D. J. Mullan

Coronal Heating by Magnetohydrodynamic Turbulence Driven by Reflected Low-Frequency Waves

A candidate mechanism for the heating of the solar corona in open field line regions is described. The interaction of Alfven waves, generated in the photosphere or chromosphere, with their reflections and the subsequent driving of quasi-two-dimensional MHD turbulence is considered. A nonlinear cascade drives fluctuations toward short wavelengths which are transverse to the mean field, thereby heating at rates insensitive to restrictive Alfven timescales. A phenomenology is presented, providing estimates of achievable heating efficiency that are most favorable.

Coronal Heating Distribution Due to Low-Frequency, Wave-driven Turbulence

The heating of the lower solar corona is examined using numerical simulations and theoretical models of magnetohydrodynamic turbulence in open magnetic regions. A turbulent energy cascade to small length scales perpendicular to the mean magnetic field can be sustained by driving with low-frequency Alfven waves reflected from mean density and magnetic field gradients. This mechanism deposits energy efficiently in the lower corona, and we show that the spatial distribution of the heating is determined by the mean density through the Alfven speed profile. This provides a robust heating mechanism which can explain observed high coronal temperatures and accounts for the significant heating (per unit volume) distribution below 2 solar radii needed in models of the origin of the solar wind. The obtained heating per unit mass, on the other hand, is much more extended, indicating that the heating on a per-particle basis persists throughout all the lower coronal region considered here. Subject headings: MHD — Sun: corona — turbulence

From Solar and Stellar Flares to Coronal Heating: Theory and Observations of How Magnetic Reconnection Regulates Coronal Conditions

There is currently no explanation of why the corona has the temperature and density it has. We present a model that explains how the dynamics of magnetic reconnection regulates the conditions in the corona. A bifurcation in magnetic reconnection at a critical state enforces an upper bound on the coronal temperature for a given density. We present observational evidence from 107 flares in 37 Sun-like stars that stellar coronae are near this critical state. The model may be important to self-organized criticality models of the solar corona.

MAGNETO-CONVECTION AND LITHIUM AGE ESTIMATES OF THE β PICTORIS MOVING GROUP

Although the means of the ages of stars in young groups determined from Li depletion often agree with mean ages determined from Hertzsprung-Russell (H-R) diagram isochrones, there are often statistically significant differences in the ages of individual stars determined by the two methods. We find that inclusion of the effects of inhibition of convection due to the presence of magnetic fields leads to consistent ages for the individual stars. We illustrate how age consistency arises by applying our results to the β Pictoris moving group (BPMG). We find that, although magnetic inhibition of convection leads to increased ages from the H-R diagram isochrones for all stars, Li ages are decreased for fully convective M stars and increased for stars with radiative cores. Our consistent age determination for BPMG of 40 Myr is larger than previous determinations by a factor of about two. We have also considered models in which the mixing length ratio is adjusted to give consistent ages. We find that our magneto-convection models, which give quantitative estimates of magnetic field strength, provide a viable alternative to models in which the effects of magnetic fields (and other processes) are accounted for by reducing the mixing length ratio.

Nonprimordial Deuterium in the Interstellar Medium

Contrary to a widespread assumption, deuterium is not simply destroyed in stars: deuterium is also synthesized in the atmospheres of active stars. This nonprimordial synthesis of D arises when protons accelerated in flares interact with the atmosphere, create a flux of free neutrons, and these neutrons then undergo radiative capture on atmospheric protons. Radiative capture does not result in excess production of Li, Be, or B. Ejection of flare-processed material contaminates the interstellar medium (ISM), as was originally suggested by Coleman & Worden. Estimates of the amount of flare-created D are subject to considerable uncertainties, but we find, using stellar parameters within permitted ranges, that flares may contribute significantly to the current ISM D content. Observational data indicate that different clouds of gas in the ISM exhibit variations in the value of D/H. We suggest that contamination of the ISM by D-enriched material ejected from stellar flares contributes to the observed D/H inhomogeneity. More precise estimates of the efficiency of D ejection from flares into the solar wind are required to evaluate this suggestion.

