Giant Molecular Clouds (GMCs) are massive, bounded, cool, dense regions containing mostly H2, but also H I, CO, and other molecules. These clouds occupy less than 1 percent of the galactic volume, but are a substantial part of the interstellar mass. They are irradiated by the high energy cosmic rays which are possibly modulated by the matter and magnetic fields within the clouds. The product of cosmic-ray flux and matter density is traced by the emission of high energy gamma-rays. A spherical cloud model is considered and the gamma ray flux from several GMCs within 1 kpc of the sun which should be detectable by the EGRET (Energetic Gamma-Ray Experimental Telescope) instrument on GRO (Gamma Ray Observatory)

Hunter, Stanley D.

Kanbach, Gottfried

English

NASA Technical Reports Server

N90-23298
GAMMA RAYS FROM GIANT MOLECULAR CLOUDS
STANLEY D. HUNTER
NASA/Goddard Space Flight Center, Greenbelt, MD 20771
GOTTFRIED KANBACH
Max-Planck-Institut fiir Extraterrestrische Physik, D-8046 Garching F.R.G.
ABSTRACT
Giant Molecular Clouds (GMCs) are massive, bounded, cool, dense regions containing
mostly H2, but also HI, CO, and other molecules. These clouds occupy < 1% of the
galactic volume, but are a substantial part of the interstellar mass. They are irradiated
by the high energy cosmic rays which are possibly modulated by the matter and magnetic
fields within the clouds. The product of cosmic-ray flfix and matter density is traced by
the emission of high energy gamma-rays. In this paper, we consider a spherical cloud
model and predict the gamma ray flux from several GMCs within 1 kpc of the sun which
should be detectable by the EGRET instrument on GRO.
I. INTRODUCTION
Interaction of cosmic rays with matter in molecular clouds, containing mostly It2, but also
Itl, CO, and other molecules, produces high energy gamma rays. These gamma rays pass nearly
unattenuated through the galaxy and thus, are a good tracer of the product of the cosmic ray flux
and the matter density at distant places in our galaxy. The density of HI in these clouds can be
observed directly. In cool, dense clouds, the II2 density can be inferred from the intensity of the
CO, J = 1 --* 0 transition. If the lh and CO radio observations accurately represent the matter
distribution in molecular clouds then the local cosmic ray flux at these clouds can be determined
from tile gamma ray flux.
The gamma ray production rate as a function of the line of sight matter density, or source
function, has been calculated for the Orion clouds by Bloelnen, et al. (1984b). They used the CO
data from Maddalena, et al. (1986), the lh data derived from Heiles and tiabing (1974, b < -10 °)
and Weaver and Williams (1973, b > -10 °) and the COS-B gamma ray data, Mayer-Ilasselwander,
et.al. (1982), with a maximum-likelihood technique to derive the parameters in the relationship
I_= A.N(III)+B-Wco+C
where A is tile gamma ray production rate per H atom, B is the average N(H2)/Wco ratio mul-
tiplied by the gamma ray production rate per I12 molecule and C is the isotropic galactic diffuse
emission and instrumental background level. The CO and II_ data were smoothed to match tile
1.5 ° point spread function of COS-B. For 198 ° < l < 222 ° and -25 ° < b < -5 ° their results are:
Energy Range aoo- 5000 [MeV] 100- 5000 [MeV]
A [Tsr -1 s-a atom -1 ]
B [7cm-2 sr -' K -1 (km/s)-']
C ITem -2 sr -1 s -1]
N(tI2)/Wco = B/2A
[molcm -2 K -1 km -1 s]
(0.52 -t- 0.13) 10 -26
(2.7 ± 1.0)10 -6
(2.0 ± 0.4) 10-s
(2.6 + 1.2) 102°
(1.70 :t: 0.25) 10-26
(10.1 -t- 1.8) 10 .6
(5.1 ± 0.4) 10 -s
(3.0 + 0.7) 1020
43
PRECEDING PAGE BLANK NOT FILMED
https://ntrs.nasa.gov/search.jsp?R=19900013982 2020-03-19T22:14:54+00:00Z
The value of A(300 - 5000 MeV) found is consistent with the average emissivity value determined by
Strong, et al. (1982) (0.594- ,,_ 10%). l0 -_ 7s -l sr -1 atom -l using medium latitude galaxy counts
and by BIoemen, et al. (1984a) (0.53 i 0.14) - 10-2s 7s -l sr-I atom-i from the radial distribution
of tile gamma ray emissivity ill the outer galaxy.
