496 research outputs found
DETERMINATION OF THE THIXOCASTING TEMPERATURES OF AZ91D AND OTHER Mg ALLOYS USING A QUENCHING METHOD
Modelling chemical abundance distributions for dwarf galaxies in the Local Group: the impact of turbulent metal diffusion
We investigate stellar metallicity distribution functions (MDFs), including
Fe and -element abundances, in dwarf galaxies from the Feedback in
Realistic Environments (FIRE) project. We examine both isolated dwarf galaxies
and those that are satellites of a Milky Way-mass galaxy. In particular, we
study the effects of including a sub-grid turbulent model for the diffusion of
metals in gas. Simulations that include diffusion have narrower MDFs and
abundance ratio distributions, because diffusion drives individual gas and star
particles toward the average metallicity. This effect provides significantly
better agreement with observed abundance distributions of dwarf galaxies in the
Local Group, including the small intrinsic scatter in [/Fe] vs.
[Fe/H] (less than 0.1 dex). This small intrinsic scatter arises in our
simulations because the interstellar medium (ISM) in dwarf galaxies is
well-mixed at nearly all cosmic times, such that stars that form at a given
time have similar abundances to within 0.1 dex. Thus, most of the scatter in
abundances at z = 0 arises from redshift evolution and not from instantaneous
scatter in the ISM. We find similar MDF widths and intrinsic scatter for
satellite and isolated dwarf galaxies, which suggests that environmental
effects play a minor role compared with internal chemical evolution in our
simulations. Overall, with the inclusion of metal diffusion, our simulations
reproduce abundance distribution widths of observed low-mass galaxies, enabling
detailed studies of chemical evolution in galaxy formation.Comment: 19 pages, 13 figures, published in MNRA
A High-Frequency Search for Pulsars Within the Central Parsec of SgrA*
We report results from a deep high-frequency search for pulsars within the
central parsec of Sgr A* using the Green Bank Telescope. The observing
frequency of 15 GHz was chosen to maximize the likelihood of detecting normal
pulsars (i.e. with periods of \,ms and spectral indices of ) close to Sgr A*, that might be used as probes of gravity in the
strong-field regime; this is the highest frequency used for such pulsar
searches of the Galactic Center to date. No convincing candidate was detected
in the survey, with a detection threshold of Jy
achieved in two separate observing sessions. This survey represents a
significant improvement over previous searches for pulsars at the Galactic
Center and would have detected a significant fraction ($\gtrsim 5%) of the
pulsars around Sgr A*, if they had properties similar to those of the known
population. Using our best current knowledge of the properties of the Galactic
pulsar population and the scattering material toward Sgr A*, we estimate an
upper limit of 90 normal pulsars in orbit within the central parsec of Sgr A*.Comment: 10 pages, 7 figures, accepted for publication in the ApJ
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Variations in the slope of the resolved star-forming main sequence: A tool for constraining the mass of star-forming regions
The correlation between galaxies' integrated stellar masses and star formation rates (the 'star formation main sequence', SFMS) is a well-established scaling relation. Recently, surveys have found a relationship between the star formation rate (SFR) and stellar mass surface densities on kpc and sub-kpc scales (the 'resolved SFMS', rSFMS). In this work, we demonstrate that the rSFMS emerges naturally in Feedback In Realistic Environments 2 (FIRE-2) zoom-in simulations of Milky Way-mass galaxies. We make SFR and stellar mass maps of the simulated galaxies at a variety of spatial resolutions and star formation averaging time-scales and fit the rSFMS using multiple methods from the literature. While the absolute value of the SFMS slope (αMS) depends on the fitting method, the slope is steeper for longer star formation time-scales and lower spatial resolutions regardless of the fitting method employed. We present a toy model that quantitatively captures the dependence of the simulated galaxies' αMS on spatial resolution and use it to illustrate how this dependence can be used to constrain the characteristic mass of star-forming clumps
On the evolution of the radio pulsar PSR J1734−3333
Recent measurements showed that the period derivative of the ‘hig
h-B’ radio pulsar PSR J1734−3333 is increasing with time. For neutron stars evolving with fallback disks, this rotational behavior is expected in certain phases of the long-term evolution. Using the same model as employed earlier to explain the evolution of anomalous X-ray pulsars and soft gamma-ray repeaters, we show that the period,the first and second period derivatives and the X-ray luminosity of this source can simultaneously acquire the observed values for a neutron star evolving with a fallback disk. We find that the required strength of the dipole field that can produce the source properties is in the range of 10^12 − 10^13 G on the pole of the neutron star. When the model source
reaches the current state properties of PSR J1734−3333, accretion onto the star has not started yet, allowing the source to operate as a regular radio pulsar. Our results imply that PSR J1734−3333 is at an age of ∼3×10^4 −2×10^5years. Such sources will have properties like the X-ray dim isolated neutron stars or transient AXPs at a later epoch of weak accretion from the diminished fallback disk
What drives the evolution of gas kinematics in star-forming galaxies?
