scholarly journals Magnetic fields of M dwarfs

2020 ◽  
Vol 29 (1) ◽  
Author(s):  
Oleg Kochukhov
Keyword(s):  
Magnetic Fields ◽  
Zeeman Effect ◽  
Mean Field ◽  
Field Measurements ◽  
Theoretical Models ◽  
Planetary Science ◽  
M Dwarfs ◽  
Stellar Magnetospheres ◽  
M Dwarf Stars ◽  
M Dwarf

AbstractMagnetic fields play a fundamental role for interior and atmospheric properties of M dwarfs and greatly influence terrestrial planets orbiting in the habitable zones of these low-mass stars. Determination of the strength and topology of magnetic fields, both on stellar surfaces and throughout the extended stellar magnetospheres, is a key ingredient for advancing stellar and planetary science. Here, modern methods of magnetic field measurements applied to M-dwarf stars are reviewed, with an emphasis on direct diagnostics based on interpretation of the Zeeman effect signatures in high-resolution intensity and polarisation spectra. Results of the mean field strength measurements derived from Zeeman broadening analyses as well as information on the global magnetic geometries inferred by applying tomographic mapping methods to spectropolarimetric observations are summarised and critically evaluated. The emerging understanding of the complex, multi-scale nature of M-dwarf magnetic fields is discussed in the context of theoretical models of hydromagnetic dynamos and stellar interior structure altered by magnetic fields.

scholarly journals Exploring the M-dwarf Luminosity–Temperature–Radius relationships using Gaia DR2

2019 ◽  
Vol 489 (2) ◽  
pp. 2615-2633 ◽  
Author(s):  
Sam Morrell ◽  
Tim Naylor
Keyword(s):  
Spectral Energy Distribution ◽  
Large Data ◽  
Theoretical Models ◽  
Spectral Energy ◽  
Data Sets ◽  
Distribution Fitting ◽  
Dwarf Stars ◽  
M Dwarf Stars ◽  
M Dwarf ◽  
Gaia Dr2

Abstract There is growing evidence that M-dwarf stars suffer radius inflation when compared to theoretical models, suggesting that models are missing some key physics required to completely describe stars at effective temperatures less than about 4000 K. The advent of Gaia DR2 distances finally makes available large data sets to determine the nature and extent of this effect. We employ an all-sky sample, comprising of >15 000 stars, to determine empirical relationships between luminosity, temperature, and radius. This is accomplished using only geometric distances and multiwave-band photometry, by utilizing a modified spectral energy distribution fitting method. The radii we measure show an inflation of $3\!-\!7{{\ \rm per\ cent}}$ compared to models, but no more than a $1\!-\!2{{\ \rm per\ cent}}$ intrinsic spread in the inflated sequence. We show that we are currently able to determine M-dwarf radii to an accuracy of $2.4{{\ \rm per\ cent}}$ using our method. However, we determine that this is limited by the precision of metallicity measurements, which contribute $1.7{{\ \rm per\ cent}}$ to the measured radius scatter. We also present evidence that stellar magnetism is currently unable to explain radius inflation in M-dwarfs.

scholarly journals ODUSSEAS: a machine learning tool to derive effective temperature and metallicity for M dwarf stars

2020 ◽  
Vol 636 ◽  
pp. A9 ◽  
Author(s):  
A. Antoniadis-Karnavas ◽  
S. G. Sousa ◽  
E. Delgado-Mena ◽  
N. C. Santos ◽  
G. D. C. Teixeira ◽  
...  
Keyword(s):  
Machine Learning ◽  
Signal To Noise Ratio ◽  
Computational Tool ◽  
M Dwarfs ◽  
Signal To Noise ◽  
Dwarf Stars ◽  
M Dwarf Stars ◽  
M Dwarf ◽  
Python Package ◽  
Machine Learning Tool

Aims. The derivation of spectroscopic parameters for M dwarf stars is very important in the fields of stellar and exoplanet characterization. The goal of this work is the creation of an automatic computational tool able to quickly and reliably derive the Teff and [Fe/H] of M dwarfs using optical spectra obtained by different spectrographs with different resolutions. Methods. ODUSSEAS (Observing Dwarfs Using Stellar Spectroscopic Energy-Absorption Shapes) is based on the measurement of the pseudo equivalent widths for more than 4000 stellar absorption lines and on the use of the machine learning Python package “scikit-learn” for predicting the stellar parameters. Results. We show that our tool is able to derive parameters accurately and with high precision, having precision errors of ~30 K for Teff and ~0.04 dex for [Fe/H]. The results are consistent for spectra with resolutions of between 48 000 and 115 000 and a signal-to-noise ratio above 20.

