16,135 research outputs found
Possible depletion of metals into dust grains in the core of the Centaurus cluster of galaxies
We present azimuthally averaged metal abundance profiles from a full,
comprehensive, and conservative re-analysis of the deep (800 ks total net
exposure) \textit{Chandra}/ACIS-S observation of the Centaurus cluster core
(NGC\,4696). After carefully checking various sources of systematic
uncertainties, including the choice of the spectral deprojection method,
assumptions about the temperature structure of the gas, and uncertainties in
the continuum modeling, we confirm the existence of a central drop in the
abundances of the `reactive' elements Fe, Si, S, Mg, and Ca, within
10 kpc. The same drops are also found when analyzing the
\textit{XMM-Newton}/EPIC data (150 ks). Adopting our most conservative
approach, we find that, unlike the central drops seen for Fe, Si, S, Mg and Ca,
the abundance of the `nonreactive' element Ar is fully consistent with showing
no central drop. This is further confirmed by the significant ()
central radial increase of the Ar/Fe ratio. Our results corroborate the
previously proposed `dust depletion scenario' , in which central metal
abundance drops are explained by the deposition of a significant fraction of
centrally cooled reactive metals into dust grains present in the central
regions of the Centaurus cluster. This is also supported by the previous
findings that the extent of the metal abundance drops in NGC\,4696 broadly
coincides with the infrared dust emission.Comment: Accepted for publication in A&A; 12 pages, 5 figures, 2 table
[O II] line ratios
Based on new calculations we reconfirm the low and high density limits on the
forbidden fine structure line ratio [O II] I(3729)/I(3726): lim_{N_ e} --> 0} =
1.5 and lim_{N_ e} --> \infty} = 0.35. Employing [O II] collision strengths
calculated using the Breit-Pauli R-matrix method we rule out any significant
deviation due to relativistic effects from these canonical values. The present
results are in substantial agreement with older calculations by Pradhan (1976)
and validate the extensive observational analysis of gaseous nebulae by Copetti
and Writzel (2002) and Wang et al (2004) that reach the same conclusions. The
present theoretical results and the recent observational analyses differ
significantly from the calculations by MacLaughlin and Bell (1998) and Keenan
et al (1999). The new maxwellian averaged effective collision strengths are
presented for the 10 transitions among the first 5 levels to enable
computations of [O II] line ratios.Comment: Submitted to MNRAS (Letters), 4 pages, 2 figures, 1 tabl
Ultra-High Energy Cosmic Rays: Some General Features, and Recent Developments Concerning Air Shower Computations
We present an introductory lecture on general features of cosmic rays, for
non-experts, and some recent developments concerning cascade equations for air
shower developments.Comment: invited talk, presented at the Hadron-RANP2004 worksho
Polynomial complexity despite the fermionic sign
It is commonly believed that in quantum Monte Carlo approaches to fermionic
many- body problems, the infamous sign problem generically implies
prohibitively large computational times for obtaining thermodynamic-limit
quantities. We point out that for convergent Feynman diagrammatic series
evaluated with the Monte Carlo algorithm of [Rossi, arXiv:1612.05184], the
computational time increases only polynomially with the inverse error on
thermodynamic-limit quantities
On helium-dominated stellar evolution: the mysterious role of the O(He)-type stars
About a quarter of all post-asymptotic giant branch (AGB) stars are
hydrogen-deficient. Stellar evolutionary models explain the carbon-dominated
H-deficient stars by a (very) late thermal pulse scenario where the
hydrogen-rich envelope is mixed with the helium-rich intershell layer.
Depending on the particular time at which the final flash occurs, the entire
hydrogen envelope may be burned. In contrast, helium-dominated post-AGB stars
and their evolution are yet not understood. A small group of very hot,
helium-dominated stars is formed by O(He)-type stars. We performed a detailed
spectral analysis of ultraviolet and optical spectra of four O(He) stars by
means of state-of-the-art non-LTE model-atmosphere techniques. We determined
effective temperatures, surface gravities, and the abundances of H, He, C, N,
O, F, Ne, Si, P, S, Ar, and Fe. By deriving upper limits for the mass-loss
rates of the O(He) stars, we found that they do not exhibit enhanced mass-loss.
The comparison with evolutionary models shows that the status of the O(He)
stars remains uncertain. Their abundances match predictions of a double helium
white dwarf merger scenario, suggesting that they might be the progeny of the
compact and of the luminous helium-rich sdO-type stars. The existence of
planetary nebulae that do not show helium enrichment around every other O(He)
star, precludes a merger origin for these stars. These stars must have formed
in a different way, for instance via enhanced mass-loss during their post-AGB
evolution or a merger within a common-envelope (CE) of a CO-WD and a red giant
or AGB star. A helium-dominated stellar evolutionary sequence exists, that may
be fed by different types of mergers or CE scenarios. It appears likely, that
all these pass through the O(He) phase just before they become white dwarfs.Comment: 29 pages, 27 figures, accepted for publication in A&
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