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12CO and 13CO J=3D2-1 Maps of the Sh 254-258 Molecular Cloud =
with
the Heinrich Hertz Telescope
Keith
Schlottman, John Bieging,
Baltasar
Vila Vilaro, & Bill Peters
Steward
Observatory, The University of Arizona
=
Introduction
Sh 254 through =
258
are a chain of 5 H II regions in the Sharpless (1959 ApJS 4, 257) catalog, ranging fr=
om
large (11' diam.) and diffuse—Sh 254-- =
to moderately compact (~3') and
bright--Sh 255 and 257--to very compact (<1')--Sh 256 and 258. The distance to this group is abo=
ut 2
kpc.
=
Previous CO and=
13CO
J=3D1-0 maps (Carpenter, Snell & Schloerb 1995 ApJ 445, 246) suggest that the
expanding H II regions are shaping or eroding the associated molecular
cloud. Such dynamical effects
might be =
expected
to initiate additional star formation by compression of the =
molecular gas.
=
We present fully
sampled maps of the Sh 254-8 region in the J=3D2-1 transitions of 12CO
and 13CO. We are us=
ing
the data to derive cloud properties and to look for evidence of interaction between =
H II
regions and molecular clouds, including evidence for induced
("sequential") star formation.![](3D"slide0001_image003.jpg")
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Observations
--fully sampled maps with the He=
inrich
Hertz Submillimeter
Telescope (Mt. Graham, Arizona)
(elevation 3200 m; 10=
-m
diameter;
beam efficiency=
of
0.82 at 230 GHz)
--spectral resolution: 1 MHz or 1.3 km/s
--lines: 12CO J=3D2-1 (230.538 GHz) and
=
;
13CO J=3D2-1 (220.399 GHz)
--technique: "on-the-fly" (OTF) mapping
-9 subfields of 15' x 20'; approx. 4 hours to complete each =
=
subfield
-total area mapped:
0.75 x 1.0 degrees in 12CO
0.5 x 0.6 degrees in 13CO
(covering brightest 12CO emission)
-maps are in galactic coordinates
--map resolution: 32"; grid spacing 16", full
sampling
--telescope pointing checked eve=
ry 1-2
hours;
pointing errors <
4“
--calibration via standard
“chopper” method, and std. sources
--data reduction in CLASS/GILDAS and MIRIAD
![](3D"slide0001_image030.gif")
=
Results
12CO images (top center and right) show two velocity components:
--Sh
254-258 clouds =
show
bright peaks associated with the visible H II regions, and weaker irregular em=
ission
filling most of the mapped area.
There is a prominent, sharply delineated "bay" devoid of CO
emission to the right of the main CO peak.
A ridge of strong emission traces the boundary of this
"bay".
--Channel maps (far right) show that 13CO is detected over much less of the mapped field than =
12CO,
but at the main peaks 13CO is nearly comparable <=
/span>to 12CO in intensity. The 12CO channel maps reveal velocity
structure in the bright ridge and cloud edge bordering the CO-empty
"bay".
--"8
kpc clouds": The integrated map (top right) shows 3 well-defined peaks, and a faint trail of 12CO
extending up toward the galactic plane from the lower left corner. The compact size of these feature=
s is
consistent with the kinematic distance , and at this longitude, shows that
significantly bright CO clouds are present at ~16 kpc from the Galactic
Center.
![](3D"slide0001_image031.gif")
=
Comparison
with other images
Optical:
The overlay =
(left) of the POSS image with <=
span
style=3D'font-family:Arial;font-size:49%;position:relative;top:-.45em;
mso-text-raise:30%'>12CO
contours s=
hows
that the large diffuse Sh
254 coincides w=
ith
the void in CO, suggesting that the expanding H II region dissociated or pu=
shed
away =
the
molecular cloud. The bright =
Sh 257 is associated with the <=
/span>indentation in the CO contours, =
also
implying a dynamical interaction or UV dissociation.&nbs=
p;
The main CO peak is between the 2 bright H II regions, suggesting that compres=
sion
of the cloud has either heated the cloud or induced the formation of massive stars.<=
span
style=3D'mso-spacerun:yes'> The compact Sh 258 lies toward the end of a ridge =
of CO
extending from the central peak. &=
#13;Mid-IR:
Figures in lower left show an MSX-satellite
Band A (6-11 micron) image overlaid with CO contours of Sh 254-258 and
"8 kpc clouds". There
is a strong correlation between CO and mid-IR peaks, implying massive young stars are
embedded in the CO cores. No=
te that the brightest mid-IR peak
coincides with the CO peak, not the adjacent bright H II regions Sh 255 and 257. A ring of IR
emission s=
urrounds
the diffuse Sh 2=
54, perhaps outlining warm dust sw=
ept up
by =
the
expanding H II region. The=
LVG
Analysis
Our data sugges=
t the
presence of a very complex density and temperature structure in Sh254-258. As shown in the Figures below-rig=
ht,
most of the emission has [12CO(2-1)/13CO(2-1)] < 5. The 3 brightest CO(2-1) peaks in =
our map (all associated with =
MSX mid-IR sources), show very low =
(i.e.,
< 2) ratio values, but have steep ratio gradients with velocity,
suggesting the presence of several components.&=
nbsp;
We have selected ten 5x5 pixel regions centered at the numbered positions shown in the =
map at
top-center. We measured the
spatially =
averaged
peak (TA*) temperatures and integrated 12CO(2-1)/13CO(2-1) ratios As can be deduc=
ed
from the Figures (below right):
-All regions have average densit=
ies
> 2000 cm-3.
=
-The highest kinetic temperature=
s are
associated with infrared sources.
-Cooler regions (e.g.
positions 3,4,5) have n(H2 ) and N(CO) values similar to warmer cores, and so are likely =
sites
for future massive star formation.
Representative
properties are in the table below:
![](3D"slide0001_image033.jpg")
![](3D"slide0001_image034.gif")
POSS red plate (false colors)
12CO channel-average map of Sh 254-258 clouds
12CO channel-average map of “8 kpc clouds&=
#8221;
PO=
SS-red
overlaid with 12CO chan-avg. contours
MSX – band A (6-11 micron) maps overlaid with 12CO contours (chan. avg’d.)
Sh
254-258 clouds
“8 kpc clouds”
12CO channel maps, Sh 254-258 clouds TA* (K)
13CO channel maps
(same velocities) TA* (K)
Average of 4 channels
Channel Maps (central region)
T<=
/span>A* (K)
T<=
/span>A* (K)
Position T<=
span
style=3D'font-family:Arial;font-size:49%;position:relative;top:.37em;mso-t=
ext-raise:
-25%'>A* (K)
12/13 ratio Tk (K) log n(H<=
span
style=3D'font-family:Arial;font-size:49%;position:relative;top:.37em;mso-t=
ext-raise:
-25%'>2) N(CO)
=
;
1 28.8 2.2 >30 >3.6 >1.0e18
=
2 14.8=
3.3 &=
nbsp;
>20
>3.3
>5.0e17
3 7.0 2.4 &=
nbsp;
>10
>3.4
>5.5e17
5 5.9 4.9 &=
nbsp;
>20
>3.2
>3.5e17
8 14.4 2.4 &=
nbsp;
>20
>3.4
>6.5e17
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1
2
8
6
7
9
10
4<=
/span>
3<=
/span>
5<=
/span>
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12CO/13CO Ratio Maps