3,029 research outputs found
Paranthropus boisei: Fifty Years of Evidence and Analysis
Paranthropus boisei is a hominin taxon with a distinctive cranial and dental morphology. Its hypodigm has been recovered from sites with good stratigraphic and chronological control, and for some morphological regions, such as the mandible and the mandibular dentition, the samples are not only relatively well dated, but they are, by paleontological standards, reasonably-sized. This means that researchers can trace the evolution of metric and nonmetric variables across hundreds of thousands of years. This paper is a detailed1 review of half a century’s worth of fossil evidence and analysis of P. boisei and traces how both its evolutionary history and our understanding of its evolutionary history have evolved during the past 50 years
The Evolution of Zinjanthropus boisei
Many people assume that OH 5, the type specimen of Paranthropus boisei, collected in 1959, was the first evidence of that taxon to be found, but OH 3, recovered in 1955, predated the discovery of OH 5 by four years. Thus, Paranthropus boisei recently celebrated the equivalent of its fiftieth birthday. This review marks that milestone by examining the way our understanding of this taxon has changed during its fifty, or so, year history
Carbon-rich dust production in metal-poor galaxies in the Local Group
We have observed a sample of 19 carbon stars in the Sculptor, Carina, Fornax,
and Leo I dwarf spheroidal galaxies with the Infrared Spectrograph on the
Spitzer Space Telescope. The spectra show significant quantities of dust around
the carbon stars in Sculptor, Fornax, and Leo I, but little in Carina. Previous
comparisons of carbon stars with similar pulsation properties in the Galaxy and
the Magellanic Clouds revealed no evidence that metallicity affected the
production of dust by carbon stars. However, the more metal-poor stars in the
current sample appear to be generating less dust. These data extend two known
trends to lower metallicities. In more metal-poor samples, the SiC dust
emission weakens, while the acetylene absorption strengthens. The bolometric
magnitudes and infrared spectral properties of the carbon stars in Fornax are
consistent with metallicities more similar to carbon stars in the Magellanic
Clouds than in the other dwarf spheroidals in our sample. A study of the carbon
budget in these stars reinforces previous considerations that the dredge-up of
sufficient quantities of carbon from the stellar cores may trigger the final
superwind phase, ending a star's lifetime on the asymptotic giant branch.Comment: ApJ, in press, 21 pages, 12 figures. Replaced Fig 12, corrected two
reference
A Post-AGB Star in the Small Magellanic Cloud Observed with the Spitzer Infrared Spectrograph
We have observed an evolved star with a rare combination of spectral
features, MSX SMC 029, in the Small Magellanic Cloud (SMC) using the
low-resolution modules of the Infrared Spectrograph on the Spitzer Space
Telescope. A cool dust continuum dominates the spectrum of MSX SMC 029. The
spectrum also shows both emission from polycyclic aromatic hydrocarbons (PAHs)
and absorption at 13.7 micron from C2H2, a juxtaposition seen in only two other
sources, AFGL 2688 and IRAS 13416-6243, both post-asymptotic giant branch (AGB)
objects. As in these sources, the PAH spectrum has the unusual trait that the
peak emission in the 7-9 micron complex lies beyond 8.0 micron. In addition,
the 8.6 micron feature has an intensity as strong as the C-C modes which
normally peak between 7.7 and 7.9 micron. The relative flux of the feature at
11.3 micron to that at 8 micron suggests that the PAHs in MSX SMC 029 either
have a low ionization fraction or are largely unprocessed. The 13-16 micron
wavelength region shows strong absorption features similar to those observed in
the post-AGB objects AFGL 618 and SMP LMC 11. This broad absorption may arise
from the same molecules which have been identified in those sources: C2H2,
C4H2, HC3N, and C6H6. The similarities between MSX SMC 029, AFGL 2688, and AFGL
618 lead us to conclude that MSX SMC 029 has evolved off the AGB in only the
past few hundred years, making it the third post-AGB object identified in the
SMC.Comment: 4 figures, Fig. 4 color; to appear in the 20 November 2006
Astrophysical Journal Letter
Spitzer Space Telescope spectral observations of AGB stars in the Fornax dwarf spheroidal galaxy
We have observed five carbon-rich AGB stars in the Fornax dwarf spheroidal
(dSph) galaxy, using the Infrared Spectrometer on board the Spitzer Space
Telescope. The stars were selected from a near-infrared survey of Fornax and
include the three reddest stars, with presumably the highest mass-loss rates,
in that galaxy. Such carbon stars probably belong to the intermediate-age
population (2-8 Gyr old and metallicity of [Fe/H] -1) of Fornax. The primary
aim of this paper is to investigate mass-loss rate, as a function of luminosity
and metallicity, by comparing AGB stars in several galaxies with different
metallicities. The spectra of three stars are fitted with a radiative transfer
model. We find that mass-loss rates of these three stars are 4-7x10^-6 Msun
yr-1. The other two stars have mass-loss rates below 1.3x10^-6 Msun yr-1. We
find no evidence that these rates depend on metallicity, although we do suggest
that the gas-to-dust ratio could be higher than at solar metallicity, in the
range 240 to 800. The C2H2 bands are stronger at lower metallicity because of
the higher C/O ratio. In contrast, the SiC fraction is reduced at low
metallicity, due to low silicon abundance. The total mass-loss rate from all
known carbon-rich AGB stars into the interstellar medium of this galaxy is of
the order of 2x10^-5 Msun yr-1. This is much lower than that of the dwarf
irregular galaxy WLM, which has a similar visual luminosity and metallicity.
The difference is attributed to the younger stellar population of WLM. The
suppressed gas-return rate to the ISM accentuates the difference between the
relatively gas-rich dwarf irregular and the gas-poor dwarf spheroidal galaxies.
Our study will be useful to constrain gas and dust recycling processes in low
metallicity galaxies.Comment: MNRAS accepte
Metal-rich carbon stars in the Sagittarius dwarf spheroidal galaxy
‘The definitive version is available at: www3.interscience.wiley.com '. Copyright Blackwell / Royal Astronomical Society. DOI: 10.1111/j.1365-2966.2009.14736.xWe present spectroscopic observations from the Spitzer Space Telescope of six carbon-rich asymptotic giant branch (AGB) stars in the Sagittarius dwarf spheroidal galaxy (Sgr dSph) and two foreground Galactic carbon stars. The band strengths of the observed C2H2 and SiC features are very similar to those observed in Galactic AGB stars. The metallicities are estimated from an empirical relation between the acetylene optical depth and the strength of the SiC feature. The metallicities are higher than those of the Large Magellanic Cloud, and close to Galactic values. While the high metallicity could imply an age of around 1 Gyr, for the dusty AGB stars, the pulsation periods suggest ages in excess of 2 or 3 Gyr. We fit the spectra of the observed stars using the dusty radiative transfer model and determine their dust mass-loss rates to be in the range 1.0–3.3 × 10−8 M⊙ yr−1 . The two Galactic foreground carbon-rich AGB stars are located at the far side of the solar circle, beyond the Galactic Centre. One of these two stars shows the strongest SiC feature in our present Local Group sample.Peer reviewe
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