A New
Component
in
the
Radio
Continua
of
PNe
S.
Casassus
, L.-≈.
Nyman
Ü,
, T.J.
Pearson
á
and
A.C.
Readhead
á
Departamento
de
Astr
onomÌa,
Univer
sidad
de
Chile
, Santia
go,
Casilla
36-D,
Chile
Ü
Eur
opean
Southern
Observatory
, Alonso
de
Cor
dova
3107,
Casilla
19001,
Santia
go
19,
Chile
Onsala
Space
Observatory
, 439
92
Onsala,
Sweden
á
Owens
Valle
y Radio
Observatory
, California
Institute
of
Technolo
gy, Pasadena,
CA
91125
Abstract.
A byproduct
of
experiments
designed
to
map
the
CMB
is the
recent
detection
of
a new
component
of
fore
ground
galactic
emission.
The
anomalous
fore
ground
at 10-30
GHz,
une
xplained
by
traditional
emission
mechanisms,
correlates
with
100
mu
m
dust
emission,
and
is thus
presumably
due
to
dust.
Is the
anomalous
fore
ground
ubiquitous
in
the
Galaxy?
I will
present
evidence
obtained
with
the
CBI
and
SIMB
A+SEST
supporting
the
existence
of
the
new
component
in
the
ISM
at
lar
ge,
and
in
specic
objects,
in
the
form
of
a 31
GHz
excess
over
free-free
emission
in
PNe.
Keyw
ords:
planetary
neb
ulae:
indi
vidual
(NGC
7293,
NGC
7009,
NGC
6369),
radio
continuum:
ISM,
radiation
mechanisms:
general,
infrared:
ISM,
ISM:
dust
PACS:
98.38.L
y;
98.58.Li;
98.38.-j;
INTR
ODUCTION
The
RING
5
M
experiment
(Leitch
et al.
1997)
disco
vered
an
anomalous
component
of
Galactic
radio
emission
in
the
direction
of
translucent
clouds
about
the
North
celes-
tial
pole.
The
14-32
GHz
and
100
μ
m
intensities
corre-
late,
as
expected
from
dust
clouds
photoionised
by
the
dif
fuse
UV
eld.
But
the
corresponding
H
a
emission
is
absent.
An
explanation
in
terms
of
a hot
plasma
at
10
6
K,
heated
by
shocks,
is
discarded
by
Draine
&
Lazarian
(1998a)
on
ener
getic
grounds.
The
anomalous
emission,
also
called
`fore
ground
X',
is also
present
in
the
COBE
data
(K
ogut
et
al.
1996)
as
a
DMR
-
DIRBE
correlation
signal.
A band-by-band
linear
model
between
the
31.5,
53,
90
GHz
DMR
bands
and
the
100
μ
m,
140
μ
m
and
240
μ
m
DIRBE
bands
gives
a
tight
correlation
slope
at
31.5
GHz
(
>
6
s
), but
loose
at
90
GHz.
The
DMR
-
DIRBE
signal
has
a at
spectrum,
with
a
+
0
:
3
0
:
4 (with
I
n
μ n
a
in
Jy),
and
is
tentati
vely
interpreted
as
a mixture
of
traditional
dust
and
free-free
emission
by
Kogut
et al.
(1996).
The
radio-IR
correlations
in
the
dif
fuse
ISM
are
con-
rmed
by
de
Oli
veira-Costa
et
al.
(1999);
Finkbeiner
et
al.
(1999);
de
Oli
veira-Costa
et
al.
(2002);
Lagache
(2003);
Finkbeiner
et
al.
(2002);
Finkbeiner
(2004).
In
particular
Finkbeiner
et
al.
(1999)
report
that
the
radio-
IR
signal
is not
the
Rayleigh-Jeans
tail
of
the
ISM
dust:
the
DMR
-
DIRBE
signal
is at
higher
levels
than
predicted
from
the
far-IR
data,
by
factors
of
1.2,
2.4
and
90
at 90,
53,
and
31
GHz,
respecti
vely
. de
Oli
veira-Costa
et
al.
