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

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

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

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&

Biogeochemical processes in sagebrush steppe: Interactions of terrain, vegetation and chemical cycles

Publications, manuscripts in various stages of progress, presentations made at scientific meetings, and undergraduate honor thesis and one Ph.D. dissertation are contained

Modelling the molecular Zeeman effect in M-dwarfs: methods and first results

We present first quantitative results of the surface magnetic field measurements in selected M-dwarfs based on detailed spectra synthesis conducted simultaneously in atomic and molecular lines of the FeH Wing-Ford $F^4\,\Delta-X^4\,\Delta$ transitions. A modified version of the Molecular Zeeman Library (MZL) was used to compute Land\'e g-factors for FeH lines in different Hund's cases. Magnetic spectra synthesis was performed with the Synmast code. We show that the implementation of different Hund's case for FeH states depending on their quantum numbers allows us to achieve a good fit to the majority of lines in a sunspot spectrum in an automatic regime. Strong magnetic fields are confirmed via the modelling of atomic and FeH lines for three M-dwarfs YZ~CMi, EV~Lac, and AD~Leo, but their mean intensities are found to be systematically lower than previously reported. A much weaker field ($1.7-2$~kG against $2.7$~kG) is required to fit FeH lines in the spectra of GJ~1224. Our method allows us to measure average magnetic fields in very low-mass stars from polarized radiative transfer. The obtained results indicate that the fields reported in earlier works were probably overestimated by about $15-30$\%. Higher quality observations are needed for more definite results.Comment: Accepted by A&A, 13 pages, 7 figures, 1 tabl

Discovery of a nearby young brown dwarf binary candidate

In near-infrared NaCo observations of the young brown dwarf 2MASS J0041353-562112, we discovered a companion a little less than a magnitude fainter than the primary. The binary candidate has a separation of 143 mas, the spectral types are M6.5 and M9.0 for the two components. Colors and flux ratios are consistent with the components being located at the same distance minimizing the probability of the secondary being a background object. The brown dwarf is known to show Li absorption constraining the age to less than ~200 Myr, and it was suspected to show ongoing accretion, indicating an age as low as ~10 Myr. We estimate distance and orbital parameters of the binary as a function of age. For an age of 10 Myr, the distance to the system is 50 pc, the orbital period is 126 yr, and the masses of the components are ~30 and ~15 MJup. The binary brown dwarf fills a so far unoccupied region in the parameters mass and age; it is a valuable new benchmark object for brown dwarf atmospheric and evolutionary models.Comment: 4 pages, 2 figures, accepted by A&

Activity-induced radial velocity jitter in a flaring M dwarf

We 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 O$_2$ 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 H$\alpha$ 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 H$\alpha$ 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.Comment: accepted by A&

The First Direct Measurements of Magnetic Fields on Very Low-Mass Stars

We present the first direct magnetic field measurements on M dwarfs cooler than spectral class M4.5. Utilizing a new method based on the effects of a field on the FeH band near 1 micron, we obtain information on whether the integrated surface magnetic flux (Bf) is low (well under 1 kilogauss), intermediate (between 1 and about 2.5 kG), or strong (greater than about 3 kG) on a set of stars ranging from M2 down to M9. We also measure the rotational broadening (vsini) and Halpha emission for more than 20 stars. Our goal is to advance the understanding of how dynamo field production varies with stellar parameters for very low-mass stars, how the field and emission activity are related, and whether there is a connection between the rotation and magnetic flux. We find that fields are produced throughout the M-dwarfs. Among the early M stars we have too few targets to yield conclusive results. In the mid-M stars, there is a clear connection between slow rotation and weak fields. In the late-M stars, rotation is always measureable, and the strongest fields go with the most rapid rotators. These very cool rapid rotators have the largest magnetic flux in the whole sample. Halpha emission is found to be a good general proxy for magnetic fields. The drop-off in fractional emission near the bottom of the main sequence is not accompanied by a drop-off in magnetic flux, lending credence to the hypothesis that it is due to atmospheric coupling to the field rather than changes in the field itself. It is clear that the methodology we have developed can be further applied to discover more about the behavior of magnetic dynamos and magnetic activity in cool and fully convective objects.Comment: 33 pages, accepted by ApJ, abstract abbreviated for astro-p

3D simulations of M star atmosphere velocities and their influence on molecular FeH lines

We present an investigation of the velocity fields in early to late M-type star hydrodynamic models, and we simulate their influence on FeH molecular line shapes. The M star model parameters range between log g of 3.0 - 5.0 and Teff of 2500 K and 4000 K. Our aim is to characterize the Teff- and log g -dependence of the velocity fields and express them in terms of micro- and macro-turbulent velocities in the one dimensional sense. We present also a direct comparison between 3D hydrodynamical velocity fields and 1D turbulent velocities. The velocity fields strongly affect the line shapes of FeH, and it is our goal to give a rough estimate for the log g and Teff parameter range in which 3D spectral synthesis is necessary and where 1D synthesis suffices. In order to calculate M-star structure models we employ the 3D radiative-hydrodynamics (RHD) code CO5BOLD. The spectral synthesis on these models is performed with the line synthesis code LINFOR3D. We describe the 3D velocity fields in terms of a Gaussian standard deviation and project them onto the line of sight to include geometrical and limb-darkening effects. The micro- and macro-turbulent velocities are determined with the "Curve of Growth" method and convolution with a Gaussian velocity profile, respectively. To characterize the log g and Teff dependence of FeH lines, the equivalent width, line width, and line depth are regarded. The velocity fields in M-stars strongly depend on log g and Teff. They become stronger with decreasing log g and increasing Teff.Comment: 14 pages, 17 figures, 3 tables, accepted by Astronomy & Astrophysic