scholarly journals Complexity of magnetic fields on red dwarfs

2019 ◽  
Vol 629 ◽  
pp. A83 ◽  
Author(s):  
N. Afram ◽  
S. V. Berdyugina
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Circular Polarization ◽  
Large Fraction ◽  
The Sun ◽  
M Dwarfs ◽  
Magnetic Studies ◽  
Molecular Lines ◽  
Magnetic Features ◽  
M Dwarf

Context. Magnetic fields in cool stars can be investigated by measuring Zeeman line broadening and polarization in atomic and molecular lines. Similar to the Sun, these fields are complex and height-dependent. Many molecular lines dominating M-dwarf spectra (e.g., FeH, CaH, MgH, and TiO) are temperature- and Zeeman-sensitive and form at different atmospheric heights, which makes them excellent probes of magnetic fields on M dwarfs. Aims. Our goal is to analyze the complexity of magnetic fields in M dwarfs. We investigate how magnetic fields vary with the stellar temperature and how “surface” inhomogeneities are distributed in height – the dimension that is usually neglected in stellar magnetic studies. Methods. We have determined effective temperatures of the photosphere and of magnetic features, magnetic field strengths and filling factors for nine M dwarfs (M1–M7). Our χ2 analysis is based on a comparison of observed and synthetic intensity and circular polarization profiles. Stokes profiles were calculated by solving polarized radiative transfer equations. Results. Properties of magnetic structures depend on the analyzed atomic or molecular species and their formation heights. Two types of magnetic features similar to those on the Sun have been found: a cooler (starspots) and a hotter (network) one. The magnetic field strength in both starspots and network is within 3–6 kG, on average it is 5 kG. These fields occupy a large fraction of M dwarf atmospheres at all heights, up to 100%. The plasma β is less than one, implying highly magnetized stars. Conclusions. A combination of molecular and atomic species and a simultaneous analysis of intensity and circular polarization spectra have allowed us to better decipher the complexity of magnetic fields on M dwarfs, including their dependence on the atmospheric height. This work provides an opportunity to investigate a larger sample of M dwarfs and L-type brown dwarfs.

scholarly journals Photometric flaring fraction of M dwarf stars from the SkyMapper Southern Survey

2019 ◽  
Vol 491 (1) ◽  
pp. 39-50 ◽  
Author(s):  
Seo-Won Chang ◽  
Christian Wolf ◽  
Christopher A Onken
Keyword(s):  
Galactic Plane ◽  
Search Algorithm ◽  
M Dwarfs ◽  
Dwarf Stars ◽  
Vertical Distance ◽  
Steep Decline ◽  
Bona Fide ◽  
Single Epoch ◽  
M Dwarf Stars ◽  
M Dwarf

ABSTRACT We present our search for flares from M dwarf stars in the SkyMapper Southern Survey DR1, which covers nearly the full Southern hemisphere with six-filter sequences that are repeatedly observed in the passbands uvgriz. This allows us to identify bona fide flares in single-epoch observations on time-scales of less than four minutes. Using a correlation-based outlier search algorithm we find 254 flare events in the amplitude range of Δu ∼ 0.1 to 5 mag. In agreement with previous work, we observe the flaring fraction of M dwarfs to increase from ∼30 to ∼1000 per million stars for spectral types M0 to M5. We also confirm the decrease in flare fraction with larger vertical distance from the Galactic plane which is expected from declining stellar activity with age. Based on precise distances from Gaia DR2, we find a steep decline in the flare fraction from the plane to 150 pc vertical distance and a significant flattening towards larger distances. We then reassess the strong type dependence in the flaring fraction with a volume-limited sample within a distance of 50 pc from the Sun: in this sample the trend disappears and we find instead a constant fraction of ∼1 650 per million stars for spectral types M1 to M5. Finally, large-amplitude flares with Δi > 1 mag are very rare with a fraction of ∼0.5 per million M dwarfs. Hence, we expect that M-dwarf flares will not confuse SkyMapper’s search for kilonovae from gravitational-wave events. proper references for those databases (or follow their guideline on citation).

scholarly journals Precise mass and radius of a transiting super-Earth planet orbiting the M dwarf TOI-1235: a planet in the radius gap?

