Magnetic fields in planetary nebulae and post‐AGB nebulae

Abstract

Magnetic fields are an important but largely unknown ingredient of planetary nebulae. They have been detected in oxygen-rich asymptotic giant branch (AGB) and post-AGB stars, and may play a role in the shaping of their nebulae. Here we present SCUBA submillimetre polarimetric observations of four bipolar planetary nebulae and post-AGB stars, including two oxygen-rich and two carbon-rich nebulae, to determine the geometry of the magnetic field by dust alignment. Three of the four sources (NGC 7027, 6537 and 6302) present a well-defined toroidal magnetic field oriented along their equatorial torus or disc. NGC 6302 may also show field lines along the bipolar outflow. CRL 2688 shows a complex field structure, where part of the field aligns with the torus, whilst an other part approximately aligns with the polar outflow. It also presents marked asymmetries in its magnetic structure. NGC 7027 shows evidence for a disorganized field in the south-west corner, where the SCUBA shows an indication for an outflow. The findings show a clear correlation between field orientation and nebular structure.

Full text

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arXiv:astro-ph/0701054v1 3 Jan 2007
Mon. Not. R. Astron. Soc. 000, 000–000 (2006) Printed 5 February 2008(MN LATEX style file v2.2)
Magnetic fields in planetary nebulae and post-AGB nebulae
L. Sabin1,2, Albert A. Zijlstra1and J. S. Greaves3
1Jodrell Bank Centre for Astrophysics, University of Manchester, P.O. Box 88, Manchester M60 1QD, UK
2Instituto de Astrof´ ısica de Canarias, C/ V´ ıa L´ actea, 38205 Laguna, Tenerife, Canary Islands, Spain
3School of Physics & Astronomy, University of St. Andrews, North Haugh, St Andrews KY16 9SS, Scotland, UK
5 February 2008
ABSTRACT
Magnetic fields are an important but largely unknown ingredient of planetary nebulae.
They have been detected in oxygen-richAGB and post-AGB stars, and may play a role in the
shaping of their nebulae. Here we present SCUBA sub-millimeter polarimetric observations
of four bipolar planetary nebulae and post-AGB stars, including two oxygen-rich and two
carbon-richnebulae,to determinethe geometryofthe magneticfield by dustalignment.Three
of the four sources (NGC 7027, NGC 6537 and NGC 6302) present a well-defined toroidal
magnetic field oriented along their equatorial torus or disk. NGC 6302 may also show field
lines along the bipolar outflow. CRL 2688 shows a complex field structure, where part of the
field aligns with the torus, whilst an other part approximately aligns with the polar outflow.
It also presents marked asymmetries in its magnetic structure. NGC 7027 shows evidence for
a disorganized field in the south-west corner, where the SCUBA shows an indication for an
outflow. The findings show a clear correlationbetween field orientation and nebular structure.
Key words: stars: AGB and post-AGB – stars: mass-loss –method: polarimetry
1 INTRODUCTION
Whether magnetic fields play a role in the shaping of planetary
nebulae (PNe) is an open question. Most of the post-AGB nebulae
appear elliptical, bipolar or even multi-polar (Balick & Frank
2002). These morphologies are amplified by the interaction
between a slow AGB wind with a faster post-AGB wind. However,
this amplification still requires an initial asymmetry in the slow
wind. This initial shaping has been mainly attributed to two
possible phenomena: binarity and magnetic fields.
Inthebinarymodel, aclosecompanion affectsthemass-losing
AGB star via common envelope evolution (De Marco & Moe
2006), mass transfer and/or tidal forces, and may lead to the
formation of an accretion disk around the companion star. The
binary orbit provides a source of angular momentum which
get carried by the wind, and leads to an equatorial disk. The
angular momentum loss by the stars may lead to a merger. The
model of shaping due to this binary interaction is quite popular
(Bond & Murdin 2000), (Ciardullo et al. 2005), partly because
of the presumed impossibility for a single star to supply the
energy necessary to create a magnetic field strong enough for
its shaping (Soker 2005). Nevertheless, there is still a lack of
observational evidence for the occurrence of close binary systems
during the AGB phase. Neither companions (Riera et al. 2003) nor
high orbital velocities of the AGB stars (Barnbaum et al. 1995)
are detected in a sufficient amount to establish the role of the
binarity as predominant (Matt et al. 2000). Evidence for binary
interactions may be found in 25–50 per cent of planetary nebulae
(Zijlstra 2006), although values of 50–100 per cent have also been
suggested (De Marco et al. 2004; De Marco & Moe 2006).
