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  • 1
    Publication Date: 2018-06-06
    Description: One of the major objectives of Laboratory Astrophysics is the optimization of data return from space missions by measuring spectra of atomic and molecular species in laboratory environments that mimic interstellar conditions (WhitePaper (2002, 2006)). Among interstellar species, PAHs are an important and ubiquitous component of carbon-bearing materials that represents a particularly difficult challenge for gas-phase laboratory studies. We present the absorption spectra of jet-cooled neutral and ionized PAHs and discuss the implications for astrophysics. The harsh physical conditions of the interstellar medium have been simulated in the laboratory. We are now, for the first time, in the position to directly compare laboratory spectra of PAHs and carbon nanoparticles with astronomical observations. This new phase offers tremendous opportunities for the data analysis of current and upcoming space missions geared toward the detection of large aromatic systems (HST/COS, FUSE, JWST, Spitzer).
    Keywords: Astrophysics
    Type: Proceedings of the NASA Laboratory Astrophysics Workshop; 243-247; NASA/CP-2006-214549
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  • 2
    Publication Date: 2019-05-21
    Description: Recent advances in laboratory spectroscopy lead to the claim of ionized Buckminsterfullerene (C60(+)) as the carrier of two diffuse interstellar bands (DIBs) in the near-infrared. However, irrefutable identication of interstellar C60(+) requires a match between the wavelengths and the expected strengths of all absorption features detectable in the laboratory and in space. Here we present Hubble Space Telescope (HST) spectra of the region covering the C60(+) 9348, 9365, 9428, and 9577 absorption bands toward seven heavily reddened stars. We focus in particular on searching for the weaker laboratory C60(+) bands, the very presence of which has been a matter for recent debate. Using the novel STIS-scanning technique to obtain ultra-high signal-to-noise spectra without contamination from telluric absorption that aficted previous ground-based observations, we obtained reliable detections of the (weak) 9365, 9428 and (strong) 9577 C60(+) bands. The band wavelengths and strength ratios are sufciently similar to those determined in the latest laboratory experiments that we consider this the rst robust identication of the 9428 band, and a conclusive conrmation of interstellar C60(+).
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN68405 , Astrophysical Journal Letters (ISSN 2041-8205) (e-ISSN 2041-8213); 875; 2; L28
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  • 3
    Publication Date: 2019-07-10
    Description: Analysis of high spectral resolution observations of the lambda6614 DIB line profile show systematic variations in the positions of the peaks in the substructure of the profile. These variations can only be understood in the framework of rotational contours of large molecules, where the variations are caused by changes in the rotational excitation temperature. We show that the rotational excitation temperature for the DIB carrier is of the order 10-40 K - much lower than the gas kinetic temperature - indicating that for this particular DIB carrier angular momentum buildup is not very efficient. The rotational constant indicates that the carrier of this DIB is smaller than previously assumed:7-22 C atoms, depending on the geometry.
    Keywords: Fluid Mechanics and Thermodynamics
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  • 4
    Publication Date: 2019-07-18
    Description: We present the results of a dedicated search for the spectral signatures in the visible range of neutral polycyclic aromatic hydrocarbons (PAHs) in astronomical observations representing various astrophysical environments, probing a total column of line of sight material corresponding to Av=50. Laboratory measurements of PAHs in simulated astrophysical conditions are now available (see contribution of Salama et al.) which provide for the first time the exact wavelengths for the spectral features of these molecules, as well as detailed information on the intrinsic line profiles and oscillator strengths. These measurements therefore allow a direct comparison to astronomical observations and an estimate of, or upper limit to, the abundance of individual PAHs in space. As the column densities for individual PAHs in interstellar or circumstellar lines of sight are expected to be very low, such a comparison and analysis requires astronomical observations at very high signal to noise. We present such a data set here for lines of sight representing diffuse clouds and circumstellar environments of carbon stars, and their comparison with gas phase spectra of a representative set of free, cold PAHs.
    Keywords: Astrophysics
    Type: AAS 206th Meeting; 29 May - 2 Jun. 2005; Minneapolis, MN; United States
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  • 5
    Publication Date: 2019-07-18
    Description: We present the gas-phase spectroscopy of neutral and ionized polycyclic aromatic hydrocarbons (PAHs) measured in the W-Visible-NIR range in an astrophysically relevant environment. These measurements provide data on PAHs and nanometer sized particles that can now be directly compared to astronomical observations. The harsh physical conditions of the IS medium - characterized by a low temperature, an absence of collisions and strong VUV radiation fields - are simulated in the laborat'ory by associating a molecular beam with an ionizing discharge to generate a cold plasma expansion. PAH ions are formed from the neutral precursors in an isolated environment at low temperature ($\sim lOO$-K). The spectra of neutral and ionized PAHs are measured using the high sensitivity methods of cavity ring down spectroscopy (CRDS) and multiplex integrated cavity output spectroscopy (MICOS). These experiments provide unique information on the spectra of free, cold large carbon molecules and ions in the gas phase. The electronic bands measured for ionized PAH are found to be intrinsically broad ($\geq$20 cm$^{-l}$) while the bands associated with the neutral precursors are narrower (of the order of 2 - 10 cm$^{-l}$). The laboratory data are discussed and compared with recent astronomical spectra of large and narrow DIBs and with the spectra of circumstellar environments of selected carbon stars (see contribution of Cami et al.) and the implications for the interstellar PAH population are derived. Preliminary results also show that carbon nanoparticles are formed during the short residence time of the precursors in the plasma. This finding holds great potential for understanding the formation process of interstellar grains.