The local cosmic ray source strength has most recently been derived by Strong, eta]. (1988),
i)y fitting the CO and 1[I survey data, inverse compton emission, isotropic background and point
source contribution to the COS-B diffuse galactic gamma ray emission. They did the fit for six
galactocentric rings, 2-4, 4-8, 8-10, l0 12, 12-15 and > 15 kpc (R® = l0 kpc), and three energy
rang(% 70-150, 150 300 and 300-5000 McV. Their results, for 8< R < 12 kpc, are
q0-{
-1
1.02:t: 0.10 x I0-26 7sr -I s -1 atom
0.65 -t- 0.06 X 10 -_s
0.62 + 0.06 x 10 -26
(70 < E < 150 MeV)
(150 < E < 300 MeV)
(300 < F < 5000 MeV)
Most source strengths are quoted for the energy range E> 100 MeV. Assuming a po_er law spectra
with index -2.1, the above results can be expressed in the form
3.7 x 10 -s 7 mg -1 s-Z sr-1q0 = 1.0 -5
(20 < E < 100 MeV)
(E > 100 MeV)
We will use the this source function and a spherically symmetric GMC model with radially
dependent density to predict the gamma ray emission of GMCs which will be observable by EGRET.
1I. SI)I[I_R.ICAI_ CLOUD MOI)EL
Following Solomon et al. (1987), to model tile gamma ray emission from GMC's, we consider
tile simplified case of a spherically symmetric cloud with radial density of tile form
p(r)[g cm-3] = Po " (Ro/r)O
where R0 is the cloud radius. The line of sight column density is given by
2 -- T 2cr(r)[gcm-2l = 2 p(r = _ + x2)dx
J0
where r is the radial distance between the cloud center and the line of sight, or "impact parameter".
Figure 1 shows the line of sight column density for a = 0, 1 and 2 normalized for clouds with
M = 106M® and R = 50pc as a function of r. A similar model for cloud density was used by
MacLaren, et al. (1988) who normalized the density at r/R0 = 0.55.
Analysis of the CO data by MacLaren, et al. (1988), shown in figure 2, suggests that a is close
to 1, but it may be almost 2 for some clouds. This can also be seen by comparing the _ = 1 column
density with the Wee contours of the Orion A and B cloud peaks in figures 6 and 7 of Maddalena,
et al. (1986). The peak at (a = 5h44 m, (_ = 0°), for example, has a fairly linear density fall-off with
radial distance from the peak and a long tail. We include a = 0 and 2 as limiting cases.
III. PREDICTED EMISSION OBSERVED BY EGRET
The diffuse galactic gamma ray spectrum above about 10 MeV is composed of an E -2's Brehm-
strahlung component and the 7r° decay spectrum centered a.t about 70 MeV. We will consider here
gamma rays with energies above about 100 MeV, where the rr° spectrum begins to dominate. The
effective area of EGRET in this energy region is about 1460 cm 2.
44
oE 150.0
am 7
-"_ I00.0
g
c
00.00.2 0.4 0.6 O.B .
Rodius/R 0
Figure 1. Column density as a function of impact parameter for a cloud with
density given by p = p0-(R0/r) _, for o = 0, 1 and 2, normalized to the same
total mass, M --- l06 M®, and radius, R = 50pc.
16 _
Column
Density I_'
12
_0 _
!
