Formation of Binary Stars
Why our Sun is a single star, while many star systems, including our nearest stellar neighbor Alpha Centauri, are in multi-star systems is an open astronomy question. The answer likely depends on the initial birth environment -- rotation, turbulence and the magnetic field. Recent simulations of collapsing magnetized cores showed that gravitational fragmentation can occur producing two or three protostars on relatively large scale (~1000 au, below left). The binaries continue to accrete gas from the shared core and gradually migrate to closer separations. Both the outflows and the stellar spins of these systems are mis-aligned, an artifact of the turbulent environment and their initial wide separations. Synthetic CO observations of the outflows launched by these binary systems statistically agree with actual binary outflows observed in the nearby Perseus cloud (below right) -- which suggests these simulations are capturing something that happens in nature.
"The Turbulent Origin of Outflow and Spin Misalignment in Multiple Star Systems." Offner et al. ApJ, 2016
Impact of Stellar Feedback on the Natal Environment of Stars
Protostars shape their environment as they form through heating and ejecting mass. At early stages they launch collimated bi-polar outflows (left, numerical simulation from Offner & Chaban 2017).
These outflows entrain and expel dense gas close to the protostar, reducing the eventual mass of the star. Depending on the strength of the local magnetic field and other gas properties, Offner & Chaban showed only 15-40% of the initial gas mass ends up in the star (right: ratio of protostar mass to initial gas mass versus time). This may partially explain why star formation is so inefficient.
syn・ the・tic ob・ser・va・tion (noun) \sin-ˈthe-tik\ \ˌäb-sər-ˈvā-shən
: a quantitative model for the emission produced by a simulation and detected assuming the simulation is a real astronomical object at some point in the sky
A simulation has complete six-dimensional information (x,y,z,vx,vy,vz) about all modeled physical properties (density, temperature, pressure, etc), so several steps are required to transform the information into the same parameter space as an observation:
- Given some temperature and density distribution, compute the radiative output. This can take the form of particular atomic and molecular lines or of broad spectrum emission from dust.
- Take into account spatial and spectral resolution, including instrumental limitations and biases (e.g. interferometry). Adopt an appropriate noise model.
- Analyze the data as if it were a true observation by employing observational fitting and post-processing techniques.
Then it is possible to compare “apples” to “apples”. Some examples from my research are below.
Synthetic integrated 12CO(1-0) emission for a simulation modeling the interaction between stellar winds and their host molecular cloud. Stars in the two top panels have winds that are 10 times weaker than the stars in the bottom panels.
Offner & Arce 2015
Synthetic 106 GHz ALMA observation of a simulated collapsing pre-stellar core. The six panels indicate increasing evolutionary time. The time and peak density appears at the top of each panel. The yellow oval indicates the beam size.
Dunhan, Offner et al 2016