U. E. Graf

U. E. Graf
Bern University of Applied Sciences | BFH · Department of Engineering and Information Technology

PhD

About

118
Publications
5,169
Reads
How we measure 'reads'
A 'read' is counted each time someone views a publication summary (such as the title, abstract, and list of authors), clicks on a figure, or views or downloads the full-text. Learn more
1,803
Citations
Citations since 2016
9 Research Items
770 Citations
2016201720182019202020212022050100150
2016201720182019202020212022050100150
2016201720182019202020212022050100150
2016201720182019202020212022050100150

Publications

Publications (118)
Preprint
Full-text available
FEEDBACK is a SOFIA legacy program dedicated to study the interaction of massive stars with their environment. It performs a survey of 11 galactic high mass star forming regions in the 158 $\mu$m (1.9 THz) line of CII and the 63 $\mu$m (4.7 THz) line of OI. We employ the 14 pixel LFA and 7 pixel HFA upGREAT instrument to spectrally resolve (0.24 MH...
Article
FEEDBACK is a SOFIA (Stratospheric Observatory for Infrared Astronomy) legacy program dedicated to study the interaction of massive stars with their environment. It performs a survey of 11 galactic high mass star-forming regions in the 158 μm (1.9 THz) line of [C II] and the 63 μm (4.7 THz) line of [O I]. We employ the 14 pixel Low Frequency Array...
Preprint
Context. The [CII] 158 {\mu}m far-infrared (FIR) fine-structure line is one of the most important cooling lines of the star-forming interstellar medium (ISM). High spectral resolution observations have shown complex structures in the line profiles of the [CII] emission. Aims. Our aim is to determine whether the complex profiles observed in [^{12}CI...
Article
Context. The [C II ] 158 μ m far-infrared fine-structure line is one of the most important cooling lines of the star-forming interstellar medium (ISM). It is used as a tracer of star formation efficiency in external galaxies and to study feedback effects in parental clouds. High spectral resolution observations have shown complex structures in the...
Article
Full-text available
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 stars). Stellar winds, supernova explosions and ionization by ultraviolet photons control the lifetimes of mol...
Article
Full-text available
We present the performance of the upGREAT heterodyne array receivers on the SOFIA telescope after several years of operations. This instrument is a multi-pixel high resolution (R > 10^7) spectrometer for the Stratospheric Observatory for Far-Infrared Astronomy (SOFIA). The receivers use 7-pixel subarrays configured in a hexagonal layout around a ce...
Article
We present a new multi-pixel high resolution (R >10^7) spectrometer for the Stratospheric Observatory for Far-Infrared Astronomy (SOFIA). The receiver uses 2 x 7-pixel subarrays in orthogonal polarization, each in an hexagonal array around a central pixel. We present the first results for this new instrument after commissioning campaigns in May and...
Article
We review the development of multi-pixel heterodyne receivers for astronomical research in the submillimeter and terahertz spectral domains. We shortly address the historical development, highlighting a few pioneering instruments. A discussion of the design concepts is followed by a presentation of the technologies employed in the various receiver...
Article
We present the first superconducting hot electron bolometer (HEB) waveguide mixer operating at 4.7 THz. The 5.5-nm-thick, 300-nm-long, and 3600-nm-wide NbN HEB microbridge is integrated into a normal metal (Au) planar circuit on a 2 m thick silicon substrate. This circuit is integrated in a 24 m 48 m 21 m waveguide cavity and a 14 m 7 m 200 m subst...
Article
Full-text available
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 1383 GHz $^2\Pi_{3/2} J=5/2-3/2$ transitions of SH towards the star-forming regions W31C, G29.96-0.02, G34.3+0.1, W49N and W51, detecting foreg...
Article
Far Infrared Astronomy is one of the last explored fields in astronomy. Several bolometer cameras have already been developed with several hundred or more pixels and there is rapid progress in this area, however high resolution spectroscopy using heterodyne receivers have still not reached that level of maturity. Indeed, until now, only single pixe...