    Keywords: Astronomy
    Type: AAS 206th Meeting; 29 May - 2 Jun. 2005; Minneapolis, MN; United States
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  • 6
    Publication Date: 2013-12-23
    Description: We searched the Spitzer Space Telescope data archive for Galactic planetary nebulae (PNe), which show the characteristic 17.4 and 18.9 μm features due to C 60 , also known as buckminsterfullerene. Out of 338 objects with Spitzer /Infrared Spectrograph data, we found eleven C 60 -containing PNe, six of which (Hen2-68, IC2501, K3-62, M1-6, M1-9 and SaSt2-3) are new detections, not known to contain C 60 prior to this work. The strongest 17.4 and 18.9 μm C 60 features are seen in Tc1 and SaSt2-3, and these two sources also prominently show the C 60 resonances at 7.0 and 8.5 μm. In the other nine sources, the 7.0 and 8.5 μm features due to C 60 are much weaker. We analysed the spectra, along with ancillary data, using the photoionization code cloudy to establish the atomic line fluxes, and determine the properties of the radiation field, as set by the effective temperature of the central star. In addition, we measured the infrared spectral features due to dust grains. We find that the polycyclic aromatic hydrocarbon (PAH) profile over 6–9 μm in these C 60 -bearing carbon-rich PNe is of the more chemically processed class A. The intensity ratio of 3.3 to 11.3 μm PAH indicates that the number of C-atoms per PAH in C 60 -containing PNe is small compared to that in non-C 60 PNe. The Spitzer spectra also show broad dust features around 11 and 30 μm. Analysis of the 30 μm feature shows that it is strongly correlated with the continuum, and we propose that a single carbon-based carrier is responsible for both the continuum and the feature. The strength of the 11 μm feature is correlated to the temperature of the dust, suggesting that it is at least partially due to a solid-state carrier. The chemical abundances of C 60 -containing PNe can be explained by asymptotic giant branch nucleosynthesis models for initially 1.5–2.5 M stars with Z  = 0.004. We plotted the locations of C 60 -containing PNe on a face-on map of the Milky Way and we found that most of these PNe are outside the solar circle, consistent with low metallicity values. Their metallicity suggests that the progenitors are an older population.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 7
    Publication Date: 2016-07-31
    Description: We performed a detailed spectroscopic analysis of the fullerene C 60 -containing planetary nebula (PN) Lin49 in the Small Magellanic Cloud (SMC) using XSHOOTER at the European Southern Observatory Very Large Telescope and the Spitzer /Infrared Spectrograph instruments. We derived nebular abundances for nine elements. We used tlusty to derive photospheric parameters for the central star. Lin49 is C-rich and metal-deficient PN ( Z ~ 0.0006). The nebular abundances are in good agreement with asymptotic giant branch nucleosynthesis models for stars with initial mass 1.25 M and metallicity Z = 0.001. Using the tlusty synthetic spectrum of the central star to define the heating and ionizing source, we constructed the photoionization model with cloudy that matches the observed spectral energy distribution (SED) and the line fluxes in the UV to far-IR wavelength ranges simultaneously. We could not fit the ~1–5 μm SED using a model with 0.005–0.1-μm-sized graphite grains and a constant hydrogen density shell owing to the prominent near-IR excess, while at other wavelengths the model fits the observed values reasonably well. We argue that the near-IR excess might indicate either (1) the presence of very small particles in the form of small carbon clusters, small graphite sheets, or fullerene precursors, or (2) the presence of a high-density structure surrounding the central star. We found that SMC C 60 PNe show a near-IR excess component to lesser or greater degree. This suggests that these C 60 PNe might maintain a structure nearby their central star.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 8
    Publication Date: 2014-08-10
    Description: We have selected a homogeneous sample of asymptotic giant branch (AGB) stars in the Galactic bulge population from the ISOGAL survey. Our target stars cover a wide range of mass-loss rates (~10 –8 –10 –4 M yr –1 ) and differ primarily by their age on the AGB. This homogeneous sample is thus ideally suited to study the dust formation process as a function of age on the AGB. We observed our sample with Spitzer -Infrared Spectrograph, and studied the overall properties of the infrared spectra of these targets. The analysis is complicated by the presence of strong and variable background emission, and the extracted infrared AGB star spectra are affected by interstellar extinction. Several stars in our sample have no detectable dust emission, and we used these ‘naked stars’ to characterize the stellar and molecular contributions to the infrared spectra of our target stars. The resulting dust spectra of our targets do indeed show significant variety in their spectral appearance, pointing to differing dust compositions for the targets. We classify the spectra based on the shape of their 10-μm emission following the scheme by Sloan & Price. We find that the early silicate emission classes associated with oxide dust are generally under-represented in our sample due to extinction effects. We also find a weak 13-μm dust feature in two of our otherwise naked star spectra, suggesting that the carrier of this feature could potentially be the first condensate in the sequence of dust condensation.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 9
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present ISO observations of bright, high-mass southern starforming regions with the Short Wavelength Spectrometer (SWS) and Long Wavelength Spectrometer (LWS). The selected targets show a range of pre-main sequence evolutionary phases. Whereas some objects still show interstellar ice features, the most evolved objects are coincident with ultra-compact HII regions and show strong emission lines from atomic species and polycyclic aromatic hydrocarbons (PAHs). We discuss the ISO spectra between 2.5 - 200 µm of selected southern star-forming regions in the context of their evolution.
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present the spectra of 5 OH/IR stars observed with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). The spectra are dominated by dust features, both in the amorphous and crystalline state and illustrate the influence of the mass loss rate on the observed spectral features.
    Type of Medium: Electronic Resource
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