Alexander G. Tielens - Leiden

Alexander G. Tielens
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Alexander G. Tielens

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Astrophysics of Galaxies (16)
Astrophysics (4)
Physics - Chemical Physics (4)
Solar and Stellar Astrophysics (4)
Cosmology and Nongalactic Astrophysics (3)
Instrumentation and Methods for Astrophysics (3)
Earth and Planetary Astrophysics (3)
Physics - Materials Science (1)
Physics - Mesoscopic Systems and Quantum Hall Effect (1)

Publications Authored By Alexander G. Tielens

Affiliations: 1University of Western Ontario, 2NASA-Ames Research Center, Entry Systems and Technology Division, 3NASA-Ames Research Center, Space Science Division, 4Leiden Observatory, 5SETI Institute, 6University of Maryland

We present 5-20 micron spectral maps of the reflection nebula NGC2023 obtained with the Infrared Spectrograph SL and SH modes on board the Spitzer Space Telescope which reveal emission from polycyclic aromatic hydrocarbons (PAHs), C60, and H2 superposed on a dust continuum. We show that several PAH emission bands correlate with each other and exhibit distinct spatial distributions revealing a spatial sequence with distance from the illuminating star. We explore the distinct morphology of the 6. Read More

The 2175 \AA\ UV extinction feature was discovered in the mid-1960s, yet its physical origin remains poorly understood. One suggestion is absorption by Polycyclic Aromatic Hydrocarbons (PAH) molecules, which is supported by theoretical molecular structure computations and by laboratory experiments. PAHs are positively detected by their 3. Read More

In this work we report on high-resolution IR absorption studies that provide a detailed view on how the peripheral structure of irregular polycyclic aromatic hydrocarbons (PAHs) affects the shape and position of their 3 {\mu}m absorption band. To this purpose we present mass-selected, high-resolution absorption spectra of cold and isolated phenanthrene, pyrene, benz[a]antracene, chrysene, triphenylene, and perylene molecules in the 2950-3150 cm-1 range. The experimental spectra are compared with standard harmonic calculations, and anharmonic calculations using a modified version of the SPECTRO program that incorporates a Fermi resonance treatment utilizing intensity redistribution. Read More

The Orion star-forming region is the nearest active high-mass star-forming region and has created a large superbubble, the Orion-Eridanus superbubble. Recent work by Ochsendorf et al. has extended the accepted boundary of the superbubble. Read More

The journey from dust particle to planetesimal involves physical processes acting on scales ranging from micrometers (the sticking and restructuring of aggregates) to hundreds of astronomical units (the size of the turbulent protoplanetary nebula). Considering these processes simultaneously is essential when studying planetesimal formation. We develop a novel, global, semi-analytical model for the evolution of the mass-dominating dust particles in a turbulent protoplanetary disk that takes into account the evolution of the dust surface density while preserving the essential characteristics of the porous coagulation process. Read More

We report on an experimental and theoretical investigation of the importance of anharmonicity in the 3 micron CH stretching region of Polycyclic Aromatic Hydrocarbon (PAH) molecules. We present mass-resolved, high-resolution spectra of the gas-phase cold (~4K) linear PAH molecules naphthalene, anthracene, and tetracene. The measured IR spectra show a surprisingly high number of strong vibrational bands. Read More

Interstellar polycyclic aromatic hydrocarbons (PAHs) are expected to be strongly processed by vacuum ultraviolet photons. Here, we report experimental studies on the ionization and fragmentation of coronene (C24H12), ovalene (C32H14) and hexa-peri-hexabenzocoronene (HBC; C42H18) cations by exposure to synchrotron radiation in the range of 8--40 eV. The results show that for small PAH cations such as coronene, fragmentation (H-loss) is more important than ionization. Read More

The coagulation of microscopic dust into planetesimals is the first step towards planet formation. The size and shape of the growing aggregates determine the efficiency of this early growth. It has been proposed that fluffy ice aggregates can grow very efficiently, suffering less from the bouncing and radial drift barriers. Read More

