Modeling of positive and negative organic magnetoresistance in organic light-emitting diodes

Copyright 2012 by the American Physical Society. Article is available at

Determining the influence of excited states on current transport in organic light emitting diodes using magnetic field perturbation

Copyright 2010 by the American Physical Society. Article is available at

Galactic AGB stars from the IPHAS survey

We present a photometric analysis of the properties of asymptotic giant branch stars identified in the INT Photometric H-alpha Survey (IPHAS) of the northern Galactic plane. Follow-up spectroscopy has revealed that the IPHAS (r - Ha) colour is a valuable diagnostic of the photospheric C/O ratio, and may be used to identify hundreds of carbon and S-type stars

Chandra observations of Cygnus OB2

Cygnus OB2 is the nearest example of a massive star forming region, containing over 50 O-type stars and hundreds of B-type stars. We have analyzed two Chandra pointings in Cyg OB2, detecting ~1700 X-ray sources, of which ~1450 are thought to be members of the association. Optical and near-IR photometry has been obtained for ~90% of these sources from recent deep Galactic plane surveys. We have performed isochrone fits to the near-IR color-magnitude diagram, deriving ages of 3.5(+0.75,-1.0) and 5.25(+1.5,-1.0) Myrs for sources in the two fields, both with considerable spreads around the pre-MS isochrones. The presence of a second population in the region, somewhat older than the present-day O-type stars, has been suggested by other authors and fits with the ages derived here. The fraction of sources with inner circumstellar disks (as traced by the K-band excess) is found to be very low, but appropriate for a population of age ~5 Myrs. We measure the stellar mass functions and find a power-law slope of Gamma = -1.09 +/- 0.13, in good agreement with the global mean value estimated by Kroupa. A steepening of the mass function at high masses is observed and we suggest this is due to the presence of the previous generation of stars that have lost their most massive members. Finally, combining our mass function and an estimate of the radial density profile of the association suggests a total mass of Cyg OB2 of ~30,000 Msun, similar to that of many of our Galaxy's most massive star forming regions.Comment: 6 pages, 4 figures, conference proceedings for JENAM 2010: Star Clusters in the Era of Large Surveys, Editors: A.Moitinho and J. Alve

The O star hinterland of the Galactic starburst, NGC 3603

The very bright and compact massive young cluster, NGC 3603, has been cited as an example of a starburst in the Milky Way and compared with the much-studied R136/30 Doradus region in the Large Magellanic Cloud. Here we build on the discovery by Mohr-Smith et al. (2017) of a large number of reddened O stars around this cluster. We construct a list of 288 candidate O stars with proper motions, in a region of sky spanning 1.5 × 1.5 square degrees centered on NGC 3603, by cross-matching the Mohr-Smith et al. (2017) catalogue with Gaia DR2 (Gaia Collaboration et al. 2018). This provides the basis for a first comprehensive examination of the proper motions of these massive stars in the halo of NGC 3603, relative to the much better studied central region. We identify up to 11 likely O star ejections – 8 of which would have been ejected between 0.60 and 0.95 Myr ago (supporting the age of ∼1 Myr that has been attributed to the bright cluster centre). Seven candidate ejections are arranged in a partial ring to the south of the cluster core spanning radii of 9–18 arcmin (18–36 pc if the cluster is 7 kpc away). We also show that the cluster has a halo of a further ∼100 O stars extending to a radius of at least 5 arcmin, adding to the picture of NGC 3603 as a scaled down version of the R136/30 Dor region

