6,263 research outputs found
Preparation of monotectic alloys having a controlled microstructure by directional solidification under dopant-induced interface breakdown
Monotectic alloys having aligned spherical particles of rods of the minor component dispersed in a matrix of the major component are prepared by forming a melt containing predetermined amounts of the major and minor components of a chosen monotectic system, providing in the melt a dopant capable of breaking down the liquid solid interface for the chosen alloy, and directionally solidfying the melt at a selected temperature gradient and a selected rate of movement of the liquid-solid interface (growth rate). Shaping of the minor component into spheres or rods and the spacing between them are controlled by the amount of dopant and the temperature gradient and growth rate values. Specific alloy systems include Al Bi, Al Pb and Zn Bi, using a transition element such as iron
Supergiant Shells and Molecular Cloud Formation in the LMC
We investigate the influence of large-scale stellar feedback on the formation
of molecular clouds in the Large Magellanic Cloud (LMC). Examining the
relationship between HI and 12CO(J=1-0) in supergiant shells (SGSs), we find
that the molecular fraction in the total volume occupied by SGSs is not
enhanced with respect to the rest of the LMC disk. However, the majority of
objects (~70% by mass) are more molecular than their local surroundings,
implying that the presence of a supergiant shell does on average have a
positive effect on the molecular gas fraction. Averaged over the full SGS
sample, our results suggest that ~12-25% of the molecular mass in supergiant
shell systems was formed as a direct result of the stellar feedback that
created the shells. This corresponds to ~4-11% of the total molecular mass of
the galaxy. These figures are an approximate lower limit to the total
contribution of stellar feedback to molecular cloud formation in the LMC, and
constitute one of the first quantitative measurements of feedback-triggered
molecular cloud formation in a galactic system.Comment: 14 pages, 6 figures. Accepted for publication in Ap
Fitting Together the HI Absorption and Emission in the SGPS
In this paper we study 21-cm absorption spectra and the corresponding
emission spectra toward bright continuum sources in the test region (326deg< l
< 333 deg) of the Southern Galactic Plane Survey. This survey combines the high
resolution of the Australia Telescope Compact Array with the full brightness
temperature information of the Parkes single dish telescope. In particular, we
focus on the abundance and temperature of the cool atomic clouds in the inner
galaxy. The resulting mean opacity of the HI, , is measured as a
function of Galactic radius; it increases going in from the solar circle, to a
peak in the molecular ring of about four times its local value. This suggests
that the cool phase is more abundant there, and colder, than it is locally.
The distribution of cool phase temperatures is derived in three different
ways. The naive, ``spin temperature'' technique overestimates the cloud
temperatures, as expected. Using two alternative approaches we get good
agreement on a histogram of the cloud temperatures, T(cool), corrected for
blending with warm phase gas. The median temperature is about 65 K, but there
is a long tail reaching down to temperatures below 20 K. Clouds with
temperatures below 40 K are common, though not as common as warmer clouds (40
to 100 K).
Using these results we discuss two related quantities, the peak brightness
temperature seen in emission surveys, and the incidence of clouds seen in HI
self-absorption. Both phenomena match what would be expected based on our
measurements of and T(cool).Comment: 50 pages, 20 figure
- …