H. Zinnecker

Universität Stuttgart, Stuttgart, Baden-Württemberg, Germany

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Publications (382)714.26 Total impact

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    ABSTRACT: The present study aims at characterizing the massive star forming region G35.20N, which is found associated with at least one massive outflow and contains multiple dense cores, one of them recently found associated with a Keplerian rotating disk. We used ALMA to observe the G35.20N region in the continuum and line emission at 350 GHz. The observed frequency range covers tracers of dense gas (e.g. H13CO+, C17O), molecular outflows (e.g. SiO), and hot cores (e.g. CH3CN, CH3OH). The ALMA 870 um continuum emission map reveals an elongated dust structure (0.15 pc long and 0.013 pc wide) perpendicular to the large-scale molecular outflow detected in the region, and fragmented into a number of cores with masses 1-10 Msun and sizes 1600 AU. The cores appear regularly spaced with a separation of 0.023 pc. The emission of dense gas tracers such as H13CO+ or C17O is extended and coincident with the dust elongated structure. The three strongest dust cores show emission of complex organic molecules characteristic of hot cores, with temperatures around 200 K, and relative abundances 0.2-2x10^(-8) for CH3CN and 0.6-5x10^(-6) for CH3OH. The two cores with highest mass (cores A and B) show coherent velocity fields, with gradients almost aligned with the dust elongated structure. Those velocity gradients are consistent with Keplerian disks rotating about central masses of 4-18 Msun. Perpendicular to the velocity gradients we have identified a large-scale precessing jet/outflow associated with core B, and hints of an east-west jet/outflow associated with core A. The elongated dust structure in G35.20N is fragmented into a number of dense cores that may form massive stars. Based on the velocity field of the dense gas, the orientation of the magnetic field, and the regularly spaced fragmentation, we interpret this elongated structure as the densest part of a 1D filament fragmenting and forming massive stars.
    06/2014;
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    ABSTRACT: The Stratospheric Observatory for Infrared Astronomy (SOFIA) has recently concluded a set of engineering flights for Observatory performance evaluation. These in-flight opportunities are viewed as a first comprehensive assessment of the Observatory's performance and are used to guide future development activities, as well as to identify additional Observatory upgrades. Pointing stability was evaluated, including the image motion due to rigid-body and flexible-body telescope modes as well as possible aero-optical image motion. We report on recent improvements in pointing stability by using an active mass damper system installed on the telescope. Measurements and characterization of the shear layer and cavity seeing, as well as image quality evaluation as a function of wavelength have also been performed. Additional tests targeted basic Observatory capabilities and requirements, including pointing accuracy, chopper evaluation and imager sensitivity. This paper reports on the data collected during these flights and presents current SOFIA Observatory performance and characterization.
    05/2014;
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    ABSTRACT: Observations from optical to centimeter wavelengths have demonstrated that multiple systems of two or more bodies is the norm at all stellar evolutionary stages. Multiple systems are widely agreed to result from the collapse and fragmentation of cloud cores, despite the inhibiting influence of magnetic fields. Surveys of Class 0 protostars with mm interferometers have revealed a very high multiplicity frequency of about 2/3, even though there are observational difficulties in resolving close protobinaries, thus supporting the possibility that all stars could be born in multiple systems. Near-infrared adaptive optics observations of Class I protostars show a lower binary frequency relative to the Class 0 phase, a declining trend that continues through the Class II/III stages to the field population. This loss of companions is a natural consequence of dynamical interplay in small multiple systems, leading to ejection of members. We discuss observational consequences of this dynamical evolution, and its influence on circumstellar disks, and we review the evolution of circumbinary disks and their role in defining binary mass ratios. Special attention is paid to eclipsing PMS binaries, which allow for observational tests of evolutionary models of early stellar evolution. Many stars are born in clusters and small groups, and we discuss how interactions in dense stellar environments can significantly alter the distribution of binary separations through dissolution of wider binaries. The binaries and multiples we find in the field are the survivors of these internal and external destructive processes, and we provide a detailed overview of the multiplicity statistics of the field, which form a boundary condition for all models of binary evolution. Finally we discuss various formation mechanisms for massive binaries, and the properties of massive trapezia.
