794 research outputs found
Angular Momentum Evolution of Young Stars: Toward a Synthesis of Observations, Theory, and Modeling
The aim of this AAS Topical Session was to update the community on the
current state of knowledge about the angular momentum evolution of young stars.
For newcomers to the subject, the session was intended to provide an
introduction and general overview and to highlight emerging issues. For
experienced workers in this field, the session provided an opportunity for
synthesizing recent developments in observations, theory, and modeling of
rotation of young stars and for identifying promising new research directions.Comment: 10 pages, conference summary, to appear in April PAS
The canonical Luminous Blue Variable AG Car and its neighbor Hen 3-519 are much closer than previously assumed
The strong mass loss of Luminous Blue Variables (LBVs) is thought to play a
critical role in massive-star evolution, but their place in the evolutionary
sequence remains debated. A key to understanding their peculiar instability is
their high observed luminosities, which often depends on uncertain distances.
Here we report direct distances and space motions of four canonical Milky Way
LBVs---AG~Car, HR~Car, HD~168607, and (candidate) Hen~3-519---from the Gaia
first data release. Whereas the distances of HR~Car and HD~168607 are
consistent with previous literature estimates within the considerable
uncertainties, Hen~3-519 and AG~Car, both at 2~kpc, are much closer than
the 6--8~kpc distances previously assumed. As a result, Hen~3-519 moves far
from the locus of LBVs on the HR Diagram, making it a much less luminous
object. For AG~Car, considered a defining example of a classical LBV, its lower
luminosity would also move it off the S~Dor instability strip. Lower
luminosities allow both AG~Car and Hen~3-519 to have passed through a previous
red supergiant phase, lower the mass estimates for their shell nebulae, and
imply that binary evolution is needed to account for their peculiarities. These
results may also impact our understanding of LBVs as potential supernova
progenitors and their isolated environments. Improved distances will be
provided in the Gaia second data release, which will include additional LBVs.
AG~Car and Hen~3-519 hint that this new information may alter our traditional
view of LBVs.Comment: 15 pages, 2 figures, 1 table. Accepted by Astronomical Journa
Magnetic Accretion and Photopolarimetric Variability in Classical T Tauri Stars
We employ a Monte Carlo radiation transfer code to investigate the multi-
wavelength photopolarimetric variability arising from a spotted T Tauri star
surrounded by a dusty circumstellar disk. Our aim is to assess the ability of
the magnetic accretion model to explain the observed photopolarimetric
variability of classical T Tauri stars, and to identify potentially useful
observational diagnostics of T Tauri star/disk/spot parameters. We model a
range of spot sizes, spot latitudes, inner disk truncation radii, and system
inclination angles, as well as multiple disk and spot geometries. We find that
the amplitude, morphology, and wavelength dependence of the photopolarimetric
variability predicted by our models are generally consistent with existing
observations; a flared disk geometry is required to reproduce the largest
observed polarization levels and variations. Our models can further explain
stochastic polarimetric variability if unsteady accretion is invoked, in which
case irregular -- but correlated -- photometric variability is predicted, in
agreement with observations.
Potentially useful observational diagnostics of system parameters of interest
are discussed. We also investigate the reliability of modeling spot parameters
via analytic fits to multi-band photometric variations.Comment: 25 pages, 8 figures To be published in ApJ, 1999, 51
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