Observational surveys are now able to detect an increasing number of transients , such as core-collapse supernovae ( SN ) and powerful non-terminal outbursts ( SN impostors ) .
Dedicated spectroscopic facilities can follow up these events shortly after detection .
Here we investigate the properties of these explosions at early times .
We use the radiative transfer code CMFGEN to build an extensive library of spectra simulating the interaction of supernovae and their progenitor ’ s winds/circumstellar medium ( CSM ) .
We consider a range of progenitor mass-loss rates ( \dot { M } = 5 \times 10 ^ { -4 } to 10 ^ { -2 } ~ { } \mathit { M } _ { \odot } { yr } ^ { -1 } ) , abundances ( solar , CNO-processed , and He-rich ) , and SN luminosities ( L = 1.9 \times 10 ^ { 8 } to 2.5 \times 10 ^ { 10 } ~ { } \mathit { L } _ { \odot } ) .
The models simulate events \simeq 1 day after explosion , and we assume a fixed location of the shock front as \mathit { R } _ { in } = 8.6 \times 10 ^ { 13 } cm .
We show that the large range of massive star properties at the pre-SN stage causes a diversity of early-time interacting SN and impostors .
We identify three main classes of early-time spectra consisting of relatively high-ionisation ( e.g .
He II and O VI ) , medium-ionisation ( e.g .
C III and N III ) , and low-ionisation lines ( e.g .
He I and Fe II/III ) .
They are regulated by L and the CSM density .
Given a progenitor wind velocity \upsilon _ { \infty } , our models also place a lower limit of \dot { M } \gtrsim 5 \times 10 ^ { -4 } ( \upsilon _ { \infty } / 150 { km s ^ { -1 } } ) % \mathit { M } _ { \odot } { yr } ^ { -1 } for detection of CSM interaction signatures in observed spectra .
Early-time SN spectra should provide clear constraints on progenitors by measuring H , He , and CNO abundances if the progenitors come from single stars .
The connections are less clear considering the effects of binary evolution .
Nevertheless , our models provide a clear path for linking the final stages of massive stars to their post-explosion spectra at early times , and guiding future observational follow-up of transients with facilities such as the Zwicky Transient Facility .