    03/2014;
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    ABSTRACT: We aim to characterize the distribution and composition of circumstellar material around young massive stars, and to investigate exactly which physical structures in these objects are probed by long-baseline mid-infrared interferometric observations. We used the two-telescope interferometric instrument MIDI of the Very Large Telescope Interferometer of the European Southern Observatory to observe a sample of 24 intermediate- and high-mass young stellar objects in the N band (8-13 micron). We had successful fringe detections for 20 objects, and present spectrally-resolved correlated fluxes and visibility levels for projected baselines of up to 128 m. We fit the visibilities with geometric models to derive the sizes of the emitting regions, as well as the orientation and elongation of the circumstellar material. Fourteen objects in the sample show the 10 micron silicate feature in absorption in the total and correlated flux spectra. For 13 of these objects, we were able to fit the correlated flux spectra with a simple absorption model, allowing us to constrain the composition and absorptive properties of the circumstellar material. Nearly all of the massive young stellar objects observed show significant deviations from spherical symmetry at mid-infrared wavelengths. In general, the mid-infrared emission can trace both disks and outflows, and in many cases it may be difficult to disentangle these components on the basis of interferometric data alone, because of the sparse spatial frequency coverage normally provided by current long-baseline interferometers. For the majority of the objects in this sample, the absorption occurs on spatial scales larger than those probed by MIDI. Finally, the physical extent of the mid-infrared emission around these sources is correlated with the total luminosity, albeit with significant scatter.
    Astronomy and Astrophysics 08/2013; · 5.08 Impact Factor
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    ABSTRACT: We provide the complete set of reduced, spectrally-resolved MIDI visibilities and differential phases for the 20 objects observed as part of the survey presented in the above publication. These interferometric data are provided in FITS format, and, more specifically, conform to the OIFITS standard (see Pauls et al., 2005PASP..117.1255P). Additionally, besides the standard OIFITS columns, the OI_VIS tables in each of the FITS files contain the columns 'CFLUX' and 'CFLUXERR', which contain the calibrated correlated flux and its estimated uncertainty (measured in Jy). (2 data files).
    VizieR Online Data Catalog. 08/2013;
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    ABSTRACT: Our general understanding of multiple star and planet formation is primarily based on observations of young multiple systems in low density regions like Tau-Aur and Oph. Since many, if not most, of the stars are born in clusters, observational constraints from young binaries in those environments are fundamental for understanding both the formation of multiple systems and planets in multiple systems throughout the Galaxy. We build upon the largest survey for young binaries in the Orion Nebula Cluster (ONC) which is based on Hubble Space Telescope observations to derive both stellar and circumstellar properties of newborn binary systems in this cluster environment. We present Adaptive Optics spatially-resolved JHKL'-band photometry and K-band R$\sim$\,5000 spectra for a sample of 8 ONC binary systems from this database. We characterize the stellar properties of binary components and obtain a census of protoplanetary disks through K-L' color excess. For a combined sample of ONC binaries including 7 additional systems with NIR spectroscopy from the literature, we derive mass ratio and relative age distributions. We compare the stellar and circumstellar properties of binaries in ONC with those in Tau-Aur and Oph from samples of binaries with stellar properties derived for each component from spectra and/or visual photometry and with a disk census obtained through K-L color excess. The mass ratio distribution of ONC binaries is found to be indistinguishable from that of Tau-Aur and, to some extent, to that of Oph in the separation range 85-560\,AU and for primary mass in the range 0.15 to 0.8\,M$_{\sun}$.A trend toward a lower mass ratio with larger separation is suggested in ONC binaries which is not seen in Tau-Aur binaries.The components of ONC binaries are found to be significantly more coeval than the overall ONC population and as coeval as components of binaries in Tau-Aur and Oph[...]
    Astronomy and Astrophysics 07/2013; · 5.08 Impact Factor
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    ABSTRACT: Binaries are the most common outcome of star formation. However, the impact of binarity on the evolution of primordial circumstellar disks, which are the birthplaces of planets, is currently only little constrained by theory and observations. Consequently, star and planet formation in binary systems may be significantly different from that in single stars. We present results from the largest coherent study of the evolution of circumstellar disks around the components of binary stars to date. 52 binaries were observed in the Orion Nebula Cluster and Chamaeleon I star-forming regions with near-infrared photometry and spectroscopy. We quantify the presence of circumstellar accretion and dust disks around the individual components of low-mass binary stars with respect to their inferred stellar (e.g. mass, luminosity, Teff), binary (binary separation, mass ratio), and cluster parameters (age, stellar density, presence of strong ionizing sources, star formation history). The results imply significantly reduced disk lifetimes when close, <100AU, stellar companions are present. This effect appears most pronounced for the less massive component of a stellar binary. At the same time, the measured mass accretion rates are of similar magnitude as those of single stars in the same cluster and other star forming regions - a counter-intuitive result because disk masses are typically smaller than around single stars and disk life-times do not appear to be shorter by the same amount. Since the studied individual components' circumstellar disks are potential birth places of planets, these results help to explain the peculiarities of the growing population of planets found in orbit around components of stellar multiples.
