Supernova 1987A What Have We Learned about Nucleosynthesis? Charles H. Atwood Mercer University, Macon. GA 31207 The supernova that was seen in the Large Magellanic Cloud on Fehruarv 23. 1987. was the first supernova visible r 1604 to the naked eye iince'the one described b y ~ e p l e in (Fig. 1).While supernovae are not exceedingly rare (usually about a dozen are detected every year), to have one occur so close (the estimated distance to the Large Magellanic Cloud is 160,000-170,000 light years) is a once-in-a-lifetime opportunity for astronomers (I). Furthermore, this supernova occurred a t a time when modern astronomical equipment was ready to record the event in detail and at the same time collect the necessary data to support or disprove the theory of nucleosynthesis in an unprecedented manner. I t is nucleosynthesis that makes the event of special concern to us chemists because, if correct, i t describes the manner in which the elements that our profession revolves around are made in stars and ultimately find their way onto our periodic charts for classroom discussions. In this paper we shall discuss the evolutionary lifetime of t h e s t a r t h a t exploded thus causing Supernova 1987A(SN1987A), the subsequent experimental data that was detected from this event, its impact on the theory of nucleosvnthesis. and what data we can reasonablv expect to see fro; ~ ~ 1 9in8the7 years ~ to come. A full expian&ion of the theorv of nucleosvnthesis is beyond the scooe of this interlsted readers are directed to the acpaper. cornpanvine paper in this issue bv Viola ( 2 ) and references ( 3 4 that & ;hart but detailed s)nopses of the theory that are ao~licableto this oaoer. It is hoped that the information detaiiid in this pape; $11 provide teachers with the necessary data and impetus to include this material in their nuclear chemistry discussions.
t ow ever
Photograph of SN1987A taken on February 26. 1987, with the European Southern Observatory's l-m Schmidt telescope.At this point in tlme the supernova was about 2000 times brighter than the progenitor star as seen In Figure 2. The cross surrounding the star is an optlcai effectcaused by the telescope's piale holder. Photograph copyrighted by and courtesy of the
Figure 1.
Evolutlon of the Progenitor Star of SN1987A In a routine survey of the stars in the Large Magellanic Cloud that was done in 1969. Nicholas Sanduleak of Case Western Reserve University observed the presence of a 12th-maenitude blue supergiant with a mass of approximately 2Esolarmasses and asurface temperature of 1;-18 X 10: K (6).This star was the 202nd star cataloged by Sanduleak in the region of stars that are 69' south of the equator. Thus it is given the name Sanduleak -69' 202 or Sk -69' 202forshort. Thereislitrleornodoubt that this was thestar that exploded, as the supernova's location has been pinoointed to thissamesoot in theskv withanaccuracsof0.05~ of arc (I). For the first time in history the pro~enitorstar of a suoernova had been observed before the exdos~on(Fig. 21. ~ i spectrum e of Sk -69' 202 showed absorGion lines d;e to H, He, C, N, 0,Mg, Si, and Ca and has been characterized as normal for a blue supergiant (7). While the fact that it was a blue supereiant that had exploded came as an initial surprise to astrdnomers (most theoretical calculations predictedthat supernovaeof this typeshould occur in redsupergiants),this observation later helped to explain some unusual properties of SN198-A (I). Many theoretical calculations of the evolutionof Sk -69" 202 have been performed (8-101, but thevall
basicallv point to the same evolutionarv oath with minor differences in details. In the following di&ssion we will use the model calculations of S. E. Woosley and co-workers that is described in refs 1 and 8. A ~ ~ r o x i m a t e10 l v million vears aeo 20 solar masses of eas hadcontracted due to pavity to the point that the intehor temoerature and densitv reached about 40 X 10" and 5 e l cm';respectively. At thib point the star began nuclear fusion reactions that fused H into He (eas 1-4) and aenerated larae amounts of heat and internal piessure that kept the s t i r from contracting further.
Thls paper was presented at the 197th National Meeting of the American Chemical Sooiety; Dallas, Texas: April 9-14, 1989.
(Stars that are in this phase of their lives are called main sequence stars, and it is the longest phase of a star's life.) Sk
European Southern Observatory.
