2,920 research outputs found
The Incidence of Debris Disks at 24 {\mu}m and 670 Myr
We use Spitzer Space Telescope 24 {\mu}m data to search for debris disks
among 122 AFGKM stars from the \sim 670 Myr clusters Hyades, Coma Ber, and
Praesepe, utilizing a number of advances in data reduction and determining the
intrinsic colors of main sequence stars. For our sample, the 1{\sigma}
dispersion about the main sequence V-K, K-[24] locus is approximately 3.1%. We
identify seven debris disks at 10% or more (\geq 3{\sigma} confidence level)
above the expected K-[24] for purely photospheric emission. The incidence of
excesses of 10% or greater in our sample at this age is 5.7 +3.1/-1.7%.
Combining with results from the literature, the rate is 7.8 +4.2/-2.1% for
early- type (B9 - F4) stars and 2.7 +3.3/-1.7% for solar-like (F5 - K9) stars.
Our primary sample has strict criteria for inclusion to allow comparison with
other work; when we relax these criteria, three additional debris disks are
detected. They are all around stars of solar-like type and hence reinforce our
conclusion that disks around such stars are still relatively common at 670 Myr
and are similar to the rate around early-type stars. The apparently small
difference in decay rates between early-type and solar-like stars is
inconsistent with the first order theoretical predictions that the later type
stellar disks would decay an order of magnitude more quickly than the earlier
type ones.Comment: 26 pages, 3 figures, accepted for publication in Ap
Crafting a rich and personal blending learning environment: an institutional case study from a STEM perspective
Institutional pressures to make optimal use of lecture halls and classrooms can be powerful motivators to identify resources to develop technology enhanced learning approaches to traditional curricula. From the academic’s perspective, engaging students in active learning and reducing the academic workload are important and complementary drivers. This paper presents a case study of a curriculum development exercise undertaken in a STEM subject area at a research-intensive UK university. A multi-skilled team of academics and learning designers have worked collaboratively to build this module which will be realised as a mix of online and face to face activities. Since the module addresses professional issues, a strong emphasis is being placed on establishing authentic learning activities and realistic use of prominent social tools.The learning designers are working for a cross-institutional initiative to support educational innovations; therefore it is important to carefully document the development process and to identify reusable design patterns which can be easily explained to other academics.<br/
A Study of H2 Emission in Three Bipolar Proto-Planetary Nebulae: IRAS 16594-4656, Hen 3-401, and Rob 22
We have carried out a spatial-kinematical study of three proto-planetary
nebulae, IRAS 16594-4656, Hen 3-401, and Rob 22. High-resolution H2 images were
obtained with NICMOS on the HST and high-resolution spectra were obtained with
the Phoenix spectrograph on Gemini-South. IRAS 16594-4656 shows a
"peanut-shaped" bipolar structure with H2 emission from the walls and from two
pairs of more distant, point-symmetric faint blobs. The velocity structure
shows the polar axis to be in the plane of the sky, contrary to the impression
given by the more complex visual image and the visibility of the central star,
with an ellipsoidal velocity structure. Hen 3-401 shows the H2 emission coming
from the walls of the very elongated, open-ended lobes seen in visible light,
along with a possible small disk around the star. The bipolar lobes appear to
be tilted 10-15 deg with respect to the plane of the sky and their kinematics
display a Hubble-like flow. In Rob 22, the H2 appears in the form of an "S"
shape, approximately tracing out the similar pattern seen in the visible. H2 is
especially seen at the ends of the lobes and at two opposite regions close to
the unseen central star. The axis of the lobes is nearly in the plane of the
sky. Expansion ages of the lobes are calculated to be approximately 1600 yr
(IRAS 16594-4656), 1100 yr (Hen 3-401), and 640 yr (Rob 22), based upon
approximate distances
What Sets the Radial Locations of Warm Debris Disks?
The architectures of debris disks encode the history of planet formation in
these systems. Studies of debris disks via their spectral energy distributions
(SEDs) have found infrared excesses arising from cold dust, warm dust, or a
combination of the two. The cold outer belts of many systems have been imaged,
facilitating their study in great detail. Far less is known about the warm
components, including the origin of the dust. The regularity of the disk
temperatures indicates an underlying structure that may be linked to the water
snow line. If the dust is generated from collisions in an exo-asteroid belt,
the dust will likely trace the location of the water snow line in the
primordial protoplanetary disk where planetesimal growth was enhanced. If
instead the warm dust arises from the inward transport from a reservoir of icy
material farther out in the system, the dust location is expected to be set by
the current snow line. We analyze the SEDs of a large sample of debris disks
with warm components. We find that warm components in single-component systems
(those without detectable cold components) follow the primordial snow line
rather than the current snow line, so they likely arise from exo-asteroid
belts. While the locations of many warm components in two-component systems are
also consistent with the primordial snow line, there is more diversity among
these systems, suggesting additional effects play a role
A Comprehensive Dust Model Applied to the Resolved Beta Pictoris Debris Disk from Optical to Radio Wavelengths
We investigate whether varying the dust composition (described by the optical
constants) can solve a persistent problem in debris disk modeling--the
inability to fit the thermal emission without over-predicting the scattered
light. We model five images of the beta Pictoris disk: two in scattered light
from HST/STIS at 0.58 microns and HST/WFC3 at 1.16 microns, and three in
