) line in the survey
Because of the large bandwidth of the AOS, covering 
, the
transition of isocyanic acid (HNCO) at
109.905573 GHz (Lovas 1992), which corresponds to a velocity
shift of -337.5 with respect to the 
line
frequency, fell into the range of the spectrometer as long as the
of the emission was not very negative.
HNCO is a molecule with a slight asymmetry caused by the protrusion of
the hydrogen atom and it has an unusual energy structure: The
K-1>0 ladders of rotational levels lie tens of degrees K above the
corresponding levels in the K-1=0 ladder. Hence, nearly all HNCO
should be in states of the K-1=0 ladder if the molecule is
collisionally excited. In this case, the two level model of
Jackson et al. (1984) indicates that the H2 density has to
be at least 
and is typically 
. Thus,
if excited collisionally HNCO is a very high density tracer. On the other
hand, the lowest K-1=1 levels are separated from the lowest
K-1=0 levels by energies which correspond to far-infrared (FIR)
wavelengths. Thus, a significant population in the K-1>0 levels is
a clear indication of radiative instead of collisional excitation. See
Winnewisser et al. (1976) for a detailed study of the
molecular parameters of HNCO.
HNCO was first detected in SgrB2 (Snyder & Buhl 1972).
Subsequent observations in the Galactic disk (Jackson et al.
1984) show the transition to be distinctly
weaker than the line. The abundance found for disk clouds
roughly agrees with chemical models (Iglesias 1977;
Huntress & Mitchell 1979), whereas the emission in SgrA
(Armstrong & Barrett 1985) and SgrB2
(Churchwell et al. 1986; Cummins et al. 1986)
is more than an order of magnitude stronger than expected. At least in
SgrB2, HNCO seems to be mainly excited by FIR radiation.
Throughout the Galactic center, there is, however, a large variation in
the 
ratio.
From a sample of 34 clouds in the inner 500 pc of the Galactic center
region, Hüttemeister (1993), Chapter 5, found both low
values, < 0.2, for this intensity ratio, similar to those found for the
Galactic Disk by Jackson et al. (1984), and high values of 3
to 5, as found in SgrA by Jackson et al. In addition,
Hüttemeister (1993) could not find any correlation between
clouds with strong HNCO emission and IRAS point sources which might be a
possible indicator for the presence of strong (local) FIR radiation.
For the full set of survey spectra refer to the presentation of the
18 spectra in Fig. 4 (click here) of
Sect. 4 (click here). These spectra includes the complete
bandwidth of the backend so that the 
)
line is also visible (with the mentioned shift in the velocity scale of
-337.5 ).
In Fig. 10 (click here), the integrated intensity is plotted as a
contour map, covering the velocity range from -37.5 to +137.5 .
This is the complete emission range of the
) line which is covered by the spectra.
Toward negative velocities this is limited by the edge of the
spectrometer. Toward positive velocities there is no confusion with the
18 emission, as can be seen in the lv-plots, in particular in
the latitude cuts, of (see
Figs. 7 (click here) and 8 (click here)) where no
) emission appears at velocities higher
than +137.5 (which corresponds to -200 in 
.
Apart from the mentioned differences, Fig. 10 (click here) is
produced in the same way as Fig. 5 (click here).
