Tracing early evolutionary stages of high-mass star formation with molecular lines

Despite its major role in the evolution of the interstellar medium, the formation of high-mass stars (M > 10 Msol) is still poorly understood. Two types of massive star cluster precursors, the so-called Massive Dense Cores (MDCs), have been observed, which differ in their mid-infrared brightness. The origin of this difference is not established and could be the result of evolution, density, geometry differences, or a combination of these. We compare several molecular tracers of physical conditions (hot cores, shocks) observed in a sample of mid-IR weak emitting MDCs with previous results obtained in a sample of exclusively mid-IR bright MDCs. The aim is to understand the differences between these two types of object. We present single-dish observations of HDO, H2O-18, SO2 and CH3OH lines at lambda = 1.3 - 3.5 mm. We study line profiles and estimate abundances of these molecules, and use a partial correlation method to search for trends in the results. The detection rates of thermal emission lines are found to be very similar between mid-IR quiet and bright objects. The abundances of H2O, HDO (1E-13 to 1E-9 in the cold outer envelopes), SO2 and CH3OH differ from source to source but independently of their mid-IR flux. In contrast, the methanol class I maser emission, a tracer of outflow shocks, is found to be strongly anti-correlated with the 12 micron source brightnesses. The enhancement of the methanol maser emission in mid-IR quiet MDCs may indicate a more embedded nature. Since total masses are similar between the two samples, we suggest that the matter distribution is spherical around mid-IR quiet sources but flattened around mid-IR bright ones. In contrast, water emission is associated with objects containing a hot molecular core, irrespective of their mid-IR brightness. These results indicate that the mid-IR brightness of MDCs is an indicator of their evolutionary stage.Comment: 15 pages, 6 figures, 11 tables, accepted for publication in A&A the 11/06/201

Magnetic frustration in the spinel compounds Ge Co_2 O_4 and Ge Ni_2 O_4

In both spinel compounds GeCo$_2$O$_4$ and GeNi$_2$O$_4$ which order antiferromagnetically (at $T_N = 23.5 K$ and $T_{N_1} = 12.13 K$, $T_{N_2} = 11.46 K$) with different Curie Weiss temperatures ($T_{CW}$=80.5 K and -15 K), the usual magnetic frustration criterion $f=|T_{CW}|/T_N>>1$ is not fulfilled. Using neutron powder diffraction and magnetization measurements up to 55 T, both compounds are found with a close magnetic ground state at low temperature and a similar magnetic behavior (but with a different energy scale), even though spin anisotropy and first neighbor exchange interactions are quite different. This magnetic behavior can be understood when considering the main four magnetic exchange interactions. Frustration mechanisms are then enlightened.Comment: submitted to Phys.Rev.B (2006

High-mass star formation in the Southern Hemisphere sky

We report on a multi-wavelength (IR to cm) and multi-resolution (1 mas to 20 arcsec) exploration of high-mass star formation regions in the Galactic plane, at longitudes observable from the Southern Hemisphere. Our source sample was originally identified through methanol masers in the Galactic plane, which exclusively trace high-mass star-forming regions. (Sub)millimetre continuum and molecular line observations were carried out with SEST/SIMBA, JCMT/SCUBA and ATNF/Mopra mm-wave telescopes and have allowed us to identify massive ($>20$ M$_{\odot}$) and luminous ($>10^3$ L$_{\odot}$) clumps in each star-forming region. We have also constrained the SED with additional archival IR data, the physical conditions ($T_{dust}$, $L$, $M$) and the chemical composition of each massive clump. Several types of objects were characterised based on the $L_{submm}/L_{bol}$ ratio, the dust temperature and the molecular line properties, ranging from class 0-like YSO clusters ($L_{sub}/L_{bol}\sim1$, T=30 K) to hot molecular clumps ($L_{sub}/L_{bol}\sim0.1$, $T=40-200$ K). Preliminary high-angular resolution observations for a subset of the sample with the ATNF/ATCA at 3 mm, the VLA at 15, 22 and 43 GHz and Gemini in MIR have revealed that several (proto)stellar objects are embedded in the massive clumps: massive protostars, hot cores and hyper-compact HII regions. We have thus identified protoclusters of massive YSOs, which are the precursors of the OB associations. This sample of Southern Hemisphere star-forming regions will be extremely valuable for the scientific preparation of the ALMA and HSO observations.Comment: 4 pages, 3 figures, conference proceeding

Numerical Study of a Mixed Ising Ferrimagnetic System

We present a study of a classical ferrimagnetic model on a square lattice in which the two interpenetrating square sublattices have spins one-half and one. This model is relevant for understanding bimetallic molecular ferrimagnets that are currently being synthesized by several experimental groups. We perform exact ground-state calculations for the model and employ Monte Carlo and numerical transfer-matrix techniques to obtain the finite-temperature phase diagram for both the transition and compensation temperatures. When only nearest-neighbor interactions are included, our nonperturbative results indicate no compensation point or tricritical point at finite temperature, which contradicts earlier results obtained with mean-field analysis.Comment: Figures can be obtained by request to [email protected] or [email protected]

