Letters of Intent for 2014

LoI 2014-130
Living Together: Planets, Stellar Binaries and Stars with Planets

Date: 8 September 2014 to 12 September 2014
Location: Litomysl, Czech Republic
Contact: Slavek Rucinski (rucinski@astro.utoronto.ca)
Coordinating division: Division V Variable Stars
Chair of SOC: Slavek Rucinski (Dept. of Astr. & Astroph., University of Torontoand Astrophysics of Toronto)
Chair of LOC: Miloslav Zejda (Inst. of Theor. Phys. & Astroph., Masaryk Univ., Brno)



1. Evolution of pre-stellar angular momentum: Formation of binary stars and of planetary systems.
2. Detection of planetary systems. New techniques and new results.
3. Results from the space missions for binary stars and exo-planets.
4. Evolution of planetary systems. Stability of planetary systems and planetary migrations.
5. Internal structure of planets. Planetary atmospheres.
6. Planetary systems in binary stars.
7. Free floating planets and their relation to planetary systems.
8. Unsolved problems in structure and evolution of binary stars.
9. Directions for further research in binary stars and exo-planets.
10. Zdenek Kopal and his legacy.



We propose to organize an IAU symposium in the Czech Republic, on the week of September 8 - 12, 2014, in conjunction with the city of Litomysl which will be honouring Zdenek Kopal, one of the foremost investigators of binary stars who was born in this city in 1914. A similar, smaller-scale conference took place in 2004 (http://var.astro.cz/kopal/). The symposium, planned for the five weekdays, will be followed over the weekend (from Friday, Sep.12, 2014) by a meeting of Czech and Slovak amateurs (variable star and exo-planet observers: http://var.astro.cz/). The local organization will be provided by the same team (LOC, led by Dr. Miloslav Zejda) which organized a successful conference "Binaries: Key to comprehension of the Universe" in Brno in 2009 (http://astro.physics.muni.cz/binkey/).

There are two main deeply-rooted astrophysical reasons for an IAU symposium: (1) binary stars and planets form and evolve through processes involving redistribution and evolution of the angular momentum of collapsing interstellar clouds through several dissipation and re-organization processes; (2) binary stars and planets are studied in similar ways, with methods and tools developed by the binary-star community over many years, but now extensively applied to study planets. The rest is in the details, but important details...

Evolution of angular momentum (AM) in collapsing pre-stellar clouds leads to several distinct outcomes. Widest binary stars form when amounts of the AM are very large; progressively tighter binaries form as the amounts of the AM decrease. The process continues to smaller scales. One of its stages is formation of accretion disks which are observed around young (<10 Myr) T Tauri type stars. Close binary stars and planets form at the later stages, but specific mechanisms appear to be quite different, when disc instabilities and/or planetary accretion processes set preferences for different eventual fates. The existence of the "brown dwarf desert" indicates that formation of planets cannot be considered as the tail-end of close-binary formation. What causes this separation? What are the details of the respective mass and mass-ratio distributions? Does stochasticity influence the outcomes? To what extent are the currently available data biased by techniques and observational limitations?

Prior to the first firm detection in 1995, extra-solar planets were assumed to exist, but their detection seemed to be in the very distant future. This was certainly true during the time of Zdenek Kopal, whose main work was internationally recognized in the field of close binary stars. Kopal's monograph "Close Binary Systems" (1959) served as a departing point for generations of astronomers studying close binary stars. He was particularly instrumental in systematizing the description of close binaries in terms of their interaction and their mutual transformations (detached, semi-detached, and contact). At that time no one doubted that our Solar System is an excellent prototype for planetary systems, so the connection between close binary stars and planetary systems was considered as remote. Now, with many detections of "hot Jupiters", interactions similar to those observed for close binaries are not irrelevant at all: The illumination effect, which was for years studied for close binary systems, has found its important analogue in planetary systems. Thus, the structure of the "hot Jupiters" is obviously different from that of "genuine Jupiters", i.e., massive planets far from their central stars. Following recent detections of rocky planets, strong efforts are being directed at building models of planets across the whole mass range, from Jupiter size down to Earth size. Many tools used in these models have already been developed for stars. But a transition from stars to planets is not a simple one: Because of the complex chemistry, multiple phase transitions in the equation of state at low temperatures, and the strong dependence on chemical composition, the structure of planets is much harder to model and understand than that of stars. Our theoretical understanding is to a large extent still very strongly dependent and driven by observational data for exo-planets which are exceedingly difficult to obtain.

