arXiv subjects

John D. Dixon

Publications and source records attributed to John D. Dixon.

6 recordsLinked to original sources

arXiv2026

The Carbon-Dependent Binary Frequency of CEMP-no Stars

Studies of the oldest and most metal-poor stars in the Milky Way confirm a common chemical signature of high carbon abundances and subsolar neutron-capture enrichment. The so-called CEMP-no stars are speculated to be bona fide population II stars, potentially tracing the nucleosynthesis of the first stars in the universe. However, constraining the binary nature of CEMP-no stars is crucial for understanding their origins. The binary fraction of CEMP-no stars has tentatively been found to vary with carbon enhancement, independent of metallicity. Here we present the results of radial-velocity monitoring of 30 CEMP-no stars over five years, increasing the total number of CEMP-no stars with a constrained binary status by ${\sim}30\%$ in order to better investigate the CEMP-no binary fraction as a function of carbon. Combining our results with literature data, we find an overall binary frequency of $50^{+13}_{-13}\%$ among high-carbon ($A(\rm{C})\ge7.3$) CEMP-no stars, compared to $18^{+5}_{-4}\%$ for low-carbon ($A(\rm{C}) < 7.3$) stars, establishing a statistically significant increase in the CEMP-no binary frequency as a function of carbon for the first time. Our results indicate no correlation between metallicity and binary frequency. Additionally, we derive orbital parameters for three new CEMP-no stars in binary systems and estimate secondary masses for these stars as well as seven binaries from the literature. We discuss these mass estimates in the context of investigating the nature of the unseen companions and the plausibility of a mass-transfer formation channel for high-carbon CEMP-no stars.

astro-ph.SR↗

arXiv2026

Boötes III is a Tidally Disrupting Ultra-Faint Dwarf Galaxy on an Eccentric Polar Orbit

We present updated systemic properties of the ultra-faint dwarf galaxy Boötes III from the Southern Stellar Stream Spectroscopic Survey (S$^5$). We identify 21 high-probability members and measure a velocity dispersion of $σ_{v} = 1.69^{+1.03}_{-0.85}$ km s$^{-1}$, about six times smaller than the previously reported $10.7 \pm 3.5$ km s$^{-1}$, and a mean metallicity of [Fe/H] $= -2.34 \pm 0.11$. The revised dispersion brings Boötes III in line with other tidally disrupting dwarfs such as Antlia II and Crater II. Orbit integrations in a Milky Way (MW) + Large Magellanic Cloud (LMC) potential confirm a highly eccentric ($e \approx 0.8$), polar ($i \approx 89.5^\circ$) orbit with a recent pericentric passage $\sim 0.14$ Gyr ago at $r_{\rm peri} \approx 9.5$ kpc. Boötes III is thus likely actively tidally disrupting, as its tidal radius at pericenter, $r_t \approx 164$ pc, is only $\sim 0.35$ of its half-light radius. The unusually low dispersion also implies that Boötes III has either lost most of its dark matter to tides or hosts a cored inner density profile, making it a probe of the nature of dark matter. Simulated tidal streams are broadly consistent with the Styx stellar stream, though the predicted track and kinematics are sensitive to the MW halo mass, LMC mass, and solar velocity. Boötes III overlaps the Typhon stream in integrals-of-motion space but has a much lower mean metallicity, suggesting the two are not the same system but may have had a common group infall origin. Sagittarius-stream contamination prevents a direct tidal-tail detection, so deep spectroscopic follow-up remains essential, both to confirm Styx as a genuine stream and to establish it as Boötes III's tidal tail.

