242 research outputs found
Magnetic Quench Antenna for MQXF Quadrupoles
High-field MQXF-series quadrupoles are presently under development by LARP and CERN for the upcoming LHC luminosity upgrade. Quench training and protection studies on MQXF prototypes require a capability to accurately localize quenches and measure their propagation velocity in the magnet coils. The voltage tap technique commonly used for such purposes is not a convenient option for the 4.2-m-long MQXF-A prototype, nor can it be implemented in the production model. We have developed and tested a modular inductive magnetic antenna for quench localization. The base element of our quench antenna is a round-shaped printed circuit board containing two orthogonal pairs of flat coils integrated with low-noise preamplifiers. The elements are aligned axially and spaced equidistantly in 8-element sections using a supporting rod structure. The sections are installed in the warm bore of the magnet, and can be stacked together to adapt for the magnet length. We discuss the design, operational characteristics and preliminary qualification of the antenna. Axial quench localization capability with an accuracy of better than 2 cm has been validated during training test campaign of the MQXF-S1 quadrupole
Discovery of another peculiar radial distribution of Blue Stragglers in Globular Clusters: The case of 47 Tuc
We have used high resolution WFPC2-HST and wide field ground-based
observations to construct a catalog of blue straggler stars (BSS) in the
globular cluster 47 Tuc spanning the entire radial extent of the cluster.
The BSS distribution is highly peaked in the cluster center, rapidly
decreases at intermediate radii, and finally rises again at larger radii. The
observed distribution closely resembles that discovered in M3 by Ferraro et al
(1993,1997). To date, complete BSS surveys covering the full radial extent (HST
in the center and wide field CCD ground based observations of the exterior)
have been performed for only these two clusters. Both show a bimodal radial
distribution, despite their different dynamical properties. BSS surveys
covering the full spatial extent of more globular clusters are clearly required
to determine how common bimodality is and what its consequence is for theories
of BSS formation and cluster dynamics.Comment: 21 pages, 6 figures, accepted for pubblication in Ap
Discovery of Carbon/Oxygen depleted Blue Straggler Stars in 47 Tucanae: the chemical signature of a mass-transfer formation process
We use high-resolution spectra obtained with the ESO Very Large Telescope to
measure surface abundance patterns of 43 Blue Stragglers stars (BSS) in 47 Tuc.
We discovered that a sub-population of BSS shows a significant depletion of
Carbon and Oxygen with respect to the dominant population. This evidence would
suggest the presence of CNO burning products on the BSS surface coming from a
deeply peeled parent star, as expected in the case of mass-transfer process.
This is the first detection of a chemical signature clearly pointing to a
specific BSS formation process in a globular cluster.Comment: Published on 2006, August 10, in ApJ 647, L5
Photometry and spectroscopy of faint candidate spectrophotometric standard DA white dwarfs
We present precise photometry and spectroscopy for 23 candidate
spectrophotometric standard white dwarfs. The selected stars are distributed in
the Northern hemisphere and around the celestial equators and are all fainter
than r ~ 16.5 mag. This network of stars, when established as standards,
together with the three Hubble Space Telescope primary CALSPEC white dwarfs,
will provide a set of spectrophotometric standards to directly calibrate data
products to better than 1%. These new faint standard white dwarfs will have
enough signal-to-noise ratio in future deep photometric surveys and facilities
to be measured accurately while still avoiding saturation in such surveys. They
will also fall within the dynamic range of large telescopes and their
instruments for the foreseeable future. This paper discusses the provenance of
the observational data for our candidate standard stars. The comparison with
models, reconciliation with reddening, and the consequent derivation of the
full spectral energy density distributions for each of them is reserved for a
subsequent paper.Comment: 31 pages, 17 figures, 10 tables, ApJ in press (accepted on December
23rd, 2018
The puzzling dynamical status of the core of the globular cluster NGC 6752
We have used high resolution WFPC2-HST and ground based Wide Field images to
determine the center of gravity and construct an extended radial density and
brightness profile of the cluster NGC 6752 including, for the first time,
detailed star counts in the very inner region. The barycenter of the 9
innermost X-ray sources detected by Chandra is located only 1.9" off the new
center of gravity. Both the density and the brightness profile of the central
region are best fitted by a double King model, suggesting that NGC 6752 is
experiencing a post-core collapse "bounce". Taking advantage from our new
optical data, we discuss the puzzling nature of the accelerations displayed by
