751 research outputs found
Development of far infrared detection techniques
This grant supports the development of a variety of advanced far infrared detection techniques that will be used in future NASA missions such as the Space Infrared Telescope Facility (SIRTF). These studies span the wavelength region of 30-200 microns and include development of focal planes and electronics that would utilize them. Efforts reported here represent collaborations among the University of Arizona's Steward Observatory; Lawrence Berkeley Laboratories (LBL); and the University of California at Berkeley. The overall goal of this program is to demonstrate extremely high performance detectors for low background applications between 30-200 microns. For the 40-120 micron region, the program is developing a 32x32 filled detector array. Previous work has demonstrated the required performance with a Z-Plane array architecture; we are now upgrading construction facilities and techniques to increase yields and reliability. We have completed the initial tradeoff analysis for the interconnects between the detectors and and readouts. We found satisfactory performance for both Flex-Cable and Tape Automated Bonding (TAB) devices, but analysis showed that an all-sapphire device would not meet our requirements. In addition, the effort continued to develop readouts that can operate close to the detector element temperature; success would substantially improve the manufacturability of the arrays. For the 100-200 micron range, previous work has demonstrated good performance with individual detector elements of stressed Ge:Ga; current efforts are to increase the quantum efficiency of these devices. Work continues to discover how to construct an alternate type of long wave detector, Ge:B Blocked Impurity Band devices. Following descoping of SIRTF, we are closing out the bolometer and refrigerator development. We documented the optical designs and approaches developed previously to meet the specific requirements of these detector types in terms of modulation of the signals for good photometric behavior within the constraints of a compact and self-contained cryogenic instrument. Further study was initiated for the Band 3 optical train, where we have concerns about complexity and the manufacturability of one component
The Intrinsic Far-infrared Continua of Type-1 Quasars
The range of currently proposed active galactic nucleus (AGN) far-infrared
templates results in uncertainties in retrieving host galaxy information from
infrared observations and also undermines constraints on the outer part of the
AGN torus. We discuss how to test and reconcile these templates. Physically,
the fraction of the intrinsic AGN IR-processed luminosity compared with that
from the central engine should be consistent with the dust-covering factor. In
addition, besides reproducing the composite spectral energy distributions
(SEDs) of quasars, a correct AGN IR template combined with an accurate library
of star-forming galaxy templates should be able to reproduce the IR properties
of the host galaxies, such as the luminosity-dependent SED shapes and aromatic
feature strengths. We develop tests based on these expected behaviors and find
that the shape of the AGN intrinsic far-IR emission drops off rapidly starting
at m and can be matched by an Elvis et al. (1994)-like template
with minor modification. Despite the variations in the near- to mid-IR bands,
AGNs in quasars and Seyfert galaxies have remarkably similar intrinsic far-IR
SEDs at - m, suggesting similar emission character of
the outermost region of the circumnuclear torus. The variations of the
intrinsic AGN IR SEDs among the type-1 quasar population can be explained by
the changing relative strengths of four major dust components with similar
characteristic temperatures, and there is evidence for compact AGN-heated dusty
structures at sub-kpc scales in the far-IR.Comment: Minor corrections to match the published version, 14 pages, 9
figures, 5 tables. The quasar intrinsic IR templates can be found at
http://u.arizona.edu/~jianwei/data/AGN_temp.ascii or in the published pape
Polar Dust, Nuclear Obscuration and IR SED Diversity in Type-1 AGNs
Despite the hypothesized similar face-on viewing angles, the infrared
emission of type-1 AGNs has diverse spectral energy distribution (SED) shapes
that deviate substantially from the well-characterized quasar templates.
