A&A 377, L1-L4 (2001)
DOI: 10.1051/0004-6361:20011168
S. Hony1 - C. Dominik1 - L.B.F.M. Waters1,2 - V. Icke3 - G. Mellema3 - R. van Boekel4,1 - A. de Koter1 - P. M. Morris5 - M. Barlow6 - P. Cox7 - H.U. Käufl4
1 -
Astronomical Institute "Anton Pannekoek'', University of Amsterdam,
Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 -
Instituut voor Sterrenkunde, Katholieke Universiteit Leuven,
Celestijnenlaan 200B, 3001 Heverlee, Belgium
3 -
Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands
4 -
European Southern Observatory, Karl-Schwarzschild Strasse 2,
85748 Garching, Germany
5 -
SIRTF Science Center, Caltech, Mail Code 314-6, 1200 East California
Boulevard, Pasadena, CA 91125, USA
6 -
University College London, Department of Physics and Astronomy,
Gower Street, WC1E 6BT London, UK
7 -
Institut d'Astrophysique Spatiale, Bât. 121,
Université de Paris Sud, 91405 Orsay Cedex, France
Received 14 May 2001 / Accepted 21 August 2001
Abstract
We report the discovery of a double ring structure in the
waist of the nebula surrounding Carinae. The rings are
detected in the mid-IR dust continuum at wavelengths of 7.9, 11.9,
12.9 and 20
m. The dust in the rings has a temperature of about
300 K. The orientation of the rings is inclined with respect to the
axis of the homunculus by either 37 or 58 degrees. The central star
is not in the projected centre of the structure defined by the two
rings. This geometry is reminiscent of that seen in SN1987A and some
planetary nebulae. We discuss several possible origins for this
remarkable geometry and its orientation.
Key words: circumstellar matter - stars: emission-line - stars:
Car - stars: massive - infrared: stars
Many studies have focused on infrared imaging of the dusty homunculus
(e.g. Gehrz et al. 1973; Smith et al. 1998; Polomski et al. 1999, hereafter
Pol99; Gehrz et al. 1999; Pantin & Le Mignant 2000, hereafter
PL00). These studies show that at near-IR wavelengths
the central star dominates while the lobes are faint. Note that
speckle polarimetry of the core of Car indicates that the
central star is not viewed directly but is obscured
(Falcke et al. 1996). At mid-IR wavelengths several equatorial
emission blobs northeast and southwest of the central star are
visible. These blobs are interpreted as evidence for an
equatorial torus (Pol99; Morris et al. 1999, hereafter
Mor99). PL00 show that the bipolar
lobes cannot be empty and suggest the presence of a second shell
interior to the optically visible homunculus. HST observations also
point to an inner structure (Ishibashi et al. 2000).
The Infrared Space Observatory (ISO) spectrum of Car (taken
Jan. 1996) shows three spectral components: a power law between 2 and
about 10
m, arising from the central point source, a
T
250 K component, attributed to the lobes, and a
T
110-130 K component, attributed to a cold,
15
equatorial torus (Mor99). It was
suggested that the massive equatorial torus was present before the
great eruption and caused the highly bipolar shape of the nebula.
In this Letter we discuss new mid-IR images of Car that
resolve the equatorial blobs and show that they have a highly
symmetric ring-like shape with a surprising orientation with respect
to the homunculus. The images may point to a drastic change in the
orientation of the plane of symmetry of the system some time after the
great eruption.
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Figure 1:
TIMMI2 image of ![]() ![]() |
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The images were reduced using a shift-and-add technique, where the
shift between individual frames was determined from a least-squares
comparison between the images. The resulting images were then
deconvolved using an empirically determined point spread function
obtained by observing Cru with the same set-up as
Car. For the Q band image we have no good empirical point
spread function. However, given that in the Q band TIMMI2 at the 3.6 m
is diffraction-limited, we have adopted a Gaussian beam of 1.25 arcsec
to deconvolve the Q band image.
We show the final images in Figs. 1 and 2.
Figure 3 shows a temperature map, derived assuming that the
total flux in the images is equal to that seen in the ISO-SWS spectrum
(Mor99). We realize that the variability of Car
may introduce errors in this calibration. However, we verified the
calibration at 7.9 and 11.9
m using
Cru and found values
of
and
Jy. The agreement between the two
methods is within 10-20 per cent. We stress that unless the shape of
the spectrum of
Car has changed considerably between 1996 and
2001, our method should result in reasonable estimates of the
temperature. Note that the strong 10
m silicate band
(e.g. Mor99) may introduce some errors (
15%)
in the temperature map.
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Figure 2:
Multi wavelength observations of the central part of the
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Figure 3:
Temperature map as derived from the 7.9, 11.9, 12.9 and 20
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The N band and Q band images give a detailed view of the innermost few
arcsec of the homunculus, where in the optical images the waist is
situated. Previous studies revealed the presence of emission blobs of
different intensity roughly 1.5 arcsec northeast and southwest of the
central point source. These emission blobs have been interpreted as
evidence for an equatorial torus (Pol99; Mor99).
This torus has been held responsible for the strongly bipolar geometry
of the homunculus (Mor99). From the ISO spectrum a
temperature of the matter in the torus of 110 to 130 K was derived,
and a dust mass of about 0.15 .
