New updates to the Very Large Telescope (VLT) in Chile have allowed a team of astronomers to detect elusive optical emissions in the remnants of three type Ia supernovae. The international team, led by Ivo Seitenzahl at the University of New South Wales in Canberra, used the improvements to observe Doppler shifts in the spectral lines emitted by highly ionized states of iron and sulphur in the gases.
Formed when white dwarf stars collapse in colossal thermonuclear explosions, type Ia supernovae are known to influence processes including star formation and galaxy evolution. Extensive optical surveys have provided astronomers with huge amounts of data about the events, allowing for reliable theoretical models to explain their formation and evolution.
Among the predictions of these models are that type Ia supernovae must be triggered by white dwarf stars above the Chandrasekhar mass limit, which can be reached by accreting material from companion stars. In addition, the models predict that different elements will be more abundant in different layers of the explosion due to the onion-like structure of the white dwarf’s progenitor star, in which heavier elements reside in layers closer to the core.
Yet despite their successes so far, these models remain plagued with uncertainties due to limitations in previous observations of the events. In the first year of a supernova, for example, its remnants are optically thick, which means that astronomers can only measure the composition of its outermost layers. While X-rays emitted by the superheated fronts of shockwaves in the remnants can reveal their composition to an extent, the limited spectral resolutions of current instruments makes them difficult to detect.
In their study, Seitenzahl’s team exploited a new spectrometer that has recently been added to the VLT to study the remnants of a supernova at visible rather than X-ray wavelengths. The MUSE spectrometer combines high spectral resolution with a wide field-of-view, which makes it possible to acquire spectra at thousands of positions at the same time.
The team used the spectrometer to search for visible wavelengths emitted by highly ionized states of iron and sulphur in the slocked, nonradiative remnants of three supernovae in the Large Magellanic Cloud. Though the light is extremely faint due to these transitions being optically forbidden, the VLT’s new setup had high enough spectral resolution to detect the characteristic transmission lines of several different ions.
Seitenzahl and colleagues then used a new technique, dubbed “supernova remnant tomography”, to relate the Doppler shifts of the spectral lines to the velocities of supernova remnants at different positions. This allowed them to test previous models of supernova explosions, and their subsequent evolution, more rigorously than ever before.
Their analysis revealed a clearly layered structure in one of the supernova remnants, with sulphur emission occurring in a region outside of one dominated by iron emissions. The team also found that one supernova appeared to have originated from a white dwarf with a lower mass than the Chandrasekhar limit. Though the dynamics of this event appeared consistent with current models, the observed spectral lines were less Doppler shifted than predicted.
Seitenzahl’s team believe that this new technique represents an important advance in supernova analysis. They now aim to use observed their observed shifts in spectral lines to update current models of supernova formation and evolution.
