1,521 research outputs found
Evolution of iron core white dwarfs
Recent measurements made by Hipparcos (Provencal et al. 1998) present
observational evidence supporting the existence of some white dwarf (WD) stars
with iron - rich, core composition. In this connection, the present paper is
aimed at exploring the structure and evolution of iron - core WDs by means of a
detailed and updated evolutionary code. In particular, we examine the evolution
of the central conditions, neutrino luminosity, surface gravity,
crystallization, internal luminosity profiles and ages. We find that the
evolution of iron - rich WDs is markedly different from that of their carbon -
oxygen counterparts. In particular, cooling is strongly accelerated as compared
with the standard case. Thus, if iron WDs were very numerous, some of them
would have had time enough to evolve at lower luminosities than that
corresponding to the fall - off in the observed WD luminosity function.Comment: 8 pages, 21 figures. Accepted for publication in MNRA
Controlled MOCVD growth of Bi2Se3 topological insulator nanoribbons
Topological insulators are a new class of materials that support
topologically protected electronic surface states. Potential applications of
the surface states in low dissipation electronic devices have motivated efforts
to create nanoscale samples with large surface-to-volume ratios and highly
controlled stoichiometry. Se vacancies in Bi2Se3 give rise to bulk conduction,
which masks the transport properties of the surface states. We have therefore
developed a new route for the synthesis of topological insulator nanostructures
using metalorganic chemical vapour deposition (MOCVD). MOCVD allows for control
of the Se/Bi flux ratio during growth. With the aim of rational growth, we vary
the Se/Bi flux ratio, growth time, and substrate temperature, and observe
morphological changes which indicate a growth regime in which nanoribbon
formation is limited by the Bi precursor mass-flow. MOCVD growth of Bi2Se3
nanostructures occurs via a distinct growth mechanism that is nucleated by gold
nanoparticles at the base of the nanowire. By tuning the reaction conditions,
we obtain either single-crystalline ribbons up to 10 microns long or thin
micron-sized platelets.Comment: Related papers at http://pettagroup.princeton.ed
Correcting for Distortions due to Ionization in the STAR TPC
Physics goals of the STAR Experiment at RHIC in recent (and future) years
drive the need to operate the STAR TPC at ever higher luminosities, leading to
increased ionization levels in the TPC gas. The resulting ionic space charge
introduces field distortions in the detector which impact tracking performance.
Further complications arise from ionic charge leakage into the main TPC volume
from the high gain anode region. STAR has implemented corrections for these
distortions based on measures of luminosity, which we present here.
Additionally, we highlight a novel approach to applying the corrections on an
event-by-event basis applicable in conditions of rapidly varying ionization
sources.Comment: 6 pages, 7 figures, proceedings of the Workshop on Tracking in High
Multiplicity Environments (TIME 05) in Zurich, Switzerland, submitted to
Nucl. Instr. and Meth.
Implementing Service Learning in Pre-service Teacher Coursework
Service learning remains a topic of interest in higher education. It has become more prevalent in teacher preparation programs with the intent of providing the opportunity for pre-service teachers to become engaged with individuals who have different life experiences than their own. Lessons can be learned through a review of the literature and the examination of existing models of service learning, including an honest discussion of the advantages and potential barriers for all stakeholders
Star formation triggered by HII regions in our Galaxy: First results for N49 from the Herschel infrared survey of the Galactic plane
It has been shown that by means of different physical mechanisms the
expansion of HII regions can trigger the formation of new stars of all masses.
This process may be important to the formation of massive stars but has never
been quantified in the Galaxy. We use Herschel-PACS and -SPIRE images from the
Herschel Infrared survey of the Galactic plane, Hi-GAL, to perform this study.
