Stellar Astrophysics

Professors Gayley and Mutel

Graduate students: Tom Brantseg, Quintin Roper, Christene Lynch, Evan Abbuhl

Radio Imaging of Stellar Coronae

Determining the spatial extent and physical properties of stellar coronae and associated magnetospheres is of fundamental importance to stellar structure and evolution studies. Although much progress has been made in x-ray and UV studies of stellar coronal emission, radio imaging (VLBI) observations are of singular importance, since they provide the only means to directly image coronal structures

Late-type binaries are among the most active stars, with x-ray and radio luminosities 104 to 106 larger than solar values. The eponymous close binary Algol (B8V+K2IV, 28 pc) is among the brightest and nearest of the Algol-class systems, and hence is well-suited for probing coronal structure. It is also an eclipsing system, in which the inactive B star occults the active K subgiant every 2.8 days, providing a convenient spatial mask. Eclipse occultations have been used to infer the locations, sizes, and densities of two large x-ray flares, both of which were located near the limb of the K subgiant with a scale height 0.1x the K-star radius. 

Professor Mutel and graduate students Evan Abbuhl and Christene Lynch have begun a program to use the HSA (High Sensitivity Array), a global network of radio telescopes linked as single interferometer, to map the radio structure of several relatively nearly active binary stellar systems. We imaged Algol's radio corona at 6 epochs from April to August 2008 at 15 GHz. The resulting images showed clear evidence for a large co-rotating magnetic loop structure oriented toward the system's center of mass. This is the first direct imaging of a stellar coronal loop (aside from the Sun), and suggests that in close binaries,  magnetic structures are significantly altered by the presence of a companion.

We are currently (late 2013) analyzing HSA observations of the active system (HR5110) to see if large co-rotating coronal loop structures are a common feature of active binaries.

Stellar Wind Studies

Stellar winds play an important role in many stellar systems. They can be hydrodynamically or radiatively driven, and exhibit significant mass-loss, or strong interactions with their environment. Prof. Gayley does research into the strong and dense winds of very hot stars.  These winds are driven by the intense radiation from the star, and can expel matter from their parent star at a rate a billion times that of the solar wind.  These winds are replete with the heavier elements that are needed for planets and life, and they stir and compress the interstellar medium, affecting the conditions for later star formation.  Prof. Gayley has looked at the fundamental mechanisms that drive these winds, how their line profiles can be used to interpret their structure, and how magnetic fields present in the winds can induce polarized wind emission.  

A recent project he has been working on with student Justin Parsons is what happens when two such winds collide in a close hot-star binary system. Such wind collisions create X-ray emitting gas and can accelerate particles to cosmic ray energies, and also serve as rotating cosmic dustmakers as they spiral and cool downstream.  This project has considered how the radiation from one star can decelerate the incident wind prior to collision, and how the shape of the bow shock can be used to infer the relative strength of the two winds, under either radiatively cooling or adiabatic conditions.  Future work will aim at including these various processes in dynamical simulations at U. of Iowa supercomputer facilities.

The photo labeled WR104  at right is an animated stack of the first well-resolved images of the dusty, infrared-luminous Wolf-Rayet star WR 104 revealed an elegant plume stretching hundreds of AU from the bright core and following a trajectory closely matched to an Archimedean spiral  [click on image to see the animation]. The origin of this geometry is simple and highly intuitive: material is embedded within a uniformly expanding spherical wind driven from the hot stars at the heart of the system, yet the orbital motion of a central binary causes a rotating wake embedded within the flow.

Recent papers

Lynch, C.; Mutel, R. L.; Güdel, M.; Ray, T.; Skinner, S. L.; Schneider, P. C.; Gayley, K. G. 2013, Very Large Array Observations of DG Tau's Radio Jet: A Highly Collimated Thermal Outflow, Ap. J. 766,53L.

Jaeger, T. R.; Osten, R. A.; Lazio, T. J.; Kassim, N.; Mutel, R. L. 2011, 325 MHz Very Large Array Observations of Ultracool Dwarfs TVLM 513-46546 and 2MASS J0036+1821104, A.J. 142,189J.

Peterson, W. M.; Mutel, R. L.; Lestrade, J.-F.; Güdel, M.; Goss, W. M. 2011, Radio Astrometry of the Triple Systems Algol and UX Arietis, Ap. J. 737,104P.

Peterson, W. Mutel, R., Gudel, M., amd Goss, M 2010,  A Large Coronal Loop in the Algol System, Nature, 463, 207.

Gayley, K. 2009, Asymptotic Opening Angles for Colliding-Wind Bow Shocks: The Characteristic-Angle Approximation, ApJ.

Tuthill, P. et al. 2009, The Prototype Colliding-wind Pinwheel WR 104, ApJ.

2M0746 pulses: Observed vs. model
HR5110 rasdio centroids
WR104 Movie