A&A 408, 237-243 (2003)
DOI: 10.1051/0004-6361:20030955
F. Mavromatakis
University of Crete, Physics Department, PO Box 2208, 710 03 Heraklion, Crete, Greece
Received 7 May 2003 / Accepted 12 June 2003
Abstract
The wide-field covered by the supernova remnant G 78.2+2.1 was observed in
the optical emission lines of HN II], [S II] and [O III].
The flux calibrated images reveal several
H II regions in the field which dominate the optical emission but
we were able to identify possible areas of shock-heated emission through
the H
N II] and [S II] images. These are mainly found to the north-east
of
Cygni as well as in the south and the morphology of the detected
emission is patchy and diffuse. A few patchy structures are also
detected in the medium ionization line image of [O III].
Long-slit spectra taken at one of the candidate positions verify that
we have detected radiation from shock-heated gas ([S II]/H
0.6).
The estimated shock velocity lies below 100 km s-1, while the
measured electron density of
700 cm-3 implies preshock
cloud densities of
20 cm-3. High resolution maps in the infrared
show that the optical emission, which may be associated with G 78.2+2.1, lies
in areas relatively free of infrared emission.
The interstellar extinction measured through the optical spectra is
compatible with current estimates of the distance to the remnant.
The optical data are in agreement with the explosion energy and
interstellar medium density estimated from the X-ray data
suggesting that the remnant is still in the adiabatic phase of its
evolution. A second set of spectra taken in the north-west suggests that we
are probably dealing with a foreground H II region.
Key words: ISM: general - ISM: supernova remnants - ISM: individual objects: G 78.2+2.1
The supernova remnant G 78.2+2.1 lies in the complex area of Cygnus where
many bright and dark nebulae are found. The first radio and optical
observations focused on the emission in the neighborhood of
the bright star
Cygni (e.g. Johnson 1974; d'Odorico &
Sabbadin 1977; Bohigas et al. 1983).
However, Higgs et al. (1977) using radio maps at
1.4 GHz and 10 GHz were able to identify a
62
diameter
shell and estimated a non-thermal spectral index
of
(
). These radio maps, as well as newer maps at various
radio frequencies, show that the most intense radiation is found in
the south-east (close to
Cygni) and in the north-west
(e.g. Zhang et al. 1997; Pineault & Chastenay 1990).
Zhang et al. (1997) combined data from different frequencies to determine
an integrated spectral index of
and also detected
systematic spatial variations of the index of the order
of
0.15. However, the origin of these variations remains an open issue.
The distance to the remnant is estimated by Landecker et al. (1980),
using a number of arguments, as
kpc. At this distance the
shell is
55 pc above the galactic plane and its radius is
14 pc. Lozinskaya et al. (2000) performed interferometric
observations in H
and [N II] in selected areas inside and outside the
extent of the remnant in an attempt to study its radial-velocity
field. Nevertheless, the authors, based on the kinematic
data, could not exclude the possibility that the optical nebulae
towards G 78.2+2.1 are H II regions. They also analyzed archival ROSAT and ASCA data
from this area of the sky and proposed that the remnant is in the
adiabatic phase of its evolution. Uchiyama et al. (2002) analyzed
the same ASCA data and found several different spectral components.
Although the details differ, both analysis suggest shock velocities
around 103 km s-1 and an age less than 104 yr.
G 78.2+2.1 is an exciting object because of its proximity to the strong
-ray source 2EG J2020+4026 (Esposito et al. 1996), while
Brazier et al. (1996) detected an X-ray source within the
-ray error circle. The
-ray and X-ray properties of the
emission lead the latter authors to propose that it originates from a pulsar
that could be associated with the remnant.
Very little information is known about the optical properties of G 78.2+2.1.
Currently, there do not exist in the literature dedicated optical
observations of this remnant with the exception of the survey of
Parker et al. (1979), while van den Bergh (1978) observed
only a narrow field in the vicinity of
Cygni.
In addition, its position on the celestial sphere makes it an ideal
target to observe from our site.
In this work we present the first flux calibrated CCD images of the
whole field of G 78.2+2.1 in the emission lines of H
N II], [S II] and [O III].
Deep long-slit spectra in two areas were also acquired.
Information about the observations and the data reduction is given
in Sect. 2. In Sects. 3 and 4 the results of the imaging and spectral
observations are presented, while in Sect. 5 the properties of
the remnant and its environment are discussed. Finally, in Sect. 6
we summarize the results of this work.
Table 1: Imaging log.
