arXiv subjects

J. MacDonald

Publications and source records attributed to J. MacDonald.

At least 19 recordsLinked to original sources

arXiv2026

New Observations of the Strongly Magnetic O-star NGC 1624-2 Reveal Its Magnetic South Pole

NCG 1624-2 has the strongest detected magnetic field of all known main-sequence O-type stars. It was originally found that its magnetospheric emission lines followed a $\sim$5 month periodicity, and the existing line-of-sight magnetic measurements were predominantly of strong positive (north) polarity. As such, the field's geometric interpretation has been a mildly tilted (with respect to the rotational axis) dipole, such that only the magnetic north pole is visible during a rotation cycle. However, S. Seadrow et al. (2026) recently reported that new magnetospheric observations no longer phased with the established ephemeris and that the period had to be decreased by a few days. S. Seadrow et al. (2026) also found that existing magnetic measurements did not rule out a period twice as long (306.56 d). This period suggests a different magnetic configuration with a larger dipolar tilt, making both magnetic poles visible over a single rotation. Because previous spectropolarimetric observations did not have sufficient phase coverage to distinguish between the geometries, both were equally viable. In this paper, we present new spectropolarimetric observations obtained specifically to resolve this ambiguity. Our new magnetic measurements have a strong negative (south) polarity, confirming that the rotational period of NGC 1624-2 is indeed nearly twice as long as previously thought. Our measurements show that both poles come within a similar angle to our line of sight and likely have roughly the same local magnetic field strength (with a dipolar strength of 15-20 kG or more depending on the inclination angle).

astro-ph.SR↗

arXiv2026

Searching for low-mass stars with magnetically-induced hyper-inflated radii

Precise empirical estimates of stellar radii have revealed that the radii of certain low-mass stars are inflated relative to stellar structure predictions: the largest inflations occur in magnetically active stars. Theoretically, the radii of magnetically active stars are in some cases found to be 'hyper-inflated' to roughly double the radius of a non-magnetic star with equal mass. Here we ask, do data exist which could allow us to search for empirical evidence in support of hyper-inflated stars? A photometric study of 44 eclipsing binaries in the Kepler field by Cruz et al. may help us in our search. The Cruz et al. study, although subject to large uncertainties, hints at the presence of hyper-inflation in some of the 88 stars in their sample. Their data enable us to set theoretical limits on the maximum strength Bc of magnetic fields inside their sample stars. According to our magneto-convective model, the average empirical inflations found from analysis of the Cruz et al. data can be replicated if Bc approx. 10 kG inside stars with masses greater than ~ 0.6 MSun. On the other hand, in stars with masses less than about 0.4 MSun, our model predicts that the average empirical inflations of the stars may approach hyper-inflated status. Such stars may require significantly stronger internal fields, i.e. Bc approx. 100-300 kG. High-resolution spectroscopy of the Kepler binaries could help to confirm or refute our conclusions.

astro-ph.SR↗

arXiv2026

New Insights from Revisiting the Rotation Period of the Strongly Magnetic O Star, NGC 1624-2

NGC 1624-2 hosts the strongest surface magnetic field found on an O star thus far. When applied across several epochs of observations, the star's currently accepted rotation period (157.99 d) does not coherently characterize the variations of spectral lines of magnetospheric origin. We analyze Lomb-Scargle periodograms produced with new and archival, multi-instrument spectroscopic time series of Balmer H and He spectral lines. We find that 153.17 $\pm$ 0.42 d and 306.56 $\pm$ 1.19 d are both equally suitable periods at phasing the spectral and magnetic time series data in a manner consistent with the Oblique Rotator Model. The 306.56 d period implies a magnetic geometry for NGC 1624-2 that is quite different from the previously accepted one, for which both magnetic poles should be observed during a full rotational cycle. If this is the case, the star's magnetic South pole has yet to be observed, and additional spectropolarimetric observations should be acquired in order to confirm whether or not the south pole is in fact observable.