We combine the Spitzer-GLIMPSE and -MIPSGAL, radio-continuum and sub-millimeter
surveys such as ATLASGAL with Hi-GAL to study Young Stellar Objects (YSOs)
observed towards Galactic HII regions. We select a representative HII region,
N49, located in the field centered on l=30 degr observed as part of the Hi-GAL
Science Demonstration Phase, to demonstrate the importance Hi-GAL will have to
this field of research. Hi-GAL PACS and SPIRE images reveal a new population of
embedded young stars, coincident with bright ATLASGAL condensations. The Hi-GAL
images also allow us, for the first time, to constrain the physical properties
of the newly formed stars by means of fits to their spectral energy
distribution. Massive young stellar objects are observed at the borders of the
N49 region and represent second generation massive stars whose formation has
been triggered by the expansion of the ionized region. Hi-GAL enables us to
detect a population of young stars at different evolutionary stages, cold
condensations only being detected in the SPIRE wavelength range. The far IR
coverage of Hi-GAL strongly constrains the physical properties of the YSOs. The
large and unbiased spatial coverage of this survey offers us a unique
opportunity to lead, for the first time, a global study of star formation
triggered by HII regions in our Galaxy.Comment: 4 pages, 2 figures, accepted by A&A (Special issue on Herschel first
results
Target Selection for the SDSS-IV APOGEE-2 Survey
APOGEE-2 is a high-resolution, near-infrared spectroscopic survey observing
roughly 300,000 stars across the entire sky. It is the successor to APOGEE and
is part of the Sloan Digital Sky Survey IV (SDSS-IV). APOGEE-2 is expanding
upon APOGEE's goals of addressing critical questions of stellar astrophysics,
stellar populations, and Galactic chemodynamical evolution using (1) an
enhanced set of target types and (2) a second spectrograph at Las Campanas
Observatory in Chile. APOGEE-2 is targeting red giant branch (RGB) and red
clump (RC) stars, RR Lyrae, low-mass dwarf stars, young stellar objects, and
numerous other Milky Way and Local Group sources across the entire sky from
both hemispheres. In this paper, we describe the APOGEE-2 observational design,
target selection catalogs and algorithms, and the targeting-related
documentation included in the SDSS data releases.Comment: 19 pages, 6 figures. Accepted to A
Herschel Observations of the W43 "mini-starburst"
Aims: To explore the infrared and radio properties of one of the closest
Galactic starburst regions. Methods: Images obtained with the Herschel Space
Observatory at wavelengths of 70, 160, 250, 350, and 500 microns using the PACS
and SPIRE arrays are analyzed and compared with radio continuum VLA data and 8
micron images from the Spitzer Space Telescope. The morphology of the
far-infrared emission is combined with radial velocity measurements of
millimeter and centimeter wavelength transitions to identify features likely to
be associated with the W43 complex. Results: The W43 star-forming complex is
resolved into a dense cluster of protostars, infrared dark clouds, and ridges
of warm dust heated by massive stars. The 4 brightest compact sources with L >
1.5 x 10^4 Lsun embedded within the Z-shaped ridge of bright dust emission in
W43 remain single at 4" (0.1 pc) resolution. These objects, likely to be
massive protostars or compact clusters in early stages of evolution are
embedded in clumps with masses of 10^3 to 10^4 Msun, but contribute only 2% to
the 3.6 x 10^6 Lsun far-IR luminosity of W43 measured in a 16 by 16 pc box. The
total mass of gas derived from the far-IR dust emission inside this region is
~10^6 Msun. Cometary dust clouds, compact 6 cm radio sources, and warm dust
mark the locations of older populations of massive stars. Energy release has
created a cavity blowing-out below the Galactic plane. Compression of molecular
gas in the plane by the older HII region near G30.684-0.260 and the bipolar
structure of the resulting younger W43 HII region may have triggered the
current mini-star burst.Comment: 5 pages, 3 figures, accepted for A&A Special Issu
SMASHing the LMC: A Tidally-induced Warp in the Outer LMC and a Large-scale Reddening Map
We present a study of the three-dimensional (3D) structure of the Large
Magellanic Cloud (LMC) using ~2.2 million red clump (RC) stars selected from
the Survey of the MAgellanic Stellar History. To correct for line-of-sight dust
extinction, the intrinsic RC color and magnitude and their radial dependence
are carefully measured by using internal nearly dust-free regions. These are
then used to construct an accurate 2D reddening map (165 square degrees with
~10 arcmin resolution) of the LMC disk and the 3D spatial distribution of RC
stars. An inclined disk model is fit to the 2D distance map yielding a best-fit
inclination angle i = 25.86(+0.73,-1.39) degrees with random errors of +\-0.19
degrees and line-of-nodes position angle theta = 149.23(+6.43,-8.35) degrees
with random errors of +/-0.49 degrees. These angles vary with galactic radius,
indicating that the LMC disk is warped and twisted likely due to the repeated
tidal interactions with the Small Magellanic Cloud (SMC). For the first time,
our data reveal a significant warp in the southwestern part of the outer disk
starting at rho ~ 7 degrees that departs from the defined LMC plane up to ~4
kpc toward the SMC, suggesting that it originated from a strong interaction
with the SMC. In addition, the inner disk encompassing the off-centered bar
appears to be tilted up to 5-15 degrees relative to the rest of the LMC disk.
These findings on the outer warp and the tilted bar are consistent with the
predictions from the Besla et al. simulation of a recent direct collision with
the SMC.Comment: 25 pages, 15 figures, published in Ap
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