Discs Andor Planets

The origin of the Solar System is one of the oldest and still one of the most fascinating problems of astronomy. Real impetus to the research was given in the 1980s when IRAS observations evidenced the presence of dust around a few main-sequence (MS) stars [1,2] and the disc of ß Pic was imaged by Smith and Terrile [3]. The next wave of interest to the problem was driven by recent discoveries of planets around other solar-type stars ([4-8]; see [9,10] for recent reviews). These intriguing discoveries have given us an opportunity to look at our Solar System from the outside and, in addition, at its past. Whereas the first offers a clearer view into the global properties of a dust disc, the latter provides insight into the process of the system's formation.

The presence of dusty discs around stars during the planet formation stage is required by all plausible theories (e.g. [11]). Being a by-product of the star formation, discs make an excellent reservoir of the material for the formation of planets and smaller bodies. "Discs, then, are the thread connecting protostars and planets" [12]. The bulk of the material left over after star formation is completed disappears by the time when the planetary system is formed. The remainder is present mostly in a solid form. The Edgeworth-Kuiper Belt (EKB; e.g. [13-15]) provides a vivid illustration of that. Further abundant evidence is found that at some evolutionary stages discs and planets may coexist: 1. The giant gas planets are thought to be formed rapidly, with a solid core being developed in is 106 yr and accretion of a massive gaseous envelope being accomplished in ~ 107 yr (see Zuckerman [16] and references therein). This is less than the ages of typical disc systems.

2. Decreasing with time, the masses of discs even around 'old' pre-main-sequence (PMS) stars are typically still higher than 0.01 Mq [12,17-19] — a minimum mass required for planet formation in a system (e.g. [11]). At the same time the total masses of circumstellar material around main-sequence stars are much less (e.g. [20-24] among others) evidencing that planets, if any, must already have been formed there.

3. A low amount of gas around MS stars is another argument in favour of the already completed planet formation (e.g. [20,22,25-28]). When compared to the standard gas to dust ratio (« 100), the gas is typically depleted by a factor of up to 100 — 1000.

4. Inner depletion zones in the discs of MS stars (see Sect. 3.1) are also thought to be a sequel of the presence of a planet.

Because of many observational restrictions the Solar System was for a long time the only example showing a co-existence both of planets and a disc. Improved observational techniques, however, made it possible to image even tenuous debris discs around nearby stars. As a result, three other solar-type stars known to be orbited by a planetary-mass companion (55 Cnc, p CrB and HD 210277) were recently found to also possess discs [29,30],

Nevertheless, the discs as tenuous as our EKB dust disc are still beyond the observational limitations [30], so that the discs accessible to observations and study are typically more massive and younger than the EKB. That is why we direct our attention now to these younger analogues of the Solar System (see Figure 1). By this the group of so-called Vega-type systems is typically meant. First recognized as a separate class of objects by IRAS observations, they have been intensively studied during the last 15 years. The four famous IRAS prototypes — ¡3 Pic , Vega, Fomalhaut and e Eri — belong to this class. Generally speaking, the group includes all MS stars surrounded by dust discs. In this sense, our Solar System may also be considered as a Vega-type system.

The discs are not, however, something that suddenly develops when the star reaches the main sequence. Just the opposite: young stars deeply embedded in residues of the protostellar clouds gradually get free of the remnant material, the clearing being particularly effective in the inner parts. This primordial material is lost from the interior of the shells well before the stars reach the main sequence (e.g. [31,32]). The discs around older stars, inevitably evidenced by different observational data, are thus second-generation, or debris, discs. They are maintained by a collisional cascade of dust grains released by comets and/or planetesimals (Sect. 3.3). It is clear now, that circumstellar shells of the 'oldest' PMS stars, though somewhat more extended and dense than the discs in Vega-type systems, are actually of the same nature. So by "young solar systems" we mean here systems, in which a MS or PMS star older than a few Myr is surrounded by debris dust material.

