436 research outputs found
SD 1313-0019 -- Another second-generation star with [Fe/H] = -5.0, observed with the Magellan Telescope
We present a Magellan/MIKE high-resolution (R ~ 35,000) spectrum of the
ancient star SD 1313-0019 which has an iron abundance of [Fe/H] = -5.0, paired
with a carbon enhancement of [C/Fe] ~ 3.0. The star was initially identified by
Allende Prieto et al. in the BOSS survey. Its medium-resolution spectrum
suggested a higher metallicity of [Fe/H] = -4.3 due to the CaII K line blending
with a CH feature which is a common issue related to the search for the most
iron-poor stars. This star joins several other, similar stars with [Fe/H] <
-5.0 that all display a combination of low iron and high carbon abundances.
Other elemental abundances of SD 1313-0019 follow that of more metal-rich halo
stars. From fitting the abundance pattern with yields of Population III
supernova, we conclude that SD 1313-0019 had only one massive progenitor star
with 20 - 30 M_sun that must have undergone a mixing and fallback episode.
Overall, there are now five stars known with [Fe/H] < -5.0 (1D LTE abundances).
This population of second-generation stars strongly suggests massive first
stars that almost exclusively produced large amounts of carbon through stellar
winds and/or their mixing and fallback supernova explosions. As a consequence,
their natal clouds -- presumably some early minihalo structures -- contained
ample amounts of carbon and oxygen that likely facilitated the formation of
these first low-mass stars.Comment: 7 pages and 3 figures, accepted by ApJ
US National Gemini Office in the NOIRLab era
This article presents an overview of the US National Gemini Office (US NGO)
and its role within the International Gemini Observatory user community.
Throughout the years, the US NGO charter changed considerably to accommodate
the evolving needs of astronomers and the observatory. The current landscape of
observational astronomy requires effective communication between stakeholders
and reliable/accessible data reduction tools and products, which minimize the
time between data gathering and publication of scientific results. Because of
that, the US NGO heavily invests in producing data reduction tutorials and
cookbooks. Recently, the US NGO started engaging with the Gemini user community
through social media, and the results have been encouraging, increasing the
observatory's visibility. The US NGO staff developed tools to assess whether
the support provided to the user community is sufficient and effective, through
website analytics and social media engagement numbers. These quantitative
metrics serve as the baseline for internal reporting and directing efforts to
new or current products. In the era of the NSF's National Optical-Infrared
Astronomy Research Laboratory (NOIRLab), the US NGO is well-positioned to be
the liaison between the US user base and the Gemini Observatory. Furthermore,
collaborations within NOIRLab programs, such as the Astro Data Lab and the Time
Allocation Committee, enhance the US NGO outreach to attract users and develop
new products. The future landscape laid out by the Astro 2020 report confirms
the need to establish such synergies and provide more integrated user support
services to the astronomical community at large.Comment: 15 pages, 8 figures, published in the Journal of Astronomical
Telescopes, Instruments, and System
Abundances of carbon-enhanced metal-poor stars as constraints on their formation
Reproduced with permission from Astronomy & Astrophysics. © 2018 ESO.Context. An increasing fraction of carbon-enhanced metal-poor (CEMP) stars is found as their iron abundance, [Fe/H], decreases below [Fe/H] =−2.0. The CEMP-s stars have the highest absolute carbon abundances, [C/H], and are thought to owe their enrichment in carbon and the slow neutron-capture (s-process) elements to mass transfer from a former asymptotic giant branch (AGB) binary companion. The most Fe-poor CEMP stars are normally single, exhibit somewhat lower [C/H] than CEMP-s stars, but show no s-process element enhancement (CEMP-no stars). Abundance determinations of CNO offer clues to their formation sites. Aims. Our aim is to use the medium-resolution spectrograph X-Shooter/VLT to determine stellar parameters and abundances for C, N, Sr, and Ba in several classes of CEMP stars in order to further classify and constrain the astrophysical formation sites of these stars. Methods. Atmospheric parameters for our programme stars were estimated from a combination of V−K photometry, model isochrone fits, and estimates from a modified version of the SDSS/SEGUE spectroscopic pipeline. We then used X-Shooter spectra in conjunction with the 1D local thermodynamic equilibrium spectrum synthesis code MOOG, 1D ATLAS9 atmosphere models to derive stellar abundances, and, where possible, isotopic 12C/13C ratios. Results. Abundances (or limits) of C, N, Sr, and Ba are derived for a sample of 27 faint metal-poor stars for which the X-Shooter spectra have sufficient signal-to-noise ratios (S/N). These moderate resolution, low S/N (~10−40) spectra prove sufficient to perform limited chemical tagging and enable assignment of these stars into the CEMP subclasses (CEMP-s and CEMP-no). According to the derived abundances, 17 of our sample stars are CEMP-s and 3 are CEMP-no, while the remaining 7 are carbon-normal. For four CEMP stars, the subclassification remains uncertain, and two of them may be pulsating AGB stars. Conclusions. The derived stellar abundances trace the formation processes and sites of our sample stars. The [C/N] abundance ratio is useful for identifying stars with chemical compositions unaffected by internal mixing, and the [Sr/Ba] abundance ratio allows us to distinguish between CEMP-s stars with AGB progenitors and the CEMP-no stars. Suggested formation sites for the latter include faint supernovae with mixing and fallback and/or primordial, rapidly-rotating, massive stars (spinstars). X-Shooter spectra have thus proved to be valuable tools in the continued search for their origin.Peer reviewe
