1,293 research outputs found
Hypervelocity Stars: Predicting the Spectrum of Ejection Velocities
The disruption of binary stars by the tidal field of the black hole in the
Galactic Center can produce the hypervelocity stars observed in the halo. We
use numerical models to simulate the full spectrum of observable velocities of
stars ejected into the halo by this binary disruption process. Our model
includes a range of parameters for binaries with 3-4 M_Solar primaries,
consideration of radial orbits of the ejected stars through an approximate mass
distribution for the Galaxy, and the impact of stellar lifetimes. We calculate
the spectrum of ejection velocities and reproduce previous results for the mean
ejection velocity at the Galactic center. The model predicts that the full
population of ejected stars includes both the hypervelocity stars with
velocities large enough to escape from the Galaxy and a comparable number of
ejected, but bound, stars of the same stellar type. The predicted median speeds
of the population of ejected stars as a function of distance in the halo are
consistent with current observations. Combining the model with the data also
shows that interesting constraints on the properties of binaries in the
Galactic Center and on the mass distribution in the Galaxy can be obtained even
with modest samples of ejected stars.Comment: 26 pages, including 6 figures, accepted for publication in the
Astrophysical Journa
Hypervelocity Stars III. The Space Density and Ejection History of Main Sequence Stars from the Galactic Center
We report the discovery of 3 new unbound hypervelocity stars (HVSs), stars
traveling with such extreme velocities that dynamical ejection from a massive
black hole (MBH) is their only suggested origin. We also detect a population of
possibly bound HVSs. The significant asymmetry we observe in the velocity
distribution -- we find 26 stars with v_rf > 275 km/s and 1 star with v_rf <
-275 km/s -- shows that the HVSs must be short-lived, probably 3 - 4 Msun main
sequence stars. Any population of hypervelocity post-main sequence stars should
contain stars falling back onto the Galaxy, contrary to the observations. The
spatial distribution of HVSs also supports the main sequence interpretation:
longer-lived 3 Msun HVSs fill our survey volume; shorter-lived 4 Msun HVSs are
missing at faint magnitudes. We infer that there are 96 +- 10 HVSs of mass 3 -
4 Msun within R < 100 kpc, possibly enough HVSs to constrain ejection
mechanisms and potential models. Depending on the mass function of HVSs, we
predict that SEGUE may find up to 5 - 15 new HVSs. The travel times of our HVSs
favor a continuous ejection process, although a ~120 Myr-old burst of HVSs is
also allowed.Comment: 10 pages, 8 figures, accepted to ApJ, minor revision
Anthropology in and of MOOCs
The suddenness with which Massive Open Online Courses, or MOOCs, sprang upon us left many within the academy grasping for interpretations. Early proponents touted them as revolutionary tools that could enhance on-campus learning while also making high-quality education accessible to a vast global population, reforming a malfunctioning university system, and producing new kinds of data on how people learn. Critics countered that behind this latest techno-utopian fad lurked an all-too-familiar conservative agenda to downsize the university; the global ambitions of a few elite, resource-rich schools; Silicon Valley corporate interests; and the disciplinary priorities of science, technology, engineering, and mathematics (the STEM fields). With some critical distance, the eight scholars in this Vital Topics Forum draw upon their experiences as anthropologists involved in MOOCs and anthropologists doing studies of MOOCs to propel us beyond such facile responses. Doing what anthropologists do best, they employ contextually rich analysis to upend received wisdom about what MOOCs mean, provide processual accounts of how they are made, and offer first-hand observations of how students are using them on the ground
Hypervelocity Stars II. The Bound Population
Hypervelocity stars (HVSs) are stars ejected completely out of the Milky Way
by three-body interactions with the massive black hole in the Galactic center.
