4,856 research outputs found

### The Onset of the Cold HI Phase in Disks of Protogalaxies

We discuss a possible delay experienced by protogalaxies with low column
density of gas in forming stars over large scales. After the hydrogen has
recombined, as the external ionizing UV flux decreases and the metal abundance
$Z$ increases, the HI, initially in the warm phase (T\simgt 5000 K), makes a
transition to the cool phase (T\simlt 100 K). The minimum abundance $Z_{min}$
for which this phase transition takes place in a small fraction of the Hubble
time decreases rapidly with increasing gas column density. Therefore in the
``anemic'' disk galaxies, where $N_{HI}$ is up to ten times smaller than for
normal large spirals, the onset of the cool HI phase is delayed. The onset of
gravitational instability is also delayed, since these objects are more likely
to be gravitationally stable in the warm phase than progenitors of today's
large spiral galaxies. The first substantial burst of star formation may occur
only as late as at redshifts $z \sim 0.5$ and give a temporary high peak
luminosity, which may be related to the ``faint blue objects". Galaxy disks of
lower column density tend to have lower escape velocities and a
starburst/galactic fountain instability which decreases the gas content of the
inner disk drastically.Comment: TeX file, 24 pages, 4 figures available upon request from
[email protected], to appear in The Astrophysical J. (Sept. 1

### Correlation Statistics of Irregular and Spiral Galaxies Mapped in HI

Several measures of galaxy size and mass obtained from the neutral hydrogen
mapping of 70 dwarf irregular galaxies presented in Paper I (Hoffman et al.
1996) are compared statistically to those for the set of all available
HI-mapped dwarfs and HI-mapped spirals distributed within the same spatial
volume to investigate variations in Tully-Fisher relations and in surface
densities as functions of galaxy size and luminosity or mass. Some ambiguities
due to the ``non-commutativity'' of the correlations among the variables are
addressed and linear regressions of logarithms of blue luminosity, HI and
optical radii, velocity profile half-width incorporating rotation and random
motions, HI mass, and indicative dynamical mass are presented and analyzed. The
surface density of HI is almost constant along the sequence of
size/mass/luminosity while surface density of blue luminosity increases with
galaxy size. For quantities not involving HI we find no evidence for a
``break'' between dwarfs and spirals, but we do find some curvature in velocity
vs. radius and in the Tully-Fisher relation. There is an indication for a
difference in the correlations involving HI mass or radius between dwarfs alone
and spirals alone, in the sense that irregulars have somewhat more HI mass or
slightly larger HI radii than spirals at a given blue luminosity, optical
radius, or velocity profile width.Comment: AASTeX, to appear in ApJ, 26 pages + 3 tables + 12 figure

### Energy input and HI spin temperatures in low pressure regions

Two recent (unpublished) HI emission/absorption studies carried out with good sensitivity using the Arecibo 21 cm beam are discussed. One study (Colgan, Salpeter and Terzian) looked for high velocity clouds of our own Galaxy in absorption in the directions of 63 of the brightest continuum sources reachable with the Arecibo telescope. HI emission mapping in the neighborhood of these directions was also carried out. The other study (Corbelli and Schneider) looked for absorption along lines of sight to about 50 weaker sources which pass within a few diameters of nearby disk galaxies. Neither study detected any absorption

### Particles, environments and possible ecologies in the Jovian atmosphere

The eddy diffusion coefficient is estimated as a function of altitude, separately for the Jovian troposphere and mesosphere. Complex organic molecules produced by the Ly alpha photolysis of methane may possibly be the absorbers in the lower mesosphere which account for the low reflectivity of Jupiter in the near ultraviolet. The optical frequency chromophores are localized at or just below the Jovian tropopause. Candidate chromophore molecules must satisfy the condition that they are produced sufficiently rapidly that convective pyrolysis maintains the observed chromophore optical depth. The condition is satisfied if complex organic chromophores are produced with high quantum yield by NH3 photolysis at less than 2,300 A. Jovian photoautotrophs in the upper troposphere satisfy this condition well, even with fast circulation, assuming only biochemical properties of comparable terrestrial organisms. An organism in the form of a thin, gas filled balloon can grow fast enough to replicate if (1) it can survive at the low mesospheric temperatures, or if (2) photosynthesis occurs in the troposphere

### On electrostatic screening of ions in astrophysical plasmas

There has been some controversy over the expression for the so-called
`interaction energy' due to screening of charged particles in a plasma. Even in
the relatively simple case of weak screening, first discussed in the context of
astrophysical plasmas by Salpeter (1954), there is disagreement. In particular,
Shaviv and Shaviv (1996) have claimed recently that by not considering
explicitly in his calculation the complete screening cloud, Salpeter obtained a
result for the interaction energy between two nuclei separated by a distance
$r$ which in the limit $r \to 0$ is only 2/3 the correct value. It appears that
this claim has arisen from a fundamental misconception concerning the dynamics
of the interaction. We rectify this misconception, and show that Salpeter's
formula is indeed correct.Comment: 17 pages, no figures, AAS Latex, to appear in The Astrophysical
Journa

### Klein-Gordon lower bound to the semirelativistic ground-state energy

For the class of attractive potentials V(r) <= 0 which vanish at infinity, we
prove that the ground-state energy E of the semirelativistic Hamiltonian
H = \sqrt{m^2 + p^2} + V(r) is bounded below by the ground-state energy e of
the corresponding Klein--Gordon problem
(p^2 + m^2)\phi = (V(r) -e)^2\phi. Detailed results are presented for the
exponential and Woods--Saxon potentials.Comment: 7 pages, 4 figure

### Selection effects and binary galaxy velocity differences

Measurements of the velocity differences (delta v's) in pairs of galaxies from large statistical samples have often been used to estimate the average masses of binary galaxies. A basic prediction of these models is that the delta v distribution ought to decline monotonically. However, some peculiar aspects of the kinematics have been uncovered, with an anomalous preference for delta v approx. equal to 72 km s(sup-1) appearing to be present in the data. The authors examine a large sample of binary galaxies with accurate redshift measurements and confirm that the distribution of delta v's appears to be non-monotonic with peaks at 0 and approx. 72 km s (exp -1). The authors suggest that the non-zero peak results from the isolation criteria employed in defining samples of binaries and that it indicates there are two populations of binary orbits contributing to the observed delta v distribution

### Systematic quantum corrections to screening in thermonuclear fusion

We develop a series expansion of the plasma screening length away from the
classical limit in powers of $\hbar^{2}$. It is shown that the leading order
quantum correction increases the screening length in solar conditions by
approximately 2% while it decreases the fusion rate by approximately $0.34%$.
We also calculate the next higher order quantum correction which turns out to
be approximately 0.05%

### Screening in Thermonuclear Reaction Rates in the Sun

We evaluate the effect of electrostatic screening by ions and electrons on
low-Z thermonuclear reactions in the sun. We use a mean field formalism and
calculate the electron density of the screening cloud using the appropriate
density matrix equation of quantum statistical mechanics. Because of well
understood physical effects that are included for the first time in our
treatment, the calculated enhancement of reaction rates does not agree with the
frequently used interpolation formulae. Our result does agree, within small
uncertainties, with Salpeter's weak screening formula. If weak screening is
used instead of the commonly employed screening prescription of Graboske et
al., the predicted $^8$B neutrino flux is increased by 7% and the predicted
chlorine rate is increased by 0.4 SNU.Comment: 15 pages, 1 figure, submitted to ApJ. Acknowledgments, a footnote,
and an explanation added. Additional information at www.sns.ias.edu/~jn

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