On the other hand, the magnetic field may act as a “squeezer”
around the central star of the PN and thereby give the dust its di-
rection (in the sense of the outflow). Magnetic fields have been de-
tected around AGB stars (Vlemmings et al. 2006) and a few post-
AGB stars (Bains et al. 2004) using radio observations of masers
(H2O, SiO, OH). Such observations measure a local value of the
strength of field, within masing high-density clumps, which may
differ from the global field. The origin of the magnetic field is
unknown: a dynamo effect resulting from an interaction between
a slow rotating envelope and a fast rotating core has been pro-
posed (Blackman et al. 2001). As magnetic fields are now known
to be present in the AGB and the post-AGB phase, their importance
should not be ruled out prematurely.
Greaves (2002) found evidence for dust alignment in two
carbon-rich objects, NGC 7027 and CRL2688 which are respec-
tivelyayoung andcompact bipolar PNandastronglybipolar proto-
planetary nebula(PPN).Thiswasbased on polarimetricScuba850-
micron observations. The data suggests the presence of toroidal
collimated magnetic fields, as would be required for the shaping.
But the presence of such a field was not conclusively proved, be-
cause of the very few detected vectors and limited spatial resolu-
tion. We present here new 850-microns and the first 450-microns
polarimetry of post-AGB stars, which allows for better resolution
and reduces angular smearing. In addition to the objects observed

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L. Sabin et al.
by Greaves (2002), we also observed NGC 6537 and NGC 6302.
The sample contains two carbon-rich and two oxygen-rich nebulae.
We show the presence of well-aligned toroidal fields in three of the
nebulae. The fourth object shows indications for both a toroidal
field and one aligned with the polar outflow. We conclude that
toroidal fields may be common in bipolar PNe, and could play a
role in the shaping of the nebulae.
2OBSERVATIONS
The polarimetric data have been obtained May 10th2005, with the
polarimeter on the Sub-millimeter Common-User Bolometer Array
(SCUBA), at the JCMT. The instrument (now decommissioned) is
described in Holland et al. (1999). The JCMT beam size is 15 arc-
sec at 850µm and 8 arcsec at 450 µm.
SCUBA contains two arrays of bolometric detectors, covering
afieldof view of2.3′indiameter. The850-µmarrayhas37 individ-
ual detectors, and the 450-µm array has 91 detectors. The two ar-
rays are used simultaneously. The spatial resolution of the detector
is 7.5” at 450µm, and 14” at 850µm. The gaps between the detec-
tors are covered by moving the telescope in sub-pixel steps, in the
so-called jiggle-map mode. The step size needs to be optimized for
the wavelength used. The polarimeter (Greaves et al. 2003) mea-
sures the linear polarization by rotating the half-waveplate in 16
steps. We used this in combination with jiggle map mode. A chop-
throw of 45 arcsec was used.
Good photometric images cannot be obtained at both wave-
lengths simultaneously. When the jiggle pattern is optimized one
wave-band, the simultaneous image obtained in the other band is
under-sampled. The size of the pixels in the final image is set
during the data reduction with the polarimetric package of Star-
link. For a jiggle pattern corresponding to a 450µm measurement,
the pixel spacing becomes 3 arcsec and for 850µm it is 6 arc-
sec. The reduction of infrared polarimetry is discussed in detail by
Hildebrand et al. (2000).
The instrumental polarization (IP) of each detector has to
be removed in order to have a correct polarization calibration. A
new IP calibration, provided on site, was used: this contains a re-
measurement of the central detector. For the other detectors, an
older IP calibration was used. The IP should not vary too much
for detectors close to the center of the array. Some tests done to
see the importance of the instrumental polarization (by changing
its value by the size of the error for example ) and if it could affect
the polarization vectors length, showed that in our case, the IP was
small (at 850µm: ∼1.20%±0.25% and at 450µm: ∼3.25%±0.25%
) and didn’t play any role in modifying the vectors. The resulting
polarimetric images were checked for different pixel binning, from
1×1 to 10×10, and we didn’t find any significant changes.
The linear polarization is measured as a percentage polariza-
tion, and a direction. The polarization is typically caused by the
alignment of spinning dust grains, with their long axis perpendicu-
lar to the local magnetic field (Greaves et al. 1999). Thus, the mea-
sured angle of polarization is 90 degrees rotated with respect to
the magnetic field. The degree of polarization does not give direct
information on the strength of the magnetic field.
