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Astron. Astrophys. 318, 472-484 (1997)

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4. Analysis of observed spectral energy distributions

The spectral energy distributions of Herbig Ae/Be stars have been fitted with models of circumstellar dust distribution of various characteristics in order to study its spatial distribution and dynamics, notably by Hillenbrand et al.  (1992, `.. massive circumstellar accretion disks') and by Hartmann et al.  (1993, `.. disks or envelopes?'). We do not try to compete with these detailed studies but occasionaly use a simple version of such models as guideline for the interpretation of our results. We do not expect that these simplified models describe the true spatial distribution of circumstellar matter but we think that they give a first approximation to the distribution of emitting dust with distance from the star.

It is mainly for the subarcsecond binaries HK Ori, V380 Ori and MWC 1080 that we obtained separate near-infrared spectral energy distributions for the components. Extrapolation of these spectral energy distributions then should allow us to associate the optical source with one of the infrared components and to estimate for both components luminosity and gross properties of circumstellar matter distribution.

We base this extrapolation on a simple model of a star surrounded by a geometrically thin disk, assuming thermal emission everywhere and calculating the optical thickness locally from the surface density of circumstellar matter. The star is characterised by [FORMULA] and [FORMULA], with values taken from the literature and modified only if necessary to obtain an acceptable fit. The stellar radius can be adjusted to fit the absolute level of the photometric data, a decrease of radius corresponding to additional neutral extinction. The disk is described by an inner and an outer radius, by its mass, by the disk surface temperature at 1 AU, and by power laws for surface density and temperature distribution, [FORMULA] r [FORMULA] and T(r) [FORMULA] r [FORMULA]. Star and disk are taken to lie behind the material causing the extinction, a simplification acceptable for the above objects which appear not heavily embedded in circumstellar material. As said above, our modelling is not to imply that all or most of the circumstellar material is in the form of thin disks. And we have also to remember that the parameters of such a disk model usually are not determined uniquely from the observations (see Thamm et al.  1994). However, our modelling at least indicates the general distribution of circumstellar matter with temperature (and hence distance from the star) around the binary components and helps us to estimate luminosities in a physically acceptable way. Detailed values of the parameters are not to be taken very seriously, and we mention only those in the text which appear relevant to us or which demonstrate the character of the fits used.

It is a weakness of our data set that data at different wavelengths often were taken at different dates. However, as far as we can tell from repeated observations of the same object at the same wavelength, the influence of brightness variations on the shape of the SEDs is not large and does not seriously affect our conclusions.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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