Abstract
We present a low-metallicity map of the Milky Way consisting of ∼110,000 metal-poor giants with −3.5 < Fe/H < −0.75, based on public photometry from the second data release of the SkyMapper ...survey. These stars extend out to ∼7 kpc 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 ∼ −3.0, and thus provides an unprecedented view into the ancient, metal-poor Milky Way. Among the more metal-rich stars in our sample (Fe/H > −2.0), 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 ∣
Z
∣ ∼ 7 kpc. We find that the metallicity distribution function (MDF) of the most metal-poor stars in our sample (−3.0 < Fe/H < −2.3) is well fit with an exponential profile with a slope of and
Fe/H = 1.52 ± 0.05, and slightly shifts to
after accounting for target selection effects. For Fe/H < −2.3, the MDF is largely insensitive to scale height ∣
Z
∣ out to ∼5 kpc, showing that very and extremely metal-poor stars are in every galactic component.
In this work, we study the chemical compositions and kinematic properties of six metal-poor stars with Fe/H < −2.5 in the Galactic halo. From high-resolution (R ∼ 110,000) spectroscopic observations ...obtained with the Lick/Automated Planet Finder, we determined individual abundances for up to 23 elements, to quantitatively evaluate our sample. We identify two carbon-enhanced metal-poor stars (J1630+0953 and J2216+0246) without enhancement in neutron-capture elements (CEMP-no stars), while the rest of our sample stars are carbon-intermediate. By comparing the light-element abundances of the CEMP stars with predicted yields from nonrotating zero-metallicity massive-star models, we find that the possible progenitors of J1630+0953 and J2216+0246 could be in the 13-25 M mass range, with explosion energies (0.3-1.8) × 1051 erg. In addition, the detectable abundance ratios of light and heavy elements suggest that our sample stars are likely formed from a well-mixed gas cloud, which is consistent with previous studies. We also present a kinematic analysis, which suggests that most of our program stars likely belong to the inner-halo population, with orbits passing as close as ∼2.9 kpc from the Galactic center. We discuss the implications of these results on the critical constraints on the origin and evolution of CEMP stars, as well as the nature of the Population III progenitors of the lowest-metallicity stars in our Galaxy.
Abstract
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 (
dv
helio
/
d
θ
1
=
−
2.6
−
2.9
+
3.0
km s
−1
kpc
−1
), reflecting a lack of evidence for tidal disruption, and derive a dynamical mass of
M
1
/
2
(
r
h
)
=
1.6
−
0.7
+
1.1
×
10
6
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.
Abstract
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 ∼280,000 stars with −3.75 ≲ Fe/H ≲ −0.75 down to
g
= 17 derived using metallicity-sensitive photometry from the second data release of the SkyMapper Southern Survey. We use the dependency of the flux through the SkyMapper
v
filter on the strength of the Ca
ii
K absorption features, in tandem with SkyMapper
u
,
g
,
i
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 we 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
σ
∼ 0.25 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 < −2.5) of our photometric metallicity determinations. In such comparisons, we find the metallicities of stars with photometric Fe/H < −2.5 in our catalog show no significant offset and a scatter of
σ
∼ 0.31 dex level relative to those in high-resolution work when considering the cooler stars (
g
−
i
> 0.65) in our sample. We also present an expanded catalog containing photometric metallicities of ∼720,000 stars as a data table for further exploration of the metal-poor Milky Way.
ABSTRACT
We present a chemo-dynamical analysis for 27 near main-sequence turnoff metal-poor stars, including 20 stars analysed for the first time. The sample spans a range in Fe/H from −2.5 to −3.6, ...with 44 per cent having Fe/H<−2.9. We derived chemical abundances for 17 elements, including strontium and barium. We derive Li abundances for the sample, which are in good agreement with the ‘Spite Plateau’ value. The lighter elements (Z < 30) generally agree well with those of other low-metallicity halo stars. This broadly indicates chemically homogeneous gas at the earliest times. We used the Sr/Ba versus Ba/Fe diagram to classify metal-poor stars into five populations based on their observed ratios. We find HE 0232 − 3755 to be a likely main r-process star, and HE 2214 − 6127 and HE 2332 − 3039 to be limited-r stars. CS30302-145, HE 2045 − 5057, and CD −24°17504 plausibly originated in long-disrupted early dwarf galaxies. We also find that the derived Sr/H and Ba/H values for CD −24°17504 are not inconsistent with the predicted yields of the s-process in massive rotating low-metallicity stars models. Further theoretical explorations will be helpful to better understand the earliest mechanisms and time scales of heavy element production for comparison with these and other observational abundance data. Finally, we investigate the orbital histories of our stars. Most display halo-like kinematics although three stars (CS 29504-018, HE 0223 − 2814, and HE 2133 − 0421) appear to be disc-like in nature. This confirms the extragalactic origin for CS 30302-145, HE 2045 − 5057, and, in particular, CD −24°17504 which likely originated from a small accreted stellar system as one of the oldest stars.
