Abstract New boron abundances or upper limits have been determined for eight early B stars in the young Galactic open cluster NGC 3293, using UV spectra obtained by the Hubble Space Telescope Cosmic ...Origins Spectrograph. With previous observations, there are now 18 early B stars in this cluster with boron measurements. Six of the newly observed stars have projected rotational velocities greater than 200 km s −1 , allowing new constraints on rotationally driven mixing in main-sequence stars. When comparing to synthetic model populations, we find that the majority of our sample stars agree well with the predicted trends of stronger boron depletion for larger rotation and for larger mass or luminosity. Based on those, a smaller than the canonical rotational mixing efficiency ( f c ≈ 0.0165 versus the more standard value of 0.033) appears to be favored. In addition, the five mostly slowly rotating stars, when considered as a group, tend to show more boron depletion than expected from rotational mixing, and we speculate that most or all of these originate from binary mergers.
The distribution of stellar abundances along the Galactic disk is an important constraint for models of chemical evolution and Galaxy formation. In this study we derive radial gradients of C, N, O, ...Mg, Al, and Si, as well as S, from abundance determinations in young OB stars. Our database is composed of a sample of 69 members of 25 open clusters, OB associations, and H II regions with Galactocentric distances between 4.7 and 13.2 kpc. An important feature of this abundance database is the fact that the abundances were derived self-consistently in non-LTE using a homogeneous set of stellar parameters. Such an uniform analysis is expected to reduce the magnitude of random errors, as well as the influence of systematics in the gradients defined by the abundance and Galactocentric distance. The metallicity gradients obtained in this study are, in general, flatter than the results from previous recent abundance studies of early-type stars. The slopes are found to be between -0.031 (for oxygen) and -0.052 dex kpc super(-1) (for magnesium). The gradients obtained for the studied elements are quite similar, and if averaged they can be represented by a single slope of -0.042 plus or minus 0.007 dex kpc super(-1). This value is generally consistent with an overall flattening of the radial gradients with time.
Abundances of fluorine (19F), as well as isotopic ratios of 16O/17O, are derived in a sample of luminous young (~107-108 yr) red giants in the Galactic center (with galactocentric distances ranging ...from 0.6-30 pc), using high-resolution infrared spectra and vibration-rotation lines of H19F near λ2.3 μm. Five of the six red giants are members of the Nuclear star cluster that orbits the central supermassive black hole. Previous investigations of the chemical evolution of 19F in Galactic thin and thick-disk stars have revealed that the nucleosynthetic origins of 19F may be rather complex, resulting from two, or more, astrophysical sites; fluorine abundances behave as a primary element with respect to Fe abundances for thick-disk stars and as a secondary element in thin-disk stars. The Galactic center red giants analyzed fall within the thin-disk relation of F with Fe, having near-solar, to slightly larger, abundances of Fe (<Fe/H> = +0.08 ± 0.04), with a slight enhancement of the F/Fe abundance ratio (<F/Fe> = +0.28 ± 0.17). In terms of their F and Fe abundances, the Galactic center stars follow the thin-disk population, which requires an efficient source of 19F that could be the winds from core-He burning Wolf-Rayet stars, or thermally pulsing AGB stars, or a combination of both. The observed increase of F/Fe with increasing Fe/H found in thin-disk and Galactic center stars is not predicted by any published chemical evolution models that are discussed, thus a quantitative understanding of yields from the various possible sources of 19F remains unknown.