    07/2013;
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    ABSTRACT: Galactic starburst clusters represent the most extreme mode of present-day star formation in the Milky Way, and are ideal laboratories for studies over the entire stellar mass range from less then 0.1 to more than 120 solar masses. We report on the results of our adaptive optics and HST high angular resolution studies comprising both multi-epoch astrometric monitoring of the cluster's internal and external dynamics, and the photometric and spectroscopic characterization of their stellar populations. Among the most surprising results are i) the distinct motions of Galactic Center starburst clusters with respect to the field, ii) the strict coevality of star formation in the spiral arm clusters NGC 3603 YC and Westerlund 1, and iii) the close agreement between dynamical and photometric mass estimates for each of the clusters (indicating that the clusters are dynamically stable and could survive for extended periods of time).
    07/2013;
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    ABSTRACT: We have made the first detection of a near-infrared counterpart associated with the disk around Radio Source "I," a massive protostar in the Kleinmann-Low Nebula in Orion using imaging with laser guide star adaptive optics on the Keck II telescope. The infrared emission is evident in images acquired using L' (3.8 microns) and Ms (4.7 microns) filters and is not detectable at K' (2.1 microns). The observed morphology strongly suggests that we are seeing some combination of scattered and thermal light emanating from the disk. The disk is also manifest in the L'/Ms flux ratio image. We interpret the near-infrared emission as the illuminated surface of a nearly edge-on disk, oriented so that only the northern face is visible; the opposite surface remains hidden by the disk. We do not see infrared radiation associated directly with the star proposed to be associated with Source "I." The data also suggest that there is a cavity above and below the disk that is oriented perpendicular to the disk, and is sculpted by the known, strong outflow from the inner disk of Source I. We compare our data to models of a protostar with a surrounding disk, envelope, and wind-blown cavity in order to elucidate the nature of the disk around Radio Source I.
    The Astrophysical Journal 04/2013; 770(2). · 6.73 Impact Factor
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    ABSTRACT: The characterization of multiple stellar systems is an important ingredient for testing current star formation models. Stars are more often found in multiple systems, the more massive they are. A complete knowledge of the multiplicity of high-mass stars over the full range of orbit separations is thus essential to understand their still debated formation process. Observations of the Orion Nebula Cluster can help to answer the question about the origin and evolution of multiple stars. Earlier studies provide a good knowledge about the multiplicity of the stars at very small (spectroscopic) and large separations (AO, speckle) and thus make the ONC a good target for such a project. We used the NIR interferometric instrument AMBER at VLTI to observe a sample of bright stars in the ONC. We complement our data set by archival NACO observations of \theta 1 Ori A to obtain more information about the orbit of the close visual companion. Our observations resolve the known multiple systems \theta 1 Ori C and \theta 1 Ori A and provide new orbit points, which confirm the predicted orbit and the determined stellar parameters for \theta 1 Ori C. Combining AMBER and NACO data for \theta 1 Ori A we were able to follow the motion of the companion from 2003 to 2011. We furthermore find hints for a companion around \theta 1 Ori D and a previously unknown companion to NU Ori. With a probability of ~90% we can exclude further companions with masses of > 3 Msun around our sample stars for separations between ~2 mas and ~110 mas. We conclude that the companion around \theta 1 Ori A is most likely physically related to the primary star. The newly discovered possible companions further increase the multiplicity in the ONC. For our sample of two O and three B-type stars we find on average 2.5 known companions per primary, which is around five times more than for low-mass stars.
