Late Spectral Evolution of the Ejecta and Reverse Shock in Sn 1987a

Abstract

We present observations with VLT and HST of the broad emission lines from the inner ejecta and reverse shock of SN 1987A from 1999 Feb. until 2012 Jan. (days 4381 -9100 after explosion). We detect broad lines from Hα, Hβ, Mg I], Na I, [O I], [Ca II] and a feature at ∼ 9220Å. We identify the latter line with Mg II λλ 9218, 9244, which is most likely pumped by Lyα fluorescence. Hα, and Hβ both have a centrally peaked component, extending to ∼ 4500 km s −1 and a very broad component extending to 11, 000 km s −1 , while the other lines have only the central component. The low velocity component comes from unshocked ejecta, heated mainly by X-rays from the circumstellar environment, whereas the broad component comes from faster ejecta passing through the reverse shock, created by the collision with the circumstellar ring. The flux in Hα from the reverse shock has increased by a factor of 4 − 6 from 2000 to 2007. After that there is a tendency of flattening of the light curve, similar to what may be seen in soft X-rays and in the optical lines from the shocked ring. The core component seen in Hα, [Ca II] and Mg II has experienced a similar increase, which is consistent with that found from HST photometry. The ring-like morphology of the ejecta is explained as a result of the X-ray illumination, depositing energy outside of the core of the ejecta. The energy deposition of the external X-rays is calculated using explosion models for SN 1987A and we predict that the outer parts of the unshocked ejecta will continue to brighten because of this. We finally discuss evidence for dust in the ejecta from line asymmetries.

Three-Dimensional Distribution of Ejecta in Supernova 1987a at 10,000 Days

“…In addition, we studied the temporal evolution of the ejecta morphology in the HST images, finding that it changes from an approximately elliptical shape before ∼5000 days to an edgebrightened, more irregular morphology thereafter. This transition coincides with and can be explained by the change in the dominant energy source powering the ejecta (from radioactive decay of 44 Ti to X-rays from the ring, see also Fransson et al 2013). …”

“…Fransson et al (2013) showed that the line profile was still consistent with this blueshift at around 4400 days. This was the last optical spectrum obtained during the epoch when the energy input to the whole ejecta was dominated by radioactivity.…”

“…It is also possible that there is a contribution from HeIl5876 blended with the NaI doublet, although Jerkstrand et al (2011) find that NaI dominates. The identification of the MgII lines is discussed in Jerkstrand et al (2011) and Fransson et al (2013). While this emission feature is outside the plotted region in Figure 2, it is seen in the upper panel of Figure 3, which shows the full 1D ejecta spectrum obtained by summing all five slits.…”

“…The brightening and change in morphology are seen in all the HST filters that were used, which together cover the wavelength interval ∼2000-Å 10000 . Fransson et al (2013) showed that the change in morphology is a natural consequence of the X-rays being absorbed at the boundary between the H envelope and the core, where the density gradient is steep and where the abundances of metals increase, resulting in a dramatic increase in the X-ray absorption.…”

“…If the "hole" were due to dust, we would expect it to become less prominent with time because the optical depth of the dust is expected to decrease ast 2 , where t is the time since explosion, contrary to what we observe. Instead, the "hole" appears at the same time as the ejecta start brightening due to the X-ray illumination, and it is a natural consequence of the absorption in the outer parts of the ejecta (L13; Fransson et al 2013). In addition, as also discussed in L13, considerable fine tuning would be required for the dust to be optically thick at m0.85 m (where the "hole" is seen in the HST/F814W filter) but not in the H band at m1.6 m. A likely explanation for the observations is that the dust is in optically thick clumps that are affecting the optical and NIR in the same way.…”