79 research outputs found
Turbulent Convection in Stellar Interiors. II. The Velocity Field
We analyze stellar convection with the aid of 3D hydrodynamic simulations,
introducing the turbulent cascade into our theoretical analysis. We devise
closures of the Reynolds-decomposed mean field equations by simple physical
modeling of the simulations (we relate temperature and density fluctuations via
coefficients); the procedure (CABS, Convection Algorithms Based on Simulations)
is terrestrially testable and is amenable to systematic improvement. We develop
a turbulent kinetic energy equation which contains both nonlocal and time
dependent terms, and is appropriate if the convective transit time is shorter
than the evolutionary time scale. The interpretation of mixing-length theory
(MLT) as generally used in astrophysics is incorrect; MLT forces the mixing
length to be an imposed constant. Direct tests show that the damping associated
with the flow is that suggested by Kolmogorov. The eddy size is approximately
the depth of the convection zone, and this dissipation length corresponds to
the "mixing length". New terms involving local heating by turbulent dissipation
should appear in the stellar evolution equations. The enthalpy flux
("convective luminosity") is directly connected to the buoyant acceleration,
and hence the velocity scale. MLT tends to systematically underestimate this
velocity scale. Quantitative comparison with a variety of 3D simulations
reveals a previously recognized consistency. Examples of application to stellar
evolution will be presented in subsequent papers in this series.Comment: 47 pages, 7 figures, accepted by Ap
Core-Collapse Supernovae and Host Galaxy Stellar Populations
We have used images and spectra of the Sloan Digital Sky Survey to examine
the host galaxies of 519 nearby supernovae. The colors at the sites of the
explosions, as well as chemical abundances, and specific star formation rates
of the host galaxies provide circumstantial evidence on the origin of each
supernova type. We examine separately SN II, SN IIn, SN IIb, SN Ib, SN Ic, and
SN Ic with broad lines (SN Ic-BL). For host galaxies that have multiple
spectroscopic fibers, we select the fiber with host radial offset most similar
to that of the SN. Type Ic SN explode at small host offsets, and their hosts
have exceptionally strongly star-forming, metal-rich, and dusty stellar
populations near their centers. The SN Ic-BL and SN IIb explode in
exceptionally blue locations, and, in our sample, we find that the host spectra
for SN Ic-BL show lower average oxygen abundances than those for SN Ic. SN IIb
host fiber spectra are also more metal-poor than those for SN Ib, although a
significant difference exists for only one of two strong-line diagnostics. SN
Ic-BL host galaxy emission lines show strong central specific star formation
rates. In contrast, we find no strong evidence for different environments for
SN IIn compared to the sites of SN II. Because our supernova sample is
constructed from a variety of sources, there is always a risk that sampling
methods can produce misleading results. We have separated the supernovae
discovered by targeted surveys from those discovered by galaxy-impartial
searches to examine these questions and show that our results do not depend
sensitively on the discovery technique.Comment: Accepted by the Astrophysical Journal (22 July 2012), conclusions not
changed, extended discussion of sample construction and updated SN
spectroscopic type
Nucleosynthesis in a Primordial Supernova: Carbon and Oxygen Abundances in SMSS J031300.36-670839.31
SMSS J031300.36-670839.3 (hereafter SM0313-6708) is a sub-giant halo star,
with no detectable Fe lines and large overabundances of C and Mg relative to
Ca. We obtained VLT-UVES spectra extending to 3060 Angstroms showing strong OH
A-X band lines enabling an oxygen abundance to be derived. The OH A-X band
lines in SM0313-6708 are much stronger than the CH C-X band lines. Spectrum
synthesis fits indicate an [O/C] ratio of 0.02 +- 0.175. Our high S/N UVES data
also enabled us to lower the Fe abundance limit to [Fe/H]{3D},NLTE < -7.52 (3
sigma). These data support our previous suggestion that the star formed from
the iron-poor ejecta of a single massive star Population III supernova.Comment: 23 pages, 4 figures, 1 tabl
Assessing the Diversity and Distribution of Apicomplexans in Host and Free-Living Environments Using High-Throughput Amplicon Data and a Phylogenetically Informed Reference Framework
Apicomplexans are a group of microbial eukaryotes that contain some of the most well-studied parasites, including the causing agents of toxoplasmosis and malaria, and emergent diseases like cryptosporidiosis or babesiosis. Decades of research have illuminated the pathogenic mechanisms, molecular biology, and genomics of model apicomplexans, but we know little about their diversity and distribution in natural environments. In this study we analyze the distribution of apicomplexans across a range of both host-associated and free-living environments. Using publicly available small subunit (SSU) rRNA gene databases, high-throughput environmental sequencing (HTES) surveys, and our own generated HTES data, we developed an apicomplexan reference database, which includes the largest apicomplexan SSU rRNA tree available to date and encompasses comprehensive sampling of this group and their closest relatives. This tree allowed us to identify and correct incongruences in the molecular identification of apicomplexan sequences. Analyzing the diversity and distribution of apicomplexans in HTES studies with this curated reference database also showed a widespread, and quantitatively important, presence of apicomplexans across a variety of free-living environments. These data allow us to describe a remarkable molecular diversity of this group compared with our current knowledge, especially when compared with that identified from described apicomplexan species. This is most striking in marine environments, where potentially the most diverse apicomplexans apparently exist, but have not yet been formally recognized. The new database will be useful for microbial ecology and epidemiological studies, and provide valuable reference for medical and veterinary diagnosis especially in cases of emerging, zoonotic, and cryptic infections
