13,677 research outputs found
Tracing Quasar Narrow-Line Regions Across Redshift: A Library of High S/N Optical Spectra
In a single optical spectrum, the quasar narrow-line region (NLR) reveals low
density, photoionized gas in the host galaxy interstellar medium, while the
immediate vicinity of the central engine generates the accretion disk continuum
and broad emission lines. To isolate these two components, we construct a
library of high S/N optical composite spectra created from the Sloan Digital
Sky Survey (SDSS-DR7). We divide the sample into bins of continuum luminosity
and Hbeta FWHM that are used to construct median composites at different
redshift steps up to 0.75. We measure the luminosities of the narrow-emission
lines [NeV]3427, [NeIII]3870, [OIII]5007, and [OII]3728 with ionization
potentials (IPs) of 97, 40, 35, and 13.6 eV respectively. The high IP lines'
luminosities show no evidence of increase with redshift consistent with no
evolution in the AGN SED or the host galaxy ISM illuminated by the continuum.
In contrast, we find that the [OII] line becomes stronger at higher redshifts,
and we interpret this as a consequence of enhanced star formation contributing
to the [OII] emission in host galaxies at higher redshifts. The SFRs estimated
from the [OII] luminosities show a flatter increase with z than non-AGN
galaxies given our assumed AGN contribution to the [OII] luminosity. Finally,
we confirm an inverse correlation between the strength of the FeII4570 complex
and both the [OIII] EW (though not the luminosity) and the width of the Hbeta
line as known from the eigenvector 1 correlations.Comment: 17 pages, colour figures, accepted for publication in MNRA
A three-dimensional hydrodynamical line profile analysis of iron lines and barium isotopes in HD140283
Heavy-elements, i.e. those beyond the iron peak, mostly form via two neutron
capture processes: the s- and r-process. Metal-poor stars should contain fewer
isotopes that form via the s-process, according to currently accepted theory.
It has been shown in several investigations that theory and observation do not
agree well, raising questions on the validity of either the methodology or the
theory. We analyse the metal-poor star HD140283, for which we have a high
quality spectrum. We test whether a 3D LTE stellar atmosphere and spectrum
synthesis code permits a more reliable analysis of the iron abundance and
barium isotope ratio than a 1D LTE analysis. Using 3D model atmospheres, we
examine 91 iron lines of varying strength and formation depth. This provides us
with the star's rotational speed. With this, we model the barium isotope ratio
by exploiting the hyperfine structure of the singly ionised 4554 resonance
line, and study the impact of the uncertainties in the stellar parameters.
HD140283's vsini = 1.65 +/- 0.05 km/s. Barium isotopes under the 3D paradigm
show a dominant r-process signature as 77 +/- 6 +/- 17% of barium isotopes form
via the r-process, where errors represent the assigned random and systematic
errors, respectively. We find that 3D LTE fits reproduce iron line profiles
better than those in 1D, but do not provide a unique abundance (within the
uncertainties). However, we demonstrate that the isotopic ratio is robust
against this shortcoming. Our barium isotope result agrees well with currently
accepted theory regarding the formation of the heavy-elements during the early
Galaxy. The improved fit to the asymmetric iron line profiles suggests that the
current state of 3D LTE modelling provides excellent simulations of fluid
flows. However, the abundances they provide are not yet self-consistent. This
may improve with NLTE considerations and higher resolution models.Comment: 16 pages, 10 figures, 5 tables. Accepted for publication in A&
Solar Flare X-ray Source Motion as a Response to Electron Spectral Hardening
Context: Solar flare hard X-rays (HXRs) are thought to be produced by
nonthermal coronal electrons stopping in the chromosphere, or remaining trapped
in the corona. The collisional thick target model (CTTM) predicts that sources
produced by harder power-law injection spectra should appear further down the
legs or footpoints of a flare loop. Therefore, hardening of the injected
power-law electron spectrum during flare onset should be concurrent with a
descending hard X-ray source.
Aims: To test this implication of the CTTM by comparing its predicted HXR
source locations with those derived from observations of a solar flare which
exhibits a nonthermally-dominated spectrum before the peak in HXRs, known as an
early impulsive event.
Methods: HXR images and spectra of an early impulsive C-class flare were
obtained using the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI).
