304 research outputs found
A Self-Consistent Model of the Circumstellar Debris Created by a Giant Hypervelocity Impact in the HD172555 System
Spectral modeling of the large infrared excess in the Spitzer IRS spectra of
HD 172555 suggests that there is more than 10^19 kg of sub-micron dust in the
system. Using physical arguments and constraints from observations, we rule out
the possibility of the infrared excess being created by a magma ocean planet or
a circumplanetary disk or torus. We show that the infrared excess is consistent
with a circumstellar debris disk or torus, located at approximately 6 AU, that
was created by a planetary scale hypervelocity impact. We find that radiation
pressure should remove submicron dust from the debris disk in less than one
year. However, the system's mid-infrared photometric flux, dominated by
submicron grains, has been stable within 4 percent over the last 27 years, from
IRAS (1983) to WISE (2010). Our new spectral modeling work and calculations of
the radiation pressure on fine dust in HD 172555 provide a self-consistent
explanation for this apparent contradiction. We also explore the unconfirmed
claim that 10^47 molecules of SiO vapor are needed to explain an emission
feature at 8 um in the Spitzer IRS spectrum of HD 172555. We find that unless
there are 10^48 atoms or 0.05 Earth masses of atomic Si and O vapor in the
system, SiO vapor should be destroyed by photo-dissociation in less than 0.2
years. We argue that a second plausible explanation for the 8 um feature can be
emission from solid SiO, which naturally occurs in submicron silicate "smokes"
created by quickly condensing vaporized silicate.Comment: Accepted to the Astrophysical Journa
Planet formation in Binaries
Spurred by the discovery of numerous exoplanets in multiple systems, binaries
have become in recent years one of the main topics in planet formation
research. Numerous studies have investigated to what extent the presence of a
stellar companion can affect the planet formation process. Such studies have
implications that can reach beyond the sole context of binaries, as they allow
to test certain aspects of the planet formation scenario by submitting them to
extreme environments. We review here the current understanding on this complex
problem. We show in particular how each of the different stages of the
planet-formation process is affected differently by binary perturbations. We
focus especially on the intermediate stage of kilometre-sized planetesimal
accretion, which has proven to be the most sensitive to binarity and for which
the presence of some exoplanets observed in tight binaries is difficult to
explain by in-situ formation following the "standard" planet-formation
scenario. Some tentative solutions to this apparent paradox are presented. The
last part of our review presents a thorough description of the problem of
planet habitability, for which the binary environment creates a complex
situation because of the presence of two irradation sources of varying
distance.Comment: Review chapter to appear in "Planetary Exploration and Science:
Recent Advances and Applications", eds. S. Jin, N. Haghighipour, W.-H. Ip,
Springer (v2, numerous typos corrected
Against all odds? Forming the planet of the HD196885 binary
HD196885Ab is the most "extreme" planet-in-a-binary discovered to date, whose
orbit places it at the limit for orbital stability. The presence of a planet in
such a highly perturbed region poses a clear challenge to planet-formation
scenarios. We investigate this issue by focusing on the planet-formation stage
that is arguably the most sensitive to binary perturbations: the mutual
accretion of kilometre-sized planetesimals. To this effect we numerically
estimate the impact velocities amongst a population of circumprimary
planetesimals. We find that most of the circumprimary disc is strongly hostile
to planetesimal accretion, especially the region around 2.6AU (the planet's
location) where binary perturbations induce planetesimal-shattering of
more than 1km/s. Possible solutions to the paradox of having a planet in such
accretion-hostile regions are 1) that initial planetesimals were very big, at
least 250km, 2) that the binary had an initial orbit at least twice the present
one, and was later compacted due to early stellar encounters, 3) that
planetesimals did not grow by mutual impacts but by sweeping of dust (the
"snowball" growth mode identified by Xie et al., 2010b), or 4) that HD196885Ab
was formed not by core-accretion but by the concurent disc instability
mechanism. All of these 4 scenarios remain however highly conjectural.Comment: accepted for publication by Celestial Mechanics and Dynamical
Astronomy (Special issue on EXOPLANETS
Circumstellar discs: What will be next?
