446 research outputs found
Pseudo-rotations of the open annulus
In this paper, we study pseudo-rotations of the open annulus, \emph{i.e.}
conservative homeomorphisms of the open annulus whose rotation set is reduced
to a single irrational number (the angle of the pseudo-rotation). We prove in
particular that, for every pseudo-rotation of angle , the rigid
rotation of angle is in the closure of the conjugacy class of . We
also prove that pseudo-rotations are not persistent in topology for any
.Comment: 25 page
Hydrogen Isocyanide in Comet 73P/Schwassmann-Wachmann (Fragment B)
We present a sensitive 3-sigma upper limit of 1.1% for the HNC/HCN abundance
ratio in comet 73P/Schwassmann-Wachmann (Fragment B), obtained on May 10-11,
2006 using Caltech Submillimeter Observatory (CSO). This limit is a factor of
~7 lower than the values measured previously in moderately active comets at 1
AU from the Sun. Comet 73P/Schwassmann-Wachmann was depleted in most volatile
species, except of HCN. The low HNC/HCN ratio thus argues against HNC
production from polymers produced from HCN. However, thermal degradation of
macromolecules, or polymers, produced from ammonia and carbon compounds, such
as acetylene, methane, or ethane appears a plausible explanation for the
observed variations of the HNC/HCN ratio in moderately active comets, including
the very low ratio in comet 73P/Schwassmann-Wachmann reported here. Similar
polymers have been invoked previously to explain anomalous 14N/15N ratios
measured in cometary CN.Comment: 6 pages, 5 figures, 2 table
Mid-Infrared Spectrophotometric Observations of Fragments B and C of Comet 73P/Schwassmann-Wachmann 3
We present mid-infrared spectra and images from the GEMINI-N (+Michelle)
observations of fragments SW3-[B] and SW3-[C] of the ecliptic (Jupiter Family)
comet 73P/Schwassmann-Wachmann 3 pre-perihelion. We observed fragment B soon
after an outburst event (between 2006 April 16 - 26 UT) and detected
crystalline silicates. The mineralogy of both fragments was dominated by
amorphous carbon and amorphous pyroxene. The grain size distribution (assuming
a Hanner modified power-law) for fragment SW3-[B] has a peak grain radius of
a_p ~ 0.5 micron, and for fragment SW3-[C], a_p ~ 0.3 micron; both values
larger than the peak grain radius of the size distribution for the dust ejected
from ecliptic comet 9P/Tempel 1 during the Deep Impact event (a_p = 0.2 micron.
The silicate-to-carbon ratio and the silicate crystalline mass fraction for the
submicron to micron-size portion of the grain size distribution on the nucleus
of fragment SW3-[B] was 1.341 +0.250 -0.253 and 0.335 +0.089 -0.112,
respectively, while on the nucleus of fragment SW3-[C] was 0.671 +0.076 -0.076
and 0.257 +0.039 -0.043, respectively. The similarity in mineralogy and grain
properties between the two fragments implies that 73P/Schwassmann-Wachmann 3 is
homogeneous in composition. The slight differences in grain size distribution
and silicate-to-carbon ratio between the two fragments likely arises because
SW3-[B] was actively fragmenting throughout its passage while the activity in
SW3-[C] was primarily driven by jets. The lack of diverse mineralogy in the
fragments SW3-[B] and SW3-[C] of 73P/Schwassmann-Wachmann 3 along with the
relatively larger peak in the coma grain size distribution suggests the parent
body of this comet may have formed in a region of the solar nebula with
different environmental properties than the natal sites where comet C/1995 O1
(Hale-Bopp) and 9P/Tempel 1 nuclei aggregated.Comment: 31 pages, 5 figure, accepted for publication in A
The chemical diversity of comets
A fundamental question in cometary science is whether the different dynamical
classes of comets have different chemical compositions, which would reflect
different initial conditions. From the ground or Earth orbit, radio and
infrared spectroscopic observations of a now significant sample of comets
indeed reveal deep differences in the relative abundances of cometary ices.
