Context. It has been suggested that the well-studied giant HII regions M 16 and M 17 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. Aims. 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 l ∼ 14°5, b ∼ +1° whose emission 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. Methods. We have obtained spectroscopy in the visible of a sample of red stars selected on the basis of their infrared colors, whose magnitudes are consistent with them being red supergiants if they are located at the distance of M 16 and M 17. Spectroscopy is needed to distinguish red supergiants from AGB stars and RGB stars, which are expected to be abundant along the line of sight. Results. 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 ∼10 and ∼30 Myr in the area. At least three of them have Gaia DR2 parallaxes consistent with them being at the same distance as M 16 and M 17. Conclusions. 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 already exploded 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 M 16 and M 17.
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.
Context. Recent studies have suggested that moving groups have a dynamic or "resonant" origin. Under this hypothesis, these kinematic structures become a powerful tool for studying the large-scale ...structure and dynamics of the Milky Way. Aims. Here we aim to characterize these structures in the U-V-age-Fe/H space and establish observational constraints that will allow us to study their origin and evolution. Methods. We apply multiscale techniques-wavelet denoising (WD)-to an extensive compendium of more than 24 000 stars in the solar neighbourhood with the best available astrometric, photometric and spectroscopic data. Results. We confirm that the dominant structures in the U-V plane are the branches of Sirius, Coma Berenices, Hyades-Pleiades and Hercules. These branches are nearly equidistant in this kinematic plane and they show a negative slope. The abrupt drops in the velocity density distribution are characterized. We find a certain dependence of these kinematic structures on Galactic position with a significant change of contrast among substructures inside the branches. A large spread of ages is observed for all branches. The Hercules branch is detected in all subsamples with ages older than similar to 2 {\rm\,Gyr} and the set of the other three branches is well established for stars >400 {\rm\,Myr}. The age-metallicity relation of each branch is examined and the relation between kinematics and metallicity is studied. Conclusions. Not all of these observational constraints are successfully explained by the recent models proposed for the formation of such kinematic structures. Simulations incorporating stellar ages and metallicities are essential for future studies. The comparison of the observed and simulated distributions obtained by WD will provide a physical interpretation of the existence of the branches in terms of local or large-scale dynamics.
Context. Observations of the galactic disk at mid-infrared and longer wavelengths reveal a wealth of structures indicating the existence of complexes of recent massive star formation. However, little ...or nothing is known about the stellar component of those complexes. Aims. We have carried out observations aiming at the identification of early-type stars in the direction of the bright infrared source RAFGL 5475, around which several interstellar medium structures usually associated with the presence of massive stars have been identified. Our observations have the potential of revealing the suspected but thus far unknown stellar component of the region around RAFGL 5475. Methods. We have carried out near-infrared imaging observations (JHKs bands) designed to reveal the presence of early-type stars based on their positions in color–color and color–magnitude diagrams centered on the location of RAFGL 5475. We took into account the possibility that candidates found might belong to a foreground population physically related either to M 16 or M 17, two giant HII regions lying midway between the Sun and RAFGL 5475. Results. The near-infrared color–color diagram shows clear evidence for the presence of a moderately obscured population of early-type stars in the region imaged. By studying the distribution of extinction in their direction and basic characteristics of the interstellar medium we show that these new early-type stars are most likely associated with RAFGL 5475. Conclusions. By investigating the possible existence of massive early-type stars in the direction of RAFGL 5475 we have discovered the existence of a new OB association. A very preliminary assessment of its contents suggests the presence of several O-type stars, some of them likely to be associated with structures in the interstellar medium. The new association is located at 4 kpc from the Sun in the Scutum-Centaurus arm.
A catalogue of eclipsing variables Malkov, O. Yu; Oblak, E.; Snegireva, E. A. ...
Astronomy & astrophysics,
02/2006, Volume:
446, Issue:
2
Journal Article
Peer reviewed
Open access
A new catalogue of 6330 eclipsing variable stars is presented. The catalogue was developed from the General Catalogue of Variable Stars (GCVS) and its textual remarks by including recently published ...information about classification of 843 systems and making corresponding corrections of GCVS data. The catalogue1 represents the largest list of eclipsing binaries classified from observations.
