Issue |
A&A
Volume 503, Number 1, August III 2009
|
|
---|---|---|
Page(s) | 129 - 136 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/200912211 | |
Published online | 22 June 2009 |
A new candidate supernova remnant G 70.5+1.9
F. Mavromatakis1 - P. Boumis2 - J. Meaburn3 - A. Caulet4
1 - Technological Education Institute of Crete, Department of Sciences, PO Box 1939, 710 04 Heraklion, Crete, Greece
2 - Institute of Astronomy & Astrophysics, National Observatory of Athens, I. Metaxa & V. Pavlou, P. Penteli, 15236 Athens, Greece
3 - Jodrell Bank Centre for Astrophysics, University of Manchester, Manchester M13 9PL, UK
4 - Department of Astronomy, University of Illinois at Urbana-Champaign, 1002 West Green Street, Urbana, IL 61801-3080, USA
Received 26 March 2009 / Accepted 18 May 2009
Abstract
A compact complex of line emission filaments in the galactic
plane has the appearance of those expected of an evolved supernova
remnant, although non-thermal radio and X-ray emission have not yet
been detected. This optical emission line region has now been
observed with deep imagery and both low and high-dispersion
spectroscopy. Diagnostic diagrams of the line intensities from the
present spectra and the new kinematical observations both point to a
supernova origin. However, several features of the nebular complex
still require an explanation within this interpretation.
Key words: ISM: general - ISM: supernova remnants - ISM: individual objects: G 70.5+1.9
1 Introduction
During observations of a new candidate supernova remnant (SNR) in the
Cygnus region, close to the known remnant CTB 80, a bright emission
line source was detected (Mavromatakis & Strom 2002). This
source, called G 70.5+1.9, is relatively compact (a few arcminutes
wide), its boundaries are rather sharp and it exhibits a tail of
diffuse emission to the east, only seen in the [O III] image. The
southern part of this structure is visible on the POSS
plates. However, no other information is available on this peculiar
optical line source. Published radio surveys do not show any strong
evidence for non-thermal emission at the position of the optical
source, while the ROSAT All-Sky survey data do not reveal any excess
X-ray emission at this position. Both could be expected if the
filamentary nebula is an evolved SNR. In order to study in more
detail the detected structures and to understand their origin, we
performed deep optical, flux calibrated imaging observations in the
low and medium ionization lines of HN II], [S II], [O II] and [O III]. We
have also obtained deep spatially-resolved low-dispersion long-slit
spectra at four different slit positions of this filamentary structure
in an effort to obtain more information about the actual physical
conditions. Spatially resolved long-slit spectra along one slit
position have also been obtained to reveal any motions typical of an
evolved SNR. Information about the observations and results are given
in Sect. 2, while in Sect. 3 we discuss the nature of these new
filamentary structures.
2 Observations and results
2.1 Imaging
The observations presented in this paper were performed with the 1.3 m
Ritchey-Cretien telescope at Skinakas Observatory, Crete, Greece.
The filamentary nebula G 70.5+1.9 was observed on July 09 and 10,
2001 with interference filters isolating the optical emission lines of
HN II], [S II], [O II], and [O III]. The 1024
1024 SITe CCD used
during the observations had a pixel size of 24
m resulting in a
8
5
8
5 field of view. Two different
pointing directions were used in order to cover the area of emission
seen in the wide field images of Mavromatakis & Strom
(2002). The first pointing concentrated on the bright
filaments to the west, while the second focused on the diffuse [O III] emission to the east. All fields were projected to a common origin on
the sky and were subsequently combined to create the final mosaics.
The astrometric solutions were calculated with IRAF routines and
utilized the HST Guide star catalogue (Lasker et al. 1999). The
log of the observations together with the filter characteristics and
the exposure times are given in Table 1. All coordinates
given in this work refer to epoch J2000.
![]() |
Figure 1:
The 8
|
Open with DEXTER |
Table 1: Imaging and spectral log.
We employed standard IRAF and MIDAS routines for the reduction of the data. Individual frames were bias subtracted and flat-field corrected using well-exposed twilight flat-fields. The spectrophotometric standard stars HR 5501, HR 7950, HR 7596, HR 9087, and HR 8634 were observed for absolute flux information (Hamuy et al. 1992, 1994).
