arXiv subjects

James MacDonald

Publications and source records attributed to James MacDonald.

At least 19 recordsLinked to original sources

arXiv2025

The Role of the Convective Kissing Instability in Cataclysmic Variable Evolution

The convective kissing instability (CKI) is postulated to occur in low mass stars around the fully convective transition. Non-equilibrium $^{3}$He burning leads to the merging of core and envelope convective zones, which causes abrupt decreases in the stellar radius. It has been suggested by van Saders & Pinsonneault (2012) that these effects may be relevant for cataclysmic variables (CVs). We have performed stellar evolution modeling to study the role of the CKI in CV evolution. We find that the CKI has no effect on normal CVs which evolve via magnetic braking and gravitational radiation above the period gap. CKI cycles either do not occur or are abruptly halted once mass transfer begins. If only gravitational radiation is considered, the CKI does occur. The abrupt radius changes can cause detachment phases which produce small period gaps with widths of a few minutes. We describe how the size of the period gaps is controlled by the $^{3}$He profiles of the secondaries. We also discuss how the results of this study apply to the evolution of strong field polars, where the magnetic field of the white dwarf is strong enough to suppress magnetic braking.

astro-ph.SR↗

arXiv2025

Early Post Asymptotic Giant Branch Instability: Does it Affect White Dwarf Hydrogen Envelope Mass?

Although most white dwarf stars have hydrogen-dominated atmospheres, a significant fraction have atmospheres in which hydrogen is spectroscopically absent, with the fraction of hydrogen-free atmospheres varying with effective temperature. Estimates of the total mass of hydrogen, MH, in the stellar envelope from either asteroseismology or spectral evolution are at odds with predicted values from theoretical stellar evolution modeling. Recent work has found that models in the early post Asymptotic Giant Branch (AGB) phase of evolution can exhibit thermally and dynamical unstable behavior. Here we investigate whether this Early Post AGB Instability (EPAGBI) can help resolve the conflict in MH values determined from white dwarf spectral evolution, analysis of DAV pulsations and canonical stellar evolution modeling, by evolving models of mass 1 and 2Msun through the AGB phases and to the white dwarf cooling track. The MH values at the end of the calculations are in the range consistent with asteroseismic determinations. The major impact of EPAGBIs is that they cause loops in the HRD, which are absent when the EPAGBI is suppressed. Such loops might be detectable in a long-term monitoring program, or by their imprint on planetary nebula morphology imparted by the cyclically varying mass loss rate. Since the characteristic timescale of the looping in the HRD depends on the stellar mass, it could provide a way to determine the stellar mass just after AGB departure. Another EPAGBI signature is the production of Li by the Cameron-Fowler process. During the EPAGBI phase the photospheric temperature is always too high for the Li I resonance line to be detected. However, 7Be is convected to the photosphere in significant amounts (up to 400 times the solar photospheric mass fraction) at various times in the EPAGBI phase, which may be detectable by observing the Be II resonance doublet.

astro-ph.SR↗

arXiv2025

The Effects of Magneto-Convection on Short Period Cataclysmic Variables

Many of the current problems related to the evolution of cataclysmic variables revolve around the magnetic nature of the main sequence secondary. It is known that magnetic fields alter the structure of low mass stars. In particular, they inhibit convection, leading to inflated radii. Here we present a simple model to demonstrate the impact of magneto-convection on the evolution of short period cataclysmic variables. We find that the inclusion of magneto-convection leads to larger secondaries, longer orbital periods and smaller mass-loss rates. When including magnetic effects, the minimum orbital period is increased by 14 minutes, indicating that this could help alleviate the period minimum problem in cataclysmic variable evolution. We also examine the effect of the white dwarf mass on the minimum period. While increasing the white dwarf mass does increase the minimum period, it is not substantial. Therefore it is unlikely that the period minimum problem can be solved with a larger white dwarf mass or with mass growth of white dwarf.

