arXiv subjects

R. Siebenmorgen

Publications and source records attributed to R. Siebenmorgen.

58 records · Page 4Linked to original sources

arXiv2000

The protostellar system HH108MMS

We probe the region around the protostar HH108MMS by deep mid infrared photometric and polarimetric imaging. The protostar is detected at 14mu in absorption against the diffuse background. Next to HH108MMS, we find a second absorbing core, named Q1, and the young stellar object IRAS18331-0035 which is more advanced in its evolution and already seen in emission at 12mu and 14mu. HH108MMS, Q1 and IRAS18331-0035 form a triplet along an extended filamentary absorption feature. From the variation of the surface brightness across the source, we derive for HH108MMS and Q1 the optical depth and density profile. As HH108MMS is also seen in the millimeter dust emission, we can derive the ratio of the dust extinction coefficients at 14mu and 1.3mm and obtain kappa(14mu) / kappa(1300mu) ~ 470. Because models for the dust in the diffuse interstellar medium predict a ratio of around 2000, our value points to fluffy composite grains which are expected to prevail in dense and cold environments. First mid infrared polarisation images of pre--stellar absorbing cores are presented. At 12mu and 14mu the polarisation of the region around HH108MMS is strong (> 15%) and tightly correlated with the source triplet. We demonstrate that the high degree of polarisation can be explained by extinction of rotationally aligned dust particles of moderate elongation.

astro-ph↗

arXiv2000

Mid infrared emission of nearby Herbig Ae/Be stars

We present mid IR spectro-photometric imaging of a sample of eight nearby ($D \leq 240$pc) Herbig Ae/Be stars. The spectra are dominated by photospheric emission (HR6000), featureless infrared excess emission (T~Cha), broad silicate emission feature (HR5999) and the infrared emission bands (HD 97048, HD 97300, TY~CrA, HD 176386). The spectrum of HD179218 shows both silicate emission and infrared emission bands (IEB). All stars of our sample where the spectrum is entirely dominated by IEB have an extended emission on scales of a few thousand AU ($\sim 10''$). We verify the derived source extension found with ISOCAM by multi--aperture photometry with ISOPHT and compare our ISOCAM spectral photometry with ISOSWS spectra.

astro-ph↗

arXiv1999

Massive Star Formation in Galaxies: Radiative transfer models of the UV to mm emission of starburst galaxies

We present illustrative models for the UV to millimeter emission of starburst galaxies which are treated as an ensemble of optically thick giant molecular clouds (GMCs) centrally illuminated by recently formed stars. The models follow the evolution of the GMCs due to the ionization-induced expansion of the HII regions and the evolution of the stellar population within the GMC according to the Bruzual & Charlot stellar population synthesis models. The effect of transiently heated dust grains/PAHs to the radiative transfer, as well as multiple scattering, is taken into account. The expansion of the HII regions and the formation of a narrow neutral shell naturally explains why the emission from PAHs dominates over that from hot dust in the near to mid-IR, an emerging characteristic of the infrared spectra of starburst galaxies. The models allow us to relate the observed properties of a galaxy to its age and star formation history. We find that exponentially decaying 10^7-10^8 yrs old bursts can explain the IRAS colours of starburst galaxies. The models are also shown to account satisfactorily for the multiwavelength data on the prototypical starburst galaxy M82 and NGC6090, a starburst galaxy recently observed by ISO. In M82 we find evidence for two bursts separated by 10^7yrs. In NGC6090 we find that at least part of the far-IR excess may be due to the age of the burst (6.4 x 10^7yrs). We also make predictions about the evolution of the luminosity of starbursts at different wavelengths which indicate that far-IR surveys may preferentially detect older starbursts than mid-IR surveys.

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arXiv1994

BD+30 3639: The Infrared Spectrum During Post-AGB Stellar Evolution

We present a radiative-transfer calculation which reproduces the infrared spectrum of the planetary nebula BD~+30$^{\circ}$3639. We calculate the transfer process through absorption and scattering in a spherical-symmetric multi-grain dust shell. The emission of transiently heated particles is taken into account, as well as polycyclic aromatic hydrocarbons. We obtain an acceptable fit to most of the spectrum, including the PAH infrared bands. At submillimetre wavelengths the observed emission is larger than the model predicts, indicating that large dust conglomerates (``f{}luffy grains'') may be needed as an additional constituent. The fit favours a distance of $\ge 2 \,$kpc, which implies that BD~+30$^\circ$3639 has evolved from a massive progenitor of several solar masses. A low dust-to-gas mass ratio is found in the ionised region. The calculations yield an original mass-loss rate of $2\times10^{-4} \msolar \peryr$ on the Asymptotic Giant Branch. Using this mass-loss rate, we calculate how the infrared spectrum has evolved during the post-AGB evolution. We show in particular the evolution of the IRAS colours during the preceding post-AGB evolution.

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