Dirk Muders

The end of star formation in Chamaeleon I ? A LABOCA census of starless and protostellar cores

Chamaeleon I is the most active region in terms of star formation in the Chamaeleon molecular cloud complex. Its population of prestellar and protostellar cores is not known and a controversy exists concerning its history of star formation. Our goal is to characterize the earliest stages of star formation in this cloud. We used the bolometer array LABOCA at APEX to map the cloud in dust continuum emission at 870 micron. The detected sources are analysed by carefully taking into account the spatial filtering inherent in the data reduction process. A search for associations with YSOs is performed using Spitzer data and the SIMBAD database. Most of the detected 870 micron emission is distributed in 5 filaments. We identify 59 starless cores, one candidate first hydrostatic core, and 21 sources associated with more evolved YSOs. The starless cores are only found above a visual extinction threshold of 5 mag. They are less dense than those detected in other nearby molecular clouds by a factor of a few on average. The core mass distribution is consistent with the IMF at the high-mass end but is overpopulated at the low-mass end. In addition, at most 17% of the cores have a mass larger than the critical Bonnor-Ebert mass. Both results suggest that a large fraction of the starless cores may not be prestellar. Based on the census of prestellar cores, Class 0 protostars, and more evolved YSOs, we conclude that the star formation rate has decreased with time in this cloud. The low fraction of candidate prestellar cores among the population of starless cores, the small number of Class 0 protostars, the high global star formation efficiency, the decrease of the star formation rate with time, and the low mass per unit length of the detected filaments all suggest that we may be witnessing the end of the star formation process in Cha I {abridged}.

APECS - The Atacama Pathfinder Experiment Control System

APECS is the distributed control system of the new Atacama Pathfinder EXperiment (APEX) telescope located on the Llano de Chajnantor at an altitude of 5107 m in the Atacama desert in northern Chile. APECS is based on Atacama Large Millimeter Array (ALMA) software and employs a modern, object-oriented design using the Common Object Request Broker Architecture (CORBA) as the middleware. New generic device interfaces simplify adding instruments to the control system. The Python based observer command scripting language allows using many existing software libraries and facilitates creating more complex observing modes. A new self-descriptive raw data format (Multi-Beam FITS or MBFITS) has been defined to store the multi-beam, multi-frequency data. APECS provides an online pipeline for initial calibration, observer feedback and a quick-look display. APECS is being used for regular science observations in local and remote mode since August 2005.

Hyperfine structure in H

The magnetic moment of the 13 C nucleus is shown to provide a potentially useful tool for analysing quiescent cold molecular clouds. We report discovery of hyperfine structure in the lowest rotational transition of H 13 CO + . The doublet splitting in H 13 CO + , observed to be of width 38.5 ± 5.2 kHz or 0.133 km s −1 , is confirmed by quantum chemical calculations which give a separation of 39.8 kHz and line strength ratio 3:1 when H and 13 C nuclear spin-rotation and spin-spin coupling between both nuclei are taken into account. We improve the spectroscopic constants of H 13 CO + and determine the hitherto uncertain frequencies of its low-J spectrum to better precision by analysing the dark cloud L 1512. Attention is drawn to potentially high optical depths (3 to 5 in L 1512) in quiescent clouds, and examples are given for the need to consider the (1-0) lines doublet nature when comparing to other molecular species, redirecting or reversing conclusions arrived at previously by single- component interpretations. We further confirm the hyperfine splitting in the (1-0) rotational transition of 13 CO that had already been theoretically predicted, and measured in the laboratory, to be of width about 46 kHz or, again, 0.13 km s −1 . By applying hyperfine analysis to the extensive data set of the first IRAM key-project we show that 13 CO optical depths can as for H 13 CO + be estimated in narrow linewidth regions without recourse to other transitions nor to assumptions on beam filling factors, and linewidth and velocity determinations can be improved. Thus, for the core of L 1512 we find an inverse proportionality between linewidth and column density, resp. linewidth and square root of optical depth, and a systematic inside-out increase of excitation temperature and of the 13 CO:C 18 O abundance ratio. Overall motion toward the innermost region is suggested.

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The NH 3 (J, K)=(1, 1) and (2, 2) line emission was mapped toward three regions with molecular outflows, L1287, AFGL 5142, and IRAS 20126+4104, using the Effelsberg 100 m telescope. Additional C 18 O (J=2→1) and CS (J=3→2) observations of L1287 were carried out with the IRAM 30 m telescope. For the three regions, the high-density molecular core, as traced by the ammonia emission, peaks very close to the position of the proposed powering sources of the outflows. In AFGL 5142 we flavor the radio continuum source proposed by Torrelles et al. (1992b) as the powering source of the outflow. The molecular cores best resolved by the telescope beam are elongated in a direction perpendicular to the outflow axis

CO

APEX, the Atacama Pathfinder Experiment, has been successfully commissioned and is in operation now. This novel submillimeter telescope is located at 5107 m altitude on Llano de Chajnantor in the Chilean High Andes, on what is considered one of the worlds outstanding sites for submillimeter astronomy. The primary reflector with 12 m diameter has been carefully adjusted by means of holography. Its surface smoothness of 17-18 μm makes APEX suitable for observations up to 200 μm, through all atmospheric submm windows accessible from the ground.

