154 research outputs found
Using late-time optical and near-infrared spectra to constrain Type Ia supernova explosion properties
The late-time spectra of Type Ia supernovae (SNe Ia) are powerful probes of
the underlying physics of their explosions. We investigate the late-time
optical and near-infrared spectra of seven SNe Ia obtained at the VLT with
XShooter at 200 d after explosion. At these epochs, the inner Fe-rich ejecta
can be studied. We use a line-fitting analysis to determine the relative line
fluxes, velocity shifts, and line widths of prominent features contributing to
the spectra ([Fe II], [Ni II], and [Co III]). By focussing on [Fe II] and [Ni
II] emission lines in the ~7000-7500 \AA\ region of the spectrum, we find that
the ratio of stable [Ni II] to mainly radioactively-produced [Fe II] for most
SNe Ia in the sample is consistent with Chandrasekhar-mass delayed-detonation
explosion models, as well as sub-Chandrasekhar mass explosions that have
metallicity values above solar. The mean measured Ni/Fe abundance of our sample
is consistent with the solar value. The more highly ionised [Co III] emission
lines are found to be more centrally located in the ejecta and have broader
lines than the [Fe II] and [Ni II] features. Our analysis also strengthens
previous results that SNe Ia with higher Si II velocities at maximum light
preferentially display blueshifted [Fe II] 7155 \AA\ lines at late times. Our
combined results lead us to speculate that the majority of normal SN Ia
explosions produce ejecta distributions that deviate significantly from
spherical symmetry.Comment: 17 pages, 12 figure, accepted for publication in MNRA
Iron and s-elements abundance variations in NGC5286: comparison with anomalous globular clusters and Milky Way satellites
We present a high resolution spectroscopic analysis of 62 red giants in the
Milky Way globular cluster NGC5286. We have determined abundances of
representative light proton-capture, alpha, Fe-peak and neutron-capture element
groups, and combined them with photometry of multiple sequences observed along
the colour-magnitude diagram. Our principal results are: (i) a broad, bimodal
distribution in s-process element abundance ratios, with two main groups, the
s-poor and s-rich groups; (ii) substantial star-to-star Fe variations, with the
s-rich stars having higher Fe, e.g. _s-rich - _s-poor ~
0.2~dex; and (iii) the presence of O-Na-Al (anti-)correlations in both stellar
groups. We have defined a new photometric index, c_{BVI}=(B-V)-(V-I), to
maximise the separation in the colour-magnitude diagram between the two stellar
groups with different Fe and s-element content, and this index is not
significantly affected by variations in light elements (such as the O-Na
anticorrelation). The variations in the overall metallicity present in NGC5286
add this object to the class of "anomalous" GCs. Furthermore, the chemical
abundance pattern of NGC5286 resembles that observed in some of the anomalous
GCs, e.g. M22, NGC1851, M2, and the more extreme Omega Centauri, that also show
internal variations in s-elements, and in light elements within stars with
different Fe and s-elements content. In view of the common variations in
s-elements, we propose the term s-Fe-anomalous GCs to describe this sub-class
of objects. The similarities in chemical abundance ratios between these objects
strongly suggest similar formation and evolution histories, possibly associated
with an origin in tidally disrupted dwarf satellites.Comment: 28 pages, 21 figures, accepted for publication in MNRA
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Insight into transcription factor gene duplication from Caenorhabditis elegans Promoterome-driven expression patterns
BACKGROUND: The C. elegans Promoterome is a powerful resource for revealing the regulatory mechanisms by which transcription is controlled pan-genomically. Transcription factors will form the core of any systems biology model of genome control and therefore the promoter activity of Promoterome inserts for C. elegans transcription factor genes was examined, in vivo, with a reporter gene approach. RESULTS: Transgenic C. elegans strains were generated for 366 transcription factor promoter/gfp reporter gene fusions. GFP distributions were determined, and then summarized with reference to developmental stage and cell type. Reliability of these data was demonstrated by comparison to previously described gene product distributions. A detailed consideration of the results for one C. elegans transcription factor gene family, the Six family, comprising ceh-32, ceh-33, ceh-34 and unc-39 illustrates the value of these analyses. The high proportion of Promoterome reporter fusions that drove GFP expression, compared to previous studies, led to the hypothesis that transcription factor genes might be involved in local gene duplication events less frequently than other genes. Comparison of transcription factor genes of C. elegans and Caenorhabditis briggsae was therefore carried out and revealed very few examples of functional gene duplication since the divergence of these species for most, but not all, transcription factor gene families. CONCLUSION: Examining reporter expression patterns for hundreds of promoters informs, and thereby improves, interpretation of this data type. Genes encoding transcription factors involved in intrinsic developmental control processes appear acutely sensitive to changes in gene dosage through local gene duplication, on an evolutionary time scale
Helium as a signature of the double detonation in Type Ia supernovae
The double detonation is a widely discussed mechanism to explain Type Ia
supernovae from explosions of sub-Chandrasekhar mass white dwarfs. In this
scenario, a helium detonation is ignited in a surface helium shell on a
carbon/oxygen white dwarf, which leads to a secondary carbon detonation.
