17 research outputs found
The Metal-Poor Metallicity Distribution of the Ancient Milky Way
We present a low metallicity map of the Milky Way consisting of 111,000
giants with [Fe/H] 0.75, based on public photometry
from the second data release of the SkyMapper survey. These stars extend out to
7kpc from the solar neighborhood and cover the main Galactic stellar
populations, including the thick disk and the inner halo. Notably, this map can
reliably differentiate metallicities down to [Fe/H] , and thus
provides an unprecedented view into the ancient, metal-poor Milky Way. Among
the more metal-rich stars in our sample ([Fe/H] ), we recover a clear
spatial dependence of decreasing mean metallicity as a function of scale height
that maps onto the thick disk component of the Milky Way. When only considering
the very metal-poor stars in our sample ([Fe/H] 2), we recover no such
spatial dependence in their mean metallicity out to a scale height of
kpc. We find that the metallicity distribution function (MDF) of the
most metal-poor stars in our sample ( [Fe/H] ) is well fit with
an exponential profile with a slope of [Fe/H] =
1.520.05, and shifts to [Fe/H] = 1.530.10 after
accounting for target selection effects. For [Fe/H] , the MDF is
largely insensitive to scale height out to kpc, showing that very
and extremely metal-poor stars are in every galactic component.Comment: 9 pages, 5 figures; accepted for publication in ApJL. Minor
corrections after acceptance addressing referee report for Chiti et al. ApJS
submitte
Metal-poor stars observed with the automated planet finder telescope. I. Discovery of five carbon-enhanced metal-poor stars from LAMOST
We report on the discovery of five carbon-enhanced metal-poor (CEMP) stars in
the metallicity range of [Fe/H] . These stars were selected from
the LAMOST DR3 low-resolution (R 2,000) spectroscopic database as
metal-poor candidates and followed-up with high-resolution spectroscopy (R110,000) with the LICK/APF. Stellar parameters and individual abundances
for 25 chemical elements (from Li to Eu) are presented for the first time.
These stars exhibit chemical abundance patterns that are similar to those
reported in other literature studies of very and extremely metal-poor stars.
One of our targets, J21140616, shows high enhancement in carbon
([C/Fe]=1.37), nitrogen ([N/Fe]= 1.88), barium ([Ba/Fe]=1.00), and europium
([Eu/Fe]=0.84). Such chemical abundance pattern suggests that J21140616 can
be classified as CEMP-r/s star. In addition, the star J1054+0528 can be
classified as a CEMP-rI star, with [Eu/Fe]=0.44 and [Ba/Fe]=0.52. The other
stars in our sample show no enhancements in neutron-capture elements and can be
classified as CEMP-no stars. We also performed a kinematic and dynamical
analysis of the sample stars based on Gaia DR2 data. The kinematic parameters,
orbits, and binding energy of these stars, show that J21140616 is member of
the outer halo population, while the remaining stars belong to the inner halo
population but with an accreted origin. Collectively, these results add
important constraints on the origin and evolution of CEMP stars as well as on
their possible formation scenarios
Stellar Metallicities from SkyMapper Photometry II: Precise photometric metallicities of 280,000 giant stars with [Fe/H] in the Milky Way
The Milky Way's metal-poor stars are nearby ancient objects that are used to
study early chemical evolution and the assembly and structure of the Milky Way.
Here we present reliable metallicities of stars with [Fe/H] down to derived using
metallicity-sensitive photometry from the second data release (DR2) of the
SkyMapper Southern Survey. We use the dependency of the flux through the
SkyMapper filter on the strength of the Ca II K absorption features, in
tandem with SkyMapper photometry, to derive photometric metallicities
for these stars. We find that metallicities derived in this way compare well to
metallicities derived in large-scale spectroscopic surveys, and use such
comparisons to calibrate and quantify systematics as a function of location,
reddening, and color. We find good agreement with metallicities from the
APOGEE, LAMOST, and GALAH surveys, based on a standard deviation of
dex of the residuals of our photometric metallicities with
respect to metallicities from those surveys. We also compare our derived
photometric metallicities to metallicities presented in a number of
high-resolution spectroscopic studies to validate the low metallicity end
([Fe/H] ) of our photometric metallicity determinations. In such
comparisons, we find the metallicities of stars with photometric [Fe/H] in our catalog show no significant offset and a scatter of
0.31dex level relative to those in high-resolution work when
considering the cooler stars () in our sample. We also present an
expanded catalog containing photometric metallicities of stars as
a data table for further exploration of the metal-poor Milky Way.Comment: 15 pages, 9 figures, 2 tables; submitted to ApJS and revised after
one round of referee feedback. Full version of Table 2 in sourc
The R-Process Alliance: The Peculiar Chemical Abundance Pattern of RAVE J183013.5-455510
We report on the spectroscopic analysis of RAVE J183013.5-455510, an
extremely metal-poor star, highly enhanced in CNO, and with discernible
contributions from the rapid neutron-capture process. There is no evidence of
binarity for this object. At [Fe/H]=-3.57, this is one of the lowest
metallicity stars currently observed, with 18 measured abundances of
neutron-capture elements. The presence of Ba, La, and Ce abundances above the
Solar System r-process predictions suggest that there must have been a
non-standard source of r-process elements operating at such low metallicities.
