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Anally Symmetrical Supernova Remnants G.S. BISNOVATYI-KOGAN,1 T.A. LOZINSKAYA,2 AND S.A. SILICH3 ABSTRACT The origin of pylindncally symmetric Supernova Remnants is discussed. The results of numerical simulations of two most distinguished barrel-like SNR SN1006 and G296.5+10.0 are presented. INTRODUCTION Recent high-resolut~on and high-sensitive observations of Supernova Remnants (SNR) have shown that radio-emitting regions generally do not have spherical symmetry. Many SNR's have a limb-brightened c`,rlindncal or barrel-like structure. There are three principal observational signs of the barrel-shaped SNR morphology: (a) there is an awns of mirror symmetry; (b) the shell has two regions of low intensity near the top and bottom of the symmetry ems; (c) there is a gradient of the radio brightness along the shell. Kesteven and Caswell (1987) have suggested that the majority of SNRs are barrel-shaped. A number of X-ray and optical remnants falls into this category as well. C;ylindncal synuneny is a distinctive feature of both young and old SNRs. ~ Institute of Space Research 2Shternberg Astronomical Institute 3 Institute olSpace Research; Main Astrophysical O~ewatory, Ukranian Academy of Sciences 19

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20 AMERICAN AND SOVIET PERSPECTIVES Possible mechanisms for generating such a structure include: (a) anisotropy of supernova explosion; (b) large-scale density gradients in the surrounding medium; (c) anisotropy of wind from the progenitor star, (d) compression of a preexisting regular interstellar magnetic field; and (e) interaction of collimated jets of relativistic particles from a central pulsar with the SNR shell It is possible that more than one of these mechanisms work simultaneously. Concentration of the ejected material in the equatorial plane is the natural consequence of magnetorotational mechanism of supernova explo- sion (Bisnovatri-Kogan 1970) or the thermonuclear explosion of a rotating presupern ova star (Bodenheimer and Woosley 1983~. Dense interstellar clouds and rarefied interstellar bubbles may affect the expanding shock fronts as well ~zinsl~ya 19863. Mass loss by a progenitor star leads to inhomogenei~ of circumstellar medium in two ways. First, mass loss by binar, systems or rotating stars is concentrated in the equatorial plane (Soker and L~vio 1989~. Second, progenitor winds generate anisotropic shells and holes in the inl~omogeneous surrounding interstellar medium. There are some difficulties in the interpretation of the barrel structure of SNRs (Roger et al. 1988) as the result of compression of a preexisting interstellar magnetic field. The energy density of the interstellar magnetic field equals approximately 10-~2 erg cm~3. This is many orders of magni- tude lower than the energy density within a typical SNR during the adiabatic stage. If the explosion energy equals 105i ergs and the radius of the SNR is 20 pc, the mean energy density within the SNR will be approximately 10-8 erg am-3. As Manchester (19g7) has pointed out, it is very difficult to see how a weak interstellar magnetic field could significantly influence the SNR morphology. It is especially difficult to apply this mechanism to a young SNR (in particular, to SN 1006), which have a radially aligned magnetic field component. It is also difficult to see how pulsars can influence the morphology of the old SNRs except in some special cases as, for example, CTB 80, which is described by Fesen et al. (19881. In this paper we examine the first three mechanisms and do not take into account magnetic field effects. We present the results of numerical simulations of two most distinguished barrel-shaped SNRs: SN 1006 and G296.6 ~ 10.0. SN 1006 AND G296~5 + 10.0 ARE IWO BEST EXAMPLES OF BARREL-LIKE SNRS The radiomaps of these two remnants (see Figure la,b in Roger ~ al. 1988) demonstrate all features of barrel morphology. Optical observations by Kirshner et al. (1987) of SN 1006 have revealed narrow and broad

