3,930 research outputs found
Relative astrometry of the J=1-0, v=1 and v=2 SiO masers towards R Leonis Minoris using VERA
Oxygen-rich Asymptotic Giant Branch (AGB) stars are intense emitters of SiO
and HO maser lines at 43 (J=1-0, v=1 and 2) and 22 GHz, respectively. VLBI
observations of the maser emission provides a unique tool to sample the
innermost layers of the circumstellar envelopes in AGB stars. Nevertheless, the
difficulties in achieving astrometrically aligned v=1 and v=2 SiO maser maps
have traditionally prevented a unique interpretation of the observations in
terms of physical underlying conditions, which depend on the nature of the SiO
pumping mechanism.
We have carried out observations of the SiO and HO maser emission towards
RLMi, using the astrometric capabilities of VERA. Due to the too-weak emission
of the reference calibrator we had to develop a special method to accurately
relate the coordinates for both transitions. We present relative
astrometrically aligned v=1 and v=2 J=1-0 SiO maser maps, at multiple epochs,
and discuss the astrophysical results. The incorporation of astrometric
information into the maps of SiO masers challenges the weak points in the
current theoretical models, which will need further refinements to address the
observations results.Comment: 17 pages, 8 figure
Hausdorff dimension and p-adic diophantine approximation
Abstract included in text
Curriculum Decisions: Assessing and Updating IS Curriculum
The increased level of accountability in higher education has prompted many vomputer-related programs to seek accreditation. As accrediting agencies continue to define and refine exactly what these degrees should entail, institutions struggle with the decisions of aligning their curriculum with a dynamic set of standards set by an outside agency, focusing on the needs identified by their own assessment data, or attempting to marry the two. This paper details how one institution completed comprehensive reviews of their curriculum under two different projects: Expanding Pathways for Educational Development and Information Technology Experiences, an NSF-funded research project investigation and examination of curriculum and regional needs and an ABET evaluation and site visit. The focus of this report is to inform others of local efforts that may be replicated on their campuses, share findings that may be of use to others in similarly situated circumstances, and add to the assessment and accreditation dialog
Cloning, preparation and preliminary crystallographic studies of penicillin V acylase autoproteolytic processing mutants
The crystallization of three catalytically inactive mutants of penicillin Vacylase (PVA) from Bacillus sphaericus in precursor and processed forms is reported. The mutant proteins crystallize in different primitive monoclinic space groups that are distinct from the crystal forms for the native enzyme. Directed mutants and clone constructs were designed to study the post-translational autoproteolytic processing of PVA. The catalytically inactive mutants will provide threedimensional structures of precursor PVA forms, plus open a route to the study of enzyme-substrate complexes for this industrially important enzyme
Physical properties of high-mass clumps in different stages of evolution
(Abridged) Aims. To investigate the first stages of the process of high-mass
star formation, we selected a sample of massive clumps previously observed with
the SEST at 1.2 mm and with the ATNF ATCA at 1.3 cm. We want to characterize
the physical conditions in such sources, and test whether their properties
depend on the evolutionary stage of the clump.
Methods. With ATCA we observed the selected sources in the NH3(1,1) and (2,2)
transitions and in the 22 GHz H2O maser line. Ammonia lines are a good
temperature probe that allow us to accurately determine the mass and the
column-, volume-, and surface densities of the clumps. We also collected all
data available to construct the spectral energy distribution of the individual
clumps and to determine if star formation is already occurring, through
observations of its most common signposts, thus putting constraints on the
evolutionary stage of the source. We fitted the spectral energy distribution
between 1.2 mm and 70 microns with a modified black body to derive the dust
temperature and independently determine the mass.
Results. The clumps are cold (T~10-30 K), massive (M~10^2-10^3 Mo), and dense
(n(H2)>~10^5 cm^-3) and they have high column densities (N(H2)~10^23 cm^-2).
All clumps appear to be potentially able to form high-mass stars. The most
massive clumps appear to be gravitationally unstable, if the only sources of
support against collapse are turbulence and thermal pressure, which possibly
indicates that the magnetic field is important in stabilizing them.
Conclusions. After investigating how the average properties depend on the
evolutionary phase of the source, we find that the temperature and central
density progressively increase with time. Sources likely hosting a ZAMS star
show a steeper radial dependence of the volume density and tend to be more
compact than starless clumps.Comment: Published in A&A, Vol. 556, A1
Hiding in the Shadows II: Collisional Dust as Exoplanet Markers
Observations of the youngest planets (1-10 Myr for a transitional disk)
will increase the accuracy of our planet formation models. Unfortunately,
observations of such planets are challenging and time-consuming to undertake
even in ideal circumstances. Therefore, we propose the determination of a set
of markers that can pre-select promising exoplanet-hosting candidate disks. To
this end, N-body simulations were conducted to investigate the effect of an
embedded Jupiter mass planet on the dynamics of the surrounding planetesimal
disk and the resulting creation of second generation collisional dust. We use a
new collision model that allows fragmentation and erosion of planetesimals, and
dust-sized fragments are simulated in a post process step including
non-gravitational forces due to stellar radiation and a gaseous protoplanetary
disk. Synthetic images from our numerical simulations show a bright double ring
at 850 m for a low eccentricity planet, whereas a high eccentricity planet
would produce a characteristic inner ring with asymmetries in the disk. In the
presence of first generation primordial dust these markers would be difficult
to detect far from the orbit of the embedded planet, but would be detectable
inside a gap of planetary origin in a transitional disk.Comment: Accepted for publication in Ap
Multi-transition study and new detections of class II methanol masers
We have used the ATNF Mopra antenna and the SEST antenna to search in the
directions of several class II methanol maser sources for emission from six
methanol transitions in the frequency range 85-115 GHz. The transitions were
selected from excitation studies as potential maser candidates. Methanol
emission at one or more frequencies was detected from five of the maser
sources, as well as from Orion KL. Although the lines are weak, we find
evidence of maser origin for three new lines in G345.01+1.79, and possibly one
new line in G9.62+0.20.
The observations, together with published maser observations at other
frequencies, are compared with methanol maser modelling for G345.01+1.79 and
NGC6334F. We find that the majority of observations in both sources are
consistent with a warm dust (175 K) pumping model at hydrogen density ~10^6
cm^-3 and methanol column density ~5 x 10^17 cm^-2. The substantial differences
between the maser spectra in the two sources can be attributed to the geometry
of the maser region.Comment: 13 pages, 6 figures, Accepted for publication in MNRA
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