1,029 research outputs found
Consequences of a Change in the Galactic Environment of the Sun
The interaction of the heliosphere with interstellar clouds has attracted
interest since the late 1920's, both with a view to explaining apparent
quasi-periodic climate "catastrophes" as well as periodic mass extinctions.
Until recently, however, models describing the solar wind - local interstellar
medium (LISM) interaction self-consistently had not been developed. Here, we
describe the results of a two-dimensional (2D) simulation of the interaction
between the heliosphere and an interstellar cloud with the same properties as
currently, except that the neutral H density is increased from the present
value of n(H) ~ 0.2 cm^-3 to 10 cm^-3. The mutual interaction of interstellar
neutral hydrogen and plasma is included. The heliospheric cavity is reduced
considerably in size (approximately 10 - 14 AU to the termination shock in the
upstream direction) and is highly dynamical. The interplanetary environment at
the orbit of the Earth changes markedly, with the density of interstellar H
increasing to ~2 cm^-3. The termination shock itself experiences periods where
it disappears, reforms and disappears again. Considerable mixing of the shocked
solar wind and LISM occurs due to Rayleigh-Taylor-like instabilities at the
nose, driven by ion-neutral friction. Implications for two anomalously high
concentrations of 10Be found in Antarctic ice cores 33 kya and 60 kya, and the
absence of prior similar events, are discussed in terms of density enhancements
in the surrounding interstellar cloud. The calculation presented here supports
past speculation that the galactic environment of the Sun moderates the
interplanetary environment at the orbit of the Earth, and possibly also the
terrestrial climate.Comment: 23 pages, 2 color plates (jpg), 3 figures (eps
Halos of Spiral Galaxies. III. Metallicity Distributions
(Abriged) We report results of a campaign to image the stellar populations in
the halos of highly inclined spiral galaxies, with the fields roughly 10 kpc
(projected) from the nuclei. We use the F814W (I) and F606W (V) filters in the
Wide Field Planetary Camera 2, on board the Hubble Space telescope. Extended
halo populations are detected in all galaxies. The color-magnitude diagrams
appear to be completely dominated by giant-branch stars, with no evidence for
the presence of young stellar populations in any of the fields. We find that
the metallicity distribution functions are dominated by metal-rich populations,
with a tail extending toward the metal poor end. To first order, the overall
shapes of the metallicity distribution functions are similar to what is
predicted by simple, single-component model of chemical evolution with the
effective yields increasing with galaxy luminosity. However, metallicity
distributions significantly narrower than the simple model are observed for a
few of the most luminous galaxies in the sample. It appears clear that more
luminous spiral galaxies also have more metal-rich stellar halos. The
increasingly significant departures from the closed-box model for the more
luminous galaxies indicate that a parameter in addition to a single yield is
required to describe chemical evolution. This parameter, which could be related
to gas infall or outflow either in situ or in progenitor dwarf galaxies that
later merge to form the stellar halo, tends to act to make the metallicity
distributions narrower at high metallicity.Comment: 20 pages, 8 figures (ApJ, in press
Chemical Evolution of the Galaxy Based on the Oscillatory Star Formation History
We model the star formation history (SFH) and the chemical evolution of the
Galactic disk by combining an infall model and a limit-cycle model of the
interstellar medium (ISM). Recent observations have shown that the SFH of the
Galactic disk violently variates or oscillates. We model the oscillatory SFH
based on the limit-cycle behavior of the fractional masses of three components
of the ISM. The observed period of the oscillation ( Gyr) is reproduced
within the natural parameter range. This means that we can interpret the
oscillatory SFH as the limit-cycle behavior of the ISM. We then test the
chemical evolution of stars and gas in the framework of the limit-cycle model,
since the oscillatory behavior of the SFH may cause an oscillatory evolution of
the metallicity. We find however that the oscillatory behavior of metallicity
is not prominent because the metallicity reflects the past integrated SFH. This
indicates that the metallicity cannot be used to distinguish an oscillatory SFH
from one without oscillations.Comment: 21 pages LaTeX, to appear in Ap
The stellar populations of spiral disks.II Measuring and modeling the radial distribution of absorption spectral indices
The radial distributions of the Mg2 and Fe5270 Lick spectral indices have
been measured to large radial distances on the disks of NGC 4303 and NGC 4535
using an imaging technique based on interference filters. These data, added to
those of NGC 4321 previously published in Paper I of this series are used to
constraint chemical (multiphase) evolutionary models for these galaxies.