Germination and emergence are critical life stages for annual plants and so their full understanding is essential for managing arable plant populations. This study investigated the most important ...species‐specific environmental cues that regulate seed germination and emergence (temperature and light) of the arable Ranunculaceae species Consolida orientalis, Consolida pubescens, Delphinium gracile, Delphinium halteratum ssp. verdunense and Nigella gallica, to propose management strategies for their preservation in agro‐ecosystems. Growth chamber and outdoor pot experiments were conducted for two consecutive seasons to analyse light (complete darkness or 12 h light) and temperature (5/10, 5/15 and 10/20°C) requirements and emergence patterns. The relative light germination requirement (ΔGₗᵢgₕₜ), which extends from −100 (complete darkness) to 100 (light), was estimated. Weibull functions were fitted to observed emergence (%) in pots. For all species, germination was higher in complete darkness than with a light regime (−60 < ΔGₗᵢgₕₜ < −95). This dark requirement indicates better germination for buried seeds. A tillage operation just after seed shed is therefore recommended. Consolida spp. germinate and emerge almost exclusively in autumn–winter, while Delphinium spp. and N. gallica can also germinate in spring. These arable plants would be able to adapt to delayed sowings, an important strategy for avoiding early‐emerging competitive weeds. Facultative winter‐germinating species could face early herbicide treatments if sufficient emergence occurs in winter–spring. These results bring new information to help develop conservation strategies for these species in agro‐ecosystems.
Effects of four tillage systems (direct drill, subsoiler, chisel plough and mouldboard plough) on the dynamics of Polygonum aviculare populations were studied over three growing seasons. Densities ...before and after herbicide application and cumulative emergence on a weekly basis were determined in each growing season in small grain crops in northeastern Spain. Cumulative emergence in chisel plough was used for the development of an emergence model for P. aviculare, based on hydrothermal time estimated with the STM2 program. The emergence model was compared to a thermal time based germination model from Argentina. Results showed that direct drilling, besides obtaining highest crop yields every season, was the unique tillage system that effectively reduced P. aviculare populations because of seed bank depletion. The model described accurately seedling emergence in different tillage systems, although it failed in direct drilling, probably due to low emergence. The comparison of the emergence model with the thermal time germination model showed that there is a delay in emergence in years with short or long drought periods, while emergence follows germination almost simultaneously in those years without droughts. The use of the germination model permitted the estimation of the minimum seed bank required for the observed emergence sequences and, consequently, an approximation to the population dynamics of P. aviculare in each tillage system could be performed. To better control this weed, direct drilling may be the best tillage option, but if this cannot be implemented, the hydrothermal time model is a practical tool that can describe the emergence and assist in the control of P. aviculare in different tillage systems.
The outskirts of Cygnus OB2 Comerón, F.; Pasquali, A.; Figueras, F. ...
Astronomy and astrophysics (Berlin),
08/2008, Volume:
486, Issue:
2
Journal Article
Peer reviewed
Context. Cygnus OB2 is one of the richest OB associations in the local Galaxy, and is located in a vast complex containing several other associations, clusters, molecular clouds, and HII regions. ...However, the stellar content of Cygnus OB2 and its surroundings remains rather poorly known largely due to the considerable reddening in its direction at visible wavelength. Aims. We investigate the possible existence of an extended halo of early-type stars around Cygnus OB2, which is hinted at by near-infrared color–color diagrams, and its relationship to Cygnus OB2 itself, as well as to the nearby association Cygnus OB9 and to the star forming regions in the Cygnus X North complex. Methods. Candidate selection is made with photometry in the 2MASS all-sky point source catalog. The early-type nature of the selected candidates is confirmed or discarded through our infrared spectroscopy at low resolution. In addition, spectral classifications in the visible are presented for many lightly-reddened stars. Results. A total of 96 early-type stars are identified in the targeted region, which amounts to nearly half of the observed sample. Most of them have featureless near-infrared spectra as expected from OB stars at the available resolution. Another 18 stars that display Brackett emission lines can be divided between evolved massive stars (most likely Be stars) and Herbig Ae/Be stars based on their infrared excesses. A component associated with Cygnus OB9/NGC 6910 is clearly identified, as well as an enhancement in the surface density of early-type stars at Cygnus X North. We also find a field population, consisting largely of early B giants and supergiants, which is probably the same as identified in recent studies of the inner $1^\circ$ circle around Cygnus OB2. The age and large extension of this population discards a direct relationship with Cygnus OB2 or any other particular association. Conclusions. Earlier claims of the possible large extent of Cygnus OB2 beyond its central, very massive aggregate seem to be dismissed by our findings. The existence of a nearly ubiquitous population of evolved stars with massive precursors suggests a massive star formation history in Cygnus having started long before the formation of the currently observed OB associations in the region.
