Early observations of the nearby Type Ia supernova SN 2015F

We present photometry and time series spectroscopy of the nearby Type Ia supernova (SN Ia) SN 2015F over -16 d to +80 d relative to maximum light, obtained as part of the Public ESO Spectroscopic Survey of Transient Objects. SN 2015F is a slightly sub-luminous SN Ia with a decline rate of Δm₁₅(B) = 1.35 ± 0.03 mag, placing it in the region between normal and SN 1991bg-like events. Our densely sampled photometric data place tight constraints on the epoch of first light and form of the early-time light curve. The spectra exhibit photospheric C _II λ6580 absorption until -4 days, and high-velocity Ca _II is particularly strong at < -10 d at expansion velocities of ≃23 000 km s^-1. At early times, our spectral modelling with SYN++ shows strong evidence for iron-peak elements (Fe _II, Cr _II, Ti _II, and V_II) expanding at velocities > 14 000 km s^-1, suggesting mixing in the outermost layers of the SN ejecta. Although unusual in SN Ia spectra, including V_II in themodelling significantly improves the spectral fits. Intriguingly, we detect an absorption feature at~6800Åthat persists until maximum light.Our favoured explanation for this line is photospheric Al _II, which has never been claimed before in SNe Ia, although detached high-velocity C_II material could also be responsible. In both cases, the absorbing material seems to be confined to a relatively narrow region in velocity space. The nucleosynthesis of detectable amounts of Al _II would argue against a low-metallicity white dwarf progenitor.We also show that this 6800 Å feature is weakly present in other normal SN Ia events and common in the SN 1991bg-like sub-class.