arXiv subjects

H. R. Neilson

Publications and source records attributed to H. R. Neilson.

3 recordsLinked to original sources

arXiv2026

Investigating the Light Curves of 5 Long Period Variable Stars: A Study Cross-Harmonics using VStar, MESA, and GYRE

Long Period Variables (LPVs) are Asymptotic Giant Branch stars with periods >100 days. Around one third to half of LPVs display a Long Secondary Period (LSP) that is 5-10 times the length of the main periods. The exact cause of LSPs are unknown, with one potential explanation lying in cross-harmonics caused by the interaction of two simultaneously occurring periods. In this paper, we use the Fourier analysis software VStar to examine the visual range photometric light curves obtained through the American Association of Variable Star Observers of a sample of 5 stars. We find that one star, V CVn, displays a secondary period over 20 times the length of its main period, inconsistent with traditional LSPs. To test if cross-harmonics can result in an LSP or VCVn's extra-long secondary period, we then construct evolutionary tracks and pulsation models of LPVs using Modules for Evolution in Stellar Astrophysics (MESA) and GYRE. After examining the interactions of the fundamental pulsation mode and the first 4 overtones with each other, we do not find evidence that LSPs or V CVn's extra-long secondary period could be caused by cross-harmonics.

astro-ph.SR↗

arXiv2019

Interferometric Fringe Visibility Null as a Function of Spatial Frequency: a Probe of Stellar Atmospheres

We introduce an observational tool based on visibility nulls in optical spectro-interferometry fringe data to probe the structure of stellar atmospheres. In a preliminary demonstration, we use both Navy Precision Optical Interferometer (NPOI) data and stellar atmosphere models to show that this tool can be used, for example, to investigate limb darkening. Using bootstrapping with either multiple linked baselines or multiple wavelengths in optical and infrared spectro-interferometric observations of stars makes it possible to measure the spatial frequency $u_0$ at which the real part of the fringe visibility ${\rm Re}(V)$ vanishes. That spatial frequency is determined by $u_0 = B_\perp/λ_0$, where $B_\perp$ is the projected baseline length, and $λ_0$ is the wavelength at which the null is observed. Since $B_\perp$ changes with the Earth's rotation, $λ_0$ also changes. If $u_0$ is constant with wavelength, $λ_0$ varies in direct proportion to $B_\perp$. Any departure from that proportionality indicates that the brightness distribution across the stellar disk varies with wavelength via variations in limb darkening, in the angular size of the disk, or both. In this paper, we introduce the use of variations of $u_0$ with $λ$ as a means of probing the structure of stellar atmospheres. Using the equivalent uniform disk diameter $θ_{\rm UD, 0}(λ_0)$, given by $θ_{\rm UD, 0} = 1.22/u_0(λ_0)$, as a convenient and intuitive parameterization of $u_0(λ_0)$, we demonstrate this concept by using model atmospheres to calculate the brightness distribution for $ν$ Ophiuchi and predict $θ_{\rm UD, 0}(λ_0)$, and then comparing the predictions to coherently averaged data from observations taken with the NPOI.

astro-ph.IM↗

arXiv2014

CHARA/MIRC observations of two M supergiants in Perseus OB1: temperature, Bayesian modeling, and compressed sensing imaging

Two red supergiants of the Per OB1 association, RS Per and T Per, have been observed in H band using the MIRC instrument at the CHARA array. The data show clear evidence of departure from circular symmetry. We present here new techniques specially developed to analyze such cases, based on state-of-the-art statistical frameworks. The stellar surfaces are first modeled as limb-darkened discs based on SATLAS models that fit both MIRC interferometric data and publicly available spectrophotometric data. Bayesian model selection is then used to determine the most probable number of spots. The effective surface temperatures are also determined and give further support to the recently derived hotter temperature scales of red su- pergiants. The stellar surfaces are reconstructed by our model-independent imaging code SQUEEZE, making use of its novel regularizer based on Compressed Sensing theory. We find excellent agreement between the model-selection results and the reconstructions. Our results provide evidence for the presence of near-infrared spots representing about 3-5% of the stellar flux.

astro-ph.SR↗