Abstract
Abundances of fluorine (
19
F), as well as isotopic ratios of
16
O/
17
O, are derived in a sample of luminous young (∼10
7
–10
8
yr) red giants in the Galactic center (with galactocentric ...distances ranging from 0.6–30 pc), using high-resolution infrared spectra and vibration-rotation lines of H
19
F near
λ
2.3
μ
m. Five of the six red giants are members of the Nuclear star cluster that orbits the central supermassive black hole. Previous investigations of the chemical evolution of
19
F in Galactic thin and thick-disk stars have revealed that the nucleosynthetic origins of
19
F may be rather complex, resulting from two, or more, astrophysical sites; fluorine abundances behave as a primary element with respect to Fe abundances for thick-disk stars and as a secondary element in thin-disk stars. The Galactic center red giants analyzed fall within the thin-disk relation of F with Fe, having near-solar, to slightly larger, abundances of Fe (〈Fe/H〉 = +0.08 ± 0.04), with a slight enhancement of the F/Fe abundance ratio (〈F/Fe〉 = +0.28 ± 0.17). In terms of their F and Fe abundances, the Galactic center stars follow the thin-disk population, which requires an efficient source of
19
F that could be the winds from core-He burning Wolf–Rayet stars, or thermally pulsing AGB stars, or a combination of both. The observed increase of F/Fe with increasing Fe/H found in thin-disk and Galactic center stars is not predicted by any published chemical evolution models that are discussed, thus a quantitative understanding of yields from the various possible sources of
19
F remains unknown.
Aims: Recent observations have challenged our understanding of rotational mixing in massive stars by revealing a population of fast- rotating objects with apparently normal surface nitrogen ...abundances. However, several questions have arisen because of a number of issues, which have rendered a reinvestigation necessary; these issues include the presence of numerous upper limits for the nitrogen abundance, unknown multiplicity status, and a mix of stars with different physical properties, such as their mass and evolutionary state, which are known to control the amount of rotational mixing.
Methods: We have carefully selected a large sample of bright, fast-rotating early-type stars of our Galaxy (40 objects with spectral types between B0.5 and O4). Their high-quality, high-resolution optical spectra were then analysed with the stellar atmosphere modelling codes DETAIL/SURFACE or CMFGEN, depending on the temperature of the target. Several internal and external checks were performed to validate our methods; notably, we compared our results with literature data for some well-known objects, studied the effect of gravity darkening, or confronted the results provided by the two codes for stars amenable to both analyses. Furthermore, we studied the radial velocities of the stars to assess their binarity.
Results: This first part of our study presents our methods and provides the derived stellar parameters, He, CNO abundances, and the multiplicity status of every star of the sample. It is the first time that He and CNO abundances of such a large number of Galactic massive fast rotators are determined in a homogeneous way.
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.
Context.
The Javalambre Photometric Local Universe Survey (J-PLUS) has obtained precise photometry in 12 specially designed filters for large numbers of Galactic stars. Deriving their precise stellar ...atmospheric parameters and individual elemental abundances is crucial for studies of Galactic structure and the assembly history and chemical evolution of our Galaxy.
Aims.
Our goal is to estimate not only stellar parameters (effective temperature,
T
eff
, surface gravity, log
g
, and metallicity, Fe/H), but also
α
/Fe and four elemental abundances (C/Fe, N/Fe, Mg/Fe, and Ca/Fe) using data from the first data release (DR1) of J-PLUS.
Methods.
By combining recalibrated photometric data from J-PLUS DR1,
Gaia
DR2, and spectroscopic labels from the Large sky Area Multi-Object fiber Spectroscopic Telescope, we designed and trained a set of cost-sensitive neural networks, the CSNet, to learn the nonlinear mapping from stellar colours to their labels. Special attention was paid to the poorly populated regions of the label space by giving different weights according to their density distribution.
Results.
We achieved precisions of
δ
T
eff
∼ 55 K,
δ
log
g
∼ 0.15 dex, and
δ
Fe/H ∼ 0.07 dex, respectively, over a wide range of temperatures, surface gravities, and metallicities. The uncertainties of the abundance estimates for
α
/Fe and the four individual elements are in the 0.04–0.08 dex range. We compare our parameter and abundance estimates with those from other spectroscopic catalogs such as the Apache Point Observatory for Galactic Evolution Experiment and the Galactic Archaeology with High Efficiency and Resolution Multi-Element Spectrograph and find an overall good agreement.
Conclusions.
Our results demonstrate the potential of well-designed, high-quality photometric data for determinations of stellar parameters as well as individual elemental abundances. Applying the method to J-PLUS DR1, we obtained the aforementioned parameters for about two million stars, providing an outstanding dataset for chemo-dynamic analyses of the Milky Way. The catalog of the estimated parameters is publicly accessible.
Context.
