324 research outputs found
Valles Marineris, Mars: Are pit chains formed by erosion and troughs by tectonism?
The origin of the Valles Marineris remains controversial. Erosional, tectonic, and hybrid processes have been proposed. To clarify these contradictions, the widths and depths of pit chains and troughs were compared and it was found that the features do not form a continuum. Rather, results are consistent with the hypothesis that pit chains formed by surficial collapse and troughs by deeper seated and coherent failure. All pit chains and linear depressions in the Valles Marineris region were classified by inspection into six morphologic categories: pit chains (linear arrays of small pits); floored chains (arrays of pit chains having flat or hummocky floors); scalloped troughs (wider linear depressions with scalloped wall segments); narrow troughs (depressions of intermediate width with straight wall segments); wide troughs (broad, linear depressions); and chaotic troughs (more irregular depressions displaying some channel morphologies). Topographic profiles were drawn across the classified depressions and the erosional width was determined between plateau margins along with the depth from the surrounding plateau level to the deepest part
Detection of Voigt Spectral Line Profiles of Hydrogen Radio Recombination Lines toward Sagittarius B2(N)
We report the detection of Voigt spectral line profiles of radio
recombination lines (RRLs) toward Sagittarius B2(N) with the 100-m Green Bank
Telescope (GBT). At radio wavelengths, astronomical spectra are highly
populated with RRLs, which serve as ideal probes of the physical conditions in
molecular cloud complexes. An analysis of the Hn(alpha) lines presented herein
shows that RRLs of higher principal quantum number (n>90) are generally
divergent from their expected Gaussian profiles and, moreover, are well
described by their respective Voigt profiles. This is in agreement with the
theory that spectral lines experience pressure broadening as a result of
electron collisions at lower radio frequencies. Given the inherent technical
difficulties regarding the detection and profiling of true RRL wing spans and
shapes, it is crucial that the observing instrumentation produce flat baselines
as well as high sensitivity, high resolution data. The GBT has demonstrated its
capabilities regarding all of these aspects, and we believe that future
observations of RRL emission via the GBT will be crucial towards advancing our
knowledge of the larger-scale extended structures of ionized gas in the
interstellar medium (ISM)
Wide-Field Infrared Imaging Polarimetry of the NGC 6334 Region: A Nest of Infrared Reflection Nebulae
We report the detection of eighteen infrared reflection nebulae (IRNe) in the
, , & linear polarimetric observations of the NGC 6334 massive
star-formation complex, of which 16 IRNe are new discoveries. Our images cover
180 square arcminutes, one of the widest near-infrared polarization data
in star-formation regions so far. These IRNe are most likely associated with
embedded young OB stars at different evolutionary phases, showing a variety of
sizes, morphologies, and polarization properties, which can be divided into
four categories. We argue the different nebula characteristics to be a possible
evolutionary sequence of circumstellar structures around young massive stars.Comment: 4 pages, 1 figur
Measurements of the 12C/13C ratio in Planetary Nebulae and implications for stellar evolution
We present the results of a study aimed at determining the 12C/13C ratio in
two samples of planetary nebuale (PNe) by means of mm-wave observations of 12CO
and 13CO. The first group includes six PNe which have been observed in the 3He+
hyperfine transition; the other group consists of 23 nebulae with rich
molecular envelopes. We have determined the isotopic ratio in 14 objects and
the results indicate a range of values between 9 and 23. In particular, three
PNe have ratios well below the value predicted by standard evolutionary models
(about 20), indicating that some extra-mixing process has occurred in these
stars. We briefly discuss the implications of our results for standard and
nonstandard stellar nucleosynthesis.Comment: 8 pages, LaTeX, 1 Postscript figure. to appear in The Primordial
Nuclei and Their Galactic Evolution, eds. N. Prantzos, M. Tosi, R. von
Steiger (Kluwer: Dordrecht
3-He in the Milky Way Interstellar Medium: Ionization Structure
The cosmic abundance of the 3-He isotope has important implications for many
fields of astrophysics. We are using the 8.665 GHz hyperfine transition of
3-He+ to determine the 3-He/H abundance in Milky Way HII regions and planetary
nebulae. This is one in a series of papers in which we discuss issues involved
in deriving accurate 3-He/H abundance ratios from the available measurements.