The adoption of no‐tillage systems in semi‐arid cereal fields in northern Europe has resulted in difficulties in controlling Bromus diandrus. However, in some fields, lower densities are observed in ...continuous long‐term no‐tillage management than in other reduced tillage systems. A cumulative effect on the seedbank could promote changes in the period of seedling emergence and in population demography. This study evaluated the effect of long‐term mouldboard plough (MbP), chisel plough (ChP), subsoiler (SS) and no‐tillage (NT) on the population dynamics of B. diandrus. The work was carried out in a barley (Hordeum vulgare)–wheat (Triticum aestivum)–barley rotation during three seasons where these soil management systems had been applied for the last 22 years. Cumulative emergence (CE) and densities of B. diandrus followed a gradient of ChP > SS > NT > MbP. This cumulative effect over time resulted in significant differences in population demography. A previous hydrothermal emergence model developed for this species estimated the percentage of emergence prior to the date of sowing to be: 71%, 92% and 53% for the seasons 2008–2009, 2009–2010 and 2010–2011 respectively. Furthermore, the reduction in CE observed was on average 53% in SS, 92% in NT and 98% in MbP in comparison with that recorded in ChP. The long‐term effect of different tillage systems tended to cause changes to soil characteristics (photo‐inhibition of germination, soil temperature, water availability) affecting B. diandrus demography, reaching equilibrium in weed densities over years, which were significantly lower in MbP and NT than in ChP or SS.
Corn poppy is the most important broad-leaved weed in winter cereals in southern Europe. It is an increasing problem due to the appearance of herbicide-resistant biotypes. Experiments were conducted ...in north-eastern Spain with three corn poppy populations to determine the level of resistance to 2,4-D and tribenuron using seed-germination tests. Field experiments were conducted at sites from where the tested populations had been collected to test the effectiveness of various herbicides applied pre-emergence (PRE) or post-emergence (POST) in winter cereals. In the seed-germination tests, two populations were found to be 2,4-D-resistant and one resistant to both 2,4-D and tribenuron. In field trials, the efficacy of 2,4-D at 900 g a.i. ha
−1 ranged from 25 to 76% of reduction of plant densities, depending on season and population, indicating that all populations were 2,4-D-resistant in field conditions. Moreover, at 15 g a.i. ha
−1 tribenuron, one of the three populations (CU2) was only reduced by 65% in two seasons, indicating also the presence of resistance to tribenuron in this population in addition to 2,4-D. Florasulam at 6.25 g a.i. ha
−1 + 2,4-D at 300 g a.i. ha
−1 resulted in poor control (75–85%) in CU2 suggesting possible cross-resistance to this triazolopyrimidine. On average, all the other herbicides provided 90–100% control of corn poppy. Only PRE herbicides and the mixture 150 g a.i. ha
−1 bromoxynil + 150 g a.i. ha
−1 ioxynil + 450 g a.i. ha
−1 mecoprop in POST performed consistently well (>90% control). The results of this study indicate that 2,4-D and tribenuron-resistant corn poppy populations in winter cereals can be controlled by application of PRE or POST herbicides with alternative modes of action.
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