Article
Full-text available
The BD+40\degree4124 region was observed with high angular and spectral resolution with the German heterodyne instrument GREAT in CO J = 13 \rightarrow 12 and [CII] on SOFIA. These observations show that the [CII] emission is very strong in the reflection nebula surrounding the young Herbig Ae/Be star BD+40\degree4124. A strip map over the nebula s...
Article
Full-text available
We describe the design and construction of GREAT, the 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 highresolution far-infrared (FIR) spectroscopy. Selected for SOFIA's Early Science demonstration, the instrumen...
Article
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 w...
Article
Aims. We aim at deriving the excitation conditions of the interstellar gas as well as the local FUV intensities in the molecular cloud surrounding NGC 3603 to get a coherent picture of how the gas is energized by the central stars. Methods. The NANTEN2-4 m submillimeter antenna is used to map the [CI] 1-0, 2-1 and CO 4-3, 7-6 lines in a 2′ × 2′ reg...
Article
The Stratospheric TeraHertz Observatory (STO) is a NASA funded, Long Duration Balloon (LDB) experiment designed to address a key problem in modern astrophysics: understanding the Life Cycle of the Interstellar Medium (ISM). STO will survey a section of the Galactic plane in the dominant interstellar cooling line [C II] (1.9 THz) and the important s...
Article
Full-text available
Aims: We aim at deriving the excitation conditions of the interstellar gas as well as the local FUV intensities in the molecular cloud surrounding NGC 3603 to get a coherent picture of how the gas is energized by the central stars. Methods: The NANTEN2-4m submillimeter antenna is used to map the [CI] 1-0, 2-1 and CO 4-3, 7-6 lines in a 2' x 2' regi...
Article
Full-text available
The Stratospheric TeraHertz Observatory (STO) is a NASA funded, Long Duration Balloon (LDB) experiment designed to address a key problem in modern astrophysics: understanding the Life Cycle of the Interstellar Medium (ISM). STO will survey a section of the Galactic plane in the dominant interstellar cooling line [C II] (1.9 THz) and the important s...
Article
Full-text available
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 int...
Article
Full-text available
NGC 2024, a sites of massive star formation, have complex internal structures caused by cal heating by young stars, outflows, and stellar winds. These complex cloud structures lead to intricate emission line shapes. The goal of this paper is to show that the complex line shapes of 12 CO lines in NGC 2024 can be explained consistently with a model,...
Article
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 oper...
Article
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 operat...
Article
Full-text available
GREAT (German REceiver for Astronomy at Terahertz frequencies) has been selected as first-light instrument for the early science flights of SOFIA, scheduled for early 2009. In its first-light configuration GREAT will allow observations in two out of three FIR bands: two low frequency channels 1.25-1.5 THz and 1.82-1.92 THz for observations of, e.g....
Article
Full-text available
Context: Star formation at earlier cosmological times took place in an interstellar medium with low metallicity. The Large Magellanic Cloud (LMC) is ideally suited to study star formation in such an environment. Aims: The physical and chemical state of the ISM in a star forming environment can be constrained by observations of submm and FIR spectra...
Article
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 ˜ 10^6...
Article
The development of powerful radiation sources and sensitive detector systems plays a key role to achieve good signal to noise ratios for THz spectroscopy. Several spectrometer setups based on heterodyne detection have been explored recently in our laboratory. % which is known as a very sensitive detection %method. % Absorption and emission spectra...
Article
Full-text available
Context.The Carina region is an excellent astrophysical laboratory for studying the feedback mechanisms of newly born, very massive stars within their natal giant molecular clouds (GMCs) at only 2.35 kpc distance.Aims.We use a clumpy PDR model to analyse the observed intensities of atomic carbon and CO and to derive the excitation conditions of the...
Article
Full-text available
Studying molecular gas in the central regions of the star burst galaxies NGC4945 and Circinus enables us to characterize the physical conditions and compare them to previous local and high-z studies. We estimate temperature, molecular density and column densities of CO and atomic carbon. Using model predictions we give a range of estimated CO/C abu...
