335 research outputs found
Analytical Description of X-Ray Peaks: Application to L X-Ray Spectra Processing of Lanthanide Elements by Means of the Electron Probe Micro-Analyzer
The shape of Lα X-ray peaks analyzed by means of a LiF (200 plane) monochromator was described by a pseudo-Voigt function:
P(λ) = 0.35 P1(λ)+ 0.65 P2(λ)
where P1(λ) and P2(λ) are a Gaussian and a Lorentzian distribution centered at the same wavelength, with the same amplitude and half-width and in relative proportion 0.35 and 0.65 respectively. For peaks occurring at wavelength greater than ≃ 0.17 nm, a Gaussian offset was added in order to correct the asymmetry of peaks resulting from the monochromator mounting within the spectrometer.
The effective wavelength resolution was obtained by quadrature addition of the instrumental resolution and the natural width of the X-ray peaks. It has been shown that the difference in peak width of the L emission peaks of the lanthanide elements resulted from their difference in their natural widths. For these elements, the Lβ2, Lγ1 and Lγ2 were found to be accompanied by non-diagram lines, Lβ14, Lγ9 and Lγ10 respectively. The wavelength separation distances Lβ14-Lβ2, Lγ9-Lγ1 and Lγ10-Lγ2 were found consistent with the distances derived from the plasmon theory
Structure and dynamics of Oxide Melts and Glasses : a view from multinuclear and high temperature NMR
Solid State Nuclear Magnetic Resonance (NMR) experiments allow characterizing
the local structure and dynamics of oxide glasses and melts. Thanks to the
development of new experiments, it now becomes possible to evidence not only
the details of the coordination state of the network formers of glasses but
also to characterize the nature of polyatomic molecular motifs extending over
several chemical bonds. We present results involving 31P homonuclear
experiments that allow description of groups of up to three phosphate units and
27Al/17O heteronuclear that allows evidencing μ3 oxygen bridges in
aluminate glasses and rediscussion of the structure of high temperature melts.Comment: Journal of Non-Crystalline Solids (2007) in press; Also available
online at: http://crmht.cnrs-orleans.fr/Intranet/Publications/?id=207
AGAPE, an experiment to detect MACHO's in the direction of the Andromeda galaxy
The status of the Agape experiment to detect Machos in the direction of the
andromeda galaxy is presented.Comment: 4 pages, 1 figure in a separate compressed, tarred, uuencoded uufile.
In case ofproblem contact [email protected]
SPB stars in the open SMC cluster NGC 371
Pulsation in beta Cep and SPB stars are driven by the kappa mechanism which
depends critically on the metallicity. It has therefore been suggested that
beta Cep and SPB stars should be rare in the Magellanic Clouds which have lower
metallicities than the solar neighborhood. To test this prediction we have
observed the open SMC cluster NGC 371 for 12 nights in order to search for beta
Cep and SPB stars. Surprisingly, we find 29 short-period B-type variables in
the upper part of the main sequence, many of which are probably SPB stars. This
result indicates that pulsation is still driven by the kappa mechanism even in
low metallicity environments. All the identified variables have periods longer
than the fundamental radial period which means that they cannot be beta Cep
stars. Within an amplitude detection limit of 5 mmag no stars in the top of the
HR-diagram show variability with periods shorter than the fundamental radial
period. So if beta Cep stars are present in the cluster they oscillate with
amplitudes below 5 mmag, which is significantly lower than the mean amplitude
of beta Cep stars in the Galaxy. We see evidence that multimode pulsation is
more common in the upper part of the main sequence than in the lower. We have
also identified 5 eclipsing binaries and 3 periodic pulsating Be stars in the
cluster field.Comment: 8 pages, 11 figures. Accepted for publication in MNRA
EROS Variable Stars : Discovery of Beat Cepheids in the Small Magellanic Cloud and the effect of metallicity on pulsation
We report the discovery of eleven beat Cepheids in the Small Magellanic
Cloud, using data obtained by the EROS microlensing survey. Four stars are
beating in the fundamental and first overtone mode (F/1OT), seven are beating
in the first and second overtone (1OT/2OT). The SMC F/1OT ratio is
systematically higher than the LMC F/1OT, while the 1OT/2OT period ratio in the
SMC Cepheids is the same as the LMC one.Comment: 4 pages, Latex file with 4 .ps figures. accepted for publication in A
A Letter
The effect of metallicity on the Cepheid distance scale and its implications for the Hubble constant () determination
Recent HST determinations of the expansion's rate of the Universe (the Hubble
constant, H_0) assumed that the Cepheid Period-Luminosity relation at V and I
