1,879 research outputs found
Chandra HETGS Multiphase Spectroscopy Of The Young Magnetic O Star Theta(1) Orionis C
We report on four Chandra grating observations of the oblique magnetic rotator theta(1) Ori C (O5.5 V), covering a wide range of viewing angles with respect to the star\u27s 1060 G dipole magnetic field. We employ line-width and centroid analyses to study the dynamics of the X-ray - emitting plasma in the circumstellar environment, as well as line-ratio diagnostics to constrain the spatial location, and global spectral modeling to constrain the temperature distribution and abundances of the very hot plasma. We investigate these diagnostics as a function of viewing angle and analyze them in conjunction with new MHD simulations of the magnetically channeled wind shock mechanism on theta(1) Ori C. This model fits all the data surprisingly well, predicting the temperature, luminosity, and occultation of the X-ray - emitting plasma with rotation phase
Chandra X-Ray Spectroscopy Of The Very Early O Supergiant HD 93129A: Constraints On Wind Shocks And The Mass-Loss Rate
We present an analysis of both the resolved X-ray emission-line profiles and the broad-band X-ray spectrum of the O-2 If* star HD 93129A, measured with the Chandra High Energy Transmission Grating Spectrometer ( HETGS). This star is among the earliest and most massive stars in the Galaxy, and provides a test of the embedded wind-shock scenario in a very dense and powerful wind. A major new result is that continuum absorption by the dense wind is the primary cause of the hardness of the observed X-ray spectrum, while intrinsically hard emission from colliding wind shocks contributes less than 10 per cent of the X-ray flux. We find results consistent with the predictions of numerical simulations of the line-driving instability, including line broadening indicating an onset radius of X-ray emission of several tenths of R-*. Helium-like forbidden-to-intercombination line ratios are consistent with this onset radius, and inconsistent with being formed in a wind-collision interface with the star\u27s closest visual companion at a distance of 100 au. The broad-band X-ray spectrum is fitted with a dominant emission temperature of just kT = 0.6 keV along with significant wind absorption. The broad-band wind absorption and the line profiles provide two independent measurements of the wind mass-loss rate:. M = 5.2(-1.5)(+1.8) x 10(-6) and 6.8(-2.2)(+2.8) x 10(-6) M-circle dot yr(-1), respectively. This is the first consistent modelling of the X-ray line-profile shapes and broad-band X-ray spectral energy distribution in a massive star, and represents a reduction of a factor of 3-4 compared to the standard H alpha mass-loss rate that assumes a smooth wind
X-ray emission from the giant magnetosphere of the magnetic O-type star NGC 1624-2
We observed NGC 1624-2, the O-type star with the largest known magnetic field
Bp~20 kG), in X-rays with the ACIS-S camera onboard the Chandra X-ray
Observatory. Our two observations were obtained at the minimum and maximum of
the periodic Halpha emission cycle, corresponding to the rotational phases
where the magnetic field is the closest to equator-on and pole-on,
respectively. With these observations, we aim to characterise the star's
magnetosphere via the X-ray emission produced by magnetically confined wind
shocks. Our main findings are:
(i) The observed spectrum of NGC 1624-2 is hard, similar to the magnetic
O-type star Theta 1 Ori C, with only a few photons detected below 0.8 keV. The
emergent X-ray flux is 30% lower at the Halpha minimum phase.
(ii) Our modelling indicated that this seemingly hard spectrum is in fact a
consequence of relatively soft intrinsic emission, similar to other magnetic
Of?p stars, combined with a large amount of local absorption (~1-3 x 10^22
cm^-2). This combination is necessary to reproduce both the prominent Mg and Si
spectral features, and the lack of flux at low energies. NGC 1624-2 is
intrinsically luminous in X-rays (log LX emission ~ 33.4) but 70-95% of the
X-ray emission produced by magnetically confined wind shocks is absorbed before
it escapes the magnetosphere (log LX ISM corrected ~ 32.5).