2020 ◽  
Vol 639 ◽  
pp. A132 ◽  
Author(s):  
P. Bluhm ◽  
R. Luque ◽  
N. Espinoza ◽  
E. Pallé ◽  
J. A. Caballero ◽  
...  
Keyword(s):  
Theoretical Models ◽  
Space Mission ◽  
High Resolution Imaging ◽  
M Dwarfs ◽  
Earth Planet ◽  
Exoplanetary Systems ◽  
Interesting Object ◽  
Resolution Imaging ◽  
Rocky Planets ◽  
M Dwarf

We report the confirmation of a transiting planet around the bright weakly active M0.5 V star TOI-1235 (TYC 4384–1735–1, V ≈ 11.5 mag), whose transit signal was detected in the photometric time series of sectors 14, 20, and 21 of the TESS space mission. We confirm the planetary nature of the transit signal, which has a period of 3.44 d, by using precise RV measurements with the CARMENES, HARPS-N, and iSHELL spectrographs, supplemented by high-resolution imaging and ground-based photometry. A comparison of the properties derived for TOI-1235 b with theoretical models reveals that the planet has a rocky composition, with a bulk density slightly higher than that of Earth. In particular, we measure a mass of Mp = 5.9 ± 0.6 M⊕ and a radius of Rp = 1.69 ± 0.08 R⊕, which together result in a density of ρp = 6.7− 1.1+ 1.3 g cm−3. When compared with other well-characterized exoplanetary systems, the particular combination of planetary radius and mass places our discovery in the radius gap, which is a transition region between rocky planets and planets with significant atmospheric envelopes. A few examples of planets occupying the radius gap are known to date. While the exact location of the radius gap for M dwarfs is still a matter of debate, our results constrain it to be located at around 1.7 R⊕ or larger at the insolation levels received by TOI-1235 b (~60 S⊕). This makes it an extremely interesting object for further studies of planet formation and atmospheric evolution.

scholarly journals CCD Parallaxes for Southern Very Low Luminosity Stars

1993 ◽  
Vol 156 ◽  
pp. 75-78
Author(s):  
Philip A. Ianna
Keyword(s):  
White Dwarfs ◽  
Late Type ◽  
M Dwarfs ◽  
Dwarf Stars ◽  
Ccd Observations ◽  
Trigonometric Parallaxes ◽  
M Dwarf Stars ◽  
M Dwarf

Trigonometric parallaxes based on CCD observations are presented here for six southern very late-type M dwarf stars and three white dwarfs. The M dwarfs RG0050-2722, ESO207-61, MH2115-4518, MH2124-4228, and LHS3003 are among the very lowest luminosity stars known.

scholarly journals Magnetic Fields in Low-Mass Stars: An Overview of Observational Biases

2013 ◽  
Vol 9 (S302) ◽  
pp. 156-163 ◽  
Author(s):  
Ansgar Reiners
Keyword(s):  
Magnetic Fields ◽  
Zeeman Effect ◽  
Field Measurements ◽  
Rotating Stars ◽  
Magnetic Field Generation ◽  
Low Mass Stars ◽  
Important Indicator ◽  
Stellar Radius ◽  
Low Mass ◽  
Rapidly Rotating Stars

AbstractStellar magnetic dynamos are driven by rotation, rapidly rotating stars produce stronger magnetic fields than slowly rotating stars do. The Zeeman effect is the most important indicator of magnetic fields, but Zeeman broadening must be disentangled from other broadening mechanisms, mainly rotation. The relations between rotation and magnetic field generation, between Doppler and Zeeman line broadening, and between rotation, stellar radius, and angular momentum evolution introduce several observational biases that affect our picture of stellar magnetism. In this overview, a few of these relations are explicitly shown, and the currently known distribution of field measurements is presented.

scholarly journals The CARMENES search for exoplanets around M dwarfs

2020 ◽  
Vol 638 ◽  
pp. A115
Author(s):  
D. Hintz ◽  
B. Fuhrmeister ◽  
S. Czesla ◽  
J. H. M. M. Schmitt ◽  
A. Schweitzer ◽  
...  
Keyword(s):  
Radiation Field ◽  
Extreme Ultraviolet ◽  
M Dwarfs ◽  
Dwarf Stars ◽  
Solar Type ◽  
Model Set ◽  
M Dwarf Stars ◽  
First Time ◽  
M Dwarf ◽  
Different Levels

