1,576 research outputs found
A Luminous Infrared Companion in the Young Triple System WL 20
We present spatially resolved near-infrared and mid-infrared (1-25 microns)
imaging of the WL 20 triple system in the nearby (d=125 pc) rho Ophiuchi
star-forming cloud core. We find WL 20 to be a new addition to the rare class
of "infrared companion systems", with WL 20:E and WL 20:W displaying Class II
(T-Tauri star) spectral energy distributions (SEDs) and total luminosities of
0.61 and 0.39 L_sun, respectively, and WL 20:S, the infrared companion, with a
Class I (embedded protostellar) SED and a luminosity of 1.0-1.8 L_sun. WL 20:S
is found to be highly variable over timescales of years, to be extended (40 AU
diameter) at mid-infrared wavelengths, and to be the source of the centimeter
emission in the system. The photospheric luminosities, estimated from our data,
allow us to compare and test current pre-main-sequence evolutionary tracks. WL
20:E and WL 20:W fall into the region of the H-R diagram in which sources may
appear up to twice as old as they actually are using non-accreting tracks, a
fact which may reconcile the co-existence of two T-Tauri stars with an embedded
protostar in a triple system. The derived masses and observed projected
separations of the components of the WL 20 triple system indicate that it is in
an unstable dynamical configuration, and may therefore provide an example of
dynamical evolution during the pre-main-sequence phase.Comment: AASTeX 5.0, 17 pages, 4 tables, 9 figures, accepted by AJ, to appear
Feb. 200
Sensor performance analysis
The theory is described and the equations required to design are developed and the performance of electro-optical sensor systems that operate from the visible through the thermal infrared spectral regions are analyzed. Methods to compute essential optical and detector parameters, signal-to-noise ratio, MTF, and figures of merit such as NE delta rho and NE delta T are developed. A set of atmospheric tables are provided to determine scene radiance in the visible spectral region. The Planck function is used to determine radiance in the infrared. The equations developed were incorporated in a spreadsheet so that a wide variety of sensor studies can be rapidly and efficiently conducted
Pacific Hake, Merluccius productus, Autecology: A Timely Review
Pacific hake, Merluccius productus, the most abundant groundfish in the California Current Large Marine Ecosystem
(CCLME), is a species of both commercial significance, supporting a large international fishery, and ecological importance, connecting other species as both predator and prey. Coastal Pacific hake migrations are characterized by movements between northern summer feeding areas and southern winter spawning areas, with variations in annual abundance, distribution, and the extent of these movements
associated with varying climate-ocean conditions. In general, warm (cool) years with enhanced (reduced) stratification and poleward (equatorward) transport are
often related to good (poor) recruitment, increased (decreased) northward distribution, and reduced (enhanced) growth. However, the classic periodic pattern of annual migration and distribution may no longer be fully representative. Based on recent advances in the understanding of climate-ocean variability off the U.S. west
coast, we hypothesize that the annual movements of Pacific hake are more responsive to climate-ocean variability than previously thought, and further, that changes observed in Pacific hake distributions may reflect long-term changes in climate-ocean conditions in the CCLME. Therefore, an updated
model of these relations is key to effective monitoring and management of this stock, as well as to devising scenarios of future change in the CCLME as a result of climate
variations. The current state of knowledge of the relationship between the Pacific hake and its environment is reviewed, highlighting emerging ideas compared to those of the past, and priorities for future research are suggested
APM 08279+5255: Keck Near- and Mid-IR High-Resolution Imaging
We present Keck high-resolution near-IR (2.2 microns; FWHM~0.15") and mid-IR
(12.5 microns; FWHM~0.4") images of APM08279+5255, a z=3.91 IR-luminous BALQSO
with a prodigious apparent bolometric luminosity of 5x10^{15} Lsun, the largest
known in the universe. The K-band image shows that this system consists of
