Extracting scaling laws from numerical dynamo models

Earth's magnetic field is generated by processes in the electrically conducting, liquid outer core, subsumed under the term `geodynamo'. In the last decades, great effort has been put into the numerical simulation of core dynamics following from the magnetohydrodynamic (MHD) equations. However, the numerical simulations are far from Earth's core in terms of several control parameters. Different scaling analyses found simple scaling laws for quantities like heat transport, flow velocity, magnetic field strength and magnetic dissipation time. We use an extensive dataset of 116 numerical dynamo models compiled by Christensen and co-workers to analyse these scalings from a rigorous model selection point of view. Our method of choice is leave-one-out cross-validation which rates models according to their predictive abilities. In contrast to earlier results, we find that diffusive processes are not negligible for the flow velocity and magnetic field strength in the numerical dynamos. Also the scaling of the magnetic dissipation time turns out to be more complex than previously suggested. Assuming that the processes relevant in the numerical models are the same as in Earth's core, we use this scaling to estimate an Ohmic dissipation of 3-8 TW for the core. This appears to be consistent with recent high CMB heat flux scenarios.Comment: 21 pages, 11 figure

Effects Of Kaluza-Klein Excited W On Single Top Quark Production At Tevatron

In extra dimension theories if the gauge bosons of the standard model propagate in the bulk of the extra dimensions then they will have Kaluza-Klein excitations that can couple to the standard model fermions. In this paper we study the effects of the first excited Kaluza-Klein mode of the W on single top production at the Tevatron. We find that the cross section for the single top production can be significantly reduced if the mass of the first Kaluza-Klein excited $W \sim 1$ TeV. Hence, a measurement of the single top production cross section smaller than the standard model prediction would not necessarily imply $V_{tb} <1$ or evidence of extra generation(s) of fermions mixed with the third generation.Comment: Text added, Latex, 16 pages, 3 figures, To appear in Phys. Lett.

Accretion, disks, and magnetic activity in the TW Hya association

We present new photometric and spectroscopic data for the M-type members of the TW Hya association with the aim of a comprehensive study of accretion, disks and magnetic activity at the critical age of ~10 Myr where circumstellar matter disappears.Comment: 4 pages, to appear in Proceedings IAU Symposium No. 314, Young Stars & Planets Near the Sun, 201

The enigmatic young brown dwarf binary FU Tau: accretion and activity

FU Tau belongs to a rare class of young, wide brown dwarf binaries. We have resolved the system in a Chandra X-ray observation and detected only the primary, FU Tau A. Hard X-ray emission, presumably from a corona, is present but, unexpectedly, we detect also a strong and unusually soft component from FU Tau A. Its X-ray properties, so far unique among brown dwarfs, are very similar to those of the T Tauri star TW Hya. The analogy with TW Hya suggests that the dominating soft X-ray component can be explained by emission from accretion shocks. However, the typical free-fall velocities of a brown dwarf are too low for an interpretation of the observed X-ray temperature as post-shock region. On the other hand, velocities in excess of the free-fall speed are derived from archival optical spectroscopy, and independent pieces of evidence for strong accretion in FU Tau A are found in optical photometry. The high X-ray luminosity of FU Tau A coincides with a high bolometric luminosity confirming an unexplained trend among young brown dwarfs. In fact, FU Tau A is overluminous with respect to evolutionary models while FU Tau B is on the 1 Myr isochrone suggesting non-contemporaneous formation of the two components in the binary. The extreme youth of FU Tau A could be responsible for its peculiar X-ray properties, in terms of atypical magnetic activity or accretion. Alternatively, rotation and magnetic field effects may reduce the efficiency of convection which in turn affects the effective temperature and radius of FU Tau A shifting its position in the HR diagram. Although there is no direct prove of this latter scenario so far we present arguments for its plausibility.Comment: Accepted for publication in MNRAS; 9 pages, 5 figure

X-Shooter spectroscopy of FU Tau A

We have analyzed a broad-band optical and near-infrared spectrum of FU Tau A, a presumed young brown dwarf in the Taurus star forming region that has intrigued both theorists and observers by its over-luminosity in the HR diagram with respect to standard pre-main sequence evolutionary models. The new data, obtained with the X-Shooter spectrograph at the Very Large Telescope, include an unprecedented wealth of information on stellar parameters and simultaneously observed accretion and outflow indicators for FU Tau A. We present the first measurements of gravity (log g = 3.5 +- 0.5), radial velocity (RV = 22.5 +- 2.9 km/s), rotational velocity (v sin(i) = 20 +- 5 km/s) and lithium equivalent width (W_Li = 430 +- 20 mAA) for FUTau A. From the rotational velocity and the published period we infer a disk inclination of i ~ 50^deg. The lithium content is much lower than theoretically expected for such a young very low mass object, adding another puzzling feature to this object's properties. We determine the mass accretion rate of FU Tau A from comparison of the luminosities of 24 emission lines to empirical calibrations from the literature and find a mean of log (dM/dt)_acc [M_sun/yr] = -9.9 +- 0.2. The accretion rate determined independently from modeling of the excess emission in the Balmer and Paschen continua is consistent with this value. The corresponding accretion luminosity is too small to make a significant contribution to the bolometric luminosity. The existence of an outflow in FU Tau A is demonstrated through the first detection of forbidden emission lines from which we obtain an estimate for the mass loss rate, log (dM/dt)_out [M_sun/yr] < -10.4. The mass outflow and inflow rates can be combined to yield (dM/dt)_out / (dM/dt)_acc ~ 0.3, a value that is in agreement with jet launching models.Comment: 12 pages, accepted for publication in A&

