5,323 research outputs found
The discovery of a low mass, pre-main-sequence stellar association around gamma Velorum
We report the serendipitous discovery of a population of low mass, pre-main
sequence stars (PMS) in the direction of the Wolf-Rayet/O-star binary system
gamma^{2} Vel and the Vela OB2 association. We argue that gamma^{2} Vel and the
low mass stars are truly associated, are approximately coeval and that both are
at distances between 360-490 pc, disagreeing at the 2 sigma level with the
recent Hipparcos parallax of gamma^{2} Vel, but consistent with older distance
estimates. Our results clearly have implications for the physical parameters of
the gamma^{2} Vel system, but also offer an exciting opportunity to investigate
the influence of high mass stars on the mass function and circumstellar disc
lifetimes of their lower mass PMS siblings.Comment: Monthly Notices of the Royal Astronomical Society, Letters - in pres
A note on dimer models and McKay quivers
We give one formulation of an algorithm of Hanany and Vegh which takes a
lattice polygon as an input and produces a set of isoradial dimer models. We
study the case of lattice triangles in detail and discuss the relation with
coamoebas following Feng, He, Kennaway and Vafa.Comment: 25 pages, 35 figures. v3:completely rewritte
Discrete complex analysis on planar quad-graphs
We develop a linear theory of discrete complex analysis on general
quad-graphs, continuing and extending previous work of Duffin, Mercat, Kenyon,
Chelkak and Smirnov on discrete complex analysis on rhombic quad-graphs. Our
approach based on the medial graph yields more instructive proofs of discrete
analogs of several classical theorems and even new results. We provide discrete
counterparts of fundamental concepts in complex analysis such as holomorphic
functions, derivatives, the Laplacian, and exterior calculus. Also, we discuss
discrete versions of important basic theorems such as Green's identities and
Cauchy's integral formulae. For the first time, we discretize Green's first
identity and Cauchy's integral formula for the derivative of a holomorphic
function. In this paper, we focus on planar quad-graphs, but we would like to
mention that many notions and theorems can be adapted to discrete Riemann
surfaces in a straightforward way.
In the case of planar parallelogram-graphs with bounded interior angles and
bounded ratio of side lengths, we construct a discrete Green's function and
discrete Cauchy's kernels with asymptotics comparable to the smooth case.
Further restricting to the integer lattice of a two-dimensional skew coordinate
system yields appropriate discrete Cauchy's integral formulae for higher order
derivatives.Comment: 49 pages, 8 figure
The TAOS Project: Upper Bounds on the Population of Small KBOs and Tests of Models of Formation and Evolution of the Outer Solar System
We have analyzed the first 3.75 years of data from TAOS, the Taiwanese
American Occultation Survey. TAOS monitors bright stars to search for
occultations by Kuiper Belt Objects (KBOs). This dataset comprises 5e5
star-hours of multi-telescope photometric data taken at 4 or 5 Hz. No events
consistent with KBO occultations were found in this dataset. We compute the
number of events expected for the Kuiper Belt formation and evolution models of
Pan & Sari (2005), Kenyon & Bromley (2004), Benavidez & Campo Bagatin (2009),
and Fraser (2009). A comparison with the upper limits we derive from our data
constrains the parameter space of these models. This is the first detailed
comparison of models of the KBO size distribution with data from an occultation
survey. Our results suggest that the KBO population is comprised of objects
with low internal strength and that planetary migration played a role in the
shaping of the size distribution.Comment: 18 pages, 16 figures, Aj submitte
Planet formation around stars of various masses: The snow line and the frequency of giant planets
We use a semi-analytic circumstellar disk model that considers movement of
the snow line through evolution of accretion and the central star to
investigate how gas giant frequency changes with stellar mass. The snow line
distance changes weakly with stellar mass; thus giant planets form over a wide
range of spectral types. The probability that a given star has at least one gas
giant increases linearly with stellar mass from 0.4 M_sun to 3 M_sun. Stars
more massive than 3 M_sun evolve quickly to the main-sequence, which pushes the
snow line to 10-15 AU before protoplanets form and limits the range of disk
masses that form giant planet cores. If the frequency of gas giants around
solar-mass stars is 6%, we predict occurrence rates of 1% for 0.4 M_sun stars
and 10% for 1.5 M_sun stars. This result is largely insensitive to our assumed
model parameters. Finally, the movement of the snow line as stars >2.5 M_sun
move to the main-sequence may allow the ocean planets suggested by Leger et.
al. to form without migration.Comment: Accepted to ApJ. 12 pages of emulateap
Collisional Cascades in Planetesimal Disks II. Embedded Planets
We use a multiannulus planetesimal accretion code to investigate the growth
of icy planets in the outer regions of a planetesimal disk. In a quiescent
minimum mass solar nebula, icy planets grow to sizes of 1000--3000 km on a
timescale t = 15-20 Myr (a/30 AU)^3 where a is the distance from the central
star. Planets form faster in more massive nebulae. Newly-formed planets stir up
leftover planetesimals along their orbits and produce a collisional cascade
where icy planetesimals are slowly ground to dust.
