arXiv subjects

D. J. Mullan

Publications and source records attributed to D. J. Mullan.

At least 19 recordsLinked to original sources

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.

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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.

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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 ΔM, the integrated mass loss on the red giant branch (RGB), for fields stars show a trend of ΔM 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 ΔM 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.

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arXiv2021

Simultaneous Multiwavelength Flare Observations of EV Lacertae

We present the first results of our ongoing project conducting simultaneous multiwavelength observations of flares on nearby active M dwarfs. We acquired data of the nearby dM3.5e star EV Lac using 5 different observatories: NASA's Transiting Exoplanet Survey Satellite (TESS), NASA's Neil Gehrels Swift Observatory (\textit{Swift}), NASA's Neutron Interior Composition Explorer (NICER), the University of Hawaii 2.2-m telescope (UH88) and the Las Cumbres Observatory Global Telescope (LCOGT) Network. During the $\sim$25 days of TESS observations, we acquired three simultaneous UV/X-ray observations using \textit{Swift} that total $\sim$18 ks, 21 simultaneous epochs totaling $\sim$98 ks of X-ray data using NICER, one observation ($\sim$ 3 hours) with UH88, and one observation ($\sim$ 3 hours) with LCOGT. We identified 56 flares in the TESS light curve with estimated energies in the range log $E_{\rm T}$ (erg) = (30.5 - 33.2), nine flares in the \textit{Swift} UVM2 light curve with estimated energies in the range log $E_{UV}$ (erg) = (29.3 - 31.1), 14 flares in the NICER light curve with estimated minimum energies in the range log $E_{N}$ (erg) = (30.5 - 32.3), and 1 flare in the LCOGT light curve with log $E_{L}$ (erg) = 31.6. We find that the flare frequency distributions (FFDs) of TESS and NICER flares have comparable slopes, $β_{T}$ = -0.67$\pm$0.09 and $β_{N}$ = -0.65$\pm$0.19, and the FFD of UVOT flares has a shallower slope ($β_{U}$ = -0.38$\pm$0.13). Furthermore, we do not find conclusive evidence for either the first ionization potential (FIP) or the inverse FIP effect during coronal flares on EV Lac.

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arXiv2020

$K2$ Ultracool Dwarfs Survey. VI. White light superflares observed on an L5 dwarf and flare rates of L dwarfs

Kepler K2 long cadence data are used to study white light flares in a sample of 45 L dwarfs. We identified 11 flares on 9 L dwarfs with equivalent durations of (1.3 - 198) hr and total (UV/optical/IR) energies of $\geq$0.9 $\times$ 10$^{32}$ erg. Two superflares with energies of $>$10$^{33}$ erg were detected on an L5 dwarf: this is the coolest object so far on which flares have been identified. The larger superflare on this L5 dwarf has an energy of 4.6$\times$ 10$^{34}$ ergs and an amplitude of $>$300 times the photospheric level: so far, this is the largest amplitude flare detected by the $Kepler/K2$ mission. The next coolest star on which we identified a flare was an L2 dwarf: 2MASS J08585891+1804463. Combining the energies of all the flares which we have identified on 9 L dwarfs with the total observation time which was dedicated by $Kepler$ to all 45 L dwarfs, we construct a composite flare frequency distribution (FFD). The FFD slope is quite shallow (-0.51$\pm$0.17), consistent with earlier results reported by Paudel et al. (2018) for one particular L0 dwarf, for which the FFD slope was found to be -0.34. Using the composite FFD, we predict that, in early and mid-L dwarfs, a superflare of energy 10$^{33}$ erg occurs every 2.4 years and a superflare of energy 10$^{34}$ erg occurs every 7.9 years. Analysis of our L dwarf flares suggests that magnetic fields of $\geq$0.13-1.3 kG are present on the stellar surface: such fields could suppress Type II radio bursts.

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arXiv2020

A transition of dynamo modes in M dwarfs: narrowing down the spectral range where the transition occurs*

Houdebine et al (2017: H17) combined CaII data with projected rotational velocities (v sin i) to construct rotation-activity correlations (RAC) in K-M dwarfs. The RAC slopes were used to argue that a transition between dynamo modes occurs at a spectral type between M2 and M3. H17 suggested that the dynamo transition corresponds to a transition to complete convection (TTCC). An independent study of GAIA data led Jao et al (2018) to suggest that the TTCC sets in near M3.0V, close to the H17 result. However, the changes in a star which cause TTCC signatures in GAIA data might not lead to changes in CaII emission at an identical spectral type: the latter are also affected by magnetic effects which depend on certain properties of convection in the core. Here, we use CaII emission fluxes in a sample of ~600 M dwarfs, and attempt to narrow down the transition from one dynamo mode to another: rather than relying on RAC slopes, we quantify how the CaII emission flux varies with spectral type to identify steps where the flux decreases significantly across a narrow range of spectral types. We suggest that the dynamo mode transition may be narrowed down to between M2.1 and M2.3. This is close to, but earlier than, the TTCC location identified by Jao et al (2018). We suggest that the transition in dynamo mode may be related to the existence of a small convective core which occurs for a finite time interval in certain low mass stars.

