5. Discussion and conclusions
In this paper, we have looked into detail at the ISO spectrum of HD 142527. Although it is currently not possible (model constraints, insufficient laboratory data, ) to derive the exact circumstellar dust properties, we are able to produce a decent fit, resulting in some ideas on how the circumstellar environment of this pre-main sequence star looks like and how it evolves.
We have shown that in the circumstellar dust disk of the young Fe-star HD 142527, at least two distinct dust populations, of which the coldest has a temperature of only 30 to 60 K, can be distinguished. The dust of the warmest component spans a temperature range from 500 to 1500 K. Not only the temperature but also the composition differs. The warm component is mainly composed of silicates, some of which are crystalline. This crystallization has not occured in the cold dust environment.
On the other hand, warm montmorillonite would display very distinct emission features in the near-IR (e.g. at 6 µm), which are not present. The crystalline silicates being warm, and the hydrous silicates being cold, should not come as a surprise, since the dust condensation temperatures are about 1400 and about 300 K respectively (Larimer & Anders 1967). Since other stars (e.g. HD 100546) do exhibit the characteristic crystalline silicate features at longer infrared wavelenghts, corresponding to much colder dust temperatures, other dust processing mechanisms must already have occurred there.
Hydrous silicates, of which montmorillonite is one example, are known to be present in solar system matter. A study of interplanetary dust particles (IDPs) by Sandford & Walker (1985) shows that IDPs can be divided in three major families, the olivines, the pyroxenes and the layer-lattice silicates. Sandford & Walker studied 26 particles, of which 11 were composed of layer-lattice silicates, such as montmorillonite. From the isotopic enrichments, they concluded that the particles were formed or during the cold molecular phase prior to solar system formation, or during the nebular-protostar phase, where they formed in cold dense regions of the nebula, possibly as a result of alteration of high temperature condensates (olivine and pyroxene) when equilibrated with (Larimer & Anders 1967; Zaikowski & Knacke, 1975). Sandford & Walker (1985) obtained a fit of the spectrum of comet Kohoutek using 50 of pyroxene IDPs and 50 layer-lattice IDPs. A nearly perfect fit of the 10 µm spectrum of comet P/Halley could be obtained as well, using a variety of IDPs, including a small amount of layer-lattice silicates (Bregman et al. 1987). By comparing the spectrum of the protostellar object W33 with the layer-lattice IDP Skywalker, Sandford & Walker (1985) demonstrated that hydrous silicates are present outside the solar system as well. Condensation of hydrous silicates in the outflow of supernova SN 1987A has also be postulated (Timmermann & Larson 1993).
In order to study the evolution circumstellar dust undergoes when a star descends towards the main sequence, we compared the dust characteristics of HD 142527 with those of HD 100546 and HD 179218, two isolated Herbig Ae/Be stars with ages of more than 107 years and about 105 years respectively, which means hundred times older and of approximately the same age of HD 142527, respectively.
The spectrum of HD 100546 (B9V) is characterized by a series of mid infrared emission peaks, which can be attributed to crystalline forsterite (), with temperatures as low as 210 K. Also a massive 50 K component is present, accounting for the 69 µm forsterite spectral feature that has been observed (Malfait et al. 1998b). Crystalline -ice is present as well in the environment of this main sequence star. The presence of a small amount of hydrous silicates is likely, some evidence for the 100 µm features is present (Waters & Waelkens, 1998). That the spectrum of HD 142527 is partially characterized by cold hydrous silicates, while the radiation of the cold dust surrounding HD 100546 is dominated by crystalline silicates is likely to be an age effect. This hypothesis is strengthened by the lack of features caused by crystalline silicates in the spectra of more embedded Herbig Ae/Be stars (Waters, van den Ancker, private communication).
HD 179218 (B9e) on the other hand, has an age similar to that of HD 142527. However, its spectrum resembles that of HD 100546 more than it resembles the spectrum of HD 142527 (Waelkens et al. 1998). Although no detailed modelling has been performed yet, the presence of cold crystalline silicates (both olivines and pyroxenes) is obvious. Since no LWS-spectrum of this star has been taken, we cannot confirm or exclude the presence of -ice (there is a peak present at 43 µm, but this might be caused by pyroxenes as well) and of hydrous silicates. IRAS photometry does not help here. From these three stars, we cannot detect a clear correlation between age and dust composition. The dust processing mechanisms or time scales on which these processes occur seem to differ from star to star.
What mechanisms do cause crystallization of the silicates? This chemical metamorphosis cannot take place at the low temperatures (T 300 K) that result from our modelling. Or is a fraction of the silicate dust already crystallized and/or hydrated before the star and disk form? If so, why do deeply embedded Herbig Ae/Be stars not show these characteristics? Is this an abundance effect? If so, how come these abundances change in time, causing a trend from amorphous towards more ordered mineral structure? Is the presence of hydrous silicates a temperature effect, and are they therefore only present in very cold circumstellar disks (like the one surrounding HD 142527)? How comes HD 142527 is already an `isolated' Haebe star, and why has the envelope already dissappeared on such a short timescale, much shorter than the age of several embedded objects (van den Ancker et al. 1998)? Does the abundance of -ice play an important role? To answer these questions, more Haebe objects (both embedded and isolated) should be studied in more detail.
We must point out that the study of only the SWS-spectrum of HD 142527 would have resulted in a totally different interpretation since no evidence for hydrous silicates would have been found. Broad band photometry (such as IRAS) in the far-IR is not sufficient to represent the dust disk continuum flux, but can almost exclusively be attributed to solid state features. As a consequence, dust modelling is necessary to derive reliable physical quantities such as the dust temperature. It turns out that discriminating between all different silicate minerals is difficult, since they are all characterized by a prominent feature at 10 µm, and often emit strongly at various longer wavelengths. This makes modelling very difficult. An overview paper ordering all laboratory research that has been done on cosmic-like dust particles, would be very useful.
The current effort that is being made to converge the knowledge of laboratory science, solar system research, earth geology and stellar astronomy will probably result in the solution of a lot of the questions remaining in this field.
© European Southern Observatory (ESO) 1999
Online publication: April 12, 1999