[13CI] Detection in Southern Star Forming Cores

Coordinator: J�rgen Stutzki, Stefan Heyminck and Rolf G�sten

We propose to use FLASH at 809 GHz (and 490 GHz in the second channel) to investigate the 13C0 abundance in [CI]-bright star forming cores. The superb transmission of the atmosphere and the favorite location of several prime sources in the sputhern sky, transiting almost overhead at the APEX site, give APEX/FLASH a previously unmatched sensitivity for these observations.

Program is available and data products can be downloaded

Scientific justification:

Due to the competing processes of stronger photo-dissociation of the (less-shielded) 13CO isotopomere and enhanced 13CO formation through the exchange reaction 13C+ + 12CO --> 12C+ + 13CO + 36 K, the 13C0 sensitively depends on the detailed physical conditions in the PDR transition, in particular the temperature of the C/CO transition layer in the photodissociation region. It thus allows to check the validity of standard PDR models. Recent large scale mapping of [CI] (Kamegai et al., 2003, ApJ 589, 378) has given new evidence, that C0 is abundant also outside PDRs, indicating a different formation mechanism during the temporal evolution of cloud and the time dependent chemistry. Given the specific fractionation mechanisms in PDRs as outlined above, the 13C0 abundance thus can give an important clue to such alternative formation pathways.
Up to now, the only [13CI] detection published is through the strongest hyperfine satellite in the 809 GHz [CI] 2-1 line in the Orion Bar (Keene et al. 1998, ApJ 494 , L107) and the two high mass star forming cores NGC 6334 A and G333.0-0.4 (and marginally in G351.6-1.3) with the large beam of theAST/RO telescope (Tieftrunk et al., 1998, A&A 375, L26; the latter sources were selected to be the brightest [CI] 2-1 809 GHz line sources from AST/RO). Detection of the second, 40% weaker hfs satellite, has been elusive. [13CI] detection through the hfs-satellites of the 492 GHz [CI] 1-0 line is possible only for very narrow line sources, as the stronger satellite is separated by only 2.2 km/s form the [12CI] line. This would for the first time probe the [CI] optical depth and hence allow a precise check on the C0 column densities derived for cold clouds.

Observations proposed
With FLASH's sensitivity of 200K at 492 GHz and 450 K at 809 GHz, 1 hour integration on close-to-zenith sources gives an rms in the 20 to 50 mK regime in a 1 MHz resolution element for a zenith opacity of 1 (likely to be reached for more than 50% of the (night-)time in July on Chajnantor.
We suggest to re-observe NGC 6334 and G333.0-0.4 with the 12"/resp. 8" 409 and 809 GHz APEX beams. A small map based on available mid-J CO maps will be used to determine the peak [CI] positions on which a deep integration of approx. 1 hour duration aims at detection of the [13CI] hfs satellites and should give the first detection of the elusive 3/2-1/2 and 3/2-3/2 hfs pair at approx. -80 km/s relative to [12CI], thus confirming the identification of the 5/2-3/2 hfs satellite at +60 km/s and given a better constraint on the [CI] optical depth and 13C0 abundance.
We also aim at the first detection of the [13CI] 1-0 transition by observing the peak [CI] positions in -Oph, where recent 492 GHz [CI] mapping by Kamegai et al. (2003, ApJ 589, 378) has identified 8 K bright and only 2 km/s wide lines, even with the 2.2 arcmin beam of he Mt.-Fuji telescope. Given the sensitivity as estimated above, the [13CI] satellite at 2.6 km/s offset should thus be identifiable as a weak shoulder on the edge of the [12CI] line (a high S/N isotopomeric 13CO line profile might be needed to get a clean intrinsic line-profile of the source).

Observing time estimate
Including overhead, the proposed observations would thus need about 2 hours per each of the for sources. Any subset of sources could be observed individually. The deep integrations proposed will give a very good test of the stability of the APEX/FLASH system.