astro-ph.SR↗

arXiv2025

Empirical measures of the largest amounts of magnetically-induced radius inflation in low-mass stars

Access to precise empirical estimates of stellar radii in recent decades has revealed that the radii of certain low-mass stars are inflated relative to stellar structure predictions. The largest inflations are found in magnetically active stars. Although various attempts have been made to incorporate magnetic effects into stellar structure codes, a major source of uncertainty is associated with our lack of knowledge as to how the field strength varies inside the star. Here, we point out that a recent study of 44 eclipsing binaries in the Kepler field by Cruz et al. may enable us for the first time to set an upper limit Bc on the field strengths inside the 88 stars in the sample. According to our magneto-convective model, the largest empirical inflations reported by Cruz et al. can be replicated if Bc is about 10 kG inside stars with masses greater than 0.65 MSun. On the other hand, in lower mass stars, especially those with masses less than 0.4 MSun, our model predicts that the largest empirical inflations may require significantly stronger fields, i.e. Bc approximately 100-300 kG.

astro-ph.SR↗

arXiv2025

A plasmoid-model for mass loss from stars on the upper red giant branch: The mass loss rate is controlled by the number of density scale heights in the convection zone

Recent asteroseismic determinations of {\Delta}M, the integrated mass loss on the red giant branch (RGB), for fields stars show a trend of {\Delta}M decreasing as metallicity increases. This trend among field stars is inconsistent with many existing models of RGB mass loss. The present paper is motivated by a 'plasmoid' model of RGB mass loss in which magnetic flux loops, generated by a shear dynamo operating below the convection zone, are buoyed up to the stellar surface starting at the evolutionary stage right after the RGB 'kink'. This model leads us to examine correlations between, on the one hand, the average post-kink RGB mass loss rate, determined from {\Delta}M and the post-kink RGB lifetime, and on the other hand, stellar properties which exist just after the end of the kink. For three distinct stellar samples, we find strong anti-correlations between the average post-kink RGB mass loss rate and the number of density scale heights in the convection zone. This leads us to propose that the number of density scale heights in the convection zone is a dominant factor in determining the rate of the mass loss process which sets in after the RGB kink.

astro-ph.SR↗

arXiv2023

Evolution of low mass population III stars from the pre-main sequence to the white dwarf cooling track

Radiation feedback from massive population III stars may have given rise to low mass star formation from primordial or nearly primordial material. If early universe low mass stars did form, some should remain locally as white dwarfs, sub-giants, or main sequence stars. In this paper, we present model calculations for the evolution of single 0.8 Msun to 3.0 Msun stars with primordial metallicity from pre-main sequence to the white dwarf cooling track, and calculations for the evolution of single 4.0 Msun to 7.0 Msun stars which conclude in the giant phase. One goal of this work is to identify potential observable markers for potential observed progenitors of first or nearly first stars. We uncover a number of seemingly peculiar evolutionary differences between that of pop III low mass stars compared with younger higher Z stars, as well as compared to other primordial evolution models. We also present an initial-final mass relationship and identify the minimum mass of a single white dwarf that could have had a population III progenitor.

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arXiv2019

Mass loss on the red giant branch: plasmoid-driven winds above the RGB bump

The onset of cool massive winds in evolved giants is correlated with an evolutionary feature on the red giant branch known as the bump. Also at the bump, shear instability in the star leads to magnetic fields that occur preferentially on small length scales. Pneuman (1983) has suggested that the emergence of small scale flux tubes in the Sun can give rise to enhanced acceleration of the solar wind as a result of plasmoid acceleration (the melon seed mechanism). In this paper, we examine the Pneuman formalism to determine if it may shed some light on the process that drives mass loss from stars above the bump. Because we do not currently have detailed information for some of the relevant physical parameters, we are not yet able to derive a detailed model. Instead, our goal in this paper is to explore a proof of concept. Using parameters that are known to be plausible in cool giants, we find that the total mass loss rate from such stars can be replicated. Moreover, we find that the radial profile of the wind speed in such stars can be steep or shallow depending on the fraction of the mass loss which is contained in the plasmoids. This is consistent with empirical data which indicate that the velocity profiles of winds from cool giants range from shallow to steep.

