Agnès Kim, PhD

October 23, 2019

Oral Presentations
Validation of Asteroseismic fitting with the new White Dwarf Evolution Code
IAU Symposium 357: White Dwarfs as probes of fundamental physics and tracers of planetary, stellar & galactic evolution
Hilo, Hawaii

September 5, 2018

Oral Presentations
Non-luminous sources of cooling in pulsating white dwarfs
Physics of Oscillating Stars
Banyuls-sur-Mer, France

October 17, 2017

Seminars
Revolutions in White Dwarf Asteroseismology
Astronomy Seminar
University Park

June 10, 2017 - June 16, 2017

Posters
Non-luminous sources of cooling in pulsating white dwarfs
Current challenges in the physics of white dwarfs
Santa Fe, NM

White dwarfs cool through several processes, including the emission of weakly interacting particles such as neutrinos. By their very nature, weakly interacting particles are difficult to observe. In pulsating white dwarfs, however, the effect they have on the cooling rate translates into an observable change in the pulsation periods. Namely, the periods of some modes vary on long time scales that are consistent with a rate of change due to cooling. Such changes in periods have been measured and have allowed us to place limits on the cooling of white dwarfs due to weakly interacting particles. We discuss the technique and present results from such studies, including an update on the mass determination of hypothetical weakly interacting particles called axions, candidate dark matter particles.

June 10, 2017 - June 16, 2017

Posters
The White Dwarf Evolution Code Revamped
Current challenges in the physics of white dwarfs
Santa Fe, NM

We present on major updates to the White Dwarf Evolution Code (WDEC). WDEC has been used in the white dwarf community for the past 4 decades for many purposes, including the construction of white dwarf luminosity functions and asteroseismology. Its main power resides in the fact that it can build a model in less than 15 seconds with a minimal amount of input and tweaking. Over the past years, WDEC was modularized and rewritten in Fortran 90 for improved readability and increased flexibility. It now interfaces with MESA (Modules for Experiments in Stellar Astrophysics), using its sate of the art equation of state tables and opacities. We present screenshots and a sample of the new (and old) capabilities of WDEC. WDEC is available on request from the author.

January 4, 2017

Oral Presentations
The connection between period spectra and constraints in white dwarf asteroseismology
229th American Astronomical Society Meeting
Grapevine, TX

White dwarfs are the end product of evolution for around 98% of the stars in our Galaxy. Buried in their interiors are the records of physical processes that take place during earlier stages in the life of the star. Not long after the discovery of the first pulsating white dwarf, the promise of asteroseismology to unveil the interior structure of white dwarfs and therefore help us constrain their prior evolution became apparent. In recent years, a well-established theory of non-radial oscillations, improved white dwarf models, year of expertise built up in the field of white dwarf asteroseismic fitting, and computing power have culminated in the asteroseismology finally delivering what it promised: a detailed map of the interior structure of white dwarfs. As always in science, new results raise new questions. We perform a number of numerical experiments to better understand the connection between a given set of periods varying in the number of periods and in the set of radial overtones and the quality of the constraints on interior structure one obtains from fitting these periods.

November 28, 2016

Posters
A systematic exploration of what pulsations modes tell us about white dwarf interiors
22nd Los Alamos Stellar Pulsation Conference Series Meeting
San Pedro de Atacama, Chile

August 1, 2016

Oral Presentations
Latest discoveries on White Dwarfs
Astrophilly
Villanova, PA

Some stars vary in brightness because of instabilities, typically tied to convection. Regardless of their source, brightness variations can be used to infer the interior structure of stars. For white dwarfs, the process is facilitated by the fact that observing the brightness variations can be done with fairly small telescopes and over a short amount of time (the variation cycles are of order of minutes). On the modeling side, white dwarfs are also easier, as they lack extended atmospheres, contract very little, and have inert interiors. In this talk I will give an overview of the field of White Dwarf Asteroseismology, what it has achieved so far and what progress is being made, especially thanks to space based observations.

