2,965 research outputs found
Cosmic stellar relics in the Galactic halo
We study the stellar population history and chemical evolution of the Milky
Way (MW) in a hierarchical LCDM model for structure formation. Using a Monte
Carlo method based on the semi-analytical EPS formalism, we reconstruct the
merger tree of our Galaxy and follow the evolution of gas and stars along the
hierarchy. Our approach allows us to compare the observational properties of
the MW with model results, exploring different properties of primordial stars,
such as their IMF and the critical metallicity for low-mass star formation,
Zcr. By matching our predictions to the Metallicity Distribution Function (MDF)
of metal-poor stars in the Galactic halo we find that: (i) supernova feedback
is required to reproduce the observed properties of the MW; (ii) stars with
[Fe/H]<-2.5 form in halos accreting Galactic Medium (GM) enriched by earlier
supernova explosions; (iii) the fiducial model (Zcr=10^-4Zsun,m_PopIII=200Msun)
provides an overall good fit to the MDF but cannot account for the two stars
with [Fe/H]<-5; the latter can be accommodated if Zcr<10^-6Zsun but such model
overpopulates the range -5.3<[Fe/H]<-4 in which no stars have been detected;
(iv) the current non-detection of metal-free stars robustly constrains either
Zcr>0 or the masses of the first stars m_PopIII>0.9Msun; (v) the statistical
impact of second generation stars, i.e stars forming out of gas polluted only
by metal-free stars, is negligible in current samples; (vi) independently of
Zcr, 60% of metals in the GM are ejected through winds by halos with masses
M<6x10^9 Msun, showing that low-mass halos are the dominant population
contributing to cosmic metal enrichment.Comment: 18 pages, 12 figures, submitted to MNRA
Bertrand-Edgeworth competition in an almost symmetric oligopoly
We analyze a Bertrand-Edgeworth game in homogeneous product industry, under effcient rationing, constant marginal cost until full capacity utilization, and identical technology across firms. We solve for the equilibrium and establish its uniqueness for capacity configurations in the mixed strategy region of the capacity space such that the capacities of the largest and smallest firm are sufficiently close.Bertrand-Edgeworth competition; mixed strategy equilibrium; almost symmetric oligopoly; Mixed strategy equilibrium
Bertrand-Edgeworth games under oligopoly with a complete characterization for the triopoly
The paper extends the analysis of price competition among capacity-constrained sellers beyond the cases of duopoly and symmetric oligopoly. We first provide some general results for the oligopoly and then focus on the triopoly, providing a complete characterization of the mixed strategy equilibrium of the price game. The region of the capacity space where the equilibrium is mixed is partitioned according to the features of the mixed strategy equilibrium arising in each subregion. Then computing the mixed strategy equilibrium becomes a quite simple task. The analysis reveals features of the mixed strategy equilibrium which do not arise in the duopoly (some of them have also been discovered by Hirata (2008)).Bertrand-Edgeworth; Price game; Oligopoly; Triopoly; Mixed strategy equilibrium
Simulating cosmic metal enrichment by the first galaxies
We study cosmic metal enrichment via AMR hydrodynamical simulations in a (10
Mpc/h) volume following the Pop III-Pop II transition and for different Pop
III IMFs. We have analyzed the joint evolution of metal enrichment on galactic
and intergalactic scales at z=6 and z=4. Galaxies account for <9% of the
baryonic mass; the remaining gas resides in the diffuse phases: (a) voids, i.e.
