Research Ideas

Here are some ideas for research projects I have bouncing around my head. Many of them can be broken down into smaller projects, appropriate for a first- or second- year project, or can be combined with other projects to lead to a more extensive body of work. If interested in these or similar projects, please come and talk to me!

Stellar Populations from Seismology + Abundances

The combination of measurments of the oscillations of stars and the atmospheric spectra of stars is an extremely powerful tool for understanding stellar populations. We can identify red clump stars from first ascent red giant stars, for example, and correlate that with abundance patterns, or kinematics, or compare with Galactic population models. We can try to identify early asymptotic giant branch stars from both spectroscopic (C, N, C isotopes) and seismic data. We can derive detailed abundance ratios for stars with known ages, masses, and evolutionary state and compared with detailed Galactic chemical evolutionary models that are just now starting to have interesting predictions. Therefore, we can see how [alpha/Fe], or [Na/Fe], etc. correlates with other properties and what this means for stellar nucleosynthesis.

This project would involve working with the APOKASC and/or COROGEE catalogs to deconstruct their stellar populations and use this information to understand the history of the Galaxy. Possibilities include identifying AGB stars in spectroscopy/seismology and This would could also (and probably will) involve theoretical discussions/work with Marc. Some recent papers to read for more information:

  • Bedding et al. "Gravity modes as a way to distinguish between hydrogen- and helium-burning red giants"
  • Miglio et al. "Galactic archaeology: mapping and dating stellar populations with asteroseismology of red-giant stars"
  • Stello et al. "Asteroseismic Classification of Stellar Populations among 13,000 Red Giants Observed by Kepler"
  • Tayar & Pinsonneault "Implications of Rapid Core Rotation in Red Giants for Internal Angular Momentum Transport in Stars"

  • The Bar-Disk Connection in the Galaxy

    Theoretically, the predictions for the Galactic Bar are well-understood: a thicker "boxy bulge" in the inner regions should slim down to a "long bar" with the same position angle. The bar should grow over time, incorporating more and more disk stars into bar orbits. At the bar-disk interface, the velocity dispersion should display a distinct pattern (see Martinez-Valpuesta & Gerhard 2011). Knowing the exact Galactic position of such effects will be critical for confirming agreement between the theoretical expectations and the observations, which so far have shown a very wide range of bar angles and bar lengths.

    This project would involved working mainly with Gail Zasowski and Jon Bird, as well as me, to investigate 1) the velocity dispersion as a function of Galactic position to detect the bar/disk interface and 2) the velocities as a function of position and metalliciity to separate the disk, bar, and bulge as a function of Galactic longitude and latitude to understand their prominence and role. The APOGEE near-infrared spectra are the perfect data to investigate this, as we have an excellent set of fields probing along the bar and continuing into the disk at low Galactic latitudes.

    Some recent papers to read for more information

  • Martinez-Vapuesta & Gerhard."Unifying A Boxy Bulge and Planar Long Bar in the Milky Way"

  • Abundances and Kinematics in the Galactic Bulge

    The central regions of spiral galaxies are the most complex, having been shaped by numerous processes since the earliest phases of galaxy formation. Possibile phenomena include the initial merging of subclumps, gaseous mergers in a clumpy and unstable early disk, bar formation and buckling, and growth of the bar in to the surrounding disk. Such processes leave kinematic and chemical signatures in the stars in the inner Galaxy. Only in the Milky Way can we trace such evolutionary steps in star-by-star detail that can then be applied to our understanding of galaxy evolution as a whole.

    This research project involves using metallicities, [alpha/Fe] ratios and radial velocities from APOGEE to trace the evolution of the inner Galaxy. If interested, there is the possibility of measuring proper motions for APOGEE stars using OGLE data to provide 6-d motions for even better discrimination.