LH+LH-2H+p++~
(1)
2H+LH-3He+y
(2)
3He+3He-4He+21H
(3)
4 'H
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'He
+ 2'2 + 2 v overall reaction
Number 9
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(4)
731
'Q+4He-ZoNe+r (7) These fusion reactions (eqs 5-7) converted He nuclei into C, 0 , and Ne nuclei. The heat generated by the reactions was intense enough to cause the external H shell of the star to expand and cool. Thus the star's external radius greatly increased to .z 108 km while the external temperature decreased and the star emitted most of its energyin the red and infrared portion of the electromagnetic spectrum. This is called the red supergiant phase by astronomers. During this ~ h a s ea ort ti on of the mass (mostly H) in the star's outer ;hell was-evaporated into the interstellar medium as a stellar wind. Near the end of this 1-million-year phase nearly 4 solar masses ~of C ~ and ~ 0 ~had accumulated a t the star's core. Once again the star's fuel source had essentially been deoleted. and it was necessarv to switch to a new fuel source. Th'is new fuel was the C n u i e i made in the red supergiant nhase. But there was a nrice to be naid for the switch over to H new fuel. Carbon n&ei have 6 protons in the nucleus rather than onlv 2 as in He. Thus the temperatures and densities requirch tofuseC musr he much higherrhnn for He ro overctrme the increased electrostatic repulsion. When the core contracted this time the temperature rose to 7 X 108K and the density reached 1.5 X lo5 g/cm3. This contraction initiated a series of reactions that fused C into Ne, Na, and Mg nuclei via the following reaction pathways (eqs 8-12). ~
~
~
~
~
~
~
-
24Mg+ y
"C
+ IZC
"C
+ IZC
"Na
+p
"C
+ lZC
'ONe
+ 'He
-
(8) (9)
(10)
1 2 1~ 1 9 ~ - 2 3 ~ "i
Figure 2. Photograph of Sk 6 9 ' 202 taken with the same telescope as used for Figure 1 in 1977. This photograph was taken with a photographic emulsion that is sensitive to ultraviolet radiation and therefore shows the nebular material in this region of the sky more clearly. Otherwise essentially the same stars producing the same photographic brightness are seen in both photographs. The brightness of the supernova is immediately obvious. Photagraph copyrighted by and courtesy of the European Southern Observatory.
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-69" 202 had beeun its life and would continue to fuse H into He for the next 9 million years. The star must burn its H fuel a t such a prodigious rate to keep the internal pressure sufficiently high to stop the gravitational collapse of the star. (As a point of comparison the sun hegan its main sequence phase some 5 billion years ago and is thought to be only halfway through this phase.) However the star is caught in a trap that as soon as it consumes a significant fraction of its fuel there will not be enough fuel left to support the star in its present condition. So as we will see the star's evolution is a competition between fuel hurning stages and gravitational contraction. Ultimately the explosion that destroyed Sk -6g0 202 was triggered by the final contraction of this supergiant. The prodigious fuel consumption also raises the external temperature of the star to the point that most of its heat is radiated into space as ultraviolet radiation rather than as blue visible light. During the 9-million-year main-sequence phase the H fusion reactions produced an amount of He roughly equal to 6 solar masses that accumulated in the star's core. The He acted like the ashes from a fire in that it no longer supported fusion a t the star's present temperature. Thus as the fusion a t the star's core began to slow down, the star's external pressure was greater than the internal pressure and gravitational contraction of the core began. As the contraction raised the internal temperature and density to 170 X 106K and 900 glcm", respectively, a new series of fusion reactions hegan that powered the star for another nearly 1 million vears. (5) (6)
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Journal of Chemical Education
the 7 108 K required to fuse c nuclei, another nuclear process came into play that robbed the star of precious energy. That process is the production of electron-positron pairs from the plentiful y rays that are emitted in the fusion reactions (eq 13). Y ei + e(13)
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A certain fraction of the electron-positron pairs annihilated one another to vroduce neutrino-antineutrino (u-V) airs that did not inteiact with the matter in the star and escaped carrying all of their energy with them. During the C fusing phase of the star's life most of the star's energy is emitted in the form of v's. The C fusion reactions fueled the star for only = 1,000 years due to the prodigious loss of energy via the u's. The switch over to C fusion also had an effect on the external appearance of the star. The luminosity (rate at which the star releases radiant energy) of the core is less during the C fusing phase of the star's life than in the He fusing phase. The decrease in luminosity is enough to prevent the star from supporting its large red supergiant envelope, and therefore the star's radius shrank, and its color shifted from the red end of the spectrum to the blue. Once again the star became a blue supergiant and remained that way until the supernova occurred. From this point on events in the core occurred so rapidly that the external structure of the star did not have time to respond. Following the end of C fusion reactions the core contracted again to a temperature and density of 1.5 X 109 K and 107 glcm" respectively. Under these new conditions Ne from the C fusing phase began to convert to 0 and Mg nuclei and fueled the star for a few years (eqs 1615).
