Stellar Astrophysics

Main Content

Questions

How does mass transfer occur in interacting binaries? How do magnetic fields affect this process?
Do most planetary nebulae (PNe) evolve from low-mass single stars, as traditional textbooks describe? Or does the evolution of these objects involve binary stars, as a growing number of astronomers propose?
Bright PNe in elliptical galaxies are not explained by either of the above two mechanisms; how do they form?
Can we reduce the errors on distance measurements of Galactic PNe to under a factor of 3?
Why is the [O III] planetary nebula luminosity function (PNLF) such an excellent standard candle?
The PN population of an elliptical galaxy is fundamentally vastly different from that of a spiral or irregular galaxy, yet the bright end of their [O III] PNLFs are identical (two within a couple of percent). What drives this, and can the PNLFs of other emission lines, such as H-alpha, shed light on the problem?
What types of stars form novae? How does the production of novae change with stellar population? Can novae be used to probe the stellar populations of galaxies?

Discoveries and Milestones

Robin Ciardullo has shown that the best explanation for the bright PNe in elliptical galaxies is through the evolution of close-binary stars which have merged while on the main sequence. These merged systems, which are presumed to be blue stragglers, have the correct number density and lifetimes to explain the extragalactic PN observations. Robin's modeling suggests that a considerable fraction of Galactic PNe may form through this blue straggler to planetary nebula scenario.
Mercedes Richards has accumulated a database of over 3000 spectra at optical (mostly H-alpha), IR, and UV wavelengths. The spectra were collected around the entire orbits of over 20 eclipsing and non-eclipsing systems, with orbital periods of up to 12 days. This is one of the largest databases of full-orbit, time dependent spectra of Algol-type binaries.
Robin Ciardullo has produced one of the largest samples of Galactic PNe with well-determined distances by using the Hubble Space Telescope to search for the PN's wide-binary companions. Many PNe should have such companions, and the vast majority of these stars will be on the main sequence. By measuring their distance via main-sequence fitting, Robin has nearly doubled the number of Galactic PNe with reliable distances. This project has also produced many extremely pretty pictures of Galactic PNe.
Mercedes Richards created the first hydrodynamic simulations to illustrate the dynamics of mass transfer in Algol-type interacting binaries.
Robin Ciardullo pioneered the use of H-alpha for extragalactic nova survey. He was also the first to measure luminosity-specific nova rates for different stellar populations and he has used H-alpha observations to explore the physics of the nova phenomenon.

Current Projects

Robin Ciardullo is continuing to explore the connection between blue straggler stars and bright PNe. In particular, the Kepler mission is providing data on the orbital parameters of very close binary stars; with these data, he is refining models of the blue straggler to PN scenario.
Mercedes Richards is using her SHELLSPEC spectral synthesis code to isolate the spectroscopic contributions of accretion disk, gas stream, hot spot, and other emission structures in 12 Algol binaries. A comparison to observations will then provide constraints on the physical properties of the components of the accretion flows.
Jason Wright and Jason Curtis are using spectra, photometry, and catalog proper motions to characterize the closest old Galactic star cluster to Earth, Ruprecht 147.
In the mid 1990s, Robin Ciardullo's HST snapshot observations provided a list of possible wide-binary companions to Galactic PNe. He is now re-observing these objects in additional colors, both to confirm their associations with proper motion measurements, and to fix the companions' stellar metallicity. These data will improve the main-sequence fitting distances to the PNe, as provide additional details about the objects' diffuse nebulae.
Mercedes Richards is using Doppler tomography to produce 2D images of close interacting binaries based on the observed spectra and difference spectra. Such images would otherwise be impossible to obtain since these binaries are unresolved even with the largest telescopes.

Participants