Abstract
We have developed a chemodynamical approach to assign 36,010 metal-poor SkyMapper stars to various Galactic stellar populations. Using two independent techniques (velocity and action space ...behavior), Gaia EDR3 astrometry, and photometric metallicities, we selected stars with the characteristics of the “metal-weak” thick-disk population by minimizing contamination by the canonical thick disk or other Galactic structures. This sample comprises 7127 stars, spans a metallicity range of −3.50 < Fe/H < −0.8, and has a systematic rotational velocity of 〈
V
ϕ
〉 = 154 km s
−1
that lags that of the thick disk. Orbital eccentricities have intermediate values between typical thick-disk and halo values. The scale length is
h
R
=
2.48
−
0.05
+
0.05
kpc, and the scale height is
h
Z
=
1.68
−
0.15
+
0.19
kpc. The metallicity distribution function is well fit by an exponential with a slope of
Δ
log
N
/
Δ
Fe
/
H
=
1.13
±
0.06
. Overall, we find a significant metal-poor component consisting of 261 SkyMapper stars with Fe/H < −2.0. While our sample contains only 11 stars with Fe/H ≲ −3.0, investigating the JINAbase compilation of metal-poor stars reveals another 18 such stars (five have Fe/H < −4.0) that kinematically belong to our sample. These distinct spatial, kinematic, and chemical characteristics strongly suggest that this metal-poor, phase-mixed kinematic sample represents an independent disk component with an accretion origin in which a massive dwarf galaxy radially plunged into the early Galactic disk. Going forward, we propose to call the metal-weak thick-disk population the Atari disk, given its likely accretion origin, and in reference to it sharing space with the Galactic thin and thick disks.
Abstract
The Milky Way has accreted many ultra-faint dwarf galaxies (UFDs), and stars from these galaxies can be found throughout our Galaxy today. Studying these stars provides insight into galaxy ...formation and early chemical enrichment, but identifying them is difficult. Clustering stellar dynamics in 4D phase space (
E
,
L
z
,
J
r
,
J
z
) is one method of identifying accreted structure that is currently being utilized in the search for accreted UFDs. We produce 32 simulated stellar halos using particle tagging with the Caterpillar simulation suite and thoroughly test the abilities of different clustering algorithms to recover tidally disrupted UFD remnants. We perform over 10,000 clustering runs, testing seven clustering algorithms, roughly twenty hyperparameter choices per algorithm, and six different types of data sets each with up to 32 simulated samples. Of the seven algorithms, HDBSCAN most consistently balances UFD recovery rates and cluster realness rates. We find that, even in highly idealized cases, the vast majority of clusters found by clustering algorithms do not correspond to real accreted UFD remnants and we can generally only recover 6% of UFDs remnants at best. These results focus exclusively on groups of stars from UFDs, which have weak dynamic signatures compared to the background of other stars. The recoverable UFD remnants are those that accreted recently,
z
accretion
≲ 0.5. Based on these results, we make recommendations to help guide the search for dynamically linked clusters of UFD stars in observational data. We find that real clusters generally have higher median energy and
J
r
, providing a way to help identify real versus fake clusters. We also recommend incorporating chemical tagging as a way to improve clustering results.
ABSTRACT We present a detailed chemical abundance and kinematic analysis of six extremely metal-poor (−4.2 ≤ Fe/H ≤−2.9) halo stars with very low neutron-capture abundances (Sr/H and Ba/H) based on ...high-resolution Magellan/MIKE spectra. Three of our stars have Sr/Ba and Sr/H ratios that resemble those of metal-poor stars in ultra-faint dwarf galaxies (UFDs). Since early UFDs may be the building blocks of the Milky Way, extremely metal-poor halo stars with low, UFD-like Sr and Ba abundances may thus be ancient stars from the earliest small galactic systems that were accreted by the proto-Milky Way. We label these objects as Small Accreted Stellar System (SASS) stars, and we find an additional 61 similar ones in the literature. A kinematic analysis of our sample and literature stars reveals them to be fast-moving halo objects, all with retrograde motion, indicating an accretion origin. Because SASS stars are much brighter than typical UFD stars, identifying them offers promising ways towards detailed studies of early star formation environments. From the chemical abundances of SASS stars, it appears that the earliest accreted systems were likely enriched by a few supernovae whose light element yields varied from system to system. Neutron-capture elements were sparsely produced and/or diluted, with r-process nucleosynthesis playing a role. These insights offer a glimpse into the early formation of the Galaxy. Using neutron-capture elements as a distinguishing criterion for early formation, we have access to a unique metal-poor population that consists of the oldest stars in the universe.
Abstract The Hercules ultrafaint dwarf galaxy (UFD) has long been hypothesized to be tidally disrupting, yet no conclusive evidence has been found for tidal disruption owing partly to difficulties in ...identifying Hercules member stars. In this work, we present a homogeneous reanalysis of new and existing observations of Hercules, including the detection of a new potential member star located ∼1° (∼1.7 kpc) west of the center of the system. In addition to measuring the line-of-sight velocity gradient, we compare predictions from dynamical models of stream formation to these observations. We report an updated velocity dispersion measurement based on 28 stars, 1.9 − 0.6 + 0.6 km s −1 , which is significantly lower than previous measurements. We find that the line-of-sight velocity gradient is 1.8 − 1.8 + 1.8 km s −1 kpc along the major axis of Hercules, consistent with zero within 1 σ . Our dynamical models of stream formation, on the other hand, can reproduce the morphology of the Hercules UFD, specifically the misalignment between the elongation and the orbital motion direction. Additionally, these dynamical models indicate that any radial velocity gradient from tidal disruption would be too small, 0.00 − 0.91 + 0.97 km s −1 kpc, to be detectable with current sample sizes. Combined with our analysis of the tidal radius evolution of the system as a function of its orbital phase, we argue that it is likely that Hercules is indeed currently undergoing tidal disruption in its extended stellar halo with a line-of-sight velocity gradient too small to be detected with current observational data sets.
We report on the discovery of five carbon-enhanced metal-poor (CEMP) stars in the metallicity range of −3.3 < Fe/H < −2.4. These stars were selected from the LAMOST DR3 low-resolution (R ∼ 2000) ...spectroscopic database as metal-poor candidates and followed up with high-resolution spectroscopy (R ∼ 110,000) 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, J2114−0616, 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 J2114−0616 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 J2114−0616 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.
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