astro-ph.GA↗

arXiv2025

Investigating non-LTE abundances of Neodymium (Nd) in metal-poor FGK stars

The dominant site(s) of the $r$-process are a subject of current debate. Ejecta from $r$-process enrichment events like kilonovae are difficult to directly measure, so we must instead probe abundances in metal-poor stars to constrain $r$-process models. This requires state-of-the-art Non-Local Thermodynamic Equilibrium (NLTE) modeling, as LTE is a poor approximation for the low-opacity atmospheres of metal-poor giants. Neodymium (Nd) is a prominent $r$-process element detected in both near-infrared kilonovae spectra and spectra of metal-poor stars, so precise Nd stellar abundances are particularly needed to model kilonovae and constrain $r$-process sites. We thus constructed a Nd I / Nd II model atom to compute NLTE abundances in FGK metal-poor stars. We obtain $\mathrm{A(Nd)}_\odot = 1.44\pm0.05$, in agreement with the meteoritic value, when calibrating the model atom with a Drawin hydrogen collision factor of $S_H=0.1$. For a sample of metal-poor $r$-process enhanced stars with observed optical and near-infrared Nd II lines, we find NLTE Nd corrections in the range $-0.3$ to $0.3$ dex. Optical and UV lines have positive NLTE corrections, whereas H band lines have negative corrections. Additionally, we compute a large grid of NLTE corrections for 122 Nd II spectral lines ranging from the UV to the H band, for stellar parameters of typical metal-poor FGK dwarfs and giants with $-3.00\le\mbox{[Fe/H]}\le-1.00$ and $-2.0\le\mathrm{A(Nd)}\le2.0$. Within this grid, we find NLTE corrections ranging from $-0.3$ to $+0.5$ dex. Deviations from LTE are found to be strongest for blue lines with low excitation potentials in the most metal-poor giants.

astro-ph.SR↗

arXiv2025

Ground-Based Reconnaissance Observations of 21 Exoplanet Atmospheres with the Exoplanet Transmission Spectroscopy Imager

One of the most prolific methods of studying exoplanet atmospheres is transmission spectroscopy, which measures the difference between the depth of an exoplanet's transit signal at various wavelengths and attempts to correlate the depth changes to potential features in the exoplanet's atmosphere. Here we present reconnaissance observations of 21 exoplanet atmospheres measured with the Exoplanet Transmission Spectroscopy Imager (ETSI), a recently deployed spectro-photometer on the McDonald Observatory Otto Struve 2.1 m telescope. ETSI measurements are mostly free of systematics through the use of a novel observing technique called common-path multi-band imaging (CMI), which has been shown to achieve photometric color precision on-par with space-based observations (300ppm or 0.03%). This work also describes the various statistical tests performed on the data to evaluate the efficacy of the CMI method and the ETSI instrument in combination. We find that none of the 8 comparisons of exoplanet atmospheres measured with ETSI and other observatories (including the Hubble Space Telescope) provide evidence that the spectra are statistically dissimilar. These results suggest that ETSI can provide initial transmission spectroscopy observations for a fraction of the observational and monetary overhead previously required to detect an exoplanet's atmosphere. Ultimately these reconnaissance observations increase the number of planets with transmission spectroscopy measurements by ~10% and provide an immediate prioritization of 21 exoplanets for future follow-up with more precious observatories, such as the James Webb Space Telescope. The reconnaissance spectra are available through the Filtergraph visualization portal at the URL: https://filtergraph.com/etsi/.

astro-ph.EP↗

arXiv2017

Recognizing cyclic matrices and a conjecture of J.G. Thompson

In 2006 J.G. Thompson conjectured: "If F is a field and A is in GL(n,F), then there is a permutation matrix P such that AP is cyclic, that is, the minimal polynomial of AP is also its characteristic polynomial" (open problem 16.95 in the Kourovka Notebook). The present note provides a simple criterion for a matrix to be cyclic and uses this to prove Thompson's conjecture. ERRATA I am indebted to Alexander Stasinski (Durham University) for the following observations. Suppose n > 2 and J is the n x n all 1's matrix over a field of characteristic not 2. Then A := J - I has the minimal polynomial (X + 1)(X - n + 1). Thus A is invertible and not cyclic even though A satisfies condition (iv) of the Proposition. The error lies in the claim towards the end of the proof that "these particular row and column errors do not change the determinants ...".

math.GR↗

arXiv2013

Longest common subsequences in binary sequences

Given two {0,1}-sequences X and Y of lengths m and n, respectively, we write L(X,Y) to denote the length of the longest common subsequence (LCS) of X and Y, and write L(m,n) to denote the expected value of L(X,Y) when X and Y are random sequences. We study the value of the function z -> lim L(nz,n)/n (as n -> infinity) and the relation of this function to the outstanding problem of computing the Chvatal-Sankoff constant lim L(n,n)/n.

math.GR↗