The dusty debris of planet formation has physical characteristics similar to
those observed in beta Pic, HR 4796A, and other debris disks. We derive dust
masses for small particles, 1 mm and smaller, and large particles, 1 mm and
larger, as a function of the initial conditions in the planetesimal disk. The
dust luminosities derived from these masses are similar to those observed in
Vega, HR 4796A, and other debris disks. The calculations produce bright rings
and dark gaps. Bright rings occur where 1000 km and larger planets have
recently formed. Dark gaps are regions where planets have cleared out dust or
shadows where planets have yet to form.Comment: to be published in the Astronomical Journal, January 2004; 7 pages of
text; 17 figures at
http://cfa-www.harvard.edu/~kenyon/pf/emb-planet-figures.pdf; 2 animations at
http://cfa-www.harvard.edu/~kenyon/pf/emb-planet-movies.htm
A Hybrid N-body--Coagulation Code for Planet Formation
We describe a hybrid algorithm to calculate the formation of planets from an
initial ensemble of planetesimals. The algorithm uses a coagulation code to
treat the growth of planetesimals into oligarchs and explicit N-body
calculations to follow the evolution of oligarchs into planets. To validate the
N-body portion of the algorithm, we use a battery of tests in planetary
dynamics. Several complete calculations of terrestrial planet formation with
the hybrid code yield good agreement with previously published calculations.
These results demonstrate that the hybrid code provides an accurate treatment
of the evolution of planetesimals into planets.Comment: Astronomical Journal, accepted; 33 pages + 11 figure
The Shortest Period Detached Binary White Dwarf System
We identify SDSS J010657.39-100003.3 (hereafter J0106-1000) as the shortest
period detached binary white dwarf (WD) system currently known. We targeted
J0106-1000 as part of our radial velocity program to search for companions
around known extremely low-mass (ELM, ~ 0.2 Msol) WDs using the 6.5m MMT. We
detect peak-to-peak radial velocity variations of 740 km/s with an orbital
period of 39.1 min. The mass function and optical photometry rule out a
main-sequence star companion. Follow-up high-speed photometric observations
obtained at the McDonald 2.1m telescope reveal ellipsoidal variations from the
distorted primary but no eclipses. This is the first example of a tidally
distorted WD. Modeling the lightcurve, we constrain the inclination angle of
the system to be 67 +- 13 deg. J0106-1000 contains a pair of WDs (0.17 Msol
primary + 0.43 Msol invisible secondary) at a separation of 0.32 Rsol. The two
WDs will merge in 37 Myr and most likely form a core He-burning single subdwarf
star. J0106-1000 is the shortest timescale merger system currently known. The
gravitational wave strain from J0106-1000 is at the detection limit of the
Laser Interferometer Space Antenna (LISA). However, accurate ephemeris and
orbital period measurements may enable LISA to detect J0106-1000 above the
Galactic background noise.Comment: MNRAS Letters, in pres
UV and X-Ray Monitoring of AG Draconis During the 1994/1995 Outbursts
The recent 1994-1995 active phase of AG Draconis has given us for the first
time the opportunity to follow the full X-ray behaviour of a symbiotic star
during two successive outbursts and to compare with its quiescence X-ray
emission. With \ros observations we have discovered a remarkable decrease of
the X-ray flux during both optical maxima, followed by a gradual recovering to
the pre-outburst flux. In the UV the events were characterized by a large
increase of the emission line and continuum fluxes, comparable to the behaviour
of AG Dra during the 1980-81 active phase. The anticorrelation of X-ray/UV flux
and optical brightness evolution is shown to very likely be due to a
temperature decrease of the hot component. Such a temperature decrease could be
produced by an increased mass transfer to the burning compact object, causing
it to slowly expand to about twice its original size.Comment: 12 pages postscript incl. figures, Proc. of Workshop on Supersoft
X-Ray Sources, to appear in Lecture Notes in Physics vol. 472 (1996
Observations and simulations of recurrent novae: U Sco and V394 CrA
Observations and analysis of the Aug. 1987 outburst of the recurrent nova V394 CrA are presented. This nova is extremely fast and its outburst characteristics closely resemble those of the recurrent nova U Sco. Hydrodynamic simulations of the outbursts of recurrent novae were performed. Results as applied to the outbursts of V394 CrA and U Sco are summarized
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