    Astronomy and Astrophysics 01/2013; · 5.08 Impact Factor
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    ABSTRACT: The Carina Nebula represents one of the largest and most active star forming regions known in our Galaxy with numerous very massive stars.Our recently obtained Herschel PACS & SPIRE far-infrared maps cover the full area (about 8.7 deg^2) of the Carina Nebula complex and reveal the population of deeply embedded young stellar objects, most of which are not yet visible in the mid- or near-infrared.We study the properties of the 642 objects that are independently detected as point-like sources in at least two of the five Herschel bands.For those objects that can be identified with apparently single Spitzer counterparts, we use radiative transfer models to derive information about the basic stellar and circumstellar parameters.We find that about 75% of the Herschel-detected YSOs are Class 0 protostars.The luminosities of the Herschel-detected YSOs with SED fits are restricted to values of <=5400 Lsun, their masses (estimated from the radiative transfer modeling) range from about 1 Msun to 10 Msun.Taking the observational limits into account and extrapolating the observed number of Herschel-detected protostars over the IMF suggest that the star formation rate of the CNC is about 0.017 Msun/yr.The spatial distribution of the Herschel YSO candidates is highly inhomogeneous and does not follow the distribution of cloud mass.Most Herschel YSO candidates are found at the irradiated edges of clouds and pillars.This provides support to the picture that the formation of this latest stellar generation is triggered by the advancing ionization fronts.The currently ongoing star formation process forms only low-mass and intermediate-mass stars, but no massive stars.The far-infrared fluxes of the famous object EtaCar are about a factor of two lower than expected from observations with the ISO obtained 15 years ago; this may be due to dynamical changes in the circumstellar dust in the Homunculus Nebula.
    Astronomy and Astrophysics 11/2012; · 5.08 Impact Factor
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    ABSTRACT: The Carina Nebula complex was observed by the Herschel satellite on December 26th, 2010. (4 data files).
    VizieR Online Data Catalog. 11/2012;
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    ABSTRACT: SOFIA, the Stratospheric Observatory for Infrared Astronomy, is an airborne observatory with a 2.7-m telescope that is under development by NASA and the German Aerospace Center DLR. From late 2010 and through the end of 2011, SOFIA conducted a series of science demonstration flights, Early Science, using FORCAST (the Faint Object InfraRed Camera for the SOFIA Telescope), HIPO (the High-speed Imaging Photometer for Occultations), and GREAT (the German REceiver for Astronomy at Terahertz frequencies). Flying at altitudes as high as 13.7 km (45,000 ft), SOFIA operates above more than 99.8% of the water vapor in the Earth’s atmosphere, opening up most of the far-infrared and sub-millimeter parts of the spectrum. During Early Science, 30 science missions were flown with results in solar system astronomy, star formation, the interstellar medium, the Galactic Center, and extragalactic studies. Many of these investigations were conducted by the first group of SOFIA General Investigators, demonstrating the operation of SOFIA as a facility for the astronomical community. This paper presents some recent highlights from Early Science.
    Proc SPIE 09/2012;
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    ABSTRACT: Aims. This study aims to determine the impact of stellar binary companions on the lifetime and evolution of circumstellar disks in the Chamaeleon I (Cha I) star-forming region by measuring the frequency and strength of accretion and circumstellar dust signatures around the individual components of T Tauri binary stars. Methods. We used high-angular resolution adaptive optics JHKsL′ -band photometry and 1.5–2.5 μm spectroscopy of 19 visual binary and 7 triple stars in Cha I – including one newly discovered tertiary component – with separations between ~25 and ~1000 AU. The data allowed us to infer stellar component masses and ages and, from the detection of near-infrared excess emission and the strength of Brackett-γ emission, the presence of ongoing accretion and hot circumstellar dust of the individual stellar components of each binary. Results. Of all the stellar components in close binaries with separations of 25–100 AU, 10+15-5% show signs of accretion. This is less than half of the accretor fraction found in wider binaries, which itself appears significantly reduced (~44%) compared with previous measurements of single stars in Cha I. Hot dust was found around 50+30-15% of the target components, a value that is indistinguishable from that of Cha I single stars. Only the closest binaries (<25 AU) were inferred to have a significantly reduced fraction (≲25%) of components that harbor hot dust. Accretors were exclusively found in binary systems with unequal component masses Msecondary/Mprimary < 0.8, implying that the detected accelerated disk dispersal is a function of mass-ratio. This agrees with the finding that only one accreting secondary star was found, which is also the weakest accretor in the sample. Conclusions. The results imply that disk dispersal is more accelerated the stronger the dynamical disk truncation, i.e., the smaller the inferred radius of the disk. Nonetheless, the overall measured mass accretion rates appear to be independent of the cluster environment or the existence of stellar companions at any separation ≳25 AU, because they agree well with observations from our previous binary study in the Orion Nebula cluster and with studies of single stars in these and other star-forming regions.
