Coronal structure of the cTTS V2129 Oph

The nature of the magnetic coupling between T Tauri stars and their disks determines not only the mass accretion process but possibly the spin evolution of the central star. We have taken a recently-published surface magnetogram of one moderately-accreting T Tauri star (V2129 Oph) and used it to extrapolate the geometry of its large-scale field. We determine the structure of the open (wind-bearing) field lines, the closed (X-ray bright) field lines and those potentially accreting field lines that pass through the equatorial plane inside the Keplerian co-rotation radius. We consider a series of models in which the stellar magnetic field is opened up by the outward pressure of the hot coronal gas at a range of radii. As this radius is increased, accretion takes place along simpler field structures and impacts on fewer sites at the stellar surface. This is consistent with the observed variation in the Ca II IRT and HeI lines which suggests that accretion in the visible hemisphere is confined to a single high-latitude spot. By determining the density and velocity of the accretion flows, we find that in order to have most of the total mass accretion rate impacting on a single high-latitude region we need disk material to accrete from approximately 7R*, close to the Keplerian co-rotation radius at 6.8R*. We also calculate the coronal density and X-ray emission measure. We find that both the magnitude and rotational modulation of the emission measure increase as the source surface is increased. For the field structure of V2129 Oph which is dominantly octupolar, the emission forms a bright, high-latitude ring that is always in view as the star rotates. Since the accretion funnels are not dense enough to cause significant scattering of coronal X-ray photons, they provide only a low rotational modulation of around 10% at most.Comment: 10 pages, 9 figure

On the Effect of Magnetic Spots on Stellar Winds and Angular Momentum Loss

We simulate the effect of latitudinal variations in the location of star spots, as well as their magnetic field strength, on stellar angular momentum loss to the stellar wind. We use the Michigan solar corona global MagnetoHydroDynamic model, which incorporates realistic relation between the magnetic field topology and the wind distribution. We find that the spots location significantly affects the stellar wind structure, and as a result, the total mass loss rate and angular momentum loss rate. In particular, we find that the angular momentum loss rate is controlled by the mass flux when spots are located at low latitudes but is controlled by an increased plasma density between the stellar surface and the Alfven surface when spots are located at high latitudes. Our results suggest that there might be a feedback mechanism between the magnetic field distribution, wind distribution, angular momentum loss through the wind, and the motions at the convection zone that generate the magnetic field. This feedback might explain the role of coronal magnetic fields in stellar dynamos

Angular Momentum Loss from Cool Stars: An Empirical Expression and Connection to Stellar Activity

We show here that the rotation period data in open clusters allow the empirical determination of an expression for the rate of loss of angular momentum from cool stars on the main sequence. One significant component of the expression, the dependence on rotation rate, persists from prior work; others do not. The expression has a bifurcation, as before, that corresponds to an observed bifurcation in the rotation periods of coeval open cluster stars. The dual dependencies of this loss rate on stellar mass are captured by two functions, $f(B-V)$ and $T(B-V)$, that can be determined from the rotation period observations. Equivalent masses and other [UBVRIJHK] colors are provided in Table 1. Dimensional considerations, and a comparison with appropriate calculated quantities suggest interpretations for $f$ and $T$, both of which appear to be related closely (but differently) to the calculated convective turnover timescale, $\tau_c$, in cool stars. This identification enables us to write down symmetrical expressions for the angular momentum loss rate and the deceleration of cool stars, and also to revive the convective turnover timescale as a vital connection between stellar rotation and stellar activity physics.Comment: 20 pages, 9 color figures; this version includes corrections listed in the associated journal erratu

ISU Percussion Ensemble

Center for the Performing Arts April 15, 2018 Sunday Evening 7:00p.m

Effect of the stellar spin history on the tidal evolution of close-in planets

We investigate how the evolution of the stellar spin rate affects, and is affected by, planets in close orbits, via star-planet tidal interactions. To do this, we used a standard equilibrium tidal model to compute the orbital evolution of single planets orbiting both Sun-like stars and 0.1 M\odot M-dwarfs. We tested two stellar spin evolution profiles, one with fast initial rotation (P=1.2 day) and one with slow initial rotation (P=8 day). We tested the effect of varying the stellar and planetary dissipation and the planet's mass and initial orbital radius. Conclusions: Tidal evolution allows to differentiate the early behaviors of extremely close-in planets orbiting either a rapidly rotating star or a slowly rotating star. The early spin-up of the star allows the close-in planets around fast rotators to survive the early evolution. For planets around M-dwarfs, surviving the early evolution means surviving on Gyr timescales whereas for Sun-like stars the spin-down brings about late mergers of Jupiter planets. In light of this study, we can say that differentiating between one spin evolution from another given the present position of planets can be very tricky. Unless we can observe some markers of former evolution it is nearly impossible to distinguish the two very different spin profiles, let alone intermediate spin profiles. Though some conclusions can still be drawn from statistical distributions of planets around fully convective M-dwarfs. However, if the tidal evolution brings about a merger late in its history it can also entail a noticeable acceleration of the star in late ages, so that it is possible to have old stars that spin rapidly. This raises the question of better constraining the age of stars

