Within 100 pc of the sun there are over a hundred cirrus clouds with masses of approx. 60 solar mass and dense molecular clouds with masses of approx. 4 solar mass. If the local interstellar density of cosmic rays is also present in these clouds, the flux of neutral pion from the decay of gamma rays from the core of a cloud at a distance of 20 pc is approx. 13 x 10(exp -8) photons/sq cm/s. The flux from the more extensive cirrus cloud is approx 4 x 10(exp -7) photons/sq cm/s. A relativistic beam of particles generated by a compact stellar object and incident upon a large, close companion can be a strong gamma ray line source if more of the beam energy is used in interactions with C and O and heavier nuclei and less with H and He. This would be the case if the companion has lost its hydrogen envelope and nucleosynthesized much of its He into C, O, and Ne. Such objects are Wolf-Rayet stars and it is believed that some Wolf-Rayet stars do, in fact, have compact companions. For a beam of protons of 10(exp 37) erg/s, the flux at 1 kpc of the 4.4 MeV C-12 line could be as high as 5 x 10(exp -6) photons/sq cm/s. The fluxes of the deexcitation lines from the spallation products of O-16 are also presented

Murphy, Ronald J.

Silberberg, R.

English

NASA Technical Reports Server

GRO SOURCE CANDIDATES: (A) NEARBY MODEST-SIZE
MOLECULAR CLOUDS, (B) PULSAR WITH WOLF-RAYET
COMPANION THAT HAS LOST ITS H-ENVELOPE
/
R. Silberberg
E.O. Hulburt Center for Space Research
Naval Research Laboratory
Washington, DC 20375-5000
R. J. Murphy
Universities Space- Research Association
Code 4154
Naval Research Laboratory
._, Washington, DC 20375-5000 i_
" - ABSTRACT //
/
/
/
Within i00 pc of the-sun there are over a hundred cirrus
clouds with masses of'_'60 i_and dense molecular cores with
masses of 4 _. If the local interstellar density of
cosmic rays is also present in these clouds, the flux of 7°-
decay gamma rays from the core of a cloud at a distance of
20 pc is -3 x 10 -8 photons 'c:_J2_ "I " The flux from the.more
extensive cirrus cloud is _4 x 10 -7 photons cm "2 s-I.
A relativistic beam of particles generated by a compact
stellar object and incident upon a large, close companion
can be a strong gamma-ray line source if more of the beam
energy is used in interactions with C and 0 and heavier
nuclei and less with H and He. This would be the case if
the companion has lost its hydrogen envelope and nucleo-
synthesized much of its He into C, 0 and Ne. Such objects
are Wolf-Rayet stars and it is believed that some Wolf-Rayet
stars do, in fact, have compact companions. For a beam of
rotons of 1037 er E s-1, the flux at 1 kpc of the 4.4 MeV
2C line could be as hlgh as 5 x 10 -6 photons cm -2 s-1. The
fluxes of the deexcitatlon lines from the spallation
products of 160 are also presented.
INTRODUCTION
The near-by cirrus clouds, although modest in size, are of particular
interest. They are responsible for a substantial contribution to the
nuclear spallatlon reactions of primary cosmic rays and thus to the
secondary cosmlc-ray nuclei observed near the earth. The bulk motion
velocity of cosmic rays is estimated to be small; hence, the principal
region of propagatlo_ of the local cosmic-rays (and probably also of the
orlglnof many of the local cosmic rays) is largely within a few hundred
parsecs. While more distant regions also contribute to the local cosmic
rays (which also clrculate in the Galactlchalo), the contribution of
the closer sources is likely to be more important. Gamma rays from the
cirrus clouds thus are probes of nuclear reactions and plon production
by cosmic rays and of the electron bremsstrahlun 8 that take place in our
Galactic neighborhood.
-(NASA-cR-16B491) GRO SOUP.CF CANOIDATES: (A)
NFARSY MODEST-SIZE M_-_LiCULAR CLCUgS; (5)
PULSAR WITH WOLF-RAYEIT COMPANION THAT HAS
LOST _Tq _-!-ENV_LOPE (Hu]burt) 7 p
N91-32021
Uncl as
G3/92 0219572
https://ntrs.nasa.gov/search.jsp?R=19910022707 2020-03-17T14:57:30+00:00Z
Gamma-ray lines from stellar objects that have undergone advanced stages
of nucleosynthesis and have unusual time- or evolution-dependent surface
compositions are of special interest. Wolf-Rayet stars are such
objects. Observations of g_-ray lines from them would provide
information on two items of interest: (I) The presence and energy
spectrum of i0 - i00 MeV particles in pulsar or black-hole beams and (2)
the Wolf-Rayet star composition which, while theoretical models exist,
is still poorly known.