    Some recent papers to read for more information:

  • Ness et al. "ARGOS - III. Stellar populations in the Galactic bulge of the Milky Way"
  • Ness et al. "ARGOS - IV. The kinematics of the Milky Way bulge"
  • Guedes et al. "Pseudobulge Formation as a Dynamical Rather than a Secular Process"
  • Martinez-Valpuesta & Gerhard. "Metallicity Gradients Through Disk Instability: A Simple Model for the Milky Way's Boxy Bulge"

  • Abundance Ratios in the Galactic Bulge

    The central regions of spiral galaxies contain the densest collections of stars and are the sites of the first star formation. As time passes and star formation in the bulge proceeds, dying stars enriched the ISM with a mixture of elements reflective of their masses and metallicities. By studying abundance ratios such [Si/Fe], [Mg/Fe], [Mn/Fe], we can investigate the star formation history of the bulge, if and how the bulge population differs from the thick disk population and whether the bulge populations are consistent with a buckling bar. We can search for signs of dispersion in abundance ratios for the lowest metallicity stars, looking for signs that the first stars in the bulge stand out from the stars in the background halo. We wish to get at the question of the early merging history of the bulge, whether chemically distinct subclumps were involved and whether the oldest stars in the Galaxy show distinct abundances patterns from the overall halo.

    Working with APOGEE data, with possible optical follow-up to get additional data

    Some recent papers to read for more information:

  • C. I. Johnson et al. "Constraints onthe Formation of the Galactic Bulge from Na, Al, and Heavy-element Abundances in Plaut's Field"
  • A. Rahimi et al. "Chemodynamical analysis of bulge stars for simulated disc galaxies"
  • A. Garcia Perez et al. "Very Metal-poor Stars in the Outer Galactic Bulge Found by the APOGEE Survey"
  • T. Bensby et al. "Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. V. Evidence for a wide age distribution and a complex MDF"

  • Comparison of the Bulge and the Inner Disk(s)

    The Milky Way features a boxy/peanut-shaped bulge, in common with a large fraction of other spiral galaxies. The origin of such a feature is likely a buckling bar in an unstable early disk. Therefore, the inner disk(s) and the bulge should resemble each other, both in overall metallicity distribution function and abundance ratios. However, such a scenario leaves little room for stars from merging subclumps in a hierarchical clustering Universe. Therefore, large samples are needed to look for any sign of such a metal-poor hot population. In addition, the bar is expected to grow over time and interact with more and more disk stars, so comparison of the bar population with the properties of the disk at a range of radii is important.

    The APOGEE data are by far the best data set for examining this question, being a homogeneous survey of these components and having a wide range of elements. Working with the APOGEE team, the first goal would be deriving MDFs for the Bulge and inner disks. Different components in metallicity in the bulge have different vertical extent, therefore it is important to compare MDFs for the subcomponents as well. Abundance ratios are another key point of comparison.

  • Bensby et al. "A First Constraint on the Thick Disk Scale Length: Differential Radial Abundances in K Giants at Galactocentric Radii 4, 8, and 12 kpc"

  • The Nature of Extremely Metal-Poor Stars from the SEGUE survey

    The nature of the first generation of stars in the Universe is important and intriguing question. At zero or very low metallicity, the initial mass function, stellar structure and evolution, and/or nucleosynthesis may be quite different than that at solar metallicity. There are several ways to approach this question. One is to continue the search for zero-metallicity stars, comparing the lack of detections with predictions from chemical evolution models. Another is to determine the nature of the second generation of stars, identified by their extremely low-metallicity. What is their metallicity distribution function? What fraction of them are carbon-rich or show other abundance anomalies that are related to nucleosynthesis in the earliest generations of stars and/or the star formation process in extremely low-metallicity clouds.

    The SDSS/SEGUE sample of stars has an extensive set of very metal-poor stars from the Galactic halo. However, these data have not yet been analyzed to produce the metallicity distribution function, which would be the best, more complete MDF at the low-metallicity end to date, and really connect the metal-poor tail to the bulk halo population. Obtaining follow-up spectra for the most metal-poor candidates, such as with LBT's PEPSI instrument, is another aspect of this project. To interpret these results, we will compare the results with models, such as with Adi Zolotov's halo models and with Brian O'Shea's models of the spatial distribution of second-generation stars to zero-metallicity stars in the pre-reionization Universe.

