Using the technique of heterodyne interferometry, measurements were made of the spatial distribution of 11 micron radiation from four late type stars. The circumstellar shells surrounding VY Canis Majoris, alpha Orionis, and alpha Scorpii were resolved, whereas that of R Leonis was only partially resolved at a fringe spacing of 0.4 sec

Betz, A. L.

Spears, D. L.

Storey, J. W. V.

Sutton, E. C.

Townes, C. H.

English

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•SPATIAL HETERODYNE INTERFEROMETRY OF VY CANIS MAJORIS,
a ORIONIS, a SCORPII, AND R LEONIS AT 11 MICRONS*
E.C. Sutton t , J.W.V. Storey, A.L. Betz, C.H. Townes
Department of Physics
University of California
Berkeley, California 9 4720
and
D.L. Spears
Lincoln Laboratory, M.I.T.
Lexington, Mass. 02173
ABSTRACT
4
JL'C 1977
<r' IN^
►T ^►
tRy _ - "
Using the technique of heterodyne interferometry,
measurements have been made of the spatial distribution of
11 micron radiation from four late type stars. The circum-
stellar shells surrounding VY Canis Majoris, a Orionis,
and a Scorpii were resolved, whereas that of R Leonis was
only partially resolved at a fringe spacing of 0.4"
SPATIAL HETERODYNE INTERFEROMETRY OF VY 	 N77-27049
CANIS KkJOR'S, ALPHA ORIONIS, ALPHA SCORPII,
AND R LEONIS AT 11 MICRONS (California
Univ.) 18 p RC k02/M F A01	 CSCL 03A	 Unclas
G3/89 35479
*This work is supported by NASA Grants NGL 05 003 272, NGR
05-003-452, and NSF Grant AST 75-20353.
tNational Science Foundaton Fellow
•
i1.
I Introduction
Circumstellar dust shells around late-type stars in
many cases emit large amounts of radiation near 10 microns
in excess of that contributed by the central stars. Woolf
and Ney (1969) suggested that this excess is characteristic
of thermal emission from silicate grains. Other evidence
for the existence of circumstellar dust grains has been found
in measurements of the intrinsic polarization of visible light
from these stars (Dyck et al., 1971 and Forrest et al., 1975).
Although work has been done to predict the density and temper-
ature distribution of this dust (Dyck and Simon, 1975), more
experimental results which measure these properties are
needed.
In this Letter observations are presented which
directly measure the spatial distribution of radiation at
11 microns for VY Canis Majoris, a Orionis, a Scorpii, and R
Leonis. This is the first use of heterodyne techniques
with separate telescopes for stellar interferometry in the
infrared. Preliminary results of this work for the first
two sources have been discussed by Sutton et al. (1976),
and non-heterodyne single-telescope measurements at some-
what lower resolution on a number of infrared sources have
been reported by McCarthy et al. (1977).
2.
II Observations
Observations were made between October 1976 and
February 1977 at Kitt Peak National Observatory* using the
twin McMath auxiliary telescopes as elements of a spatial
interferometer. These telescopes provided 81 cm apertures
separated by a 5.5 m east-west baseline. The interferometer
was similar to that described by Johnson, et al. (1974) and
Johnson (1974) except for a factor of 5 improvement in
sensitivity due to the use o HgCdTe photodiode detectors
(Spears, 1977). A detector mounted at each telescope
operated as a heterodyne receiver using a CO 2 laser local
oscillator, and the two detected outputs were correlated
electrically. All observations were made at a central wave-
length of 11.106 u and with an infrared bandwidth of approxi-
mately 0.1 cm-1 . The receivers each had a sensitivity of
approximately 5 x 10 -15 W after a 1 second integration period,
corresj.onding to an effective system quantum efficiency of 0.2.
For each source, fringe visibilities were measured over
the range of spatial frequencies made possible by the fore-
shortening of the interferometer baseline as the hour angle
of the source changed. Highest spatial resolution for each
source occurred at the time of transit and corresponded to
a spacing of 0.4" between lobes of the interference pattern.
*
Operated by the Association of Universities for Research in
Astronomy, Inc., under contract with the National Science
Foundation.
•
3.
The coarsest lobe spacing at which observations could be
made without looking too near the horizon was determined
by the declination of the source. Only for VY Canis Majoris
were such observations attempted very near the horizon.
Typically a measurement consisted of a 1000 or 2000 second
integration on the interference signal preceded and followed
by calibration measurements of the source intensity in each
telescope separately. The fringe visibility is simply the
amplitude of the interference signal normalized to these
single telescope signals.
The data discussed in the following sections are pre-
sented in Figures 1 through 4. Error bars are ± a and rep-
resent statistical errors only. The determination of the
