111 research outputs found
Fast-neutron induced background in LaBr3:Ce detectors
The response of a scintillation detector with a cylindrical 1.5-inch LaBr3:Ce
crystal to incident neutrons has been measured in the energy range En = 2-12
MeV. Neutrons were produced by proton irradiation of a Li target at Ep = 5-14.6
MeV with pulsed proton beams. Using the time-of-flight information between
target and detector, energy spectra of the LaBr3:Ce detector resulting from
fast neutron interactions have been obtained at 4 different neutron energies.
Neutron-induced gamma rays emitted by the LaBr3:Ce crystal were also measured
in a nearby Ge detector at the lowest proton beam energy. In addition, we
obtained data for neutron irradiation of a large-volume high-purity Ge detector
and of a NE-213 liquid scintillator detector, both serving as monitor detectors
in the experiment. Monte-Carlo type simulations for neutron interactions in the
liquid scintillator, the Ge and LaBr3:Ce crystals have been performed and
compared with measured data. Good agreement being obtained with the data, we
present the results of simulations to predict the response of LaBr3:Ce
detectors for a range of crystal sizes to neutron irradiation in the energy
range En = 0.5-10 MeVComment: 28 pages, 10 figures, 4 Table
The EROS2 search for microlensing events towards the spiral arms: the complete seven season results
The EROS-2 project has been designed to search for microlensing events
towards any dense stellar field. The densest parts of the Galactic spiral arms
have been monitored to maximize the microlensing signal expected from the stars
of the Galactic disk and bulge. 12.9 million stars have been monitored during 7
seasons towards 4 directions in the Galactic plane, away from the Galactic
center. A total of 27 microlensing event candidates have been found. Estimates
of the optical depths from the 22 best events are provided. A first order
interpretation shows that simple Galactic models with a standard disk and an
elongated bulge are in agreement with our observations. We find that the
average microlensing optical depth towards the complete EROS-cataloged stars of
the spiral arms is , a number that is
stable when the selection criteria are moderately varied. As the EROS catalog
is almost complete up to , the optical depth estimated for the
sub-sample of bright target stars with () is easier to interpret. The set of microlensing events
that we have observed is consistent with a simple Galactic model. A more
precise interpretation would require either a better knowledge of the distance
distribution of the target stars, or a simulation based on a Galactic model.
For this purpose, we define and discuss the concept of optical depth for a
given catalog or for a limiting magnitude.Comment: 22 pages submitted to Astronomy & Astrophysic
Search for new resonant states in 10C and 11C and their impact on the cosmological lithium problem
The observed primordial 7Li abundance in metal-poor halo stars is found to be
lower than its Big-Bang nucleosynthesis (BBN) calculated value by a factor of
approximately three. Some recent works suggested the possibility that this
discrepancy originates from missing resonant reactions which would destroy the
7Be, parent of 7Li. The most promising candidate resonances which were found
include a possibly missed 1- or 2- narrow state around 15 MeV in the compound
nucleus 10C formed by 7Be+3He and a state close to 7.8 MeV in the compound
nucleus 11C formed by 7Be+4He. In this work, we studied the high excitation
energy region of 10C and the low excitation energy region in 11C via the
reactions 10B(3He,t)10C and 11B(3He,t)11C, respectively, at the incident energy
of 35 MeV. Our results for 10C do not support 7Be+3He as a possible solution
for the 7Li problem. Concerning 11C results, the data show no new resonances in
the excitation energy region of interest and this excludes 7Be+4He reaction
channel as an explanation for the 7Li deficit.Comment: Accepted for publication in Phys. Rev. C (Rapid Communication
Gamma ray production cross sections in proton induced reactions on natural Mg, Si and Fe targets over the proton energy range 30 up to 66 MeV
Gamma-ray excitation functions have been measured for 30, 42, 54 and 66 MeV
proton beams accelerated onto C + O (Mylar), Mg, Si, and Fe targets of
astrophysical interest at the separate-sector cyclotron of iThemba LABS in
Somerset West (Cape Town, South Africa). A large solid angle, high energy
resolution detection system of the Eurogam type was used to record Gamma-ray
energy spectra. Derived preliminary results of Gamma-ray line production cross
sections for the Mg, Si and Fe target nuclei are reported and discussed. The
current cross section data for known, intense Gamma-ray lines from these nuclei
consistently extend to higher proton energies previous experimental data
measured up to Ep ~ 25 MeV at the Orsay and Washington tandem accelerators.
