Background

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). 

Methods

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.

Results (TBD)

Dixon, J. D., Rana Ezzeddine, Yangyang Li, Thibault Merle, Manuel Bautista, Yanjun Guo. 2025. Investigating Non-LTE Abundances of Neodymium in Metal-poor FGK stars. ApJ, 994, 44.

Background

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.

Methods

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.

Results

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.