Research Interests
Star Cluster Formation
.Our Sun formed in a cluster together with thousands of other stars. Forming stars affect each other and their birth enviornment: they shape stellar masses, the rate at which stars form and the lifetime of the natal cloud. The STARFORGE project using the Gizmo code allows us to model the full complexity of star formation and disentangle the impact of different physical processes on the outcome. Our recent work showed that the distribution of initial stellar masses depends on the high-velocity outflows that occur during the accretion process. These jets expel gas at high-velocities, pushing away gas and halting stellar accretion.
STARFORGE: the effects of protostellar outflows on the IMF. Guzejnov et al. 2021.
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 (see "The Origin and Evolution of Multiple Star Systems, " Offner et al. 2023 for a review). 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
See also "Effects of the enviornment on the multiplicity properties of stars in the STARFORGE simulations", Guszejnov et al. MNRAS, 2023.
Impact of Stellar Feedback on the Natal Environment of Stars
Stellar Winds
Massive stars produce winds that signficantly shape their environment by sweeping up surrounding gas into large bubbles. A number of these bubbles are visable in nearby star-forming regions, where they are thought to inject significant energy (below: Spitzer images of two bubbles, identified using machine learning in Xu & Offner 2017).
A second impact of these winds that is difficult to observe directly is that the expanding shells can "pluck" on the local magnetic field and excite magnetic waves (video on the right). These waves propagate beyond the wind boundary, helping to drive turbulent motions (Offner & Liu 2018, Nature Astronomy). Stellar feedback like this can help to explain the properties and lifetimes of star-forming regions.
Protostellar Outflows
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.
Synthetic Observations
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
