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D. Coffey

Publications and source records attributed to D. Coffey.

34 records · Page 2Linked to original sources
arXiv2018

The circumstellar environment of HD50138 revealed by VLTI/AMBER at high angular resolution

HD50138 is a Herbig B[e] star with a circumstellar disc detected at IR and mm wavelength. Its brightness makes it a good candidate for NIR interferometry observations. We aim to resolve, spatially and spectrally, the continuum and hydrogen emission lines in the 2.12-2.47 micron region, to shed light on the immediate circumstellar environment of the star. VLTI/AMBER K-band observations provide spectra, visibilities, differential phases, and closure phases along three long baselines for the continuum, and HI emission in Br$γ$ and five high-n Pfund lines. By computing the pure-line visibilities, we derive the angular size of the different line-emitting regions. A simple LTE model was created to constrain the physical conditions of HI emitting region. The continuum region cannot be reproduced by a geometrical 2D elongated Gaussian fitting model. We estimate the size of the region to be 1 au. We find the Br$γ$ and Pfund lines come from a more compact region of size 0.4 au. The Br$γ$ line exhibits an S-shaped differential phase, indicative of rotation. The continuum and Br$γ$ line closure phase show offsets of $\sim$-25$\pm$5 $^o$ and 20$\pm$10$^o$, respectively. This is evidence of an asymmetry in their origin, but with opposing directions. We find that we cannot converge on constraints for the HI physical parameters without a more detailed model. Our analysis reveals that HD50138 hosts a complex circumstellar environment. Its continuum emission cannot be reproduced by a simple disc brightness distribution. Similarly, several components must be evoked to reproduce the interferometric observables within the Br$γ$, line. Combining the spectroscopic and interferometric data of the Br$γ$ and Pfund lines favours an origin in a wind region with a large opening angle. Finally, our results point to an evolved source.

astro-ph.SR↗
arXiv2017

SPIDERS: Selection of spectroscopic targets using AGN candidates detected in all-sky X-ray surveys

SPIDERS (SPectroscopic IDentification of eROSITA Sources) is an SDSS-IV survey running in parallel to the eBOSS cosmology project. SPIDERS will obtain optical spectroscopy for large numbers of X-ray-selected AGN and galaxy cluster members detected in wide area eROSITA, XMM-Newton and ROSAT surveys. We describe the methods used to choose spectroscopic targets for two sub-programmes of SPIDERS: X-ray selected AGN candidates detected in the ROSAT All Sky and the XMM-Newton Slew surveys. We have exploited a Bayesian cross-matching algorithm, guided by priors based on mid-IR colour-magnitude information from the WISE survey, to select the most probable optical counterpart to each X-ray detection. We empirically demonstrate the high fidelity of our counterpart selection method using a reference sample of bright well-localised X-ray sources collated from XMM-Newton, Chandra and Swift-XRT serendipitous catalogues, and also by examining blank-sky locations. We describe the down-selection steps which resulted in the final set of SPIDERS-AGN targets put forward for spectroscopy within the eBOSS/TDSS/SPIDERS survey, and present catalogues of these targets. We also present catalogues of ~12000 ROSAT and ~1500 XMM-Newton Slew survey sources which have existing optical spectroscopy from SDSS-DR12, including the results of our visual inspections. On completion of the SPIDERS program, we expect to have collected homogeneous spectroscopic redshift information over a footprint of ~7500 deg$^2$ for >85 percent of the ROSAT and XMM-Newton Slew survey sources having optical counterparts in the magnitude range 17<r<22.5, producing a large and highly complete sample of bright X-ray-selected AGN suitable for statistical studies of AGN evolution and clustering.

astro-ph.GA↗
arXiv2016

ALMA observations of the Th 28 protostellar disk - A new example of counter-rotation between disk and optical jet