astro-ph.SR↗

arXiv2019

The umbral--penumbral boundary in sunspsots in the context of magneto-convection

Jurcak et al (2018) have reported that, in a sample of more than 100 umbral cores in sunspots, the umbral-penumbral boundary (UPB) is characterized by a remarkably narrowly-defined numerical value (1867 G) of the vertical component of the magnetic field. Gough and Tayler (1966), in their study of magneto-convection, showed that the onset of convection in the presence of a magnetic field is controlled by a parameter δ which also depends on the vertical component of the field. Combining the Jurcak et al result with various empirical models of sunspots leads us to propose the following hypothesis: the UPB occurs where the vertical field is strong enough to increase the effective adiabatic temperature gradient by at least 100% above its non-magnetic value.

astro-ph.SR↗

arXiv2019

Two-Colour Interferometry and Thomson Scattering Measurements of a Plasma Gun

We present experimental measurements of a pulsed plasma gun, using two-colour imaging laser interferometry and spatially resolved Thomson scattering. Interferometry measurements give an electron density $n_e\approx2.7\times10^{17}$ cm$^{-3}$ at the centre of the plasma plume, at 5 mm from the plasma gun nozzle. The Thomson scattered light is collected from two probing angles allowed us to simultaneously measure the collective and non-collective spectrum of the electron feature from the same spatial locations. The inferred electron densities from the location of the electron plasma waves is in agreement with interferometry. The electron temperatures inferred from the two spectra are not consistent, with $T_e\approx 10$ eV for non-collective scattering and $T_e\approx 30$ eV for collective scattering. We discuss various broadening mechanisms such as finite aperture effects, density gradients within the collective volume and collisional broadening to account for some of this discrepancy. We also note the significant red/blue asymmetry of the electron plasma waves in the collective scattering spectra, which could relate to kinetic effects distorting the distribution function of the electrons.

physics.plasm-ph↗

arXiv2018

Magnetic Fields on the Flare Star Trappist-1: Consequences for Radius Inflation and Planetary Habitability

We construct evolutionary models of Trappist-1 in which magnetic fields impede the onset of convection according to a physics-based criterion. In the models that best fit all observational constraints, the photospheric fields in Tr-1 are found to be in the range 1450-1700 G. These are weaker by a factor of about 2 than the fields we obtained in previous magnetic models of two other cool dwarfs (GJ65A/B). Our results suggest that Tr-1 possesses a global poloidal field which is some one hundred times stronger than in the Sun. In the context of exoplanets in orbit around Tr-1, the strong poloidal fields on the star may help to protect the planets from the potentially destructive effects of coronal mass ejections. This, in combination with previous arguments about beneficial effects of flare photons in ultraviolet and visible portions of the spectrum, suggests that conditions on Tr-1 are not necessarily harmful to life on a planet in the habitable zone of Tr-1.

astro-ph.SR↗

arXiv2016

Mechanical energy fluxes associated with saturated coronal heating in M dwarfs: comparison with predictions of a turbulent dynamo

Empirically, the X-ray luminosity LX from M dwarfs has been found to have an upper limit of about 0.2% of the bolometric flux Lbol. In the limit where magnetic fields in M dwarfs are generated in equipartition with convective motions, we use stellar models to calculate the energy flux of Alfven waves FA as a function of depth in the sub-surface convection zone. Since Alfven waves have the optimal opportunity for wave modes to reach the corona, we suggest that FA sets an upper limit on the mechanical flux Fmech which causes coronal heating. This suggestion accounts quantitatively for the saturated values of LX/Lbol which have been reported empirically for M dwarfs.

astro-ph.SR↗

arXiv2015

The effects of convection criteria on the evolution of population III stars and the detectability of their supernovae