July 27, 2016

Oral Presentations
Calibrating white dwarf asteroseismic fitting techniques
20th European Workshop on White Dwarfs
Warwick, England

The main goal of looking for intrinsic variability in stars is the unique opportunity to study their internal structure. Once we have extracted independent modes from the data, it appears to be a simple matter of comparing the period spectrum with those from theoretical model grids to learn the inner structure of that star. However, asteroseismology is much more complicated than this simple description. We must account not only for observational uncertainties in period determination, but most importantly for the limitations of the model grids, coming from the uncertainties in the constitutive physics, and of the fitting techniques. In this presentation, we will discuss results of numerical experiments where we used different independently calculated model grids (white dwarf cooling models WDEC and fully evolutionary LPCODE-PUL) and fitting techniques to fit synthetic stars. The advantage of using synthetic stars is that we know the details of their interior structure so we can assess how well our models and fitting techniques are able to recover the interior structure, as well as the stellar parameters.

July 27, 2016

Oral Presentations
Semi-chaotic behaviors observed in the asteroseismic fitting of GD358
20th European Workshop on White Dwarfs
Warwick, England

The analysis of 33 years worth of data on GD358 allowed us to assemble a spectrum of 15 l=1 modes, 13 of which constitute a consecutive sequence. This is the longest consecutive sequence of modes we fit formally so far, with high resolution grids. There were some hints before that some periods may be highly sensitive to some parameters in certain areas of parameter space. With the long consecutive sequence of same ell modes in GD358’s pulsation spectrum, we can study the effect such behaviors can have on asteroseismic fitting. We show some examples of large changes in the calculated period spectra due to small changes in a given parameter and discuss the implications for pipeline fitting of white dwarfs.

June 3, 2016

Oral Presentations
Lessons Learned from Fitting GD358
4th Delaware Workshop on White Dwarfs & Compact Objects
Newark, DE

The analysis of 33 years worth of data on GD358 allowed us to assemble a spectrum of 15 ell=1 modes, 13 of which constitute a consecutive sequence. This is the longest consecutive sequence of modes we fit formally so far, with high resolution grids. There were some hints before that some periods may be highly sensitive to some parameters in certain areas of parameter space. With the long consecutive sequence of same ell modes in GD358’s pulsation spectrum, we can study the effect such behaviors can have on asteroseismic fitting. We show some examples of sudden changes in the calculated period spectra and discuss the implications on pipeline fitting of white dwarfs.

August 12, 2015

Posters
Asteroseismology of GD358 with complex core carbon and oxygen abundance profiles
International Astronomical Union XXIX General Assembly "Advances in Stellar Physics from Asteroseismology"
Honolulu, Hawaii

GD 358 is the brightest (Mv = 13.7) helium atmosphere white dwarf pulsator. We present an analysis based on over 1000 hours of observations spanning 2007-2014 as well as archival data going back to 1982. From the complete data set, we identified a total of 27 independent frequencies and fit 14 of them as m=0 modes in our asteroseismic analysis. We add GD358 to a set of helium atmosphere white dwarfs fitted with similar models. With this consistent set, we can see a trend in the thickness of the pure helium layer that are quantitatively consistent with time-dependent diffusion calculations.

June 15, 2015

Posters
Signatures of a common-envelope phase in the structure of a pulsating whitedwarf
KASC8/TASC1 workshop
Aarhus, Denmark

We present a follow-up study of a pulsating white dwarf in a binary system, observed for more than 77 days during the first full campaign of the extended Kepler mission. SDSS J1136+0409 is made up of a ZZ-Ceti star and an M-dwarf companion. We previously published an observational paper on the physical parameters of the system, along with a preliminary asteroseismic analysis of the ZZ-Ceti star (Hermes et al., submitted 2015). One scientific question we are trying to answer with the data gathered on this system and further modeling is whether the white dwarf has a thinner than normal hydrogen layer, that would have resulted from a common-envelope phase. Such a phase in the evolution of the system is expected, as it is a close binary, with a 1.70 solar radius orbital separation (roughly 9 times the radius of the M-dwarf). We present an asteroseismic fit based on 7 periods extracted from the light curve of the white dwarf, with physically motivated carbon-oxygen profiles, and varying helium and hydrogen layer masses.