are independent of metallicity (Freedman, et al., 1996, Saha et al., 1996,
Tanvir et al., 1995). The three groups obtain different vales for H_0. We note
that most of this discrepancy stems from the asumption (by both groups) that
the Period-Luminosity relation is independent of metallicity. We come to this
conclusion as a result of our study of the Period-Luminosity relation of 481
Cepheids with 3 millions two colour measurements in the Large Magellanic Cloud
and the Small Magellanic Cloud obtained as a by-product of the EROS
microlensing survey. We find that the derived interstellar absorption
corrections are particularly sensitive to the metallicity and when our result
is applied to recent estimates based on HST Cepheids observations it makes the
low-H_0 values higher and the high-H_0 value lower, bringing those discrepant
estimates into agrement around .Comment: 4 pages, Latex, with 2 .ps accepted for publication astronomy and
astrophysics Letter
Observational Limits on Machos in the Galactic Halo
We present final results from the first phase of the EROS search for
gravitational microlensing of stars in the Magellanic Clouds by unseen
deflectors (machos: MAssive Compact Halo Objects). The search is sensitive to
events with time scales between 15 minutes and 200 days corresponding to
deflector masses in the range 1.e-7 to a few solar masses. Two events were
observed that are compatible with microlensing by objects of mass of about 0.1
Mo. By comparing the results with the expected number of events for various
models of the Galaxy, we conclude that machos in the mass range [1.e-7, 0.02]
Mo make up less than 20% (95% C.L.) of the Halo dark matter.Comment: 4 pages, 3 Postscript figures, to be published in Astronomy &
Astrophysic
Towards A Census of Earth-mass Exo-planets with Gravitational Microlensing
Thirteen exo-planets have been discovered using the gravitational
microlensing technique (out of which 7 have been published). These planets
already demonstrate that super-Earths (with mass up to ~10 Earth masses) beyond
the snow line are common and multiple planet systems are not rare. In this
White Paper we introduce the basic concepts of the gravitational microlensing
technique, summarise the current mode of discovery and outline future steps
towards a complete census of planets including Earth-mass planets. In the
near-term (over the next 5 years) we advocate a strategy of automated follow-up
with existing and upgraded telescopes which will significantly increase the
current planet detection efficiency. In the medium 5-10 year term, we envision
an international network of wide-field 2m class telescopes to discover
Earth-mass and free-floating exo-planets. In the long (10-15 year) term, we
strongly advocate a space microlensing telescope which, when combined with
Kepler, will provide a complete census of planets down to Earth mass at almost
all separations. Such a survey could be undertaken as a science programme on
Euclid, a dark energy probe with a wide-field imager which has been proposed to
ESA's Cosmic Vision Programme.Comment: 10 pages. White Paper submission to the ESA Exo-Planet Roadmap
Advisory Team. See also "Inferring statistics of planet populations by means
of automated microlensing searches" by M. Dominik et al. (arXiv:0808.0004
Observation of periodic variable stars towards the galactic spiral arms by EROS II
We present the results of a massive variability search based on a photometric
survey of a six square degree region along the Galactic plane at (, ) and (, ). This
survey was performed in the framework of the EROS II (Exp\'erience de Recherche
d'Objets Sombres) microlensing program. The variable stars were found among
1,913,576 stars that were monitored between April and June 1998 in two
passbands, with an average of 60 measurements. A new period-search technique is
proposed which makes use of a statistical variable that characterizes the
overall regularity of the flux versus phase diagram. This method is well suited
when the photometric data are unevenly distributed in time, as is our case.
1,362 objects whose luminosity varies were selected. Among them we identified 9
Cepheids, 19 RR Lyrae, 34 Miras, 176 eclipsing binaries and 266 Semi-Regular
stars. Most of them are newly identified objects. The cross-identification with
known catalogues has been performed. The mean distance of the RR Lyrae is
estimated to be kpc undergoing an average absorption of
magnitudes. This distance is in good agreement with the one
of disc stars which contribute to the microlensing source star population.Our
catalogue and light curves are available electronically from the CDS,
Strasbourg and from our Web site http://eros.in2p3.fr.Comment: 15 pages, 11 figures, accepted in A&A (april 2002
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