The Extreme Hosts of Extreme Supernovae
We use GALEX ultraviolet (UV) and optical integrated photometry of the hosts
of seventeen luminous supernovae (LSNe, having peak M_V < -21) and compare them
to a sample of 26,000 galaxies from a cross-match between the SDSS DR4 spectral
catalog and GALEX interim release 1.1. We place the LSNe hosts on the galaxy
NUV-r versus M_r color magnitude diagram (CMD) with the larger sample to
illustrate how extreme they are. The LSN hosts appear to favor low-density
regions of the galaxy CMD falling on the blue edge of the blue cloud toward the
low luminosity end. From the UV-optical photometry, we estimate the star
formation history of the LSN hosts. The hosts have moderately low star
formation rates (SFRs) and low stellar masses (M_*) resulting in high specific
star formation rates (sSFR). Compared with the larger sample, the LSN hosts
occupy low-density regions of a diagram plotting sSFR versus M_* in the area
having higher sSFR and lower M_*. This preference for low M_*, high sSFR hosts
implies the LSNe are produced by an effect having to do with their local
environment. The correlation of mass with metallicity suggests that perhaps
wind-driven mass loss is the factor that prevents LSNe from arising in
higher-mass, higher-metallicity hosts. The massive progenitors of the LSNe
(>100 M_sun), by appearing in low-SFR hosts, are potential tests for theories
of the initial mass function that limit the maximum mass of a star based on the
SFR.Comment: 8 pages, 3 figures, 2 tables, accepted to ApJ, amended references and
updated SN designation
Exploring critical parameters of electrode fabrication in polymer electrolyte membrane fuel cells
Microstructure and electrochemical properties of the cathode catalyst layers (CCL) of a polymer electrolyte
membrane fuel cells (PEMFC) have great impact on the performance and durability of the cell. The aim of this
work is to establish a link between CCL structure and fuel cell performance. To obtain CCLs with unique
structures six types of electrodes were prepared, each with a different coating technique but with the same Pt
loading. The coating techniques are airbrush, screen printing, inkjet printing, dry spraying, doctor blade and
drop casting. Moreover, intrinsic properties of PEMFC electrodes such as porosity, permeability, diffusivity as
well as ionomer distribution are determined by Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) and
Atomic Force Microscopy (AFM). Overall, 12 parameters have been evaluated. Generally, CCLs with low fractions of uncovered Pt/C show higher performance at low current densities. In this case the more homogeneous ionomer distribution leads to a higher catalyst utilization. At high current densities transport properties of the
CCL have to be considered in addition to the catalyst utilization to explain their performance. The CCL prepared by screen printing shows a low fraction of uncovered Pt/C in combination with good transport properties, leading to the best performance at high currents
On the detection of supermassive primordial stars
The collapse of supermassive primordial stars in hot, atomically-cooled halos
may have given birth to the first quasars at 15 - 20. Recent numerical
simulations of these rapidly accreting stars reveal that they are cool, red
hypergiants shrouded by dense envelopes of pristine atomically-cooled gas at
6,000 - 8,000 K, with luminosities 10 L. Could
such luminous but cool objects be detected as the first stage of quasar
formation in future near infrared (NIR) surveys? We have now calculated the
spectra of supermassive primordial stars in their birth envelopes with the
Cloudy code. We find that some of these stars will be visible to JWST at 20 and that with modest gravitational lensing Euclid and WFIRST could
detect them out to 10 - 12. Rather than obscuring the star, its
accretion envelope enhances its visibility in the NIR today by reprocessing its
short-wavelength flux into photons that are just redward of the Lyman limit in
the rest frame of the star.Comment: 6 pages, 4 figures, accepted by ApJ
Accreting Black Holes
This chapter provides a general overview of the theory and observations of
black holes in the Universe and on their interpretation. We briefly review the
black hole classes, accretion disk models, spectral state classification, the
AGN classification, and the leading techniques for measuring black hole spins.
We also introduce quasi-periodic oscillations, the shadow of black holes, and
the observations and the theoretical models of jets.Comment: 41 pages, 18 figures. To appear in "Tutorial Guide to X-ray and
Gamma-ray Astronomy: Data Reduction and Analysis" (Ed. C. Bambi, Springer
Singapore, 2020). v3: fixed some typos and updated some parts. arXiv admin
note: substantial text overlap with arXiv:1711.1025
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