Recently, differences in Doppler shifts across the base of four close classical T Tauri star jets have been detected with the HST in optical and near-ultraviolet (NUV) emission lines, and these Doppler shifts were interpreted as rotation signatures under the assumption of steady state flow. To support this interpretation, it is necessary that the underlying disks rotate in the same sense. Agreement between disk rotation and jet rotation determined from optical lines has been verified in two cases and rejected in one case. Meanwhile, the NUV lines, which may trace faster and more collimated inner spines of the jet than optical lines, either agree or show no clear indication. We propose to perform this test on the fourth system, Th 28. We present ALMA high angular resolution Band 7 continuum, 12CO(3-2) and 13CO(2-1) observations of the circumstellar disk around the T Tauri star Th 28. We were able to detect, in CO and continuum, clear signatures of a disk in Keplerian rotation around Th28. The 12CO emission is resolved, allowing us to derive estimates of disk position angle and inclination. The large velocity separation of the peaks in 12CO, combined with the resolved extent of the emission, indicate a central stellar mass in the range 1-2 Msun. The rotation sense of the disk is well detected in both 13CO and 12CO emission lines, and this direction is opposite to that implied by the transverse Doppler shifts measured in the optical lines of the jet. The Th 28 system is the second system where counter-rotation between the disk and the optical jet is detected. These findings imply either that optical transverse velocity gradients detected with HST do not trace jet rotation or that modeling the flow with the steady assumption is not valid. In both cases jet rotation studies that rely solely on optical lines are not suitable to derive the launching radius of the jet.

astro-ph.SR↗
arXiv2014

Physical properties of the jet from DG Tauri on sub-arcsecond scales with HST/STIS

We derive the physical properties at the base of the jet from DG Tau both along and across the flow and as a function of velocity. We analysed seven optical spectra of the DG Tau jet, taken with the Hubble Space Telescope Imaging Spectrograph. The spectra were obtained by placing a long-slit parallel to the jet axis and stepping it across the jet width. The resulting position-velocity diagrams in optical forbidden emission lines allowed access to plasma conditions via calculation of emission line ratios. We find at the base of the jet high electron density, $n_e \sim $ 10$^5$, and very low ionisation, $x_e \sim 0.02-0.05$, which combine to give a total density up to $n_H \sim $ 3 10$^6$. This analysis confirms previous reports of variations in plasma parameters along the jet, (i.e. decrease in density by several orders of magnitude, increase of $x_e$ from 0.05 to a plateau at 0.7 downstream at 2$''$ from the star). Furthermore, a spatial coincidence is revealed between sharp gradients in the total density and supersonic velocity jumps. This strongly suggests that the emission is caused by shock excitation. The position-velocity diagrams indicate the presence of both fast accelerating gas and slower, less collimated material. We derive the mass outflow rate, $\dot{M}_j$, in the blue-shifted lobe in different velocity channels, that contribute to a total of $\dot{M}_j \sim$ 8 $\pm$ 4 10$^{-9}$ M$_\odot$ yr$^{-1}$. We estimate that a symmetric bipolar jet would transport at the low and intermediate velocities probed by rotation measurements, an angular momentum flux of $\dot{L}_j \sim$ 2.9 $\pm$ 1.5 10$^{-6}$ M$_\odot$ yr$^{-1}$ AU km s$^{-1}$. The derived properties of the DG Tau jet are demonstrated to be consistent with magneto-centrifugal theory. However, non-stationary modelling is required in order to explain all of the features revealed at high resolution.

astro-ph.SR↗
arXiv2009

Collapse of the hyperfine magnetic field at the Ru site in ferromagnetic rare earth intermetallics

The Mössbauer Effect(ME) is frequently used to investigate magnetically ordered systems. One usually assumes that the magnetic order induces a hyperfine magnetic field, $B_{hyperfine}$, at the ME active site. This is the case in the ruthenates, where the temperature dependence of $B_{hyperfine}$ at $^{99}$Ru sites tracks the temperature dependence of the ferromagnetic or antiferromagnetic order. However this does not happen in the rare-earth intermetallics, GdRu$_2$ and HoRu$_2$. Specific heat, magnetization, magnetic susceptibility, Mössbauer effect, and neutron diffraction have been used to study the nature of the magnetic order in these materials. Both materials are found to order ferromagnetically at 82.3 and 15.3 K, respectively. Despite the ferromagnetic order of the rare earth moments in both systems, there is no evidence of a correspondingly large $B_{hyperfine}$ in the Mössbauer spectrum at the Ru site. Instead the measured spectra consist of a narrow peak at all temperatures which points to the absence of magnetic order. To understand the surprising absence of a transferred hyperfine magnetic field, we carried out {\it ab initio} calculations which show that spin polarization is present only on the rare-earth site. The electron spin at the Ru sites is effectively unpolarized and, as a result, $B_{hyperfine}$ is very small at those sites. This occurs because the 4$d$ Ru electrons form broad conduction bands rather than localized moments. These 4$d$ conduction bands are polarized in the region of the Fermi energy and mediate the interaction between the localized rare earth moments.

