1,081 research outputs found
On the kinematic evolution of young local associations and the Sco-Cen complex
Context: Over the last decade, several groups of young (mainly low-mass)
stars have been discovered in the solar neighbourhood (closer than ~100 pc),
thanks to cross-correlation between X-ray, optical spectroscopy and kinematic
data. These young local associations -- including an important fraction whose
members are Hipparcos stars -- offer insights into the star formation process
in low-density environments, shed light on the substellar domain, and could
have played an important role in the recent history of the local interstellar
medium. Aims: To study the kinematic evolution of young local associations and
their relation to other young stellar groups and structures in the local
interstellar medium, thus casting new light on recent star formation processes
in the solar neighbourhood. Methods: We compiled the data published in the
literature for young local associations. Using a realistic Galactic potential
we integrated the orbits for these associations and the Sco-Cen complex back in
time. Results: Combining these data with the spatial structure of the Local
Bubble and the spiral structure of the Galaxy, we propose a recent history of
star formation in the solar neighbourhood. We suggest that both the Sco-Cen
complex and young local associations originated as a result of the impact of
the inner spiral arm shock wave against a giant molecular cloud. The core of
the giant molecular cloud formed the Sco-Cen complex, and some small cloudlets
in a halo around the giant molecular cloud formed young local associations
several million years later. We also propose a supernova in young local
associations a few million years ago as the most likely candidate to have
reheated the Local Bubble to its present temperature.Comment: 27 pages, 13 figures, 14 tables. Accepted for publication in
Astronomy & Astrophysic
On the possible common origin of M16 and M17
It has been suggested that the well-studied giant HII regions M16 and M17 may
have had a common origin, being an example of large-scale triggered star
formation. While some features of the distribution of the interstellar medium
in the region support this interpretation, no definitive detection of an
earlier population of massive stars responsible for the triggering has been
made thus far. We have carried out observations looking for red supergiants in
the area covered by a giant shell seen in HI and CO centered on galactic
coordinates , that peaks near the same
radial velocity as the bulk of the emission from both giant HII regions, which
are located along the shell. Red supergiants have ages in the range expected
for the parent association whose most massive members could have triggered the
formation of the shell and of the giant HII regions along its rim. Out of a
sample of 37 bright red stars, we identify four red supergiants that confirm
the existence of massive stars in the age range between and ~Myr in the area. At least three of them have Gaia DR2 parallaxes consistent
with them being at the same distance as M16 and M17. The evidence of past
massive star formation within the area of the gaseous shell lends support to
the idea that it was formed by the combined action of stellar winds and
ionizing radiation of the precursors of the current red supergiants. These
could be the remnants of a richer population, whose most massive members have
exploded already as core-collapse supernovae. The expansion of the shell
against the surrounding medium, perhaps combined with the overrun of
preexisting clouds, is thus a plausible trigger of the formation of a second
generation of stars currently responsible for the ionization of M16 and M17.Comment: 7 pages, 3 figures; accepted for publication in Astronomy and
Astrophysic
"Non-Target-Site" resistance mechanisms of "Papaver rhoeas" L. biotypes with multiple resistance
Hasta el momento en España no se han desarrollado estudios a fin de evaluar los mecanismos de resistencia “Non-Target-Site” en “Papaver rhoeas”. Tras constatar la resistencia múltiple de los biotipos empleados en este ensayo, los resultados de absorción y translocación obtenidos para 2,4-D y tribenuron-metil parecen arrojar conclusiones interesantes. Sobre todo, en el caso de la auxina sintética, donde una destacada menor translocación detectada en biotipos resistentes respecto a los sensibles está en sintonía con las más recientes teorías de mecanismos de resistencia a herbicidas hormonales.So far in Spain studies to evaluate the mechanisms of resistance “Non-Target-Site” on “Papaver rhoeas” have not been done. After finding multiple resistance in the biotypes used in this trial, absorption and translocation results obtained for 2,4-D and tribenuron-methyl seem to throw interesting findings. Mainly in the case of the synthetic auxin, where a lack of translocation was detected in resistant biotypes compared to the sensitive one. This could be related with the latest theories of mechanisms of resistance to synthetic auxin
The Nature of the Gould Belt from a Fractal Analysis of its Stellar Population
The Gould Belt (GB) is a system of gas and young, bright stars distributed
along a plane that is inclined with respect to the main plane of the Milky Way.
Observational evidence suggests that the GB is our closest star formation
complex, but its true nature and origin remain rather controversial. In this
work we analyze the fractal structure of the stellar component of the GB. In
order to do this, we tailor and apply an algorithm that estimates the fractal
dimension in a precise and accurate way, avoiding both boundary and small data
set problems. We find that early OB stars (of spectral types earlier than B4)
in the GB have a fractal dimension very similar to that of the gas clouds in
our Galaxy. On the contrary, stars in the GB of later spectral types show a
larger fractal dimension, similar to that found for OB stars of both age groups
in the local Galactic disk (LGD). This result seems to indicate that while the
younger OB stars in the GB preserve the memory of the spatial structure of the
cloud where they were born, older stars are distributed following a similar
morphology as that found for the LGD stars. The possible causes for these
differences are discussed.Comment: 20 pages including 7 figures and 1 table. ApJ (in press
- …