672 research outputs found
Multiwavelength Observations of the Low Metallicity Blue Compact Dwarf Galaxy SBS 0335-052
New infrared and millimeter observations from Keck, Palomar, ISO, and OVRO
and archival data from the NRAO VLA and IRAS are presented for the low
metallicity blue compact dwarf galaxy SBS 0335-052. Mid-infrared imaging shows
this young star-forming system is compact (0.31"; 80 pc) at 12.5 microns. The
large Br-gamma equivalent width (235 Angstroms) measured from integral field
spectroscopy is indicative of a ~5 Myr starburst. The central source appears to
be optically thin in emission, containing both a warm (~80 K) and a hot (~210
K) dust component, and the overall interstellar radiation field is quite
intense, about 10,000 times the intensity in the solar neighborhood. CO
emission is not detected, though the galaxy shows an extremely high global H I
gas-to-dust mass ratio, high even for blue compact dwarfs. Finally, the
galaxy's mid-infrared-to-optical and mid-to-near-infrared luminosity ratios are
quite high, whereas its far-infrared-to-radio and far-infrared-to-optical flux
ratios are surprisingly similar to what is seen in normal star-forming
galaxies. The relatively high bolometric infrared-to-radio ratio is more easily
understood in the context of such a young system with negligible nonthermal
radio continuum emission. These new lines of evidence may outline features
common to primordial galaxies found at high redshift.Comment: 28 pages including 6 figures; accepted for publication in the
Astronomical Journa
Studies of aging and HV break down problems during development and operation of MSGC and GEM detectors for the Inner Tracking System of HERA-B
The results of five years of development of the inner tracking system of the
HERA-B experiment and first experience from the data taking period of the year
2000 are reported. The system contains 184 chambers, covering a sensitive area
of about 20 * 20 cm2 each. The detector is based on microstrip gas counters
(MSGCs) with diamond like coated (DLC) glass wafers and gas electron
multipliers (GEMs). The main problems in the development phase were gas
discharges in intense hadron beams and aging in a high radiation dose
environment. The observation of gas discharges which damage the electrode
structure of the MSGC led to the addition of the GEM as a first amplification
step. Spurious sparking at the GEM cannot be avoided completely. It does not
affect the GEM itself but can produce secondary damage of the MSGC if the
electric field between the GEM and the MSGC is above a threshold depending on
operation conditions. We observed that aging does not only depend on the dose
but also on the spot size of the irradiated area. Ar-DME mixtures had to be
abandoned whereas a mixture of 70% Ar and 30% CO2 showed no serious aging
effects up to about 40 mC/cm deposited charge on the anodes. X-ray measurements
indicate that the DLC of the MSGC is deteriorated by the gas amplification
process. As a consequence, long term gain variations are expected. The Inner
Tracker has successfully participated in the data taking at HERA-B during
summer 2000.Comment: 29 pages, 22 figure
Stellar age versus mass of early-type galaxies in the Virgo Cluster
The flux excess of elliptical galaxies in the far-ultraviolet can be
reproduced by population synthesis models when accounting for the population of
old hot helium-burning subdwarf stars. This has been achieved by Han and
coworkers through a quantitative model of binary stellar evolution. Here, we
compare the resulting evolutionary population synthesis model to the GALEX
far-near ultraviolet colors (FUV-NUV) of Virgo cluster early-type galaxies that
were published by Boselli and coworkers. FUV-NUV is reddest at about the
dividing luminosity of dwarf and giant galaxies, and becomes increasingly blue
for both brighter and fainter luminosities. This behavior can be easily
explained by the binary model with a continuous sequence of longer duration and
later truncation of star formation at lower galaxy masses. Thus, in contrast to
previous conclusions, the GALEX data do not require a dichotomy between the
stellar population properties of dwarfs and giants. Their apparently opposite
behavior in FUV-NUV occurs naturally when the formation of hot subdwarfs
through binary evolution is taken into account.Comment: 8 pages, 4 figures. Accepted for publication in Ap
The violent past of Cygnus X-2
Cygnus X-2 appears to be the descendant of an intermediate-mass X-ray binary
(IMXB). Using Mazzitelli's (1989) stellar code we compute detailed evolutionary
sequences for the system and find that its prehistory is sensitive to stellar
input parameters, in particular the amount of core overshooting during the
main-sequence phase. With standard assumptions for convective overshooting a
case B mass transfer starting with a 3.5 M_sun donor star is the most likely
evolutionary solution for Cygnus X-2. This makes the currently observed state
rather short-lived, of order 3 Myr, and requires a formation rate > 1e-7 - 1e-6
per yr of such systems in the Galaxy. Our calculations show that neutron star
IMXBs with initially more massive donors (> 4 M_sun) encounter a delayed
dynamical instability; they are unlikely to survive this rapid mass transfer
phase. We determine limits for the age and initial parameters of Cygnus X-2 and
calculate possible dynamical orbits of the system in a realistic Galactic
potential, given its observed radial velocity. We find trajectories which are
consistent with a progenitor binary on a circular orbit in the Galactic plane
inside the solar circle that received a kick velocity < 200 km/s at the birth
of the neutron star. The simulations suggests that about 7% of IMXBs receiving
an arbitrary kick velocity from a standard kick velocity spectrum would end up
in an orbit similar to Cygnus X-2, while about 10% of them reach yet larger