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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.

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arXiv2019

The Mass-Activity relationships in M and K dwarfs. I. Stellar parameters of our sample of M and K dwarfs

Empirical correlations between stellar parameters such as rotation or radius and magnetic activity diagnostics require estimates of the effective temperatures and the stellar radii. The aim of this study is to propose simple methods that can be applied to large samples of stars in order to derive estimates of the stellar parameters. Good empirical correlations between Red/Infra-Red colours (e.g. (R-I)$_C$) and effective temperatures have been well established for a long time (e.g. Veeder 1974, Bessell 1979, Leggett 1992). The more recent (R-I)$_C$ colour-$T_{eff}$ correlation using the data of Mann et al. (2015: hereafter M15) and Boyajian et al. (2012: hereafter B12) shows that this colour can be applied as a temperature estimate for large samples of stars. We find that the mean scatter in $T_{eff}$ relative to the (R-I)$_C$-$T_{eff}$ relationship of B12 and M15 data is only $\pm 3σ=$44.6~K for K dwarfs and $\pm 3σ=$39.4~K for M dwarfs. These figures are small and show that the (R-I)$_C$ colour can be used as a first guess effective temperature estimator for K and M dwarfs. We derive effective temperatures for about 1910 K and M dwarfs using the calibration of (R-I)$_C$ colour-$T_{eff}$ from B12 and M15 data. We also compiled $T_{eff}$ and metallicity measurements available in the literature using the VizieR database. We determine $T_{eff}$ for 441 stars with previously unknown effective temperatures. We also identified 21 new spectroscopic binaries and 1 triple system from our high resolution spectra.

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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.

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arXiv2019

Origin of radio-quiet coronal mass ejections in flare stars

Type II radio bursts are observed in the Sun in association with many coronal mass ejections (CME's. In view of this association, there has been an expectation that, by scaling from solar flares to the flares which are observed on M dwarfs, radio emission analogous to solar Type II bursts should be detectable in association with M dwarf flares. However, several surveys have revealed that this expectation does not seem to be fulfilled. Here we hypothesize that the presence of larger global field strengths in low-mass stars, suggested by recent magneto-convective modeling, gives rise to such large Alfven speeds in the corona that it becomes difficult to satisfy the conditions for the generation of Type II radio bursts. As a result, CME's propagating in the corona/wind of a flare stars are expected to be "radio-quiet" as regards Type II bursts. In view of this, we suggest that, in the context of Type II bursts, scaling from solar to stellar flares is of limited effectiveness.

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arXiv2018

K2 Ultracool Dwarfs Survey. V. High superflare rates on rapidly rotating late-M dwarfs

We observed strong superflares (defined as flares with energy in excess of 10^33 erg) on three late-M dwarfs: 2MASS J08315742+2042213 (hereafter 2M0831+2042; M7 V), 2MASS J08371832+2050349 (hereafter 2M0837+2050; M8 V) and 2MASS J08312608+2244586 (hereafter 2M0831+2244; M9 V). 2M0831+2042 and 2M0837+2050 are members of the young (~700 Myr) open cluster Praesepe. The strong superflare on 2M0831+2042 has an equivalent duration (ED) of 13.7 hr and an estimated energy of 1.3 X 10^35 erg. We observed five superflares on 2M0837+2050, on which the strongest superflare has an ED of 46.4 hr and an estimated energy of 3.5 X 10^35 erg. This energy is larger by 2.7 orders of magnitude than the largest flare observed on the older (7.6 Gyr) planet-hosting M8 dwarf TRAPPIST-1. Furthermore, we also observed five superflares on 2M0831+2244 which is probably a field star. The estimated energy of the strongest superflare on 2M0831+2244 is 6.1 X 10^34 erg. 2M0831+2042, 2M0837+2050 and 2MASS J0831+2244 have rotation periods of 0.556\pm0.002, 0.193\pm0.000 and 0.292\pm0.001 d respectively, which are measured by using K2 light curves. We compare the flares of younger targets with those of TRAPPIST-1 and discuss the possible impacts of such flares on planets in the habitable zone of late-M dwarfs.

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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.

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arXiv2018

Photosynthesis on a planet orbiting an M dwarf: enhanced effectiveness during flares

On planets near M dwarfs, photosynthesis (PS) will occur with an effectiveness which depends on the supply of visible photons with wavelengths between 400 and 700 nm. In this paper, we quantify the effectiveness of PS in two contexts which are relevant for M dwarfs. First, using photons from an M dwarf in its quiescent non-flaring state, we find that PS on an M dwarf planet in the HZ of its parent star is less effective than on Earth by a factor of 10 for a flare star with mid-M spectral type. For a flare star with late-M spectral type, PS effectiveness is smaller than on Earth by a factor of 100 or more. Second, using photons which are incident on the HZ planet during flares, we find that PS effectiveness can increase by factors of 5-20 above the quiescent values. In the case of a flare star with mid-M spectral type, we find that the PS effectiveness during a flare can increase up to as much as 50-60 percent of the values on Earth. However, for a late-M flare star, even during flares, the PS effectiveness remains almost one order of magnitude smaller than on Earth. We suggest that for biological processes on M dwarf planets, the stellar activity cycle may replace the orbital period as the year.

