arXiv subjects

Th. Henning

Publications and source records attributed to Th. Henning.

At least 469 records · Page 26Linked to original sources

arXiv2011

Dynamics in Young Star Clusters: From Planets to Massive Stars

The young star clusters we observe today are the building blocks of a new generation of stars and planets in our Galaxy and beyond. Despite their fundamental role we still lack knowledge about the conditions under which star clusters form and the impact of these often harsh environments on the evolution of their stellar and substellar members. We demonstrate the vital role numerical simulations play to uncover both key issues. Using dynamical models of different star cluster environments we show the variety of effects stellar interactions potentially have. Moreover, our significantly improved measure of mass segregation reveals that it can occur rapidly even for star clusters without substructure. This finding is a critical step to resolve the controversial debate on mass segregation in young star clusters and provides strong constraints on their initial conditions.

astro-ph.GA↗

arXiv2011

Multi-wavelength observations of the young binary system Haro 6-10: The case of misaligned discs

Context. We present a multi-wavelength, high-resolution observational survey of the young binary system Haro 6-10 (GV Tau, IRAS 04263+2426), which is harbouring one of the few known infrared companions. Aims. The primary goal of this project is to determine the physical and geometrical properties of the circumstellar and circumbinary material in the Haro 6-10 system. Methods. High-resolution optical (HST/WFPC2) and near-infrared (VLT/NACO) images in different bands were analysed to investigate the large-scale structures of the material around the binary.Mid-infrared interferometry (VLTI/MIDI) and spectroscopy (TIMMI2 at the 3.6m ESO telescope) were carried out to determine the structure and optical depth of the circumstellar material around the individual components. Results. The multi-wavelength observations suggest that both components of the binary system Haro 6-10 are embedded in a common envelope. The measured extinction indicates a dust composition of the envelope similar to that of the interstellar medium. Each component of the system has a circumstellar disc-like structure typical of young stars. The discs are highly misaligned: the northern component is seen almost edge-on and the southern component is an almost face-on disc. Conclusions. The two main formation scenarios of binary systems with misaligned discs are the gravitational capture of a passing object in a dense environment, and the fragmentation of the collapsing molecular cloud. Given the low-density environment of the Taurus-Aurigae star-forming region, the first scenario is unlikely for Haro 6-10. The binary system most probably formed via fragmentation of two different parts of the collapsing molecular cloud combined with other dynamical processes related to the cloud and/or the protostars. This can be the explanation also for other binary systems with an infrared companion.

astro-ph.SR↗

arXiv2011

The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals? III. Sedimentation driven coagulation inside the snow-line

We investigate dust growth due to settling in a 1D vertical column of a protoplanetary disk. It is known from the observed 10 micron feature in disk SEDs, that small micron-sized grains are present at the disk atmosphere throughout the lifetime of the disk. We hope to explain such questions as what process can keep the disk atmospheres dusty for the lifetime of the disk and how does the particle properties change as a function of height above the midplane. We use a Monte Carlo code to follow the mass and porosity evolution of the particles in time. The used collision model is based on laboratory experiments performed on dust aggregates. As the experiments cannot cover all possible collision scenarios, the largest uncertainty of our model is the necessary extrapolations we had to perform. We simultaneously solve for the particle growth and motion. Particles can move vertically due to settling and turbulent mixing. We assume that the vertical profile of the gas density is fixed in time and only the solid component evolves. We find that the used collision model strongly influences the masses and sizes of the particles. The laboratory experiment based collision model greatly reduces the particle sizes compared to models that assume sticking at all collision velocities. We find that a turbulence parameter of alpha = 10^-2 is needed to keep the dust atmospheres dusty, but such strong turbulence can produce only small particles at the midplane which is not favorable for planetesimal formation models. We also see that the particles are larger at the midplane and smaller at the upper layers of the disk. At 3-4 pressure scale heights micron-sized particles are produced. These particle sizes are needed to explain the 10 micron feature of disk SEDs. Turbulence may therefore help to keep small dust particles in the disk atmosphere.

