV (p = .04), and the median APV Ctrough was reduced compared to reported controls. Conclusion: Our data do not support an additional benefit in virologic reduction of double boosting with APV and LPV/r relative to LPV/r alone in salvage ARV therapy. Our study's limitations include its retrospective nature and the imbalance between the two groups potentially confounding the results. Although these factors were adjusted for in the multivariate analysis, a prospective randomized controlled trial is warranted to confirm our findings
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
The Open Cluster NGC 7789: I. Radial Velocities for Giant Stars
A total of 597 radial-velocity observations for 112 stars in the ~1.6 Gyr old
open cluster NGC 7789 have been obtained since 1979 with the radial velocity
spectrometer at the Dominion Astrophysical Observatory. The mean cluster radial
velocity is -54.9 +/- 0.12 km/s and the dispersion is 0.86 km/s, from 50
constant-velocity stars selected as members from this radial-velocity study and
the proper motion study of McNamara and Solomon (1981). Twenty-five stars (32%)
among 78 members are possible radial-velocity variable stars, but no orbits are
determined because of the sparse sampling. Seventeen stars are radial-velocity
non-members, while membership estimates of six stars are uncertain.
There is a hint that the observed velocity dispersion falls off at large
radius. This may due to the inclusion of long-period binaries preferentially in
the central area of the cluster. The known radial-velocity variables also seem
to be more concentrated toward the center than members with constant velocity.
Although this is significant at only the 85% level, when combined with similar
result of Raboud and Mermilliod (1994) for three other clusters, the data
strongly support the conclusion that mass segregation is being detected.Comment: 16 pages (including 3 figures) and 3 table
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
An HST/WFPC2 Survey for Brown Dwarf Binaries in the alpha Per and the Pleiades Open Clusters
We present the results of a high-resolution imaging survey for brown dwarf
(BD) binaries in two open clusters. The observations were carried out with
WFPC2 onboard HST. Our sample consists of 8 BD candidates in the alpha Per
cluster and 25 BD candidates in the Pleiades. We have resolved 4 binaries in
the Pleiades with separations in the range 0".094--0".058, corresponding to
projected separations between 11.7~AU and 7.2~AU. No binaries were found among
the alpha Per targets. Three of the binaries have proper motions consistent
with cluster membership in the Pleiades cluster, and for one of them we report
the detection of Halpha in emission and LiI absorption obtained from
Keck~II/ESI spectroscopy. One of the binaries does not have a proper motion
consistent with Pleiades membership. We estimate that BD binaries wider than
12~AU are less frequent than 9% in the alphaPer and Pleiades clusters. This is
consistent with an extension to substellar masses of a trend observed among
stellar binaries: the maximum semimajor axis of binary systems decreases with
decreasing primary mass. We find a binary frequency of 2 binaries over 13 BDs
with confirmed proper motion membership in the Pleiades, corresponding to a
binary fraction of 15%(1 sigma error bar +15%/-5%). These binaries are limited
to the separation range 7-12~AU and their mass ratios are larger than 0.7. The
relatively high binary frequency (>10%), the bias to separations smaller than
about 15 AU and the trend to high mass ratios (q>0.7) are fundamental
properties of BDs. Current theories of BD formation do not appear to provide a
good description of all these properties.Comment: Accepted by ApJ (scheduled publication in volume 594, September 1,
2003
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