402 research outputs found
The Metallicity of the Monoceros Stream
We present low-resolution MMT Hectospec spectroscopy of 594 candidate
Monoceros stream member stars. Based on strong color-magnitude diagram
overdensities, we targeted three fields within the stream's footprint, with 178
deg < l < 203 deg and -25 deg < b < 25 deg. By comparing the measured iron
abundances with those expected from smooth Galactic components alone, we
measure, for the first time, the spectroscopic metallicity distribution
function for Monoceros. We find the stream to be chemically distinct from both
the thick disk and halo, with [Fe/H] = -1, and do not detect a trend in the
stream's metallicity with Galactic longitude. Passing from b = +25 deg to b =
-25 deg the median Monoceros metallicity trends upward by 0.1 dex, though
uncertainties in modeling sample contamination by the disk and halo make this a
marginal detection. In each field, we find Monoceros to have an intrinsic
[Fe/H] dispersion of 0.10-0.22 dex. From the CaII K line, we measure [Ca/Fe]
for a subsample of metal poor program stars with -1.1 < [Fe/H] < -0.5. In two
of three fields, we find calcium deficiencies qualitatively similar to
previously reported [Ti/Fe] underabundances in Monoceros and the Sagittarius
tidal stream. Further, using 90 spectra of thick disk stars in the Monoceros
pointings with b ~ +/-25 deg, we detect a 0.22 dex north/south metallicity
asymmetry coincident with known stellar density asymmetry at R_GC ~ 12 kpc and
|Z| ~ 1.7 kpc. Our median Monoceros [Fe/H] = -1.0 and its relatively low
dispersion naturally fit the expectation for an appropriately luminous M_V ~
-13 dwarf galaxy progenitor.Comment: accepted for publication in Ap
SD 1313-0019 -- Another second-generation star with [Fe/H] = -5.0, observed with the Magellan Telescope
We present a Magellan/MIKE high-resolution (R ~ 35,000) spectrum of the
ancient star SD 1313-0019 which has an iron abundance of [Fe/H] = -5.0, paired
with a carbon enhancement of [C/Fe] ~ 3.0. The star was initially identified by
Allende Prieto et al. in the BOSS survey. Its medium-resolution spectrum
suggested a higher metallicity of [Fe/H] = -4.3 due to the CaII K line blending
with a CH feature which is a common issue related to the search for the most
iron-poor stars. This star joins several other, similar stars with [Fe/H] <
-5.0 that all display a combination of low iron and high carbon abundances.
Other elemental abundances of SD 1313-0019 follow that of more metal-rich halo
stars. From fitting the abundance pattern with yields of Population III
supernova, we conclude that SD 1313-0019 had only one massive progenitor star
with 20 - 30 M_sun that must have undergone a mixing and fallback episode.
Overall, there are now five stars known with [Fe/H] < -5.0 (1D LTE abundances).
This population of second-generation stars strongly suggests massive first
stars that almost exclusively produced large amounts of carbon through stellar
winds and/or their mixing and fallback supernova explosions. As a consequence,
their natal clouds -- presumably some early minihalo structures -- contained
ample amounts of carbon and oxygen that likely facilitated the formation of
these first low-mass stars.Comment: 7 pages and 3 figures, accepted by ApJ
Linking dwarf galaxies to halo building blocks with the most metal-poor star in Sculptor
Current cosmological models indicate that the Milky Way's stellar halo was
assembled from many smaller systems. Based on the apparent absence of the most
metal-poor stars in present-day dwarf galaxies, recent studies claimed that the
true Galactic building blocks must have been vastly different from the
surviving dwarfs. The discovery of an extremely iron-poor star (S1020549) in
the Sculptor dwarf galaxy based on a medium-resolution spectrum cast some doubt
on this conclusion. However, verification of the iron-deficiency and
measurements of additional elements, such as the alpha-element Mg, are
mandatory for demonstrating that the same type of stars produced the metals
