arXiv subjects

S. Thorwirth

Publications and source records attributed to S. Thorwirth.

36 records · Page 2Linked to original sources

arXiv2008

The empirical equilibrium structure of diacetylene

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, HCCCCH. 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 Almloef and Taylor. An empirical equilibrium structure based on experimental rotational constants for thirteen isotopic species of diacetylene and computed zero-point vibrational corrections is determined (r_e^emp: rC-H=1.0615 A, rCtripleC=1.2085 A, rC-C = 1.3727 A) and in good agreement with the best theoretical structure (CCSD(T)/cc-pCV5Z: rC-H=1.0617 Angstrom, rCtripleC=1.2083 A, rC-C=1.3737 A). In addition, the computed fundamental vibrational frequencies are compared with the available experimental data and found in satisfactory agreement.

physics.chem-ph↗

arXiv2008

ATCA 3mm observations of NGC6334I and I(N): dense cores, outflows and an UCHII region

Aims: Investigation of the dense gas, the outflows and the continuum emission from the massive twin cores NGC6334I and I(N) at high spatial resolution. Methods: We imaged the region with the Australia Telescope Compact Array (ATCA) at 3.4mm wavelength in continuum as well as CH3CN(5_K-4_K) and HCN(1-0) spectral line emission. Results: While the continuum emission in NGC6334I mainly traces the UCHII region, toward NGC6334I(N) we detect line emission from four of the previously identified dust continuum condensations that are of protostellar or pre-stellar nature. The CH3CN(5_K-4_K) lines are detected in all K-components up to energies of 128K above ground toward two protostellar condensations in both regions. We find line-width increasing with increasing K for all sources, which indicates a higher degree of internal motions closer to the central protostars. Toward the main mm and CH3CN source in NGC6334I we identify a velocity gradient approximately perpendicular to the large-scale molecular outflow. This may be interpreted as a signature of an accretion disk, although other scenarios, e.g., an unresolved double source, could produce a similar signature as well. No comparable signature is found toward any of the other sources. HCN does not trace the dense gas well but it is dominated by the molecular outflows. While the outflow in NGC6334I exhibits a normal Hubble-law like velocity structure, the data indicate a precessing outflow close to the plane of the sky for NGC6334I(N). Furthermore, we observe a wide (~15.4km/s) HCN absorption line, much broader than the previously observed CH3OH and NH3 absorption lines. Several explanations for the difference are discussed.

astro-ph↗

arXiv2007

New ammonia masers towards NGC6334I

We report the detection of new ammonia masers in the non-metastable (8,6) and (11,9) transitions towards the massive star forming region NGC6334I. Observations were made with the ATCA interferometer and the emitting region appears unresolved in the 2.7" x 0.8" beam, with deconvolved sizes less than an arcsecond. We estimate peak brightness temperatures of 7.8 x 10^5 and 1.2 x 10^5 K for the (8,6) and (11,9) transitions, respectively. The masers appear coincident both spatially and in velocity with a previously detected ammonia (6,6) maser. We also suggest that emission in the (10,9), (9,9) and (7,6) transitions may also be masers, based on their narrow line widths and overlapping velocity ranges with the above masers, as observed with the single-dish Mopra radiotelescope.

astro-ph↗

arXiv2007

A multi-transition submillimeter water maser study of evolved stars - detection of a new line near 475 GHz

Context: Maser emission from the H2O molecule probes the warm, inner circumstellar envelopes of oxygen-rich red giant and supergiant stars. Multi-maser transition studies can be used to put constraints on the density and temperature of the emission regions. Aims: A number of known H2O maser lines were observed toward the long period variables R Leo and W Hya and the red supergiant VY CMa. A search for a new, not yet detected line near 475 GHz was conducted toward these stars. Methods: The Atacama Pathfinder Experiment telescope was used for a multi-transition observational study of submillimeter H2O lines. Results: The 5_33-4_40 transition near 475 GHz was clearly detected toward VY CMa and W Hya. Many other H2O lines were detected toward all three target stars. Relative line intensity ratios and velocity widths were found to vary significantly from star to star. Conclusions: Maser action is observed in all but one line for which it was theoretically predicted. In contrast, one of the strongest maser lines, in R Leo by far the strongest, the 437 GHz 7_53-6_60 transition, is not predicted to be inverted. Some other qualitative predictions of the model calculations are at variance with our observations. Plausible reasons for this are discussed. Based on our findings for W Hya and VY CMa, we find evidence that the H2O masers in the AGB star W Hya arise from the regular circumstellar outflow, while shock excitation in a high velocity flow seems to be required to excite masers far from the red supergiant VY CMa.

