APEX submillimeter-wave line studies of NGC6334I

Coordinator: S. Thorwirth, C. Comito, S. Leurini and P. Schilke

Program is available and data products can be downloaded

Scientific justification:

At a distance of 1.7 kpc, the NGC6334 giant molecular cloud is an 11-pclong filament containing a remarkable chain of five luminousstar-forming regions (NGC6334I to V). Among these, NGC6334I is ofparticular interest because it has proven to be extremely rich inmolecular line emission (e.g. Thorwirth et al., 2003),comparable in chemical complexity to high-mass star-forming regionssuch as Orion-KL and SgrB2. Moreover, line widths in NGC6334I aresignificantly smaller compared to, e.g., the prototypicalstar-formingregion Sgr B2 - 6 vs. 10-15 km s-1 - making accidental lineblending a much less severe issue. This is shown strikingly in Figure 1that displays part of an unbiased millimeter-wave survey of NGC6334Itaken with the SEST (Thorwirth et al., 2003) against the same spectrumtaken towards SgrB2(N).
As a consequence of these narrow lines, many more (especiallyweaker) spectroscopic features can be resolved in NGC6334I making thissource a very good target for studies of the chemistry associated withhigh-mass star-formation. Here we propose, as part of the APEX scienceverification program and as a pathfinder experiment for futurehigh-frequency spectroscopic studies with APEX and the HERSCHELmission, to employ the FLASH-460/810 receiver combination in order torecord line emission in selected bands at 350 and 600 microns. The datawill also be very valuable in comparison to the recently reported CSOsurvey of Orion-KL at 350 microns by Comito et al. (2005) whichrepresents the only systematic line study at these high frequenciesreported so far.

Observational Setup and Time Estimate
We propose toobserve selected line-rich bands at submillimeter wavelengths. First ofall, we plan to cover a range of a few (2-3) GHz around 820 GHz. Weknow from Comito et al. (2005) that this band displays a large numberof transitions from simple and complex molecules, such as C2 H5 CN, SO2, C2 H5 OH, C34 S, andmethanol (CH3 OH). Recently, Leurini et al. (2004) have investigatedthe potential of CH3 OH as tracer of physical conditions in starforming regions; their main result is that while at millimeterwavelengths methanol transitions are mainly density probes, in thesubmillimeter regime, where high J bands of transitions with a broadexcitation range are found, the information on kinetic temperature canbe also recovered. Moreover, the torsionally excited transitions arepure temperature tracers as, with very high critical densities (1010-1011 cm-3 ) and high level energies (T 300 K), they are hardlypopulated by collisions, but trace the IR field instead. The 17k 16k vt= 0, 1 CH3 OH bands around 820 GHz are therefore an ideal tool toinvestigate the physical conditions of the hot, dense gas in starforming regions: by covering such a broad excitation range (Elow 200 K- 1050 K), they will allow independent determinations of both thespatial density and the kinetic temperature in the gas phase.
At the same time, we will observe the frequency band around 495GHz, which is expected to show a plethora of lines from CH_3OH, SO_2 ,C_2H_3CN, HCOOCH_3 (cf. White et al., 2003, for a corresponding surveyof Orion-KL). Moreover, observation of the J = 27 - 26 transition ofCH_3CN at 496 GHz will allow an independent measurement of the kinetictemperature that will be compared against the value obtained forCH_3OH.
In total, we request 12 hours of observing time in the LST interval from 10h to 22h.

Comito, C., Schilke, P., Phillips, T. G., Lis, D. C., Motte, F., & Mehringer, D. 2005, Astrophys. J. Suppl. S., 156, 127
Leurini, S., Schilke, P., Menten, K. M., Flower, D. R., Pottage, J. T., & Xu, L.-H. 2004, Astron. Astrophys., 422, 573
Thorwirth, S., Winnewisser, G., Megeath, S. T., & Tieftrunk, A.R. 2003, in ASP Conf. Ser. 287: Galactic Star Formation Across theStellar Mass Spectrum, 257­260
White, G. J., Araki, M., Greaves, J. S., Ohishi, M., & Higginbottom, N. S. 2003, Astron. Astrophys., 407, 589