A&A 461, 991-997 (2007)
DOI: 10.1051/0004-6361:20054786
F. Mavromatakis1 - E. M. Xilouris2 - P. Boumis2
1 - Technological Education Institute of Crete, General Department of Applied
Scienses, PO Box 1939, 710 04 Heraklion, Crete, Greece
2 -
Institute of Astronomy & Astrophysics, National Observatory of Athens,
I. Metaxa & V. Pavlou, P. Penteli, 152 36 Athens, Greece
Received 29 December 2005 / Accepted 25 August 2006
Abstract
Flux-calibrated CCD images of HB 21 are presented, along
with flux calibrated spectra of moderate resolution. The low ionization images
reveal filamentary structures in the east, while the emission in the central to
west areas may be either filamentary or patchy.
The filaments in the east are correlated very well with
the radio emission, but a smaller degree of correlation exists for the
patchy emission. An area in the west exhibits strong sulfur
emission and is probably associated to the remnant.
Uncatalogued H II regions are present in the area of the remnant,
mainly in the south.
We find that an ellipse extending for
incorporates most of the radio and optical emission.
The actual physical extent of the remnant depends on the distance, which
is not well determined since it ranges from 0.8 to 1.7 kpc.
Deep long-slit spectra were also acquired at a number of locations.
The H
emission is generally weak, typically below 15
10-17 erg s-1 cm-2 arcsec-2. The
H
emission was not always detected, suggesting substantial attenuation
of the light through the interstellar medium. The medium ionization line of
[O III]5007 Å was not detected indicating low shock velocities into the
interstellar clouds whose density is estimated to be a few atoms per cm3.
Key words: ISM: general - ISM: supernova remnants - ISM: individual objects: G 89.0+4.7
The supernova remnant HB 21 was discovered more than 50 years
ago in the radio band (Brown & Hazard 1953).
Current estimates of the distance to the source differ significantly
ranging from 0.8 kpc to 1.7 kpc (e.g. Tatematsu et al. 1990;
Byun et al. 2006). A number of observations
at different frequencies suggest a spectral index of 0.4, clearly
identifying
the non-thermal nature of the radio emission (e.g.
Reich et al. 2003, and references therein).
Strong radio emission is located in the inner areas of the remnant, while
filamentary emission is found at specific locations along its periphery
(e.g. Uyaniker et al. 2003).
The 856 MHz image presented by Reich et al. (2003) reveals a well-defined
boundary at an angular resolution of 14
5.
Molecular, atomic, and radio continuum observations led Tatematsu et al. (1990) to propose the interaction of the remnant with the surrounding material, mainly along its eastern boundary. The atomic gas displays a smoother distribution compared to the clumpy nature of the CO emission (their Fig. 6a). It is believed that some of the detected clumps belong to the Cyg OB7 association. Koo et al. (2001) performed higher resolution molecular observations of the eastern half of the remnant. In the northern and southern areas the detected CO broad emission lines were attributed to fast-moving clumps overtaken by the blast wave. They also analyzed IRAS HIRES images to show that the infrared emission is due to shocks propagating into them.
HB 21 was originally detected in the 0.2-4.0 keV band by the Einstein satellite
(Leahy 1987) and was also detected in the soft X-ray band by
ROSAT (Leahy & Aschenbach 1996) during the All Sky survey.
It has been proposed that this object belongs to the class of mixed morphology
remnants due to its centrally brightened X-ray emission and shell like radio
morphology (Pannuti et al. 2006).
Subsequent observations of the brightest emission regions were performed
by the ASCA satellite (Lee et al. 2001). The spectral fits point to the
thermal nature of the X-ray emission with temperatures around 0.6 keV and
hydrogen column densities in the range of
cm-2.
Similar temperatures (0.6-0.7 keV) and hydrogen column densities have been
obtained by Pannuti et al. (2006).
Even though this supernova is well-observed in radio wavelengths, less
data are available on its optical morphological and spectral properties.
Images in the H
and [O III] lines were acquired by Lozinskaya (1972)
but were not shown.
Interferometric observations in the H
line by Lozinskaya (1972,
1975, 1980) lead to the determination of an
expanding shell velocity of
25 km s-1, while the highest observed
velocities reached 60-80 km s-1.
Willis (1973) presented an optical image
of HB 21 taken from deep red plates. However, the quality of the reproduced
photograph does not permit a detailed analysis, and the author commented that
the radio and optical features correspond poorly.
He also suggested that absorption increases from the west towards the east
parts of the remnant.
However, van den Bergh (1978) did not detect any optical nebulosity
in deep H
and [S II] plate images, related to HB 21, obtained with the
1.2 m Schmidt telescope at Palomar mountain.
