1,157 research outputs found
Alignement experience in STAR
The STAR experiment at RHIC uses four layers of silicon strip and silicon drift detectors for secondary vertex reconstruction. An attempt for a direct charm meson measurement put stringent requirements on alignment and calibration. We report on recent alignment and drift velocity calibration work performed on the inner silicon tracking system
Massive runaway stars in the Large Magellanic Cloud
The origin of massive field stars in the Large Magellanic Cloud (LMC) has
long been an enigma. The recent measurements of large offsets (~100 km/s)
between the heliocentric radial velocities of some very massive (O2-type) field
stars and the systemic LMC velocity provides a possible explanation of this
enigma and suggests that the field stars are runaway stars ejected from their
birth places at the very beginning of their parent cluster's dynamical
evolution. A straightforward way to prove this explanation is to measure the
proper motions of the field stars and to show that they are moving away from
one of the nearby star clusters or OB associations. This approach however is
complicated by the large distance to the LMC, which makes accurate proper
motion measurements difficult. We use an alternative approach for solving the
problem, based on the search for bow shocks produced by runaway stars. The
geometry of detected bow shocks would allow us to infer the direction of
stellar motion and thereby to determine their possible parent clusters. In this
paper we present the results of a search for bow shocks around six massive
field stars which were suggested in the literature as candidate runaway stars.
Using archival (Spitzer Space Telescope) data, we found a bow shock associated
with one of our program stars, the O2 V((f*)) star BI 237, which is the
first-ever detection of bow shocks in the LMC. Orientation of the bow shock
suggests that BI 237 was ejected from the OB association LH 82 (located at ~120
pc in projection from the star). A by-product of our search is the detection of
bow shocks generated by four OB stars in the field of the LMC and an arc-like
structure attached to the candidate luminous blue variable R81 (HD 269128). The
geometry of two of these bow shocks is consistent with the possibility that
their associated stars were ejected from the 30 Doradus star forming complex.Comment: 5 pages, 7 figures, accepted for publication in A&
Optical Observations of the Binary Pulsar System PSR B1718-19: Implications for Tidal Circularization
We report on Keck and Hubble Space Telescope optical observations of the
eclipsing binary pulsar system PSR B1718-19, in the direction of the globular
cluster NGC 6342. These reveal a faint star (; Vega
system) within the pulsar's 0\farcs5 radius positional error circle. This may
be the companion. If it is a main-sequence star in the cluster, it has radius
\rcomp\simeq0.3 \rsun, temperature \teff\simeq3600 K, and mass
\mcomp\simeq0.3 \msun. In many formation models, however, the pulsar (spun up
by accretion or newly formed) and its companion are initially in an eccentric
orbit. If so, for tidal circularization to have produced the present-day highly
circular orbit, a large stellar radius is required, i.e., the star must be
bloated. Using constraints on the radius and temperature from the Roche and
Hayashi limits, we infer from our observations that \rcomp\simlt0.44 \rsun
and \teff\simgt3300 K. Even for the largest radii, the required efficiency of
tidal dissipation is larger than expected for some prescriptions.Comment: 10 pages, 2 figures, aas4pp2.sty. Accepted for publication in Ap
Supernova remnant S147 and its associated neutron star(s)
The supernova remnant S147 harbors the pulsar PSR J0538+2817 whose
characteristic age is more than an order of magnitude greater than the
kinematic age of the system (inferred from the angular offset of the pulsar
from the geometric center of the supernova remnant and the pulsar proper
motion). To reconcile this discrepancy we propose that PSR J0538+2817 could be
the stellar remnant of the first supernova explosion in a massive binary system
and therefore could be as old as its characteristic age. Our proposal implies
that S147 is the diffuse remnant of the second supernova explosion (that
disrupted the binary system) and that a much younger second neutron star (not
necessarily manifesting itself as a radio pulsar) should be associated with
S147. We use the existing observational data on the system to suggest that the
progenitor of the supernova that formed S147 was a Wolf-Rayet star (so that the
supernova explosion occurred within a wind bubble surrounded by a massive
shell) and to constrain the parameters of the binary system. We also restrict
the magnitude and direction of the kick velocity received by the young neutron
star at birth and find that the kick vector should not strongly deviate from
the orbital plane of the binary system.Comment: 9 pages, 5 figures, revised version accepted for publication in A&
The infrared supernova rate in starburst galaxies
We report the results of our ongoing search for extincted supernovae (SNe) at
near-infrared wavelengths. We have monitored at 2.2 micron a sample of 46
Luminous Infrared Galaxies and detected 4 SNe. The number of detections is
still small but sufficient to provide the first estimate of supernova rate at
near-infrared wavelengths. We measure a SN rate ofv 7.6+/-3.8 SNu which is an
order of magnitude larger than observed in quiescent galaxies. On the other
hand, the observed near-infrared rate is still a factor 3-10 smaller than that
estimated from the far-infrared luminosity of the galaxies. Among various
possibilities, the most likely scenario is that dust extinction is so high
(Av>30) to obscure most SNe even in the near-IR.
