2,827 research outputs found
Constraining Collapsar r-Process Models through Stellar Abundances
We use observations of heavy elements in very metal-poor stars ([Fe/H] <
-2.5) in order to place constraints on the viability of collapsar models as a
significant source of the r-process. We combine bipolar explosion
nucleosynthesis calculations with recent disk calculations to make predictions
of the observational imprints these explosions would leave on very metal-poor
stars. We find that a source of low (~ 0.1-0.5 ) Fe mass which also
yields a relatively high (> 0.08 ) r-process mass would, after
subsequently mixing and forming new stars, result in [r/Fe] abundances up to
three orders of magnitude higher than those seen in stars. In order to match
inferred abundances, 10-10 of Fe would need be efficiently
incorporated into the r-process ejecta. We show that Fe enhancement and hence
[r/Fe] dilution from other nearby supernovae is not able to explain the
observations unless significant inflow of pristine gas occurs before the ejecta
are able to form new stars. Finally, we show that the inferred [Eu/Fe]
abundances require levels of gas mixing which are in conflict with other
properties of r-process enhanced metal-poor stars. Our results suggest that
early r-process production is likely to be spatially uncorrelated with Fe
production, a condition which can be satisfied by neutron star mergers due to
their large kick velocities and purely r-process yields.Comment: 6 pages, 2 figures, accepted for publication in ApJ
The Stars in M15 Were Born with the r-process
High-resolution spectroscopy of stars on the red giant branch (RGB) of the globular cluster M15 has revealed a large (~1 dex) dispersion in the abundances of r-process elements such as Ba and Eu. Neutron star mergers (NSMs) have been proposed as a major source of the r-process. However, most NSM models predict a delay time longer than the timescale for cluster formation. One possibility is that a NSM polluted the surfaces of stars in M15 long after the cluster finished forming. In this case, the abundances of the polluting elements would decrease in the first dredge-up as stars turn on to the RGB. We present Keck/DEIMOS abundances of Ba in 66 stars along the entire RGB and the top of the main sequence. The Ba abundances have no trend with stellar luminosity (evolutionary phase). Therefore, the stars were born with the Ba that they have today, and Ba did not originate in a source with a delay time longer than the timescale for cluster formation. In particular, if the source of Ba was a NSM, it would have had a very short delay time. Alternatively, if Ba enrichment took place before the formation of the cluster, an inhomogeneity of a factor of 30 in Ba abundance needs to be able to persist over the length scale of the gas cloud that formed M15, which is unlikely
Accretion Disk Assembly During Common Envelope Evolution: Implications for Feedback and LIGO Binary Black Hole Formation
During a common envelope episode in a binary system, the engulfed companion
spirals to tighter orbital separations under the influence of drag from the
surrounding envelope material. As this object sweeps through material with a
steep radial gradient of density, net angular momentum is introduced into the
flow, potentially leading to the formation of an accretion disk. The presence
of a disk would have dramatic consequences for the outcome of the interaction
because accretion might be accompanied by strong, polar outflows with enough
energy to unbind the entire envelope. Without a detailed understanding of the
necessary conditions for disk formation during common envelope, therefore, it
is difficult to accurately predict the population of merging compact binaries.
This paper examines the conditions for disk formation around objects embedded
within common envelopes using the `wind tunnel' formalism developed by MacLeod
et al. (2017). We find that the formation of disks is highly dependent on the
compressibility of the envelope material. Disks form only in the most
compressible of stellar envelope gas, found in envelopes' outer layers in zones
of partial ionization. These zones are largest in low-mass stellar envelopes,
but comprise small portions of the envelope mass and radius in all cases. We
conclude that disk formation and associated accretion feedback in common
envelope is rare, and if it occurs, transitory. The implication for LIGO black
hole binary assembly is that by avoiding strong accretion feedback, common
envelope interactions should still result in the substantial orbital tightening
needed to produce merging binaries.Comment: 12 pages, 10 figures, submitted to Ap
r-process enrichment of ultra-faint dwarf galaxies by fast merging double neutron stars
The recent aLIGO/aVirgo discovery of gravitational waves from the neutron
star merger (NSM) GW170817 and the follow up kilonova observations have shown
that NSMs produce copious amount of r-process material. However, it is
difficult to reconcile the large natal kicks and long average merging times of
Double Neutron Stars (DNSs), with the levels of r-process enrichment seen in
ultra faint dwarf (UFD) galaxies such as Reticulum II and Tucana III. Assuming
that such dwarf systems have lost a significant fraction of their stellar mass
through tidal stripping, we conclude that contrary to most current models, it
is the DNSs with rather large natal kicks but very short merging timescales
that can enrich UFD-type galaxies. These binaries are either on highly
eccentric orbits, or form with very short separations due to an additional
mass-transfer between the first-born neutron star and a naked helium star,
progenitor of the second-born neutron star. These DNSs are born with a
frequency that agrees with the statistics of the r-process UFDs, and merge well
within the virial radius of their host halos, therefore contributing
significantly to their r-process enrichment.Comment: Accepted for publication in Ap
Nephrotic syndrome and kidney failure due to immunocomplex-mediated renal damage in a patient with Waldenström's Macroglobulinemia: a case report
The use of fibre optic sensing technology with intraluminal impedance catheter for functional gastrointestinal motility disorders
Author accepted manuscript made available with permission from Engineers Australia.We present a composite impedance fibre optic manometry catheter for monitoring functional gastrointestinal disorders (FGID). The catheter uses a dual lumen silicone extrusion to separate each technology and has been validated in ex-vivo animal models
In Vivo Evaluation of (-)-Zampanolide Demonstrates Potent and Persistent Antitumor Efficacy When Targeted to the Tumor Site.
Microtubule-stabilizing agents (MSAs) are a class of compounds used in the treatment of triple-negative breast cancer (TNBC), a subtype of breast cancer where chemotherapy remains the standard-of-care for patients. Taxanes like paclitaxel and docetaxel have demonstrated efficacy against TNBC in the clinic, however new classes of MSAs need to be identified due to the rise of taxane resistance in patients. (-)-Zampanolide is a covalent microtubule stabilizer that can circumvent taxane resistance in vitro but has not been evaluated for in vivo antitumor efficacy. Here, we determine that (-)-zampanolide has similar potency and efficacy to paclitaxel in TNBC cell lines, but is significantly more persistent due to its covalent binding. We also provide the first reported in vivo antitumor evaluation of (-)-zampanolide where we determine that it has potent and persistent antitumor efficacy when delivered intratumorally. Future work on zampanolide to further evaluate its pharmacophore and determine ways to improve its systemic therapeutic window would make this compound a potential candidate for clinical development through its ability to circumvent taxane-resistance mechanisms
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