astro-ph.EP↗

arXiv2011

The Coalsack near and far

Context: The large Coalsack dark cloud is one of the rare molecular clouds without clear signs of star formation. Aims: We investigate the dynamical properties of the gas within the Coalsack. Methods: The two highest extinction regions were mapped with the APEX telescope in 13CO(2-1) comprising a region of ~1 square degree. Results: In addition to the well-known, nearby gas component around -4km/s, we identified additional molecular gas components - in particular a second extended molecular cloud at a velocity of ~-30km/s and an estimated distance of ~3.1kpc - that dominate the column density and visual extinction distributions in the northeastern part of the Coalsack. Although comprising ~2600Msun, the mass of this distant cloud is distributed over an extent of ~73pc, much larger than typical high-mass infrared dark clouds. Its filamentary structure is consistent with a compressible gaseous self-gravitating cylinder, and its low mass per length indicates that it may be stable against gravitational collapse. We find barely any mid-infrared emission in archival MSX data, which is indicative of almost no star-formation activity in the near and far cloud complexes. The nearby clouds have narrow, almost thermal velocity dispersions with median values between 0.2 and 0.4km/s, which is also consistent with low star-formation activity. Only Tapia's Globule 2 exhibits a velocity dispersion increase toward the extinction peak and peak-velocity gradients over the core, which is indicative of a state of elevated dynamical properties. Conclusions: The Coalsack is not one single coherent structure, but consists of several cloud complexes nearby as well as at several kpc distance. All studied clouds appear as starless low-turbulence regions that may not even collapse in the future. Only one globule exhibits more dynamical signatures and is a good candidate for present/future star formation.

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arXiv2011

A highly efficient measure of mass segregation in star clusters

Investigations of mass segregation are of vital interest for the understanding of the formation and dynamical evolution of stellar systems on a wide range of spatial scales. Our method is based on the minimum spanning tree (MST) that serves as a geometry-independent measure of concentration. Compared to previous such approaches we obtain a significant refinement by using the geometrical mean as an intermediate-pass. It allows the detection of mass segregation with much higher confidence and for much lower degrees of mass segregation than other approaches. The method shows in particular very clear signatures even when applied to small subsets of the entire population. We confirm with high significance strong mass segregation of the five most massive stars in the Orion Nebula Cluster (ONC). Our method is the most sensitive general measure of mass segregation so far and provides robust results for both data from simulations and observations. As such it is ideally suited for tracking mass segregation in young star clusters and to investigate the long standing paradigm of primordial mass segregation by comparison of simulations and observations.

astro-ph.GA↗

arXiv2011

Low abundance, strong features: Window-dressing crystalline forsterite in the disk wall of HD 100546

Forsterite is one of the crystalline dust species that is often observed in protoplanetary disks and solar system comets. Being absent in the interstellar medium, it must be produced during the disk lifetime. It can therefore serve as a tracer of dust processing and disk evolution, which can lead to a better understanding of the physical processes occurring in the disk, and possibly planet formation. However, the connection of these processes with the overall disk crystallinity remains unclear. We aim to characterize the forsterite abundance and spatial distribution in the disk of the Herbig Be star HD 100546, to investigate if a connection exists with the large disk gap. We use a 2D radiative transfer code, MCMax, to model the circumstellar dust around HD 100546. We use VISIR Q-band imaging to probe the outer disk geometry and mid-infrared features to model the spatial distribution of forsterite. The temperature-dependent shape of the 69 micron feature observed with Herschel PACS is used as a critical tool to constrain this distribution. We find a crystalline mass fraction of 40 - 60 %, located close to the disk wall between 13 and 20 AU, and possibly farther out at the disk surface. The forsterite is in thermal contact with the other dust species. We put an upper limit on the iron content of forsterite of 0.3 %. Optical depth effects play a key role in explaining the observed forsterite features, hiding warm forsterite from view at short wavelengths. The disk wall acts as a showcase: it displays a localized high abundance of forsterite, which gives rise to a high observed crystallinity, while the overall mass fraction of forsterite is a factor of ten lower.