April 25, 2015

Oral Presentations
Insights into the evolution of stars in a close binary system 
Penn State Science Division Meeting
UPark

September 25, 2014

Seminars
Pulsating White dwarfs: The poster child and the little devil
Colloquia and Seminars @ CRAQ
Montreal, Canada

August 11, 2014

Oral Presentations
Patterns emerging from the asteroseismic studies of DBVs
19th European Workshop on White Dwarfs
Montreal, Canada

The discovery and analysis of KIC 8626021, a DBV found in the field of view of the first Kepler mission, brings the total of DB pulsators fitted using White Dwarf Evolution Code models to four. We summarize results from the study of GD358, CBS114 and EC20058, and present results from the asteroseismic analysis of KIC 8626021 based on seven periods. Even if more work is required to compare the results of the analysis of each individual star with those of spectroscopy and stellar evolution calculations in absolute terms, interesting patterns emerge. For instance, we confirm a trend from small to larger helium layer mass with decreasing effective temperature, as is expected from the outward diffusion of helium as DBs cool. KIC 8626021, with exquisite data acquired by Kepler, is particularly useful in solidifying some of these trends by allowing us to precisely determine the ell identification of several of the modes from multiplet structure.

July 6, 2014

Posters
Seven-Period Asteroseismic Fit of KIC 8626021
The Space Photometry Revolution CoRoT Symposium 3, Kepler KASC-7 joint meeting
Toulouse, France

We present a new, better-constrained asteroseismic analysis of the helium-atmosphere (DB) white dwarf discovered in the field of view of the original Kepler mission. Observations obtained over the course of two years yield at least seven independent modes, two more than were found in the discovery paper for the object. With several triplets and doublets, we are able to fix the ell and m identification of several modes before performing the fitting, greatly reducing the number of assumptions we must make about mode identification. We find a very thin helium layer for this relative hot DB, which adds evidence to the hypothesis that helium diffuses outward during DB cooling.

June 23, 2014

Posters
Constraints on stellar evolution from white dwarf asteroseismology
IAU Symposium 307 "New Windows on massive stars: asteroseismology, interferometry and spectropolarimetry"
Geneva, Switzerland

High mass and low mass stars follow a similar evolution until the inert core phase that follows the end of the core helium burning stage. In particular, one common phase of stellar evolution is the alpha capture reaction that turns carbon into oxygen in the core. We can obtain constraints on this reaction rate by studying the remnants of low mass stars, as this is the ultimate reaction that occurs in their core. We also present results that allow us to test the time dependent calculations of diffusion in dense interiors.

May 20, 2014

Oral Presentations
Taking the pulse of dead stars
Stars: Old, Young and variable
Ames, IA

May 3, 2014

Oral Presentations
Taking the pulse of dead stars
Penn State Science Division Meeting
UPark

April 15, 2014

Seminars
The Kepler DBV: another chance to measure plasmon neutrino rates?
Physics Seminar
University of Delaware

The interior conditions inside hot helium atmosphere pulsating white dwarfs are just right for the abundant production of plasmon neutrinos. Because these conditions are so unique - not reproducible in the lab and not present in the Sun, plasmon neutrinos remain to this day in the realm of theory. Furthermore, if the star behaves, it is possible to use its pulsations to measure its cooling rate. Such a measurement, combined with our knowledge of radiative cooling of white dwarfs, can allow us to detect any "unseen" source of energy loss from the star. We may have found such a well-behaved object in the original field of view of the Kepler satellite. In this talk, I present what the star has revealed to us so far.

January 6, 2014

Oral Presentations
Update on the asteroseismology of the Kepler field hot pulsating white dwarf
223rd American Astronomical Society Meeting
Washington, DC

In 2012, asteroseismic studies of a pulsating helium atmosphere white dwarf discovered in the Kepler field (KIC 862621) revealed that the star was one of the hottest in its class. Data gathered by Kepler also revealed that a number of the pulsation modes observed in the star were stable over time. Such stable modes can be used to measure a rate of cooling for the star. Combined with interior models of the object, such measurement can help constrain plasmon neutrino emission rates. KIC 862621 is in a temperature range where neutrino cooling contributes to more than half its luminosity. We report on the modeling of the object. The original studies were based on the 5 pulsation periods available at the time. Such a small number of modes only allowed partial constraints on the interior structure. Since then, through continuous observations with Kepler up until the end of the mission, 5 additional independent modes were discovered. We present a new, better constrained asteroseismic study of KIC 862621 based on 10 pulsation periods. Ground observations of the object will continue, yielding a measurement of the rate of cooling. We will then have the elements in place to help us constrain plasmon neutrino emission rates.

