Research & Ongoing Projects
Research & Ongoing Projects
Carbon-enhanced metal-poor (CEMP) stars in the Milky Way halo are excellent tools for probing the origins of heavy elements, due to their pristine nature. A subset of these stars lack significant enhancement of neutron-capture elements; these are called CEMP-no stars, in contrast with CEMP-s stars which contain s-process elements, and CEMP-r/s stars which have both r- and s- process signatures. The CEMP-s signature is known to arise due to binary mass transfer from an evolved companion, but the CEMP-no signature is not as well understood, and recent observations suggest that the binary frequency of CEMP-no stars may depend strongly on the absolute carbon abundance A(C).
We compiled a list of known CEMP-no binary statuses from past literature, building from previous literature and radial velocity monitoring experiments including Hansen et al. (2016), Yoon et al. (2016), and Arentsen et al. (2019). Additionally, we are collecting more radial velocity data for CEMP-no stars whose binary status has not been confirmed, in order to better constrain and reduce the uncertainty on the CEMP-no binary fraction.
We find that there is most likely a correlation between A(C) and binary frequency for CEMP-no stars, although a bit more analysis is needed to confirm this. The binary frequency changes around A(C) ≈ 6.8. The population of CEMP-no stars with A(C) < 6.8 have a binary fraction of ~15%, while the population of CEMP-no stars with A(C) > 6.8 have a binary fraction of ~40%. This implies that there may be multiple formation pathways for CEMP-no stars; lower A(C) stars form from carbon enrichment of stellar natal clouds as expected, but some higher A(C) stars may form due to binary mass transfer from an evolved companion after birth.
(submitted to AAS journals) Dixon, J. D., Rana Ezzeddine, Yangyang Li, Thibault Merle, Manuel Bautista, Yanjun Guo. 2025. Investigating non-LTE abundances of Neodymium (Nd) in metal-poor FGK stars.
The origins of r-process elements in the early universe are highly debated, but neutron star mergers are suspected to be one of the dominant sites. Precise abundance analyses of r-process enhanced metal-poor stars are crucial for constraining models of r-process enrichment; this requires state-of-the-art non-LTE (NLTE) modeling of stellar atmospheres, as LTE is a poor approximation for the low-opacity atmospheres of metal-poor stars. Neodymium (Z = 60) in particular is a crucial r-process element for constraining models of kilonovae as several Nd II lines have been found in the H band which allow us to more accurately measure abundances of stars within the galactic plane.
I generated a large grid of NLTE departure coefficients for 122 prominent Nd II lines from an up-to-date model atom, using the University of Florida's HiPerGator 3.0 and the radiative transfer code MULTI. With this grid, I created curves of growth for LTE and NLTE, allowing me to calculate and assemble a 6-dimensional grid of NLTE corrections ΔANLTE = A(Nd)NLTE - A(Nd)LTE, as a function of stellar parameters (Teff, log g, [Fe/H], and vt), LTE Nd abundance (ALTE), and spectral line wavelength.
We find that the non-LTE effects on neodymium vary significantly, with corrections ranging from approximately -0.3 to +0.5 depending on both stellar parameters and spectral line properties. For many lines, the trend in ΔANLTE as a function of stellar parameters in the optical range is the opposite of the trend in the infrared, which may potentially explain significant discrepancies in previous LTE abundance analysis between optical and H-band Nd II lines.
Astronomical data are often displayed visually due to the nature of the science. However, graphical representations of spectra may cause confusion for younger students and create unnecessary barriers of entry to the field for blind or low-vision (BLV) scientists. Thus, it is worth exploring methods of auditory representation both to convey spectral information to a wider audience and to increase accessibility of spectroscopic data for BLV researchers.
With the help and expertise of Dr. Tina Tallon, I used Pure Data (Pd) to create an interactive patch with a simple user interface that allows for the sonification of stellar spectra. This program requires only a single input file containing stellar parameters and spectral line intensities from a real or synthetic spectrum. See PIANISSIMO for more details.
Through the distribution of a short lesson and qualitative survey, I found that supplementing traditional visual methods with the auditory spectra generated by PIANISSIMO is generally useful for conveying the basic concepts of stellar spectroscopy to an audience with no prior knowledge of astronomy. The PureData patch will be released shortly for public use, and I encourage for any dependency issues or other bugs to be reported to me via email or the contact form on this website.