arXiv subjects

Niall Miller

Publications and source records attributed to Niall Miller.

6 recordsLinked to original sources

arXiv2026

Custom Colors: A Module for Computing Synthetic Photometry On-the-Fly in Stellar Evolution Calculations, Integrated with MESA and Usable with Other Stellar Evolution Codes

Stellar evolution simulations predict physical quantities such as luminosity, whereas broadband photometric observations measure flux in specific bandpasses; converting the predicted stellar state to magnitudes requires interpolating a stellar atmosphere model at the surface parameters, convolving the resulting spectral energy distribution (SED) with filter transmission curves, and applying a photometric zero-point correction. We introduce Custom Colors, which performs this conversion from the stellar model. It is integrated into MESA as MESA Custom Colors, and available as the Python package SED_Model for external applications. At each timestep, the module interpolates a user-specified atmosphere grid at the current T_eff, log g, [M/H], applies geometric dilution at a user-specified distance, convolves the SED with each requested filter transmission curve, and appends observer-frame magnitudes to standard MESA output files. We demonstrate the module across six diverse use cases: TP-AGB evolution, nonlinear RR Lyrae pulsation, starspot-modified photospheres, white dwarf cooling, blue-loop evolution, and rotational spot modulation based on an external YREC SPOTS grid. These cases draw on Kurucz/ATLAS9, BT-Settl, and Koester DA atmosphere grids and filter systems including Roman WFI, LSST ugrizy, Gaia, and extended Johnson UBVRIJHKLMN. Two of these demonstrations compare the module directly with observational data: a 0.6M_dot DA white dwarf cooling track follows the broad Gaia DR3 cooling locus from M_G ~ 8.5 to 15mag without intermediate color transformation, and an RSP model recovers the Kepler light curve of FN Lyr (KIC6936115) to 1.1% in period and 0.2% in amplitude.

astro-ph.SR↗

arXiv2025

The Two-infall Model Revisited: Constraints on Milky Way Bulge Assembly from >30,000 Galactic Chemical Evolution Models and Machine Learning

We constrain the formation history of the Milky Way bulge using a two-infall galactic chemical evolution (GCE) algorithm implemented in the N'OMEGA+ code. We recover a best-fit scenario in which the bulge forms through an early, rapid starburst ($t_1 \sim 0.1$ Gyr, $\tau_1 \sim 0.09$ Gyr, and star formation efficiency (SFE) $\sim 3~\mathrm{Gyr}^{-1}$), followed by a delayed, lower-mass second infall ($t_2 \sim 5.1$ Gyr, $\tau_2 \sim 1.7$ Gyr, and $\sigma_2 \sim 0.69$). Our model adopts mass- and metallicity-dependent nucleosynthetic yields from modern stellar grids and explores a wide GCE parameter space in infall timing, SFE, mass partitioning, initial mass function upper mass, and type Ia supernova normalization, optimized via a hybrid genetic algorithm with Markov Chain Monte Carlo refinement. The later infall features a reduced SFE ($\Delta\mathrm{SFE} \sim 0.72$), reproducing the metal-rich peak of the bulge metallicity distribution function (MDF) and the decline in [$\alpha$/Fe] at high [Fe/H]. Our model naturally favors the M. Joyce et al. age--metallicity relation over the ages in T. Bensby et al. Degeneracy and principal component analyses show that the infall history, SFE, and mass partitioning are strongly covariant---the bulge's observed MDF, abundance trends, and age distribution constrain only their combinations, not each parameter independently. The results support a composite bulge origin---an early, rapid collapse builds the majority of the mass, while a younger component is required to match the late-stage enrichment.