2.1.1 The H
N II] and [S II] line images
The higher angular resolution images of G70.5+1.9 allow us to study
in more detail the network of filaments. A basic characteristic of
these filaments is their brightness in HN II] (Fig. 1) and
[S II] (Fig. 2). The bulk of the emission seems to be
bounded by two very sharp filaments. These are separated by a typical
distance of
2
along the declination axis. The southern
filament is found at an almost constant declination
(
33
53
30
), while the northern is inclined
by
15
with respect to the east-west direction. Diffuse
emission and several shorter filamentary structures are detected
between the north and south boundaries (Figs. 1 and
2). The detected emission line structures are not bounded in
the east-west direction and weak diffuse as well as filamentary
emission is detected further to west in an arc-like shape
(Fig. 3,
20
00
40
,
33
53
). There is also a long filamentary
structure originating roughly from
20
01
07
,
33
55
20
and extending down to
20
00
42
and
33
50
02
,
after
a long gap of
3
.
Finally, a patchy structure is present
around
20
01
05
and
33
52
15
to the south of the main
emission area. The east field was also observed in these filters as
reported in Table 1 but it is not shown here since any
emission is below our detection limit.
![]() |
Figure 2:
The image of G70.5+1.9 in the low
ionization line of [S II] without continuum subtraction so that a
reference star field is clear. The long dashed rectangles show the
projection of the low-dispersion slits 1-4 on the sky, while the
small, individual rectangles along these represent the areas where
line strengths were extracted. Slit position 5 is that part of the
high-dispersion slit for which the position-velocity arrays of line
profiles are shown in Fig. 5. The shadings run linearly from to 0 to
8 |
Open with DEXTER |
![]() |
Figure 3:
The 8
|
Open with DEXTER |
![]() |
Figure 4:
The [O III] image of the whole network of filaments after
subtraction of the continuum image. The north and south filamentary
boundaries are also prominent in the medium ionization line of [O III]5007 Å. However, this line reveals diffuse emission further to the
east which is not detected in the lower ionization images. The shading
run linearly from 0 to 5 |
Open with DEXTER |
2.1.2 The [O II] and [O III] line mosaics
The detected structures are also quite strong in the low ionization
line of [O II]3727 Å (Fig. 3). The morphology in this
emission line is generally similar to that of the low ionization lines
discussed above. The south, bright filament (
20
01
,
33
53
30
)
is characterized by a surface
brightness up to
28
10-16 erg s-1 cm-2 arcsec-2, while the northern
filament displays a surface brightness in the range of 12-19
10-16 erg s-1 cm-2 arcsec-2. The weak but arc-like shaped emission to the west is
detected at a level of 1-2
10-16 erg s-1 cm-2 arcsec-2. The typical full width
at half maximum (fwhm) of the filament to the south is found in the
range of
3
-6
,
while the northern filament
(
20
01
10
,
33
55
20
)
is
broader with a fwhm of
8
.
The weaker filaments seen to
the interior of the bright emission are characterized by a fwhm of
3
5.
Table 2: Relative line fluxes.
The morphology of the structure as recorded through the [O III] interference filter (Fig. 4) differs from the morphology of
the low ionization images, including the [O II] image. The variety of
structures detected in the HN II], [S II] and [O II] images between the
south and north filaments are not clearly present in the medium
ionization line of [O III]. In fact, some structures may not be present
at all or their morphology has now become so diffuse that does not
allow the direct identification of the corresponding structures seen
in the low ionization images. The north boundary filament appears
quite sharp in this emission line, while it appears less well defined
in H
N II]. The inverse holds in the south-west, i.e. the filament is
sharp in the H
N II] image but diffuse emission is seen in the [O III] image. A major difference between the low and medium ionization line
images is the greater extent to the east of the north and south
filaments (Fig. 4). The north [O III] filament extends for
43
further to the east from the tip of the corresponding
H
N II] filament, while the south [O III] filament extends for
34
further to the east compared to the H
N II] emission
(around 20
01
15
).
The images of these regions provide direct evidence for the presence
of incomplete shock structures, i.e. areas where part of the
recombination zone is missing, allowing high [O III]/H
ratios.
Finally, the [O III] image displays a ``tail'' of mainly diffuse
emission further to the east for
8
.
The typical width of
this diffuse emission is less than
2
,
while its typical
brightness is
1.5-2.0
10-16 erg s-1 cm-2 arcsec-2. This diffuse component is
not detected in the low ionization images and it is not clear if it is
related to the filamentary emission in the west.
