1,689 research outputs found
The First Direct Distance and Luminosity Determination for a Self-Luminous Giant Exoplanet: The Trigonometric Parallax to 2MASS1207334-393254Ab
We present the first trigonometric parallax and distance for a young
planetary mass object. A likely TW Hya cluster member, 2MASSW J1207334-393254Ab
(hereafter 2M1207Ab) is an M8 brown dwarf with a mid to late L type planetary
mass companion. Recent observations of spectral variability have uncovered
clear signs of disk accretion and outflow, constraining the age of the system
to <10 Myr. Because of its late spectral type and the clearly youthful nature
of the system, 2M1207b is very likely a planetary mass object. We have measured
the first accurate distance and luminosity for a self-luminous planetary mass
object. Our parallax measurements are accurate to <2 mas (1sigma) for 2M1207Ab.
With 11 total epochs of data taken from January 2006 through April 2 007 (475
images for 2M1207Ab), we determine a distance of 58.8+-7.0 pc (17.0{+2.3}{-1.8}
mas, 1.28sigma) to 2M1207Ab and a calculated luminosity of 0.68-2.2x10^-5 Lsun
for 2M1207b. Hence 2M1207Ab is a clear member of the TW Hya cluster in terms of
its distance, proper motions, and youthful nature. However, as previously noted
by Mohanty and co-workers, 2M1207b's luminosity appears low compared to its
temperature according to evolutionary models.Comment: 12 pages, 3 figures, accepted to ApJ Letter
Science capabilities of the VERITAS array of 10m imaging atmospheric Cherenkov gamma-ray detectors
Journal ArticleThe Very Energetic Radiation Imaging Telescope Array System (VERITAS) is an array of seven 10m aperture telescopes used for gamma-ray astronomy in the 50 GeV to 50 TeV (1 TeV= 101 2 electron Volt) energy range. The gamma rays are detected by measuring the optical Cherenkov light emitted by the cascade of electromagnetic particles that is generated by interactions of the high energy gamma-ray with the Earth's Atmosphere. This paper describes the science goals of the VERITAS array, a description of the array, and expected performance of the instrument
The very nearby M/T dwarf binary SCR 1845-6357
The recently discovered star SCR 1845-6357 is the first late M/T dwarf binary
discovered. SCR 1845 is a particular object due to its tight orbit (currently
around 4 AU) and its proximity to the Sun (3.85 pc). We present spatially
resolved VLT/NACO images and low resolution spectra of SCR 1845 in the J, H and
K near-infrared bands. Since the T dwarf companion, SCR 1845B, is so close to
the primary SCR 1845A, orbital motion is evident even within a year. Following
the orbital motion, the binary's mass can be measured accurately within a
decade, making SCR 1845B a key T-dwarf mass-luminosity calibrator. The NIR
spectra allow for accurate determination of spectral type and also for rough
estimates of the object's physical parameters. The spectral type of SCR 1845B
is determined by direct comparison of the flux calibrated JHK spectra with T
dwarf standard template spectra and also by NIR spectral indices obtained from
synthetic photometry. Constrained values for surface gravity, effective
temperature and metallicity are derived by comparison with model spectra. Our
data prove that SCR 1845B is a brown dwarf of spectral type T6 that is
co-moving with and therefore gravitationally bound to the M8.5 primary. Fitting
the NIR spectrum of SCR 1845B to model spectra yields an effective temperature
of about 950K and a surface gravity log(g)=5.1 (cgs) assuming solar
metallicity. Mass and age of SCR 1845B are in the range 40 to 50 Jupiter masses
and 1.8 to 3.1 Gyr.Comment: 5 pages, 4 figures, accepted for publication in Astronomy &
Astrophysic
Accounting for parameter uncertainty in large-scale stochastic simulations with correlated inputs
This paper considers large-scale stochastic simulations with correlated inputs having normal-to-anything (NORTA) distributions with arbitrary continuous marginal distributions. Examples of correlated inputs include processing times of workpieces across several workcenters in manufacturing facilities and product demands and exchange rates in global supply chains. Our goal is to obtain mean performance measures and confidence intervals for simulations with such correlated inputs by accounting for the uncertainty around the NORTA distribution parameters estimated from finite historical input data. This type of uncertainty is known as the parameter uncertainty in the discrete-event stochastic simulation literature. We demonstrate how to capture parameter uncertainty with a Bayesian model that uses Sklar's marginal-copula representation and Cooke's copula-vine specification for sampling the parameters of the NORTA distribution. The development of such a Bayesian model well suited for handling many correlated inputs is the primary contribution of this paper. We incorporate the Bayesian model into the simulation replication algorithm for the joint representation of stochastic uncertainty and parameter uncertainty in the mean performance estimate and the confidence interval. We show that our model improves both the consistency of the mean line-item fill-rate estimates and the coverage of the confidence intervals in multiproduct inventory simulations with correlated demands. © 2011 INFORMS
Quantifying input uncertainty in an assemble-to-order system simulation with correlated input variables of mixed types
