I. Gosachinskiy " HI Distribution in the Region of the Supernova
Remnant G78.2+2.1 ".
Based on RATAN-600 21cm line observations with an angular
resolution of 2.4' over a wide range of radial velocities, we analyze
the neutral-hydrogen distribution in the region of the supernova remnant
( SNR ) G78.2+2.1.
In addition to an HI shell at low radial velocities
immediately surrounding the radio remnant, we detected an extended
expanding HI shell, 3 dg. in diameter, at a radial velocity of -25 km/s,
which closely coincides in coordinates and angular sizes with the
outer X-ray shell discovered by Lozinskaya et al. ( 2000 ).
The Halpha emission studied by these authors
in the SNR region also has a secondary peak at radial velocities
from -45 to -20 km/s. Since the radial velocities of these two
objects differ markedly, their distances can be assumed to differ
as well; i.e., a chance projection of two distinct objects is observed.
The well-known shell-type supernova remnant ( SNR ) G78.2+2.1 see Green
( 1996 ), Gosachinskiy et al. ( 1999 ), and Lozinskaya et al. ( 2000 ) for
references is projected onto the region of the Cygnus X radio source.
More than half a hundred various objects, mostly compact H II regions
( Piepenbrink and Wendker 1988 ), are located there in a small area of the
sky ( a mere 6-7 dg. in diameter ). Only three of these objects, judging
from their H110alpha radial velocities measured by the
above authors, are projected by chance onto this region from more distant
At the same time, the interpretation of observations in this region
is complicated by uncertainties in distances, especially kinematic
ones, because of the small dv/dr, and by the fact that the line of sight
here passes along the Orion-Cygnus spiral arm.
The interstellar medium in this region has been studied by dozens of
authors over various wavelength ranges and in various spectral lines.
Gosachinskiy et al. ( 2000 ) discovered two annular HI structures,
which morphologically resemble expanding shells.
One of them, with a larger angular size, is located around the
Cygnus X radio source, and the other, at a mean radial velocity of +3 km/s,
immediately surrounds SNR G78.2+2.1. A quantitative analysis shows
that it could be produced both by the SNR shock wave and by the
progenitor star's stellar wind. Here, we also give references to
previous studies of neutral hydrogen in the SNR region and
critically analyze the results of some of them.
The archival ROSAT and ASCA X-ray data were analyzed by Lozinskaya
et al. ( 2000 ). Their analysis revealed two clumpy nested X-ray shells,
which may be associated with this SNR. The surprising thing is that,
whereas one of them, composed of three clouds, is only slightly larger
than the SNR, the angular diameter of the second, weaker shell is three
times that of the SNR.
Bearing in mind that this region is very rich in various objects,
a chance projection of X-ray features onto one another and onto
the SNR cannot be ruled out either.
On the other hand, high-angular-resolution observations are known
to commonly reveal shells or cavities in the HI distribution where
active objects with high energy release are located:
SNRs, H II regions, or stellar-wind-associated shells,let alone X-ray
For this reason, when investigating the relationship of the HI
distribution to SNR G78.2+2.1 and other objects, it makes sense not
to restrict the analysis to radial velocities near zero, which
corresponds to the SNR distance, but consider the entire HI-line
range. Below, we present our results.
INSTRUMENTATION AND TECHNIQUES
We used 18 cuts in right ascension at 0.6 dg. intervals in declination
obtained with the RATAN-600 radio telescope to investigate the
interstellar-gas distribution in the region of the Cygnus X radio source
and SNR G78.2+2.1.
The instrumentation and techniques are detailed in Gosachinskiy et
al. ( 1999 ), but, for convenience, we give their brief description.
In this elevation range, the RATAN-600 antenna at 21 cm has
an angular resolution of 2' x 12', an effective area of 875 m2,
and substantial ( up to 40% ) losses in brightness temperature.
The latter is attributable to the peculiar antenna design and to
the fact that the observations were carried out near zenith.
The system noise temperature is 60 K; the 39-channel filter spectrum
analyzer has a channel bandwidth of 30 kHz ( 6.3 km/s ).
The cut at each declination consists of two series of three
observations each obtained by shifting the receiver tuning frequency
by half the channel bandwidth. As a result, each cut has 78 channels
that follow at intervals of 3.15 km/s.
This observing technique also allows an effective noise cleaning.
