1,077 research outputs found
Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. IX. The giant LMC HII region complex N11
We present maps and a catalogue containing the J=1-0 12CO parameters of 29
individual molecular clouds in the second-brightest LMC star formation complex,
N11. In the southwestern part of N11, molecular clouds occur in a ring or shell
surrounding the major OB star association LH9. In the northeastern part, a
chain of molecular clouds delineates the rim of one of the so-called supergiant
shells in the LMC. The well-defined clouds have dimensions close to those of
the survey beam (diameters of 25 pc or less). Some of the clouds were also
observed in J=2-1 12CO, and in the lower two transitions of 13CO. Clouds mapped
with a twice higher angular resolution in J=2-1 12CO show substructure with
dimensions once again comparable to those of the mapping beam. The few clouds
for which we could model physical parameters have fairly warm (T(kin) = 60 -
150 K) and moderately dense (n(H2) = 3000 cm-3) gas. The northeastern chain of
CO clouds, although lacking in diffuse intercloud emission, is characteristic
of the more quiescent regions of the LMC and appears to have been subject to
relatively little photo-processing. The clouds forming part of the southwestern
shell or ring, however, are almost devoid of diffuse intercloud emission and
also exhibit other characteristics of an extreme photon-dominated region (PDR).Comment: 14 pages; accepted for publication in A&
Fine structure in the gamma-ray sky
The EGRET results for gamma-ray intensities in and near the Galactic Plane
have been analysed in some detail. Attention has been concentrated on energies
above 1 GeV and the individual intensities in a longitude bin have
been determined and compared with the large scale mean found from a nine-degree
polynomial fit.
Comparison has been made of the observed standard deviation for the ratio of
these intensities with that expected from variants of our model. The basic
model adopts cosmic ray origin from supernova remnants, the particles then
diffusing through the Galaxy with our usual 'anomalous diffusion'. The variants
involve the clustering of SN, a frequency distribution for supernova explosion
energies, and 'normal', rather than 'anomalous' diffusion.
It is found that for supernovae of unique energy, and our usual anomalous
diffusion, clustering is necessary, particularly in the Inner Galaxy. An
alternative, and preferred, situation is to adopt the model with a frequency
distribution of supernova energies. The results for the Outer Galaxy are such
that no clustering is required.Comment: 10 pages, 4 figures, 1 table, accepted for publication in J.Phys.G:
Nucl.Part.Phy
Si and Fe depletion in Galactic star-forming regions observed by the Spitzer Space Telescope
We report the results of the mid-infrared spectroscopy of 14 Galactic
star-forming regions with the high-resolution modules of the Infrared
Spectrograph (IRS) on board the Spitzer Space Telescope. We detected [SiII]
35um, [FeII] 26um, and [FeIII] 23um as well as [SIII] 33um and H2 S(0) 28um
emission lines. Using the intensity of [NII] 122um or 205um and [OI] 146um or
63um reported by previous observations in four regions, we derived the ionic
abundance Si+/N+ and Fe+/N+ in the ionized gas and Si+/O0 and Fe+/O0 in the
photodissociation gas. For all the targets, we derived the ionic abundance of
Si+/S2+ and Fe2+/S2+ for the ionized gas. Based on photodissociation and HII
region models the gas-phase Si and Fe abundance are suggested to be 3-100% and
<8% of the solar abundance, respectively, for the ionized gas and 16-100% and
2-22% of the solar abundance, respectively, for the photodissociation region
gas. Since the [FeII] 26um and [FeIII] 23um emissions are weak, the high
sensitivity of the IRS enables to derive the gas-phase Fe abundance widely in
star-forming regions. The derived gas-phase Si abundance is much larger than
that in cool interstellar clouds and that of Fe. The present study indicates
that 3-100% of Si atoms and <22% of Fe atoms are included in dust grains which
are destroyed easily in HII regions, probably by the UV radiation. We discuss
possible mechanisms to account for the observed trend; mantles which are
photodesorbed by UV photons, organometallic complexes, or small grains.Comment: 43 pages with 7 figures, accepted in Astrophysical Journa
The RMS Survey: 13CO observations of candidate massive YSOs in the southern hemisphere
Abridged: The Red MSX Source (RMS) survey is an ongoing multi-wavelength
observational programme designed to return a large, well-selected sample of
massive young stellar objects (MYSOs). Here we present 13CO observations made
towards 854 MYSOs candidates located in the 3rd and 4th quadrants. We detected
13CO emission towards a total of 751 of the 854 RMS sources observed (~88%). In
total 2185 emission components are detected above 3 level. Multiple
emission profiles are observed towards the majority of these sources - 455
sources (~60%) - with an average of ~4 molecular clouds along the line of
sight. These multiple emission features make it difficult to assign a kinematic
velocity to many of our sample. We have used archival CS (J=2-1) and maser
velocities to resolved the component multiplicity towards 82 sources and have
derived a criterion which is used to identify the most likely component for a
further 202 multiple component sources. Combined with the single component
detections we have obtained unambiguous kinematic velocities towards 580
sources (~80% of the detections). The 171 sources for which we have not been
able to determine the kinematic velocity will require additional line data.