2. CIRCUMSTELLAR DUST: OBSERVATIONAL MANIFESTATIONS

The presence of dust around MS stars was first indicated by the mid- and far-IR excesses detected by IRAS for four sources [1,2]. Later on the excesses were found for many other MS stars (e.g. [33-38]). The excesses are typically seen in far-IR, although in a few cases mid-IR emission is also detectable. The younger the system, the larger is the excess in

Spectra) Class

Figure 1. Classification of'young solar systems'

Age, yr

Spectra) Class

Figure 1. Classification of'young solar systems'

the mid-IR, and PMS objects show strong emission already in the near-IR. Up till now thermal emission of dust has remained the main criterion for the selection of Vega-type candidates [39-41],

Spectroscopic observations in the IR region provide another good means of studying circumstellar dust. Whereas warmed dust emits mostly in the continuum, warm silicate particles produce also the well-known 10 pm emission feature which is observed in many dusty objects (see Sect. 3.2 for more details). The presence of silicates in crystalline form is also evidenced by emission features in the 20 —70 pm region [42-45]. A family of emission bands (3.29 pm and associated 3.4 — 3.6 pm features, 6.2,7.7,8.6 and 11.3 pm) from small carbon-rich particles, generally attributed to polycyclic aromatic hydrocarbons (PAHs), is also often observed [24,43-47],

It is well known that small dust particles are also effective absorbers and scatterers of stellar radiation. The amount of dust around Vega-type stars is, however, insufficient to cause observable circumstellar extinction. The optical depth of the discs is always much less than unity even in the visual. A crude measure of the optical depth is offered by the fractional luminosity of dust relative to the star L/r/L» (see e.g. [31]). For Vega-type stars, it is commonly less than 10-2 (see Table 1). Being dustier (which is well seen from comparison of the ages and values L/r/L»), younger (PMS) objects have, on average, more material along the line of sight, so that the circumstellar extinction is observable. Note, that the material around PMS stars may not be confined to a flat disc, but rather to a more extended envelope (see discussions by Pezzuto et al. [48], Waters and Waelkens [49] and Miroshnichenko et al. [50]).

Circumstellar dust also shows up in the observed polarization of stellar light. To cite an example, an optical polarimetrical monitoring of 18 Vega-type and post-T Tau stars undertaken by Oudmaijer et al. [51] has shown that the radiation of 15 of them was polarized. In Figure 2 the observed polarization of Vega-type stars and their precursors is plotted as a function of the observed fractional luminosity of dust. The tendency is clear: the integral polarization of star-disc systems is proportional to the disc optical depths. It is also proportional to the albedo of dust particles and depends on the inclination angle of the disc with respect to the line of sight [52,53]. This should and does introduce some dispersion into the observed values. For the oldest objects with tenuous discs, the integral polarization is, unfortunately, very low and close to the detection limits, which makes the study difficult. Further uncertainty is introduced by the interstellar component of the polarization. Fortunately, in the case of Vega-type systems this component is normally not large, as the objects are fairly close. Obtaining polarization maps instead of measuring the integral polarization may be more advantageous, as in this case, polarization in an optically thin disc is independent of the (typically poorly known) optical depth and is predominantly determined by the dust properties.

Imaging is, of course, a direct and the best way to probe circumstellar dust — both in thermal emission in the IR and in the scattered stellar light in the visual (including polarization maps). Indisputable leader among the systems with available 'portraits' is (3 Pic . Being one of the closest and with the favourable edge-on orientation, it is still dense enough to be imaged at different wavelengths from optical [3,63,65,67,70,96,97] through near- [68] and mid-IR [66,98] to the submillimetre region [23]. A number of other Vega-type sources have been recently imaged in different wavelength regions (see e.g. [30,100-102,136] and Table 1). The discs/shells of PMS stars are dustier, which should make them easier to detect. These objects are, however, more distant, so that it

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