Letters of Intent received in 2015

LoI 2017-273
150 Years of Emission Line Stars

Topics

- Circumstellar matter: stellar disks and winds
- Spectroscopy, interferometry and polarimetry
- Pre-main sequence stars
- Be stars
- Supergiants, luminous blue variables, Wolf-Rayet stars
- Interacting binaries
- Physical processes: line driving, viscosity, and magnetism
- Survey and archival mining: GAIA, KEPLER, K2, CoRoT, etc.

Rationale

It is now 150 years since the first stellar emission lines formed in circumstellar environments were discovered, first in 1866 by A. Secchi in a Be star and then in 1867 by Wolf & Rayet in WR stars. As we now know, most barionic matter in the Universe is at some point part of a circumstellar environment, first during star and planet formation, and then some smaller, but most important fraction, again during stellar evolution and the final stages of stellar life.

Despite this long time, a surprisingly large number of even basic questions have only recently been answered, or are still being investigated, in the age of high precision observational data and computationally intensive numerical models.

Stellar winds are a good example. They are, when it comes their general principles, fairly well understood, but as one goes into the details the limits of our current understanding become obvious. Theoretical prescriptions to predict the mass-loss rates, for instance, differ between themselves by a large factor, as do the observational determinations. In addition to the well-established and widely used technique of spectroscopy, new techniques are emerging that have already provided invaluable information about the physical conditions and processes present in these structures. For example, interferometry of the resolved wind emission lines is becoming more and more common. Spectrally resolved linear polarimetry displayed a baffling scenario in which the observed phenomena surpass the initial theoretical expectations by far. Finally, spectrally resolved circular polarimetry, finding magnetism in about
10% of even non-convective stars, has opened an entirely new dimension along which stellar winds differ from their non-magnetic counterparts. New computational techniques, such as the ability to do non-LTE Monte Carlo simulations of the circumstellar structure, and soon maybe even the line formation of metals, evolve hand in hand with more powerful hardware, such as the use of GPUs for spectral integration, and shared memory units to solve the hydrodynamic equations. The recent literature abounds in examples of the fantastic synergy between high quality observations and quite advanced numerical modeling.

Stellar disks are at a similar state as winds are. After much earlier debate the questions on their basic structure and governing principles have mostly been settled since about the mid 1990s. Viscosity has been identified as the main driver of the disk evolution in disks around both young and main sequence stars (Be stars are the only know examples of the last case). Now, however, we are at a point where the above-mentioned advancements, together with the availability of huge wealth of data through professional archives, and nearly as much through the dedication of amateur spectroscopists, allow us to explore new domains in the disk physics. For example, what is the value of the viscosity parameter, alpha? While accretion disks and disks of cataclysmic variables seem to settle on a value of an alpha parameter of 0.02 to 0.2, the disks around Be stars seem to work at much higher values closer to, and even at unity. What is the physical processes giving rise to such a high viscosity? Are the known instabilities sufficient to explain the full spectrum of known variability, and what might these instabilities tell us on the problem of fragmentation, on either pre-stellar or pre-planetary scale?

Interacting binaries, finally, are important players in stellar evolution, and, as it seems, the more massive the stars, the more important the impact of binarity. The key moment in the lifetime of these binaries is when mass-transfer happens; however, while many examples are known among the low and intermediate mass stars, we lack direct observations for the massive ones. Our understanding of, and even constraints on, non-conservative mass transfer is found strongly wanting, to the point that it has even been invoked as one hypothesis (out of very many) for the LBV giant eruptions, and the role of binarity in the formation of planetary nebulae is lively discussed.

In light of the above, the proposed conference aims at bringing together astronomers that work in these fields to discuss the advances on the underlying physics and observational techniques and to allow these advances to be communicated between the subfields. Here, "emission lines", as an observational phenomenon linked to several physical processes happening in quite diverse astrophysical situations, will be the driver that will bring astronomers of different fields to a common arena. A second important purpose is to allow both observers and theoreticians to explore the challenges and find the best gateways between advances in computational techniques and hardware to the new observational techniques, vast data archives, and as well unprecedented data quality.