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

From solar-like to anti-solar differential rotation in cool stars

Stellar differential rotation can be separated into two main regimes: solar-like when the equator rotates faster than the poles and anti-solar when the polar regions rotate faster than the equator. We investigate the transition between these two regimes with 3-D numerical simulations of rotating spherical shells. We conduct a systematic parameter study which also includes models from different research groups. We find that the direction of the differential rotation is governed by the contribution of the Coriolis force in the force balance, independently of the model setup (presence of a magnetic field, thickness of the convective layer, density stratification). Rapidly-rotating cases with a small Rossby number yield solar-like differential rotation, while weakly-rotating models sustain anti-solar differential rotation. Close to the transition, the two kinds of differential rotation are two possible bistable states. This study provides theoretical support for the existence of anti-solar differential rotation in cool stars with large Rossby numbers.Comment: 5 pages, 6 figures, accepted for publication in MNRA

Detecting Planets Around Very Low Mass Stars with the Radial Velocity Method

The detection of planets around very low-mass stars with the radial velocity method is hampered by the fact that these stars are very faint at optical wavelengths where the most high-precision spectrometers operate. We investigate the precision that can be achieved in radial velocity measurements of low mass stars in the near infrared (nIR) Y-, J-, and H-bands, and we compare it to the precision achievable in the optical. For early-M stars, radial velocity measurements in the nIR offer no or only marginal advantage in comparison to optical measurements. Although they emit more flux in the nIR, the richness of spectral features in the optical outweighs the flux difference. We find that nIR measurement can be as precise than optical measurements in stars of spectral type ~M4, and from there the nIR gains in precision towards cooler objects. We studied potential calibration strategies in the nIR finding that a stable spectrograph with a ThAr calibration can offer enough wavelength stability for m/s precision. Furthermore, we simulate the wavelength-dependent influence of activity (cool spots) on radial velocity measurements from optical to nIR wavelengths. Our spot simulations reveal that the radial velocity jitter does not decrease as dramatically towards longer wavelengths as often thought. The jitter strongly depends on the details of the spots, i.e., on spot temperature and the spectral appearance of the spot. Forthcoming nIR spectrographs will allow the search for planets with a particular advantage in mid- and late-M stars. Activity will remain an issue, but simultaneous observations at optical and nIR wavelengths can provide strong constraints on spot properties in active stars.Comment: accepted by ApJ, v2 accepted revision with new precision calculations, abstract abride

What controls the large-scale magnetic fields of M dwarfs?

Observations of active M dwarfs show a broad variety of large-scale magnetic fields encompassing dipole-dominated and multipolar geometries. We detail the analogy between some anelastic dynamo simulations and spectropolarimetric observations of 23 M stars. In numerical models, the relative contribution of inertia and Coriolis force in the global force balance -estimated by the so-called local Rossby number- is known to have a strong impact on the magnetic field geometry. We discuss the relevance of this parameter in setting the large-scale magnetic field of M dwarfs.Comment: 4 pages, 3 figures, conference proceeding, IAUS 302 'Magnetic Fields Throughout the Stellar Evolution', (26-30 Aug 2013, Biarritz, France

Differential rotation in rapidly rotating F-stars

We obtained high quality spectra of 135 stars of spectral types F and later and derived ``overall'' broadening functions in selected wavelength regions utilizing a Least Squares Deconvolution (LSD) procedure. Precision values of the projected rotational velocity $v \sin{i}$ were derived from the first zero of the Fourier transformed profiles and the shapes of the profiles were analyzed for effects of differential rotation. The broadening profiles of 70 stars rotating faster than $v \sin{i} = 45$ km s$^{-1}$ show no indications of multiplicity nor of spottedness. In those profiles we used the ratio of the first two zeros of the Fourier transform $q_2/q_1$ to search for deviations from rigid rotation. In the vast majority the profiles were found to be consistent with rigid rotation. Five stars were found to have flat profiles probably due to cool polar caps, in three stars cuspy profiles were found. Two out of those three cases may be due to extremely rapid rotation seen pole on, only in one case ($v \sin{i} = 52$ km s$^{-1}$) solar-like differential rotation is the most plausible explanation for the observed profile. These results indicate that the strength of differential rotation diminishes in stars rotating as rapidly as v \sin{i} \ga 50 km s$^{-1}$.Comment: 10 pages, accepted for publication in A&

What controls the magnetic geometry of M dwarfs?

