14,227 research outputs found
Pressure and isotope effect on the anisotropy of MgB
We analyze the data for the pressure and boron isotope effect on the
temperature dependence of the magnetization near . Invoking the
universal scaling relation for the magnetization at fixed magnetic field it is
shown that the relative shift of , induced by pressure or boron isotope
exchange, mirrors essentially that of the anisotropy. This uncovers a novel
generic property of anisotropic type II superconductors, inexistent in the
isotropic case. For MgB it implies that the renormalization of the Fermi
surface topology due to pressure or isotope exchange is dominated by a
mechanism controlling the anisotropy.Comment: 7 pages, 3 figure
Theory and practice of microlensing lightcurves around fold singularities
Among all galactic microlensing events, those involving a passage of the
observed source star over the caustic created by a binary lens are particularly
useful in providing information about stellar atmospheres, the dynamics of
stellar populations in our own and neighbouring galaxies, and the statistical
properties of stellar and sub-stellar binaries. This paper presents a
comprehensive guide for modelling and interpreting the lightcurves obtained in
events involving fold-caustic crossings. A new general, consistent, and optimal
choice of parameters provides a deep understanding of the involved features,
avoids numerical difficulties and minimizes correlations between model
parameters. While the photometric data of a microlensing event around a caustic
crossing itself do not provide constraints on the characteristics of the
underlying binary lens and does not allow predictions of the behaviour of other
regions of the lightcurve, vital constraints can be obtained in an efficient
way if these are combined with a few simple characteristics of data outside the
caustic crossings. A corresponding algorithm containing some improvements over
an earlier approach which takes into account multi-site observations is
presented and discussed in detail together with the arising parameter
constraints paying special attention to the role of source and background
fluxes.Comment: 19 pages with 7 EPS figures embedded, LaTeX2e using mn2e.cls. Final
version, tables clarifying meaning and constraints on parameters added. This
is a preprint of an Article accepted for publication in Monthly Notices of
the Royal Astronomical Society, (C) 2004 The Royal Astronomical Societ
Crowdscanning: The future of open innovation and artificial intelligence
Open innovation will take on a new meaning as AI will scan internal and open data to find the best ideas, write Alessandro di Fiore and Simon Schneide
On the alumina dust production in the winds of O-rich Asymptotic Giant Branch stars
The O-rich Asymptotic Giant Branch (AGB) stars experience strong mass loss
with efficient dust condensation and they are major sources of dust in the
interstellar medium. Alumina dust (AlO) is an important dust component
in O-rich circumstellar shells and it is expected to be fairly abundant in the
winds of the more massive and O-rich AGB stars. By coupling AGB stellar
nucleosynthesis and dust formation, we present a self-consistent exploration on
the AlO production in the winds of AGB stars with progenitor masses
between 3 and 7 M and metallicities in the range 0.0003 Z
0.018. We find that AlO particles form at radial distances from
the centre between and 4 R (depending on metallicity), which is in
agreement with recent interferometric observations of Galactic O-rich AGB
stars. The mass of AlO dust is found to scale almost linearly with
metallicity, with solar metallicity AGBs producing the highest amount (about
10 M) of alumina dust. The AlO grain size decreases
with decreasing metallicity (and initial stellar mass) and the maximum size of
the AlO grains is 0.075 for the solar metallicity models.