The non-dipolar magnetic fields of accreting T Tauri stars

Models of magnetospheric accretion on to classical T Tauri stars often assume that stellar magnetic fields are simple dipoles. Recently published surface magnetograms of BP Tau and V2129 Oph have shown, however, that their fields are more complex. The magnetic field of V2129 Oph was found to be predominantly octupolar. For BP Tau the magnetic energy was shared mainly between the dipole and octupole field components, with the dipole component being almost four times as strong as that of V2129 Oph. From the published surface maps of the photospheric magnetic fields we extrapolate the coronal fields of both stars, and compare the resulting field structures with that of a dipole. We consider different models where the disc is truncated at, or well-within, the Keplerian corotation radius. We find that although the structure of the surface magnetic field is particularly complex for both stars, the geometry of the larger scale field, along which accretion is occurring, is somewhat simpler. However, the larger scale field is distorted close to the star by the stronger field regions, with the net effect being that the fractional open flux through the stellar surface is less than would be expected with a dipole magnetic field model. Finally, we estimate the disc truncation radius, assuming that this occurs where the magnetic torque from the stellar magnetosphere is comparable to the viscous torque in the disc.Comment: 14 pages, 8 figures. Figures are reduced resolutio

A Search for Star-Disk Interaction Among the Strongest X-ray Flaring Stars in the Orion Nebula Cluster

The Chandra Orion Ultradeep Project observed hundreds of young, low-mass stars undergoing highly energetic X-ray flare events. The 32 most powerful cases have been modeled with the result that the magnetic structures responsible for these flares can be many stellar radii in extent. In this paper, we model the observed spectral energy distributions of these 32 stars in order to determine, in detail for each star, whether there is circumstellar disk material situated in sufficient proximity to the stellar surface for interaction with the large magnetic loops inferred from the observed X-ray flares. Our spectral energy distributions span the wavelength range 0.3-8 um (plus 24 um for some stars), allowing us to constrain the presence of dusty circumstellar material out to >10 AU from the stellar surface in most cases. For 24 of the 32 stars in our sample the available data are sufficient to constrain the location of the inner edge of the dusty disks. Six of these (25%) have spectral energy distributions consistent with inner disks within reach of the observed magnetic loops. Another four stars may have gas disks interior to the dust disk and extending within reach of the magnetic loops, but we cannot confirm this with the available data. The remaining 14 stars (58%) appear to have no significant disk material within reach of the large flaring loops. Thus, up to ~40% of the sample stars exhibit energetic X-ray flares that possibly arise from a magnetic star-disk interaction, and the remainder are evidently associated with extremely large, free-standing magnetic loops anchored only to the stellar surface.Comment: Accepted to the ApJ; 26 pages, 6 tables, 6 figure

RACE-OC Project: Rotation and variability in the epsilon Chamaeleontis, Octans, and Argus stellar associations

We aim at determining the rotational and magnetic-related activity properties of stars at different stages of evolution. We focus our attention primarily on members of young stellar associations of known ages. Specifically, we extend our previous analysis in Paper I (Messina et al. 2010, A&A 520, A15) to 3 additional young stellar associations beyond 100 pc and with ages in the range 6-40 Myr: epsilon Chamaeleontis (~6 Myr), Octans (~20 Myr), and Argus (~40 Myr). Additional rotational data of eta Chamaeleontis and IC2391 clusters are also considered. Rotational periods were determined from photometric time-series data obtained by the All Sky Automated Survey (ASAS) and the Wide Angle Search for Planets (SuperWASP) archives. With the present study we have completed the analysis of the rotational properties of the late-type members of all known young loose associations in the solar neighborhood. Considering also the results of Paper I, we have derived the rotation periods of 241 targets: 171 confirmed, 44 likely, 26 uncertain. The period of the remaining 50 stars known to be part of loose associations still remains unknown. This rotation period catalogue, and specifically the new information presented in this paper at ~6, 20, and 40 Myr, contributes significantly to a better observational description of the angular momentum evolution of young stars.Comment: Accepted by Astronomy & Astrophysics. Onlines figures will be available at CD

Mass Loss in Pre-main-sequence Stars via Coronal Mass Ejections and Implications for Angular Momentum Loss

We develop an empirical model to estimate mass-loss rates via coronal mass ejections (CMEs) for solar-type pre-main-sequence (PMS) stars. Our method estimates the CME mass-loss rate from the observed energies of PMS X-ray flares, using our empirically determined relationship between solar X-ray flare energy and CME mass: log ( M CME [g]) = 0.63 × log ( E flare [erg]) – 2.57. Using masses determined for the largest flaring magnetic structures observed on PMS stars, we suggest that this solar-calibrated relationship may hold over 10 orders of magnitude in flare energy and 7 orders of magnitude in CME mass. The total CME mass-loss rate we calculate for typical solar-type PMS stars is in the range 10 –12 -10 –9 M ☉ yr –1 . We then use these CME mass-loss rate estimates to infer the attendant angular momentum loss leading up to the main sequence. Assuming that the CME outflow rate for a typical ~1 M ☉ T Tauri star is <10 –10 M ☉ yr –1 , the resulting spin-down torque is too small during the first ~1 Myr to counteract the stellar spin-up due to contraction and accretion. However, if the CME mass-loss rate is ##IMG## [http://ej.iop.org/icons/Entities/gsim.gif] {gsim 10 –10 M ☉ yr –1 , as permitted by our calculations, then the CME spin-down torque may influence the stellar spin evolution after an age of a few Myr.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98581/1/0004-637X_760_1_9.pd

The role of magnetic fields in governing the angular momentum evolution of solar-type stars

I review the development of ideas regarding the angular momentum evolution of solar-type stars, from the early 60's to the most recent years. Magnetic fields are the central agent that dictates the rotational evolution of solar-type stars, both during the pre-main sequence, through star-disk magnetic coupling, and during the main sequence, through magnetized winds. Key theoretical developments as well as important observational results are summarized in this review.Comment: Stellar Magnetism, eds. C. Neiner, J.-P. Zahn, EAS Publication Series 200