arXiv subjects

S. Hony

Publications and source records attributed to S. Hony.

79 records · Page 5Linked to original sources

arXiv2002

The rich 6 to 9 um spectrum of interstellar PAHs

Here, we present 6-9 um spectra of a sample of RNe, HII regions, YSOs, evolved stars and galaxies that show strong unidentified infrared bands, obtained with the ISO-SWS. The IR emission features show pronounced variations. 1) The 6.2 um feature shifts and clearly shows profile variations. 2) The 7.7 um complex is comprised of at least two subpeaks peaking at 7.6 and one longwards of 7.7 um. In some cases the main peak can apparently shift up to 8 um. Two sources do not exhibit a 7.7 um complex but instead show a broad emission feature at 8.22 um. 3) The 8.6 um feature has a symmetric profile in all sources and some sources exhibit this band at slightly longer wavelengths. For the 6.2, 7.7 and 8.6 um features, the sources have been classified independently based on their profile and peak position. The classes derived for these features are directly linked with each other. Furthermore, the observed 6--9 um spectrum depends on the type of object. We attribute the observed 6.2 um profile and peak position to the combined effect of a PAH family and anharmonicity with pure PAHs representing the 6.3 um component and substituted/complexed PAHs representing the 6.2 um component. The 7.6 um component is well reproduced by both pure and substituted/complexed PAHs but the 7.8 um component remains an enigma. In addition, the exact identification of the 8.22 um feature remains unknown. The observed variations in the characteristics of the IR emission bands are linked to the local physical conditions. Possible formation and evolution processes that may influence the interstellar PAH class are highlighted.

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arXiv2002

The carrier of the "30" micron emission feature in evolved stars. A simple model using magnesium sulfide

We present 2-45 micron spectra of a large sample of carbon-rich evolved stars in order to study the ``30'' micron feature. We find the ``30'' micron feature in sources in a wide range of sources: low mass loss carbon stars, extreme carbon-stars, post-AGB objects and planetary nebulae. We extract the profiles from the sources by using a simple systematic approach to model the continuum. We find large variations in the wavelength and width of the extracted profiles of the ``30'' micron feature. We modelled the whole range of profiles in a simple way by using magnesium sulfide (MgS) dust grains with a MgS grain temperature different from the continuum temperature. The systematic change in peak positions can be explained by cooling of MgS grains as the star evolves off the AGB. In several sources we find that a residual emission excess at ~26 micron can also be fitted using MgS grains but with a different grains shape distribution. The profiles of the ``30'' micron feature in planetary nebulae are narrower than our simple MgS model predicts. We discuss the possible reasons for this difference. We find a sample of warm carbon-stars with very cold MgS grains. We discuss possible causes for this phenomenon. We find no evidence for rapid destruction of MgS during the planetary nebula phase and conclude that the MgS may survive to be incorporated in the ISM.

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arXiv2002

An atlas of 2.4 to 4.1 microns ISO/SWS spectra of early-type stars

We present an atlas of spectra of O- and B-type stars, obtained with the Short Wavelength Spectrometer (SWS) during the Post-Helium program of the Infrared Space Observatory (ISO). This program is aimed at extending the Morgan & Keenan classification scheme into the near-infrared. Later type stars will be discussed in a seperate publication. The observations consist of 57 SWS Post-Helium spectra from 2.4 to 4.1 microns, supplemented with 10 spectra acquired during the nominal mission with a similar observational setting. For B-type stars, this sample provides ample spectral converage in terms of subtype and luminosity class. For O-type stars,the ISO sample is coarse and therefore is complemented with 8 UKIRT L'-band observations. In terms of the presence of diagnostic lines, the L'-band is likely the most promising of the near-infrared atmospheric windows for the study of the physical properties of B stars. Specifically, this wavelength interval contains the Brackett alpha, Pfund gamma, and other Pfund lines which are probes of spectral type, luminosity class and mass loss. Here, we present simple empirical methods based on the lines present in the 2.4 to 4.1 microns interval that allow the determination of: the spectral type of B dwarfs and giants to within two subtypes; the luminosity class of B stars to within two classes; the mass-loss rate of O stars and B supergiants to within 0.25 dex.