One plausible explanation is that this enhancement originates from material
ejected at unusually fast velocities in a neutron star merger event. We also
explore the possibility that the neutron-capture elements were produced during
the evolution and explosion of a rotating massive star. In addition, based on
comparisons with yields from zero-metallicity faint supernova, we speculate
that RAVE J1830-4555 was formed from a gas cloud pre-enriched by both
progenitor types. From analysis based on Gaia DR2 measurements, we show that
this star has orbital properties similar to the Galactic metal-weak thick-disk
stellar population.Comment: Accepted for publication in Ap
SPLUS J142445.34-254247.1: An R-Process Enhanced, Actinide-Boost, Extremely Metal-Poor star observed with GHOST
We report on the chemo-dynamical analysis of SPLUS J142445.34-254247.1, an
extremely metal-poor halo star enhanced in elements formed by the rapid
neutron-capture process. This star was first selected as a metal-poor candidate
from its narrow-band S-PLUS photometry and followed up spectroscopically in
medium-resolution with Gemini South/GMOS, which confirmed its low-metallicity
status. High-resolution spectroscopy was gathered with GHOST at Gemini South,
allowing for the determination of chemical abundances for 36 elements, from
carbon to thorium. At [Fe/H]=-3.39, SPLUS J1424-2542 is one of the lowest
metallicity stars with measured Th and has the highest logeps(Th/Eu) observed
to date, making it part of the "actinide-boost" category of r-process enhanced
stars. The analysis presented here suggests that the gas cloud from which SPLUS
J1424-2542 was formed must have been enriched by at least two progenitor
populations. The light-element (Z<=30) abundance pattern is consistent with the
yields from a supernova explosion of metal-free stars with 11.3-13.4 Msun, and
the heavy-element (Z>=38) abundance pattern can be reproduced by the yields
from a neutron star merger (1.66Msun and 1.27Msun) event. A kinematical
analysis also reveals that SPLUS J1424-2542 is a low-mass, old halo star with a
likely in-situ origin, not associated with any known early merger events in the
Milky Way.Comment: 26 pages, 11 figures, accepted for publication on Ap
Metal-poor Stars Observed with the Automated Planet Finder Telescope. III. CEMP-no Stars are the Descendant of Population III Stars
In this study, we report a probabilistic insight into the stellar mass and
supernovae (SNe) explosion energy of the possible progenitors of five CEMP-no
stars. This was done by a direct comparison between the abundance ratios [X/Fe]
of the light-elements and the predicted nucleosynthetic yields of SN of
high-mass metal-free stars. This comparison suggests possible progenitors with
stellar mass range of 11 - 22\,M and explosion energies of \,erg. The coupling of the chemical abundances with kinematics
derived from DR2 suggests that our sample do not enter the outer-halo
region. In addition, we suggest that these CEMP-no stars are not -Sausage
nor -Sequoia remnant stars, but another accretion event might be
responsible for the contribution of these stars to the Galactic Halo of the
Milky-Way
Detailed Chemical Abundances of Stars in the Outskirts of the Tucana II Ultrafaint Dwarf Galaxy
We present chemical abundances and velocities of five stars between 0.3 and 1.1 kpc from the center of the Tucana II ultrafaint dwarf galaxy (UFD) from high-resolution Magellan/MIKE spectroscopy. We find that every star is deficient in metals (−3.6 < [Fe/H] < −1.9) and in neutron-capture elements as is characteristic of UFD stars, unambiguously confirming their association with Tucana II. Other chemical abundances (e.g., C, iron peak) largely follow UFD trends and suggest that faint core-collapse supernovae (SNe) dominated the early evolution of Tucana II. We see a downturn in [ α /Fe] at [Fe/H] ≈ −2.8, indicating the onset of Type Ia SN enrichment and somewhat extended chemical evolution. The most metal-rich star has strikingly low [Sc/Fe] = −1.29 ± 0.48 and [Mn/Fe] = −1.33 ± 0.33, implying significant enrichment by a sub-Chandrasekhar mass Type Ia SN. We do not detect a radial velocity gradient in Tucana II ( km s ^−1 kpc ^−1 ), reflecting a lack of evidence for tidal disruption, and derive a dynamical mass of M _⊙ . We revisit formation scenarios of the extended component of Tucana II in light of its stellar chemical abundances. We find no evidence that Tucana II had abnormally energetic SNe, suggesting that if SNe drove in situ stellar halo formation, then other UFDs should show similar such features. Although not a unique explanation, the decline in [ α /Fe] is consistent with an early galactic merger triggering later star formation. Future observations may disentangle such formation channels of UFD outskirts