astro-ph.SR↗

arXiv2024

Using ZDI maps to determine magnetic forces and torques at the photospheres of Early-type stars

We use the magnetic field components measured by Zeeman Doppler imaging (ZDI) to calculate the stellar surface force and torque due to magnetic stresses for the fast rotators $\sigma$ Ori E, 36 Lyn and CU Vir, and the slow rotator $\tau$ Sco. If we assume the stars have spherical photospheres, the estimated torques give spin down time scales no larger than $7 \times 10^5$ yr. For $\sigma$ Ori E, the predicted spin down time scale, $\simeq 6000$ yr, is much less than the observationally measured time scale of $\simeq 10^6$ yr. However, for CU Vir, we find that the spin down time scale from its ZDI map is $7 \times 10^5$ yr in good agreement with its average rate of spin down from 1960 to 2010. With the exception of $\tau$ Sco, the net force due to magnetic stresses at the stellar surface are large compared to the surface-integrated pressure. We discuss possible reasons for the large values of the forces (and torques), and suggest that the likely explanation is that rotation and the magnetic stresses create significant departures from spherical symmetry.

astro-ph.SR↗

arXiv2020

Designing knowledge plane to optimize leaf and spine data center

In the last few decades, data center architecture evolved from the traditional client-server to access-aggregation-core architectures. Recently there is a new shift in the data center architecture due to the increasing need for low latency and high throughput between server-to-server communications, load balancing and, loop-free environment. This new architecture, known as leaf and spine architecture, provides low latency and minimum packet loss by enabling the addition and deletion of network nodes on demand. Network nodes can be added or deleted from the network based on network statistics like link speed, packet loss, latency, and throughput. With the maturity of Open Virtual Switch (OvS) and OpenFlow based Software Defined Network (SDN) controllers, network automation through programmatic extensions has become possible based on network statistics. The separation of the control plane and data plane has enabled automated management of network and Machine Learning (ML) can be applied to learn and optimize the network. In this publication, we propose the design of an ML-based approach to gather network statistics and build a knowledge plane. We demonstrate that this knowledge plane enables data center optimization using southbound APIs and SDN controllers. We describe the design components of this approach - using a network simulator and show that it can maintain the historical patterns of network statistics to predict future growth or decline. We also provide an open-source software that can be utilized in a leaf and spine data center to provide elastic capacity based on load forecasts.

cs.NI↗

arXiv2019

Magnetic inhibition of convection in O star envelopes

It has been suggested that the absence of macroturbulence in the atmosphere of NGC 1624 - 2 is due its strong magnetic field (the strongest known for a massive O star) suppressing convection in its outer layers, removing the mechanism thought responsible for the observed macroturbulence in stars with lower field strengths. Here, we develop and apply a criterion for a uniform magnetic field to suppress convection in stellar envelopes in which radiation pressure is a significant contributor to hydrostatic balance. We find upper mass limits of ~55 Msun and ~30 Msun for magnetic suppression to be possible in Zero Age Main Sequence and Terminal Age Main Sequence stars, respectively. For evolved stars, magnetic suppression of convection can significantly alter the stars' evolution. For NGC 1624 - 2, we find a polar dipole strength of 16.5 +/- 5.9 kG is required to suppress convection, in good agreement with the value ~20 kG measured by spectropolarimetry.

astro-ph.SR↗

arXiv2018

An explanation for the gap in the Gaia HRD for M dwarfs

We show that the recently discovered narrow gap in the Gaia Hertzsprung - Russell Diagram near MG =10 can be explained by standard stellar evolution models and results from a dip in the luminosity function associated with mixing of 3He during merger of envelope and core convection zones that occurs for a narrow range of masses.