Because the integrated light of a stellar disk is a time average over the
history of the galaxy weighted by the star formation rate, these constraints
complement the information on chemical gradients provided by the study of HII
regions which, by themselves, can only provide the alpha-elements abundance
accumulate over the life of the galaxy. The agreement between the observations
and the model predictions shown here lends confidence to the models which are
then used to describe the time evolution of galaxy parameters such as star
formation rates, chemical gradients, and gradients in the mean age of the
stellar population.Comment: to be published in Astrophysical Journa
XMM-Newton observation of PSR B2224+65 and its jet
We have investigated the pulsar PSR B2224+65 and its X-ray jet with
XMM-Newton. Apart from the long X-ray jet which is almost perpendicular to the
direction of proper motion, a putative extended feature at the pulsar position,
which oriented in the opposite direction of the proper motion, is also
suggested by this deep X-ray imaging. Non-detection of any coherent X-ray
pulsation disfavors the magnetospheric origin of the X-rays observed from the
position of PSR B2224+65 and hence suggest that the interpretation of pulsar
wind nebula is more viable. We have also probed the origin of PSR B2224+65 and
identified a runaway star, which possibly originated from the Cygnus OB9
association, as a candidate for the former binary companion of the neutron
star's progenitor.Comment: 24 pages, 8 figures, 3 tables, accepted for publication in Ap
Lambda-Cold Dark Matter, Stellar Feedback, and the Galactic Halo Abundance Pattern
(Abridged) The hierarchical formation scenario for the stellar halo requires
the accretion and disruption of dwarf galaxies, yet low-metallicity halo stars
are enriched in alpha-elements compared to similar, low-metallicity stars in
dwarf spheroidal (dSph) galaxies. We address this primary challenge for the
hierarchical formation scenario for the stellar halo by combining chemical
evolution modelling with cosmologically-motivated mass accretion histories for
the Milky Way dark halo and its satellites. We demonstrate that stellar halo
and dwarf galaxy abundance patterns can be explained naturally within the LCDM
framework. Our solution relies fundamentally on the LCDM model prediction that
the majority of the stars in the stellar halo were formed within a few
relatively massive, ~5 x 10^10 Msun, dwarf irregular (dIrr)-size dark matter
halos, which were accreted and destroyed ~10 Gyr in the past. These systems
necessarily have short-lived, rapid star formation histories, are enriched
primarily by Type II supernovae, and host stars with enhanced [a/Fe]
abundances. In contrast, dwarf spheroidal galaxies exist within low-mass dark
matter hosts of ~10^9 Msun, where supernovae winds are important in setting the
intermediate [a/Fe] ratios observed. Our model includes enrichment from Type Ia
and Type II supernovae as well as stellar winds, and includes a
physically-motivated supernovae feedback prescription calibrated to reproduce
the local dwarf galaxy stellar mass - metallicity relation. We use
representative examples of the type of dark matter halos we expect to host a
destroyed ``stellar halo progenitor'' dwarf, a surviving dIrr, and a surviving
dSph galaxy, and show that their derived abundance patterns, stellar masses,
and gas masses are consistent with those observed for each type of system.Comment: 10 pages, 3 figures, version accepted by Ap
Galactic chemical evolution : Carbon through zinc
Copyright © 2006. The American Astronomical Society. Permission for further reuse of this content should be sought from the publisher or the rights holder. This is the final published version of the work, which was originally published at https://doi.org/10.1086/508914We calculate the evolution of heavy-element abundances from C to Zn in the solar neighborhood, adopting our new nucleosynthesis yields. Our yields are calculated for wide ranges of metallicity (Z = 0-Z circle dot) and the explosion energy (normal supernovae and hypernovae), based on the light-curve and spectra fitting of individual supernovae. The elemental abundance ratios are in good agreement with observations. Among the alpha-elements, O, Mg, Si, S, and Ca show a plateau at [Fe/H] <= -1, while Ti is underabundant overall. The observed abundance of Zn ([Zn/Fe] similar to 0) can be explained only by the high-energy explosion models, as it requires a large contribution of hypernovae. The observed decrease in the odd-Z elements (Na, Al, and Cu) toward low [Fe/H] is reproduced by the metallicity effect on nucleosynthesis. The iron-peak elements (Cr, Mn, Co, and Ni) are consistent with the observed mean values at -2.5 less than or similar to [Fe/H] less than or similar to -1, and the observed trend at the lower metallicity can be explained by the energy effect. We also show the abundance ratios and the metallicity distribution functions of the Galactic bulge, halo, and thick disk. Our results suggest that the formation timescale of the thick disk is similar to 1-3 Gyr.Peer reviewe
Corotation: its influence on the chemical abundance pattern of the Galaxy
A simple theory for the chemical enrichment of the Galaxy which takes into
account the effects of spiral arms on heavy elements output was developed. In
the framework of the model with the corotation close to the position of the Sun
in the Galaxy the observed abundance features are explained.Comment: LaTeX, 6 pages, 5 jpg figures, uses aastex.sty, submitted to ApJ Let
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