ALBERT

Description: We present photometric and spectroscopic models of the Classical T Tauri star AA Tau. Photometric and spectroscopic variability present in observations of AA Tau is attributed to a magnetically induced warp in the accretion disc, periodically occulting the photosphere on an 8.2 d time-scale. Emission line profiles show signatures of both infall, attributed to magnetospherically accreting material, and outflow. Using the radiative transfer code torus , we have investigated the geometry and kinematics of AA Tau's circumstellar disc and outflow, which is modelled here as a disc wind. Photometric models have been used to constrain the aspect ratio of the disc, the offset angle of the magnetosphere dipole with respect to the stellar rotation axis, and the inner radius of the circumstellar disc. Spectroscopic models have been used to constrain the wind and magnetosphere temperatures, wind acceleration parameter, and mass-loss rate. We find that observations are best fitted by models with a mass accretion rate of 5 10 –9 M  yr –1 , a dipole offset of between 10° and 20°, a magnetosphere that truncates the disc from 5.2 to 8.8 R * , a mass-loss-rate to accretion-rate ratio of ~0.1, a magnetosphere temperature of 8500–9000 K, and a disc-wind temperature of 8000 K.

Description: When they first appear in the HR diagram, young stars rotate at a mere 10 per cent of their break-up velocity. They must have lost most of the angular momentum initially contained in the parental cloud, the so-called angular momentum problem. We investigate here a new mechanism by which large amounts of angular momentum might be shed from young stellar systems, thus yielding slowly rotating young stars. Assuming that planets promptly form in circumstellar discs and rapidly migrate close to the central star, we investigate how the tidal and magnetic interactions between the protostar, its close-in planet(s), and the inner circumstellar disc can efficiently remove angular momentum from the central object. We find that neither the tidal torque nor the variety of magnetic torques acting between the star and the embedded planet are able to counteract the spin-up torques due to accretion and contraction. Indeed, the former are orders of magnitude weaker than the latter beyond the corotation radius and are thus unable to prevent the young star from spinning up. We conclude that star–planet interaction in the early phases of stellar evolution does not appear as a viable alternative to magnetic star–disc coupling to understand the origin of the low angular momentum content of young stars.

Description: We calculate the flux received from a binary system obscured by a circumbinary disc. The disc is modelled using two-dimensional hydrodynamical simulations, and the vertical structure is derived by assuming it is isothermal. The gravitational torque from the binary creates a cavity in the disc's inner parts. If the line of sight along which the system is observed has a high inclination I , it intersects the disc and some absorption is produced. As the system is not axisymmetric, the resulting light curve displays variability. We calculate the absorption and produce light curves for different values of the dust disc aspect ratio H / r and mass of dust in the cavity M dust . This model is applied to the high inclination ( $I=85\circ$ ) eclipsing binary CoRoT 223992193, which shows 5–10 per cent residual photometric variability after the eclipses and a spot model are subtracted. We find that such variations for $I \sim 85\circ$ can be obtained for H / r = 10 –3 and M dust ≥ 10 –12 M . For higher H / r, M dust would have to be close to this lower value and I somewhat less than $85\circ$ . Our results show that such variability in a system where the stars are at least 90 per cent visible at all phases can be obtained only if absorption is produced by dust located inside the cavity. If absorption is dominated by the parts of the disc located close to or beyond the edge of the cavity, the stars are significantly obscured.

Description: Spectropolarimetric observations have been used to map stellar magnetic fields, many of which display strong bands of azimuthal fields that are toroidal. A number of explanations have been proposed to explain how such fields might be generated though none are definitive. In this paper, we examine the toroidal fields of a sample of 55 stars with magnetic maps, with masses in the range 0.1–1.5 M . We find that the energy contained in toroidal fields has a power-law dependence on the energy contained in poloidal fields. However the power index is not constant across our sample, with stars less and more massive than 0.5 M having power indices of 0.72 ± 0.08 and 1.25 ± 0.06, respectively. There is some evidence that these two power laws correspond to stars in the saturated and unsaturated regimes of the rotation-activity relation. Additionally, our sample shows that strong toroidal fields must be generated axisymmetrically. The latitudes at which these bands appear depend on the stellar rotation period with fast rotators displaying higher latitude bands than slow rotators. The results in this paper present new constraints for future dynamo studies.