2MASS J18082002-5104378: The brightest (V=11.9) ultra metal-poor star
Context. The most primitive metal-poor stars are important for studying the
conditions of the early galaxy and are also relevant to big bang
nucleosynthesis. Aims. Our objective is to find the brightest (V<14) most
metal-poor stars. Methods. Candidates were selected using a new method, which
is based on the mismatch between spectral types derived from colors and
observed spectral types. They were observed first at low resolution with EFOSC2
at the NTT/ESO to obtain an initial set of stellar parameters. The most
promising candidate, 2MASS J18082002-5104378 (V=11.9), was observed at high
resolution (R=50 000) with UVES at the VLT/ESO, and a standard abundance
analysis was performed. Results. We found that 2MASS J18082002-5104378 is an
ultra metal-poor star with stellar parameters Teff = 5440 K, log g = 3.0 dex,
vt = 1.5 km/s, [Fe/H] = -4.1 dex. The star has [C/Fe]<+0.9 in a 1D analysis, or
[C/Fe]<=+0.5 if 3D effects are considered; its abundance pattern is typical of
normal (non-CEMP) ultra metal-poor stars. Interestingly, the star has a binary
companion. Conclusions. 2MASS J1808-5104 is the brightest (V=11.9) metal-poor
star of its category, and it could be studied further with even higher S/N
spectroscopy to determine additional chemical abundances, thus providing
important constraints to the early chemical evolution of our Galaxy.Comment: A&A Letter
The role of binaries in the enrichment of the early Galactic halo. II. Carbon-Enhanced Metal-Poor Stars - CEMP-no stars
The detailed composition of most metal-poor halo stars has been found to be
very uniform. However, a fraction of 20-70% (increasing with decreasing
metallicity) exhibit dramatic enhancements in their abundances of carbon - the
so-called carbon-enhanced metal-poor (CEMP) stars. A key question for Galactic
chemical evolution models is whether this non-standard composition reflects
that of the stellar natal clouds, or is due to local, post-birth mass transfer
of chemically processed material from a binary companion; CEMP stars should
then all be members of binary systems. Our aim is to determine the frequency
and orbital parameters of binaries among CEMP stars with and without
over-abundances of neutron-capture elements - CEMP-s and CEMP-no stars,
respectively - as a test of this local mass-transfer scenario. This paper
discusses a sample of 24 CEMP-no stars, while a subsequent paper will consider
a similar sample of CEMP-s stars. Most programme stars exhibit no statistically
significant radial-velocit variation over this period and appear to be single,
while four are found to be binaries with orbital periods of 300-2,000 days and
normal eccentricity; the binary frequency for the sample is 17+-9%. The single
stars mostly belong to the recently-identified ``low-C band'', while the
binaries have higher absolute carbon abundances. We conclude that the
nucleosynthetic process responsible for the strong carbon excess in these
ancient stars is unrelated to their binary status; the carbon was imprinted on
their natal molecular clouds in the early Galactic ISM by an even earlier,
external source, strongly indicating that the CEMP-no stars are likely bona
fide second-generation stars. We discuss potential production sites for carbon
and its transfer across interstellar distances in the early ISM, and
implications for the composition of high-redshift DLA systems. Abridged.Comment: 16 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
Population Studies. XIII. A New Analysis of the Bidelman-MacConnell "Weak-Metal" Stars - Confirmation of Metal-Poor Stars in the Thick Disk of the Galaxy
A new set of very high signal-to-noise (S/N > 100/1), medium-resolution
(R~3000) optical spectra have been obtained for 302 of the candidate
"weak-metal" stars selected by Bidelman & MacConnell. We use these data to
calibrate the recently developed generalization of the SEGUE Stellar Parameter
Pipeline, and obtain estimates of the atmospheric parameters (Teff, log g , and
[Fe/H]) for these non-SDSS/SEGUE data; we also obtain estimates of [C/Fe]. The
new abundance measurements are shown to be consistent with available
high-resolution spectroscopic determinations, and represent a substantial
improvement over the accuracies obtained from the previous photometric
estimates reported in Paper I of this series. The apparent offset in the
photometric abundances of the giants in this sample noted by several authors is
confirmed by our new spectroscopy; no such effect is found for the dwarfs. The
presence of a metal-weak thick-disk (MWTD) population is clearly supported by
these new abundance data. Some 25% of the stars with metallicities -1.8 <
[Fe/H] <= -0.8 exhibit orbital eccentricities e < 0.4, yet are clearly
separated from members of the inner-halo population with similar metallicities
by their location in a Lindblad energy vs. angular momentum diagram. A
comparison is made with recent results for a similar-size sample of RAVE stars
from Ruchti et al. We conclude, based on both of these samples, that the MWTD
is real, and must be accounted for in discussions of the formation and
evolution of the disk system of the Milky Way.Comment: 45 pages, 14 figures; accepted for publication in Ap
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