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HIGH-ENERGY ASTROPHYSICS 21 components of the Ha emission. The ratio of the intensities of these components implies a shock velocity in the range 2800-3900 knits. Distance estimations for SN 1006 made by different methods give 1.5 - 2.1 kpc. Mean value of 1.8 kpc leads to a radius of 7.8 pc and Z = 450 pc. At this distance from the galactic plane ambient gas is dominated by the diffuse component with a number density no < 0.1 cm~3. This estimation is close to the value O.O5cm~3, obtained in the X-ray model of Hamilton et al. (1986~. The distance and physical parameters of G296.5~10.0 are not well de- termined. Recent ~-D distance estimations with account of Zcorrection, yield a value 1.1 - 1.9 kpc. The dominant feature of the radio emission from G296.5 ~ 10.0 is two ridges perpendicular to the galactic plane. The relation of the large axis to the small ems is approximated 1.5:1. The mean distance of around 1.5 kpc results in a linear radii of about 24 pc and 16 pc and Z ~ 260 pc. It is suggested that there is a connection of the G296.5+10.0 with a depression in HI distn~ution at the velocities V!,SR = -11 - -17 km/s and with weak SNR G300.1 ~ 9.4 (Dubner e' al. 1986~. The x-ray's remarkable feature is the compact source near the center of the SNR. That point source has a spectrum harder than that of the SNR, but is characterized by a similar absorbing column density and most probably represents the neutron star remnant of the SN explosion. Strong oxygen lines in the optical spectra of G296.5 + 10.0 (Ruts 1983) could indicate on an SNR belonging to O-rich SNRs, which usually are consider results of explosions of massive stars. The mean ambient number density near the G296.5 + 10.0 from x-ray data is estimated to be 0.24 - 0.08 am~3. Density of the optically emitting filaments Is about 5 cm~3. High galactic latitude and low ambient gas densities are the common features of the described above SNRs. One can expect therefore that their evolution is highly influenced by the initial conditions: possible explosion asymmetry and interaction of progenitors with ambient interstellar medium. my NUMERICAL SCHEME AND INITIAL CONDITIONS We have assumed cylindrical symmetry in all calculations and used cylindrical coordinate system R. is, . We have used the numerical hydrody- namical code deserted by BisnovaWi-Kogan et al. (1982, 1989), based on thin layer approximation. The main assumptions of this method are that all ejected and swept-up gas collapses into an infinitely thin shell and that gas pressure is uniform inside the cavity. As a main parameter we adopt explosion energy Eel, the temperature To and densit r distribution p = pof(R,Z3 of undisturbed gas, initial ejecta mass Mej and its ratio to the swept-up interstellar gas mass Me. We also

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22 AMERICAN AND SOVIET PERSPECTIVES i, ~ J.1 30' is' -41 40' - z o IS J -41 50' IJJ -42 00' -52 20' _ -52 40' - - z o F -52 00' AS An J C) Lo -52 20' -52 40' 53 040' 's-VC\ it ~O/ \ , ,~C>o ~ n i7J,~(~) \! /-\ ~ 0 BEAM ~ o \ '/~ Alto of ~ / ~ 15 01 15 00 1 4h 59m 1 4h Gem RIGHT ASCENSION (1950) ~ o of b) Vie "~'\B O ~ oA ~ o . A ~. ..v ~ a, o ., Coo - LD.~4 Us. 3) ~ O ~\ 121 Om 1 2h 08m 1 2h 06m 1 2h 04m Rl GHT ASCENSION (1950) FIGURE 1 lbe 843 MHz maps of G327.6 + 14.6(a) and G2965 ~ 10.0(b).

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HIGH-ENERGY ASTROPHYSICS 23 define the distribution of surface density ~ in the shell and the shares of kinetic Ek and thermal energy E' at the initial time to. We start from the spherical shell of the radius R _ ~ e Mej Me \~47r pO Mej J (I) with constant expansion velocity UO = (2Ek/Mej)~/2. Lagrangian coordi- nates at the onset of calculation are defined by the expressions: R = Re siIlA' Z = Re cOs ~ (2) Anistropy of explosion implies inhomogeneous distribution of the surface density All of ejected material along the shell. We assume that at the onset of calculations aej = ~O(Asin2 ~ + Bsin) + C). (3) We adopt normalization A ~ B = 1 for convenience. Then constant C can be expressed as C = aP/~e 1-ap/ae (4) Integrating initial surface density Eel (~) by A, we obtain the expression for initial mass Mel. Then constant ~0 may be defined as follows: Mel ~ = 4~R2 (2A + ~ B ~ C) (5) Fling into account surface density of the swept-up interstellar gas, we obtain the relation for initial surface density of the shell: Am= PO3 e [1 + 2A ei,,/B+C(Asin23+ BsinA+C)] (6) The radio luminosity of the remnants does not arise directly from our hydrodynamical calculations. We assume that radio luminosity is higher as the surface density of the shell is greater. Therefore in this paper we present the results of calculations of the shape and surface density distn~ution of the shell. RESULTS AND DISCUSSION SN 1006 is a young, almost spherical SNR. It seems to us that it is difficult to interpret its radiobrightness distribution by the influence of the

