arXiv subjects

Stephan Hachinger

Publications and source records attributed to Stephan Hachinger.

22 records · Page 2Linked to original sources

arXiv2013

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.

astro-ph.SR↗

arXiv2012

Spectral analysis of the 91bg-like Type Ia SN 2005bl: Low luminosity, low velocities, incomplete burning

The properties of underluminous type Ia supernovae (SNe Ia) of the 91bg subclass have yet to be theoretically understood. Here, we take a closer look at the structure of the dim SN Ia 2005bl. We infer the abundance and density profiles needed to reproduce the observed spectral evolution between -6 d and +12.9 d with respect to B maximum. Initially, we assume the density structure of the standard explosion model W7; then we test whether better fits to the observed spectra can be obtained using modified density profiles with different total masses and kinetic energies. Compared to normal SNe Ia, we find a lack of burning products especially in the rapidly-expanding outer layers (v>~15000 km/s). The zone between ~8500 and 15000 km/s is dominated by oxygen and includes some amount of intermediate mass elements. At lower velocities, intermediate mass elements dominate. This holds down to the lowest zones investigated in this work. This fact, together with negligible-to-moderate abundances of Fe-group elements, indicates large-scale incomplete Si burning or explosive O burning, possibly in a detonation at low densities. Consistently with the reduced nucleosynthesis, we find hints of a kinetic energy lower than that of a canonical SN Ia: The spectra strongly favour reduced densities at >~13000 km/s compared to W7, and are very well fitted using a rescaled W7 model with original mass (1.38 M_sun), but a kinetic energy reduced by ~30 % (i.e. from 1.33e51 erg to 0.93e51 erg).

astro-ph.HE↗

arXiv2008

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 λ5972 and λ6355 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 λ5972, because SiII λ6355 is saturated. In less luminous objects, SiII λ5972 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 λ5972 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.

astro-ph↗

arXiv2007

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.

astro-ph↗