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J/A+A/499/149     1.2mm maps of southern Infrared Dark Clouds (Vasyunina+, 2009)

Physical properties of southern Infrared Dark Clouds. Vasyunina T., Linz H., Henning Th., Stecklum B., Klose S., Nyman L.-A. <Astron. Astrophys. 499, 149 (2009)> =2009A&A...499..149V
ADC_Keywords: Molecular clouds ; Infrared sources ; Millimetric/submm sources Keywords: ISM: dust, extinction - ISM: clouds - infrared: ISM - radio continuum: ISM - stars: Formation Abstract: What are the mechanisms by which massive stars form? What are the initial conditions for these processes? It is commonly assumed that cold and dense Infrared Dark Clouds (IRDCs) likely represent the birth sites massive stars. Therefore, this class of objects gets increasing attention, and their analysis offers the opportunity to tackle the above mentioned questions. To enlarge the sample of well-characterised IRDCs in the southern hemisphere, where ALMA will play a major role in the near future, we have set up a program to study the gas and dust of southern infrared dark clouds. The present paper aims at characterizing the continuum properties of this sample of IRDCs. We cross-correlated 1.2 mm continuum data from SIMBA@SEST with Spitzer/GLIMPSE images to establish the connection between emission sources at millimeter wavelengths a nd the IRDCs we see at 8µm in absorption against the bright PAH background. Analysing the dust emission and extinction leads to a determination of masses and column densities, which are important quantities in characterizing the initial conditions of massive star formation. We also evaluated the limitations of the emission and extinction methods. The morphology of the 1.2mm continuum emission is in all cases in close agreement with the mid-infrared extinction. The total masses of the IRDCs were found to range from 150 to 1150M (emission data) and from 300 to 1750M (extinction data). We derived peak column densities between 0.9 and 4.6x1022cm-2 (emission data) and 2.1 and 5.4x1022cm-2 (extinction data). We demonstrate that the extinction method fails for very high extinction values (and column densities) beyond AV values of roughly 75mag according to the Weingartner & Draine (2001ApJ...548..296W) extinction relation RV=5.5 model B (around 200mag when following the common Mathis (1990ARA&A..28...37M) extinction calibration). By taking the spatial resolution effects into account and restoring the column densities derived from the dust emission back to a linear resolution of 0.01pc, peak column densities of 3.0x1023cm-2 are obtained, much higher than typic al values for low-mass cores. The derived column densities, taking into account the spatial resolution effects, are beyond the column density threshold of 3.0x1023cm-2 required by theoretical considerations for massive star formation. We conclude that the values for column densities derived for the selected IRDC sample make these objects excellent candidates for objects in the earliest stages of massive star formation. Description: The 1.2mm continuum observations were carried out with the 37-channel bolometer array SIMBA (Nyman et al., 2001Msngr.106...40N) at the SEST on La Silla, Chile between July 16-18, 2003. SIMBA is a hexagonal array in which the HPBW of a single element is about 24" and the separation between elements on the sky is 44". The observations were made using a fast mapping technique without a wobbling secondary (Weferling et al., 2002A&A...383.1088W). Maps of Uranus were taken to check the flux calibration of the resulting data. To correct for the atmospheric opacity, skydips were performed every 2-3 hours. Despite the occurrence of some thin clouds, the observing conditions were good which is reflected in zenith opacity values of 0.16-0.18. The pointing was checked roughly every two hours and proved to be better than 6". The combination of typically three maps with sizes of 560"x900" resulted in a residual noise of about 22-28mJy/beam (rms) in the center of the mapped region. The 1.2mm data for the IRDC regions from SIMBA at the SEST telescope were reduced using the MOPSI package (developed by R. Zylka, IRAM). All maps were reduced by applying the atmospheric opacity corrections, fitting and subtracting a baseline, and removing the correlated sky noise. Thereby, we followed a three-stage approach as suggested in the SIMBA manual. After a first iteration using all data for the sky noise removal, the map regions showing source emission are neglected for sky noise removal in the second iteration. From this second interim map a source model is derived which is being included in the third iteration. The resulting maps were flux-calibrated using the conversion factor obtained from observations of Uranus. For our July 2003 observations, this factor was around 60mJy/beam per count. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file list.dat 54 12 List of maps map3/* 0 12 Individual FITS files
Byte-by-byte Description of file: list.dat
Bytes Format Units Label Explanations
1- 4 A4 --- --- [IRDC] 6- 16 A11 --- IRDC Source name ( 18- 19 I2 h RAh Right ascension (J2000.0) 21- 22 I2 min RAm Right ascension (J2000.0) 24- 28 F5.2 s RAs Right ascension (J2000.0) 30 A1 --- DE- Declination sign (J2000.0) 31- 32 I2 deg DEd Declination (J2000.0) 34- 35 I2 arcmin DEm Declination (J2000.0) 37- 40 F4.1 arcsec DEs Declination (J2000.0) 42- 54 A13 --- FileName Name of the FITS file in subdirectory map3
Acknowledgements: Tatiana Vasyunina, vasyunina(at)
(End) Tatiana Vasyunina [MPIA], Patricia Vannier [CDS] 25-May-2009
The document above follows the rules of the Standard Description for Astronomical Catalogues.From this documentation it is possible to generate f77 program to load files into arrays or line by line

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