Differential Rotation in F Stars

Differential 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.Comment: 4 pages, overview talk at the 5th Potsdam thinkshop, accepted by A

Strong latitudinal shear in the shallow convection zone of a rapidly rotating A-star

We have derived the mean broadening profile of the star V102 in the region of the open cluster IC4665 from high resolution spectroscopy. At a projected equatorial rotation velocity of vsini = (105 +- 12)km/s we find strong deviation from classical rotation. We discuss several scenarios, the most plausible being strong differential rotation in latitudinal direction. For this scenario we find a difference in angular velocity of DeltaOmega = 3.6 +- 0.8 rad/d (DeltaOmega/Omega = 0.42 +- 0.09). From the Halpha line we derive a spectral type of A9 and support photometric measurements classifying IC4665 V102 as a non-member of IC4665. At such early spectral type this is the strongest case of differential rotation observed so far. Together with three similar stars, IC4665 V102 seems to form a new class of objects that exhibit extreme latitudinal shear in a very shallow convective envelope.Comment: accepted for A&A Letter

A Volume-limited Sample of 63 M7-M9.5 Dwarfs II. Activity, magnetism, and the fade of the rotation-dominated dynamo

In 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 Halpha line. We confirm earlier results that the mean level of normalized Halpha luminosity decreases with lower temperature, and we find that the scatter among Halpha luminosities is larger at lower temperature. We measure average magnetic fields between 0 and 4kG with no indication for a dependence on temperature between M7 and M9.5. For a given temperature, Halpha 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 Halpha emission, 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 1kG in stars rotating faster than ~3km/s, 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 type.Comment: accepted by Ap

Using HERON at the University of Lincoln – a pilot study

Report of a pilot using HERON digitisation and copyright-clearance services at the University of Lincoln, to provide 100 second-year Media Production undergraduates with a selection of digitised reading material

The effects of inclination, gravity darkening and differential rotation on absorption profiles of fast rotators

Mechanisms influencing absorption line profiles of fast rotating stars can be sorted into two groups; (i) intrinsic variations sensitive to temperature and pressure, and (ii) global effects common to all spectral lines. I present a detailed study on the latter effects focusing on gravity darkening and inclination for various rotational velocities and spectral types. It is shown that the line shapes of rapidly and rigidly rotating stars mainly depend on the equatorial velocity $v_{\rm e}$, not on the projected rotational velocity $v \sin{i}$ which determines the lines width. The influence of gravity darkening and spectral type on the line profiles is shown. The results demonstrate the possibility of determining the inclination angle $i$ of single fast rotators, and they show that constraints on gravity darkening can be drawn for stellar samples. While significant line profile deformation occurs in stars rotating as fast as v_{\rm e} \ga 200 km s$^{-1}$, for slower rotators profile distortion are marginal. In these cases spectral signatures induced by, e.g., differential rotation are not affected by gravity darkening and the methods applicable to slow rotators can be applied to these faster rotators, too.Comment: 7 pages, accepted by A&

Observations of Cool-Star Magnetic Fields

Cool 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.Comment: Published version at http://www.livingreviews.org/lrsp-2012-

Rotation- and temperature-dependence of stellar latitudinal differential rotation

More than 600 high resolution spectra of stars with spectral type F and later were obtained in order to search for signatures of differential rotation in line profiles. In 147 stars, the rotation law could be measured, 28 of them are found to be differentially rotating. Comparison to rotation laws in stars of spectral type A reveals that differential rotation sets in at the convection boundary in the HR-diagram; no star that is significantly hotter than the convection boundary exhibits the signatures of differential rotation. Four late A-/early F-type stars close to the convection boundary and at vsini~100 km/s show extraordinarily strong absolute shear at short rotation periods around one day. It is suggested that this is due to their small convection zone depth and that it is connected to a narrow range in surface velocity. Detection frequencies of differential rotation were analyzed in stars with varying temperature and rotation velocity. Measurable differential rotation is more frequent in late-type stars and slow rotators. The strength of absolute shear and differential rotation are examined as functions of the stellar effective temperature and rotation period. The strongest shear is found at rotation periods between two and three days. In slower rotators, the strongest shear at a given rotation rate is given approximately by DOmega_max ~ P^{-1}. In faster rotators, alpha_max and DOmega_max diminish less rapidly. A comparison with differential rotation measurements in stars of later spectral type shows that F-stars exhibit stronger shear than cooler stars do, the upper boundary in absolute shear DOmega with temperature is consistent with the temperature scaling law found in Doppler Imaging measurements.Comment: 15 pages, accepted for publication in A&A, typos correcte

Spectral type dependent rotational braking and strong magnetic flux in three components of the late-M multiple system LHS 1070

We show individual high resolution spectra of components A, B, and C of the nearby late-M type multiple system LHS 1070. Component A is a mid-M star, B and C are known to have masses at the threshold to brown dwarfs. From our spectra we measure rotation velocities and the mean magnetic field for all three components individually. We find magnetic flux on the order of several kilo-Gauss in all components. The rotation velocities of the two late-M objects B and C are similar (vsini = 16km/s), the earlier A component is spinning only at about half that rate. This suggests weakening of net rotational braking at late-M spectral type, and that the lack of slowly rotating late-M and L dwarfs is real. Furthermore, we found that magnetic flux in the B component is about twice as strong as in component C at similar rotation rate. This indicates that rotational braking is not proportional to magnetic field strength in fully convective objects, and that a different field topology is the reason for the weak braking in low mass objects.Comment: accepted for publication as A&A Lette