December 16, 2012

Seminars
Asteroseismology of the Kepler hot white dwarf
Astronomy Seminar
La Plata, Argentina

A talk about the discovery of a DBV in the Kepler satellite's field of view and the analysis of the data.

November 25, 2012

Oral Presentations
Decoding EC14012's rich pulsation spectrum
The 62nd Fujihara seminar: Progress in solar/stellar physics with helio- and asteroseismology
Hakone, Japan

EC14012-1446 (from hereon “EC14012”) is a ZZ Ceti star (DAV) that was the object of a Whole Earth Telescope run in 2008. The extended coverage run provided a detailed and well resolved period spectrum for the star, confirming and revealing 19 independent modes of vibration, including 1 triplet and a few more incomplete triplets (doublets). The data and partial mode identification were published in Provencal et al. in 2012 (ApJ, 751, 91). With a large number of modes (for pulsating white dwarfs) and good clues for some of the mode identification from independent methods, EC14012 is a good candidate for high speed asteroseismology, were we try to infer interior structure based mainly on available data (spectroscopy and pulsation spectrum). With the asteroseismic analysis of EC14012 using this method, a consistent picture of its stellar properties and internal structure emerges. We also present preliminary results on the ZZ Ceti star that was discovered in the field of view of the Kepler satellite.

April 23, 2012

Seminars
Asteroseismology of the Kepler hot white dwarf
Astronomy Seminar
University of Delaware

A talk about the discovery of a DBV in the Kepler satellite's field of view and the analysis of the data.

March 26, 2012

Seminars
Asteroseismology of the Kepler hot white dwarf
Physics and Astronomy Colloquium
Georgia Southern

A talk about the discovery of a DBV in the Kepler satellite's field of view and the analysis of the data.

January 9, 2012

Oral Presentations
Asteroseismology Of The Kepler DBV - It’s a Hot One!
219th American Astronomical Society Meeting
Austin, TX

We present an asteroseismic analysis of the DBV recently found in the field of view of the Kepler satellite. We analyze the 5-mode pulsation spectrum that was produced based on one month of high cadence Kepler data. The pulsational characteristics of the star and the asteroseismic analysis strongly suggest that the star is hotter (29200 K) than the 24900 K suggested by model fits to the low S/N survey spectrum of the object. This result has profound and exciting implications for tests of the Standard Model of particle physics. Hot DBVs are expected to lose over half of their energy through the emission of plasmon neutrinos. Continuous monitoring of the star with the Kepler satellite over the course of 3 to 5 years is not only very likely to yield more modes to help constrain the asteroseismic fits, but also allow us to obtain a rate of change of any stable mode and therefore measure the emission of plasmon neutrinos.

January 9, 2012

Posters
High Speed White Dwarf Asteroseismology with the Herty Hall Cluster
219 American Astronomical Society Meeting
Austin, TX

December 1, 2011

Seminars
Asteroseismology of the Kepler hot white dwarf
Physics and Astronomy Colloquium
Clemson University

A talk about the discovery of a DBV in the Kepler satellite's field of view and the analysis of the data.

November 12, 2011

Oral Presentations
Asteroseismology of the Kepler DBV - It's a hot one!
Georgia Regional Astronomers Meeting
Agnes Scott College, Decatur, GA

A talk about the discovery of a DBV in the Kepler satellite's field of view and the analysis of the data.

March 15, 2011

Oral Presentations
Asteroseismology of rich pulsating white dwarfs
The 61st Fujihara seminar: Progress in solar/stellar physics with helio- and asteroseismology
Hakone, Japan

White dwarf asteroseismology provides a powerful test of the predictions of stellar evolution.The best targets to test the predictions of stellar evolution with asteroseismology are DAV’s and DBV’s that exhibit a dozen or more pulsation modes. We present some results of such analyses for DBV EC20058 and DAV EC14012.

November 6, 2010

Oral Presentations
Astronomy at GCSU
GA Regional Astronomers Meeting at Georgia Tech
Georgia Tech, Atlanta