The first stars continue to elude modern telescopes, but much has been accomplished in observing the glow of the first galaxies. As detection capabilities improve we will eventually resolve these galaxies, but hopes of observing an individual star remains dim for the foreseeable future. However, our first view of an individual first star may be possible due to its explosion. In this work, we present evolution calculations for Population III (Pop III) stars and their subsequent supernovae explosions. Our evolution models include a mass range of 15 - 100 M$_\odot$, each with initial heavy element abundance Z = 10-14. Our models are evolved from pre-main sequence through formation of an iron core, and thus near to core collapse. We find that modelling the evolution of these stars is very sensitive to the choice of convection criterion; here we provide evolution results using both the Schwarzschild and Ledoux criteria. We also use the final structure from our models for numerical simulation of their supernovae light curves using a radiation hydrodynamics code. In doing so, we estimate a lower bound of initial model mass that may be possible to observe in the near future. We also find that our 40 M$_\odot$ Schwarzschild evolution model produces the brightest supernova peak and statistically should be the most frequently observed. At our highest redshift z= 15, only the 60 M$_\odot$ Schwarzschild model at peak magnitude starts to rival the 40 M$_\odot$ model in brightness.

astro-ph.SR↗

arXiv2015

Magneto-convective models of red dwarfs: constraints imposed by the lithium abundance

Magnetic fields impede the onset of convection, thereby altering the thermal structure of a convective envelope in a low mass star: this has an effect on the amount of lithium depletion in a magnetized star. In order to quantify this effect, we have applied a magneto-convective model to two low mass stars for which lithium abundances and precise structural parameters are known: YY Gem and CU Cnc. For both stars, we have obtained models which satisfy empirical constraints on the following parameters: R, L, surface magnetic field strength, and Li abundance. In the case of YY Gem, we have obtained a model which satisfies the empirical constraints with an internal magnetic field of several megagauss: such a field strength is within the range of a dynamo where the field energy is in equipartition with rotational energy deep inside the convection zone. However, in the case of CU Cnc, the Li requires an internal magnetic field which is probably too strong for a dynamo origin: we suggest possible alternatives which might account for the reported Li abundance in CU Cnc.

astro-ph.SR↗

arXiv2014

M dwarf search for pulsations within Kepler GO program

We present the analysis of four M dwarf stars -plus one M giant that seeped past our selection criteria- observed in Cycle 3 of Kepler Guest Observer program (GO3) in a search for intrinsic pulsations. Stellar oscillations in M dwarfs were theoretically predicted by Rodríguez-López et al. (2012) to be in the range ~20-40 min and ~4-8 h, depending on the age and the excitation mechanism. We requested Kepler short cadence observations to have an adequate sampling of the oscillations. The targets were chosen on the basis of detectable rotation in the initial Kepler results, biasing towards youth.The analysis reveals no oscillations attributable to pulsations at a detection limit of several parts per million, showing that either the driving mechanisms are not efficient in developing the oscillations to observable amplitudes, or that if pulsations are driven, the amplitudes are very low. The size of the sample, and the possibility that the instability strip is not pure, allowing the coexistence of pulsators and non-pulsators, prevent us from deriving definite conclusions. Inmediate plans include more M dwarfs photometric observations of similar precision with Kepler K2 mission and spectroscopic searches already underway within the Cool Tiny Beats Project (Anglada-Escudé et al. 2014, Berdiñas et al. 2014) with the high-resolution spectrographs HARPS and HARPS-N.

astro-ph.SR↗

arXiv2013

The theoretical instability strip of M dwarf stars

The overstability of the fundamental radial mode in M dwarf models was theoretically predicted by Rodríguez-López et al. (2012). The periods were found to be in the ranges ~25-40 min and ~4-8 h, depending on stellar age and excitation mechanism. We have extended our initial M dwarf model grid in mass, metallicity, and mixing length parameter. We have also considered models with boundary conditions from PHOENIX NextGen atmospheres to test their influence on the pulsation spectra. We find instability of non-radial modes with radial orders up to k=3, degree l=0-3, including p and g modes, with the period range extending from 20 min up to 11 h. Furthermore, we find theoretical evidence of the potential of M dwarfs as solar-like oscillators.

