Abstract
We present a chemical abundance analysis of the faint halo metal-poor main-sequence star J0023+0307, with Fe/H < −6.3, based on a high-resolution (
R
∼ 35,000)
Magellan
/MIKE spectrum. The ...star was originally found to have Fe/H < −6.6 based on a Ca
ii
K measurement in an
R
∼ 2500 spectrum. No iron lines could be detected in our MIKE spectrum. Spectral lines of Li, C, Na, Mg, Al, Si, and Ca were detected. The Li abundance is close to the Spite Plateau,
(Li) = 1.7, not unlike that of other metal-poor stars, although in stark contrast to the extremely low value found, e.g., in HE 1327–2326 at a similar Fe/H value. The carbon G-band is detected and indicates strong C-enhancement, as is typical for stars with low Fe abundances. Elements from Na through Si show a strong odd–even effect, and J0023+0307 displays the second-lowest known Ca/H abundance. Overall, the abundance pattern of J0023+0307 suggests that it is a second-generation star that formed from gas enriched by a massive Population III first star exploding as a fallback supernova. The inferred dilution mass of the ejecta is 10
5±0.5
M
⊙
of hydrogen, strongly suggesting J0023+0307 formed in a recollapsed minihalo. J0023+0307 is likely very old because it has a very eccentric orbit with a pericenter in the Galactic bulge.
Abstract
Hot Jupiters orbiting rapidly rotating stars on inclined orbits undergo tidally induced nodal precession measurable over several years of observations. The Hot Jupiters WASP-33 b and KELT-9 ...b are particularly interesting targets because they are among the hottest planets found to date, orbiting relatively massive stars. Here, we analyze archival and new data that span 11 and 5 yr for WASP-33 b and KELT-9 b, respectively, in order to model and improve upon their tidal precession parameters. Our work confirms the nodal precession for WASP-33 b and presents the first clear detection of the precession of KELT-9 b. We determine that WASP-33 and KELT-9 have gravitational quadrupole moments
(
6.3
−
0.8
+
1.2
)
×
10
−
5
and
(
3.26
−
0.80
+
0.93
)
×
10
−
4
, respectively. We estimate the planets’ precession periods to be
1460
−
130
+
170
yr and
890
−
140
+
200
yr, respectively, and that they will cease to transit their host stars around the years
2090
−
10
+
17
CE and
2074
−
10
+
12
CE, respectively. Additionally, we investigate both planets’ tidal and orbital evolution, suggesting that a high-eccentricity tidal migration scenario is possible to produce both system architectures and that they will most likely not be engulfed by their hosts before the end of their main-sequence lifetimes.
Abstract
The ultrafaint dwarf galaxy Reticulum II was enriched by a single rare and prolific
r
-process event. The
r
-process content of Reticulum II thus provides a unique opportunity to study metal ...mixing in a relic first galaxy. Using multi-object high-resolution spectroscopy with VLT/GIRAFFE and Magellan/M2FS, we identify 32 clear spectroscopic member stars and measure abundances of Mg, Ca, Fe, and Ba where possible. We find
72
−
12
+
10
%
of the stars are
r
-process-enhanced, with a mean
Ba
/
H
=
−
1.68
±
0.07
and unresolved intrinsic dispersion
σ
Ba/H
<0.20. The homogeneous
r
-process abundances imply that Ret II’s metals are well mixed by the time the
r
-enhanced stars form, which simulations have shown requires at least 100 Myr of metal mixing in between bursts of star formation to homogenize. This is the first direct evidence of bursty star formation in an ultrafaint dwarf galaxy. The homogeneous dilution prefers a prompt and high-yield
r
-process site, such as collapsar disk winds or prompt neutron star mergers. We also find evidence from Ba/H and Mg/Ca that the
r
-enhanced stars in Ret II formed in the absence of substantial pristine gas accretion, perhaps indicating that ≈70% of Ret II stars formed after reionization.
The Most Metal-poor Stars in the Inner Bulge Reggiani, Henrique; Schlaufman, Kevin C.; Casey, Andrew R. ...
