Scientific Results in Press

Topical Review: SIS and bolometer mixers for terahertz frequencies

K.H. Gundlach(¹) and M. Schicke(¹)
(¹)Institut de Radio Astronomie Millimétrique, Domaine Universitaire de Grenoble, F-38406 St. Martin d'Hères, France

Abstract:
The need for extremely sensitive heterodyne receivers in the astronomical community has created strong efforts to develop appropriate frequency mixers. Nb/Al-Al_xO_y-Nb tunnel junctions with Nb matching circuits lead to best results up to approximately 700 GHz, the energy-gap frequency of Nb. For higher frequencies the Nb in the matching circuit becomes lossy and is replaced beyond 800 GHz by Al (which operates in the normal conducting state). The gap frequency of NbN is as high as 1.2 THz, but NbN technology is not yet mature. Practical films still have substantial radio frequency losses and the barrier of NbN tunnel junctions is too leaky. More recently, NbTiN, for which the gap frequency is also about 1.2 THz, was found to have very low losses, and is therefore a good choice for tuning circuits. New interesting tunnel junctions for mixers around 1 THz are NbTiN-MgO-NbTiN and Nb/Al-AlN_x-NbTiN. Excellent performance from about 1 THz up to several terahertz can be expected from hot-electron transition-edge bolometer mixers. They consist of NbN or Nb microbridges, they do not need matching circuits and their frequency limit is not determined by the gap frequency.

Appeared in: Supercond. Sci. Technol. 13 (2000), R171-R187

Atmospheric Phase Correction for Connected-Element Interferometry and for VLBI

M. Bremer(¹)
(¹)Institut de Radio Astronomie Millimétrique (IRAM), 300 rue de la Piscine, Domaine Universitaire de Grenoble, 38406 Saint Martin d'Hères, France

Abstract:
In observations with the IRAM Interferometer, a clear sky atmospheric phase correction has been applied since 1995. Currently only the improved amplitudes of the interferometric visibilities are used, as total power assisted phase tracking between calibrator and target is not yet reliable. Setting the modeled phases to zero average on a source observation can improve the results, and may even come close to an absolute technique (i.e. phase tracking during source changes). The potential of this method is illustrated with an example of a map without and with phase correction.

An absolute phase correction scheme with cloud correction based on radiometric measurements at the 22 GHz water vapor line is now under development at IRAM. A prototype receiver for this purpose has been built and tested. Some specially adapted stability criteria are described which were found useful to qualify the instrument.

At the IRAM 30m telescope on Pico Veleta, Spain, the 200 GHz sky emission has been used for phase monitoring during recent intercontinental VLBI experiments. Examples of corrected and uncorrected observed phases indicate that the method is operational.

To appear in: IAU Site 2000 Workshop Proceedings, ASP Conference Series, in prep.

Anatomy of the counterrotating molecular disk in the spiral NGC 3593. ¹²CO(1-0) interferometer observations and numerical simulations

García-Burillo, S.(¹); Sempere, M.J.(²); Combes, F.(³); Hunt, L.K.(⁴); Neri, R.(⁵)
(¹)Observatorio Astronomico Nacional, Ap. 1143, 28800 Alcala de Henares, Spain, (²)Instituto de Estructura de la Materia, CSIC, Madrid, Spain, (³)Observatoire de Paris, DEMIRM, 61, Avenue de l'Observatoire, Paris, France, (⁴)Centro per l'Astronomia Infrarossa e lo Studio del Mezzo Interstellare-CNR, Largo E. Fermi 5, 50125 Firenze, Italy, (⁵)IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France

Abstract:
This paper presents high-resolution (4 $''\times 3''$) interferometer observations of the inner disk of the starburst spiral NGC 3593 made in the (1-0) line of ¹²CO. NGC3593 is an early-type system known to possess two counter-rotating stellar disks of markedly different scale lengths and masses. The CO emission comes from a highly structured molecular gas disk of $M_{\rm gas} \sim 3\,10^8\,M_\odot$, and total radial extent $r \sim 35''$. The observed CO kinematics indicates that the molecular gas is counterrotating at all radii with respect to the most massive stellar disk (disk I).