(2004)
dispro
ve the
synchrotron
interpretation
by
Ben-
nett
et al.
(2003)
of
the
radio-IR
correlation
as
a
WMAP
fore
ground.
The
idea
that
char
ged
very
small
grains
(
VSG
s)
may
rotate
at
frequencies
reaching
observ
able
radio
frequen-
cies
originated
with
Erickson
(1957)
and
Ho
yle
&
Wick-
ramasinghe
(1970),
and
was
revived
in an
accurate
model
by
Draine
&
Lazarian
(1998b)
to
explain
the
RING
5
M
experiment.
The
spectral
ener
gy
distrib
ution
(
SED
) of
spinning
dust
rises
steeply
with
frequenc
y (
a
3),
and
then
drops-of
f with
a Boltzmann
factor
. Draine
&
Lazar
-
ian
(1998b)
obtain
a peak
at
25ñ30
GHz
and
intensity
levels
slightly
short
of
the
anomalous
fore
ground
in
the
RING
5
M
experiment.
The
spinning
dust
emission
mechanism
may
be
un-
derstood
in
crude
terms
by
assuming
equipartition
in
the
rotational
degrees
of
freedom:
1
2
I
w
2
=
kT
. For
a spher
-
ical
and
homogeneous
grain,
with
I
=
2
5
mR
2
, a radius
R
=
N
10
10
m,
and
a mass
density
r
=
1 kg
m
3
, the
rotation
frequenc
y is
n
=
2 10
3
p
T
=
N
5
GHz.
A
typical
dust
temperature
is
T
25
K , and
a very
small
grain
can-
not
be
much
less
than
100
atoms
in
radius
(so
N
=
10
2
),
therefore
n
30
GHz.
As
explained
by
Draine
&
Lazarian
(1999)
dust
emis-
sion
traditionally
refers
to
thermal
uctuations
in
the
char
ge
distrib
ution
(or
vibr
ational
emission
).
Ma
gnetic
dipole
emission
arises
from
uctuations
in the
grain
mag-
netization.
Draine
& Lazarian
(1999)
sho
w that
the
inclu-
sion
of
the
grain
response
to
the
magnetic
eld
enhances
the
grain
emissi
vities
at
radio-frequencies
by
adding
a term
to
the
grain
cross-section,
j
n
=
n
grains
(
C
e
(
n
) +
C
m
(
n
))
B
n
(
T
)
, to
an
extent
that
rivals
spinning-dust
for
the
case
of
ferromagnetic
grains.
Unlik
e spinning
dust,
magnetic
dipole
emission
at
radio-frequencies
stems
from
the
lar
ge
grains.
204
Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp
We report
on
observ
ations
obtained
with
the
CBI
sup-
porting
the
existence
of
the
new
component
in
specic
tar
gets.
The
CBI
is a radio-interferometer
operating
in
10
channels
over
26ñ36
GHz.
Its
45
arcmin
primary
beam
and
4 arcmin
resolution
is well
suited
to
map
extended
sources,
such
as
the
Helix
neb
ula
and
the
dark-cloud
LDN
1622.
The
CBI
also
allo
ws
measuring
the
31
GHz
ux
densities
from
compact
and
isolated
objects,
such
as
young
planetary
neb
ulae
(
PN
e),
which
we
complement
with
SIMBA
250
GHz
data.
A RADIO-BRIGHT
DARK
CLOUD:
LDN
1622
LDN
1622
is the
rst
specic
tar
get
where
the
anomalous
emission
was
detected.
Finkbeiner
et al.
(2002)
reported
the
mar
ginal
detection
of
a positi
ve 5ñ10
GHz
inde
x,
and
Finkbeiner
(2004)
detected
a negati
ve inde
x abo
ve
30
GHz
using
the
WMAP
data.