    Astronomy and Astrophysics 06/2012; 554:43. · 5.08 Impact Factor
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    ABSTRACT: Molecular globules and pillars are spectacular features, found only in the interface region between a molecular cloud and an H II-region. Impacting far-ultraviolet (FUV) radiation creates photon-dominated regions (PDRs) on their surfaces that can be traced by typical cooling lines. With the GREAT receiver onboard SOFIA we mapped and spectrally resolved the [C II] 158 $mu$m atomic fine-structure line and the highly excited 12CO J = 11 → 10 molecular line from three objects in Cygnus X (a pillar, a globule, and a strong IRAS source). We focus here on the globule and compare our data with existing Spitzer data and recent Herschel open-time PACS data. Extended [C II] emission and more compact CO-emission was found in the globule. We ascribe this emission mainly to an internal PDR, created by a possibly embedded star-cluster with at least one early B-star. However, external PDR emission caused by the excitation by the Cyg OB2 association cannot be fully excluded. The velocity-resolved [C II] emission traces the emission of PDR surfaces, possible rotation of the globule, and high-velocity outflowing gas. The globule shows a velocity shift of ~2 km s-1 with respect to the expanding H II-region, which can be understood as the residual turbulence of the molecular cloud from which the globule arose. This scenario is compatible with recent numerical simulations that emphazise the effect of turbulence. It is remarkable that an isolated globule shows these strong dynamical features traced by the [C II]-line, but it demands more observational studies to verify if there is indeed an embedded cluster of B-stars. Appendices are available in electronic form at http://www.aanda.org
    Astronomy and Astrophysics 05/2012; 542:L18. · 5.08 Impact Factor
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    ABSTRACT: The Becklin-Neugebauer/Kleinmann-Low (BN/KL) region of the Orion Nebula is the nearest region of high-mass star formation in our galaxy. As such, it has been the subject of intense investigation at a variety of wavelengths, which have revealed it to be brightest in the infrared to submillimeter wavelength regime. Using the newly commissioned SOFIA airborne telescope and its 5-40 μm camera FORCAST, images of the entire BN/KL complex have been acquired. The 31.5 and 37.1 μm images represent the highest resolution observations (4'') ever obtained of this region at these wavelengths. These observations reveal that the BN object is not the dominant brightness source in the complex at wavelengths ≥ 31.5 μm and that this distinction goes instead to the source IRc4. It was determined from these images and derived dust color temperature maps that IRc4 is also likely to be self-luminous. A new source of emission has also been identified at wavelengths ≥ 31.5 μm that coincides with the northeastern outflow lobe from the protostellar disk associated with radio source I.
    The Astrophysical Journal Letters 03/2012; 749(2):L23. · 6.35 Impact Factor
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    ABSTRACT: The BN/KL region of the Orion Nebula is the nearest region of high mass star formation in our galaxy. As such, it has been the subject of intense investigation at a variety of wavelengths, which have revealed it to be brightest in the infrared to sub-mm wavelength regime. Using the newly commissioned SOFIA airborne telescope and its 5-40 micron camera FORCAST, images of the entire BN/KL complex have been acquired. The 31.5 and 37.1 micron images represent the highest resolution observations (<=4") ever obtained of this region at these wavelengths. These observations reveal that the BN object is not the dominant brightness source in the complex at wavelengths >31.5 microns, and that this distinction goes instead to the source IRc4. It was determined from these images and derived dust color temperature maps that IRc4 is also likely to be self-luminous. A new source of emission has also been identified at wavelengths >31.5 microns that coincides with the northeastern outflow lobe from the protostellar disk associated with radio source I.
