THE GALACTIC SUPERNOVAE OF THE SECOND MILLENNIUM A.D.*

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1 Pub. Astron. Soc. Pacific, Volume 85, June 1973 THE GALACTIC SUPERNOVAE OF THE SECOND MILLENNIUM A.D.* SIDNEY VAN DEN BERGH David Dunlap Observatory, University of Toronto, Richmond Hill, Ontario, Canada Received 26 February 1973 This paper discusses the five most recent known galactic Supernovae and their remnants. The possible importance of circumstellar shells ejected before the explosion of Kepler s supernova and the Cas A supernova is emphasized. It is pointed out that the optical remnant of Kepler s supernova (which was of type I) is expanding with a velocity that is comparable to the expansion velocity of the Crab nebula. This observation weakens the main argument that has been given against the idea that the Crab nebula was produced by a supernova of type I. Some new observations are presented of the optical remnants of the supernova of 1604 and of Cas A. Key words: supernovae supernova remnants I. Introduction Supernovae occupy a central role in the astrophysics of the second half of the twentieth century. Reasons for this are: (1) many heavy elements are thought to have been produced in supemovae so that the study of supernova remnants provides direct evidence for nucleosynthesis, (2) supernova explosions stir up the interstellar medium, (3) some supemovae produce pulsars, others might possibly form black holes, (4) supernova remnants generate X-rays, radio waves, and cosmic ray particles, and (5) supernovae provide a powerful tool for the calibration of the extragalactic distance scale. Recent estimates, which are based on studies of radio sources associated with supernova remnants (Caswell 1970; Milne 1970; Downes 1971; Ilovaisky and Lequeux 1972), yield a mean time interval r = 50 ± 25 years between galactic supernova outbursts. A rate of 20 supernovae per millennium is comparable to the supernova rate in giant extragalactic spiral nebulae of Hubble-type intermediate between Sb and Sc (Tammann 1970; van den Bergh 1972). The actual rate at which supemovae have been observed in the Galaxy is much smaller than two per century. The reason for this is that most distant supernova explosions, which take place close to the galactic plane, are hidden by dense clouds of interstellar dust. (A type I supernova * Based in part on observations obtained at the Hale, and Lick Observatories. 335 exploding in the galactic center would have an apparent magnitude V 10.) Some observational data on the known supernovae of the last millennium are summarized in Table I and plotted in Figure 1. This figure shows that at least four, and possibly all five (the distance to Kepler s supernova is very uncertain), of these supemovae occurred in the quadrant of the Galaxy that contains the sun. The data in Table I are therefore entirely consistent with the assumption that the tme supernova frequency in the Galaxy is about 20 per millennium. Only one of the supemovae observed during the last 1000 years was of type II, whereas three were of type I. This predominance of type I supemovae is probably due to the fact that these objects are not as strongly concentrated to the galactic disk (where absorption is high) as are the supemovae of type II. Name Lupus Crab Tycho Kepler Cas A TABLE I Galactic Supernovae Year V(max) D(kpc) : -5: >0 *H = 100 km sec -1 Mpc -1 assumed. 3* 2 5 6* 3 Type I:? I I II

2 336 SIDNEY van den BERGH Fig. 1 Positions of the five Supernovae seen during the last millennium projected on the Galactic plane. All observed Supernovae are seen to be located in the same galactic quadrant as the sun. II. Discussion of Individual Supernovae A. The Lupus Supernova of1006. Medieval observations of the Lupus supernova have been discussed by Goldstein (1965) and by Goldstein and Yoke (1965). According to Goldstein this supernova appeared on 1 May 1006 and had an apparent magnitude similar to (or possibly brighter than) Venus, i.e. V(max) < 4. Minkowski (1966) has suggested that the supernova of 1006 was of type I and that it reached 10 < V(max) < 8. For V(max) = 10 the supernova would have to be located at a distance of only about 0.3 kpc. The a priori probability that a supernova should occur so close to the sun in a galaxy in which the supernova rate is two Supernovae per century is only ~ 0.01 per millennium. We shall therefore (more or less arbitrarily) adopt V(max) = 5 for the Lupus supernova. According to Kowal (1973) Aiy(max) = log (H/100) for supemovae of type I. Adopting H 100 km sec -1 Mpc -1, A v 0.7 and V(max) = 5 yields (m M) 0 = 12.6 corresponding to a distance D = 3.3 kpc. At this distance die supernova explosion occurred at 0. 