Mei-Yin Chou

A 2MASS ALL-SKY VIEW OF THE SAGITTARIUS DWARF GALAXY. V. VARIATION OF THE METALLICITY DISTRIBUTION FUNCTION ALONG THE SAGITTARIUS STREAM

We present reliable measurements of the metallicity distribution function (MDF) at different points along the tidal stream of the Sagittarius (Sgr) dwarf spheroidal (dSph) galaxy, based on high-resolution, echelle spectroscopy of candidate M giant members of the Sgr system. The Sgr MDF is found to evolve significantly from a median [Fe/H] ~ -0.4 in the core to ~-1.1 dex over a Sgr leading arm length representing ~2.5-3.0 Gyr of dynamical (i.e., tidal stripping) age. This is direct evidence that there can be significant chemical differences between current dSph satellites and the bulk of the stars they have contributed to the halo. Our results suggest that Sgr experienced a significant change in binding energy over the past several gigayears, which has substantially decreased its tidal boundary across a radial range over which there must have been a significant metallicity gradient in the progenitor galaxy. By accounting for MDF variation along the debris arms, we approximate the MDF Sgr would have had several gigayears ago. We also analyze the MDF of a moving group of M giants we previously discovered toward the north Galactic cap. These objects have the opposite radial velocities to the infalling Sgr leading arm stars there, and we propose that most of them represent Sgr trailing arm stars overlapping the Sgr leading arm in this part of the sky. If so, these trailing arm stars further demonstrate the strong MDF evolution within the Sgr stream.

A Unified Model for Bipolar Outflows from Young Stars

We develop a unified model for molecular outflows in star formation. The model incorporates essential features expected of the primary wind, which is thought to be driven magnetocentrifugally from close to the central stellar object, and the ambient core material shaped by anisotropic magnetic support. The primary wind is modeled as a toroidally magnetized fast outflow moving radially away from the origin, with an angle-dependent density distribution: a dense axial jet surrounded by a more tenuous wide-angle wind, as expected in the X-wind model. If dynamically significant magnetic fields are present, the star-forming core will settle faster along the field lines than across, forming a toroid-like structure. We approximate the structure with a singular isothermal toroid whose density distribution can be obtained analytically. The interaction of the laterally stratified wind and the ambient toroid is followed using the Zeus2D magnetohydrodynamics (MHD) code. We find that the lobes produced by the interaction resemble many systematics observed in molecular outflows from very young stars, ranging from Class 0 to I sources. In particular, both the dense axial jet and the wide-angle wind participate in the wind-ambient interaction. In our model, the jet- and wind-driven pictures of molecular outflows are unified. We discuss the observational implications of the unified picture, including the possibility of detecting the primary jet/wind directly.

TIME VARIABILITY OF EMISSION LINES FOR FOUR ACTIVE T TAURI STARS. I. OCTOBER–DECEMBER IN 2010*

We present optical spectrophotometric monitoring of four active T Tauri stars (DG Tau, RY Tau, XZ Tau, RW?Aur?A) at high spectral resolution (R 1 ? 104), to investigate the correlation between time variable mass ejection seen in the jet/wind structure of the driving source and time variable mass accretion probed by optical emission lines. This may allow us to constrain the understanding of the jet/wind launching mechanism, the location of the launching region, and the physical link with magnetospheric mass accretion. In 2010, observations were made at six different epochs to investigate how daily and monthly variability might affect such a study. We perform comparisons between the line profiles we observed and those in the literature over a period of decades and confirm the presence of time variability separate from the daily and monthly variability during our observations. This is so far consistent with the idea that these line profiles have a long-term variability (3-20?yr) related to episodic mass ejection suggested by the structures in the extended flow components. We also investigate the correlations between equivalent widths and between luminosities for different lines. We find that these correlations are consistent with the present paradigm of steady magnetospheric mass accretion and emission line regions that are close to the star.

STABLE AND UNSTABLE REGIMES OF MASS ACCRETION ONTO RW AUR A

We present monitoring observations of the active T Tauri star RW Aur, from 2010 October to 2015 January, using optical high-resolution (R>10000) spectroscopy with CFHT-ESPaDOnS. Optical photometry in the literature shows bright, stable fluxes over most of this period, with lower fluxes (by 2-3 mag.) in 2010 and 2014. In the bright period our spectra show clear photospheric absorption, complicated variation in the Ca II 8542 A emission}profile shapes, and a large variation in redshifted absorption in the O I 7772 and 8446 A and He I 5876 A lines, suggesting unstable mass accretion during this period. In contrast, these line profiles are relatively uniform during the faint periods, suggesting stable mass accretion. During the faint periods the photospheric absorption lines are absent or marginal, and the averaged Li I profile shows redshifted absorption due to an inflow. We discuss (1) occultation by circumstellar material or a companion and (2) changes in the activity of mass accretion to explain the above results, together with near-infrared and X-ray observations from 2011-2015. Neither scenario can simply explain all the observed trends, and more theoretical work is needed to further investigate their feasibilities.

Chemical Fingerprinting and Chemical Analysis of Galactic Halo Substructure

We present high-resolution spectroscopic measurements of the abundances of the αlike element titanium (Ti) and s-process elements yttrium (Y) and lanthanum (La) for M giant candidates of (a) the Sagittarius (Sgr) dwarf spheroidal + tidal tail system, (b) the TriangulumAndromeda (TriAnd) Star Cloud, and (c) the Galactic Anticenter Stellar Structure (GASS, or Monoceros Stream). All three systems show abundance patterns unlike the Milky Way but typical of dwarf galaxies. The Sgr system abundance patterns resemble those of the Large Magellanic Cloud. GASS/Mon chemically resembles Sgr but is distinct from TriAnd, a result that does not support previous suggestions that TriAnd is a piece of the Monoceros Stream.