Search

We present a home-built chirped-pulse Fourier transform millimeter wave (CP-FTMMW) spectrometer. The setup is devoted to the sensitive recording of high-resolution molecular spectroscopy in the W band between 75 and 110 GHz. We describe the experimental setup in detail, including a characterization of the chirp excitation source, the optical beam path, and the receiver. The receiver is a further development of our 100 GHz emission spectrometer. The spectrometer is equipped with a pulsed jet expansion and a DC discharge. Spectra of methyl cyanide as well as hydrogen cyanide (HCN) and hydrogen isocyanide (HNC) products from the DC discharge of this molecule are recorded to characterize the performance of the CP-FTMMW instrument. The formation of the HCN isomer is favored by a factor of 63 with respect to HNC. Hot/cold calibration measurements enable a direct comparison of the signal and noise levels of the CP-FTMMW spectra to those of the emission spectrometer. For the CP-FTMMW instrument, we find many orders of magnitude of signal enhancement and a much stronger noise reduction due to the coherent detection scheme. [ABSTRACT FROM AUTHOR]

Massive stars inject mechanical and radiative energy into the surrounding environment, which stirs it up, heats the gas, produces cloud and intercloud phases in the interstellar medium, and disrupts molecular clouds (the birth sites of new stars1,2). Stellar winds, supernova explosions and ionization by ultraviolet photons control the lifetimes of molecular clouds3-7. Theoretical studies predict that momentum injection by radiation should dominate that by stellar winds8, but this has been difficult to assess observationally. Velocity-resolved large-scale images in the fine-structure line of ionized carbon ([C II]) provide an observational diagnostic for the radiative energy input and the dynamics of the interstellar medium around massive stars. Here we report observations of a one-square-degree region (about 7 parsecs in diameter) of Orion molecular core 1—the region nearest to Earth that exhibits massive-star formation—at a resolution of 16 arcseconds (0.03 parsecs) in the [C II] line at 1.9 terahertz (158 micrometres). The results reveal that the stellar wind originating from the massive star θ1 Orionis C has swept up the surrounding material to create a 'bubble' roughly four parsecs in diameter with a 2,600-solar-mass shell, which is expanding at 13 kilometres per second. This finding demonstrates that the mechanical energy from the stellar wind is converted very efficiently into kinetic energy of the shell and causes more disruption of the Orion molecular core 1 than do photo-ionization and evaporation or future supernova explosions. Wind from the most massive star in the Trapezium cluster in Orion has carved out a large and expanding cavity around the cluster, bounded by a thin, 2,600-solar-mass shell. [ABSTRACT FROM AUTHOR]

We present first results on a newly built broadband emission spectrometer for the laboratory making use of a double sideband (DSB) heterodyne receiver. The new spectrometer is perfectly suited for high-resolution emission spectroscopy of molecules of astrophysical importance. The current SIS receiver operates at RF frequencies between 270 and 390 GHz, coincident with Band 7 of the ALMA telescope. The instantaneous bandwidth is 5 GHz (DSB). In this work the full spectrometer and its components are described. Its performance, in particular its sensitivity, stability, reproducibility and systematic errors, is characterized in detail. For this purpose very broad band emission spectra of methyl cyanide have been recorded and compared to theoretical spectra. Isotopic variants are found in natural abundance and features attributed to vibrationally excited species are all recorded in the same spectrum. The performance of the new spectrometer is compared extensively to that of a traditional FM-absorption spectrometer and to recent versions of chirped-pulse spectrometers operated in the mm-wave regime. Further applications and future advancements of the current instrument are discussed. [ABSTRACT FROM AUTHOR]

