arXiv subjects

Th. Henning

Publications and source records attributed to Th. Henning.

At least 541 records · Page 30Linked to original sources

arXiv2008

The structure of the protoplanetary disk surrounding three young intermediate mass stars. II. Spatially resolved dust and gas distribution

[Abridged] We present the first direct comparison of the distribution of the gas, as traced by the [OI] 6300 AA emission, and the dust, as traced by the 10 micron emission, in the protoplanetary disk around three intermediate-mass stars: HD 101412, HD 135344 B and HD 179218. N-band visibilities were obtained with VLTI/MIDI. Simple geometrical models are used to compare the dust emission to high-resolution optical spectra in the 6300 AA [OI] line of the same targets. The disks around HD 101412 and HD 135344 B appear strongly flared in the gas, but self-shadowed in the dust beyond ~ 2 AU. In both systems, the 10 micron emission is rather compact (< 2 AU) while the [OI] brightness profile shows a double peaked structure. The inner peak is strongest and is consistent with the location of the dust, the outer peak is fainter and is located at 5-10 AU. Spatially extended PAH emission is found in both disks. The disk around HD 179218 is flared in the dust. The 10 micron emission emerges from a double ring-like structure with the first ring peaking at ~ 1 AU and the second at ~ 20 AU. No dust emission is detected between ~ 3 -- 15 AU. The oxygen emission seems also to come from a flared structure, however, the bulk of this emission is produced between ~ 1 -- 10 AU. This could indicate a lack of gas in the outer disk or could be due to chemical effects which reduce the abundance of OH -- the parent molecule of the observed [OI] emission -- further away from the star. The three systems, HD 179218, HD 135344 B and HD 101412, may form an evolutionary sequence: the disk initially flared becomes flat under the combined action of gas-dust decoupling, grain growth and dust settling.

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arXiv2008

Mid-infrared interferometry of massive young stellar objects

The very inner structure of massive young stellar objects (YSOs) is difficult to trace. With conventional observational methods we identify structures still several hundreds of AU in size. However, the (proto-)stellar growth takes place at the innermost regions (<100 AU) where the actual mass transfer onto the forming high-mass star occurs. We present results from our programme toward massive YSOs at the VLTI, utilising the two-element interferometer MIDI. To date, we observed 10 well-known massive YSOs down to scales of 20 mas (typically corresponding to 20 - 40 AU for our targets) in the 8-13 micron region. We clearly resolve these objects which results in low visibilities and sizes in the order of 30-50 mas. For two objects, we show results of our modelling. We demonstrate that the MIDI data can reveal decisive structure information for massive YSOs. They are often pivotal in order to resolve ambiguities still immanent in model parameters derived from sole SED fitting.

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arXiv2008

Molecular Emission Line Formation in Prestellar Cores

We investigate general aspects of molecular line formation under conditions which are typical of prestellar cores. Focusing on simple linear molecules, we study formation of their rotational lines by radiative transfer simulations. We present a thermalization diagram to show the effects of collisions and radiation on the level excitation. We construct a detailed scheme (contribution chart) to illustrate the formation of emission line profiles. This chart can be used as an efficient tool to identify which parts of the cloud contribute to a specific line profile. We show how molecular line characteristics for uniform model clouds depend on hydrogen density, molecular column density, and kinetic temperature. The results are presented in a 2D plane to illustrate cooperative effects of the physical factors. We also use a core model with a non-uniform density distribution and chemical stratification to study the effects of cloud contraction and rotation on spectral line maps. We discuss the main issues that should be taken into account when dealing with interpretation and simulation of observed molecular lines.

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arXiv2008

Turbulent AGN tori

Recently, the MID-infrared Interferometric instrument (MIDI) at the VLTI has shown that dust tori in the two nearby Seyfert galaxies NGC 1068 and the Circinus galaxy are geometrically thick and can be well described by a thin, warm central disk, surrounded by a colder and fluffy torus component. By carrying out hydrodynamical simulations with the help of the TRAMP code (Klahr et al. 1999), we follow the evolution of a young nuclear star cluster in terms of discrete mass-loss and energy injection from stellar processes. This naturally leads to a filamentary large scale torus component, where cold gas is able to flow radially inwards. The filaments open out into a dense and very turbulent disk structure. In a post-processing step, we calculate observable quantities like spectral energy distributions or images with the help of the 3D radiative transfer code MC3D (Wolf 2003). Good agreement is found in comparisons with data due to the existence of almost dust-free lines of sight through the large scale component and the large column densities caused by the dense disk.

