Year: 2020
Type: article
Abstract: M-dwarf stars below a certain mass are convective from their cores to their photospheres. These fully convective objects are extremely numerous, very magnetically active, and the likely hosts of many exoplanets. Here we study, for the first time, dynamo action in simulations of stratified, rotating, fully convective M-dwarf stars. Importantly, we use new techniques to capture the correct full ball geometry down to the center of the star. We find surprising dynamo states in these systems, with the global-scale mean fields confined strongly to a single hemisphere, in contrast to prior stellar dynamo solutions. These hemispheric-dynamo stars are likely to have profoundly different interactions with their surroundings, with important implications for exoplanet habitability and stellar spindown.
Authors: Benjamin P. Brown, Jeffrey S. Oishi, Geoffrey M. Vasil, Daniel Lecoanet, Keaton J. Burns
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: hybrid
Estimated APC paid: $2,400
Awards: THE M-TYPE STARS ARE BECOMING DOMINANT TARGETS IN SEARCHES FOR EARTH-LIKE PLANETS THAT COULD OCCUPY THEIR HABITABLE ZONES. THE LOW MASSES AND LUMINOSITIES OF M-DWARF CENTRAL STARS MAKE THEM VERY ATTRACTIVE FOR SUCH EXOPLANETARY HUNTS. THE HABITABLE ZONE OF M DWARFS IS CLOSE TO THE STAR DUE TO THEIR LOW LUMINOSITY. THUS POSSIBLY HABITABLE PLANETS WILL HAVE SHORT ORBITAL PERIODS MAKING THEIR DETECTION FEASIBLE BOTH WITH THE TRANSIT METHOD (USED BY KEPLER K2 AND SOON WITH TESS) AND WITH THE RADIAL VELOCITY APPROACHES. YET HABITABILITY ON A PLANET LIKELY REQUIRES BOTH SOLID SURFACES AND ATMOSPHERES BUT ALSO A FAVORABLE RADIATION ENVIRONMENT. IT IS HERE THAT THE M-DWARF CENTRAL STARS RAISE MAJOR THEORETICAL PUZZLES FOR MANY OF THEM EXHIBIT REMARKABLY INTENSE AND FREQUENT FLARING DESPITE THEIR MODEST INTRINSIC LUMINOSITIES. THE SUPER-FLARES RELEASE THEIR ENERGY BOTH IN WHITE LIGHT AND IN X-RAYS AND CAN BE THOUSANDS OF TIMES BRIGHTER THAN THE STRONGEST SOLAR FLARES. SUCH STRIKING EVENTS MUST HAVE MAGNETIC ORIGINS LIKELY FROM FIELDS BUILT BY CONVECTIVE DYNAMOS OPERATING IN THEIR INTERIORS. FURTHER RECENT OBSERVATIONS SUGGEST THAT THE SURFACE OF SOME M STARS IS CARPETED WITH MAGNETIC FIELDS OF 3 KG OR MORE. SUCH FIELD STRENGTHS ARE REMINISCENT OF A SUNSPOT BUT HERE INSTEAD COVER MUCH OF THE STELLAR SURFACE. WITH M STARS NOW TAKING CENTER STAGE IN THE SEARCH FOR EARTH-LIKE PLANETS IT IS CRUCIAL TO BEGIN TO UNDERSTAND HOW CONVECTIVE DYNAMOS MAY BE ABLE TO BUILD INTENSE MAGNETIC FIELDS INVOLVED WITH SUPER-FLARES AND VAST