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3 Results

3.1 HI in the environs of 3C 400.2

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 $\simeq$ (-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 $\simeq + 45$ km s-1  are forbidden; however, as it is clear from this figure, strong HI emission is still detected up to $\simeq +70$ km s-1. Emission at negative velocities should correspond to very distant gas, beyond the Solar Circle ($d \ge$ 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.

\includegraphics [width=17.5cm]{001sl.eps}
 \end{figure} Figure 3: Grayscale images of 3C 400.2 in the neutral hydrogen $\lambda$21 cm line in the velocity range v=-54.4 to +74.2 km s-1. The grayscale goes linearly from -10 to +15 K. Each image is an average of three channels. The central velocity is indicated in the upper left corner. Superimposed on the HI images are the 2, 6 and 10 mJy/beam contours of the radio continuum emission of 3C 400.2 at 1465 MHz as taken from Dubner et al. (1994)

\includegraphics [width=17.5cm]{002sl.eps}

 \end{figure} Figure 3: continued

\includegraphics [width=17.5cm]{003sl.eps}

 \end{figure} Figure 3: continued
\includegraphics [width=17.5cm]{004sl.eps}

\vspace{1cm} \end{figure} Figure 4: Grayscale images of 3C 400.2 in the neutral hydrogen $\lambda$21 cm line in the velocity range v=+14.8 to +41.2 km s-1 each 1.65 km s-1. The grayscale goes linearly from 0 to 20 K. The central velocity is indicated in the upper right corner. The 1465 MHz continuum image has been included on every image

\includegraphics [width=17.5cm]{005sl.eps}

 \end{figure} Figure 4: continued
Each HI line map was obtained by averaging 3 individual maps, producing images with a velocity resolution of $\sim$5.0 km s-1, and convolved to a beam size of 3 arcmin. Superimposed on the HI images are the 2, 6 and 10 mJy/beam contours of the radio continuum emission of 3C 400.2 at 1465 MHz as taken from Dubner et al. (1994). The central velocity of each map is indicated in the upper left corner. The same grayscale is used for the whole velocity interval to help a quick visual identification of maxima and minima in the emission along the line of sight.

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 $\rm 19^{h} 39^{m} 30^{s},
+17^{\circ} 20'$ (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 $v \simeq +50$ 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;

\includegraphics [height=16.3cm]{ds7671f5.eps}
 \end{figure} 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$\alpha$ +[NII] CCD image of 3C 400.2, as obtained by Winkler et al. (1993), with some contours of the HI distribution at v=+26.4 km s-1

(2) at v= +26.4 and +28 km s-1  a hole in the HI distribution is noticeable centered near $\rm 19^{h} 38^{m} 26^{s}, +17^{\circ} 15'$. 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$_{\alpha}$emission (greyscale) as taken from Winkler et al. (1993) with a few HI contourlines overlapped. It can be appreciated that most of the H$_{\alpha}$ 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 $v\simeq + 31$ 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 $v \simeq +27$ 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).

3.2 The distance to 3C 400.2

Current estimates of the distance to 3C 400.2 based on the $\Sigma$ -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 $R_{\odot}$ = 8.5 Kpc and $V_{\odot}$ = 220 km s-1), we obtain the kinematical distances of $\simeq$ 2.3 kpc and $\simeq$ 7.7 kpc. An uncertainty of $\simeq$ 0.8 kpc can be estimated assuming non-circular motions of about $\pm$ 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 $\pm$ 0.8 kpc the linear sizes of 3C 400.2 are 15 $\pm$ 5 pc for the "large shell" and 9.4 $\pm$ 3 pc for the "small NW shell".

3.3 Physical parameters

According to the above scenario, the supernova explosion took place near $\rm 19^{h}38\hbox{$.\!\!^{\rm m}$}4$, $17^{\circ} 15'$ 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 $v \simeq$ +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 $M_{\odot}$ 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 $220~M_{\odot}$ 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 $540~M_{\odot}$.

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 $4 \ 10^{49}$ 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 $v_{\rm out}/v_{\rm in}= 4.22 -1.89(\log (n_{\rm out}/n_{\rm in}) +2)$,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 $n_{\rm in}/n_{\rm out} \simeq$ 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 $v_{\rm out}/v_{\rm
in} \simeq$ 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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