506 research outputs found
Discovery of extreme asymmetry in the debris disk surrounding HD 15115
We report the first scattered light detection of a dusty debris disk
surrounding the F2V star HD 15115 using the Hubble Space Telescope in the
optical, and Keck adaptive optics in the near-infrared. The most remarkable
property of the HD 15115 disk relative to other debris disks is its extreme
length asymmetry. The east side of the disk is detected to ~315 AU radius,
whereas the west side of the disk has radius >550 AU. We find a blue optical to
near-infrared scattered light color relative to the star that indicates grain
scattering properties similar to the AU Mic debris disk. The existence of a
large debris disk surrounding HD 15115 adds further evidence for membership in
the Beta Pic moving group, which was previously argued based on kinematics
alone. Here we hypothesize that the extreme disk asymmetry is due to dynamical
perturbations from HIP 12545, an M star 0.5 degrees (0.38 pc) east of HD 15115
that shares a common proper motion vector, heliocentric distance, galactic
space velocity, and age.Comment: 4 pages, 3 figures, ApJ Letters, accepte
STIS Coronagraphic Imaging of Fomalhaut: Main Belt Structure and the Orbit of Fomalhaut b
We present new optical coronagraphic data of the bright star Fomalhaut
obtained with the HST in 2010/2012 using STIS. Fomalhaut b is recovered at both
epochs to high significance. The observations include the discoveries of
tenuous nebulosity beyond the main dust belt detected to at least 209 AU
projected radius and a ~50 AU wide azimuthal gap in the belt northward of Fom
b. The morphology of Fomalhaut b appears elliptical in the STIS detections. We
show that residual noise in the processed data can plausibly result in point
sources appearing extended. A MCMC analysis demonstrates that the orbit of Fom
b is highly eccentric, with e=0.8+/-0.1, a=177+/-68 AU, and q = 32+/-24 AU. Fom
b is apsidally aligned with the belt and 90% of allowed orbits have mutual
inclination 36 deg or less. Fomalhaut b's orbit is belt-crossing in projection,
but only 12% of possible orbits have nodes within a 25 AU wide belt annulus
(133-158 AU). The high e invokes a dynamical history where Fom b may have
experienced a significant dynamical interaction with a hypothetical planet
Fomalhaut c, and the current orbital configuration may be relatively
short-lived. The new value for the periastron distance diminishes the Hill
radius of Fom b and any weakly bound satellite system surrounding a planet
would be sheared and dynamically heated at periapse. We argue that Fom b's
minimum mass is that of a dwarf planet in order for a circumplanetary satellite
system to remain bound to a sufficient radius from the planet to be consistent
with the dust scattered light hypothesis. Fom b may be optically bright because
the recent passage through periapse and/or the ascending node has increased the
erosion rates of planetary satellites. In the coplanar case, Fomalhaut b will
collide with the main belt around 2032, and the subsequent emergent phenomena
may help determine its physical nature.Comment: 49 Pages, 33 Figures, 5 Tables; Submitted to ApJ, Dec. 31, 201
Modeling Self-Subtraction in Angular Differential Imaging: Application to the HD 32297 Debris Disk
We present a new technique for forward-modeling self-subtraction of spatially
extended emission in observations processed with angular differential imaging
(ADI) algorithms. High-contrast direct imaging of circumstellar disks is
limited by quasi-static speckle noise and ADI is commonly used to suppress
those speckles. However, the application of ADI can result in self-subtraction
of the disk signal due to the disk's finite spatial extent. This signal
attenuation varies with radial separation and biases measurements of the disk's
surface brightness, thereby compromising inferences regarding the physical
processes responsible for the dust distribution. To compensate for this
attenuation, we forward-model the disk structure and compute the form of the
self-subtraction function at each separation. As a proof of concept, we apply
our method to 1.6 and 2.2 micron Keck AO NIRC2 scattered-light observations of
the HD 32297 debris disk reduced using a variant of the "locally optimized
combination of images" (LOCI) algorithm. We are able to recover disk surface
brightness that was otherwise lost to self-subtraction and produce simplified
models of the brightness distribution as it appears with and without
self-subtraction. From the latter models, we extract radial profiles for the
disk's brightness, width, midplane position, and color that are unbiased by
self-subtraction. Our analysis of these measurements indicates a break in the
brightness profile power law at r~110 AU and a disk width that increases with
separation from the star. We also verify disk curvature that displaces the
midplane by up to 30 AU towards the northwest relative to a straight fiducial
midplane.Comment: Accepted for publication in ApJ, 20 pages, 10 figures, 1 tabl
Insights on the dynamical history of the Fomalhaut system - Investigating the Fom c hypothesis
The eccentric shape of the debris disk observed around Fomalhaut was first
attributed to Fom b, a companion detected near the belt inner-edge, but new
constraints on its orbit revealed that it is belt-crossing, highly eccentric
, and can hardly account for the shape of the belt. The best
scenario to explain this paradox is that there is another massive body in this
system, Fom c, which drives the debris disk shape. The resulting planetary
system is highly unstable, which hints at a dynamical scenario involving a
recent scattering of Fom b on its current orbit, potentially with the putative
Fom c.
Our goal is to give insights on the probability for Fom b to have been set on
its highly eccentric orbit by a close-encounter with the putative Fom c. We aim
to study in particular the part played by mean-motion resonances with Fom c,
which could have brought Fom b sufficiently close to Fom c for it to be
scattered on its current orbit, but also delay this scattering event.
Using N-body simulations, we found that the generation of orbits similar to
that of Fom b, either in term of dimensions or orientation, is a robust process
involving a scattering event and a further secular evolution of inner material
with an eccentric massive body such as the putative Fom c. We found in
particular that mean-motion resonances can delay scattering events, and thus
the production of Fom b-like orbits, on timescales comparable to the age of the
system, thus explaining the witnessing of an unstable configuration.