fringe visibility scale relies on the internal calibration
of the instrument since none of the brightest 11 V sources
was deemed, a priori, to be a suitable point source. The
accuracy of this calibration is estimated to be better than
10%. In the case of a Orionis, the visibilities determined
are in reasonable agreement with the independent measurements
of McCarthy et al. (1977) who report visibilities of 0.76
at lobe spacings of 1.14" and 0.76". This is the only
source for which a direct comparison with these authors can
be made.
a) VY Canis Majoris
Herbig (1970 b) has described VY Canis Majoris as an
M3-5 star surrounded by an optically thick shell of dust.
4.
In his model the shell extends to a diameter of 2" about
the central source with dust temperatures near 1700 K-on
the inside of the shell and 350 K on the outer surface.
The optical depth in the silicate feature is not known,
so a detailed prediction of the distribution of radiation
at 11 u can not be made. The interpretation of the geo-
metry of this source is further complicated by the sug-
gestion (Herbig, 1970 a; Snyder and Buhl, 1975; van Blerkom
and Auer, 1976) that the gas and dust are confined to a
rotating disk. This suggestion was made in order to explain
the multiple components of SiO maser lines in the radio
region.
The data in figure 1 show that VY Canis Majoris is
resolved at fringe spacings between 0.40" and 0.65". If a
uniform circular emitting region is r'losen as a simplified
model to the flux distribution, the present data can be fit
with a shell diameter of 0.60" t 0.05". However this inter-
pretation does not agree with the measurement of McCarthy et
al. (1977) within their stated errors nor does it fit the
present data particularly well. On the other hand, the
diameter of 0.78" t 0.04" given by McCarthy et al., also for
a uniform circular intensity distribution model, predicts
visibilities lower than those seen in these present measure-
ments at higher spatial frequencies. A circular model of
uniform brightness is thus inadequate to explain the obser-
vations. Since the dust shell should gradually become cooler
and optically thinner with increasing radius, a more tapered
0 S.
form of the brightness distribution is likely. For example,
a simple Gaussian intensity distribution fits the data better
than a uniform circular distribution. The data reported here
together with that of McCarthy et al. are fit by a Gaussian
flux distribution whose full width to 1!e intensity is 0.58"
± 0.04" and which is augmented by an unresolved component
containing 8% of the flux. There are other distributions
which are possible, but differentiation between more complex
radial distributions or models which are not spherically
symmetric will require further observations.
b) a Orionis
a Ori is an M2 supergiant whose circumstellar material
has been extensively studied. Bernat and Lambert (1975)
have observed the gas shell of a Ori out to distances of
100 R* . In addition, the infrared photometry has been inter-
preted by Dyck and Simon (1975) in terms of a dust shell
extending to 250 R * . But many details are still lacking,
such as the conditions necessary for the formation of cir-
cumstellar grains and hence their likely distribution..
It is known from the broadband photometric measurements
reported by Gillett et al. (1971) and Dyck et al. (1971)
that a Ori is one magnitude brighter at 11 u than at 3.5 V.
Assuming the photosphere of a Ori is at a temperature of
3250 K this excess indicates that 55% of the flux at 11 u
represents photospheric emission and the remaining 45% is
contributed by circumstellar mate y
 ial. This ratio is
6.
confirmed by spectrophotometric observations in the 11 u
region (Merrill and Stein, 1976).
The data presented in figure 2 show a visibility of
65% for a Ori with essentially no variation. for fringe
spacings between 0.41" and 0.76". If a diameter of 0.05"
is adopted for the photosphere of a Ori (Bonneau and Labeyrie,
1973; Currie et al., 1974; Lynds et al., 1976) the flatness
of the visibility curve indicates that less than 20% of the
excess circumstellar radiation at 11 U is emitted by material
between 6 R* and 12 Re from a Ori. The discrepancy between .
the observed visibility and the predicted photospheric con-
tribution is comparable to the errors assigned to these
quantities, indicating that the dust shell may be fully
resolved in these measurements.
If any dust were present within 12 R *
 of a Ori it
would be at relatively high temperature and therefore would
contribute strongly to the radiation at 11 V. At this dis-
tance the equilibrium temperature of a blackbody grain is
approximately 700 K, which is cooler than the temperature
generally assumed to be necessary for the condensation of
grains (Gilman, 1969). Thus althoigh the outflowing cir-