While studies of planetary systems have evolved into a mature but very diversified field of astrophysics, more detections are needed. Extensive efforts are underway to detect planetary systems by: (1) radial-velocity reflex motions in stars (the Doppler method), (2) photometric transits, (3) gravitational micro-lensing, (4) direct detection (including interferometric methods), (5) astrometry, and (6) timing of transits of already detected planets. They have different strengths and biases so inter-comparison is an absolute necessity. The micro-lensing technique has recently led to the detection of free-floating planets, sometimes also called "sub-brown dwarfs". How common are they? How do they form? Are they products of binary and/or planetary formation that have been ejected from bound systems through dynamical interactions? Mechanisms of binary/planetary formation definitely lead to formation of multiple systems, but subsequent evolution of such systems is a complex process with many possible outcomes. Planets may move within their systems through inward migration (including swallowing by the star) but can also be lost through ejections. How many have been sent into space that way? Do they form in the same way as brown (or sub-brown) dwarfs? Are stars into which planets have fallen any different (in terms of metallicities, rotational angular momentum) than the "immaculate" ones?

A particularly interesting subset of the vast field of dynamical stability of binary stars and planets are planets in stellar binary and multiple systems. Much attention is directed at the detection of circumbinary and circumstellar planets in binary stars. This is the field where binary and planetary system research fully intersect and enrich each other. Planetary systems in binaries may not only give essential clues on formation mechanisms, but may provide crucial age estimates for stars outside clusters for which ages are currently one of the most uncertain parameters (binaries as two-object "mini-clusters").

While many ground-based observational projects are now underway to detect planets, space missions will continue to contribute significantly to the characterization of planetary systems, and the connection between planets and binaries. Of note are the satellite missions CoRoT, Spitzer, and particularly the Kepler satellite (now extended to 2016), which by now has detected more that 2,300 planetary candidates. Even though its main objective is planets, Kepler detects thousands of interesting binary stars enabling the first unbiased statistical studies of their various subclasses. By the time of the Kopal 2014 symposium, the GAIA mission (launch in 2013) will be well on its way to producing its first results. This satellite will totally revolutionize the astrophysics of binary stars and planets by providing parallax and proper motion data for over 10^9 stars, i.e. for about 1% of the entire Galaxy. GAIA will detect planets astrometrically and - with parallel radial velocity measurements for many of its targets - will remove many of the systematic limitations of current distance and age estimates for stars (and their attending planets).

While concerted efforts are now directed at detection, characterization and understanding of planetary systems, there are challenges in many areas of traditional stellar astrophysics that can only be solved by continued studies of binary stars. The most difficult (thus most interesting) areas are at both ends of the mass range. The most massive stars are of crucial importance for chemical evolution of matter, yet our ideas are still rather sketchy on how their various classes (OB stars, supergiants, WR stars, supernovae) relate, how their mutual transformations depend on the chemical composition (including the important epoch of the zero-metallicity Universe), and what the role is of binarity and of massive winds in their evolution. At the other extreme of the mass range, well-established discrepancies in the mass-radius and mass-temperature diagrams between observations for low-mass eclipsing binaries and current stellar evolution models are clear indications that our understanding in this regime is still far from complete. While magnetic activity appears to be the culprit, a self-consistent prescription for including this new physics in models is still lacking. Also, despite numerous investigations over decades, solar-type contact binaries of the W UMa-type (one of the cherished subjects of Kopal's work) still lack a solid theory of formation, evolution and destruction.

The symposium will bring together experts on topics relating to binary star and exo-planet research in the context of a nearly complete overlap of observational methods for both fields. It will be organized with the goals of emphasizing similarities, highlighting current problems, and encouraging further exchanges of ideas between the exo-planet and binary-star communities.