1,721,265 research outputs found
Von der Astrologie zur Astrophysik: Kopernikanische Wende und unser Verständnis des Universums
No full textObservations of Cool-Star Magnetic Fields
Full text linkCool stars like the Sun harbor convection zones capable of producing substantial surface magnetic fields leading to stellar magnetic activity. The influence of stellar parameters like rotation, radius, and age on cool-star magnetism, and the importance of the shear layer between a radiative core and the convective envelope for the generation of magnetic fields are keys for our understanding of low-mass stellar dynamos, the solar dynamo, and also for other large-scale and planetary dynamos. Our observational picture of cool-star magnetic fields has improved tremendously over the last years. Sophisticated methods were developed to search for the subtle effects of magnetism, which are difficult to detect particularly in cool stars. With an emphasis on the assumptions and capabilities of modern methods used to measure magnetism in cool stars, I review the different techniques available for magnetic field measurements. I collect the analyses on cool-star magnetic fields and try to compare results from different methods, and I review empirical evidence that led to our current picture of magnetic fields and their generation in cool stars and brown dwarfs.Deutsche Forschungsgemeinschaft [RE 1664/4-1, RE 1664/9-1
Activity-induced radial velocity jitter in a flaring M dwarf
Full text linkWe investigate the effect of stellar activity and flares on short-term radial velocity measurements in the mid-M flare star CN Leo. Radial velocity variations are calculated from 181 UVES
spectra obtained during three nights. We searched for spectral orders that contain very few atmospheric absorption lines and calibrated them against the telluric A-band from O2 in the
Earth's atmosphere. One giant flare occurred during our observations, which has a very strong effect on radial velocity. The apparent radial velocity shift due to the flare is several hundred m s-1 and clearly correlated with Hα emission. Outside the flare, only spectral orders containing the most prominent emission lines of H, He, and Ca show a correlation to chromospheric activity together with a radial velocity jitter exceeding a few 10 m s-1. We identify a number of spectral orders that are free of strong emission lines and show no flaring-related radial velocity jitter, although flares occurred as strong as 0.4 dex in normalized Hα luminosity. The mean radial velocity jitter due to moderate flaring is less than 10 m s-1. Strong flares are easily recognized directly in the spectra and should be neglected
for planet searches
Differential rotation in F stars
No full textDifferential rotation can be detected in single line profiles of stars rotating more rapidly than about v sin i = 10 km s(-1) with the Fourier transform technique. This allows to search for differential rotation in large samples to look for correlations between differential rotation and other stellar parameters. I analyze the fraction of differentially rotating stars as a function of color, rotation, and activity in a large sample of F-type stars. Color and rotation exhibit a correlation with differential rotation in the sense that more stars are rotating differentially in the cooler, less rapidly rotating stars. Effects of rotation and color, however, cannot be disentangled in the underlying sample. No trend with activity is found. (c) 2007 WHEY VCH Verlag GmbH & Co. KGaA, Weinheim
The narrowest M-dwarf line profiles and the rotation-activity connection at very slow rotation
No full textContext.The rotation-activity connection explains stellar activity in terms
of rotation and convective overturn time. It is well established
in stars of spectral types F–K as well as in M-type stars of
young clusters, in which rotation is still very rapid even among
M-dwarfs. The rotation-activity connection is not established in
field M-dwarfs, because they rotate very slowly, and detecting
rotation periods or rotational line broadening is a challenge. In
field M-dwarfs, saturation sets in below vrot =
5 km s-1, hence they are expected to populate the
non-saturated part of the rotation-activity connection.
Aims.This work for the first time shows intrinsically resolved spectral
lines of slowly rotating M-dwarfs and makes a first comparison to
estimates of convective velocities. By measuring rotation
velocities in a sample of mostly inactive M-dwarfs, the
unsaturated part of the rotation-activity connection is followed
into the regime of very low activity.
Methods.Spectra of ten M-dwarfs are taken at a resolving power of at the CES in the near infrared region where molecular
FeH has strong absorption bands. The intrinsically very narrow
lines are compared to model calculations of convective flows, and
rotational broadening is measured.
