1,860 research outputs found
Spectroscopy of horizontal branch stars in Omega Centauri
We analyze the reddening, surface helium abundance and mass of 115 horizontal
branch (HB) and blue hook (BH) stars in OmegaCentauri, spanning the HB from the
blue edge of the instability strip to Teff~50000K. The mean cluster reddening
is E(B-V)=0.115+-0.004, in good agreement with previous estimates, but we
evidence a pattern of differential reddening in the cluster area. The stars in
the western half are more reddened than in the southwest quadrant by 0.03-0.04
magnitudes. We find that the helium abundances measured on low-resolution
spectra are systematically lower by ~0.25 dex than the measurements based on
higher resolution. No difference in helium abundance is detected between
OmegaCentauri and three comparison clusters, and the stars in the range
11500-20000K follow a trend with temperature, which probably reflects a
variable efficiency of the diffusion processes. There is mild evidence that two
families of extreme HB (EHB) stars (Teff>20000K) could exist, as observed in
the field, with ~15% of the objects being helium depleted by a factor of ten
with respect to the main population. The distribution of helium abundance above
30000K is bimodal, but we detect a fraction of He-poor objects lower than
previous investigations. The observations are consistent with these being stars
evolving off the HB. Their spatial distribution is not uniform, but this
asymmetric distribution is only marginally significative. We also find that EHB
stars with anomalously high spectroscopic mass could be present in
OmegaCentauri, as previously found in other clusters. The derived
temperature-color relation reveals that stars hotter than 11000K are fainter
than the expectations of the canonical models in the U band, while no anomaly
is detected in B and V. This behavior, not observed in NGC6752, is a new
peculiarity of OmegaCentauri HB stars.Comment: Accepted for publication in A&
The M4 Core Project with HST - IV. Internal Kinematics from Accurate Radial Velocities of 2771 Cluster Members
We present a detailed study of the internal kinematics of the Galactic
Globular Cluster M 4 (NGC 6121), by deriving the radial velocities from 7250
spectra for 2771 stars distributed from the upper part of the Red Giant Branch
down to the Main Sequence. We describe new approaches to determine the
wavelength solution from day-time calibrations and to determine the radial
velocity drifts that can occur between calibration and science observations
when observing with the GIRAFFE spectrograph at VLT. Two techniques to
determine the radial velocity are compared, after a qualitative description of
their advantages with respect to other commonly used algorithm, and a new
approach to remove the sky contribution from the spectra obtained with
fibre-fed spectrograph and further improve the radial velocity precision is
presented. The average radial velocity of the cluster is km s with an average dispersion of km
s. Using the same dataset and the same statistical approach of previous
analyses, 20 additional binary candidates are found, for a total of 87
candidates. A new determination of the internal radial velocity dispersion as a
function of cluster distance is presented, resulting in a dispersion of
km s within 2 from the center of cluster and steadily
decreasing outward. We statistically confirm the small amplitude of the cluster
rotation, as suggested in the past by several authors. This new analysis
represents a significant improvement with respect to previous results in
literature and provides a fundamental observational input for the modeling of
the cluster dynamics.Comment: 17 pages, 17 figures. Accepted for publication in MNRAS on September
15, 201
Absolute proper motion of the Galactic open cluster M67
We derived the absolute proper motion (PM) of the old, solar-metallicity
Galactic open cluster M67 using observations collected with CFHT (1997) and
with LBT (2007). About 50 galaxies with relatively sharp nuclei allow us to
determine the absolute PM of the cluster. We find (mu_alpha
cos(delta),mu_delta)_J2000.0 = (-9.6+/-1.1,-3.7+/-0.8) mas/yr. By adopting a
line-of-sight velocity of 33.8+/-0.2 km/s, and assuming a distance of 815+/-50
pc, we explore the influence of the Galactic potential, with and without the
bar and/or spiral arms, on the galactic orbit of the cluster.Comment: 7 pages, 5 figures, and 3 tables. Published in Astronomy and
Astrophysics, Volume 513, id.A51
A hot horizontal branch star with a close K-type main-sequence companion
Dynamical interactions in binary systems are thought to play a major role in
the formation of extreme horizontal branch stars (EHBs) in the Galactic field.