This prospective chapter gives our view on the evolution of the study of
circumstellar discs within the next 20 years from both observational and
theoretical sides. We first present the expected improvements in our knowledge
of protoplanetary discs as for their masses, sizes, chemistry, the presence of
planets as well as the evolutionary processes shaping these discs. We then
explore the older debris disc stage and explain what will be learnt concerning
their birth, the intrinsic links between these discs and planets, the hot dust
and the gas detected around main sequence stars as well as discs around white
dwarfs.Comment: invited review; comments welcome (32 pages
Invasions and Extinctions Reshape Coastal Marine Food Webs
The biodiversity of ecosystems worldwide is changing because of species loss due to human-caused extinctions and species gain through intentional and accidental introductions. Here we show that the combined effect of these two processes is altering the trophic structure of food webs in coastal marine systems. This is because most extinctions (∼70%) occur at high trophic levels (top predators and other carnivores), while most invasions are by species from lower trophic levels (70% macroplanktivores, deposit feeders, and detritivores). These opposing changes thus alter the shape of marine food webs from a trophic pyramid capped by a diverse array of predators and consumers to a shorter, squatter configuration dominated by filter feeders and scavengers. The consequences of the simultaneous loss of diversity at top trophic levels and gain at lower trophic levels is largely unknown. However, current research suggests that a better understanding of how such simultaneous changes in diversity can impact ecosystem function will be required to manage coastal ecosystems and forecast future changes
TrpC3 Regulates Hypertrophy-Associated Gene Expression without Affecting Myocyte Beating or Cell Size
Pathological cardiac hypertrophy is associated with an increased risk of heart failure and cardiovascular mortality. Calcium (Ca2+) -regulated gene expression is essential for the induction of hypertrophy, but it is not known how myocytes distinguish between the Ca2+ signals that regulate contraction and those that lead to cardiac hypertrophy. We used in vitro neonatal rat ventricular myocytes to perform an RNA interference (RNAi) screen for ion channels that mediate Ca2+-dependent gene expression in response to hypertrophic stimuli. We identified several ion channels that are linked to hypertrophic gene expression, including transient receptor potential C3 (TrpC3). RNAi-mediated knockdown of TrpC3 decreases expression of hypertrophy-associated genes such as the A- and B-type natriuretic peptides (ANP and BNP) in response to numerous hypertrophic stimuli, while TrpC3 overexpression increases BNP expression. Furthermore, stimuli that induce hypertrophy dramatically increase TrpC3 mRNA levels. Importantly, whereas TrpC3-knockdown strongly reduces gene expression associated with hypertrophy, it has a negligible effect on cell size and on myocyte beating. These results suggest that Ca2+ influx through TrpC3 channels increases transcription of genes associated with hypertrophy but does not regulate the signaling pathways that control cell size or contraction. Thus TrpC3 may represent an important therapeutic target for the treatment of cardiac hypertrophy and heart failure
Inorganic Polyphosphate Modulates TRPM8 Channels
Polyphosphate (polyP) is an inorganic polymer built of tens to hundreds of phosphates, linked by high-energy phosphoanhydride bonds. PolyP forms complexes and modulates activities of many proteins including ion channels. Here we investigated the role of polyP in the function of the transient receptor potential melastatin 8 (TRPM8) channel. Using whole-cell patch-clamp and fluorescent calcium measurements we demonstrate that enzymatic breakdown of polyP by exopolyphosphatase (scPPX1) inhibits channel activity in human embryonic kidney and F-11 neuronal cells expressing TRPM8. We demonstrate that the TRPM8 channel protein is associated with polyP. Furthermore, addition of scPPX1 altered the voltage-dependence and blocked the activity of the purified TRPM8 channels reconstituted into planar lipid bilayers, where the activity of the channel was initiated by cold and menthol in the presence of phosphatidylinositol 4,5-biphosphate (PtdIns(4,5)P2). The biochemical analysis of the TRPM8 protein also uncovered the presence of poly-(R)-3-hydroxybutyrate (PHB), which is frequently associated with polyP. We conclude that the TRPM8 protein forms a stable complex with polyP and its presence is essential for normal channel activity
- …