Then the core heated up to 2.1 X 109K and the fusing of 0 to Si and S nuclei fueled the star for another few years (eqs 1619).
160
+ 160-22s
"0
+ Ifi0
-
+? O ' Si
(17)
+2p
(18)
32S+2n-"S (19) By the end of these two short phases the core is rich in S and Si isotopes, primarily ZSSi, 3W, 32S, and 3%. Then the core heated up to 3.5 X lo9 K and attained a density of 10Sg/ cm3. For a few days some of the Si nuclei fell apart into free ar particles, neutrons, and protons that then fused to the remaining S and Si nuclei and in a series of steps ultimately formed nuclei in the Fe-Ni region of the periodic chart (eqs 20-24). 32S+?-%+n (20)
-
+ + + -
32s+ y 32S 32S
7
'He
36Ar 'He
+p "Si + 4He 3'P
36Ar+ y 'OCa
+ 7 and SO forth up to Fe-Ni region
(21)
(22) (23)
(24)
This phase of the star's life ultimatelv produced about 1.4 solar masses of Fe-Ni nuclei. ~ o w e v e r i h i time s the star had finally consumed its last supplv of fuel. Fusion to form nuclei with masses greater than t h e - ~ e - ~nuclei i is an endoergic process rather than an exoergic process (11). Thus there was no energy to be gained from furiher fusion. However gravity was still operational and it initiated the final death throes of Sk -69' 202. At the temperature of the core the electrons that surround each atom were stripped off and no longer provided any support to the star's gases plus the intense y radiation riooed aoart the nuclei that the star had built over thelast 10 &iflion ;ears. Thus when the core gravitationally contracted for the last time there were few nuclear or atomic forces to impede it. In a matter of only a few tenths of a second a core that was roughly the radius of the earth shrank to a radius of a 50 km, and some parts of the collapsing core attained velocities of nearly 70,000 kmh. The density inside the collapsing core zoomed up to over 2 X l O I 4 g/cm3. At this density the strong nuclear binding force (that normally is attractive and binds protons and neutrons into a nucleus) became repulsive and halted the collapse in the inner part of the core. However the outer part of the core continued to collapse and ran into the hard central part of the core, this set up a shock wave with an energy of = 1O6I ergs that was propagated out into the star and began the explosion that we would see 160,000 to 170,000 years later as SN1987A. As this shock wave passed through the layers of the star, it initiated yet more nucleosynthesis as the temperature and density of the surrounding layers of the star were raised and new nuclear reactions occurred, this last stage of nucleosynthesis has been called "exolosive nucIeosvnthesis". The core collapse also triggered another phenomenon that we would see.The Fe-Xi nuclei in thecoredisintegrated into protons and neutrons and the protons captured electrons to form more neutrons (ea 25). The formation of neutrons from protons alsoliberatdd'a burst of v's carrying with them a total energy release of ergs. p+e--n+v (25) Some fraction of this energy was channeled into the explosion hut most of i t was carried away by the v's as they began their journey through the star and into space. Behind them the u's left a tightly packed mass of neutrons that will he the final remnant of SN1987A, a neutron star.