The
CBI
has
ner
spatial
resolution
than
WMAP
,
which
allo
ws
us
to use
morphological
information
to link
the
31
GHz
emission
with
the
VSG
s.
Fig.
1 sho
ws
an
overlay
of
a super
-resolv
ed
CBI
image
on
the
four
IRAS
bands.
The
images
are
MEM
models
taylored
to
the
CBI
,
and
the
CBI
uv
-plane
has
been
simulated
on
the
IRAS
templates.
Inspection
of
Fig.
1 sho
ws
that
the
CBI
image
re-
sembles
most
the
IRAS
12
μ
m
and
25
μ
m
band
im-
ages,
and
not
the
100
μ
m
one.
VSG
s dominate
the
shorter
-w
avelength
IRAS
bands,
and
contrib
ute
little
to
the
100
μ
m
band.
We conclude
that
the
31
GHz
emis-
sion
is
link
ed
to
the
VSG
population,
as
expected
from
spinning-dust.
THE
HELIX
NEB
ULA
The
CBI
observ
ations
of
the
Helix
PN
sho
w
an
excess
at
31
GHz
at
about
3
s
, which
cannot
be
explained
by
free-free
emission.
In
Fig.
2,
tak
en
from
Casassus
et
al.
(2004),
we
set
up
a
SED
for
the
Helix,
including
upper
limits
at
250
GHz
obtained
with
SIMBA
on
SEST
. The
SED
sho
ws
a low
frequenc
y rise
that
requires
excesi
ve
emission
measure
for
optically
thick
free-free
emission.
The
Helix
is reno
wned
for
its
cometary
knot
comple
x.
With
a turn-o
ver
frequenc
y of
n
T
1 GHz,
the
knots
would
be
optically
thin
and
stand
out
at
31
GHz,
and
yet
be
optically
thick
and
faint
at
0.408
GHz.
But
the
required
lling-f
actor
is
e
1
=
1000,
while
comparing
the
radio
and
H
b
ux
es
gives
e
1.
The
morphology
of
the
31
GHz
emission
also
supports
an
origin
other
than
free-free
emission.
In
Fig.
3 the
CBI
image
resembles
a mixture
of
the
far-
IR
emission
and
the
FIGURE
1.
Intensity
units
are
MJy
sr
1
, and
the
contours
follo
w the
CBI
31
GHz
MEM
model.
FIGURE
2.
SED
of
the
Helix
PN.
The
solid
line
is a theo-
retical
SED
designed
to
t
the
measurement
at
408
MHz
and
the
IRAS
and
ISOPHOT
data.
The
dashed
line
is a model
SED
designed
to
t
the
CBI
points,
dra
wn
to
emphasize
the
dis-
agreement
with
low
frequenc
y observ
ations.
The
two
SIMBA
points
correspond
to
two levels
of
rigor
in
the
data
reduction.
The
black-body
curv
es
in
dotted
lines
rule-out
cold
grains
as
responsible
for
the
31
GHz
excess.
H
b
maps.
To obtain
comparable
maps
we
simulate
CBI
observ
ations
on
dust
and
free-free
emission
templates
(see
Casassus
et
al.
2004).
H
b
should
be
proportional
to
free-free
emission
if the
electronic
temperature
T
e
is
constant,
and
if extinction
is negligible
at H
b
(the
Helix
has
negligible
Balmer
extinction,
O'Dell
1998;
Henry
et
al.
1999).
T
e
variations
cannot
explain
the
anomalous
emission
map
in
Fig.
3.
For
a free-free
continuum
at
31
GHz,
we
can
produce
the
T
e
map
sho
wn
on
Fig.
4 by
dividing
the
205
Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp
FIGURE
3.
The
Helix
at 31
GHz,
100
μ
m,
180
μ
m,
60
μ
m,
and
H
b
, as
seen
by
the
CBI
:
CBI
observ
ations
have been
simulated
on
the
templates
for
a faithful
comparison.
