5,422 research outputs found
Quadri-tilings of the plane
We introduce {\em quadri-tilings} and show that they are in bijection with
dimer models on a {\em family} of graphs arising from rhombus
tilings. Using two height functions, we interpret a sub-family of all
quadri-tilings, called {\em triangular quadri-tilings}, as an interface model
in dimension 2+2. Assigning "critical" weights to edges of , we prove an
explicit expression, only depending on the local geometry of the graph ,
for the minimal free energy per fundamental domain Gibbs measure; this solves a
conjecture of \cite{Kenyon1}. We also show that when edges of are
asymptotically far apart, the probability of their occurrence only depends on
this set of edges. Finally, we give an expression for a Gibbs measure on the
set of {\em all} triangular quadri-tilings whose marginals are the above Gibbs
measures, and conjecture it to be that of minimal free energy per fundamental
domain.Comment: Revised version, minor changes. 30 pages, 13 figure
Anisotropic KPZ growth in 2+1 dimensions: fluctuations and covariance structure
In [arXiv:0804.3035] we studied an interacting particle system which can be
also interpreted as a stochastic growth model. This model belongs to the
anisotropic KPZ class in 2+1 dimensions. In this paper we present the results
that are relevant from the perspective of stochastic growth models, in
particular: (a) the surface fluctuations are asymptotically Gaussian on a
sqrt(ln(t)) scale and (b) the correlation structure of the surface is
asymptotically given by the massless field.Comment: 13 pages, 4 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
Combined ultraviolet studies of astronomical sources
Topics addressed include: Cygnus Loop; P Cygni profiles in dwarf novae; YY Gem; nova shells; HZ Herculis; activity cycles in cluster giants; Alpha Ori; metal deficient giant stars; ultraviolet spectra of symbiotic stars detected by the Very Large Array; time variability in symbiotic stars; blue galaxies; and quasistellar objects with X-ray spectra
Optical and infrared properties of V1647 Orionis during the 2003-2006 outburst. I The reflection nebula
Aims: The recent outburst of the young eruptive star V1647 Orionis has
produced a spectacular appearance of a new reflection nebula in Orion (McNeil's
nebula). We present an optical/near infrared investigation of McNeil's nebula.
This analysis is aimed at determining the morphology, temporal evolution and
nature of the nebula and its connection to the outburst.
Method: We performed multi epoch B, V, R, I, z, and K imaging of McNeil's
nebula and V1647 Ori as well as K_S imaging polarimetry. The multiband imaging
allows us to reconstruct the extinction map inside the nebula. Through
polarimetric observations we attempt to disentangle the emission from the
nebula from that of the accretion disk around V1647 Ori. We also attempt to
resolve the small spatial scale structure of the illuminating source.
Results: The energy distribution and temporal evolution of McNeil's nebula
mimic that of the illuminating source. The extinction map reveals a region of
higher extinction in the direction of V1647 Ori. Excluding foreground
extionction, the optical extinction due to McNeil's nebula in the direction of
V1647 Ori is A_V ~ 6.5 mag. The polarimetric measurement shows a compact high
polarization emission around V1647 Ori. The percentage of K_S band linear
polarization goes from 10 -- 20 %. The vectors are all well aligned with a
position angle of 90 +/- 9 degree East of North. This may correspond to the
orientation of a possible accretion disk around V1647 Ori. These findings
suggest that the appearance of McNeil's nebula is due to reflection of light by
pre-existing material in the surroundings of V1647 Ori. We also report on the
discovery of a new candidate brown dwarf or protostar in the vicinity of V1647
Ori as well as the presence of clumpy structure within HH 22A.Comment: 8 pages, 7 figures, in pres
Ultra‐sensitive transition‐edge sensors (TESs) for far-IR/submm space‐borne spectroscopy
We have built surface micromachined thin‐film metallized Si_xN_y optical absorbers for transition‐edge sensors (TESs) suitable for the Background‐Limited far‐IR/Submm Spectrograph (BLISS). BLISS is a broadband (38 μm–433 μm), grating spectrometer consisting of five wavebands each with a modest resolution of R ∼1000. Because BLISS requires the effective noise equivalent power (NEP) of the TES to be below 10^(-19) W/Hz^(1/2), our TESs consist of four long (1000 μm), narrow (0.4 μm), and thin (0.25 μm) Si_xN_y support beams that reduce the thermal conductance G between the substrate and the optical absorber. To reduce the heat capacity of the absorber and make the response time τ fast enough for BLISS, our absorbers are patterned into a mesh geometry with a fill factor of less than 10%. We use a bilayer of Ti/Au to make the effective impedance of the absorber match the impedance of the incoming radiation for each band. Measurements of the response time of the metallized absorbers to heat pulses show that their heat capacity exceeds the predictions of the Debye model. Our results are suggestive that the surface of the low pressure chemical vapor deposition (LPCVD) Si_xN_y used in the absorbers’ construction is the source of microstates that dominate the heat capacity
Optical spectra of selected Chamaeleon I young stellar objects
We present optical spectra of eight candidate brown dwarfs and a previously
known T Tauri star (Sz 33) of the Chamaeleon I dark cloud. We derived spectral
types based on the strength of the TiO or VO absorption bands present in the
spectra of these objects as well as on the PC3 index of Martin et al. (1999).
Photometric data from the literature are used to estimate the bolometric
luminosities for these sources. We apply D'Antona & Mazzitelli (1997) pre-main
sequence evolutionary tracks and isochrones to derive masses and ages. Based on
the presence of Halpha in emission, we confirm that most of the candidates are
young objects. Our sample however includes two sources for which we can only
provide upper limits for the emission in Halpha; whereas these two objects are
most likely foreground/background stars, higher resolution spectra are required
to confirm their true nature. Among the likely cloud members, we detect one new
sub-stellar object and three transition stellar/sub-stellar sources.Comment: 22 pages - manuscript forma
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