The bulk of the CO emission arises from a ringed circumnuclear disk (CND) of radius $r \sim 10''$ and mass $M_{\rm gas} \sim 1.5\,10^8\,M_\odot$, which hosts a nuclear starburst. The link between the starburst and the CND is corroborated by high-resolution observations of other star formation tracers (H$\alpha$, Pa$\alpha$and J-K color index maps). The starburst episode is fueling the less massive counterrotating stellar disk (disk II). We find extinctions $A_V \sim 1$mag in the CND based on optical and near-infrared recombination lines, but find >5mag from the CO and 100 $\mu$m fluxes.

Out of the CND, molecular gas is distributed in a one-arm spiral feature which winds up tightly from the edges of the CND ( $r \sim 10''$) up to $r \sim 35''$. The CO one-arm spiral is leading with respect to the gas flow in the southern half of the disk. There is a secondary trailing spiral arc in the northern half. The analysis of streaming motions linked with the passage of the CO one-arm spiral indicates that the southern feature would be a stationary m=1instability (pattern speed $\Omega_p \sim 0$).

To account for the observed gas response in the disk of NGC3593, we have run self-consistent numerical simulations, including the stellar and the gaseous components, in a physical scenario which approximates this case of study. We discuss the rapidly changing response of the disk, which evolves from a transitory regime, in which all instabilities are m=1 waves leading with respect to the counter-rotating gas, towards a stationary regime, in which m=1 are mixed with m=2 features, trailing with respect to the gas flow at all radii. In the light of the present simulations, NGC3593 might be starting to change from the transitory towards the stationary regime.

Appeared in A&A 2000, 363, 869

Methylpolyynes and Small Hydrocarbons in CRL 618

Cernicharo, J.(¹),(²); Heras, A.M.(³); Pardo, J.R.(¹),(⁴); Tielens, A.G.G.M.(⁵); Guélin, M.(⁶); Dartois, E.(⁶); Neri, R.(⁶); Waters, L.B.F.M.(⁷)
(¹)Instituto de Estructura de la Materia, Departamento de Fisica Molecular, CSIC, Serrano 121, E-28006 Madrid, Spain, (²)Visiting scientist at the Division of Physics, Mathematics, and Astronomy, California Institute of Technology, MS 320-47, Pasadena, CA 91125, (³)Astrophysics Division, Space Science Department of ESA, ESTEC, P.O.Box 299, 2200 AG Noordwijk, Netherlands, (⁴)Division of Physics, Mathematics, and Astronomy, California Institute of Technology, MS 320-47, Pasadena, CA 91125, (⁵)Kapteyn Astronomical Institute, P.O.Box 800, 9700 AV Groningen, Netherlands, (⁶)IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France, (⁷)University of Amsterdam, Astronomical Institute Anton Pannekoek, Kruislaan, 403, 1098 SJ Amsterdam, Netherlands

Abstract:
We report on the detection with the Infrared Space Observatory of strong infrared absorption from NH₃ and C₂H₄ in CRL618. The observed NH₃ and C₂H₄ bands arise from a region with kinetic temperatures $\simeq 200\,$ K, i.e. the dense gas in the photodissociation region associated to the dense torus surrounding the central star, as was the case for the polyynes and cyanopolyynes. Several absorption bands, probably arising from small gas-phase hydrocarbons, are observed between 5.5 and 11$\mu$m. Two of these species have been identified with the 30m IRAM telescope as the methylpolyynes CH₃C₂H and CH₃C₄H. However, the absorption around 6.2$\mu$m is particularly broad and could arise from the combination of these small hydrocarbons and from the aromatic CC stretching of polycyclic aromatic hydrocarbons of moderate size. These bands and those associated to the polyynes, cyanopolyynes, methylpolyynes, and benzene are not present in the infrared spectrum of the asymptotic giant branch star IRC+10216.

