1,635 research outputs found
Helium and Hydrogen Line Ratios and The Stellar Content of Compact HII Regions
We present observations and models of the behaviour of the HI and HeI lines
between 1.6 and 2.2um in a small sample of compact HII regions. As in our
previous papers on planetary nebulae, we find that the `pure' 1.7007um
4^3D-3^3P and 2.16475um 7^(3,1)G-4^(3,1)F HeI recombination lines behave
approximately as expected as the effective temperature of the central exciting
star(s) increases. However, the 2.058um 2^1P-2^1S HeI line does not behave as
the model predicts, or as seen in planetary nebulae. Both models and planetary
nebulae showed a decrease in the HeI 2^1P-2^1S/HI Br gamma ratio above an
effective temperature of 40000K. The compact HII regions do not show any such
decrease. The problem with this line ratio is probably due to the fact that the
photoionisation model does not account correctly for the high densities seen in
these HII regions, and that we are therefore seeing more collisional excitation
of the 2^1P level than the model predicts. It may also reflect some deeper
problem in the assumed model stellar atmospheres. In any event, although the
normal HeI recombination lines can be used to place constraints on the
temperature of the hottest star present, the HeI 2^1P-2^1S/HI Br gamma ratio
should not be used for this purpose in either Galactic HII regions or in
starburst galaxies, and conclusions from previous work using this ratio should
be regarded with extreme caution. We also show that the combination of the near
infrared `pure' recombination line ratios with mid-infrared forbidden line data
provides a good discriminant of the form of the far ultraviolet spectral energy
distribution of the exciting star(s). From this we conclude that CoStar models
are a poor match to the available data for our sources, though the more recent
WM-basic models are a better fit.Comment: Accepted for publication in MNRA
Antiferromagnetic ordering and dipolar interactions of YbAlO
In this paper we report low-temperature magnetic properties of the rare-earth
perovskite material YbAlO. Results of elastic and inelastic neutron
scattering experiment, magnetization measurements along with the crystalline
electrical field (CEF) calculations suggest that the ground state of Yb moments
is a strongly anisotropic Kramers doublet, and the moments are confined in the
-plane, pointing at an angle of to the
-axis. With temperature decreasing below K, Yb moments
order into the coplanar, but non-collinear antiferromagnetic (AFM) structure
, where the moments are pointed along their easy-axes. In addition, we
highlight the importance of the dipole-dipole interaction, which selects the
type of magnetic ordering and may be crucial for understanding magnetic
properties of other rare-earth orthorhombic perovskites. Further analysis of
the broad diffuse neutron scattering shows that one-dimensional interaction
along the -axis is dominant, and suggests YbAlO as a new member of one
dimensional quantum magnets.Comment: 8 pages, 6 figure
Magneto-structural coupling and harmonic lattice dynamics in CaFeAs probed by M\"ossbauer spectroscopy
In this paper we present detailed M\"ossbauer spectroscopy study of
structural and magnetic properties of the undoped parent compound
CaFeAs single crystal. By fitting the temperature dependence of the
hyperfine magnetic field we show that the magneto-structural phase transition
is clearly first-order in nature and we also deduced the compressibility of our
sample to be . Within the Landau's theory of phase
transition, we further argue that the observed phase transition may stem from
the strong magneto-structural coupling effect. Temperature dependence of the
Lamb-M\"ossbauer factor show that the paramagnetic phase and the
antiferromagnetic phase exhibit similar lattice dynamics in high frequency
modes with very close Debye temperatures, 270\,K.Comment: 6 pages,5 figures Accepted by J. Phys.: Condens. Matte
Static and Dynamic Magnetism in Underdoped Superconductor BaFeCoAs
We report neutron scattering measurements on single crystals of
BaFeCoAs. The magnetic Bragg peak intensity is reduced by
6 % upon cooling through T. The spin dynamics exhibit a gap of 8 meV with
anisotropic three-dimensional (3d) interactions. Below T additional
intensity appears at an energy of 4.5(0.5) meV similar to previous
observations of a spin resonance in other Fe-based superconductors. No further
gapping of the spin excitations is observed below T for energies down to 2
meV. These observations suggest the redistribution of spectral weight from the
magnetic Bragg position to a spin resonance demonstrating the direct
competition between static magnetic order and superconductivity.Comment: 4 pages, 4 figure
Completeness in Photometric and Spectroscopic Searches for Clusters
We investigate, using simulated galaxy catalogues, the completeness of
searches for massive clusters of galaxies in redshift surveys or imaging
surveys with photometric redshift estimates, i.e. what fraction of clusters
(M>10^14/h Msun) are found in such surveys. We demonstrate that the matched
filter method provides an efficient and reliable means of identifying massive
clusters even when the redshift estimates are crude. In true redshift surveys
the method works extremely well. We demonstrate that it is possible to
construct catalogues with high completeness, low contamination and both varying
little with redshift.Comment: ApJ in press, 15 pages, 10 figure
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