1,087 research outputs found
Subarcsecond Submillimeter Imaging of the Ultracompact HII Region G5.89-0.39
We present the first subarcsecond submillimeter images of the enigmatic
ultracompact HII region (UCHII) G5.89-0.39. Observed with the SMA, the 875
micron continuum emission exhibits a shell-like morphology similar to longer
wavelengths. By using images with comparable angular resolution at five
frequencies obtained from the VLA archive and CARMA, we have removed the
free-free component from the 875 micron image. We find five sources of dust
emission: two compact warm objects (SMA1 and SMA2) along the periphery of the
shell, and three additional regions further out. There is no dust emission
inside the shell, supporting the picture of a dust-free cavity surrounded by
high density gas. At subarcsecond resolution, most of the molecular gas tracers
encircle the UCHII region and appear to constrain its expansion. We also find
G5.89-0.39 to be almost completely lacking in organic molecular line emission.
The dust cores SMA1 and SMA2 exhibit compact spatial peaks in optically-thin
gas tracers (e.g. 34SO2), while SMA1 also coincides with 11.9 micron emission.
In CO(3-2), we find a high-velocity north/south bipolar outflow centered on
SMA1, aligned with infrared H2 knots, and responsible for much of the maser
activity. We conclude that SMA1 is an embedded intermediate mass protostar with
an estimated luminosity of 3000 Lsun and a circumstellar mass of ~1 Msun.
Finally, we have discovered an NH3 (3,3) maser 12 arcsec northwest of the UCHII
region, coincident with a 44 GHz CH3OH maser, and possibly associated with the
Br gamma outflow source identified by Puga et al. (2006).Comment: 41 pages, 11 figures, published in The Astrophysical Journal (2008)
Volume 680, Issue 2, pp. 1271-1288. An error in the registration of the
marker positions in Figure 11 has been corrected in this versio
Chemical Abundances of Planetary Nebulae in the Sagittarius Dwarf Elliptical Galaxy
Spectrophotometry and imaging of the two planetary nebulae He2-436 and
Wray16-423, recently discovered to be in the Sagittarius dwarf elliptical
galaxy, are presented. Wray16-423 is a high excitation planetary nebula (PN)
with a hot central star. In contrast He2-436 is a high density PN with a cooler
central star and evidence of local dust, the extinction exceeding that for
Wray16-423 by E(B-V)=0.28. The extinction to Wray16-423, (E(B-V)=0.14), is
consistent with the extinction to the Sagittarius (Sgr) Dwarf. Both PN show
Wolf-Rayet features in their spectra, although the lines are weak in
Wray16-423. Images in [O III] and H-alpha+[N II], although affected by poor
seeing, yield a diameter of 1.2'' for Wray16-423 after deconvolution; He~2-436
was unresolved. He2-436 has a luminosity about twice that of Wray16-423 and its
size and high density suggest a younger PN. In order to reconcile the differing
luminosity and nebular properties of the two PN with similar age progenitor
stars, it is suggested that they are on He burning tracks
The abundance pattern is very similar in both nebulae and shows an oxygen
depletion of -0.4 dex with respect to the mean O abundance of Galactic PN and
[O/H] = -0.6. The Sgr PN progenitor stars are representative of the higher
metallicity tail of the Sgr population. The pattern of abundance depletion is
similar to that in the only other PN in a dwarf galaxy companion of the Milky
Way, that in Fornax, for which new spectra are presented. However the
abundances are larger than for Galactic halo PN suggesting a later formation
age. The O abundance of the Sgr galaxy deduced from its PN, shows similarities
with that of dwarf ellipticals around M31, suggesting that this galaxy was a
dwarf elliptical before its interaction with the Milky Way.Comment: 24 pages, Latex (aas2pp4.sty) including 5 postscript figures. To
appear in Ap
Double logarithms, , and the NLO DGLAP evolution for the non-singlet component of the nucleon spin structure function,
Theoretical predictions show that at low values of Bjorken the spin
structure function, is influenced by large logarithmic corrections,
, which may be predominant in this region. These corrections are
also partially contained in the NLO part of the standard DGLAP evolution. Here
we calculate the non-singlet component of the nucleon structure function,
, and its first moment, using a unified evolution
equation. This equation incorporates the terms describing the NLO DGLAP
evolution and the terms contributing to the resummation. In order
to avoid double counting in the overlapping regions of the phase-space, a
unique way of including the NLO terms into the unified evolution equation is
proposed. The scheme-independent results obtained from this unified evolution
are compared to the NLO fit to experimental data, GRSV'2000. Analysis of the
first moments of shows that the unified evolution including the
resummation goes beyond the NLO DGLAP analysis. Corrections
generated by double logarithms at low influence the -dependence of the
first moments strongly.Comment: 13 pages, latex, 2 figures; Appendix adde
Spitzer Space Telescope spectral observations of AGB stars in the Fornax dwarf spheroidal galaxy
We have observed five carbon-rich AGB stars in the Fornax dwarf spheroidal
(dSph) galaxy, using the Infrared Spectrometer on board the Spitzer Space
Telescope. The stars were selected from a near-infrared survey of Fornax and
include the three reddest stars, with presumably the highest mass-loss rates,
in that galaxy. Such carbon stars probably belong to the intermediate-age
population (2-8 Gyr old and metallicity of [Fe/H] -1) of Fornax. The primary
aim of this paper is to investigate mass-loss rate, as a function of luminosity
and metallicity, by comparing AGB stars in several galaxies with different
metallicities. The spectra of three stars are fitted with a radiative transfer
model. We find that mass-loss rates of these three stars are 4-7x10^-6 Msun
yr-1. The other two stars have mass-loss rates below 1.3x10^-6 Msun yr-1. We
find no evidence that these rates depend on metallicity, although we do suggest
that the gas-to-dust ratio could be higher than at solar metallicity, in the
range 240 to 800. The C2H2 bands are stronger at lower metallicity because of
the higher C/O ratio. In contrast, the SiC fraction is reduced at low
metallicity, due to low silicon abundance. The total mass-loss rate from all
known carbon-rich AGB stars into the interstellar medium of this galaxy is of
the order of 2x10^-5 Msun yr-1. This is much lower than that of the dwarf
irregular galaxy WLM, which has a similar visual luminosity and metallicity.