We consider an assemble-to-order production system where the product demands and the time since the last customer arrival are not independent. The simulation of this system requires a multivariate input model that generates random input vectors with correlated discrete and continuous components. In this paper, we capture the dependence between input variables in an undirected graphical model and decouple the statistical estimation of the univariate input distributions and the underlying dependence measure into separate problems. The estimation errors due to finiteness of the real-world data introduce the so-called input uncertainty in the simulation output. We propose a method that accounts for input uncertainty by sampling the univariate empirical distribution functions via bootstrapping and by maintaining a posterior distribution of the precision matrix that corresponds to the dependence structure of the graphical model. The method improves the coverages of the confidence intervals for the expected profit the per period. © 2014 IEEE
On the origin of planets at very wide orbits from the re-capture of free floating planets
In recent years several planets have been discovered at wide orbits (>100 AU)
around their host stars. Theoretical studies encounter difficulties in
explaining their formation and origin. Here we propose a novel scenario for the
production of planetary systems at such orbits, through the dynamical recapture
of free floating planets (FFPs) in dispersing stellar clusters. This process is
a natural extension of the recently suggested scenario for the formation of
wide stellar binaries. We use N-body simulations of dispersing clusters with
10-1000 stars and comparable numbers of FFPs to study this process. We find
that planets are captured into wide orbits in the typical range ~100-10^6 AU,
and have a wide range of eccentricities (thermal distribution). Typically, 3-6
x (f_FFP/1) % of all stars capture a planetary companion with such properties
(where f_FFP is the number of FFP per star). The planetary capture efficiency
is comparable to that of capture-formed stellar-binaries, and shows a similar
dependence on the cluster size and structure. It is almost independent of the
specific planetary mass; planets as well as sub-stellar companions of any mass
can be captured. The capture efficiency decreases with increasing cluster size,
and for a given cluster size the it increases with the host/primary mass. More
than one planet can be captured around the same host and planets can be
captured into binary systems. Planets can also be captured into pre-existing
planetary and into orbits around black holes and massive white dwarfs, if these
formed early enough before the cluster dispersal. In particular, stellar black
holes have a high capture efficiency (>50 % and 5-10 x (f_FFP/1) % for capture
of stars and planetary companions, respectively) due to their large mass.
Finally, although rare, two FFPs or brown dwarfs can become bound and form a
FFP-binary system with no stellar host.Comment: ApJ, in press. Added two figure
Hot Gas Structure in the Elliptical Galaxy NGC 4472
We present X-ray spectroscopic and morphological analyses using Chandra ACIS
and ROSAT observations of the giant elliptical galaxy NGC 4472 in the Virgo
cluster. We discuss previously unobserved X-ray structures within the extended
galactic corona. In the inner 2' of the galaxy, we find X-ray holes or cavities
with radii of ~2 kpc, corresponding to the position of radio lobes. These holes
were produced during a period of nuclear activity that began 1.2 x 10^7 years
ago and may be ongoing. We also find an asymmetrical edge in the galaxy X-ray
emission 3' (14 kpc) northeast of the core and an ~8' tail (36 kpc) extending
southwest of the galaxy. These two features probably result from the
interaction of NGC 4472 gas with the Virgo gas, which produces compression in
the direction of NGC 4472's infall and an extended tail from ram pressure
stripping. Assuming the tail is in pressure equilibrium with the surrounding
gas, we compute its angle to our line of sight and estimate that its true
extent exceeds 100 kpc. Finally, in addition to emission from the nucleus
(first detected by Soldatenkov, Vikhlinin & Pavlinsky), we detect two small
extended sources within 10'' of the nucleus of the galaxy, both of which have
luminosities of ~7 x 10^38 erg/s.Comment: 25 pages, 11 figures, accepted by Ap
Cascading on extragalactic background light
High-energy gamma-rays propagating in the intergalactic medium can interact
with background infrared photons to produce e+e- pairs, resulting in the
absorption of the intrinsic gamma-ray spectrum. TeV observations of the distant
blazar 1ES 1101-232 were thus recently used to put an upper limit on the
infrared extragalactic background light density. The created pairs can
upscatter background photons to high energies, which in turn may pair produce,
thereby initiating a cascade. The pairs diffuse on the extragalactic magnetic
field (EMF) and cascade emission has been suggested as a means for measuring
its intensity. Limits on the IR background and EMF are reconsidered taking into
account cascade emissions. The cascade equations are solved numerically.
Assuming a power-law intrinsic spectrum, the observed 100 MeV - 100 TeV
spectrum is found as a function of the intrinsic spectral index and the
intensity of the EMF. Cascades emit mainly at or below 100 GeV. The observed
TeV spectrum appears softer than for pure absorption when cascade emission is
taken into account. The upper limit on the IR photon background is found to be
robust. Inversely, the intrinsic spectra needed to fit the TeV data are
uncomfortably hard when cascade emission makes a significant contribution to
the observed spectrum. An EMF intensity around 1e-8 nG leads to a
characteristic spectral hump in the GLAST band. Higher EMF intensities divert
the pairs away from the line-of-sight and the cascade contribution to the
spectrum becomes negligible.Comment: 5 pages, to be published as a research note in A&
- …