A model is presented for the dust emission from the Sagittarius B2 molecular cloud core which reproduces the observed spectrum between 30 and 1300 micron, as well as the distribution of the emission at 1300 micron. The model is based on the assumption that Sgr B2(N) continuum source is located behind the dust cloud associated with Sgr B2(M) continuum source. The fact that Sgr B2(N) is stronger at 1300 micron can be attributed to a local column density maximum at the position of this source. Absence of a 53 micron emission peak at the position of Sgr B2(N) suggests that the luminosity of the north source is lower than that of the middle source

Goldsmith, Paul F.

Lis, Dariusz C.

English

NASA Technical Reports Server

N91-1494  (hj._
DUST EMISSION IN THE SAGITTARIUS B2 MOLECULAR CLOUD CORE
DARIUSZ C. LIS AND PAUL F. GOLDSMITH
FIVE COLLEGE RADIO ASTRONOMY OBSERVATORY, DEPARTMENT OF PHYSICS AND
ASTRONOMY, UNIVERSITY OF MASSACHUSE'Vrs, AMHERST, MA 01003
ABSTRACT
We present a model for the dust emission from the Sagittarius B2 molecular cloud
core which reproduces the observed spectrum between 30 g and 1300 It, as well as the
distribution of the emission at 1300 It. The model is based on the assumption that Sgr
B2(N) continuum source is located behind the dust cloud associated with Sgr B2(M)
continuum source. The fact that Sgr B2(N) is stronger at 1300 It can be attributed to a
local column density maximum at the position of this source. Absence of a 53 It
emission peak at the position of Sgr B2(N) suggests that the luminosity of the north
source is lower than that of the middle source.
I. OBSERVATIONS
We have observed the 1300 It continuum emission in the Sgr B2 molecular cloud
core, using the 14 m FCRAO radome enclosed telescope at New Salem, Massachusetts.
The receiver system comprised two cooled Schottky diode receivers sensitive to
orthogonal linear polarizations with an IF bandwidth of approximately 500 MHz. The
LO frequency of 230.2 GHz was selected to be free of any strong line emission. The data
were taken on three days in February and March 1988 and consist of 45 positions with
15" spacing and 16 positions with 20" spacing centered at the position of Sgr B2(M)
continuum source (0q950 = 17 h44mlO.5s, _1950 = -28°22'05" ). The size of the mapped
region is 1' × 2'. The receivers were tuned for double sideband operation. A typical
double sideband system temperature referred to above the Earth's atmosphere at the low
elevation characteristic of Sgr B2 was 1900 K. A 4 minute integration time gave an
205
https://ntrs.nasa.gov/search.jsp?R=19910005634 2020-03-19T19:34:20+00:00Z
observedr.m.s,fluctuationlevel of about40mK (lc). Saturnwasusedfor pointing and
calibration. The FWHM beamsizewasmeasuredto be23", andthebeamefficiencywas
about 0.1 . The resulting conversionfrom the observedantennatemperatureto the
unpolarizedflux density in 23" beamis 185Jy K-1. A contour mapof the emission,
presentedin Fig. 1,showsaqualitativeagreementwith thefirst datasetobtainedin 1986
(Goldsmith,Snell andLis 1987),exceptof a small differencein declinationof thepeak
of theemission. Eachof thethreedatasetswhich areaveragedin Fig. 1showsthesame
distributionof theemission. Thepeakantennatemperatureof 0.31K (equivalentto the
flux densityof 58 Jy) is observedat the(0,40")positioncorrespondingapproximatelyto
theSgr B2(N) continuumsource.The antennatemperatureobservedat the positionof
theSgrB2 (M) sourceis0.24K (equivalentto 45Jy).
II. MODEL OF THE SOURCE
An acceptable model of the source should be able to reproduce the observed spectrum
of the continuum emission, as well as to explain the absence of the 53 [.t emission peak
coincident with the maximum of the 1300 I1 emission at the position of Sgr B2(N)
(Harvey, Campbell, and Hoffmann 1977). We have modeled the source structure using
the radiation transfer code of Egan, Leung, and Spagna (1988). We first find an
equilibrium temperature distribution for Sgr B2(N) and Sgr B2(M) assuming spherical
symmetry and no interaction between both sources. Then we compute the observed flux
convolved with a Gaussian beam assuming that Sgr B2(N) is located behind the dust
cloud associated with Sgr B2(M), and the emission from this source is therefore
attenuated by cold foreground dust. The observations of the J=l---_0 transition of C180
(Lis and Goldsmith 1988) suggest that the molecular cloud associated with Sgr B2(M)
has a radius of about 22.5 pc and consists of a constant density envelope with an H 2
density of about 2200 cm -3 surrounding a central region with a power law density
distribution with an exponent of-2 extending inward to about 1.25 pc. The average
density within inner 1.25 pc is about 5.7 x 104 cm -3. We have used this as a starting
