167 research outputs found
Free-floating planets in stellar clusters?
We have simulated encounters between planetary systems and single stars in various clustered environments. This allows us to estimate the fraction of systems liberated, the velocity distribution of the liberated planets, and the separation and eccentricity distributions of the surviving bound systems. Our results indicate that, for an initial distribution of orbits that is flat in log space and extends out to 50 au, 50 per cent of the available planets can be liberated in a globular cluster, 25 per cent in an open cluster, and less than 10 per cent in a young cluster. These fractions are reduced to 25, 12 and 2 per cent if the initial population extends only to 20 au. Furthermore, these free-floating planets can be retained for longer than a crossing time only in a massive globular cluster. It is therefore difficult to see how planets, which by definition form in a disc around a young star, could be subsequently liberated to form a significant population of free-floating substellar objects in a cluste
Planetary dynamics in stellar clusters
We investigate how the formation and evolution of extrasolar planetary systems can be affected by stellar encounters that occur in the crowded conditions of a stellar cluster. Using plausible estimates of cluster evolution, we show how planet formation may be suppressed in globular clusters while planets wider than ≳0.1 au that do form in such environments can be ejected from their stellar system. Less crowded systems such as open clusters have a much reduced effect on any planetary system. Planet formation is unaffected in open clusters and only the wider planetary systems will be disrupted during the cluster's lifetime. The potential for free-floating planets in these environments is also discusse
Development of a Direct Fabrication Technique for Full-Shell X-Ray Optics
Future astrophysical missions will require fabrication technology capable of producing high angular resolution x-ray optics. A full-shell direct fabrication approach using modern robotic polishing machines has the potential for producing high resolution, light-weight and affordable x-ray mirrors that can be nested to produce large collecting area. This approach to mirror fabrication, based on the use of the metal substrates coated with nickel phosphorous alloy, is being pursued at MSFC. The design of the polishing fixtures for the direct fabrication, the surface figure metrology techniques used and the results of the polishing experiments are presented
The enigmatic pair of dwarf galaxies Leo IV and Leo V: coincidence or common origin?
We have obtained deep photometry in two 1x1 degree fields covering the close
pair of dwarf spheroidal galaxies (dSph) Leo IV and Leo V and part of the area
in between. We find that both systems are significantly larger than indicated
by previous measurements based on shallower data and also significantly
elongated. With half-light radii of r_h=4'.6 +- 0'.8 (206 +- 36 pc) and
r_h=2'.6 +- 0'.6 (133 +- 31 pc), respectively, they are now well within the
physical size bracket of typical Milky Way dSph satellites. Their ellipticities
of epsilon ~0.5 are shared by many faint (M_V>-8) Milky Way dSphs. The large
spatial extent of our survey allows us to search for extra-tidal features with
unprecedented sensitivity. The spatial distribution of candidate red giant
branch and horizontal branch stars is found to be non-uniform at the ~3 sigma
level. This substructure is aligned along the direction connecting the two
systems, indicative of a possible `bridge' of extra-tidal material. Fitting the
stellar distribution with a linear Gaussian model yields a significance of 4
sigma for this overdensity, a most likely FWHM of ~16 arcmin and a central
surface brightness of ~32 mag arcsec^{-2}. We investigate different scenarios
to explain the close proximity of Leo IV and Leo V and the possible tidal
bridge between them. Orbit calculations demonstrate that they are unlikely to
be remnants of a single disrupted progenitor, while a comparison with
cosmological simulations shows that a chance collision between unrelated
subhalos is negligibly small. Leo IV and Leo V could, however, be a bound
`tumbling pair' if their combined mass exceeds 8 +- 4 x 10^9 M_sun. The
scenario of an internally interacting pair appears to be the most viable
explanation for this close celestial companionship. (abridged)Comment: 9 pages, 8 figures, small number of minor textual changes, accepted
for publication in Astrophysical Journa
Inorganic speciation of dissolved elements in seawater: the influence of pH on concentration ratios
