arXiv subjects

H. Feuchtgruber

Publications and source records attributed to H. Feuchtgruber.

60 records · Page 4Linked to original sources

arXiv2000

Excitation of H_2 and HD in shocks and PDRs

Photodissociation regions (PDRs) and shocks give rise to conspicuous emission from rotationally and vibrationally excited molecular hydrogen. This line emission has now been studied with ISO and from the ground in great detail. A remarkable discovery has been that toward the Orion outflow and other shock-excited regions, the H_2 level populations show a very high excitation component. We suggest that these high-excitation populations may arise from non-thermal pumping processes, such as H_2 formation and high-velocity ion-molecule collision in partially dissociative shocks. In PDRs such as NGC 7023 however, formation pumping is always less important than fluorescent pumping. We furthermore present two HD emission line detections toward Orion Peak 1. This enables the first comparison of the H_2 and the HD excitation, which surprisingly turn out to be identical.

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arXiv2000

Detection of interstellar CH_3

Observations with the Short Wavelength Spectrometer (SWS) onboard the {\it Infrared Space Observatory} (ISO) have led to the first detection of the methyl radical ${\rm CH_3}$ in the interstellar medium. The $ν_2$ $Q-$branch at 16.5 $μ$m and the $R$(0) line at 16.0 $μ$m have been unambiguously detected toward the Galactic center SgrA$^*$. The analysis of the measured bands gives a column density of (8.0$\pm$2.4)$\times10^{14}$ cm$^{-2}$ and an excitation temperature of $(17\pm 2)$ K. Gaseous ${\rm CO}$ at a similarly low excitation temperature and ${\rm C_2H_2}$ are detected for the same line of sight. Using constraints on the ${\rm H_2}$ column density obtained from ${\rm C^{18}O}$ and visual extinction, the inferred ${\rm CH_3}$ abundance is $(1.3{{+2.2}\atop{-0.7}}) \times 10^{-8}$. The chemically related ${\rm CH_4}$ molecule is not detected, but the pure rotational lines of ${\rm CH}$ are seen with the Long Wavelength Spectrometer (LWS). The absolute abundances and the ${\rm CH_3/CH_4}$ and ${\rm CH_3/CH}$ ratios are inconsistent with published pure gas-phase models of dense clouds. The data require a mix of diffuse and translucent clouds with different densities and extinctions, and/or the development of translucent models in which gas-grain chemistry, freeze-out and reactions of ${\rm H}$ with polycyclic aromatic hydrocarbons and solid aliphatic material are included.

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arXiv2000

ISO-SWS observations of pure rotational water absorption lines toward Orion-IRc2

First detections of thermal water vapor absorption lines have been made toward Orion IRc2 using the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). Grating spectra covering wavelengths 25-45 micron yield 19 pure rotational lines, originating from energy levels 200-750 K above ground. Fabry-Perot spectra of 5 transitions resolve the line profiles and reveal the water gas kinematics. The fact that all lines are seen in absorption is in striking contrast with data from the ISO Long Wavelength Spectrometer (LWS), where the water lines appear in emission. At least one line displays a P-Cygni type profile, which suggests that the water is located in an expanding shell centered on or near IRc2. The expansion velocity is 18 km per second, in agreement with the value inferred from water maser observations by Genzel et al. (1981). Because the continuum is intense and likely formed in or near the water-containing gas, the excitation of the observed transitions is dominated by radiative processes. A simple, generalised curve-of-growth method is presented and used to analyze the data. A mean excitation temperature of 72 K and a total water column density of 1.5e18 molecules per cm squared are inferred, each with an estimated maximum uncertainty of 20%. Combined with the molecular hydrogen column density derived from ISO observations of the pure rotational lines, and an assumed temperature of 200-350 K, the inferred water abundance is 2e-4 to 5e-4 in the warm shocked gas. This abundance is similar to that found recently by Harwit et al. (1998) toward Orion using data from the LWS, but higher than that found for most other shocked regions by, for example, Liseau et al. (1996).

