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.
This study assesses the performance of the Weather Research and Forecasting (WRF) model to represent the near-surface weather conditions and the precipitable water vapour (PWV) in the Chajnantor ...plateau, in the north of Chile, from 2007 April to December. The WRF model shows a very good performance forecasting the near-surface temperature and zonal wind component, although it overestimates the 2 m water vapour mixing ratio and underestimates the 10 m meridional wind component. The model represents very well the seasonal, intraseasonal and the diurnal variation of PWV. However, the PWV errors increase after the 12 h of simulation. Errors in the simulations are larger than 1.5 mm only during 10 per cent of the study period, they do not exceed 0.5 mm during 65 per cent of the time and they are below 0.25 mm more than 45 per cent of the time, which emphasizes the good performance of the model to forecast the PWV over the region. The misrepresentation of the near-surface humidity in the region by the WRF model may have a negative impact on the PWV forecasts. Thus, having accurate forecasts of humidity near the surface may result in more accurate PWV forecasts. Overall, results from this, as well as recent studies, supports the use of the WRF model to provide accurate weather forecasts for the region, particularly for the PWV, which can be of great benefit for astronomers in the planning of their scientific operations and observing time.
Context. Sites of massive star formation have complex internal structures. Local heating by young stars and kinematic processes, such as outflows and stellar winds, generate large temperature and ...velocity gradients. Complex cloud structures lead to intricate emission line shapes. CO lines from high mass star forming regions are rarely Gaussian and show often multiple peaks. Furthermore, the line shapes vary significantly with the quantum number Jup, due to the different probed physical conditions and opacities. Aims. The goal of this paper is to show that the complex line shapes of 12CO and 13CO in NGC 2024 showing multiple emission and absorption features, which vary with rotational quantum number J can be explained consistently with a model, whose temperature and velocity structure are based on the well-established scenario of a PDR and the “Blister model”. Methods. We present velocity-resolved spectra of seven 12CO and 13CO lines ranging from $J_{\rm up}=3$ to $J _{\rm up}=13$. We combined these data with 12CO high-frequency data from the ISO satellite and analyzed the full set of CO lines using an escape probability code and a one-dimensional full radiative transfer code. Results. We find that the bulk of the molecular cloud associated with NGC 2024 consists of warm (75 K) and dense ($9\times 10^5$ cm-3) gas. An additional hot (~300 K) component, located at the interface of the HII region and the molecular cloud, is needed to explain the emission of the high-J CO lines. Deep absorption notches indicate that very cold material (~20 K) exists in front of the warm material, too. Conclusions. A temperature and column density structure consistent with those predicted by PDR models, combined with the velocity structure of a “Blister model”, appropriately describes the observed emission line profiles of this massive star forming region. This case study of NGC 2024 shows that, with physical insights into these complex regions and careful modeling, multi-line observations of 12CO and 13CO can be used to derive detailed physical conditions in massive star forming regions.
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 spectral lines of the main carbon carrying species, CO, C I and C II, which originate in the surface layers of molecular clouds illuminated by the UV radiation of the newly formed, young stars. Methods. We present high-angular resolution sub- millimeter observations in the N159W region in the LMC obtained with the NANTEN2 telescope of the super(12) CO J = 4 \to 3, J = 7 \to 6, and super(13) CO J = 4 \to 3 rotational and C I super(3) P sub(1)- super(3) P sub(0) and super(3) P sub(2)- super(3) P sub(1) fine-structure transitions. The super(13) CO J = 4 \to 3 and C I super(3) P sub(2)- super(3) P sub(1) transitions are detected for the first time in the LMC. We derive the physical and chemical properties of the low-metallicity molecular gas using an escape probability code and a self-consistent solution of the chemistry and thermal balance of the gas in the framework of a clumpy cloud PDR model. Results. The separate excitation analysis of the submm CO lines and the carbon fine structure lines shows that the emitting gas in the N159W region has temperatures of about 80 K and densities of about 10 super(4) cm super(-3). The estimated C to CO abundance ratio close to unity is substantially higher than in dense massive star-forming regions in the Milky Way. The analysis of all observed lines together, including the C II line intensity reported in the literature, in the context of a clumpy cloud PDR model constrains the UV intensity to about \chi \approx 220 and an average density of the clump ensemble of about 10 super(5) cm super(-3), thus confirming the presence of high density material in the LMC N159W region.
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 substrate as an antireflection (AR) layer. The refractive index of the AR-layer depends on the incident wave mode (TE/TM), i.e., the AR-layer is a birefringent medium. A short theoretical description is given.
Simulation of the AR-layer was done by using a transmission line analogue and the
Scatter-program written by Padman. FTS measurements yield a transmittance greater than 96% at 2.1
THz and a band width of 400
GHz (1.9–2.3
THz) with ⩾90% transmission.
The 1.9THz 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 analyzed. The LO is designed as a stand-alone system and fits into GREAT’s local-oscillator compartments. It produces more than 1.5μW of stable output power to pump the hot electron bolometer mixers of the GREAT instrument.
Context. Studying molecular gas in the central regions of the star burst galaxies NGC 4945 and Circinus enables us to characterize the physical conditions and compare them to previous local and ...high-z studies. Aims.We estimate temperature, molecular density and column densities of CO and atomic carbon. Using model predictions we give a range of estimated CO/C abundance ratios. Methods.Using the new NANTEN2 4 m sub-millimeter telescope in Pampa La Bola, Chile, we observed for the first time CO 4–3 and $\ion{C}{i}$ $\rm ^3P_1{-}^3P_0$ at the centers of both galaxies at linear scale of 682 pc and 732 pc respectively. We compute the cooling curves of 12CO and 13CO using radiative transfer models and estimate the physical conditions of CO and CI. Results.The centers of NGC 4945 and Circinus are very $\ion{C}{i}$ bright objects, exhibiting $\ion{C}{i}$ $\rm ^3P_1{-}^3P_0$ luminosities of 91 and 67 K km s-1 kpc2, respectively. The $\ion{C}{i}$ $\rm ^3P_1{-}^3P_0$/CO 4–3 ratio of integrated intensities are large at 1.2 in NGC 4945 and 2.8 in Circinus. Combining previous CO $J= 1{-}0$, 2–1 and 3–2 and 13CO $J= 1{-}0$, 2–1 studies with our new observations, the radiative transfer calculations give a range of densities, $n(\rm H_{2})=10^{3}{-}3 \times 10^{4}$ cm-3, and a wide range of kinetic temperatures, $T_{\rm kin}= 20{-}100$ K, depending on the density. To discuss the degeneracy in density and temperature, we study two representative solutions. In both galaxies the estimated total CI cooling intensity is stronger by factors of ~$1{-}3$ compared to the total CO cooling intensity. The CO/C abundance ratios are 0.2-2, similar to values found in Galactic translucent clouds. Conclusions.Our new observations enable us to further constrain the excitation conditions and estimate the line emission of higher-J CO- and the upper CI-lines. For the first time we give estimates for the CO/C abundance ratio in the center regions of these galaxies. Future CO $J= 7{-}6$ and CI 2–1 observations will be important to resolve the ambiguity in the physical conditions and confirm the model predictions.
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