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arXiv2008

Absorption Line Survey of H3+ toward the Galactic Center Sources II. Eight Infrared Sources within 30 pc of the Galactic Center

Infrared absorption lines of H3+, including the metastable R(3,3)l line, have been observed toward eight bright infrared sources associated with hot and massive stars located in and between the Galactic Center Cluster and the Quintuplet Cluster 30 pc to the east. The absorption lines with high velocity dispersion arise in the Galaxy's Central Molecular Zone (CMZ) as well as in foreground spiral arms. The temperature and density of the gas in the CMZ, as determined from the relative strengths of the H3+ lines, are T=200-300K and n=50-200cm^-3. The detection of high column densities of H3+ toward all eight stars implies that this warm and diffuse gaseous environment is widespread in the CMZ. The products of the ionization rate and path length for these sight lines are 1000 and 10 times higher than in dense and diffuse clouds in the Galactic disk, respectively, indicating that the ionization rate, zeta, is not less than 10^-15 s^-1 and that L is at least on the order of 50 pc. The warm and diffuse gas is an important component of the CMZ, in addition to the three previously known gaseous environments: (1) cold molecular clouds observed by radio emission of CO and other molecules, (2) hot (T=10^4-10^6K) and highly ionized diffuse gas (n_e=10-100cm^-3) seen in radio recombination lines, far infrared atomic lines, and radio-wave scattering, and (3) ultra-hot (T=10^7-10^8K) X-ray emitting plasma. Its prevalence significantly changes the understanding of the environment of the CMZ. The sight line toward GC IRS 3 is unique in showing an additional H3+ absorption component, which is interpreted as due to either a cloud associated with circumnuclear disk or the "50 km s^-1 cloud" known from radio observations. An infrared pumping scheme is examined as a mechanism to populate the (3,3) metastable level in this cloud.

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arXiv2008

Planetesimal formation around the snow line in MRI-driven turbulent protoplanetary disks

The formation of planetesimals in protoplanetary disks due to collisional sticking of smaller dust aggregates has to face at least two severe obstacles, namely the rapid loss of material due to radial inward drift and particle fragmentation due to destructive collisions. We present a scenario to circumvent these two hurdles. Our dust evolution model involves two main mechanisms. First, we consider a disk with a dead zone. In an almost laminar region close to the midplane, the relative velocities of the turbulent particles are comparatively small, which decreases the probability of destructive particle collisions. Second, turbulence is not the only source of violent relative particle velocities, because high radial drift speeds can also lead to boulder fragmentation. For this reason, we focus additionally on the snow line. Evaporation fronts can be associated with gas pressure maxima in which radial drift basically vanishes. This implies that particle fragmentation becomes even less likely. Our simulation results suggest that particles can overcome the fragmentation barrier. We find that boulders of several 100 m can form within only a few thousand years.

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arXiv2008

The characteristics of the IR emission features in the spectra of Herbig Ae stars: Evidence for chemical evolution

Herbig Ae/Be stars are a class of young pre-main sequence stellar objects of intermediate mass and are known to have varying amounts of natal cloud material still present in their direct vicinity. We characterise the IR emission bands, due to fluorescence by PAH molecules, in the spectra of Herbig Ae/Be stars and link observed variations to spatial aspects of the mid-IR emission. We analysed two PAH dominated spectra from a sample of 15 Herbig Ae/Be stars observed with Spitzer and derive profiles of the major PAH bands. The shape and the measured band characteristics show pronounced variations between the two Spitzer spectra. Those variations parallel those found between three ISO spectra of other, well-studied, Herbig Ae/Be stars. The derived profiles are compared to those from a broad sample of sources. The Spitzer and ISO spectra exhibit characteristics commonly interpreted respectively as interstellar matter-like (ISM), non-ISM-like, or a combination of the two. We argue that the PAH emission detected from the sources exhibiting a combination of ISM-like and non-ISM-like characteristics indicates the presence of two dissimilar, spatially separated, PAH families. As the shape of the individual PAH band profiles reflects the composition of the PAH molecules involved, this demonstrates that PAHs in subsequent, evolutionary linked stages of star formation are different from those in the general ISM, implying active chemistry. None of the detected PAH emission can be associated with the (unresolved) disk and is thus associated with the circumstellar cloud. This implies that chemical changes may already occur in the natal cloud and not necessarily in the disk.

