12,057 research outputs found
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The ParsecâScale Accretion Disk in NGC 3393
We present a Very Long Baseline Interferometry image of the water maser emission in the nuclear region of NGC 3393. The maser emission has a linear distribution oriented at a position angle of ~â34°, perpendicular to both the kiloparsec-scale radio jet and the axis of the narrow-line region. The position-velocity diagram displays a red-blue asymmetry about the systemic velocity and the estimated dynamical center, and is thus consistent with rotation. Assuming Keplerian rotation in an edge-on disk, we obtain an enclosed mass of ( within 0.36 ± 0.02 pc (1.48 ± 0.06 mas), which corresponds to a mean mass density of . We also report the measurement with the Green Bank Telescope of a velocity drift, a manifestation of centripetal acceleration within the disk, of in the maser feature, which is most likely located along the line of sight to the dynamical center of the system. From the acceleration of this feature, we estimate a disk radius of 0.17 ± 0.02 pc, which is smaller than the inner disk radius (0.36 ± 0.02 pc) of emission that occurs along the midline (i.e., the line of nodes). The emission along the line of sight to the dynamical center evidently occurs much closer to the center than the emission from the disk midline, contrary to the situation in the archetypal maser systems NGC 4258 and NGC 1068. The outer radius of the disk as traced by the masers along the midline is about 1.5 pc.Astronom
Turning hardened steel using coated carbide at high cutting speeds
The present work studies some aspects of the turning process applied on hardened steel using multilayer coated carbide tools at high cutting speeds. The influence of cutting parameters (vc, f, and depth of cut - d.o.c.) on tool temperature, tool wear, cutting forces, and surface roughness were analyzed. The current literature reports many studies using PcBN on hardened steel, but it is also important to know the results when using coated carbide tools, mainly for economical reasons. Temperature was measured by a thermocouple positioned at the lowest insert face, underneath it. Temperature near the rake face was calculated using the measured gradient within the insert thickness. To measure the gradient a special technique was used with one embedded thermocouple near the rake face and one underneath. Tool wear measurements demonstrated the capability of such tools in turning hardened steel with reasonable tool life. Forces measured resulted in relatively low values, being the radial component the largest of all. For the different cutting conditions studied, the doc has the greatest influence on force and temperature. Additionally, the best surface roughness values were smaller than 0,4 ”m Ra
Colligative properties of solutions: II. Vanishing concentrations
We continue our study of colligative properties of solutions initiated in
math-ph/0407034. We focus on the situations where, in a system of linear size
, the concentration and the chemical potential scale like and
, respectively. We find that there exists a critical value \xit such
that no phase separation occurs for \xi\le\xit while, for \xi>\xit, the two
phases of the solvent coexist for an interval of values of . Moreover, phase
separation begins abruptly in the sense that a macroscopic fraction of the
system suddenly freezes (or melts) forming a crystal (or droplet) of the
complementary phase when reaches a critical value. For certain values of
system parameters, under ``frozen'' boundary conditions, phase separation also
ends abruptly in the sense that the equilibrium droplet grows continuously with
increasing and then suddenly jumps in size to subsume the entire system.
Our findings indicate that the onset of freezing-point depression is in fact a
surface phenomenon.Comment: 27 pages, 1 fig; see also math-ph/0407034 (both to appear in JSP
Probing the Magnetic Field at Sub-Parsec Radii in the Accretion Disk of NGC 4258
We present an analysis of polarimetric observations at 22 GHz of the water
vapor masers in NGC 4258 obtained with the VLA and the GBT. We do not detect
any circular polarization in the spectrum indicative of Zeeman-induced
splitting of the maser lines of water, a non-paramagnetic molecule. We have
improved the 1-sigma upper limit estimate on the toroidal component of the
magnetic field in the circumnuclear disk of NGC 4258 at a radius of 0.2 pc from
300 mG to 90 mG. We have developed a new method for the analysis of spectra
with blended features and derive a 1-sigma upper limit of 30 mG on the radial
component of the magnetic field at a radius of 0.14 pc. Assuming thermal and
magnetic pressure balance, we estimate an upper limit on the mass accretion
rate of ~10^(-3.7) M_sun/yr for a total magnetic field of less than 130 mG. We
discuss the ramifications of our results on current maser models proposed to
explain the observed maser emission structure and the consequences for current
accretion theories. We find from our magnetic field limits that the thin-disk
model and the jet-disk model are better candidates for accounting for the
extremely low-luminosity nature of NGC 4258, than models that include
advection-dominated accretion flows.Comment: 20 pages, including 10 figures and 2 tables. Accepted for publication
in the Astrophysical Journa
Proper Motion of Water Masers Associated with IRAS 21391+5802: Bipolar Outflow and an AU-Scale Dusty Circumstellar Shell
We present VLBA observations of water maser emission associated with the