The mean square of fluctuations in spectral channels on an averaged
scan is 0.2 K. The antenna and equipment parameters were checked in
each series of observations by measuring a set of reference sources
( Venger et al. 1979 ).
Subsequently, we subtracted an extended background obtained by spline
interpolation at lower brightness-distribution level from the transit
curves in each spectral channel and then reduced the transit curves
containing only features of small angular sizes.
The subtracted background part of the transit curves includes:
It should be noted that subtracting the background by the above
method could result in an underestimation of the brightness and angular
sizes of the remaining small-scale features, whose parameters were
determined by using a code of Gauss analysis.
- (1) large-scale features of the interstellar-gas
distribution, such as spiral arms or giant complexes;
- (2) emission from the intercloud medium, if present;
- (3) features of small angular sizes unresolved by the RATAN-600 beam;
- (4) and spurious large-scale background produced by distant side lobes
and the RATAN-600 stray field.
The measured parameters have the following errors. The radial
velocity of an isolated medium-brightness HI feature is measured with
an accuracy of 1 km/s. In several cases noted below, the accuracy
is reduced because of the difficulties in separating the object from
the background or from neighboring features.
Given the antenna calibration errors, the measurement error
of the HI line brightness temperature is about 0.5 K, while the error
of the estimated angular sizes in right ascension is 0.1 dg.
The same qualifications as those for the radial velocities hold in
the latter case. The antenna resolution in declination is much lower,
and, accordingly, the accuracy of measuring angular sizes is lower.
The accuracy of estimating distances depends on the method of their
determination, and it should be considered separately in each case.
As a result, the accuracy of estimating the HI mass turns out to be
no higher than 0.5-1 order of magnitude.
OBSERVATIONS AND THEIR DISCUSSION
The brightness distribution of HI features at a radial velocity of
+10 km/s after the extended radio-line emission background in the
region of the Cygnus X radio source and SNR G78.2+2.1 has been subtracted
is shown in Fig. 1.
The declinations of the cuts are indicated on the right;
the vertical and horizontal scales slightly differ. The annular HI
structures that can be correlated with the above objects, as was done
by Gosachinskiy et al. ( 1999 ), are represented by thin lines.
A schematic 21cm radio image of the SNR from Higgs et al. ( 1977 ) is
shown in Fig. 2 ( heavy circle ) together with X-ray ( 0.5-2.0 keV )
isophotes from Lozinskaya et al. ( 2000 ). In this figure, the vertical
and horizontal scales are approximately the same.
Gosachinskiy et al. ( 1999 ) found that the smaller annular HI
structure immediately surrounds the radio remnant and determined the
following observed parameters of the HI shell around the SNR:
When identifying the HI shell, we carefully allowed for the effect
of the HI absorption line, because the SNR continuum emission was intense.
We also compared the detected structure with the data of other authors
who analyzed the H I distribution in this region.
We assumed the SNR distance to be 1.5 kpc ( Landecker et al. 1980 )
when calculating the following physical parameters of the HI shell around
- Coordinates of the center R.A. alpha ( 2000.0 ) = 20h21.3m,
delta ( 2000.0 ) = +40.8 dg.;
- Angular sizes 2.8 dg. x 3.5 dg. ( inner ) and 2.0 dg. x 2.5 dg. ( outer );
- Mean brightness temperature of the line 8 +- 0.5 K;
- Mean radial velocity +3 km/s;
- The radial-velocity range in which the HI shell was observed > 20 km/s.
If the HI shell around SNR G78.2+2.1 results from the impact of the
shock wave generated by a supernova explosion on the interstellar gas,
then the initial explosion energy and the SNR age can be estimated from
observed HI-shell parameters.
The required density of the ambient interstellar medium,
n0 = 1.6 cm-3, was estimated by Gosachinskii et al.
( 1999 ) by assuming that the HI-shell gas was initially "spread"
over its entire volume.
The presence of an expanding HI shell around the SNR was also assumed
to suggest that the remnant is in the radiative phase. In this case,
using relations from Wheeler et al. ( 1980 ), we obtained the initial
explosion energy and the SNR age:
- Large-scale velocity of radial motions > 10 km/s;
- Outer diameter 75 pc;
- Inner diameter 55 pc;
- Gas density 2.5 cm-3;
- Shell mass 8.1 x 103 M of the Sun.
