116 research outputs found
Detection of isolated population III stars with the James Webb Space Telescope
The first population III stars are predicted to form in minihalos at a
redshift of approximately 10-30. The James Webb Space Telescope (JWST),
tentatively scheduled for launch in 2018, will probably be able to detect some
of the first galaxies, but whether it will also be able to detect the first
stars remains more doubtful. Here, we explore the prospects of detecting an
isolated population III star or a small cluster of population III stars down to
redshift 2 in either lensed or unlensed fields. Our calculations are based on
realistic stellar atmospheres and take into account the potential flux
contribution from the surrounding HII region. We find that unlensed population
III stars are beyond the reach of JWST, and that even lensed population III
stars will be extremely difficult to detect. However, the main problem with the
latter approach is not necessarily that the lensed stars are too faint, but
that their surface number densities are too low. To detect even one population
III star of 60 solar masses when pointing JWST through the galaxy cluster MACS
J0717.5+3745, the lensing cluster with the largest Einstein radius detected so
far, the cosmic star formation rate of population III stars would need to be
approximately an order of magnitude higher than predicted by the most
optimistic current models.Comment: 8 pages, 6 figures, 1 table, published in MNRAS. The main change in
version 2 is the inclusion of lower redshifts, down to 2. There was also one
more SFR comparison model (Tornatore 2007) adde
A search for Population III galaxies in CLASH. I. Singly-imaged candidates at high redshift
Population III galaxies are predicted to exist at high redshifts and may be
rendered sufficiently bright for detection with current telescopes when
gravitationally lensed by a foreground galaxy cluster. Population III galaxies
that exhibit strong Lya emission should furthermore be identifiable from
broadband photometry because of their unusual colors. Here, we report on a
search for such objects at z > 6 in the imaging data from the Cluster Lensing
And Supernova survey with Hubble (CLASH), covering 25 galaxy clusters in 16
filters. Our selection algorithm returns five singly-imaged candidates with
Lya-like color signatures, for which ground-based spectroscopy with current
8-10 m class telescopes should be able to test the predicted strength of the
Lya line. None of these five objects have been included in previous CLASH
compilations of high-redshift galaxy candidates. However, when large grids of
spectral synthesis models are applied to the study of these objects, we find
that only two of these candidates are significantly better fitted by Population
III models than by more mundane, low-metallicity stellar populations.Comment: 18 pages, 10 figures, accepted by Ap
Progress on single barrier varactors for submillimeter wave power generation
Theoretical work on Single Barrier Varactor (SBV) diodes, indicate that the efficiency for a multiplier has a maximum for a considerably smaller capacitance variation than previously thought. The theoretical calculations are performed, both with a simple theoretical model and a complete computer simulation using the method of harmonic balance. Modeling of the SBV is carried out in two steps. First, the semiconductor transport equations are solved simultaneously using a finite difference scheme in one dimension. Secondly, the calculated I-V, and C-V characteristics are input to a multiplier simulator which calculates the optimum impedances, and output powers at the frequencies of interest. Multiple barrier varactors can also be modeled in this way. Several examples on how to design the semiconductor layers to obtain certain characteristics are given. The calculated conversion efficiencies of the modeled structures, in a multiplier circuit, are also presented. Computer simulations for a case study of a 750 GHz multiplier show that InAs diodes perform favorably compared to GaAs diodes. InAs and InGaAs SBV diodes have been fabricated and their current vs. voltage characteristics are presented. In the InAs diode, was the large bandgap semiconductor AlSb used as barrier. The InGaAs diode was grown lattice matched to an InP substrate with InAlAs as a barrier material. The current density is greatly reduced for these two material combinations, compared to that of GaAs/AlGaAs SBV diodes. GaAs based diodes can be biased to higher voltages than InAs diodes
Erratum: Observational constraints on supermassive dark stars
No abstract is available for this article
A case of Mycobacterium goodii prosthetic valve endocarditis in a non-immunocompromised patient: use of 16S rDNA analysis for rapid diagnosis
Background: Mycobacterium goodii is a rare cause of significant infection. M. goodii has mainly been associated with lymphadenitis, cellulitis, osteomyelitis, and wound infection. Case presentation: A case of a 76-year-old Caucasian female is presented. The patient developed a prosthetic valve endocarditis caused by M. goodii. She had also suffered from severe neurological symptoms related to a septic emboli that could be demonstrated as an ischemic lesion found on CT of the brain. Transesophageal echocardiography verified a large vegetation attached to the prosthetic valve. Commonly used blood culture bottles showed growth of the bacteria after 3 days. Conclusions: Although M. goodii is rarely involved in these kinds of severe infections, rapidly growing mycobacteria should be recognized during conventional bacterial investigations and identified by molecular tools such as analysis of 16S rDNA. Species identification of nontuberculous mycobacteria is demanding and is preferably done in collaboration with a mycobacterial laboratory. An early diagnosis provides the opportunity for adequate treatment. In the present case, prolonged antimicrobial treatment and surgery with replacement of the prosthetic valve was successful
Finding high-redshift dark stars with the James Webb Space Telescope
The first stars in the history of the Universe are likely to form in the
dense central regions of 10^5-10^6 Msolar cold dark matter halos at z=10-50.
The annihilation of dark matter particles in these environments may lead to the
formation of so-called dark stars, which are predicted to be cooler, larger,
more massive and potentially more long-lived than conventional population III
stars. Here, we investigate the prospects of detecting high-redshift dark stars
with the upcoming James Webb Space Telescope (JWST). We find that dark stars at
z>6 are intrinsically too faint to be detected by JWST. However, by exploiting
foreground galaxy clusters as gravitational telescopes, certain varieties of
cool (Teff < 30000 K) dark stars should be within reach at redshifts up to
z=10. If the lifetimes of dark stars are sufficiently long, many such objects
may also congregate inside the first galaxies. We demonstrate that this could
give rise to peculiar features in the integrated spectra of galaxies at high
redshifts, provided that dark stars make up at least 1 percent of the total
stellar mass in such objects.Comment: 12 pages, 7 figures; v2: matches published versio
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