I was in UlaanBaatar, Mongolia during 1-5 September 2014 for the conference on “Binary systems, their evolution and environments”.
Here is the summary of the meeting, which I tried to compile with the help of the closing speech of Charles Bailyn.
It was a tradition updated, with covering most basic topics such as common envelope, basic evolutionary scenarios, population and binary synthesis, multiplicity, the three body problem and cluster dynamics. In particular, one of the things that many talks have been about is common envelope evolution, especially alpha parameter (This is a term which was introduced in 1976 by Paczynski and there were five separate talks (by Rebassa Mansergas, Onno Pols, Roberto Iaconi, Michael Politano, and Noam Soker) explaining why this is a bad idea). It was not just these five speakers but any discussion on binary synthesis or of compact objects in interacting binaries, necessarily refers back to common envelope evolution. Also it was stated that the basic physics is insufficient, the simulations do not work, and the connection to observations is inadequate. At this point it is necessary to remember that, this alpha parameter is not actually a physical theory, it is a fitting formula designed simply to connect the pre and post orbital separation.
There are related issues of other situations which are hard to describe and hard to model such as non-conservative mass transfer, wind hydrodynamics and mergers. All of which were discussed by Romain Deschamps, Thomas Madura, Evert Glebbeek, Michael Politano and Noam Soker. This raised a general problem of which is the hydrodynamics or even magnetohydrodynamics, does not really solve the problem you wanted to solve. It only solves that particular situation and it is connected through many different time scales and many different length scales to much more general problems and only serves as an input to other kinds of simulations.
On the other hand, the basic approaches of binary evolution (Case A, B, C by Podsiadlowski) and mass transfer seem to be holding up quite well. Ulrich Kolb showed that the period gap and the low period limit of CVs are still very much intact and in fact that is strongly supported by current observations. The dynamical creation and disruption of binaries in clusters and other situations has given rise to all sort of interesting new results and seems to be holding up. There are some exceptions as on symbiotic stars do not seem to work out but in general the basic concepts of binary evolution that have been around for half a century still seem to be the center of the field and seem to be holding up.
Similarly population of binary synthesis was a hot topic. The synthesis codes are increasingly sophisticated, contain more and more physics, more and more astrophysics and one of the difficulties that this present is that as one produces grids from these things the dimensionality of these grids increases with the number of parameters. You have so many nobs to turn, it is now perhaps a little less surprising that you can get things that work. Also the interpretation of these codes, one can learn and one has learnt, becomes increasingly challenging.
Another topic that has been studied in length was the relationship between binary stars and chemistry. The influence of binaries in chemistry in astrophysics occurs that all size scales from galaxies to star clusters, to individual stars and binary systems. One of the striking facts was that the galaxy simulations require clumpiness to get them to work and so did the cluster simulations, as Aaron Geller pointed out. The star clusters are polluted perhaps with ejecta of various sorts which can explain some of the very peculiar recent observations and a wide variety of stars have their chemistry influenced by the winds, mergers and mixtures caused by binary stars. There was a discussion session which pointed to many partially solved in this area; the r process, rotation, population synthesis, and what is going on in clusters. This kind of partially solved problems indicates a field that is in the process of rapidly advancing.
Another problem what comes from the back in the 1970s and we still hear about today are the x-ray sources where binary accretion on to compact objects turns out to create the most intense x-ray sources in the galaxy. We heard about the high mass sources (Sylvain Chaty), low mass x-ray binaries (Gregorz Wiktorowicz) and of course the sources in star clusters of various kind, presumably created or evolved by dynamical processes.
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| Me and my paper/poster |
We have also heard about tidal forces, and in particular, the Kosai three body problem from 1960s was brought up into the present day. Beyond duplicity and triplicity, we heard about multiplicity from Andrei Tokovinin on the first day about triples, quads and larger configurations. We have heard about binaries in clusters, in the star forming regions. Also in the discussion session Tokovinin pointed the importance of two plus two quads, particularly as in result of a disintegration of unstable star forming regions. We heard at some length about the importance of binaries to cluster dynamics, in particular the talks by Chengyuan Li and Aaron Geller, in which the some very peculiar observational results heading very clean explanations from the theory
These were all old topics but there were also new technology and new ideas, both observational and theoretical. The new instrumentation has provided us for the very first time the full separation range of binary stars from very close binaries to widest possible binaries is now available. We had a number of talks that pointed to the importance of analysing the full range of binary separations. The formation and evolution mechanisms that we are familiar with appeared to cover some 10th orders of magnitude separation and still have some validity. Although the intermediate periods did seem to require a new ‘intermediate mechanism’, namely ‘wind-fed Roche Lobe Over Flow’ which is of course an oxymoron but nevertheless seems to be required to explain the period distributions of many of the stars.
