Sven Thorwirth

The Cologne Database for Molecular Spectroscopy, CDMS

The unambiguous assignment of atomic or molecular lines in the interstellar or circumstellar medium (ISM or CSM) depends critically on the availability of laboratory data with appropriate accuracy. Above approximately 500 GHz the amount of transitions measured in the laboratory decreases fast. However, with upcoming missions such as the Stratospheric Observatory For Infrared Astronomy (SOFIA), the Herschel Space Observatory, or the Atacama Large Millimeter Array (ALMA) the need for accurate rest frequencies up to at least 2 THz increases considerably. The catalog section of the Cologne Database for Molecular Spectroscopy (CDMS) has been created to provide the astronomical community with line frequencies of atoms and molecules of astronomical interest. The CDMS is available on the internet free of charge via http://www.ph1.uni-koeln.de/vorhersagen/ or via the short-cut http://www.cdms.de/.

Millimeter multiplicity in NGC 6334 I and I(N)

Using the Submillimeter Array (SMA), we have imaged the 1.3 mm continuum emission at the centers of the massive star-forming regions NGC 6334 I and I(N). In both regions, the SMA observations resolvethe emission into multiple millimeter sources, with most of the sources clustered into areas only 10,000 AU in diameter. Toward NGC 6334I, wefindfourcompactsources:thetwo brightest (I-SMA1andI-SMA2)areassociatedwithpreviouslyknown ammonia cores; I-SMA3 coincides with the peak of the compact H ii region (NGC 6334 F), and I-SMA4 is a newly discovered object. While I-SMA3 exhibits a mixture of free-free and dust emission, the rest of the objects are dust cores.TowardNGC6334I(N),sevencompactdustcoresarefound,oneofwhichisassociatedwithafaintcentimeter source. With the exception of I-SMA3, none of the millimeter sources have infrared counterparts in Spitzer Space Telescope3‐8� mimages.Usingasimplephysicalmodelforthedustcontinuumemission,we estimatethatthemass of the interstellar material toward each of these compact objects is in the range of 3‐66 M� . The total mass in the compact objects appears to be similar in I and I(N). The small size of these groups of sources suggest that these objects are proto-Trapezia forming in the centers of clusters of low- to intermediate-mass stars.

Deuterium chemistry in the Orion Bar PDR. "Warm" chemistry starring CH2D+

Context. High levels of deuterium fractionation in gas-phase molecules are usually associated with cold regions, such as prestellar cores. Significant fractionation ratios are also observed in hot environments such as hot cores or hot corinos, where they are believed to be produced by the evaporation of the icy mantles surrounding dust grains, and are thus remnants of a previous cold (either gasphase or grain surface) chemistry. The recent detection of DCN towards the Orion Bar, in a clump at a characteristic temperature of 70 K, has shown that high deuterium fractionation can also be detected in PDRs. The Orion Bar clumps thus appear to be a good environment for the observational study of deuterium fractionation in luke warm gas, allowing us to validate chemistry models for a different temperature range, where dominating fractionation processes are predicted to differ from those in cold gas (< 20K). Aims. We aimed to study observationally in detail the chemistry at work in the Orion Bar PDR, to understand whether DCN is either produced by ice mantle evaporation or is the result of warm gas-phase chemistry, involving the CH_2D^+ precursor ion (which survives higher temperatures than the usual H_2D^+ precursor). Methods. Using the APEX and the IRAM 30m telescopes, we targeted selected deuterated species towards two clumps in the Orion Bar. Results. We confirmed the detection of DCN and detected two new deuterated molecules (DCO^+ and HDCO) towards one clump in the Orion Bar PDR. Significant deuterium fractionations are found for HCN and H_2CO, but we measured a low fractionation in HCO^+. We also provide upper limits to other molecules relevant to deuterium chemistry. Conclusions. We argue that grain evaporation in the clumps is unlikely to be a dominant process, and we find that the observed deuterium fractionation ratios are consistent with predictions of pure gas-phase chemistry models at warm temperatures (T ~ 50K). We show evidence that warm deuterium chemistry driven by CH_2D^+ is at work in the clumps.

