Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...271....9r&link_type=abstract
Astronomy and Astrophysics, Vol.271, NO. 1/APRI, P. 9, 1993
Astronomy and Astrophysics
Astrophysics
121
Scientific paper
We discuss some of the results of observations of CO in the SW "Bar" region of the Small Magellanic Cloud (SMC), made with the SEST telescope at 45" resolution. These observations are reported in a companion paper (Rubio et al. 1992; Paper II). For two resolved CO clouds associated with H II regions comparable to the Orion nebula, the intensities in the four observed CO lines imply that they are clumpy with a surface filling factor lower than for Galactic clouds, and that their temperature is comparatively high. For a variety of CO structures with a wide range of radii R (10 to 200 pc), the ^12^CO (1-0) line width {DELTA}V is proportional to R^1/2^, as for Galactic molecular clouds and complexes. However, the relations between the CO luminosity L_CO_ and {DELTA}V or the virial mass M_vir_ are different in the SMC and in the Galaxy. At the smallest scales that we can resolve (R ~ 10 pc) the SMC structures are less luminous in CO than Galactic molecular structures of the same size by a factor of only a few (about 1.5 when compared to clouds in the outer Galaxy), while at large scales (R ~ 200 pc) they are much less luminous, by a factor of 10-20. This property as well as the properties of two clouds discussed in details, can be understood as resulting from a higher rate of photodissociation of CO in the SMC through the combined effects of a higher UV radiation field, of a lower dust/gas ratio and of a lower abundance of carbon. In the SMC, contrary to the Galaxy, CO molecules can only survive in dense clumps, while they are photodissociated in the diffuse medium and even in the interclump medium of individual clouds, if any. As a result, the largest interstellar entities in the SMC, which are dominated by diffuse gas, contain little CO: CO is found only in dense regions. Even these denser cores (the "molecular clouds') are permeated by UV radiation so that CO exists only in the densest clumps. Presumably most of the interstellar hydrogen in the SMC is atomic rather than molecular except in these dense regions. Assuming that the interstellar structures are in virial equilibrium at all observed scales, we derive a preliminary estimate for the calibration coefficient for determining the total column density of gas N(H_2_ + 2H) from the ^12^CO (1-0) line intensity I(CO). This coefficient X_SMC_ is larger than the canonical galactic value X_gal_ = 2.3 10^20^ mol cm^-2^ (K km s^-1^)' and depends on the scale R: X_SMC_ ~ 9 10^20^ (R/10 pc)^0.7^ mol cm^-2^(K km s^-1^)^-1^. This explains why it has been so difficult to detect CO in irregular galaxies at low linear resolutions, while its detection is easier at higher linear resolutions.
Boulanger Francois
Lequeux James
Rubio M'onica
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