astro-ph↗

arXiv2007

Millimeter wave spectral line surveys and line mapping studies of NGC6334I and I(N)

NGC6334I and I(N) have been observed with the Swedish-ESO Submillimetre Telescope, SEST, at wavelengths of 3, 2, and 1.3 mm. Especially NGC6334 I shows rich emission from many different molecules, comparable in line density to prototypical hot cores such as Orion-KL and SgrB2(N). In addition, a 4' by 4' region enfolding NGC 6334 I and I(N) has been mapped at a wavelength of 3 mm (75 to 116 GHz) with the Mopra telescope.

astro-ph↗

arXiv2007

APEX survey of southern high mass star forming regions

A systematic study of a large sample of sources, covering a wide range in galactocentric distances, masses and luminosities, is a fast and efficient way of obtaining a good overview of the different stages of high-mass star formation. With these goals in mind, we have started a survey of 40 color selected IRAS sources south of -20 degrees declination with the APEX telescope on Chajnantor, Chile. Our first APEX results already demonstrate that the selection criteria were successful, since some of the sources are very rich in molecular lines.

astro-ph↗

arXiv2007

Hot ammonia in NGC6334I & I(N)

Aims: The massive twin cores NGC6334I 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 407K above ground. This is particularly surprising for NGC6334I(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 NGC6334I, 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 NGC6334I. 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 NGC6334I(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. The data to Figures 3 to 7 are also available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/.

astro-ph↗

arXiv2006

APEX 1 mm line survey of the Orion Bar

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. The first results of an ongoing millimeter-wave survey obtained towards the Orion Bar are reported. The APEX telescope in combination with the APEX-2A facility receiver was employed in this investigation. 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.

astro-ph↗

arXiv2006

Revealing the environs of the remarkable southern hot core G327.3-0.6

We present a submm study of the massive hot core G327.3-0.6 that constrains its physical parameters and environment. The APEX telescope was used to image CO and N2H+ emission, to observe lines from other molecules toward a hot and a cold molecular core, and to measure the continuum flux density of the hot core. In the C18O J=3-2 line, two clumps were found, one associated with the HII region G327.3-0.5 and the other associated with the hot core. An additional cold clump is found 30 arcsec (0.4 pc) northeast of the hot core in bright N2H+ emission. From the the continuum data, we calculate a mass of 420 Msol and a size of 0.1 pc for the hot core. A new, more accurate position of the hot core is reported, which allows the association of the core with a bright mid-infrared source. The luminosity of the hot core is estimated to be between 5 and 15 10^4 Lsol. This study revealed several different evolutionary stages of massive star formation in the G327.3-0.6 region.

astro-ph↗

arXiv2006

Submillimeter spectroscopy of southern hot cores: NGC6334(I) and G327.3-0.6

High-mass star-forming regions are known to have a rich molecular spectrum from many species. Some of the very highly excited lines are emitted from very hot and dense gas close to the central object(s). The physics and chemistry of the inner cores of two high mass star forming regions, NGC6334(I) and G327.3-0.6, shall be characterized. Submillimeter line surveys with the APEX telescope provide spectra which sample many molecular lines at high excitation stages. Partial spectral surveys were obtained, the lines were identified, physical parameters were determined through fitting of the spectra. Both sources show similar spectra that are comparable to that of the only other high mass star forming region ever surveyed in this frequency range}, Orion-KL, but with an even higher line density. Evidence for very compact, very hot sources is found.

