12,986 research outputs found
SR-FTiR microscopy and FTIR imaging in the earth sciences
During the last decades, several books have been devoted to the application
of spectroscopic methods in mineralogy. Several short courses and meetings have
addressed particular aspects of spectroscopy, such as the analysis of hydrous
components in minerals and Earth materials. In these books, complete treatment
of the infrared theory and practical aspects of instrumentation and methods,
along with an exhaustive list of references, can be found. The present chapter
is intended to cover those aspects of infrared spectroscopy that have been
developed in the past decade and are not included in earlier reviews such as
Volume 18 of Reviews in Mineralogy. These new topics involve primarily: (1) the
use of synchrotron radiation (SR), which, although not a routine method, is now
rather extensively applied in infrared studies, in particular those requiring
ultimate spatial and time resolution and the analysis of extremely small
samples (a few tens of micrometers); (2) the development of imaging techniques
also for foreseen time resolved studies of geo-mineralogical processes and
environmental studies.Comment: 36 pages, 24 figures - Reviews in Mineralogy & Geochemistry - Vol. 78
(2013) in pres
Modelling the closest double degenerate system RXJ0806.3+1527 and its decreasing period
In the hypothesis that the 5.4m binary RXJ0806.3+1527 consists of a low mass
helium white dwarf (donor) transferring mass towards its more massive white
dwarf companion (primary), we consider as possible donors white dwarfs which
are the result of common envelope evolution occurring when the helium core mass
of the progenitor giant was still very small (~ 0.2Msun), so that they are
surrounded by a quite massive hydrogen envelope (~1/100Msun or larger), and
live for a very long time supported by proton--proton burning. Mass transfer
from such low mass white dwarfs very probably starts during the hydrogen
burning stage, and the donor structure will remain dominated by the burning
shell until it loses all the hydrogen envelope and begins transferring helium.
We model mass transfer from these low mass white dwarfs, and show that the
radius of the donor decreases while they shed the hydrogen envelope. This
radius behavior, which is due to the fact that the white dwarf is not fully
degenerate, has two important consequences on the evolution of the binary: 1)
the orbital period decreases, with a timescale consistent with the period
decrease of the binary RXJ0806.3+1527; 2) the mass transfer rate is a factor of
about 10 smaller than from a fully degenerate white dwarf, easing the problem
connected with the small X-ray luminosity of this object. The possibility that
such evolution describes the system RXJ0806.3+1527 is also consistent with the
possible presence of hydrogen in the optical spectrum of the star, whose
confirmation would become a test of the model.Comment: 13 pages, 4 figures, accepted for publication on ApJ, main journa
The oxygen vs. sodium (anti)correlation(s) in omega Cen
Recent exam of large samples of omega Cen giants shows that it shares with
mono-metallic globular clusters the presence of the sodium versus oxygen
anticorrelation, within each subset of stars with iron content in the range
-1.9<~[Fe/H]<~-1.3. These findings suggest that, while the second generation
formation history in omega Cen is more complex than that of mono-metallic
clusters, it shares some key steps with those simpler cluster. In addition, the
giants in the range -1.3<[Fe/H]<~-0.7 show a direct O--Na correlation, at
moderately low O, but Na up to 20 times solar. These peculiar Na abundances are
not shared by stars in other environments often assumed to undergo a similar
chemical evolution, such as in the field of the Sagittarius dwarf galaxy. These
O and Na abundances match well the yields of the massive asymptotic giant
branch stars (AGB) in the same range of metallicity, suggesting that the stars
at [Fe/H]>-1.3 in omega Cen are likely to have formed directly from the pure
ejecta of massive AGBs of the same metallicities. This is possible if the
massive AGBs of [Fe/H]>-1.3 in the progenitor system evolve when all the
pristine gas surrounding the cluster has been exhausted by the previous star
formation events, or the proto--cluster interaction with the Galaxy caused the
loss of a significant fraction of its mass, or of its dark matter halo, and the
supernova ejecta have been able to clear the gas out of the system. The absence
of dilution in the metal richer populations lends further support to a scenario
of the formation of second generation stars in cooling flows from massive AGB
progenitors. We suggest that the entire formation of omega Cen took place in a
few 10^8yr, and discuss the problem of a prompt formation of s--process
elements.Comment: The Astrophysical Journal, in pres
A unique model for the variety of multiple populations formation(s) in globular clusters: a temporal sequence
We explain the multiple populations recently found in the 'prototype'
Globular Cluster (GC) NGC 2808 in the framework of the asymptotic giant branch
(AGB) scenario. The chemistry of the five -or more- populations is
approximately consistent with a sequence of star formation events, starting
after the supernovae type II epoch, lasting approximately until the time when
the third dredge up affects the AGB evolution (age ~90-120Myr), and ending when
the type Ia supernovae begin exploding in the cluster, eventually clearing it
from the gas. The formation of the different populations requires episodes of
star formation in AGB gas diluted with different amounts of pristine gas. In
the nitrogen-rich, helium-normal population identified in NGC 2808 by the UV
Legacy Survey of GCs, the nitrogen increase is due to the third dredge up in
the smallest mass AGB ejecta involved in the star formation of this population.