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arXiv2018

K2 Ultracool Dwarfs Survey. IV. Monster flares observed on the young brown dwarf CFHT-BD-Tau 4

We present photometric measurements of two superflares observed on a very young brown dwarf CFHT-BD-Tau 4, observed during Campaign 13 of the \textit{Kepler K2} mission. The stronger of the two superflares brightened by a factor of $\sim$48 relative to the quiescent photospheric level, with an increase in \textit{Kepler} magnitude $Δ\tilde{K_{p}}$ = -4.20. It has an equivalent duration of $\sim$107 hour, a flare duration of 1.7 day, and an estimated total bolometric (ultraviolet/optical/infrared) energy up to 2.1 $\times$ 10$^{38}$ erg. The weaker of the two superflares is a complex (multipeaked) flare with an estimated total bolometric (UV/optical/IR) energy up to 4.7 $\times$ 10$^{36}$ erg. They are the strongest flares observed on any brown dwarf so far. The flare energies are strongly dependent on the value of visual extinction parameter $A_{V}$ used for extinction correction. If we apply a solar flare-model to interpret the two superflares, we find that the magnetic fields are required to be stronger by as much as an order of magnitude than previous reports of field measurements in CFHT-BD-Tau 4 by Reiners et al. (2009b). On the other hand, if we interpret our data in terms of accretion, we find that the requisite rate of accretion for the stronger superflare exceeds the rates which have been reported for other young brown dwarfs.

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arXiv2018

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 1σ. 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.

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arXiv2018

K2 Ultracool Dwarfs Survey. III. White Light Flares are Ubiquitous in M6-L0 Dwarfs

We report the white light flare rates for 10 ultracool dwarfs (UCDs) using \textit{Kepler K2} short cadence data. Among our sample stars, two have spectral type M6, three are M7, three are M8 and two are L0. Most of our targets are old low mass stars. We identify a total of 283 flares in all of the stars in our sample, with \textit{Kepler} energies in the range log \textit{E$_{Kp}$} $\sim$(29 - 33.5) erg. Using the maximum-likelihood method of line fitting, we find that the flare frequency distribution (FFD) for each star in our sample follows a power law with slope -$α$ in range -(1.3-2.0). We find that cooler objects tend to have shallower slopes. For some of our targets, the FFD follows either a broken power law, or a power law with an exponential cutoff. For the L0 dwarf 2MASS J12321827-0951502, we find a very shallow slope (-$α$ $=$ -1.3) in the \textit{Kepler} energy range (0.82-130)$\times$10$^{30}$ erg: this L0 dwarf has flare rates which are comparable to the rates of high energy flares in stars of earlier spectral types. In addition, we report photometry of two superflares: one on the L0 dwarf 2MASS J12321827-0951502 and another on the M7 dwarf 2MASS J08352366+1029318. In case of 2MASS J12321827-0951502, we report a flare brightening by a factor of $\sim$144 relative to the quiescent photospheric level. Likewise, for 2MASS J08352366+1029318, we report a flare brightening by a factor of $\sim$60 relative to the quiescent photospheric level. These two superflares have bolometric (UV/optical/infrared) energies 3.6 $\times$ 10$^{33}$ erg and 8.9 $\times$ 10$^{33}$ erg respectively, while the FWHM time scales are very short, $\sim$2 minutes. We find that the M8 star TRAPPIST-1 is more active than the M8.5 dwarf: 2M03264453+1919309, but less active than another M8 dwarf (2M12215066-0843197).

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arXiv2018

Frequencies of flare occurrence: Interaction Between convection and coronal loops

Observations of solar and stellar flares have revealed the presence of power law dependences between the flare energy and the time interval between flares. Various models have been proposed to explain these dependences, and to explain the numerical value of the power law indices. Here, we propose a model in which convective flows in granules force the foot-points of coronal magnetic loops, which are frozen-in to photospheric gas, to undergo a random walk. In certain conditions, this can lead to a twist in the loop, which drives the loop unstable if the twist exceeds a critical value. The possibility that a solar flare is caused by such a twist-induced instability in a loop has been in the literature for decades. Here, we quantify the process in an approximate way with a view to replicating the power-law index. We find that, for relatively small flares, the random walk twisting model leads to a rather steep power law slope which agrees very well with the index derived from a sample of 56,000+ solar X-ray flares reported by the GOES satellites. For relatively large flares, we find that the slope of the power law is shallower. The empirical power law slopes reported for flare stars also have a range which overlaps with the slopes obtained here. We suggest that in the coolest stars, a significant change in slope should occur when the frozen-flux assumption breaks down due to low electrical conductivity.

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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).

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