2.2 Spectroscopy
2.2.1 Low dispersion
Long-slit low-dispersion spectra were obtained on June 22 and 23,
2001 using the 1.3 m Ritchey-Cretien telescope at Skinakas
Observatory. The data were taken with a 1300 line mm-1 grating
and a 800
2000 SITe CCD covering the range of 4750-6815 Å. The slit had a width of 7
7 and a length of 7
9
and, in all cases, was oriented in the south-north direction. The
coordinates of the slit centers along with the number of spectra and
the total exposure times are given in Table 1. A spectrum
taken on August 3, 2000 with the same hardware configuration and
presented by Mavromatakis & Strom (2002) is also used here
with different apertures extracted. The spectrophotometric standard
stars HR 5501, HR 7596, HR 7950, HR 9087, and HR 718 were observed for
absolute flux information (Hamuy et al. 1992, 1994).
Long-slit spectra were obtained at four different positions.
The relatively small size of the source (3)
compared with the
slit length of 7
9 allowed very good background subtraction. In
addition, we have the possibility to study the nature of the diffuse
emission detected between the bright filaments. The absolute H
flux
ranges from 0.5 to 7
10-16 erg s-1 cm-2 arcsec-2 (Table 2). The high
[S II]/H
ratios clearly demonstrate that the detected emission
originates from shock heated gas ([S II]/H
0.5-1.6;
Table 2). The fluxes of the individual sulfur lines may
provide data on the electron density of the emitting gas (Osterbrock
& Ferland 2006). However, all sulfur line ratios approach the
high end of the allowable range of values suggesting low electron
densities. Using the nebular package within the IRAF software (Shaw
& Dufour 1995) we find that all electron densities are
estimated to lie below
120 cm-3. Therefore we cannot directly
estimate the preshock cloud densities but can only place upper limits,
provided that there is no magnetic field to halt the compression.
The log(H/[N II]) versus log(H
/[S II]) intensities, from
Table 2, corrected for interstellar extinction, are
compared with those of other well-defined phenomena in
Fig. 6 (following Sabbadin et al. 1977; Cantó 1981).
2.2.2 High dispersion
Observations of G70.5+1.9 were made with the Manchester Echelle
Spectrometer (MES-SPM - see Meaburn et al. 1984,
2003) combined with the 2.1-m San Pedro Martir telescope on
23 August, 2005. A SITe CCD was the detector with
,
24
m pixels although
binning was employed throughout
the observations.
Spatially resolved, long-slit H,
[N II] & [O III] line profiles were
obtained with the MES-SPM along the line marked 5 in
Fig. 2. This is only a partial length of the full NS slit as
relevant emission only occurred over small sections of the full slit
length. The increments along the slit length each corresponds to
0
63.
In this spectroscopic mode, MES-SPM has no
cross-dispersion, consequently, for the present observations, a
filter of 90 Å bandwidth was used to isolate the 87th
echelle order containing the H
and [N II] nebular emission lines and
one of 60 Å bandwidth for [O III]. The slit width was always 150
m which is
1.9
on the sky and 10 km s-1spectral halfwidth. Each integration time was 1800 s.
The longslit spectra were cleaned of cosmic rays and calibrated in
wavelength to 1 km s-1 accuracy in the usual way using
STARLINK FIGARO software. The greyscale representation of the
position-velocity (pv) arrays of H
,
[N II] & [O III] line profiles
for the partial slit length shown in Fig. 2 for Slit 5 are
shown in Fig. 5. As no standard star was observed, the
absolute surface brightnesses are unreliable and will not be presented
here.
![]() |
Figure 5:
Greyscale representations of the position-velocity arrays of
the H |
Open with DEXTER |
![]() |
Figure 6: Diagnostic diagram (Sabbadin et al. 1977; Cantó 1981), where the positions of line ratios listed in Table 2, from Areas 1a to 4f, are shown as black squares. |
Open with DEXTER |
3 Discussion
Detailed optical observations have been performed in an attempt to
understand the nature of G70.5+1.9. The lower ionization images in
HN II], [S II], [O II] and the higher ionization image in [O III] reveal
several filamentary structures. In addition, velocity resolved
profiles have been obtained along a specific slit position. The
current data point to a shock heated origin of the optical emission.