CIRRUS CLOUDS AND THEIR MOLECULAR CORES
Cirrus clouds in the solar system neighborhood have been explored by
Turner, Rickard and Ping (1989). They report that over i00 cirrus
clouds with a mean mass of about 60 M 0 are within i00 pc of the sun.
These clouds have dense molecular cores with masses of -4 MO, densities
of ~4 x 104 atoms cm "3, and sizes of -0.15 pc. Molecules like H2CO ,
C3H 2 and HC3N have been observed. Due to their close distance, many of
these clouds are at high galactic latitude and were seen with IRAS.
The flux of gamma-rays from a cloud-core of mass 4 M e was calculated
assuming that the density of cosmic rays in the cloud is the same as in
local interstellar space. We use the formula given by Issa etal.
(1981) for the flux, #:
= 2.8 x i0 -12 M / dk2 photons cm -2 s -I
where M is the cloud mass in units of M e and dk is the distance to the
cloud in kpc. The flux at 20 pc (0.02 kpc) of gamma rays from the decay
of 7 ° produced in a 4 M e core is then ~3 x 10 -8 photons cm -2 s -I and,
from a 60 M e extended cirrus cloud, is -4 x 10 -7 photons cm "2 s-I.
Depending on the detector threshold, the gamma rays produced by electron
bremsstrahlung could yield a similar flux from these sources.
In addition to the relatively small clouds within i00 pc, there are a
few large clouds somewhat beyond i00 pc from whlch gamma rays have
already been detected with COS-B. These clouds are probably also within
the principal propagation region of the locally-observed cosmic rays.
The distances and masses of these clouds and the observed gamma-ray
fluxes are given by Issa etal. (1981) and are reproduced here.
Possible reasons for the deviation of observed fluxes from expected
values (such as enhanced cosmlc-ray intensity in clouds or additional
ganmm-ray production by stars in clouds) are discussed by Issa etal.
(1981).
distance
(pc)
mass flux
(103 M e ) (10 -6 ph cm "2 s "1)
p Oph 160 6 1.1
Taurus 140 200 0.5
Cot. Aust. 150 i00 0.4
WOLF-RAYET STARS WITH BINARY COMPANIONS
Nuclear excitation and spallation lines have been observed from the sun.
In particular, the flare of 27 April 1981 (discussed by Forrest 1983 and
analysed for elemental abundances by Murphy etal. 1985a,b and for
accelerated-particle anisotropies by Murphy etal. 1990) has shown what
can be learned from ganmm ray lines from astrophysical sources. _e
shall explore here whether such lines can he observed from typical
galactic distances. Ramaty, Kozlovsky and Lingenfelter (1979)
calculated the gamma-ray line intensities generated by cosmic-ray
interactions with the galactic gas, adopting rather optimistic
assumptions. Recent background estimates by Schonfelder, Ballmoos and
Diehl (1987) and line intensity estimates by Higdon (1987) suggest that
only the narrow-line emission of 160, with an intensity of about 2 to 8
x 10 -6 photons cm "2 s"I tad -I, appears to be a promising candidate when
a large, hlgh-resolution detector is used. Morfill and Meyer (1981)
explored ganmm-ray line production from supernova remnants hidden in
clouds. At 5 kpc, the fluxes of the 4.4 MeV 12C and 6.1 MeV 160 lines
from such a remnant would be about 6 x 10 -5 photons cm -2 s-I.
The observation of stellar excitation and spallatlon gamma-ray lines
from galactic distances appears possible only with the combination of a
powerful energy source for proton acceleration (1037 to 1038 erg s'l),
like a young pulsar or an accreting X-ray binary, and a target object
consistin 8 mainly of heavier nuclei (atomic numbers Z Z 6). The Wolf-
Rayet stars are such objects. Many (about I/3) of these are in binary
systems with massive OB star companions that are about 2 to 5 times more
massive that the Wolf-Rayet star (Vanbeveren 1987, Schulte-Ladbeck
1988). Abbott and Conti (1987) state in their review paper that there
should also be Wolf-Rayet stars with collapsed (neutron-star or black-
hole) companions. Some X-ray
binary systems consisting of an 0
star and a collapsed companion
could have the 0-star evolve into
a Wolf-Rayet star. In fact, Wolf- I
Rayet stars with periods of a few
days (which would imply the 10-'
presence of a compact companion)
have been observsdT the object HD
197406 is believed to be a Wolf- Io_
Rayet star with a black hole w
companion. A possible scenario x
could begin in a binary consisting I°7
of two massive stars. One enters
the helium burning phase, expands . _o.4
to a red gian t , transfers its
hydrogen envelope to the
companion, and becomes a Wolf- _
Rayet star with most of the mass
of the system now in the OB
companion. The _olf-Rayet Star
becomes a supernova and,
ultimately, a pulsar, while the OB
companion expands to a giant star,
loses its hydrogen envelope, and
Figure I. Wolf-Rayet star surface
mass fractions (from Mewaldt 1989).