cumstellar material may have begun to form dust grains close
to the star, the density of grains is still rather low out
to 12 R*.
c) a Scorpii
a Sco is an oxygen-rich supergiant similar to a Ori in
. 7.
spectral type and luminosity class. However, it has a
weaker silicate emission feature accounting for only 30% of
the total flux at 11 p (Merrill and Stein, 1976). The data
in figure 3, though limited, are sufficient to show that this
dust shell is also resolved at fringe spacings of 0.4". The
dust is found to exist outside of 12 Rs from a Sco, as was
the case also for a Ori.
d) R Leonis
R Leo is a late type Mira variable with a photospheric
temperature of approximately 2400 . K. Visible diameters for
the photosphere have been reported which range from 0.03" to
0.07" (31axit, et al., 1977; Nather and Wild, 1973). The
silicate excess in this source accounts for 35% of the 11 u
flux (Merrill and Stein, 1976).
The data in figure 4 show that, in contrast to a Ori
and a Sco, much of this excess emission remains to be resolved.
The data are fit by a uniform circular shell of diameter
0.28" ± 0:09" contributing the 35% of the flux corresponding
to the silicate excess. Although the errors on this measure-
ment are large, the result is in reasonable agreement with
the diameter of 0.34" t 0.09" for the silicate shell previously
determined by Neugebauer et al. (1972) and Zappala (1977).
This diameter implies that the silicate emission
comes predominantly from material within about 5 R* from R Leo.
For a Ori and a Sco a lower limit of 12 R*
 was set for the
distance to the emitting region. It is likely that this
difference is related to the lower temperature of R Leo
-	 which enables the grains to condense on the average closer
to the star.
8.
49.
III Summary
The circumstellar dust shells around the sources
reported in this Letter are seen to have quite different
characteristics. The shell around VY Canis Majoris is
optically thick and has been well resolved; most of the
radiation can be modeled by a Gaussian intensity distribution
with full width to lle intensity of 0.58 11 . The remaining
three sources are surrounded by optically thin dust, at
different distances from the central star. The coolest
stir, the Mira variable R Leo, emits the majority of its
excess 11 V radiation near 5 Re while the supergiants a Ori
aid a Sco emit tht majority of their excess from outside
of 12 R*.
The use of spatial heterodyne interferometry to
successfully measure the sizes of circumstellar dust shells
demonstrates the potential of this technique for examining at
high resolution the spatial distribution of infrared emission
from stars. The resolution possible using two well-separated
telescopes can be much greater than that possible on any
single existing telescope; further measurements using this
technique promise to provide detailed information about
circumstellar material which would be difficult to obtain
in any other way.
We are grateful for the substantial and continued support
provided by the staff of Kitt Peak National Observatory. We
also appreciate occasional loans of electronic equipment by
10.
the National Radio Astronomy Observatory. Finally, we
thank E. Wollman, H. Smith, and F. Baas for their assistance
in making these observations.
T
i.
11.
FIGURE CAPTIONS:
FIGURE 1: Fringe visibility plot for VY Canis Majoris.
Open circles represent the measurements reported
in this Letter and the filled circle is from
McCarthy et al. (1977). The dashed curve shows
the fringe visibility expected for a uniform
circular disc 0.6" in diameter. The solid curve
is a Gaussian flux distribution with 0.58 1' full
width to lle intensity containing 92% of the flux
with the remainder in an unresolved component.
FIGURE 2: Fringe visibility measurements for a Orionis.
FIGURE 3: Fringe visibility measurements for a Scorpii.
FIGURE 4: Fringe visibility measurements for R Leonis.
The solid curve shows the fit to a uniform diet
shell 0.28" in diameter contributing 35% of 1-he
flux with the remainder in an unresolved component.
Spatial frequency, 10 5 rod-'
0	 1	 2	 3	 4 5
a
Rol
1.0
VY Canis Maioris
	
0.s	 r a
'> 0.f.
03
	
i
`_ 0.4
0.2
	
0 
eot;9
2	 1	 0.5
Lobe spacing, seconds of arc
Figure 1.
2 ---	 1	 0.5
Lobe spacing, seconds -of arc
Figure 2.
0.2
0
Spatial frequency, 11 5 rod—'
0	 1	 2	 3	 d
	
S
1.0
a Orionis
0.8
:s3
> 0.6
cu
m
L
u_. 0.4
Spatial frequency, 10 5 rad-'
0	 1	 2	 3	 4	 5
1.0
a Scorpii
0.8
^> 0.6
(D
rnc
.L-
U- 
0.4
0.2
0
2	 1	 0.5
Lob e spacing, seconds of arc
Figure 3.
R Leonis
0.2
>1 0.8
^> 0.60
0)c
•L
"" 0.4
Spatial frequency, 10 5 rod`
0	 1	 2	 3	 4
	
5
1.0
0
2	 1	 0.5
Lobe spacing, seconds- of arc
Figure 4.
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Spatial heterodyne interferometry of VY Canis Major's, alpha Orionis, alpha Scorpii, and R leonis at 11 microns

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