In this work, we present flux-calibrated CCD images of
this remnant in the emission lines of HN II] and [S II].
Deep long-slit spectra were
also acquired in a number of selected locations. In Sect. 2 information about
the observations and the data analysis is provided, while in Sect. 3 we
report on the results of the imaging and spectral observations. In Sect. 4, we
discuss the optically-related properties of this remnant, and summarize
the results of the current in Sect. 5.
Table 1: Journal of the observations.
The 0.3 m Schmidt-Cassegrain telescope at Skinakas Observatory, Crete, Greece,
was used for the imaging observations.
Multiple pointings, typically two per field, were performed from July 31
to August 02, 2003 in order to map this extended remnant.
A
Thomson CCD was used resulting in a
field of view and an image scale
of 4
per pixel.
A detailed log of the observations is given in Table 1.
The peak transmittances of the H
N II] and [S II] filters occur at 6560 and 6708 Å, and their FWHM are 75 and 20 Å, respectively.
All data frames were projected to an ra-dec grid in J2000 coordinates
with the aid of the HST Guide star catalogue (Lasker et al. 1999).
Standard IRAF
and MIDAS
routines were utilized to reduce the data.
The data were bias subtracted and flat-field corrected using a series of
well exposed twilight flat-fields. The spectrophotometric standard stars
HR 5501, HR 7596, HR 7950, HR 8634, HR 9087, and HR 718 were observed in order to
provide absolute flux calibration of the data
(Hamuy et al. 1992, 1994).
The long-slit spectra were acquired with the 1.3 m Ritchey-Cretien telescope
at Skinakas Observatory from July 30 to August 02, 2003.
The spectral log (Table 2) lists
the coordinates of the slit centers, the number of available spectra at
each location, and the total exposure time of each spectrum.
The data were taken with a 1300 line mm-1 grating
and a
SITe CCD allowing the range of 4750 Å-6815 Å
to be explored. The angular width of the slit is 7
7 (east-west
direction), and its length extends for 7
9 in the south-north direction.
The current setup results in a resolution of
7 and
11 Å in the red and blue wavelengths, respectively.
The spectrophotometric standard stars HR 5501, HR 7596, HR 9087, HR 718,
and HR 7950 were observed for the spectral calibration.
Table 2: Spectral log.
The images in the HN II] and [S II] filters of the area of HB 21
are presented in Figs. 1 and 2, respectively.
The field appears somewhat complex due to the presence of a significant
number of diffuse emission structures. Some of these patches are found
outside the radio emission boundaries of HB 21 in the
south-west and are thus not related to the remnant.
However, weaker and diffuse emission patches are located in its central,
western areas (Fig. 1). A search in the SIMBAD database did not
reveal any known bright nebulae. The closest recorded bright nebula is
LBN 381 (Lynds 1965), which is located at a declination of
48
30
.
Generally, H II regions display [S II]/H
ratio of less than 0.35,
typically around 0.2
(e.g. Hunter 1992), while in supernova remnants this ratio
exceeds the value of 0.4 and is usually greater than 0.5 (e.g.
Smith et al. 1993). Thus, areas of stronger sulfur emission might imply
a possible association to the remnant. The overall diffuse emission, mainly
present in the H
N II] image, may not be related to the remnant given the
significant attenuation of the emission in the [S II] image.
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Figure 1:
The field of HB 21 in the H![]() ![]() |
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Figure 2:
Areas of strong sulfur emission [S II] are good candidates
for an association with the remnant because the sulfur filter
suppresses the background emission, as well as the emission from
HII regions.
The image has been smoothed to suppress the residuals
from the imperfect continuum subtraction, while the
shadings run linearly from 1 to 9 ![]() |
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Figure 3: The field of HB 21 in the [S II] filter (background image) with the 4850 MHz contours overlaid. The contours scale linearly, every 0.022 Jy/beam, from 0.01 to 0.14 Jy/beam (Condon et al. 1994). The letters A to D refer to areas of emission that may be associated to the remnant, while the ellipse drawn encompasses the majority of the radio and optical emission. |
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Figure 4: Typical long-slit spectra from locations I, II, and VII in the area of the remnant HB 21. More details can be found in Sect. 3.2 and Table 3. |
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The average sulfur line ratio is 1.35 over the area of the remnant,
indicative of low electron densities. Given the relatively low significance of
the measured fluxes, the one-sigma upper limits on the electron densities
are quoted in Table 3. The wide range of electron densities does
not allow us to draw secure conclusions about the shock velocity and/or the
strength of the magnetic field.
The measured [S II]/H
ratios exceed 0.53, with the exception of locations IV
and V, suggesting the presence of shock-heated gas.