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arXiv2001

The discovery of the "21" micron and "30" micron emission features in Planetary Nebulae with Wolf-Rayet central stars

We report the discovery of the "21" micron and "30" micron features in the planetary nebulae around the hydrogen-deficient stars HD 826 and HD 158269. The carriers of these features are known to be produced in outflows around carbon-rich stars. This discovery demonstrates that the bulk of the dust in these nebulae has been produced during a carbon-rich phase before the atmospheres of these stars became hydrogen poor. This is the first time that the "21" micron feature has been detected in any planetary nebula. It shows that once formed its carrier can survive the formation of the nebula and the exposure to the UV radiation of the hot central star. This means that the carrier of "21" micron feature is not transient: the absence of the feature sets limits on the production of its carrier.

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arXiv2001

Discovery of a double ring in the core of eta Carinae

We report the discovery of a double ring structure in the waist of the nebula surrounding eta Carinae. The rings are detected in the mid-IR dust continuum at wavelengths of 7.9, 11.9, 12.9 and 20 micronmeter. The dust in the rings has a temperature of about 300 K. The orientation of the rings is inclined with respect to the axis of the homunculus by either 37 or 58 degrees. The central star is not in the projected centre of the structure defined by the two rings. This geometry is reminiscent of that seen in SN1987A and some planetary nebulae. We discuss several possible origins for this remarkable geometry and its orientation.

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arXiv2001

The CH out-of-plane bending modes of PAH molecules in astrophysical environments

We present 10-15 micron spectra of a sample of H II regions, YSOs and evolved stars that show strong unidentified infrared emission features, obtained with the ISO/SWS spectrograph on-board ISO. These spectra reveal a plethora of emission features with bands at 11.0, 11.2, 12.0, 12.7, 13.5 and 14.2 micron. These features are observed to vary considerably in relative strength to each-other from source to source. In particular, the 10-15 micron spectra of the evolved stars are dominated by the 11.2 micron band while for H II regions the 12.7 is typically as strong as the 11.2 micron band. Analysing the ISO data we find a good correlation between the 11.2 micron band and the 3.3 micron band, and between the 12.7 micron and the 6.2 micron band. There is also a correlation between the ratio of the UIR bands to the total dust emission and the 12.7 over 11.2 micron ratio. Bands in the 10-15 micron spectral region are due to CH out-of-plane (OOP) bending modes of polycyclic aromatic hydrocarbons (PAHs). We summarise existing laboratory data and theoretical quantum chemical calculations of these modes for neutral and cationic PAHs. Due to mode coupling, the exact peak position of these bands depends on the number of adjacent CH groups and hence the observed interstellar 10-15 micron spectra can be used to determine the molecular structure of the interstellar PAHs emitting in the different regions. We conclude that evolved stars predominantly inject compact ~100-200 C-atom PAHs into the ISM where they are subsequently processed, resulting in more open and uneven PAH structures.

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arXiv1999

The infrared spectrum of the Be star gamma Cassiopeiae

We present the 2.4-45 micrometer ISO-SWS spectrum of the Be star gamma Cas (B0.5 IVe). The spectrum is characterised by a thermal continuum which can be well fit by a power-law S_nu ~ nu^0.99 over the entire SWS wavelength range. For an isothermal disc of ionized gas with constant opening angle, this correponds to a density gradient rho(r) ~ r^(-2.8). We report the detection of the Humphreys bound-free jump in emission at 3.4 micrometer. The size of the jump is sensitive to the electron temperature of the gas in the disc, and we find T~9000 K, i.e. much lower than the stellar effective temperature (25000-30000 K). The spectrum is dominated by numerous emission lines, mostly from HI, but also some HeI lines are detected. Several spectral features cannot be identified. The line strengths of the HI{\sc i} emission lines do not follow case B recombination line theory. The line strengths and widths suggest that many lines are optically thick and come from an inner, high density region with radius 3-5 R_star and temperature above that of the bulk of the disc material. Only the alpha, beta and gamma transitions of the series lines contain a contribution from the outer regions. The level populations deviate significantly from LTE and are highly influenced by the optically thick, local (disc) continuum radiation field. The inner disk may be rotating more rapidly than the stellar photosphere.

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