Galactocentric distances.Comment: 9 pages, 12 figures, accepted for publication in MNRA
Keck Spectroscopy of Candidate Proto-globular Clusters in NGC 1275
Keck spectroscopy of 5 proto-globular cluster candidates in NGC 1275 has been
combined with HST WFPC2 photometry to explore the nature and origin of these
objects and discriminate between merger and cooling flow scenarios for globular
cluster formation. The objects we have studied are not HII regions, but rather
star clusters, yet their integrated spectral properties do not resemble young
or intermediate age Magellanic Cloud clusters or Milky Way open clusters. The
clusters' Balmer absorption appears to be too strong to be consistent with any
of the standard Bruzual & Charlot evolutionary models at any metallicity. If
these models are adopted, an IMF which is skewed to high masses provides a
better fit to the data. A truncated IMF with a mass range of 2-3 Mo reproduces
the observed Balmer equivalent widths and colors at about 450 Myr. Formation in
a continuous cooling flow appears to be ruled out since the age of the clusters
is much larger than the cooling time, the spatial scale of the clusters is much
smaller than the cooling flow radius, and the deduced star formation rate in
the cooling flow favors a steep rather than a flat IMF. A merger would have to
produce clusters only in the central few kpc, presumably from gas in the
merging galaxies which was channeled rapidly to the center. Widespread shocks
in merging galaxies cannot have produced these clusters. If these objects are
confirmed to have a relatively flat, or truncated, IMF it is unclear whether or
not they will evolve into objects we would regard as bona fide globular
clusters.Comment: 30 pages (AAS two column style, including 9 tables and 7 figures) to
appear in the AJ (August issue), also available at
http://www.ucolick.org/~mkissler/Sages/sages.html (with a full resolution
Fig.1) Revised Version: previous posted version was an uncorrect ealier
iteration, parts of the text, tables and figures changed. The overall
conclusions remain unchange
Molecular analysis endophytic bacteria from the genus Bacillus isolated from tropical maize (Zea mays L.).
Endophytíc bacteria play an important role in agriculture by improving plant performance and adaptation against biotic and abiotic stresses. In the present study molecular methods were used for identifying Bacillus endophytic bacteria isolated from Brazilian sweet corn. SDS-PAGE of whole-cell protein extract of forty-two isolates revealed a high number of scrutinable bands. Twenty-four isolates were identified in nine different groups of duplicated bacteria and eighteen were identified as unique. Some high-accumulated polipeptides with variable length were observed in almost isolates. Partial sequencing of 16S ribosonal gene revealed that all isolates are Bacillus sp. and among thirteen isolates with similar protein profiles, two were different strains. Among the forty-two isolates identified by rDNA sequencing. Bacillus. subitilis and B. pumilus were the most frequenty species (15 and 12 isolates. respectively) followed by B. licheniforms (7 isolates), B. cereus (5 isolates) and B. uniloliquefascelns (3 isolates). According to present results. SDS-P AGE techínique could be used as a fast and cheap first tool for identifying inter-specific variation in maize endophytic bacterial collections while rDNA sequencing could be applied for analyzing intra-specific variation among isolates with similar protein profile as well as for taxonomic studies
Graft preservation in heart transplantation: current approaches
Heart transplantation (HTx) represents the current best surgical treatment for patients affected by end-stage heart failure. However, with the improvement of medical and interventional therapies, the population of HTx candidates is increasingly old and at high-risk for mortality and complications. Moreover, the use of “extended donor criteria” to deal with the shortage of donors could increase the risk of worse outcomes after HTx. In this setting, the strategy of donor organ preservation could significantly affect HTx results. The most widely used technique for donor organ preservation is static cold storage in ice. New techniques that are clinically being used for donor heart preservation include static controlled hypothermia and machine perfusion (MP) systems. Controlled hypothermia allows for a monitored cold storage between 4°C and 8°C. This simple technique seems to better preserve the donor heart when compared to ice, probably avoiding tissue injury due to sub-zero °C temperatures. MP platforms are divided in normothermic and hypothermic, and continuously perfuse the donor heart, reducing ischemic time, a well-known independent risk factor for mortality after HTx. Also, normothermic MP permits to evaluate marginal donor grafts, and could represent a safe and effective technique to expand the available donor pool. However, despite the increasing number of donor hearts preserved with these new approaches, whether these techniques could be considered superior to traditional CS still represents a matter of debate. The aim of this review is to summarize and critically assess the available clinical data on donor heart preservation strategies employed for HTx
The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background: IV. Cosmological Implications
In this paper we examine the cosmological constraints of the recent DIRBE and
FIRAS detection of the extragalactic background light between 125-5000 microns
on the metal and star formation histories of the universe.Comment: 38 pages and 9 figures. Accepted for publications in The
Astrophysical Journa
Stochastic association of neighboring replicons creates replication factories in budding yeast
Peer reviewedPublisher PD
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