astro-ph.SR↗

arXiv2011

Near-infrared spectroscopy of EX Lupi in outburst

EX Lup is the prototype of the EXor class of young eruptive stars: objects showing repetitive brightenings due to increased accretion from the circumstellar disk to the star. In this paper, we report on medium-resolution near-infrared spectroscopy of EX\,Lup taken during its extreme outburst in 2008, as well as numerical modeling with the aim of determining the physical conditions around the star. We detect emission lines from atomic hydrogen, helium, and metals, as well as first overtone bandhead emission from carbon monoxide. Our results indicate that the emission lines are originating from gas located in a dust-free region within ~ 0.2 AU of the star. The profile of the CO bandhead indicates that the CO gas has a temperature of 2500 K, and is located in the inner edge of the disk or in the outer parts of funnel flows. The atomic metals are probably co-located with the CO. Some metallic lines are fluorescently excited, suggesting direct exposure to ultraviolet photons. The Brackett series indicates emission from hot (10000 K) and optically thin gas. The hydrogen lines display a strong spectro-astrometric signal, suggesting that the hydrogen emission is probably not coming from an equatorial boundary layer; a funnel flow or disk wind origin is more likely. This picture is broadly consistent with the standard magnetospheric accretion model usually assumed for normally accreting T Tauri stars. Our results also set constraints on the eruption mechanism, supporting a model where material piles up around the corotation radius and episodically falls onto the star.

astro-ph.SR↗

arXiv2011

Sequential star formation in IRAS 06084-0611 (GGD 12-15): From intermediate-mass to high-mass stars

Context. The formation and early evolution of high- and intermediate-mass stars towards the main sequence involves the interplay of stars in a clustered and highly complex environment. To obtain a full census of this interaction, the Formation and Early evolution of Massive Stars (FEMS) collaboration studies a well-selected sample of 10 high-mass star-forming regions. Aims. In this study we examine the stellar content of the high-mass star-forming region centered on IRAS 06084-0611 in the Monoceros R2 cloud. Methods. Using the near-infrared H- and K-band spectra from the VLT/SINFONI instrument on the ESO Very Large Telescope (VLT)and photometric near-infrared NTT/SOFI, 2MASS and Spitzer/IRAC data, we were able to determine the spectral types for the most luminous stars in the cluster. Results. Two very young and reddened massive stars have been detected by SINFONI: a massive Young Stellar Object (YSO) con- sistent with an early-B spectral type and a Herbig Be star. Furthermore, stars of spectral type G and K are detected while still in the Pre-Main Sequence (PMS) phase. We derive additional properties such as temperatures, extinctions, radii and masses. We present a Hertzsprung-Russell diagram and find most objects having intermediate masses between \sim1.5-2.5 M\odot. For these stars we derive a median cluster age of \sim4 Myr. Conclusions. Using Spitzer/IRAC data we confirm earlier studies that the younger class 0/I objects are centrally located while the class II objects are spread out over a larger area, with rough scale size radii of \sim0.5 pc and \sim1.25 pc respectively. Moreover, the presence of a massive YSO, an ultracompact H ii region and highly reddened objects in the center of the cluster suggest a much younger age of < 1 Myr. A possible scenario for this observation would be sequential star formation along the line of sight; from a cluster of intermediate-mass to high-mass stars.

astro-ph.SR↗

arXiv2011

Turbulence and Steady Flows in 3D Global Stratified MHD Simulations of Accretion Disks

