4,189 research outputs found
Chandra Observations of a Young Embedded Magnetic B Star in the rho Ophiuchus Cloud
This paper reports on an analysis of two Chandra X-ray observations of the
young magnetic B star rho Ophiuchus S1. X-ray emission from the star was
detected in both observations. The average flux was almost the same in both,
but during each observation the flux showed significant time variations by a
factor of two on timescales of 20-40 ks. Each spectrum could be fit by either
an absorbed power-law model with a photon index of ~3 or a thin-thermal plasma
model with a temperature of ~2 keV and an extremely low metal abundance (<~0.1
solar). The spectrum of the first observation has a weak-line feature at about
6.8 keV, which might correspond to highly ionized iron Kalpha. In contrast, the
spectrum of the second observation apparently shows a weak edge absorption
component at E~4 keV. The continuum emission and log LX/Lbol ~ -6 are similar
to those of young intermediate-mass stars (Herbig Ae/Be stars), although the
presence of a strong magnetic field (inferred from the detection of non-thermal
radio emission) has drawn an analogy between rho Ophiuchus S1 and magnetic
chemically peculiar (MCP) stars. If the X-ray emission is thermal, the small
abundances that we derived might be related to the inverse first-ionization
potential (FIP) effect, though there is no significant trend as a function of
FIP from our model fits. If the emission is non-thermal, it might be produced
by high-energy electrons in the magnetosphere.Comment: 13 pages, 6 figures, To appear in the October issue of PASJ (vol.55,
No. 5
Laser Guide Star Adaptive Optics Integral Field Spectroscopy of a Tightly Collimated Bipolar Jet from the Herbig Ae star LkHa 233
We have used the integral field spectrograph OSIRIS and laser guide star
adaptive optics at Keck Observatory to obtain high angular resolution (0.06"),
moderate spectral resolution (R ~ 3800) images of the bipolar jet from the
Herbig Ae star LkHa 233, seen in near-IR [Fe II] emission at 1.600 & 1.644
microns. This jet is narrow and tightly collimated, with an opening angle of
only 9 degrees, and has an average radial velocity of ~ 100 km/s. The jet and
counterjet are asymmetric, with the red-shifted jet much clumpier than its
counterpart at the angular resolution of our observations. The observed
properties are in general similar to jets seen around T Tauri stars, though it
has a relatively large mass flux of (1.2e-7 +- 0.3e-7) M_sun/year, near the
high end of the observed mass flux range around T Tauri stars. We also
spatially resolve an inclined circumstellar disk around LkHa 233, which
obscures the star from direct view. By comparison with numerical radiative
transfer disk models, we estimate the disk midplane to be inclined i = 65 +- 5
degrees relative to the plane of the sky. Since the star is seen only in
scattered light at near-infrared wavelengths, we detect only a small fraction
of its intrinsic flux. Because previous estimates of its stellar properties did
not account for this, either LkHa 233 must be located closer than the
previously believed, or its true luminosity must be greater than previously
supposed, consistent with its being a ~4 M_sun star near the stellar birthline.Comment: Accepted for publication in the Ap
An Appreciative Approach to Managerial Coaching: Words Matter When Increasing Employee Engagement
Employers set out to create positive cultures where employees can thrive. Despite this effort, engagement surveys find more than 60% of employees are just going through the motions at work. Disengagement affects such key workplace factors as productivity, customer satisfaction, absenteeism, safety, and turnover. What will it take to shift employees from being disengaged to bringing their best self to work? The field of positive psychology may offer promising possibilities. Mounting evidence on the use of appreciative inquiry, strengths-based development, and self-determination theory in the workplace illuminate pathways to initiate and sustain greater well-being and productivity. Managers and others who coach employees are a critical to creating and sustaining this enhanced work environment. This paper examines how the findings of current positive psychology research points to potential ways coaching conversations can foster higher levels of motivation, cultivate a sense that one’s work is valued, and strengthen a commitment to goals. This literature review identified a number of evidence-based practices managers may use when coaching to constructively develop individuals in ways that are aligned with personal values and motivation. A discussion of future directions for this work is proposed through a positive psychology coaching intervention aimed at increasing employee engagement
Modeling the RXTE light curve of Carinae from a 3-D SPH simulation of its binary wind collision
The very massive star system Carinae exhibits regular 5.54-year
(2024-day) period disruptive events in wavebands ranging from the radio to
X-ray. There is a growing consensus that these events likely stem from
periastron passage of an (as yet) unseen companion in a highly eccentric
() orbit. This paper presents three-dimensional (3-D)
Smoothed Particle Hydrodynamics (SPH) simulations of the orbital variation of
the binary wind-wind collision, and applies these to modeling the X-ray light
curve observed by the Rossi X-ray Timing Explorer (RXTE). By providing a global
3-D model of the phase variation of the density of the interacting winds, the
simulations allow computation of the associated variation in X-ray absorption,
presumed here to originate from near the apex of the wind-wind interaction
cone. We find that the observed RXTE light curve can be readily fit if the
observer's line of sight is within this cone along the general direction of
apastron. Specifically, the data are well fit by an assumed inclination for the orbit's polar axis, which is thus consistent with orbital
angular momentum being along the inferred polar axis of the Homunculus nebula.
