39 research outputs found
Helium in Double-Detonation Models of Type Ia Supernovae
The double-detonation explosion model has been considered a candidate for
explaining astrophysical transients with a wide range of luminosities. In this
model, a carbon-oxygen white dwarf star explodes following detonation of a
surface layer of helium. One potential signature of this explosion mechanism is
the presence of unburned helium in the outer ejecta, left over from the surface
helium layer. In this paper we present simple approximations to estimate the
optical depths of important He I lines in the ejecta of double-detonation
models. We use these approximations to compute synthetic spectra, including the
He I lines, for double-detonation models obtained from hydrodynamical explosion
simulations. Specifically, we focus on photospheric-phase predictions for the
near-infrared 10830 \AA~and 2 m lines of He I. We first consider a double
detonation model with a luminosity corresponding roughly to normal SNe Ia. This
model has a post-explosion unburned He mass of 0.03 and our
calculations suggest that the 2 m feature is expected to be very weak but
that the 10830 \AA~feature may have modest opacity in the outer ejecta.
Consequently, we suggest that a moderate-to-weak He I 10830 \AA~feature may be
expected to form in double-detonation explosions at epochs around maximum
light. However, the high velocities of unburned helium predicted by the model
(~km~s) mean that the He I 10830 \AA~feature may be
confused or blended with the C I 10690~\AA~line forming at lower velocities. We
also present calculations for the He I 10830 \AA~and 2 m lines for a lower
mass (low luminosity) double detonation model, which has a post-explosion He
mass of 0.077 . In this case, both the He I features we consider are
strong and can provide a clear observational signature of the double-detonation
mechanism.Comment: 12 pages, 11 figures, accepted by A&
Spectral luminosity indicators in SNe Ia - Understanding the R(SiII) line strength ratio and beyond
SNe Ia are good distance indicators because the shape of their light curves,
which can be measured independently of distance, varies smoothly with
luminosity. This suggests that SNe Ia are a single family of events. Similar
correlations are observed between luminosity and spectral properties. In
particular, the ratio of the strengths of the SiII \lambda 5972 and \lambda
6355 lines, known as R(SiII), was suggested as a potential luminosity
indicator. Here, the physical reasons for the observed correlation are
investigated. A Monte-Carlo code is used to construct a sequence of synthetic
spectra resembling those of SNe with different luminosities near B maximum. The
influence of abundances and of ionisation and excitation conditions on the
synthetic spectral features is investigated. The ratio R(SiII) depends
ssentially on the strength of SiII \lambda 5972, because SiII \lambda 6355 is
saturated. In less luminous objects, SiII \lambda 5972 is stronger because of a
rapidly increasing SiII/SiIII ratio. Thus, the correlation between R(SiII) and
luminosity is the effect of ionisation balance. The SiII \lambda 5972 line
itself may be the best spectroscopic luminosity indicator for SNe Ia, but all
indicators discussed show scatter which may be related to abundance
distributions.Comment: 10 pages, 16 figures. Accepted for publication in MNRA
Investigating regional source and sink patterns of Alpine CO2 and CH4 concentrations based on a back trajectory receptor model
How much H and He is "hidden" in SNe Ib/c? -- II. Intermediate-mass objects: a 22 M progenitor case study
Stripped envelope supernovae are a sub-class of core collapse supernovae
showing several stages of H/He shell stripping that determines the class:
H-free/He-poor SNe are classified as Type Ic, H-poor/He-rich are Type Ib, and
H/He-rich are Type IIb. Stripping H/He with only stellar wind requires
significantly higher mass loss rates than observed while binary-involved mass
transfer may usually not strip enough to produce H/He free SNe. Type Ib/c SNe
are sometimes found to include weak H/He transient lines as a product of a
trace amount of H/He left over from stripping processes. The extent and mass of
the H/He required to produce these lines is not well known. In this work, a 22
M progenitor model is stripped of the H/He shells in five steps prior
to collapse and then exploded at four explosion energies. Requiring both
optical and NIR He I lines for helium identification does not allow much He
mass to be hidden in SE--SNE. Increasing the mass of He above the CO core
delays the visibility of O I 7774 in early spectra. Our SN Ib-like models are
capable of reproducing the spectral evolution of a set of observed SNe with
reasonable estimated accuracy. Our SN\,IIb-like models can
partially reproduce low energy observed SN IIb, but we find no observed
comparison for the SN IIb-like models with high .Comment: 19 pages, 15 figures. Accepted by MNRAS, awaiting publicatio
Application of trajectory clustering and source attribution methods for investigating regional CO2 and CH4 concentrations at Germany's highest mountain site
Analysis of total column COâ and CHâ measurements in Berlin with WRF-GHG