Description: We report results of a spectropolarimetric and photometric monitoring of the weak-line T Tauri stars (wTTSs) V819 Tau and V830 Tau within the MaTYSSE (Magnetic Topologies of Young Stars and the Survival of close-in giant Exoplanets) programme, involving the ESPaDOnS spectropolarimeter at the Canada–France–Hawaii Telescope. At ~=3 Myr, both stars dissipated their discs recently and are interesting objects for probing star and planet formation. Profile distortions and Zeeman signatures are detected in the unpolarized and circularly polarized lines, whose rotational modulation we modelled using tomographic imaging, yielding brightness and magnetic maps for both stars. We find that the large-scale magnetic fields of V819 Tau and V830 Tau are mostly poloidal and can be approximated at large radii by 350–400 G dipoles tilted at ~=30° to the rotation axis. They are significantly weaker than the field of GQ Lup, an accreting classical T Tauri star (cTTS) with similar mass and age which can be used to compare the magnetic properties of wTTSs and cTTSs. The reconstructed brightness maps of both stars include cool spots and warm plages. Surface differential rotation is small, typically ~=4.4 times smaller than on the Sun, in agreement with previous results on wTTSs. Using our Doppler images to model the activity jitter and filter it out from the radial velocity (RV) curves, we obtain RV residuals with dispersions of 0.033 and 0.104 km s –1 for V819 Tau and V830 Tau, respectively. RV residuals suggest that a hot Jupiter may be orbiting V830 Tau, though additional data are needed to confirm this preliminary result. We find no evidence for close-in giant planet around V819 Tau.

Description: Spectropolarimetric observations have been used to map stellar magnetic fields, many of which display strong bands of azimuthal fields that are toroidal. A number of explanations have been proposed to explain how such fields might be generated though none are definitive. In this paper, we examine the toroidal fields of a sample of 55 stars with magnetic maps, with masses in the range 0.1–1.5 M . We find that the energy contained in toroidal fields has a power-law dependence on the energy contained in poloidal fields. However the power index is not constant across our sample, with stars less and more massive than 0.5 M having power indices of 0.72 ± 0.08 and 1.25 ± 0.06, respectively. There is some evidence that these two power laws correspond to stars in the saturated and unsaturated regimes of the rotation-activity relation. Additionally, our sample shows that strong toroidal fields must be generated axisymmetrically. The latitudes at which these bands appear depend on the stellar rotation period with fast rotators displaying higher latitude bands than slow rotators. The results in this paper present new constraints for future dynamo studies.

Description: The surface rotation rates of young solar-type stars vary rapidly with age from the end of the pre-main sequence through the early main sequence. Important changes in the dynamos operating in these stars may result from this evolution, which should be observable in their surface magnetic fields. Here we present a study aimed at observing the evolution of these magnetic fields through this critical time period. We observed stars in open clusters and stellar associations of known ages, and used Zeeman Doppler imaging to characterize their complex magnetic large-scale fields. Presented here are results for 15 stars, from five associations, with ages from 20 to 250 Myr, masses from 0.7 to 1.2 M , and rotation periods from 0.4 to 6 d. We find complex large-scale magnetic field geometries, with global average strengths from 14 to 140 G. There is a clear trend towards decreasing average large-scale magnetic field strength with age, and a tight correlation between magnetic field strength and Rossby number. Comparing the magnetic properties of our zero-age main-sequence sample to those of both younger and older stars, it appears that the magnetic evolution of solar-type stars during the pre-main sequence is primarily driven by structural changes, while it closely follows the stars’ rotational evolution on the main sequence.