cond-mat.str-el↗
arXiv2005

Adiabatic and Non-Adiabatic Contributions to the Free Energy from the Electron-Phonon Interaction for Na, K, Al, and Pb

We calculate the adiabatic contributions to the free energy due to the electron--phonon interaction at intermediate temperatures, $0 \leqslant k_{B} T < ε_{F}$ for the elemental metals Na, K, Al, and Pb. Using our previously published results for the nonadiabatic contributions we show that the adiabatic contribution, which is proportional to $T^{2}$ at low temperatures and goes as $T^{3}$ at high temperatures, dominates the nonadiabatic contribution for temperatures above a cross--over temperature, $T_{c}$, which is between 0.5 and 0.8 $T_{m}$, where $T_{m}$ is the melting temperature of the metal. The nonadiabatic contribution falls as $T^{-1}$ for temperatures roughly above the average phonon frequency.

cond-mat.other↗
arXiv2002

Magnetic Anisotropy in the Molecular Complex V15

We apply degenerate perturbation theory to investigate the effects of magnetic anisotropy in the magnetic molecule V15. Magnetic anisotropy is introduced via Dzyaloshinskii-Moriya (DM) interaction in the full Hilbert space of the system. Our model provides an explanation for the rounding of transitions in the magnetization as a function of applied field at low temperature, from which an estimate for the DM interaction is found. We find that the calculated energy differences of the lowest energy states are consistent with the available data. Our model also offers a novel explanation for the hysteretic nature of the time-dependent magnetization data.

cond-mat.mes-hall↗
arXiv2001

Accurate Results from Perturbation Theory for Strongly Frustrated $S=1/2$ Heisenberg Spin Clusters

We investigate the use of perturbation theory in finite sized frustrated spin systems by calculating the effect of quantum fluctuations on coherent states derived from the classical ground state. We first calculate the ground and first excited state wavefunctions as a function of applied field for a 12-site system and compare with the results of exact diagonalization. We then apply the technique to a 20-site system with the same three fold site coordination as the 12-site system. Frustration results in asymptotically convergent series for both systems which are summed with Padé approximants. We find that at zero magnetic field the different connectivity of the two systems leads to a triplet first excited state in the 12-site system and a singlet first excited state in the 20-site system, while the ground state is a singlet for both. We also show how the analytic structure of the Padé approximants at $|λ| \simeq 1$ evolves in the complex $λ$ plane at the values of the applied field where the ground state switches between spin sectors and how this is connected with the non-trivial dependence of the $ $ number on the strength of quantum fluctuations. We discuss the origin of this difference in the energy spectra and in the analytic structures. We also characterize the ground and first excited states according to the values of the various spin correlation functions.

cond-mat.str-el↗
arXiv1999

Evolution of Fermi Liquid Behavior with Doping in the Hubbard Model

We calculate the single-particle Green's function for the tight-binding band structure, $ξ_{\vec p}=-2t\cos p_x-2t\cos p_y -μ$, with a function of chemical potential $μ$ for square-lattice system. The form of the single-particle self-energy, $Σ({\vec p}, E)$, is determined by the density-density correlation function, $χ({\vec q}, ω)$, which develops two peaks for $μ\gtrsim -2.5t$ unlike parabolic band case. Near half filling $χ({\vec q}, ω)$ becomes independent of $ω$, one dimensional behavior, at intermediate values of $ω$ which leads to one dimensional behavior in $Σ({\vec p},E)$. However $μ\leq -0.1t$ there is no influence on the Fermi Liquid dependences from SDW instability. The strong $\vec p$ and $E$ dependence of the off-shell self-energy, $Σ(p,E)$, found earlier for the parabolic band is recovered for $μ\lesssim -t$ but deviations from this develop for $μ\gtrsim -0.1t$. The resonance peak width of the spectral function, $A({\vec p}, E)$ has linear dependence in $ξ_{\vec p}$ due to the $E$ dependence of the imaginary part of $Σ({\vec p}, E)$. We point out that an accurate detailed form for $Σ({\vec p},E)$ would be very difficult to recover from ARPES data for the spectral density.

cond-mat.str-el↗
arXiv1997

Strong-Coupling Features Due to Quasiparticle Interaction in Two Dimensional Superconductors