The possibly-iron-rich small population in NGC 2808 may be a result of
contamination by a single type Ia supernova. The NGC 2808 case is used to build
a general framework to understand the variety of 'second generation' stars
observed in GCs. Cluster-to-cluster variations are ascribed to differences in
the effects of the many processes and gas sources which may be involved in the
formation of the second generation. We discuss an evolutionary scheme, based on
pollution by delayed type II supernovae, which accounts for the properties of
s-Fe-anomalous clusters.Comment: 20 pages, 7 figures, in press on MNRA
Gas and dust from solar metallicity AGB stars
We study the asymptotic giant branch (AGB) evolution of stars with masses
between . We focus on stars with a solar chemical
composition, which allows us to interpret evolved stars in the Galaxy. We
present a detailed comparison with models of the same chemistry, calculated
with a different evolution code and based on a different set of physical
assumptions. We find that stars of mass experience hot
bottom burning at the base of the envelope. They have AGB lifetimes shorter
than yr and eject into their surroundings gas contaminated
by proton-capture nucleosynthesis, at an extent sensitive to the treatment of
convection. Low mass stars with become
carbon stars. During the final phases the C/O ratio grows to . We find
a remarkable agreement between the two codes for the low-mass models and
conclude that predictions for the physical and chemical properties of these
stars, and the AGB lifetime, are not that sensitive to the modelling of the AGB
phase. The dust produced is also dependent on the mass: low-mass stars produce
mainly solid carbon and silicon carbide dust, whereas higher mass stars produce
silicates and alumina dust. Possible future observations potentially able to
add more robustness to the present results are also discussed.Comment: 27 pages, 24 figures; accepted for publication in MNRA
The helium spread in the Globular cluster 47 Tuc
Spectroscopy has shown the presence of the CN band dicothomy and the Na-O
anticorrelations for 50--70% of the investigated samples in the cluster 47 Tuc,
otherwise considered a "normal" prototype of high metallicity clusters from the
photometric analysis. Very recently, the re-analysis of a large number of
archival HST data of the cluster core has been able to put into evidence the
presence of structures in the Sub Giant Branch: it has a brighter component
with a spread in magnitude by 0.06 mag and a second one, made of about
10% of stars, a little fainter (by 0.05 mag). These data also show that
the Main Sequence of the cluster has an intrinsic spread in color which, if
interpreted as due to a small spread in helium abundance, suggests
Y0.027. In this work we examine in detail whether the Horizontal
Branch morphology and the Sub Giant structure provide further independent
indications that a real --although very small-helium spread is present in the
cluster. We re--analyze the HST archival data for the Horizontal Branch of 47
Tuc, obtaining a sample of 500 stars with very small photometric errors,
and build population synthesis based on new models to show that its particular
morphology can be better explained by taking into account a spread in helium
abundance of 2% in mass. The same variation in helium is able to explain the
spread in luminosity of the Sub Giant Branch, while a small part of the second
generation is characterized by a small C+N+O increase and provides an
explanation for the fainter Sub Giant Branch. We conclude that three
photometric features concur to form the paradigm that a small but real helium
spread is present in a cluster that has no spectacular evidence for multiple
populations like those shown by other massive clusters.Comment: Accepted for publication in the MNRAS on 2010 June 8. Received 2010
May 19; in original form 2010 February 9. 7 pages and 3 figures. No table
The formation of multiple populations in the globular cluster 47 Tuc
We use the combination of photometric and spectroscopic data of 47 Tuc stars
to reconstruct the possible formation of a second generation of stars in the
central regions of the cluster, from matter ejected from massive Asymptotic
Giant Branch stars, diluted with pristine gas. The yields from massive AGB
stars with the appropriate metallicity (Z=0.004, i.e. [Fe/H]=-0.75) are
compatible with the observations, in terms of extension and slope of the
patterns observed, involving oxygen, nitrogen, sodium and aluminium. Based on
the constraints on the maximum helium of 47 Tuc stars provided by photometric
investigations, and on the helium content of the ejecta, we estimate that the
gas out of which second generation stars formed was composed of about one-third
of gas from intermediate mass stars, with M>= 5Mo and about two-thirds of
pristine gas. We tentatively identify the few stars whose Na, Al and O
abundances resemble the undiluted AGB yields with the small fraction of 47 Tuc
stars populating the faint subgiant branch. From the relative fraction of first
and second generation stars currently observed, we estimate that the initial FG
population in 47 Tuc was about 7.5 times more massive than the cluster current
total mass.Comment: Accepted for publication in MNRA
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