Though they cover less than 3â% of the global land area, urban areas are responsible for over 70â% of the global greenhouse gas (GHG) emissions and contain 55â% of the global population. A quantitative tracking of GHG emissions in urban areas is therefore of great importance, with the aim of accurately assessing the amount of emissions and identifying the emission sources. The Weather Research and Forecasting model (WRF) coupled with GHG modules (WRF-GHG) developed for mesoscale atmospheric GHG transport can predict column-averaged abundances of CO2 and CH4 (XCO2 and XCH4). In this study, we use WRF-GHG to model the Berlin area at a high spatial resolution of 1âkm. The simulated wind and concentration fields were compared with the measurements from a campaign performed around Berlin in 2014 (Hase et al., 2015). The measured and simulated wind fields mostly demonstrate good agreement. The simulated XCO2 shows quite similar trends with the measurement but with approximately 1âppm bias, while a bias in the simulated XCH4 of around 2.7â% is found. The bias could potentially be the result of relatively high background concentrations, the errors at the tropopause height, etc. We find that an analysis using differential column methodology (DCM) works well for the XCH4 comparison, as corresponding background biases are then canceled out. From the tracer analysis, we find that the enhancement of XCH4 is highly dependent on human activities. The XCO2 enhancement in the vicinity of Berlin is dominated by anthropogenic behavior rather than biogenic activities. We conclude that DCM is an effective method for comparing models to observations independently of biases caused, e.g., by initial conditions. It allows us to use our high-resolution WRF-GHG model to detect and understand major sources of GHG emissions in urban areas
The Outermost Ejecta of Type Ia Supernovae
The properties of the highest velocity ejecta of normal Type Ia supernovae
(SNe Ia) are studied via models of very early optical spectra of 6 SNe. At
epochs earlier than 1 week before maximum, SNe with a rapidly evolving Si II
6355 line velocity (HVG) have a larger photospheric velocity than SNe with a
slowly evolving Si II 6355 line velocity (LVG). Since the two groups have
comparable luminosities, the temperature at the photosphere is higher in LVG
SNe. This explains the different overall spectral appearance of HVG and LVG
SNe. However, the variation of the Ca II and Si II absorptions at the highest
velocities (v >~ 20,000 km/s) suggests that additional factors, such as
asphericity or different abundances in the progenitor white dwarf, affect the
outermost layers. The C II 6578 line is marginally detected in 3 LVG SNe,
suggesting that LVG undergo less intense burning. The carbon mass fraction is
small, only less than 0.01 near the photosphere, so that he mass of unburned C
is only <~ 0.01 Msun. Radioactive 56Ni and stable Fe are detected in both LVG
and HVG SNe. Different Fe-group abundances in the outer layers may be one of
the reasons for spectral diversity among SNe Ia at the earliest times. The
diversity among SNe Ia at the earliest phases could also indicate an intrinsic
dispersion in the width-luminosity relation of the light curve.Comment: 13 pages, 10 figures, Accepted for publication in The Astrophysical
Journa
Spectral modelling of the "Super-Chandra" Type Ia SN 2009dc - testing a 2 M_sun white dwarf explosion model and alternatives
Extremely luminous, super-Chandrasekhar (SC) Type Ia Supernovae (SNe Ia) are
as yet an unexplained phenomenon. We analyse a well-observed SN of this class,
SN 2009dc, by modelling its photospheric spectra with a spectral synthesis
code, using the technique of 'Abundance Tomography'. We present spectral models
based on different density profiles, corresponding to different explosion
scenarios, and discuss their consistency. First, we use a density structure of
a simulated explosion of a 2 M_sun rotating C-O white dwarf (WD), which is
often proposed as a possibility to explain SC SNe Ia. Then, we test a density
profile empirically inferred from the evolution of line velocities
(blueshifts). This model may be interpreted as a core-collapse SN with an
ejecta mass ~ 3 M_sun. Finally, we calculate spectra assuming an interaction
scenario. In such a scenario, SN 2009dc would be a standard WD explosion with a
normal intrinsic luminosity, and this luminosity would be augmented by
interaction of the ejecta with a H-/He-poor circumstellar medium. We find that
no model tested easily explains SN 2009dc. With the 2 M_sun WD model, our
abundance analysis predicts small amounts of burning products in the
intermediate-/high-velocity ejecta (v > 9000 km/s). However, in the original
explosion simulations, where the nuclear energy release per unit mass is large,
burned material is present at high v. This contradiction can only be resolved
if asymmetries strongly affect the radiative transfer or if C-O WDs with masses
significantly above 2 M_sun exist. In a core-collapse scenario, low velocities
of Fe-group elements are expected, but the abundance stratification in SN
2009dc seems 'SN Ia-like'. The interaction-based model looks promising, and we
have some speculations on possible progenitor configurations. However,
radiation-hydro simulations will be needed to judge whether this scenario is
realistic at all.Comment: 22 pages, 12 figures, published in MNRAS. V2: several small
corrections (typos, style
Chapter 4 Data System and Data Management in a Federation of HPC/ Cloud Centers
Artificial Intelligence, Deep Learning, Machine Learning, Supercomputin