astro-ph.GA↗

arXiv2025

VVV-WIT-13: an eruptive young star with cool molecular features

Here we investigate an infrared eruptive source, identified from the decade-long VISTA Variables in the Via Lactea survey (VVV). We named this target after a group of variable sources discovered by VVV, as VVV-WIT-13, with WIT standing for "What Is This?", due to its unique photometric variation behaviour and the mysterious origin of the outburst. This target exhibited an outburst with a 5.7 mag amplitude in the Ks-band, remained on its brightness plateau for 3.5 years, and then rapidly faded to its pre-eruptive brightness afterwards. We aim to reveal the variable nature and outburst origin of VVV-WIT-13 by presenting our follow-up photometric and spectroscopic observations along with theoretical models. We gathered photometric time series in both near- and mid-infrared wavelengths. We obtained near-infrared spectra during the outburst and decaying stages on XSHOOTER/VLT and FIRE/Magellan, and then fitted the detected molecular absorption features using models from ExoMol. We applied 2D numerical simulations to re-create the observables of the eruptive phenomenon. We observe deep AlO absorption bands in the infrared spectra of VVV-WIT-13, during the outburst stage, along with other more common absorption bands (e.g. CO). Our best-fit model suggests a 600 K temperature of the AlO absorption band. In the decaying stage, the AlO bands disappeared, whilst broad blue-shifted H2 lines arose, a common indicator of stellar wind and outflow. The observational evidence suggests that the CO and TiO features originate from an outflow or a wind environment. We find that VVV-WIT-13 is an eruptive young star with instability occurring in the accretion disk. One favoured theoretical explanation of this event is a disrupted gas clump at a distance of 3 au from the source. If confirmed, this would be the first such event observed in real time.

astro-ph.SR↗

arXiv2024

The verification of periodicity with the use of recurrent neural networks

The ability to automatically and robustly self-verify periodicity present in time-series astronomical data is becoming more important as data sets rapidly increase in size. The age of large astronomical surveys has rendered manual inspection of time-series data less practical. Previous efforts in generating a false alarm probability to verify the periodicity of stars have been aimed towards the analysis of a constructed periodogram. However, these methods feature correlations with features that do not pertain to periodicity, such as light curve shape, slow trends and stochastic variability. The common assumption that photometric errors are Gaussian and well determined is also a limitation of analytic methods. We present a novel machine learning based technique which directly analyses the phase folded light curve for its false alarm probability. We show that the results of this method are largely insensitive to the shape of the light curve, and we establish minimum values for the number of data points and the amplitude to noise ratio.

astro-ph.IM↗

arXiv2023

A survey for variable young stars with small telescopes: VI -- Analysis of the outbursting Be stars NSW284, Gaia19eyy, and VES263

This paper is one in a series reporting results from small telescope observations of variable young stars. Here, we study the repeating outbursts of three likely Be stars based on long-term optical, near-infrared, and mid-infrared photometry for all three objects, along with follow-up spectra for two of the three. The sources are characterised as rare, truly regularly outbursting Be stars. We interpret the photometric data within a framework for modelling light curve morphology, and find that the models correctly predict the burst shapes, including their larger amplitudes and later peaks towards longer wavelengths. We are thus able to infer the start and end times of mass loading into the circumstellar disks of these stars. The disk sizes are typically 3-6 times the areas of the central star. The disk temperatures are ~40%, and the disk luminosities are ~10% of those of the central Be star, respectively. The available spectroscopy is consistent with inside-out evolution of the disk. Higher excitation lines have larger velocity widths in their double-horned shaped emission profiles. Our observations and analysis support the decretion disk model for outbursting Be stars.

astro-ph.SR↗

arXiv2021

A survey for variable young stars with small telescopes: IV -- Rotation Periods of YSOs in IC5070

Studying rotational variability of young stars is enabling us to investigate a multitude of properties of young star-disk systems. We utilise high cadence, multi-wavelength optical time series data from the Hunting Outbursting Young Stars citizen science project to identify periodic variables in the Pelican Nebula (IC5070). A double blind study using nine different period-finding algorithms was conducted and a sample of 59 periodic variables was identified. We find that a combination of four period finding algorithms can achieve a completeness of 85% and a contamination of 30% in identifying periods in inhomogeneous data sets. The best performing methods are periodograms that rely on fitting a sine curve. Utilising GaiaEDR3 data, we have identified an unbiased sample of 40 periodic YSOs, without using any colour or magnitude selections. With a 98.9% probability we can exclude a homogeneous YSO period distribution. Instead we find a bi-modal distribution with peaks at three and eight days. The sample has a disk fraction of 50%, and its statistical properties are in agreement with other similarly aged YSOs populations. In particular, we confirm that the presence of the disk is linked to predominantly slow rotation and find a probability of 4.8$\times$10$^{-3}$ that the observed relation between period and presence of a disk has occurred by chance. In our sample of periodic variables, we also find pulsating giants, an eclipsing binary, and potential YSOs in the foreground of IC5070.

astro-ph.SR↗