0 t
\
\
p = const
X
0 02 03 0_. 05 06 07 OB 09 ]0
fir c
Figure 2. Column density of hydrogen (arbitrary units) derived from CO
data vs. normalized cloud radius for several local molecular clouds. The
solid curves are column densities for spherical clouds with density given by
p = p0.(R0/r) _, for (_ = 0, 1 and 2, normalized at r/Ro = 0.55. Orion(A+B),
(+); Taurus, (o); Perseus, (× and a). From MacLaren et al. (1988).
ORIGINAL PAGE IS
OF POOR QUALITY
45
ORIGINAL PAGE IS
OF POOR QUALITY
1100 110O
1000
gO0
8O0
_soo.
Q
°5O0
t-
O
0 400
I1.
300
2OO
-t
• I • 1 ! •
1 n 2-0 3.0 4.0 6.0 8.0 7.0 O.D 4.0 7.0
Cloud Radius (dGg)
=_" "_''"
! • ! • I • 1 •
1.0 2.0 3.0 4.0 5.0
Cloud Radius (deg)
Figure 3. Number of gamma rays observed from a hypothetical GMC with
cloud density given by p = P0 • (Ro/r) '_, for (_ = 0, 1, and 2, M = 106 M®,
Ro = 20 pc at D = 500 pc (left) and 1000pc (right). The total counts for
Tobs -----6 × 10ss are given per 0.5 ° ring.
To match the EGRET instrument angular resolution of about 0.5 ° for gamma ray energies
above 100 MeV, it is appropriate to sum the total number of gamma-rays observed from a cloud
into rings of width 0.5 ° . Thus, the total line of sight mass of a ring with inner radius rt and outer
radius r_ is given by
1
If all the sources of gamma ray production in the cloud can be expressed in terms of a single source
function, such as that derived by Strong, et al. (1988), then the approximate gamma ray flux from
the ring is
Counts/ring[Tsr -t s-ll = f'qo" S(rl, r2)
where q0 [_,g-1 sr-1 s-l] iS the local cosmic ray source function and ]'is the relative cosmic ray
intensity at the cloud. The calculation of the number of observed gamma ray from a GMC requires
knowledge of the total mass of the cloud and the distance to the cloud in addition to the sensitivity
or effective area of the detector.
Countsobs/ring = Aeff • Tobs • 4r/4rD 2 • Counts/ring
w'here Aeer [cm 2] is the energy dependent effective area, Tobs [s] is the observation time and D[cm]
is the distance to the cloud.
In figure 3 the number of counts, E > 100 MeV, per 0.5 ° ring are shown for a hypothetical
106 M® cloud with 50 pc radius immersed in cosmic rays of the local density (f = 1) for an
observation time of 6 x 10 s s (effective time for a two week observation) at a distance of 500 and
1000 pc respectively. The diffuse galactic and extra-galactic gamma ray emission has not been
included. Significant differences in the count profile can be seen for the different values of a. For
(_ = 0 the cloud appears very much as a ring with a sharp edge. For a = 1 the ring structure is
still visible, but the edge is softer• For a = 2 the cloud appears as a bright spot with a soft edge.
46
We havecalculatedthe numberof gammarayswhich EGR.ETcanexpect to observein a
typical two weekobservationfor thecloudslistedin table 1. These clouds, selected from the work
by Dame et al. (1987), are more than 25 pc above or below tile galactic plane, within 1 kpc of the
sun a_ld have masses greater than 1 × 104 Mo, see figure 4 .
TABLE 1
l,_in l°t_: b°m b°,_x D[pc] M[10 s M®]
Cygnus OB7 87.0 99.0
Cepheus 100.0 120.0
Taurus 163.0 178.0
Orion B 202.5 208.0
Orion A 208.5 218.0
Cltamaeleon 295.0 305.0
From Dame et al. (!987)
-3.0 8.0 800 7.5
11.0 22.0 450 1.9
-22.0 -9.5 140 0.3
-21.0 -6.0 500 1.7
-21.0 -14.5 500 1.6
-20.0 -12.0 215 0.1
Dtslonce |tom
Golocllc Plone
] Z < -250¢
[] Izl< zspc
[_] 2' >+25pc
270 °
MOSS
0 I04 MO
105 M 0
Q_ 106 M 0
160 °
Per 082
,_= 0 °
I kpc
90 °
Figure 4. The distribution of GMCs with mass greater than 1 x 104 M®
within 1 kpc of the sun. Adapted from Dame et al. (1987).