The He I infrared (IR) line at a vacuum wavelength of 10 833 Å is a diagnostic for the investigation of atmospheres of stars and planets orbiting them. For the first time, we study the behavior of the He I IR line in a set of chromospheric models for M-dwarf stars, whose much denser chromospheres may favor collisions for the level population over photoionization and recombination, which are believed to be dominant in solar-type stars. For this purpose, we use published PHOENIX models for stars of spectral types M2 V and M3 V and also compute new series of models with different levels of activity following an ansatz developed for the case of the Sun. We perform a detailed analysis of the behavior of the He I IR line within these models. We evaluate the line in relation to other chromospheric lines and also the influence of the extreme ultraviolet (EUV) radiation field. The analysis of the He I IR line strengths as a function of the respective EUV radiation field strengths suggests that the mechanism of photoionization and recombination is necessary to form the line for inactive models, while collisions start to play a role in our most active models. Moreover, the published model set, which is optimized in the ranges of the Na I D2, Hα, and the bluest Ca II IR triplet line, gives an adequate prediction of the He I IR line for most stars of the stellar sample. Because especially the most inactive stars with weak He I IR lines are fit worst by our models, it seems that our assumption of a 100% filling factor of a single inactive component no longer holds for these stars.

scholarly journals Why do we find ourselves around a yellow star instead of a red star?

2017 ◽  
Vol 17 (1) ◽  
pp. 77-86 ◽  
Author(s):  
Jacob Haqq-Misra ◽  
Ravi Kumar Kopparapu ◽  
Eric T. Wolf
Keyword(s):  
Bayesian Inference ◽  
Habitable Zone ◽  
M Dwarfs ◽  
Dwarf Stars ◽  
Habitable Planets ◽  
Chance Event ◽  
The Future ◽  
M Dwarf Stars ◽  
M Dwarf

AbstractM-dwarf stars are more abundant than G-dwarf stars, so our position as observers on a planet orbiting a G-dwarf raises questions about the suitability of other stellar types for supporting life. If we consider ourselves as typical, in the anthropic sense that our environment is probably a typical one for conscious observers, then we are led to the conclusion that planets orbiting in the habitable zone of G-dwarf stars should be the best place for conscious life to develop. But such a conclusion neglects the possibility that K-dwarfs or M-dwarfs could provide more numerous sites for life to develop, both now and in the future. In this paper we analyse this problem through Bayesian inference to demonstrate that our occurrence around a G-dwarf might be a slight statistical anomaly, but only the sort of chance event that we expect to occur regularly. Even if M-dwarfs provide more numerous habitable planets today and in the future, we still expect mid G- to early K-dwarfs stars to be the most likely place for observers like ourselves. This suggests that observers with similar cognitive capabilities as us are most likely to be found at the present time and place, rather than in the future or around much smaller stars.

scholarly journals Differences in radio emission from similar M dwarfs in the binary system Ross 867-8

2020 ◽  
Vol 633 ◽  
pp. A130
Author(s):  
L. H. Quiroga-Nuñez ◽  
H. T. Intema ◽  
J. R. Callingham ◽  
J. Villadsen ◽  
H. J. van Langevelde ◽  
...  
Keyword(s):  
Binary System ◽  
Radio Emission ◽  
Stellar System ◽  
M Dwarfs ◽  
Dwarf Stars ◽  
X Ray ◽  
Peak Flux ◽  
Stellar Properties ◽  
M Dwarf Stars ◽  
M Dwarf

Serendipitously, we rediscovered radio emission from the binary system Ross 867 (M4.5V) and Ross 868 (M3.5V) while inspecting archival Giant Metrewave Radio Telescope (GMRT) observations. The binary system consists of two M-dwarf stars that share common characteristics such as spectral type, astrometric parameters, age, and emission at infrared, optical, and X-ray frequencies. The GMRT data at 610 MHz taken on July 2011 shows that the radio emission from Ross 867 is polarized and highly variable on hour timescales with a peak flux of 10.4 ± 0.7 mJy beam−1. Additionally, after reviewing archival data from several observatories (VLA, GMRT, JVLA, and LOFAR), we confirm that although the two stars are likely coeval, only Ross 867 was detected, while Ross 868 remains undetected at radio wavelengths. As the stars have a large orbital separation, this binary stellar system provides a coeval laboratory to examine and constrain the stellar properties linked to radio activity in M dwarfs. We speculate that the observed difference in radio activity between the dwarfs could be due to vastly different magnetic field topologies or that Ross 867 has an intrinsically different dynamo.

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