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arXiv2008

Is the silicate emission feature only influenced by grain size?

The flattening of the 10mu silicate emission feature observed in the spectra of T Tauri and Herbig Ae/Be stars is usually interpreted as an indicator of grain growth. We show in this paper that a similar behaviour of the feature shape occurs when the porosity of composite grains varies. The fluffy aggregates, having inclusions of different sizes, were modeled by multi-layered spheres consisting of amorphous carbon, amorphous silicate and vacuum. It is also found that the inclusion of crystalline silicates in composite porous particles can lead to a shift of the known resonances and production of new ones.

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arXiv2008

Dissecting Massive YSOs with Mid-Infrared Interferometry

The very inner structure of massive YSOs is difficult to trace. With conventional observational methods we often identify structures still several hundreds of AU in size. But we also need information about the innermost regions where the actual mass transfer onto the forming high-mass star occurs. An innovative way to probe these scales is to utilise mid-infrared interferometry. Here, we present first results of our MIDI GTO programme at the VLTI. We observed 10 well-known massive YSOs down to scales of 20 mas. We clearly resolve these objects which results in low visibilities and sizes in the order of 30 - 50 mas. Thus, with MIDI we can for the first time quantify the extent of the thermal emission from the warm circumstellar dust and thus calibrate existing concepts regarding the compactness of such emission in the pre-UCHII region phase. Special emphasis will be given to the BN-type object M8E-IR where our modelling is most advanced and where there is indirect evidence for a strongly bloated central star.

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arXiv2008

Probing the Early Evolution of Young High-Mass Stars

Near-infrared imaging surveys of high-mass star-forming regions reveal an amazingly complex interplay between star formation and the environment (Churchwell et al. 2006; Alvarez et al. 2004). By means of near-IR spectroscopy the embedded massive young stars can be characterized and placed in the context of their birth site. However, so far spectroscopic surveys have been hopelessly incomplete, hampering any systematic study of these very young massive stars. New integral field instrumentation available at ESO has opened the possibility to take a huge step forward by obtaining a full spectral inventory of the youngest massive stellar populations in star-forming regions currently accessible. Simultaneously, the analysis of the extended emission allows the characterization of the environmental conditions. The Formation and Early Evolution of Massive Stars (FEMS) collaboration aims at setting up a large observing campaign to obtain a full census of the stellar content, ionized material, outflows and PDR's over a sample of regions that covers a large parameter space. Complementary radio, mm and infrared observations will be used for the characterization of the deeply embedded population. For the first eight regions we have obtained 40 hours of SINFONI observations. In this contribution, we present the first results on three regions that illustrate the potential of this strategy.

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arXiv2008

The formation and evolution of planetary systems: Grain growth and chemical processing of dust in T Tauri systems

This paper is one in a series presenting results obtained within the Formation and Evolution of Planetary Systems (FEPS) Legacy Science Program on the Spitzer Space Telescope. Here we present a study of dust processing and growth in seven protoplanetary disks. Our spectra indicate that the circumstellar silicate dust grains have grown to sizes at least 10 times larger than observed in the interstellar medium, and show evidence for a non-negligible (~5 % in mass fractions) contribution from crystalline species. These results are similar to those of other studies of protoplanetary disks. In addition, we find a correlation between the strength of the amorphous silicate feature and the shape of the spectral energy distribution. This latter result is consistent with the growth and subsequent gravitational settling of dust grains towards the disk mid-plane. Further, we find a change in the relative abundance of the different crystalline species: more enstatite relative to forsterite is observed in the inner warm dust population at ~1 AU, while forsterite dominates in the colder outer regions at ~5 to 15 AU. This change in the relative abundances argues for a localized crystallization process rather than a radial mixing scenario where crystalline silicates are being transported outwards from a single formation region in the hot inner parts of the disk. Last, we report the detection of emission from polycyclic aromatic hydrocarbon molecules in five out of seven sources. We find a tentative PAH band at 8.2 micron, previously undetected in the spectra of disks around low-mass pre-main-sequence stars.

