The brightness distribution of IRC +10216 at a wavelength of 11 microns was measured in detail using a spatial interferometer. This brightness distribution appears to have azimuthal symmetry; an upper limit of 1.1 may be set to the ellipticity at 11 microns if the object has a major axis oriented either along or perpendicular to the major axis of the optical image. The radial distribution shows both compact and extended emission. The extended component, which is due to thermal emission from circumstellar dust, contributes 91% of the total flux and has a 1/e diameter of 0.90 minutes. The tapered shape of this component is consistent with a l/r squared dust density dependence. The compact component is unresolved (less than 0.2 minutes in diameter) and represents emission from the central star seen through the circumstellar envelope

Betz, A. L.

Spears, D. L.

Storey, J. W. V.

Sutton, E. C.

English

NASA Technical Reports Server

The Brightness Distribution of IRC +10216 at 11 Microns*
E. C. Sutton, A. L. Betz, and J. W. V. Storey
Department of Physics
University of California, Berkeley
and
D. L. Spears
Lincoln Laboratory
Massachusetts Institute of Technology, Lexington, Massachusetts
Received
ABSTRACT
The brightness distribution of IRC +10216 at a wave-
length of 11 microns has been measured in detail using a
spatial interferometer. This brightness distribution appears
to have azimuthal symmetry; an upper limit of 1.1 may be set
to the ellipticity at 11 microns if the object has a major
axis oriented either along or perpendicular to the major axis
of the optical image. The radial distribution shows both com-
pact and extended emission. The extended component, which is
due to thermal emission.from circumstellar dust, contributes
s<*
91% of the total flux and has a 1/e diameter of 0.90". The
2
tapered shape of this component is consistent with a 1/r
dust density dependence. The compact component is unresolved
/\
(less than 0.2" in diameter) and represents emission from
the central star seen through the circumstellar envelope.
*This work is supported in part by NASA Grant NGL 05-003-272
and NSP Grant AST77-12256. c_
IHASA-CR-157646P THE "BRIGHT;NESi N79-16750
DISTRIBUTION OF IBC +10216 AT 11 MICEONS
{California Oniv.) 15 p HC A02/MF A01
CSCI 03A Onclas
G3/89 43589
https://ntrs.nasa.gov/search.jsp?R=19790008579 2020-03-22T01:38:18+00:00Z
I. Introduction
The bright infrared source IRC +10216 has an extensive
circumstellar envelope of gas and dust. The dust component
of this envelope is important in determining the overall
spectral distribution of the radiation. The spectrum of
IRC +10216 peaks in the infrared near 5 microns and is
similar to that of a 650 K blackbody (Becklin et al. 1969).
Dust which is optically thick at visible wavelengths pre-
sumably absorbs virtually all of the radiation from the
central star and reradiates the energy in the infrared.
Since the central object in IRC +10216 is thought to be a
carbon star, the dust grains are likely to be composed of
graphite and SiC. The presence of SiC grains in this object
is inferred from the observation of an emission feature of
these grains at 11.5 microns (Treffers and Cohen, 1974;
Merrill and Stein, 1976).
Some studies of the size and shape of this object at
infrared wavelengths have been carried out in order to
determine the spatial distribution of the dust. The first
size measurements were made by Toombs et al. (1972) who
observed lunar occultations of IRC +10216 at four different
infrared wavelengths.. The results at 2.2, 3.5, and 4.8
microns were all fit by uniform circular disks approximately
s\
0.4" in diameter. At 10 microns the source was spatially
more extended and a more elaborate model was required. The
authors chose to describe it in terms of two uniform cir-
cular components which were 0.4" and 2" in diameter and
3which contributed equally to the 10 micron flux. Since then
these two components have often been interpreted as discrete
shells of material ejected from the central star, although
the data were not sufficient to justify such a distinct
division between the two components. Recently Selby and
Wade (1979) have measured the size of IRC +10216 at
2.2 microns using a technique involving spatial-frequency
/*»
filtering. They report a diameter1 of 0.35" as the equivalent
width of a Gaussian model fit to their data, in agreement with
the size reported by Toombs et al. at this wavelength.
