13CO(J=1-0) On-the-fly Mapping of the Giant HII Region NGC 604:
  Variation in Molecular Gas Density and Temperature due to Sequential Star
  Formation
  We present 13CO(J=1-0) line emission observations with the Nobeyama 45-m
telescope toward the giant HII region NGC 604 in the spiral galaxy M 33. We
detected 13CO(J=1-0) line emission in 3 major giant molecular clouds (GMCs)
labeled as GMC-A, B, and C beginning at the north. We derived two line
intensity ratios, 13CO(J=1-0)/12CO(J =1-0), R13/12, and 12CO(J=3-2)/12CO(J
=1-0), R31, for each GMC at an angular resolution of 25" (100 pc). Averaged
values of R13/12 and R31 are 0.06 and 0.31 within the whole GMC-A, 0.11 and
0.67 within the whole GMC-B, and 0.05 and 0.36 within the whole GMC-C,
respectively. In addition, we obtained R13/12=0.09\pm0.02 and R31=0.76\pm0.06
at the 12CO(J=1-0) peak position of the GMC-B. Under the Large Velocity
Gradient approximation, we determined gas density of 2.8 \times10^3 cm^-3 and
kinetic temperature of 33+9-5 K at the 12CO(J=1-0) peak position of the GMC-B.
Moreover, we determined 2.5 \times10^3 cm^-3 and 25\pm2 K as averaged values
within the whole GMC-B. We concluded that dense molecular gas is formed
everywhere in the GMC-B because derived gas density not only at the peak
position of the GMC but also averaged over the whole GMC exceeds 10^3 cm^-3. On
the other hand, kinetic temperature averaged over the whole GM-B, 25 K, is
significantly lower than that at the peak position, 33 K. This is because HII
regions are lopsided to the northern part of the GMC-B, thus OB stars can heat
only the northern part, including the 12CO(J=1-0) peak position, of this GMC.