star-forming region IRAS 21391+5802, which is embedded in a bright rimmed
cometary globule in IC1396. The angular resolution of the maps is about 0.8
mas, corresponding to a spatial resolution of about 0.6 AU, at an estimated
distance of 750 pc. Proper motions are derived for 10 maser features identified
consistently over three epochs, which were separated by intervals of about one
month. The masers appear in four groups, which are aligned linearly on the sky,
roughly along a northeast-southwest direction, with a total separation of about
520 AU (about 0.7 arcseconds). The 3-D velocities of the masers have a maximum
value of about 42 km/s (about 9 AU/yr). The average error on the derived proper
motions is about 4 km/s. The overall pattern of proper motions is indicative of
a bipolar outflow. Proper motions of the masers in a central cluster, with a
projected extent of about 20 AU, show systematic deviations from a radial
outflow. However, we find no evidence of Keplerian rotation, as has been
claimed elsewhere. A nearly circular loop of masers lies near the middle of the
cluster. The radius of this loop is 1 AU and the line-of-sight velocities of
the masers in the loop are within 2 km/s of the systemic velocity of the
region. These masers presumably exist at the radial distance where significant
dust condensation occurs in the outflow emanating from the star.Comment: 15 pages, 4 figures. Accepted for publication in the Astrophysical
Journal. Version 2.12.00: Astrometric coordinates of maser revise
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Visitor Support for Recreation Fees in Georgia State Parks
Abstract
Public land managers are experiencing increasing pressure to create parks that are self-sustaining. Recreation fees can help managers achieve this goal, but tradeoffs associated with fees often produce undesired consequences (i.e., declining visitation) that warrant further investigation. This study used intercept surveys (n = 1049) to examine support for recreation fees among visitors to Georgia state parks. Participants were evenly split regarding their willingness to pay to enter state parks, with a mean willing to pay increase about 5 parking fee. Proposed fee increases negatively affected projected visitation among all groups, particularly low-income visitors. Place attachment ratings were the strongest predictor of support for fees. Results suggest that, to accommodate a diverse clientele, managers could work on strengthening visitorsâ attachment to and reliance upon state parks while developing flexible pricing schemes that appeal to a range of potential users
A Binary Millisecond Pulsar in Globular Cluster NGC6544
We report the detection of a new 3.06 ms binary pulsar in the globular
cluster NGC6544 using a Fourier-domain ``acceleration'' search. With an implied
companion mass of ~0.01 solar masses and an orbital period of only P_b~1.7
hours, it displays very similar orbital properties to many pulsars which are
eclipsed by their companion winds. The orbital period is the second shortest of
known binary pulsars after 47 Tuc R. The measured flux density of 1.3 +/- 0.4
mJy at 1332 MHz indicates that the pulsar is almost certainly the known
steep-spectrum point source near the core of NGC6544.Comment: Accepted by ApJ Letters on 11 October 2000, 5 page
The purpose of mess in action research: building rigour though a messy turn
Mess and rigour might appear to be strange bedfellows. This paper argues that the purpose of mess is to facilitate a turn towards new constructions of knowing that lead to transformation in practice (an action turn). Engaging in action research - research that can disturb both individual and communally held notions of knowledge for practice - will be messy. Investigations into the 'messy area', the interface between the known and the nearly known, between knowledge in use and tacit knowledge as yet to be useful, reveal the 'messy area' as a vital element for seeing, disrupting, analysing, learning, knowing and changing. It is the place where long-held views shaped by professional knowledge, practical judgement, experience and intuition are seen through other lenses. It is here that reframing takes place and new knowing, which has both theoretical and practical significance, arises: a 'messy turn' takes place
Local awareness of and attitudes toward pygmy hippopotamus conservation in the Moa River Island Complex, Sierra Leone.
Abstract The pygmy hippopotamus Choeropsis liberiensis is an Endangered species found only in the Upper Guinea rainforests of West Africa. Using a two-phase approach, with initial semi-structured interviews followed by more extensive questionnaires, we examined local residentsâ awareness of and attitudes towards the pygmy hippopotamus along the Moa River near Tiwai Island Wildlife Sanctuary in Sierra Leone. The interviews and questionnaires addressed humanâhippopotamus interactions, local knowledge and awareness of pygmy hippopotamus ecology and behaviour, and public attitudes towards hippopotamus conservation. Overall, 22% of questionnaire respondents acknowledged benefits related to hippopotamus conservation; factors affecting the perception of benefits included age, livestock ownership, distance from Tiwai Island and exposure to conservation programmes. The results of this study could be used to inform the conservation of the pygmy hippopotamus and highlight the critical role of local support in the management of threatened species in biodiversity hotspots
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