57 research outputs found
The Effects of Stellar Rotation. I. Impact on the Ionizing Spectra and Integrated Properties of Stellar Populations
We present a sample of synthetic massive stellar populations created using
the Starburst99 evolutionary synthesis code and new sets of stellar
evolutionary tracks, including one set that adopts a detailed treatment of
rotation. Using the outputs of the Starburst99 code, we compare the
populations' integrated properties, including ionizing radiation fields,
bolometric luminosities, and colors. With these comparisons we are able to
probe the specific effects of rotation on the properties of a stellar
population. We find that a population of rotating stars produces a much harder
ionizing radiation field and a higher bolometric luminosity, changes that are
primarily attributable to the effects of rotational mixing on the lifetimes,
luminosities, effective temperatures, and mass loss rates of massive stars. We
consider the implications of the profound effects that rotation can have on a
stellar population, and discuss the importance of refining stellar evolutionary
models for future work in the study of extragalactic, and particularly
high-redshift, stellar populations.Comment: 13 pages, 8 figures; accepted for publication in the Astrophysical
Journa
Red Supergiants in the Andromeda Galaxy (M31)
Red supergiants are a short-lived stage in the evolution of moderately
massive stars (10-25Mo), and as such their location in the H-R diagram provides
an exacting test of stellar evolutionary models. Since massive star evolution
is strongly affected by the amount of mass-loss a star suffers, and since the
mass-loss rates depend upon metallicity, it is highly desirable to study the
physical properties of these stars in galaxies of various metallicities. Here
we identify a sample of red supergiants in M31 (the most metal-rich of the
Local Group galaxies) and derive their physical properties by fitting MARCS
atmosphere models to moderate resolution optical spectroscopy, and from V-K
photometry.Comment: Accepted for publication in the Astrophysical Journa
Yellow and Red Supergiants in the Large Magellanic Cloud
Due to their transitionary nature, yellow supergiants provide a critical
challenge for evolutionary modeling. Previous studies within M31 and the SMC
show that the Geneva evolutionary models do a poor job at predicting the
lifetimes of these short-lived stars. Here we extend this study to the LMC
while also investigating the galaxy's red supergiant content. This task is
complicated by contamination by Galactic foreground stars that color and
magnitude criteria alone cannot weed out. Therefore, we use proper motions and
the LMC's large systemic radial velocity (\sim278 km/s) to separate out these
foreground dwarfs. After observing nearly 2,000 stars, we identified 317
probable yellow supergiants, 6 possible yellow supergiants and 505 probable red
supergiants. Foreground contamination of our yellow supergiant sample was
\sim80%, while that of the the red supergiant sample was only 3%. By placing
the yellow supergiants on the H-R diagram and comparing them against the
evolutionary tracks, we find that new Geneva evolutionary models do an
exemplary job at predicting both the locations and the lifetimes of these
transitory objects.Comment: Accepted for publication in the Ap
Silencing of Amyloid Precursor Protein Expression Using a New Engineered Delta Ribozyme
Alzheimer's disease (AD) etiological studies suggest that an elevation in amyloid-β peptides (Aβ) level contributes to aggregations of the peptide and subsequent development of the disease. The major constituent of these amyloid peptides is the 1 to 40–42 residue peptide (Aβ40−42) derived from amyloid protein precursor (APP). Most likely, reducing Aβ levels in the brain may block both its aggregation and neurotoxicity and would be beneficial for patients with AD. Among the several possible ways to lower Aβ accumulation in the cells, we have selectively chosen to target the primary step in the Aβ cascade, namely, to reduce APP gene expression. Toward this end, we engineered specific SOFA-HDV ribozymes, a new generation of catalytic RNA tools, to decrease APP mRNA levels. Additionally, we demonstrated that APP-ribozymes are effective at decreasing APP mRNA and protein levels as well as Aβ levels in neuronal cells. Our results could lay the groundwork for a new protective treatment for AD
The Yellow and Red Supergiants of M33
Yellow and red supergiants are evolved massive stars whose numbers and
locations on the HR diagram can provide a stringent test for models of massive
star evolution. Previous studies have found large discrepancies between the
relative number of yellow supergiants observed as a function of mass and those
predicted by evolutionary models, while a disagreement between the predicted
and observed locations of red supergiants on the HR diagram was only recently
resolved. Here we extend these studies by examining the yellow and red
supergiant populations of M33. Unfortunately, identifying these stars is
difficult as this portion of the color-magnitude diagram is heavily
contaminated by foreground dwarfs. We identify the red supergiants through a
combination of radial velocities and a two-color surface gravity discriminant
and, after re-characterizing the rotation curve of M33 with our newly selected
red supergiants, we identify the yellow supergiants through a combination of
radial velocities and the strength of the OI 7774 triplet. We examine
~1300 spectra in total and identify 121 yellow supergiants (a sample which is
unbiased in luminosity above log(L/L\odot) ~ 4.8) and 189 red supergiants.