becomesa Wolf-Rayet star. (The
pulsar may deflect the hydrogen
envelope, or absorb _t, becoming a
black hole; a mass-accretlng black
hole can also generate a
relativistic particle beam.)
The composition of Wolf-Rayet
stars is not yet well known. In
the C- and O-rich WC and WO stars,
hydrogen has been depleted and
12C, 160, 22Ne, 25Mg and 26Mg have
been formed as a result of helium
burning and the He + 14N reaction•
The possible evolution of
composition has been reviewed by
Mewaldt (1989). We reproduce his
Figure 14 (which is based on eOe-V
Prantzos etal. 1986) in Figure i.
A proton beam incident on a Wolf-
Rayet star with such evolved _.__v
composition is a much stronger
ganmm-ray llne source than when o
the beam is incident on a star of =
0 5_-7
solar composition (by a factor of
~i00) The beam energy is not
•
wasted in proton-Hydrogen v
ionization losses and the relative _ _7,-7
abundance of the line-producing
nuclei (C, O, etc.) are enhanced.
We have carried out calculations
of ganmm ray llne fluxes using the
Monte Carlo technique developed by
Ramaty, Kozlovsky and Lingenfelter
(1979) which incorporates
laboratory measurements of the
energy- and angle-dependent
reaction cross sections and takes
r..
>
OJ 58e-7
U_
Woif-Rayet star
Interaction site
Proton bea_ 'Observati°n nile" Sob"
Pulsar (neutron star) To observer
Figure 2. Binary geometry.
55* -7
Figure 3.
I I I I I
isotropic
s - 2.5, E¢ - 25 MeV
I I I I .I
0 20 40 6O 8O lO0
Observation angle. Oobj
5.2 HeY line-complex flux.
into account Doppler shifts and relativistic beaming. The calculations
were performed for thick-target interactions in which energetic protons
produce the nuclear reactions while they slow down and stop in the
ambient medium (e.g., see Ramaty and Murphy, 1987 ) . Compton-scattering
of the emerging photons wal neglected. For the ambient composition, we
use the abundancei of Figure 1 at 5.3 x I06 years. For the energetic
_proton number kinetic energy spectrums we assume a power law with
various values for the spectral index s and with low-energy cutoffs (due
either: to effects at the pulsar or to some hello-magnetic effect at the
Wolf-Rayet star) of E c - 25, 50, 100 and 200 MeV. We assume a total
beam power of 1037 errs s-1. The calculations were made for energetic
protons collimated into a beam with beam-to-observer angles (8oh s) of
0e, 45 e and 90e. This angle is illustrated in Figure 2. For
comparison, the fluxes from Isotroplc proton interactions have also been
calculated. The fluxes given are the instantaneous values during the
interaction time of the pulsar beam with the star. The flux value
averaged over the rotation period _.-6
of the pulsar would depend on the
duty cycle of beam exposure. We --
focus on the 4.4 MeV llne from 12C
deexcitation and the -5.2 HeY line E 4,-8
O
complex from deexcitation of 14N,
15N and 150.
o
o
_3e-6
X
D
>
2e-B
Deexcitatlon line emission is
quite isotroplc regardless of the
angular distribution of the
accelerated particles. This i_
illustrated in Figure 3 where the
~5.2 MeV flux for a beam of
energetic protons with s - 2.5 and _
Ec - 25 HeY is plotted as a
function of Oob s (together with i.,-6
the flux from an isotropic
distribution). This weak (-41)
dependence is due to the
relativistic beaming of the
photons emitted by the recoil 18
excited nuclei. The cross
sections for gamma-ray production ,e
generally exhibit a fairly sharp
peak at an energy of -10-30 MeV o
-J 14
with the result that, in the
thick-target limit, most of the 712
interactions occur at these
proton energies. The velocities >
of the recoil nuclei are therefore _1o
small and the beaming effect is
correspondingly small. _ e
O
6
The 4.4 HeV line flux is shown in
Figure 4 for 8ob s = 45" as a
function of the proton kinetic
energy spectrum low-energy cutoff,
E c, for both s - 2.5 and 3. The
flux exhibits a significant
sensitivity to E c. This is also
true of the -5.2 MeV flux. For
Figure 4.