We explore the stellar content of the Javalambre Photometric Local Universe Survey (J-PLUS) Data Release 2 and show its potential for identifying low-metallicity stars using the Stellar ...Parameters Estimation based on Ensemble Methods (SPEEM) pipeline.
Aims.
SPEEM is a tool used to provide determinations of atmospheric parameters for stars and separate stellar sources from quasars based on the unique J-PLUS photometric system. The adoption of adequate selection criteria allows for the identification of metal-poor star candidates that are suitable for spectroscopic follow-up investigations.
Methods.
SPEEM consists of a series of machine-learning models that use a training sample observed by both J-PLUS and the SEGUE spectroscopic survey. The training sample has temperatures,
T
eff
, between 4800 K and 9000 K, values of log
g
between 1.0 and 4.5, as well as −3.1 < Fe/H < +0.5. The performance of the pipeline was tested with a sample of stars observed by the LAMOST survey within the same parameter range.
Results.
The average differences between the parameters of a sample of stars observed with SEGUE and J-PLUS, obtained with the SEGUE Stellar Parameter Pipeline and SPEEM, respectively, are Δ
T
eff
~ 41 K, Δlog
g
~ 0.11 dex, and ΔFe/H ~ 0.09 dex. We define a sample of 177 stars that have been identified as new candidates with Fe/H < −2.5, with 11 of them having been observed with the ISIS spectrograph at the
William Herschel
Telescope. The spectroscopic analysis confirms that 64% of stars have Fe/H < −2.5, including one new star with Fe/H < −3.0.
Conclusions.
Using SPEEM in combination with the J-PLUS filter system has demonstrated their potential in estimating the stellar atmospheric parameters (
T
eff
, log
g
, and Fe/H). The spectroscopic validation of the candidates shows that SPEEM yields a success rate of 64% on the identification of very metal-poor star candidates with Fe/H < −2.5.
Aims. Recent observations have challenged our understanding of rotational mixing in massive stars by revealing a population of fast-rotating objects with apparently normal surface nitrogen ...abundances. However, several questions have arisen because of a number of issues, which have rendered a reinvestigation necessary; these issues include the presence of numerous upper limits for the nitrogen abundance, unknown multiplicity status, and a mix of stars with different physical properties, such as their mass and evolutionary state, which are known to control the amount of rotational mixing. Methods. We have carefully selected a large sample of bright, fast-rotating early-type stars of our Galaxy (40 objects with spectral types between B0.5 and O4). Their high-quality, high-resolution optical spectra were then analysed with the stellar atmosphere modelling codes DETAIL/SURFACE or CMFGEN, depending on the temperature of the target. Several internal and external checks were performed to validate our methods; notably, we compared our results with literature data for some well-known objects, studied the effect of gravity darkening, or confronted the results provided by the two codes for stars amenable to both analyses. Furthermore, we studied the radial velocities of the stars to assess their binarity. Results. This first part of our study presents our methods and provides the derived stellar parameters, He, CNO abundances, and the multiplicity status of every star of the sample. It is the first time that He and CNO abundances of such a large number of Galactic massive fast rotators are determined in a homogeneous way.
Sulfur abundances are derived for a sample of 10 B main-sequence star members of the Orion association. The analysis is based on LTE plane-parallel model atmospheres and non-LTE line formation theory ...by means of a self-consistent spectrum synthesis analysis of lines from two ionization states of sulfur, S II and S III. The observations are high-resolution spectra obtained with the ARCES spectrograph at the Apache Point Observatory. The abundance distribution obtained for the Orion targets is homogeneous within the expected errors in the analysis: A(S) = 7.15 +/- 0.05. This average abundance result is in agreement with the recommended solar value (both from modeling of the photospheres in one-dimensional and three-dimensional, and meteorites) and indicates that little, if any, chemical evolution of sulfur has taken place in the last ~4.5 billion years. The sulfur abundances of the young stars in Orion are found to agree well with results for the Orion Nebulae, and place strong constraints on the amount of sulfur depletion onto grains as being very modest or nonexistent. The sulfur abundances for Orion are consistent with other measurements at a similar galactocentric radius: combined with previous results for other OB-type stars produce a relatively shallow sulfur abundance gradient with a slope of -0.037 +/- 0.012 dex kpc-1.
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