The fits also constrain the position angle that an orbital-plane
projection makes with the apastron side of the semi-major axis, strongly
excluding positions along or to the retrograde side of the
axis, with the best fit position given by . Overall the
results demonstrate the utility of a fully 3-D dynamical model for constraining
the geometric and physical properties of this complex colliding-wind binary
system.Comment: 5 pages, 4 figures, accepted to MNRAS Letter
X-ray properties of the young open clusters HM1 and IC2944/2948
Using XMM data, we study for the first time the X-ray emission of HM1 and
IC2944/2948. Low-mass, pre-main-sequence objects with an age of a few Myr are
detected, as well as a few background or foreground objects. Most massive stars
in both clusters display the usual high-energy properties of that type of
objects, though with log(Lx/Lbol) apparently lower in HM1 than in IC2944/2948.
Compared with studies of other clusters, it seems that a low signal-to-noise
ratio at soft energies, due to the high extinction, may be the main cause of
this difference. In HM1, the two Wolf-Rayet stars show contrasting behaviors:
WR89 is extremely bright, but much softer than WR87. It remains to be seen
whether wind-wind collisions or magnetically confined winds can explain these
emissions. In IC2944/2948, the X-ray sources concentrate around HD101205; a
group of massive stars to the north of this object is isolated, suggesting that
there exist two subclusters in the field-of-view.Comment: 29 pages in total with 10 figures (12 pages paper + supplementary
material), accepted by A&
In Hot Pursuit of the Hidden Companion of Eta Carinae: An X-ray Determination of the Wind Parameters
We present X-ray spectral fits to a recently obtained Chandra grating
spectrum of Eta Carinae, one of the most massive and powerful stars in the
Galaxy and which is strongly suspected to be a colliding wind binary system.
Hydrodynamic models of colliding winds are used to generate synthetic X-ray
spectra for a range of mass-loss rates and wind velocities. They are then
fitted against newly acquired Chandra grating data. We find that due to the low
velocity of the primary wind (~500 km/s), most of the observed X-ray emission
appears to arise from the shocked wind of the companion star. We use the
duration of the lightcurve minimum to fix the wind momentum ratio at 0.2. We
are then able to obtain a good fit to the data by varying the mass-loss rate of
the companion and the terminal velocity of its wind. We find that Mdot ~ 1e-5
Msol/yr and v ~ 3000 km/s. With observationally determined values of ~500-700
km/s for the velocity of the primary wind, our fit implies a primary mass-loss
rate of Mdot ~ 2.5e-4 Msol/yr. This value is smaller than commonly inferred,
although we note that a lower mass-loss rate can reduce some of the problems
noted by Hillier et al. (2001) when a value as high as 1e-3 Msol/yr is used.
The wind parameters of the companion are indicative of a massive star which may
or may not be evolved. The line strengths appear to show slightly sub-solar
abundances, although this needs further confirmation. Based on the
over-estimation of the X-ray line strengths in our model, and re-interpretation
of the HST/FOS results, it appears that the homunculus nebula was produced by
the primary star.Comment: 12 pages, 7 figures, accepted by A&
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