Figure 2 serves to illustrate the general HI emission in a region
around 3C 400.2. Significant
HI emission is detected in the velocity interval (-70, +70)
km s-1. The emission which dominates the spectrum in the range
-10 km s-1 to +15 km s-1 is usually associated with the local spiral arm;
while emission at higher velocities can be related
to the Carina-Sagittarius spiral arm, which is seen
longitudinally in this direction of the Galaxy (Georgelin & Georgelin
1976). According to circular galactic rotation models
(e.g. Brand & Blitz 1993), positive
velocities higher than
km s-1 are forbidden; however, as it is
clear from this figure, strong
HI emission is still detected up to
km s-1. Emission at negative velocities
should correspond to very distant gas, beyond the Solar Circle (
10 kpc).
We examined in detail the HI 21-cm line data in the whole observed velocity interval looking for traces of expanding events like holes, shell-like features, etc. The HI data can also give information about the existence of neighbouring neutral clouds that may have been collided by a strong SN shock or are part of the population of cloudlets that are currently being evaporated, resulting in the observed thermal X-ray emission.
Figure 3 displays the HI emission distribution in the velocity interval -55 km s-1 to +74 km s-1.
![]() |
Figure 3:
Grayscale images of 3C 400.2 in the neutral hydrogen
![]() |
Each HI line map was obtained by averaging 3 individual maps, producing images with a velocity resolution of
In the images at -49.5 and -44.5 km s-1 a small patch of emission is seen projected onto the center of the remnant. Based on the positional coincidence with the remnant and on the fact that this feature is unique, we propose that it is a cloud that may have been accelerated by the SN shock.
At v= -39.6 and -34.6 km s-1 the emission is patchy; no particular structure is likely to be associated with the SNR. From v= -29.7 to -19.7 km s-1 the HI emission distribution appears almost perpendicular to the galactic plane orientation. Again its relation to the remnant is not evident. Between -14.8 and -4.9 km s-1 a thin filament of neutral gas can be seen running through 3C 400.2 in a direction almost parallel to the galactic plane.
At positive velocities, the galactic background contribution is important,
and the HI structures that are candidates for coupling with the radio continuum
emission of 3C 400.2 need to
be disentangled from the field emission.
From 0 to +9.9 km s-1 clumpy emission is detected in the whole observed field.
In the velocity range +14.8 to +29.7 km s-1the most outstanding feature is the
extended HI cloud, brighter at +19.8 km s-1, which overlaps the west and north portions of
3C 400.2. Part of this cloud appears also delineating the northeast radio
maximum of the remnant centered around (see Fig. 1).
From +34.6 to +44.5 km s-1 the HI emission forms an incomplete ring-shaped feature surrounding the southern and western border of the "large shell".
For velocities greater than km s-1 the only neutral features
likely to be related to the
remnant are a very weak cloudlet
seen projected onto the center of the SNR at v=+69.3 km s-1, and
a small flattened cloud that appears "touching" the western border of
3C 400.2 at +64.3 and +69.3 km s-1.
In our search for HI features associated with 3C 400.2, the structures seen between +15 and +42 km s-1 appear particularly promising, since they show several morphological correspondences with the radio continuum emission. Figure 4 gives a more detailed view of the neutral gas distribution in the local neighbourhood of 3C 400.2 in the mentioned velocity range. The HI emission is displayed with a velocity resolution of 1.65 km s-1 using the same grayscale for the whole set of images. Again the 2, 6 and 10 mJy/beam contours of the radio continuum emission are superimposed.
From this figure, it can be observed that the overall HI distribution embraces most of the periphery of 3C 400.2, opening in the direction opposite to the galactic plane.