We conclude that Fom b probably originated from an inner resonance with Fom
c, which is at least Neptune-Saturn size, and was set on its current orbit by a
scattering event with Fom c. Since Fom b could not have formed from material in
resonance, our scenario also hints at former migration processes in this
planetary system
First optical images of circumstellar dust surrounding the debris disk candidate HD 32297
Near-infrared imaging with the Hubble Space Telescope recently revealed a
circumstellar dust disk around the A star HD 32297. Dust scattered light is
detected as far as 400 AU radius and the linear morphology is consistent with a
disk ~10 degrees away from an edge-on orientation. Here we present the first
optical images that show the dust scattered light morphology from 560 to 1680
AU radius. The position angle of the putative disk midplane diverges by 31
degrees and the color of dust scattering is most likely blue. We associate HD
32297 with a wall of interstellar gas and the enigmatic region south of the
Taurus molecular cloud. We propose that the extreme asymmetries and blue disk
color originate from a collision with a clump of interstellar material as HD
32297 moves southward, and discuss evidence consistent with an age of 30 Myr or
younger.Comment: 5 pages; Accepted for publication in ApJ Letter
Discovery of Reflection Nebulosity Around Five Vega-like Stars
Coronagraphic optical observations of six Vega-like stars reveal reflection
nebulosities, five of which were previously unknown. The nebulosities
illuminated by HD 4881, HD 23362, HD 23680, HD 26676, and HD 49662 resemble
that of the Pleiades, indicating an interstellar origin for dust grains. The
reflection nebulosity around HD 123160 has a double-arm morphology, but no
disk-like feature is seen as close as 2.5 arcsec from the star in K-band
adaptive optics data. We demonstrate that uniform density dust clouds
surrounding HD 23362, HD 23680 and HD 123160 can account for the observed
12-100 micron spectral energy distributions. For HD 4881, HD 26676, and HD
49662 an additional emission source, such as from a circumstellar disk or
non-equilibrium grain heating, is required to fit the 12-25 micron data. These
results indicate that in some cases, particularly for Vega-like stars located
beyond the Local Bubble (>100 pc), the dust responsible for excess thermal
emission may originate from the interstellar medium rather than from a
planetary debris system.Comment: The Astrophysical Journal, in press for March, 2002 (32 pages, 13
figures
Hubble Space Telescope High Resolution Imaging of Kepler Small and Cool Exoplanet Host Stars
High resolution imaging is an important tool for follow-up study of exoplanet
candidates found via transit detection with the Kepler Mission. We discuss here
HST imaging with the WFC3 of 23 stars that host particularly interesting Kepler
planet candidates based on their small size and cool equilibrium temperature
estimates. Results include detections, exclusion of background stars that could
be a source of false positives for the transits, and detection of
physically-associated companions in a number of cases providing dilution
measures necessary for planet parameter refinement. For six KOIs, we find that
there is ambiguity in which star hosts the transiting planet(s), with
potentially strong implications for planetary characteristics. Our sample is
evenly distributed in G, K, and M spectral types. Albeit with a small sample
size, we find that physically-associated binaries are more common than expected
at each spectral type, reaching a factor of 10 frequency excess at M. We
document the program detection sensitivities, detections, and deliverables to
the Kepler follow-up program archive.Comment: Accepted for the Astronomical Journal; 13 pages with 9 figure
Building mathematical knowledge with programming: insights from the ScratchMaths project
The ScratchMaths (SM) project sets out to exploit the recent commitment to programming in schools in England for the benefit of mathematics learning and reasoning. This design research project aims to introduce students (age 9-11 years) to computational thinking as a medium for exploring mathematics following a constructionist approach. This paper outlines the project and then focuses on two tensions related to (i) the tool and learning, and (ii) direction and discovery, which can arise within constructionist learning environments and describes how these tensions were addressed through the design of the SM curriculum
Discovery of an extended debris disk around the F2V star HD 15745
Using the Advanced Camera for Surveys aboard the Hubble Space Telescope, we
have discovered dust-scattered light from the debris disk surrounding the F2V
star HD 15745. The circumstellar disk is detected between 2.0" and 7.5" radius,
corresponding to 128 - 480 AU radius. The circumstellar disk morphology is
asymmetric about the star, resembling a fan, and consistent with forward
scattering grains in an optically thin disk with an inclination of ~67 degrees
to our line of sight. The spectral energy distribution and scattered light
morphology can be approximated with a model disk composed of silicate grains
between 60 and 450 AU radius, with a total dust mass of 10E-7 M_sun (0.03
M_earth) representing a narrow grain size distribution (1 - 10 micron).
Galactic space motions are similar to the Castor Moving Group with an age of
~10E+8 yr, although future work is required to determine the age of HD 15745
using other indicators.Comment: 7 pages, 4 figures, ApJ Letters, in pres
First scattered light images of debris disks around HD 53143 and HD 139664
We present the first scattered light images of debris disks around a K star
(HD 53143) and an F star (HD 139664) using the coronagraphic mode of the
Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST). With
ages 0.3 - 1 Gyr, these are among the oldest optically detected debris disks.
HD 53143, viewed ~45 degrees from edge-on, does not show radial variation in
disk structure and has width >55 AU. HD 139664 is seen close to edge-on and has
belt-like morphology with a dust peak 83 AU from the star and a distinct outer
boundary at 109 AU. We discuss evidence for significant diversity in the radial
architecture of debris disks that appears unconnected to stellar spectral type
or age. HD 139664 and possibly the solar system belong in a category of narrow
belts 20-30 AU wide. HD 53143 represents a class of wide disk architecture with
characteristic width >50 AU.Comment: 7 pages, 3 figure
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