The medium ionization line of oxygen at 5007 Å is not detected in our spectra pointing to low shock velocities and possibly to a neutral cloudy medium. Emission from neutral oxygen at 6300 Å is detected in the majority of the measured spectra but is a factor of 2-3 less than the [N II]6548, 6584 Å flux. These two observational data suggest that slow shocks (<100 km s-1) propagate into the interstellar clouds.
Table 3: Relative line fluxes.
Flux-calibrated images and spectra of the extended remnant HB 21 are presented for the first time. Even though this remnant is well observed in radio wavelengths, little information is available about its optical properties, with the exception of its kinematic behavior (Lozinskaya 1980, and references therein).
The low-ionization images reveal filamentary emission in the east and patchy emission in the central to west areas of the remnant. The optical filaments in the east display a high degree of correlation with the radio emission, while the patchy emission shows a reduced degree of correlation. It is possible that the diffuse-patchy emission covering an extended area is not related to the remnant since such emission is quite suppressed in the [S II] image. The detection of strong sulfur emission in the majority of the observed locations clearly suggests that this optical emission is associated to the remnant (Table 3). The morphology of the optical emission is either filamentary or patchy. The filamentary nature may be related to the viewing geometry, while the patchy appearance may be attributed to the effects of turbulent magnetic fields (e.g. Raymond & Curiel 1995).
Flux-calibrating the HN II] and [S II] images can identify areas of
emission which may be associated to the remnant.
Emission in the areas A, B, C, and D may be associated to the remnant given
their spatial correlation with the radio emission (Fig. 3).
We also show the low-ionization images as an RG composite to allow for an
easier identification of any emission areas possibly related to the remnant
(Fig. 6). The reddish areas of emission indicate areas prominent
in the [S II] emission.
The correlation in area B is further supported by the characteristics of the
long-slit spectrum at location III.
A promising candidate is the emission detected in area D
(Fig. 3). The sulfur emission in this area
is, at least, a factor of 2 stronger than in other areas where the long-slit
spectra clearly identify the shock-heated nature of the detected emission.
Furthermore, the sulfur emission is still strong, even when compared
to the corresponding H
N II] emission. Considering the [S II]/H
N II] ratios
measured in the low ionization images and the [S II]/H
values from the
spectra, we propose that the emission in area D is associated to the remnant.
This suggestion is supported by radio data, at a frequency of 4850 MHz
and a resolution of 7
available from the Green Bank survey
(Condon et al. 1994). Despite this low resolution, the correlation
between the radio and the optical emission is immediately evident both in
area D and location VII.
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Figure 5:
The field of HB 21 in the [S II] filter (background
image) with the 60 ![]() |
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Figure 6:
This figure shows an RG composite based on the
H![]() ![]() |
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To compare the [S II] optical emission from HB 21 with the infrared emission
from this region, we made use of the IRAS resolution-enchanced
(HiRes; Aumann et al. 1990) images of this area at 60 m.
The comparison is shown in Fig. 5 with the [S II] emission
on the background and the 60
m emission presented with contours.
It shows that the infrared emission quite nicely traces
most of the structures observed in the north, as well as in the south.
In their analysis, Koo et al. (2001) argue that the enchanced
far-infrared color and the morphological correlation between the infrared
and the CO emission suggest that the infrared emission is caused by shocks
propagating into the clouds. The correlation between the optical
and the infrared emission that we find supports this argument.
The failure to detect oxygen emission at 5007 Å indicates low shock
velocities around or below 100 km s-1,
as Cox & Raymond (1985) have shown in their work.
Using the relation of Fesen & Kirshner (1980),
![]() |
(1) |
McKee & Cowie (1975) obtained the relation
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(2) |
HB 21 is an interesting object for further studies because it appears to be evolved, and it exhibits filamentary optical emission in the east and also in the west, while radio and X-ray emission are detected as well. Optical kinematic data at the positions of the detected filaments would allow the cloud velocities to be determined and compared with those estimated from X-ray data and those estimated from models. Our knowledge for remnants beyond the adiabatic phase of their evolution could then be advanced.
The extended supernova remnant HB 21 was observed for the first time
with deep CCD
images and spectra. Filamentary and patchy structures were detected that
appear correlated with the radio emission.
The flux-calibrated images indicate that
the H,
[N II], and [S II] emission is weak. Deep long-slit spectra also
suggest significant attenuation of the optical emission by the interstellar gas.
The H
line emission is either non-detectable or weakly present.
The strength
of the oxygen line at 5007 Å points to slow shock propagating into
the interstellar clouds. The density of these clouds is estimated to be a few
particles per cubic centimeter.
Acknowledgements
The authors would like to thank the referee for comments and suggestions that significantly improved this work. 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 obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center.