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Abstract High-level quantum-chemical calculations are reported at the MP2 and CCSD(T) levels of theory for the equilibrium structure and the harmonic and anharmonic force fields of diacetylene, H C C C C H. The calculations were performed employing Dunning’s hierarchy of correlation-consistent basis sets cc-pVXZ, cc-pCVXZ, and cc-pwCVXZ, as well as the ANO2 basis set of Almlof and Taylor. An empirical equilibrium structure based on experimental rotational constants for 13 isotopic species of diacetylene and computed zero-point vibrational corrections is determined ( r e emp : r C H = 1.0615 A , r C C = 1.2085 A , r C C = 1.3727 A ) and in good agreement with the best theoretical structure (CCSD(T)/cc-pCV5Z: r C H = 1.0617 A , r C C = 1.2083 A , r C C = 1.3737 A ). In addition, the computed fundamental vibrational frequencies are compared with the available experimental data and found in satisfactory agreement.

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We present CO(2−1), 13 CO(2−1), CO(6−5), CO(7−6), and SO(65−54) line observations made with the IRAM 30 m and Atacama Pathfinder Experiment (APEX) radiotelescopes and the Submillimeter Array (SMA) toward the highly collimated (11 ◦ ) and extended (∼2 � ) southwest lobe of the bipolar outflow Ori-S6 located in the Orion South region. We report for all these lines, the detection of velocity asymmetries about the flow axis with velocity differences roughly on the order of 1 km s −1 over distances of about 5000 AU, 4k m s −1 over distances of about 2000 AU, and close to the source of between 7 and 11 km s −1 over smaller scales of about 1000 AU. The redshifted gas velocities are located to the southeast of the outflow’s axis, the blueshifted ones to the northwest. We interpret these velocity differences as a signature of rotation, but also discuss some alternatives which we recognize as unlikely in view of the asymmetries’ large downstream continuation. In particular, any straightforward interpretation by an ambient velocity gradient does not seem viable. This rotation across the Ori-S6 outflow is observed out to (projected) distances beyond 2.5 × 10 4 AU from the flow’s presumed origin. Comparison of our large-scale (single dish) and small-scale (SMA) observations suggests the rotational velocity to decline not faster than 1/R with distance R from the axis; in the innermost few arcsecs an increase of rotational velocity with R is even indicated. The magnetic field lines threading the inner rotating CO shell may well be anchored in a disk of a radius of ∼50 AU; the field lines further out need a more extended rotating base. Our high angular resolution SMA observations also suggest this outflow to be energized by the compact millimeter radio source 139-409, a circumbinary flattened ring that is located in a small cluster of very young stars associated with the extended and bright source FIR4.

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The modeling and analysis generic interface for external numerical codes (MAGIX) is a model optimizer developed under the framework of the coherent set of astrophysical tools for spectroscopy (CATS) project. The MAGIX package provides a framework of an easy interface between existing codes and an iterating engine that attempts to minimize deviations of the model results from available observational data, constraining the values of the model parameters and providing corresponding error estimates. Many models (and, in principle, not only astrophysical models) can be plugged into MAGIX to explore their parameter space and find the set of parameter values that best fits observational/experimental data. MAGIX complies with the data structures and reduction tools of ALMA (Atacama Large Millimeter Array), but can be used with other astronomical and with non-astronomical data.

CO: Measurement, analysis, and consequences for the study of dark clouds

We report on developments of submillimeter heterodyne arrays for high resolution spectroscopy with APEX. Shortly, we will operate state-of-the-art instruments in all major atmospheric windows accessible from Llano de Chajnantor. CHAMP+, a dual-color 2×7 element heterodyne array for operation in the 450 μm and 350 μm atmospheric windows is in operation since late 2007. With its state-of-the-art SIS detectors and wide tunable local oscillators, its cold optics with single sideband filters and with 3 GHz of processed IF bandwidth per pixel, CHAMP+ does provide outstanding observing capabilities. The Large APEX sub-Millimeter Array (LAsMA) is in the final design phase, with an installation goal in 2009. The receiver will operate 7 and 19 pixels in the lower submillimeter windows, 285-375 GHz and 385-520 GHz, respectively. The front-ends are served by an array of digital wideband Fast Fourier Transform spectrometers currently processing up to 32×1.5 (optionally 1.8) GHz of bandwidth. For CHAMP+, we process 2.8 GHz of instantaneous bandwidth (in 16.4 k channels) for each of the 14 pixels.

The molecular cores in the L1287, AFGL 5142, and IRAS 20126 + 4104 regions

We present a fully sampled map covering the Orion Hot Core and dense molecular ridge, in the sub-millimeter J=6-5 rotational transition of 13CO, at 0.45 mm with a resolution of 13 arcsec and 0.5 km s^-1. The map covers 3 arc min by 2arc min . The profile centered on the Hot Core peaks at 8.5 km s^-1 and has a peak intensity of 40 K, corrected antenna temperature. It shows line wings from 30 km s^-1 to -20 km s^-1. The map of intensity, integrated from 0 to +18 km s^-1, shows a prominent maximum <5 arcsec from the center of the Orion Hot Core. The FWHP is 3 arcsec, larger than the regions containing complex molecules. Single dish measurements of lines from the J=2-1 or J=1-0 transitions of CO isotopes show no such distinct maximum. Correcting for optical depth 1.5 in the J=6-5 line of 13CO, and assuming that the level populations are thermalized at 150 K, the beam averaged column density between 0 to +18 km s^-1 is N(13CO )=6.8 10^17 cm^-2 and N(CO)=5.2 10^19 cm-2. When combined with published dust emission data, the CO/ H2 number ratio is 2 {\cdot} 10^-5, a factor of ~5 lower than the canonical value, 10^-4. For the Orion South and Orion Ridge region, the column density of CO is <25% of that found for the Hot Core but CO/H2 ratios are similar. Models of Photodissociation Regions, PDRs, predict that CO lines from PDRs are only marginally optically thick. Thus our map traces warm and dense molecular gas rather than PDRs.