2,882 research outputs found
The pre-main sequence spectroscopic binary UZ Tau East: improved orbital parameters and accretion phase dependence
We present radial-velocity measurements obtained using high- and
intermediate-resolution spectroscopic observations of the classical T Tauri
star UZ Tau East obtained from 1994 to 1996. We also provide measurements of
H equivalent widths and optical veiling. Combining our radial-velocity
data with those recently reported by Prato et al. (2002), we improve the
orbital elements for this spectroscopic binary. The orbital period is
18.9790.007 days and the eccentricity is e=0.14. We find variability in
the H emission and veiling, signposts of accretion, but at periastron
passage the accretion is not as clearly enhanced as in the case of the binary
DQ Tau. The difference in the behaviour of these two binaries is consistent
with the hydrodynamical models of accretion from circumbinary disks because UZ
Tau East has lower eccentricity than DQ Tau. It seems that enhanced periastron
accretion may occur only in systems with very high eccentricity (e0.5).Comment: accepted for publication in A&
Young Binary Stars and Associated Disks
The typical product of the star formation process is a binary star. Binaries
have provided the first dynamical measures of the masses of pre-main-sequence
(PMS) stars, providing support for the calibrations of PMS evolutionary tracks.
Surprisingly, in some star-forming regions PMS binary frequencies are higher
than among main-sequence solar-type stars. The difference in PMS and
main-sequence binary frequencies is apparently not an evolutionary effect;
recent attention has focussed on correlations between binary frequency and
stellar density or cloud temperatures. Accretion disks are common among young
binary stars. Binaries with separations between 1 AU and 100 AU have
substantially less submillimeter emission than closer or wider binaries,
suggesting that they have truncated their disks. Evidence of dynamical clearing
has been seen in several binaries. Remarkably, PMS binaries of all separations
show evidence of circumstellar disks and continued accretion. This suggests
that the circumstellar disks are replenished from circumbinary disks or
envelopes. The frequent presence of disks suggests that planet formation can
occur in binary environments, and formation of planets in wide binaries is
already established by their discovery. Circumbinary disk masses around very
short period binaries are ample to form planetary systems such as our own. The
nature of planetary systems among the most common binaries, with separations
between 10 AU and 100 AU, is less clear given the observed reduction in disk
mass, though they may have disk masses adequate for the formation of
terrestrial-like planets.Comment: 32 pages, including 6 Postscript figures (TeX, uses psfig.sty); to
appear in "Protostars & Planets IV". Gif figures with captions and high-res
Postscript color figure available at
http://hven.swarthmore.edu/~jensen/preprints/ppiv.htm
Linear Haskell: practical linearity in a higher-order polymorphic language
Linear type systems have a long and storied history, but not a clear path
forward to integrate with existing languages such as OCaml or Haskell. In this
paper, we study a linear type system designed with two crucial properties in
mind: backwards-compatibility and code reuse across linear and non-linear users
of a library. Only then can the benefits of linear types permeate conventional
functional programming. Rather than bifurcate types into linear and non-linear
counterparts, we instead attach linearity to function arrows. Linear functions
can receive inputs from linearly-bound values, but can also operate over
unrestricted, regular values.
To demonstrate the efficacy of our linear type system - both how easy it can
be integrated in an existing language implementation and how streamlined it
makes it to write programs with linear types - we implemented our type system
in GHC, the leading Haskell compiler, and demonstrate two kinds of applications
of linear types: mutable data with pure interfaces; and enforcing protocols in
I/O-performing functions
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