McCarthy (1979) has made size measurements at a number of
infrared wavelengths which are also in qualitative agreement
with Toombs et al. Information on the azimuthal shape of
IRC +10216 has come primarily from less direct evidence.
Shawl and Zellner (1970) found that the 1 micron radiation
from this object was highly polarized along a position angle
of 120°. In addition, the visual photograph published by
Becklin et al. (1969) showed a diffuse image elongated along
a position angle of 30°, which is perpendicular to the direc-
tion of polarization in this source. The ellipticity of the
image (the ratio of lengths of major and minor axes) was
approximately 2. Both of these pieces of evidence suggest
that the short wavelength radiation is primarily light
scattered by an asymmetric circumstellar envelope. At longer
wavelengths the radiation comes from thermal dust emission
instead of scattering. McCarthy (1979) has measured the
shape of IRC +10216 at a wavelength of 5 microns and reports
it to be extended north-south. This direction, however,
bears no obvious relationship to the axis determined from
the scattered light. Thus a number of questions remain about
the brightness distribution of this source. Among these are
whether the 10 micron radiation comes from discrete shells or
rather a continuous distribution of material and whether the
azimuthal distribution of the infrared radiation has the same
symmetry as is observed in the visible. The interferometric
measurements reported here provide answers to these questions.
II. Observations
Observations of IRC +10216 were made at Kitt Peak National
Observatory using the two bl cm McMath auxiliary telescopes.
These telescopes are used as part of a heterodyne spatial
interferometer as previously described by Sutton et al.
(1977, 1978). The wavelength band of the observations,centered
at 11.106 microns and 0.1 cm" in width,was contained within
the 11.5 micron SiC emission feature commonly seen in carbon .
stars.
The two telescopes of the interferometer were separated
by 5.5 meters in an east-west direction. Determination of a
Operated by the Association of Universities for Research in
Astronomy, Inc., under contract with the National Science
Foundation
source's brightness distribution requires that the source be
measured with a variety of interferometer baselines. Where
it is not otherwise possible to change the baseline, the
earth's rotation may be used to provide variation both in the
length and orientation of the baseline as projected into the
plane of the sky. For a source on the meridian the projected •
baseline always appears in an east-west direction in the sky
and is the full 5-5 meters in length. Off of the meridian the
baseline appears both foreshortened and rotated. These
effects are the same in the eastern and western halves of the
sky except that the sense of the baseline rotation is reversed.
Thus by comparing observations at equal distances to the
east and west of the meridian, an object can be viewed with
identical resolutions but at different orientations. The
range of baseline lengths which are practical for sources at
the declination of IRC +10216 extends as short as approximately
2 meters, and the range in baseline position angles is
roughly, from 45° to 135°.
Fringe visibility measurements of IRC +10216 are presented
in Figure 1. The data are divided into two parts corresponding
to observations west and east of the meridian. The observa-
tions west of the meridian are from February and April, 1978
and are plotted in Figure la. Those east of the meridian are
from November and December, 1978 and are plotted in-Figure Ib.
III. Results
a) Azimuthal symmetry
The degree of azimuthal symmetry of the 11 micron thermal
emission from IRC +10216 may be determined by comparing the
two parts of Figure 1. The data measured to the east of the
meridian, although of somewhat poorer quality, reproduce the
visibility curve of the data from west of the meridian.
Thus there is no evidence for asymmetry in this source at
11 microns. The most sensitive indication of angular shape
comes from a comparison of the data taken at the extremes of
baseline position angle, which are approximately 45° and 135°.
From this,various upper limits may be set to the ellipticity
at 11 microns depending on the assumed orientation of the
major axis. If the position angle of the major axis is in
the range from 30° to 60° or from 120° to 150°, the ellip-
•
ticity must be less than 1.1. This range includes the direc-