Search of X-ray emission from roAp stars: The case of gamma Equulei

The detection of X-ray emission from Ap stars can be an indicator for the presence of magnetic activity and dynamo action, provided different origins for the emission, such as wind shocks and close late-type companions, can be excluded. Here we report on results for gamma Equu, the only roAp star for which an X-ray detection is reported in ROSAT catalogs. We use high resolution imaging in X-rays with Chandra and in the near-infrared with NACO/VLT that allow us to spatially resolve companions down to ~1" and ~0.06" separations, respectively. The bulk of the X-ray emission is associated with a companion of gamma Equu identified in our NACO image. Assuming coevality with the primary roAp star (~900 Myr), the available photometry for the companion points at a K-type star with ~0.6 M_sun. Its X-ray properties are in agreement with the predictions for its age and mass. An excess of photons with respect to the expected background and contribution from the nearby companion is observed near the optical position of gamma Equu. We estimate an X-ray luminosity of log L_x [erg/s] = 26.6 and log(L_x/L_bol) = -7.9 for this emission. A small offset between the optical and the X-ray image leaves some doubt on its association with the roAp star. The faint X-ray emission that we tentatively ascribe to the roAp star is difficult to explain as a solar-like stellar corona due to its very low L_x/L_bol level and the very long rotation period of gamma Equu. It could be produced in magnetically confined wind shocks implying a mass loss rate of ~10^(-14) M_sun/yr or from an additional unknown late-type companion at separation ~0.4". If confirmed by future deeper X-ray observations this emission could point at the origin for the presence of radioactive elements on some roAp stars.Comment: Accepted for publication in Astronomy & Astrophysics (5 pages

An extensive VLT/X-Shooter library of photospheric templates of pre-main sequence stars

Studies of the formation and evolution of young stars and their disks rely on the knowledge of the stellar parameters of the young stars. The derivation of these parameters is commonly based on comparison with photospheric template spectra. Furthermore, chromospheric emission in young active stars impacts the measurement of mass accretion rates, a key quantity to study disk evolution. Here we derive stellar properties of low-mass pre-main sequence stars without disks, which represent ideal photospheric templates for studies of young stars. We also use these spectra to constrain the impact of chromospheric emission on the measurements of mass accretion rates. The spectra in reduced, flux-calibrated, and corrected for telluric absorption form are made available to the community. We derive the spectral type for our targets by analyzing the photospheric molecular features present in their VLT/X-Shooter spectra by means of spectral indices and comparison of the relative strength of photospheric absorption features. We also measure effective temperature, gravity, projected rotational velocity, and radial velocity from our spectra by fitting them with synthetic spectra with the ROTFIT tool. The targets have negligible extinction and spectral type from G5 to M8. We perform synthetic photometry on the spectra to derive the typical colors of young stars in different filters. We measure the luminosity of the emission lines present in the spectra and estimate the noise due to chromospheric emission in the measurements of accretion luminosity in accreting stars. We provide a calibration of the photospheric colors of young PMS stars as a function of their spectral type in a set of standard broad-band optical and near-infrared filters. For stars with masses of ~ 1.5Msun and ages of ~1-5 Myr, the chromospheric noise converts to a limit of measurable mass accretion rates of ~ 3x10^-10 Msun/yr.Comment: Accepted for publication on Astronomy & Astrophysics. The spectra of the photospheric templates will be uploaded to Vizier, but are already available on request. Abstract shortened for arxiv constraints. Language edited versio

Extracting scaling laws from numerical dynamo models

Earth's magnetic field is generated by processes in the electrically conducting, liquid outer core, subsumed under the term ‘geodynamo'. In the last decades, great effort has been put into the numerical simulation of core dynamics following from the magnetohydrodynamic equations. However, the numerical simulations are far from Earth's core in terms of several control parameters. Different scaling analyses found simple scaling laws for quantities like heat transport, flow velocity, magnetic field strength and magnetic dissipation time. We use an extensive data set of 116 numerical dynamo models compiled by Christensen and co-workers to analyse these scalings from a rigorous model selection point of view. Our method of choice is leave-one-out cross-validation which rates models according to their predictive abilities. In contrast to earlier results, we find that diffusive processes are not negligible for the flow velocity and magnetic field strength in the numerical dynamos. Also the scaling of the magnetic dissipation time turns out to be more complex than previously suggested. Assuming that the processes relevant in the numerical models are the same as in Earth's core, we use this scaling to estimate an Ohmic dissipation of 3-8 TW for the core. This appears to be consistent with recent high core-mantle boundary heat flux scenario