Investigating the rotational evolution of young, low mass stars using Monte Carlo simulations

We investigate the rotational evolution of young stars through Monte Carlo simulations. We simulate 280,000 stars, each of which is assigned a mass, a rotational period, and a mass accretion rate. The mass accretion rate depends on mass and time, following power-laws indices 1.4 and -1.5, respectively. A mass-dependent accretion threshold is defined below which a star is considered as diskless, which results in a distribution of disk lifetimes that matches observations. Stars are evolved at constant angular spin rate while accreting and at constant angular momentum when they become diskless. We recover the bimodal period distribution seen in several young clusters. The short period peak consists mostly of diskless stars and the long period one is mainly populated by accreting stars. Both distributions present a long tail towards long periods and a population of slowly rotating diskless stars is observed at all ages. We reproduce the observed correlations between disk fraction and spin rate, as well as between IR excess and rotational period. The period-mass relation we derive from the simulations exhibits the same global trend as observed in young clusters only if we release the disk locking assumption for the lowest mass stars. We find that the time evolution of median specific angular momentum follows a power law index of -0.65 for accreting stars and of -0.53 for diskless stars, a shallower slope that results from a wide distribution of disk lifetimes. Using observationally-documented distributions of disk lifetimes, mass accretion rates, and initial rotation periods, and evolving an initial population from 1 to 12 Myr, we reproduce the main characteristics of pre-main sequence angular momentum evolution, which supports the disk locking hypothesis. (abridged)Comment: 11 pages, 14 figures, accepted for publication in A&

Rotational Mixing and Lithium Depletion

I review basic observational features in Population I stars which strongly implicate rotation as a mixing agent; these include dispersion at fixed temperature in coeval populations and main sequence lithium depletion for a range of masses at a rate which decays with time. New developments related to the possible suppression of mixing at late ages, close binary mergers and their lithium signature, and an alternate origin for dispersion in young cool stars tied to radius anomalies observed in active young stars are discussed. I highlight uncertainties in models of Population II lithium depletion and dispersion related to the treatment of angular momentum loss. Finally, the origins of rotation are tied to conditions in the pre-main sequence, and there is thus some evidence that enviroment and planet formation could impact stellar rotational properties. This may be related to recent observational evidence for cluster to cluster variations in lithium depletion and a connection between the presence of planets and stellar lithium depletion.Comment: 6 pages, 1 figure, to appear in proceedings of IAU Symp. 268, in pres

The Magnetic Fields of Classical T Tauri Stars

We report new magnetic field measurements for 14 classical T Tauri stars (CTTSs). We combine these data with one previous field determination in order to compare our observed field strengths with the field strengths predicted by magnetospheric accretion models. We use literature data on the stellar mass, radius, rotation period, and disk accretion rate to predict the field strength that should be present on each of our stars according to these magnetospheric accretion models. We show that our measured field values do not correlate with the field strengths predicted by simple magnetospheric accretion theory. We also use our field strength measurements and literature X-ray luminosity data to test a recent relationship expressing X-ray luminosity as a function of surface magnetic flux derived from various solar feature and main sequence star measurements. We find that the T Tauri stars we have observed have weaker than expected X-ray emission by over an order of magnitude on average using this relationship. We suggest the cause for this is actually a result of the very strong fields on these stars which decreases the efficiency with which gas motions in the photosphere can tangle magnetic flux tubes in the corona.Comment: 25 pages, 5 figure

MHD simulations of disk-star interaction

We discuss a number of topics relevant to disk-magnetosphere interaction and how numerical simulations illuminate them. The topics include: (1) disk-magnetosphere interaction and the problem of disk-locking; (2) the wind problem; (3) structure of the magnetospheric flow, hot spots at the star's surface, and the inner disk region; (4) modeling of spectra from 3D funnel streams; (5) accretion to a star with a complex magnetic field; (6) accretion through 3D instabilities; (7) magnetospheric gap and survival of protoplanets. Results of both 2D and 3D simulations are discussed.Comment: 12 pages, 10 figures, Star-Disk Interaction in Young Stars, Proceedings of the International Astronomical Union, IAU Symposium, Volume 243. See animations at http://astro.cornell.edu/~romanova/projects.htm and at http://astro.cornell.edu/us-rus

Low-Mass Star Formation and the Initial Mass Function in the Rho Ophiuchi Cloud Core

