Here is a list of all the publications for which I’ve had a significant impact in and a brief description of the paper, highlighting my specific contributions to the work.
Some acronyms that will be helpful:
Modelling Hydrogen-deficient Carbon stars in MESA— The effects of total mass and mass ratio
C. L. Crawford, N. Nikultsev et al. (2024, in prep)
I am in the final drafting stages of a paper that began as my student’s Honours thesis. This paper describes the generation and analysis of 35 new post-double-white-dwarf-merger models which represent the real, rare phenomenon of the HdC stars. These 35 models vary the initial merger parameters, specifically the total mass and mass ratio of the progenitor binary system. In doing so, we show evidence that the dLHdC stars are likely formed via lower mass mergers than the RCB stars.
Decline Frequency in RCB Stars
C. L. Crawford et al. (2024, in prep)
Due to the proliferation of known RCB stars in recent years, and the culmination of many years of large-field photometric surveys, there is an unprecedented amount of light curve data available for the RCB type variables that has heretofore not been studied. We have been studied the statistics and properties of the dust-decline events seen in the RCB light curves to characterize the dust formation properties of these stars. We find that the cooler RCB stars have more active dust production than the warmer members, which has been posited previously using different data. This paper also contains the work of many undergraduate student projects over the years.
The highest mass Kepler red giants— II. Spectroscopic parameters of 48 stars
C. L. Crawford et al. (2024, in prep)
I have been working on a series of papers studying the highest mass red giants from the Kepler sample. I am currently preparing the second of this set of papers, for which we have taken high-resolution spectra from the Keck HIRES instrument. In this paper, I will focus on discussing the atmospheric parameters and abundances of these 48 stars with masses potentially greater than 3 solar masses. In general, we do not find any particularly odd abundances in our sample, however we do find one potential halo star in our sample, and do not see any evidence for binary interactions in the abundances.
The highest mass Kepler red giants - I. Global asteroseismic parameters of 48 stars
C. L. Crawford et al. (2024)
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This is the first of a series of papers I have been working on studying the highest mass red giants from the Kepler sample. These are interesting as they are rare and relatively unstudied. In this paper I define the sample as those stars with masses greater than 3 solar masses. I report their global asteroseismic parameters, and discuss various other asteroseismic measurements that can be made with the Kepler data. I show that these stars are most likely to be core helium burning, that the majority of them show weak or suppressed dipole modes, and that they have fairly large oscillation widths.
Identifying 850 δ Scuti pulsators in a narrow Gaia colour range with TESS 10-min full-frame images
A. Reid et al. (2024)
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This work is on the systematic identification of 850 delta Scuti pulsators using the Gaia and TESS data, in order to study the pulsation fraction near the center of the instability strip. I contributed many isochrones, stellar evolution tracks, and advice to the student who ran this project.
A simple method to measure numax for asteroseismology: application to 16,000 oscillating Kepler red giants
K. R. Sreenivas et al. (2024)
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This work defines a new way to measure the global asteroseismic parameter called nu_max without fitting the background noise profile for the star. This work was performed by a PhD student who I help to supervise.
HdC and EHe stars through the prism of Gaia DR3: 3D distribution and Gaia’s chromatic PSF effects
P. Tisserand et al. (2024a)
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This paper is one of two works we put out analyzing the new Gaia data for the HdC stars. This paper focuses on the 3D distribution we find for the stars, as well as an analysis of how well Gaia is actually handling dusty star measurements. I provided the 3D transformation of the stellar coordinates.
HdC and EHe stars through the prism of Gaia DR3: Evolution of RV amplitude and dust formation rate with effective temperature
P. Tisserand et al. (2024b)
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This paper is the second of two works we put out analyzing the new Gaia data for the HdC stars. This work focuses on the radial velocity measurements for the stars and how we can use that information to make inferences on the dust production and pulsation properties of these stars. I provided all of the RV measurements for these stars that came from the WiFeS spectrograph.
