108 research outputs found
Exploring the spectroscopic diversity of Type Ia Supernovae
The velocities and equivalent widths (EWs) of a set of absorption features
are measured for a sample of 28 well-observed Type Ia supernovae (SN Ia)
covering a wide range of properties. The values of these quantities at maximum
are obtained through interpolation/extrapolation and plotted against the
decline rate, and so are various line ratios. The SNe are divided according to
their velocity evolution into three classes defined in a previous work of
Benetti et al.: low velocity gradient (LVG), high velocity gradient (HVG) and
FAINT. It is found that all the LVG SNe have approximately uniform velocities
at B maximum, while the FAINT SNe have values that decrease with increasing
Delta m_15(B), and the HVG SNe have a large spread. The EWs of the Fe-dominated
features are approximately constant in all SNe, while those of Intermediate
mass element (IME) lines have larger values for intermediate decliners and
smaller values for brighter and FAINT SNe. The HVG SNe have stronger Si II
6355-A lines, with no correlation with Delta m_15(B). It is also shown that the
Si II 5972 A EW and three EW ratios, including one analogous to the R(Si II)
ratio introduced by Nugent et al., are good spectroscopic indicators of
luminosity. The data suggest that all LVG SNe have approximately constant
kinetic energy, since burning to IME extends to similar velocities. The FAINT
SNe may have somewhat lower energies. The large velocities and EWs of the IME
lines of HVG SNe appear correlated with each other, but are not correlated with
the presence of high-velocity features in the Ca II infrared triplet in the
earliest spectra for the SNe for which such data exist.Comment: 24 pages, 22 figures, updated (typo and style corrections). MNRAS, in
pres
Multi-layered Spectral Formation in SNe Ia Around Maximum Light
We use the radiative transfer code PHOENIX to study the line formation of the
wavelength region 5000-7000 Angstroms. This is the region where the SNe Ia
defining Si II feature occurs. This region is important since the ratio of the
two nearby silicon lines has been shown to correlate with the absolute blue
magnitude. We use a grid of LTE synthetic spectral models to investigate the
formation of line features in the spectra of SNe Ia. By isolating the main
contributors to the spectral formation we show that the ions that drive the
spectral ratio are Fe III, Fe II, Si II, and S II. While the first two strongly
dominate the flux transfer, the latter two form in the same physical region
inside of the supernova. We also show that the naive blackbody that one would
derive from a fit to the observed spectrum is far different than the true
underlying continuum.Comment: 35 pages, 15 figures, ApJ (2008) 684 in pres
Diversity of Decline-Rate-Corrected Type Ia Supernova Rise Times: One Mode or Two?
B-band light-curve rise times for eight unusually well-observed nearby Type
Ia supernovae (SNe) are fitted by a newly developed template-building
algorithm, using light-curve functions that are smooth, flexible, and free of
potential bias from externally derived templates and other prior assumptions.
From the available literature, photometric BVRI data collected over many
months, including the earliest points, are reconciled, combined, and fitted to
a unique time of explosion for each SN. On average, after they are corrected
for light-curve decline rate, three SNe rise in 18.81 +- 0.36 days, while five
SNe rise in 16.64 +- 0.21 days. If all eight SNe are sampled from a single
parent population (a hypothesis not favored by statistical tests), the rms
intrinsic scatter of the decline-rate-corrected SN rise time is 0.96 +0.52
-0.25 days -- a first measurement of this dispersion. The corresponding global
mean rise time is 17.44 +- 0.39 days, where the uncertainty is dominated by
intrinsic variance. This value is ~2 days shorter than two published averages
that nominally are twice as precise, though also based on small samples. When
comparing high-z to low-z SN luminosities for determining cosmological
parameters, bias can be introduced by use of a light-curve template with an
unrealistic rise time. If the period over which light curves are sampled
depends on z in a manner typical of current search and measurement strategies,
a two-day discrepancy in template rise time can bias the luminosity comparison
by ~0.03 magnitudes.Comment: As accepted by The Astrophysical Journal; 15 pages, 6 figures, 2
tables. Explanatory material rearranged and enhanced; Fig. 4 reformatte
Type Ic supernova of a 22 M⊙ progenitor
