262,990 research outputs found
Observations of Cool-Star Magnetic Fields
No full textCool 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
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
A New Enantioselective Synthetic Approach to ß-Aminothio-compounds via Enantioselective Reduction of N,S-Heterocyclic Imines
No full textReiners I, Gröger H, Martens J. A New Enantioselective Synthetic Approach to ß-Aminothio-compounds via Enantioselective Reduction of N,S-Heterocyclic Imines. J. Prakt. Chem. 1997;339:541-546
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
Tradition oder Plagiat? : die ‚Stilkunst‘ von Ludwig Reiners und die ‚Stilkunst‘ von Eduard Engel im Vergleich
No full textZugl.: Bamberg, Univ., Diss., 2014Ist Ludwig Reiners’ Stilkunst ein Plagiat? Hat er aus Eduard Engels (1851-1938) Stilkunst abgeschrieben? Oder reiht sich Reiners (1896-1957) in eine lange Traditionslinie ein, die dadurch gekennzeichnet ist, dass die Autoren griffige Zitate und schmissige Beispiele von ihren Vorgängern übernehmen, ohne auf die Quelle zu verweisen?
Engels Stilkunst erschien erstmals 1911 – sie erlebte 31. Auflagen. 1933 erteilten die Nationalsozialisten dem Autor – er war jüdischer Herkunft – Publikationsverbot. Reiners war Mitglied der NSDAP. Die Schriftstellerei war sein Hobby; im Hauptberuf war er Kaufmann. Seine Stilkunst erschien 1944 und ist noch heute im Handel erhältlich.
In ihren Sprachratgebern stellen Wustmann, Engel, Reiners und Co. Regeln für einen guten Stil auf. Sie zeigen Unsicherheiten im Sprachgebrauch auf und präsentieren ihren Lesern überzeugend, aber meist ohne wissenschaftliche Argumente eine Lösung. Aufgrund ihres normativen Charakters werden Stilratgeber der Laienlinguistik zugeschrieben.
Das Thema ‚Plagiat‘ hat stets Konjunktur, wenn Personen des öffentlichen Interesses betroffen sind. Der Umgang mit Plagiaten wird kontrovers diskutiert. Stilratgeber konnten sich dieser Diskussion bisher weitgehend entziehen und ihre „Abschreibtradition“ ungehindert pflegen.
Vor diesem Hintergrund wird betrachtet, inwiefern Engel und Reiners ihren Vorgängern verhaftet sind. Zentral ist schließlich die Frage, ob Reiners diese sprachberatende Tradition fortführt oder er die Stilkunst seines Vorgängers Engel ausgeschlachtet hat. Reiners hat einem Spiegel-Artikel von 1956 zufolge über den Umgang mit seinen Quellen gesagt: „Das lasse ich hemmungslos abschreiben. Dafür besitze ich Kinder und Sekretärinnen.“ Bezeichnend ist dieser Ausspruch deshalb, weil in Reiners’ Stilkunst hunderte von Übereinstimmungen mit Engels Werk nachgewiesen werden können. Die Frage, wie viel die nächsten Generationen der Stillehren (z.B. von Wolf Schneider) davon übernommen haben, ist offen.Is Ludwig Reiners’s Stilkunst a case of plagiarism? Did he copy Eduard Engel’s (1851-1938) Stilkunst? Or is Reiners (1896-1957) part of a long tradition of writers characterized by simply adopting catchy quotations and snappy examples of their predecessors without citing their sources?
Originally released in 1911, Engel’s Stilkunst has been published in 31 editions since then. In 1933 the National Socialist regime banned him from publishing his works due to his Jewish background. Reiners was a member of the NSDAP. Writing was his hobby; his regular profession was being a merchant. His Stilkunst was published in 1944 and is still available commercially.
Wustmann, Engel, Reiners and Co. established stylistic rules in their usage guides. They point out linguistic uncertainties and offer their readers solutions in a persuasive way but without scholarly reasoning. Because of their normative character style guides are classified as works of lay linguistics.
The topic "plagiarism" is always popular whenever people of public interest are affected. How to deal with plagiarism is subject to controversial debate. Authors of style guides have been able to avoid this debate for the most part and maintain their "copy-and-paste tradition" unimpeded.
