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 YbAlO3_3

In this paper we report low-temperature magnetic properties of the rare-earth perovskite material YbAlO$_3$. 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 $ab$-plane, pointing at an angle of $\varphi = \pm 23.5^{\circ}$ to the $a$-axis. With temperature decreasing below $T_{\rm N}=0.88$ K, Yb moments order into the coplanar, but non-collinear antiferromagnetic (AFM) structure $AxGy$, 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 $c$-axis is dominant, and suggests YbAlO$_3$ as a new member of one dimensional quantum magnets.Comment: 8 pages, 6 figure

Magneto-structural coupling and harmonic lattice dynamics in CaFe2_2As2_2 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 CaFe$_2$As$_2$ 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 $1.67\times10^{-2}\,GPa^{-1}$. 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, $\Theta_D \sim$270\,K.Comment: 6 pages,5 figures Accepted by J. Phys.: Condens. Matte

Static and Dynamic Magnetism in Underdoped Superconductor BaFe1.92_{1.92}Co0.08_{0.08}As2_2

We report neutron scattering measurements on single crystals of BaFe$_{1.92}$Co$_{0.08}$As$_2$. The magnetic Bragg peak intensity is reduced by 6 % upon cooling through T$_C$. The spin dynamics exhibit a gap of 8 meV with anisotropic three-dimensional (3d) interactions. Below T$_C$ additional intensity appears at an energy of $\sim$4.5(0.5) meV similar to previous observations of a spin resonance in other Fe-based superconductors. No further gapping of the spin excitations is observed below T$_C$ 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