Evidence for a cool wind from the K2 dwarf in the detached binary V471 Tauri

Evidence for mass loss from the K2 dwarf in V471 Tauri is found in the form of discrete absorption features in lines of various elements (Mg, Fe, Cr, Mn) and ionization stages (Mg I, Mg II, Fe I, Fe II). Resonant Mg II absorption indicates a mass loss rate of at least 10 to the -11th solar masses per year. The wind appears to be cool (no more than a few times 10,000 K). 6 refs.

STRUCTURAL EFFECTS OF MAGNETIC FIELDS IN BROWN DWARFS

In the brown dwarf (BD) binary 2M0535 - 05, Stassun et al. have reported that the more massive primary has a lower T{sub eff} than the less massive secondary. Here, we report results obtained by an evolutionary code in which the criterion for the onset of convection in the primary is modified in the presence of a magnetic field. Structural alterations to the primary lead to a lower T{sub eff} and a larger radius than would occur in a non-magnetic BD of the same age mass and age. The observed value of T{sub eff} can be explained if the field in the primary increases in strength from 120-320 G at the surface to 5-13 MG at the center. With zero field in the secondary, our models indicate that both components can be co-eval with an age of 1.0-1.3 Myr. Because the binary is so young, the components have not yet had time to synchronize their rotations: differences in angular velocity may explain why one component has developed a field while the other has not.

SURFACE MAGNETIC FIELD STRENGTHS: NEW TESTS OF MAGNETOCONVECTIVE 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. In a recent paper, Torres has identified four 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 by several percent. These four systems potentially provide the most challenging tests of precision evolutionary models of cool dwarfs at the present time. 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. In the present paper, we apply our magnetoconvective approach to the four prime systems in the Torres list. Going a step beyond what we have already modeled in CM Dra (one of the four Torres systems), we note that new constraints on magnetoconvective models of M dwarfs are now available from empirical estimates of magnetic field strengths on the surfaces of these stars. In the present paper, we consider how well our magnetoconvective models succeed when confronted with this new test of surface magnetic field strengths. Among the systems listed by Torres, we find that plausible magnetic models work well for CM Dra, YY Gem, and CU Cnc. (The fourth system in Torress list does not yet have enough information to warrant magnetic modeling.) Our magnetoconvection models of CM Dra, YY Gem, and CU Cnc yield predictions of the magnetic fluxes on the stellar surface which are consistent with the observed correlation between magnetic flux and X-ray luminosity.

Winds from OB Stars: A Two-Component Scenario?

X-ray spectroscopy of several OB stars with massive winds has revealed that many X-ray line profiles exhibit unexpectedly small blueshifts and are almost symmetric. Moreover, the hottest X-ray lines appear to originate closest to the star. These properties appear to be inconsistent with the standard model of X-rays originating in shocked material in line-driven spherically symmetric winds. Here we raise the question, can the X-ray line data be understood in terms of a two-component wind? We consider a scenario in which one component of the wind is a standard line-driven wind that emerges from a broad range of latitudes centered on the equator. The second component of the wind emerges from magnetically active regions in extensive polar caps. The existence of such polar caps is suggested by a recent model of dynamo action in massive stars. We describe how the two-component model is consistent with a variety of observational properties of OB star winds.

Short-Period Magnetic Fluctuations in Advanced Composition Explorer Solar Wind Data: Evidence for Anticorrelation with Alfvén Speed

Short-period rms fluctuations δBrms in the interplanetary magnetic field have been derived from Advanced Composition Explorer/MAG data for 50 solar rotations between 1998 and 2001. We find that in many cases, individual maxima in δBrms are associated with local minima in the Alfven speed VAlf. To determine whether this anticorrelation persists in large data sets, we compute the correlation coefficient between the temporal gradients (in the spacecraft frame) δrms and Alf for each of the 50 solar rotations in the data set. We find that, in 48 of the 50 rotations, δrms is anticorrelated with Alf. The confidence level of the anticorrelation is greater than 90% in 35 rotations and is greater than 99.95% in 14 rotations. We find that in some low-VAlf regions of solar wind, there is a pronounced enhancement of non-Alfvenic (compressive) fluctuations. We suggest that refraction effects operating on MHD waves contribute to these aspects of the magnetic field fluctuation properties of the solar wind.