5,279 research outputs found
Spectroscopic characterization of X-ray emitting young stars associated with the Sh 2-296 nebula
We studied a sample of stars associated with the Sh 2-296 nebula, part of the
reflection nebulae complex in the region of Canis Major (CMa R1). Our sample
corresponds to optical counterparts of X-ray sources detected from observations
with the XMM-Newton satellite, which revealed dozens of possible low-mass young
stars not yet known in this region.
A sample of 58 young star candidates were selected based on optical spectral
features, mainly H{\alpha} and lithium lines, observed with multi-objects
spectroscopy performed by the Gemini South telescope. Among the candidates, we
find 41 confirmed T Tauri and 15 very likely young stars. Based on the
H{\alpha} emission, the T Tauri stars were distinguished between classical
(17%) and weak-lined (83%), but no significant difference was found in the age
and mass distribution of these two classes.
The characterization of the sample was complemented by near- and mid-infrared
data, providing an estimate of ages and masses from the comparison with
pre-main-sequence evolutionary models. While half of the young stars have an
age of 1-2 Myrs or less, only a small fraction (~25%) shows evidence of IR
excess revealing the presence of circumstellar discs. This low fraction is
quite rare compared to most young star-forming regions, suggesting that some
external factor has accelerated the disc dissipation
Star formation history of Canis Major OB1 - II. A bimodal X-ray population revealed by XMM-Newton
The Canis Major OB1 Association has an intriguing scenario of star formation,
especially in the Canis Major R1 (CMa R1) region traditionally assigned to a
reflection nebula, but in reality an ionized region. This work is focused on
the young stellar population associated to CMa R1, for which our previous
results from ROSAT, optical and near-infrared data had revealed two stellar
groups with different ages, suggesting a possible mixing of populations
originated from distinct star-formation episodes. The X-ray data allow the
detected sources to be characterized according to hardness ratios, light curves
and spectra. Estimates of mass and age were obtained from the 2MASS catalogue,
and used to define a complete subsample of stellar counterparts, for
statistical purposes. A catalogue of 387 XMM-Newton sources is provided, 78%
being confirmed as members or probable members of the CMa R1 association.
Flares were observed for 13 sources, and the spectra of 21 bright sources could
be fitted by a thermal plasma model. Mean values of fits parameters were used
to estimate X-ray luminosities. We found a minimum value of log(L[erg/s]) =
29.43, indicating that our sample of low-mass stars (M 0.5
M), being faint X-ray emitters, is incomplete. Among the 250 objects
selected as our complete subsample (defining our best sample), 171 are found to
the East of the cloud, near Z CMa and dense molecular gas, 50% of them being
young ( 10 Myr). The opposite happens to the
West, near GU CMa, in areas lacking molecular gas: among 79 objects, 30% are
young and 50% are older. These findings confirm that a first episode of
distributed star formation occurred in the whole studied region ~10 Myr ago and
dispersed the molecular gas, while a second, localized episode (< 5 Myr) took
place in the regions where molecular gas is still present.Comment: 38 pages, 21 figures, accepted for A&
Selected Morphological Characteristics, Lead Uptake and Phytochelatin Synthesis by Coffeeweed (Sesbania exaltata Raf.) Grown in Elevated Levels of Lead-Contaminated Soil
Remediation of lead-contaminated soil is significant due to the inherent toxicity of lead (Pb), and the quantity of Pb discharged into the soil. One of the most cost-effective and environmentally sound technologies for the cleanup of metal-contaminated soils is through the use of plants. While much is known about the ecological evolution of metal tolerance in plants, the physiological, biochemical, and genetic mechanisms of tolerance is not well understood in the majority of resistant ecotypes such as the legume, Sesbania exaltata Raf. This study was therefore conducted to determine the morphological and physiological characteristics of Sesbania that had been grown in Pb-contaminated soil, and to assess phytochelatin synthesis as a way of elucidating its relative Pb tolerance. Sesbania plants were grown in the greenhouse and exposed to various levels of Pb: 0, 1000, and 2000 mg Pb/kg soil. Plants were harvested after 6, 8, and 10 weeks of growth and morphological characteristics (e.g., root and shoot biomass, root length, number of root nodules, shoot height, number of leaves, number of flowers, number and length of pods) were recorded. Generally, there were no statistical differences in morphological characteristics among the treatments. Further, no discernible phytotoxic symptoms, such as chlorosis, wilting, or necrotic lesions, in neither roots nor shoots were observed. We concluded that while Sesbania did not fit the model of a hyperaccumulator, the plant was, nonetheless, tolerant to elevated Pb levels. Our assessment for phytochelatin synthesis as a tolerance mechanism was inconclusive and further investigations of tolerance mechanisms are warranted
Politics and power in national REDD+ policy processes
National Strategy and Policy Options Maria Brockhaus Arild Angelsen ... depend on existing institutions and legal structures, current political and economic processes, the distribution of power and wealth, and the REDD+ actions appropriate to ..
Unexpectedly large mass loss during the thermal pulse cycle of the red giant R Sculptoris!
The asymptotic giant branch star R Sculptoris is surrounded by a detached
shell of dust and gas. The shell originates from a thermal pulse during which
the star undergoes a brief period of increased mass loss. It has hitherto been
impossible to constrain observationally the timescales and mass-loss properties
during and after a thermal pulse - parameters that determine the lifetime on
the asymptotic giant branch and the amount of elements returned by the star.
Here we report observations of CO emission from the circumstellar envelope and
shell around R Sculptoris with an angular resolution of 1.3 arcsec. What was
hitherto thought to be only a thin, spherical shell with a clumpy structure, is
revealed to contain a spiral structure. Spiral structures associated with
circumstellar envelopes have been seen previously, from which it was concluded
that the systems must be binaries. Using the data, combined with hydrodynamic
simulations, we conclude that R Sculptoris is a binary system that underwent a
thermal pulse approximately 1800 years ago, lasting approximately 200 years.
About 0.003 Msun of mass was ejected at a velocity of 14.3 km s-1 and at a rate
approximately 30 times higher than the prepulse mass-loss rate. This shows that
approximately 3 times more mass is returned to the interstellar medium during
and immediately after a pulse than previously thought.Comment: Accepted by Natur
Lactarius purpureobadius Malençon ex Basso, en España
Lactarius purpureobadius Malençon ex Basso, a Espanya. Es descriu, comenta i
iconografia un tà xon molt interessant, no citat prÚviament a Espanya.Lactarius purpureobadius Malençon ex Basso, in Spain. One interesting taxon,
previously unrecorded in Spain, is described, commented and illustrated.Lactarius purpureobadius Malençon ex Basso, en España. Se describe, comenta e
ilustra un taxon muy interesante, no citado previamente en España
- âŠ