However, it is still unclear if the same mechanisms are at work in globular
clusters, where EHBs are predominantly single stars. Here we report on the
discovery of a unique close binary system (period ~1.61 days) in the globular
cluster NGC6752, comprising an EHB and a main-sequence companion of 0.63+-0.05
Msun. Such a system has no counterpart among nearly two hundred known EHB
binaries in the Galactic field. Its discovery suggests that either field
studies are incomplete, missing this type of systems possibly because of
selection effects, or that a particular EHB formation mechanism is active in
clusters but not in the field
Spectroscopic search for binaries among EHB stars in globular clusters
We performed a spectroscopic search for binaries among hot Horizontal Branch
stars in globular clusters. We present final results for a sample of 51 stars
in NGC6752, and preliminary results for the first 15 stars analyzed in M80. The
observed stars are distributed along all the HBs in the range 8000 < Teff <
32000 K, and have been observed during four nights. Radial velocity variations
have been measured with the cross-correlation technique. We carefully analyzed
the statistical and systematic errors associated with the measurements in order
to evaluate the statistical significance of the observed variations. No close
binary system has been detected, neither among cooler stars nor among the
sample of hot EHB stars (18 stars with Teff > 22000 K in NGC6752). The data
corrected for instrumental effects indicate that the radial velocity variations
are always below the 3sigma level of ~15 km/s. These results are in sharp
contrast with those found for field hot subdwarfs, and open new questions about
the formation of EHB stars in globular clusters, and possibly of the field
subdwarfs.Comment: To appear in Baltic Astronomy. Proceedings of the 2nd meeting on Hot
Subdwarf Stars, La Palma, June 2005. 4 pages, 2 figure
Multiple stellar populations in three rich Large Magellanic Cloud star clusters
We present deep colour-magnitude diagrams for three rich intermediate-age
star clusters in the Large Magellanic Cloud, constructed from archival ACS
F435W and F814W imaging. All three clusters exhibit clear evidence for peculiar
main-sequence turn-offs. NGC 1846 and 1806 each possess two distinct turn-off
branches, while the turn-off for NGC 1783 shows a much larger spread in colour
than can be explained by the photometric uncertainties. We demonstrate that
although all three clusters contain significant populations of unresolved
binary stars, these cannot be the underlying cause of the observed turn-off
morphologies. The simplest explanation is that each cluster is composed of at
least two different stellar populations with very similar metal abundances but
ages separated by up to ~300 Myr. The origin of these unusual properties
remains unidentified; however, the fact that at least three massive clusters
containing multiple stellar populations are now known in the LMC suggests a
potentially significant formation channel.Comment: To appear in ApJ Letters, replaced with accepted versio
Robust and Biocompatible Functionalization of ZnS Nanoparticles by Catechol-Bearing Poly(2-Methyl-2-Oxazoline)s.
Zinc sulfide (ZnS) nanoparticles (NPs) are particularly interesting materials for their electronic and luminescent properties. Unfortunately, their robust and stable functionalization and stabilization, especially in aqueous media, has represented a challenging and not yet completely accomplished task. In this work, we report the synthesis of colloidally stable, photoluminescent and biocompatible core\u2013polymer shell ZnS and ZnS:Tb NPs by employing a water-in-oil miniemulsion (ME) process combined with surface functionalization via catechol-bearing poly-2-methyl-2-oxazoline (PMOXA) of various molar masses. The strong binding of catechol anchors to the metal cations of the ZnS surface, coupled with the high stability of PMOXA against chemical degradation, enable the formation of suspensions presenting excellent colloidal stability. This feature, combined with the assessed photoluminescence and biocompatibility, make these hybrid NPs suitable for optical bioimaging
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