8,976 research outputs found
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
Ion acceleration in "dragging field" of a light-pressure-driven piston
We propose a new acceleration scheme that combines shock wave acceleration
(SWA) and light pressure acceleration (LPA). When a thin foil driven by light
pressure of an ultra-intense laser pulse propagates in underdense background
plasma, it serves as a shock-like piston, trapping and reflecting background
protons to ultra-high energies. Unlike in SWA, the piston velocity is not
limited by the Mach number and can be highly relativistic. Background protons
can be trapped and reflected forward by the enormous "dragging field" potential
behind the piston which is not employed in LPA. Our one- and two-dimensional
particle-in-cell simulations and analytical model both show that proton
energies of several tens to hundreds of GeV can be obtained, while the
achievable energy in simple LPA is below 10 GeV.Comment: submitte
The Chemical Imprint of Silicate Dust on the Most Metal-Poor Stars
We investigate the impact of dust-induced gas fragmentation on the formation
of the first low-mass, metal-poor stars (< 1Msun) in the early universe.
Previous work has shown the existence of a critical dust-to-gas ratio, below
which dust thermal cooling cannot cause gas fragmentation. Assuming the first
dust is silicon-based, we compute critical dust-to-gas ratios and associated
critical silicon abundances ([Si/H]crit). At the density and temperature
associated with protostellar disks, we find that a standard Milky Way grain
size distribution gives [Si/H]crit = -4.5 +/- 0.1, while smaller grain sizes
created in a supernova reverse shock give [Si/H]crit = -5.3 +/- 0.1. Other
environments are not dense enough to be influenced by dust cooling. We test the
silicate dust cooling theory by comparing to silicon abundances observed in the
most iron-poor stars ([Fe/H] < -4.0). Several stars have silicon abundances low
enough to rule out dust-induced gas fragmentation with a standard grain size
distribution. Moreover, two of these stars have such low silicon abundances
that even dust with a shocked grain size distribution cannot explain their
formation. Adding small amounts of carbon dust does not significantly change
these conclusions. Additionally, we find that these stars exhibit either high
carbon with low silicon abundances or the reverse. A silicate dust scenario
thus suggests that the earliest low-mass star formation in the most metal-poor
regime may have proceeded through two distinct cooling pathways: fine structure
line cooling and dust cooling. This naturally explains both the carbon-rich and
carbon-normal stars at extremely low [Fe/H].Comment: 14 pages, 8 figures; accepted to Ap
Complete element abundances of nine stars in the r-process galaxy Reticulum II
We present chemical abundances derived from high-resolution Magellan/MIKE
spectra of the nine brightest known red giant members of the ultra-faint dwarf
galaxy Reticulum II. These stars span the full metallicity range of Ret II
(-3.5 < [Fe/H] < -2). Seven of the nine stars have extremely high levels of
r-process material ([Eu/Fe]~1.7), in contrast to the extremely low
neutron-capture element abundances found in every other ultra-faint dwarf
galaxy studied to date. The other two stars are the most metal-poor stars in
the system ([Fe/H] < -3), and they have neutron-capture element abundance
limits similar to those in other ultra-faint dwarf galaxies. We confirm that
the relative abundances of Sr, Y, and Zr in these stars are similar to those
found in r-process halo stars but ~0.5 dex lower than the solar r-process
pattern. If the universal r-process pattern extends to those elements, the
stars in Ret II display the least contaminated known r-process pattern. The
abundances of lighter elements up to the iron peak are otherwise similar to
abundances of stars in the halo and in other ultra-faint dwarf galaxies.
However, the scatter in abundance ratios is large enough to suggest that
inhomogeneous metal mixing is required to explain the chemical evolution of
this galaxy. The presence of low amounts of neutron-capture elements in other
ultra-faint dwarf galaxies may imply the existence of additional r-process
sites besides the source of r-process elements in Ret II. Galaxies like Ret II
may be the original birth sites of r-process enhanced stars now found in the
halo.Comment: 14 pages, 5 figures, 5 tables. Accepted to Ap
High-resolution spectroscopy of extremely metal-poor stars in the least evolved galaxies: Bootes II
We present high-resolution Magellan/MIKE spectra of the four brightest
confirmed red giant stars in the ultra-faint dwarf galaxy Bootes II (Boo II).
These stars all inhabit the metal-poor tail of the Boo II metallicity
distribution function. The chemical abundance pattern of all detectable
elements in these stars is consistent with that of the Galactic halo. However,
all four stars have undetectable amounts of neutron-capture elements Sr and Ba,
with upper limits comparable to the lowest ever detected in the halo or in
other dwarf galaxies. One star exhibits significant radial velocity variations
over time, suggesting it to be in a binary system. Its variable velocity has
likely increased past determinations of the Boo II velocity dispersion. Our
four stars span a limited metallicity range, but their enhanced
{\alpha}-abundances and low neutron-capture abundances are consistent with the
interpretation that Boo II has been enriched by very few generations of stars.
The chemical abundance pattern in Boo II confirms the emerging trend that the
faintest dwarf galaxies have neutron-capture abundances distinct from the halo,
suggesting the dominant source of neutron-capture elements in halo stars may be
different than in ultra-faint dwarfs.Comment: 10 pages, 5 figures, 4 tables. Updated to match ApJ accepted versio
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