APEX SV projects

APEX submillimeter-wave line studies of NGC6334I

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

Data:
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-pc long filament containing a remarkable chain of five luminous star-forming regions (NGC6334I to V). Among these, NGC6334I is of particular interest because it has proven to be extremely rich in molecular line emission (e.g. Thorwirth et al., 2003), comparable in chemical complexity to high-mass star-forming regions such as Orion-KL and SgrB2. Moreover, line widths in NGC6334I are significantly smaller compared to, e.g., the prototypical star-formingregion Sgr B2 - 6 vs. 10-15 km s-1 - making accidental line blending a much less severe issue. This is shown strikingly in Figure 1 that displays part of an unbiased millimeter-wave survey of NGC6334I taken with the SEST (Thorwirth et al., 2003) against the same spectrum taken towards SgrB2(N).
As a consequence of these narrow lines, many more (especially weaker) spectroscopic features can be resolved in NGC6334I making this source a very good target for studies of the chemistry associated with high-mass star-formation. Here we propose, as part of the APEX science verification program and as a pathfinder experiment for future high-frequency spectroscopic studies with APEX and the HERSCHEL mission, to employ the FLASH-460/810 receiver combination in order to record line emission in selected bands at 350 and 600 microns. The data will also be very valuable in comparison to the recently reported CSO survey of Orion-KL at 350 microns by Comito et al. (2005) which represents the only systematic line study at these high frequencies reported so far.

Observational Setup and Time Estimate
We propose to observe selected line-rich bands at submillimeter wavelengths. First of all, we plan to cover a range of a few (2-3) GHz around 820 GHz. We know from Comito et al. (2005) that this band displays a large number of transitions from simple and complex molecules, such as C2 H5 CN, SO2 , C2 H5 OH, C34 S, and methanol (CH3 OH). Recently, Leurini et al. (2004) have investigated the potential of CH3 OH as tracer of physical conditions in star forming regions; their main result is that while at millimeter wavelengths methanol transitions are mainly density probes, in the submillimeter regime, where high J bands of transitions with a broad excitation range are found, the information on kinetic temperature can be also recovered. Moreover, the torsionally excited transitions are pure temperature tracers as, with very high critical densities (1010 -1011 cm-3 ) and high level energies (T 300 K), they are hardly populated 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 to investigate the physical conditions of the hot, dense gas in star forming regions: by covering such a broad excitation range (Elow 200 K - 1050 K), they will allow independent determinations of both the spatial density and the kinetic temperature in the gas phase.
At the same time, we will observe the frequency band around 495 GHz, 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 survey of Orion-KL). Moreover, observation of the J = 27 - 26 transition of CH_3CN at 496 GHz will allow an independent measurement of the kinetic temperature that will be compared against the value obtained for CH_3OH.
In total, we request 12 hours of observing time in the LST interval from 10h to 22h.

References
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 the Stellar Mass Spectrum, 257260
White, G. J., Araki, M., Greaves, J. S., Ohishi, M., & Higginbottom, N. S. 2003, Astron. Astrophys., 407, 589