Figure 10: The integrated intensity of the Galactic center region in
). The velocity over which the
intensity is integrated ranges from -37.5 to +137.5 . The solid
contour levels are 2.4, 5.0, 7.5, and from 10.0 to 42.0 in steps of
4.0 K where the lowest level is the 3
-value. The dashed
contour is at 1.6 K which is the 
-value. The circle in
the lower left corner of the plot indicates the beam size of
9
2
Most notably, the ) emission is much
more restricted to the Galactic plane than the
emission. In general, it appears abruptly at b = -0
15 but is
not visible at more negative latitudes. Similarly, it disappears
abruptly at latitudes more positive than b = 00; only in the
area of SgrD some emission appears at higher latitudes. The strongest
source by far is SgrB2. In relation to the other sources, SgrB2 is
even more prominent in HNCO than in 
. It is not clear to what
degree systematic effects at the edge of the spectrometer hide emission
features. Therefore, even though no emission at negative velocities was
found, some emission might be present. This could be the reason for the
seemingly complete absence of SgrC and could also have weakened the
emission of SgrA. In addition, this could, at least partly, be the
reason for the smaller b-extension of HNCO; as one can see from
Fig. 6 (click here), the b-extension of
at negative velocities is larger than at positive velocities. Similar
to , SgrD (the l=15-complex of Bally
et al. 1988) peaks at about (l=10, b=00) and has an
arc-like extension to (l=135, b=+04). In the area of
l=15 to 185 and b=-025 to +01, there appears a
fairly strong emission feature which peaks at (l=165, b=00).
This feature -- hereafter called HNCO-1.65-0.0 -- is rather
interesting because it has only a very weak and barely detected
counterpart in peaking at (l=175, b=-015).
Especially at the peak position of the HNCO emission, no is
visible.
Figure 11: The integrated intensity of the Galactic center region, in
), in velocity intervals of 50
width. The solid contour levels start at 1.3 K, which is the
-level, continue with 2.7, 4.2, 6.0, and 9.0 K and
then increase in steps of 5.0 K. The dashed contour is at
0.8667 K which is the -value. The circle in the lower
left corner of the plots indicates the beam size of 92. In the
top panel the integrated intensity of the velocity range from -25.0
to +25.0 is plotted, in the middle panel the velocity ranges
from +25.0 to +75.0 , and at the bottom the velocity range from
+75 to +125.0 is displayed
Figure 12: Longitude-velocity plots showing the velocity distribution of
the ) emission. a) The velocity
distribution in the latitude range from -030 to +030
which covers the complete HNCO emission area. The solid contour levels
range from 0.018, 0.04 to 0.22 in steps of 0.03 Karcdeg where the
lowest level is the -value. The dashed contour is at
0.012 Karcdeg which is the -value. b) The velocity
distribution in the latitude range from -015 to +015
which covers the major HNCO emission area. The solid contour levels
range from 0.0141, 0.03 to 0.205 in steps of 0.025 Karcdeg where the
lowest level is the -value. The dashed contour is at
0.0094 Karcdeg which is the -value
In Fig. 11 (click here), the integrated intensity of the
[4] ) line is plotted in
velocity intervals of 50 width. Because of the limitation in
velocity due to the edge of the spectrometer band toward negative
velocities and due to the lack of emission at higher positive
velocities only the channels of 0 , +50 , and +100 are
plotted. Apart from that, the plots are produced as in
Fig. 6 (click here).
Figure 13: Longitude-velocity plots of cuts at single b showing the
(smoothed) spectra of the ) emission in
a contour map representation. The solid contour levels start at
0.0081 Karcdeg (corresponding to 0.054 K), which is the
-level, continue with 0.017 and 0.028 Karcdeg, and then
increase from 0.04 Karcdeg in steps of 0.02 Karcdeg. The dashed
contour is at 0.0054 Karcdeg (corresponding to 0.036 K) which is the
-value. a) In the upper left panel, the lv-plot at b
= -015 is shown. b) In the upper right panel, the
lv-plot at b = 00 is shown. c) In the lower left
panel, the lv-plot at b = +015 is shown. d) In the
lower right panel, the lv-plot at b = +030 is shown
At 0 , only little HNCO is visible, in contrast to C18O which
is widespread and uniformly distributed at this velocity. However,
distinct but limited emission regions are visible at SgrA
(l=00, b=00) and at (l=075, b=-015)
somewhat east of SgrB2. The latter region has an extension to
(l=05, b=00). These regions coincide with the strongest
emission regions in C18O. Because of the reasons mentioned above,
the emission from SgrC and a larger b-extension might be
systematically suppressed. However, this cannot explain the more
limited extension in l of the HNCO emission at this velocity, because
the emission at higher positive longitudes tends to have positive
velocities. Thus, this point supports the idea that HNCO is more
restricted to limited areas than . Emission related to Clump 2
seems to appear in some isolated spots, but it seems possible that
these features which are just barely rising above the 3-level
are introduced by effects caused by the edge of the backend.