ORIGINAL PA(_E IS
OF POOR QUALITY
Combining the emission data of Strong et al. (1988) with the mass, radius and distance data
in Table 1, we can calculate the high energy (E > 100 MeV) gamma ray emission from these clouds.
Figure 5 , a, b and c, shows the estimated number of gamma rays (E > 100 MeV which will be
observed from each of these clouds during a two week observation (6 x l0 s s) summed into 0.5 °
circular bins. In each figure, the diagonal dotted line is the estimated number of counts from
47
tile diffuse emission for tile same observation. The diffuse gamma ray emission (E > 100MeV
was estimated using the data given by Fichtel et al. (1978). The appropriate longitude data was
averaged over the width of the cloud in latitude.
We now consider the statisticaJ significance of these observations in two ways. The significance
of the gamma ray observed in a ring over the background reaches a broad maximum at about 30%
of tile cloud radius and falls to zero at the cloud edge, as defined by the model. The significance of
the total number of gamma, rays observed from a cloud over the background reaches a.maximum
at about 82% of the cloud radius and then starts to fall a.s the background begins to dominate the
signal. These values are model dependent, however, it is clear that knowledge of the cloud radii will
be important in tile analysis of a wide field of view gamma ray observation to determine tile gamma
rays which are produced in molecular clouds. 'Fable 2 gives the modeled detection significance (in
standard deviations) of tile ring of maximum significance and of the total cloud for tile six clouds
listed in table 1.
TAIILE 2
Significance of
Maximum Ring Total Cloud
Cygnus O B7 2.2 5.5
Cepheus 14.0 39.6
Taurus 23.5 64.0
Orion B 21.4 47.1
Orion A 24.4 50.7
Chain aeleon 6.9 15.1
(in stalldar(t deviations)
IV. CONCI, USION
Clouds with masses greater than about 105 M o and closer than about 1 kpc should t)e detectible
with the EGRET instrument in a two week observation. In addition, local and cloud-to-cloud
variations in the product of the column density and the cosmic ray density should be resolvable
for many of these clouds. We expect to also be able to detect some clouds to distances of a several
kpc, and should be able to use these clouds ms tracers of the cosmic ray density in more distant
parts of the galaxy.
V. REFERENCES
Bloemen, J.B.G.M., Bennett, K., Bignami, G.F., Blitz, L., Caraveo, P.A., Gottwald, M., ]lermsen,
W., Lebrun, F., Mayer-hazselwander, H.A., Strong, A.W.: 1984a, Astron. Astrophys. 135, 12
BIoemen, J.B.G.M., Caraveo, P.A., llermsen, W., Lebrun, F., Strong, A.W., Thaddeus, I).: 1984b,
Ap. J. 139, 37
Dame, T.M., Ungerechts, H., Cohen, R.S., de Geus, E.J., Grenier, I.A., May, J., Murphy, I).C.,
Nyman, L.-/_., Thaddeus, P.: 1987, Ap. J. 322, 706
Fichtel, C.E., Simpson, G.A., Thompson, D.J.: 1978, Ap. J. 222,833
Heiles, C. Habing, H.J.: 1974, Astron. Astrophys. Suppl. 14, 1
MacLaren, I., Richardson, K.M., Wolfendale, A.W.: 1988, _lp. J. 333,821
Maddalena, R.J., Morris, M., Moscowitz, J., Thaddeus, P.: 1986, Ap. J. 303,375
48
Predicted Emission from Cephus
M-1.9x10_Mo . R-6.0pc, D-4.5Opc, Tob,,-6xl05',,.
300 ,
=:.1
I
2o04
"'/ l
0 -_ , ] , ] , 1 • ! • i • _ • ] ,
O.O 1.0 2.O 5.0 ¢.0 5.0 6.0 7.0 8.0
C_o,ud Radius (deg)
Predicted Emission from Taurus
M-3.0xlO4M o, R-171:¢. D",1401_. Tob.-6xlOSs
300
_2o0
=¢
n,,.
