7 research outputs found
The R-Process Alliance: Chemical Abundances for a Trio of R-Process-Enhanced Stars -- One Strong, One Moderate, One Mild
We present detailed chemical abundances of three new bright (V ~ 11),
extremely metal-poor ([Fe/H] ~ -3.0), r-process-enhanced halo red giants based
on high-resolution, high-S/N Magellan/MIKE spectra. We measured abundances for
20-25 neutron-capture elements in each of our stars. J1432-4125 is among the
most r-process rich r-II stars, with [Eu/Fe]= +1.44+-0.11. J2005-3057 is an r-I
star with [Eu/Fe] = +0.94+-0.07. J0858-0809 has [Eu/Fe] = +0.23+-0.05 and
exhibits a carbon abundance corrected for evolutionary status of [C/Fe]_corr =
+0.76, thus adding to the small number of known carbon-enhanced r-process
stars. All three stars show remarkable agreement with the scaled solar
r-process pattern for elements above Ba, consistent with enrichment of the
birth gas cloud by a neutron star merger. The abundances for Sr, Y, and Zr,
however, deviate from the scaled solar pattern. This indicates that more than
one distinct r-process site might be responsible for the observed
neutron-capture element abundance pattern. Thorium was detected in J1432-4125
and J2005-3057. Age estimates for J1432-4125 and J2005-3057 were adopted from
one of two sets of initial production ratios each by assuming the stars are
old. This yielded individual ages of 12+-6 Gyr and 10+-6 Gyr, respectively.Comment: 30 pages, includes a long table, 5 figure
A Panchromatic Study of Massive Stars in the Extremely Metal-poor Local Group Dwarf Galaxy Leo A*
We characterize massive stars (M > 8 M⊙) in the nearby (D ∼ 0.8 Mpc) extremely metal-poor (Z ∼ 5% Z⊙) galaxy Leo A using Hubble Space Telescope ultraviolet (UV), optical, and near-infrared (NIR) imaging along with Keck/Low-Resolution Imaging Spectrograph and MMT/Binospec optical spectroscopy for 18 main-sequence OB stars. We find that: (a) 12 of our 18 stars show emission lines, despite not being associated with an H ii region, suggestive of stellar activity (e.g., mass loss, accretion, binary star interaction), which is consistent with previous predictions of enhanced activity at low metallicity; (b) six are Be stars, which are the first to be spectroscopically studied at such low metallicity—these Be stars have unusual panchromatic SEDs; (c) for stars well fit by the TLUSTY nonlocal thermodynamic equilibrium models, the photometric and spectroscopic values of and agree to within ∼0.01 dex and ∼0.18 dex, respectively, indicating that near-UV/optical/NIR imaging can be used to reliably characterize massive (M ∼ 8–30 M⊙) main-sequence star properties relative to optical spectroscopy; (d) the properties of the most-massive stars in H II regions are consistent with constraints from previous nebular emission line studies; and (e) 13 stars with M > 8M⊙ are >40 pc from a known star cluster or H II region. Our sample comprises ∼50% of all known massive stars at Z ≲ 10% Z⊙with derived stellar parameters, high-quality optical spectra, and panchromatic photometry
The R-process alliance: discovery of the first metal-poor star with a combined r- and s-process element signature
We present a high-resolution (R ∼ 35,000), high signal-to-noise ratio (S/N > 200) Magellan/MIKE spectrum of the star RAVE J094921.8-161722, a bright (V = 11.3) metal-poor red giant star with [Fe/H] = -2.2, identified as a carbon-enhanced metal-poor (CEMP) star from the RAVE survey. We report its detailed chemical abundance signature of light fusion elements and heavy neutron-capture elements. We find J0949-1617 to be a CEMP star with s-process enhancement that must have formed from gas enriched by a prior r-process event. Light neutron-capture elements follow a low-metallicity s-process pattern, while the heavier neutron-capture elements above Eu follow an r-process pattern. The Pb abundance is high, in line with an s-process origin. Thorium is also detected, as expected from an r-process origin, as Th is not produced in the s-process. We employ nucleosynthesis model predictions that take an initial r-process enhancement into account, and then determine the mass transfer of carbon and s-process material from a putative