The Astronomical journal,
10/2020, Letnik:
160, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Abstract
The bulge is the oldest component of the Milky Way. Since numerous simulations of Milky Way formation have predicted that the oldest stars at a given metallicity are found on tightly bound ...orbits, the Galaxy’s oldest stars are likely metal-poor stars in the inner bulge with small apocenters (i.e.,
R
apo
≲ 4 kpc). In the past, stars with these properties have been impossible to find due to extreme reddening and extinction along the line of sight to the inner bulge. We have used the mid-infrared metal-poor star selection of Schlaufman & Casey (2014) on Spitzer/Galactic Legacy Infrared Mid-Plane Survey Extraordinaire data to overcome these problems and target candidate inner bulge metal-poor giants for moderate-resolution spectroscopy with Anglo-Australian Telescope/AAOmega. We used those data to select three confirmed metal-poor giants (Fe/H = −3.15, −2.56, −2.03) for follow-up high-resolution Magellan/Magellan Inamori Kyocera Echelle spectroscopy. A comprehensive orbit analysis using Gaia DR2 astrometry and our measured radial velocities confirms that these stars are tightly bound inner bulge stars. We determine the elemental abundances of each star and find high titanium and iron-peak abundances relative to iron in our most metal-poor star. We propose that the distinct abundance signature we detect is a product of nucleosynthesis in the Chandrasekhar-mass thermonuclear supernova of a CO white dwarf accreting from a helium star with a delay time of about 10 Myr. Even though chemical evolution is expected to occur quickly in the bulge, the intense star formation in the core of the nascent Milky Way was apparently able to produce at least one Chandrasekhar-mass thermonuclear supernova progenitor before chemical evolution advanced beyond
.
Abstract
We report the discovery of RAVE J203843.2−002333, a bright (
V
= 12.73), very metal-poor (
= −2.91),
r
-process-enhanced (
= +1.64 and
= −0.81) star selected from the RAVE survey. This star ...was identified as a metal-poor candidate based on its medium-resolution (
R
∼ 1600) spectrum obtained with the KPNO/Mayall Telescope, and followed up with high-resolution (
R
∼ 66,000) spectroscopy with the
Magellan
/Clay Telescope, allowing for the determination of elemental abundances for 24 neutron-capture elements, including thorium and uranium. RAVE J2038−0023 is only the fourth metal-poor star with a clearly measured U abundance. The derived chemical abundance pattern exhibits good agreement with those of other known highly
r
-process-enhanced stars, and evidence suggests that it is not an actinide-boost star. Age estimates were calculated using U/X abundance ratios, yielding a mean age of 13.0 ± 1.1 Gyr.
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 demonstrate that using up to seven stellar abundance ratios can place observational constraints on the star formation histories (SFHs) of Local Group dSphs, using Sculptor dSph as a test ...case. We use a one-zone chemical evolution model to fit the overall abundance patterns of
α
elements (which probe the core-collapse supernovae that occur shortly after star formation),
s
-process elements (which probe AGB nucleosynthesis at intermediate delay times), and iron-peak elements (which probe delayed Type Ia supernovae). Our best-fit model indicates that Sculptor dSph has an ancient SFH, consistent with previous estimates from deep photometry. However, we derive a total star formation duration of ∼0.9 Gyr, which is shorter than photometrically derived SFHs. We explore the effect of various model assumptions on our measurement and find that modifications to these assumptions still produce relatively short SFHs of duration ≲1.4 Gyr. Our model is also able to compare sets of predicted nucleosynthetic yields for supernovae and AGB stars, and can provide insight into the nucleosynthesis of individual elements in Sculptor dSph. We find that observed Mn/Fe and Ni/Fe trends are most consistent with sub-
M
Ch
Type Ia supernova models, and that a combination of “prompt” (delay times similar to core-collapse supernovae) and “delayed” (minimum delay times ≳50 Myr)
r
-process events may be required to reproduce observed Ba/Mg and Eu/Mg trends.
