We analyze the dynamical properties of ∼1500 very metal-poor (VMP; Fe/H −2.0) halo stars, based primarily on medium-resolution spectroscopic data from the HK and Hamburg/ESO surveys. These data, ...collected over the past 30 yr, are supplemented by a number of calibration stars and other small samples, along with astrometric information from Gaia DR2. We apply a clustering algorithm to the 4D energy-action space of the sample, and identify a set of 38 dynamically tagged groups (DTGs), containing between 5 and 30 member stars. Many of these DTGs can be associated with previously known prominent substructures such as Gaia-Sausage/Enceladus (GSE), Sequoia, Helmi Stream (HStr), and Thamnos. Others are associated with previously identified smaller dynamical groups of stars and streams. We identify 10 new DTGs as well, many of which have strongly retrograde orbits. We also investigate possible connections between our DTGs and ∼300 individual r-process-enhanced (RPE) stars from a recent literature compilation. We find that several of these objects have similar dynamical properties to GSE (5), the HStr (4), Sequoia (1), and Rg5 (1), indicating that their progenitors might have been important sources of RPE stars in the Galaxy. Additionally, a number of our newly identified DTGs are shown to be associated with at least two RPE stars each (DTG-2: 3, DTG-7: 2; DTG-27: 2). Taken as a whole, these results are consistent with ultra-faint and/or dwarf spheroidal galaxies as birth environments in which r-process nucleosynthesis took place, and then were disrupted by the Milky Way.
Abstract
We report on the discovery of
SPLUS J210428.01−004934.2
, an ultra metal-poor (UMP) star first identified from the narrowband photometry of the Southern Photometric Local Universe Survey ...(S-PLUS) Data Release 1, in the SDSS Stripe 82 region. Follow-up medium- and high-resolution spectroscopy (with Gemini South and Magellan-Clay, respectively) confirmed the effectiveness of the search for low-metallicity stars using the S-PLUS narrowband photometry. At Fe/H = −4.03,
SPLUS J2104−0049
has the lowest
detected
carbon abundance,
A
(C) = +4.34, when compared to the 34 previously known UMP stars in the literature, which is an important constraint on its stellar progenitor and also on stellar evolution models at the lowest metallicities. Based on its chemical abundance pattern, we speculate that
SPLUS J2104−0049
could be a bona fide second-generation star, formed from a gas cloud polluted by a single metal-free ∼ 30
M
⊙
star. This discovery opens the possibility of finding additional UMP stars directly from narrowband photometric surveys, a potentially powerful method to help complete the inventory of such peculiar objects in our Galaxy.
Abstract
We combine spectroscopic, photometric, and astrometric information from APOGEE data release 17 and Gaia early data release 3 to perform a self-consistent characterization of ...Gaia-Sausage/Enceladus (GSE), the remnant of the last major merger experienced by the Milky Way, considering stars and globular clusters (GCs) altogether. Our novel set of chemodynamical criteria to select genuine stars of GSE yields a metallicity distribution function with a median Fe/H of −1.22 and 0.23 dex dispersion. Stars from GSE present an excess of Al/Fe and Mg/Mn (also Mg/Fe) in comparison to surviving Milky Way dwarf satellites, which can be explained by differences in star formation efficiencies and timescales between these systems. However, stars from Sequoia, another proposed accreted halo substructure, essentially overlap the GSE footprint in all analyzed chemical-abundance spaces, but present lower metallicities. Among the probable GCs of GSE with APOGEE observations available, we find no evidence for atypical Fe/H spreads with the exception of
ω
Centauri (
ω
Cen). Under the assumption that
ω
Cen is a stripped nuclear star cluster, we estimate the stellar mass of its progenitor to be
M
⋆
≈ 1.3 × 10
9
M
⊙
, well within literature expectations for GSE. This leads us to envision GSE as the best available candidate for the original host galaxy of
ω
Cen. We also take advantage of Gaia's photometry and APOGEE metallicities as priors to determine fundamental parameters for eight high-probability (>70%) GC members of GSE via statistical isochrone fitting. Finally, the newly determined ages and APOGEE Fe/H values are utilized to model the age–metallicity relation of GSE.