We observed four targets: NGC 6537, CRL 2688, NGC 6302
and NGC 7027. They were observed in jiggle map observing mode
at both wave-bands, 450µm and 850µm. For each object, the mean
direction and angle of polarization are listed in Table 1.
The figures below show the direction of the magnetic field, i.e.
thevectorsareperpendicular tothedirectionof thegrainalignment.
3 THE RESULTS
3.1NGC 6537
NGC 6537 is also known as the Red Spider nebula. It is a bipolar
planetary nebula with a very hot central star (1.5–2.5×105K). The
nebula suffers extinction by circumstellar dust. This extinction is
localized mainly in a compact circumstellar shell. An extinction
map (Matsuura et al. 2005) reveals a compact dust shell with a
roughly spherical inner radius of about 3 arcsec, with a minimum
towards the central star (Matsuura et al. 2005; Cuesta et al. 1995).
The polar outflows (traced by high velocity winds of about
300km/s: Corradi & Schwarz 1993) extend 2 arcminutes along
the NE-SW direction. The Scuba 850-µm continuum map (Fig.
1) shows elongation perpendicular to the outflow direction, with
a major-axis diameter of approximately 20 arcsec. The extinction
map is now seen to represent the inner edge of a more extended,
and possibly toroidal, structure. The equatorial plane may be
oriented a little closer to the EW direction, based on the Scuba
map.
The best polarimetric results obtained with SCUBA are ob-
tained at 850µm. The consistent orientation of the polarization vec-
tors shows that the magnetic field (hereafter?B) has a dominant
direction along the equatorial plane, approximately perpendicular
to the outflow direction. The dust alignment is therefore directed in
the same sense as the outflow. The length of the 18 different vectors
does not show large variations, with a degree of polarization vary-
ing from 8 to 14%, suggesting a consistent magnetic field. More-
over the absence of smaller polarization vectors toward the center
of the nebula indicates that there is no change in geometry of?B
towards the core (Greaves 2002). This supports a location some
distance from the star (i.e. in a detached shell), since otherwise av-
eraging of vectors in different directions within the JCMT beam
would reduce the detected net polarization at the central position
(beam depolarization).
These observations indicate the presence of a consistent
toroidal magnetic field, located along the equatorial plane of NGC
6537, in a circumstellar torus. Compared to the size of the outflow
lobes, the field is located relatively close to the star. The presence
of a?B-field had already been suspected, based on the occurrence
of filaments near the central star (Huggins & Manley 2005).
Theextinctionmap obtained byMatsuura et al.(2005) (Fig.1-
Bottom panel) shows an asymmetry, in that the extinction is higher
on the western side (>∼2 mag versus<∼1.6 mag towards the east).
This asymmetry is also seen in the Scuba data, with a larger ex-
tension and more polarization vectors on this side. This is a further
indicationthat themagneticfieldislocatedwithinthedetached dust
shell.
There is no strong indication for magnetic fields along the spi-
der lobes. There is a slight trend for the vectors to curve, but this
is caused by only a few of the vectors and would need confirma-
tion. We cannot state with confidence whether the lobes also carry
a magnetic field.
3.2NGC 7027
The 450µm jiggle map of the young planetary nebula NGC 7027
(Fig. 2) and its near environment covers a field of about 40×36
arcsec2. The central star is surrounded by an ionized area of ∼283
arcsec2, which is in turn enclosed by a thin atomic and molecu-
lar layer that is seen for instance in the H2 emission observed by

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Magnetic fields in PNe
3
Figure 1. Scuba 850µm results on NGC 6537. North on the top and East on
the left. The axes give the image scale in arcseconds. Top panel: the 850µm
continuum map of NGC 6537, with contours at 1%, 2% ,5% and 10% of
the peak. Middle panel: Magnetic field orientation. The general outflow di-
rection of the nebula is indicated by A and the equatorial plane by B. The
polarization vector scale on the left is set at 10%. Bottom panel: Extinction
map presented by Matsuura et al. (2005). The highest levels of extinction
occur at ∼ 4 arcsec from the central star with E(Hβ − Hα) > 2mag, co-
incident with the dust emission in the 850µm map. The polarization vector
scale is set at 10%. The magnetic field which coincides with the area of in-
ternal extinction, is mostly aligned along the equatorial plane, indicating a
toroidal field. The dust alignment is perpendicular to the vectors displayed.