tion of linear polarization at 1 micron and its perpendicular,
the major axis of the optical image, which are the two most
likely axes for elongation. A less stringent limit on the
ellipticity may be set if the major axis is assumed to lie
outside of this range, although the limit remains less than
1.5 except for orientations within 5° of exact east-west or
north-south. A north-south elongation such as that suggested
by McCarthy (1979) at 5 microns can not be ruled out, and its
presence would complicate the analysis which follows.
7b) Radial brightness distribution
The radial dependence of the 11 micron brightness distri-
bution may be determined from the data in Figure la. A good
empirical fit is obtained with a simple two component model.
In this model one component, which contributes the majority
of the flux, has a Gaussian distribution with a full-width to
/\
1/e intensity of 0.90". The other component has a diameter
s^
less than or equal to the resolution limit of 0.20". The
fractional contribution of the smaller component to the
11 micron flux is somewhat dependent on its size. If it is
/\
as large as possible (0.20" in diameter) it contributes lM%
^
of the flux; if it is significantly smaller (0.10" or less in
diameter) its contribution drops to 9% of the flux. The
remaining flux (in the latter case, 91% of the flux) is
.^
associated with the 0.90" component. In comparison with
this, the model of Toombs et al. (1972) is distinctly differ-
ent and does not fit the present visibility data as is illus-
trated in Figure •!. Nevertheless, their model is qualitatively
correct in that their characteristic size for the object, as
/\ *
determined by an average of their 0.4" and 2" components, is
/N
comparable to the 0.90" diameter determined from this work.
Also their use of two uniform circular components provides a
rough approximation to the tapered shape of the Gaussian
distribution used in this model. An alternative method for
interpreting visibility data which avoids the artificial
assumptions about component shapes inherent to model fitting
ais given by a procedure similar to that used for "cleaning"
radio interferometry maps (HSgbom, 197*O. The results of
this procedure are shown in Figure 2. This picture of the
brightness distribution is virtually the same as that obtained
when the model described above is convolved with the synthe-
sized beam used in making this map.
In the outer parts of the distribution there is no
evidence for a concentration of dust at any unique radius.
Instead, the surface brightness drops off smoothly indicating
a continuous distribution of material. This implies a steady
rate of mass loss instead of a series of discrete events in
which shells of material were ejected from the central star.
If the rate of mass loss and the outflow velocity are both
2
constant, a 1/r density dependence is expected in the cir-
cumstellar envelope. Crabtree and Martin (1979) have shown
that a model based on such a density dependence is consistent
with the previous lunar occultation and interferometry data.
Such a model may also be used to provide a reasonable fit
to the surface brightness observed here.
The brightness distribution of the circumstellar envelope
may be calculated on the basis of a dust temperature depen-
dence similar to that given by Jones and Merrill (1976) and
2
a 1/r density dependence outside of some inner radius rQ
at which the grains are formed. No reasonable models have
been found which fit the data and have dust extending in
very close to the star. The data are best fit with an inner
^ /\
radius of about 0.2" (5 R* for a stellar radius of 0.04"). At
this radius the equilibrium temperature of graphite grains'
would be about 1000 K, which is lower than the 1^800 K
condensation temperature for such grains. In this model, the
optical depth of the circumstellar envelope at 11 microns must
be approximately 0.5 in order to account for the total
11 micron flux observed from this object.
The central part of the brightness distribution contains
an unresolved peak which in the two component model was
^described as being less than or equal to 0.2" in diameter and
contributing from 9% to i4% of the total flux from IRC +10216.
Additional evidence for a compact central component at 11 microns
has been provided by the spectroscopic work of Betz, McLaren,
and Spears (1979) who observed infrared lines of NH-, in absorp-