Abstract
We explore the kinematic and chemical properties of the Monoceros stellar overdensity by combining data from the Two Micron All Sky Survey, Wide-field Infrared Survey Explorer, APOGEE, and ...Gaia. Monoceros is a structure located toward the Galactic anticenter and close to the disk. We have identified that its stars have azimuthal velocity in the range of 200 <
v
ϕ
(km s
−1
) < 250. Combining their kinematics and spatial distribution, we designed a new method to select stars from this overdensity. This method allows us to easily identify the structure in both hemispheres and estimate their distances. Our analysis was supported by comparison with simulated data from the entire sky generated by the
Galaxia
code. Furthermore, we characterized, for the first time, the Monoceros overdensity in several chemical abundance spaces. Our results confirm its similarity to stars found in the thin disk of the Galaxy and suggest an in situ formation. Furthermore, we demonstrate that the southern and northern regions of Monoceros exhibit indistinguishable chemical compositions.
Abstract
In this work, we study the phase-space and chemical properties of the Sagittarius (Sgr) stream, the tidal tails produced by the ongoing destruction of the Sgr dwarf spheroidal (dSph) galaxy, ...focusing on its very metal-poor (VMP; Fe/H < −2) content. We combine spectroscopic and astrometric information from SEGUE and Gaia EDR3, respectively, with data products from a new large-scale run of the
StarHorse
spectrophotometric code. Our selection criteria yield ∼1600 stream members, including >200 VMP stars. We find the leading arm (
b
> 0°) of the Sgr stream to be more metal-poor, by ∼0.2 dex, than the trailing one (
b
< 0°). With a subsample of turnoff and subgiant stars, we estimate this substructure’s stellar population to be ∼1 Gyr older than the thick disk’s. With the aid of an
N
-body model of the Sgr system, we verify that simulated particles stripped earlier (>2 Gyr ago) have present-day phase-space properties similar to lower metallicity stream stars. Conversely, those stripped more recently (<2 Gyr) are preferentially akin to metal-rich (Fe/H > −1) members of the stream. Such correlation between kinematics and chemistry can be explained by the existence of a dynamically hotter, less centrally concentrated, and more metal-poor population in Sgr dSph prior to its disruption, implying that this galaxy was able to develop a metallicity gradient before its accretion. Finally, we identified several carbon-enhanced metal-poor (C/Fe > +0.7 and Fe/H ≤ −1.5) stars in the Sgr stream, which might be in tension with current observations of its remaining core where such objects are not found.
Abstract We present a comprehensive recalibration of narrowband/medium-band and broadband photometry from the Southern Photometric Local Universe Survey (S-PLUS) by leveraging two approaches: an ...improved Gaia XP synthetic photometry (XPSP) method with corrected Gaia XP spectra, and the stellar color regression (SCR) method with corrected Gaia Early Data Release 3 photometric data and spectroscopic data from LAMOST Data Release 7. Through the use of millions of stars as standards per band, we demonstrate the existence of position-dependent systematic errors, up to 23 mmag for the main survey region, in the S-PLUS iDR4 photometric data. A comparison between the XPSP and SCR methods reveals minor differences in zero-point offsets, typically within the range of 1–6 mmag, indicating the accuracy of the recalibration, and a twofold to threefold improvement in the zero-point precision. During this process, we also verify and correct for systematic errors related to CCD position. The corrected S-PLUS iDR4 photometric data will provide a solid data foundation for conducting scientific research that relies on high-precision calibration. Our results underscore the power of the XPSP method in combination with the SCR method, showcasing their effectiveness in enhancing calibration precision for wide-field surveys when combined with Gaia photometry and XP spectra, to be applied for other S-PLUS subsurveys.
Abstract
We present a chemodynamical study of the Triangulum–Andromeda overdensity (TriAnd) employing a sample of 31 candidate stars observed with the GRACES high-resolution (
R
= 40,000) ...spectrograph at the Gemini North (8.1 m) telescope. TriAnd is a stellar substructure found toward the outer disk of the Milky Way, located at
R
GC
∼ 18 kpc from the Sun, toward Galactic latitude
b
∼ 25°. Most stars in our sample have dynamical properties compatible with a disk stellar population. In addition, by applying an eccentricity cut, we are able to detect a stellar contamination that seems to be consistent with an accreted population. In chemical abundance space, the majority of our TriAnd candidates are similar to the outer thin-disk population, suggesting that the overdensity has an in situ origin. Finally, the found accreted halo interlopers spatially overlapping with TriAnd should explain the historical discussion of the overdensity’s nature due to its complex chemical patterns.
