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Theoretical models show the main sequence gap is a result of the mixing of 3He during the merger of envelope and core convection zones. Unlike stars the either side of the gap, stars in a narrow mass range will go through instability phases, where their dynamos could switch between the αΩ dynamo like the Sun and Ω2 dynamo like late M dwarfs. At the same time, they show radial pulsation and their fluxes fluctuate, which resemble the pulsations observed in evolved stars like red giants and asymptotic giant branch stars. Consequently, they are a unique type of dwarf like no other on the main sequence. In this work, we would like to know 1) will the unstable interior structures result in observable characteristics such as flaring and spots, and 2) what is the mass range for these stars observationally? Here we present our preliminary results: 1) stars in the gap have higher percentage rate of activities than their adjacent regions, and 2) high resolution speckle results yield promising close binaries to yield dynamical masses in the future.
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Comprising three out of every four stars, the M dwarfs form a unique sample that can host companions orbiting at Solar System scales and spanning a factor of 100,000 in mass. Targeting 120 M dwarf binaries within 25 parsecs, we are determining the period vs. eccentricity distribution for M dwarf stellar companions with orbital periods up to 6 years and semimajor axes up to 5 AU. This range is enabled by our combination of multiple observational methods: long-term astrometry from our RECONS program at the CTIO/SMARTS 0.9m is characterizing orbits on decades-long timescales, while our speckle interferometry survey at SOAR with HRCAM+SAM maps shorter orbits of systems identified from Gaia DR2, while also providing resolutions and masses for our long-period astrometric binaries. We will supplement these results with orbits from the literature, from both radial velocity and high-resolution imaging surveys, to ensure that our sample is rich with companions of all types orbiting within 5 AU. Initial results of this work so far suggest a notable paucity of M dwarf stellar companions with circular orbits greater than 5 years in period, showcasing the additional leverage provided by this combination of long-term astrometry and high-resolution imaging. Ultimately, when compared to the orbits of brown dwarf and planetary companions, such structures will be critical to understanding the formation mechanisms of these systems.
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Presentation #205.10 in the session Binary Stellar System - iPoster Session.
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We present an investigation into the rotation and stellar activity of four fully convective M dwarf “twin” wide binaries. Components in each pair have (1) astrometry confirming they are common-proper-motion binaries, (2) Gaia BP, RP, and 2MASS J, H, and K s magnitudes matching within 0.10 mag, and (3) presumably the same age and composition. We report long-term photometry, rotation periods, multiepoch Hα equivalent widths, X-ray luminosities, time series radial velocities, and speckle observations for all components. Although it might be expected for the twin components to have matching magnetic attributes, this is not the case. Decade-long photometry of GJ 1183 AB indicates consistently higher spot activity on A than B, a trend matched by A appearing 58% ± 9% stronger in L X and 26% ± 9% stronger in Hα on average—this is despite similar rotation periods of A = 0.86 day and B = 0.68 day, thereby informing the range in activity for otherwise identical and similarly rotating M dwarfs. The young β Pic Moving Group member 2MA 0201+0117 AB displays a consistently more active B component that is 3.6 ± 0.5 times stronger in L X and 52% ± 19% stronger in Hα on average, with distinct rotation at A = 6.01 days and B = 3.30 days. Finally, NLTT 44989 AB displays remarkable differences with implications for spindown evolution—B has sustained Hα emission while A shows absorption, and B is ≥39 ± 4 times stronger in L X, presumably stemming from the surprisingly different rotation periods of A = 38 days and B = 6.55 days. The last system, KX Com, has an unresolved radial velocity companion, and is therefore not a twin system.
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Presentation #305.15 in the session Stars, Cool Dwarfs, Brown Dwarfs — iPoster Session.
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The RECONS (REsearch Consortium On Nearby Stars, www.recons.org) team continues to explore the solar neighborhood by evaluating the nearest stars, both individually and as a population. Key points are becoming clear: we now know that 86% of all stars are K and M dwarfs, and we need to reach to 50 pc and 25 pc, respectively, to create samples of 5000 and 3000 primaries each. These two sizable samples allow us to understand the outcome of the star formation process across a factor of ten in mass as never before. Here we focus on one crucial area of research --- stellar companions --- with results of our surveys combining radial velocities, astrometry, high-resolution imaging, and trawls of catalogs and the literature. The surveys are carried out primarily at the CTIO/SMARTS 0.9m and 1.5m, the SOAR 4.1m, and both Gemini 8.1m telescopes. We reveal companions at separations from less than 1 AU to more than 1000 AU from the K and M dwarfs, with the key result that these stellar partners are found most often at separations similar to our Solar System. Thus, the star and planet formation processes work on the same spatial scales ... a fact that we must keep in mind as our solar neighborhood becomes enriched with planetary discoveries at distances comparable to where stellar companions are found. This work has been supported by NSF grants AST-0507711, AST-0908402, AST-1109445, AST-1411206, and AST-1715551, AST-1910130, and the SMARTS Consortium.
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