Johannes Schmid-Burgk

Orion KL: The hot core that is not a "Hot Core"

We present sensitive high angular resolution submillimeter and millimeter observations of torsionally/vibrationally highly excited lines of the CH3OH, HC3N, SO2 ,a nd CH 3CN molecules and of the continuum emission at 870 and 1300 μm from the Orion KL region, made with the Submillimeter Array (SMA). These observations and SMA CO J = 3− 2a ndJ = 2−1 imaging of the explosive flow originating in this region suggest that the molecular Orion “hot core” is a pre-existing density enhancement heated from the outside by the explosive event. Unlike in other hot cores, we do not find any self-luminous submillimeter, radio, or infrared source embedded in the hot molecular gas, nor observe filamentary CO flow structures or “fingers” in the shadow of the hot core pointing away from the explosion center. The low-excitation CH3CN emission shows the typical molecular heart-shaped structure, traditionally named the hot core, and is centered close to the dynamical origin of the explosion. The highest excitation CH3CN lines all originate from the northeast lobe of the heart-shaped structure, i.e. from the densest and most highly obscured parts of the extended ridge. The torsionally excited CH3OH and vibrationally excited HC3N lines appear to form a shell around the strongest submillimeter continuum source. All of these observations suggest that the southeast and southwest sectors of the explosive flow have impinged on a pre-existing very dense part of the extended ridge, thus creating the bright Orion KL hot core. However, additional theoretical and observational studies are required to test this new heating scenario.

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.

\mathsf{^{13}}

The paper reports the discovery of a 120 arcsec long jet of CO in OMC-1. The feature, which is redshifted up to 15 km s-1 from the ambient CO gas, was presumably ejected from CS3/FIR4 60 arcsec southwest of the Trapezium and 110 arcsec south of IRc2. It might be related to a local bipolar source recently found there in SiO. The jet has very narrow width (not greater than 8 arcsec) and shows no sign of angular divergence; its axis is a straight line. The matter is accelerated all along, with the largest accelerations observed on the central axis. Lower velocity material envelops the high-speed core. Halfway along the jet, there is a sharp break at which the higher velocities terminate abruptly. Beyond the break, acceleration continues without change of direction or strength; the flows final disappearance might be caused by accelerative dilution. The jet is probably denser than the ambient cloud; its temperature (about 45 K) approaches ambient values. Blueshifted emission of narrow lateral extent is seen over the full length of the redshifted jet. 15 refs.

CO

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.

\mathsf{^{+}}

During their infancy, stars are well known to expel matter violently in the form of well-defined, collimated outflows. A fairly unique exception is found in the Orion Becklin-Neugebauer/Kleinmann-Low star-forming region where a poorly collimated and somewhat disordered outflow composed of numerous elongated finger-like structures was discovered more than 30 years ago. In this Letter, we report the discovery in the same region of an even more atypical outflow phenomenon. Using 13CO(2-1) line observations made with the Submillimeter Array, we have identified there a 500-1000 year old, expanding, roughly spherically symmetric bubble whose characteristics are entirely different from those of known outflows associated with young stellar objects. The center of the bubble coincides with the initial position of a now defunct massive multiple stellar system suspected to have disintegrated 500 years ago and with the center of symmetry of the system of molecular fingers surrounding the Kleinmann-Low nebula. We hypothesize that the bubble is made up of gas and dust that used to be part of the circumstellar material associated with the decayed multiple system. The Orion hot core, recently proposed to be the result of the impact of a shock wave onto a massive dense core, is located toward the southeast quadrant of the bubble. The supersonic expansion of the bubble and/or the impact of some low-velocity filaments provide a natural explanation for its origin.

and

Molecular cloud studies allow determinations kinetic temperatures, H2 densities, B fields and abundances. These results allow a determination of the chemical and isotopic content of the the interstellar medium (ISM). Data for clouds in our galaxy, especially those near the galactic center are needed to interpret results obtained for other galaxies. Molecular clouds are the birthplaces of stars. For investigations of star formation and the interaction of young stars with molecular clouds, measurements of abundant polar molecules, such as carbon monoxide, on scales of <1015 cm are needed.

\mathsf{^{13}}

We have recently discovered a large-scale (200”) outflow system in the core of OMC-1 (fig. 1), centered about 100” South of IRc2 and extending over some 120” (red lobe) resp. 60” (blue) along a position angle of —31°Schmid-Burgk et al. 1990). The blue lobe which might actually trude into the Ell region M42is poorly defined in CO 2-1, but the red lobe reveals a number of remarkable properties which we summarize here:

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

Abstract Radio lines, emitted as a consequence of H + and He + recombinations in HII regions, provide nearly ideal tools for the determination of He/H abundance ratios all across the galaxy. After applying appropriate corrections for some unseen He o , and taking into account the post-big bang fraction of He derived from radio and optical observations of oxygen, recent 100-m radio telescope measurements give a primordial He mass fraction Yp of between 0.21 and 0.234. This puts stringent limits on some elementary particle scenarios in standard cosmology.

A highly collimated outflow in the core of OMC-1

Contrary to previous expectations based on plane-stratified model atmospheres, calculations of photospheres taking spherical extension properly into account indicate that surface temperatures of late M type stars of low mass and high luminosity fall below the condensation temperature of A1203. Therefore, the formation of the observed circumstellar dust in M giants and supergiants with normal solartype 0/C ratio (≃2 by number) might occur in these upper photospheres.