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arXiv2000

Jupiter's hydrocarbons observed with ISO-SWS: vertical profiles of C2H6 and C2H2, detection of CH3C2H

We have analysed the ISO-SWS spectrum of Jupiter in the 12-16 micron range, where several hydrocarbons exhibit rovibrational bands. Using temperature information from the methane and hydrogen emissions, we derive the mixing ratios (q) of acetylene and ethane at two independent pressure levels. For acetylene, we find $q=(8.9^{+1.1}_{-0.6})\times10^{-7}$ at 0.3 mbar and $q=(1.1^{+0.2}_{-0.1})\times10^{-7}$ at 4 mbar, giving a slope $-d\ln q / d\ln P=0.8\pm0.1$, while for ethane $q=(1.0\pm0.2)\times10^{-5}$ at 1 mbar and $q=(2.6^{+0.5}_{-0.6})\times10^{-6}$ at 10 mbar, giving $-d\ln q / d\ln P=0.6\pm0.2$. The ethane slope is consistent with the predictions of Gladstone et al. (1996), but that predicted for acetylene is larger than we observe. This disagreement is best explained by an overestimation of the acetylene production rate compared to that of ethane in the Gladstone et al. (1996) model. At 15.8 micron, methylacetylene is detected for the first time at low jovian latitudes, and a stratospheric column density of $(1.5\pm0.4)\times10^{15}$ molecule.cm-2 is inferred. We also derive an upper limit for the diacetylene column density of $7\times10^{13}$ molecule.cm-2.

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arXiv2000

ISO-SWS spectra of galaxies: continuum and features

We present an inventory of mid-infrared spectral features detected in high resolution 2.4 to 45um ISO-SWS spectra of the galaxies M82, NGC253, Circinus, NGC1068, and the 30Doradus region of the LMC. We discuss their identifications and highlight possible relations between these features and the physical state of the interstellar medium in galaxies. In the 5--13 um region, emission from unidentified infrared bands (UIBs), usually ascribed to aromatic molecules, and apparent silicate absorption dominate the spectrum. We demonstrate the danger of overestimating the amount of silicate absorption by comparing the spectrum of M82 to the (absorption free) spectrum of the reflection nebula NGC7023. No strong silicate absorption is present in M82. The (very small grain) dust continuum under the UIB emission in our starburst templates can be modeled by a simple power law, starting at wavelengths between 8 and 9um. The spectra are valuable templates for future mid-infrared missions. We smooth our data to simulate low resolution spectra as obtained with ISOCAM-CVF, ISOPHOT-S, and in the future with the low resolution mode of SIRTF-IRS, and use our high spectral resolution information to highlight possible identification problems at low resolving power that are caused by coincidences of lines and features.

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arXiv1999

The central stars of the planetary nebulae NGC 7027 and NGC 6543

Infrared spectra of NGC 7027 and NGC 6543 ranging from 2.4 to 45 micron were obtained with the Short Wavelength Spectrometer on board the Infrared Space Observatory. A first analysis of these spectra, with the aid of photo-ionization models, is presented. We report the first detection of the Ar VI 4.53 micron and Ne VI 7.65 micron lines in the spectrum of NGC 7027. When compared with older observations it is clear that the Ar VI line and possibly also other lines have increased in strength since 1981. We argue that a likely explanation for this variability is a change in the spectral energy distribution of the central star, possibly an increase in effective temperature. However, this result needs to be confirmed by further observations. We also report a non-detection of the O IV 25.9 micron line and the first detection of the Na III 7.32 micron line in the spectrum of NGC 6543. The non-detection is not expected based on a blackbody approximation for the spectrum of the central star. The ionization threshold for O(3+) is just beyond the He II limit, and the absence of this line shows that the stellar flux drops at least by a factor 350 at the He II limit. Modeling the O IV line may prove to be a valuable test for atmosphere models.

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