updated 10/2019

Research

"Well, Fred, you'd look pretty simple too from ten parsecs." -R.O. Redman

Stars are anything but "simple" and they often violate the predictions of standard stellar evolutionary models. Never has this been more apparent than today, as photometric variations and chemical abundances are measured with unprecedented precision and we expand our exploration to investigate stars that would have been considered too dim or too active given our prior technological limitations. This well-sampled parameter space will soon be enhanced with forthcoming missions like the LSST and WFIRST. 

Now is the time to reexamine the "simple" star in all its true complexity.

 

I'm interested in observational and theoretical topics relevant to the stellar spatial scale at z=0. This includes stellar winds; using astroseismology to probe the internal stellar structure; the impact of stellar evolution on angular momentum and composition; the effects produced in stars due to companions (stellar & substellar); and even the planetary architectures that stars foster.

A layer of complexity is missing from the prevailing standard stellar evolution model. The linchpin to this refinement is the population of peculiar stellar sources, particularly those found in stellar clusters. Peculiar stars include those with odd pulsational signatures, unusual chemical abundances, and unexpected rotation rates.

Below I describe some of the ways I detect and model such sources. 

Peculiar Chemical Abundances

Observations of lithium-enriched giant stars are perplexing and demand explanation. 

Would the engulfment of a substellar companion create such a signature? 

In an upcoming publication, we find that such events can enhance surface lithium abundances at a 5-sigma confidence level and that these signatures can survive for up to 1 Gyr. We reveal target regions in the HR diagram where observers should search for such engulfment sites.

Why is it important to study engulfment?

  • Engulfment sites offer a forensic analysis of cannibalized companions. Spectral analysis of these stars permits the direct measurement of a planet's bulk composition. While transit spectroscopy reveals the outer planetary composition, the bulk composition is unattainable. Knowledge of the bulk composition would help constrain planet formation models.

  • Exploring the rotation rates of cannibal stars would provide information regarding the efficacy of angular momentum transfer. 

  • Eliminating the viability of the engulfment mechanism in some regions of the HR diagram indicates that another process is responsible for enriched sources found therein. These processes can shed light on the internal stellar structure and stellar rotation history.

Engulfment-Derived Orbital Decay Power 

Last year, our team calculated the orbital decay power supplied to a host star by an engulfed companion. We found that this power supply can exceed the luminosity of the star by a factor of 10,000. Such an event would produce a significant energetic disturbance to the stellar host.  We revealed target regions in the HR diagram where significant energetic disturbances are expected.

Periodic Variable Searches in the Open Clusters M35 & NGC 2158

I developed a K2-specific image-subtraction pipeline to produce open cluster catalogs. In an upcoming publication (recently submitted), I revealed more than 1,000 variables in the K2C0 super stamp containing two open clusters.  

Why is this important?

Stellar clusters serve as invaluable astrophysical laboratories, harboring an assemblage of coeval stars that share similar distances and chemical compositions. Variables found in these environments can be used to probe a number of open questions. I describe just a few examples of the immense value of these variables below.

​Rotating Variables:

  • Cluster magnitude-rotation curves may be used to explore stellar angular moment evolution.

  • Rapidly rotating giants are particularly compelling.

    • What causes this rapid rotation?

    • Could angular momentum from substellar engulfment explain a non-negligible fraction? 

Pulsating Variables:

  • Astroseismology of cluster stars provide constraints to test and refine stellar evolution models. To do this, we must sample clusters across a wide range of ages and metallicities.

    • Pulsating variables provide mass, radius, and luminosity estimates, which are requisite parameters to explore the relationships between rotation, stellar activity, age, and mass.

    • Mass, radius, and luminosity estimates also place constraints on the underlying models of stellar pulsation.

Eclipsing Binaries (EBs):

  • EBs provide invaluable measurements of the masses, radii, ages, atmospheres, and interiors of stars.

  • Cluster EBs provide more stringent tests of stellar evolution theory than field binaries, as models must match the binary properties and the radiative properties associated with cluster members of a single chemical composition and age.

  • The study of  EBs reveals vital information regarding close binaries formation and evolution.

  • Changes in the eclipse timing measurements provide insight regarding close binary evolution, mass exchange, mass loss, apsidal motion for eccentric systems. Timing variations may also provide indirect detections of unseen 3rd bodies.

  • Cluster EBs provide accurate constraints on both the cluster distance and age.

Transiting Exoplanets:

  • To date, few planets have been found in stellar clusters. Given that all stars are believed to form in such systems, this is puzzling. Using techniques like image subtraction, we can probe these systems to help discern if the paucity of planets is due to observational bias.

  • Are planet architectures similar in cluster environments to the field? If they are different, why? We need more data to adequately answer this question. 

  • Metallicity and age are not highly correlated in open clusters. As such, we can use planet occurrence rates in these cluster environments to discern the significance of such priors. 

Click to enlarge figures

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The blending of stellar light in these densely populated regions poses a formidable challenge to the generation of high-precision light curves. To mitigate these concerns, our team produced an image-subtraction-based reduction pipeline that is tailored to the systematics of the K2 mission. We apply our technique on a super stamp in the K2 field, demonstrating that an image subtraction process is required to fully exploit many of the K2 stellar cluster. For some sources, like the lightcurve shown below, image subtraction outperforms other techniques. This method was applied to Galactic bulge observations carried out during K2C9 and the globular cluster M4.