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arXiv2008

Three-dimensional radiative transfer models of clumpy tori in Seyfert galaxies

Tori of Active Galactic Nuclei are made up of a mixture of hot and cold gas, as well as dust. In order to protect the dust grains from destruction by the hot gas as well as by the energetic radiation of the accretion disk, the dust is often assumed to be distributed in clouds. In our new 3D model of AGN dust tori, the torus is modelled as a wedge-shaped disk in which dusty clouds are randomly distributed, by taking the dust density distribution of the corresponding continuous model into account. We especially concentrate on the differences between clumpy and continuous models in terms of the temperature distributions, the surface brightness distributions and interferometric visibilities, as well as spectral energy distributions. To this end, we employ radiative transfer calculations with the help of the 3D Monte Carlo code MC3D. In a second step, interferometric visibilities are calculated from the simulated surface brightness distributions, which can be directly compared to observations with the MIDI instrument. The radial temperature distributions of clumpy models possess significantly enhanced scatter compared to the continuous cases. Even at large distances, clouds can be heated directly by the central accretion disk. The existence of the silicate 10 micron-feature in absorption or in emission depends sensitively on the distribution, the size and optical depth of clouds in the innermost part of the torus, due to shadowing effects of clouds there. This explains failure and success of previous modelling efforts of clumpy tori. After adapting the parameters of our clumpy standard model to the circumstances of the Seyfert 2 Circinus galaxy, it can qualitatively explain recent mid-infrared interferometric observations performed with MIDI, as well as high resolution spectral data.

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arXiv2008

Ethynyl (C2H) in massive star formation: Tracing the initial conditions?

APEX single-dish observations at sub-millimeter wavelengths toward a sample of massive star-forming regions reveal that C2H is almost omni-present toward all covered evolutionary stages from Infrared Dark Clouds via High-Mass Protostellar Objects to Ultracompact HII regions. High-resolution data from the Submillimeter Array toward one hot-core like High-Mass Protostellar Object show a shell-like distribution of C2H with a radius of ~9000AU around the central submm peak position. These observed features are well reproduced by a 1D cloud model with power-law density and temperature distributions and a gas-grain chemical network. The reactive C2H radical (ethynyl) is abundant from the onset of massive star formation, but later it is rapidly transformed to other molecules in the core center. In the outer cloud regions the abundance of C2H remains high due to constant replenishment of elemental carbon from CO being dissociated by the interstellar UV photons. We suggest that C2H may be a molecule well suited to study the initial conditions of massive star formation.

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arXiv2008

Chemical and thermal structure of protoplanetary disks as observed with ALMA

We predict how protoplanetary disks around low-mass young stars would appear in molecular lines observed with the ALMA interferometer. Our goal is to identify those molecules and transitions that can be used to probe and distinguish between chemical and physical disk structure and to define necessary requirements for ALMA observations. Disk models with and without vertical temperature gradient as well as with uniform abundances and those from a chemical network are considered. As an example, we show the channel maps of HCO$^+$(4-3) synthesized with a non-LTE line radiative transfer code and used as an input to the GILDAS ALMA simulator to produce noise-added realistic images. The channel maps reveal complex asymmetric patterns even for the model with uniform abundances and no vertical thermal gradient. We find that a spatial resolution of $0.2-0.5\arcsec$ and 0.5--10 hours of integration time will be needed to disentangle large-scale temperature gradients and the chemical stratification in disks in lines of abundant molecules.

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arXiv2007

A Search for Mid-Infrared Molecular Hydrogen Emission from Protoplanetary Disks

We observed the Herbig Ae/Be stars UX Ori, HD 34282, HD 100453, HD 101412, HD 104237 and HD 142666, and the T Tauri star HD 319139 and searched for H2 0-0 S(2) emission at 12.278 micron and H2 0-0 S(1) emission at 17.035 micron with VISIR, ESO-VLT's high-resolution MIR spectrograph. None of the sources present evidence for H2 emission. Stringent 3sigma upper limits to the integrated line fluxes and the mass of optically thin warm gas in the disks are derived. The disks contain less than a few tenths of Jupiter mass of optically thin H2 gas at 150 K at most, and less than a few Earth masses of optically thin H2 gas at 300 K and higher temperatures. We compare our results to a Chiang and Goldreich (1997, CG97) two-layer disk model. The upper limits to the disk's optically thin warm gas mass are smaller than the amount of warm gas in the interior layer of the disk, but they are much larger than the amount of molecular gas in the surface layer. We present a calculation of the expected thermal H2 emission from optically thick disks, assuming a CG97 disk structure, a gas-to-dust ratio of 100 and Tgas = Tdust. The expected H2 thermal emission fluxes from typical disks around Herbig Ae/Be stars (10^-16 to 10^-17 erg/s/cm2 at 140 pc) are much lower than the detection limits of our observations (5*10^-15 erg/s/cm2). H2 emission levels are very sensitive to departures from the thermal coupling between the molecular gas and dust. Additional sources of heating of gas in the disk's surface layer could have a major impact on the expected H2 disk emission. In the observed sources the molecular gas and dust in the surface layer have not significantly departed from thermal coupling (Tgas/Tdust< 2) and that the gas-to-dust ratio in the surface layer is very likely lower than 1000.

