2,612 research outputs found
Microwave emission from spinning dust in circumstellar disks
In the high density environments of circumstellar disks dust grains are
expected to grow to large sizes by coagulation. Somewhat unexpectedly, recent
near-IR observations of PAH features from disks around Herbig Ae/Be stars
demonstrate that substantial amount of dust mass in these disks (up to several
tens of per cent of the total carbon content) can be locked up in particles
with sizes ranging from several to tens of nanometers. We investigate the
possibility of detecting the electric dipole emission produced by these
nanoparticles as they spin at thermal rates (tens of GHz) in cold gas. We show
that such emission peaks in the microwave range and dominates over the thermal
disk emission at \nu 5 % of the
total carbon abundance is locked up in nanoparticles. We test the sensitivity
of this prediction to various stellar and disk parameters and show that if the
potential contamination of the spinning dust component by the free-free and/or
synchrotron emission can be removed, then the best chances of detecting this
emission would be in disks with small opacity, having SEDs with steep sub-mm
slopes (which minimizes thermal disk emission at GHz frequencies). Detection of
the spinning dust emission would provide important evidence for the existence,
properties, and origin of the population of small dust particles in
protoplanetary disks, with possible ramifications for planet formation.Comment: 9 pages, 3 figures, submitted to Ap
A New Family of Planets ? "Ocean Planets"
A new family of planets is considered which is between rochy terrestrial
planets and gaseous giant ones: "Ocean-Planets". We present the possible
formation, composition and internal models of these putative planets, including
that of their ocean, as well as their possible Exobiology interest. These
planets should be detectable by planet detection missions such as Eddington and
Kepler, and possibly COROT (lauch scheduled in 2006). They would be ideal
targets for spectroscopic missions such as Darwin/TPF.Comment: 15 pages, 3 figures submitted to Icarus notes (10 july 2003
Rationale for the use of color information on Eddington
For the Eddington mission, the intrinsic stellar variability can be a major
source of noise in the detection of extrasolar planets by the transit method.
We derive that most detections of terrestrial planets (1-2 R_Earth) will occur
around G or K stars with 15-16th magnitude. When these stars are 7-12 times
more variable than the Sun on a 10 hour timescale, we demonstrate that the
detection can be performed with a higher S/N provided composite lightcurves
obtained with the combination of two colors are used instead of white ones. The
level of 10 hour variability for K stars is quite uncertain. We make two
"guess-estimates" of it and find that it could be several times larger than the
solar value. If these estimates were relevant, the color information would not
provide a significant advantage. Although we do not demonstrate a need for
colors, we point out the risk of an unpleasant surprise regarding the 10 hour
stellar variability. Indeed, there is presently no qualified proxy for this
variability. Besides, if Eddington were designed to provide this information at
the cost of added complexity but not sensitivity, white photometry by channel
summation would still be as efficient. Considering the risk that 10 hour
variability is higher than estimated, the Precaution Imperative points to a
study of practical implementations of photometry in different colors before
taking irreversible decisions about the Eddington instrument.Comment: Submitted to A&A as a Research Not
Infrared Spectroscopy of the Diffuse Ionized Halo of NGC 891
We present infrared spectroscopy from the Spitzer Space Telescope at one disk
position and two positions at a height of 1 kpc from the disk in the edge-on
spiral NGC 891, with the primary goal of studying halo ionization. Our main
result is that the [Ne III]/[Ne II] ratio, which provides a measure of the
hardness of the ionizing spectrum free from the major problems plaguing optical
line ratios, is enhanced in the extraplanar pointings relative to the disk
pointing. Using a 2D Monte Carlo-based photo-ionization code which accounts for
the effects of radiation field hardening, we find that this trend cannot be
reproduced by any plausible photo-ionization model, and that a secondary source
of ionization must therefore operate in gaseous halos. We also present the
first spectroscopic detections of extraplanar PAH features in an external
normal galaxy. If they are in an exponential layer, very rough emission
scale-heights of 330-530 pc are implied for the various features. Extinction
may be non-negligible in the midplane and reduce these scale-heights
significantly. There is little significant variation in the relative emission
from the various features between disk and extraplanar environment. Only the
17.4 micron feature is significantly enhanced in the extraplanar gas compared
to the other features, possibly indicating a preference for larger PAHs in the
halo.Comment: 35 pages in ApJ preprint format, 8 figures, accepted for publication
in ApJ. Minor change to Introduction to give appropriate credit to earlier,
related wor
Infrared dust emission in the outer disk of M51
We examine faint infrared emission features detected in Spitzer Space
Telescope images of M51, which are associated with atomic hydrogen in the outer
disk and tidal tail at R greater than R_25 (4.9', ~14 kpc at d=9.6 Mpc). The
infrared colors of these features are consistent with the colors of dust
associated with star formation in the bright disk. However, the star formation
efficiency (as a ratio of star formation rate to neutral gas mass) implied in
the outer disk is lower than that in the bright disk of M51 by an order of
magnitude, assuming a similar relationship between infrared emission and star
formation rate in the inner and outer disks.Comment: 13 pages in manuscript form, 2 figures; download PDF of manuscript
with original-resolution Figure 1 at
http://www.eg.bucknell.edu/physics/thornley/thornleym51.pd
IRAC Excess in Distant Star-Forming Galaxies: Tentative Evidence for the 3.3m Polycyclic Aromatic Hydrocarbon Feature ?