Article
The CO N+ Deuterium Observations Receiver, CONDOR, is one of the very few existing THz heterodyne receivers in the world. It covers a frequency range between 1.25 and 1.53 THz (240mum - 196mum). CONDOR was built with a focus on observing star forming regions, using high-J CO lines H2D+ and N+ In November 2005 we tested CONDOR on the APEX telescope,...
Article
We present the first observations of emission from the CO J=13-12 transition from the Orion KL region in OMC-1. The observations were made possible with the CO N+ Deuterium Observations Receiver (CONDOR) on the APEX telescope. CONDOR is a heterodyne receiver specifically designed to make velocity-resolved observations of spectral lines between 1.25...
Article
Full-text available
The Carina region is an excellent astrophysical laboratory for studying the feedback mechanisms of newly born, very massive stars within their natal giant molecular clouds (GMCs) at only 2.35 kpc distance. We use a clumpy PDR model to analyse the observed intensities of atomic carbon and CO and to derive the excitation conditions of the gas. The NA...
Article
GREAT, the German REceiver for Astronomy at Terahertz frequencies, is a first generation SOFIA dual channel heterodyne PI-instrument for high resolution spectroscopy. The system is developed by a consortium of German research institutes. The receiver will allow simultaneous observations in two out of the following three far-infrared frequency bands...
Article
The 1.9 THz local-oscillator (LO) of the GREAT heterodyne receiver is presented. The LO is based on a frequency tripled backward-wave oscillator source. The frequency stabilization system is described and an astigmatic imaging system, developed for improved beam coupling, is presented. Allan variances and temperature dependent power drifts are anal...
Article
Full-text available
In Gaussian Optics, beams with a circular cross section can easily be matched by using off-axis mirrors. Since the E-field distribution of a Gaussian beam mode is entirely independent in the two directions perpendicular to its axis of propagation, it is also possible to employ the formalism for asymmetric beams. Conventional off-axis mirrors, howev...
Article
Full-text available
We present an overview of a high-mass star formation region through the major (sub-)mm, and far-infrared cooling lines to gain insight into the physical conditions and the energy budget of the molecular cloud. We used the KOSMA 3m telescope to map the core ($10'\times 14'$) of the Galactic star forming region DR 21/DR 21 (OH) in the Cygnus X region...
Article
The CO N+ Deuterium Observations Receiver (CONDOR) is an astronomical heterodyne receiver that successfully operates between 1.25-1.53 THz. The instrument follows the standard heterodyne design, using a solid state local oscillator and a superconducting NbTiN hot electron bolometer (HEB) as a mixer. For easy maintenance at the telescope CONDOR is t...
Article
This paper presents a method to manufacture a low loss window applicable for THz frequencies. The window is made out of high resistivity silicon (3 kΩ cm, n = 3.42, and α = 0.1/cm). Reflective loss due to the impedance mismatch between the substrate and free space is overcome by etching (Bosch-process) rectangular grooves of depth λ/4 into the subs...
Article
Full-text available
The CO N+ 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 H[2]D^+ emission. The instrument follows the standard heterodyne design. It uses a solid state local oscillator (LO), whose signal is overlaid with that of the sky usi...
Article
Full-text available
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 (200-240m) atmospheric windows. CONDOR's first light observations were made with the APEX telescope in November 2005. The CONDOR beam on APEX (at = 1.5 THz) was expected to consist of a 4.3 main b...
Article
Full-text available
Development of a dual-color heterodyne instrument for use with the Atacama Pathfinder EXperiment. Commissioning of the APEX began in mid 2004, and regular science operation has been performed since July 2005. Verification of the telescope required a dual-channel receiver operating at (short) submillimeter wavelengths. It was important for the chara...
Conference Paper
GREAT, the German REceiver for Astronomy at Terahertz frequencies, is a first generation SOFIA dual channel heterodyne PI-instrument for high resolution spectroscopy. The system is developed by a cosortium of German research institutes. The receiver will allow simultaneous observations in two out of the following three far-infrared frequency bands:...
