Research

The research I've pursued during my time in graduate school has been diverse, but in general I tend to be most interested in things related to the formation and evolution of protoplanetary disks and planets. My thesis work is on planet migration in a magnetically active protoplanetary disks with advisor Dr. Arieh Konigl.

I outlined in more detail the things I like to spend my time on below, in fairly friendly not-an-astronomer terms. If you are an astronomer and want even more detail, check out my ADS library here.

Research Topics:

Protplanetary Disks: Outflows and Structure
Exoplanets: Planet Migration and Exomoons
Other Interests

Protplanetary Disks: Outflows and Structure


An important way we can further understand planetary systems is by looking at the structure and evolution of their birthplaces:protoplanetary disks. Protoplanetary disks are rotating disks of gas and dust surrounding forming stars. These disks last around forming stars for millions of years, and eventually through complicated (not yet completely understood) processes the dust and gas form planets. Since the planets directly come from the disk material, it is crucial to understand the physics of these disks, how they evolve, and what outflow processes they may undergo to move gas and dust to from one place to another. This last point is something I focus on: How outflows shape what disks looks like and how these outflows change the dynamics of disk material.

The outflows I work most on are known as magneto-rotationally driven disk winds (a la Blandford & Payne 1982) , they are caused from the radially bent magnetic field lines that thread the disk and fling material up and out of the system. (You can think of the field lines as wire and the matter that couples to the field, as a bead on that wire. As you rotate that wire ,the bead by centrifugal acceleration will be flung up and out. ) Here's a cartoon of what this particular type of outflow might look like around Herbig Ae stars:

wind cartoon

The wind has both a gaseous component and a dusty component outside the sublimation radius (the radius where grains evaporate), and it could be relatively narrow due to self-shielding of far ultraviolet photons (see Bans & Konigl 2012 for details). The dust in the wind gives rise to many unique observational features, so one thing I've focused on is radiative transfer modeling to test whether the dusty disk wind could explain some of the perplexing observations of Herbig Ae and Be systems. Some for the results for Herbig Ae systems are shown below:

model sed model visibility
a) To the left is an example spectral energy distrubtion (or SED) for a Herbig Ae object known as AB Auriga. The black dots and gray lines are observational data and the red and green line are some of our outflow models that fit the data nicely. For more details on this plot see Bans & Konigl (2012) and all the references therein.
b) This is an example of a common measurement that comes from interferometers known as the visibility curve (basically the visibility curve tells you something about the size and morphology of the object you are observing). The different points are data from various interferometers and the green and red curves are some of our models for the Herbig Ae star MWC 275. Even though this type of measurement was hard to explain for Herbig Ae objects, the wind model does a pretty good job fitting the data. For more details see Bans & Konigl (2012) and references therein.


As you can see from the examples above, a dusty disk wind does a good job explaining the observational data for Herbig Ae systems, but I still have a lot more questions I'd like to answer! Right now I'm wondering if dusty winds around brighter forming stars (namely Herbig Be objects) produce different observational diagnostics than the Herbig Ae case. Radiation pressure in bright protostars can push out material radially which may actual bend the field lines, producing a different wind morphology.

I'm also hoping to study how these outflows transport material from one region of the disk to another. Meteorites in our own solar system tell us that widespread transport was common, so likely these winds could play an important role in shaping forming planetary systems.


Exoplanets: Planet Migration and Exomoons


One perplexing feature of observed planetary systems is the presence of 'hot Jupiters'; Jupiter-sized planets found very close to their host stars. Since there probably is not enough protoplanetary disk material to form these hot Jupiters where we find them, the most likely scenario is that they formed further out in the disk and then migrated in. There are many ways a planet can move/migrate within the disk, one likely ubiquitous method of moving planets is through the tidal interaction between the forming planet as the gas in the disk. Usually, an analysis of this tidal interaction leads to the rapid inward migration of a forming planet, which is in sharp contradiction to all those planets we find, that unlike hot Jupiters, are not sitting right next to their star (an example is of course the planets in own solar system). It is clear that this disk migration cannot always be a definite inward process. One possible solution that we are currently developing is that the presence of magnetic fields, like the ones responsible for the outflow described above, may alter the direction and duration of planet migraiton (this work is a large part of my PhD thesis).

Though not yet observed, moons around extrasolar gas giant planets (like those hot Jupiters) could be commonplace due to the prevalence and diversity of moons around the gas giants in our own solar system. It is especially tantilizing to study exomoons because though a Jupiter close to its parent star is not habitable, a terrestrial moon around it could be. However, there are many issues that complicate that idea. One thing I've worked on, is studying the fate of moons around migrating planets. Generally the fate isn't good, but occasionally if the mass ratio of the moon to planet is large (as it maybe could be if the moon was captured by the planet) moons can survive the migration process. The figure below shows the evolution of a Neptune planet and an Earth-massed moon as the system migrated inward around a low mass M-star. This configuration would be rare if not very unlikely to occur in nature, which illustrates the difficulty in keeping moons in any sort of planet migriaiton.

moon 1
This graph shows both the evolution of the moon's distance from the planet (red) and the planet's spin evolution (green) with time, for the configuration described above. Though the moon begins to fall in towards the planet, it soon reaches a stable orbital distance. The planet in this case started spinning more rapidly until is was synchronous with the moon's orbital period.





Even if moons were to survive around a Jupiter close to its star, they might have strange climates due to the fact they would be periodically eclipsed from the Sun by their host planet. The figure below shows what some of these eclipses would look like:

moon 2 This graph shows the stellar flux that opposite sides of a moon around a planet (like Jupiter) receives versus time. The moon would be tidally locked to its host planet, meaning the an observer from the Jupiter would only see one face of the moon. This Jupiter-facing side of the moon undergoes eclipses during the middle of its day and in general receives less flux than the other side of the moon.







Other Interests


While protoplanetary disks and planets are my main focus, I've pursued some other research over the years that still catch my interest. As an undergraduate student and REU student (at Nantucket's Maria Mitchell Observatory) I've studied the chemistry of the dusty and gaseous ejecta of a particularly awesome massive star: Eta Carina. Also at Maria Mitchell Observatory, as an advisor to REU students, I studied the chemistry on another perculiar object, a massive protostellar disk that happens to be the source of a hydrogren maser --MWC 349A. Finally, in graduate school I also focused on the multi-wavelength analysis of a strongly-lensed luminous infrared galaxy. This galaxy was at a redshift of z=.82, the fact that it was strongly lensed gave us an opportunity to study a very distant galaxy with unmatched resolution.