We were also pointed to some of the fantastic new data coming out of the planetary missions COROT and KEPLER. There is an extraordinary precision in large numbers of these things, and we are only just barely starting to figure out what is possible to learn from them.
Looking toward other fields of astronomy, we have of course a close connection to the people in the star formation field. We have discovered that all massive stars are duplicitous. Hughes Sana and Thijs Kouvenhoven, both said this and we have a lot to contribute to star formation, particularly in massive stars. The break up of clusters and mergers are key in star formation, explained by Tokovinin in a discussion session. As two plus two formation of quads, there is clues to the formation of clusters as well. On the last day of the meeting it was also pointed out that there is a potential moment of meeting with colleagues in the star formation field in meeting associated with next year’s IAU conference, that will be a moment that both sides can really benefit from.
Moving on to another adjoining field; cosmology. However binary stars have a way to insinuating themselves into this and other situations, in particular topic this happens with Type Ia supernova, which are of course the products of binary evolution one way or another. The single degenerate hypothesis seems to be facing some difficulties in these days and the cosmologists really should pay attention to this because you hear them explaining Ia SN why it works. Here is why it works; all of these exploding stars are precisely the same mass, namely the Chandrasekhar Limit. That’s of course only true in the single degenerate hypothesis. If you got double degenerates coming together, you can have a wide variety of masses and this whole wonderful idea requires further explanation. There are of course other types of SN, but the cosmologists do not care about them any more, though they are important for chemistry and for the evolution of galaxy and we heard from a number of people pointing toward the basic problem of whether these things really are binary stars or weather they are Wolf-Rayet winds. This has gone back and forth and in this group it seems like the binary stars are triumphing.
Another adjacent field is the exoplanets. We have a lot of techniques and concepts and problems in collaboration with them and they have and will provided enormous amount of useful data for us, already with COROT and KEPLER, but with continuing onward. Interestingly during the talks, I heard only one specific reference back to planets, Tokovinin pointing out his co-planer quad which looked very much like a planetary system. In the discussions, these issues came up frequently; formation, evolution, the chemistry and the relationship of those things to exoplanets. I think one of the things that would come up over the next years is probably to try and make more explicit connections between what we do and what the exoplanet people are up to, because it really can not be that far apart. As it was discussed on the last day of the meeting, if you have a 40 solar mass and 1 solar mass star and you scale it down by a factor of 40, you got something that looks rather like a solar mass star and a super-jupiter, which are observed in these days in abundance.
When we look to the future, the giant databases are the biggest issue. We have started this with Kepler and COROT; GAIA will provide with literally millions of sources. LSST will provide us with an equivalent number of things. That’s why one of the technical skills that might wish to be acquired is skills with giant databases. That’s going to become more and more important across the astrophysics but particularly in our field. And we got a little bit a hint of that as we were looking at the increase in the number of for example of binary planetary nebulae and the constraints that started to pose on traditional evolutionary scenarios. In a similar way, there will be new simulations: The all-in cluster simulations which simulate everything that can happen in a star cluster, are moving forward rapidly. There is an increasing number of truly 3-D magnetohydrodynamics simulations; these can be printed out in 3-D printers, and even can be distributed in LaTeX forms to participants. There are also new ideas such as wind-fed RLOF and we are going to need some new ideas about how to deal with common envelopes.
Besides the conference topics, it was a very good opportunity for me to consider some other places than I have been looking, such as China. I see that most of the important people of the field is accumulating over there and it might be wise to consider to apply China for post-doc. In addition, I learnt some other funding institutions which might be very well fitting for me as a female scientist from a developing country. I also had some contacts and exchanged ideas for my post-doc plans. As a whole, this conference allowed me to have a different aspect to post-doc projects and applications.
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