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OCR for page 19
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
OCR for page 20
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
OCR for page 21
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
OCR for page 22
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).
OCR for page 23
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
OCR for page 24
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
OCR for page 25
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
OCR for page 26
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
OCR for page 27
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
Representative terms from entire chapter:
equatorial plane
