APEX SV projects

Study of an extragalactic hot core: N159W

Coordinator: S. Leurini, P. Schilke, C. Comito, A. Belloche

Data:
Program is available and data products can be downloaded

Scientific justification:
The Small and Large Magellanic Clouds (hereafter LMC and SMC) have considerably differ- ent characteristics from those of the Milky Way; they both have low metallicities, low dust-to-gas ratio and intense radiation at ultraviolet and far-ultraviolet wavelengths, while the opposite is found in the Galaxy. The strong UV radiation is explained by intense star formation activity in some parts of the LMC and SMC, which, given their low metallicity, must be characterized by a different chemistry than in the Milky Way. Their proximity enables studies of individual clouds, even at single dish resolution. Therefore, by comparison with similar studies in our Galaxy, the Magellanic Clouds are an ideal target to understand the role of the environment in star formation and in the evolution of molecular clouds. Moreover, their low metallicity makes them a "laboratory" to study the physical conditions in the early universe.
A powerful diagnostic tool to investigate the effects of the environment in star formation is found in the analysis of molecular spectra, which can be seen as fingerprints of a molecular cloud, as they carry the information on its chemistry, and therefore on its history, on its physical conditions and on its dynamics and kinematics. Recently, Leurini et al. (2004) have discussed the tracing properties of CH3 OH, reaching the conclusion that methanol is indeed very useful as a probe of physical conditions in star forming regions in all mass regimes, since it has a very reach spectrum of transitions spread throughout all the centimeter, millimeter and submillimeter spectral windows. Their main results are that transitions in the millimeter spectrum are mainly density probes, while submillimeter lines are often sensitive to both the spatial density and the kinetic temperature of the gas. Therefore, with a multi-transition analysis of the methanol spec- trum, covering different energies and excitation ranges, the physical conditions of the interstellar medium in molecular clouds (kinetic temperature and spatial density) can be derived.
The N159 area in the LMC contains several Hii regions; Heikkila et al. (1999) detected several hot core molecules (e.g. CH3 OH, CH3 CCH) in N159W, which has also the highest observed CO (J=3-2, 2-1, 1-0) integrated intensity in the LMC and it is therefore associated with prominent star formation. From a study on several molecules, they derived typical 20-30 K temperature. However, their results are a global averaging over a beam that ranges from 50 at 3 mm to 20 at 1 mm and are not sensitive to small scale variations, which are likely to happen in individual clouds. The detection of hot core molecules (e.g. CH3 OH, CH3 CCH) and high excitation transitions reveals dense, hot gas in the region.
With a beam of 18 at 340 GHz, we here propose to observe the inner region of the N159W clump in the 7k 6K CH3 OH band at 338 GHz with the APEX telescope, to study the gas in the active star forming area. By comparing the line intensities of the CH3 OH lines in the millimeter regime (Heikkila et al. (1999)) with the ones from the Orion KL line (Turner (1989), Schilke et al. (1997)), line intensities in the 7k 6k band are expected to be in the range 0.2-1 K. A total of 21 CH3 OH lines falls in the 1 GHz bandwidth of the receiver, half of which is expected to have intensities close to 1 K. Based on the Orion KL survey from Schilke et al. (1997), by tuning the receiver at 338 GHz in the lower side band, we expect to detect 184,14 183,15 SO2 in the lower side band ( = 338.306 GHz) and 102 10-1 , 40 3-1 CH3 OH (350.5 GHz and 350.7 GHz) and 7/2 5/2 NO (350.6 GHz) in the upper side band. Assuming a system temperature of 250 K and an observing efficiency of 30% after accounting for slewing times and times spend on focus checks, pointing and flux calibration measurements this project will need a total time of 8 hours to reach an r.m.s. of 5 mK, one period in the 2-10 LST range.

Source	R.A.(2000)	Dec.(2000)	v_LSR [km.s-1]
N159W	05:39:36.1	-69:45:35	240

References
Heikkila, A., Johansson, L. E. B., & Olofsson, H. 1999, A&A, 344, 817
Leurini, S., Schilke, P., Menten, K. M., Flower, D. R., Pottage, J. T., & Xu, L.-H. 2004, A&A,422, 573
Schilke, P., Groesbeck, T. D., Blake, G. A., & Phillips, T. G. 1997, ApJS, 108, 301
Turner, B. E. 1989, ApJS, 70, 539