The rotational spectra, electric dipole moments and molecular structures of anisole and benzaldehyde

The rotational spectra of anisole and of benzaldehyde were investigated in supersonic expansion at frequencies up to 41 GHz, and at room temperature in the millimetre-wave region, from 170 to 330 GHz. Accurate spectroscopic constants for the parent isotopomers in the ground vibrational state and for the first excited torsional state were determined for both molecules. The supersonic expansion spectrum allowed measurement, in natural abundance, of all singly substituted 13C isotopomers, as well as of the 18O isotopomer for both anisole and benzaldehyde. The rotational constants were used to determine the r(s) and the r(m)(1) gas-phase geometries, which are found to be consistent with prediction of bond length alternation in the phenyl ring induced by the asymmetric substituent. Stark measurements were made on the supersonic expansion spectrum resulting in electric dipole moment determination, /mu a/ = 2.9061(22) D, /mu b/ = 1.1883(10) D, /mu tOt/ = 3.1397(24) D for benzaldehyde and /mu a/ = 0.6937(12) D, /mu b/ = 1.0547(8) D, mu tOt = 1.2623(14) D for anisole. During the investigation it was found that use of a carrier gas mixture consisting of 30% Ar in He carries significant advantages for studies of weak lines, and pertinent experimental details are reported.

DIGGING INTO NGC 6334 I(N): MULTIWAVELENGTH IMAGING OF A MASSIVE PROTOSTELLAR CLUSTER

We present a high-resolution, multi-wavelength study of the massive protostellar cluster NGC 6334 I(N) that combines new spectral line data from the Submillimeter Array (SMA) and VLA with a re-analysis of archival VLA continuum data, Two Micron All Sky Survey and Spitzer images. As shown previously, the brightest 1.3 mm source SMA1 contains substructure at subarcsecond resolution, and we report the first detection of SMA1b at 3.6 cm along with a new spatial component at 7 mm (SMA1d). We find SMA1 (aggregate of sources a, b, c, and d) and SMA4 to be comprised of free-free and dust components, while SMA6 shows only dust emission. Our 15 resolution 1.3 mm molecular line images reveal substantial hot-core line emission toward SMA1 and to a lesser degree SMA2. We find CH3OH rotation temperatures of 165 ± 9 K and 145 ± 12 K for SMA1 and SMA2, respectively. We estimate a diameter of 1400 AU for the SMA1 hot-core emission, encompassing both SMA1b and SMA1d, and speculate that these sources comprise a 800 AU separation binary that may explain the previously suggested precession of the outflow emanating from the SMA1 region. Compact line emission from SMA4 is weak, and none is seen toward SMA6. The LSR velocities of SMA1, SMA2, and SMA4 all differ by 1-2 km s–1. Outflow activity from SMA1, SMA2, SMA4, and SMA6 is observed in several molecules including SiO(5-4) and IRAC 4.5 μm emission; 24 μm emission from SMA4 is also detected. Eleven water maser groups are detected, eight of which coincide with SMA1, SMA2, SMA4, and SMA6, while two others are associated with the Sandell source SM2. We also detect a total of 83 Class I CH3OH 44 GHz maser spots which likely result from the combined activity of many outflows. Our observations paint the portrait of multiple young hot cores in a protocluster prior to the stage where its members become visible in the near-infrared.

APEX 1 mm line survey of the Orion Bar

Context. Unbiased molecular line surveys are a powerful tool for analyzing the physical and chemical parameters of astronomical objects and are the only means for obtaining a complete view of the molecular inventory for a given source. The present work stands for the first such investigation of a photon-dominated region. Aims. The first results of an ongoing millimeter-wave survey obtained towards the Orion Bar are reported. Methods. The APEX telescope in combination with the APEX-2A facility receiver was employed in this investigation. Results. We derived the physical parameters of the gas through LVG analyses of the methanol and formaldehyde data. Information on the sulfur and deuterium chemistry of photon-dominated regions is obtained from detections of several sulfur-bearing molecules and DCN.