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arXiv2007

A Search for NIR Molecular Hydrogen Emission in the CTTS LkHa 264 and the debris disk 49 Cet

We report on the first results of a search for H2 emission from protoplanetary disks using CRIRES, ESO's new VLT high resolution NIR spectrograph. We observed the CTTS LkHa 264 and the debris disk 49 Cet, and searched for the 1-0 S(1), 1-0 S(0) and 2-1 S(1) H2 emission lines. The H2 line at 2.1218 micron is detected in LkHa 264. Our CRIRES spectra reveal the previously observed but not detected H2 line at 2.2233 micron in LkHa 264. An upper limit on the 2-1 S(1) H2 line flux in LkHalpha 264 is derived. These observations are the first simultaneous detection of 1-0 S(1) and 1-0 S(0) H2 emission from a protoplanetary disk. 49 Cet does not exhibit H2 emission in any of the three observed lines. There are a few lunar masses of optically thin hot H2 in the inner disk (~0.1 AU) of LkHa 264, and less than a tenth of a lunar mass of hot H2 in the inner disk of 49 Cet. The measured 1-0 S(0)/1-0 S(1) and 2-1 S(1)/1-0 S(1) line ratios in LkHa 264 indicate that the H2 emitting gas is at T<1500 K and that the H2 is most likely thermally excited by UV photons. Modeling of the shape of the line suggests that the disk should be seen close to face-on (i<35). A comparative analysis of the physical properties of CTTS in which the H2 1-0 S(1) line has been detected and non-detected indicates that the presence of H2 emission is correlated with the magnitude of the UV excess and the strength of the Halpha line. The lack of H2 emission in the NIR spectra of 49 Cet and the absence of Halpha emission suggest that the gas in the inner disk of 49 Cet has dissipated. The disk surrounding 49 Cet should have an inner hole.

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arXiv2007

Coagulation, fragmentation and radial motion of solid particles in protoplanetary disks

The growth of solid particles towards meter sizes in protoplanetary disks has to circumvent at least two hurdles, namely the rapid loss of material due to radial drift and particle fragmentation due to destructive collisions. In this paper, we present the results of numerical simulations with more and more realistic physics involved. Step by step, we include various effects, such as particle growth, radial/vertical particle motion and dust particle fragmentation in our simulations. We demonstrate that the initial dust-to-gas ratio is essential for the particles to overcome the radial drift barrier. If this value is increased by a factor of 2 compared with the canonical value for the interstellar medium, km-sized bodies can form in the inner disk <2 AU within 10 thousand years. However, we find that solid particles get destroyed through collisional fragmentation. Only with the unrealistically high-threshold velocities needed for fragmentation to occur (>30 m/s), particles are able to grow to larger sizes in low turbulent disks. We also find that less than 5% of the small dust grains remain in the disk after 1 Myrs due to radial drift, no matter whether fragmentation is included in the simulations or not. In this paper, we also present considerable improvements to existing algorithms for dust-particle coagulation, which speed up the coagulation scheme by a factor of 10 thousand.

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arXiv2007

High-resolution polarimetry of Parsamian 21: revealing the structure of an edge-on FU Ori disc

We present the first high spatial resolution near-infrared direct and polarimetric observations of Parsamian 21, obtained with the VLT/NACO instrument. We complemented these measurements with archival infrared observations, such as HST/WFPC2 imaging, HST/NICMOS polarimetry, Spitzer IRAC and MIPS photometry, Spitzer IRS spectroscopy as well as ISO photometry. Our main conclusions are the following: (1) we argue that Parsamian 21 is probably an FU Orionis-type object; (2) Parsamian 21 is not associated with any rich cluster of young stars; (3) our measurements reveal a circumstellar envelope, a polar cavity and an edge-on disc; the disc seems to be geometrically flat and extends from approximately 48 to 360 AU from the star; (4) the SED can be reproduced with a simple model of a circumstellar disc and an envelope; (5) within the framework of an evolutionary sequence of FUors proposed by Green et al. (2006) and Quanz et al. (2007), Parsamian 21 can be classified as an intermediate-aged object.

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