Article
Full-text available
The THz atmospheric windows centered at roughly 1.3 and 1.5~THz, contain numerous spectral lines of astronomical importance, including three high-J CO lines, the N+ line at 205 microns, and the ground transition of para-H2D+. The CO lines are tracers of hot (several 100K), dense gas; N+ is a cooling line of diffuse, ionized gas; the H2D+ line is a...
Article
Full-text available
The CO N + Deuterium Observations Receiver (CONDOR) is a modular heterodyne receiver for 1.25 to 1.5 THz frequencies. It will be used on the Atacama Pathfinder EXperiment (APEX), a 12 m telescope in Chile, and the Stratospheric Observatory For Infrared Astronomy (SOFIA), a 2.7 m telescope mounted in a high-altitude aircraft, as a low frequency exte...
Article
We present a project overview and science goals of the upcoming NANTEN2 project. This is an upgrade of the 4-m mm telescope, NANTEN, which was operated at Las Campanas Observatory, Chile. We have been carrying out extensive molecular cloud surveys in the Galaxy as well as toward the Magellanic system for seven years. The upgrade started by moving N...
Article
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...
Article
Since August 2005 the Atacama Pathfinder EXperiment (APEX), a novel 12m submillimeter telescope is in science operation. As a powerful mapping tool for the 345 GHz and the 460 GHz atmospheric windows we launched the development of a dual-color 7-pixel heterodyne array receiver. The beam pattern is planed to be hexagonal while both colors will be ob...
Conference Paper
We give a review on the present status and perspectives of THz-receiver development for the new observatories going to cover this very important, but little explored frequency range. The rapid advances in all components along the signal chain, starting from the receiver optics and the THz-local oscillators and continuing with the heterodyne mixers,...
Article
Full-text available
We are developing a new submillimeter heterodyne array receiver for the KOSMA 3m telescope on Gornergrat, Switzerland. In order to facilitate assembly and maintenance, the receiver consists of two self-contained 3×3 beam cartridge-type receiver modules. They employ new focal plane array optics which are both fully reflective and scalable in frequen...
Article
Full-text available
The availability of large scale, high angular resolution Galactic continuum surveys from cm to near-IR wave lengths makes complementary spectroscopic surveys increasingly more important. The velocity information provided by spectral line surveys is crucial not only for the study of internal cloud dynamics, but also to determine distances, masses, s...
Article
We are developing a 1.4 THz receiver to explore the far infrared universe, such as probing high mass star forming regions using, e.g., the high J transitions of CO, investigating the warm interstellar medium in N+ or probing cold and dense stellar cores in H2D+. Due to the poor atmospheric transmission at these frequencies we are planning to use th...
Article
We present the first astronomical results from DesertSTAR, a 7 pixel heterodyne array receiver designed for operation in the astrophysically rich 345 GHz atmospheric window. DesertSTAR was constructed for the 10m Heinrich Hertz Telescope located at 3150m elevation on Mt. Graham, Arizona. This receiver promises to increase mapping speed at the HHT b...
Conference Paper
We present the first astronomical results from DesertSTAR, a 7 pixel heterodyne array receiver designed for operation in the astrophysically rich 345 GHz atmospheric window. DesertSTAR was constructed for the 10m Heinrich Hertz Telescope located at 3150m elevation on Mt. Graham, Arizona. This receiver promises to increase mapping speed at the HHT b...
Article
The German REceiver for Astronomy at Terahertz frequencies (GREAT) is a first generation PI instrument for the SOFIA telescope, developed by a collaboration between the MPIfR, KOSMA, DLR, and the MPAe. The first three institutes each contribute one heterodyne receiver channel to operate at 1.9, 2.7 and 4.7 THz, respectively. A later addition of a e...
Article
We are developing a 1.4 THz receiver to explore the far infrared universe, such as probing high mass star forming regions using, e.g., the high J transitions of CO, investigating the warm interstellar medium in N+ or probing cold and dense stellar cores in H2D+. Due to the poor atmospheric transmission at these frequencies we are planning to use th...