126 research outputs found
Germanium, Arsenic, and Selenium Abundances in Metal-Poor Stars
The elements germanium (Ge, Z=32), arsenic (As, Z=33), and selenium (Se,
Z=34) span the transition from charged-particle or explosive synthesis of the
iron-group elements to neutron-capture synthesis of heavier elements. Among
these three elements, only the chemical evolution of germanium has been studied
previously. Here we use archive observations made with the Space Telescope
Imaging Spectrograph on board the Hubble Space Telescope and observations from
several ground-based facilities to study the chemical enrichment histories of
seven stars with metallicities -2.6 < [Fe/H] < -0.4. We perform a standard
abundance analysis of germanium, arsenic, selenium, and several other elements
produced by neutron-capture reactions. When combined with previous derivations
of germanium abundances in metal-poor stars, our sample reveals an increase in
the [Ge/Fe] ratios at higher metallicities. This could mark the onset of the
weak s-process contribution to germanium. In contrast, the [As/Fe] and [Se/Fe]
ratios remain roughly constant. These data do not directly indicate the origin
of germanium, arsenic, and selenium at low metallicity, but they suggest that
the weak and main components of the s-process are not likely sources.Comment: Accepted for publication in the Astrophysical Journal. (12 pages, 5
figures
Detailed Abundances of 15 Stars in the Metal-Poor Globular Cluster NGC 4833
We have observed 15 red giant stars in the relatively massive, metal-poor
globular cluster NGC 4833 using the Magellan Inamori Kyocera Echelle
spectrograph at Magellan. We calculate stellar parameters for each star and
perform a standard abundance analysis to derive abundances of 43 species of 39
elements, including 20 elements heavier than the iron group. We derive
= -2.25 +/- 0.02 from Fe I lines and = -2.19 +/- 0.013 from Fe II
lines. We confirm earlier results that found no internal metallicity spread in
NGC 4833, and there are no significant star-to-star abundance dispersions among
any elements in the iron group (19 <= Z <= 30). We recover the usual abundance
variations among the light elements C, N, O, Na, Mg, Al, and possibly Si. The
heavy-element distribution reflects enrichment by r-process nucleosynthesis
([Eu/Fe] = +0.36 +/- 0.03), as found in many other metal-poor globular
clusters. We investigate small star-to-star variations found among the
neutron-capture elements, and we conclude that these are probably not real
variations. Upper limits on the Th abundance, log epsilon (Th/Eu) < -0.47 +/-
0.09, indicate that NGC 4833, like other globular clusters where Th has been
studied, did not experience a so-called "actinide boost."Comment: Accepted for publication in MNRAS. Version 2 adds final publication
referenc
Heavy Element Dispersion in the Metal-Poor Globular Cluster M92
Dispersion among the light elements is common in globular clusters (GCs),
while dispersion among heavier elements is less common. We present detection of
r-process dispersion relative to Fe in 19 red giants of the metal-poor GC M92.