Against this background it has to be considered how much Engel and Reiners adhere to their predecessors. The central question is whether Reiners continues their tradition of language advice or simply exploits the Stilkunst of his predecessor Engel. According to an 1956 article in the magazine Spiegel, Reiners said the following about his sources: "I unashamedly allow copying to be made. This is what I have children and secretaries for." This quote is significant due to the fact that there are hundreds of proven matches between Reiners’s Stilkunst und Engel’s work. The question of how many of these the next generation of style guide authors have adopted, is still to be answered
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
On detectability of Zeeman broadening in optical spectra of F- and G-dwarfs
No full textWe investigate the detectability of Zeeman broadening in optical Stokes I spectra of slowly rotating sun-like stars. To this end, we apply the LTE spectral line inversion package SPINOR to very-high quality CES data and explore how fit quality depends on the average magnetic field, Bf. One-component (OC) and two-component (TC) models are adopted. In OC models, the entire surface is assumed to be magnetic. Under this assumption, we determine formal 3 sigma upper limits on the average magnetic field of 200G for the Sun, and 150 G for 61 Vir (G6V). Evidence for an average magnetic field of similar to 500 G is found for 59 Vir (G0V), and of similar to 1000G for HD68456 (F6V). A distinction between magnetic and non-magnetic regions is made in TC models, while assuming a homogeneous distribution of both components. In our TC inversions of 59 Vir, we investigate three cases: both components have equal temperatures; warm magnetic regions; cool magnetic regions. Our TC model with equal temperatures does not yield significant improvement over OC inversions for 59 Vir. The resulting Bf values are consistent for both. Fit quality is significantly improved, however, by using two components of different temperatures. The inversions for 59 Vir that assume different temperatures for the two components yield results consistent with 0-450 G at the formal 3 sigma confidence level. We thus find a model dependence of our analysis and demonstrate that the influence of an additional temperature component can dominate over the Zeeman broadening signature, at least in optical data. Previous comparable analyses that neglected effects due to multiple temperature components may be prone to the same ambiguities
On the magnetic topology of partially and fully convective stars
No full textWe compare the amount of magnetic flux measured in Stokes V
and Stokes I in a sample of early- and mid-M stars around the
boundary to full convection (~M 3.5). Early-M stars possess a
radiative core, mid-M stars are fully convective. While Stokes V is
sensitive to the net polarity of magnetic flux arising mainly from
large-scale configurations, Stokes I measurements can see the total
mean flux. We find that in early-M dwarfs, only ~6% of the
total magnetic flux is detected in Stokes V. This ratio is more than
twice as large, ~14%, in fully convective mid-M dwarfs. The
bulk of the magnetic flux on M-dwarfs is not seen in Stokes V. This
is presumably because magnetic flux is mainly stored in small scale
components. There is also more to learn about the effect of the
weak-field approximation on the accuracy of strong field detections.
In our limited sample, we see evidence for a change in magnetic
topology at the boundary to full convection. Fully convective stars
store a 2–3 times higher fraction of their flux in fields visible
to Stokes V. We estimate the total magnetic energy detected in
Stokes I and compare it to results from Stokes V. We find that in
early-M dwarfs only ~0.5% of the total magnetic energy is
detected in Stokes V while this fraction is ~2.5% in mid-M
dwarfs
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
THE ORIGIN OF ENHANCED ACTIVITY IN THE SUNS OF M67
No full textWe report the results of the analysis of high-resolution photospheric line spectra obtained with the UVES instrument on the VLT for a sample of 15 solar-type stars selected from a recent survey of the distribution of H and K chromospheric line strengths in the solar-age open cluster M67. We find upper limits to the projected rotation velocities that are consistent with solar-like rotation (i. e., v sin i less than or similar to 2-3 km s(-1)) for objects with Ca II chromospheric activity within the range of the contemporary solar cycle. Two solar-type stars in our sample exhibit chromospheric emission well in excess of even solar maximum values. In one case, Sanders 1452, we measure a minimum rotational velocity of v sin i = 4 +/- 0.5 km s(-1), or over twice the solar equatorial rotational velocity. The other star with enhanced activity, Sanders 747, is a spectroscopic binary. We conclude that high activity in solar-type stars in M67 that exceeds solar levels is likely due to more rapid rotation rather than an excursion in solar-like activity cycles to unusually high levels. We estimate an upper limit of 0.2% for the range of brightness changes occurring as a result of chromospheric activity in solar-type stars and, by inference, in the Sun itself. We discuss possible implications for our understanding of angular momentum evolution in solar-type stars, and we tentatively attribute the rapid rotation in Sanders 1452 to a reduced braking efficiency
- …
