400 research outputs found
Planet formation around low mass stars: the moving snow line and super-Earths
We develop a semi-analytic model for planet formation during the pre-main
sequence contraction phase of a low mass star. During this evolution, the
stellar magnetosphere maintains a fixed ratio between the inner disk radius and
the stellar radius. As the star contracts at constant effective temperature,
the `snow line', which separates regions of rocky planet formation from regions
of icy planet formation, moves inward. This process enables rapid formation of
icy protoplanets that collide and merge into super-Earths before the star
reaches the main sequence. The masses and orbits of these super-Earths are
consistent with super-Earths detected in recent microlensing experiments.Comment: accepted by ApJ Letter
The Arctic Game
Since outsiders first visited the Arctic, they have believed in man\u27s ability to conquer the region. Today\u27s Arctic conquest is not one of heroic exploration, but rather one of ownership and exploitation. This paper illustrates contestation in the Arctic through the metaphor of a game, with attendant prizes, players, and rules. It focuses on how to prevent the future destruction of the Arctic given the interactions of the Arctic\u27s landscape, prizes, players, and current management frameworks. In the wake of renewed resource exploitation and escalating climate change impacts, the current frameworks and mindsets are inadequate to support the precarious balance of cooperation and competition in the region. The presence of an indigenous population is a defining characteristic of the Arctic landscape, requiring a change from traditional policy methods as an appropriate management tool. Turning toward leadership from northern indigenous populations and following the example of cooperation initiated by the natural science community may be the best way forward to prevent a dystopian future for the Arctic
Massive Protoplanetary Disks in the Trapezium Region
(abridged) We determine the disk mass distribution around 336 stars in the
young Orion Nebula cluster by imaging a 2.5' x 2.5' region in 3 mm continuum
emission with the Owens Valley Millimeter Array. For this sample of 336 stars,
we observe 3 mm emission above the 3-sigma noise level toward ten sources, six
of which have also been detected optically in silhouette against the bright
nebular background. In addition, we detect 20 objects that do not correspond to
known near-IR cluster members. Comparisons of our measured fluxes with longer
wavelength observations enable rough separation of dust emission from thermal
free-free emission, and we find substantial dust emission toward most objects.
For the ten objects detected at both 3 mm and near-IR wavelengths, eight
exhibit substantial dust emission. Excluding the high-mass stars and assuming a
gas-to-dust ratio of 100, we estimate circumstellar masses ranging from 0.13 to
0.39 Msun. For the cluster members not detected at 3 mm, images of individual
objects are stacked to constrain the mean 3 mm flux of the ensemble. The
average flux is detected at the 3-sigma confidence level, and implies an
average disk mass of 0.005 Msun, comparable to the minimum mass solar nebula.
The percentage of stars in Orion surrounded by disks more massive than ~0.1
Msun is consistent with the disk mass distribution in Taurus, and we argue that
massive disks in Orion do not appear to be truncated through close encounters
with high-mass stars. Comparison of the average disk mass and number of massive
dusty structures in Orion with similar surveys of the NGC 2024 and IC 348
clusters constrains the evolutionary timescales of massive circumstellar disks
in clustered environments.Comment: 27 pages, including 7 figures. Accepted by Ap
CEN34 -- High-Mass YSO in M17 or Background Post-AGB Star?
We investigate the proposed high-mass young stellar object (YSO) candidate
CEN34, thought to be associated with the star forming region M17. Its optical
to near-infrared (550-2500 nm) spectrum reveals several photospheric absorption
features, such as H{\alpha}, Ca triplet and CO bandheads but lacks any emission
lines. The spectral features in the range 8375-8770{\AA} are used to constrain
an effective temperature of 5250\pm250 (early-/mid-G) and a surface gravity of
2.0\pm0.3 (supergiant). The spectral energy distribution of CEN34 resembles the
SED of a high-mass YSO or an evolved star. Moreover, the observed temperature
and surface gravity are identical for high-mass YSOs and evolved stars. The
radial velocity relative to LSR (V_LSR) of CEN34 as obtained from various
photospheric lines is of the order of -60 km/s and thus distinct from the +25
km/s found for several OB stars in the cluster and for the associated molecular
cloud. The SED modeling yields ~ 10^{-4} M_sun of circumstellar material which
contributes only a tiny fraction to the total visual extinction (11 mag). In
the case of a YSO, a dynamical ejection process is proposed to explain the
V_LSR difference between CEN34 and M17. Additionally, to match the temperature
and luminosity, we speculate that CEN34 had accumulated the bulk of its mass
with accretion rate > 4x10^{-3} M_sun/yr in a very short time span (~ 10^3
yrs), and currently undergoes a phase of gravitational contraction without any
further mass gain. However, all the aforementioned characteristics of CEN34 are
compatible with an evolved star of 5-7 M_sun and an age of 50-100 Myrs, most
likely a background post-AGB star with a distance between 2.0 kpc and 4.5 kpc.