We present full 2 Pi global 3-D stratified MHD simulations of accretion disks. We interpret our results in the context of proto-planetary disks. We investigate the turbulence driven by the magneto-rotational instability (MRI) using the PLUTO Godunov code in spherical coordinates with the accurate and robust HLLD Riemann solver. We follow the turbulence for more than 1500 orbits at the innermost radius of the domain to measure the overall strength of turbulent motions and the detailed accretion flow pattern. We find that regions within two scale heights of the midplane have a turbulent Mach number of about 0.1 and a magnetic pressure two to three orders of magnitude less than the gas pressure, while outside three scale heights the magnetic pressure equals or exceeds the gas pressure and the turbulence is transonic, leading to large density fluctuations. The strongest large-scale density disturbances are spiral density waves, and the strongest of these waves has m=5. No clear meridional circulation appears in the calculations because fluctuating radial pressure gradients lead to changes in the orbital frequency, comparable in importance to the stress gradients that drive the meridional flows in viscous models. The net mass flow rate is well-reproduced by a viscous model using the mean stress distribution taken from the MHD calculation. The strength of the mean turbulent magnetic field is inversely proportional to the radius, so the fields are approximately force-free on the largest scales. Consequently the accretion stress falls off as the inverse square of the radius.

astro-ph.EP↗

arXiv2011

Electronic spectroscopy of FUV-irradiated diamondoids: A combined experimental and theoretical study

Irradiation with high energy photons (10.2 - 11.8 eV) was applied to small diamondoids isolated in solid rare gas matrices at low temperature. The photoproducts were traced via UV absorption spectroscopy. We found that upon ionization the smallest of these species lose a peripheral H atom to form a stable closed-shell cation. This process is also likely to occur under astrophysical conditions for gas phase diamondoids and it opens the possibility to detect diamond-like molecules using their rotational spectrum since the dehydrogenated cations possess strong permanent dipole moments. The lowest-energy electronic features of these species in the UV were found to be rather broad, shifting to longer wavelengths with increasing molecular size. Calculations using time-dependent density functional theory support our experimental findings and extend the absorption curves further into the vacuum ultraviolet. The complete sigma - sigma* spectrum displays surprisingly strong similarities to meteoritic nanodiamonds containing 50 times more C atoms.

physics.chem-ph↗

arXiv2011

Absorption Line Survey of H3+ toward the Galactic Center Sources III. Extent of the Warm and Diffuse Clouds

We present follow-up observations to those of Geballe & Oka (2010), who found high column densities of H3+ ~100 pc off of the Galactic center (GC) on the lines of sight to 2MASS J17432173-2951430 (J1743) and 2MASS J17470898-2829561 (J1747). The wavelength coverages on these sightlines have been extended in order to observe two key transitions of H3+, R(3,3)l and R(2,2)l, that constrain the temperatures and densities of the environments. The profiles of the H3+ R(3,3)l line, which is due only to gas in the GC, closely matches the differences between the H3+ R(1,1)l and CO line profiles, just as it does for previously studied sightlines in the GC. Absorption in the R(2,2)l line of H3+ is present in J1747 at velocities between -60 and +100 km/s. This is the second clear detection of this line in the interstellar medium after GCIRS 3 in the Central Cluster. The temperature of the absorbing gas in this velocity range is 350 K, significantly warmer than in the diffuse clouds in other parts of the Central Molecular Zone. This indicates that the absorbing gas is local to Sgr B molecular cloud complex. The warm and diffuse gas revealed by Oka et al. (2005) apparently extends to ~100 pc, but there is a hint that its temperature is somewhat lower in the line of sight to J1743 than elsewhere in the GC. The observation of H3+ toward J1747 is compared with the recent Herschel observation of H2O+ toward Sgr B2 and their chemical relationship and remarkably similar velocity profiles are discussed.