9 that the Lupus supernova occurred in the galactic halo lends support to Minkowski's suggestion that this object was a supernova of type 1. An optical search for the remnant of the supernova of 1006 (Minkowski 1965) has so far remained unsuccessful. The fact that the supernova of 1006 exploded high above the galactic plane (where the density of interstellar material is very low) probably accounts for the lack of an optical remnant. The area in the direction of the Lupus supernova has been studied at radio wavelengths by Gardner and Milne (1965) and by Milne (1971). Two nonthermal sources are located close to the position at which the supernova of 1006 occurred. One of these is a large diffuse shell source, the Lupus Loop". The other is a more compact source MSH The Lupus loop has an angular diameter of ~ 270 \ Atan assumed distance of 3.3 kpc this corresponds to a linear diameter of 260 pc. To attain such a large radius in ( ) = 967 years would require an expansion velocity ~1.3 X 10 5 km sec -1, which is ruled out by the observed widths of the emission (or absorption) features in the spectra of type I supemovae. At a distance of 3.3 kpc the 25' angular size of MSH corresponds to a diameter of 24 pc. Such a diameter requires an expansion velocity ~~ 12,000 km sec -1, which is quite consistent with other observations of supemovae of type I (Mustel 1972). B. The Crab Nebula. The Crab nebula is the remnant of the supernova of This remarkable object consists of two distinct components: (1) a system of filaments that emits line radiation and (2) an amorphous component which emits synchrotron radiation. The radio radiation from the Crab comes from an elongated region, which is brightest near its center. The fact that the Crab is not a shell source is no doubt due to the fact that the energy radiated by the Crab is ultimately derived from the pulsar at the center of the nebula. According to Woltjer (1958) the Crab is a prolate spheroid located at a distance of 2 kpc. With this distance the observed proper motions in the remnant yield space velocities of between 1500 and 2200 km sec -1. It has been argued that the Crab nebula cannot

3 GALACTIC SUPERNOVAE 337 be the remnant of a supernova of type I because these objects are known to have expansion velocities in the range 10,000-30,000 km sec -1 (Mustel 1972). This argument is, however, considerably weakened by recent observations (see D) of the remnant of the type I supernova of These observations show that the optical remnant of Keplers supernova is expanding with a velocity similar to that of the Crab. In the case of Kepler s supernova the radio shell is an order of magnitude larger than the optical remnant. It would be interesting to know if the Crab nebula is also embedded in a much larger/ami shell of radio emission. Woltjer (1958) has shown that helium is overabundant relative to hydrogen in the Crab nebula by at least a factor of two. An even larger overabundance of helium is obtained by Davidson and Tucker (1970). It should, however, be emphasized that these results are uncertain (Davidson 1973) because of stratification effects in the filaments of the Crab nebula and because much of the cooling of the filaments takes place in the Cm] A1909 line which has not yet been observed. Studies of the composition of the Crab are of obvious importance because the Crab nebula and Cassiopeia A are the only supernova remnants in which direct evidence for nucleosynthesis in supemovae has so far been obtained. References to the extensive literature on the Crab nebula may be found in I AU Symposium No. 46 (Davies and Smith 1971). C. Tycho's Supernova. Baade (1945) has been able to reconstruct the light curve of Tycho s supernova from 16th century observations. This light curve, which is plotted in Figure 2, shows that B Cassiopeiae was a typical supernova of type I. The remnant of Tycho a supernova of 1572 consists of a small number of very faint long filaments (van den Bergh 1971a). These filaments are strung out along the outer rim of the radio shell (Baldwin 1967). This suggests that the optical filaments consist of emitting sheets that are seen almost edge-on. This conclusion is strengthened by Minkowski s (1959) observations which show that the radial velocity of the brightest filament (in which only Ha was observed) is low. Intercomparison of four 200-inch Fig. 2 Light curves of Kepler s and of Tycho s supernovae, as reconstructed by Baade (1945). Scale on right is for the