tion against a central continuum source contributing approxi-
mately this fraction of the total 11 micron flux. This unre-
solved component is distinctly smaller than the region of
dust emission and represents photospheric emission
from the central star. In order for the central star to be
seen, the dust can not be too optically thick at 11 microns,
consistent with the optical depth of 0.5 determined above.
This does not conflict with the view that the circumstellar
envelope is optically thick at visible wavelengths due to
the rapid drop in opacity expected with increasing wavelength.
A carbon star with a temperature of 1800 K and a radius of
A
 ]_ij
O.Oil" (1.2 x 10 cm at a distance of 200 pc) could account for
the flux observed in the unresolved component. These parameters
10
were chosen so that the luminosity of the star agreed with the
observed luminosity of IRC +10216.
IV. Summary
The distribution of dust around IRC +10216 has been
studied through measurements of the spatial distribution of
the 11 micron radiation from this source. The circumstellar
envelope probably has a high degree of spherical symmetry.
In particular, the ellipticity of the object at 11 microns
must be less than 1.1 if it has a major axis either parallel
or perpendicular to.the major axis of the optical image. The
radial distribution does not consist of a few discrete shells
2
of material, but instead may be characterized by a 1/r
2
density dependence. This 1/r distribution starts at an
inner radius of about 5 R#. The optical depth of the dust is
approximately 0.5 at 11 microns, and the central star may be
seen through the circumstellar shell at this wavelength.
The authors wish to thank C. H. Townes for his helpful
comments and suggestions, A. Harris for his assistance with
the observations, and the staff of Kitt Peak National
Observatory for their continued hospitality.
11
References
Becklin, E. E., Frogel, J. A., Hyland, A. R., Kristian, J.
and Neugebauer, G. 1969, Ap. J. (Letters), .158, L133.
Betz, A. L., McLaren, R. A., and Spears, D. L. 1979, submitted
to Ap. J. (Letters).
Crabtree, D. R. and Martin, P. G. 1979, Ap. J.. 227, to be
published.
Jones, T. W. and Merrill, K. M. 1976, Ap. J., 209, 509-
Hogbom, J. A. 197^, Astron. and Astrophys. Suppl. , 15, il!7.
McCarthy, D. W. 1979, Proc. IAU Colloq. No. 50 (1978), to be
published.
Merrill, K. M. and Stein, W. A. 1976, Publ. Astron. Soc. Pac.,,
88_, 294. __
Selby, M. J. and Wade, R. 1979, Proc. IAU Colloq. No. 50
(1978), to be published.
Shawl, S. J. and Zellner, B. 1970, Ap.J.(Letters), 162, L19.
Sutton, E. C., Storey, J. W. V., Betz, A. L., Townes, C. H.,
'and Spears, D. L. 1977, Ap. J. (Letters), 217, L97•
Sutt'on, E. C.," Storey, J. W. V., Townes, C. H., and Spears, D. L.
1978, Ap. J. (Letters), 22_4, L123-
Toombs, R. I., Becklin, E. E., Frogel, J. A., Law, S. K.,
Porter, F. C. and Westphal, J. A. 1972, Ap. J. (Letters),
173, L71.
Treffers, R. and Cohen, M. 197H, Ap. J., 188,
12
Figure Captions
Figure 1. Fringe visibility data on IRC +10216. The solid
curves plotted with the data in parts a) and b)
are identical and are derived from a two component
model of the brightness distribution fit to just
the data in part a). The dashed curves represent
the fringe visibility calculated for the model of
Toombs et al. (1972). In these graphs the value
of the abscissa is proportional to the length of
the projected baseline at the time of each observa-
tion. The angular orientation of the .baseline may
be read off of the graphs directly below the data.
Figure 2. Radial dependence of the: surface brightness of
IRC +10216 at 11 microns. The source intensity is
-1
normalized to 10
the actual surface brightness distribution convolved
with the synthesized interferometer beam, whose
half-width-half-maximum is shown. Since the width
of the central peak in this map is comparable to
the resolution, the actual brightness distribution
contains a bright and compact central source.
-2 -1
cm vim . This map represents
Baseline position angle,
degrees
13
Fringe visibility
oo
o
CO
o o
p
k>
p
<>
p
bo
•o
o
o
5"
<
nO
0_
0
•n
o
p
In
CO
T3Q
c
a>
o
Oi
-1
a
o_
Figure 1
lit
CM
CU
f-,
•3
faO
•H
'ssau4i|6uc|
15
A. L. BETZ
J. W. V. STOREY
E. C. SUTTON
Department of Physics, University of California,
Berkeley, CA 9^720
D. L. SPEARS
Lincoln Laboratory, Massachusetts Institute of Technology,
Lexington, MA 02173


The brightness distribution of IRC +10216 at 11 microns

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server