I calculate the effect of interactions among superconducting quasiparticles in two-dimensional(2D) a superconductor at T=0. The strength of the effective interaction among the quasiparticles is essentially given by the screened Coulomb interaction which has strength at low frequency because of the gapless nature of the plasmon. This is in contrast to three dimensions where the effective interaction has negligible weight at frequencies $\sim Δ$, the superconducting gap. The quasiparticle interactions give rise to strong-coupling effects in experimental quantities which are beyond the conventional Eliashberg treatment of superconductivity. The present calculation offers an explanation of why these effects are much larger in 2D than in 3D superconductors and, in particular, why the analogous strong-coupling effects due to quasiparticle interactions are seen in data on the quasi-2D cuprate superconductors. the strong-coupling features seen in data on the cuprates are discussed in light of the present calculation.

cond-mat.supr-con↗
arXiv1997

Quasiparticle Interactions in Two and Three Dimensional Superconductors

I investigate the difference between the quasiparticle properties in two dimensional(2D)and three dimensional(3D) s-wave superconductors. Using the original BCS model for the pairing interaction and direct Coulomb interaction I show that quasiparticle interactions lead to a stronger energy dependence in the single-particle self-energies in 2D than in 3D superconductors. This difference arises from the presence of the low lying collective mode of the order parameter in the 2D case which ensures that oscillator strength in the response function is at low frequencies, $\sim Δ$. This strong quantitative difference between 2D and 3D superconductors points to the importance of treating quasiparticle interactions in low dimensional superconductors rather than assuming that renormalizations remain unchanged from the normal state.

cond-mat.supr-con↗
arXiv1997

Single-Particle Properties of a Two-Dimensional Fermi Liquid at finite Frequencies and Temperatures

We review the leading momentum, frequency and temperature dependences of the single particle self-energy and the corresponding term in the entropy of a two dimensional Fermi liquid (FL) with a free particle spectrum. We calculate the corrections to these leading dependences for the paramagnon model and the electron gas and find that the leading dependences are limited to regions of energy and temperature which decrease with decreasing number density of fermions. This can make it difficult to identify the frequency and temperature dependent characteristics of a FL ground state in experimental quantities in low density systems even when complications of band structure and other degrees of freedom are absent. This is an important consideration when the normal state properties of the undoped cuprate superconductors are analyzed.

cond-mat.str-el↗
arXiv1993

Non-Analytic Contributions to the Self-Energy and the Thermodynamics of Two-Dimensional Fermi Liquids

We calculate the entropy of a two-dimensional Fermi Liquid(FL) using a model with a contact interaction between fermions. We find that there are $T^2$ contributions to the entropy from interactions separate from those due to the collective modes. These $T^2$ contributions arise from non-analytic corrections to the real part of the self-energy which may be calculated from the leading log dependence of the imaginary part of the self-energy through the Kramers-Kronig relation. We find no evidence of a breakdown in Fermi Liquid theory in 2D and conclude that FL in 2D are similar to 3D FL's.

cond-mat↗
arXiv1992

Correlations for the S=1/2 Antiferromagnet on a Truncated Tetrahedron

The antiferromagnetic Heisenberg Hamiltonian is investigated on a truncated tetrahedron, which is a closed 12 site system. We find that the ground state has many similarities to that of $C_{60}$. We study 2- and 4-spin correlations in the classical ground state of the truncated tetrahedron and calculate the same correlations in the exact S=1/2 ground state. We find that the classical correlations survive for a range of bond strengths in the Heisenberg Hamiltonian and that one can construct a good trial wavefunction based on the classical ground state. This suggests that the correlations present in the classical ground state of $C_{60}$ also survive in the exact ground state of that system, for a range of bond strengths about the physically relevant $J_2 \approx J_1$. We calculate the momentum-space correlation function $S ( q )$, which is measurable by neutron scattering, for both $C_{12}$ and $C_{60}$. We also calculate correlations at finite temperature.

cond-mat↗
arXiv1992

Magnetic Properties of Undoped $C_{60}$

The Heisenberg antiferromagnet, which arises from the large $U$ Hubbard model, is investigated on the $C_{60}$ molecule and other fullerenes. The connectivity of $C_{60}$ leads to an exotic classical ground state with nontrivial topology. We argue that there is no phase transition in the Hubbard model as a function of $U/t$, and thus the large $U$ solution is relevant for the physical case of intermediate coupling. The system undergoes a first order metamagnetic phase transition. We also consider the S=1/2 case using perturbation theory. Experimental tests are suggested.

cond-mat↗