No Slide Title

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.

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

= Results

12CO images (top center and right) show two velocity components: ~8 km/s associated with Sh 254-258; and = ~25 km/s, which implies a kinematic distance of 8 kpc for a flat rotation curve.

--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.

= 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 "8 kpc clouds" also ha= ve mid-IR counterparts, consistent with embedded luminous stars (at 16 kpc galactic radius).<= /span>

= 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 (see table below). We also obtained corresponding va= lues for 12CO(1-0)/13CO(1-0) from Carpenter et al (1995).<= span style=3D'mso-spacerun:yes'> Data are plotted in the figure to= the right. = Large Velocity Gradient single-cloud models have been produced for kinetic = temperatures Tk =3D 10, 20, 40, 60, 100 K, hydrogen molecule densities log(n)=3D1.0-7.0, Z(CO)/(dv/dr)=3D1.e-4 (standard CO/H2 abundances) and [12CO]/[13CO]=3D80 (since Rgal =3D 10 kpc for the Sh 254-8 cloud)= .

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:

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)

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

4<= /span>

3<= /span>

5<= /span>

12CO/13CO Ratio Maps

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