Appeared in ApJ 2001, 546, L127

Distribution and Properties of the Molecular Clouds in M 31

M. Guélin(¹), C. Nieten(²), N. Neininger(³), S. Muller(¹), R. Lucas(¹), H. Ungerechts(¹), R. Wielebinski(²)
(¹)IRAM, 300 rue de la Piscine , 38406 St. Martin d'Hères, France, (²)Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany, (³)Radioastronomisches Institut der Universität Bonn, Germany

Abstract:
Although most of our knowledge on the interstellar medium (ISM) comes from the Milky Way, some basic questions on its structure and evolution can be addressed only by studying external galaxies. This is particularly true for the large gas structures, such as spiral arms which are hard to discern in the Milky Way disk and for the properties of distant clouds, which are confused by foreground gas. The Andromeda galaxy, the closest spiral galaxy to our own (D= 0.8 Mpc), is probably the best object where to study these questions. It has been mapped recently in the mm lines of CO with the IRAM 30-m telescope. Figure 7 shows the map observed in the CO 1-0 line with the IRAM 30-m telescope (Nieten et al in preparation, see also Neininger et al. 1998, Nature 395, 871). It was observed `On-The-Fly' by using a basket-weaving procedure: The telescope beam (FWHM= 21'') was moved back and forth in the Y direction with steps in X of 9'', until an unit area of 20 $'\times 20'$ was entirely covered. Then, the same area was scanned in the perpendicular direction, by inverting the X and Y coordinates. The two sets of maps were averaged and de-striped by using the Emerson and Gräve (1988, AAp 190, 353-358) algorithm adapted to spectral line observations. The fully sampled data cube, which consists of 11 unit areas and 10⁵ spectra, has been smoothed from 23'' to 45'' to show more clearly the spiral structure.

Compared to the HI arms, the molecular arms traced by CO are thinner and extend more into the central region (see the discussion by Nieten et al., this conference). This allows to recognize more easily the spiral pattern. The question of M 31's spiral structure is an old one which had no clear answer until now. The most elaborate model was proposed by Braun (1991, ApJ 372, 54) from a detailed analysis of the HI interferometric data. Braun's model, a two armed spiral pattern with a pitch angle changing with radius, is interesting for the outer regions where HI is strong and the gaseous disk is warped. The arms are not constrained to lie in a plane, but rise above, or drop below the mid-plane. Braun's model explains the apparent crowding of the HI arms near the minor axis, but fails to reproduce the main arm features in the southwest part of the disk.

Because CO emission hardly extends to the warped part of the disk, we have fitted our CO map with a very simple two-armed pattern. This pattern is suggested by the spiral pattern of M 81, a galaxy of the the same morphological type as M 31. The model arms are trailing logarithmic spirals with a constant pitch angle; they are located in a plane inclined at $i= 78^\circ$ with respect to the plane of the sky with a position angle $PA=\, 37.7^\circ$ (the values of i and PA are derived from the kinematics of the HI gas in M 31's inner disk - Brinks & Shane 1984, AApS 55, 179). Surprisingly, this simple pattern with few free parameters accounts fairly well for most of the arm segments detected in CO. The agreement is particularly good for the first spiral arm which follows the CO emission over 400$^\circ$ with only minor deviations. The fit is less good for the second arm, mostly because the CO-HI lane is kinked inwards in the vicinity of the giant stellar association NGC 206 ( X=-42', Y=0). The origin of the two spirals coincides with M 31's nucleus within few arcmin. The pitch angle is close to 7$^\circ$.

  

Figure: The ¹²CO (1-0) line integrated intensity, observed with the 30-m telescope, smoothed to a resolution of 45'' (Nieten et al., in preparation). The spiral arms drawn here are two trailing logarithmic spirals with pitch angles of 6$^\circ$ and 8$^\circ$.

Published in: The interstellar medium in M 31 and M 33, Proc. 232th WE-Heraeus Seminar, eds. E.M. Berkhuijsen, R. Beck, R.A.M. Walterbos, Shaker Verlag, Aachen 2000.
Reprints are available from: guelin@iram.fr