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Figure 1:
Diffuse and patchy emission dominates the field of G 78.2+2.1.
The shadings run linearly from 0 to
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Figure 2:
The same field as in Fig. 1 but now seen
through the sulfur filter. The shadings run linearly from
0 to
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Table 2: Relative line fluxes.
The data were reduced using standard IRAF and MIDAS routines. The available frames were bias subtracted and flat-field corrected using a series of well exposed twilight flat-fields. The absolute flux calibration was performed through observations of a series of spectrophotometric standard stars (HR 5501, HR 7596, HR 7950, HR 9087, HR 718, and HR 8634; Hamuy et al. 1992, 1994).
The low-ionization emission in HN II] from the area of G 78.2+2.1 is shown
in Fig. 1. The bright nebulae dominate the emission in this
field, while dark nebulae mark their presence through absorption of
background emission. The open cluster NGC 6910 lies in the
north-east area of the image, while the 2.2 mag star
Cygni in the
south-east is heavily overexposed in these deep exposures.
Filamentary structures are not immediately seen in this complex field,
instead patches of bright emission are present in the north-west and
the east.
In Fig. 1 we also overlay the radio-continuum contours at
1420 MHz from the Canadian Galactic Plane Survey (CGPS -
Taylor et al. 2003) and discover that the patchy optical emission
shows some degree of correlation with the radio emission.
It is not clear if the radio emission at the correlation
areas is non-thermal or thermal, i.e. if the corresponding optical
emission is due to shock-heating or photoionization.
Sulfur line emission from H II regions is generally suppressed compared
to emission from a shock-heated gas. The [S II] image, shown in
Fig. 2, displays the same overall morphology as the H
N II] image,
however certain areas emit less intense sulfur line radiation.
In an attempt to identify shock-heated emission, we use the ratio of these
two low-ionization images since they are flux calibrated.
The H
N II] filter transmits equally well the H
and [N II]6548, 6584 Å lines, while the sulfur filter transmits 100% of the 6716 Å
line and 18% of the 6731 Å line. Assuming the ratio of these two
latter lines to be
1 (see Sect. 4) and that the H
flux is
1-1.5 of the [N II] flux, we find that
([S II]/H
)
3 ([S II]/H
N II])
.
The bulk of the emission in the
west, north-west seems to originate from photoionized gas since
a ratio of 0.33(
0.04) is measured (e.g. Hunter et al. 1992),
while in the south, around
20
,
00
,
we find a ratio of
0.59(
0.03; Smith et al. 1993).
The bright emission is the east is mainly due to
photoionization. However, there are specific locations where ratios
of 0.5-0.7 are measured. One of those coincides with the location
where a long-slit spectrum was taken and the more accurately
measured [S II]/H
ratio turns out to be 0.65 (Table 2).
Another structure easily identified in the [S II] image is found
around
23
20
and
27
,
at
an average flux level of
erg s-1 cm-2 arcsec-2.
Interestingly, this source appears less prominent in the H
N II] image due
to the ambient patchy emission which is almost as strong.
The [S II]/H
ratios measured along this structure are in the range of
0.5-0.6 suggesting that the observed emission can have a shock-heated
origin.
It is evident that the use of the flux calibrated images provides
rough estimates of the [S II]/H
ratio but it does show that it may
vary by a factor of 2 within the field of the remnant.
In Fig. 2 the contours of infrared emission (IR) at 8.28
m
are overlaid to the [S II] image. These show that the locations with
high [S II]/H
ratios are found in areas free of IR emission or with weak IR
emission.
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Figure 3:
The emission in the medium ionization line of
[O III] 5007 Å is shown in this figure. The dashed lines
mark the detected small scale structures in the east and north-west.
The shadings run linearly from 0 to
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We were able to extract four different apertures from pos. 1. All of these
spectra suggest that the emitting gas is of photoionized origin
([S II]/H
0.2-0.3; Table 2). In this table we list only one
of the extracted spectra, that from the brightest patch of emission at
19
15
and
40
44
.
The electron density is calculated with the temden task of the
nebular package (Shaw & Dufour 1995)
and is found to be
630 cm-3.
The [O III]5007 Å flux is quite weak amounting
to only 50% of the H
flux.
Two apertures were extracted from pos. 2 and both are listed in
Table 2. One of them displays the characteristic signature of
emission from shock-heated gas ([S II]/
), while the second seems
to point to emission from photoionized gas ([S II]/
). The electron
density in the recombination zone is
750 cm-3 as the sulfur lines ratio
of 0.95 suggests. In addition, the H
/H
ratio is
10, in contrast
to the H
/H
ratio of
7 measured in the second spectrum from the
same area (pos. 2).
