4,321 research outputs found
Detectability of Exoplanets in the Beta Pic Moving Group with the Gemini Planet Imager
We model the detectability of exoplanets around stars in the Beta Pic Moving
Group (BPMG) using the Gemini Planet Imager (GPI), a coronagraphic instrument
designed to detect companions by imaging. Members of the BPMG are considered
promising targets for exoplanet searches because of their youth (~12 MY) and
proximity (median distance ~35 pc). We wrote a modeling procedure to generate
hypothetical companions of given mass, age, eccentricity, and semi-major axis,
and place them around BPMG members that fall within the V-band range of the
GPI. We count as possible detections companions lying within the GPI's field of
view and H-band fluxes that have a host-companion flux ratio placing them
within its sensitivity. The fraction of companions that could be detected
depends on their brightness at 12 Myr, and hence formation mechanism, and on
their distribution of semi-major axes. We used brightness models for formation
by disk instability and core-accretion. We considered the two extreme cases of
the semi-major axis distribution - the log-normal distribution of the nearby F
and G type stars and a power-law distribution indicated by the exoplanets
detected by the radial velocity technique. We find that the GPI could detect
exoplanets of all the F and G spectral type stars in the BPMG sample with a
probability that depends on the brightness model and semi-major axis
distribution. At spectral type K to M1, exoplanet detectability depends on
brightness and hence distance of the host star. GPI will be able to detect the
companions of M stars later than M1 only if they are closer than 10 pc. Of the
four A stars in BPMG sample, only one has V-band brightness in the range of
GPI; the others are too bright.Comment: Accepted for publication in the Astronomical Journa
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
Truthful Matching with Online Items and Offline Agents
We study truthful mechanisms for welfare maximization in online bipartite matching. In our (multi-parameter) setting, every buyer is associated with a (possibly private) desired set of items, and has a private value for being assigned an item in her desired set. Unlike most online matching settings, where agents arrive online, in our setting the items arrive online in an adversarial order while the buyers are present for the entire duration of the process. This poses a significant challenge to the design of truthful mechanisms, due to the ability of buyers to strategize over future rounds. We provide an almost full picture of the competitive ratios in different scenarios, including myopic vs. non-myopic agents, tardy vs. prompt payments, and private vs. public desired sets. Among other results, we identify the frontier up to which the celebrated e/(e-1) competitive ratio for the vertex-weighted online matching of Karp, Vazirani and Vazirani extends to truthful agents and online items
Dimension of the repeller for a piecewise expanding affine map
In this paper, we study the dimension theory of a class of piecewise affine systems in euclidean spaces suggested by Barnsley, with some applications to the fractal image compression. It is a more general version of the class considered in the work of Keane, Simon and Solomyak and can be considered as the continuation of the works by the authors. We also present some applications of our results for generalized Takagi functions and fractal interpolation functions
On the dimension of triangular self-affine sets
As a continuation of a recent work [Bárány et al, On the dimension of self-affine sets and measures with overlaps. Proc. Amer. Math. Soc. 144 (2016) 4427–4440] of the same authors, in this note we study the dimension theory of diagonally homogeneous triangular planar self-affine iterated function systems
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