Using the rotation curve of Brand and Blitz (1993) and their radial velocities
we calculate kinematic distances for all components detected.Comment: Summitted to A&A, the resolution of figure 1 has been reduced,
samples of Table 3 and Figure 11 are now included but the full version will
only be available in the online version of the journa
Tropospheric Phase Calibration in Millimeter Interferometry
We review millimeter interferometric phase variations caused by variations in
the precipitable water vapor content of the troposphere, and we discuss
techniques proposed to correct for these variations. We present observations
with the Very Large Array at 22 GHz and 43 GHz designed to test these
techniques. We find that both the Fast Switching and Paired Array calibration
techniques are effective at reducing tropospheric phase noise for radio
interferometers. In both cases, the residual rms phase fluctuations after
correction are independent of baseline length for b > b_{eff}. These techniques
allow for diffraction limited imaging of faint sources on arbitrarily long
baselines at mm wavelengths. We consider the technique of tropospheric phase
correction using a measurement of the precipitable water vapor content of the
troposphere via a radiometric measurement of the brightness temperature of the
atmosphere. Required sensitivities range from 20 mK at 90 GHz to 1 K at 185 GHz
for the MMA, and 120 mK for the VLA at 22 GHz. The minimum gain stability
requirement is 200 at 185 GHz at the MMA assuming that the astronomical
receivers are used for radiometry. This increases to 2000 for an uncooled
system. The stability requirement is 450 for the cooled system at the VLA at 22
GHz. To perform absolute radiometric phase corrections also requires knowledge
of the tropospheric parameters and models to an accuracy of a few percent. It
may be possible to perform an `empirically calibrated' radiometric phase
correction, in which the relationship between fluctuations in brightness
temperature differences with fluctuations in interferometric phases is
calibrated by observing a celestial calibrator at regular intervals.Comment: AAS LATEX preprint format. to appear in Radio Science 199
Topological Orthoalgebras
We define topological orthoalgebras (TOAs) and study their properties. While
every topological orthomodular lattice is a TOA, the lattice of projections of
a Hilbert space is an example of a lattice-ordered TOA that is not a toplogical
lattice. On the other hand, we show that every compact Boolean TOA is a
topological Boolean algebra. We also show that a compact TOA in which 0 is an
isolated point is atomic and of finite height. We identify and study a
particularly tractable class of TOAs, which we call {\em stably ordered}: those
in which the upper-set generated by an open set is open. This includes all
topological OMLs, and also the projection lattices of Hilbert spaces. Finally,
we obtain a topological version of the Foulis-Randall representation theory for
stably ordered TOAsComment: 16 pp, LaTex. Minor changes and corrections in sections 1; more
substantial corrections in section
Search for massive protostellar candidates in the southern hemisphere: I. Association with dense gas
(Abridged) We have observed CS and C17O lines, and 1.2 mm cont. emission
towards a sample of 130 high-mass protostellar candidates with DEC<-30 deg.
This is the first step of the southern extension of a project started more than
a decade ago aimed at the identification of massive protostellar candidates. We
selected from the IRAS PSC 429 sources which potentially are compact molecular
clouds. The sample is divided into two groups: the 298 sources with
[25-12]>0.57 and [60-12]>1.30 we call 'High' sources, the remaining 131 we call
'Low' sources. In this paper, we check the association with dense gas and dust
in 130 'Low' sources. We find a detection rate of ca. 85% in CS, demonstrating
a tight association with dense molecular clumps. Among the sources detected in
CS, ca. 76% have also been detected in C17O and ca. 93% in the 1.2 mm cont.
Mm-cont. maps show the presence of clumps with diameters 0.2-2 pc and masses
from a few Msun to 10^5 Msun; H2 volume densities lie between ca. 10^{4.5} and
10^{5.5} cm^{-3}. The L(bol) are 10^3-10^6 Lsun, consistent with embedded
high-mass objects. Based on our results and those found in the literature for
other samples, we conclude that our sources are massive objects probably in a
stage prior to the formation of an HII region. We propose a scenario in which
'High' and 'Low' sources are both made of a massive clump hosting a high-mass
protostellar candidate and a nearby stellar cluster. The difference might be
due to the fact that the IRAS 12mu flux, the best discriminant between the two
groups, is dominated by the emission from the cluster in 'Lows' and from the
massive protostellar object in 'Highs'.Comment: Accepted for publication in Astron. & Astroph.; 34 pages (incl. 14
figures and 8 tables
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