7,586 research outputs found
Circumstellar Disk Evolution: Constraining Theories of Planet Formation
Observations of circumstellar disks around stars as a function of stellar
properties such as mass, metallicity, multiplicity, and age, provide
constraints on theories concerning the formation and evolution of planetary
systems. Utilizing ground- and space-based data from the far-UV to the
millimeter, astronomners can assess the amount, composition, and location of
circumstellar gas and dust as a function of time. We review primarily results
from the Spitzer Space Telescope, with reference to other ground- and
space-based observations. Comparing these results with those from exoplanet
search techniques, theoretical models, as well as the inferred history of our
solar system, helps us to assess whether planetary systems like our own, and
the potential for life that they represent, are common or rare in the Milky Way
galaxy.Comment: To appear in IAU Symposium No. 258, Eds. E. Mamajek, D.R. Soderblom,
and R.F.G. Wys
Unsupervised learning of overlapping image components using divisive input modulation
This paper demonstrates that nonnegative matrix factorisation is mathematically related to a class of neural networks that employ negative feedback as a mechanism of competition. This observation inspires a novel learning algorithm which we call Divisive Input Modulation (DIM). The proposed algorithm provides a mathematically simple and computationally efficient method for the unsupervised learning of image components, even in conditions where these elementary features overlap considerably. To test the proposed algorithm, a novel artificial task is introduced which is similar to the frequently-used bars problem but employs squares rather than bars to increase the degree of overlap between components. Using this task, we investigate how the proposed method performs on the parsing of artificial images composed of overlapping features, given the correct representation of the individual components; and secondly, we investigate how well it can learn the elementary components from artificial training images. We compare the performance of the proposed algorithm with its predecessors including variations on these algorithms that have produced state-of-the-art performance on the bars problem. The proposed algorithm is more successful than its predecessors in dealing with overlap and occlusion in the artificial task that has been used to assess performance
The nearby population of M dwarfs with WISE: A search for warm circumstellar dust
Circumstellar debris disks are important because of their connection to
planetary systems. An efficient way to identify these systems is through their
infrared excess. Most studies so far concentrated on early-type or solar-type
stars, but less effort has gone into investigating M dwarfs.
We characterize the mid-infrared photometric behavior of M dwarfs and search
for infrared excess in nearby M dwarfs taken from the volume-limited RECONS
sample using data from the WISE satellite and the 2MASS catalog. Our sample
consists of 85 sources encompassing 103 M dwarfs. We derive empirical infrared
colors from these data and discuss their errors. Based on this, we check the
stars for infrared excess and discuss the minimum excess we would be able to
detect.
Other than the M8.5 dwarf SCR 1845-6357 A, where the excess is produced by a
known T6 companion, we detect no excesses in any of our sample stars. The
limits we derive for the 22um excess are slightly higher than the usual
detection limit of 10-15% for Spitzer studies, but including the [12]-[22]
color in our analysis allows us to derive tight constraints on the fractional
dust luminosity L_dust/L_star. We show that this result is consistent with M
dwarf excesses in the mid-inrared being as frequent as excesses around
earlier-type stars. The low detection rate could be an age effect. We also
present a tentative excess detection at 22um around the known cold debris disk
M dwarf AU Mic, which is not part of our statistical sample.
There is still no clear detection of a mid-infrared excess around any old
(>30 Myr) main-sequence M dwarf. It is unclear whether this is due to a
different dust evolution around M dwarfs or whether this is an age effect
combined with the diffculties involved in searching M dwarfs for infrared
excesses. A significantly larger sample of well-studied M dwarfs is required to
solve this question.Comment: Accepted for publication in A&A, 15 pages, 7 figure
Interleukin-l regulates synthesis of nerve growth factor in non-neuronal cells of rat sciatic nerve
On the gap-opening criterion of migrating planets in protoplanetary disks
We perform two-dimensional hydrodynamical simulations to quantitatively
explore the torque balance criterion for gap-opening (as formulated by Crida et
al. 2006) in a variety of disks when considering a migrating planet. We find
that even when the criterion is satisfied, there are instances when planets
still do not open gaps. We stress that gap-opening is not only dependent on
whether a planet has the ability to open a gap, but whether it can do so
quickly enough. This can be expressed as an additional condition on the
gap-opening timescale versus the crossing time, i.e. the time it takes the
planet to cross the region which it is carving out. While this point has been
briefly made in the previous literature, our results quantify it for a range of
protoplanetary disk properties and planetary masses, demonstrating how crucial
it is for gap-opening. This additional condition has important implications for
the survival of planets formed by core accretion in low mass disks as well as
giant planets or brown dwarfs formed by gravitational instability in massive
disks. It is particularly important for planets with intermediate masses
susceptible to Type III-like migration. For some observed transition disks or
disks with gaps, we expect that estimates on the potential planet masses based
on the torque balance gap-opening criterion alone may not be sufficient. With
consideration of this additional timescale criterion theoretical studies may
find a reduced planet survivability or that planets may migrate further inwards
before opening a gap.Comment: Accepted by ApJ, 22 pages, 13 figures, 6 table
- …