At 50 , SgrB2 is, as in , by far the most prominent
emission feature. It peaks at (l=07, b=-01) which is
somewhat (but not significantly) further to the southeast than in the
respective plot (Fig. 6 (click here)). The integrated
intensity of more than 30 K at this peak position is twice as
high as in the plot. The second strongest source is
HNCO-1.65-0.0 which appears only within this velocity interval. As in
the plot, SgrA and D are present with clear emission
signatures, but in comparison to SgrB2, they seem rather weak. In
addition, the eastern part of SgrD is missing due to an emission free
valley at l=12. SgrC is not visible, but weak emission could
be hidden due to effects caused by the edge of the backend. In contrast
to , Clump 2 is not visible in HNCO.
At 100 , SgrD with the arc-like extension (l=15-complex)
to (l=135, b=+04) is, as in , the dominant
feature. The shape and extension are nearly the same as in ,
only the extension toward SgrB2 is absent. Also as in ,
emission is visible slightly north of SgrA. In contrast to ,
Clump 2 is not visible in HNCO.
In Fig. 12 (click here), we have plotted the intensity integrated
over two latitude ranges showing the velocity distribution of the
) emission. In part a) of the figure,
the latitude ranges from -030 to +030 covering the
complete b-extension of the HNCO emission. In part b) of the figure,
the latitude integral from -015 to +015, which covers the
major HNCO emission area, is shown.
In general, both plots show the same features. Because the HNCO
emission is concentrated toward the plane, the emission features are
more prominent in plot b). There is no emission near = 0 .
The 3-kpc-arm (Rougoor & Oort 1960) is beyond the edge of
the spectrometer band. The diagonal chain of emission centers which is very
prominent in is only represented by SgrA, SgrB2, and
SgrD. In HNCO, this feature resembles a spiral, starting in the weak
emission extending from SgrA to (
05, 
), continuing through SgrA itself with a peak at (
15, 
), turning toward positive longitudes
and velocities for SgrB2 (7, 
), to SgrD (
0, 
),
and turning back toward lower velocities for HNCO-1.65-0.0 at
(65, 
). The lines of weak
emission which appear at -20 , +40 , and +75 in the
longitude range from 225 to 350 (Clump 2 area) are dubious
because these are visible neither in the contour plot of the total
integrated intensity (Fig. 10 (click here)) nor in the channel maps
(Fig. 11 (click here)). In addition, such streamers of emission at
these velocities are not found for any other molecular line.
In Fig. 13 (click here), we show the intensity at selected
latitudes, analogous to Fig. 6 (click here). The latitude cuts
range from b = -015 to b = +030 covering the most
interesting features.
At b = -015, the emission features show similarity to the
features integrated over latitude in Fig. 12 (click here) but some
distinct deviations are visible. SgrB2 appears as a rift-like
ellipsoidal maximum at l=+075, with v ranging from +30
to +50 . There is no connection between SgrB2 and the SgrA
features. The maximum of SgrD is shifted to (9,
), and the connection to HNCO-1.65-0.0 is rather
diffuse and only barely tracing a spiral.
At b = 00, the emission features are very close to what is
visible in Fig. 12 (click here) but more prominent. Especially the
spiral structure is more emphasized, and it is clear from this plot
that the SgrA emission at (15, ) is not part of the spiral-like structure in the lv-space.
At b = +015, nearly all )
emission has disappeared. Only in the area near SgrD (the
1.5-complex) there is some weak but widespread emission present. In
addition, SgrB2 is marginally visible.
At b = +030, almost no HNCO emission is present. It is not
clear if the extended line of emission at l=+135, ranging in
v from +50 to +120 , is real but there is no obvious
reason to doubt it. In addition, a weak counterpart in
exists.