astro-ph.GA↗

arXiv2011

High-mass star formation at high luminosities: W31 at >10^6 L_sun

Context: High-mass star formation has been a very active field over the last decade, however, most studies targeted regions of luminosities between 10^4 and 10^5 L_sun. Methods: We selected the W31 star-forming complex with a total luminosity of ~6x10^6 L_sun for a multi-wavelength spectral line and continuum study covering wavelengths from the near- and mid-infrared via (sub)mm wavelength observations to radio data in the cm regime. Results: While the overall structure of the multi-wavelength continuum data resembles each other well, there are several intriguing differences. The 24mum emission stemming largely from small dust grains follows tightly the spatial structure of the cm emission tracing the ionized free-free emission. Hence warm dust resides in regions that are spatially associated with the ionized hot gas (~10^4 K) of the HII regions. Furthermore, we find several evolutionary stages within the same complexes, ranging from infrared-observable clusters, via deeply embedded regions associated with active star formation traced by 24\,$μ$m and cm emission, to at least one high-mass gas clump devoid of any such signature. The 13CO(2-1) and C18O(2-1) spectral line observations reveal a large kinematic breadth in the entire region with a total velocity range of approximately 90 km/s. While the average virial mass ratio for W31 is close to unity, the line width analysis indicates large-scale evolutionary differences between the southern and northern sub-regions (G10.2-0.3 and G10.3-0.1) of the whole W31 complex. The clump mass function - tracing cluster scales and not scales of individual stars - derived from the 875mum continuum data has a slope of 1.5+-0.3, consistent with previous cloud mass functions.

astro-ph.SR↗

arXiv2011

UV-controlled physical and chemical structure of protoplanetary disks

We study details of the UV radiation transfer in a protoplanetary disk, paying attention to the influence of dust growth and sedimentation on the disk density and temperature. Also, we show how the dust evolution affects photoreaction rates of key molecules, like CN and CS.

astro-ph.EP↗

arXiv2011

Abundances of PAHs in the ISM: Confronting Observations with Experimental Results

We present recent UV laboratory spectra of various polycyclic aromatic hydrocarbons (PAHs) and explore the potential of these molecules as carriers of the DIBs. From a detailed comparison of gas-phase and Ne-matrix absorption spectra of anthracene, phenanthrene, pyrene, 2,3-benzofluorene, benzo[ghi]perylene, and hexabenzocoronene with new interstellar spectra, we infer upper limits in the abundance of these PAHs in the interstellar medium. Upper limits in the column densities of anthracene of $0.8 - 2.8 \times 10^{12}$\ cm$^{-2}$ and of pyrene and 2,3-benzofluorene ranging from $2 - 8 \times 10^{12}$\ cm$^{-2}$ are inferred. Upper limits in the column densities of benzo[ghi]perylene are $0.9 - 2.4 \times 10^{13}$ and $10^{14}$ cm$^{-2}$ for phenanthrene. The measurements indicate fractional abundances of anthracene, pyrene, and 2,3-benzofluorene of a few times $10^{-10}$. Upper limits in the fractional abundance of benzo[ghi]perylene of a few times $10^{-9}$ and of phenanthrene of few times $10^{-8}$ are inferred. {Toward CPD $-32^\circ 1734$, we found near 3584 Å an absorption line of OH$^+$, which was discovered in the interstellar medium only very recently. The fractional abundances of PAHs inferred here are up to two orders of magnitude lower than estimated total PAH abundances in the interstellar medium. This indicates that either neutral PAHs are not abundant in translucent molecular clouds, or that a PAH population with a large variety of molecules is present.

astro-ph.GA↗

arXiv2010

Fundamental Vibrational Transition of CO during the Outburst of EX Lupi in 2008

We report monitoring observations of the T Tauri star EX Lupi during its outburst in 2008 in the CO fundamental band at 4.6-5.0 um. The observations were carried out at the VLT and the Subaru Telescope at six epochs from April to August 2008, covering the plateau of the outburst and the fading phase to a quiescent state. The line flux of CO emission declines with the visual brightness of the star and the continuum flux at 5 um, but composed of two subcomponents that decay with different rates. The narrow line emission (50 km s-1 in FWHM) is near the systemic velocity of EX Lupi. These emission lines appear exclusively in v=1-0. The line widths translate to a characteristic orbiting radius of 0.4 AU. The broad line component (FWZI ~ 150 km s-1) is highly excited upto v<=6. The line flux of the component decreases faster than the narrow line emission. Simple modeling of the line profiles implies that the broad-line emitting gas is orbiting around the star at 0.04-0.4 AU. The excitation state, the decay speed of the line flux, and the line profile, indicate that the broad-line emission component is physically distinct from the narrow-line emission component, and more tightly related to the outburst event.