The
image
on
the
lower
right
results
after
subtracting
the
appropriately
scaled
H
b
template
from
the
CBI
image
(see
Casassus
et al.
(2004)),
and
sho
ws
the
anomalous
emission.
The
beam
ellipse
sho
wn
on
the
31
GHz
image
is the
same
for
all
images.
31
GHz
and
H
b
images.
The
T
e
variations
do
not
follo
w
the
anomalous
emission
map.
T
e
peaks
on
the
neb
ular
ring,
while
the
photoionisation
models
in
Henry
et
al.
(1999)
and
the
arguments
given
by
O'Dell
(1998)
point
at a radially
decreasing
T
e
.
31
GHZ
EXCESS
IN
COMP
ACT
PNE
We set
up
the
SED
s of
a sample
of
compact
PN
e using
the
CBI
and
SIMBA
ux
densities.
A subset
of
the
sample
sho
ws
a drop
at
250
GHz
compared
to
the
free-free
continuum
that
can
be
extrapolated
from
the
CBI
and
literature
data.
We give two examples
in Fig.
5 and
Fig.
6.
The
250
GHz
ux
es
fall
well
belo
w the
expected
level of
free-free
emission
in
the
PN
e
NGC
3242
(9
s
deviation),
NGC
6369
(7
s
),
NGC
3918
(6
s
),
NGC
7009
(5.5
s
),
NGC
6572
(3.5
s
).
The
SIMBA
+
SEST
data
points
at
250
GHz
were
ob-
tained
in
3
dif
ferent
observing
runs
in
the
case
of
NGC
7009,
and
two for
NGC
6369.
The
SIMBA
bolome-
FIGURE
4.
T
e
map
in
gray
scale,
with
an
overlay
of
the
anomalous
31
GHz
emission
from
Fig.
3 in
contours.
Units
are
10
4
K.
206
Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp
FIGURE
5.
SED
s of
the
compact
PN
NGC
7009.
The
dot-
dashed
and
dashed
lines
correspond
to
a core-halo
free-free
t
to
the
circled
data.
FIGURE
6.
SED
s of
the
compact
PN
NGC
6369.
ter
maps
conrm
pre
vious
heterodyne
data.
Additionally
,
we
corroborate
our
250
GHz
ux
measurements
by
com-
parison
with
Hoare
et
al.
(1992)
within
20%
uncertain-
ties,
for
the
three
objects
we
have in common
(
NGC
6572,
NGC
6302,
and
NGC
6537).
An
interpretation
of
the
drop-of
f abo
ve 30
GHz
in
terms
of
extinction
due
to cm
or
mm-sized
grains
implies
absurd
dust-to-gas
mass
ratios
(of
order
1).
Fitting
the
NGC
6369
SED
with
a synchrotron
component
requires
that
all
of
the
5-30
GHz
emission
be
synchrotron,
modu-
lated
by
an
absurdly
cold
free-free
screen
at
100
K.
CONCLUSION
The
Helix
neb
ula
pro
vides
evidence
for
a new
emission
mechanism
at
31
GHz,
other
than
synchrotron
or
free-
free
emission.
Since
VSG
s are
not
expected
to
survi
ve
the
PN
phase,
and
since
the
dust
in
the
Helix
is relati
vely
hot,
this
result
supports
magnetic
dipole
emission
as
a
candidate
mechanism
for
the
31
GHz
excess.
We
obtained
a 30
GHz
image
of
the
dark
cloud
LDN
1622,
which
is indicati
ve of
spinning
dust
emission.
The
compact
PN
e pro
vide
the
strongest
evidence
for
the
existence
of
the
new
component
in
the
conte
xt
of
photoionised
neb
ulae.
Free-free
represents
50%
of
the
30
GHz
ux
in
the
extreme
case
of
NGC
6369.
ACKNO
WLEDGMENTS
S.C
ackno
wledges
support
from
Fondec
yt
grant
1030805,
and
from
the
Chilean
Center
for
Astrophysics
FOND
AP
15010003.
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