Molecular cores and excitation temperature in the Eagle Nebula's fingers

Coordinator: F. Schuller, K. M. Menten

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Scientific justification:

The Eagle Nebula, M16, is a prominent Hii region located at a distanceof 2.2 kpc from the Sun. It is associated with the NGC 6611 starcluster, which contains several hundred intermediate to high mass starsthat formed over the last few million years. To the south of thiscluster, the radiation from hot cluster stars has sculpted columns ofdense obscuring material on a few arcmin scales, usually referred to as"fingers" (or, dramatically, "Pillars of God"), whose breath-taking HSTimages even entered popular culture (Hester et al. 1996).
Indications of present-day star formation near the tips of thefingers are seen e.g. in the infrared (Thompson et al. 2002), while thethree fingers have been mapped in molecular lines and submm continuumradiation byWhite et al. (1999) using the JCMT 15 m and OSO 20 m radiotelescopes.They report CO(2-1) and CO(3-2) peak brightness temperatures above 40 Kand 60 K, respectively, and a factor 2 - 3 lower temperature in theCO(1-0) line, which they interpreted as effects of depletion onto thecoldest phase of the dusty material (although that appears difficult tounderstand to us). The CO(2-1) transition was also observed by Andersenet al. (2004) using the SEST 15 m telescope. Their map cover an areafurther to the south, at the base of the pillars, in the vicinity ofthe Herbig-Haro object HH216.
The dust and molecular gas inside the columns are dense enough tobe opaque to optical emission, but may not have reached yet a densitysufficient to produce significant star formation. Here we propose tomap that region in the higher excitation lines CO(4-3) and CO(7-6) tobetter constrain its geometry, and to derive(using LVG modelling) temperatures and densities in the dense molecularcores. The improved spatial resolution provided by APEX at 807 GHz willallow us to precisely localise the warmer and denser regions of themolecular cores and to address the issue of cloud fragmentation. Inaddition, given that there is a boundary between the Hii region and thedense molecular gas, this source is probably a textbook example of aPhotodissociation Region (PDR). We would like to probe this PDR withthe 1-0 and 2-1 transitions of atomic carbon.

Proposed observations
We propose to perform rastermapping, covering 1'×1' in CO to map the tip of the northern mostfinger, and 1'×30" in [C I] lines at the top of this finger, toprobe the interface between the molecular gas and the Hii region.
Integration time

Assuming CO(4-3) is as bright as CO(2-1), and CO(7-6) twicefainter, i.e. peak at 20 K, detecting 10% peak at 3- requires an rms of0.7 K at 810 GHz. The FFTS backend with 16k channels provides avelocity resolution 23 m/s at 810 GHz, and can be smoothed to 0.5 km/s(20 channels) without any loss of structure according to the maps inWhite et al. (1999). Therefore an rms of 3 K per spectral element wouldgive a sufficient sensitivity. With a system temperature of 2500 K at810 GHz, an integration time of 20 s per point with standard rastermapping is well suited.
To map the proposed 3'×1' area with a sampling of half thebeam at 807 GHz (i.e. 4") would then require 16×16 points, or 1.4hours of integration, which translates to 4 hours with overheads.
Plume et al. (1999) observed four molecular complexes in [C I]3 P1-3 P0 and CO(2-1) lines. They report peaks in integrated lineintensities 5 to 15 times fainter in [C I] than in CO(2-1), withcomparable linewidths. Using a ratio of ten, we can expect a peakbrightness temperature of order 4 K in [C I]. Then, to reach 20% of thepeak at 3- requires an rms below 0.3 K. This rms can be reached bysmoothing to a resolution of 0.5 km/s so that an rms of 1.2 K perchannel will be enough. An integration time of 140 s per point willprovide this sensitivity at 809 GHz. Then, mapping 1 ×30 with 6sampling (11×6 points) takes 2.6 hours of integration, or 7.7hours with overheads. Thus, we are asking for a total of 12 hours ofobserving time to complete thisproject.

Possible extensions
This exploratory project can later be complemented by observing:
- a larger area (e.g. 6.5 ×4.5 ) covering the three fingersfrom their tips to their bases well inside the nebula in the same COlines
- the same area in [C I] lines
- dust emission in a larger ( half degree) region using the Laboca and 350 µm bolometer arrays


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White G.J., Nelson R.P., Holland W.S., et al., 1999, A&A 342, 233