Figure 5.
i ! ! I
| I [ l
50 IOO ;50 200
Ec (MeV)
4.4 HeY line flux.
I I 1 !
Ec s
I I I
3 4 S
Power law index, S
4.4-to-5.2 MeV flux ratio.
the compositions and parameters
considered here, the 4.4 HeY llne flux could be as high as -5 x 10 -6
photons cm -2 s-I. The 4.4 Mev line is produced primarily by direct
x a o 12 16 se Cit tion f ambient C nuclei, the contribution from 0 pallatlon
being _30l. The -5.2 MeV llne complex, however, is produced completely
by spallation of 160 to 14N, 15N and 150. Since the cross sections for
spallatlon reactions generally have higher energy thresholds and extend
to higher energies than those for direct excitation, the ratio of the
4.4 MeV to -5.2 MeV fluxes is sensitive to the energetic proton kinetic
energy spectrum in the -10-100 MeV range where the cross sections are
significant. This can be seen in Figure 5 where this ratio for E c - 25
MeV and 8ob s - 45 ° is plotted as a function of the power-law index. The
ratio is also sensitive to the low energy cutoff, as can be seen in
Figure 6 where it is plotted as a
function of Ec for s - 2.5 and
8oh s - 45°. If theoretical
considerations concerning the
generation of the proton beam can
constrain either s or Ec, then
calculations such as these can be
used to determine the other
spectral parameter.
SUMMARY
We have calculated the ga=mm-ray
flux expected from two possible
sources: nearby, modest-sized
molecular clouds and a pulsar-Wolf
Rayet binary. Cosmlc-ray
interactions with the dense core
of a molecular cloud at a distance
of 20 pc could produce a flux of
O
C_
>
o5
C_
uS
O
m'4
= 2.,5. e=_ - 45"
3 L 1 I ) ,
0 50 tO0 t50 200
Ec (MeV)
Figure 6. 4.4-to-5.2 HeY flux ratio.
f°-decay gamma rays of ~3 x 10 -8
photons cm -2 s-I if the local cosmic-ray intensity is present also in
the clouds. The flux from the extended cloud could be -4 x 10 -7 photons
cm-Z s-l.
A relativistic beam of particles generated by a compact stellar object
and incident upon a large, close companion can be a strong gamma-ray
line source if more of the beam energy is used in interactions with C
and 0 and heavier nuclei and less in ionization losses with H and He.
Wolf-Rayet stars have lost their hydrogen envelopes and have nucleo-
synthesized much of the He into C, 0 and Ne. Pulsar-Wolf-Rayet binaries
are therefore good candidates for strong gamma-ray line production. We
have calculated the fluxes in the 4.4 MeV 12C line and in the -5.2 MeV
160 spallation llne complex resulting from interactions of a proton beam
with a stellar atmosphere of typical Wolf-Rayet composition. We find
that the 4.6 MeV line flux at 1 kpc could be as high as 5 x 10 -6 photons
cm -2 s-1. In addition, the ratio of the 4.4 MeV flux to the -5.2 MeV
flux is sensitive to the spectral shape of the energetic protons in the
10-100 MeV range. It could thus be used as a probe of pulsar
acceleration mechanisms.
_'_ ..... - ..... REFERENCES
Abbott, D. C., and Conti, P. S. 1987, Ann, Rev. Astron. Ao., 25, iii.
Forrest, D. J. 1983, in "Positron and Electron Pairs in Astrophysics',
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Higdon, J. C. 1987, 20th Internat. Cosmic Ray Conf. P_vers (Moscow), I,
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Issa, M. R., Riley, P. A., Ti-Pei, Li, and Wolfendale, A. W. i981, 17th
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Mewaldt, R. A. 1989, in "Cosmic Abundances of Matter', ed. C. 3.
vi,
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Morfill, G. E., and Meyer, P. 1981, 17th Internal. Cosmic RaT Conf.
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GRO source candidates: (A) Nearby modest-size molecular clouds; (B) Pulsar with Wolf-Rayet companion that has lost its H-envelope

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