241 research outputs found
On non-singular renewal kernels with an application to a semigroup of transition kernels
AbstractWe study the non-singularity and limit properties of the renewal kernel R=∑K∗n associated with a positive convolution kernel K(x,dy×dt) defined on a general measurable space (E, E). The principal tool is the use of embedded renewal measures. As an application we consider continuous parameter semigroups (Rt(x,dy);t⩾0) of transition kernels on (E, E)
Superposition with First-class {B}ooleans and Inprocessing Clausification
International audienceWe present a complete superposition calculus for first-order logic with an interpreted Boolean type. Our motivation is to lay the foundation for refutationally complete calculi in more expressive logics with Booleans, such as higher-order logic, and to make superposition work efficiently on problems that would be obfuscated when using clausification as preprocessing. Working directly on formulas, our calculus avoids the costly axiomatic encoding of the theory of Booleans into first-order logic and offers various ways to interleave clausification with other derivation steps. We evaluate our calculus using the Zipperposition theorem prover, and observe that, with no tuning of parameters, our approach is on a par with the state-of-the-art approach
The largest oxigen bearing organic molecule repository
We present the first detection of complex aldehydes and isomers in three
typical molecular clouds located within 200pc of the center of our Galaxy.
We find very large abundances of these complex organic molecules (COMs) in
the central molecular zone (CMZ), which we attribute to the ejection of COMs
from grain mantles by shocks. The relative abundances of the different COMs
with respect to that of CH3OH are strikingly similar for the three sources,
located in very different environments in the CMZ. The similar relative
abundances point toward a unique grain mantle composition in the CMZ. Studying
the Galactic center clouds and objects in the Galactic disk having large
abundances of COMs, we find that more saturated molecules are more abundant
than the non-saturated ones. We also find differences between the relative
abundance between COMs in the CMZ and the Galactic disk, suggesting different
chemical histories of the grain mantles between the two regions in the Galaxy
for the complex aldehydes. Different possibilities for the grain chemistry on
the icy mantles in the GC clouds are briefly discussed. Cosmic rays can play an
important role in the grain chemistry. With these new detections, the molecular
clouds in the Galactic center appear to be one of the best laboratories for
studying the formation of COMs in the Galaxy.Comment: 20 pages, 4 figures, accepted in Ap
Revealing the environs of the remarkable southern hot core G327.3-0.6
We present a submm study of the massive hot core G327.3-0.6 that constrains
its physical parameters and environment. The APEX telescope was used to image
CO and N2H+ emission, to observe lines from other molecules toward a hot and a
cold molecular core, and to measure the continuum flux density of the hot core.
In the C18O J=3-2 line, two clumps were found, one associated with the HII
region G327.3-0.5 and the other associated with the hot core. An additional
cold clump is found 30 arcsec (0.4 pc) northeast of the hot core in bright N2H+
emission. From the the continuum data, we calculate a mass of 420 Msol and a
size of 0.1 pc for the hot core. A new, more accurate position of the hot core
is reported, which allows the association of the core with a bright
mid-infrared source. The luminosity of the hot core is estimated to be between
5 and 15 10^4 Lsol. This study revealed several different evolutionary stages
of massive star formation in the G327.3-0.6 region.Comment: APEX A&A special issue, accepte
Submillimeter continuum observations of Sagittarius B2 at subarcsecond spatial resolution
We report the first high spatial resolution submillimeter continuum
observations of the Sagittarius B2 cloud complex using the Submillimeter Array
(SMA). With the subarcsecond resolution provided by the SMA, the two massive
star-forming clumps Sgr B2(N) and Sgr B2(M) are resolved into multiple compact
sources. In total, twelve submillimeter cores are identified in the Sgr B2(M)
region, while only two components are observed in the Sgr B2(N) clump. The gas
mass and column density are estimated from the dust continuum emission. We find
that most of the cores have gas masses in excess of 100 M and column
densities above 10 cm. The very fragmented appearance of Sgr
B2(M), in contrast to the monolithic structure of Sgr B2 (N), suggests that the
former is more evolved. The density profile of the Sgr B2(N)-SMA1 core is well
fitted by a Plummer density distribution. This would lead one to believe that
in the evolutionary sequence of the Sgr B2 cloud complex, a massive star forms
first in an homogeneous core, and the rest of the cluster forms subsequently in
the then fragmenting structure.Comment: 4 pages, 2 figures, accepted by A&A letter
Submillimeter spectroscopy of southern hot cores: NGC6334(I) and G327.3-0.6
High-mass star-forming regions are known to have a rich molecular spectrum
from many species. Some of the very highly excited lines are emitted from very
hot and dense gas close to the central object(s). The physics and chemistry of
the inner cores of two high mass star forming regions, NGC6334(I) and
G327.3-0.6, shall be characterized. Submillimeter line surveys with the APEX
telescope provide spectra which sample many molecular lines at high excitation
stages. Partial spectral surveys were obtained, the lines were identified,
physical parameters were determined through fitting of the spectra. Both
sources show similar spectra that are comparable to that of the only other high
mass star forming region ever surveyed in this frequency range}, Orion-KL, but
with an even higher line density. Evidence for very compact, very hot sources
is found.Comment: APEX A&A special issue, accepte
Large scale grain mantle disruption in the Galactic Center
We present observations of C2H5OH toward molecular clouds in Sgr A, Sgr B2
and associated with thermal and non-thermal features in the Galactic center.
