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Register a new userThe Late-time Expansion of the Ejecta of SN 1987A
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The evolution of the shape and size of the ejecta of SN 1987A is analyzed over a period of ~ 8 years based on HST images and spectra taken between 1278 and 4336 days after the supernova outburst. We combine both proprietary and archival HST data obtained with the FOC, WFPC2 and STIS. The low resolution near-UV prism FOC spectrum obtained at day 3043 has not been described previously. Although the FWHM of the ejecta grew linearly over the time span studied, the appearance of the SN envelope also changed markedly with wavelength. At visible wavelengths (lambda ~ 5000 Angstrom) the ejecta became progressively more elongated, reaching an ellipticity epsilon ~ 0.25 by day 4000. In the near-UV (lambda ~ 2500 AA), the ejecta remained closely circular (epsilon <= 0.1) and ~ 50% larger in angular extent than in the visible. The FOC prism observations show that the large extent of the SN envelope is confined to a grouping of resonance lines spanning Mg I 2852, Mg II 2795,2802 and several Fe II multiplets -- thereby confirming that the larger size of the debris in the near-UV is due to scattering in these optically thick transitions compared to the optically thin forbidden and semi-forbidden transitions that dominate the visible spectrum. The available data are not of sufficient quality to detect the slight deviation from linear expansion expected for the outermost regions of the near-UV images as predicted by Chugai et al. (1997).
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We have used the Faint Object Spectrograph on the Hubble Space Telescope to observe the spectra of SN 1987A over the wavelength range 2000 -- 8000 AA on dates 1862 and 2210 days after the supernova outburst. Even these pre-COSTAR observations avoid much of the contamination from the bright stars nearby and provide a very useful set of line strengths and shapes for analysis. The spectrum is formed in an unusual physical setting: cold gas which is excited and ionized by energetic electrons from the radioactive debris of the supernova explosion. The spectra of SN 1987A at this phase are surprisingly similar to those of the nova shells of CP Puppis and T Pyxidis decades after outburst. SN 1987A and the novae are characterized by emission from material with electron temperatures of only a few hundred degrees Kelvin, and show narrow Balmer continuum emission and strong emission lines from O$^+$. The Balmer continuum shape requires the electron temperature in the supernova ejecta to be as low as 500 K on day 1862 and 400 K on day 2210 after outburst. The OIIUV doublet is surprisingly strong and is plausibly powered by collisional ionization of neutral oxygen to excited states of O$^+$. The line intensity ratio of the OID doublet obtained from Gaussian fits of the line profiles is 1.8$pm0.2$, contrary to the optically thin limit of 3. This ratio is {it not} due to an optical depth effect, but rather is an artifact of assuming a Gaussian profile to fit the OID doublet profile. Specifying the line ratio $R, = , F([{rm OI}]6300)/F([{rm OI}]6364)$ = 3 is consistent with the data and allows a calculation of the decomposed line profile. All the observed strong lines are found to be blueshifted by a similar amount
We present high angular resolution (~80 mas) ALMA continuum images of the SN 1987A system, together with CO $J$=2 $!rightarrow!$ 1, $J$=6 $!rightarrow!$ 5, and SiO $J$=5 $!rightarrow!$ 4 to $J$=7 $!rightarrow!$ 6 images, which clearly resolve the ejecta (dust continuum and molecules) and ring (synchrotron continuum) components. Dust in the ejecta is asymmetric and clumpy, and overall the dust fills the spatial void seen in H$alpha$ images, filling that region with material from heavier elements. The dust clumps generally fill the space where CO $J$=6 $!rightarrow!$ 5 is fainter, tentatively indicating that these dust clumps and CO are locationally and chemically linked. In these regions, carbonaceous dust grains might have formed after dissociation of CO. The dust grains would have cooled by radiation, and subsequent collisions of grains with gas would also cool the gas, suppressing the CO $J$=6 $!rightarrow!$ 5 intensity. The data show a dust peak spatially coincident with the molecular hole seen in previous ALMA CO $J$=2 $!rightarrow!$ 1 and SiO $J$=5 $!rightarrow!$ 4 images. That dust peak, combined with CO and SiO line spectra, suggests that the dust and gas could be at higher temperatures than the surrounding material, though higher density cannot be totally excluded. One of the possibilities is that a compact source provides additional heat at that location. Fits to the far-infrared--millimeter spectral energy distribution give ejecta dust temperatures of 18--23K. We revise the ejecta dust mass to $mathrm{M_{dust}} = 0.2-0.4$M$_odot$ for carbon or silicate grains, or a maximum of $<0.7$M$_odot$ for a mixture of grain species, using the predicted nucleosynthesis yields as an upper limit.
