S. Philipp

The APEX digital Fast Fourier Transform Spectrometer

Context. We present the technology and rst scientic results of a new generation of very e xible and sensitive spectrometers, well-suited for the needs of spectral-line radio and (sub)millimeter astronomy: Fast Fourier Transform Spectrometers (FFTS), which are in operation at the Atacama Pathnder EXperiment (APEX) telescope. Aims. The FFTS for APEX is a novel high-resolution 2 1 GHz bandwidth digital spectrometer backend. Due to its high frequency resolution, and the demonstrated capability of operating at high altitude, the FFTS became the facility spectrometer for spectral line observations at APEX. Methods. The FFTS is based on one of the currently most powerful digitizer/analyzer boards available from Acqiris, Switzerland. The board incorporates two 1 Gsamples/s analog-to-digital converters (ADCs) with 8-bit resolution which feed an on-board complex eld programmable gate array (FPGA) chip. The enormous processing power by today’s FPGAs allow for a complete real-time FFT signal processing pipeline to decompose a 1 GHz band into 16384 spectral channels in just one chip. Results. Since May 2005 an MPIfR FFTS has been extensively used in all regular spectroscopic science observations. The performance at APEX was demonstrated to be very reliable and as good as measured in the rst laboratory tests which nally led to the request to provide a second, facility type FFTS for APEX. The unit was delivered and commissioned in March this year. Conclusions. Using a commercially available digitizer board, it was possible to develop a complete FFTS in only a few months. Successful observations at APEX demonstrate that this new generation of FPGA-based spectrometers easily matching and superseding the performance of older technology spectrometers and can built up much more easily. Furthermore, the by now available class of new high-speed ADCs and the continuous increase of FPGA processing power makes it very likely that FFTS can be pushed to broader bandwidth and even more spectral channels in the near future.

First observations with CONDOR, a 1.5 THz heterodyne receiver

Context. 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 II] line at 205 µm, and the ground transition of para-H2D + . The CO lines are tracers of hot (several 100 K), dense gas; [N II] is a cooling line of diffuse, ionized gas; the H2D + line is a non-depleting tracer of cold (∼20 K), dense gas. Aims. As the THz lines benefit the study of diverse phenomena (from high-mass star-forming regions to the WIM to cold prestellar cores), we have built the CO N + Deuterium Observations Receiver (CONDOR) to further explore the THz windows by ground-based observations. Methods. CONDOR was designed to be used at the Atacama Pathfinder EXperiment (APEX) and Stratospheric Observatory For Infrared Astronomy (SOFIA). CONDOR was installed at the APEX telescope, and test observations were made to characterize the instrument. Results. The combination of CONDOR on APEX successfully detected THz radiation from astronomical sources. CONDOR operated with typical Trec = 1600 K and spectral Allan variance times of ∼30 s. CONDOR’s “first light” observations of CO 13−12 emission from the hot core Orion FIR4 revealed a narrow line with TMB ≈ 210 K and ∆V ≈ 5. 4k m s −1 . A search for [N II] emission from the ionization front of the Orion Bar resulted in a non-detection. Conclusions. The successful deployment of CONDOR at APEX demonstrates the potential for making observations at THz frequencies from ground-based facilities.

Submillimeter vibrationally excited water emission from the peculiar red supergiant VY Canis Majoris

Context. Vibrationally excited emission from the SiO and H2O molecules probes the innermost circumstellar envelopes of oxygenrich red giant and supergiant stars. VY CMa is the most prolific known emission source in these molecules. Aims. Observations were made to search for rotational lines in the lowest vibrationally excited state of H2O. Methods. The APEX telescope was used for observations of H2O lines at frequencies around 300 GHz. Results. Two vibrationally excited H2O lines were detected, a third one could not be found. In one of the lines we find evidence for weak maser action, similar to known (sub)millimeter ν2 = 1 lines. We find that the other line’s intensity is consistent with thermal excitation by the circumstellar infrared radiation field. Several SiO lines were detected together with the H2O lines.

APEX: the Atacama Pathfinder EXperiment

APEX, the Atacama Pathfinder Experiment, has been successfully commissioned and is in operation now. This novel submillimeter telescope is located at 5107 m altitude on Llano de Chajnantor in the Chilean High Andes, on what is considered one of the worlds outstanding sites for submillimeter astronomy. The primary reflector with 12 m diameter has been carefully adjusted by means of holography. Its surface smoothness of 17-18 μm makes APEX suitable for observations up to 200 μm, through all atmospheric submm windows accessible from the ground.

Water production in comet 81P/Wild 2 as determined by Herschel/HIFI

The high spectral resolution and sensitivity of Herschel/HIFI allows for the detection of multiple rotational water lines and accurate determinations of water production rates in comets. In this Letter we present HIFI observations of the fundamental 1_(10)–1_(01) (557 GHz) ortho and 1_(11)–0_(00) (1113 GHz) para rotational transitions of water in comet 81P/Wild 2 acquired in February 2010. We mapped the extent of the water line emission with five point scans. Line profiles are computed using excitation models which include excitation by collisions with electrons and neutrals and solar infrared radiation. We derive a mean water production rate of 1.0 × 10^(28) molecules s^(−1) at a heliocentric distance of 1.61 AU about 20 days before perihelion, in agreement with production rates measured from the ground using observations of the 18-cm OH lines. Furthermore, we constrain the electron density profile and gas kinetic temperature, and estimate the coma expansion velocity by fitting the water line shapes.