The difference is attributed to the younger stellar population of WLM. The
suppressed gas-return rate to the ISM accentuates the difference between the
relatively gas-rich dwarf irregular and the gas-poor dwarf spheroidal galaxies.
Our study will be useful to constrain gas and dust recycling processes in low
metallicity galaxies.Comment: MNRAS accepte
Hipparcos period-luminosity relations for Miras and semiregular variables
We present period-luminosity diagrams for nearby Miras and semiregulars,
selecting stars with parallaxes better than 20 per cent and well-determined
periods. Using K-band magnitudes, we find two well-defined P-L sequences, one
corresponding to the standard Mira P-L relation and the second shifted to
shorter periods by a factor of about 1.9. The second sequence only contains
semiregular variables, while the Mira sequence contains both Miras and
semiregulars. Several semiregular stars show double periods in agreement with
both relations. The Whitelock evolutionary track is shown to fit the data,
indicating that the semiregulars are Mira progenitors. The transition between
the two sequences may correspond to a change in pulsation mode or to a change
in the stellar structure. Large amplitude pulsations leading to classical Mira
classification occur mainly near the tip of the local AGB luminosity function.Comment: 10 pages with figures, accepted by ApJ Letter
Scale Dependence of Polarized DIS Asymmetries
We compare the dependence of the polarized deep inelastic scattering
proton asymmetry, driven by the leading order Altarelli Parisi evolution
equations, to those arising from fixed order and
approximations. It is shown that the evolution effects associated with gluons,
which are not properly taken into account by the leading order approximation,
cannot be neglected in the analysis of the most recent experimental data.Comment: Latex file, (9 figures in postcript available from
[email protected]
Neon and Sulfur Abundances of Planetary Nebulae in the Magellanic Clouds
The chemical abundances of neon and sulfur for 25 planetary nebulae (PNe) in
the Magellanic Clouds are presented. These abundances have been derived using
mainly infrared data from the Spitzer Space Telescope. The implications for the
chemical evolution of these elements are discussed. A comparison with similarly
obtained abundances of Galactic PNe and HII regions and Magellanic Clouds HII
regions is also given. The average neon abundances are 6.0x10(-5) and
2.7x10(-5) for the PNe in the Large and Small Magellanic Clouds respectively.
These are ~1/3 and 1/6 of the average abundances of Galactic planetary nebulae
to which we compare. The average sulfur abundances for the LMC and SMC are
respectively 2.7x10(-6) and 1.0x10(-6). The Ne/S ratio (23.5) is on average
higher than the ratio found in Galactic PNe (16) but the range of values in
both data sets is similar for most of the objects. The neon abundances found in
PNe and HII regions agree with each other. It is possible that a few (3-4) of
the PNe in the sample have experienced some neon enrichment, but for two of
these objects the high Ne/S ratio can be explained by their very low sulfur
abundances. The neon and sulfur abundances derived in this paper are also
compared to previously published abundances using optical data and
photo-ionization models.Comment: 13 pages, 4 tables, 5 figures. Accepted for publication in Ap
MHz Unidirectional Rotation of Molecular Rotary Motors
A combination of cryogenic UV-vis and CD spectroscopy and transient absorption spectroscopy at ambient temperature is used to study a new class of unidirectional rotary molecular motors. Stabilization of unstable intermediates is achieved below 95 K in propane solution for the structure with the fastest rotation rate, and below this temperature measurements on the rate limiting step in the rotation cycle can be performed to obtain activation parameters. The results are compared to measurements at ambient temperature using transient absorption spectroscopy, which show that behavior of these motors is similar over the full temperature range investigated, thereby allowing a maximum rotation rate of 3 MHz at room temperature under suitable irradiation conditions
Mapping localized surface plasmons within silver nanocubes using cathodoluminescence hyperspectral imaging
Localized surface plasmons within silver nanocubes less than 50 nm in size are investigated using high resolution cathodoluminescence hyperspectral imaging. Multivariate statistical analysis of the multidimensional luminescence dataset allows both the identification of distinct spectral features in the emission and the mapping of their spatial distribution. These results show a 490 nm peak emitted from the cube faces, with shorter wavelength luminescence coming from the vertices and edges; this provides direct experimental confirmation of theoretical predictions
Survey of Planetary Nebulae at 30 GHz with OCRA-p
We report the results of a survey of 442 planetary nebulae at 30 GHz. The
purpose of the survey is to develop a list of planetary nebulae as calibration
sources which could be used for high frequency calibration in future. For 41
PNe with sufficient data, we test the emission mechanisms in order to evaluate
whether or not spinning dust plays an important role in their spectra at 30
GHz.
The 30-GHz data were obtained with a twin-beam differencing radiometer,
OCRA-p, which is in operation on the Torun 32-m telescope. Sources were scanned
both in right ascension and declination. We estimated flux densities at 30 GHz
using a free-free emission model and compared it with our data.
The primary result is a catalogue containing the flux densities of 93
planetary nebulae at 30 GHz. Sources with sufficient data were compared with a
spectral model of free-free emission. The model shows that free-free emission
can generally explain the observed flux densities at 30 GHz thus no other
emission mechanism is needed to account for the high frequency spectra.Comment: 10 pages, 7 Postscript figures, to be published in A&
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