Comparison between HI emission distribution and radio continuum emission show several good correspondences:
(1) from v=+16.5 to +28 km s-1, the HI closely surrounds the NE, N and W borders of 3C 400.2. In order to remark all the interesting matchings, in Fig. 5a we display the HI integrated between +18 and +21 km s-1 in contourlines with 3C 400.2 in grayscale. A perfect fitting between radio features and surrounding gas is observed towards both, the NE and NW radio continuum maxima;
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Figure 5:
a) Overlay of the 1465 radio continuum image of
3C 400.2 as obtained by Dubner et al. (1994), with some
contours of the HI distribution integrated between +18 and +21 km s-1. b)
Overlay of the H![]() |
(2) at v= +26.4 and +28 km s-1
a hole in the HI distribution is
noticeable centered near
. This minimum coincides with the
interior of the "small NW shell" of 3C 400.2. Moreover, the SE wall of the
HI cavity exactly overlaps the SE border of that radio shell (see
particularly the image at v= +26.4 km s-1). This suggested association
between the HI features and the remnant, is reinforced by the fact that the
HI features also match the optical and the X-ray emission observed to be
associated with 3C 400.2. Figure 5b displays the H
emission (greyscale) as taken from Winkler et al. (1993)
with a few HI contourlines overlapped. It can be appreciated that most of
the H
filaments correspond with regions of higher compression in
the HI contours, and the center of the HI void coincides with the
geometrical center of the optical shell. Also, from the visual comparison
between the X-ray image of 3C 400.2 (see for example, Fig. 2 in
Saken et al. 1995) and the HI emission, particularly at
v=+28 km s-1, it is found that the HI closely surrounds the X-ray emission
along the eastern side; and
(3) from to +42 km s-1 the most interesting feature
is the band of neutral gas that exactly embraces the west and south rim of
the "large shell". This excellent
correspondence allows one to conclude that they are in physical contact.
Note that in this last velocity range the "small NW shell" is increasingly
covered by the HI cloud and only the "large shell" appears interacting with
the HI.
Based on the present HI results, we propose that 3C 400.2 is the
complex remnant resulting from a SN explosion occurring in the border of a
dense HI cloud. In the proposed scenario the "small NW shell" is
penetrating into the dense material, while the "large shell" is breaking
out into a lower density medium. Thus, the expanding
shock front pushed away neutral gas from the surroundings, transferring mechanical energy
to the environs.
We choose km s-1 as the systemic
velocity of the remnant
based on the fact that the HI void
is better seen at v= +26.4 and
+28 km s-1.
Then, all the associated HI features
seen between +14 and +27 km s-1 should represent approaching gas. In this
range we showed the existence of HI correlated with both the "small" and
the "large" shells. The receding gas is seen at velocities between
+27 and +42 km s-1, and correlates only with the "large shell"
(mainly towards the south).
Current estimates of the distance to 3C 400.2 based on the -D relationship,
range from 3.8 to 6.3 kpc (Clark & Caswell 1976;
Caswell & Lerche 1979; Milne 1979;
Dubner et al. 1994); however this method is known to have
large intrinsic dispersion. Rosado (1983) derived a distance
of 6.7 kpc from the kinematics of the optical filaments, but her estimate is
highly uncertain because it is based on interferograms obtained in a small
portion of the SNR. From our HI observations, adopting +27 km s-1 as the
systemic velocity of the SNR and using the galactic flat rotation curve by
Fich et al. (1989), (with
= 8.5 Kpc
and
= 220 km s-1), we obtain the kinematical distances of
2.3 kpc and
7.7 kpc. An uncertainty of
0.8 kpc
can be estimated assuming non-circular motions of about
7 km s-1
(Burton 1992). Since the last value would correspond to an
interarm region (Georgelin & Georgelin 1976), we adopt
the near kinematical distance as the most reliable for this SNR. At a
distance of 2.3
0.8 kpc the linear sizes of 3C 400.2 are 15
5 pc
for the "large shell" and 9.4
3 pc for the "small NW shell".
According to the above scenario, the supernova explosion took place near
,
and the shock front travelling
towards the NW and W directions was slowed down by the presence of the HI
cloud detected between +14 and +42 km s-1 forming the 9.4 pc "small NW
shell". The "large shell" expanded into a lower density medium towards the
SE.
To estimate the volumetric density of both the high and low density regions, we integrated
the HI emission in the (+14, +42) km s-1 velocity range and assumed that the gas is optically thin.
In a three-dimensional picture we adopted for the HI dense cloud a depth at least equal to the diameter
of the "large shell", since as it was shown above there are signs of physical interaction
between the "large shell" and the external cloud at +40 km s-1.
Using this model we obtained values of about 21 cm-3 and 4 cm-3 for the volume
atomic densities for the dense cloud and the lower density medium
respectively.
