Type Ia supernovae (SNe Ia) are explosions of white dwarfs and remain leading probes of cosmological expansion history. With uncertainties on the dark energy equation of state parameter being reduced to 5% and that on the Hubble constant to nearly 1%, we find ourselves well and truly in the era of precision SN cosmology. Recent analyses of large SN samples have shown that poor understanding of the astrophysics of the SN explosions, the relationships between their brightness and their host galaxy environment, and dust along their line of sight have become the largest source of uncertainty in cosmological measurements. SNe Ia brightnesses are currently standardized according to their light curve shape and colour, but even after this process, residual correlations remain between the inferred distances to SNe and their host galaxy properties, which require further correction. In some samples, the magnitude of the so called `mass step' appears to be larger for redder SNe, which can be modelled by different dust extinction laws either side of the stellar mass divide. However, it is unlikely that galaxy stellar mass itself drives the dust extinction law along the SN line of sight. Open questions include: is the dust that is causing most of the extinction circumstellar (and thus affected by the progenitor and explosion) or interstellar? What is the truly causing a change in dust properties, and what is the best observational tracer of this driver? Do these dust laws evolve with redshift? Differing dust laws may not be able to explain the entire `mass step': in recent analyses a step remains in blue (`dust free') SNe, and appears to be more defined when comparing SNe in galaxies with different colours or specific star-formation rates. If true, this evidence points to SNe with progenitors of different ages having different intrinsic luminosities. What causes this difference is probably caused by the progenitor system and explosion mechanism which are both the focus of intense ongoing theoretical and observational study. Could age be tracing white dwarf mass? Metallicity? Could it signify a switch in dominant progenitor channel between single and double degenerate scenarios?

Two key avenues in the pursuit of understanding SN Ia progenitors are their light curve characteristics, and the rate of their occurrence. The modelling early excesses and late time evolution, and links between these properties and host environments, may shed light on SN environmental relationships. The rates of SNe with different properties, their delay-time distributions, and their correlation with environment, are additional probes of progenitor and explosion scenarios.

In this symposium we seek to bring together theorists, observers, and cosmologists alike to address these outstanding issues: to share new results and promising models of SN explosions; to devise tests of the dust extinction affecting SNe Ia, and to look ahead to the unprecedented data sets from the era of Rubin/LSST.

Programme

• Observations of astrophysical biases in SN Ia cosmology
• Dust in the line of sight to SNe
• Observing dust properties in SN host galaxies
• Progenitors of thermonuclear SNe: how do reproduce observed diversity
...

Invited speakers

• Lisa Kelsey(ICG Portsmouth)
• Mattia Bulla(University of Ferrara)
• Mark Magee (University of Warwick)
• Matt Grayling (University of Cambridge)
• Lindsey Kwok (Rutgers University)

Scientific organisers

• Phil Wiseman (Southampton)
• Maxime Deckers (Trinity College Dublin/ ICG Portsmouth)
• Brodie Popovic (IN2P3)
• Mat Smith (Lancaster)
• Marcus Toy (Southampton)
• Maria Vincenzi (Duke, Hubble Fellow)

Contact

p.s.wiseman @ soton.ac.uk