We have observed five sulphur-bearing molecules in foreground diffuse molecular clouds lying along the sight-lines to five bright continuum sources. We have used the GREAT instrument on SOFIA to observe the SH 1383 GHz 2Π3/2 ← = 5/2 = 3/2 lambda doublet toward the star-forming regions W31C, G29.96-0.02, G34.3+0.1, W49N and W51, detecting foreground absorption towards all five sources; and the EMIR receivers on the IRAM 30 m telescope at Pico Veleta to detect the H2S 110-101 (169 GHz), CS J = 2-1 (98 GHz) and SO 32-21 (99 GHz) transitions. Upper limits on the H3S+ 10-00 (293 GHz) transition were also obtained at the IRAM 30 m. In nine foreground absorption components detected towards these sources, the inferred column densities of the four detected molecules showed relatively constant ratios, with N(SH)/N(H2S) in the range 1.1-3.0, N(CS)/N(H2S) in the range 0.32-0.61, and N(SO)/N(H2S) in the range 0.08-0.30. The column densities of the sulphur-bearing molecules are very well correlated amongst themselves, moderately well correlated with CH (a surrogate tracer for H2), and poorly correlated with atomic hydrogen. The observed SH/H2 ratios - in the range 5 to 26 × 10-9 - indicate that SH (and other sulphur-bearing molecules) account for «1% of the gas-phase sulphur nuclei. The observed abundances of sulphur-bearing molecules, however, greatly exceed those predicted by standard models of cold diffuse molecular clouds, providing further evidence for the enhancement of endothermic reaction rates by elevated temperatures or ion-neutral drift. We have considered the observed abundance ratios in the context of shock and turbulent dissipation region (TDR) models. Using the TDR model, we find that the turbulent energy available at large scale in the diffuse ISM is sufficient to explain the observed column densities of SH and CS. Standard shock and TDR models, however, fail to reproduce the column densities of H2S and SO by a factor of about 10; more elaborate shock models - in which account is taken of the velocity drift, relative to H2, of SH molecules produced by the dissociative recombination of H3S+ - reduce this discrepancy to a factor ~3. [ABSTRACT FROM AUTHOR]

Context. We analyze the NGC 2024 Ho region and molecular cloud interface using [12CII] and [13 CII] observations. Alma. We attempt to gain insight into the physical structure of the interface layer between the molecular cloud and the HII region. Methods. Observations of [`12CII] and [13Co] emission at 158 μm with high spatial and spectral resolution allow us to study the detailed structure of the ionization front and estimate the column densities and temperatures of the ionized carbon layer in the photon- dominated region. Results. The [12CII] emission closely follows the distribution of the 8 μm continuum. Across most of the source, the spectral lines have two velocity peaks similar to lines of rare CO isotopes. The [13;CII] emission is detected near the edge-on ionization front. It has only a single velocity component, which implies that the [°Cti] line shape is caused by self-absorption. An anomalous hyperfine line-intensity ratio observed in [13 CII] cannot yet be explained. Conclusions. Our analysis of the two isotopes results in a total column density of N(H) 1 .6 x 1023 cm-2 in the gas emitting the [CII] line. A large fraction of this gas has to beat a temperature of several hundred K. The self-absorption is caused by a cooler (T ≤ 100 K) foreground component containing a column density of N(H) ≈ 1022 cm-2. [ABSTRACT FROM AUTHOR]

We describe the design and construction of GREAT (German REceiver for Astronomy at Terahertz frequencies) operated on the Stratospheric Observatory For Infrared Astronomy (SOFIA). GREAT is a modular dual-color heterodyne instrument for high- resolution far-infrared (FIR) spectroscopy, Selected for SOFIA's Early Science demonstration, the instrument has successfully per- formed three Short and more than a dozen Basic Science flights since first light was recorded on its April 1, 2011 commissioning flight. We report on the in-flight performance and operation of the receiver that - in various flight configurations, with three different detector channels - observed in several science-defined frequency windows between 1.25 and 2.5 THz. The receiver optics was verified to be diffraction-limited as designed, with nominal efficiencies; receiver sensitivities are state-of-the-art, with excellent system stability. The modular design allows for the continuous integration of latest technologies; we briefly discuss additional channels under development and ongoing improvements for Cycle 1 observations. GREAT it a principal investigator instrument, developed by a consortium of four German research institutes, available to the SOFIA users on a collaborative basis. [ABSTRACT FROM AUTHOR]

By observing radiation-affected gas in the Cepheus B molecular cloud, we probe whether the sequential star formation in this source is triggered by the radiation from newly formed stars. We used the dual band receiver GREAT onboard SOFIA to map [C II] and CO 13-12 and 11-10 in Cep B and compared the spatial distribution and the spectral profiles with complementary ground-based data of low-i transitions of CO isotopes, atomic carbon, and the radio continuum. The interaction of the radiation from the neighboring OB association creates a large photon-dominated region (PDR) at the surface of the molecular cloud traced through the photoevaporation of C+. Bright internal PDRs of hot gas are created around the embedded young stars, where we detect evidence of the compression of material and local velocity changes; however, on the global scale we find no indications that the dense molecular material is dynamically affected. [ABSTRACT FROM AUTHOR]