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24 AMERICAN AND SOVIET PERSPECTIVES external factors only. A more convenient interpretation of this observation is that the SN explosion was highly anisotropic with most of the Recta confined to an equatorial plane. Using our code we investigated the evolution of SNR caused by an asymmetrical SN explosion. The initial surface density of the shell has been taken from formula (3) with Pie = 0.1 - 0.2, ambient density no = 0.05~.1 cm~3, the mass ejecta Mej = 0~5- 2.5M~, parameter A has been taken in the range 0.~-1.0. The ejected mass has been 250-1000 times greater than the swept-up mass at the onset of the calculations. We have assumed that the shell freely expands with a constant velocity up to the initial moment of calculations. The energy of explosion has been taken as 105t ergs. Our calculations show that evolution of SNR caused by asymmetric explosion in homogeneous medium is characterized by elongation of the shock front in Zdirection during the first hundred years. At a later time the material at the shell's poles that has been accelerated by the internal gas pressure begins to decelerate. Expansion velocity of the shell's equatorial region becomes greater than velocities at the poles due to a larger initial mass and momentum. This phase is accompanied by the stretching of the shell in the equatorial plane. Then the shell becomes spherical Our calculations of the evolution of SNRs caused by anisotropic explosion show many examples of the appearance of apple-like shapes. The reason for the development of such unusual shaping of SNRs is that due to initial surface density distribution of the Lagrangian layers, which, placed between poles and equator (but not at the poles), have a maximum Zcomponent of momentum. A long time after explosion, the surface density of the shell remains nonuniform with the maximum at the equatorial plane. The configuration formed by an axisymmetncal explosion on the edge of the gas layer with density enhancement is presented in Figure ~ Initial parameters for this variant have been chosen as follows: ejected mass was equal to 2.0M<3, total energy of the explosion was 105i ergs with 85% in the form of kinetic energy; initial ratio of surface densities was ~p/ae = 0.1; initial radius was 0.7 pa, parameters A and B from formula (3) were A = 0.g7, B = 0.03. Calculations with ejected mass 1.5-ZOM<3 and almost the same parameters give the best coincidence with observed properties of SN 1006. The density of surrounding gas was taken in the form 2 1-~ Z~J (~7) The atomic concentration in the point of explosion was taken as no = O.OScm~3, the density difference in interstellar media is characterized in (7) by parameters ~ = 3 and ZO = lpc. It is clear from Figure 2 that 1000 years after the explosion the surface density distn~ution remains strongh,r nonuniform. The maximum of the surface density is shifted relative to the

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HIGH-ENER~ MTROP~ICS a) ~ ~1 1 1 , , 1 11 -6 -4 -2 25 b) 0 1 2 i 6' -2 ~ :_ o~ O - 2 l 2 4 6 ~ 1 8 ~ 10 '6-1 R(PC) G/aO - FIGURE 2 Shape (a) and surf~ce densi~ distnbution (b) of SNR, caused py an axLsym- metneal e~losion on the edge of the gas layer. Mej = 2.0M<3, t = 981 yr. equatorial plane of the explosion Z = 0. The shape of the remnant is close to the spherical one, but apple-like features are present. The radius of the rennin ant is equal to 6pc and it is situated on the transition phase from free expansion to an adiabatic one. The velocities of the shock waves on the poles are 3900 keys (up) and 2700 knits (down) and on the equator that velocity is equal to 5100 knits. These values agree with the data of Washier e' al. (1986) whose measurements have been made in the north (upper) part of the remnant. When ejected mass is equal to l.5M~, the radius of the remnant increases up to appro~nmatel~r 7 pc for the same age and initial energies. The radii 6-7 pc determine the distances to the remnant 1.4- 1.6 kpc. The radioremnant G296.5 ~ 10.0 has dimensions much greater than SN 1006 and the gas density in its vicinity Is higher. Our calculations have shown that it is impossible to obtain the observed shape and surface density distribution for the explosion in the uniform media using only the asymmetry of the explosion. The observed shape with two extended radioarcs lay rather strong restrictions on the possible gas distribution in the vicinity of the explosion. From several tens of variants for which calculations have been made, the best coincidence with observations has been obtained for the explosion in the tunnel where density falls with increasing Z and the point of the explosion on the axis of symmetry is shifted from the symmetry place. The density distribution in the viciIiit~r of the explosion point (R = O. Z = Z) was given by the following formulas

