Letters of Intent received in 2016

LoI 2018-1942
Physics of star-disk interactions in OB stars

Topics

- Mass-ejection and disk formation process governed by pulsation
- Transition of the disk from its formation/dynamical/asymmetric state to its hydrostatic/viscous phase
- Wind-disk interaction (such as ablation)
- Disks affected by binary companions
-- HMXRBs/Disk variability in Algol binaries/Structural response to tidal forces
- Recent/upcoming/proposed observational facilities (ALMA/BRITE/TESS/GRAVITY/Arago/...)

Rationale

After much earlier debate the questions on the basic structure of disks, in particular disks around Oe/Be stars and HMXRBs, this question was mostly settled in the last decade. Now we are at a point where these advancements, together with the availability of huge wealth of data through professional archives, and nearly as much through the dedication of amateur spectroscopists, allow to explore new domains in disk physics, namely in particular the dynamical behaviour of disks.

Disk physics has occasionally been included in IAUS in the past (e.g. IAUS 272 & 282) but little attention has been given to variability, the dynamics that cause it, and the extent to which this activity influences the parameters of a (quasi)stable disk. One of the most important parameters governing any astrophysical disk is the viscosity. In spite of that, dynamical determinations of the viscosity and its behaviour throughout a disk are very rare, indeed most such works rely on quasi steady-state conditions. Observations of the dynamical response of a disk to a disturbance (e.g. to a stellar outburst at the inner edge of the disk or to a periastron passage of a companion at its outer edge) become increasingly possible thanks to new facilities and techniques, as well as thanks to dedicated observing campaigns both by the professional and amateur community. Any better determination and understanding of viscosity is of fundamental interest for a wide range of astronomical fields, from star and planet formations to the behaviour of AGN and quasars.

One of the most outstanding recent improvements of the observational capabilities is the space based long-term photometric monitoring with high cadence. This enables a hitherto impossible near-real time identification of otherwise unpredictable events. Until now observations of such dynamical events in the disks were severely limited: They are too rare and short-lived to warrant continuous observation with more expensive techniques (spectroscopy, spectropolarimetry, interferometry), yet such data is needed for unambiguous modelling. It is noteworthy that the Austrian community, with the participation in the BRITE-constellation of micro-satellites, has made a major contribution here: The identification of a dynamical event early enough requires the precision of space-based photometry, here BRITE, as well as an operational structure able to react quickly, provided by the Austrian ground-support team in Graz and Vienna.

About half of the stars in our galaxy have one or more close companions, and the percentage is thought to be far greater for early-type stars. Companions have been confirmed a=for many bright Be stars, and in several cases there is evidence that the secondary produces a tidal wake as it moves through the periphery of the massive, extended disk. Similarly, the disks in Algol binaries with early B primaries respond to variations in mass transfer. Comparison between the variability in the disks of binary and (apparently) non-binary early type stars will be discussed.

The program will include a balance between observations and theoretical modeling, already reflected in the composition of the current core SOC (which is to be expanded, also taking into account suggestions we receive in response to this LoI).