Assessments of inorganic elemental speciation in seawater span the past four decades. Experimentation, compilation and critical review of equilibrium data over the past forty years have, in particular, considerably improved our understanding of cation hydrolysis and the complexation of cations by carbonate ions in solution. Through experimental investigations and critical evaluation it is now known that more than forty elements have seawater speciation schemes that are strongly influenced by pH. In the present work, the speciation of the elements in seawater is summarized in a manner that highlights the significance of pH variations. For elements that have pH-dependent species concentration ratios, this work summarizes equilibrium data (S = 35, t = 25°C) that can be used to assess regions of dominance and relative species concentrations. Concentration ratios of complex species are expressed in the form log[A]/[B] = pH - C where brackets denote species concentrations in solution, A and B are species important at higher (A) and lower (B) solution pH, and C is a constant dependent on salinity, temperature and pressure. In the case of equilibria involving complex oxy-anions (MO(x)(OH)(y)) or hydroxy complexes (M(OH)(n)), C is written as pK(n )= -log K(n )or pK(n)* = -log K(n)* respectively, where K(n )and K(n)* are equilibrium constants. For equilibria involving carbonate complexation, the constant C is written as pQ = -log(K(2)(l)K(n )[HCO(3)(-)]) where K(2)(l )is the HCO(3 )(- )dissociation constant, K(n )is a cation complexation constant and [HCO(3)(-)] is approximated as 1.9 × 10(-3 )molar. Equilibrium data expressed in this manner clearly show dominant species transitions, ranges of dominance, and relative concentrations at any pH
Planetary dynamics in stellar clusters
We investigate how the formation and evolution of extrasolar planetary
systems can be affected by stellar encounters that occur in the crowded
conditions of a stellar cluster. Using plausible estimates of cluster
evolution, we show how planet formation may be supressed in globular clusters
while planets wider than 0.1 AU that do form in such environments can be
ejected from their stellar system. Less crowded systems such as open clusters
have a much reduced effect on any planetary system. Planet formation is
unaffected in open clusters and only the wider planetary systems will be
disrupted during the cluster's lifetime. The potential for free-floating
planets in these environments is also discussed.Comment: 8, pages, 6 figures. accepted by MNRA
Detection of the Neupert Effect in the Corona of an RS CVn Binary System by XMM-Newton and the VLA
The RS CVn-type binary Geminorum was observed during a large,
long-duration flare simultaneously with {\it XMM-Newton} and the VLA. The light
curves show a characteristic time dependence that is compatible with the
Neupert effect observed in solar flares: The time derivative of the X-ray light
curve resembles the radio light curve. This observation can be interpreted in
terms of a standard flare scenario in which accelerated coronal electrons reach
the chromosphere where they heat the cool plasma and induce chromospheric
evaporation. Such a scenario can only hold if the amount of energy in the fast
electrons is sufficient to explain the X-ray radiative losses. We present a
plausibility analysis that supports the chromospheric evaporation model.Comment: Accepted for publication in Ap
Development of a Direct Fabrication Technique for Full-Shell X-Ray Optics
No abstract availabl
Discovery of an Extraordinarily Massive Cluster of Red Supergiants
We report the discovery of an extraordinarily massive young cluster of stars
in the Galaxy, having an inferred total initial cluster mass comparable to the
most massive young clusters in the Galaxy. Using {\it IRMOS}, {\it 2MASS}, and
{\it Spitzer} observations, we conclude that there are 14 red supergiants in
the cluster, compared with five, in what was previously thought to be the
richest Galactic cluster of such stars. We infer spectral types from
near-infrared spectra that reveal deep CO bandhead absorption that can only be
fit by red supergiants. We identify a gap of {\it K}4
magnitudes between the stars and the bulk of the other stars in the region that
can only be fit by models if the brightest stars in the cluster are red
supergiants. We estimate a distance of 5.8~\kpc to the cluster by associating
an OH maser with the envelope of one of the stars. We also identify a
``yellow'' supergiant of G6~I type in the cluster. Assuming a Salpeter IMF, we
infer an initial cluster mass of 20,000 to 40,000~\Msun for cluster ages of
7-12~\Myr. Continuing with these assumptions, we find 80% of the intial mass
and 99% of the number of stars remain at the present time. We associate the
cluster with an x-ray source (detected by {\it ASCA} and {\it Einstein}), a
recently discovered very high energy -ray source (detected by {\it
INTEGRAL} and {\it HESS}), and several non-thermal radio sources, finding that
these objects are likely related to recent supernovae in the cluster. In
particular, we claim that the cluster has produced at least one recent
supernova remnant with properties similar to the Crab nebula. It is not
unlikely to find such a source in this cluster, given our estimated supernova
rate of one per 40,000 to 80,000~{\it yr}.Comment: ApJ, accepte
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