The role of type Ia SNe is also discussed within this context. We derive the
type Ia SN rate as a function of the stellar mass of the galaxy and find a
sharp increase toward galaxies with higher activity of star formation. This
suggests that a significant fraction of type Ia SNe are associated with young
stellar populations.
Finally, as a by-product, we give the average K-band light curve of
core-collapse SNe based on all the existing data, and review the relation
between SN rate and far-infrared luminosity.Comment: A&A, in press, 13 page
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&
Illumination in symbiotic binary stars: Non-LTE photoionization models. II. Wind case
We describe a non-LTE photoionization code to calculate the wind structure
and emergent spectrum of a red giant wind illuminated by the hot component of a
symbiotic binary system. We consider spherically symmetric winds with several
different velocity and temperature laws and derive predicted line fluxes as a
function of the red giant mass loss rate, \mdot. Our models generally match
observations of the symbiotic stars EG And and AG Peg for \mdot about 10^{-8}
\msunyr to 10^{-7} \msunyr. The optically thick cross- section of the red giant
wind as viewed from the hot component is a crucial parameter in these models.
Winds with cross-sections of 2--3 red giant radii reproduce the observed
fluxes, because the wind density is then high, about 10^9 cm^{-3}. Our models
favor winds with acceleration regions that either lie far from the red giant
photosphere or extend for 2--3 red giant radii.Comment: 51 pages, LaTeX including three tables, requires 15 Encapsulated
Postscript figures, to appear in Ap
Overview Of The Inner Silicon Detector Alignment Procedure And Techniques In The Rhic/star Experiment
The STAR experiment was primarily designed to detect signals of a possible phase transition in nuclear matter. Its layout, typical for a collider experiment, contains a large Time Projection Chamber (TPC) in a solenoid magnet, a set of four layers of combined silicon strip and silicon drift detectors for secondary vertex reconstruction, plus other detectors. In this presentation, we will report on recent global and individual detector element alignment as well as drift velocity calibration work performed on this STAR inner silicon tracking system. We will show how attention to details positively impacts the physics capabilities of STAR and explain the iterative procedure conducted to reach such results in low, medium and high track density and detector occupancy. © 2008 IOP Publishing Ltd.1193STAR detector overview, H.K.Ackerman et al, NIM A499: 624,2003The STAR time projection chamber: a unique tool for studying high multiplicity events at RHIC, M.Anderson et al, NIM A499: 659,2003The laser system for the STAR time projection chamber, J. Abele et al, NIM A499: 692,2003Correcting for distortions due to ionization in the STAR TPC, G. Van Buren et al.,NIM A566:22-25,2006The STAR Silicon Vertex Tracker A large area Silicon Drift Detector, R.Bellwied et Al., NIM A499: 640, 2003The STAR silicon strip-detector (SSD), L.Arnold et al, NIM 2003 A499: 652, 2003SVT Alignment, STAR SVT review, 2004, Marcelo G. Munhoz, private communicationSensor Alignment by Tracks, V.Karimaki et al,CMS CR-2004/009 (presented at CHEP 2003)Alignment Strategy for the SMT Barrel Detectors, D.Chakborty, J.D.Hobbs, October 13, 1999. DO Note (unpublished
A comparison of statistical hadronization models
We investigate the sensitivity of fits of hadron spectra produced in heavy
ion collisions to the choice of statistical hadronization model. We start by
giving an overview of statistical model ambiguities, and what they tell us
about freeze-out dynamics. We then use Montecarlo generated data to determine
sensitivity to model choice. We fit the statistical hadronization models under
consideration to RHIC data, and find that a comparison fits can shed
light on some presently contentious questions.Comment: Proceedings for SQM2003 [7th Int. Conf. on Strangeness in Quark
Matter (Atlantic Beach, NC, USA, Mar 12-17, 2003)], to be published in
Journal of Physics G (Typos corrected, reference added
4U 1907+09: a HMXB running away from the Galactic plane
We report the discovery of a bow shock around the high-mass X-ray binary
(HMXB) 4U 1907+09 using the Spitzer Space Telescope 24 m data (after Vela
X-1 the second example of bow shocks associated with HMXBs). The detection of
the bow shock implies that 4U 1907+09 is moving through the space with a high
(supersonic) peculiar velocity. To confirm the runaway nature of 4U 1907+09, we
measured its proper motion, which for an adopted distance to the system of 4
kpc corresponds to a peculiar transverse velocity of km/s,
meaning that 4U 1907+09 is indeed a runaway system and supporting the general
belief that most of HMXBs possess high space velocities. The direction of
motion of 4U 1907+09 inferred from the proper motion measurement is consistent
with the orientation of the symmetry axis of the bow shock, and shows that the
HMXB is running away from the Galactic plane. We also present the Spitzer
images of the bow shock around Vela X-1 (a system similar to 4U 1907+09) and
compare it with the bow shock generated by 4U 1907+09.Comment: accepted for publication in A&
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