E0 = 3 x 1050 erg and t = 1.1 x 106
Although the derived initial explosion energy is in good agreement with
the universally accepted value, the age proves to be too large for an
X-ray emitting remnant.
As an alternative explanation, Gosachinskiy et al. ( 1999 ) assumed
that an extended slow HI shell could be produced by the supernova
progenitor's stellar wind. Using the calculations by Weaver et al.
( 1977 ), we then estimated the stellar-wind intensity and
duration required for the formation of an HI shell from its observed
parameters to be:
Lw = 1036 erg and t = 2.3 x 106 years.
Such an intensity is characteristic of the winds from OB stars with
Minit > 8 M of the Sun, which produce supernova explosions,
while the wind duration is an order of magnitude shorter than the
main-sequence lifetimes of these stars. This confirms that the outer HI
shell around SNR G78.2+2.1 could be produced by the supernova progenitor's
Data on the soft X-ray emission from the G78.2+2.1 region ( Lozinskaya
et al. 2000 ) ( see Fig. 2 ) confuse the picture further still. The
presence of the so-called inner X-ray shell, which consists of three
extended clouds and coincides almost exactly with the radio remnant in
coordinates and angular sizes, can still be explained somehow if the
remnant is still in the adiabatic phase (see Lozinskaya et al. ( 2000 )
for a detailed discussion.
The weak outer clumpy X-ray shell ( of course, if it actually
exists ) is offset by almost 1 dg. northwest of the radio-remnant center.
In addition, its angular size is almost triple that of the remnant,
which also completely disagree with the sizes and location of the
HI shell detected by Gosachinskiy et al. ( 1999 ).
Note, however, that, in accordance with the assumed distance to the
with the SNR only at nearly zero radial velocities. Clearly, the pattern
of HI distribution should be studied over a much wider range of radial
The possible association of HI features with the above objects in the
radio, optical, and X-ray ranges was analyzed in the radial-velocity
range -125 to +35 km/s. It turned out that, apart from the features at
positive radial velocities noted by Gosachinskiy et al. ( 1999 ),
there is an interesting gas structure in the range -15 to -35 km/s.
The distribution of HI features at radial velocities in the range
-20 to -30 km/s is shown in Fig. 3.
A comparison of this map with Fig. 2 reveals a distinct cavity or even
an HI shell, which closely coincides in coordinates and angular sizes
with the outer X-ray shell from Lozinskaya et al. ( 2000 ).
This shell has the following observed parameters:
At lower and higher radial velocities, the angular sizes of the
annular structure decrease, while the center slightly displaces in
right ascension. Such an isophotal structure is characteristic of a
shell with a large-scale radial velocity component (expansion)
Nevertheless, it should be noted that the cavity is not quite
circular and is elongated from north to south. This, it cannot be
reliably interpreted as an expanding shell. However, in the
radial-velocity range studied, we failed to detect any other HI
features whose morphology would correspond so closely to the outer
X-ray shell found by Lozinskaya et al. (2000).
Whether the annular structure that we detected in the HI
distribution is real can be verified, for example, by using data from
a survey made with the 90-m NRAO radio telescope with an angular
resolution of 12' and a radial-velocity resolution of 2 km/s
(Westerhout and Wendlandt 1982).
Data from this survey are now accessible via the Internet in the
ADS archive (Strasburg). Figure 4 shows the distributions of HI-line
brightness temperature in Galactic longitude taken from this survey at
the source latitude b = +2.1 dg. in the radial-velocity range
-20.0 to -40.0 km/s.
The heavy and thin vertical lines mark, respectively, the position
of the SNR center and its sizes at "zero" continuum radio brightness
as inferred by Reich et al. (1990). There is a clear reduction in
HI-line intensity southwest of the SNR at longitudes 76-77 dg.
This effect is at a maximum at radial velocities of about -30 km/s.
Of course, the H I distribution from Westerhout and Wendlandt
(1982) differs in details from our distribution, particularly the mean
radial velocity of the cavity on the profiles in Fig. 4.
This can be explained, first, by a marked difference between the
antenna beam shapes and a different radial-velocity resolution and,
second, by the fact that Westerhout and Wendlandt (1982 provided data
only for the southeastern part of the cavity up to
b = +2.4 dg.