Data for new Gamma-ray lines observed for the first time in this work are also
reported.Comment: 11 pages, 6 figures. IOP Institute of Physics Conference Nuclear
Physics in Astrophysics VII, 28th EPF Nuclear Physics Divisional Conference,
May 18-22 2015, York, U
Discovery of a peculiar Cepheid-like star towards the northern edge of the Small Magellanic Cloud
For seven years, the EROS-2 project obtained a mass of photometric data on
variable stars. We present a peculiar Cepheid-like star, in the direction of
the Small Magellanic Cloud, which demonstrates unusual photometric behaviour
over a short time interval. We report on data of the photometry acquired by the
MARLY telescope and spectroscopy from the EFOSC instrument for this star,
called EROS2 J005135-714459(sm0060n13842), which resembles the unusual Cepheid
HR 7308. The light curve of our target is analysed using the Analysis of
Variance method to determine a pulsational period of 5.5675 days. A fit of
time-dependent Fourier coefficients is performed and a search for proper motion
is conducted. The light curve exhibits a previously unobserved and spectacular
change in both mean magnitude and amplitude, which has no clear theoretical
explanation. Our analysis of the spectrum implies a radial velocity of 104 km
s and a metallicity of -0.40.2 dex. In the direction of right
ascension, we measure a proper motion of 17.46.0 mas yr using EROS
astrometry, which is compatible with data from the NOMAD catalogue. The nature
of EROS2 J005135-714459(sm0060n13842) remains unclear. For this star, we may
have detected a non-zero proper motion for this star, which would imply that it
is a foreground object. Its radial velocity, pulsational characteristics, and
photometric data, however, suggest that it is instead a Cepheid-like object
located in the SMC. In such a case, it would present a challenge to
conventional Cepheid models.Comment: Correction of typos in the abstrac
Galactic Bulge Microlensing Optical Depth from EROS-2
We present a new EROS-2 measurement of the microlensing optical depth toward
the Galactic Bulge. Light curves of clump-giant stars
distributed over of the Bulge were monitored during seven Bulge
seasons. 120 events were found with apparent amplifications greater than 1.6
and Einstein radius crossing times in the range 5 {\rm d}.
This is the largest existing sample of clump-giant events and the first to
include northern Galactic fields. In the Galactic latitude range
1.4\degr<|b|<7.0\degr, we find with . These results are in good
agreement with our previous measurement, with recent measurements of the MACHO
and OGLE-II groups, and with predictions of Bulge models.Comment: accepted A&A, minor revision
R Coronae Borealis stars in the Galactic Bulge discovered by EROS-2
Rare types of variable star may give unique insight into short-lived stages
of stellar evolution. The systematic monitoring of millions of stars and
advanced light curve analysis techniques of microlensing surveys make them
ideal for discovering also such rare variable stars. One example is the R
Coronae Borealis (RCB) stars, a rare type of evolved carbon-rich supergiant. We
have conducted a systematic search of the EROS-2 database for the Galactic
catalogue Bulge and spiral arms to find Galactic RCB stars. The light curves of
100 million stars, monitored for 6.7 years (from July 1996 to February
2003), have been analysed to search for the main signature of RCB stars, large
and rapid drops in luminosity. Follow-up spectroscopy has been used to confirm
the photometric candidates. We have discovered 14 new RCB stars, all in the
direction of the Galactic Bulge, bringing the total number of confirmed
Galactic RCB stars to about 51. After reddening correction, the colours and
absolute magnitudes of at least 9 of the stars are similar to those of
Magellanic RCB stars. This suggests that these stars are in fact located in the
Galactic Bulge, making them the first RCB stars discovered in the Bulge. The
localisation of the 5 remaining RCBs is more uncertain: 4 are either located
behind the Bulge at an estimated maximum distance of 14 kpc or have an unusual
thick circumstellar shell; the other is a DY Per RCB which may be located in
the Bulge, even if it is fainter than the known Magellanic DY Per. From the
small scale height found using the 9 new Bulge RCBs,
pc (95% C.L.), we conclude that the RCB stars follow a disk-like distribution
inside the Bulge.Comment: 20 pages, 26 figures, Accepted in A&
New Magellanic Cloud R Coronae Borealis and DY Per type stars from the EROS-2 database: the connection between RCBs, DYPers and ordinary carbon stars
R Coronae Borealis stars (RCB) are a rare type of evolved carbon-rich
supergiant stars that are increasingly thought to result from the merger of two
white dwarfs, called the Double degenerate scenario. This scenario is also
studied as a source, at higher mass, of type Ia Supernovae (SnIa) explosions.