After placing these objects on the HR diagram, we find that the latest
generation of Geneva evolutionary tracks show excellent agreement with the
observed locations of our red and yellow supergiants, the observed relative
number of yellow supergiants with mass and the observed red supergiant upper
mass limit. These models therefore represent a drastic improvement over
previous generations.Comment: Accepted for publication in the Astrophysical Journa
On the interaction of acetone with electrophilic metallocavitands having extended cavities
We report the synthesis and characterization of tantalum–boronate trimetallic clusters of general formula {[Cp*Ta]3(μ2-RB(O)2)3(μ2-OH)(μ2-O)2(μ3-OH)} (R= 4-(C6H5)(C6H4) (Ta3-4Ph), 4-(C6H5O)(C6H4) (Ta3-4OPh), 4-(C7H7O)(C6H4) (Ta3-4OBn), 4-(C8H5)(C6H4) (Ta3-4PhEt), and 4-(C12H7)(C6H4) (Ta3-4Napht)). All complexes have been characterized by NMR spectroscopy and X-ray diffraction. The trimetallic species feature a large Lewis acid type cavity allowing for substrate binding in both the solid and the liquid state using a unique electrostatic interaction and a hydrogen bond. ΔH° and ΔS° values for association of acetone with the complexes vary between −2.0 and −4.1 kcal·mol–1 and −3 and 2 cal·mol–1·K–1, respectively, showing weaker binding than smaller cavitands of the same type. The barrier for acetone exchange at equilibrium is similar for all complexes, and ΔH‡ values vary between 8.2 and 11.4 kcal·mol–1
Evolution of Massive Stars at Low Metallicity
This paper reports the contributions made on the occasion of the Special Session entitled "Evolution of Massive Stars at Low Metallicity” which was held on Sunday, December 9, 2007 in Kauai (USA
Yellow Supergiants in the Small Magellanic Cloud (SMC): Putting Current Evolutionary Theory to the Test
The yellow supergiant content of nearby galaxies provides a critical test of
massive star evolutionary theory. While these stars are the brightest in a
galaxy, they are difficult to identify because a large number of foreground
Milky Way stars have similar colors and magnitudes. We previously conducted a
census of yellow supergiants within M31 and found that the evolutionary tracks
predict a yellow supergiant duration an order of magnitude longer than we
observed. Here we turn our attention to the SMC, where the metallicity is 10x
lower than that of M31, which is important as metallicity strongly affects
massive star evolution. The SMC's large radial velocity (~160 km/s) allows us
to separate members from foreground stars. Observations of ~500 candidates
yielded 176 near-certain SMC supergiants, 16 possible SMC supergiants, along
with 306 foreground stars and provide good relative numbers of yellow
supergiants down to 12Mo. Of the 176 near-certain SMC supergiants, the
kinematics predicted by the Besancon model of the Milky Way suggest a
foreground contamination of >4%. After placing the SMC supergiants on the H-R
diagram and comparing our results to the Geneva evolutionary tracks, we find
results similar to those of the M31 study: while the locations of the stars on
the H-R diagram match the locations of evolutionary tracks well, the models
over-predict the yellow supergiant lifetime by a factor of ten. Uncertainties
about the mass-loss rates on the main-sequence thus cannot be the primary
problem with the models.Comment: Accepted by the Ap
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