    02/2012;
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    ABSTRACT: We present new mid-infrared images of the central region of the Orion Nebula using the newly commissioned SOFIA airborne telescope and its 5 -- 40 micron camera FORCAST. The 37.1 micron images represent the highest resolution observations (<4") ever obtained of this region at these wavelengths. After BN/KL (which is described in a separate letter in this issue), the dominant source at all wavelengths except 37.1 micron is the Ney-Allen Nebula, a crescent-shaped extended source associated with theta 1D. The morphology of the Ney-Allen nebula in our images is consistent with the interpretation that it is ambient dust swept up by the stellar wind from theta 1D, as suggested by Smith et al. (2005). Our observations also reveal emission from two "proplyds" (proto-planetary disks), and a few embedded young stellar objects (YSOs; IRc9, and OMC1S IRS1, 2, and 10). The spectral energy distribution for IRc9 is presented and fitted with standard YSO models from Robitaille et al. (2007) to constrain the total luminosity, disk size, and envelope size. The diffuse, nebular emission we observe at all FORCAST wavelengths is most likely from the background photodissociation region (PDR) and shows structure that coincides roughly with H_alpha and [N II] emission. We conclude that the spatial variations in the diffuse emission are likely due to undulations in the surface of the background PDR.
    The Astrophysical Journal Letters 02/2012; 749(2). · 6.35 Impact Factor
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    ABSTRACT: The massive star forming region W3 was observed with the faint object infrared camera for the SOFIA telescope (FORCAST) as part of the Short Science program. The 6.4, 6.6, 7.7, 19.7, 24.2, 31.5 and 37.1 \um bandpasses were used to observe the emission of Polycyclic Aromatic Hydrocarbon (PAH) molecules, Very Small Grains and Big Grains. Optical depth and color temperature maps of W3A show that IRS2 has blown a bubble devoid of gas and dust of $\sim$0.05 pc radius. It is embedded in a dusty shell of ionized gas that contributes 40% of the total 24 \um emission of W3A. This dust component is mostly heated by far ultraviolet, rather than trapped Ly$\alpha$ photons. This shell is itself surrounded by a thin ($\sim$0.01 pc) photodissociation region where PAHs show intense emission. The infrared spectral energy distribution (SED) of three different zones located at 8, 20 and 25\arcsec from IRS2, show that the peak of the SED shifts towards longer wavelengths, when moving away from the star. Adopting the stellar radiation field for these three positions, DUSTEM model fits to these SEDs yield a dust-to-gas mass ratio in the ionized gas similar to that in the diffuse ISM. However, the ratio of the IR-to-UV opacity of the dust in the ionized shell is increased by a factor $\simeq$3 compared to the diffuse ISM.
    The Astrophysical Journal Letters 02/2012; 749(2). · 6.35 Impact Factor
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    ABSTRACT: The inner three square arcminute area of the Orion nebula, centered on the Trapezium and including the BN/KL region, was observed as a part of Early Science observations with SOFIA. Using the 5-40 micron camera FORCAST, we imaged this region at 6.6, 7.7, 19.7, 31.5, and 37.1 microns. These observations provide the highest resolution images ever obtained for these latter two wavelengths with a resolution of FWHM 3 arcseconds. Here we present data on the whole area, concentrating on the results obtained for the BN/KL region, where the mid-infrared intensities and dust color temperature distribution help determine which sources are internally and externally heated. In particular, our observations show that the BN object itself is not a dominant luminosity source at wavelengths >30 microns, and that the brightest source at our longest wavelengths is IRc4, which appears to be self-luminous.
    01/2012;

Publication Stats

3k Citations
714.26 Total Impact Points

Institutions

  • 2011
    • Universität Stuttgart
      Stuttgart, Baden-Württemberg, Germany
    • Fisk University
      • Department of Physics
      Nashville, Tennessee, United States
  • 1990–2007
    • University of Wuerzburg
      Würzburg, Bavaria, Germany
  • 2000–2005
    • Max Planck Institute for Radio Astronomy
      Bonn, North Rhine-Westphalia, Germany
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States
    • Friedrich-Schiller-University Jena
      Jena, Thuringia, Germany
  • 2003
    • Cornell University
      Ithaca, New York, United States
    • The University of Arizona
      Tucson, Arizona, United States
  • 2002
    • Russian Academy of Sciences
      • Institute of Astronomy
      Moscow, Moscow, Russia
  • 1998
    • Leibniz Institute for Astrophysics Potsdam
      Potsdam, Brandenburg, Germany
  • 1995–1997
    • Max Planck Institute for Extraterrestrial Physics
      Arching, Bavaria, Germany
  • 1994
    • University of Leicester
      Leiscester, England, United Kingdom
  • 1989
    • Joint Astronomy Centre
      Hilo, Hawaii, United States