Context: observations of rapidly rotating M dwarfs show a broad variety of large-scale magnetic fields encompassing dipole-dominated and multipolar geometries. In dynamo models, the relative importance of inertia in the force balance -- quantified by the local Rossby number -- is known to have a strong impact on the magnetic field geometry. Aims: we aim to assess the relevance of the local Rossby number in controlling the large-scale magnetic field geometry of M dwarfs. Methods: we explore the similarities between anelastic dynamo models in spherical shells and observations of active M-dwarfs, focusing on field geometries derived from spectropolarimetric studies. To do so, we construct observation-based quantities aimed to reflect the diagnostic parameters employed in numerical models. Results: the transition between dipole-dominated and multipolar large-scale fields in early to mid M dwarfs is tentatively attributed to a Rossby number threshold. We interpret late M dwarfs magnetism to result from a dynamo bistability occurring at low Rossby number. By analogy with numerical models, we expect different amplitudes of differential rotation on the two dynamo branches.Comment: 4 pages, 4 figures, accepted for publication in A&

Multi-wavelength observations of Proxima Centauri

We report simultaneous observations of the nearby flare star Proxima Centauri with VLT/UVES and XMM-Newton over three nights in March 2009. Our optical and X-ray observations cover the star's quiescent state, as well as its flaring activity and allow us to probe the stellar atmospheric conditions from the photosphere into the chromosphere, and then the corona during its different activity stages. Using the X-ray data, we investigate variations in coronal densities and abundances and infer loop properties for an intermediate-sized flare. The optical data are used to investigate the magnetic field and its possible variability, to construct an emission line list for the chromosphere, and use certain emission lines to construct physical models of Proxima Centauri's chromosphere. We report the discovery of a weak optical forbidden Fe xiii line at 3388 AA during the more active states of Proxima Centauri. For the intermediate flare, we find two secondary flare events that may originate in neighbouring loops, and discuss the line asymmetries observed during this flare in H i, He i, and Ca ii lines. The high time-resolution in the H alpha line highlights strong temporal variations in the observed line asymmetries, which re-appear during a secondary flare event. We also present theoretical modelling with the stellar atmosphere code PHOENIX to construct flaring chromospheric models.Comment: 19 pages, 22 figures, accepted by A&

On a transition from solar-like coronae to rotation-dominated jovian-like magnetospheres in ultracool main-sequence stars

For main-sequence stars beyond spectral type M5 the characteristics of magnetic activity common to warmer solar-like stars change into the brown-dwarf domain: the surface magnetic field becomes more dipolar and the evolution of the field patterns slows, the photospheric plasma is increasingly neutral and decoupled from the magnetic field, chromospheric and coronal emissions weaken markedly, and the efficiency of rotational braking rapidly decreases. Yet, radio emission persists, and has been argued to be dominated by electron-cyclotron maser emission instead of the gyrosynchrotron emission from warmer stars. These properties may signal a transition in the stellar extended atmosphere. Stars warmer than about M5 have a solar-like corona and wind-sustained heliosphere in which the atmospheric activity is powered by convective motions that move the magnetic field. Stars cooler than early-L, in contrast, may have a jovian-like rotation-dominated magnetosphere powered by the star's rotation in a scaled-up analog of the magnetospheres of Jupiter and Saturn. A dimensional scaling relationship for rotation-dominated magnetospheres by Fan et al. (1982) is consistent with this hypothesis

Periodic Radio and H-alpha Emission from the L Dwarf Binary 2MASSW J0746425+200032: Exploring the Magnetic Field Topology and Radius of an L Dwarf

[Abridged] We present an 8.5-hour simultaneous radio, X-ray, UV, and optical observation of the L dwarf binary 2MASSW J0746+20. We detect strong radio emission, dominated by short-duration periodic pulses at 4.86 GHz with P=124.32+/-0.11 min. The stability of the pulse profiles and arrival times demonstrates that they are due to the rotational modulation of a B~1.7 kG magnetic field. A quiescent non-variable component is also detected, likely due to emission from a uniform large-scale field. The H-alpha emission exhibits identical periodicity, but unlike the radio pulses it varies sinusoidally and is offset by exactly 1/4 of a phase. The sinusoidal variations require chromospheric emission from a large-scale field structure, with the radio pulses likely emanating from the magnetic poles. While both light curves can be explained by a rotating mis-aligned magnetic field, the 1/4 phase lag rules out a symmetric dipole topology since it would result in a phase lag of 1/2 (poloidal field) or zero (toroidal field). We therefore conclude that either (i) the field is dominated by a quadrupole configuration, which can naturally explain the 1/4 phase lag; or (ii) the H-alpha and/or radio emission regions are not trivially aligned with the field. Regardless of the field topology, we use the measured period along with the known rotation velocity (vsini=27 km/s), and the binary orbital inclination (i=142 deg), to derive a radius for the primary star of 0.078+/-0.010 R_sun. This is the first measurement of the radius of an L dwarf, and along with a mass of 0.085+/-0.010 M_sun it provides a constraint on the mass-radius relation below 0.1 M_sun. We find that the radius is about 30% smaller than expected from theoretical models, even for an age of a few Gyr.Comment: Submitted to Ap

Geometry Technology Module (GTM). Volume 1: Engineering description and utilization manual

The geometry technology module (GTM) is described as a system of computerized elements residing in the engineering design integration system library developed for the generation, manipulation, display, computation of mass properties, and data base management of panelled geometry. The GTM is composed of computer programs and associated data for performing configuration analysis on geometric shapes. The program can be operated in batch or demand mode and is designed for interactive use