The SNR origin of the proposed candidate remnant is strongly suggested
by the positions of the line ratios in Fig. 6 compared with
those of Herbig-Haro objects (HH-objects), H II regions and
planetary nebulae (PNe). They follow closely the shape of those
observed for those of shock ionized evolved SNRs. The [O III]/H
ratio is a very usefull diagnostic tool for complete or incomplete
shock structures (Raymond et al. 1988). All spectra suggest
complete shock structures except that from Area 1a with an
[O III]/H
ratio of
26. Typically, this ratio is below
6 (Cox & Raymond 1985; Hartigan et al. 1987). However, this limit is easily exceeded in case of
shocks with incomplete recombination zones like in Area 1a. Since the
long-slit spectra do not cover the full extent of the source, we can
use our flux calibrated images to map areas with incomplete shock
structures. Since our H
N II] filter transmits equally the [N II]6548,
6584 Å, and H
lines we can estimate the H
flux as
1/2 of
the flux measured in the H
N II] filter. Assuming that the H
/H
ratio is
4 all over this source, as the long-slit spectra
suggest, we estimate the [O III]/H
ratio as
8 [O III]/H
N II].
Interestingly, we find that the area further to the east, i.e. between
20
01
08
and 20
01
14
is dominated by such
structures.
The motions of the filaments as seen in Fig. 5 also suggest an evolved SNR origin. The line profiles are single over the bright filaments, where expansive motions are expected to be tangential along the line of sight, but become split, albeit by only a few tens of km s-1 towards the fainter regions.
It remains possible that this isolated and irregular group of
filaments is part of a wider structure but is being seen through holes in
intervening clouds, leading to patchy optical interstellar
extinction. The correlation of these filaments with the structures
reported by Mavromatakis & Strom (2002) is also not
clear. The current data alone are not sufficient to claim a
correlation. The H/H
ratios in Table 2 can be used to estimate
the variations in logarithmic extinction coefficient c over this
source, assuming an intrinsic ratio of 3 and the interstellar
extinction curve of Kaler (1976) as implemented in the nebular
package (Shaw & Dufour 1995) within the IRAF software. The
signal to noise weighted average of the observed H
/H
ratios in
Table 2 is 4.0 (
0.1) derived from all
apertures and spectra available. The logarithmic extinction c is then
0.40, which is equivalent to an
of 0.8 and an E(B-V) of
0.27. Here we have assumed
E(B-V) = 0.664 c (Kaler 1976;
Aller 1984). E(B-V) values are listed in Table 2
along with other parameters, where it can be seen that statistically
significant H
/H
ratios vary from 2.9 (area 4f) to 4.5 (area 4d).
The average hydrogen column density derived from the statistical
relation of Predehl & Schmidt (1995) is 1.4
1021cm-3, while the total galactic hydrogen column density in the
direction of the candidate remnant is around 1
1022 cm-3(Kalberla et al. 2005; and Dickey & Lockman 1990). It
is clear that the value based on the optical data and a statistical
relation is lower by a factor of
6 than the estimated total
N H. This implies that the detected structures are closer to us
than the total distance to the outer part of the galaxy in that
specific direction. The use of the code given by Hakkila et
al. (1997) to calculate the visual extinction in the direction
of G70.5+1.9 supports the above suggestion, given our measurements of
optical extintion (with a typical color excess of 0.27). It is likely
that the distance to the proposed candidate remnant is less than 1
kpc.
A crucial issue concerns the existence of radio emission. Radio
emission in the area of the optical structures is detected in the low
resolution (7)
4850 MHz images of the Green Bank survey
(Gregory & Condon 1991). Given the low resolution, it is very
hard to state any spatial correlation. We have also examined the
higher resolution CGPS data at 1420 MHz (Taylor et al. 2003)
but no prominent emission was detected. The 3
upper limit
obtained is
4 mJy/beam. We note here that there are also other
SNRs that do not display radio emission, at least at the detection
level of the corresponding observations (e.g. Dickel et al. 2001; Filipovic et al. 2008; Stupar et al. 2008). Radio observations in different wavelengths
should be performed to provide conclusive evidence of the nature of
the source as an evolved SNR. Finally, we have searched ROSAT data
for X-rays but none have been detected in the area of the optical
emission.
Acknowledgements
We would like to thank the referee Prof. Dickel J. for his comments and S. Akras for his help on spectral fluxes calculations. 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 has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center.