However, no obvious correlation with dynamical classes is found. Further
results come, or are expected, from space exploration. Such investigations, by
nature limited to a small number of objects, are unfortunately focussed on
short-period comets (mainly Jupiter-family). But these in situ studies provide
"ground truth" for remote sensing. We discuss the chemical differences in
comets from our database of spectroscopic radio observations, which has been
recently enriched by several Jupiter-family and Halley-type comets.Comment: In press in Earth, Moon and Planets (proceedings of the workshop
"Future Ground-based Solar System Research: Synergies with Space Probes and
Space Telescopes", Portoferraio, Isola d'Elba, Livorno (Italy), 8-12
September 2008). 6 pages with 2 figure
A Turbulent Origin for Flocculent Spiral Structure in Galaxies: II. Observations and Models of M33
Fourier transform power spectra of azimuthal scans of the optical structure
of M33 are evaluated for B, V, and R passbands and fit to fractal models of
continuum emission with superposed star formation. Power spectra are also
determined for Halpha. The best models have intrinsic power spectra with 1D
slopes of around -0.7pm0.7, significantly shallower than the Kolmogorov
spectrum (slope =-1.7) but steeper than pure noise (slope=0). A fit to the
power spectrum of the flocculent galaxy NGC 5055 gives a steeper slope of
around -1.5pm0.2, which could be from turbulence. Both cases model the optical
light as a superposition of continuous and point-like stellar sources that
follow an underlying fractal pattern. Foreground bright stars are clipped in
the images, but they are so prominent in M33 that even their residual affects
the power spectrum, making it shallower than what is intrinsic to the galaxy. A
model consisting of random foreground stars added to the best model of NGC 5055
fits the observed power spectrum of M33 as well as the shallower intrinsic
power spectrum that was made without foreground stars. Thus the optical
structure in M33 could result from turbulence too.Comment: accepted by ApJ, 13 pages, 10 figure
HI power spectrum of the spiral galaxy NGC628
We have measured the HI power spectrum of the nearly face-on spiral galaxy
NGC628 (M74) using a visibility based estimator. The power spectrum is well
fitted by a power law , with over the
length scale . The slope is found to be
independent of the width of the velocity channel. This value of the slope is a
little more than one in excess of what has been seen at considerably smaller
length scales in the Milky-Way, Small Magellanic Cloud (LMC), Large Magellanic
Cloud (SMC) and the dwarf galaxy DDO210. We interpret this difference as
indicating a transition from three dimensional turbulence at small scales to
two dimensional turbulence in the plane of the galaxy's disk at length scales
larger than galaxy's HI scale height.
The slope measured here is similar to that found at large scales in the LMC.
Our analysis also places an upper limit to the galaxy's scale height at $800\
{\rm pc}$ .Comment: 4 Pages, 2 Figures, 1 Table. Accepted for Publication in MNRAS
LETTER
The Millennium Arecibo 21-CM Absorption Line Survey. II. Properties of the Warm and Cold Neutral Media
We use the Gaussian-fit results of Paper I to investigate the properties of
interstellar HI in the Solar neighborhood. The Warm and Cold Neutral Media (WNM
and CNM) are physically distinct components. The CNM spin temperature histogram
peaks at about 40 K. About 60% of all HI is WNM. At z=0, we derive a volume
filling fraction of about 0.50 for the WNM; this value is very rough. The
upper-limit WNM temperatures determined from line width range upward from about
500 K; a minimum of about 48% of the WNM lies in the thermally unstable region
500 to 5000 K. The WNM is a prominent constituent of the interstellar medium
and its properties depend on many factors, requiring global models that include
all relevant energy sources, of which there are many. We use Principal
Components Analysis, together with a form of least squares fitting that
accounts for errors in both the independent and dependent parameters, to
discuss the relationships among the four CNM Gaussian parameters. The spin
temperature T_s and column density N(HI) are, approximately, the two most
important eigenvectors; as such, they are sufficient, convenient, and
physically meaningful primary parameters for describing CNM clouds. The Mach
number of internal macroscopic motions for CNM clouds is typically 2.5, but
there are wide variations. We discuss the historical tau-T_s relationship in
some detail and show that it has little physical meaning. We discuss CNM
morphology using the CNM pressure known from UV stellar absorption lines.