Rotational spectroscopy and equilibrium structures of S3 and S4

The sulfur molecules thiozone S3 and tetrasulfur S4 have been observed in a supersonic molecular beam in the centimeter-wave band by Fourier transform microwave spectroscopy, and in the millimeter- and submillimeter-wave bands in a low-pressure glow discharge. For S3 over 150 rotational transitions between 10 and 458 GHz were measured, and for S4 a comparable number between 6 and 271 GHz. The spectrum of S3 is reproduced to within the measurement uncertainties by an asymmetric top Hamiltonian with three rotational and 12 centrifugal distortion constants; ten distortion constants, but an additional term to account for very small level shifts caused by interchange tunneling, are required to reproduce to comparable accuracy the spectrum of S4. Empirical equilibrium (r(e)(emp)) structures of S3 and S4 were derived from experimental rotational constants of the normal and sulfur-34 species and vibrational corrections from coupled-cluster theory calculations. Quantum chemical calculations show that interchange tunneling occurs because S4 automerizes through a transition state with D2h symmetry which lies about 500 cm(-1) above the two equivalent C2upsilon minima on the potential energy surface.

Hot ammonia in NGC 63341 & I(N)

Aims. The massive twin cores NGC 6334I and I(N) are in different evolutionary stages and hence ideal targets to study evolutionary variations within the same larger-scale environment. Here, we study the warm, compact gas components. Methods. We imaged the two regions with the Australia Telescope Compact Array (ATCA) at high angular resolution in the NH3(3, 3) to (6, 6) inversion lines. Results. Compact emission is detected toward both regions in all observed inversion lines with energy levels up to 407 K above ground. This is particularly surprising for NGC 6334I(N) since it lacks bright infrared emission and is considered a massive cold core at an early evolutionary stage. High optical depth and multiply-peaked line profiles complicate rotation temperature estimates, and we can only conclude that gas components with temperatures >100K are present in both regions. Toward NGC 6334I, we confirm previous reports of NH3(3, 3) maser emission toward the outflow bow-shocks. Furthermore, we report the first detection of an NH3(6, 6) maser toward the central region of NGC 6334I. This maser is centered on the second millimeter (mm) peak and elongated along the outflow axis, indicating that this mm continuum core harbors the driving source of the molecular outflow. Toward the main mm peak in NGC 6334I(N), we detect a double-horn line profile in the NH3(6, 6) transition. The current data do not allow us to differentiate whether this double-horn profile is produced by multiple gas components along the line of sight, or whether it may trace a potential underlying massive accretion disk.

Physical Conditions in the Proto-Planetary Nebula CRL 618 Derived from Observations of Vibrationally Excited HC3N

We used the Effelsberg 100 m and IRAM 30 m telescopes to observe vibrationally excited cyanoacetylene (HC3N) in several rotational transitions toward the proto-planetary nebula CRL 618. Lines from nine different vibrationally excited states, with energies ranging up to 1600 K above ground, were detected. The lines show P Cygni profiles indicating that the HC3N emission originates from an expanding and accelerating molecular envelope. The HC3N rotational temperature varies with velocity, peaks at 520 K, 3 km s-1 blueshifted from the systemic velocity, and decreases with higher blueshift of the gas. The column density of the absorbing HC3N is (3-6) × 1017 cm-2. We modeled spectra based on spherical models of the expanding envelope that provided an excellent fit to the observations and discuss the implications of the models. Additionally, lines from 13C substituted cyanoacetylene were observed. They can be used to constrain the 12C/13C ratio in this source to 10 ± 2.

ROTATIONAL SPECTRA OF SMALL PAHs: ACENAPHTHENE, ACENAPHTHYLENE, AZULENE, AND FLUORENE

Pure rotational spectra of four small polycyclic aromatic hydrocarbons have been observed by Fourier transform microwave spectroscopy of a molecular beam in the frequency range from 7 to 37 GHz. Initial searches for acenaphthene (C12H10), acenaphthylene (C12H8), and fluorene (C13H10) were guided by quantum chemical calculations performed at the B3LYP/cc-pVTZ level of theory. All three molecules exhibit b-type rotational spectra and are calculated to be moderately polar, with dipole moments of 0.3-0.9 D. Close agreement (to better than 1%) between the calculated equilibrium and experimentally derived ground-state rotational constants is achieved. Selected transitions of acenaphthene and fluorene have also been measured in the 3 mm region by conventional free-space absorption spectroscopy, as have transitions of the previously studied azulene (C10H8). The data presented here facilitate deep radio astronomical searches with large radio telescopes.