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Data from SN1987A
Chronologically the first evidence of SN1987A's explosion to arrive a t earth was the u's from the collapsing core. They
arrived nearly 3 h before the supernova was first seen visual1" 1 I). The v'i were detected independently by two lahoratories at two different locations on earth. Ele\,en v's were seen by the Kamiokande I1 detector in Japan and (within the limits of the two experiments), simultaneously, another 8 v's were seen by the IMB detector near Cleveland, Ohio. The v's detected by these two experiments came from the direction of SN1987A and are consistent with the first direct observation of the gravitational collapse of a stellar core and the production of a neutron star (12,13). This is the only physical evidence that was detected of a v burst estimated at nearly 50 billion v's per square centimeter that passed through the earth on February 23, 1987 (1).(It should he noted that both detectors are in the Northern Hemisphere and, since the supernova occurred in the Southern Hemisphere, all of the detected u's had made the journey through space from Sk -69' 202 and through the earth before detection.) The v burst was not reported until March because the experiments were not designed to signal the experimenters that an abnormally large number of u's had been detected. Thus the visual siehtine of the suoernova oromoted the two laboratories to sifCthr~;~h their data in search dfthe u burst, which thev subseauentlv found. In the lone run the detection of v's-associated with SN1987A may &ll be the single most significant discovery from the supernova. This event provided astronomers with the key clue that their theory of the mechanism for a supernova explosion was essentiallv correct and began a new &a of u astronomy. Immediately after Ian Shelton of the Las Campanas Observatory in Chile announced his visual aighting of Sh'l987A, nearly every observatory in the Southern Hemisohere trained their telescooes on Sk -6g0 202. A routine dart of their observations was recording the bolometric light curve (Fig. 3). This graph show6 the total energy radiated in
37 0
200 400 600 800 Days since 1987 February 23.32
Figure 3. Bolometrlo llght curve of Superma 1987A taken over the wavelengths 0.35 to 5.0 pm by the South African Astronomical Observatory ISAADI. Note the steeo . droo . in luminositv . earlv. In the suoernova's evolution followed OY an ncresse n bmlnositr tpreodmao y o ~ to e raoioact vs aecay of lehuand then the near ponon of tne graph bom = 120 oeys after Me explos on ~ nI t 400 days after the erplos on Th r linear PaR at me grepn which Is drawn an semilogarithmlc paper, tracks the exponential radioactive decayofSeCowltha 77.ldsy half-life. Duringthe early portion of the supernova. up until about day 240, essentially the entire flux of the supernova was in this range of wavelengths. Since then more and more of the flux is in other portions of the electromagnetic spectrum. At present Only 20% of the total flux are represented on the latter portions of this graph.
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the 0.35- to 5.0-pm wavelengths as a function of time after the explosion. Initially the explosion fueled the holometric light curve as the roughly ergs of energy ripped through the star but this energy quickly dissipated and the curve dropped to a 1.78 X 1041ergls. This amount of energy loss is roughly a factor of 10 less than predicted for a "normal" supernova of this type (1). The fact that Sk -69' 202 was a blue supergiant, rather than a red supergiant, is the most likely explanation of the light curve's dramatic decline. Blue supergiants are much more compact and dense than red supergiants. Thus more of the explosion's energy is expended in blowing the star apart, and there is less energy available to radiate in the infrared, visible and microwave portions of the spectrum (1.7). Following the light curve's decline, it began to rise and in 90 days reached a maximum of a 8.32 X IO4l erg/% Part of this maximum came from the expansion of the star's H envelope and presumably the remainder was fueled by the p+ decay of SfiNito WOwith a 6 day ti12 (1,6,7). (The sfiNiis an explosive nucleosynthesis product. Model calculations indicate that 0.07 solar masses of 5Wi were made in SN1987A (1, a).) After 120 days the light curve began an exponential decay with a t1,2 of 77.1 days, which corresponds precisely with the tll2 of the P+ decay of W o to 56Fe.This observation indicates that SN1987A produced 56Ni during the explosion and initiated the following decay chain (eq 26). mNi
qCaheFe
(26)
This decay chain probably provided the energy source for the holometric light curve for around 400 days after the explosion. According to D. D. Clayton, "If every supernova did this.. thev could have made all the iron in the universe" (14). Infrared spectra of SN1987A were taken in April and November of 1987 using NASA's Kuiper Airborne Ohservatory (a large airplane equipped with a variety of telescopes including an infrared telescope that is effective in the high altitudes provided by the airplane) (15). The April spectrum showed basically a large amount of H that was heated to nearly 5,000 K. This is consistent with an expanding ball of gas 6110wing the explosion. Huwrrer hy Nm.eml)er the outer sphere of the supernova had hecome somewhar transparent t i infrared and good bit of structure in the spectrum was evident. Several prominent lines due to Ni, Ar, and Co were detected as well as hints of