more massive companion onto the observed star. The resulting abundances agree well with the observed pattern. We conclude that J0949-1617 is the first bonafide CEMP-r + s star identified. This class of objects has previously been suggested to explain stars with neutron-capture element patterns that originate from neither the r- nor the s-process alone. We speculate that J0949-1617 formed in an environment similar to those of ultra-faint dwarf galaxies like Tucana III and Reticulum II, which were enriched in r-process elements by one or multiple neutron star mergers at the earliest times. ©2018NSF CAREER Grant (AST-1255160)National Science Foundation (Grant no. PHY-1430152)NASA Hubble Fellowship Grant (HST-HF2-51393.001
R-process-rich Stellar Streams in the Milky Way*
Abstract
We present high-resolution Magellan/MIKE spectra of 22 bright (9 < V < 13.5) metal-poor stars (−3.18 < [Fe/H] < −1.37) in three different stellar streams, the Helmi debris stream, the Helmi trail stream, and the ω Centauri progenitor stream. We augment our Helmi debris sample with results for 10 stars by Roederer et al. for a total of 32 stars. Detailed chemical abundances of light elements as well as heavy neutron-capture elements have been determined for our 22 stars. All three streams contain carbon-enhanced stars. For 13 stars, neutron-capture element lines were detectable, and they all show signatures in agreement with the scaled solar r-process pattern, albeit with a large spread of −0.5 < [Eu/Fe] < +1.3. Eight of these stars show an additional small s-process contribution superposed onto their r-process pattern. This could be discerned because of the relatively high signal-to-noise ratio of the spectra given that the stars are close by in the halo. Our results suggest that the progenitors of these streams experienced one or more r-process events early on, such as a neutron star merger or another prolific r-process source. This widely enriched these host systems before their accretion by the Milky Way. The small s-process contribution suggests the presence of asymptotic giant branch stars and associated local (inhomogeneous) enrichment as part of the ongoing chemical evolution by low-mass stars. Stars in stellar streams may thus be a promising avenue for studying the detailed history of large dwarf galaxies and their role in halo assembly with easily accessible targets for high-quality spectra of many stars.</jats:p
The R-Process Alliance:Discovery of the First Metal-poor Star with a Combined r- and s-process Element Signature
We present a high-resolution (R~35,000), high signal-to-noise (S/N>200)
Magellan/MIKE spectrum of the star RAVE J094921.8-161722, a bright (V=11.3)
metal-poor red giant star with [Fe/H] = -2.2, identified as a carbon-enhanced
metal-poor (CEMP) star from the RAVE survey. We report its detailed chemical
abundance signature of light fusion elements and heavy neutron-capture
elements. We find J0949-1617 to be a CEMP star with s-process enhancement that
must have formed from gas enriched by a prior r-process event. Light
neutron-capture elements follow a low-metallicity s-process pattern, while the
heavier neutron-capture elements above Eu follow an r-process pattern. The Pb
abundance is high, in line with an s-process origin. Thorium is also detected,
as expected from an r-process origin, as Th is not produced in the s-process.
We employ nucleosynthesis model predictions that take an initial r-process
enhancement into account, and then determine the mass transfer of carbon and
s-process material from a putative more massive companion onto the observed
star. The resulting abundances agree well with the observed pattern. We
conclude that J0949-1617 is the first bonafide CEMP-r+s star identified. This
class of objects has previously been suggested to explain stars with
neutron-capture element patterns that originate from neither the r- or
s-process alone. We speculate that J0949-1617 formed in an environment similar
to those of ultra-faint dwarf galaxies like Tucana III and Reticulum II, which
were enriched in r-process elements by one or multiple neutron star mergers at
the earliest times.aComment: accepted for publication in Ap