Chapillon E.,MPIfR |
Chapillon E.,University of Bordeaux Segalen |
Chapillon E.,French National Center for Scientific Research |
Guilloteau S.,University of Bordeaux Segalen |
And 5 more authors.
Astronomy and Astrophysics | Year: 2012
Context. The chemistry of protoplanetary disks is thought to be dominated by two major processes: photodissociation near the disk surface and depletion on dust grains in the disk midplane, resulting in a layered structure with molecules located in a warm layer above the disk midplane. Aims. We attempt here to confront this warm molecular layer model prediction with the distribution of two key molecules for dissociation processes: CN and HCN Methods. Using the IRAM Plateau de Bure interferometer, we obtained high spatial and spectral resolution images of the CN J = 2-1 and HCN J = 1-0 lines in the disks surrounding the two T Tauri DM Tau and LkCa 15 and the Herbig Ae MWC 480. We have derived disk properties by assuming power-law distributions. The hyperfine structure of the observed transitions allowed us to constrain the line opacities and excitation temperatures. We compare the observational results with predictions from existing chemical models, and used a simple PDR model (without freeze-out of molecules on grains and surface chemistry) to illustrate dependencies on UV field strength, grain size, and gas-to-dust ratio. We also evaluated the impact of Ly α radiation. Results. The temperature ordering follows the trend found from CO lines, with DM Tau the coldest object and MWC 480 the warmest. Although CN indicates somewhat higher excitation temperatures than HCN, the derived values in the T Tauri disks are very low (8-10 K). They agree with results obtained from C 2H, and contradict thermal and chemical model predictions. These very low temperatures, as well as geometrical constraints, suggest that substantial amounts of CN and HCN remain in the gas phase close to the disk midplane and that this midplane is quite cold. The observed CN/HCN ratio (≈5-10) is in better agreement with the existence of large grains and possibly also with a substantial contribution of Ly α radiation. © 2012 ESO.
Seelmann-Eggebert M.,Fraunhofer Institute for Applied Solid State Physics |
Schafer F.,MPIfR |
Leuther A.,Fraunhofer Institute for Applied Solid State Physics |
Massler H.,Fraunhofer Institute for Applied Solid State Physics
IEEE MTT-S International Microwave Symposium Digest | Year: 2010
A versatile scalable small signal model for high electron mobility transistors (HEMTs) of gate length 50 nm and 100 nm has been developed. The model covers a large bias range and includes the temperature dependence from 300 K to 15 K. Especially, it is capable to predict the noise behaviour of the transistor in dependence of ambient temperature and frequency. © 2010 IEEE.
Prieto M.A.,IAC |
Reunanen J.,University of Turku |
Tristram K.R.W.,MPIfR |
Neumayer N.,ESO |
And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010
Spectral energy distributions (SEDs) of the central few tens of parsec region of some of the nearest, most well-studied, active galactic nuclei (AGN) are presented. These genuine AGN-core SEDs, mostly from Seyfert galaxies, are characterized by two main features: an infrared (IR) bump with the maximum in the 2-10 μm range and an increasing X-ray spectrum with frequency in the 1 to ∼200 keV region. These dominant features are common to Seyfert type 1 and 2 objects alike. In detail, type 1 AGN are clearly distinguished from type 2 by their high spatial resolution SEDs: type 2 AGN exhibit a sharp drop shortwards of 2 μm, with the optical to UV region being fully absorbed; type 1s instead show a gentle 2 μm drop ensued by a secondary, partially absorbed optical to UV emission bump. On the assumption that the bulk of optical to UV photons generated in these AGN is reprocessed by dust and re-emitted in the IR in an isotropic manner, the IR bump luminosity represents ≳70 per cent of the total energy output in these objects, and the second energetically important contribution is the high energies above 20 keV.Galaxies selected by their warm IR colours, i.e. presenting a relatively flat flux distribution in the 12-60 μm range, have often being classified as AGN. The results from these high spatial resolution SEDs question this criterion as a general rule. It is found that the intrinsic shape of the infrared SED of an AGN and inferred bolometric luminosity largely depart from those derived from large aperture data. AGN luminosities can be overestimated by up to two orders of magnitude if relying on IR satellite data. We find these differences to be critical for AGN luminosities below or about 1044 erg s-1. Above this limit, AGN tend to dominate the light of their host galaxy regardless of the integration aperture size used. Although the number of objects presented in this work is small, we tentatively mark this luminosity as a threshold to identify galaxy-light-dominated versus AGN-dominated objects. © 2010 The Authors. Journal compilation © 2010 RAS.
