THE 13CO-AV RELATION AT HIGH EXTINCTIONS - THE PARA-OPHIUCHI COMPLEX

We have carried out a study of the 13CO-/AV relation at high visual extinctions in a 40′ × 40′ section of the ρ Oph molecular cloud complex. The molecular data consist of LTE column density maps based on J = 1 → 0 CO and 13CO observations made with the 14 m FCRAO radio telescope. The optical data are derived from star counts at λ ∼ 8000 Å made from deep, wide-field photographic plates obtained with the Cerro Tololo 4 m telescope. By establishing our own calibrated stellar surface density curves, we avoid many of the difficulties associated with using star counts to determine very high extinctions. LTE 13CO column density is found to be well correlated with visual extinction on spatial scales of ∼3′ in the range 0 ≤ AV ≤ 10 mag. A least-squares fit gives the result N13fit= (2.16 ± 0.12)AV - (3.06 ± 0.90), where N13fit is in units of 1015 cm-2. The slope of this relation is consistent with our own and other previous work except in the case of the Taurus clouds, where a significantly lower slope has been found. The nonzero intercept in the relation above indicates that there is an extinction threshold of 1.4 mag in the ρ Oph clouds which must be exceeded in order for there to be observable 13CO emission. We argue that the threshold is produced by a diffuse, foreground layer of the main Ophiuchus cloud, in which densities are too low, and photodissociation processes too strong to permit significant carbon monoxide formation. As such, the molecular mass of the ρ Oph clouds should be well established by simply using the slope of the relation above and ignoring the offset entirely. However, envelopes considerably denser than that of the Oph cloud are likely to be common and precautions should therefore be taken to account for their potential contribution to molecular cloud masses.