thermal emission from Spitzer/MIPS at 24 microns, Herschel/PACS at 70 microns,
and ALMA at 870 microns. The WFC3 and MIPS data are published here for the
first time. We focus our modeling on the outer part of this disk, consisting of
a parent body ring and a halo of small grains. First, we confirm that a model
using astronomical silicates cannot simultaneously fit the thermal and
scattered light data. Next, we use a simple, generic function for the optical
constants to show that varying the dust composition can improve the fit
substantially. Finally, we model the dust as a mixture of the most plausible
debris constituents: astronomical silicates, water ice, organic refractory
material, and vacuum. We achieve a good fit to all datasets with grains
composed predominantly of silicates and organics, while ice and vacuum are, at
most, present in small amounts. This composition is similar to one derived from
previous work on the HR 4796A disk. Our model also fits the thermal SED,
scattered light colors, and high-resolution mid-IR data from T-ReCS for this
disk. Additionally, we show that sub-blowout grains are a necessary component
of the halo.Comment: 23 pages, 20 figures, accepted to Ap
Spitzer IRS Spectroscopy of the 10 Myr-old EF Cha Debris Disk: Evidence for Phyllosilicate-Rich Dust in the Terrestrial Zone
We describe Spitzer IRS spectroscopic observations of the 10 Myr-old star, EF
Cha. Compositional modeling of the spectra from 5 {\mu}m to 35 {\mu}m confirms
that it is surrounded by a luminous debris disk with LD/L\star ~ 10-3,
containing dust with temperatures between 225 K and 430 K characteristic of the
terrestrial zone. The EF Cha spectrum shows evidence for many solid-state
features, unlike most cold, low-luminosity debris disks but like some other
10-20 Myr-old luminous, warm debris disks (e.g. HD 113766A). The EF Cha debris
disk is unusually rich in a species or combination of species whose
emissivities resemble that of finely powdered, laboratory-measured
phyllosilicate species (talc, saponite, and smectite), which are likely
produced by aqueous alteration of primordial anhydrous rocky materials. The
dust and, by inference, the parent bodies of the debris also contain abundant
amorphous silicates and metal sulfides, and possibly water ice. The dust's
total olivine to pyroxene ratio of ~ 2 also provides evidence of aqueous
alteration. The large mass volume of grains with sizes comparable to or below
the radiation blow-out limit implies that planetesimals may be colliding at a
rate high enough to yield the emitting dust but not so high as to devolatize
the planetesimals via impact processing. Because phyllosilicates are produced
by the interactions between anhydrous rock and warm, reactive water, EF Cha's
disk is a likely signpost for water delivery to the terrestrial zone of a young
planetary system.Comment: 21 pages, 10 figures, accepted for publication in The Astrophysical
Journa
The first 40 million years of circumstellar disk evolution: the signature of terrestrial planet formation
We characterize the first 40 Myr of evolution of circumstellar disks through
a unified study of the infrared properties of members of young clusters and
associations with ages from 2 Myr up to ~ 40 Myr: NGC 1333, NGC 1960, NGC 2232,
NGC 2244, NGC 2362, NGC 2547, IC 348, IC 2395, IC 4665, Chamaeleon I, Orion
OB1a and OB1b, Taurus, the \b{eta} Pictoris Moving Group, \r{ho} Ophiuchi, and
the associations of Argus, Carina, Columba, Scorpius-Centaurus, and
Tucana-Horologium. Our work features: 1.) a filtering technique to flag noisy
backgrounds, 2.) a method based on the probability distribution of deflections,
P(D), to obtain statistically valid photometry for faint sources, and 3.) use
of the evolutionary trend of transitional disks to constrain the overall
behavior of bright disks. We find that the fraction of disks three or more
times brighter than the stellar photospheres at 24 {\mu}m decays relatively
slowly initially and then much more rapidly by ~ 10 Myr. However, there is a
continuing component until ~ 35 Myr, probably due primarily to massive clouds
of debris generated in giant impacts during the oligarchic/chaotic growth
phases of terrestrial planets. If the contribution from primordial disks is
excluded, the evolution of the incidence of these oligarchic/chaotic debris
disks can be described empirically by a log-normal function with the peak at 12
- 20 Myr, including ~ 13 % of the original population, and with a post-peak
mean duration of 10 - 20 Myr.Comment: accepted for publication, the Astrophysical Journal (2017
Herschel Observations and Updated Spectral Energy Distributions of Five Sunlike Stars with Debris Disks
Observations from the Herschel Space Observatory have more than doubled the
number of wide debris disks orbiting Sunlike stars to include over 30 systems
with R > 100 AU. Here we present new Herschel PACS and re-analyzed Spitzer MIPS
photometry of five Sunlike stars with wide debris disks, from Kuiper belt size
to R > 150 AU. The disk surrounding HD 105211 is well resolved, with an angular
extent of >14" along the major axis, and the disks of HD 33636, HD 50554, and
HD 52265 are extended beyond the PACS PSF size (50% of energy enclosed within
radius 4.23"). HD 105211 also has a 24-micron infrared excess that was
previously overlooked because of a poorly constrained photospheric model.
Archival Spitzer IRS observations indicate that the disks have small grains of
minimum radius ~3 microns, though the minimum grain gradius is larger than the
radiation pressure blowout size in all systems. If modeled as
single-temperature blackbodies, the disk temperatures would all be <60 K. Our
radiative transfer models predict actual disk radii approximately twice the
radius of model blackbody disks. We find that the Herschel photometry traces
dust near the source population of planetesimals. The disk luminosities are in
the range 0.00002 <= L/L* <= 0.0002, consistent with collisions in icy
planetesimal belts stirred by Pluto-size dwarf planets.Comment: Accepted for publication in ApJ. 18 pages, including 10 figures and 3
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