A spectral classification system for hydrogen-deficient carbon stars
Courtney L. Crawford et al. (2023)
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This is my doctoral thesis. In the same style as the MK stellar classification system we all know (e.g. OBAFGKM classifications), I’ve created a stellar classification system for RCB and HdC stars using modern techniques (principal component analysis for dimensionality reductuion and K-means clustering). The system classifies all known HdC stars which have been observed at maximum light on their temperature and carbon molecular strengths. I additionally provide commentary on many of the unique optical spectroscopic traits of HdC stars.
TESS Observations of the Pleiades Cluster: A Nursery for δ Scuti Stars
T. Bedding et al. (2023)
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This letter shows the observations of delta Scutis in the Pleiades cluster and the pulsator fraction of this cluster. I provided the isochrones used in this work.
Peculiar R Coronae Borealis Stars: Strontium-Rich Stars and the s-Process
Courtney L. Crawford et al. (2022)
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In this work I looked at all of the spectra we have collected of the RCB and HdC stars and discovered a subclass of these stars which show strongly enhanced Strontium spectral features. Three of these stars have the strongest Strontium lines of any known stars. I also explored the potential reason behind the unique s-processing that would have had to occur in these stars.
The dawn of a new era for dustless HdC stars with GAIA eDR3
P. Tisserand et al. (2022)
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We created a new method of discovering dLHdC stars using IR colors and therefore we were able to discover 27 new dLHdC stars and 2 new EHe stars. This is really exciting as these are very rare subclasses of stars. I specifically classified all candidate spectra as either members of the dLHdC or the EHe class.
R Coronae Borealis and dustless hydrogen-deficient carbon stars likely have different oxygen isotope ratios
V. Karambelkar et al. (2022)
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This work studies the Infrared spectra of HdC stars, especially the oxygen-18 abundances. We find that the dLHdCs seem to have larger isotopic ratios, which may point to them being from different merger populations. I provided much of the discussion of these results during meetings, but did not provide any direct measurements or writing on the paper.
R Coronae Borealis Star Evolution: Simulating 3D Merger Events to 1D Stellar Evolution Including Large-scale Nucleosynthesis
B. Munson et al. (2021)
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In this work, my colleague, Bradley Munson, was able to transfer a 3D hydrodynamical model of a double white dwarf merger into the MESA 1D framework and evolve the post-merger-object into the RCB region of the HR diagram. This is the first work of its kind. I assisted in making sure the abundances for the resultant model were consistent with other RCBs and RCB models.
Modelling R Coronae Borealis stars: effects of He-burning shell temperature and metallicity
Courtney L. Crawford et al. (2020)
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I created 18 MESA models of double white dwarf post-merger objects, where we varied the temperature of the helium burning shell and the metallicity of the star. Through this, we were able to explore how the parameters of a double white dwarf merger will effect the evolution of the resultant star. Additionally, I was able to figure out that the reason RCB stars show partial helium burning products on their surfaces was because of a unique pattern in the convection of the envelope of the star. This same result was shown later in a different type of model, implying that this is likely inherent to the evolution of the post-merger-object and not specific to the type of model that we created.
The age of the Galactic stellar halo from Gaia white dwarfs
M. Kilic et al. (2020)
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This was an improved version of my undergraduate thesis at the University of Oklahoma. Through fitting the SEDs of the coolest white dwarfs in the local solar neighborhood, we were able to get an estimate of the temperature and age of the star, which gives us estimates for the age of the Galactic population for which that star belongs.
TASC7/KASC14 2023 - Poster
The Highest Mass Kepler Red Giants
Link to the Poster
ASA ASM 2023 - Poster
Spectral Classification of Hydrogen-deficient Carbon Stars using Principal Component Analysis
ASA ASM Poster Archive
Link to the Poster
AAS 240 - June 2022 - Dissertation Talk (Online)
A Spectral Classification System for R Coronae Borealis Variables
AAS 238 - June 2021 - Talk (Online)
3-d Mapping The Distribution Of R Coronae Borealis Stars
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AAS 236 - June 2020 - Poster (Online)
The Effect of Helium Burning Shell Temperature on Models of R Coronae Borealis stars in MESA
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