astro-ph↗

arXiv2006

{Millimeter Multiplicity in NGC 6334 I and I(N)

Using the Submillimeter Array (SMA), we have imaged the 1.3 millimeter continuum emission at the center of the massive star-forming regions NGC 6334 I and I(N). In both regions, the SMA observations resolve the emission into multiple millimeter sources, with most of the sources clustered into areas only 10,000 AU in diameter. Toward NGC 6334 I, we find four compact sources: the two brightest (I-SMA1 and I-SMA2) are associated with previously-known ammonia cores; I-SMA3 is coincident with the peak of the compact HII 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. Toward NGC 6334 I(N), seven compact dust cores are found, one of which is associated with a faint centimeter source. With the exception of I-SMA3, none of the millimeter sources have infrared counterparts in Spitzer Space Telescope 3-8 micron images. Using a simple physical model for the dust continuum emission, the estimated mass of interstellar material toward each of these compact objects is in the range of 3 to 66 solar masses. 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.

astro-ph↗

arXiv2006

Discovery of Interstellar CF+

We discuss the first astronomical detection of the CF+ (fluoromethylidynium) ion, obtained by observations of the J=1-0 (102.6 GHz), J=2-1 (205.2 GHz) and J=3-2 (307.7 GHz) rotational transitions toward the Orion Bar region. Our search for CF+, carried out using the IRAM 30m and APEX 12m telescopes, was motivated by recent theoretical models that predict CF+ abundances of a few times 1.E-10 in UV-irradiated molecular regions where C+ is present. The CF+ ion is produced by exothermic reactions of C+ with HF. Because fluorine atoms can react exothermically with H2, HF is predicted to be the dominant reservoir of fluorine, not only in well-shielded regions but also in the surface layers of molecular clouds where the C+ abundance is large. The observed CF+ line intensities imply the presence of CF+ column densities of at least 1.E+12 cm-2 over a region of size at least ~ 1 arcmin, in good agreement with theoretical predictions. They provide support for our current theories of interstellar fluorine chemistry, which suggest that hydrogen fluoride should be ubiquitous in interstellar gas clouds and widely detectable in absorption by future satellite and airborne observatories.

astro-ph↗

arXiv2005

Rotational Spectroscopy of PAHs: Acenaphthene, Acenaphthylene and Fluorene

Pure rotational spectra of three polycyclic aromatic hydrocarbons - acenaphthene, acenaphthylene and fluorene - have been obtained by Fourier transform microwave spectroscopy of a molecular beam and subsequently by millimeter wave absorption spectroscopy for acenaphthene and fluorene. The data presented here will be useful for deep radio astronomical searches for PAHs employing large radio telecopes.

astro-ph↗

arXiv2005

First astronomical detection of the CF+ ion

We report the first astronomical detection of the CF+ (fluoromethylidynium) ion obtained by recent observations of its J = 1 - 0 (102.6 GHz), J = 2 - 1 (205.2 GHz), and J = 3 - 2 (307.7 GHz) pure rotational emissions toward the Orion Bar. Our search for CF+, carried out using the IRAM 30m and APEX 12m telescopes, was motivated by recent theoretical models that predict CF+ abundances of a few x E-10 in UV-irradiated molecular regions where C+ is present. The measurements confirm the predictions. They provide support for our current theories of interstellar fluorine chemistry, which suggest that hydrogen fluoride should be ubiquitous in interstellar gas clouds.

astro-ph↗

arXiv2005

High-spatial-resolution observations of NH3 and CH3OH towards the massive twin cores NGC6334 I & I(N)