STAR SPOTS AND HOW THEY DEPEND UPON THE MASS AND ROTATION RATE OF THESE STARS. WE PROPOSE TO USE MAJOR 3-D MHD SIMULATIONS WITH OUR ANELASTIC SPHERICAL HARMONIC (ASH) CODE TO STUDY THE COUPLING OF TURBULENT CONVECTION ROTATION AND MAGNETISM WITHIN FULL SPHERICAL DOMAINS SUCH AS THE INTERIOR OF AN M DWARF. THIS PERMITS THE EXPLORATION OF THE MAGNETIC DYNAMOS THAT MUST BE RESPONSIBLE FOR THE EVOLVING MAGNETISM AND INTENSE ACTIVITY OF MANY M DWARFS. WE BRING TO THIS OUR PRIOR EXPERIENCE WITH STUDYING DYNAMO PROCESSES IN THE OUTER CONVECTIVE ENVELOPES OF G- (THE SUN) AND F-TYPE STARS BRIEFLY OF M DWARFS AND IN FULL CONVECTIVE CORES WITHIN MORE MASSIVE A- AND B-TYPE STARS. OUR PREVIOUS WORK SUGGESTS THAT M DWARFS COULD DISPLAY A BROAD RANGE OF DYNAMO BEHAVIOR FROM CYCLIC REVERSALS TO MORE CHAOTIC VARIATIONS AND FURTHER TO BOTH WEAK AND STRONG DYNAMO STATES. WE WILL FOCUS ON THE LATTER EXPLORING HOW SUPER-EQUIPARTITION MAGNETIC FIELDS COULD BE ACHIEVED BY DYNAMO ACTION IN M DWARFS AS ARE LIKELY NEEDED TO ENERGIZE SUPER-FLARES AND HUGE ACTIVE REGIONS AND WHAT LIMITS THE PEAK FIELD STRENGTHS. M-TYPE STARS ARE DISTINCTIVE IN BECOMING FULLY CONVECTIVE WITH DECREASING MASS AT ABOUT M3.5 IN SPECTRAL TYPE (OR ABOUT 0.35 SOLAR MASSES). AT THIS TRANSITION A STEEP RISE IN THE FRACTION OF MAGNETICALLY ACTIVE STARS IS OBSERVED THAT IS ACCOMPANIED BY AN INCREASING ROTATIONAL VELOCITY. CLEARLY HOW MASS-LOSS AND SPIN-DOWN CAN LEAD TO THIS IS OF INTEREST IN ITSELF. HOWEVER HERE WE PROPOSE TO STUDY THE MANNER IN WHICH DYNAMOS OPERATING IN FULLY CONVECTIVE M DWARF INTERIORS BEYOND THE TRANSITION MAY BE ABLE TO ACHIEVE VERY STRONG MAGNETIC FIELDS AND HOW FIELD STRENGTHS AND APPARENT MAGNETIC ACTIVITY INCREASES WITH ROTATION RATE AS SUGGESTED BY OBSERVATIONS. WE BELIEVE THAT GLOBAL CONNECTIVITY OF FLOWS AND FIELDS ACROSS THE CORE CENTER WILL ADMIT NEW CLASSES OF STRONG BEHAVIOR AS REVEALED BY OUR B STAR CORE DYNAMOS NOT REALIZED WHEN A CONVECTIVE ENVELOPE IS BOUNDED BELOW BY A TACHOCLINE. THESE IDEAS NEED TO BE TESTED IN A SELFCONSISTENT MANNER WITH GLOBAL ASH SIMULATIONS TO GAIN THEORETICAL INSIGHTS INTO WHAT IS THE ORIGIN OF THE FIERCE MAGNETIC ACTIVITY IN SOME OF M DWARFS THAT MAY BE POTENTIAL HOSTS TO EARTH-LIKE PLANETS. SUCH 3-D MHD SIMULATIONS THOUGH CHALLENGING ARE NOW FEASIBLE AND WOULD COMPLEMENT THE INTENSIVE OBSERVATIONAL SEARCHES UNDER WAY., MAGNETISM IS A UBIQUITOUS FEATURE OF STARS LIKE OUR SUN. STARS ON THE LOWER MAIN SEQUENCE GENERATE THEIR MAGNETIC FIELDS BY DYNAMO ACTION IN THEIR SU, G3059513448, G422218593
OpenAlex ID: W3047146013