point for our calculations. From the small source size at 1300 g it is clear that the
emission originates from a small high density core of the cloud. Due to small beam
filling factor and high temperature this core does not show up in the C180 emission. We
have assumed the core to be a Gaussian and have taken the FWHM size (DN) and central
density (nN) as parameters of the model. For Sgr B2(N) we have assumed a simple
Gaussian core and a constant density envelope of the same density as for Sgr B2(M). For
simplicity we have assumed the same radii of the envelopes. Density distributions for
206
Fig. i. Fig. 2.
1.5
1
!.°
0
-.5
_1,,,i,,,i,,,i,,,I _-
,-_ I i ,, I,,, I ,_,1,,, I _
.4 .2 0 -.2 -.4
Aa(arcmin)
Fig. 3.
1.5
1
!.°
0
--°5
4--I J I l I J I I I J I I I J I I I J
.4 .2 0 -.2 -.4
Aa(_cmin)
,.w0
+
+
2
1 1.5 2 2.5 3 3.5
log X _)
207
both sources as described above have been then scaled to give a required optical depth at
100 It. Other important input parameters of the model are luminosities and temperatures
of the central objects (LN, LM, TN, TM). We have used standard grain model with 0.1 It
radius and density of 3 g cm -3 . The grain emissivity varies as _-1 between 0.1 and 100 It
and as Xa for wavelengths longer than 100 It, o_ being again a parameter varied in the
computations. Based on a series of test runs we conclude that the wavelength of the peak
of the convolved spectrum is sensitive mainly to the optical depth of the middle source
(XM). The fact that the peak occurs at the wavelength of about 100 It suggests that the
optical depth at this frequency is relatively high. The flux at the peak is a function of the
luminosity of the middle source. The 1300 It flux is not very sensitive neither to the
luminosity nor the temperature of the central star. It is mainly a function of the column
density (or x) and t_. The 100 It optical depth of the north source (XN) is, therefore, given
by the optical depth of the middle source and the north-to-middle peak flux ratio at 1300
It. The absence of the peak of the 53 It emission at the position of Sgr B2(N ) imposes an
upper limit for the luminosity of the north source.
In this paper we present a model that successfully predicts distribution of the
emission at 1300 It, observed spectrum between 20 and 1300 It and is consistent with the
C180 data. The range of relevant parameters for which a good fit to the data is obtained
has yet to be determined. The parameters of our model are presented in Table 1. Figure
2 presents distribution of the emission from our model cloud at 1300 It, and Figure 3 the
observed flux in 60" beam as a function of wavelength.
III. CONCLUSION
We have presented a model for the dust emission from the Sgr B2 molecular cloud
core. The model successfully predicts the observed spectrum of the continuum emission
between 30 It and 1300 It and the observed distribution of the emission at 1300 It. Sgr
B2(N) is less luminous than Sgr B2(M) and the peak of the 1300 It emission at the
position of the north source is a result of a local column density maximum. This
together with the fact that Sgr B2(N) is located behind the dust cloud associated with Sgr
B2(M) continuum source explains why no peak of the 53 It emission is detected at the
position of Sgr B2(N). Observations of the 350 It emission would be especially useful to
test our model, because it predicts the peak fluxes from both sources to be approximately
equal at this frequency.
208
Table 1. - Model parameters.
Parameter M N
L(L®) a ........... 1.0 × 10 7 5.1 × 10 6
T(K) b .............. 4. x 104 4. x 104
,qoo(O') c ......... 2.2 2.1
Xloo(23") ......... 1.7
Z100(60") ......... 0.9
Dcore(pC) a ....... 0.5 1.1
ngrain(cm-3) e .. 7.0 x 10-6 3.5 x 10-6
nt/2(cm-3) f ..... 2.5 x 106 1.3 x 106
o_g ................... 1.4 1.4
NH2(O') h ........ 5.4 x 1024
NH2(2y') ........ 2.0 x 1024
NH2(60") ........ 1.1 x 1024
Nn2(core) i ...... 4.1 x 1024 4.4 x 1024
aTotal luminosity of the central star.
bTemperature of the central star.
c 100 IXoptical depth for different beam sizes.
aFWHM size of the core.
eGrain density at the center.
fH2 density at the center assuming standard gas to dust ratio of 100 by mass.
gGrain emissivity law slope for _ > 100 It.
hH 2 column density for different beam sizes.
iI-I2 column density through the core.
REFERENCES
Goldsmith, P.F., Snell, R.L., and Lis, D.C. 1987, Ap. J., 313, L5.
Egan, M.P, Leung, C.M., and Spagna, G.F 1988, Comput. Phys. Commun., 48, 271.
Harvey, P.M., Campbell, M.F., and Hoffmann, W.F. 1977, Ap. J., 211,786. 271.
Lis, D.C., and Goldsmith, P.F. 1988, Ap. J., in press.
2O9



Dust emission in the Sagittarius B2 molecular cloud core

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server