Description: We report results of a spectropolarimetric and photometric monitoring of the weak-line T Tauri star LkCa 4 within the Magnetic Topologies of Young Stars and the Survival of close-in giant Exoplanets (MaTYSSE) programme, involving ESPaDOnS at the Canada–France–Hawaii Telescope. Despite an age of only 2 Myr and a similarity with prototypical classical T Tauri stars, LkCa 4 shows no evidence for accretion and probes an interesting transition stage for star and planet formation. Large profile distortions and Zeeman signatures are detected in the unpolarized and circularly polarized lines of LkCa 4 using Least-Squares Deconvolution (LSD), indicating the presence of brightness inhomogeneities and magnetic fields at the surface of LkCa 4. Using tomographic imaging, we reconstruct brightness and magnetic maps of LkCa 4 from sets of unpolarized and circularly polarized LSD profiles. The large-scale field is strong and mainly axisymmetric, featuring a ~=2 kG poloidal component and a ~=1 kG toroidal component encircling the star at equatorial latitudes – the latter making LkCa 4 markedly different from classical T Tauri stars of similar mass and age. The brightness map includes a dark spot overlapping the magnetic pole and a bright region at mid-latitudes – providing a good match to the contemporaneous photometry. We also find that differential rotation at the surface of LkCa 4 is small, typically ~=5.5 times weaker than that of the Sun, and compatible with solid-body rotation. Using our tomographic modelling, we are able to filter out the activity jitter in the radial velocity curve of LkCa 4 (of full amplitude 4.3 km s –1 ) down to an rms precision of 0.055 km s –1 . Looking for hot Jupiters around young Sun-like stars thus appears feasible, even though we find no evidence for such planets around LkCa 4.

Description: We present the results of a major high-resolution spectropolarimetric BCool project magnetic survey of 170 solar-type stars. Surface magnetic fields were detected on 67 stars, with 21 classified as mature solar-type stars, a result that increases by a factor of 4 the number of mature solar-type stars on which magnetic fields have been observed. In addition, a magnetic field was detected for 3 out of 18 of the subgiant stars surveyed. For the population of K-dwarfs, the mean value of | B l | (| B l | mean ) was also found to be higher (5.7 G) than | B l | mean measured for the G-dwarfs (3.2 G) and the F-dwarfs (3.3 G). For the sample as a whole, | B l | mean increases with rotation rate and decreases with age, and the upper envelope for | B l | correlates well with the observed chromospheric emission. Stars with a chromospheric S-index greater than about 0.2 show a high magnetic field detection rate and so offer optimal targets for future studies. This survey constitutes the most extensive spectropolarimetric survey of cool stars undertaken to date, and suggests that it is feasible to pursue magnetic mapping of a wide range of moderately active solar-type stars to improve our understanding of their surface fields and dynamos.

Description: We present the results of an optical photometry and high-resolution spectroscopy campaign for a modest sample of X-ray selected stars in the Chamaeleon and Rho Ophiuchus star-forming regions. With R ~ 50 000 optical spectra, we establish kinematic membership of the parent association and confirm stellar youth for each star in our sample. With the acquisition of new standardized BVIc photometry, in concert with near-infrared data from the literature, we derive age and mass from stellar positions in model-dependent Hertzsprung–Russell diagrams. We compare isochronal ages derived using colour-dependent extinction values finding that, within error bars, ages are the same irrespective of whether E ( B – V ), E ( V – Ic ), E ( J – H ) or E ( H – K ) is used to establish extinction, although model ages tend to be marginally younger for redder E colour values. For Cham I and Cham members, we derive ages of 5–6 Myr, whereas our three Cha candidates are more consistent with a 25 Myr post-T Tauri star population. In Rho Ophiuchus, most stars in our sample have isochronal ages 〈10 Myr. Five objects show evidence of strong infrared excess ( Av 〉 5) in the Two Micron All Sky Survey colour–colour diagram, however in terms of Hα emission, all stars except RXJ1625.6–2613 are consistent with being weak-lined T-Tauri stars. Spectral energy distributions (SEDs) over the range ~=4000 Å 〈 〈 1000 μm, show that only one Chamaeleon star (RXJ1112.7 –7637) and three Rho Ophiuchus stars (ROXR1 13, RXJ1625.6–2613 & RXJ1627.1–2419) reveal substantial departures from a bare photosphere.