Motivated by the commonly-seen UV-optical obscuration and the discovery of
parsec-scale mid-IR polar dust emission in some nearby AGNs, we develop
semi-empirical SED libraries for reddened type-1 AGNs built on the quasar
intrinsic templates, assuming low-level extinction caused by an extended
distribution of large dust grains. We demonstrate that this model can reproduce
the nuclear UV-to-IR SED and the strong mid-IR polar dust emission of NGC 3783,
the type-1 AGN with the most relevant and robust observational constraints. In
addition, we compile 64 low- Seyfert-1 nuclei with negligible mid-IR star
formation contamination and satisfactorily fit the individual IR SEDs as well
as the composite UV to mid-IR composite SEDs. Given the success of these fits,
we characterize the possible infrared SED of AGN polar dust emission and
utilize a simple but effective strategy to infer its prevalence among type-1
AGNs. The SEDs of high- peculiar AGNs, including the extremely red quasars,
mid-IR warm-excess AGNs, and hot dust-obscured galaxies, can be also reproduced
by our model. These results indicate that the IR SEDs of most AGNs, regardless
of redshift or luminosity, arise from similar circumnuclear torus properties
but differ mainly due to the optical depths of extended obscuring dust
components.Comment: 37 pages, 22 figures, 5 tables; accepted for publication in The
Astrophysical Journal; the AGN templates can be retrieved from
https://github.com/karlan/AGN_template
The Correlation Between Metallicity and Debris Disk mass
We find that the initial dust masses in planetary debris disks are correlated
with the metallicities of their central stars. We compiled a large sample of
systems, including Spitzer, the Herschel DUNES and DEBRIS surveys, and WISE
debris disk candidates. We also merged 33 metallicity catalogs to provide
homogeneous [Fe/H] and values. We analyzed this merged
sample, including 222 detected disks (74 warm and 148 cold) around a total of
187 systems (some with multiple components) and 440 disks with only upper
limits (125 warm and 315 cold), around a total of 360 systems. The disk dust
masses at a common early evolutionary point in time were determined using our
numerical disk evolutionary code, evolving a unique model for each of the 662
disks backward to an age of 1 Myr. We find that disk-bearing stars seldom have
metallicities less than [Fe/H] = -0.2 and that the distribution of warm
component masses lacks examples with large mass around stars of low metallicity
([Fe/H] < -0.085). Previous efforts to find a correlation have been largely
unsuccessful; the primary improvements supporting our result are: 1.) basing
the study on dust masses, not just infrared excess detections; 2.) including
upper limits on dust mass in a quantitative way; 3.) accounting for the
evolution of debris disk excesses as systems age; 4.) accounting fully for the
range of uncertainties in metallicity measurements; and 5.) having a
statistically large enough sample.Comment: 13 pages, 7 figures, accepted for publication to Ap
The Collisional Evolution of Debris Disks
We explore the collisional decay of disk mass and infrared emission in debris
disks. With models, we show that the rate of the decay varies throughout the
evolution of the disks, increasing its rate up to a certain point, which is
followed by a leveling off to a slower value. The total disk mass falls off ~
t^-0.35 at its fastest point (where t is time) for our reference model, while
the dust mass and its proxy -- the infrared excess emission -- fades
significantly faster (~ t^-0.8). These later level off to a decay rate of
M_tot(t) ~ t^-0.08 and M_dust(t) or L_ir(t) ~ t^-0.6. This is slower than the ~
t^-1 decay given for all three system parameters by traditional analytic
models.
We also compile an extensive catalog of Spitzer and Herschel 24, 70, and 100
micron observations. Assuming a log-normal distribution of initial disk masses,
we generate model population decay curves for the fraction of debris disk
harboring stars observed at 24 micron and also model the distribution of
measured excesses at the far-IR wavelengths (70-100 micron) at certain age
regimes. We show general agreement at 24 micron between the decay of our
numerical collisional population synthesis model and observations up to a Gyr.
We associate offsets above a Gyr to stochastic events in a few select systems.
We cannot fit the decay in the far infrared convincingly with grain strength
properties appropriate for silicates, but those of water ice give fits more
consistent with the observations.Comment: 32 pages, 16 figures, emulateapj format, Accepted for publication in
Ap
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