Therefore a better understanding of RCBs composition would help to constrain
simulations of such events. We searched for and studied RCB stars in the EROS
Magellanic Clouds database. We also extended our research to DY Per type stars
(DYPers) that are expected to be cooler RCBs (T~3500 K) and much more numerous
than their hotter counterparts. The light curves of ~70 millions stars have
been analysed to search for the main signature of RCBs and DYPers: a large drop
in luminosity. Follow-up optical spectroscopy was used to confirm each
photometric candidate found. We have discovered and confirmed 6 new Magellanic
Cloud RCB stars and 7 new DYPers, but also listed new candidates: 3 RCBs and 14
DYPers. We estimated a range of Magellanic RCB shell temperatures between 360
and 600 K. We confirm the wide range of absolute luminosity known for RCB
stars, M_V~-5.2 to -2.6. Our study further shows that mid-infrared surveys are
ideal to search for RCB stars, since they have thinner and cooler circumstellar
shells than classical post-AGB stars. In addition, by increasing the number of
known DYPers by ~400%, we have been able to shed light on the similarities in
the spectral energy distribution between DYPers and ordinary carbon stars. We
also observed that DYPer circumstellar shells are fainter and hotter than those
of RCBs. This suggests that DYPers may simply be ordinary carbon stars with
ejection events, but more abundance analysis is necessary to give a status on a
possible evolutionnary connexion between RCBs and DYPers.Comment: 22 pages, 38 figures, Accepted for publication in A&
Removing the Microlensing Blending-Parallax Degeneracy Using Source Variability
Microlensing event MACHO 97-SMC-1 is one of the rare microlensing events for
which the source is a variable star, simply because most variable stars are
systematically eliminated from microlensing studies. Using observational data
for this event, we show that the intrinsic variability of a microlensed star is
a powerful tool to constrain the nature of the lens by breaking the degeneracy
between the microlens parallax and the blended light. We also present a
statistical test for discriminating the location of the lens based on the
\chi^2 contours of the vector \Lambda, the inverse of the projected velocity.
We find that while SMC self lensing is somewhat favored over halo lensing,
neither location can be ruled out with good confidence.Comment: 15 text pages + 2 tables + 7 figures. Published in the Astrophysical
Journa
Detection of Extrasolar Planets by Gravitational Microlensing
Gravitational microlensing provides a unique window on the properties and
prevalence of extrasolar planetary systems because of its ability to find
low-mass planets at separations of a few AU. The early evidence from
microlensing indicates that the most common type of exoplanet yet detected are
the so-called "super-Earth" planets of ~10 Earth-masses at a separation of a
few AU from their host stars. The detection of two such planets indicates that
roughly one third of stars have such planets in the separation range 1.5-4 AU,
which is about an order of magnitude larger than the prevalence of gas-giant
planets at these separations. We review the basic physics of the microlensing
method, and show why this method allows the detection of Earth-mass planets at
separations of 2-3 AU with ground-based observations. We explore the conditions
that allow the detection of the planetary host stars and allow measurement of
planetary orbital parameters. Finally, we show that a low-cost, space-based
microlensing survey can provide a comprehensive statistical census of
extrasolar planetary systems with sensitivity down to 0.1 Earth-masses at
separations ranging from 0.5 AU to infinity.Comment: 43 pages. Very similar to chapter 3 of Exoplanets: Detection,
Formation, Properties, Habitability, John Mason, ed. Springer (April 3, 2008
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