We describe 643 new spectroscopic observations from our targeted survey for
HVSs. We find a significant (3.5 sigma) excess of B-type stars with large
velocities +27510 kpc that are most plausibly
explained as a new class of HVSs: stars ejected from the Galactic center on
bound orbits. If a Galactic center ejection origin is correct, the distribution
of HVSs on the sky should be anisotropic for a survey complete to a fixed
limiting apparent magnitude. The unbound HVSs in our survey have a marginally
anisotropic distribution on the sky, consistent with the Galactic center
ejection picture.Comment: 8 pages, accepted to Ap
Hypervelocity Stars: From the Galactic Center to the Halo
Hypervelocity stars (HVS) traverse the Galaxy from the central black hole to
the outer halo. We show that the Galactic potential within 200 pc acts as a
high pass filter preventing low velocity HVS from reaching the halo. To trace
the orbits of HVS throughout the Galaxy, we construct two forms of the
potential which reasonably represent the observations in the range 5--100,000
pc, a simple spherically symmetric model and a bulge-disk-halo model. We use
the Hills mechanism (disruption of binaries by the tidal field of the central
black hole) to inject HVS into the Galaxy and compute the observable spatial
and velocity distributions of HVS with masses in the range 0.6--4 Msun. These
distributions reflect the mass function in the Galactic Center, properties of
binaries in the Galactic Center, and aspects of stellar evolution and the
injection mechanism. For 0.6--4 Msun main sequence stars, the fraction of
unbound HVS and the asymmetry of the velocity distribution for their bound
counterparts increases with stellar mass. The density profiles for unbound HVS
decline with distance from the Galactic Center approximately as r^{-2} (but are
steeper for the most massive stars which evolve off the main sequence during
their travel time from the Galactic Center); the density profiles for the bound
ejecta decline with distance approximately as r^{-3}. In a survey with a
limiting visual magnitude V of 23, the detectability of HVS (unbound or bound)
increases with stellar mass.Comment: 32 pages of text, 5 tables, 12 figures, ApJ, accepted; revisions:
corrected typos, added references, clarified some aspects of potential model
and physical processes affecting relative frequency of HVS
JWST Reveals Star Formation Across a Spiral Arm in M33
Young stellar objects (YSOs) are the gold standard for tracing star formation
in galaxies but have been unobservable beyond the Milky Way and Magellanic
Clouds. But that all changed when the James Webb Space Telescope was launched,
which we use to identify YSOs in the Local Group galaxy M33, marking the first
time that individual YSOs have been identified at these large distances. We
present MIRI imaging mosaics at 5.6 and 21 microns that cover a significant
portion of one of M33's spiral arms that has existing panchromatic imaging from
the Hubble Space Telescope and deep ALMA CO measurements. Using these MIRI and
Hubble Space Telescope images, we identify point sources using the new DOLPHOT
MIRI module. We identify 793 candidate YSOs from cuts based on colour,
proximity to giant molecular clouds (GMCs), and visual inspection. Similar to
Milky Way GMCs, we find that higher mass GMCs contain more YSOs and YSO
emission, which further shows YSOs identify star formation better than most
tracers that cannot capture this relationship at cloud scales. We find evidence
of enhanced star formation efficiency in the southern spiral arm by comparing
the YSOs to the molecular gas mass.Comment: 12 pages, 10 figures, 1 tables, accepted for publication at MNRA
The Bolocam Galactic Plane Survey IV: 1.1 and 0.35 mm Dust Continuum Emission in the Galactic Center Region
The Bolocam Galactic Plane Survey (BGPS) data for a six square degree region
of the Galactic plane containing the Galactic center is analyzed and compared
to infrared and radio continuum data. The BGPS 1.1 mm emission consists of
clumps interconnected by a network of fainter filaments surrounding cavities, a
few of which are filled with diffuse near-IR emission indicating the presence
of warm dust or with radio continuum characteristic of HII regions or supernova
remnants. New 350 {\mu}m images of the environments of the two brightest
regions, Sgr A and B, are presented. Sgr B2 is the brightest mm-emitting clump
in the Central Molecular Zone and may be forming the closest analog to a super
star cluster in the Galaxy. The Central Molecular Zone (CMZ) contains the
highest concentration of mm and sub-mm emitting dense clumps in the Galaxy.