the innermost millisecond pulsars detected in this cluster. We discuss two
possible origins to the accelerations: 1) the overall cluster gravitational
potential which would require a central projected mass to light ratio of order
6-7 and the existence of a few thousand solar masses of low-luminosity matter
within the inner 0.08 pc of NGC6752; 2) the existence of a local perturber(s)
of the pulsar dynamics, such as a recently proposed binary black hole of
intermediate (100-$200 Msun) mass.Comment: 24 pages, 5 figures (accepted on ApJ
Past and present star formation in the SMC: NGC 346 and its neighborhood
In the quest of understanding how star formation occurs and propagates in the
low metallicity environment of the Small Magellanic Cloud (SMC), we acquired
deep F555W (~V), and F814W (~I) HST/ACS images of the young and massive star
forming region NGC 346. These images and their photometric analysis provide us
with a snapshot of the star formation history of the region. We find evidence
for star formation extending from ~10 Gyr in the past until ~150 Myr in the
field of the SMC. The youngest stellar population (~3 +/- 1 Myr) is associated
with the NGC 346 cluster. It includes a rich component of low mass pre-main
sequence stars mainly concentrated in a number of sub-clusters, spatially co-
located with CO clumps previously detected by Rubio et al. (2000). Within our
analysis uncertainties, these sub-clusters appear coeval with each other. The
most massive stars appear concentrated in the central sub-clusters, indicating
possible mass segregation. A number of embedded clusters are also observed.
This finding, combined with the overall wealth of dust and gas, could imply
that star formation is still active. An intermediate age star cluster, BS90,
formed ~4.3 +/-0.1 Gyr ago, is also present in the region. Thus, this region of
the SMC has supported star formation with varying levels of intensity over much
of the cosmic time.Comment: 38 pages, 13 figures, 3 tables; AJ accepte
The Unimodal Distribution Of Blue Straggler Stars in M75 (NGC 6864)
We have used a combination of multiband high-resolution and wide-field
ground-based observations to image the Galactic globular cluster M75 (NGC
6864). The extensive photometric sample covers the entire cluster extension,
from the very central regions out to the tidal radius, allowing us to determine
the center of gravity and to construct the most extended star density profile
ever published for this cluster. We also present the first detailed star counts
in the very inner regions. The star density profile is well re-produced by a
standard King model with core radius r_c ~ 5.4" and intermediate-high
concentration c ~ 1.75. The present paper presents a detailed study of the BSS
population and its radial distribution. A total number of 62 bright BSSs (with
m_F255W < 21, corresponding to m_F555W < 20) has been identified, and they have
been found to be highly segregated in the cluster core. No significant upturn
in the BSS frequency has been observed in the outskirts of M75, in contrast to
several other clusters studied with the same technique. This observational fact
is quite similar to what has been found in M79 (NGC 1904) by Lanzoni et al.
(2007a). Indeed the BSS radial distributions in the two clusters is
qualitatively very similar, even if in M75 the relative BSS frequency seems to
decrease significantly faster than in M79: indeed it decreases by a factor of 5
(from 3.4 to 0.7) within 1 r_c. Such evidence indicate that the vast majority
of the cluster heavy stars (binaries) have already sunk to the core.Comment: ApJ accepted, 10 pages, 11 figures, 2 table
HST astrometry in the 30 Doradus region: II. Runaway stars from new proper motions in the Large Magellanic Cloud
We present a catalog of relative proper motions for 368,787 stars in the 30
Doradus region of the Large Magellanic Cloud (LMC), based on a dedicated
two-epoch survey with the Hubble Space Telescope (HST) and supplemented with
proper motions from our pilot archival study. We demonstrate that a relatively
short epoch difference of 3 years is sufficient to reach a 0.1 mas
yr level of precision or better. A number of stars have relative proper
motions exceeding a 3-sigma error threshold, representing a mixture of Milky
Way denizens and 17 potential LMC runaway stars. Based upon 183 VFTS OB-stars
with the best proper motions, we conclude that none of them move faster than
0.3 mas yr in each coordinate -- equivalent to 70 km
s. Among the remaining 351 VFTS stars with less accurate proper motions,
only one candidate OB runaway can be identified. We rule out any OB star in our
sample moving at a tangential velocity exceeding 120 km s. The
most significant result of this study is finding 10 stars over wide range of
masses, which appear to be ejected from the massive star cluster R136 in the
tangential plane to angular distances from out to
, equivalent to 8-98 pc. The tangential velocities of these
runaways appear to be correlated with apparent magnitude, indicating a possible
dependence on the stellar mass.Comment: 45 pages (in referee format), 12 figures, 3 tables. Submitted to AJ.