supernova in IC The light curves show that the supemovae of 1572 and 1604 were of type I. (5 m) plates taken during the interval (van den Bergh 1971a) shows that the brightest filaments exhibit structural changes on a time scale ~~ 10 years. From its known age and present radius it follows that the optical remnant of Tycho s supernova must have expanded with a mean proper motion p ~ 0'.'5 per year. This motion is significantly larger than the value pt 0'/2 per year that van den Bergh (1971a) obtained for the present proper motions of the brightest filaments in Tycho s remnant. It follows that the expansion of the remnant of Tycho s supernova has been decelerated by interstellar material. The observations are consistent with the assumption that the expansion of the remnant of B Cas can be described by a Sedov similarity solution. For such a solution to be valid the shell ejected by Tycho s supernova must have swept up an amount of interstellar matter several times larger than its own mass. For an assumed distance of 5 kpc (Minkowski 1970) the mean expansion velocity of the remnant of B Cas during the last 400 years is about 12,000 km sec -1. The velocity at the time of the explosion of B Cas must have been considerably greater than this. No optical remnant of Tycho s supernova has so far been identified. A moderately bright star near the geometrical center of the radio shell has been found to have a late-type spectrum. Attempts to obtain image-tube spectra of the faint bluish companion to this star with the Hale telescope have so far remained tantalizingly unsuccessful. An attempt to observe possible rapid fluctuations of this object is currently being

4 338 SIDNEY VAN DEN BERGH undertaken in collaboration with Professor Groth of Princeton University. D. Keplers Supernova = S. N. Ophiuchi Seventeenth century observations (Baade 1943) show that Keplers star of 1604 was a supernova of type I (see Fig. 2). At the time of its greatest brilliance it reached an apparent magnitude V(max) = 2.2. From data given by Minkowski (1964) and van den Bergh (1970) A v 2 2 so that V 0 (max) = 4 4. From equation (1) (Kowal 1973) My(max) = log (HI 100) for supemovae of type I. With H = 100 km sec -1 Mpc -1 this yields a distance of 6.0 kpc and Z = 0.7 kpc. (For H = 50 km sec -1 Mpc -1 Keplers supernova is located in the galactic halo beyond the nucleus at a distance of 12.1 kpc and at a height Z = 1.4 kpc above the galactic plane. ) The optical remnant of Keplers supernova consists of a fan-shaped region containing a number of bright knots. A few fainter filaments are also present. Intercomparison of 200-inch (5 m) plates taken during the last 20 years shows that the bright fan-shaped nebula, which constitutes the most prominent part of the remnant of Keplers supernova, is moving bodily towards the northwest with a velocity of ~~ 0'/03 per year. At an assumed distance of 10 kpc this corresponds to a tangential velocity of ~ 1400 km sec -1. A recent radial velocity observation of this fan-shaped nebula obtained with the nebular spectrograph at the prime focus of the Lick 120-inch (3 m) telescope yields a radial velocity V = 230 km sec -1. This result implies that this nebula must be located quite close to the outer edge of the shell of Kepler s supernova. The total space velocity of the remnant of Kepler s supernova, which is obtained by combining the observed radial and tangential velocity components, is surprisingly low. The fact that this supernova remnant is located at ~~ 1 kpc above the galactic plane makes it virtually certain that deceleration by interstellar gas cannot account for this low expansion velocity. Possibly these observations can be understood by assuming that the slow-moving material that constitutes the optical remnant of Kepler s supernova was formed from a shell that was ejected at low velocity before the main explosion of Kepler s supernova took place. (Alternatively it might be assumed that the supernova shell transferred momentum to a stationary circumstellar shell.) If the latter explanation is correct one might, however, wonder why the system of quasistationary flocculi associated with Cas A does not also exhibit a systematic outward motion (van den Bergh 1971fo). Detailed studies of the expansion of the optical remnant of Kepler s supernova have been greatly hampered by the fact that this object is located low in the southern sky, where seeing conditions are usually poor for northern observers. None of my 200-inch (5 m) direct plates taken in recent years can rival Baade s beautiful red plate taken in Hazard and Sutton (1971) have used lunar occultation observations to study the structure of the radio source 3C 358, which is associated with Kepler s supernova. On the basis of these observations they conclude that the radio remnant of Kepler s supernova consists of a broken shell comprising two components or two concentric shell-shaped sources. The diameter of the radio source is 3' so that the expansion velocity of the radio shell must be 90"/( ) 0'/24 per year. At an assumed distance of 10 kpc this corresponds to an expansion velocity of ~1.2 X 10 4 km sec -1, i.e. an order of magnitude greater than the velocity of the optical knots. It is rather surprising that the supemovae of 1572 and 1604, which were both of type I (see Fig. 2 and Minkowski 1966), have produced such very different optical remnants. In particular the remnant of Tycho s supernova does not contain any nebulosity that resembles the slowly expanding fan of flocculi associated with Kepler s supernova. E. Cassiopeiae A. The optical remnant of this bright radio source, which was discovered by Baade and Minkowski (1954), consists of two distinct components: (1) a system of fast-moving knots and (2) a number of quasi-stationary flocculi. On plates taken in good seeing about 100 fastmoving knots are visible. Most of these knots are concentrated in an irregular arc along the northern edge of the remnant. At an assumed distance of 3 kpc most individual knots are found to have space velocities between 4000 and 9000 km sec -1. These moving knots exhibit

5 GALACTIC SUPERNOVAE 339 emission lines of [Oi], [Oui], [Su], and [Arm] (van den Bergh 1971fo). In these knots oxygen, argon, and sulphur are overabundant with respect to hydrogen and nitrogen by at least a factor of 30. This shows that the bright knots in Cas A cannot have swept up a significant amount of (hydrogen-rich) interstellar material. The overall appearance of the bright arc of nebulosity forming the northern rim of the shell of Cas A has changed but little during the last two decades, even though individual knots are found to have lifetimes of only ~10 years. The southern part of the remnant, which was essentially free of moving nebulosity 20 years ago, now exhibits a few fast-moving knots. Very fastmoving knots continue to appear near the extreme northeastern comer of the remnant. These knots lie outside the radio shell of Cas A (Rosenberg 1970a,b). For the fastest of these knots van den Bergh (1971fo) finds a velocity ~ 10,000 km sec -1. A system of faint radial streaks appears to be associated with the very fast moving knots in the northeastern part of Cas A. The brightest filament in Cas A, which has an internal velocity dispersion of ~ 3000 km sec -1, has recently broken up into a number of distinct knots. The origin of the velocity differences between the different knots that make up this filament is not understood. The filament could not have survived for 300 years with such a large internal velocity dispersion. On the other hand, the absence of hydrogen in the spectra of these knots clearly shows that deceleration by interstellar material cannot be responsible for the observed velocity differences. It therefore appears that some mysterious mechanism is still at work in this filament which accelerates (or decelerates) individual knots of nebulosity to velocities well in excess of 1000 km sec -1. Annual spectroscopic observations of this filament are being obtained with the Hale telescope on Palomar Mountain in the hope of obtaining a better understanding of the behavior of these knots. The fact that moving knots appear and disappear on a time scale of a decade or so indicates that the recombination time scale must be smaller than or equal to ten years. This result implies that the electron density n e > 10 3 in the brighter filaments. Such a high value is confirmed by the observed intensity ratio of the [Sn] lines AA6717, 6731 which is sensitive to electron density. The total mass of the moving knots which are visible at the present time amounts to only ~~ W 0 (Peimbert and van den Bergh 1971). This observation does not place any useful limits on the mass of the star that produced Cas A. According to van den Bergh and Dodd (1970) the explosion of Cas A took place in a.d ± 8. Presumably the fact that this outburst was not observed implies that V(max) ^ 0. With A v = 4 3 (Searle 1971) and a distance of 2.8 kpc this yields (m M) v 16^5, so that Aiy(max) > 16^5. This low luminosity is just barely consistent with the hypothesis that Cas A was produced by a supernova of type II. A type I supernova is clearly ruled out. No stellar