Results.In one star, an upper limit of = 1 km s-1 was
found, significant rotation was detected in the other nine
objects. All inactive stars show rotation below or equal to
2 km s-1. In the two active stars AD Leo and YZ CMi,
rotation velocities are found to be 40–50% below the results
from earlier studies.
Conclusions. The rotation activity connection holds in field early-M stars,
too. Activity and rotation velocities of the sample stars are
well in agreement with the relation found in earlier and younger
stars. The intrinsic absorption profiles of molecular FeH lines
are consistent with calculations from atomic Fe lines.
Investigation of FeH line profiles is a very promising tool to
measure convection patterns at the surfaces of M-stars
Ultra‐cool and extra‐vigorous: Rotation and activity in Mand L dwarfs
No full textThe study of rotation and activity in low-mass stars or brown dwarfs of spectral classes M and L has seen enormous progress during the last years. I summarize the results from different works that measured activity, rotation, and sometimes magnetic fields. The generation of magnetic activity seems to be unchanged at the threshold to completely convective stars, i.e. no change in the efficiency of the magnetic dynamos is observed. On the other hand, a sudden change in the strength of rotational braking appears at the threshold mass to full convection, and strong evidence exists for rotational braking weakening with lower mass. A probable explanation is that the field topology changes from dipolar to small scale structure as the objects become fully convective. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
A VOLUME-LIMITED SAMPLE OF 63 M7-M9.5 DWARFS. II. ACTIVITY, MAGNETISM, AND THE FADE OF THE ROTATION-DOMINATED DYNAMO
No full textIn a volume-limited sample of 63 ultracool dwarfs of spectral type M7-M9.5, we have obtained high-resolution spectroscopy with UVES at the Very Large Telescope and HIRES at Keck Observatory. In this second paper, we present projected rotation velocities, average magnetic field strengths, and chromospheric emission from the H alpha line. We confirm earlier results that the mean level of normalized H alpha luminosity decreases with lower temperature, and we find that the scatter among H alpha luminosities is larger at lower temperature. We measure average magnetic fields between 0 and 4 kG with no indication for a dependence on temperature between M7 and M9.5. For a given temperature, H alpha luminosity is related to magnetic field strength, consistent with results in earlier stars. A few very slowly rotating stars show very weak magnetic fields and H alpha emission, and all stars rotating faster than our detection limit show magnetic fields of at least a few hundred Gauss. In contrast to earlier-type stars, we observe magnetic fields weaker than 1 kG in stars rotating faster than similar to 3 km s(-1), but we find no correlation between rotation and magnetic flux generation among them. We interpret this as a fundamental change in the dynamo mechanism; in ultracool dwarfs, magnetic field generation is predominantly achieved by a turbulent dynamo, while other mechanisms can operate more efficiently at earlier spectral types.DFG [RE 1664/4-1]; NSF [AST06-06748
A VOLUME-LIMITED SAMPLE OF 63 M7-M9.5 DWARFS. I. SPACE MOTION, KINEMATIC AGE, AND LITHIUM
No full textIn a volume-limited sample of 63 ultracool dwarfs of spectral type M7-M9.5, we have obtained high-resolution spectroscopy with UVES at the Very Large Telescope and HIRES at Keck Observatory. In this first paper we introduce our volume-complete sample from DENIS and 2MASS targets, and we derive radial velocities and space motion. Kinematics of our sample are consistent with the stars being predominantly members of the young disk. The kinematic age of the sample is 3.1 Gyr. We find that six of our targets show strong Li lines implying that they are brown dwarfs younger than several hundred million years. Five of the young brown dwarfs were unrecognized before. Comparing the fraction of Li detections to later spectral types, we see a hint of an unexpected local maximum of this fraction at spectral type M9. It is not yet clear whether this maximum is due to insufficient statistics, or to a combination of physical effects including spectral appearance of young brown dwarfs, Li line formation, and the star formation rate at low masses.DFG [RE1664/4-1]; NSF [AST06-06748
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