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 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 2001ApJ...548..296W) extinction relation
RV=5.5 model B (around 200mag when following the common Mathis
(1990ARA&A..28...37M 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 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 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:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
list.dat 54 12 List of maps
map3/* . 12 Individual FITS files
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 4 A4 --- --- [IRDC]
6- 16 A11 --- IRDC Source name (LLL.ll+B.bb)
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
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Acknowledgements:
Tatiana Vasyunina, vasyunina(at)mpia-hd.mpg.de
(End) Tatiana Vasyunina [MPIA], Patricia Vannier [CDS] 25-May-2009