32,808 research outputs found
VLT observations of the highly ionized nebula around Brey2
We present the first high resolution HeII 4686 images of the high excitation
nebula around the WR star Brey 2 in the LMC. This nebula presents a striking
morphology: a small arc-like feature some 3.6pc in radius is particularly
prominent in the HeII 4686 line. We further discover a previously unknown faint
HeII emission that extends over an area of 22*17 pc^2. An even fainter HeII
emission is apparently associated with the interstellar bubble blown by the
progenitor of Brey2. The total HeII flux corresponds to an ionizing flux of
4*10^{47} photons/s. Halpha, [OIII], and HeI 5876 images and long-slit spectra
are also examined in this letter, enabling us to investigate the detailed
physical properties at various locations of the nebula.Comment: 4 pages, 3 figures (2 in jpg), accepted by A&A Letters, also
available from http://vela.astro.ulg.ac.be/Preprints/P80/index.htm
XMM-Newton Detection of Hot Gas in the Eskimo Nebula: Shocked Stellar Wind or Collimated Outflows?
The Eskimo Nebula (NGC 2392) is a double-shell planetary nebula (PN) known
for the exceptionally large expansion velocity of its inner shell, ~90 km/s,
and the existence of a fast bipolar outflow with a line-of-sight expansion
velocity approaching 200 km/s. We have obtained XMM-Newton observations of the
Eskimo and detected diffuse X-ray emission within its inner shell. The X-ray
spectra suggest thin plasma emission with a temperature of ~2x10^6 K and an
X-ray luminosity of L_X = (2.6+/-1.0)x10^31 (d/1150 pc)^2 ergs/s, where d is
the distance in parsecs. The diffuse X-ray emission shows noticeably different
spatial distributions between the 0.2-0.65 keV and 0.65-2.0 keV bands.
High-resolution X-ray images of the Eskimo are needed to determine whether its
diffuse X-ray emission originates from shocked fast wind or bipolar outflows.Comment: 4 pages, 2 figures, accepted in Astronomy and Astrophysics Letter
Solidification mechanism of highly undercooled metal alloys
Experiments were conducted on metal droplet undercooling, using Sn-25wt%Pb and Ni-34wt%Sn alloys. To achieve the high degree of undercooling, emulsification treatments were employed. Results show the fraction of supersaturated primary phase is a function of the amount of undercooling, as is the fineness of the structures. The solidification behavior of the tin-lead droplets during recalescence was analyzed using three different hypotheses; (1) solid forming throughout recalescence is of the maximum thermodynamically stable composition; (2) partitionless solidification below the T sub o temperature, and solid forming thereafter is of the maximum thermodynamically stable composition; and (3) partitionless solidification below the T sub o temperature with solid forming thereafter that is of the maximum thermodynamically metastable composition that is possible. The T sub o temperature is calculated from the equal molar free energies of the liquid solid using the regular solution approximation
Strong spin-lattice coupling in multiferroic HoMnO: Thermal expansion anomalies and pressure effect
Evidence for a strong spin-lattice coupling in multiferroic HoMnO_3 is
derived from thermal expansion measurements along a- and c-axis. The
magnetoelastic effect results in sizable anomalies of the thermal expansivities
at the antiferromagnetic (T_N) and the spin rotation (T_{SR}) transition
temperatures as well as in a negative c-axis expansivity below room
temperature. The coupling between magnetic orders and dielectric properties
below T_N is explained by the lattice strain induced by the magnetoelastic
effect. At T_{SR} various physical quantities show discontinuities that are
thermodynamically consistent with a first order phase transition
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