It is shown that the Ni-56-Fe-56 observed in SN 1987A argues that core collapse supernovae may be responsible for more that 50 percent of the iron in the galaxy. Furthermore it is argued that the time averaged rate of thermonuclear driven Type I supernovae may be at least an order of magnitude lower than the average rate of core collapse supernovae. The present low rate of Type II supernovae (below their time averaged rate of approx. 1/10 yr) is either because the past rate was much higher because many core collapse supernovae are dim like SN 1987A. However, even in this latter case they are only an order of magnitude dimmer that normal Type II's due to the contribution of Ni-56 decay to the light curve

Arnett, W. David

Schramm, David N.

Truran, James W.

English

NASA Technical Reports Server

1 a Fermi National Accelerator Laboratory 
F E ~ I ~ - P u b - 8 8 / 1 1 8 - A  
A u g u s t  1 9 8 8  3 F  cr 
On Relative Supernova Rates 
and Nucleosynthesis Roles 
ORWNAL PAGE tS 
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W. David Arnett1'2, David N. Schramm 1 
James W. Truran 3 
and 
ABSTRACT 
It is shown that the 56Ni-56Fe observed in SN 1987A 
argues that core collapse supernovae may be responsible 
for more than 50% of the iron in the galaxy. 
Furthermore it is argued that the time averaged rate of 
thermonuclear driven Type I supernovae may be at least 
an order of magnitude lower than the average rate of 
core collapse supernovae. The present low rate of 
Type I1 supernovae (below their time averaged rate of 
"1/10 yr) is either because the past rate was much 
higher or because many core collapse supernovae are dim 
like SN 1 9 8 7 A .  However, even in this latter case they 
are only an order of magnitude dimmer than normal Type 
II's, due t-d the contribution of 56Ni decay to the 
light curve. 
The University of Chicago and Fermilab. 
The University of Illinois. 
* The University of Arizona. 
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W 
Oporrted by Universities Research Association Inc. under contract with the United States Department of Energy 
https://ntrs.nasa.gov/search.jsp?R=19890002302 2020-03-20T05:45:46+00:00Z
INTRODUCTION 
ORKYNAL PAGE fS 
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Recent observational and theoretical developments with respect 
to supernovae have provided interesting and important constraints on 
the contributions of supernovae of Types I and I1 to galactic 
nucleosynthesis. In particular, theoretical modelling of the light 
curves of Type I supernovae has led to the conclusion that 
approximately 0.7 M, must be ejected in the form of 
(Arnett 1988; Woosley 1988). More recently, observations of 
56Fe(56Ni) 
supernovae 1987A indicate an exponentially falling light curve with 
a lifetime compatible with 56C0 decay; the observed luminosity and 
the distance dictate that approximately 0.07 Ma of matter was 
ejected in the form of nuclei of mass A=56  in this Type I1 
supernova event. We have available, for the first time, a measure 
of the relative contributions of supernovae of Types I and I1 to the 
abundance of 56Fe in galactic matter. We briefly explore some 
interesting possible constraints these mass estimates permit us to 
impose on galactic chemical evolution and on the rates of supernova 
activity over the histories of our Galaxy and other galaxies. 
DEFINITIONS 
To avoid many-of the  misconceptions that persist in supernova 
rate discussions, it is imperative that we define terms. In 
particular, we want to isolate the internal physics of an event from 
the traditional astronomical classification based on the character 
of the external optical outburst. 
The optical outbursts have traditionally been split i n t o  two 
major categories. Type I (no hydrogen, L "10 9 La, Vrms IO^^^/^^^) 
" 5 0 0 0  km/sec). and Type I1 (hydrogen, 10 Lo , Vrms a 
Physically, Type 11's are associated with young massive 
( M > 1 0  Mo ) stars undergoing core collapse. Standard Type 1's are 
associated with the thermonuclear detonation or deflagration of a 
C/O white dwarf. Rather than get into the inelegant details of 
traditional classification schemes (Type IBIS, etc.,) we will 
classify supernova in this paper by the physics, namely 
1. Thermonuclear explosion 
2 .  Core collapse 
The surface "weather", composition and exterior structure which 
determine how a traditional astronomer classifies supernovae will be 
ignored, except when discussing observational predictions. 
Note that from the physics point of view, a core collapse model 
produces a dense remnant (either a neutron star or a black hole) and 
ejects large amounts of the heavy elements from oxygen to iron. 
However, the Fe ejecta comes from near the " 1 . 4  Mo core where all 
massive stars have similar structures, while the oxygen and other 
intermediate elements come from the mantle where the mass of each 
region varies with the mass of the star. Therefore the O/Fe ratio 
will vary with the mass of the star. A s  mentioned above, from 
SN 1987A we know that about 0.07 MQ This 
amount should hold approximately for all core collapse models. The 