regions with extremely low density (<1), (b) the true intergalactic
medium (IGM, 1<<10) and (c) the circumgalactic medium (CGM,
10<), the interface between the IGM and galaxies. By z=6 a
galactic mass-metallicity relation is established. At z=4, galaxies with a
stellar mass show log(O/H)+12=8.19, consistent with
observations. The total amount of heavy elements rises from
at z=6 to 8.05 at z=4. Metals in
galaxies make up to ~0.89 of such budget at z=6; this fraction increases to
~0.95 at z=4. At z=6 (z=4) the remaining metals are distributed in
CGM/IGM/voids with the following mass fractions: 0.06/0.04/0.01
(0.03/0.02/0.01). Analogously to galaxies, at z=4 a density-metallicity
(-Z) relation is in place for the diffuse phases: the IGM/voids have a
spatially uniform metallicity, Z~Zsun; in the CGM Z steeply rises
with density up to ~Zsun. In all diffuse phases a considerable
fraction of metals is in a warm/hot (T>K) state. Due to these
physical conditions, CIV absorption line experiments can probe only ~2% of the
total carbon present in the IGM/CGM; however, metal absorption line spectra are
very effective tools to study reionization. Finally, the Pop III star formation
history is almost insensitive to the chosen Pop III IMF. Pop III stars are
preferentially formed in truly pristine (Z=0) gas pockets, well outside
polluted regions created by previous star formation episodes.Comment: 23 pages, 18 figures, 3 tables, Accepted for publication in MNRA
Initial mass function of intermediate mass black hole seeds
We study the Initial Mass Function (IMF) and host halo properties of
Intermediate Mass Black Holes (IMBH, 10^{4-6} Msun) formed inside metal-free,
UV illuminated atomic cooling haloes (virial temperature T_vir > 10^4 K) either
via the direct collapse of the gas or via an intermediate Super Massive Star
(SMS) stage. We achieve this goal in three steps: (a) we derive the gas
accretion rate for a proto-SMS to undergo General Relativity instability and
produce a direct collapse black hole (DCBH) or to enter the ZAMS and later
collapse into a IMBH; (b) we use merger-tree simulations to select atomic
cooling halos in which either a DCBH or SMS can form and grow, accounting for
metal enrichment and major mergers that halt the growth of the proto-SMS by gas
fragmentation. We derive the properties of the host halos and the mass
distribution of black holes at this stage, and dub it the "Birth Mass
Function"; (c) we follow the further growth of the DCBH due to accretion of
leftover gas in the parent halo and compute the final IMBH mass.We consider two
extreme cases in which minihalos (T_vir < 10^4 K) can (fertile) or cannot
(sterile) form stars and pollute their gas leading to a different IMBH IMF. In
the (fiducial) fertile case the IMF is bimodal extending over a broad range of
masses, M= (0.5-20)x10^5 Msun, and the DCBH accretion phase lasts from 10 to
100 Myr. If minihalos are sterile, the IMF spans the narrower mass range M=
(1-2.8)x10^6 Msun, and the DCBH accretion phase is more extended (70-120 Myr).
We conclude that a good seeding prescription is to populate halos (a) of mass
7.5 < log (M_h/Msun) < 8, (b) in the redshift range 8 < z < 17, (c) with IMBH
in the mass range 4.75 < log (M_BH/Msun) < 6.25.Comment: MNRAS, in press. Comments welcom
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A new CH carbonaceous chondrite from Acfer, Algeria
A single stone weighing 1456 g was found in November 2002 in the Acfer area, Algeria. Oxygen isotope, chondrules-matrix ratio as well as other petrographic features point to a classification as CH carbonaceous chondrite
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A new CO carbonaceous chondrite from Acfer, Algeria
Many small fragments, totally weighing 118 g were found in the Acfer area by an Italian dealer. Chondrules size and types (predominance of granular olivine type), occurrence of twinned clinoenstatite and absence of plagioclase suggested a classification as CO carbonaceous chondrite
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Textural and minerochemical features of NWA 1807 and 2180, two new CV3 chondrites from northwest Africa
A textural and minerochemical study has been performed on NWA 1807 and 2180 chondrites in order to determine sizes and typologies of chondrules, relative abundances of mineral phases, compositions of main phases and bulk oxygen isotope composition
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A new CK carbonaceous chondrite from Hammada Al Hamra, Libya
A single stone weighing 198 g was found in 2001 in the Hammada al Hamra region of Libya. Petrographic features (mean chondrules dimensions, coarse grained matrix and presence of AOIs and CAIs) point to a classification as CK carbonaceous chondrite
Ultimate strength of adjustable telescopic steel props according to standard EN 1065
The European standard EN 1065 - to which most current European production appears not to have
been upgraded - addresses the design and the manufacturing of adjustable telescopic steel props, an interest
bearing economic sector. The present work aims at showing an efficient strategy for the assessment of
the ultimate strength of steel props according to EN 1065 in order to identify the ideal cost-performance
ratio in the real life production
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