994 research outputs found
Reconstructing the cosmic evolution of the chemical elements
The chemical elements are created in nuclear fusion processes in the hot and
dense cores of stars. The energy generated through nucleosynthesis allows stars
to shine for billions of years. When these stars explode as massive supernovae,
the newly made elements are expelled, chemically enriching the surrounding
regions. Subsequent generations of stars are formed from gas that is slightly
more element enriched than that from which previous stars formed. This chemical
evolution can be traced back to its beginning soon after the Big Bang by
studying the oldest and most metal-poor stars still observable in the Milky Way
today. Through chemical analysis, they provide the only available tool for
gaining information about the nature of the short-lived first stars and their
supernova explosions more than thirteen billion years ago. These events set in
motion the transformation of the pristine universe into a rich cosmos of
chemically diverse planets, stars, and galaxies.Comment: 13 pages, 2 figures. To appear in "From Atoms to the Stars", a
special issue of Daedalus (Fall 2014, vol. 143, no. 4
Near-Field Cosmology with Metal-Poor Stars
The oldest, most metal-poor stars in the Galactic halo and satellite dwarf
galaxies present an opportunity to explore the chemical and physical conditions
of the earliest star forming environments in the Universe. We review the fields
of stellar archaeology and dwarf galaxy archaeology by examining the chemical
abundance measurements of various elements in extremely metal-poor stars. Focus
on the carbon-rich and carbon-normal halo star populations illustrates how
these provide insight into the Population III star progenitors responsible for
the first metal enrichment events. We extend the discussion to near-field
cosmology, which is concerned with the formation of the first stars and
galaxies and how metal-poor stars can be used to constrain these processes.
Complementary abundance measurements in high-redshift gas clouds further help
to establish the early chemical evolution of the Universe. The data appear
consistent with the existence of two distinct channels of star formation at the
earliest times.Comment: 126 pages, 12 figures, Annual Review of Astronomy and Astrophysics
(ARA&A), in pres
From actinides to zinc: Using the full abundance pattern of the brightest star in Reticulum II to distinguish between different r-process sites
The ultra-faint dwarf galaxy Reticulum II was enriched by a rare and prolific
r-process event, such as a neutron star merger. To investigate the nature of
this event, we present high-resolution Magellan/MIKE spectroscopy of the
brightest star in this galaxy. The high signal-to-noise allows us to determine
the abundances of 41 elements, including the radioactive actinide element Th
and first ever detections of third r-process peak elements (Os and Ir) in a
star outside the Milky Way. The observed neutron-capture element abundances
closely match the solar r-process component, except for the first r-process
peak which is significantly lower than solar but matches other r-process
enhanced stars. The ratio of first peak to heavier r-process elements implies
the r-process site produces roughly equal masses of high and low electron
fraction ejecta, within a factor of 2. We compare the detailed abundance
pattern to predictions from nucleosynthesis calculations of neutron star
mergers and magneto-rotationally driven jet supernovae, finding that nuclear
physics uncertainties dominate over astrophysical uncertainties. We measure
\log\mbox{Th/Eu} = -0.84 \pm 0.06\,\text{(stat)} \pm 0.22\,\text{(sys)},
somewhat lower than all previous Th/Eu observations. The youngest age we derive
from this ratio is Gyr,
indicating that current initial production ratios do not well describe the
r-process event in Reticulum II. The abundance of light elements up to Zn are
consistent with extremely metal-poor Milky Way halo stars. They may eventually
provide a way to distinguish between neutron star mergers and
magneto-rotationally driven jet supernovae, but this would require more
detailed knowledge of the chemical evolution of Reticulum II.Comment: 23 pages, 7 figures, accepted to Ap
CD -24_17504 revisited: a new comprehensive element abundance analysis
With [Fe/H] ~ -3.3, CD -24_17504 is a canonical metal-poor main sequence
turn-off star. Though it has appeared in numerous literature studies, the most
comprehensive abundance analysis for the star based on high resolution, high
signal-to-noise spectra is nearly 15 years old. We present a new detailed
abundance analysis for 21 elements based on combined archival Keck-HIRES and
VLT-UVES spectra of the star that is higher in both spectral resolution and
signal-to-noise than previous data. Our results for many elements are very
similar to those of an earlier comprehensive study of the star, but we present
for the first time a carbon abundance from the CH G-band feature as well as
improved upper limits for neutron-capture species such as Y, Ba and Eu. In
particular, we find that CD -24_17504 has [Fe/H] = -3.41, [C/Fe] = +1.10,
[Sr/H] = -4.68 and [Ba/H] <= -4.46, making it a carbon enhanced metal-poor star
with neutron-capture element abundances among the lowest measured in Milky Way
halo stars.Comment: Accepted to ApJ. 24 pages, 13 figures, 7 table
Segue 1 - A Compressed Star Formation History Before Reionization
Segue 1 is the current best candidate for a "first galaxy", a system which
experienced only a single short burst of star formation and has since remained
unchanged. Here we present possible star formation scenarios which can explain
its unique metallicity distribution. While the majority of stars in all other
ultra-faint dwarfs (UFDs) are within 0.5 dex of the mean [Fe/H] for the galaxy,
5 of the 7 stars in Segue 1 have a spread of [Fe/H] dex. We show
that this distribution of metallicities canot be explained by a gradual
build-up of stars, but instead requires clustered star formation. Chemical
tagging allows the separate unresolved delta functions in abundance space to be
associated with discrete events in space and time. This provides an opportunity
to put the enrichment events into a time sequence and unravel the history of
the system. We investigate two possible scenarios for the star formation
history of Segue 1 using Fyris Alpha simulations of gas in a M
dark matter halo. The lack of stars with intermediate metallicities
[Fe/H] can be explained either by a pause in star formation caused by
supernova feedback, or by the spread of metallicities resulting from one or two
supernovae in a low-mass dark matter halo. Either possibility can reproduce the
metallicity distribution function (MDF), as well as the other observed
elemental abundances. The unusual MDF and the low luminosity of Segue 1 can be
explained by it being a first galaxy that originated with
~M at .Comment: 12 pages, 6 figures, ApJ, Accepte
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