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Figure 2:
From a) to d): images of the H I data (interferometric
only) between LSR velocities ![]() ![]() ![]() ![]() ![]() |
Figure 2 (from (a) to (d)) displays in contour lines the H I distribution
within
the velocity interval
(-52, +30) km s-1,
the range over
which significative H I emission is detected. To produce this image we
have used the interferometric data (smoothed with a 5
FWHM Gaussian)
in order
to easily recognize the occurrence of small scale structure. All calculations
are, however, carried out using the combined (interferometric plus single dish) data.
Each H I line image was obtained by
averaging 5 consecutive channel images, thus spanning 4 km s-1.
The velocity
corresponding to the first channel is shown in the top right corner of
each image.
These contour plots, with isophotes ranging from 2.1 K to 21 K,
provide both a qualitative and
quantitative image of the H I distribution in an extended field around
SN 1006. The MOST image of the radio continuum emission of SN 1006 is
included in greyscale in all the images for comparison.
At high negative velocities (
km s-1) the
most conspicuous feature is the extended H I concentration located
near the S-SE border of SN 1006.
From
km s-1 to
km s-1,
the presence of an H I concentration projected onto the
center of the SNR is quite striking.
Between
km s-1 and
km s-1,
SN 1006 appears surrounded by smooth, tenuous gas, with a few
H I concentrations scattered over the field.
At
km s-1 an extended H I structure
encompass SN 1006 along its E, N and NW sides. Within this
structure, a bright H I concentration is observed towards the NW, in
the direction where the brightest optical filaments are detected. This
feature has two maxima centered
near (15
01
00
,
-41
45
)
and (15
02
30
,
-41
35
).
Part of this feature can still be detected at
km s-1.
At this last velocity, an H I feature delineates
the E-NE border of SN 1006.
At
km s-1, a bright H I concentration is detected to the SW of the SNR, with a maximum centered near
15
01
20
,
-42
10
.
The outer contour of this feature
mimics the shape of the SW limb of the radio remnant.
At
km s-1, this feature acquires an elongated
NS morphology.
At +5.8 km s-1 and at +9.9 km s-1, the maximum of this feature appears
adjoining the SNR outer border in coincidence with
a small indentation of the radio continuum.
At these velocities the presence of an H I concentration adjacent
to the flat NW
border of SN 1006, peaking near
15
02
,
-41
40
,
is also
apparent. Because
this is the direction where the bright Balmer filaments are detected,
we will analyze this feature despite the anomalous positive velocity.
At
km s-1 and +14 km s-1, a small H I
emission feature is observed projected onto the center of SN 1006.
From
km s-1 to
km s-1, the
only noticeable feature is the concentration detected to the SE,
which appears to overlap a small
portion of the SE corner of SN 1006.
In order to understand the peculiar characteristics of SN 1006 (e.g.
the bilateral appearance, the position of the brightest optical
filaments, and the reason why the TeV ray emission originates
only on the NE lobe) it is important to estimate the preshock
conditions towards different directions around SN 1006, looking
for inhomogeneities and/or anisotropies in the H I distribution.
A fundamental issue in establishing a physical association
between galactic H I and
SN 1006 is the determination of the systemic velocity for this SNR.
Several estimates for the distance to SN 1006 have been carried out
using different methods: d=1.4-2.1 kpc (Kirshner et al. 1987);
kpc (Willingale et al. 1996);
kpc (Schaefer 1996); d=2 kpc (Winkler & Long 1997);
kpc (Burleigh et al. 2000) and d=1.4 kpc (Allen et al.
2001). Collectively, these determinations suggest that the distance to
SN 1006 lies in the range 1.4-2 kpc.
The Galactic circular rotation model (Fich et al. 1989) suggests
a
between
-16 and
-25 km s-1 for this distances range, with possible uncertainties of
the order of
7 km s-1 because of peculiar or non-circular motions (Burton 1992).
However, one must be very cautious when applying circular rotation
models for sources located far from the Galactic plane, since these
models are only strictly valid for low Galactic latitudes. Moreover,
the warping of the Galactic plane towards negative latitudes in the
fourth Galactic quadrant (Burton 1976) increases the height above the
plane of an object located at a positive latitude. Here, we analyze
the H I information based on morphological comparisons among the H I radiocontinuum, X-rays and optical emission from SN 1006, irrespective
of the H I velocity.
As mentioned before, between
km s-1 and
km s-1, an H I concentration appears projected
onto the center of SN 1006. X-ray
observations indicate that the interior of SN 1006 is almost free of
neutral hydrogen, thus this H I feature is either in front or behind
SN 1006. Based on H I observations, there is no way to accurately
locate this feature along the line of sight, because the hollow center
of this SNR precludes the use of absorption tests. If
this H I cloud lies behind the SNR, an upper limit of
-20 km s-1
can be derived for the systemic velocity of SN 1006, thus suggesting
an upper limit of 1.7 kpc for the distance to the remnant. Within the
large uncertainties involved in the models, this limit for the distance
is compatible with the average of the
various distance estimates for SN 1006, as summarized above. If, on
the other hand, the
central H I feature would be localized in front of SN 1006, then the systemic
velocity of the SNR would be any value more negative than
-31 km s-1, more difficult to reconcile with the distance estimated for SN 1006 based on independent methods. We thus conclude that adopting
km s-1 and
kpc for the possible
systemic velocity and distance to SN 1006, respectively, is a plausible
hypothesis.