  • Schoerck et al. "The stellar content of the Hamburg/ESO survey. V. The metallicity distribution function of the Galactic halo"
  • Salvadori et al. "Cosmic stellar relics in the Galactic halo"
  • Caffau et al. "An extremely primitive star in the Galactic halo"

  • Galactic Halo Metallicity Distribution with APOGEE, SEGUE and/or LBT

    The Galactic Halo has both stars formed in situ and stars accreted from satellites. Galaxy simulations suggest that the fraction obtained by each method may change as a function of radius. In addition, the outer parts of the current Galactic halo may be preferentially populated by smaller, more metal-poor galaxies. Therefore, the halo of the Galaxy may shows trends in population with Galactocentric radius. However, halo accretion for an individual galaxy also has a random element, so different galaxy halos may have strikingly different radial trends. What does our Galaxy look like? There is evidence both for (e.g. Carollo et al. 2010) and against (e.g. Ma et al., in prep) the idea that our Galaxy has an "inner" and "outer" halo, reflecting different populations. Determining the properties of the Galaxy halo near and far is also critical for evaluating whether the metal-poor stars found close to the Galactic center are an extension of the halo or whether any of them could belong to the bulge and be formed in situ there. Such questions could be probed by measuring the metallicity distribution function and density of the inner inner halo and by close comparison of abundance ratios for inner halo and metal-poor stars in the inner 1 kpc.

    Several possible data sets:
    APOGEE has many inner halo stars, SEGUE has outer halo stars, and there is SEGUE/APOGEE overlap, so a comprehensive view of the halo can be constructed. In addition, Paul Harding and I have a program to get metallicities for "inner inner" halo stars (Rgc < 4 kpc). These will be used to calibrate extensive Washington photometry of these fields and develop the first metallicity distribution function and abundance ratios so close to the inner Galaxy. All of these data/insights can be used to compare with the low-metallicity stars currently in the bulge from APOGEE and other surveys (such as SkyMapper).

    Some recent papers to read for more information:

  • Garcia-Perez et al. "Very Metal-poor Stars in the Outer Galactic Bulge Found by the APOGEE Survey"
  • Carollo et al. "Structure and Kinematics of the Stellar Halos and Thick Disks of the Milky Way Based on Calibration Stars from Sloan Digital Sky Survey DR7"
  • Johnston et al. "Tracing Galaxy Formation with Stellar Halos. II. Relating Substructure in Phase and Abundance Space to Accretion Histories"
  • Cooper et al. "Galactic stellar haloes in the CDM model"
  • Tissera et al. "Chemical signatures of formation processes in the stellar populations of simulated galaxies"

  • Abundance ratios in the thick and thin disk with APOGEE

    Range of Chemical Compositions of Rocky Planets

    Ground-based Asteroseismic Observations of Luminous Globular Cluster Stars

    Ratio of Type Ia to core collapse SNe as a function of galaxy type (and maybe redshift?)

    This project would first involve determining how well we can type supernovae using light-curves alone, as we are unlikely to get the extensive follow-up devoted to a small fraction of the "best" Type Ia's. Then, using photometry from SN survey, such as e.g. the SDSS-2 SN survey, we will determine the fraction of Type Ia and core-collapse SNe and compare with galaxy formation simulations and chemical evolution models.

    Origin of the r-process: Heavy Elements in the Globular Cluster M92

    This project would involve a careful differential analysis of groups of stars with using archival Keck data

  • Roederer Sneden "Heavy-element Dispersion in the Metal-poor Globular Cluster M92"
  • Cohen "No Heavy-element Dispersion in the Globular Cluster M92 "
  • Sneden et al. "Neutron-Capture Element Abundances in the Globular Cluster M15 "

  • Analysis of Microlensed Bulge Dwarf Stars

    These are fascinating objects

    Some recent results:

  • Gould et al. "MOA-2010-BLG-523: "Failed Planet" = RS CVn Star"
  • Bensby et al.

  • Mass Transfer and AGB Nucleosynthesis We will be looking at systems with a known white dwarf and companion with lines we can actually analyze. Also possible: the Galex/SDSS matches.