astro-ph.SR↗

arXiv2013

The extremely low metallicity star SDSS J102915+172927: a subgiant scenario

Spectroscopic analysis of the Galactic halo star SDSS J102915+172927 has shown it to have a very low heavy element abundance, Z < 7.4 10-7, with [Fe/H] = -4.89 plus/minus 0.10 and an upper limit on the C abundance of [C/H] < -4.5. The low C/Fe ratio distinguishes this object from most other extremely metal poor stars. The effective temperature and surface gravity have been determined to be Teff = 5811 plus/minus 150 K and log g = 4.0 plus/minus 0.5. The surface gravity estimate is problematical in that it places the star between the main sequence and the subgiants in the Hertzsprung-Russell diagram. If it is assumed that the star is on the main sequence, its mass and are estimated to be M = 0.72 plus/minus 0.06 Msun and L = 0.45 plus/minus 0.10 Lsun, placing it at a distance of 1.35 plus/minus 0.16 kpc. The upper limit on the lithium abundance, A(Li) < 0.9, is inconsistent with the star being a dwarf, assuming that mixing is due only to convection. In this paper, we propose that SJ102915 is a sub-giant that formed with significantly higher Z than currently observed, in agreement with theoretical predictions for the minimum C and/or O abundances needed for low mass star formation. In this scenario, extremely low Z and low Li abundance result from gravitational settling on the main sequence followed by incomplete convective dredge-up during subgiant evolution. The observed Fe abundance requires the initial Fe abundance to be enhanced compared to C and O, which we interpret as formation of SJ102915 occurring in the vicinity of a type Ia supernova.

astro-ph.SR↗

arXiv2012

An Empirical Measure of the Rate of White Dwarf Cooling in 47 Tucanae

We present an empirical determination of the white dwarf cooling sequence in the globular cluster 47 Tucanae. Using spectral models, we determine temperatures for 887 objects from Wide Field Camera 3 data, as well as 292 objects from data taken with the Advanced Camera for Surveys. We make the assumption that the rate of white dwarf formation in the cluster is constant. Stellar evolution models are then used to determine the rate at which objects are leaving the main sequence, which must be the same as the rate at which objects are arriving on the white dwarf sequence in our field. The result is an empirically derived relation between temperature ($T_{eff}$) and time ($t$) on the white dwarf cooling sequence. Comparing this result to theoretical cooling models, we find general agreement with the expected slopes between 20,000K and 30,000K and between 6,000K and 20,000K, but the transition to the Mestel cooling rate of $T_{eff} \propto t^{-0.4}$ is found to occur at hotter temperatures, and more abruptly than is predicted by any of these models.

astro-ph.SR↗

arXiv2011

Pulsations in M dwarf stars

We present the results of the first theoretical non-radial non-adiabatic pulsational study of M dwarf stellar models with masses in the range 0.1 to 0.5M_solar. We find the fundamental radial mode to be unstable due to an εmechanism caused by deuterium (D-) burning for the young 0.1 and 0.2M_solar models, by non-equilibrium He^3 burning for the 0.2 and 0.25M_solar models of 10^4Myr, and by a flux blocking mechanism for the partially convective 0.4 and 0.5M_solar models once they reach the age of 500Myr. The periods of the overstable modes excited by the D-burning are in the range 4.2 to 5.2h for the 0.1M_solar models and is of order 8.4h for the 0.2M_solar models. The periods of the modes excited by He^3 burning and flux blocking are in the range 23 to 40min. The more massive and oldest models are more promising for the observational detection of pulsations, as their ratio of instability e-folding time to age is more favourable.

astro-ph.SR↗