News Article | February 25, 2016
Astronomers all over the world have been surveying the sky for years now, even looking beyond our galaxy. Now, the Milky Way is seen in a dramatic new way in a mesmerizing new image produced by astronomers using the Apex observatory. The Apex Telescope Large Area Survey of the Galaxy (ATLASGAL) project recorded the image of our home galaxy. Due to the unique nature of this instrument, astronomers recorded the presence of cold gas scattered throughout our Milky Way galaxy. The Apex telescope is located on Chajnantor Plateau, 5,100 meters (16,700 feet) above sea level in Chile. It boasts a reflector 468 inches in diameter. Data from the massive ground-based telescope was pieced together with observations from the Planck space-based telescope, operated by the European Space Agency (ESA). "ATLASGAL provides exciting insights into where the next generation of high-mass stars and clusters form. By combining these with observations from Planck, we can now obtain a link to the large-scale structures of giant molecular clouds," said Timea Csengeri of the Max Planck Institute for Radio Astronomy (MPIfR), located in Bonn, Germany. From side to side, this new image covers 140 degrees in length, and 3 degrees across. This is nearly 1,700 times larger than the full moon as seen from Earth. One instrument utilized in the Apex survey is the Large Bolometer Camera (LABOCA), an ultra-sensitive camera capable of recording a change of temperature in its detectors driven by cool bands of dust obscuring light from distant stars. The Apex telescope has been in operation for just over a decade, complimenting the Atacama Large Millimeter/submillimeter Array (Alma), also operating on the Chajnantor Plateau. Data gathered from the Apex observations of the southern half of our galaxy will be examined in greater detail by astronomers using the Alma observatory. This southern part of the galaxy includes the galactic core, making it a rich field of data for astronomers. The northern half of the Milky Way was mapped by researchers using the James Clerk Maxwell Telescope. Frequencies of electromagnetic energy studied in the Apex observations sit between infrared and radio wavelengths. This is the second Atlasgal photograph released by astronomers, and data from the program has already spawned nearly 70 scientific articles published in journals. Future research will examine gas in the Milky Way using other instruments in an effort to view the galaxy in a variety of wavelengths. "The new release of the full survey opens up the possibility to mine this marvellous dataset for new discoveries. Many teams of scientists are already using the ATLASGAL data to plan for detailed ALMA follow-up," said Leonardo Testi of the European Southern Observatory.
Ao Y.,MPIfR |
Ao Y.,Chinese Academy of Sciences |
Henkel C.,MPIfR |
Braatz J.A.,U.S. National Radio Astronomy Observatory |
And 3 more authors.
Astronomy and Astrophysics | Year: 2011
We present the detection of the ammonia (NH 3) (J,K)=(1,1) to (4,4) and (6,6) inversion lines toward the prototypical Seyfert 2 galaxy NGC 1068, made with the Green Bank Telescope (GBT). This is the first detection of ammonia in a Seyfert galaxy. The ortho-to-para-NH 3 abundance ratio suggests that the molecule was formed in a warm medium of at least 20K. For the NH 3 column density and fractional abundance, we find (1.09 ± 0.23) × 10 14 cm -2 and (2.9 ± 0.6) × 10 -8, respectively, from the inner ∼1.2 kpc of NGC 1068. The kinetic temperature can be constrained to 80 ± 20 K for the bulk of the molecular gas, while some fraction has an even higher temperature of 140 ± 30 K. © 2011 ESO.