astro-ph.SR↗

arXiv2017

The Magnetic Binary GJ 65: A Test of Magnetic Diffusivity Effects

GJ 65 is a M dwarf binary system consisting of the two flare stars BL Cet (GJ 65A) and UV Cet (GJ 65B). Two teams of investigators have recently reported total magnetic fluxes corresponding to fields of 4.5 and 5.2 kG for GJ65A, and 5.8 and 6.7 kG for GJ65B: for each component, the magnetic results obtained by the two teams agree with each other within 1{\sigma}. For the first time, we can directly compare the predictions of our magneto-convective models, based on fitting observed stellar parameters, with measured field strengths. We find that our models agree with the observed field strengths provided the effects of finite conductivity are accounted for. Thus, GJ65 provides us an opportunity to use observations of field strengths to distinguish between the predictions of our models that assume perfect electrical conductivity and those that allow for finite conductivity.

astro-ph.SR↗

arXiv2017

Magnetic modeling of inflated low-mass stars using interior fields no larger than ~10 kilogauss

We have previously reported on models of low-mass stars in which the presence of inflated radii is ascribed to magnetic fields which impede the onset of convection (e.g. MacDonald & Mullan [2017a] and citations therein). Some of our magneto-convection models have been criticized because, when they were first reported by Mullan & MacDonald (2001), the deep interior fields were found to be very large (50-100 MG). Such large fields are now known to be untenable. For example, Browning et al. (2016) used stability arguments to suggest that interior fields in low-mass stars cannot be larger than ~1 MG. Moreover, 3D models of turbulent stellar dynamos suggest that fields generated in low-mass interiors may be not much stronger than 10-20 kG (Browning 2008). In the present paper, we present magneto-convective models of inflated low-mass stars in which the interior fields are not permitted to be stronger than 10 kG. These models are used to fit empirical data for 15 low-mass stars for which precise masses and radii have been measured. We show that our 10 kG magneto-convective models can replicate the empirical radii and effective temperatures for 14 of the stars. In the case of the remaining star (in the Praesepe cluster), two different solutions have been reported in the literature. We find that one of these solutions (by Gillen et al. 2017) can be fitted well with our model using the nominal age of Praesepe (800 Myr). However, the second solution (by Kraus et al. 2017) cannot be fitted unless the star's age is assumed to be much younger (~ 150 Myr).

astro-ph.SR↗

arXiv2017

High-cadence spectroscopy of M-dwarfs - II. Searching for stellar pulsations with HARPS

Stellar oscillations appear all across the Hertzsprung-Russell diagram. Recent theoretical studies support their existence also in the atmospheres of M dwarfs. These studies predict for them short periodicities ranging from 20~min to 3~h. Our Cool Tiny Beats (CTB) programme aims at finding these oscillations for the very first time. With this goal, CTB explores the short time domain of M dwarfs using radial velocity data from the HARPS-ESO and HARPS-N high-precision spectrographs. Here we present the results for the two most long-term stable targets observed to date with CTB, GJ~588 and GJ~699 (i.e. Barnard's star). In the first part of this work we detail the correction of several instrumental effects. These corrections are specially relevant when searching for sub-night signals. Results show no significant signals in the range where M dwarfs pulsations were predicted. However, we estimate that stellar pulsations with amplitudes larger than $\sim0.5\,\mathrm{m\,s}^{-1}$ can be detected with a 90% completeness with our observations. This result, along with the excess of power regions detected in the periodograms, open the possibility of non-resolved very low amplitude pulsation signals. Next generation more precise instrumentation would be required to detect such oscillations. However, the possibility of detecting pulsating M-dwarf stars with larger amplitudes is feasible due to the short size of the analysed sample. This motivates the need for completeness of the CTB survey.

astro-ph.SR↗

arXiv2017

LSPM J1314: An oversized magnetic star with constraints on the radio emission mechanism