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26 a) T-1 6800 MO-2.5 30 20- 15- \l 10- \ 5- 1 ~\!1 c 1 1 1 ~11 -30 ~20 -15 -10 -S 0 \ 1 -5- W 1 -15- _ / 1 / i i/ ~ I I i 5 10 15 20 25 30 . / AMERICAN AND SOVIET PERSPECTIVES b) 1 1 C - -20- -25- -30- R(PC) 30- 25- 20 15- 10 o 5 _ -10- ~ 5 _ -20 - _ -25 - _ -30 ~1~e - \ 1 1 1 1 1 ~1 1 2 4 6 8 10 12 ~16 FIGURE 3 Shape (a) and surface density distribution (by of SNR after the explosion in the gas tunnel for Mej = Z5 ME at t = 16800 yr. The center of the coordinate system coincides with the point of explosion. The s3 mmetry plane of the gas distribution is situated 7.5 pc below this point. Dashed lines represent the form and symmetry planes of the gas tunnel. _ ~ P(R1 Z) = Po/ (R (z) - 1) +~21 t(Z ) J L p(R, Z) = Po/ {42 (zzo)2 + 1 Ro(Z) = Ro [1 + (Z/Rc)2]. L] + 1 I } , R < Ro(Z) | }, R > Ro(Z) (8.1) (8.2) (8.3) The results of our calculations are presented in Figure 3, where the ma~n- mum density in the plane of symmetry Is n(Ro, Z = 0) = lam~3; the density difference in all layers is n(ROZ)/n(O, Z) = 3; and the characterisitic scale of density change along Z axis is Z = 10 pc. The radius of the tunnel Is equal to Ro = 10 pc in the symmetry plane Z = ~ and increases with characteristic scale Rc = 20 pc for larger A, and the point of the explosion is shifted up from the summery plane by Zc = 7,5 pc. The initial energy of the explosion is equal to 105i ergs with 75% in the form of ldnetic energy and the mass ejected in the explosion is equal lo 2.5M~. The age of the remnant in Figure 3 is about 17,000 years, but it is still in the adiabatic stage. The velocities of the shock wave are equal to 1100 km/s on the upper pole, 440 km/s on the lower pole and 250 lan/s in the plane of maximum surface density. The distribution of the gas described by (~.1~-~.3) may be a result

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HIGH-ENERGY ASTROPHYSICS 27 of partial merging of two old SNR,1 or the tunnel may be formed by the proge~tor~s mass loss into the nonuniform gas layer. CONCLUSIONS 1. The supernova remnants with axial symmetry may be formed by anisotropic supernova explosions with most of ejecta confined to an equatorial plane as well as a result of the explosions in nonuniform media. The first mechanism determines the asymme- t~y of a majority of young SNRs, and the second determines the morphology of the older ones. 2. SN 1006 is formed by the anisotropic explosion and corresponds to the stage of transition from free expansion to the adiabatic stage. The distance to the remnant Is equal to 1.~1.6 kpc and corresponds to the lower boundary of observational estimations. 3. The morphology of SNR G296.5~10.0 may be explained if the explosion had occurred in the tunnel with the density falling with increasing Z The explosion point is situated on the symmetry am but is shifted up from the symmetry plane. REFERENCES Bisnovatyi-Kogan, G.S. 1970. Astron. Zh. 47:813. Bisnovatyi-Kogan, G.S., and S.I. Blinnikov. 1982 Astron. Zh. 59:876. Bisnovatyi-Kogan, G.S., S.I. Blinnikov, and SW Silich. 1989. Astrophys. Space Sci. 154:229. Bodenheimer, P. and S.E. Wooster. 19B3. Astrophys. J. 269 281. Dubaer, G.M., F.R Colomb, and E.B. Giacani. 1986. A J. 91: 343. Fesen, R^, J.M. Shull, and J.M. Saken. 1988. Nature 334:229. Hamilton, AJ.S., CL Sarazin, and NE. Szymkowia~ 1986. Astrophys. J. 300:698. Kesteven, MJ., and JO Caswell. 1987. Astron. Astrophys. 183:11& Kirshner, RP., P.F. Wrinkler, and RN Chevalier. 1987. Astrophys. J. 315:L135. Lozinskaya, I:A 1986. Supernovae and Stellar Wind: Interaction with the Galactic Gas. Nauka, Moscow. Manchester, R.N. 1987. Astron. Astrophys. 171:21)5. Roger, AS., D.K Milne, MJ. Kesteven, KJ. Wellington, and RF. Haynes. 1988. Astrophys. J. 332:940. Ruis, M.T 1983. Astron. J. 88:1210. Soker, N., and M. Rio. 1989. Astrophys J. 339:268. 1To one of us (1WL^) this possibility seems unrealistic