1,768 research outputs found
Merger of Black Holes in the Galactic Center
We present the results of three body simulations focused on understanding the
fates of intermediate mass black holes (IBH) that drift within the central 0.5
pc of the Galaxy. In particular, we modeled the interactions between pairs of
black holes as they orbit a central blac k hole of mass
. The simulations performed assume a
Schwarzschild geometry and account for Chandrasekhar dynamical friction as well
as acceleration resulting from energy lost due to gravitational radiation.
We found the branching ratio for one of the orbiting IBHs to merge with the
CBH was 0.95 and is independent of the inner IBH's initial eccentricity as well
as the rate of sinking. This, coupled with an infall rate of yrs
for an IBH to drift into the Galactic center, results in an IBH-CBH merger
every Myrs. Lastly we found that the IBH-IBH-CBH triple body
system ``resets'' itself, in the sense that a system with an inner I BH with an
initially circular orbit generally left behind an IBH with a large
eccentricity, whereas a system in which the inner IBH had a high eccentricity
() usually left a remnant with low eccentricity. Branching ratios
for different outcomes are also similar in the two cases.Comment: Official paper to appear in November 2008 issue of Ap
Rapid formation of exponential disks and bulges at high redshift from the dynamical evolution of clump cluster and chain galaxies
Many galaxies at high redshift have peculiar morphologies dominated by
10^8-10^9 Mo kpc-sized clumps. Using numerical simulations, we show that these
"clump clusters" can result from fragmentation in gravitationally unstable
primordial disks. They appear as "chain galaxies" when observed edge-on. In
less than 1 Gyr, clump formation, migration, disruption, and interaction with
the disk cause these systems to evolve from initially uniform disks into
regular spiral galaxies with an exponential or double-exponential disk profile
and a central bulge. The inner exponential is the initial disk size and the
outer exponential is from material flung out by spiral arms and clump torques.
A nuclear black hole may form at the same time as the bulge from smaller black
holes that grow inside the dense cores of each clump. The properties and
lifetimes of the clumps in our models are consistent with observations of the
clumps in high redshift galaxies, and the stellar motions in our models are
consistent with the observed velocity dispersions and lack of organized
rotation in chain galaxies. We suggest that violently unstable disks are the
first step in spiral galaxy formation. The associated starburst activity gives
a short timescale for the initial stellar disk to form.Comment: ApJ Accepted, 13 pages, 9 figure
The use of pediatric flexible bronchoscopy in the COVID-19 pandemic era
On March 11, 2020, the World Health Organization (WHO) declared the pandemic because of a novel coronavirus, called severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In January 2020, the first transmission to healthcare workers (HCWs) was described. SARS-CoV-2 is transmitted between people because of contact, droplets, and airborne. Airborne transmission is caused by aerosols that remain infectious when suspended in air over long distances and time. In the clinical setting, airborne transmission may occur during aerosol generating procedures like flexible bronchoscopy. To date, although the role of children in the transmission of SARS-CoV-2 is not clear the execution of bronchoscopy is associated with a considerably increased risk of SARS-CoV-2 transmission to HCWs. The aim of this overview is to summarize available recommendations and to apply them to pediatric bronchoscopy. We performed systematic literature searches using the MEDLINE (accessed via PubMed) and Scopus databases. We reviewed major recommendations and position statements published at the moment by the American Association for Bronchology and Interventional Pulmonology, WHO, European Center for Disease Prevention and Control and expert groups on the management of patients with COVID-19 to limit transmission among HCWs. To date there is a lack of recommendations for safe bronchoscopy during the pandemic period. The main indications concern adults and little has been said about children. We have summarized available recommendations and we have applied them to pediatric bronchoscopy
Mid-infrared diagnostics of starburst galaxies: clumpy, dense structures in star-forming regions in the Antennae (NGC 4038/4039)
Recently, mid-infrared instruments have become available on several large
ground-based telescopes, resulting in data sets with unprecedented spatial
resolution at these long wavelengths. In this paper we examine
'ground-based-only' diagnostics, which can be used in the study of star-forming
regions in starburst galaxies. By combining output from the stellar population