Astronomical detection of the free radical SiCN

M. Guélin(¹), S. Muller(¹), J. Cernicharo(²), A.J. Apponi(³), M.C. Mc Carthy(³), C.A. Gottlieb(³), and P. Thaddeus(³)
(¹)IRAM, 300 rue de la Piscine, F-38406 S$^{\rm t}$ Martin d'Hères, France, (²)Instituto de Estructura de la Materia, C/Serrano 121, 28006 Madrid, Spain, (³)Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Abstract:
We report the detection of the SiCN radical in an astronomical source, the envelope of the C star IRC+10216/CW Leo. The microwave spectrum of SiCN was recently studied by four of us in the laboratory and the rotational transition frequencies were accurately measured. The ground fine structure state, $^2\Pi_{1/2}$, has three rotational transitions, each with $\lambda$-doubling in the 80-116 GHz atmospheric window ( $J=7.5 \rightarrow 6.5$, $8.5\rightarrow 7.5$ and $9.5\rightarrow 8.5$, at 83.0, 94.0, and 105.1 GHz). The three $\lambda$-doublets (six components) are detected at a level of 5 mK with the IRAM 30-m telescope. Judging from the cusped shape of the line profiles, SiCN is largely confined to the outer molecular envelope, like most other radicals. Its abundance relative to H₂is estimated to be 4 10^-9, a factor of 20 lower than that of MgNC.

The isoelectronic radical SiCCH was not detected. We confirm our previous tentative detections of the carbon chain H₂C₆ and of NP in IRC+10216. We report the detection of the SiCN radical in an astronomical source, the envelope of the C star IRC+10216/CW Leo. The microwave spectrum of SiCN was recently studied by four of us in the laboratory and the rotational transition frequencies were accurately measured. The ground fine structure state, $^2\Pi_{1/2}$, has three rotational transitions, each with $\lambda$-doubling in the 80-116 GHz atmospheric window ( $J=7.5 \rightarrow 6.5$, $8.5\rightarrow 7.5$ and $9.5\rightarrow 8.5$, at 83.0, 94.0, and 105.1 GHz). The three $\lambda$-doublets (six components) are detected at a level of 5 mK with the IRAM 30-m telescope (Fig. 8). Judging from the cusped shape of the line profiles, SiCN is largely confined to the outer molecular envelope, like most other radicals. Its abundance relative to H₂is estimated to be 4 10^-9, a factor of 20 lower than that of MgNC.

The isoelectronic radical SiCCH was not detected. We confirm our previous tentative detections of the carbon chain H₂C₆ and of NP in IRC+10216.

  

Figure: Spectra from the 30-m telescope of three successive rotational transitions of SiCN. The position of the SiCN $\Lambda$-doublet components, at the center of each spectrum, and those of other identified lines are indicated by arrows. The lines of C₄H at 105174 MHz and 105231 MHz are the fine-structure components of the $N= 11\rightarrow 10$ rotational transition in the $\nu _7=2, \, l=0$ bending state. The broad feature around 82880 MHz is a blend of transitions of C₈H and of vibrationnally excited HC₅N ($\nu _{11}$=2); note that the noise is larger on the left side of the bottom spectrum, because the integration time was a factor of 3 smaller below 82960 MHz, than above.

Appeared in A&A Letters, 363, L9

Millimetric Observations of Plerionic Supernova Remnants

R.Bandiera (¹), R.Neri (²), R.Cesaroni (¹)
(¹)Osservatorio Astrofisico di Arcetri, Largo Fermi 5, 50125 Firenze, Italy, (²)IRAM, 300 rue de la Piscine, 38406 St-Martin D'Hères, France

Abstract:
We present results of observations of the Crab Nebula and G21.5-0.9 performed at 1.3mm with the MPIfR bolometer arrays at the IRAM 30m telescope. In the Crab Nebula we measure spatial variations of the average spectral index between 20cm and 1.3mm. Since the electrons emitting at mm wavelengths are affected by negligible synchrotron losses, such variations imply the presence of at least two different populations of injected particles. By subtracting the emission extrapolated from the radio a residual component appears, similar in size and shape to the soft X-ray map as well as to the flatter-spectrum optical component. Moreover near the major synchrotron filaments we measure a spectral bending consistent with a break at a frequency lower than the average break frequency in the Nebula: this indicates that near the filaments the magnetic field is typically 6 times higher than the average. For G21.5-0.9 we derive a spectral break at $\sim 540$GHz, in contrast with the previously accepted value of 40GHz. Therefore this object does not strictly belong to the class of plerions with a low-frequency break.

To appear in the College Park Conference Series on ``Young Supernova Remnants'', Oct. 16-18 2000, Eds. S.S.Holt and U.Hwang