Explosion simulations predict high abundances of unburnt helium in the ejecta,
however, radiative transfer simulations have not been able to fully address
whether helium spectral features would form. This is because helium can not be
sufficiently excited to form spectral features by thermal processes, but can be
excited by collisions with non-thermal electrons, which most studies have
neglected. We carry out a full non-local thermodynamic equilibrium (non-LTE)
radiative transfer simulation for an instance of a double detonation explosion
model, and include a non-thermal treatment of fast electrons. We find a clear
He I {\lambda} 10830 feature which is strongest in the first few days after
explosion and becomes weaker with time. Initially this feature is blended with
the Mg II {\lambda} 10927 feature but over time separates to form a secondary
feature to the blue wing of the Mg II {\lambda} 10927 feature. We compare our
simulation to observations of iPTF13ebh, which showed a similar feature to the
blue wing of the Mg II {\lambda} 10927 feature, previously identified as C I.
Our simulation shows a good match to the evolution of this feature and we
identify it as high velocity He I {\lambda} 10830. This suggests that He I
{\lambda} 10830 could be a signature of the double detonation scenario.Comment: 7 pages, accepted by MNRA
Augmented Reality in Astrophysics
Augmented Reality consists of merging live images with virtual layers of
information. The rapid growth in the popularity of smartphones and tablets over
recent years has provided a large base of potential users of Augmented Reality
technology, and virtual layers of information can now be attached to a wide
variety of physical objects. In this article, we explore the potential of
Augmented Reality for astrophysical research with two distinct experiments: (1)
Augmented Posters and (2) Augmented Articles. We demonstrate that the emerging
technology of Augmented Reality can already be used and implemented without
expert knowledge using currently available apps. Our experiments highlight the
potential of Augmented Reality to improve the communication of scientific
results in the field of astrophysics. We also present feedback gathered from
the Australian astrophysics community that reveals evidence of some interest in
this technology by astronomers who experimented with Augmented Posters. In
addition, we discuss possible future trends for Augmented Reality applications
in astrophysics, and explore the current limitations associated with the
technology. This Augmented Article, the first of its kind, is designed to allow
the reader to directly experiment with this technology.Comment: 15 pages, 11 figures. Accepted for publication in Ap&SS. The final
publication will be available at link.springer.co
Self-consistent 3D radiative transfer for kilonovae: directional spectra from merger simulations
We present three-dimensional radiative transfer calculations for the ejecta
from a neutron star merger that include line-by-line opacities for tens of
millions of bound-bound transitions, composition from an r-process nuclear
network, and time-dependent thermalization of decay products from individual
and decay reactions. In contrast to expansion opacities and
other wavelength-binned treatments, a line-by-line treatment enables us include
fluorescence effects and associate spectral features with the emitting and
absorbing lines of individual elements. We find variations in the synthetic
observables with both the polar and azimuthal viewing angles. The spectra
exhibit blended features with strong interactions by Ce III, Sr II, Y II, and
Zr II that vary with time and viewing direction. We demonstrate the importance
of wavelength-calibration of atomic data using a model with calibrated Sr, Y,
and Zr data, and find major differences in the resulting spectra, including a
better agreement with AT2017gfo. The synthetic spectra for near-polar
inclination show a feature at around 8000 A, similar to AT2017gfo. However,
they evolve on a more rapid timescale, likely due to the low ejecta mass (0.005
M) as we take into account only the early ejecta. The comparatively
featureless spectra for equatorial observers gives a tentative prediction that
future observations of edge-on kilonovae will appear substantially different
from AT2017gfo. We also show that 1D models obtained by spherically averaging
the 3D ejecta lead to dramatically different direction-integrated luminosities
and spectra compared to full 3D calculations.Comment: 12 pages, 5 figures. Accepted by ApJ
Monte Carlo radiative transfer for the nebular phase of Type Ia supernovae
