Luminosity Indicators in the Ultraviolet Spectra of Type Ia Supernovae

TL;DR

This study analyzes ultraviolet spectra of Type Ia supernovae to identify spectral features and parameters, revealing a strong correlation between the UV ratio at maximum light and supernova luminosity, which could improve distance measurements.

Contribution

It introduces the UV ratio as a new spectral parameter and demonstrates its correlation with supernova luminosity, enhancing existing distance estimation methods.

Findings

UV ratio at maximum light correlates with SN Ia luminosity.

Fe II 3250 line velocity does not correlate with light-curve shape.

Distinct EW behavior for slow and fast-declining supernovae.

Abstract

We present a complete sample of International Ultraviolet Explorer and Hubble Space Telescope ultraviolet (UV) spectra of Type Ia supernovae (SNe Ia) through 2004. We measure the equivalent width (EW) and blueshifted velocity of the minimum of the one strong UV feature, Fe II 3250. We also quantify the slope of the near-UV spectra using a new parameter, the ``UV ratio.'' We find that the velocity of the Fe II line does not correlate with light-curve shape, while the EW shows distinct behavior for the slow and fast-declining objects. Using precise Cepheid and surface brightness fluctuation distance measurements of 6 objects with UV spectra observed near maximum light (a total of 12 spectra), we determine that the UV ratio at maximum light is highly correlated with SN Ia luminosity. A larger sample of UV spectra is necessary to increase the statistical certainty of these luminosity indicators and whether they can be combined with light-curve shape to improve measured SN Ia distances.