We have obtained moderate-resolution (R=800-1200) K-band spectra for ~100 stars within and surrounding the cloud core of rho Oph. We have measured spectral types and continuum veilings and have combined this information with results from new deep imaging. The IMF peaks at about 0.4 M_sun and slowly declines to the hydrogen burning limit with a slope of ~-0.5 in logarithmic units (Salpeter is +1.35). Our lower limits on the numbers of substellar objects demonstrate that the IMF probably does not fall more steeply below the hydrogen burning limit, at least down to ~0.02 M_sun. We then make the first comparison of mass functions of stars and pre-stellar clumps (Motte, Andre, & Neri) measured in the same region. The similar behavior of the two mass functions in rho Oph supports the suggestion of Motte et al. and Testi & Sargent that the stellar mass function in young clusters is a direct product of the process of cloud fragmentation. After considering the effect of extinction on the SED classifications of the sample, we find that ~17% of the rho Oph stars are Class I, implying ~0.1 Myr for the lifetime of this stage. In spectra separated by two years, we observe simultaneous variability in the Br gamma emission and K-band continuum veiling for two stars, where the hydrogen emission is brighter in the more heavily veiled data. This behavior indicates that the disk may contribute significantly to continuous K-band emission, in contrast to the proposal that the infalling envelope always dominates. Our detection of strong 2 micron veiling (r_K=1-4) in several Class II and III stars, which should have disks but little envelope material, further supports this proposition.Comment: 35 pages, 14 figures, accepted to Ap

The effect of landfills on rural residential property values: Some empirical evidence.

Abstract The question of whether solid waste landfills affect residential property values has long been a subject of debate. Past research has resulted in mixed conclusions. The current study examines six landfills, which differ in size, operating status, and history of contamination. The effect of each landfill is estimated by the use of multiple regression. In five of the landfills, no statistically significant evidence of an effect was found. In the remaining case, evidence of an effect was found, indicating that houses in close proximity to this landfill suffered an average loss of about six percent in value

A 10-micron Search for Inner-Truncated Disks Among Pre-Main-Sequence Stars With Photometric Rotation Periods

We use mid-IR (primarily 10 $\mu$m) photometry as a diagnostic for the presence of disks with inner cavities among 32 pre-main sequence stars in Orion and Taurus-Auriga for which rotation periods are known and which do not show evidence for inner disks at near-IR wavelengths. Disks with inner cavities are predicted by magnetic disk-locking models that seek to explain the regulation of angular momentum in T Tauri stars. Only three stars in our sample show evidence for excess mid-IR emission. While these three stars may possess truncated disks consistent with magnetic disk-locking models, the remaining 29 stars in our sample do not. Apparently, stars lacking near-IR excesses in general do not possess truncated disks to which they are magnetically coupled. We discuss the implications of this result for the hypothesis of disk-regulated angular momentum. Evidently, young stars can exist as slow rotators without the aid of present disk-locking, and there exist very young stars already rotating near breakup velocity whose subsequent angular momentum evolution will not be regulated by disks. Moreover, we question whether disks, when present, truncate in the manner required by disk-locking scenarios. Finally, we discuss the need for rotational evolution models to take full account of the large dispersion of rotation rates present at 1 Myr, which may allow the models to explain the rotational evolution of low-mass pre-main sequence stars in a way that does not depend upon braking by disks.Comment: 20 pages, 4 figure

X-ray to NIR emission from AA Tauri during the dim state - Occultation of the inner disk and gas-to-dust ratio of the absorber

AA Tau is a well-studied, nearby classical T Tauri star, which is viewed almost edge-on. A warp in its inner disk periodically eclipses the central star, causing a clear modulation of its optical light curve. The system underwent a major dimming event beginning in 2011 caused by an extra absorber, which is most likely associated with additional disk material in the line of sight toward the central source. We present new XMM-Newton X-ray, Hubble Space Telescope FUV, and ground based optical and near-infrared data of the system obtained in 2013 during the long-lasting dim phase. The line width decrease of the fluorescent H$_2$ disk emission shows that the extra absorber is located at $r>1\,$au. Comparison of X-ray absorption ($N_H$) with dust extinction ($A_V$), as derived from measurements obtained one inner disk orbit (eight days) after the X-ray measurement, indicates that the gas-to-dust ratio as probed by the $N_H$ to $A_V$ ratio of the extra absorber is compatible with the ISM ratio. Combining both results suggests that the extra absorber, i.e., material at $r>1\,$au, has no significant gas excess in contrast to the elevated gas-to-dust ratio previously derived for material in the inner region ($\lesssim0.1\,$au).Comment: 16 pages, 12 figures, accepted by A&