Carsten Dominik

Understanding the spectra of isolated Herbig stars in the frame of a passive disk model

We discuss spectral energy distributions of a sample of Herbig Ae/Be stars in the context of a passive irradiated disk model. The data have been presented earlier by Meeus et al. (2001), and preliminary interpretations ofthese data were given in that paper. While the spectra of Herbig Ae stars all show similarities, there is significant variation between the spectra, in particular in the shape of the mid-IR rise and in the presence or absence of a silicate feature. We explore the hypothesis that all these different spectra can be interpreted as pure disk spectra without additional components. Using the model of Dullemond et al. (2001) we deduce the disk parameters of a number of the sources, and find that for a large fraction of investigated sources, satisfactory fits can be obtained. The derived model parameters show that some group Ia sources can only be fit with radially increasing surface densities, indicating the presence of depleted inner disk regions. The steep-sloped SEDs of group IIa sources can be fit with very compact disks, probably representing disks with collapsed outer regions. The largest difficulties arise from sources that do not show significant silicate emission features. Our attempts to explain these objects with a pure geometric effect are only partially successful. It seems that these stars indeed require a strong depletion of small silicate grains.

Age Dependence of the Vega Phenomenon: Observations

We study the time dependency of Vega-like excesses using infrared studies obtained with the imaging photopolarimeter ISOPHOT on board of ISO. We review the different studies published on this issue, and critically check and revise ages and fractional luminosities in the different samples. The conclusions of our study differ significantly from those obtained by other authors (e.g. Holland et al. 1998; Spangler et al. 2001) who suggested that there is a global powerlaw governing the amount of dust seen in debris disks as a function of time. Our investigations lead us to conclude that (i) for stars at most ages, a large

Disappearance of stellar debris disks around main-sequence stars after 400 million years

Almost 5 billion years ago, the Sun formed in a local contraction of a cloud of molecular gas. A rotating disk of gas and dust is believed to have fed material onto the proto-Sun for the first few million years of its life, and to have formed the planets, comets and other Solar System objects. Similar disks, but with less mass, have been observed around a few main-sequence stars such as Vega. The dust particles orbiting stars like Vega will be removed on timescales of the order of 1u2009Myr (Vega is about 350u2009Myr old), and therefore must be resupplied, at least for a time. But earlier surveys lacked the sensitivity to determine how many nearby stars have dust disks, and to investigate how long such disks survive. Here we report infrared observations indicating that most stars younger than 300u2009Myr have dust disks, while most older than 400u2009Myr do not: ninety per cent of the disks disappear when the star is between 300 and 400u2009Myr old. Several events that are related to the ‘clean up’ of debris in the early history of our Solar System have a similar timescale.