1,315 research outputs found
High energy photons and neutrinos from cosmic sources
High energy gamma and X-ray photons and neutrinos from cosmic sources - galactic radiatio
Detection of Pristine Gas Two Billion Years after the Big Bang
In the current cosmological model, only the three lightest elements were
created in the first few minutes after the Big Bang; all other elements were
produced later in stars. To date, however, heavy elements have been observed in
all astrophysical environments. We report the detection of two gas clouds with
no discernible elements heavier than hydrogen. These systems exhibit the lowest
heavy-element abundance in the early universe and thus are potential fuel for
the most metal poor halo stars. The detection of deuterium in one system at the
level predicted by primordial nucleosynthesis provides a direct confirmation of
the standard cosmological model. The composition of these clouds further
implies that the transport of heavy elements from galaxies to their
surroundings is highly inhomogeneous.Comment: 32 pages, 11 figures, SOM included. To appear in Scienc
Galactic X-ray Sources
Bremsstrahlung and synchrotron hypotheses considered as possible mechanisms for galactic X-ray productio
Pneumococcal conjugate vaccine given shortly after birth stimulates effective antibody concentrations and primes immunological memory for sustained infant protection.
BACKGROUND: In developing countries, newborn immunization with pneumococcal conjugate vaccines (PCVs) could protect young infants who are at high risk of invasive pneumococcal disease (IPD) but might lead to immune tolerance. METHODS: In a randomized trial, young infants received 7-valent PCV at 6, 10, and 14 weeks (Expanded Programme on Immunization [EPI] group) or 0, 10, and 14 weeks (newborn group). Safety was monitored actively at 2-7 days and then passively. Serum samples obtained at birth and 6, 10, 14, 18, 36, and 37 weeks were assayed by enzyme-linked immunosorbent assay for anticapsular immunoglobulin G concentration and avidity. Infants were boosted with either 7-valent PCV or one-fifth dose of pneumococcal polysaccharide vaccine at 36 weeks. Nasopharyngeal swab samples were obtained at 18 and 36 weeks. RESULTS: Three-hundred neonates and young infants were enrolled. Newborn vaccination was well tolerated. Adverse events occurred equally in each group; none was related to immunization. One infant, immunized at birth, died of unrelated neonatal sepsis. At 18 weeks, protective concentrations (≥0.35 μg/mL) were achieved against each serotype by ≥87% of infants with no significant differences between groups. Geometric mean concentrations were higher in the EPI group for serotypes 4, 9V, 18C, and 19F at 18 weeks and for serotype 4 at 36 weeks. Avidity was greater in the newborn group for serotypes 4, 6B, and 19F at 18 weeks and for serotype 19F at 36 weeks. Booster responses and vaccine-type/nonvaccine-type carriage prevalence did not differ between groups. CONCLUSIONS: PCV was safe, immunogenic, and primed for memory when given at birth. There was no evidence of immune tolerance. Vaccination beginning at birth offers an alternative to control IPD in vulnerable young infants
Chemical Evolution of the Juvenile Universe
Only massive stars contribute to the chemical evolution of the juvenile
universe corresponding to [Fe/H]<-1.5. If Type II supernovae (SNe II) are the
only relevant sources, then the abundances in the interstellar medium of the
juvenile epoch are simply the sum of different SN II contributions. Both
low-mass (~8-11M_sun) and normal (~12-25M_sun) SNe II produce neutron stars,
which have intense neutrino-driven winds in their nascent stages. These winds
produce elements such as Sr, Y, and Zr through charged-particle reactions
(CPR). Such elements are often called the light r-process elements, but are
considered here as products of CPR and not the r-process. The observed absence
of production of the low-A elements (Na through Zn including Fe) when the true
r-process elements (Ba and above) are produced requires that only low-mass SNe
II be the site if the r-process occurs in SNe II. Normal SNe II produce the CPR
elements in addition to the low-A elements. This results in a two-component
model that is quantitatively successful in explaining the abundances of all
elements relative to hydrogen for -3<[Fe/H]<-1.5. This model explicitly
predicts that [Sr/Fe]>-0.32. Recent observations show that there are stars with
[Sr/Fe]<-2 and [Fe/H]<-3. This proves that the two-component model is not
correct and that a third component is necessary to explain the observations.
This leads to a simple three-component model including low-mass and normal SNe
II and hypernovae (HNe), which gives a good description of essentially all the
data for stars with [Fe/H]<-1.5. We conclude that HNe are more important than
normal SNe II in the chemical evolution of the low-A elements, in sharp
distinction to earlier models. (Abridged)Comment: 10 pages, 9 figures, to appear in Pub. Astron. Soc. Australi
Do Quasars Lens Quasars?
If the unexpectedly high frequency of quasar pairs with very different
component redshifts is due to the lensing of a population of background quasars
by the foreground quasar, typical lens masses must be \sim10^{12}M_{\sun} and
the sum of all such quasar lenses would have to contain times the
closure density of the Universe. It then seems plausible that a very high
fraction of all \sim10^{12} M_{\sun} gravitational lenses with redshifts
contain quasars. Here I propose that these systems have evolved to
form the present population of massive galaxies with M and M
>5\times10^{11} M_{\sun}.Comment: 6 pages, aas style, ams symbols, ApJL (accepted
The Distribution of Redshifts in New Samples of Quasi-stellar Objects
Two new samples of QSOs have been constructed from recent surveys to test the
hypothesis that the redshift distribution of bright QSOs is periodic in
. The first of these comprises 57 different redshifts among all
known close pairs or multiple QSOs, with image separations 10\arcsec,
and the second consists of 39 QSOs selected through their X-ray emission and
their proximity to bright comparatively nearby active galaxies. The redshift
distributions of the samples are found to exhibit distinct peaks with a
periodic separation of in identical to that claimed
in earlier samples but now extended out to higher redshift peaks and 4.47, predicted by the formula but never seen before. The periodicity
is also seen in a third sample, the 78 QSOs of the 3C and 3CR catalogues. It is
present in these three datasets at an overall significance level -
, and appears not to be explicable by spectroscopic or similar
selection effects. Possible interpretations are briefly discussed.Comment: submitted for publication in the Astronomical Journal, 15 figure
Ejection of Matter and Energy from NGC 4258
It has been claimed that the megamaser observations of the nucleus of NGC
4258 show that a massive black hole is present in its center (Miyoshi et al.
1995, Greenhill et al. 1995). We show that the evidence of ejection of gas,
radio plasma, and X-ray emitting QSOs from this nucleus all show that the
ejection is coming from the center in a curving flow within a cone with angle
~40 degrees, centered at P.A. 100 degrees. This is close to the direction in
which the velocities from the megamaser have been measured, so that the
evidence taken as a whole suggests that the masering gas also is being ejected
in the same direction at velocities +/- 900 km/sec and not rotating about a
massive black hole. Thus it does not provide evidence for a black hole in the
center.
Subject headings: galaxies: nuclei: individual (NGC 4258) -- black holes --
masersComment: 7 pages, 1 Figure, LaTex using epsf.tex, submitted to Ap.J.Letter
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