210 research outputs found
Status of the High Field Cable Test Facility at Fermilab
Fermi National Accelerator Laboratory (FNAL) and Lawrence Berkeley National
Laboratory (LBNL) are building a new High Field Vertical Magnet Test Facility
(HFVMTF) for testing superconducting cables in high magnetic field. The
background magnetic field of 15 T in the HFVMTF will be produced by a magnet
provided by LBNL. The HFVMTF is jointly funded by the US DOE Offices of
Science, High Energy Physics (HEP), and Fusion Energy Sciences (FES), and will
serve as a superconducting cable test facility in high magnetic fields and a
wide range of temperatures for HEP and FES communities. This facility will also
be used to test high-field superconducting magnet models and demonstrators,
including hybrid magnets, produced by the US Magnet Development Program (MDP).
The paper describes the status of the facility, including construction,
cryostat designs, top and lambda plates, and systems for powering, and quench
protection and monitoring
Energy deposition studies of block-coil quadrupoles for the LHC luminosity upgrade
At the LHC upgrade luminosity of 10{sup 35} cm{sup -2} s{sup -1}, collision product power in excess of a kW is deposited in the inner triplet quadrupoles. The quadrupole field sweeps secondary particles from pp-collisions into the superconducting (SC) coils, concentrating the power deposition at the magnetic mid-planes. The local peak power density can substantially exceed the conductor quench limits and reduce component lifetime. Under these conditions, block-coil geometries may result in overall improved performance by removing the superconductor from the magnetic mid-planes and/or allowing increased shielding at such locations. First realistic energy deposition simulations are performed for an interaction region based on block-coil quadrupoles with parameters suitable for the LHC upgrade. Results are presented on distributions of power density and accumulated dose in the inner triplet components as well as on dynamic heat loads on the cryogenic system. Optimization studies are performed on configuration and parameters of the beam pipe, cold bore and cooling channels. The feasibility of the proposed design is discussed
Time-domain Study of the Young Massive Cluster Westerlund 2 with the Hubble Space Telescope. I
Time-domain studies of pre-main-sequence (PMS) stars have long been used to investigate star properties during their early evolutionary phases and to trace the evolution of circumstellar environments. Historically these studies have been confined to the nearest, low-density, star-forming regions. We used the Wide Field Camera 3 on board the Hubble Space Telescope to extend, for the first time, the study of PMS variability to one of the few young massive clusters in the Milky Way, Westerlund 2. Our analysis reveals that at least one-third of the intermediate- and low-mass PMS stars in Westerlund 2 are variable. Based on the characteristics of their light curves, we classified ~11% of the variable stars as weak-line T Tauri candidates, ~52% as classical T Tauri candidates, ~5% as dippers, and ~26% as bursters. In addition, we found that 2% of the stars below 6 M ⊙ (~6% of the variables) are eclipsing binaries, with orbital periods shorter than 80 days. The spatial distribution of the different populations of variable PMS stars suggests that stellar feedback and UV radiation from massive stars play an important role in the evolution of circumstellar and planetary disks
Massive runaway stars in the Small Magellanic Cloud
Using archival Spitzer Space Telescope data, we identified for the first time
a dozen runaway OB stars in the Small Magellanic Cloud (SMC) through the
detection of their bow shocks. The geometry of detected bow shocks allows us to
infer the direction of motion of the associated stars and to determine their
possible parent clusters and associations. One of the identified runaway stars,
AzV 471, was already known as a high-velocity star on the basis of its high
peculiar radial velocity, which is offset by ~40 km/s from the local systemic
velocity. We discuss implications of our findings for the problem of the origin
of field OB stars. Several of the bow shock-producing stars are found in the
confines of associations, suggesting that these may be "alien" stars
contributing to the age spread observed for some young stellar systems. We also
report the discovery of a kidney-shaped nebula attached to the early WN-type
star SMC-WR3 (AzV 60a). We interpreted this nebula as an interstellar structure
created owing to the interaction between the stellar wind and the ambient
interstellar medium.Comment: Accepted by A&
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
STEP: the VST survey of the SMC and the Magellanic Bridge - I : Overview and first results