References
- Aller, L. H. 1984, Physics of thermal gaseous nebulae (D. Reidel Publishing Company) (In the text)
- Cantó, J. 1981, in Investigating the Universe (Dordrecht: Reidel), 95 (In the text)
- Cox, D. P., & Raymond, J. C. 1985, ApJ, 298, 651 [NASA ADS] [CrossRef] (In the text)
- Dickel, J. R., Williams, R. M., Carter, L. M., et al. 2001, AJ, 122, 849 [NASA ADS] [CrossRef] (In the text)
- Dickey, J. M., & Lockman, F. J. 1990, ARA&A, 28, 215 [NASA ADS] [CrossRef] (In the text)
- Filipovic, M. D., Haberl, F., Winkler, P. F., et al. 2008, A&A, 485, 63 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Gregory, P. C., & Condon, J. J., 1991, ApJS, 75, 1011 [NASA ADS] [CrossRef] (In the text)
- Hakkila, J., Myers, J. M., Stidham, B. J., & Hartmann, D. H. 1997, AJ, 114, 2043 [NASA ADS] [CrossRef] (In the text)
- Hamuy, M., Walker, A. R., Suntzeff, N. B., et al. 1992, PASP, 104, 533 [NASA ADS] [CrossRef] (In the text)
- Hamuy, M., Suntzeff, N. B., Heathcote, S. R., et al. 1994, PASP, 106, 566 [NASA ADS] [CrossRef] (In the text)
- Hartigan, P., Raymond, J., & Hartmann, L. 1987, ApJ, 316, 323 [NASA ADS] [CrossRef] (In the text)
- Kalberla, P. M. W., Burton, W. B., Hartmann, D., et al. 2005, A&A, 440, 775 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Kaler, J. B. 1976, ApJS, 31, 517 [NASA ADS] [CrossRef] (In the text)
- Lasker, B. M., Russel, J. N., & Jenkner, H., 1999, in the HST Guide Star Catalog, version 1.1-ACT, The Association of Universities for Research in Astronomy, Inc. (In the text)
- Mavromatakis F., & Strom, R. G. 2002, A&A, 382, 291 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- McKee, C. F., & Cowie, L. 1975, ApJ, 195, 715 [NASA ADS] [CrossRef]
- Meaburn, J., Blundell, B., Carling, R., et al. 1984, MNRAS, 210, 463 [NASA ADS] (In the text)
- Meaburn, J., López, J. A., Gutiérrez, L., et al. 2003, RMxAA, 39, 185 [NASA ADS] (In the text)
- Osterbrock, D. E., & Ferland, G. J., 2006, Astrophysics of gaseous nebulae and AGN (US: University Science Books) (In the text)
- Predehl, P., & Schmitt, J. H. M. M. 1995, A&A, 293, 889 [NASA ADS] (In the text)
- Raymond, J. C., Hester, J. J., Cox, D., et al. 1988, ApJ, 324, 869 [NASA ADS] [CrossRef] (In the text)
- Sabbadin, F., Minello, S., & Bianchini, A. 1977, A&A, 60, 147 [NASA ADS] (In the text)
- Shaw, R. A., & Dufour, R. J. 1995, PASP, 107, 896 [NASA ADS] [CrossRef] (In the text)
- Stupar, M., Parker, Q. A., & Filipovic, M. D. 2008, MNRAS, 390, 1037 [NASA ADS] [CrossRef] (In the text)
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All Tables
Table 1: Imaging and spectral log.
Table 2: Relative line fluxes.
All Figures
![]() |
Figure 1:
The 8
|
Open with DEXTER | |
In the text |
![]() |
Figure 2:
The image of G70.5+1.9 in the low
ionization line of [S II] without continuum subtraction so that a
reference star field is clear. The long dashed rectangles show the
projection of the low-dispersion slits 1-4 on the sky, while the
small, individual rectangles along these represent the areas where
line strengths were extracted. Slit position 5 is that part of the
high-dispersion slit for which the position-velocity arrays of line
profiles are shown in Fig. 5. The shadings run linearly from to 0 to
8 |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
The 8
|
Open with DEXTER | |
In the text |
![]() |
Figure 4:
The [O III] image of the whole network of filaments after
subtraction of the continuum image. The north and south filamentary
boundaries are also prominent in the medium ionization line of [O III]5007 Å. However, this line reveals diffuse emission further to the
east which is not detected in the lower ionization images. The shading
run linearly from 0 to 5 |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
Greyscale representations of the position-velocity arrays of
the H |
Open with DEXTER | |
In the text |
![]() |
Figure 6: Diagnostic diagram (Sabbadin et al. 1977; Cantó 1981), where the positions of line ratios listed in Table 2, from Areas 1a to 4f, are shown as black squares. |
Open with DEXTER | |
In the text |
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