Molecular hydrogen beyond the optical edge of an isolated spiral galaxy

We know little about the outermost portions of galaxies because there is little light coming from them. We do know that in many cases atomic hydrogen (HI) extends well beyond the optical radius \cite{Casertano91}. In the centers of galaxies, however, molecular hydrogen (H2) usually dominates by a large factor, raising the question of whether H2 is abundant also in the outer regions but hitherto unseen.Here we report the detection of emission from carbon monoxide (CO), the most abundant tracer of H2, beyond the optical radius of the nearby galaxy NGC 4414. The molecular clouds probably formed in the regions of relatively high HI column density and in the absence of spiral density waves. The relative strength of the lines from the two lowest rotational levels indicates that both the temperature and density of the H2 are quite low compared to conditions closer to the center. The inferred surface density of the molecular material continues the monotonic decrease from the inner regions. We conclude that while molecular clouds can form in the outer region of this galaxy, there is little mass associated with them.Comment: 3 Nature page

Dissociation of the benzene molecule by UV and soft X-rays in circumstellar environment

Benzene molecules, present in the proto-planetary nebula CRL 618, are ionized and dissociated by UV and X-ray photons originated from the hot central star and by its fast wind. Ionic species and free radicals produced by these processes can lead to the formation of new organic molecules. The aim of this work is to study the photoionization and photodissociation processes of the benzene molecule, using synchrotron radiation and time of flight mass spectrometry. Mass spectra were recorded at different energies corresponding to the vacuum ultraviolet (21.21 eV) and soft X-ray (282-310 eV) spectral regions. The production of ions from the benzene dissociative photoionization is here quantified, indicating that C6H6 is more efficiently fragmented by soft X-ray than UV radiation, where 50% of the ionized benzene molecules survive to UV dissociation while only about 4% resist to X-rays. Partial ion yields of H+ and small hydrocarbons such as C2H2+, C3H3+ and C4H2+ are determined as a function of photon energy. Absolute photoionization and dissociative photoionization cross sections have also been determined. From these values, half-life of benzene molecule due to UV and X-ray photon fluxes in CRL 618 were obtained.Comment: The paper contains 8 pages, 9 figures and 4 tables. Accepted to be published on MNRAS on 2008 November 2

Dynamics of a ferromagnetic domain wall and the Barkhausen effect

We derive an equation of motion for the the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium and we study the associated depinning transition. The long-range dipolar interactions set the upper critical dimension to be $d_c=3$, so we suggest that mean-field exponents describe the Barkhausen effect for three-dimensional soft ferromagnetic materials. We analyze the scaling of the Barkhausen jumps as a function of the field driving rate and the intensity of the demagnetizing field, and find results in quantitative agreement with experiments on crystalline and amorphous soft ferromagnetic alloys.Comment: 4 RevTex pages, 3 ps figures embedde

Master crossover behavior of parachor correlations for one-component fluids

The master asymptotic behavior of the usual parachor correlations, expressing surface tension $\sigma$ as a power law of the density difference $\rho_{L}-\rho_{V}$ between coexisting liquid and vapor, is analyzed for a series of pure compounds close to their liquid-vapor critical point, using only four critical parameters $(\beta_{c})^{-1}$, $\alpha_{c}$, $Z_{c}$ and $Y_{c}$, for each fluid. ... The main consequences of these theoretical estimations are discussed in the light of engineering applications and process simulations where parachor correlations constitute one of the most practical method for estimating surface tension from density and capillary rise measurements

Fluctuations of elastic interfaces in fluids: Theory and simulation

We study the dynamics of elastic interfaces-membranes-immersed in thermally excited fluids. The work contains three components: the development of a numerical method, a purely theoretical approach, and numerical simulation. In developing a numerical method, we first discuss the dynamical coupling between the interface and the surrounding fluids. An argument is then presented that generalizes the single-relaxation time lattice-Boltzmann method for the simulation of hydrodynamic interfaces to include the elastic properties of the boundary. The implementation of the new method is outlined and it is tested by simulating the static behavior of spherical bubbles and the dynamics of bending waves. By means of the fluctuation-dissipation theorem we recover analytically the equilibrium frequency power spectrum of thermally fluctuating membranes and the correlation function of the excitations. Also, the non-equilibrium scaling properties of the membrane roughening are deduced, leading us to formulate a scaling law describing the interface growth, W^2(L,T)=L^3 g[t/L^(5/2)], where W, L and T are the width of the interface, the linear size of the system and the temperature respectively, and g is a scaling function. Finally, the phenomenology of thermally fluctuating membranes is simulated and the frequency power spectrum is recovered, confirming the decay of the correlation function of the fluctuations. As a further numerical study of fluctuating elastic interfaces, the non-equilibrium regime is reproduced by initializing the system as an interface immersed in thermally pre-excited fluids.Comment: 15 pages, 11 figure

Low temperature spin fluctuations in geometrically frustrated Yb3Ga5O12

In the garnet structure compound Yb3Ga5O12, the Yb3+ ions (ground state effective spin S' = 1/2) are situated on two interpenetrating corner sharing triangular sublattices such that frustrated magnetic interactions are possible. Previous specific heat measurements evidenced the development of short range magnetic correlations below 0.5K and a lambda-transition at 54mK (Filippi et al. J. Phys. C: Solid State Physics 13 (1980) 1277). From 170-Yb M"ossbauer spectroscopy measurements down to 36mK, we find there is no static magnetic order at temperatures below that of the lambda-transition. Below 0.3K, the fluctuation frequency of the short range correlated Yb3+ moments progressively slows down and as the temperature tends to 0, the frequency tends to a quasi-saturated value of 3 x 10^9 s^-1. We also examined the Yb3+ paramagnetic relaxation rates up to 300K using 172-Yb perturbed angular correlation measurements: they evidence phonon driven processes.Comment: 6 pages, 5 figure