CO(4-3) and CO(7-6) maps of the nucleus of NGC 253

Context. Molecular line excitation studies of the nuclei of nearby starburst galaxies yield important information on the starburst phenomena, in particular on the temperature and density of the star-forming gas. Such studies also provide templates for high redshift galaxies with even more extreme star formation. Aims. Fundamental constraints on the physical properties in the nuclear regions of external galaxies can be derived from the spectral energy distribution (i.e., integrated flux density vs. rotational quantum number) of CO rotational emission arising from warm gas. Methods. The resolution and sensitivity of the APEX telescope makes it feasible to perform spatially resolved studies of submillimeter (submm) CO emission from the warm, dense gas in nearby starburst nuclei. Using the FLASH dual-channel heterodyne receiver we mapped emission in the CO J = 4-3 and 7-6 lines toward the archetypical nuclear starburst galaxy NGC 253. Results. Combining our new observations with data from the literature, we derive the CO line SED in the central 250 pc of NGC 253, which peaks near the 6-5 transition and has a shape very similar to that of M 82. All CO transitions in the central region can well be fitted with a single temperature/density Large Velocity Gradient (LVG) model. A good match to the observations is found by assuming kinetic gas temperatures that are comparable to the dust temperature (T kin ≈ 60 K) and a H 2 density of order 10 4 cm -3 . Conclusions. Our very first APEX submm study of a nearby starburst nucleus (NGC 253) meaningfully constrains the physical properties of the star-forming molecular gas it contains. With broader band spectrometers and a chopping secondary coming soon, the impact of APEX on extragalactic astrophysics will be foreseeably significant.

The distribution of ND2H in LDN 1689N

Aims. Finding tracers of the innermost regions of prestellar core s is important for understanding their chemical and dynamical evolution before the onset of gravitational collapse. While classical molec ular tracers, such as CO and CS, have been shown to be strongly depleted in cold, dense gas by condensation on grain mantles, it has been a subject of discussion to what extent nitrogen-bearing species, such as ammonia, are affected by this process. As deuterium fractionation is effi cient in cold, dense gas, deuterated species are excellent t racers of prestellar cores. A comparison of the spatial distribution of neutral and ioniz ed deuterated species with the dust continuum emission can thus provide important insights into the physical and chemical structure of such regions. Methods. We study the spatial distribution of the ground-state 335.5 GHz line of ND2H in LDN1689N, using APEX, and compare it with the distribution of the DCO + (3‐2) line, as well as the 350 µm dust continuum emission observed with the SHARC II bolometer camera at CSO. Results. While the distribution of the ND2H emission in LDN1689N is generally similar to that of the 350 µm dust continuum emission, the peak of the ND2H emission is offset by ∼10 ′′ to the East from the dust continuum and DCO + emission peak. ND2H and ND3 share the same spatial distribution. The observed offset between the ND2H and DCO + emission is consistent with the hypothesis that the deuterium peak in LDN1689N is an interaction region between the outflow shock from IRAS16293‐2422 and the dense ambient gas. We detect the J = 4 → 3 line of H 13 CO + at 346.998 GHz in the image side band serendipitously. This line shows the same spatial distribution as DCO + (3‐2), and peaks close to the 350 µm emission maximum which provides further support for the shock interaction scenario.

CHAMP + : a powerful array receiver for APEX

CHAMP+, a dual-color 2 × 7 element heterodyne array for operation in the 450 μm and 350 μm atmospheric windows is under development. The instrument, which is currently undergoing final evaluation in the laboratories, will be deployed for commissioning at the APEX telescope in August this year. With its state-of-the-art SIS detectors and wide tunable local oscillators, its cold optics with SSB filters and with 2 GHz of usable IF bandwidth per pixel, CHAMP+ will provide unmatched observing capabilities for the APEX community. The optics allows for simultaneous observations in both colors. For both sub-arrays a hexagonal arrangement with closest feasible spacing of the pixels on sky (2×Θmb) was chosen, which, in scanning mode, will provide data sampled with half-beam spacing. The front-end is connected to a flexible autocorrelator array with a total bandwidth of 32 GHz and 32768 spectral channels, subdivided into 32 IF bands of 1 GHz and 1024 channels each.

Galactic Center Molecular Clouds

At a distance of 8 kpc only, the center of the Galaxy provides a unique testbed for studies about the physics of the interstellar medium and star formation in the nuclei of galaxies. Since it shares many properties of other, more spectacular nuclei, a detailed investigation of the Galactic Center is a necessary step to gain better understanding of the physical processes governing galactic nuclei in general

Discovery of water vapour in the carbon star V Cygni from observations with Herschel/HIFI

We report the discovery of water vapour toward the carbon star V Cygni. We have used Herschels HIFI instrument, in dual beam switch mode, to observe the 1(11)-0(00) para-water transition at 1113.3430 GHz in the upper sideband of the Band 4b receiver. The observed spectral line profile is nearly parabolic, but with a slight asymmetry associated with blueshifted absorption, and the integrated antenna temperature is 1.69 +/- 0.17 K km s(-1). This detection of thermal water vapour emission, carried out as part of a small survey of water in carbon-rich stars, is only the second such detection toward a carbon-rich AGB star, the first having been obtained by the Submillimeter Wave Astronomy Satellite toward IRC+ 10216. For an assumed ortho-to-para ratio of 3 for water, the observed line intensity implies a water outflow rate similar to 3-6 x 10(-5) Earth masses per year and a water abundance relative to H-2 of similar to 2-5 x 10(-6). This value is a factor of at least 10(4) larger than the expected photospheric abundance in a carbon-rich environment, and - as in IRC+ 10216 - raises the intriguing possibility that the observed water is produced by the vapourisation of orbiting comets or dwarf planets. However, observations of the single line observed to date do not permit us to place strong constraints upon the spatial distribution or origin of the observed water, but future observations of additional transitions will allow us to determine the inner radius of the H2O-emitting zone, and the H2O ortho-to-para ratio, and thereby to place important constraints upon the origin of the observed water emission.