We present the first quantitative spectroscopic modeling of an early-time supernova that interacts with its progenitor wind .
Using the radiative transfer code CMFGEN , we investigate the recently-reported 15.5 h post-explosion spectrum of the type IIb SN 2013cu .
For the first time , we are able to directly measure the chemical abundances of a SN progenitor and find a relatively H-rich wind , with H and He abundances ( by mass ) of X = 0.46 \pm 0.2 and Y = 0.52 \pm 0.2 , respectively .
The wind is enhanced in N and depleted in C relative to solar values ( mass fractions of 8.2 \times 10 ^ { -3 } and 1.0 \times 10 ^ { -5 } , respectively ) .
We obtain that a dense wind/circumstellar medium , with a mass-loss rate of \dot { M } \simeq 3 \times 10 ^ { -3 } ~ { } \mathit { M } _ { \odot } { yr } ^ { -1 } and \mathit { v } _ { \mathrm { wind } } \simeq 100 ~ { } { km s ^ { -1 } } , surrounds the precursor at the pre-SN stage .
These values are lower than previous analytical estimates , although we find \dot { M } / \upsilon _ { \infty } consistent with previous work .
We also compute a CMFGEN model to constrain the progenitor spectral type and find that the high \dot { M } and low \mathit { v } _ { \mathrm { wind } } imply that the star had an effective temperature of \simeq~ { } 8000 ~ { } \mathrm { K } immediately before the SN explosion .
Our models suggest that the progenitor was either an unstable luminous blue variable or a yellow hypergiant undergoing an eruptive phase , and rule out a WR star .
We classify the post-explosion spectra at 15.5 h as XWN5 ( h ) and advocate for the use of the prefix ‘ X ’ ( eXplosion ) to avoid confusion between post-explosion , non-stellar spectra with those of massive stars .
We show that the progenitor spectral type is significantly different than the early post-explosion spectral type owing to the huge differences in the ionization structure before and after the SN event .
We find the following temporal evolution : LBV/YHG \rightarrow XWN5 ( h ) \rightarrow SN IIb .
Future early-time spectroscopy in the UV will give access to additional spectroscopic diagnostics and further constrain the properties of SN precursors , such as their metallicities .