We extend the range of validity of the ARTIS 3D radiative transfer code up to hundreds of days after explosion, when Type Ia supernovae (SNe Ia) are in their nebular phase. To achieve this, we add a non-local thermodynamic equilibrium population and ionization solver, a new multifrequency radiation field model, and a new atomic data set with forbidden transitions. We treat collisions with non-thermal leptons resulting from nuclear decays to account for their contribution to excitation, ionization, and heating. We validate our method with a variety of tests including comparing our synthetic nebular spectra for the well-known one-dimensional W7 model with the results of other studies. As an illustrative application of the code, we present synthetic nebular spectra for the detonation of a sub-Chandrasekhar white dwarf (WD) in which the possible effects of gravitational settling of 22Ne prior to explosion have been explored. Specifically, we compare synthetic nebular spectra for a 1.06 M☉ WD model obtained when 5.5 Gyr of very efficient settling is assumed to a similar model without settling. We find that this degree of 22Ne settling has only a modest effect on the resulting nebular spectra due to increased 58Ni abundance. Due to the high ionization in sub-Chandrasekhar models, the nebular [Ni II] emission remains negligible, while the [Ni III] line strengths are increased and the overall ionization balance is slightly lowered in the model with 22Ne settling. In common with previous studies of sub-Chandrasekhar models at nebular epochs, these models overproduce [Fe III] emission relative to [Fe II] in comparison to observations of normal SNe Ia
Towards inferring the geometry of kilonovae
Recent analysis of the kilonova, AT2017gfo, has indicated that this event was
highly spherical. This may challenge hydrodynamics simulations of binary
neutron star mergers, which usually predict a range of asymmetries, and
radiative transfer simulations show a strong direction dependence. Here we
investigate whether the synthetic spectra from a 3D kilonova simulation of
asymmetric ejecta from a hydrodynamical merger simulation can be compatible
with the observational constraints suggesting a high degree of sphericity in
AT2017gfo. Specifically, we determine whether fitting a simple P-Cygni line
profile model leads to a value for the photospheric velocity that is consistent
with the value obtained from the expanding photosphere method. We would infer
that our kilonova simulation is highly spherical at early times, when the
spectra resemble a blackbody distribution. The two independently inferred
photospheric velocities can be very similar, implying a high degree of
sphericity, which can be as spherical as inferred for AT2017gfo, demonstrating
that the photosphere can appear spherical even for asymmetrical ejecta. The
last-interaction velocities of radiation escaping the simulation show a high
degree of sphericity, supporting the inferred symmetry of the photosphere. We
find that when the synthetic spectra resemble a blackbody the expanding
photosphere method can be used to obtain an accurate luminosity distance
(within 4-7 per cent).Comment: 11 pages, submitted to MNRA
Opacities of Singly and Doubly Ionised Neodymium and Uranium for Kilonova Emission Modeling
Even though the electromagnetic counterpart AT2017gfo to the binary neutron
star merger GW170817 is powered by the radioactive decay of r-process nuclei,
only few tentative identifications of light r-process elements have been made
so far. One of the major limitations for the identification of heavy nuclei is
incomplete or missing atomic data. While substantial progress has been made on
lanthanide atomic data over the last few years, for actinides there has been
less emphasis, with the first complete set of opacity data only recently
published. We perform atomic structure calculations of neodymium as
well as the corresponding actinide uranium . Using two different codes
(FAC and HFR) for the calculation of the atomic data, we investigate the
accuracy of the calculated data (energy levels and electric dipole transitions)
and their effect on kilonova opacities. For the FAC calculations, we optimise
the local central potential and the number of included configurations and use a
dedicated calibration technique to improve the agreement between theoretical
and available experimental atomic energy levels (AELs). For ions with vast
amounts of experimental data available, the presented opacities agree quite
well with previous estimations. On the other hand, the optimisation and
calibration method cannot be used for ions with only few available AELs. For
these cases, where no experimental nor benchmarked calculations are available,
a large spread in the opacities estimated from the atomic data obtained with
the various atomic structure codes is observed.We find that the opacity of
uranium is almost double the neodymium opacity.Comment: 20 pages, 13 figures. Accepted by MNRA
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