The H
/H
ratio of 10 corresponds to a
logarithmic extinction c of 1.6(
0.2), adopting the interstellar
extinction curve of Kaler (1976) as implemented in the redcorr
task within the nebular package.
The corresponding color excess E(B-V) is then 1.1(
0.2) assuming that
(Kaler 1976; Aller 1984). We can now calculate the hydrogen
column density
,
using the statistical relation of Predehl
& Schmitt (1995).
It is found that the equivalent hydrogen column density is
cm-2, which is substantially lower than the
estimated total galactic
of
cm-2
(Dickey & Lockmann 1990).
We have also used the code of Hakkila et al. (1997) to estimate the color
excess as a function of distance in the direction of G 78.2+2.1 and the following
pairs of distance and color excess were obtained:
(1.0 kpc,
), (1.5 kpc,
),
(2.0 kpc,
), (2.5 kpc,
).
Although the uncertainties are not negligible, these results show that our
measurement of the color excess is compatible with the range of distances
estimated for G 78.2+2.1 (Landecker et al. 1980).
Note here that the signal to noise ratios given in Table 2 do not
incorporate errors due to the calibration process which are
10%.
Table 3: Spectral log.
The low-ionization images of HN II] and [S II] display similar morphologies
although the overall emission is somewhat suppressed in the latter image.
Filamentary structures are not prominent and the diffuse emission
is the major characteristic of the field of G 78.2+2.1.
The [O III] emission line image reveals three elongated structures, one in the
north-west and two in the east (Fig. 3).
The low-ionization images provide
evidence for emission from shock-heated gas for one of the structures in the
east. This does not necessarily imply that the other two are not associated
to G 78.2+2.1, given the complexity of the environment, since they appear oriented
along contours of radio emission. We have obtained estimates
of the [S II]/H
ratio which reveal possible areas of emission from
shock-heated gas. One of those is found in the south where diffuse emission
dominates, while two filamentary structures in the east seem to be good
candidates to associate with the remnant.
Especially, the filaments in the east coincide with the intense filamentary
1400 MHz emission in this area, while in the south the radio emission is not
filamentary and overlaps the diffuse optical emission.
We were able to show, through long-slit spectroscopy, that a faint filament
in the east displays a [S II]/H
ratio typical
to supernova remnants (Sect. 4; Table 2). The low [O III]/H
ratio
suggests low shock velocities, probably below 90 km s-1 (e.g.
Cox & Raymond 1985). At these shock
velocities the helium is basically neutral, while hydrogen can be
50%
ionized, assuming preshock ionization equilibrium (e.g. Hartigan et al. 1987). The [O I] flux, amounting to only
5% of the H
flux, can
be accounted for by the low shock velocity. Furthermore, the weak [O III] emission in this part of G 78.2+2.1 suggests complete recombination zones,
i.e. the post-shock gas is able to cool down to very low temperatures
(e.g. Raymond et al. 1988).
The measured electron density
of
750 cm-3, at the specific location, allows us to estimate a preshock
cloud density of the order of 20 cm-3, in case of negligible magnetic field
(Fesen & Kirshner 1980).
The H
/H
ratio of 10.2 (
1.7) is the highest measured in all spectra
and may indicate that the H II regions are closer to us and seen in projection
against the emission from the supernova remnant. Although a statistical
nature of this difference cannot be excluded, we note that all four spectra
from pos. 1 and the second spectrum from pos. 2 have systematically lower
H
/H
ratios.
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Figure 4:
The total molecular CO emission in the field of G 78.2+2.1
with contours of the radio emission at 1400 MHz.
Weak CO emission is mainly seen in the north-east,
the west and towards the central areas of G 78.2+2.1.
The strongest molecular emission seems to be projected to the
south-east of ![]() |
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Saken et al. (1992) reported the detection of a partial shell in the
IR using data from the IRAS satellite. However, the moderate
resolution of these data (2
)
does not permit a detailed comparison
with the optical and radio data. New IR data
were obtained by the Midcourse Space Experiment (MSX) in four bands and are
of sufficient resolution (
20
)
to compare with the optical and
radio images. Substantial IR emission is present to the south and to the
east of
Cygni (Fig. 2). Weaker emission extends further to
west, north-west characterized by several patchy structures which seem
to have the same curvature as the radio emission, while the central and
north parts of the remnant are practically free of IR emission.