Comments are welcom
Galaxies at z = 6 - 9 from the WFC3/IR imaging of the HUDF
We present the results of a systematic search for galaxies in the redshift
range z = 6 - 9, within the new, deep, near-infrared imaging of the Hubble
Ultra Deep Field provided by the Wide Field Camera 3 (WFC3) on HST. We have
performed full SED fitting to the optical+infrared photometry of all
high-redshift galaxy candidates detected at greater than 5-sigma in at least
one of the WFC3/IR broad-band filters. After rejection of contaminants, the
result is a sample of 49 galaxies with primary redshift solutions z > 5.9. Our
sample, selected without recourse to specific colour cuts, re-selects all but
the faintest one of the 16 z-drops selected by Oesch et al. (2009), recovers
all 5 of the Y-drops reported by Bouwens et al. (2009), and adds a further 29
galaxy candidates, of which 12 lie beyond z = 6.3, and 4 lie beyond z = 7. We
also present confidence intervals on our photometric redshift estimates, and
caution that acceptable low-redshift (z < 2) solutions exist for 28 out of the
37 galaxies at z > 6.3, and for all 8 galaxy candidates at z > 7.5.
Nevertheless, the very highest redshift candidates appear to be strongly
clustered in the field. We derive new estimates of the ultraviolet galaxy
luminosity function at z = 7 and z = 8. Where our results are most robust, at a
characteristic luminosity M(1500) ~ -19.5 (AB), we find that the comoving
number density of galaxies declines by a factor of ~ 2.5 between z = 6 and z =
7, and by a further factor of ~ 2 by z = 8. These results suggest that it is
difficult for the observed population of high-redshift star-forming galaxies to
achieve reionisation by z ~ 6 without a significant contribution from galaxies
well below the detection limits, plus alterations in the escape fraction of
ionising photons and/or continued vigorous star formation at z > 15.Comment: 25 Pages, Accepted for publication in MNRA
Star Formation Histories of the LEGUS Dwarf Galaxies (I): recent History of NGC1705, NGC4449 and Holmberg II
We use HST observations from the Legacy Extragalactic UV Survey to
reconstruct the recent star formation histories (SFHs) of three actively
star-forming dwarf galaxies, NGC4449, Holmberg II and NGC1705, from their UV
color-magnitude diagrams (CMDs). We apply a CMD fitting technique using two
independent sets of stellar isochrones, PARSEC-COLIBRI and MIST, to assess the
uncertainties related to stellar evolution modelling. Irrespective of the
adopted stellar models, all the three dwarfs are found to have had almost
constant star formation rates (SFRs) in the last 100-200 Myr, with modest
enhancements (a factor of 2) above the 100 Myr-averaged-SFR. Significant
differences among the three dwarfs are found in the overall SFR, the timing of
the most recent peak and the SFRarea. The Initial Mass Function (IMF) of
NGC1705 and Holmberg II is consistent with a Salpeter slope down to 5
M, whereas it is slightly flatter, s, in NGC4449. The SFHs
derived with the two different sets of stellar models are consistent with each
other, except for some quantitative details, attributable to their input
assumptions. They also share the drawback that all synthetic diagrams predict a
clear separation in color between upper main sequence and helium burning stars,
which is not apparent in the data. Since differential reddening, significant in
NGC4449, or unresolved binaries don't appear to be sufficient to fill the gap,
we suggest this calls for a revision of both sets of stellar evolutionary
tracks.Comment: 22 pages, 17 figures, accepted for publication on Ap
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