three components, all of which are likely to be the gravitationally lensed
images of the same background object, and the 12.5 micron image shows a
morphology consistent with such an image configuration. Our lens model suggests
that the magnification factor is ~100 from the restframe UV to mid-IR, where
most of the luminosity is released. The intrinsic bolometric luminosity and IR
luminosity of APM08279+5255 are estimated to be 5x10^{13} Lsun and 1x10^{13}
Lsun, respectively. This indicates that APM 08279+5255 is intriniscally
luminous, but it is not the most luminous object known. As for its dust
contents, little can be determined with the currently available data due to the
uncertainties associated with the dust emissivity and the possible effects of
differential magnification. We also suggest that the lensing galaxy is likely
to be a massive galaxy at z~3.Comment: 32 pages, 4 tables, 11 figures; Accepted for publication in Ap
Mid-infrared Imaging of a Circumstellar Disk Around HR 4796: Mapping the Debris of Planetary Formation
We report the discovery of a circumstellar disk around the young A0 star, HR
4796, in thermal infrared imaging carried out at the W.M. Keck Observatory. By
fitting a model of the emission from a flat dusty disk to an image at
lambda=20.8 microns, we derive a disk inclination, i = 72 +6/-9 deg from face
on, with the long axis of emission at PA 28 +/-6 deg. The intensity of emission
does not decrease with radius as expected for circumstellar disks but increases
outward from the star, peaking near both ends of the elongated structure. We
simulate this appearance by varying the inner radius in our model and find an
inner hole in the disk with radius R_in = 55+/-15 AU. This value corresponds to
the radial distance of our own Kuiper belt and may suggest a source of dust in
the collision of cometesimals. By contrast with the appearance at 20.8 microns,
excess emission at lambda = 12.5 microns is faint and concentrated at the
stellar position. Similar emission is also detected at 20.8 microns in residual
subtraction of the best-fit model from the image. The intensity and ratio of
flux densities at the two wavelengths could be accounted for by a tenuous dust
component that is confined within a few AU of the star with mean temperature of
a few hundred degrees K, similar to that of zodiacal dust in our own solar
system. The morphology of dust emission from HR 4796 (age 10 Myr) suggests that
its disk is in a transitional planet-forming stage, between that of massive
gaseous proto-stellar disks and more tenuous debris disks such as the one
detected around Vega.Comment: 9 pages, 4 figures as LaTex manuscript and postscript files in
gzipped tar file. Accepted for publication in Astrophysical Journal Letters.
http://upenn5.hep.upenn.edu/~davidk/hr4796.htm
In situ structural investigations of the (MoVW)<sub>5</sub>O<sub>14</sub> type mixed molybdenum oxide catalysts
In situ investigations of structure-activity correlations of mixed molybdenum oxide catalysts
High Resolution Infrared Imaging of the Compact Nuclear Source in NGC4258
We present high resolution imaging of the nucleus of NGC4258 from 1 micron to
18 microns. Our observations reveal that the previously discovered compact
source of emission is unresolved even at the near-infrared resolution of about
0.2 arcsec FWHM which corresponds to about 7 pc at the distance of the galaxy.
This is consistent with the source of emission being the region in the
neighborhood of the purported 3.5*10^7 M_sun black hole. After correcting for
about 18 mags of visual extinction, the infrared data are consistent with a
F_nu \propto nu^(-1.4+/-0.1) spectrum from 1.1 micron to 18 micron, implying a
non-thermal origin. Based on this spectrum, the total extinction corrected
infrared luminosity (1-20 micron) of the central source is 2*10^8 L_sun. We
argue that the infrared spectrum and luminosity of the central source obviates
the need for a substantial contribution from a standard, thin accretion disk at
these wavelengths and calculate the accretion rate through an advection
dominated accretion flow to be Mdot \sim 10^(-3) M_sun/yr. The agreement
between these observations and the theoretical spectral energy distribution for
advection dominated flows provides evidence for the existence of an advection
dominated flow in this low luminosity AGN.Comment: 21 pages, 5 figures, Appearing in Mar 2000 ApJ vol. 53
- …