Interestingly, the location of pos. 2 and its neighbor location are
areas where the IR emission is, at least, a factor of 5 lower than that
measured close to
Cygni. This may indicate that the dust clouds are in
the foreground of the remnant possibly obscuring optical emission from
these areas that otherwise would have been detected.
Diffuse infrared emission is present in the south where the morphology
of the [S II] emission, that could be related to G 78.2+2.1, is also diffuse.
The column density
is a measure of the absorption towards
a source and is mainly determined through X-ray observations.
The overall difference between the optical and X-ray determined
column density can be accounted for by the presence of molecular
gas which mainly affects the X-rays
(see also Fesen & Hurford 1995).
Examination of CO emission maps (Fig. 4) shows that the
equivalent molecular column
density in the direction of G 78.2+2.1 is
cm-2 (Dame et al. 2001; Leung & Thaddeus 1992). Although, the spatial resolution
of this survey is
8
,
we can measure a column density of
cm-2 at our pos. 2, while the minimum of
cm-2 is found in the west, in the neighborhood of our
pos. 1. The median value of the column density over the entire remnant is
cm-2 with the maximum of
cm-2 measured in a single bin to the south-east of
Cygni.
We have also examined the velocity resolved neutral hydrogen maps of
Hartmann & Burton (1997). A column density of
cm-2 is measured in the direction of this
remnant and for local standard of rest velocities appropriate to the Local
Arm.
Thus, the average total column density should not be more than
cm-2 according to these surveys. The average
neutral hydrogen density of the interstellar medium towards
G 78.2+2.1, and for a distance of 1.5 kpc, is a factor of
5 higher than the
ambient density of
0.2-0.3 cm-3 implied by X-ray and infrared
observations. These densities are not unreasonable (see also
Wendker et al. 1991; Landecker et al. 1980), if we realize that the
remnant lies
55 pc above the galactic plane, and may imply that
most of the absorption the X-rays suffer, takes place in the foreground.
The remnant evolves in a medium of low density, typically
0.3 cm-3,
encompassing interstellar "clouds'' of higher densities (
20 cm-3).
These clouds are responsible for the optical emission and a typical
projected length scale is
0.7 pc.
The density contrast of
70 under the assumption of pressure
equilibrium between the cloud and intercloud regions,
implies a primary shock velocity of
750 km s-1 which agrees,
within the errors, with the lower temperature measured in the east by
Uchiyama et al. (2002).
Adopting the formulae of Hailey & Craig (1994 and references therein),
we estimate an explosion energy of
erg and an age of
7000 yr, for a radius of 15 pc and an interstellar
medium density of
0.3 cm-3 (Uchiyama et al. 2002;
Lozinskaya et al. 2000). Note also that the Sedov-Taylor
solution for G 78.2+2.1 implies an E51/
for a radius of 15 pc and a shock velocity
900 km s-1
(Lozinskaya et al. 2000; Uchiyama et al. 2002)
independently of the actual age. Here E51 stands for the explosion
energy in units of 1051 erg and
is the density
of the ambient interstellar medium.
Thus higher densities would imply accordingly higher energies or vice versa.
The above estimates of the explosion energy and age are in very good
agreement with those obtained from X-ray measurements analyzed
by the aforementioned authors considering the uncertainties involved
and the assumptions made. According to these calculations,
shell formation would occur at
72 000 yr (Cioffi et al. 1988).
However, radiative losses will manifest themselves earlier at an
age of
26 000 yr which is a lot larger than the current age of
the remnant. It can also be shown that for a primary shock velocity
around 800 km s-1, it is not possible to obtain realistic values of the
energy and ISM density assuming the age of the pressure driven snow-plow
phase to be equal to the current age of the remnant. Thus, it seems that
G 78.2+2.1 is currently in the adiabatic phase of its evolution.
Acknowledgements
The author would like to thank N. Kylafis, T. Landecker, J. Papamastorakis and the referee for their insightful and helpful comments. Skinakas Observatory is a collaborative project of the University of Crete, the Foundation for Research and Technology-Hellas and the Max-Planck-Institut für Extraterrestrische Physik. This research made use of data products from the Midcourse Space Experiment. Processing of the data was funded by the Ballistic Missile Defense Organization with additional support from NASA Office of Space Science. This research has also made use of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The data from the Canadian Galactic Plane Survey were obtained from the Canadian Astronomy Data Centre (where the author is a guest user) which is operated by the Herzberg Institute of Astrophysics of the National Research Council Canada.