Most 1.1 mm features at positive longitudes are seen in silhouette against the
3.6 to 24 {\mu}m background observed by the Spitzer Space Telescope. However,
only a few clumps at negative longitudes are seen in absorption, confirming the
hypothesis that positive longitude clumps in the CMZ tend to be on the
near-side of the Galactic center, consistent with the suspected orientation of
the central bar in our Galaxy. Some 1.1 mm cloud surfaces are seen in emission
at 8 {\mu}m, presumably due to polycyclic aromatic hydrocarbons (PAHs). A
~0.2\degree (~30 pc) diameter cavity and infrared bubble between l \approx
0.0\degree and 0.2\degree surrounds the Arches and Quintuplet clusters and Sgr
A. The bubble contains several clumpy dust filaments that point toward Sgr
A\ast; its potential role in their formation is explored. [abstract truncated]Comment: 76 pages, 22 figures, published in ApJ:
http://iopscience.iop.org/0004-637X/721/1/137
UWISH2 -- The UKIRT Widefield Infrared Survey for H2
We present the goals and preliminary results of an unbiased, near-infrared,
narrow-band imaging survey of the First Galactic Quadrant (10deg<l<65deg ;
-1.3deg<b<+1.3deg). This area includes most of the Giant Molecular Clouds and
massive star forming regions in the northern hemisphere. The survey is centred
on the 1-0S(1) ro-vibrational line of H2, a proven tracer of hot, dense
molecular gas in star-forming regions, around evolved stars, and in supernova
remnants. The observations complement existing and upcoming photometric surveys
(Spitzer-GLIMPSE, UKIDSS-GPS, JCMT-JPS, AKARI, Herschel Hi-GAL, etc.), though
we probe a dynamically active component of star formation not covered by these
broad-band surveys. Our narrow-band survey is currently more than 60% complete.
The median seeing in our images is 0.73arcsec. The images have a 5sigma
detection limit of point sources of K=18mag and the surface brightness limit is
10^-19Wm^-2arcsec^-2 when averaged over our typical seeing. Jets and outflows
from both low and high mass Young Stellar Objects are revealed, as are new
Planetary Nebulae and - via a comparison with earlier K-band observations
acquired as part of the UKIDSS GPS - numerous variable stars. With their
superior spatial resolution, the UWISH2 data also have the potential to reveal
the true nature of many of the Extended Green Objects found in the GLIMPSE
survey.Comment: 14pages, 8figures, 2tables, accepted for publication by MNRAS, a
version with higher resolution figures can be found at
http://astro.kent.ac.uk/~df
Breast cancer early detection : a phased approach to implementation
Q1Q1When breast cancer is detected and treated early, the chances of survival are very high. However, women in many settings face complex barriers to early detection, including social, economic, geographic, and other interrelated factors, which can limit their access to timely, affordable, and effective breast health care services. Previously, the Breast Health Global Initiative (BHGI) developed resource-stratified guidelines for the early detection and diagnosis of breast cancer. In this consensus article from the sixth BHGI Global Summit held in October 2018, the authors describe phases of early detection program development, beginning with management strategies required for the diagnosis of clinically detectable disease based on awareness education and technical training, history and physical examination, and accurate tissue diagnosis. The core issues address include finance and governance, which pertain to successful planning, implementation, and the iterative process of program improvement and are needed for a breast cancer early detection program to succeed in any resource setting. Examples are presented of implementation, process, and clinical outcome metrics that assist in program implementation monitoring. Country case examples are presented to highlight the challenges and opportunities of implementing successful breast cancer early detection programs, and the complex interplay of barriers and facilitators to achieving early detection for breast cancer in real-world settings are considered.https://scholar.google.com/citations?user=xFiKCkMAAAAJ&hl=eshttp://scienti.colciencias.gov.co:8081/cvlac/visualizador/generarCurriculoCv.do?cod_rh=0000264474Revista Nacional - Indexad
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