Context. The H IL region/PDR,/molecular cloud complex S 106 is excited by a single 0-star. The full extent of the warm and dense gas close to the star has not been mapped in spectrally resolved high-i CO or [Cu] lines, to the kinematics of the warm, partially ionized gas, see unknown. Whether the prominent dark lane bisecting the hourglass-shaped nebula is due solely to the shadow cast by a small disk around the exciting star or also to extinction in high column foreground gas was an open question until now. Aims. We disentangle the morphology and kinematics of warm neutral and ionized gas close to the star, study their relation to the hulk of she molecular gas, and we investigate the nature of the dark lane. Methods. We used the heterodyne receiver GREAT onboard on SOFIA to observe velocity resolved spectral lines of [Cu] and CO 11 → 10 in comparison with so far unpublished submm continuum data at 350 μm (SHARC-II) and complementary molecular line data. Results. The high angular and spectral resolution observations show a very complex morphology and kinematics of the inner 5106 region, with many different components at different excitation conditions contributing to the observed emission. The [C II] lines are found so he bright and very broad, tracing high velocity gas clone to the interface of molecular cloud and Hit region. CO 11→10 emission is more confined, high spatially and in velocity, to the immediate surroundings of S106 IR showing the presence of warm, high density (clumpy) gas. Our high angular resolution subm.sn continuum observations rule out the scenario where she dark tune separating the two lobes is due solely to the shadow cast by a small disk close to the star. The lane is clearly seen also as warm, high column density gas at the boundary of the molecular cloud and H u region. [ABSTRACT FROM AUTHOR]

The BD+40°4124 region was observed with high angular and The BD+40°4124 region was observed with high angular and spectral resolution with the German heterodyne instrument GREAT in CO J = 13 → 12 and [C II] on SOFIA. These observations show that the [C II] emission is very strong in the reflection nebula surrounding the young Herbig Ae/Be star BD+40°4124. A strip map over the nebula shows that the [CII] emission approximately coincides with the optical nebulosity. The strongest [C II] emission is centered on the B2 star and a deep spectrum shows that it has faint wings, which suggests that the ionized gas is expanding. We also see faint CO J = 13 → 12 at the position of BD+40°4124, which suggests that the star may still be surrounded by an accretion disk. We also detected [C II] emission and strong CO J = 13 → 12 toward V 1318 Cyg. Here the [C1] emission is fainter than in BD+40°4124 and appears to come from the outflow, since it shows red and blue wings with very little emission at the systemic velocity, where the CO emission is quite strong. It therefore appears that in the broad ISO beam the [CII] emission was dominated by the reflection nebula surrounding BD+4fF4124, while the high J CO lines originated from the adjacent younger and more deeply embedded binary system V 1318 Cyg. [ABSTRACT FROM AUTHOR]

The GREAT observations need frequency-selective calibration across the passband for the residual atmospheric opacity at flight altitude. At these altitudes the atmospheric opacity has both narrow and broad spectral features. To determine the atmospheric transmission at high spectral resolution, GREAT compares the observed atmospheric emission with atmospheric model predictions, and therefore depends on the validity of the atmospheric models. We discuss the problems identified in this comparison with respect to the observed data and the models, and describe the strategy used to calibrate the science data from GREAT/SOFIA during the first observing periods. [ABSTRACT FROM AUTHOR]

We present 1.9 THz [Cii] and 4.7 THz [Oi] measurements of the star forming region NGC 2024 observed with GREAT on SOFIA. Velocity resolved spectroscopy reveals the strong foreground absorption, which conceals most of the intrinsic line brightness of the dense PDR region. Modelling of the line shapes allows disentangling the foreground absorption from the strong background emission. [ABSTRACT FROM AUTHOR]

We describe a focal-plane optics for millimeter- and submillimeter-wave array receivers which are both fully reflective—thus avoiding the absorption and reflection losses of dielectric lenses—and expandable to an arbitrary number of pixels. The optics unit cell consists of two mirrors and a feedhorn optimized for near-field operation. Employing an integrated optics approach the mirror setup consists of only three mechanical parts, independent of the number of pixels, and requires no internal optical alignment. With a 345 GHz 3×3-beam prototype a Gaussicity of >=98% and a focal-plane beam separation of 3.6 waist radii was obtained. In this article we present the optics design as well as numerical simulations and measured beam patterns. [ABSTRACT FROM AUTHOR]