mass of ejected oxygen goes from perhaps "0.1 M for the extreme 
of Fe gets ejected. 
e 
ORJGJNAL PAGE IS 
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low end of the mass range to "10 Mo 
collapse events. 
for the most massive core 
Core collapse is the prime nucleosynthetic event; thermonuclear 
explosions skip the bulk of the intermediate mass nuclei on their 
way to creating iron. It is now widely believed that this almost 
complete conversion of burned fuel to Fe is what powers 
traditional Type I light curves, with their long decay time. Such 
powering necessarily demands that >60% of the core is converted to 
iron. 
INTEGRAL RATE OF CORE COLLAPSES IN THE GALAXY 
Since the bulk of the heavy elements are ejected in core 
collapse events, the "average" rate of nucleosynthesis in the Galaxy 
gives us a good estimate of the average rate of core collapse 
events. 
11 Taking the mass of the disk of our Galaxy as 10 Mo 
and its age as lolo yr and the mass fraction of oxygen in the disk 
as " 8 . 5 ~ 1 0 - ~  , one estimates the total mass of oxygen in the disk 
as "8.5~10 Mo . The oxygen yields of massive stars increase 
sharply with the mass of the star. Detailed yield estimates have 
been carried out in a continuous series of calculations by Arnett . 
and Weaver and Woosley and their collaborators (c.f. Thielemann & 
Arnett 1985 and Woosley & Weaver, 1982 as well as Arnett 1988 and 
Woosley 1988). Although some difference exist from year-to-year and 
between the two groups the numbers we will use in our arguments are 
8 
L"R!GINAL PAGE IS 
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a reasonable representation of the results and their spread. i 
steeply falling initial mass function of the Salpeter type would 
argue that the typical core collapse supernova is one with a mass 
near the lower mass cutoff (c.f. Arnett & Schramm 1973). Such an 
object ejects 
Salpeter mass function is slightly higher due to the higher oxygen 
yields of the more massive stars. The actual value is sensitive to 
the high mass cutoff on the mass function and to mass loss 
assumptions. For reasonable choices, the mass averaged yield ranges 
from 0.7 to 1.5 MQ 
the averaged yields are correspondingly lower and can almost 
approach the yield of the "typical" supernova selected by number. To 
within the uncertainties, it seems that to produce 
oxygen would require about 10 supernovae. Since these occurred 
over the 
collapse supernova rate of "1/10 yr over the history of the Galaxy. 
Although there is about a factor of 2 uncertainty in this 
"0.5 Mo oxygen. The mass averaged yield using a 
of oxygen. For the steeper Scalo mass function 
" l o 9  Me of 
9 
10" yr galactic lifetime, this implies an averaue core 
average rate, it seems clear that the average is higher than the 
currently observed visible Type I1 supernova rate from external Sb 
and Sc galaxies which is 1/50 yr (c.f. Trimble 1988, Vandenberg 
1988, McClure 61 Evans 1987) also to a factor of "2 accuracy. 
The reason for this difference may be either that the past rate 
was much higher or that the observed Type I1 rate does not include 
a large fraction of the actual core collapse events. This latter 
possibility would occur if many core collapse events were no more 
ORlGlNAL PAGE IS 
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luminous than SN 1987A or if they were embedded in dense obscuring 
regions, such as is the case for CasA. 
Alternative approaches to finding supernova rates, such as 
remnant statistics, pulsar statistics or X-ray heating of the disk 
are plagued by model dependencies on lifetimes, beaming factors, hot 
X-ray chimneys, etc. and may be less reliable than the 
nucleosynthesis arguments. 
Galactic evolution models can accommodate a present rate either 
equal to or below the average rate as long as the average is 
maintained overall. For example, constant rate models can utilize 
infall, outflow or variable initial mass function to fit abundance 
versus age data (c.f. Tinsley 1976) while high early rate models 
have the star formation rate decrease with time to fit the data. 
Standard one-zone galactic evolution, models in the absence of 
infall or X-ray outflow with a constant IMF, 
decreasing rates as the available gas is used up. However, it 
should be remembered that a comparison of nucleosynthetic rates 
averaged over nuclearchronometers of different lifetimes argues that 
rates have not changed by more than a factor of " 2  (Meyer and 
Schramm 1986). 
do predict such 
For the future it is clear that supernova searches need to be 
refined so that low luminosity events such as SN 1987A can be seen 
in external galaxies. Note that because of the 
decay, SN 1987A eventually reached a peak luminosity that was within 
an order of magnitude of a "normal" Type 11, so with appropriate 
56Ni,56 C0-t 56 Fe 
search techniques even such faint ones might be able to be observed 
in more distant galaxies. Of C O L - S ~ ,  observations will be more 
difficult in high luminosity galaxies than in low luminosity dwarfs 
so care must be taken to avoid biases. 
This minimal luminosity from SN 1987A shows us that even with a 