Extensive early observations proved that the ejecta of supernova 1987A (SN 1987A) are aspherical. Fifteen years after the supernova explosion, the Hubble Space Telescope has resolved the rapidly expanding ejecta. The late-time images and spectroscopy provide a geometrical picture that is consistent with early observations and suggests a highly structured, axially symmetric geometry. We present here a new synthesis of the old and new data. We show that the Bochum event, presumably a clump of $^{56}$Ni, and the late-time image, the locus of excitation by $^{44}$Ti, are most naturally accounted for by sharing a common position angle of about 14degree, the same as the mystery spot and early speckle data on the ejecta, and that they are both oriented along the axis of the inner circumstellar ring at 45degree to the plane of the sky. We also demonstrate that the polarization represents a prolate geometry with the same position angle and axis as the early speckle data and the late-time image and hence that the geometry has been fixed in time and throughout the ejecta. The Bochum event and the Doppler kinematics of the [Ca II]/[O II] emission in spatially resolved HST spectra of the ejecta can be consistently integrated into this geometry. The radioactive clump is deduced to fall approximately along the axis of the inner circumstellar ring and therefore to be redshifted in the North whereas the [Ca II]/[O II] 7300 AA emission is redshifted in the South. We present a jet-induced model for the explosion and argue that such a model can account for many of the observed asymmetries. In the jet models, the oxygen and calcium are not expected to be distributed along the jet, but primarily in an expanding torus that shares the plane and northern blue shift of the inner circumstellar ring.
Most massive stars end their lives in core-collapse supernova explosions and enrich the interstellar medium with explosively nucleosynthesized elements. Following core collapse, the explosion is subject to instabilities as the shock propagates outwards through the progenitor star. Observations of the composition and structure of the innermost regions of a core-collapse supernova provide a direct probe of the instabilities and nucleosynthetic products. SN 1987A in the Large Magellanic Cloud (LMC) is one of very few supernovae for which the inner ejecta can be spatially resolved but are not yet strongly affected by interaction with the surroundings. Our observations of SN 1987A with the Atacama Large Millimeter/submillimeter Array (ALMA) are of the highest resolution to date and reveal the detailed morphology of cold molecular gas in the innermost regions of the remnant. The 3D distributions of carbon and silicon monoxide (CO and SiO) emission differ, but both have a central deficit, or torus-like distribution, possibly a result of radioactive heating during the first weeks (nickel heating). The size scales of the clumpy distribution are compared quantitatively to models, demonstrating how progenitor and explosion physics can be constrained.
We present a study of the morphology of the ejecta in Supernova 1987A based on images and spectra from the HST as well as integral field spectroscopy from VLT/SINFONI. The HST observations were obtained between 1994 - 2011 and primarily probe the outer hydrogen-rich zones of the ejecta. The SINFONI observations were obtained in 2005 and 2011 and instead probe the [Si I]/[Fe II] emission from the inner regions. We find a strong temporal evolution of the morphology in the HST images, from a roughly elliptical shape before ~5,000 days, to a more irregular, edge-brightened morphology thereafter. This transition is a natural consequence of the change in the dominant energy source powering the ejecta, from radioactive decay before ~5,000 days to X-ray input from the circumstellar interaction thereafter. The [Si I]/[Fe II] images display a more uniform morphology, which may be due to a remaining significant contribution from radioactivity in the inner ejecta and the higher abundance of these elements in the core. Both the H-alpha and the [Si I]/[Fe II] line profiles show that the ejecta are distributed fairly close to the plane of the inner circumstellar ring, which is assumed to define the rotational axis of the progenitor. The H-alpha emission extends to higher velocities than [Si I]/[Fe II] as expected. There is no clear symmetry axis for all the emission and we are unable to model the ejecta distribution with a simple ellipsoid model with a uniform distribution of dust. Instead, we find that the emission is concentrated to clumps and that the emission is distributed somewhat closer to the ring in the north than in the south. This north-south asymmetry may be partially explained by dust absorption. We compare our results with explosion models and find some qualitative agreement, but note that the observations show a higher degree of large-scale asymmetry.
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