115 research outputs found
Terahertz Quantum Cascade Laser With Efficient Coupling and Beam Profile
Quantum cascade lasers (QCLs) are unipolar semiconductor lasers, where the wavelength of emitted radiation is determined by the engineering of quantum states within the conduction band in coupled multiple-quantum-well heterostructures to have the desired energy separation. The recent development of terahertz QCLs has provided a new generation of solid-state sources for radiation in the terahertz frequency range. Terahertz QCLs have been demonstrated from 0.84 to 5.0 THz both in pulsed mode and continuous wave mode (CW mode). The approach employs a resonant-phonon depopulation concept. The metal-metal (MM) waveguide fabrication is performed using Cu-Cu thermo-compression bonding to bond the GaAs/AlGaAs epitaxial layer to a GaAs receptor wafer
Spitzer View of Massive Star Formation in the Tidally Stripped Magellanic Bridge
The Magellanic Bridge is the nearest low-metallicity, tidally stripped
environment, offering a unique high-resolution view of physical conditions in
merging and forming galaxies. In this paper we present analysis of candidate
massive young stellar objects (YSOs), i.e., {\it in situ, current} massive star
formation (MSF) in the Bridge using {\it Spitzer} mid-IR and complementary
optical and near-IR photometry. While we definitely find YSOs in the Bridge,
the most massive are , found in the Large
Magellanic Cloud (LMC). The intensity of MSF in the Bridge also appears
decreasing, as the most massive YSOs are less massive than those formed in the
past. To investigate environmental effects on MSF, we have compared properties
of massive YSOs in the Bridge to those in the LMC. First, YSOs in the Bridge
are apparently less embedded than in the LMC: 81% of Bridge YSOs show optical
counterparts, compared to only 56% of LMC sources with the same range of mass,
circumstellar dust mass, and line-of-sight extinction. Circumstellar envelopes
are evidently more porous or clumpy in the Bridge's low-metallicity
environment. Second, we have used whole samples of YSOs in the LMC and the
Bridge to estimate the probability of finding YSOs at a given \hi\ column
density, N(HI). We found that the LMC has higher probability than
the Bridge for N(HI) cm, but the trend reverses at
lower N(HI). Investigating whether this lower efficiency relative to HI is due
to less efficient molecular cloud formation, or less efficient cloud collapse,
or both, will require sensitive molecular gas observations.Comment: 41 pages, 20 figures, 6 tables; accepted for publication in ApJ;
several figures are in low resolution due to the size limit here and a high
resolution version can be downloaded via
http://www.astro.virginia.edu/~cc5ye/ms_bridge20140215.pd
Expansion of W 3(OH)
A direct measurement of the expansion of W 3(OH) is made by comparing Very
Large Array images taken about 10 yr apart. The expansion is anisotropic with a
typical speed of 3 to 5 km/s, indicating a dynamical age of only 2300 yr. These
observations are inconsistent with either the freely expanding shell model or a
simple bow shock model. The most favored model is a slowly expanding shell-like
HII region, with either a fast rarefied flow or another less massive diffuse
ionized region moving towards the observer. There is also a rapidly evolving
source near the projected center of emission, perhaps related to the central
star.Comment: LaTeX file, 28 pages, includes 8 figures. To appear in ApJ in
December 10 (1998) issue. Also available at
http://www.submm.caltech.edu/~kawamura/w3oh_pp.p
Probing ISM Structure in Trumpler 14 & Carina I Using The Stratospheric Terahertz Observatory 2
We present observations of the Trumpler 14/Carina I region carried out using
the Stratospheric Terahertz Observatory 2 (STO2). The Trumpler 14/Carina I
region is in the west part of the Carina Nebula Complex, which is one of the
most extreme star-forming regions in the Milky Way. We observed Trumpler
14/Carina I in the 158 m transition of [C\,{\sc ii}] with a spatial
resolution of 48 and a velocity resolution of 0.17 km s. The
observations cover a 0.25 by 0.28 area with central position
{\it l} = 297.34, {\it b} = -0.60. The kinematics show that
bright [C\,{\sc ii}] structures are spatially and spectrally correlated with
the surfaces of CO clouds, tracing the photodissociation region and ionization
front of each molecular cloud. Along 7 lines of sight that traverse Tr 14 into
the dark ridge to the southwest, we find that the [C\,{\sc ii}] luminosity from
the HII region is 3.7 times that from the PDR. In same los we find in the PDRs
an average ratio of 1:4.1:5.6 for the mass in atomic gas:dark-CO gas: molecular
gas traced by CO. Comparing multiple gas tracers including HI 21cm, [C\,{\sc
ii}], CO, and radio recombination lines, we find that the HII regions of the
Carina Nebula Complex are well-described as HII regions with one-side freely
expanding towards us, consistent with the champagne model of ionized gas
evolution. The dispersal of the GMC in this region is dominated by EUV
photoevaporation; the dispersal timescale is 20-30 Myr.Comment: ApJ accepte
Spitzer View of Young Massive Stars in the LMC HII Complexes. II. N159
The HII complex N159 in the Large Magellanic Cloud (LMC) is used to study
massive star formation in different environments, as it contains three giant
molecular clouds (GMCs) that have similar sizes and masses but exhibit
different intensities of star formation. We identify candidate massive young
stellar objects (YSOs) using infrared photometry, and model their SEDs to
constrain mass and evolutionary state. Good fits are obtained for less evolved
Type I, I/II, and II sources. Our analysis suggests that there are massive
embedded YSOs in N159B, a maser source, and several ultracompact HII regions.
