Reka Konyves-Toth
A new class of exploding stars, the so-called superluminous supernovae (SLSNe), was discovered and intensely studied in the past two decades. The brightness of these extremely luminous events exceed ~100 times the brightness of the normal supernova explosions. Like classical supernova types, SLSNe are divided into two main groups: the hydrogen-rich Type II SLSNe and the hydrogen-poor SLSN-I classes. In our study, we focus on SLSNe-I, which has also been divided into two subgroups: the fast evolving SLSN-I have an average light curve rise time of ~28 days, and the slow evolving SLSNe-I with rise time of ~52 days.
Here, we present a comparative spectroscopic examination of the recently discovered and relatively close Fast SLSNe-I, SN1019neq with the well-observed Slow SLSNe-I, SN2010kd. Our main goal is to explore the differences between the two subgroups of SLSNe-I apart from the dissimilarity in their light curve evolution timescale, and the differences in their spectral evolution. It is a crucial question, because the differences in the spectrum imply different ejecta as well.
We examine the chemical evolution and ejecta composition of SN2019neq and SN2010kd by identifying the elements and ionization states in their spectra using a spectral synthesis code. Based on the spectrum modeling, we classify SN2019neq as a fast evolving SLSN-I having one of the highest photospheric velocity gradients observed for a SLSN-I. Furthermore, we give constraints on the ejecta mass and find, that both SLSNe have at least one order of magnitude higher ejecta mass than normal Type Ia supernovae (, which is ~1.45 solar masses), while the fast-evolving SN2019neq has a factor of 2 lower ejecta mass compared to the slow-evolving SN2010kd. These mass estimates suggest the existence of a possible correlation between the evolution timescale and the ejected mass of SLSNe-I.
Like classical supernova types, superluminous supernovae (SLSNe) are divided into two main groups: the hydrogen-rich Type II SLSNe, and the hydrogen-poor SLSNe-I classes. SLSNe-II are separated into the following subclasses: SLSNe-IIn, with a luminosity evolution powered by an interaction with a massive circumstellar medium, and normal SLSNe-II, ostensibly without interaction. SLSNe-I has also been divided into two subgroups: the fast evolving, and the slow evolving.
Slide2
Since there is no single, unblended feature in the spectra shown on the left, a spectrum synthesis code, named SYN++ was necessary to determine the chemical composition of the ejecta reliably.
These figures are illustrating the role of some of the model parameters in the spectrum formation. In these plots, there are 4 curves, each showing a spectrum that belongs to a particular parameter value. The yellow curve denotes the smallest value of the parameter, than orange, red, and black curves represent the increasing values respectively.
Slide3
We modeled the spectra of SN2019neq, taken at 3 epochs. The figures on the left show the modeling of the -4 days phase spectrum of SN2019neq. It is interesting, that alternatively, this spectrum can also be fitted assuming different chemical elements. This shows that the chemical composition of the SN is difficult to determine.
In the figures to the right, the first spectrum of SN2019neq taken at phase -4d is compared to that of SN2005ap taken at similar phase (-2d). The similarity of the two spectra is apparent. The lack of hydrogen features in the spectrum provides significant evidence, that SN2019neq is a SLSN-I. But is it slow or fast? To decide, we compared the +29d spectrum of SN2019neq to that of SN2010kd taken at +85d phase. The features of the two spectra are quite similar, in spite of their different phases. SN2010kd is a slowly evolving SLSN-I, and SN2019neq reached the same physical stage at ~30 days as SN2010kd at +85d phase, illustrating the fast spectral evolution of SN2019neq.
Slide4
The evolution of the photospheric velocities are shown as a function of the phase since maximum. It is seen that SN2019neq shows a factor of ~2 higher velocity than SN2010kd around maximum light, that quickly decreases to ~12000 km/s by +30d phase. This very fast velocity decline is probably caused by the quick decrease of the density in the outer ejecta, which may suggest a different density profile and somewhat lower ejecta mass for SN2019neq compared to SN2010kd.
The optical depth of ionized carbon is the same order of magnitude for SN2010kd and SN2019neq. It quickly decreases in both SNe after maximum light. This behavior is consistent with the observations of other SLSNe, where the carbon features can be found only before or around maximum, and they quickly diminish in post-maximum phases.
The optical depth of FeII seems to be different for the two objects: SN2010kd shows nearly constant values after maximum, while the FeII optical depth of SN2019neq rises rapidly in this phase. This is related to the strengthening of the FeII features with decreasing temperature, as seen e.g. in the post-maximum spectra of Type SNe Ia during the FeII-phase.
Finally, we gave lower limits to the ejecta mass of these object. We predicted inferred a ~48M☉ mass for the Slow SLSN-I, SN2010kd, which is more than a factor of 2 higher than the value belonging to the fast evolving one, SN2019neq ~23M☉. They exceed the typical SN-Ia ejecta mass by at least one order of magnitude. Furthermore, we suggest a possible correlation between the ejecta mass and evolution time scale of SLSNe: faster evolving SLSNe may have lower ejecta mass.