LSPM J1314+1320 (=NLTT 33370) is a binary star system consisting of two nearly identical pre-main sequence stars of spectral type M7. The system is remarkable among ultracool dwarfs for being the most luminous radio emitter over the widest frequency range. Masses and luminosities are at first sight consistent with the system being coeval at age ~80 Myr according to standard (non-magnetic) evolutionary models. However, these models predict an average effective temperature, 2950 +/- 5 K, which is 180 K hotter than the empirical value. Thus, the empirical radii are oversized relative to the standard models by ~13%. We demonstrate that magnetic stellar models can account quantitatively for the oversizing. As a check on our models, we note that the radio emission limits the surface magnetic field strengths: the limits depend on identifying the radio emission mechanism. We find that the field strengths required by our magnetic models are too strong to be consistent with gyrosynchrotron emission, but are consistent with electron cyclotron maser emission.

astro-ph.SR↗

arXiv2016

Apparent Non-Coevality among the Stars in Upper Scorpio: Resolving the Problem using a Model of Magnetic Inhibition of Convection

Two eclipsing binaries in the USco association have recently yielded precise values of masses and radii for 4 low-mass members of the association. Standard evolution models would require these dM4.5 - dM5 stars to have ages which are younger than the ages of more massive stars in the association by factors which appear (in extreme cases) to be as large as ~3. Are the stars in the association therefore non-coeval? We suggest that the answer is No: by incorporating the effects of magnetic inhibition of convective onset, we show that the stars in USco can be restored to coevality provided that the 4 low-mass member stars have vertical surface fields in the range 200 - 700 G. Fields of such magnitude have already been measured on the surface of certain solar-type stars in other young clusters.

astro-ph.SR↗

arXiv2016

The Age of the KIC 7177553 System

KIC 7177553 is a quadruple system containing two binaries of orbital periods 16.5 and 18 d. All components have comparable masses and are slowly rotating, non-evolved stars of spectral type near G2V. The longer period binary is eclipsing with component masses and radii, M1 = 1.043 +/- 0.014 M_sun, R1 = 0.940 +/- 0.005 R_sun, and M2 = 0.986 +/- 0.015 M_sun, R2 = 0.941 +/- 0.005 R_sun. The essentially equal radii measurements are inconsistent with the two stars being on the main sequence at the same age using standard stellar evolution models. Instead a consistent scenario is found if the stars are in their pre-main sequence phase of evolution and have age 33 - 36 Myr. Such a young age has important implications for the detectability of the massive planet indicated by eclipse time variations.

astro-ph.SR↗

arXiv2013

Lithium abundance and surface magnetic fields: new constraints in magnetic models of M dwarfs

Precision modeling of M dwarfs has become worthwhile in recent years due to the increasingly precise values of masses and radii which can be obtained from eclipsing binary studies. Torres (2013) has identified 4 prime M dwarf pairs with the most precise empirical determinations of masses and radii. The measured radii are consistently larger than standard stellar models predict. We have previously modeled M dwarfs in the context of a criterion due to Gough & Tayler in which magnetic fields inhibit the onset of convection according to a physics-based prescription. New constraints on the models of M dwarfs are now provided by measurements of lithium abundances. The key aspect of Li in terms of setting constraints on magnetic modeling is that Li burning starts at T = 2.5 MK, and temperatures of just such magnitude are associated with the base of the convection zone: magnetic inhibition of convective onset can shift this base slightly closer to the surface, i.e. to slightly lower temperatures, thereby reducing the amount of Li depletion compared to a non-magnetic model. In the present paper, we consider how our magneto-convection models handle the new test of stellar structure provided by Li measurements. Among the prime systems listed by Torres, we find that plausible magnetic models work well for CM Dra and YY Gem but not for CU Cnc. (The fourth system in Torres's list does not yet have enough information to warrant magnetic modeling.) For CU Cnc, we suggest that the observed lithium may have been accreted from a circumstellar disk. We find that our magneto-convection models of CM Dra, YY Gem and CU Cnc yield results which are consistent with the observed correlation between magnetic flux and X-ray luminosity.