synthesis code Starburst 99 with the photoionization code Mappings, we model
stellar clusters and their surrounding interstellar medium, focusing on the
evolution of emission lines in the N- and Q-band atmospheric windows (8-13 and
16.5-24.5 micron respectively) and those in the near-infrared. We address the
detailed sensitivity of various emission line diagnostics to stellar population
age, metallicity, nebular density, and ionization parameter. Using our model
results, we analyze observations of two stellar clusters in the overlap region
of the Antennae galaxies obtained with VLT Imager and Spectrometer for mid
Infrared (VISIR). We find evidence for clumpy, high density, ionized gas. The
two clusters are young (younger than 2.5 and 3 Myr respectively), the
surrounding interstellar matter is dense (10^4 cm^-3 or larger) and can be
characterized by a high ionization parameter (logU > -1.53). Detailed analysis
of the mid-infrared spectral features shows that a (near-)homogeneous medium
cannot account for the observations, and that complex structure on scales below
the resolution limit, containing several young stellar clusters embedded in
clumpy gas, is more likely.Comment: 24 pages, 16 figures (3 in color), accepted for publication in Ap
Minimum Conductivity and Evidence for Phase Transitions in Ultra-clean Bilayer Graphene
Bilayer graphene (BLG) at the charge neutrality point (CNP) is strongly
susceptible to electronic interactions, and expected to undergo a phase
transition into a state with spontaneous broken symmetries. By systematically
investigating a large number of singly- and doubly-gated bilayer graphene (BLG)
devices, we show that an insulating state appears only in devices with high
mobility and low extrinsic doping. This insulating state has an associated
transition temperature Tc~5K and an energy gap of ~3 meV, thus strongly
suggesting a gapped broken symmetry state that is destroyed by very weak
disorder. The transition to the intrinsic broken symmetry state can be tuned by
disorder, out-of-plane electric field, or carrier density
Targeted shock-and-kill HIV-1 gene therapy approach combining CRISPR activation, suicide gene tBid and retargeted adenovirus delivery
Infections with the human immunodeficiency virus type 1 (HIV-1) are incurable due the long-lasting, latent viral reservoir. The shock-and-kill cure approach aims to activate latent proviruses in HIV-1 infected cells and subsequently kill these cells with strategies such as therapeutic vaccines or immune enhancement. Here, we combined the dCas9-VPR CRISPR activation (CRISPRa) system with gRNA-V, the truncated Bid (tBid)-based suicide gene strategy and CD3-retargeted adenovirus (Ad) delivery vectors, in an all-in-one targeted shock-and-kill gene therapy approach to achieve specific elimination of latently HIV-1 infected cells. Simultaneous transduction of latently HIV-1 infected J-Lat 10.6 cells with a CD3-retargeted Ad-CRISPRa-V and Ad-tBid led to a 57.7 ± 17.0% reduction of productively HIV-1 infected cells and 2.4-fold ± 0.25 increase in cell death. The effective activation of latent HIV-1 provirus by Ad-CRISPRa-V was similar to the activation control TNF-α. The strictly HIV-1 dependent and non-leaky killing by tBid could be demonstrated. Furthermore, the high transduction efficiencies of up to 70.8 ± 0.4% by the CD3-retargeting technology in HIV-1 latently infected cell lines was the basis of successful shock-and-kill. This novel targeted shock-and-kill all-in-one gene therapy approach has the potential to safely and effectively eliminate HIV-1 infected cells in a highly HIV-1 and T cell specific manner
Generation of photovoltage in graphene on a femtosecond time scale through efficient carrier heating
Graphene is a promising material for ultrafast and broadband photodetection.
Earlier studies addressed the general operation of graphene-based
photo-thermoelectric devices, and the switching speed, which is limited by the
charge carrier cooling time, on the order of picoseconds. However, the
generation of the photovoltage could occur at a much faster time scale, as it
is associated with the carrier heating time. Here, we measure the photovoltage
generation time and find it to be faster than 50 femtoseconds. As a
proof-of-principle application of this ultrafast photodetector, we use graphene
to directly measure, electrically, the pulse duration of a sub-50 femtosecond
laser pulse. The observation that carrier heating is ultrafast suggests that
energy from absorbed photons can be efficiently transferred to carrier heat. To
study this, we examine the spectral response and find a constant spectral
responsivity between 500 and 1500 nm. This is consistent with efficient
electron heating. These results are promising for ultrafast femtosecond and
broadband photodetector applications.Comment: 6 pages, 4 figure
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