Proper motions of OB stars in the far Carina Arm

ABSTRACT In large-scale maps of the Galactic disc, the Carina Arm stands out as a clear spiral feature, hosting prominent star clusters and associations rich in massive stars. We study the proper motions of 4199 O and early B most likely in the far Carina Arm, at distances mainly in excess of 4 kpc from the Sun, within the sky region, 282° &amp;lt; ℓ &amp;lt; 294° and −3° &amp;lt; b &amp;lt; +1° (Galactic coordinates). The sample is constructed by extending an existing blue-selected catalogue, and cross-matching with Gaia EDR3 astrometry. The observed pattern of proper motions is modulated into a saw-tooth pattern, with full amplitude approaching 1 mas yr−1, recurring roughly every 2–3 degrees of longitude (200–300 pc at the median OB-star distance of 5.8 kpc). Kinematic perturbation of underlying circular rotation is most likely present. The data also reveal a moving group containing &amp;gt;50 OB stars at ℓ ∼ 286°, b ∼ −1.4° behind the main run of the far arm. An analysis of relative proper motions is performed that yields an incidence of runaway O stars of at least 10 per cent (potentially &amp;gt;20 per cent when full space motions become available). To map where runaways have run away from, we set up simulations for the region that assume linear trajectories and test for trajectory impact parameter in order to identify likely ejection hot spots. We find the method currently gives good results for times of flight of up to ∼4 Myr. It shows convincingly that only NGC 3603 and Westerlund 2 have ejected OB stars in significant numbers. Indeed, both clusters have experienced intense spells of ejection between 0.6–0.9 and 0.5–0.8 Myr ago, respectively.</jats:p

Effects of climate, species interactions, and dispersal on decadal colonization and extinction rates of Iberian tree species

We studied the relative importance of climate, abundance of potentially competing species, and dispersal in explaining local colonization and extinction rates of tree species throughout mainland Spain. We used a Bayesian framework to parameterize a patch occupancy model to 23 species censused in 46,596 permanent plots in a 1 × 1 km grid across most Spanish forests. For most species, dispersal was the single best predictor of colonization, whereas climate and dispersal were equally important as predictors of extinction. Precipitation was positively correlated with the colonization rate of 12 out of 13 deciduous broad-leaved species, and negatively correlated with the extinction rate of nine of them. In contrast, precipitation equally decreased colonization and extinction of five out of eight of needle-leaved species (Juniperus and Pinus spp.). There was, however, marked variation among species in the magnitude of these effects, with some species exhibiting contrasting patterns for the colonization and the extinction process. Abundance of competing tree species (= summed plot basal area) was consistently correlated with decreased colonization of all needle-leaved species, and it increased the extinction rate of 6 out of 8 of these species. It had, nonetheless, weak facilitative effect on some broad-leaved species by promoting colonization (3 of 13 species) and decreasing extinction (7 of 13 species). With local colonization and extinction data, non-equilibrial and dynamic species distribution modelling can be improved by incorporating measures of biotic interactions and dispersal effects, along with traditional climate variables.Ministerio de Economía y CompetitividadComunidad de Madri

Effects of climate, species interactions, and dispersal on decadal colonization and extinction rates of Iberian tree species

We studied the relative importance of climate, abundance of potentially competing species, and dispersal in explaining local colonization and extinction rates of tree species throughout mainland Spain. We used a Bayesian framework to parameterize a patch occupancy model to 23 species censused in 46,596 permanent plots in a 1 × 1 km grid across most Spanish forests. For most species, dispersal was the single best predictor of colonization, whereas climate and dispersal were equally important as predictors of extinction. Precipitation was positively correlated with the colonization rate of 12 out of 13 deciduous broad-leaved species, and negatively correlated with the extinction rate of nine of them. In contrast, precipitation equally decreased colonization and extinction of five out of eight of needle-leaved species (Juniperus and Pinus spp.). There was, however, marked variation among species in the magnitude of these effects, with some species exhibiting contrasting patterns for the colonization and the extinction process. Abundance of competing tree species (= summed plot basal area) was consistently correlated with decreased colonization of all needle-leaved species, and it increased the extinction rate of 6 out of 8 of these species. It had, nonetheless, weak facilitative effect on some broad-leaved species by promoting colonization (3 of 13 species) and decreasing extinction (7 of 13 species). With local colonization and extinction data, non-equilibrial and dynamic species distribution modelling can be improved by incorporating measures of biotic interactions and dispersal effects, along with traditional climate variables.Ministerio de Economía y CompetitividadComunidad de Madri