the presence of CO and SiO molecules. This provided the first unambiguous detection of the soectra of radioactive elements formed in a T .w.e I1 supernova explosion (15). In a seuarate infrared studs of SN1987A, the Anglo-Australian observatory detectedthe prescence of CO ~ o l e c u l e s as early as 112 days after the explosion. These molecules were believed to he in the ejecta of SN1987A, not in some previously existingenvelope of Sk -6g0 202, and their detection is another first in the annals of supernova history (16). Apparently not only do supernovae create elements, but they are also a site for the formation of molecules. The optical spectrum of SN1987A has been studied a t several sites throueh out the Southern Hemisphere (17-20). Spectra taken ah&t 24 h after the e~~losionbhowed broad emission lines of H and He that were strongly hlue-shifted. The blue shifts indicate that these gases Geie blown away from Sk -6g0 202 and toward the earth with velocities as high as 30,000 kmls (1). Approximately 2 months later the optical spectrum had changed significantly, and the structure of the expanding star was more apparent. Lines that were assigned to Fe, Na, Ca, and Ba were now dominant in the spectrum. The presence of Ba at so early a stage in the supernova's evolution was quite a surprise for the astronomers because Ba is made through the "s process" of nucleosynthesis (2-4). Consequently it should have been buried much deeper in the star's interior. I t is believed that the Ba was probably dredged up from the star's core by a mixing 734
Journal of Chemical Education
process before the explosion and consequently appeared earthe exnlodine star had lier than exnected (I). . . Bv.Seotember . an optical spectrum that was similar in structure to a nebula and strongly showed the presence of 0 , Na, and Ca atoms 1\-,. 1) X-ray spectra of SN1987A were taken in early August through September of 1987 by two separate experiments aboard the Soviet space station Mir and the Japanese satellite Ginga (21,22). Both observations showed a series of relatively hard X-rays having an energy of 20 keV to nearly 300 keV. The origin of these X-rays could be either the degradation via Compton scattering of y rays from the radioactive decay of W o , radiation from a young pulsar, heat radiation from a neutron star, or mass collecting on the collapsed star (21). However astronomers were reasonably certain that the origin of the X-rays was the degradation of t h e y rays from SGCo decay as the 7 rays worked their way out of the expanding star's shell (1,23,24). A firm confirmation of the astronomer's belief in W o decay being the primary source of the X-rays came almost simultaneously with the X-rays' detection. Using the y ray spectrometer aboard NASA's Solar Maximum Mission satellite over the period August 1to October 31, 1987, the 847and 1238-keV y rays from the decay of S C O to W e were detected (25). This experimental evidence showed that the idea of the explosive nucleosynthesis of W o is factually based. SN1987A had logged yet another first in the annals of supernova astronomy.
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What Is Still on Its Way Here? Many crucial pieces of information on the evolution of a supernova and the nucleosvnthesis that took place in the star before the explosion a r e still making the 160,000170.000-year journey to the earth from Sk -fig0 202. Astronomers will study this region of the sky for decades, perhaps centuries, to come as this exploding star unfolds and reveals its secrets. Some information that should arrive in another 2 to 3 years is the spectra of the truly heavy elements such as U and Th. If the theory of the nucleosynthesis of these elements is correct.. thev. are made in the ex~losionbv the r nrocess (rapid neutron absorption before-a or 6- aecay) (2). The detection of these elements would provide yet another vital piece of information for the theorists. One of the most voorly understood nucleosvnthetic processes is the so called p process (proton captuie reacti&s). This process theoretically builds the very rare nuclei like 19F, 124Xe, and '"Ba. Almost any information on the presence or abundance of these nuclei would greatly increase the understanding of this process. The spectra of these rare nuclei will be mixed in with the spectra of many other nuclei. Hopefully we will be able to detect and seoarate their soectra thus providing us with valuable clues to the p-process pathways. The abundances of the elements produced in SN1987A are a vital clue to the theorists as tb the validity of their models. The abundances can he discerned from the spectra of the supernova (as we have previously seen) and used as parameters to fit the theoretical models of supernovae explosive nucleosynthesis. A phenomenon that several astronomers are intensely lookkg for is the evidence of dust grain formation as the ejecta cool and condense into silicate minerals. (Remember that the eiecta are fairlv rich in Si and 0 from the previous nucleosynthesis and those elements are the ones necessary to make silicates (14)). This event should be marked by the blocking of the optical spectrum of the supernova by the dust and a subsequent enhancement of the infrared spectrum as the dust reemits this energy in the infrared. For dust formation to begin it is necessary that the ejecta cool to about 1000 OC. Estimates of the time necessary for this cooling to occur extend to as long as 10 years after the explosion (26). This phenomenon is interesting because as-