100
0.0 1,0 2.0 3.0 4,0 5.0 15.0 7.0 8.0
Cloud Rodiul (deg)
Figure 5.a Estimated gamma ray counts (E > 100MeV) from GMCs in Ce-
phus (left) and Taurus (right) summed into 0.5 ° circles for a two week ob-
servation. The dotted line is the estimated diffuse emission for the same
observations.
Predicted Emission from Chamaeleon
.OxlO4"Wo, R-17pc, D-215pc, Toba-6xlOSs
Predicted Emission from Cygnus OB71
M--7.SIlO4Mo , R=,,8O{:>c, D--800_c, Tobg.BKl05 s I
M-I
3()0 ,
o_ !
20o4
" Oiffusl
o . "' ' Ernlsa6)nc)
I
,. ..:
E loo-I ..
_3
0 t • i , _ • ! i . ! . ] •
D.O 1 .O 2.0 3.0 4-.0 5.0 6,0 7,0 8.0
Cloud Radius (deg)
_. 2OO
8
&-
_o0
0
Em[lalon
o _ , ', !
o.o 1.o 2.0 &o ,4.o s.o B.o 7.0
Cloud RodiuJ (da9)
B.O
Figure 5.b Estimated gamma ray counts (E > 100MeV) from GMCs in Cha-
maeleon (left) and Cygnus (right) summed into 0.5 ° circles for a two week
observation. Tile dotted line is tile estimated diffuse emission for tile same
observations.
ORIGINAL PAGE IS
49 OF POOR QUALITY
Predicted Emission from Orion A
M-3.0xl O4Mo , R- 1 71>c. D- 14Op<, Tob_m-6Xl 05s
3O0
o_
_. 200,
o_
oE 100-
0
¢ 1
i
i"(_mUS e
" ": E_ii_'O_
..1
0 • i • i • "-; ! • 1 • 1 •
0.0 1.40 2.0 ,5.n _.0 5.0 (%0 7.0
O<x,.<l Rodius (deg)
8.0
30O
_200
oo
o_
tr
oEloo
0
Predicted Emission from Orion B
M--I.7xlO 5, R--4Op¢, D--5(X)p_. Toba--SxlOSa
a" !
S i"
....-
0 • i • i • i • i ,I • i • ! •
0.0 1-0 2.0 5.0 4.0 5.0 6.0 7.0
Cloud RodiuJ (deg)
8.0
Figure 5.c Estimated gamma ray counts (E > 100MeV) from Orion_A (left)
and Orion_B (right) summed into 0.5 ° circles for a two week observation.
The dotted line is the estimated diffuse emission for the same observations.
Mayer-llasselwander, II.A., Bennett, K., Bignami, G.F., Buccheri, l't., Caraveo, P.A., Hermsen,
W., Kanbach, G., Lebrun, F., Lichti, G.G., Masnou, J.L., Paul, J.A., Pinkau, K., Sacco, B.,
Scarsi, L., Swanenburg, B.N., Wills, R.D.: 1982, Astron. Astrophys. 105, 164
Solomon, P.M., Rivolo, A.R., Barrett, J., Yahil, A.: 1987, Ap. J. 319, 730
Strong, A.W., Bloemen, J.B.C.M., Dame, T.M., Grenier, I.A., Ilermsen, W., Lebrun, F., Nyman,
L.-_., Pollock, A.M.T., Thaddeus, P.: 1988, Astron. Astrophys. 207, 1
Strong, A.W., Bignami, G.F., Bloemen, J.B.G.M., Buccheri, R., Caraveo, P.A., Hermsen, W.,
Kanbach, G., Lebrun, F., Mayer-llasselwander, H.A., Paul, J.A., Wills, R.D.: 1982, Astron.
Astrophys. 115,404
Weaver, H., Williams, D.R.W.: 1973, Astron. Astvophys. Suppl. 8, 1
5O


Gamma rays from giant molecular clouds

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server