Searching for the footprint of shock excitation
in wind-driven nebulae
David Martín-Gordón(1), José M. Vílchez(1), Angels Riera(2,3) & Daniel Reverte-Payá(1)
dmg@iaa.es, jvm@iaa.es, angels.riera@upc.es, drp@iaa.es
(1) Instituto de Astrofísica de Andalucía, CSIC, Granada – Spain; (2) Departament de Física i Enginyeria Nuclear, Universitat Politècnica de Catalunya, Vilanova i la Geltrú – Spain; (3) Department d’Astronomia i Meteorologia, Universitat de Barcelona, Barcelona - Spain
NGC 6888
NGC 6543
MOTIVATIONS
OF THE STUDY
The study of the environments of stars provides
important clues to their evolution. The history of a
changing stellar output of winds and ejecta is
written in the close environments of massive and
intermediate mass stars. Wolf-Rayet Ring Nebulae
(WRN), surrounding the evolved descendant of
massive OB stars, and multiple shell planetary
nebulae (MSPNe), constitute ideal laboratories to
study the history of the stellar mass loss in these
nebulae. These Planetary Nebulae are surrounded
by faint structures such as shells, haloes and other
irregular outer structures. These outer haloes are
of great interest as they are presumably the
remains of the original stages of mass loss from
the progenitor stars. WR Ring nebulae typically
show a wind-blown bubble of interstellar material
that, in some cases, presents ejecta of matter
processed by the star.
W
N
Figure 1. The composite image of NGC 6888 made with the Hα + [NII] (Red), [OII] (Blue) and
[OIII] (Green) emission line images displayed with linear intensity. This false-color image is made of
three (RGB) 8-bit image. This image shows the fine-scale structures of the halo. Fig. 1a shows parts
of the halo of NGC 6888. Fig. 1b shows the northwest Bubble Boundary and Fig. 1c shows a dust
cloud that seems to be physically associated to the nebula.
Figure 3. Spatial cut along the northest Bubble Boundary of NGC 6888 as indicated (left).
The spatial profiles of the flux (erg cm-2 s-1) of the emission lines: Hα + [NII] (Red), [OIII] (Green),
[OII] (Blue), [SII] (Yellow) are shown. Note the striking differences between the [OIII] profile and
the profiles of the rest of the lines, implying the presence of high excitation gas which is in
accordance with the expectations from recent X-ray results (Gruendl et al. 2003) .
Figure 5. Maps show the [OIII]/(Hα + [NII]) (left) and [OII]/[OIII] (right) quotients of
NGC 6888. Note how the oxygen ionization structure delineates a nearly spherical high
excitation "cavity" centered in the WR star of NGC 6888, a geometry which is not
apparent in Halpha and [SII]. The squares indicate the location of the measurements
used in the diagnostic diagram shown in Fig. 7.
Figure 7. Diagnostic diagram for
NGC 6888: [OII]/[OIII] versus
[OIII]/(Hα + [NII]). Models
predictions for HII regions (green
symbols), from Stasinska et al. (1982);
planeparallel shock models (black
symbols, Hartigan et al. (1987)
(crosses) and Riera private
communication. Red triangles
corresponds to our observations for
NGC 6888, obtained in the blown
cavity zones as indicated in Fig. 5
(right).
It has been claimed that the haloes of some
MSPNe (e.g. NGC 6543, NGC 6826, and NGC
7662) are hotter than their respective cores
(Manchado & Pottasch 1989, Meaburn et al. 1991,
Middlemass et al. 1989, Middlemass et al. 1991).
Middlemass et al. have shown that photo-electric
heating is not
able to reproduce the high
temperatures observed in the halo of NGC 6543,
and an extra heating source is required. A
candidate for the extra energy source is the fast
wind passing the cold filamentary knots observed
in the haloes of these Planetary Nebulae. A similar
scenario is expected to operate in the WR nebulae
(Chu et al. 2003, García-Segura et al. 1996,
Langer et al. 1998), though in this case, physical
processes such as the socalled mass-loaded flows
(Hartquist et al. 1986) can play an important role.
Both scenarios of nebulae formation open the door
to the presence of shock excitation of their gaseous
material.
The aim of this work is the detection and study
of substructures of these nebulae that are good
candidates to host shock-heated gas, in
particular in their haloes. Our new approach to
this problem makes use of the 2D information of
the ionization structure of the haloes and outer
shells of a selected group of nebulae.
OBSERVATIONS
AND DATA
REDUCTION
This work is part of an observational program
aimed to study the ionization structure and the
fine-scale structures in the haloes of MSPNe and
WRNe. As a first approach to this subject, we have
obtained narrow band images of a selected group
of MSPNe and WRNe, using narrow-band filters
including the emission from low-ionization (e.g. [S
II]) to high ionization species (e.g. [O III]). The
comparison of the emission from different states of
ionization or different emissivity conditions
provides direct evidence for fine-scale structure
within the haloes of the nebulae (see Figures 1 and
2). Narrow-band images were obtained at the 2.5
m INT (La Palma) with the Wide Field Camera
(WFC), and at the 2.2 m Telescope at Calar Alto
using the Bonn University Simultaneous Camera
(BUSCA) during three runs (2002 from July to
December). In total we have obtained images of 14
nebulae (including MSPNe and WRNe).
We followed standard image reduction techniques
for
the bias-substracting, flat-fielding, and
removing cosmic rays using the IRAF packages.
The study of the haloes of these nebulae requires
the determination of the scattered light from the
bright cores (see Corradi et al. 2003). We
determined the underlying scattered continuum at
the location of the fine-scale structure of the
haloes, which was substracted accordingly. Next,
the flux calibration was accomplished using
spectrophotometric standard stars. Sequence of
images with different times were obtained in order
to cover the large dynamic range of these nebulae.
W
N
Figure 2. The composite image of NGC 6543 made with the Hα + [NII] (Red), [OII] (Blue) and
[OIII] (Green) emission line images displayed with linear intensity. This false-color image is made of
three (RGB) 8-bit image. This image shows the fine-scale structures of the halo. Figures 2a, 2b and
2c show parts of the halo of NGC 6543 with the false-color combination as in Fig. 2. Figure 2d shows
a close-up of the concentric rings observed in NGC 6543 (e.g. Balick, Wilson & Hajian 2001).
Figure 4. Spatial cut along the structure of the brilliant knot (shown in Fig.2a) of NGC 6543 as
indicated (left). The spatial profiles of the flux (erg cm-2 s-1) of the emission lines: Hα + [NII] (Red),
[OIII] (Green), [OII] (Blue), [SII] (Yellow) are shown (right) to illustrate the striking dependence
of the ionization structure with relative position. Lines from higher ionization potential ions
appear to be more displaced in the direction of the central star.
Figure 6. Detail of the ionization structure of the "BOW“ of NGC 6543 (see Fig. 2b). Maps
show the [OIII]/(Hα + [NII]) (left) and [OII]/[OIII] (right) quotients for this structure
illustrating how the relatively low ionization filament is embedded in a higher ionization,
tenuous gas component. Note the interest of this high excitation gas which is located at a large
distance from the center, in the halo. Spectroscopical confirmation is urgently needed to
ascertain its nature.
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