Interstellar molecules are thought to build up in the shielded environment of molecular clouds or in the envelope of evolved stars. This follows many sequential reaction steps of atoms and simple molecules in the gas phase and/or on (icy) grain surfaces. However, these chemical routes are highly inefficient for larger species in the tenuous environment of space as many steps are involved and, indeed, models fail to explain the observed high abundances. Read More

Recent studies have confirmed the presence of buckminsterfullerene (C$_{60}$) in different interstellar and circumstellar environments. However, several aspects regarding C$_{60}$ in space are not well understood yet, such as the formation and excitation processes, and the connection between C$_{60}$ and other carbonaceous compounds in the interstellar medium, in particular polycyclic aromatic hydrocarbons (PAHs). In this paper we study several photodissociation regions (PDRs) where C$_{60}$ and PAHs are detected and the local physical conditions are reasonably well constrained, to provide observational insights into these questions. Read More

We report a Herschel detection of high-J rotational CO lines from a dense knot in the supernova remnant Cas A. Based on a combined analysis of these rotational lines, and previously observed ro-vibrational CO lines, we find the gas to be warm (two components at 400 and 2000 K) and dense (1e6-7 cm-3), with a CO column density of 5e17 cm-2. This, along with the broad line widths (400 kms-1), suggests that the CO emission originates in the post-shock region of the reverse shock. Read More

Optical and infrared emission lines from HII regions are an important diagnostic used to study galaxies, but interpretation of these lines requires significant modeling of both the internal structure and dynamical evolution of the emitting regions. Most of the models in common use today assume that HII region dynamics are dominated by the expansion of stellar wind bubbles, and have neglected the contribution of radiation pressure to the dynamics, and in some cases also to the internal structure. However, recent observations of nearby galaxies suggest that neither assumption is justified, motivating us to revisit the question of how HII region line emission depends on the physics of winds and radiation pressure. Read More

The emission line ratios [OIII]5007/H-beta and [NII]6584/H-alpha have been adopted as an empirical way to distinguish between the fundamentally different mechanisms of ionization in emission-line galaxies. However, detailed interpretation of these diagnostics requires calculations of the internal structure of the emitting HII regions, and these calculations depend on the assumptions one makes about the relative importance of radiation pressure and stellar winds. In this paper we construct a grid of quasi-static HII region models to explore how choices about these parameters alter HII regions' emission line ratios. Read More

Proplyds are photodissociation region (PDR)-like cometary cocoons around young stars which are thought to originate through photo-evaporation of the central protoplanetary disk by external UV radiation from the nearby OB stars. This letter presents spatially resolved mid-infrared imaging and spectroscopy of the proplyd HST10 obtained with the VLT/VISIR instrument. These observations allow us to detect Polycyclic Aromatic Hydrocarbons (PAH) emission in the proplyd photodissociation region and to study the general properties of PAHs in proplyds for the first time. Read More

Over the last few years, the chemistry of molecules other than CO in the planet-forming zones of disks is starting to be explored with Spitzer and high-resolution ground-based data. However, these studies have focused only on a few simple molecules. The aim of this study is to put observational constraints on the presence of more complex organic and sulfur-bearing molecules predicted to be abundant in chemical models of disks and to simulate high resolution spectra in view of future missions. Read More

We present 15-20 um spectral maps towards the reflection nebula NGC2023 obtained with the Infrared Spectrograph in short-wavelength, high-resolution mode on board the Spitzer Space Telescope. These spectra reveal emission from PAHs, C60, and H2 superposed on a dust continuum. These emission components exhibit distinct spatial distributions: with increasing distance from the illuminating star, we observe the PAH emission followed by the dust continuum emission and the H2 emission. Read More

Buckminsterfullerene (C60) was recently confirmed to be the largest molecule identified in space. However, it remains unclear how, and where this molecule is formed. It is generally believed that C60 is formed from the build up of small carbonaceous compounds, in the hot and dense envelopes of evolved stars. Read More

