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Register a new userComplex Modulation of Rapidly Rotating Young M Dwarfs: Adding Pieces to the Puzzle
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Added by
Maximilian G\\\"unther
Publication date
2020
fields
Physics
and research's language is
English
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New sets of young M dwarfs with complex, sharp-peaked, and strictly periodic photometric modulations have recently been discovered with Kepler/K2 and TESS data. All of these targets are part of young star-forming associations. Suggested explanations range from accretion of dust disks to co-rotating clouds of material to stellar spots getting periodically occulted by spin-orbit-misaligned dust disks. Here we provide a comprehensive overview of all aspects of these hypotheses, and add more observational constraints in an effort to understand these objects with photometry from TESS and the SPECULOOS Southern Observatory (SSO). We scrutinize the hypotheses from three different angles: (1) we investigate the occurrence rates of these scenarios through existing young star catalogs; (2) we study the longevity of these features using over one year of combined photometry from TESS and SSO; and (3) we probe the expected color dependency with multi-color photometry from SSO. In this process, we also revisit the stellar parameters accounting for activity effects, study stellar flares as activity indicators over year-long time scales, and develop toy models to imitate typical morphologies. We identify which parts of the hypotheses hold true or are challenged by these new observations. So far, none of the hypotheses stand out as a definite answer, and each come with limitations. While the mystery of these complex rotators remains, we here add valuable observational pieces to the puzzle for all studies going forward.
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We have searched for short periodicities in the light curves of stars with $T_{rm eff}$ cooler than 4000 K made from 2-minute cadence data obtained in TESS sectors 1 and 2. Herein we report the discovery of 10 rapidly rotating M-dwarfs with highly structured rotational modulation patterns among 10 M dwarfs found to have rotation periods less than 1 day. Star-spot models cannot explain the highly structured periodic variations which typically exhibit between 10 and 40 Fourier harmonics. A similar set of objects was previously reported following K2 observations of the Upper Scorpius association (Stauffer et al. 2017). We examine the possibility that the unusual structured light-curves could stem from absorption by charged dust particles that are trapped in or near the stellar magnetosphere. We also briefly explore the possibilities that the sharp structured features in the lightcurves are produced by extinction by coronal gas, by beaming of the radiation emitted from the stellar surface, or by occultations of spots by a dusty ring that surrounds the star. The latter is perhaps the most promising of these scenarios. Most of the structured rotators display flaring activity, and we investigate changes in the modulation pattern following the largest flares. As part of this study, we also report the discovery of 371 rapidly rotating M-dwarfs with rotational periods below 4 hr, of which the shortest period is 1.63 hr.
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 0.556pm0.002, 0.193pm0.000 and 0.292pm0.001 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.
L dwarfs exhibit low-level, rotationally-modulated photometric variability generally associated with heterogeneous, cloud-covered atmospheres. The spectral character of these variations yields insight into the particle sizes and vertical structure of the clouds. Here we present the results of a high precision, ground-based, near-infrared, spectral monitoring study of two mid-type L dwarfs that have variability reported in the literature, 2MASS J08354256-0819237 and 2MASS J18212815+1414010, using the SpeX instrument on the Infrared Telescope Facility. By simultaneously observing a nearby reference star, we achieve <0.15% per-band sensitivity in relative brightness changes across the 0.9--2.4um bandwidth. We find that 2MASS J0835-0819 exhibits marginal (< ~0.5% per band) variability with no clear spectral dependence, while 2MASS J1821+1414 varies by up to +/-1.5% at 0.9 um, with the variability amplitude declining toward longer wavelengths. The latter result extends the variability trend observed in prior HST/WFC3 spectral monitoring of 2MASS J1821+1414, and we show that the full 0.9-2.4 um variability amplitude spectrum can be reproduced by Mie extinction from dust particles with a log-normal particle size distribution with a median radius of 0.24 um. We do not detect statistically significant phase variations with wavelength. The different variability behavior of 2MASS J0835-0819 and 2MASS J1821+1414 suggests dependencies on viewing angle and/or overall cloud content, underlying factors that can be examined through a broader survey.
We present a library of near-infrared (1.1-2.45 microns) medium-resolution (R~1500-2000) integral field spectra of 15 young M6-L0 dwarfs, composed of companions with known ages and of isolated objects. We use it to (re)derive the NIR spectral types, luminosities and physical parameters of the targets, and to test (BT-SETTL, DRIFT-PHOENIX) atmospheric models. We derive infrared spectral types L0+-1, L0+-1, M9.5+-0.5, M9.5+-0.5, M9.25+-0.25, M8+0.5-0.75, and M8.5+-0.5 for AB Pic b, Cha J110913-773444, USco CTIO 108B, GSC 08047-00232 B, DH Tau B, CT Cha b, and HR7329B, respectively. BT-SETTL and DRIFT-PHOENIX models yield close Teff and log g estimates for each sources. The models seem to evidence a 600-300+600 K drop of the effective temperature at the M-L transition. Assuming the former temperatures are correct, we derive new mass estimates which confirm that DH Tau B, USco CTIO 108B, AB Pic b, KPNO Tau 4, OTS 44, and Cha1109 lay inside or at the boundary of the planetary mass range. We combine the empirical luminosities of the M9.5-L0 sources to the Teff to derive semi-empirical radii estimates that do not match hot-start evolutionary models predictions at 1-3 Myr. We use complementary data to demonstrate that atmospheric models are able to reproduce the combined optical and infrared spectral energy distribution, together with the near-infrared spectra of these sources simultaneously. But the models still fail to represent the dominant features in the optical. This issue casts doubts on the ability of these models to predict correct effective temperatures from near-infrared spectra alone. We advocate the use of photometric and spectroscopic data covering a broad range of wavelengths to study the properties of very low mass young companions to be detected with the planet imagers (Subaru/SCExAO, LBT/LMIRCam, Gemini/GPI, VLT/SPHERE).
We image 104 newly identified low-mass (mostly M-dwarf) pre-main sequence members of nearby young moving groups with Magellan Adaptive Optics (MagAO) and identify 27 binaries with instantaneous projected separation as small as 40 mas. 15 were previously unknown. The total number of multiple systems in this sample including spectroscopic and visual binaries from the literature is 36, giving a raw multiplicity rate of at least $35^{+5}_{-4}%$ for this population. In the separation range of roughly 1 - 300 AU in which infrared AO imaging is most sensitive, the raw multiplicity rate is at least $24^{+5}_{-4}%$ for binaries resolved by the MagAO infrared camera (Clio). The M-star sub-sample of 87 stars yields a raw multiplicity of at least $30^{+5}_{-4}%$ over all separations, $21^{+5}_{-4}%$ for secondary companions resolved by Clio from 1 to 300 AU ($23^{+5}_{-4}%$ for all known binaries in this separation range). A combined analysis with binaries discovered by the Search for Associations Containing Young stars shows that multiplicity fraction as a function of mass and age over the range of 0.2 to 1.2 $M_odot$ and 10 - 200 Myr appears to be linearly flat in both parameters and across YMGs. This suggests that multiplicity rates are largely set by 100 Myr without appreciable evolution thereafter. After bias corrections are applied, the multiplicity fraction of low-mass YMG members ($< 0.6 M_odot$) is in excess of the field.
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