(iii) The high X-ray luminosity, its variation with stellar rotation, and its
large attenuation are all consistent with a large dynamical magnetosphere with
magnetically confined wind shocks.Comment: Accepted in MNRAS 13 pages, 10 figures, 4 table
Compensation profiles among private sector employees in Sweden: Differences in work-related and health-related outcomes
How experiences and perceptions of pay and pay setting relate to employees’ job performance, willingness to remain in the organization, and health has been the subject of much debate. Previous research has typically used a variable-centered approach to investigate associations between different pay-related factors and such outcomes. In contrast, we used latent profile analysis to explore combinations of compensation characteristics (pay level, perceived horizontal pay dispersion, and procedural quality, i.e., transactional leadership and procedural pay-setting justice), combining relevant theories on the subject. Based on a nationally representative sample of private sector employees in Sweden (N = 1,146), our study identified six compensation profiles. Our key findings show, first, that higher levels of pay were generally associated with better performance, lower turnover intention, better self-rated health, and lower work-related exhaustion, especially when combined with perceptions of high procedural quality. Second, in terms of perceived horizontal pay dispersion, the results indicate that pay compression may be associated with beneficial outcomes, particularly when combined with high procedural quality. Third, procedural quality was generally associated with favorable work-related and health-related outcomes, although such positive effects may be contingent upon pay level and perceived horizontal pay dispersion. In conclusion, while pay level, perceptions of horizontal pay dispersion, and procedural quality may all matter for employee outcomes, it is important to consider their combinations
Relative Reactivity of the Metal-Amido versus Metal-Imido Bond in Linked Cp-Amido and Half-Sandwich Complexes of Vanadium
Treatment of (η5-C5H4C2H4NR)V(N-t-Bu)Me (R = Me, i-Pr) and CpV(N-p-Tol)(N-i-Pr2)Me (Cp = η5-C5H5) with B(C6F5)3 or [Ph3C][B(C6F5)4] results in formation of the corresponding cations, [(η5-C5H4C2H4NR)V(N-t-Bu)]+ and [CpV(N-p-Tol)(N-i-Pr2)]+. The latter could also be generated as its N,N-dimethylaniline adduct by treatment of the methyl complex with [PhNMe2H][BAr4] (Ar = Ph, C6F5). Instead, the analogous reaction with the linked Cp-amido precursor results in protonation of the imido-nitrogen atom. Sequential cyclometalation of the amide substituents gave cationic imine complexes [(η5-C5H4C2H4NCR'2)V(NH-t-Bu)]+ (R' = H, Me) and methane. Reaction of cationic [(η5-C5H4C2H4NR)V(N-t-Bu)]+ with olefins affords the corresponding olefin adducts, whereas treatment with 1 or 2 equiv of 2-butyne results in insertion of the alkyne into the vanadium-nitrogen single bond, affording the mono- and bis-insertion products [(η5-C5H4C2H4N(i-Pr)C2Me2)V(N-t-Bu)]+ and [(η5-C5H4C2H4N(i-Pr)C4Me4)V(N-t-Bu)]+. The same reaction with the half-sandwich compound [CpV(N-p-Tol)(N-i-Pr2)]+ results in a paramagnetic compound that, upon alcoholysis, affords sec-butylidene-p-tolylamine, suggesting an initial [2+2] cycloaddition reaction. The difference in reactivity between the V-N bond versus the V=N bond was further studied using computational methods. Results were compared to the isoelectronic titanium system CpTi(NH)(NH2). These studies indicate that the kinetic product in each system is derived from a [2+2] cycloaddition reaction. For titanium, this was found as the thermodynamic product as well, whereas the insertion reaction was found to be thermodynamically more favorable in the case of vanadium.
Aerosol charging state at an urban site: new analytical approach and implications for ion-induced nucleation
The charging state of aerosol populations was determined using an Ion-DMPS in Helsinki, Finland between December 2008 and February 2010. We extrapolated the charging state and calculated the ion-induced nucleation fraction to be around 1.3 % ± 0.4 % at 2 nm and 1.3 % ± 0.5 % at 1.5 nm, on average. We present a new method to retrieve the average charging state for a new particle formation event, at a given size and polarity. We improve the uncertainty assessment and fitting technique used previously with an Ion-DMPS. We also use a new theoretical framework that allows for different concentrations of small ions for different polarities (polarity asymmetry). We extrapolate the ion-induced fraction using polarity symmetry and asymmetry. Finally, a method to calculate the growth rates from the behaviour of the charging state as a function of the particle diameter using polarity symmetry and asymmetry is presented and used on a selection of new particle formation events
Global X-ray properties of the O and B stars in Carina
The key empirical property of the X-ray emission from O stars is a strong
correlation between the bolometric and X-ray luminosities. In the framework of
the Chandra Carina Complex Project, 129 O and B stars have been detected as
X-ray sources; 78 of those, all with spectral type earlier than B3, have enough
counts for at least a rough X-ray spectral characterization. This leads to an
estimate of the Lx/Lbol ratio for an exceptional number of 60 O stars belonging
to the same region and triples the number of Carina massive stars studied
spectroscopically in X-rays. The derived log(Lx/Lbol) is -7.26 for single
objects, with a dispersion of only 0.21dex. Using the properties of hot massive
stars listed in the literature, we compare the X-ray luminosities of different
types of objects. In the case of O stars, the Lx/Lbol ratios are similar for
bright and faint objects, as well as for stars of different luminosity classes
or spectral types. Binaries appear only slightly harder and slightly more
luminous in X-rays than single objects; the differences are not formally
significant (at the 1% level), except for the Lx/Lbol ratio in the medium
(1.0--2.5keV) energy band. Weak-wind objects have similar X-ray luminosities
but they display slightly softer spectra compared to "normal" O stars with the
same bolometric luminosity. Discarding three overluminous objects, we find a
very shallow trend of harder emission in brighter objects. The properties of
the few B stars bright enough to yield some spectral information appear to be
different overall (constant X-ray luminosities, harder spectra), hinting that
another mechanism for producing X-rays, besides wind shocks, might be at work.
However, it must be stressed that the earliest and X-ray brightest amongst
these few detected objects are similar to the latest O stars, suggesting a
possibly smooth transition between the two processes.Comment: 30 pages, 9 figures. Accepted for the ApJS Special Issue on the
Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011.
All 16 CCCP Special Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
leas
The development of the representation of conceptual knowledge in memory and the design of instruction
An integrative SDT-based investigation of the relation between financial incentives, motivation, and performance
To this day, researchers are debating the adequacy of using financial incentives to bolster performance in work settings. Our goal was to contribute to current understanding by considering the moderating role of distributive justice in the relation between financial incentives, motivation, and performance. Based on self-determination theory, we hypothesized that when bonuses are fairly distributed, using financial incentives makes employees feel more competent and autonomous, which in turn fosters greater autonomous motivation and lower controlled motivation, and better work performance. Results from path analyses in three samples supported our hypotheses, suggesting that the effect of financial incentives is contextual, and that compensation plans using financial incentives and bonuses can be effective when properly managed
- …