© 2020 The Author(s). Type Ic supernovae (SNe Ic) are a sub-class of core-collapse SNe that exhibit no helium or hydrogen lines in their spectra. Their progenitors are thought to be bare carbon-oxygen cores formed during the evolution of massive stars that are stripped of their hydrogen and helium envelopes sometime before collapse. SNe Ic present a range of luminosities and spectral properties, from luminous GRB-SNe with broad-lined spectra to less luminous events with narrow-line spectra. Modelling SNe Ic reveals a wide range of both kinetic energies, ejecta masses, and 56Ni masses. To explore this diversity and how it comes about, light curves and spectra are computed from the ejecta following the explosion of an initially 22 M⊙ progenitor that was artificially stripped of its hydrogen and helium shells, producing a bare CO core of ∼5 M⊙, resulting in an ejected mass of ∼4 M⊙, which is an average value for SNe Ic. Four different explosion energies are used that cover a range of observed SNe. Finally, 56Ni and other elements are artificially mixed in the ejecta using two approximations to determine how element distribution affects light curves and spectra. The combination of different explosion energy and degree of mixing produces spectra that roughly replicate the distribution of nearpeak spectroscopic features of SNe Ic. High explosion energies combined with extensive mixing can produce red, broad-lined spectra, while minimal mixing and a lower explosion energy produce bluer, narrow-lined spectra
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
Studying the diversity of Type Ia supernovae in the ultraviolet: comparing models with observations
In the ultraviolet (UV), Type Ia supernovae (SNe Ia) show a much larger diversity in their properties than in the optical. Using a stationary Monte Carlo radiative transfer code, a grid of spectra at maximum light was created varying bolometric luminosity and the amount of metals in the outer layers of the SN ejecta. This model grid is then compared to a sample of high-redshift SNe Ia in order to test whether the observed diversities can be explained by luminosity and metallicity changes alone. The dispersion in broad-band UV flux and colours at approximately constant optical spectrum can be readily matched by the model grid. In particular, the UV1 − b colour is found to be a good tracer of metal content of the outer ejecta, which may in turn reflect on the metallicity of the SN progenitor. The models are less successful in reproducing other observed trends, such as the wavelengths of key UV features, which are dominated by reverse fluorescence photons from the optical, or intermediate-band photometric indices. This can be explained in terms of the greater sensitivity of these detailed observables to modest changes in the relative abundances. Specifically, no single element is responsible for the observed trends. Due to their complex origin, these trends do not appear to be good indicators of either luminosity or metallicity
Interannuelle Variabilität des hochalpinen CO2- und CH4-Haushalts auf Basis atmosphärischer Messreihen 2011-2015
Photometric and spectroscopic observations, and abundance tomography modelling of the Type Ia supernova SN 2014J located in M82
Spectroscopic and photometric observations of the nearby Type Ia Supernova (SN Ia) SN 2014J are presented. Spectroscopic observations were taken −8 to +10 d relative to Bband maximum, using FRODOSpec, a multipurpose integral-field unit spectrograph. The observations range from 3900 to 9000 Å. SN 2014J is located in M82 which makes it the
closest SN Ia studied in at least the last 28 yr. It is a spectroscopically normal SN Ia with high-velocity features.Wemodel the spectra of SN 2014J with a Monte Carlo radiative transfer code, using the abundance tomography technique. SN 2014J is highly reddened, with a host
galaxy extinction of E(B − V) = 1.2 (RV = 1.38). It has a �m15(B) of 1.08 ± 0.03 when corrected for extinction. As SN 2014J is a normal SN Ia, the density structure of the classical W7 model was selected. The model and photometric luminosities are both consistent with B-band maximum occurring on JD 245 6690.4 ± 0.12. The abundance of the SN 2014J behaves like other normal SN Ia, with significant amounts of silicon (12 per cent by mass)and sulphur (9 per cent by mass) at high velocities (12 300 km s−1) and the low-velocity ejecta (v < 6500 km s−1) consists almost entirely of 56Ni.
Key words: radiative transfer – techniques: spectroscopic – supernovae: general – supernovae: individual: SN 2014J
A massive, energetic model for the luminous transitional Type Ib/IIb SN 2020cpg
Using a combined spectral and light-curve modelling approach, we fit a massive and energetic explosion model to the luminous Type Ib/IIb SN 2020cpg. This model has an ejected mass of ∼(7 ± 2) M⊙ with a final explosion energy of ∼(6 ± 1) × 1051 erg with MNi = 0.27 ± 0.05 M⊙. The early spectra are hot and blue with weak He I lines, and a complicated Hα region suggested to be a multicomponent feature. Modelling the spectra required ∼0.08 M⊙ of H at velocities >11 000 km s−1 and a total He mass of ∼1.0 M⊙ at velocities >9500 km s−1 above CO-rich ejecta. This model has a ratio of kinetic energy and ejected mass of 0.85+0.5−0.3 foe M⊙−1. The high luminosity and explosion energy results in a broadened Hα line that is blended with Si II, C II, and He I, which led to the initial classification of SN 2020cpg as a Type Ib. We instead classify SN 2020cpg as a bright transitional event between the Type Ib and Type IIb classes. Comparing our model parameters to stellar evolution models, a progenitor mass of 25–30 M⊙, i.e. stripped of most of the hydrogen shell and of some of the helium shell prior to collapse produces a He core of comparable mass. The excess 56Ni production in SN 2020cpg as compared to objects of similar ejected mass may suggest evidence of additional energy sources such as a failed GRB or weak magnetar energy injection, or a smaller remnant mass
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