blue envelope structure, the resulting collapse event yields an I 
I 
I appreciable luminosity. Therefore, searches sensitive to one order 
I 
of magnitude fainter should be successful regardless of whether or 
not the progenitor is red or blue, unless the object is embedded in 
a dense cloud. In any case, this avoids the complication as to 
whether or not red or blue progenitors are more or less likely in 
galactic disks. 
Fe YIELDS 
Notice that, with an average core collapse rate of 1/10 yr and 
the assumption that all core collapses produce O.07M of Fe as did 
SN 1987A, core collapse events alone can produce about 10 M of Fe. 
This is sufficient to explain half the Galactic Fe abundance. 
Therefore, within the uncertainties, thermonuclear event iron 
production may even be unnecessary to the Galactic iron production. 
Most likely it is only comparable to the yields from core collapse 
events. Since thermonuclear Type 1's produce O.7M of Fe this 
argues that the average Galactic rate of such events must be no more 
than their present extragalactic observed rate of 1/70 yr (factor 
of 2 accuracy) (c.f. Vandenberg, McClure and Evans 1987)) or they 
0 
8 
0 
8 
I 
would over produce Fe. In other words, the rate of thermonuclear 
events (Type I t s )  must either be constant or decrease as one looks 
into the past (high red shift) whereas the rate of core collapse 
events must either be much, much higher in the past than the present 
observed Type I1 rate or the present collapse rate itself is about a 
factor of 5 higher than the observed Type I1 rate. In either case 
high redshift studies should find at least an order of magnitude 
more collapse events than thermonuclear events. 
Obviously an important check on these arguments is to verify 
that other core collapse events such as CasA and "normal" Type 11's 
also produced about 0.07 Ma of iron, 
CONCLUSIONS 
(1) The integrated Type I (thermonuclear event) rate by number is 
"10 percent of the corresponding rate of Type I1 (collapse) 
events. 
(2) The observed Type I thermonuclear rate todav is "10 percent of 
the average collapse event rate. This implies that the Type I 
rate can at best have remained constant in time over the course 
of galactic history, and might even have been lower in the 
past. 
(3) From SN 1987A light curve characteristics, we can now identify 
collapse events with the production of approximately 
of 56Ni. 
red or blue), the luminosity should reach values within one 
0.1 Ma 
Independent of surface "weather conditions" (whether 
magnitude of the values characteristic of Type I1 supernovae at 
maximum optical light.(Typical Type 11's have Mv"-16.5, 
SN 1987a peaked at Mv"-15.5.) 
Je emphasize the importance of a survey aimed at the 
establishment of the rate of 1987A-like events in our Galaxy 
and other galaxies. It is this rate (generalized Type I1 rate) 
that is relevant for neutrino searches. 
We also emphasize the need for a survey of Type I1 light curves 
to determine whether a yield of 56Ni of the order of 0.7 M 
is statistically accurate for "normal" Type 11's. 
We predict that the rate of Type I events in galaxies at 
increasing redshift should not increase, but rather remain 
constant or perhaps even decrease. 
The origin of about half (or more) of the iron in the disk of 
the galaxy lies in collapse events (Type I1 supernovae) and 
thus iron tracks massive star deaths. 
Metal abundance anomalies in extreme Population I1 objects 
might be attributed to massive star collapse events alone, 
since the O/Fe ratio in the ejecta is expected to vary with 
initial stellar mass. The contribution from massive stars 
2 30 Ma 
while stars of mass M X 20 MQ 
O/Fe (SN 1987A). 
are characterized by O/Fe ratios several times solar, 
yield approximately solar 
56 The presence or absence of 56Ni ( Co), as indicated by the 
light curves, provides a potential probe of the correctness of 
our arguments. 
The ratio of the required integrated rate of nucleosynthesis 
events to the present rate of collapse events also holds 
important implications for galactic evolution. 
Nucleochrometer arguments imply constancy to factor of “ 2  
accuracy. 
ACKNOWLEDGEMENTS 
The authors acknowledge the hospitality of the Aspen Center for 
Physics where this work was initiated and finalized. This work was 
supported in part by the NSF at the University of Chicago and at the 
University of Illinois and by the NASA/Fermilab astrophysics 
program. 
REFERENCES 
W. D. Arnett 1988 - University of Chicago preprint. 
W. D. Arnett 61 D. N. Schramm 1973 - ApJ, 185 L47. 
B. Meyer and D. N. Schramm 1986 - ApJ, 311, 406 
B. M. Tinsley 1976 - ApJ. 108, 797. 
F. Theilemann and W. D. Arnett 1985 in Nucleosynthesis, ed. N. 
D. Arnett and J. Truran, Univ. of Chicago Press 
V. Trimble 1988 - Proc. Yanada Conference, Tokyo. 
S. Vandenberg, R. McClure and R. Evans 1987 - ApJ, 323, 44. 
S. Woosley 1988 - University of California, Santa Cruz preprint. 
S. Woosley and T. Weaver 1982 in Essavs in Nuclear Astrophvsics, 
ed. C. Barnes, D. Clayton and D. N. Schramm, Cambridge Univ. 
Press 


On relative supernova rates and nucleosynthesis roles

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