C2H5OH emission in Sgr A and Sgr B2 is widespread, but not uniform. C2H5OH
emission is much weaker or it is not detected in some molecular clouds in both
complexes, in particular those with radial velocities between 70 and 120 km/s.
While most of the clouds associated with the thermal features do not show
C2H5OH emission, that associated with the Non-Thermal Radio Arc shows emission.
The fractional abundance of C2H5OH in most of the clouds with radial velocities
between 0 and 70 km/s in Sgr A and Sgr B2 is relatively high, of few 1e-8. The
C2H5OH abundance decreases by more than one order of magnitude (aprox. 1e-9) in
the clouds associated with the thermal features. The large abundance of C2H5OH
in the gas-phase indicates that C2H5OH has formed in grains and released to
gas-phase by shocks in the last aprox. 1e5 years.Comment: In press in Astrophysical Journal Letters 7 pages, 1 table, 1 figur
Infrared spectroscopy of solid CO-CO2 mixtures and layers
The spectra of pure, mixed and layered CO and CO2 ices have been studied
systematically under laboratory conditions using infrared spectroscopy. This
work provides improved resolution spectra (0.5 cm-1) of the CO2 bending and
asymmetric stretching mode, as well as the CO stretching mode, extending the
existing Leiden database of laboratory spectra to match the spectral resolution
reached by modern telescopes and to support the interpretation of the most
recent data from Spitzer. It is shown that mixed and layered CO and CO2 ices
exhibit very different spectral characteristics, which depend critically on
thermal annealing and can be used to distinguish between mixed, layered and
thermally annealed CO-CO2 ices. CO only affects the CO2 bending mode spectra in
mixed ices below 50K under the current experimental conditions, where it
exhibits a single asymmetric band profile in intimate mixtures. In all other
ice morphologies the CO2 bending mode shows a double peaked profile, similar to
that observed for pure solid CO2. Conversely, CO2 induces a blue-shift in the
peak-position of the CO stretching vibration, to a maximum of 2142 cm-1 in
mixed ices, and 2140-2146 cm-1 in layered ices. As such, the CO2 bending mode
puts clear constraints on the ice morphology below 50K, whereas beyond this
temperature the CO2 stretching vibration can distinguish between initially
mixed and layered ices. This is illustrated for the low-mass YSO HH46, where
the laboratory spectra are used to analyse the observed CO and CO2 band
profiles and try to constrain the formation scenarios of CO2.Comment: Accepted in A&
Rotational spectra of isotopic species of methyl cyanide, CHCN, in their ground vibrational states up to terahertz frequencies
Methyl cyanide is an important trace molecule in star-forming regions. It is
one of the more common molecules used to derive kinetic temperatures in such
sources. As preparatory work for Herschel, SOFIA, and in particular ALMA we
want to improve the rest frequencies of the main as well as minor isotopologs
of methyl cyanide. The laboratory rotational spectrum of methyl cyanide in
natural isotopic composition has been recorded up to 1.63 THz. Transitions with
good signal-to-noise ratio could be identified for CHCN, CHCN,
CHCN, CHCN, CHDCN, and CHCN in their
ground vibrational states up to about 1.2 THz. The main isotopic species could
be identified even in the highest frequency spectral recordings around 1.6 THz.
The highest quantum numbers included in the fit are 64 for
CHCN and 89 for the main isotopic species. Greatly improved
spectroscopic parameters have been obtained by fitting the present data
together with previously reported transition frequencies. The present data will
be helpful to identify isotopologs of methyl cyanide in the higher frequency
bands of instruments such as the recently launched Herschel satellite, the
upcoming airplane mission SOFIA or the radio telescope array ALMA.Comment: 13 pages, 2 figures, article appeared; CDMS links update
Effects of CO2 on H2O band profiles and band strengths in mixed H2O:CO2 ices
H2O is the most abundant component of astrophysical ices. In most lines of
sight it is not possible to fit both the H2O 3 um stretching, the 6 um bending
and the 13 um libration band intensities with a single pure H2O spectrum.
Recent Spitzer observations have revealed CO2 ice in high abundances and it has
been suggested that CO2 mixed into H2O ice can affect relative strengths of the
3 um and 6 um bands. We used laboratory infrared transmission spectroscopy of
H2O:CO2 ice mixtures to investigate the effects of CO2 on H2O ice spectral
features at 15-135 K. We find that the H2O peak profiles and band strengths are
significantly different in H2O:CO2 ice mixtures compared to pure H2O ice. In
all H2O:CO2 mixtures, a strong free-OH stretching band appears around 2.73 um,
which can be used to put an upper limit on the CO2 concentration in the H2O
ice. The H2O bending mode profile also changes drastically with CO2
concentration; the broad pure H2O band gives way to two narrow bands as the CO2
concentration is increased. This makes it crucial to constrain the environment
of H2O ice to enable correct assignments of other species contributing to the
interstellar 6 um absorption band. The amount of CO2 present in the H2O ice of
B5:IRS1 is estimated by simultaneously comparing the H2O stretching and bending
regions and the CO2 bending mode to laboratory spectra of H2O, CO2, H2O:CO2 and
HCOOH.Comment: 12 pages, 11 figures, accepted by A&
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