astro-ph.SR↗

arXiv2013

Magnetic effects and oversized M dwarfs in the young open cluster NGC 2516

By combining rotation periods with spectroscopic determinations of projected rotation velocity, Jackson, Jeffries & Maxted (2009) have found that the mean radii for low-mass M-dwarfs in the young, open cluster NGC 2516 are larger than model predictions at a given absolute I magnitude or I - K color and also larger than measured radii of magnetically inactive M-dwarfs. The relative radius difference is correlated with magnitude, increasing from a few per cent at MI = 7 to greater than 50 per cent for the lowest luminosity stars in their sample at MI about 9.5. Jackson et al (2009) have suggested that a two-temperature star spot model is capable of explaining the observations, but their model requires spot coverage fractions of at least 50 per cent in rapidly rotating M-dwarfs. Here we examine these results in terms of stellar models that include the inhibiting effects of magnetic fields on convective energy transport, with and without the effects of star spots. We find that a pure spot model is inconsistent with the color - magnitude diagram. The observations of radii versus color and radii versus absolute magnitude in NGC 2516 are consistent with models which include only magnetic inhibition or a combination of magnetic inhibition and spots. At a given mass we find a large dispersion in the strength of the vertical component of the magnetic field in the stellar photosphere but the general trend is that the vertical field increases with decreasing mass from a few hundred Gauss at 0.65 Msun to 600 - 900 Gauss, depending on spot coverage, in the lowest mass stars in the sample at 0.25 Msun.

astro-ph.SR↗

arXiv2011

Precision modeling of M dwarf stars: the magnetic components of CM Draconis

The eclipsing binary CM Dra contains nearly identical red dwarfs of spectral class dM4.5. Their masses and radii have been reported with unprecedentedly small statistical errors. When compared with standard stellar models of appropriate age (\approx4 Gyr), the empirical results indicate that both components are larger in R, and lower in luminosity L, by several standard deviations. Here, we attempt at first to model the components in the context of standard stellar models using differ

astro-ph.SR↗

arXiv2004

Synthetic stellar populations: single stellar populations, stellar interior models and primordial proto-galaxies

We present a new set of stellar interior and synthesis models for predicting the integrated emission from stellar populations in star clusters and galaxies of arbitrary age and metallicity. This work differs from existing spectral synthesis codes in a number of important ways, namely (1) the incorporation of new stellar evolutionary tracks, with sufficient resolution in mass to sample rapid stages of stellar evolution; (2) a physically consistent treatment of evolution in the HR diagram, including the approach to the main sequence and the effects of mass loss on the giant and horizontal-branch phases. Unlike several existing models, ours yield consistent ages when used to date a coeval stellar population from a wide range of spectral features and colour indexes. We rigorously discuss degeneracies in the age-metallicity plane and show that inclusion of spectral features blueward of 4500 AA, suffices to break any remaining degeneracy and that with moderate S/N spectra (10 per 20AA, resolution element) age and metallicity are not degenerate. We also study sources of systematic errors in deriving the age of a single stellar population and conclude that they are not larger than 10-15%. We illustrate the use of single stellar populations by predicting the colors of primordial proto-galaxies and show that one can first find them and then deduce the form of the IMF for the early generation of stars in the universe. Finally, we provide accurate analytic fitting formulas for ultra fast computation of colors of single stellar populations. The models can be found at http://www.physics.upenn.edu/~raulj

astro-ph↗

arXiv2003

The cosmic production of Helium

We estimate the cosmic production rate of helium relative to metals ($ΔY/ΔZ$) using K dwarf stars in the Hipparcos catalog with accurate spectroscopic metallicities. The best fitting value is $ΔY/ΔZ=2.1 \pm 0.4$ at the 68% confidence level. Our derived value agrees with determinations from HII regions and with theoretical predictions from stellar yields with standard assumptions for the initial mass function. The amount of helium in stars determines how long they live and therefore how fast they will enrich the insterstellar medium with fresh material.

astro-ph↗