Abstract The growing number of Milky Way satellites detected in recent years has introduced a new focus for stellar abundance analysis. Abundances of stars in satellites have been used to probe the ...nature of these systems and their chemical evolution. However, for most satellites, only centrally located stars have been examined. This paper presents an analysis of three stars in the Tucana V system, one in the inner region and two at ∼10′ (7–10 half-light radii) from the center. We find a remarkable chemical diversity between the stars. One star exhibits enhancements in rapid neutron-capture elements (an r -I star), and another is highly enhanced in C, N, and O but with low neutron-capture abundances (a CEMP-no star). The metallicities of the stars analyzed span more than 1 dex from Fe/H = −3.55 to −2.46. This, combined with a large abundance range of other elements like Ca, Sc, and Ni, confirms that Tuc V is an ultrafaint dwarf (UFD) galaxy. The variation in abundances, highlighted by Mg/Ca ratios ranging from +0.89 to −0.75, among the stars demonstrates that the chemical enrichment history of Tuc V was very inhomogeneous. Tuc V is only the second UFD galaxy in which stars located at large distances from the galactic center have been analyzed, along with Tucana II. The chemical diversity seen in these two galaxies, driven by the composition of the noncentral member stars, suggests that distant member stars are important to include when classifying faint satellites and that these systems may have experienced more complex chemical enrichment histories than previously anticipated.
Abstract
We use deep narrowband CaHK (F395N) imaging taken with the Hubble Space Telescope (HST) to construct the metallicity distribution function (MDF) of Local Group ultra-faint dwarf galaxy ...Eridanus
II
(Eri
II
). When combined with archival F475W and F814W data, we measure metallicities for 60 resolved red giant branch stars as faint as
m
F475W
∼ 24 mag, a factor of ∼4× more stars than current spectroscopic MDF determinations. We find that Eri
II
has a mean metallicity of Fe/H = −2.50
−
0.07
+
0.07
and a dispersion of
σ
Fe
/
H
=
0.42
−
0.06
+
0.06
, which are consistent with spectroscopic MDFs, though more precisely constrained owing to a larger sample. We identify a handful of extremely metal-poor star candidates (EMP; Fe/H < −3) that are marginally bright enough for spectroscopic follow-up. The MDF of Eri
II
appears well described by a leaky box chemical evolution model. We also compute an updated orbital history for Eri
II
using Gaia eDR3 proper motions, and find that it is likely on first infall into the Milky Way. Our findings suggest that Eri
II
underwent an evolutionary history similar to that of an isolated galaxy. Compared to MDFs for select cosmological simulations of similar mass galaxies, we find that Eri
II
has a lower fraction of stars with Fe/H < −3, though such comparisons should currently be treated with caution due to a paucity of simulations, selection effects, and known limitations of CaHK for EMPs. This study demonstrates the power of deep HST CaHK imaging for measuring the MDFs of UFDs.
Abstract
We present the first detailed comparison of populations of dwarf galaxy stellar streams in cosmological simulations and the Milky Way. In particular, we compare streams identified around 13 ...Milky Way analogs in the FIRE-2 simulations to streams observed by the Southern Stellar Stream Spectroscopic Survey (
S
5
). For an accurate comparison, we produce mock Dark Energy Survey (DES) observations of the FIRE streams and estimate the detectability of their tidal tails and progenitors. The number and stellar mass distributions of detectable stellar streams is consistent between observations and simulations. However, there are discrepancies in the distributions of pericenters and apocenters, with the detectable FIRE streams, on average, forming at larger pericenters (out to >110 kpc) and surviving only at larger apocenters (≳40 kpc) than those observed in the Milky Way. We find that the population of high-stellar-mass dwarf galaxy streams in the Milky Way is incomplete. Interestingly, a large fraction of the FIRE streams would only be detected as intact satellites in DES-like observations, since their tidal tails have too low surface brightness to be detectable. We thus predict a population of yet-undetected tidal tails around Milky Way satellites, as well as a population of fully undetected low-surface-brightness stellar streams, and estimate their detectability with the Rubin Observatory. Finally, we discuss the causes and implications of the discrepancies between the stream populations in FIRE and the Milky Way, and explore future avenues for tests of satellite disruption in cosmological simulations.
PDF