Interestingly, the strong depletion of gaseous Al observed in the
low-metallicity HBB AGB star HV 2576 seems to be consistent with the formation
of AlO dust as predicted by our models. We suggest that the content of
Al may be used as a mass (and evolutionary stage) indicator in AGB stars
experiencing HBB.Comment: 13 pages, 8 figures, accepted for publication in MNRA
AGB stars in the SMC: evolution and dust properties based on Spitzer observations
We study the population of asymptotic giant branch (AGB) stars in the Small
Magellanic Cloud (SMC) by means of full evolutionary models of stars of mass
1Msun < M < 8Msun, evolved through the thermally pulsing phase. The models also
account for dust production in the circumstellar envelope. We compare Spitzer
infrared colours with results from theoretical modelling. We show that ~75% of
the AGB population of the SMC is composed by scarcely obscured objects, mainly
stars of mass M < 2.5Msun at various metallicity, formed between 700 Myr and 5
Gyr ago; ~ 70% of these sources are oxygen--rich stars, while ~ 30% are
C-stars. The sample of the most obscured AGB stars, accounting for ~ 25% of the
total sample, is composed almost entirely by carbon stars. The distribution in
the colour-colour ([3.6]-[4.5], [5.8]-[8.0]) and colour-magnitude ([3.6]-[8.0],
[8.0]) diagrams of these C-rich objects, with a large infrared emission, traces
an obscuration sequence, according to the amount of carbonaceous dust in their
surroundings. The overall population of C-rich AGB stars descends from
1.5-2Msun stars of metallicity Z=0.004, formed between 700 Myr and 2 Gyr ago,
and from lower metallicity objects, of mass below 1.5Msun, 2-5 Gyr old. We also
identify obscured oxygen-rich stars (M ~ 4-6Msun) experiencing hot bottom
burning. The differences between the AGB populations of the SMC and LMC are
also commented.Comment: 18, pages, 11 figures, accepted for publication on MNRA
The dust production rate of AGB stars in the Magellanic Clouds
We compare theoretical dust yields for stars with mass 1 Msun < mstar < 8
Msun, and metallicities 0.001 < Z < 0.008 with observed dust production rates
(DPR) by carbon- rich and oxygen-rich Asymptotic Giant Branch (C-AGB and O-AGB)
stars in the Large and Small Magellanic Clouds (LMC, SMC). The measured DPR of
C-AGB in the LMC are reproduced only if the mass loss from AGB stars is very
efficient during the carbon-star stage. The same yields over-predict the
observed DPR in the SMC, suggesting a stronger metallicity dependence of the
mass-loss rates during the carbon- star stage. DPR of O-AGB stars suggest that
rapid silicate dust enrichment occurs due to efficient hot-bottom-burning if
mstar > 3 Msun and Z > 0.001. When compared to the most recent observations,
our models support a stellar origin for the existing dust mass, if no
significant destruction in the ISM occurs, with a contribution from AGB stars
of 70% in the LMC and 15% in the SMC.Comment: 12 pages, 8 figures, accepted by MNRA
Dust from AGBs: relevant factors and modelling uncertainties
The dust formation process in the winds of Asymptotic Giant Branch stars is
discussed, based on full evolutionary models of stars with mass in the range
MMM, and metallicities .
Dust grains are assumed to form in an isotropically expanding wind, by growth
of pre--existing seed nuclei. Convection, for what concerns the treatment of
convective borders and the efficiency of the schematization adopted, turns out
to be the physical ingredient used to calculate the evolutionary sequences with
the highest impact on the results obtained. Low--mass stars with MM produce carbon type dust with also traces of silicon carbide. The
mass of solid carbon formed, fairly independently of metallicity, ranges from a
few M, for stars of initial mass M, to
M for MM; the size of dust
particles is in the range mm. On the contrary,
the production of silicon carbide (SiC) depends on metallicity. For the size of SiC grains varies in the range m, while the mass of SiC formed is
. Models of
higher mass experience Hot Bottom Burning, which prevents the formation of
carbon stars, and favours the formation of silicates and corundum. In this case
the results scale with metallicity, owing to the larger silicon and aluminium
contained in higher--Z models. At Z= we find that the most
massive stars produce dust masses M, whereas models of
smaller mass produce a dust mass ten times smaller. The main component of dust
are silicates, although corundum is also formed, in not negligible quantities
().Comment: Paper accepted for publication in Monthly Notices of the Royal
Astronomical Society Main Journal (2014 January 4
Dissecting the Spitzer color-magnitude diagrams of extreme LMC AGB stars
We trace the full evolution of low- and intermediate-mass stars () during the Asymptotic Giant Branch (AGB) phase in the
{\it Spitzer} two-color and color-magnitude diagrams. We follow the formation
and growth of dust particles in the circumstellar envelope with an
isotropically expanding wind, in which gas molecules impinge upon pre--existing
seed nuclei, favour their growth. These models are the first able to identify
the main regions in the {\it Spitzer} data occupied by AGB stars in the Large
Magellanic Cloud (LMC). The main diagonal sequence traced by LMC extreme stars
in the [3.6]-[4.5] vs. [5.8]-[8.0] and [3.6]-[8.0] vs. [8.0] planes are nicely
fit by carbon stars models; it results to be an evolutionary sequence with the
reddest objects being at the final stages of their AGB evolution. The most
extreme stars, with [3.6]-[4.5] 1.5 and [3.6]-[8.0] 3, are 2.5-3
stars surrounded by solid carbon grains. In higher mass () models dust formation is driven by the extent of Hot Bottom Burning
(HBB) - most of the dust formed is in the form of silicates and the maximum
obscuration phase by dust particles occurs when the HBB experienced is
strongest, before the mass of the envelope is considerably reduced.Comment: 5 pages, 2 figures, accepted for publication in MNRAS Letter
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