To investigate the average H I density in the environs
of SN 1006, we have calculated the atomic density
in different directions around the remnant and in each
velocity plane within the interval
km s-1,
assuming that the systemic velocity of SN 1006 lies somewhere
within this range, as suggested by the present and previous results.
An average density of
n0 = 0.5 d-1 cm-3 is obtained, where
d is the distance in kpc. If
kpc, then
cm-3. To carry out these calculations we have assumed that the gas
is optically
thin, a good approximation for gas at
,
pc. The determined value is an upper limit since
contribution from unrelated distant
gas with the same kinematical velocities may be included due to the
distance ambiguity in this direction of the Galaxy.
The features to the north-west:
As discussed above, the flat NW border of SN 1006 is faint in
radio continuum and in X-rays, but is delimited by sharp, bright optical
filaments with strong Balmer lines (Long et al. 1988;
Smith et al. 1991; Winkler & Long
1997). For these H
filaments
proper motions of the order of 0.30§ yr-1 (Long et al. 1988) and
shock velocities within the range 2200-3500 km s-1 (Smith et al. 1991),
have been estimated. Winkler & Long (1997) also detected fainter
emission 2
beyond the bright filaments to the NW.
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Figure 3:
a) Comparison of the H I distribution (contours) at
![]() ![]() |
Balmer-dominated filaments are characterized by a spectrum of Balmer lines with little or none of the characteristic forbidden-line emission of radiative filaments (such as [SII], [NII] or [OIII]). Chevalier & Raymond (1978) explained the nature of this spectrum as due to a collisionless shock moving into partially neutral interstellar material. Winkler & Long (1997) conclude that the bright optical filaments observed in SN 1006 are, in fact, thin sheets observed nearly edge-on, which originate where the shock encounters a "wall'' with density of about 1 cm-3 (half of which would be ionized), oriented nearly parallel to the line of sight. The fainter filaments are formed where the SN shell encounters a lower density region. The fact that the northwest portion of SN 1006 appears flattened and expands less rapidly than elsewhere (Moffett et al. 1993; Winkler & Long 1997) is consistent with the existence of denser preshock material in this direction.
We can model a neutral "wall'' with the geometry and density
suggested by Winkler & Long (1997): a narrow sheet of gas with
a width of 2,
comparable to the separation between bright and
faint filaments (about 1 pc at the adopted distance of 1.7 kpc),
a length of 15
,
which is the angular size of the bright optical
filaments (
7.5 pc at
kpc), and a depth
of 30
along the line of sight, similar to the diameter of SN 1006
(
15 pc at
kpc), with a neutral gas density of
0.5 cm-3. This neutral
hydrogen feature
has a column density of
cm-2, and the correspondent brightness temperature would be
15 K. Therefore, the present
H I images should reveal the presence of the predicted
feature, even taking into
account the possibility that the signal may suffer from beam dilution
and/or is spread over several channels.
From an inspection of the entire observed H I cube (from -250 to
+400 km s-1), we conclude that H I emission enhancements towards
the NW occur only from
km s-1 to
km s-1 and from
km s-1 to
km s-1 (Figs. 2b and 2c). One of these two
concentrations must be responsible for the particular formation of
bright optical filaments exclusively on this side of the SNR. In
Fig. 3 we display
the H I distribution overlaid on the radio continuum and
the optical emission (towards the NW) at
these velocity ranges (Figs. 3a and 3b respectively).
From the morphological point of view, the best agreement is found near
+9.9 km s-1, where there is an excellent correspondence between the H I
and the optical emission. The major problem with this
association is the discrepancy between this velocity of
+10 km s-1
and the probable systemic velocity of SN 1006 of
-20 km s-1.
Colomb & Dubner (1982) have shown the existence of two concentric
expanding H I shells of swept up interstellar matter around the
SNR Lupus Loop (G330.0+15.0). Therefore, in spite of the good morphological
agreement, it is possible that the observed features near +9.9 km s-1
are part of the slowly expanding external shell around Lupus Loop and
are located at
300-500 pc.
On the other hand, for the NW feature observed near -5 km s-1,
the existence of a second coincidence between the shape of the H I features
and the radio continuum emission reinforces
the hypothesis of association between the H I and the SNR.
In effect, near 15
03
30
,
-41
45
,
an H I concentration
is seen apparently adjacent to SN 1006 at the position where the radio shell is
broken and appears to branch off to the interior of the SNR. Of course,
the LSR velocity of this H I concentration is still anomalous if the
systemic velocity of SN 1006 is close to -20 km s-1, and some
kinematical perturbation has to be assumed.
If there is a physical link between the NW cloud and the SNR, then
an atomic
density n = 0.8 d-1 cm-3 is derived (n = 0.5 cm-3 at
a distance of
1.7 kpc).
In order to calculate this density we have assumed for the elongated
feature a dimension along the line of sight equal to the diameter of
the SNR, and integrated the brightness
temperature between -7 and -4 km s-1. This value is in good
agreement with the density proposed by Winkler & Long (1997)
for the neutral component of the "wall'' based on shock model
fitting to the
optical and X-ray data. The origin of the anomalous kinematical
velocity remains an unsolved problem.
Unfortunately, there is little kinematic evidence to support the physical association of the H I with the SNR; projection effects and chance alignments cannot be ruled out. Thus, a quantitative analysis of the kinematical effects of this young SNR on the surrounding ISM, is ruled out.
In Fig. 4 we show the spatial distribution of the ,
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