Molecular line observations of NH3 (J,K)=(1,1), (2,2) and CH3OH at 24.93GHz taken with the Australian Telescope Compact Array (ATCA) toward the massive twin cores NGC6334 I & I(N) reveal significant variations in the line emission between the two massive cores. The UCHII region/hot core NGC6334 I exhibits strong thermal NH3 and CH3OH emission adjacent to the UCHII region and coincident with two mm continuum peaks observed by Hunter et al. (in prep.). In contrast, we find neither compact NH3 nor thermal CH3OH line emission toward NGC6334 I(N). There, the NH3 emission is distributed over a broad region (>1') without a clear peak, and we find Class I CH3OH maser emission with peak brightness temperatures up to 7000K. The maser emission peaks appear to be spatially associated with the interfaces between the molecular outflows and the ambient dense gas. Peak NH3(1,1) line brightness temperatures >= 70K in both regions indicate gas temperatures of the same order. NH3 emission is also detected toward the outflow in NGC6334 I resulting in an estimated rotational temperature of Trot~19K. Furthermore, we observe CH3OH and NH3 absorption toward the UCHII region, the velocity structure is consistent with expanding molecular gas around the UCHII region. Thermal and kinematic effects possibly imposed from the UCHII region on the molecular core are also discussed.

astro-ph↗

arXiv2002

Detection of HCN Direct l-Type Transitions Probing Hot Molecular Gas in the Proto-Planetary Nebula CRL 618

We report the detection of direct l-type transitions towards the proto-planetary nebula CRL 618 during a study of vibrationally excited carbon chains. The J=8,10,11,12,13,14 DJ=0 transitions of HCN in its first excited bending mode v2=1 were detected in absorption against the continuum of the central HII region making use of the Effelsberg 100m telescope and the Very Large Array. Additionally, the J=9 direct l-type transition was detected in emission presumably indicating a weak maser. All lines are blueshifted with respect to the systemic velocity of CRL 618 indicating that the lines originate from a hot, expanding circumstellar envelope. The HCN column density along the line of sight in front of the continuum is 2E18 cm^-2.

astro-ph↗

arXiv2002

Physical conditions in the Protoplanetary Nebula CRL 618 derived from observations of vibrationally excited HCCCN

We used the Effelsberg 100m and IRAM 30m telescopes to observe vibrationally excited cyanoacetylene (HCCCN) in several rotational transitions toward the proto-planetary nebula CRL618. Lines from 9 different vibrationally excited states with energies ranging up to 1600 K above ground were detected. The lines show P Cygni profiles indicating that the HCCCN emission originates from an expanding and accelerating molecular envelope. The HCCCN rotational temperature varies with velocity, peaks at 520 K, 3 km/s blue-shifted from the systemic velocity and decreases with higher blueshift of the gas. The column density of the absorbing HCCCN is 3-6 x 1E17 cm^2. We modeled spectra based on spherical models of the expanding envelope which provide 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.

astro-ph↗

arXiv2001

High-Mass Stars as Early Signpost of Cluster Formation

The connection between high-mass stars and young stellar clusters has been well established by near-IR observations showing young massive stars in HII regions surrounded by clusters of lower mass stars. Mm-wave observations show that these clusters form in approx. 1000 Msol dense cores. Interestingly, the mm-wave observations of very active cluster-forming regions also reveal the presence of seemingly quiescent dense massive cores, which contain few signs of ongoing star formation. In the W3 Main region we find an approx. 1000 Msol mass dense core which contains no mid-IR or cm-continuum sources. Near-IR imaging has failed to detect any embedded sources. Yet, we suggest that this region is at an early stage of cluster formation. This is given support by comparative observations of the NGC 6334 GMC. Here, we find another quiescent massive 1000 Msol core without mid-IR sources, HII regions, or a detectable near-IR cluster. Motivated by the presence of CH3OH and H2O masers in this IR-quiet core, we have searched for and detected a bipolar outflow driven by a young massive star deeply embedded in the core. The presence of an embedded high-mass star strengthens our arguments that IR-quiet cores are at an early stage of cluster formation. We propose that masers and outflows from very deeply embedded (and consequently IR-quiet) high-mass stars are among the first incipient signposts of cluster formation in massive cores. Accordingly, we have undertaken a SEST survey of 46 CH3OH maser clusters which are not coincident with IRAS sources or HII regions. From this sample, we detected outflows in transitions of SiO toward 15 of the maser clusters. These sources have now been observed at 1200 micron with SIMBA, the 37-multichannel bolometer at SEST. Each one is associated with a bright 1 mm point source.

astro-ph↗