Massive O-type YSOs are found in GMCs N159-E and N159-W, which are associated
with ionized gas, i.e., where massive stars formed a few Myr ago. The third
GMC, N159-S, has neither O-type YSOs nor evidence of previous massive star
formation. This correlation between current and antecedent formation of massive
stars suggests that energy feedback is relevant. We present evidence that
N159-W is forming YSOs spontaneously, while collapse in N159-E may be
triggered. Finally, we compare star formation rates determined from YSO counts
with those from integrated H-alpha and 24 micron luminosities and expected from
gas surface densities. Detailed dissection of extragalactic GMCs like the one
presented here is key to revealing the physics underlying commonly used star
formation scaling laws.Comment: 60 pages, 11 figures. Accepted for publication in Astrophysical
Journa
Probing seed black holes using future gravitational-wave detectors
Identifying the properties of the first generation of seeds of massive black
holes is key to understanding the merger history and growth of galaxies.
Mergers between ~100 solar mass seed black holes generate gravitational waves
in the 0.1-10Hz band that lies between the sensitivity bands of existing
ground-based detectors and the planned space-based gravitational wave detector,
the Laser Interferometer Space Antenna (LISA). However, there are proposals for
more advanced detectors that will bridge this gap, including the third
generation ground-based Einstein Telescope and the space-based detector DECIGO.
In this paper we demonstrate that such future detectors should be able to
detect gravitational waves produced by the coalescence of the first generation
of light seed black-hole binaries and provide information on the evolution of
structure in that era. These observations will be complementary to those that
LISA will make of subsequent mergers between more massive black holes. We
compute the sensitivity of various future detectors to seed black-hole mergers,
and use this to explore the number and properties of the events that each
detector might see in three years of observation. For this calculation, we make
use of galaxy merger trees and two different seed black hole mass distributions
in order to construct the astrophysical population of events. We also consider
the accuracy with which networks of future ground-based detectors will be able
to measure the parameters of seed black hole mergers, in particular the
luminosity distance to the source. We show that distance precisions of ~30% are
achievable, which should be sufficient for us to say with confidence that the
sources are at high redshift.Comment: 14 pages, 6 figures, 2 tables, accepted for proceedings of 13th GWDAW
meetin
The sub-mm J=6-5 line of 13CO in Orion
We present a fully sampled map covering the Orion Hot Core and dense
molecular ridge, in the sub-millimeter J=6-5 rotational transition of 13CO, at
0.45 mm with a resolution of 13 arcsec and 0.5 km s^-1. The map covers 3 arc
min by 2arc min . The profile centered on the Hot Core peaks at 8.5 km s^-1 and
has a peak intensity of 40 K, corrected antenna temperature. It shows line
wings from 30 km s^-1 to -20 km s^-1. The map of intensity, integrated from 0
to +18 km s^-1, shows a prominent maximum <5 arcsec from the center of the
Orion Hot Core. The FWHP is 3 arcsec, larger than the regions containing
complex molecules. Single dish measurements of lines from the J=2-1 or J=1-0
transitions of CO isotopes show no such distinct maximum. Correcting for
optical depth 1.5 in the J=6-5 line of 13CO, and assuming that the level
populations are thermalized at 150 K, the beam averaged column density between
0 to +18 km s^-1 is N(13CO )=6.8 10^17 cm^-2 and N(CO)=5.2 10^19 cm-2. When
combined with published dust emission data, the CO/ H2 number ratio is 2
{\cdot} 10^-5, a factor of ~5 lower than the canonical value, 10^-4. For the
Orion South and Orion Ridge region, the column density of CO is <25% of that
found for the Hot Core but CO/H2 ratios are similar. Models of
Photodissociation Regions, PDRs, predict that CO lines from PDRs are only
marginally optically thick. Thus our map traces warm and dense molecular gas
rather than PDRs.Comment: 23 pages total, including 5 figure
Nonuniform Cardiac Denervation Observed by 11C-meta-Hydroxyephedrine PET in 6-OHDA-Treated Monkeys