found in dwarf galaxies and the Galactic halo. Only then can dwarf galaxy stars
be conclusively linked to early stellar halo assembly. Here we report
high-resolution spectroscopic abundances for 11 elements in S1020549,
confirming the iron abundance of less than 1/4000th that of the Sun, and
showing that the overall abundance pattern mirrors that seen in low-metallicity
halo stars, including the alpha-elements. Such chemical similarity indicates
that the systems destroyed to form the halo billions of years ago were not
fundamentally different from the progenitors of present-day dwarfs, and
suggests that the early chemical enrichment of all galaxies may be nearly
identical.Comment: 16 pages, including 2 figures. Accepted for publication in Nature. It
is embargoed for discussion in the press until formal publication in Natur
Actinide-rich and Actinide-poor -Process Enhanced Metal-Poor Stars do not Require Separate -Process Progenitors
The astrophysical production site of the heaviest elements in the universe
remains a mystery. Incorporating heavy element signatures of metal-poor,
-process enhanced stars into theoretical studies of -process production
can offer crucial constraints on the origin of heavy elements. In this study,
we introduce and apply the "Actinide-Dilution with Matching" model to a variety
of stellar groups ranging from actinide-deficient to actinide-enhanced to
empirically characterize -process ejecta mass as a function of electron
fraction. We find that actinide-boost stars do not indicate the need for a
unique and separate -process progenitor. Rather, small variations of neutron
richness within the same type of -process event can account for all observed
levels of actinide enhancements. The very low-, fission-cycling ejecta of
an -process event need only constitute 10-30% of the total ejecta mass to
accommodate most actinide abundances of metal-poor stars. We find that our
empirical distributions of ejecta are similar to those inferred from
studies of GW170817 mass ejecta ratios, which is consistent with neutron-star
mergers being a source of the heavy elements in metal-poor, -process
enhanced stars.Comment: 14 pages, 11 figures, Submitted to Ap
The R-Process Alliance: A Comprehensive Abundance Analysis of HD 222925, a Metal-Poor Star with an Extreme R-Process Enhancement of [Eu/H] = -0.14
We present a detailed abundance analysis of the bright (V = 9.02), metal-poor
([Fe/H] = -1.47 +/- 0.08) field red horizontal-branch star HD 222925, which was
observed as part of an ongoing survey by the R-Process Alliance. We calculate
stellar parameters and derive abundances for 46 elements based on 901 lines
examined in a high-resolution optical spectrum obtained using the Magellan
Inamori Kyocera Echelle spectrograph. We detect 28 elements with 38 <= Z <= 90;
their abundance pattern is a close match to the Solar r-process component. The
distinguishing characteristic of HD 222925 is an extreme enhancement of
r-process elements ([Eu/Fe] = +1.33 +/- 0.08, [Ba/Eu] = -0.78 +/- 0.10) in a
moderately metal-poor star, so the abundance of r-process elements is the
highest ([Eu/H] = -0.14 +/- 0.09) in any known r-process-enhanced star. The
abundance ratios among lighter (Z <= 30) elements are typical for metal-poor
stars, indicating that production of these elements was dominated by normal
Type II supernovae, with no discernible contributions from Type Ia supernovae
or asymptotic giant branch stars. The chemical and kinematic properties of HD
222925 suggest it formed in a low-mass dwarf galaxy, which was enriched by a
high-yield r-process event before being disrupted by interaction with the Milky
Way.Comment: Accepted for publication in the Astrophysical Journal (17 pages, 4
figures, 3 tables
Metal-Poor Stars Observed with the Magellan Telescope. III. New Extremely and Ultra Metal-Poor Stars from SDSS/SEGUE and Insights on the Formation of Ultra Metal-Poor Stars
We report the discovery of one extremely metal-poor (EMP; [Fe/H]<-3) and one
ultra metal-poor (UMP; [Fe/H]<-4) star selected from the SDSS/SEGUE survey.