Our new images show for the first time the detailed geometry of the
material emitting at these wavelengths down to very low brightness
levels. The images show that the mid-IR emission is not due to
limb-brightening of a torodial dust distribution seen edge-on (in
Mor99 assumed to be co-spatial with the distribution
of the massive cold material). Rather, the blobs reported by previous
studies turn out to be two arcs of emission. A careful inspection of
the 11.9 m image shows that the arc southwest of the central star
is a closed ring. We will refer to these two structures as "the two
rings'' in what follows, because both arcs have the same size and
inclination. The southwest ring passes through the central point
source, so this point source cannot be at the centre of the rings. The
northeast structure has an irregular surface intensity with a strong
intensity maximum in the north. The axis connecting both rings is not
aligned with the projection of the long axis of the homunculus on the
sky (see below). We note that one of the two rings can be recognised
in the 20
m images published by
Pol99; Mor99 and PL00.
However, the factor of two better spatial resolution at 10
m
compared to 20
m together with the high sensitivity allows for a
much clearer view of these structures.
The temperature map shows that the two rings have roughly similar
temperatures of 280-380 K. These temperatures agree well with the
values derived by previous studies (Smith et al. 1998;
Pol99; PL00). If we assume that there is no
large difference in the foreground extinction towards both rings, this
shows that the dust in the two rings is heated similarly. The
question arises what the location of the 110-130 K massive dust
component, inferred from the ISO observations is. Since this spectral
component peaks at 30 m and no flux jumps due to SWS aperture
transitions are seen, it must fit within the SWS band 3A beam. The low
temperature implies a different physical component from that in the
lobes or rings, such as a torus, as previously suggested by
Mor99. Davidson & Smith (2000) have shown that
this cold component should have a minumum projected area of 37
arcsec2 in order to reproduce the required flux levels. An inclined
torus of that size (which would not show limb-brightening) fits easily
within the SWS beam.
We have constructed a geometrical model for the mid-IR emission of the
equatorial regions (sketched in Fig. 4). The model
assumes the presence of two circular rings. We varied the position
angle on the sky as well as the inclination angle until a good match
with the observations was obtained. The result is shown in
Fig. 4. We find that the rings have a diameter of
1.8 arcsec and a de-projected distance between the two rings of 3
arcsec. The structure is rotated by 117 or 297 degrees in the plane
of the sky with respect to north-south (depending on which of the two
rings is in front). The inclination between the major axis of the
homunculus and of the axis connecting the centres of the two rings is
either 37 or 58 degrees, using an inclination of the axis of the
homunculus to the plane of the sky of 40 degrees. The error on these angles is about 10 to 15 degrees, mostly determined by the wide range of values given in the
literature for the inclination of the homunculus
(e.g. Hillier & Allen 1992). Note that the region in our model
where the projection of both rings overlap on the sky coincides with
an intensity maximum in the observed images, again supporting our two
ring model.
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Figure 4:
Simple geometrical model. The left panel sketches the
position of the rings viewed from above the plane defined by the
main axis of the homunculus and the line through the centers of
the rings. There are two solutions because in the projection seen
from Earth it is unknown which ring is in front. The right panel
shows the projected rings overlaid on the 11.9 ![]() |
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PL00 show that the inner homunculus structure
seen at 20 m represents regions of increased column density. If
indeed the rings trace a density enhancement, then they could be
denser rings in a bipolar nebula, similar to the rings in SN1987A
(Burrows et al. 1995). We also note that this geometry shows
strong resemblance to that seen in PNe such as He2-113
(Sahai et al. 2000) and Hb 12 (Welch et al. 1999)
including the two rings, the misalignment between the bipolar
structure and the rings, and the offset of the central star with
respect to the ring structure. It seems reasonable to conclude that
the physical mechanism causing these structures is generic and acts in
high mass as well as in low mass objects.
A number of mechanisms to produce double rings in bipolar nebulae have
been proposed for PNe (Icke 1988) and
SN1987A (Crotts & Heathcote 2000), but which one applies where has not
been established. In all cases the rings are perpendicular to the
major axis of the nebula, which for Car implies that the major
axis of this inner nebula is at a significant angle (37 or 58 degrees)
to the major axis of the homunculus.
It seems difficult to avoid the conclusion that there must have been a
change in the orientation of the outflow between the moment of
production of the homunculus and the creation of the double ringed
structure. This strongly favours the binary model for the Car
system. The shredded appearance of the skirt in the HST images and the
proper motion of the condensations indicate that the equatorial
regions were highly perturbed by the great eruption. It is therefore
likely that the rings were produced after the great eruption.
1) The change of orientation could result from an asymmetry in the
mass loss during the great eruption. 2) It could be due to tidal
interaction of the eccentric binary with material in its environment.
The required mass for such a process can be estimated in the following
crude way. A gravitational perturbation can act most easily in the
apocenter. In a Keplerian motion about a mass M* with
eccentricity e and semi-major axis a, the apocenter distance and
velocity are given by r=a(1+e) and
.
The required acceleration to change the orbital inclination by about 1
radian is of the order of
v2/a(1+e). Since the great eruption of
1840, about
orbital periods have passed. In
order to produce the required total change in N steps, a disturbing
mass
at distance R would have to fulfil the condition
,
with M* the
reduced mass of the binary. Therefore with e=0.6 and
,
then
.
A moderate amount of mass
close to the binary could already be sufficient to explain the
observed change in the system orientation.
Acknowledgements
We thank N. Ageorges for the excellent help with data-acquisition. VI thanks A. van Genderen for discussions. This work was supported by a NWO Pionier grant to LBFMW and a NWO Spinoza grant to E. P. J. van den Heuvel.