We present evidence for the existence of an IRAC excess in the spectral
energy distribution (SED) of 5 galaxies at 0.6<z<0.9 and 1 galaxy at z=1.7.
These 6 galaxies, located in the Great Observatories Origins Deep Survey field
(GOODS-N), are star forming since they present strong 6.2, 7.7, and 11.3 um
polycyclic aromatic hydrocarbon (PAH) lines in their Spitzer IRS mid-infrared
spectra. We use a library of templates computed with PEGASE.2 to fit their
multiwavelength photometry and derive their stellar continuum. Subtraction of
the stellar continuum enables us to detect in 5 galaxies a significant excess
in the IRAC band pass where the 3.3 um PAH is expected. We then assess if the
physical origin of the IRAC excess is due to an obscured active galactic
nucleus (AGN) or warm dust emission. For one galaxy evidence of an obscured AGN
is found, while the remaining four do not exhibit any significant AGN activity.
Possible contamination by warm dust continuum of unknown origin as found in the
Galactic diffuse emission is discussed. The properties of such a continuum
would have to be different from the local Universe to explain the measured IRAC
excess, but we cannot definitively rule out this possibility until its origin
is understood. Assuming that the IRAC excess is dominated by the 3.3 um PAH
feature, we find good agreement with the observed 11.3 um PAH line flux arising
from the same C-H bending and stretching modes, consistent with model
expectations. Finally, the IRAC excess appears to be correlated with the
star-formation rate in the galaxies. Hence it could provide a powerful
diagnostic for measuring dusty star formation in z>3 galaxies once the
mid-infrared spectroscopic capabilities of the James Webb Space Telescope
become available.Comment: 25 pages, 4 figures, accepted by Ap
An experimental testbed for NEAT to demonstrate micro-pixel accuracy
NEAT is an astrometric mission proposed to ESA with the objectives of
detecting Earth-like exoplanets in the habitable zone of nearby solar-type
stars. In NEAT, one fundamental aspect is the capability to measure stellar
centroids at the precision of 5e-6 pixel. Current state-of-the-art methods for
centroid estimation have reached a precision of about 4e-5 pixel at Nyquist
sampling. Simulations showed that a precision of 2 micro-pixels can be reached,
if intra and inter pixel quantum efficiency variations are calibrated and
corrected for by a metrology system. The European part of the NEAT consortium
is designing and building a testbed in vacuum in order to achieve 5e-6 pixel
precision for the centroid estimation. The goal is to provide a proof of
concept for the precision requirement of the NEAT spacecraft. In this paper we
give the basic relations and trade-offs that come into play for the design of a
centroid testbed and its metrology system. We detail the different conditions
necessary to reach the targeted precision, present the characteristics of our
current design and describe the present status of the demonstration.Comment: SPIE proceeding
Polycylcic Aromatic Hydrocarbons (PAH's) in dense cloud chemistry
Virtually all detailed gas-phase models of the chemistry of dense
interstellar clouds exclude polycyclic aromatic hydrocarbons (PAH's). This
omission is unfortunate because from the few studies that have been done on the
subject, it is known that the inclusion of PAH's can affect the gas-phase
chemistry strongly. We have added PAH's to our network to determine the role
they play in the chemistry of cold dense cores. In the models presented here,
we include radiative attachment to form PAH-, mutual neutralization between PAH
anions and small positively-charged ions, and photodetachment. We also test the
sensitivity of our results to changes in the size and abundance of the PAH's.
Our results confirm that the inclusion of PAH's changes many of the calculated
abundances of smaller species considerably. In TMC-1, the general agreement
with observations is significantly improved contrary to L134N. This may
indicate a difference in PAH properties between the two regions. With the
inclusion of PAH's in dense cloud chemistry, high-metal elemental abundances
give a satisfactory agreement with observations. As a result, we do not need to
decrease the observed elemental abundances of all metals and we do not need to
vary the elemental C/O ratio in order to produce large abundances of carbon
species in TMC-1 (CP).Comment: Accepted to ApJ. Astrophysical Journal (2008) accepte
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