Parkinson's disease presents nonmotor complications such as autonomic dysfunction that do not respond to traditional anti-parkinsonian therapies. The lack of established preclinical monkey models of Parkinson's disease with cardiac dysfunction hampers development and testing of new treatments to alleviate or prevent this feature. This study aimed to assess the feasibility of developing a model of cardiac dysautonomia in nonhuman primates and preclinical evaluations tools. Five rhesus monkeys received intravenous injections of 6-hydroxydopamine (total dose: 50 mg/kg). The animals were evaluated before and after with a battery of tests, including positron emission tomography with the norepinephrine analog 11C-meta-hydroxyephedrine. Imaging 1 week after neurotoxin treatment revealed nearly complete loss of specific radioligand uptake. Partial progressive recovery of cardiac uptake found between 1 and 10 weeks remained stable between 10 and 14 weeks. In all five animals, examination of the pattern of uptake (using Logan plot analysis to create distribution volume maps) revealed a persistent region-specific significant loss in the inferior wall of the left ventricle at 10 (P<0.001) and 14 weeks (P<0.01) relative to the anterior wall. Blood levels of dopamine, norepinephrine (P<0.05), epinephrine, and 3,4-dihydroxyphenylacetic acid (P<0.01) were notably decreased after 6-hydroxydopamine at all time points. These results demonstrate that systemic injection of 6-hydroxydopamine in nonhuman primates creates a nonuniform but reproducible pattern of cardiac denervation as well as a persistent loss of circulating catecholamines, supporting the use of this method to further develop a monkey model of cardiac dysautonomia
Mouse Genome-Wide Association and Systems Genetics Identify Asxl2 As a Regulator of Bone Mineral Density and Osteoclastogenesis
Significant advances have been made in the discovery of genes affecting bone mineral density (BMD); however, our understanding of its genetic basis remains incomplete. In the current study, genome-wide association (GWA) and co-expression network analysis were used in the recently described Hybrid Mouse Diversity Panel (HMDP) to identify and functionally characterize novel BMD genes. In the HMDP, a GWA of total body, spinal, and femoral BMD revealed four significant associations (−log10P>5.39) affecting at least one BMD trait on chromosomes (Chrs.) 7, 11, 12, and 17. The associations implicated a total of 163 genes with each association harboring between 14 and 112 genes. This list was reduced to 26 functional candidates by identifying those genes that were regulated by local eQTL in bone or harbored potentially functional non-synonymous (NS) SNPs. This analysis revealed that the most significant BMD SNP on Chr. 12 was a NS SNP in the additional sex combs like-2 (Asxl2) gene that was predicted to be functional. The involvement of Asxl2 in the regulation of bone mass was confirmed by the observation that Asxl2 knockout mice had reduced BMD. To begin to unravel the mechanism through which Asxl2 influenced BMD, a gene co-expression network was created using cortical bone gene expression microarray data from the HMDP strains. Asxl2 was identified as a member of a co-expression module enriched for genes involved in the differentiation of myeloid cells. In bone, osteoclasts are bone-resorbing cells of myeloid origin, suggesting that Asxl2 may play a role in osteoclast differentiation. In agreement, the knockdown of Asxl2 in bone marrow macrophages impaired their ability to form osteoclasts. This study identifies a new regulator of BMD and osteoclastogenesis and highlights the power of GWA and systems genetics in the mouse for dissecting complex genetic traits
The naked truth:a comprehensive clarification and classification of current 'myths' in naked mole-rat biology
The naked mole-rat (Heterocephalus glaber) has fascinated zoologists for at least half a century. It has also generated considerable biomedical interest not only because of its extraordinary longevity, but also because of unusual protective features (e.g. its tolerance of variable oxygen availability), which may be pertinent to several human disease states, including ischemia/reperfusion injury and neurodegeneration. A recent article entitled 'Surprisingly long survival of premature conclusions about naked mole-rat biology' described 28 'myths' which, those authors claimed, are a 'perpetuation of beautiful, but falsified, hypotheses' and impede our understanding of this enigmatic mammal. Here, we re-examine each of these 'myths' based on evidence published in the scientific literature. Following Braude et al., we argue that these 'myths' fall into four main categories: (i) 'myths' that would be better described as oversimplifications, some of which persist solely in the popular press; (ii) 'myths' that are based on incomplete understanding, where more evidence is clearly needed; (iii) 'myths' where the accumulation of evidence over the years has led to a revision in interpretation, but where there is no significant disagreement among scientists currently working in the field; (iv) 'myths' where there is a genuine difference in opinion among active researchers, based on alternative interpretations of the available evidence. The term 'myth' is particularly inappropriate when applied to competing, evidence-based hypotheses, which form part of the normal evolution of scientific knowledge. Here, we provide a comprehensive critical review of naked mole-rat biology and attempt to clarify some of these misconceptions
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