mmsoares AT wisc.edu
Stars often violate the predictions of standard stellar evolutionary models. Never has this been more apparent than today. Photometric variations and chemical abundances are measured with unprecedented precision. Moreover, technical innovations allow us to investigate stars that would have been considered too dim or too active in the past. This well-sampled parameter space will be enhanced with forthcoming initiatives like the Vera Rubin Observatory and the Nancy Grace Roman Space Telescope.
Now is the time to reexamine the "simple" star in all its true complexity.
The linchpin to refining these models can be found among anomalous stars, particularly those found in stellar clusters. This includes stars with odd pulsational signatures, unusual chemical abundances, and rapid rotation rates.
Below I describe some of the ways I detect and investigate these 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 Open Clusters
I developed a Kepler and K2-specific image-subtraction pipeline to produce open cluster catalogs. In a recent publication, I revealed more than 1,000 variables in the K2C0 super stamp containing two open clusters. I am continuing this work on open clusters NGC 6791 and NGC 6819, which were observed by the original Kepler mission.
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 Variable Stars:
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 Variable Stars:
Astroseismology of cluster stars provides 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 necessary 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 the formation and evolution of close-orbiting binaries.
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.
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
K2 Campaign 0 super-stamp containing open clusters M35 and NGC 2158. The left panel displays the raw composite frame before our image subtraction reduction pipeline is applied. The right panel is an example of a single subtracted frame for an arbitrary cadence.
Phase-folded LC of an M35 EB source (UCAC4-572-025138) with a period of 4.9 days. Epoch is provided in BJD-2454833. Plotted along the y-axis is the Kp magnitude. This source is an ambiguous blend with two field EBs. An offset from 0.5 in phase is seen in the secondary eclipse. This M35 variable was not reported in prior analyses.
The rotation period-magnitude relation is obvious among our M35 variables. This is a chief rotation feature of intermediate aged stars. The upper x-axis shows the corresponding masses, produced using isochrone fits to the cluster parameters (age and metallicity) and the absolute V magnitude.
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 clusters. For some sources, like the light curve shown below, image subtraction outperforms other techniques. This method has been applied to sources near the Galactic bulge (K2C9) and members of the globular cluster M4.