of 4
1972ApJ...173L..71T
THE
ASTROPHYSICAL
JouRNAL,
173:L71-L74,
1972
April
15
©
1972.
The
University
of
Chicago.
All
rights
reserved.
Printed
in
U.S.A.
INFRARED
DIAMETER
OF
IRC+10216
DETERMINED
FROM
LUNAR
OCCULTATIONS
R.
I.
TooMBs,*t
E.
E.
BECKLIN,*t
J.
A.
FRoGEL,*§
S.
K.
LAw,*
F.
C.
PORTER,*
AND
J.
A.
WESTPHALtll
Received
1972
February
9
ABSTRACT
Lunar
occultations
of
IRC+10216
have
been
used
to
determine
the
size
of
the
regions
emitting
2.2-,
3.5-,
4.8-,
and
10-μ
radiation.
The
results
are
interpreted
as
the
first
direct
measurement
of
the
physical
size
of
a
dust
shell
surrounding
a late-type
star.
During
the
winter
of
1970-1971,
two
lunar
occultations
of
the
bright
infrared
object
IRC+10216
(Becklin
et
al.
1969)
were
observed
from
sites
in
California
and
Arizona.
These
observations
have
resulted
in
determinations
of
the
angular
size
of
this
object
at
2.2
μ(~>..
=
0.4
μ),
3.5
μ(~>..
=
0.6
μ),
4.8
μ(~>..
=
0.6
μ),and
10
μ(~>..
=
5
μ).
The
3.5-,
4.8-,
and
10-μ
observations
were
made
on
1970
November
20
on
the
24-
and
60-inch
(61
and
152
cm)
telescopes
at
Mount
Wilson
Observatory
and
the
200-inch
(508
cm)
Hale
Telescope
at
Palomar
Observatory,
respectively.
The
2.2-μ
observation
was
made
on
1971
January
14
on
the
60-inch
Tillinghast
telescope
at
Mount
Hopkins
Observatory.
Conventional
infrared
photometers
(Becklin
and
Neugebauer
1968)
and
data
systems
were
modified
to
allow
chopping
frequencies
higher
than
25
Hz
and
time
resolutions
shorter
than
50
ms.
Table
1 summarizes
the
relevant
parameters
of
these
observations;
because
IRC+
10216
is
variable,
the
magnitudes
at
the
time
of
the
oc-
cultations
are
also
given.
The
four
occultation
curves
obtained
as
IRC+10216
emerged
from
behind
the
dark
limb
of
the
Moon
are
shown
in
figure
1.
For
comparison,
figure
1 also
shows
the
data
from
the
occultation
of
the
M4
star
HD
76830
observed
at
2.2
μon
the
24-inch
telescope
on
1970
November
19.
All
sets
of
data
were
normalized
to
uniform
spatial
resolution
by
taking
a running
average
over
100-ms
time
intervals;
the
time
coordinate
can
be
con-
verted
to
angular
distance
in
the
sky
by
using
the
apparent
rate
and
direction
of
lunar
motion.
For
the
observations
of
IRC+
10216,
1 second
of
time
in
figure
1 corresponds
to
0
1
:34
±
0
1
:01
in
the
sky;
for
HD
76830,
1 second
of
time
corresponds
to
0
1
:29
±
0'~2.
The
10-μ
measurement
of
IRC+
10216
was
affected
by
the
fact
that
the
image
was
only
two-thirds
within
the
10"
aperture
upon
reappearance
from
behind
the
Moon
while
the
seeing
disk
was
greater
than
5".
At
4.8
μa
series
of
recurrent
noise
spikes
did
not
allow
the
complete
determination
of
the
reappearance
curve.
An
accurate
measurement
of
the
time
of
reappearance
at
2.2
μwas
made
at
Mount
Hopkins,
allowing
a determination
of
the
position
of
the
infrared
source
along
the
path
of
the
Moon
which
was
in
a southeast
direction.
The
position
of
the
optical
image
(Becklin
et
al.
1969)
relative
to
the
infrared
position
is
indicated
in
figure
1.
The
data
in
figure
1 clearly
show
that
IRC+
10216
is
extended.
Several
models
for
*Division
of
Physics,
Mathematics,
and
Astronomy,
California
Institute
of
Technology.
t
Present
address:
Jet
Propulsion
Laboratory,
California
Institute
of
Technology.
t
Staff
Associate,
Hale
Observatories,
California
Institute
of
Technology,
Carnegie
Institution
of
Washington.
§Present
address:
Harvard
College
Observatory.
II
Division
of
Geological
and
Planetary
Sciences,
California
Institute
of
Technology.
L71
©
American
Astronomical
Society
Provided
by
the
NASA
Astrophysics
Data
System