STEP (the SMC in Time: Evolution of a Prototype interacting late-type dwarf galaxy) is a Guaranteed Time Observation survey being performed at the VST (the ESO VLT Survey Telescope). STEP will image an area of 74 sq. deg. covering the main body of the Small Magellanic Cloud (32 sq. deg.), the Bridge that connects it to the Large Magellanic Cloud (30 sq. deg.) and a small part of the Magellanic Stream (2 sq. deg.). Our g, r, i, Hα photometry is able to resolve individual stars down to magnitudes well below the main-sequence turn-off of the oldest populations. In this first paper, we describe the observing strategy, the photometric techniques and the upcoming data products of the survey. We also present preliminary results for the first two fields for which data acquisition is completed, including some detailed analysis of the two stellar clusters IC 1624 and NGC 419.Peer reviewedFinal Accepted Versio
The VLT-FLAMES Tarantula Survey XVIII. Classifications and radial velocities of the B-type stars
We present spectral classifications for 438 B-type stars observed as part of the VLT-FLAMES Tarantula Survey (VFTS) in the 30 Doradus region of the Large Magellanic Cloud. Radial velocities are provided for 307 apparently single stars, and for 99 targets with radial-velocity variations which are consistent with them being spectroscopic binaries. We investigate the spatial distribution of the radial velocities across the 30 Dor region, and use the results to identify candidate runaway stars. Excluding potential runaways and members of two older clusters in the survey region (SL 639 and Hodge 301), we determine a systemic velocity for 30 Dor of 271.6 ± 12.2 kms-1 from 273 presumed single stars. Employing a 3σ criterion we identify nine candidate runaway stars (2.9% of the single stars with radial-velocity estimates). The projected rotational velocities of the candidate runaways appear to be significantly different to those of the full B-type sample, with a strong preference for either large (≥345 kms-1) or small (≤65 kms-1) rotational velocities. Of the candidate runaways, VFTS 358 (classified B0.5: V) has the largest differential radial velocity (−106.9 ± 16.2 kms-1), and a preliminary atmospheric analysis finds a significantly enriched nitrogen abundance of 12 + log (N/H) ≳ 8.5. Combined with a large rotational velocity (ve sin i = 345 ± 22 kms-1), this is suggestive of past binary interaction for this star
Insertion Magnets
Chapter 3 in High-Luminosity Large Hadron Collider (HL-LHC) : Preliminary
Design Report. The Large Hadron Collider (LHC) is one of the largest scientific
instruments ever built. Since opening up a new energy frontier for exploration
in 2010, it has gathered a global user community of about 7,000 scientists
working in fundamental particle physics and the physics of hadronic matter at
extreme temperature and density. To sustain and extend its discovery potential,
the LHC will need a major upgrade in the 2020s. This will increase its
luminosity (rate of collisions) by a factor of five beyond the original design
value and the integrated luminosity (total collisions created) by a factor ten.
The LHC is already a highly complex and exquisitely optimised machine so this
upgrade must be carefully conceived and will require about ten years to
implement. The new configuration, known as High Luminosity LHC (HL-LHC), will
rely on a number of key innovations that push accelerator technology beyond its
present limits. Among these are cutting-edge 11-12 tesla superconducting
magnets, compact superconducting cavities for beam rotation with ultra-precise
phase control, new technology and physical processes for beam collimation and
300 metre-long high-power superconducting links with negligible energy
dissipation. The present document describes the technologies and components
that will be used to realise the project and is intended to serve as the basis
for the detailed engineering design of HL-LHC.Comment: 19 pages, Chapter 3 in High-Luminosity Large Hadron Collider (HL-LHC)
: Preliminary Design Repor
Chemical Abundances of Seven Irregular and Three Tidal Dwarf Galaxies in the M81 Group
We have derived nebular abundances for 10 dwarf galaxies belonging to the M81
Group, including several galaxies which do not have abundances previously
reported in the literature. For each galaxy, multiple H \ii regions were
observed with GMOS-N at the Gemini Observatory in order to determine abundances
of several elements (oxygen, nitrogen, sulfur, neon, and argon). For seven
galaxies, at least one H \ii region had a detection of the temperature
sensitive [OIII] 4363 line, allowing a "direct" determination of the
oxygen abundance. No abundance gradients were detected in the targeted galaxies
and the observed oxygen abundances are typically in agreement with the well
known metallicity-luminosity relation. However, three candidate "tidal dwarf"
galaxies lie well off this relation, UGC 5336, Garland, and KDG 61. The nature
of these systems suggests that UGC 5336 and Garland are indeed recently formed
systems, whereas KDG 61 is most likely a dwarf spheroidal galaxy which lies
along the same line of sight as the M81 tidal debris field. We propose that
these H \ii regions formed from previously enriched gas which was stripped from
nearby massive galaxies (e.g., NGC 3077 and M81) during a recent tidal
interaction.Comment: 37 pages, 10 figures, accepted for publication in ApJ. Slit positions
in Table 2 have been update
- …