The Small Magellanic Cloud (SMC) provides a unique laboratory for the study of the lifecycle of dust given its low metallicity (~1/5 solar) and relative proximity (~60 kpc). This motivated the SAGE-SMC (Surveying the Agents of Galaxy Evolution in the Tidally-Stripped, Low Metallicity Small Magellanic Cloud) Spitzer Legacy program with the specific goals of studying the amount and type of dust in the present interstellar medium, the sources of dust in the winds of evolved stars, and how much dust is consumed in star formation. This program mapped the full SMC (30 sq. Read More


We report the detection of absorption lines by the reactive ions OH+, H2O+ and H3O+ along the line of sight to the submillimeter continuum source G10.6$-$0.4 (W31C). Read More

This white paper is submitted to the Astronomy and Astrophysics 2010 Decadal Survey (Astro2010)1 Committee on the State of the Profession to emphasize the potential of the Stratospheric Observatory for Infrared Astronomy (SOFIA) to contribute to the training of instrumentalists and observers, and to related technology developments. This potential goes beyond the primary mission of SOFIA, which is to carry out unique, high priority astronomical research. SOFIA is a Boeing 747SP aircraft with a 2. Read More

A solid-state feature was detected at around 2175 cm-1 towards 30 embedded young stellar objects in spectra obtained using the ESO VLT-ISAAC. We present results from laboratory studies of CO adsorbed at the surface of Zeolite wafers, where absorption bands were detected at 2177 and 2168 cm-1 (corresponding to CO chemisorbed at the Zeolite surface), and 2130 cm-1 (corresponding to CO physisorbed at the Zeolite surface), providing an excellent match to the observational data. We propose that the main carrier of the 2175-band is CO chemisorbed at bare surfaces of dust grains in the interstellar medium. Read More

Affiliations: 1Laboratoire d'Astrophysique de l'Observatoire de Grenoble, 2Laboratoire d'Astrophysique de l'Observatoire de Grenoble, 3Institut de Radio Astronomie Millimetrique, 4Institute for Astronomy, Hawaii, 5Centre d'Etude Spatiale des Rayonnements, 6Osservatorio Astrofisico d'Arcetri, 7Laboratoire d'Astrophysique de l'Observatoire de Grenoble, 8Laboratoire d'Astrophysique de l'Observatoire de Grenoble, 9Kapteyn Astronomical Institute
Category: Astrophysics

Complex organic molecules have previously been discovered in solar type protostars, raising the questions of where and how they form in the envelope. Possible formation mechanisms include grain mantle evaporation, interaction of the outflow with its surroundings or the impact of UV/X-rays inside the cavities. In this Letter we present the first interferometric observations of two complex molecules, CH3CN and HCOOCH3, towards the solar type protostar IRAS16293-2422. Read More

Affiliations: 1Laboratoire d'Astrophysique de l'Observatoire de Grenoble, 2Laboratoire d'Astrophysique de l'Observatoire de Grenoble, 3Laboratoire d'Astrophysique de l'Observatoire de Grenoble, 4Institute for Astronomy, Hawaii, 5Observatoire de Bordeaux, 6Centre d'Etude Spatiale des Rayonnements, 7Osservatorio Astrofisico d'Arcetri, 8Laboratoire d'Astrophysique de l'Observatoire de Grenoble, 9Centre d'Etude Spatiale des Rayonnements, 10Kapteyn Astronomical Institute
Category: Astrophysics

We report the detection of complex molecules (HCOOCH_3, HCOOH and CH_3CN), signposts of a "hot core" like region, toward the low mass, Class 0 source NGC1333-IRAS4A. This is the second low mass protostar where such complex molecules have been searched for and reported, the other source being IRAS16293-2422. It is therefore likely that compact (few tens of AUs) regions of dense and warm gas, where the chemistry is dominated by the evaporation of grain mantles, and where complex molecules are found, are common in low mass Class 0 sources. Read More

We present a survey of the formaldehyde emission in a sample of eight Class 0 protostars obtained with the IRAM and JCMT telescopes. The data have been analyzed with three different methods with increasing level of sophistication. We first analyze the observed emission in the LTE approximation, and derive rotational temperatures between 11 and 40 K, and column densities between 1 and 20 x 10^13 cm^-2. Read More