Using spectra obtained with the Hydra multi-object spectrograph on the WIYN
Telescope at Kitt Peak National Observatory, we derive differential abundances
for 21 species of 19 elements. The Fe-group elements, plus Y and Zr, are
homogeneous at a level of 0.07-0.16 dex. The heavy elements La, Eu, and Ho
exhibit clear star-to-star dispersion spanning 0.5-0.8 dex. The abundances of
these elements are correlated with one another, and we demonstrate that they
were produced by r-process nucleosynthesis. This r-process dispersion is not
correlated with the dispersion in C, N, or Na in M92, indicating that r-process
inhomogeneities were present in the gas throughout star formation. The
r-process dispersion is similar to that previously observed in the metal-poor
GC M15, but its origin in M15 or M92 is unknown at present.Comment: Accepted for publication in the Astronomical Journal (22 pages, 12
figures). v2: references update
Detailed Abundance Analysis of the Brightest Star in Segue 2, the Least Massive Galaxy
We present the first high resolution spectroscopic observations of one red
giant star in the ultra-faint dwarf galaxy Segue 2, which has the lowest total
mass (including dark matter) estimated for any known galaxy. These observations
were made using the MIKE spectrograph on the Magellan II Telescope at Las
Campanas Observatory. We perform a standard abundance analysis of this star,
SDSS J021933.13+200830.2, and present abundances of 21 species of 18 elements
as well as upper limits for 25 additional species. We derive [Fe/H] = -2.9, in
excellent agreement with previous estimates from medium resolution
spectroscopy. Our main result is that this star bears the chemical signatures
commonly found in field stars of similar metallicity. The heavy elements
produced by neutron-capture reactions are present, but they are deficient at
levels characteristic of stars in other ultra-faint dwarf galaxies and a few
luminous dwarf galaxies. The otherwise normal abundance patterns suggest that
the gas from which this star formed was enriched by metals from multiple Type
II supernovae reflecting a relatively well-sampled IMF. This adds to the
growing body of evidence indicating that Segue 2 may have been substantially
more massive in the past.Comment: Accepted for publication in MNRAS. 13 pages, 7 figures, 3 tables,
including 1 long machine-readable table availabl
The diverse origins of neutron-capture elements in the metal-poor star HD 94028 : possible detection of products of i-process nucleosynthesis
We present a detailed analysis of the composition and nucleosynthetic origins of the heavy elements in the metal-poor ([Fe/H] = −1.62 ± 0.09) star HD 94028. Previous studies revealed that this star is mildly enhanced in elements produced by the slow neutron-capture process (s process; e.g., [Pb/Fe] = +0.79 ± 0.32) and rapid neutron-capture process (r process; e.g., [Eu/Fe] = +0.22 ± 0.12), including unusually large molybdenum ([Mo/Fe] = +0.97 ± 0.16) and ruthenium ([Ru/Fe] = +0.69 ± 0.17) enhancements. However, this star is not enhanced in carbon ([C/Fe] = −0.06 ± 0.19). We analyze an archival near-ultraviolet spectrum of HD 94028, collected using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, and other archival optical spectra collected from ground-based telescopes. We report abundances or upper limits derived from 64 species of 56 elements. We compare these observations with s-process yields from low-metallicity AGB evolution and nucleosynthesis models. No combination of s- and r-process patterns can adequately reproduce the observed abundances, including the super-solar [As/Ge] ratio (+0.99 ± 0.23) and the enhanced [Mo/Fe] and [Ru/Fe] ratios. We can fit these features when including an additional contribution from the intermediate neutron-capture process (i process), which perhaps operated through the ingestion of H in He-burning convective regions in massive stars, super-AGB stars, or low-mass AGB stars. Currently, only the i process appears capable of consistently producing the super-solar [As/Ge] ratios and ratios among neighboring heavy elements found in HD 94028. Other metal-poor stars also show enhanced [As/Ge] ratios, hinting that operation of the i process may have been common in the early Galaxy
Neutron-Capture Nucleosynthesis in the First Stars
Recent studies suggest that metal-poor stars enhanced in carbon but
containing low levels of neutron-capture elements may have been among the first
to incorporate the nucleosynthesis products of the first generation of stars.
We have observed 16 stars with enhanced carbon or nitrogen using the MIKE
Spectrograph on the Magellan Telescopes at Las Campanas Observatory and the
Tull Spectrograph on the Smith Telescope at McDonald Observatory. We present
radial velocities, stellar parameters, and detailed abundance patterns for
these stars. Strontium, yttrium, zirconium, barium, europium, ytterbium, and
other heavy elements are detected. In four stars, these heavy elements appear
to have originated in some form of r-process nucleosynthesis. In one star, a
partial s-process origin is possible. The origin of the heavy elements in the
rest of the sample cannot be determined unambiguously. The presence of elements
heavier than the iron group offers further evidence that zero-metallicity
rapidly-rotating massive stars and pair instability supernovae did not
contribute substantial amounts of neutron-capture elements to the regions where
the stars in our sample formed. If the carbon- or nitrogen-enhanced metal-poor
stars with low levels of neutron-capture elements were enriched by products of
zero-metallicity supernovae only, then the presence of these heavy elements
indicates that at least one form of neutron-capture reaction operated in some
of the first stars.Comment: Accepted for publication in the Astrophysical Journal (36 pages, 26
figures
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