Figure 2. Scuba 450µm results on NGC 7027. North on the top and East
on the left. The axes give the image scale in arcseconds. Top panel: The
450µm continuum map of NGC 7027. Contours are set from 1 to 5% and
10% of the peak. Middle panel: Magnetic field distribution. The general
outflow direction is indicated by A and the equatorial plane by B. The po-
larization vector scale (showing the degree of polarization) is set at 10%.
Bottom panel: The Scuba polarization vectors are shown on an continuum
subtracted H2(color-inverted) map of NGC7027 (North onthe top and East
on the left). The field is mostly directed along the equatorial plane. The cen-
tral region shows much reduced polarization and the field orientation differs
on the extreme western side.

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L. Sabin et al.
Cox et al. (2002). A thin dark ring delineates the equatorial plane
in optical images. The structure is better seen in the HST image
(Fig. 2-bottom), which shows both the surrounding molecular layer
(in H2) and the inner ionized region. Latter et al. (2000) shows that
the NW lobe is blue-shifted (closer) and the SW lobe is red-shifted.
The dominant direction of the magnetic field coincides with
the equatorial plane. But this behaviour is mainly seen on the
North-East side while the south-west part seems to be disturbed:
the magnetic field may be “broken”. The degree of polarization is
8.9%±0.9%) towards the NE direction(or lobe) and 7.6%±1.3%)
towards the SW. The degree of polarisation is much reduced in the
center of the nebula and lacks a coherent direction here. This effect
was also noted by Greaves (2002) and may indicate that coherence
is lost in the ionized region.
3.3 CRL 2688
The proto-planetary nebula CRL 2688 (or Egg Nebula) is a bright
carbon-rich bipolar object characterized by two pairs of searchlight
beams superposed on a reflection nebula. The origin of the light
beams has been suggested to be a very close stellar companion
(Sahai et al. 1998; Kastner & Soker 2004), or the presence of dust
layers reflecting the light of the central star (Goto et al. 2002). The
bipolar reflection nebula shows a dark equatorial lane where a large
amount of dust may by present.
The 850-µm is presented in Fig. 3-Top panels. It shows elon-
gation in the direction of the polar outflows. The equatorial extent
is less but at the lowest contour the torus is more extended on the
eastern side. The extent is ∼60”×40” although it is not clear pre-
cisely where the emission ends. The bright core at 850µm shows a
FWHM of 16×14 arcsec2, at a position angle of 25 degrees which
is in the same direction as the outflow. We could not measure the
core elongation at 450µm due to the under-sampling. The spatial
resolution is insufficient to separate the two light beams, but the
width of the emission suggests that the dust traces the larger lobes
which the light beams illuminate, and that these beams themselves
are not present in the far-infrared data. (The elongation of the core
is in fact along the line of one of the two beams only.)
CRL 2688 presents the highest number of polarization vectors
inour sample. Forthisnebula, theunder-sampled 450µm mapgives
some additional and useful indications (even if we observe alack of
vectors due to this under-sampling). The polarization vectors cover
the full nebular extent (seen at both wavelengths), as they do in
NGC 7027. The degree of polarization is not uniform and strongly
decreases near the center. This phenomenon is more visible in the
850µm map, which suggests that beam depolarization may play
a role. At 850µm, the mean value of polarization of the region
containing the bipolar lobes is about 3.2%, the outer region has a
mean value of 8.8%, and the region of the dark lane shows a mean
degree of polarization of 1.4%.
If we draw a line passing through the nebula in a longitudinal
direction (NNE-SSW fig.3 bottom-left panel), we can see distinct
behaviours of the magnetic field on either side of this line: on the
eastern side, the orientation of?B is mainly in the direction of this
line, while on the western side, the magnetic field appears perpen-
dicular to it.
The 450-µm map (Fig. 3-bottom-right panel) shows the mag-
netic field at higher resolution, confirming the bimodal distribution.
This map is under-sampled and only some positions in the nebula
are covered. The map gives a suggestion of a superposition of a
toroidal field and one aligned with the polar outflows. The field
becomes less ordered towards the tip of the outflow direction, but
the emission here is faint and the uncertainties on the polarization
vectors are larger.
Thecomplicated magnetic morphology makes itlikelythat the
dynamics have shaped significant parts of the field, rather than the
magnetic field shaping the nebula. The structure may show the su-
perposition of two components over most of the area of the source.
The lack of polarization in the centre suggests that the bright core
is not polarized, or its polarization is averaged out over the JCMT
resolution. The outermost field vectors in the longitudinal direction
are along the outflow, and this may indicate a field carried along
by the outflow. This is most apparent in the north when looking at
the outer vectors direction in the figure 3-bottom-left, in the south
the outer vectors point towards the corresponding outflow so we
assume that the magnetic field may be carried by this outflow.