We present observations of five 12CO and 13CO lines in NGC 2024, including the first velocity resolved observations of lines above 1 THz in this object (J = 13-12). We analysed the observations using radiative transfer codes and derived a more consistent model of this complex source. We found that the high-J CO emission stems from a dense (n ~ 106 cm-3) and hot (~300 K) component, located at the interface of the HII region and the molecular cloud. [ABSTRACT FROM AUTHOR]

We present a preliminary analysis of the self-absorbed [CII]-spectra observed with SOFIA/GREAT towards NGC 2024. Together with the detected [13CII] hyperfine satellites, the observed spectra require surprisingly high column densities of C+, both in the warm core and the foreground absorption component. Such high column densities are a challenge to explain with present state-of-the-art PDR models of the UV/molecular cloud interaction. [ABSTRACT FROM PUBLISHER]

The CO N+ Deuterium Observations Receiver (CONDOR) is one of the very few heterodyne receivers operational between 1.25 and 1.53 THz. It has successfully obtained first-light observations on APEX and is currently being modified to fly as the low frequency channel of GREAT on SOFIA. [ABSTRACT FROM AUTHOR]

The CON+Deuterium Observations Receiver (CONDOR) is a heterodyne receiver that operates between 1250–1530 GHz. Its primary goal is to observe star-forming regions in CO, N+, and H2D+ emission. [ABSTRACT FROM PUBLISHER]

CONDOR, the CO, N+, Deuterium Observations Receiver, is designed to make velocity-resolved observations of the CO, [NII], and p-H2D+ lines in the 1.4 THz (200-240μm) atmospheric windows. CONDOR's first light observations were made with the APEX telescope in November 2005. The CONDOR beam on APEX (at ν = 1.5 THz) was expected to consist of a 4.3″ main beam and a 73″ error beam; this beam structure was verified from scans of Mars. The pointing accuracy, also determined from Mars scans, was better than 7″. The average atmospheric transmission during our Orion observations (elev~57°) was 19 ± 4% along the line-of-sight. A forward efficiency of Feff = 0.8 was determined from sky dips, and observations of the Moon and Mars were used to couple the CONDOR beam to sources of different sizes (ηc = 0.40 and ~0.10, respectively). For more information, see Wiedner et al. 2006. [ABSTRACT FROM PUBLISHER]

We demonstrate for the first time the closure of an electronic phase lock loop for a continuous-wave quantum cascade laser (QCL) at 1.5 THz. The QCL is operated in a closed cycle cryo cooler. We achieved a frequency stability of better than 100 Hz, limited by the resolution bandwidth of the spectrum analyser. The PLL electronics make use of the intermediate frequency (IF) obtained from a hot electron bolometer (HEB) which is downconverted to a PLL IF of 125 MHz. The coarse selection of the longitudinal mode and the fine tuning is achieved via the bias voltage of the QCL. Within a QCL cavity mode, the free-running QCL shows frequency fluctuations of about 5 MHz, which the PLL circuit is able to control via the Stark-shift of the QCL gain material. Temperature dependent tuning is shown to be nonlinear, and of the order of -16 MHz/K. Additionally we have used the QCL as local oscillator (LO) to pump an HEB and perform, again for the first time at 1.5 THz, a heterodyne experiment, and obtain a receiver noise temperature of 1741 K.

We have fabricated large, coarsely ruled, echelle patterns on silicon wafers by using photolithography and chemical-etching techniques. The grating patterns consist of 142-microm-wide, V-shaped grooves with an opening angle of 70.6 degrees, blazed at 54.7 degrees. We present a detailed description of our grating-fabrication techniques and the results of extensive testing. We have measured peak diffraction efficiencies of 70% at lambda = 632.8 nm and conclude that the gratings produced by our method are of sufficient quality for use in high-resolution spectrographs in the visible and near IR (lambda approximately = 500-5000 nm).

We evaluated the optical performance of an IR echelle grating produced on a silicon wafer with anisotropic etching techniques. We measured the diffraction efficiency of a sample with a 55° blaze angle and 25-µm groove spacing. We also calculated the efficiency for typical triangular and trapezoidal groove profiles of etched gratings. The diffraction efficiency for unpolarized light can be approximately as high as the efficiency of right-angle groove gratings. The great potential of the etched silicon grating lies in its ease of fabrication, its excellent surface quality, and the high reproducibility of the production process. Compact high-resolution diffraction gratings can be produced by etching the grating pattern into the rear side of a transparent prism. When used in internal reflection, this increases the resolving power of the grating by a factor equal to the refractive index of the prism over a front surface grating of the same length.