Lozinskaya et al. (2000) measured the Halpha radial
velocities in the G78.2+2.1 region. It turned out that there is a
secondary peak of the line profile at radial velocities in the range -20
to -45 km/s outside the bright remnant, but inside the outer weak X-ray
shell, which closely corresponds to the velocity range of the H I shell
we detected and confirms that it is real. However, Lozinskaya et al.
(2000) point out that the above velocity range is also observed in the
Halpha profiles in the broader region of the Cygnus X source.
The question arises as to whether the two objects, which can be
characterized as a supernova remnant, on the one hand, and as an outer
X-ray shell of a large diameter, on the other, are interrelated. In the
range of Galactic longitudes under study, the objects in the
Orion-Cygnus arm have radial velocities of Galactic rotation within
5 km/s for any Galactic-rotation models (see Burton and Gordon 1978;
Kerr and Linden Bell 1986).
Given the peculiar velocities and systematic noncircular motions,
this range extends to +- 15 km/s. The radial velocity -25 km/s is
well outside this range and formally corresponds to a distance of 6 kpc
from the Sun in the most compact Galactic-rotation model by
Gluskova et al. (1998).
Of course, we can assume that these objects spatially coincide and
that the radial-velocity difference is explained by their internal
kinematics, for example, by radial motions. However, there are no clear
traces of interaction between these objects in the HI distribution at
radial velocities between +3 and -25 km/s.
Therefore, we believe a chance projection of these objects to be
most likely. This comes as no surprise in a region that is so rich in
objects of different nature: the SNR G78.2+2.1 itself is projected
onto the Cygnus X thermal complex, while the well-known gaseous
nebula around the star gamma Cyg is projected onto the SNR shell.
- Coordinates of the center alpha (2000.0) = 20h20.0m,
delta (2000.0) = +40.8 dg.;
- Angular sizes 3.7 dg. x 4.7 dg. (inner) and 2.5 dg. x 3.5 dg. (outer);
- Mean brightness temperature of the line 12 +- 0.5 K;
- Mean radial velocity -25 km/s;
- The radial-velocity range in which the HI shell was observed +- 10 km/s.
- Note also that the angular sizes of the shell are at a maximum at a
radial velocity of -25 km/s; this velocity can thus be considered to
- This velocity can thus be considered to be the mean for the object.
A thorough analysis of the HI distribution in the G78.2+2.1 region
over a wide radial-velocity range has revealed an extended expanding
elliptical HI shell, 3 dg. in diameter, at radial velocities from -10
to -25 km/s, which closely coincides in coordinates and angular sizes
with the outer X-ray shell from Lozinskaya et al. (2000).
The Halpha emission investigated by the above authors
in the SNR region also has a secondary peak inside the outer shell at
radial velocities from -45 to -20 km/s.
Since the radial velocities of the SNR with the inner X-ray shell
and of the outer X-ray shell with the HI shell and
Halpha-emitting regions differ markedly, their distances
can be assumed to differ as well; i.e., we observe a chance projection
of two distinct objects, which is not surprising for the Cygnus region
Of course, if subsequent observations in all ranges will reveal
unquestionable evidence for a physical association of these remarkable
objects, then this simplest interpretation will have to be rejected.
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- Figure 1. The brightness distribution of H I features at a radial velocity
of +10 km/s after an extended background in the region of the Cygnus
X radio source and SNR G78.2+2.1 has bee subtracted. The declinations of
HI cuts are indicated on the right; the vertical and horizontal scales
are different. The thin lines represent the annular H I structures
related to the above objects (Gosachinskii et al. 1999).
- Figure 2. A schematic image of SNR G78.2+2.1 as constructed
from the 21cm data of Higgs et al. (1977) (heavy circle) and X-ray (0.5-2.0 keV)
isophotes from Lozinskaya et al. (2000). The vertical and horizontal
scales and approximately the same.
- Figure 3. Same as Fig. 1 at radial velocities of -20, -25 and -30 km/s.
The thin lines represent the cavity that coincides in coordinates
with the outer X-ray shell found by Lozinskaya et al. (2000) and shown
in Fig. 2.
- Figure 4. The distribution of HI-line brightness temperature in the
vicinity of G78.2+2.1 as constructed from data of the survey by
Westerhout and Wendlandt (1982) with an angular resolution of 12' and a
radial-velocity resolution of 2 km/s1. The source center (heavy
lines) and the sizes at "zero" brightness temperature as inferred by
Reich et al. (1990) at 11 cm are indicated.
- Translated by V. Astakhov