astro-ph.SR↗

arXiv2010

Structure and evolution of debris disks around F-type stars: I. Observations, database and basic evolutionary aspects

Although photometric and spectroscopic surveys with the Spitzer Space Telescope increased remarkably the number of well studied debris disks around A-type and Sun-like stars, detailed analyzes of debris disks around F-type stars remained less frequent. Using the MIPS camera and the IRS spectrograph we searched for debris dust around 82 F-type stars with Spitzer. We found 27 stars that harbor debris disks, nine of which are new discoveries. The dust distribution around two of our stars, HD 50571 and HD 170773, was found to be marginally extended on the 70um MIPS images. Combining the MIPS and IRS measurements with additional infrared and submillimeter data, we achieved excellent spectral coverage for most of our debris systems. We have modeled the excess emission of 22 debris disks using a single temperature dust ring model and of 5 debris systems with two-temperature models. The latter systems may contain two dust rings around the star. In accordance with the expected trends, the fractional luminosity of the disks declines with time, exhibiting a decay rate consistent with the range of model predictions. We found the distribution of radial dust distances as a function of age to be consistent with the predictions of both the self stirred and the planetary stirred disk evolution models. A more comprehensive investigation of the evolution of debris disks around F-type stars, partly based on the presented data set, will be the subject of an upcoming paper.

astro-ph.SR↗

arXiv2010

MESS (Mass-loss of Evolved StarS), a Herschel Key Program

MESS (Mass-loss of Evolved StarS) is a Guaranteed Time Key Program that uses the PACS and SPIRE instruments on board the Herschel Space Observatory to observe a representative sample of evolved stars, that include asymptotic giant branch (AGB) and post-AGB stars, planetary nebulae and red supergiants, as well as luminous blue variables, Wolf-Rayet stars and supernova remnants. In total, of order 150 objects are observed in imaging and about 50 objects in spectroscopy. This paper describes the target selection and target list, and the observing strategy. Key science projects are described, and illustrated using results obtained during Herschel's science demonstration phase. Aperture photometry is given for the 70 AGB and post-AGB stars observed up to October 17, 2010, which constitutes the largest single uniform database of far-IR and sub-mm fluxes for late-type stars.

astro-ph.SR↗

arXiv2010

Chemistry in Infrared Dark Clouds

Massive stars play an important role in shaping the structure of galaxies. Infrared dark clouds (IRDCs), with their low temperatures and high densities, have been identified as the potential birthplaces of massive stars. In order to understand the formation processes of massive stars the physical and chemical conditions in infrared dark clouds have to be characterized. The goal of this paper is to investigate the chemical composition of a sample of southern infrared dark clouds. One important aspect of the observations is to check, if the molecular abuncances in IRDCs are similar to the low-mass pre-stellar cores, or whether they show signatures of more evolved evolutionary stages. We performed observations toward 15 IRDCs in the frequency range between 86 and 93 GHz using the 22-m Mopra radio telescope. We detect HNC, HCO$^+$ and HNC emission in all clouds and N$_2$H$^+$ in all IRDCs except one. In some clouds we detect SiO emission. Complicated shapes of the HCO$^+$ emission line profile are found in all IRDCs. Both signatures indicates the presence of infall and outflow motions and beginning of star formation activity, at least in some parts of the IRDCs. Where possible, we calculate molecular abundances and make a comparison with previously obtained values for low-mass pre-stellar cores and high-mass protostellar objects (HMPOs). We show a tendency for IRDCs to have molecular abundances similar to low-mass pre-stellar cores rather than to HMPOs abundances on the scale of our single-dish observations.

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