remnant V < 22 5 is visible within eight standard deviations of the center of expansion of Cas A (van den Bergh and Dodd 1970). Adopting a distance of 2.8 kpc and A v = 4 3 this yields M v > +6 for any stellar remnant of this supernova. On plates taken in good seeing, approximately 30 quasi-stationary flocculi are visible in the remnant of Cas A. Only Ha and AA6748, 6784 of [N u] have so far been observed in the spectra of these flocculi. For temperatures T > 6200 K Peimbert and van den Bergh (1971) find that the nitrogen-to-oxygen ratio in the quasi-stationary flocculi in Cas A is higher than that prevailing in the Orion nebula. Taken at face value this result implies that the quasi-stationary flocculi cannot represent interstellar gas that was trapped by the expanding supernova shell. The observed overabundance of nitrogen might be understood by assuming that the flocculi were formed by the compression of a preexisting circumstellar shell. Such a shell might have been enriched in 14 N that was produced in the CNO bi-cycle (Fowler and Caughlan 1962). The quasi-stationary flocculi occur in all parts of the remnant of this supernova but are more frequent in the arc of nebulosity along the northern rim of Cas A than they are in other parts of the remnant. In no case is there any evidence for a direct connection between fast-moving knots and quasi-stationary flocculi. Individual flocculi exhibit brightness changes on a time scale of decades. Intercomparison of plates taken during

6 340 SIDNEY van den BERGH the last 20 years does not give evidence for any systematic outward motions of these flocculi. F. Other Recent Supernovae. It is not yet clear if objects like tj Carinae should be classified as supemovae. Such variables appear to be intermediate in luminosity between average Hubble-Sandage variables and ordinary supemovae of type II. In fact the Hubble-Sandage variable No. 12 in NGC 2403 (Tammann and Sandage 1968) is now classified as supernova 1954J (Kowal and Sargent 1971). Probably there exists a continuum of young variable stars that range in luminosity from the brightest main-sequence stars to typical supernovae of type II. For more general discussions on supernova remnants, the reader is referred to excellent reviews by Minkowski (1964, 1968), Shklovsky (1968), and Woltjer (1972). It is a pleasure to thank the directors of the Hale and Lick Observatories for their hospitality. Special thanks are also due to Maarten Schmidt and Margaret Burbidge for instructing me in the use of the spectrographic equipment on the Hale 5 m and Lick 3 m telescopes, respectively. REFERENCES Baade, W. 1943, Ap. J. 97, , ibid. 102, 309. Baade, W., and Minkowski, R. 1954, Ap. J. 119, 206. Baldwin, J. E. 1967, in Radio Astronomy and the Galactic System, I.A.U. Symposium No. 31, H. van Woerden, ed. (London, New York: Academic Press), p Bergh, S. van den 1970, Nature 225, a, Ap. J. 168, fc, ibid. 165, , Astr. and Ap. 20, 469. Bergh, S. van den, and Dodd, W. W. 1970, Ap. J. 162, 485. Caswell, J. L. 1970, Astr. and Ap. 7, 59. Davidson, K. 1973, Bull. A.A.S. 5,19. Davidson, K., and Tucker, W. 1970, Ap. J. 161, 437. Davies, R. D., and Smith, F. G. 1971, The Crab Nebula (Dordrecht: D. Reidel Publishing Co.). Downes, D. 1971, A.]. 76,305. Fowler, W. A., and Caughlan, G. R. 1962, Ap. J. 136, 453. Gardner, F. F., and Milne, D. K. 1965, A.]. 70, 754. Goldstein, B. R. 1965, A.]. 70, 105. Goldstein, B. R., and Yoke, H. P. 1965, A.J. 70, 748. Hazard, C., and Sutton, J. 1971, Ap. Letters 7, 179. Ilovaisky, S. A., and Lequeux, J. 1972, Astr. and Ap. 18, 169. Kowal, C. T. 1973, Bull. A.A.S. 5, 28. Kowal, C. T., and Sargent, W. L. W. 1971, A.J. 76, 756. Milne, D. K. 1970, Austr. J. Phys. 23, , ibid. 24, 757. Minkowski, R. 1959, in Paris Symposium on Radio Astronomy, R. N. Brace well, ed. (Stanford: Stanford University Press), p , Annual Rev. of Astr. and Astrophysics 2, , A.J. 70, , ibid 71, , in Nebulae and Interstellar Matter, B. M. Middlehurst and L. H. Aller, eds. (Chicago: University of Chicago Press), p , in Supernovae and Their Remnants, P. J. Brancazio and A. G. W. Cameron, eds. (New York: Gordon and Breach), p. 23. Mustel, E. R. 1972, Soviet Astronomy (A.J.) 16, 10. Peimbert, M., and Bergh, S. van den 1971, Ap. J. 170, 261. Rosenberg, I. 1970a, M.N.R.A.S. 147, h, ibid. 151, 109. Searle, L. 1971, Ap.J. 168, 41. Shklovsky, I. S. 1968, Supernovae (New York: John Wiley and Sons), p Tammann, G. A. 1970, Astr. and Ap. 8, 458. Tammann, G. A., and Sandage, A. R. 1968, Ap. J. 151, 825. Woltjer, L. 1958, B.A.N. 14, , Annual Rev. of Astr. and Astrophysics 10, 129.