Images were reconstructed to produce HXR source height evolutions for three
energy bands. Spatially-integrated spectral analysis was performed to isolate
nonthermal emission, and to determine the power-law index of the electron
injection spectrum. The observed height-time evolutions were then fit with
CTTM-based simulated heights for each energy.
Results: A good match between model and observed source heights was reached,
requiring a density model that agreed well with previous studies of flare loop
densities.
Conclusions: The CTTM has been used to produce a descent of model HXR source
heights that compares well with observations of this event. Based on this
interpretation, downward motion of nonthermal sources should indeed occur in
any flare where there is spectral hardening in the electron distribution during
a flare. However, this would often be masked by thermal emission associated
with flare plasma pre-heating.Comment: 8 pages, 5 figure
Strongly correlated gases of Rydberg-dressed atoms: quantum and classical dynamics
We discuss techniques to generate long-range interactions in a gas of
groundstate alkali atoms, by weakly admixing excited Rydberg states with laser
light. This provides a tool to engineer strongly correlated phases with reduced
decoherence from inelastic collisions and spontaneous emission. As an
illustration, we discuss the quantum phases of dressed atoms with dipole-dipole
interactions confined in a harmonic potential, as relevant to experiments. We
show that residual spontaneous emission from the Rydberg state acts as a
heating mechanism, leading to a quantum-classical crossover.Comment: 4 pages, 4 figure
Recommended from our members
Perinatal aspects on the covid-19 pandemic: a practical resource for perinatal-neonatal specialists.
BackgroundLittle is known about the perinatal aspects of COVID-19.ObjectiveTo summarize available evidence and provide perinatologists/neonatologists with tools for managing their patients.MethodsAnalysis of available literature on COVID-19 using Medline and Google scholar.ResultsFrom scant data: vertical transmission from maternal infection during the third trimester probably does not occur or likely it occurs very rarely. Consequences of COVID-19 infection among women during early pregnancy remain unknown. We cannot conclude if pregnancy is a risk factor for more severe disease in women with COVID-19. Little is known about disease severity in neonates, and from very few samples, the presence of SARS-CoV-2 has not been documented in human milk. Links to websites of organizations with updated COVID-19 information are provided. Infographics summarize an approach to the pregnant woman or neonate with suspected or confirmed COVID-19.ConclusionsAs the pandemic continues, more data will be available that could lead to changes in current knowledge and recommendations
An in-depth spectroscopic examination of molecular bands from 3D hydrodynamical model atmospheres I. Formation of the G-band in metal-poor dwarf stars
Recent developments in the three-dimensional (3D) spectral synthesis code
Linfor3D have meant that, for the first time, large spectral wavelength
regions, such as molecular bands, can be synthesised with it in a short amount
of time. A detailed spectral analysis of the synthetic G-band for several dwarf
turn-off-type 3D atmospheres (5850 <= T_eff [K] <= 6550, 4.0 <= log g <= 4.5,
-3.0 <= [Fe/H] <= -1.0) was conducted, under the assumption of local
thermodynamic equilibrium. We also examine carbon and oxygen molecule formation
at various metallicity regimes and discuss the impact it has on the G-band.
Using a qualitative approach, we describe the different behaviours between the
3D atmospheres and the traditional one-dimensional (1D) atmospheres and how the
different physics involved inevitably leads to abundance corrections, which
differ over varying metallicities. Spectra computed in 1D were fit to every 3D
spectrum to determine the 3D abundance correction. Early analysis revealed that
the CH molecules that make up the G-band exhibited an oxygen abundance
dependency; a higher oxygen abundance leads to weaker CH features. Nitrogen
abundances showed zero impact to CH formation. The 3D corrections are also
stronger at lower metallicity. Analysis of the 3D corrections to the G-band
allows us to assign estimations of the 3D abundance correction to most dwarf
stars presented in the literature. The 3D corrections suggest that A(C) in CEMP
stars with high A(C) would remain unchanged, but would decrease in CEMP stars
with lower A(C). It was found that the C/O ratio is an important parameter to
the G-band in 3D. Additional testing confirmed that the C/O ratio is an equally
important parameter for OH transitions under 3D. This presents a clear
interrelation between the carbon and oxygen abundances in 3D atmospheres
through their molecular species, which is not seen in 1D.Comment: 19 pages, 13 figures, 4 tables. Accepted for publication in A&
- …