16,557 research outputs found
Massive vector fields on the Schwarzschild spacetime: quasinormal modes and bound states
We study the propagation of a massive vector or Proca field on the
Schwarzschild spacetime. The field equations are reduced to a one-dimensional
wave equation for the odd-parity part of the field and two coupled equations
for the even-parity part of the field. We use numerical techniques based on
solving (scalar or matrix-valued) three-term recurrence relations to compute
the spectra of both quasi-normal modes and quasi-bound states, which have no
massless analogue, complemented in the latter case by a forward-integration
method. We study the radial equations analytically in both the near-horizon and
far-field regions and use a matching procedure to compute the associated
spectra in the small-mass limit. Finally, we comment on extending our results
to the Kerr geometry and its phenomenological relevance for hidden photons
arising e.g. in string theory compactifications.Comment: 15 pages, 8 figures; minor corrections, to be published in Phys. Rev.
Scaling and correlations in the dynamics of forest-fire occurrence
Forest-fire waiting times, defined as the time between successive events
above a certain size in a given region, are calculated for Italy. The
probability densities of the waiting times are found to verify a scaling law,
despite that fact that the distribution of fire sizes is not a power law. The
meaning of such behavior in terms of the possible self-similarity of the
process in a nonstationary system is discussed. We find that the scaling law
arises as a consequence of the stationarity of fire sizes and the existence of
a non-trivial ``instantaneous'' scaling law, sustained by the correlations of
the process.Comment: Not a long paper, but many figures (but no large size in kb
Galactic Extinction from Colors and Counts of Field Galaxies in WFPC2 Frames: An Application to GRB 970228
We develop the ``simulated extinction method'' to measure average foreground
Galactic extinction from field galaxy number-counts and colors. The method
comprises simulating extinction in suitable reference fields by changing the
isophotal detection limit. This procedure takes into account selection effects,
in particular, the change in isophotal detection limit (and hence in isophotal
magnitude completeness limit) with extinction, and the galaxy color--magnitude
relation.
We present a first application of the method to the HST WFPC2 images of the
gamma-ray burster GRB 970228. Four different WFPC2 high-latitude fields,
including the HDF, are used as reference to measure the average extinction
towards the GRB in the F606W passband. From the counts, we derive an average
extinction of A_V = 0.5 mag, but the dispersion of 0.4 mag between the
estimates from the different reference fields is significantly larger than can
be accounted by Poisson plus clustering uncertainties. Although the counts
differ, the average colors of the field galaxies agree well. The extinction
implied by the average color difference between the GRB field and the reference
galaxies is A_V = 0.6 mag, with a dispersion in the estimated extinction from
the four reference fields of only 0.1 mag. All our estimates are in good
agreement with the value of 0.81\pm0.27 mag obtained by Burstein & Heiles, and
with the extinction of 0.78\pm0.12 measured by Schlegel et al. from maps of
dust IR emission. However, the discrepancy between the widely varying counts
and the very stable colors in these high-latitude fields is worth
investigating.Comment: 14 pages, 2 figures; submitted to the Astrophysical Journa
Star formation in the giant HII regions of M101
The molecular components of three giant HII regions (NGC 5461, NGC 5462, NGC
5471) in the galaxy M101 are investigated with new observations from the James
Clerk Maxwell Telescope, the NRAO 12-meter, and the Owens Valley millimeter
array. Of the three HII regions, only NGC 5461 had previously been detected in
CO emission.
We calculate preliminary values for the molecular mass of the GMCs in NGC
5461 by assuming a CO-to-H_2 factor (X factor) and then compare these values
with the virial masses. We conclude that the data in this paper demonstrate for
the first time that the value of X may decrease in regions with intense star
formation.
The molecular mass for the association of clouds in NGC 5461 is approximately
3x10^7 Mo and is accompanied by 1-2 times as much atomic mass. The observed CO
emission in NGC 5461 is an order of magnitude stronger than in NGC 5462, while
it was not possible to detect molecular gas toward NGC 5471 with the JCMT. An
even larger ratio of atomic to molecular gas in NGC 5471 was observed, which
might be attributed to efficient conversion of molecular to atomic gas.
The masses of the individual clouds in NGC 5461, which are gravitationally
bound, cover a range of (2-8) x 10^5 Mo, comparable with the masses of Galactic
giant molecular clouds (GMCs). Higher star forming efficiencies, and not
massive clouds, appear to be the prerequisite for the formation of the large
number of stars whose radiation is required to produce the giant HII regions in
M101.Comment: 32 pages, 5 figures, accepted for publication in the Astrophysical
Journa
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