These stars were identified as EMP candidates based on their medium-resolution
(R~2,000) spectra, and were followed-up with high-resolution (R~35,000)
spectroscopy with the Magellan-Clay Telescope. Their derived chemical
abundances exhibit good agreement with those of stars with similar
metallicities. We also provide new insights on the formation of the UMP stars,
based on comparison with a new set of theoretical models of supernovae
nucleosynthesis. The models were matched with 20 UMP stars found in the
literature, together with one of the program stars (SDSS J1204+1201), with
[Fe/H]=-4.34. From fitting their abundances, we find that the supernovae
progenitors, for stars where carbon and nitrogen are measured, had masses
ranging from 20.5 M_sun to 28 M_sun and explosion energies from 0.3 to
0.9x10^51 erg. These results are highly sensitive to the carbon and nitrogen
abundance determinations, which is one of the main drivers for future
high-resolution follow-up of UMP candidates. In addition, we are able to
reproduce the different CNO abundance patterns found in UMP stars with a single
progenitor type, by varying its mass and explosion energy.Comment: 15 pages, 12 figures; accepted for publication in Ap
Exploring the Universe with Metal-Poor Stars
The early chemical evolution of the Galaxy and the Universe is vital to our
understanding of a host of astrophysical phenomena. Since the most metal-poor
Galactic stars (with metallicities down to [Fe/H]\sim-5.5) are relics from the
high-redshift Universe, they probe the chemical and dynamical conditions of the
Milky Way and the origin and evolution of the elements through nucleosynthesis.
They also provide constraints on the nature of the first stars, their
associated supernovae and initial mass function, and early star and galaxy
formation. The Milky Way's dwarf satellites contain a large fraction (~30%) of
the known most metal-poor stars that have chemical abundances that closely
resemble those of equivalent halo stars. This suggests that chemical evolution
may be universal, at least at early times, and that it is driven by massive,
energetic SNe. Some of these surviving, ultra-faint systems may show the
signature of just one such PopIII star; they may even be surviving first
galaxies. Early analogs of the surviving dwarfs may thus have played an
important role in the assembly of the old Galactic halo whose formation can now
be studied with stellar chemistry. Following the cosmic evolution of small
halos in simulations of structure formation enables tracing the cosmological
origin of the most metal-poor stars in the halo and dwarf galaxies. Together
with future observations and additional modeling, many of these issues,
including the reionization history of the Milky Way, may be constrained this
way. The chapter concludes with an outlook about upcoming observational
challenges and ways forward is to use metal-poor stars to constrain theoretical
studies.Comment: 34 pages, 11 figures. Book chapter to appear in "The First Galaxies -
Theoretical Predictions and Observational Clues", 2012 by Springer, eds. V.
Bromm, B. Mobasher, T. Wiklin
R-process enrichment from a single event in an ancient dwarf galaxy
Elements heavier than zinc are synthesized through the (r)apid and (s)low
neutron-capture processes. The main site of production of the r-process
elements (such as europium) has been debated for nearly 60 years. Initial
studies of chemical abundance trends in old Milky Way halo stars suggested
continual r-process production, in sites like core-collapse supernovae. But
evidence from the local Universe favors r-process production mainly during rare
events, such as neutron star mergers. The appearance of a europium abundance
plateau in some dwarf spheroidal galaxies has been suggested as evidence for
rare r-process enrichment in the early Universe, but only under the assumption
of no gas accretion into the dwarf galaxies. Cosmologically motivated gas
accretion favors continual r-process enrichment in these systems. Furthermore,
the universal r-process pattern has not been cleanly identified in dwarf
spheroidals. The smaller, chemically simpler, and more ancient ultra-faint
dwarf galaxies assembled shortly after the first stars formed, and are ideal
systems with which to study nucleosynthesis events such as the r-process.
Reticulum II is one such galaxy. The abundances of non-neutron-capture elements
in this galaxy (and others like it) are similar to those of other old stars.
Here, we report that seven of nine stars in Reticulum II observed with
high-resolution spectroscopy show strong enhancements in heavy neutron-capture
elements, with abundances that follow the universal r-process pattern above
barium. The enhancement in this "r-process galaxy" is 2-3 orders of magnitude
higher than that detected in any other ultra-faint dwarf galaxy. This implies
that a single rare event produced the r-process material in Reticulum II. The
r-process yield and event rate are incompatible with ordinary core-collapse
supernovae, but consistent with other possible sites, such as neutron star
mergers.Comment: Published in Nature, 21 Mar 2016:
http://dx.doi.org/10.1038/nature1742
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