Here we describe the ionization correction we use to convert the 3-He+/H+
abundance, y3+, to the 3-He/H abundance, y3. In principle the nebular
ionization structure can significantly influence the y3 derived for individual
sources. We find that in general there is insufficient information available to
make a detailed ionization correction. Here we make a simple correction and
assess its validity. The correction is based on radio recombination line
measurements of H+ and 4-He+, together with simple core-halo source models. We
use these models to establish criteria that allow us to identify sources that
can be accurately corrected for ionization and those that cannot. We argue that
this effect cannot be very large for most of the sources in our observational
sample. For a wide range of models of nebular ionization structure we find that
the ionization correction factor varies from 1 to 1.8. Although large
corrections are possible, there would have to be a conspiracy between the
density and ionization structure for us to underestimate the ionization
correction by a substantial amount.Comment: 36 pages, 4 figures To appear Astrophysical Journal, 20 August 2007,
vol 665, no
Beyond the Heisenberg time: Semiclassical treatment of spectral correlations in chaotic systems with spin 1/2
The two-point correlation function of chaotic systems with spin 1/2 is
evaluated using periodic orbits. The spectral form factor for all times thus
becomes accessible. Equivalence with the predictions of random matrix theory
for the Gaussian symplectic ensemble is demonstrated. A duality between the
underlying generating functions of the orthogonal and symplectic symmetry
classes is semiclassically established
Herschel Observations of the W43 "mini-starburst"
Aims: To explore the infrared and radio properties of one of the closest
Galactic starburst regions. Methods: Images obtained with the Herschel Space
Observatory at wavelengths of 70, 160, 250, 350, and 500 microns using the PACS
and SPIRE arrays are analyzed and compared with radio continuum VLA data and 8
micron images from the Spitzer Space Telescope. The morphology of the
far-infrared emission is combined with radial velocity measurements of
millimeter and centimeter wavelength transitions to identify features likely to
be associated with the W43 complex. Results: The W43 star-forming complex is
resolved into a dense cluster of protostars, infrared dark clouds, and ridges
of warm dust heated by massive stars. The 4 brightest compact sources with L >
1.5 x 10^4 Lsun embedded within the Z-shaped ridge of bright dust emission in
W43 remain single at 4" (0.1 pc) resolution. These objects, likely to be
massive protostars or compact clusters in early stages of evolution are
embedded in clumps with masses of 10^3 to 10^4 Msun, but contribute only 2% to
the 3.6 x 10^6 Lsun far-IR luminosity of W43 measured in a 16 by 16 pc box. The
total mass of gas derived from the far-IR dust emission inside this region is
~10^6 Msun. Cometary dust clouds, compact 6 cm radio sources, and warm dust
mark the locations of older populations of massive stars. Energy release has
created a cavity blowing-out below the Galactic plane. Compression of molecular
gas in the plane by the older HII region near G30.684-0.260 and the bipolar
structure of the resulting younger W43 HII region may have triggered the
current mini-star burst.Comment: 5 pages, 3 figures, accepted for A&A Special Issu
Radio Recombination Lines in Galactic HII Regions
We report radio recombination line (RRL) and continuum observations of a
sample of 106 Galactic HII regions made with the NRAO 140 Foot radio telescope
in Green Bank, WV. We believe this to be the most sensitive RRL survey ever
made for a sample this large. Most of our source integration times range
between 6 and 90 hours which yield typical r.m.s. noise levels of 1.0--3.5
milliKelvins. Our data result from two different experiments performed,
calibrated, and analyzed in similar ways. A CII survey was made at 3.5 cm
wavelength to obtain accurate measurements of carbon radio recombination lines.
When combined with atomic (CI) and molecular (CO) data, these measurements will
constrain the composition, structure, kinematics, and physical properties of
the photodissociation regions that lie on the edges of HII regions. A second
survey was made at 3.5 cm wavelength to determine the abundance of 3He in the
interstellar medium of the Milky Way. Together with measurements of the 3He+
hyperfine line we get high precision RRL parameters for H, 4He, and C. Here we
discuss significant improvements in these data, with both longer integrations
and newly observed sources.Comment: LaTeX, 50 pages with 11 figures. Accepted for publication in The
Astrophysical Journal Supplement Serie
The Chemical Evolution Carousel of Spiral Galaxies : Azimuthal Variations of Oxygen Abundance in NGC1365
19 pages, 13 figures. Accepted to ApJThe spatial distribution of oxygen in the interstellar medium of galaxies is the key to understanding how efficiently metals that are synthesized in massive stars can be redistributed across a galaxy. We present here a case study in the nearby spiral galaxy NGC1365 using 3D optical data obtained in the TYPHOON Program. We find systematic azimuthal variations of the HII region oxygen abundance imprinted on a negative radial gradient. The 0.2 dex azimuthal variations occur over a wide radial range of 0.3 to 0.7 R25 and peak at the two spiral arms in NGC1365. We show that the azimuthal variations can be explained by two physical processes: gas undergoes localized, sub-kpc scale self-enrichment when orbiting in the inter-arm region, and experiences efficient, kpc scale mixing-induced dilution when spiral density waves pass through. We construct a simple chemical evolution model to quantitatively test this picture and find that our toy model can reproduce the observations. This result suggests that the observed abundance variations in NGC1365 are a snapshot of the dynamical local enrichment of oxygen modulated by spiral-driven, periodic mixing and dilution.Peer reviewedFinal Published versio
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