Abstract
We report photometric estimates of effective temperature,
T
eff
, metallicity, Fe/H, carbonicity, C/Fe, and absolute carbon abundances,
A
(C), for over 700,000 stars from the Southern ...Photometric Local Universe Survey (S-PLUS) Data Release 2, covering a substantial fraction of the equatorial Sloan Digital Sky Survey Stripe 82. We present an analysis for two stellar populations: (1) halo main-sequence turnoff stars and (2) K-dwarf stars of mass 0.58 <
M
/
M
⊙
< 0.75 in the Solar Neighborhood. Application of the Stellar Photometric Index Network Explorer (
SPHINX
) to the mixed-bandwidth (narrow- plus wide-band) filter photometry from S-PLUS produces robust estimates of the metallicities and carbon abundances in stellar atmospheres over a wide range of temperatures, 4250 <
T
eff
(K) < 7000. The use of multiple narrow-band S-PLUS filters enables
SPHINX
to achieve substantially lower levels of “catastrophic failures” (i.e., large offsets in metallicity estimates relative to spectroscopic determinations) than previous efforts using a single metallicity-sensitive narrow-band filter. We constrain the exponential slope of the Milky Way’s K-dwarf halo metallicity distribution function (MDF),
λ
10,Fe/H
= 0.85 ± 0.21, over the metallicity range −2.5 < Fe/H < −1.0; the MDF of our local-volume K-dwarf sample is well-represented by a gamma distribution with parameters
α
= 2.8 and
β
= 4.2. S-PLUS photometry obtains absolute carbon abundances with a precision of ∼0.35 dex for stars with
T
eff
< 6500 K. We identify 364 candidate carbon-enhanced metal-poor stars, obtain assignments of these stars into the Yoon–Beers morphological groups in the
A
(C)-Fe/H space, and we derive the CEMP frequencies.
Abstract
We identified eight additional stars as members of the Helmi stream (HStr) in the combined GALAH+ DR3 and Gaia EDR3 catalog. By consistently reevaluating claimed members from the literature, ...we consolidate a sample of 22 HStr stars with parameters determined from high-resolution spectroscopy and spanning a considerably wider (by ∼0.5 dex) metallicity interval (− 2.5 ≲ Fe/H < − 1.0) than previously reported. Our study focuses on
α
(Mg and Ca) and neutron-capture (Ba and Eu) elements. We find that the chemistry of HStr is typical of dwarf spheroidal (dSph) galaxies, in good agreement with previous
N
-body simulations of this merging event. Stars of HStr constitute a clear declining sequence in
α
/Fe for increasing metallicity up to Fe/H ∼ −1.0. Moreover, stars of HStr show a median value of +0.5 dex for Eu/Fe with a small dispersion (±0.1 dex). Every star analyzed with Fe/H < −1.2 belongs to the
r
-process enhanced (Eu/Fe > +0.3 and Ba/Eu < 0.0) metal-poor category, providing remarkable evidence that, at such a low-metallicity regime, stars of HStr experienced enrichment in neutron-capture elements predominantly via
r
-process nucleosynthesis. Finally, the extended metallicity range also suggests an increase in Ba/Eu for higher Fe/H, in conformity with other surviving dwarf satellite galaxies of the Milky Way.
Abstract
We present an updated sample of blue horizontal-branch (BHB) stars selected from the photometric and spectroscopic data from Sloan Digital Sky Survey and its associated project Sloan ...Extension for Galactic Understanding and Exploration (SEGUE). With these data, we selected candidates for A-type stars in the color–color space and then a mixture modeling technique was implemented in order to distinguish between BHB and main-sequence/blue-straggler stars based on their surface gravity values (
log
g
) estimated by the SEGUE Stellar Parameter Pipeline. Our robust approach allows us to attribute individual probabilities of each star truly being in the BHB stage. Hence, our method is advantageous in comparison to previous SEGUE BHB selections that adopted simple
log
g
cuts. We also revisit the color–magnitude relation for these stars and propose two calibrations, based on updated distances for Galactic globular clusters, to estimate absolute magnitudes with (
g
−
r
)
0
and (
u
−
r
)
0
colors.
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