One important result from recent large integral field spectrograph (IFS) surveys is that the intrinsic velocity dispersion of galaxies traced by star-forming gas increases with redshift. Massive, rotation-dominated discs are already in place at z ∼ 2, but they are dynamically hotter than spiral galaxies in the local Universe. Although several plausible mechanisms for this elevated velocity dispersion (e.g. star formation feedback, elevated gas supply, or more frequent galaxy interactions) have been proposed, the fundamental driver of the velocity dispersion enhancement at high redshift remains unclear. We investigate the origin of this kinematic evolution using a suite of cosmological simulations from the FIRE (Feedback In Realistic Environments) project. Although IFS surveys generally cover a wider range of stellar masses than in these simulations, the simulated galaxies show trends between intrinsic velocity dispersion (σ intr ), SFR, and z in agreement with observations. In both observations and simulations, galaxies on the star-forming main sequence have median σ intr values that increase from z ∼ 0 to z ∼ 1–1.5, but this increasing trend is less evident at higher redshift. In the FIRE simulations, σ intr can vary significantly on time-scales of 100 Myr. These variations closely mirror the time evolution of the SFR and gas inflow rate (M gas ). By cross-correlating pairs of σ intr, M gas, and SFR, we show that increased gas inflow leads to subsequent enhanced star formation, and enhancements in σ intr tend to temporally coincide with increases in M gas and SFR
Mean-flux Regulated PCA Continuum Fitting of SDSS Lyman-alpha Forest Spectra
Continuum fitting is an important aspect of Lyman-alpha forest science, since
errors in the estimated optical depths scale with the fractional continuum
error. However, traditional methods of estimating continua in noisy and
moderate-resolution spectra (S/N < 10 pixel^-1 and R ~ 2000, respectively, such
as SDSS) using power-law extrapolation or the mean spectrum, achieve no better
than ~ 10-15% RMS accuracy. To improve on this, we introduce mean-flux
regulated/principal component analysis (MF-PCA) continuum fitting. In this
technique, PCA fitting is carried out redwards of the quasar Lyman-alpha line
in order to provide a prediction for the shape of the Lyman-alpha forest
continuum. The slope and amplitude of this continuum prediction is then
corrected using external constraints for the Lyman-alpha forest mean-flux. From
tests on mock spectra, we find that MF-PCA reduces the errors to 8% RMS in S/N
~ 2 spectra, and 5. The residual Fourier power
in the continuum is decreased by a factor of a few in comparison with dividing
by the mean continuum, enabling Lyman-alpha flux power spectrum measurements to
be extended to ~2x larger scales. Using this new technique, we make available
continuum fits for 12,069 z>2.3 Lyman-alpha forest spectra from SDSS DR7 for
use by the community. This technique is also applicable to future releases of
the ongoing BOSS survey, which is obtaining spectra for ~ 150,000 Lyman-alpha
forest spectra at low signal-to-noise (S/N ~ 2).Comment: 14 pages; 11 figures; submitted to AJ. Continua publicly available
via anonymous FTP or Data Conservancy repositor
The High Time Resolution Universe Survey VI: An Artificial Neural Network and Timing of 75 Pulsars
We present 75 pulsars discovered in the mid-latitude portion of the High Time
Resolution Universe survey, 54 of which have full timing solutions. All the
pulsars have spin periods greater than 100 ms, and none of those with timing
solutions are in binaries. Two display particularly interesting behaviour; PSR
J1054-5944 is found to be an intermittent pulsar, and PSR J1809-0119 has
glitched twice since its discovery.
In the second half of the paper we discuss the development and application of
an artificial neural network in the data-processing pipeline for the survey. We
discuss the tests that were used to generate scores and find that our neural
network was able to reject over 99% of the candidates produced in the data
processing, and able to blindly detect 85% of pulsars. We suggest that
improvements to the accuracy should be possible if further care is taken when
training an artificial neural network; for example ensuring that a
representative sample of the pulsar population is used during the training
process, or the use of different artificial neural networks for the detection
of different types of pulsars.Comment: 15 pages, 8 figure
Effects of the environment on the multiplicity properties of stars in the STARFORGE simulations
Most observed stars are part of a multiple star system, but the formation of
such systems and the role of environment and various physical processes is
still poorly understood. We present a suite of radiation-magnetohydrodynamic
simulations of star-forming molecular clouds from the STARFORGE project that
include stellar feedback with varied initial surface density, magnetic fields,
level of turbulence, metallicity, interstellar radiation field, simulation
geometry and turbulent driving. In our fiducial cloud the raw simulation data
reproduces the observed multiplicity fractions for Solar-type and higher mass
stars, similar to previous works. However, after correcting for observational
incompleteness the simulation under-predicts these values. The discrepancy is
likely due to the lack of disk fragmentation, as the simulation only resolves
multiples that form either through capture or core fragmentation. The raw mass
distribution of companions is consistent with randomly drawing from the initial
mass function for the companions of stars, however,
accounting for observational incompleteness produces a flatter distribution
similar to observations. We show that stellar multiplicity changes as the cloud
evolves and anti-correlates with stellar density. This relationship also
explains most multiplicity variations between runs, i.e., variations in the
initial conditions that increase stellar density (increased surface density,
reduced turbulence) decrease multiplicity. While other parameters, such as
metallicity, interstellar radiation, and geometry significantly affect the star
formation history or the IMF, varying them produces no clear trend in stellar
multiplicity properties.Comment: 20 pages, 21 figures, submitted to MNRA
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