Thegrain alignment, whichisperpendicular tothedirection of
the magnetic field, is in the direction of the outflows on the western
side. On the eastern side, the grain alignment is toroidal.
High resolution molecular images in the literature include
IRAM CO J=2-1 data (Cox et al. 2000) and HST infrared H2 im-
ages (Sahai et al. 1998). The continuum-subtracted H2 image in
Fig. 4, shows multiple jets, located both in the equatorial plane
and towards the polar directions. The CO data show these jets to
be the tip of flows originating much closer to the star. These jets
all fall within the Scuba 850-µm core, indicating that indeed struc-
ture is present on scales much smaller than the JCMT beam size.
Thus, beam dilution and beam-depolarization of the magnetic field
is likely.
We note that the elongation visible in the Scuba image, on the
eastern side, is in the same direction as the largest molecular sub-
jet E2 in Fig.4, or A1 in Cox et al. (2000) (but the Scuba structure
is much larger). The polarization vectors seem unaffected. On the
western side, both the molecular and the Scuba images indicate a
smaller extent of the envelope. The polar outflows as seen in the
molecular lines do not show the light beams, but instead show rel-
atively well-collimated lobes. The lobes are brighter in the north.
The polar lobes in the Scuba image are consistent with this, both
in direction and in brightness, but again are much larger than seen
in the molecules. The molecular emission shows the wind-blown
cavities, while the Scuba emission arises in the surrounding shells.
3.4 NGC 6302
This oxygen-rich planetary nebula (Pottasch & Beintema 1999) is
a proto-typical butterfly-type nebula. It shows two extended lobes
that result from a bipolar outflow, with a dark lane in the center.
It has been studied by Meaburn & Walsh (1980), Meaburn et al.
(2005), Matsuura et al. (2005) and Casassus et al. (2000).
The Scuba observations were optimized for 450-µm. They
show a bright core, of FWHM 14 × 12 arcsec2(larger than the
beam), with the long axis in the north-south direction, with an ex-
tension towards the south-southeast (Fig.5). We underline the fact
that the long horizontal line in the eastern part of the nebula and
the long vertical line in the South-West (Fig.5 bottom panel), are
artifacts from the HST image and not Scuba polarization vectors.
The arcs seen at 25 arcsec are likely part of the diffraction pattern
from the very bright core. The polarimetric data obtained at 450µm
showsonly fivepolarization vectors. Theydo not lineupwitheither
the dark lane or the outflow. However, they are fairly well aligned
with the ellipsoidal radio source in the center of the nebula. The
radio nebula shows the inner ionized region, confined by the dense
torus: the elongation is perpendicular to the torus.
The difference in position angle of the inner torus with the

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Magnetic fields in PNe
5
Figure 3. Scuba 850 and 450µm results on CRL2688. North on the top and East on the left. The axes give the image scale in arcseconds. Top left panel: The
850µm map of CRL 2688, with contours from 1 to 5% and 10% of the peak. Top right panel: The 850-µm vector polarization showing the magnetic field
orientation. The general outflow direction is indicated by A and the equatorial plane by B. The polarization vector scale (showing the degree of polarization)
is set at 10%. Bottom panels: HST images (WFPC2, filter F606W) with the overlaid magnetic field corresponding to the 850µm map (left) and 450µm
map (right). The dust continuum is elongated along the outflows. The field shows a complicated structure with components along the outflow and along the
equatorial plane. The polarization vector scale (showing the degree of polarization) is set at 20% for both maps.
dark lane is interpreted as a warped disk (Matsuura et al. 2005).
Further from the center the outflows have different position angles,
and eventually become east-west in the outer bipolar lobes. The
450-µm polarization indicates that the magnetic field may be ori-
ented in the direction of the inner outflow.
We have no data for magnetic fields elsewhere in the outflows
even at 850µm.
4 DISCUSSION AND CONCLUSION
The sub-millimeter observations of the four post-AGB objects re-
veal new information regarding the distribution of the magnetic
fields, the dust emission and the link between the two components.
Extended dust emission was seen for all four nebulae. For
CRL 2688, the large polar lobes have been detected. The orien-
tation of its polar lobes is along only one of the two light beams.
The equatorial emission is also extended, and shows an asymmetry
which is correlated with the appearance of the (much more com-
pact) equatorial jets. These jets originate close to the star and it is