A total HI mass for the dense cloud of about 3000
was calculated by adding all the emission associated to 3C 400.2 for each channel between
+14 and +42 km s-1and after subtracting an appropriate background level for
each channel. This mass, which is very approximate includes gas swept up by the SN shock and probably
also ambient gas not yet reached by the blast wave. A rough estimation of the mass
swept up by 3C 400.2 can be derived by assuming that a 9.4 pc diameter shell (the
"small NW shell") expanded spherically into a uniformly distributed 21 cm-3 density ambient gas.
In this way, the "small shell" swept up about
of neutral
hydrogen. Concerning the "large shell", it apparently expanded to the SE into a
4 cm-3 density medium and partially to the NW into the 21 cm-3 medium.
Under these assumptions the "large shell" swept up roughly
.
The ambient density discontinuity which this remnant encounters will certainly affect the expansion velocity: the large shell is expected to attain larger velocities than the smaller one. This situation is in fact observed. We have shown in Fig. 4 the presence of HI features related to different portions of the "large shell" all along the velocity range, from +14 to +42 km s-1; whereas for the "small NW shell", we detected associated features only between +16 and +32 km s-1. From these evidences, we adopt as the expansion velocity for the "large shell" portion of the remnant, 14 km s-1, while for the "small shell", 8 km s-1. From the calculated masses and these partial expansion velocities, the kinetic energy transferred by 3C 400.2 to the surrounding neutral gas turns out to be about 1.2 1048 ergs. This is a small portion of the probable initial energy of the explosion; however, we have to bear in mind that a good deal of energy is being presently radiated in form of optical filaments and in thermal X-ray. Also the derived masses and velocities are lower limits because neither molecular nor ionized gas contribution were included, and the determination of the extreme velocities can be seriously limited by galactic gas confusion.
If, on the other hand, we consider that the high velocity HI clouds seen projected onto the
center of the remnant between -49.5 and -44.5 km s-1 and probably the
faint feature shown at +69 km s-1, have been accelerated by the expanding
shock front, then the expansion velocity turns out to be of the order of 70 km s-1. In this
case the kinetic energy would be roughly ergs.
Tenorio-Tagle et al. (1985) investigated the dynamical
evolution of SN explosions occurring near or inside molecular clouds. They
predict that the morphology of the SNR depend upon the distance of the
explosion site from the cloud boundary. The model which better fits the
morphology observed in 3C 400.2 corresponds to a SN exploding close to the
border of the cloud. In this case, two hemispherical remnants with
different radii are produced: the smaller one within the cloud and a
spherical large shell outside the cloud. Tenorio-Tagle et al.
(1985) predict that for a breakout SNR in the Sedov phase the
velocity ratio is related to the density ratio through the relationship
,where out refers to the portion of SN expanding outwards from the cloud,
and in points to the shock propagating inwards into the cloud. In the case
of 3C 400.2, there are regions that may have already become radiative as
shown by the presence of optical filaments. We can however assume that the
SNR is globally still in the adiabatic phase with cooling shock waves in
denser clumps producing the optical emission. In this case, for a density
contrast
5.2, the velocity of the shock
propagating into the cloud will be smaller by a factor of 1.8, than that of
the shock expanding outwards into the more diffuse ambient. This is in
very good agreement with the observed velocity contrast
1.75.
It has been suggested that a SNR expanding into a cloudy ISM is a plausible mechanism to produce centrally condensed X-ray morphology as observed in 3C 400.2 (White & Long 1991). Our HI observations have revealed that the ambient interstellar medium where the remnant evolves has indeed a clumpy structure; but the observed contrast in densities between the cloud and intercloud media is much smaller than that assumed in the theoretic calculations. On the other hand, in order to explain the observed X-ray luminosity in 3C 400.2, Long et al. (1991) found that the intercloud density should be 0.04 cm-3. Our HI observations have demonstrated that the density of atomic gas in the vicinity of the SNR is at least two orders of magnitude higher, and the model fitting to the X-ray observations should be revised. An alternative scenario to produce similar morphology in the X-ray band was developed by Shelton (See review by Jones et al. 1998). This model assumes saturated thermal conduction as a mechanism to transport energy outwards from the very hot center. This may well be the mechanism at work in 3C 400.2.
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