Knowing the pressure allows us to show that CNM structures cannot be isotropic
but instead are sheetlike, with length-to-thickness aspect ratios ranging up to
about 280. We present large-scale maps of two regions where CNM lies in very
large ``blobby sheets''.Comment: Revised submission to Ap.J. Changes include: (1) correction of
turbulent Mach number in equation 16 and figure 12; the new typical value is
1.3 versus the old, incorrect value 2.5. (2) smaller typeface for the
astro-ph version to conserve paper. 60 pages, 16 figure
Measuring Molecular, Neutral Atomic, and Warm Ionized Galactic Gas Through X-Ray Absorption
We study the column densities of neutral atomic, molecular, and warm ionized
Galactic gas through their continuous absorption of extragalactic X-ray spectra
at |b| > 25 degrees. For N(H,21cm) < 5x10^20 cm^-2 there is an extremely tight
relationship between N(H,21cm) and the X-ray absorption column, N(xray), with a
mean ratio along 26 lines of sight of N(xray)/N(H,21cm) = 0.972 +- 0.022. This
is significantly less than the anticpated ratio of 1.23, which would occur if
He were half He I and half He II in the warm ionized component. We suggest that
the ionized component out of the plane is highly ionized, with He being mainly
He II and He III. In the limiting case that H is entirely HI, we place an upper
limit on the He abundance in the ISM of He/H <= 0.103.
At column densities N(xray) > 5x10^20 cm^-2, which occurs at our lower
latitudes, the X-ray absorption column N(xray) is nearly double N(H,21cm). This
excess column cannot be due to the warm ionized component, even if He were
entirely He I, so it must be due to a molecular component. This result implies
that for lines of sight out of the plane with |b| ~ 30 degrees, molecular gas
is common and with a column density comprable to N(H,21cm).
This work bears upon the far infrared background, since a warm ionized
component, anticorrelated with N(H,21cm), might produce such a background. Not
only is such an anticorrelation absent, but if the dust is destroyed in the
warm ionized gas, the far infrared background may be slightly larger than that
deduced by Puget et al. (1996).Comment: 1 AASTeX file, 14 PostScript figure files which are linked within the
TeX fil
Can Reflection from Grains Diagnose the Albedo?
By radiation transfer models with a realistic power spectra of the projected
density distributions, we show that the optical properties of grains are poorly
constrained by observations of reflection nebulae. The ISM is known to be
hierarchically clumped from a variety of observations (molecules, H I,
far-infrared). Our models assume the albedo and phase parameter of the dust,
the radial optical depth of the sphere averaged over all directions, and random
distributions of the dust within the sphere. The outputs are the stellar
extinction, optical depth, and flux of scattered light as seen from various
viewing angles. Observations provide the extinction and scattered flux from a
particular direction.
Hierarchical geometry has a large effect on the flux of scattered light
emerging from a nebula for a particular extinction of the exciting star. There
is a very large spread in both scattered fluxes and extinctions for any
distribution of dust. Consequently, an observed stellar extinction and
scattered flux can be fitted by a wide range of albedos. With hierarchical
geometry it is not completely safe to determine even relative optical constants
from multiwavelength observations of the same reflection nebula. The geometry
effectively changes with wavelength as the opacity of the clumps varies. Limits
on the implications of observing the same object in various wavelengths are
discussed briefly.
Henry (2002) uses a recipe to determine the scattered flux from a star with a
given extinction. It is claimed to be independent of the geometry. It provides
considerably more scattering than our models, probably leading to an
underestimate of the grain albedos from the UV Diffuse Galactic Light.Comment: 27 pages, including 7 figures. Accepted by Ap
Velocity and density spectra of the Small Magellanic Cloud
This paper reports results on the statistical analysis of HI turbulence in
the Small Magellanic Cloud (SMC). We use 21 cm channel maps, obtained with the
Australia Telescope Compact Array and the Parkes telescope, and analyze the
spectrum of observed intensity fluctuations as a function of the velocity slice
thickness. We confirm predictions by Lazarian & Pogosyan (2000) on the change
of the power law index and establish the spectra of 3-D density and velocity.
The obtained spectral indices, -3.3 and -3.4, are slightly more shallow than
the predictions for the Kolmogorov spectrum. This contrasts to the predictions
for the shock-type spectra that are steeper than the Kolmogorov one. The nature
of the energy injection in the SMC is unclear as no distinct energy injection
scales are observed up to the entire scale of the SMC.Comment: 5 pages, 1 figure, accepted for publication in the ApJ Letter
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