arXiv subjects

E. Peeters

Publications and source records attributed to E. Peeters.

59 records · Page 4Linked to original sources

arXiv2002

The stellar content, metallicity and ionization structure of HII regions

Observations of infrared fine-structure lines provide direct information on the metallicity and ionization structure of HII regions and indirectly on the hardness of the radiation field ionizing these nebulae. We have analyzed a sample of Galactic and Magellanic Cloud HII regions observed by the Infrared Space Observatory (ISO) to examine the interplay between stellar content, metallicity and the ionization structure of HII regions. The observed [SIV]10.5/[SIII]18.7 mum and [NeIII]15.5/[NeII]12.8 mum line ratios are shown to be highly correlated over more than two orders of magnitude. We have compared the observed line ratios to the results of photoionization models using different stellar energy distributions. The derived characteristics of the ionizing star depend critically on the adopted stellar model as well as the (stellar) metallicity. We have compared the stellar effective temperatures derived from these model studies for a few well-studied HII regions with published direct spectroscopic determinations of the spectral type of the ionizing stars. This comparison supports our interpretation that stellar and nebular metallicity influences the observed infrared ionic line ratios. We can explain the observed increase in degree of ionization, as traced by the [SIV]\[SIII] and [NeIII]\[NeII] line ratios, by the hardening of the radiation field due to the decrease of metallicity. The implications of our results for the determination of the ages of starbursts in starburst galaxies are assessed.

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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

A photoionization model of the compact HII region G29.96-0.02

We present a detailed photoionization model of G29.96-0.02 (hereafter G29.96), one of the brightest Galactic Ultra Compact HII (UCHII) regions in the Galaxy. This source has been observed extensively at radio and infrared wavelengths. The most recent data include a complete ISO (SWS and LWS) spectrum, which displays a remarkable richness in atomic fine-structure lines. The number of observables is twice as much as the number available in previous studies. In addition, most atomic species are now observed in two ionization stages. The radio and infrared data on G29.96 are best reproduced using a nebular model with two density components: a diffuse (n_e~680cm-3) extended (~1 pc) component surrounding a compact (~0.1 pc) dense (n_e~57000cm-3) core. The properties of the ionizing star were derived using state-of-the-art stellar atmosphere models. CoStar models yield an effective temperature of \~30^{+2}_{-1} kK whereas more recent non-LTE line blanketed atmospheres with stellar winds indicate somewhat higher values, Teff~32--38 kK. This range in Teff is compatible with all observational constraints, including near-infrared photometry and bolometric luminosity. The range 33-36 kK is also compatible with the spectral type O5-O8 determined by Watson and Hanson (97) when recent downward revisions of the effective temperature scale of O stars are taken into account. The age of the ionizing star of G29.96 is found to be a few 10^6 yr, much older than the expected lifetime of UCHII regions. Accurate gas phase abundances are derived with the most robust results being Ne/S=7.5 and N/O=0.43 (1.3 and 3.5 times the solar values, respectively).

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arXiv2001

ISO spectroscopy of compact HII regions in the Galaxy. II Ionization and elemental abundances

Based on the ISO spectral catalogue of compact HII regions by Peeters et al. (2001), we present a first analysis of the hydrogen recombination and atomic fine-structure lines originated in the ionized gas. The sample consists of 34 HII regions located at galactocentric distances between Rgal = 0 and 15 kpc. The SWS HI recombination lines between 2 and 8 mum are used to estimate the extinction law at these wavelengths for 14 HII regions. An extinction in the K band between 0 and $\sim$ 3 mag. has been derived. The fine-structure lines of N, O, Ne, S and Ar are detected in most of the sources. Most of these elements are observed in two different ionization stages probing a range in ionization potential up to 41 eV. The ISO data, by itself or combined with radio data taken from the literature, is used to derive the elemental abundances relative to hydrogen. The present data thus allow us to describe for each source its elemental abundance, its state of ionization and to constrain the properties of the ionizing star(s).

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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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