We consider the latter classification as the more likely interpretation.
Further discrimination between the two possible scenarios should come from the
more strict confinement of CEN34's distance.Comment: 8 pages, 8 figures, 2 tables; accepted by A&
VLT near- to mid-IR imaging and spectroscopy of the M17 UC1-IRS5 region
We investigate the surroundings of the hypercompact HII region M17 UC1 to
probe the physical properties of the associated young stellar objects and the
environment of massive star formation. Five of the seven point sources in this
region show -band excess emission. Geometric match is found between the H_2
emission and near-IR polarized light in the vicinity of IRS5A, and between the
diffuse mid-IR emission and near-IR polarization north of UC1. The H_2 emission
is typical for dense PDRs, which are FUV pumped initially and repopulated by
collisional de-excitation. The spectral types of IRS5A and B273A are B3-B7
V/III and G4-G5 III, respectively. The observed infrared luminosity L_IR in the
range 1-20 micron is derived for three objects; we obtain 2.0x10^3 L_\sun for
IRS5A, 13 L_\sun for IRS5C, and 10 L_\sun for B273A. IRS5 might be a young
quadruple system. Its primary star IRS5A is confirmed to be a high-mass
protostellar object (~ 9 M_\sun, ~1x10^5 yrs); it might have terminated
accretion due to the feedback from the stellar activities (radiation pressure,
outflow) and the expanding HII region of M17. UC1 might also have terminated
accretion because of the expanding hypercompact HII region ionized by itself.
The disk clearing process of the low-mass YSOs in this region might be
accelerated by the expanding HII region. The outflows driven by UC1 are running
in south-north with its northeastern side suppressed by the expanding
ionization front of M17; the blue-shifted outflow lobe of IRS5A is seen in two
types of tracers along the same line of sight in the form of H_2 emission
filament and mid-emission. The H_2 line ratios probe the properties of M17 SW
PDR, which is confirmed to have a clumpy structure with two temperature
distributions: warm, dense molecular clumps with n_H>10^5 cm^-3 and T~575 K and
cooler atomic gas with n_H~3.7x10^3-1.5x10^4 cm-3 and T~50-200 K.Comment: accepted for publication in A&A, 19 pages, 15 figures, 5 table
Ar luz razão certa: João Cabral vendo Franz Weissmann
Este artigo propõe uma breve análise de “Exposição Franz Weissmann”, texto publicado por João Cabral de Melo Neto na década de 1960 e recuperado em Museu de tudo (1975), a fim de compreender a reflexão do poeta sobre a produção do artista plástico. Tendo em vista a similaridade entre os projetos estéticos de Weissmann e João Cabral, o artigo também pretende repensar o lugar específico do poema na própria obra do autor como um momento de problematização interna de seu ideário construtivista.This article intends an analysis of “Exposição Franz Weissmann”, published by João Cabral de Melo Neto in 1960’s and recovers in Museu de tudo (1975), in order to understand the poet’s thought on the visual artist’s production. Keeping in mind the resemblance between aesthetic projects of Weissmann and João Cabral, the article will reconsider the specific place of this poem as questioning of the constructive ideology inside Cabral’s own work
João Cabral em dois autos
Esse artigo propõe uma breve análise de Morte e vida severina em contraposição ao Auto do frade, ambos poemas de João Cabral de Melo Neto. Se, na década de 1960, o auto de natal pernambucano alcançou grande êxito no meio teatral, é curioso constatar que a peça da década de 1980, centrada na figura histórica de frei Caneca, não obteve o mesmo sucesso. Tendo isso em vista, o artigo pretende refletir sobre os motivos que justificam a distância entre as peças, atravessada por duas décadas de ditadura civil-militar no Brasil
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