7,164 research outputs found
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
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
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&
Virus-transformed pre-B cells show ordered activation but not inactivation of immunoglobulin gene rearrangement and transcription
Virus-transformed pre-B cells undergo ordered immunoglobulin (Ig) gene rearrangements during culture. We devised a series of highly sensitive polymerase chain reaction assays for Ig gene rearrangement and unrearranged Ig gene segment transcription to study both the possible relationship between these processes in cultured pre-B cells and the role played by heavy (H) chain (mu) protein in regulating gene rearrangement. Our analysis of pre-B cell cultures representing various stages of maturity revealed that transcription of each germline Ig locus precedes or is coincident with its rearrangement. Cell lines containing one functional rearranged H chain allele, however, continue to transcribe and to rearrange the allelic, unrearranged H chain locus. These cell lines appear to initiate but not terminate rearrangement events and therefore provide information about the requirements for activating rearrangement but not about allelic exclusion mechanisms
Interactions in Massive Colliding Wind Binaries
The most massive stars (M> 60 Solar Mass) play crucial roles in altering the chemical and thermodynamic properties of their host galaxies. Stellar mass is the fundamental stellar parameter that determines their ancillary properties and which ultimately determines the fate of these stars and their influence on their galactic environs. Unfortunately, stellar mass becomes observationally and theoretically less well constrained as it increases. Theory becomes uncertain mostly because very massive stars are prone to strong, variable mass loss which is difficult to model. Observational constraints are uncertain too. Massive stars are rare, and massive binary stars (needed for dynamical determination of mass) are rarer still: and of these systems only a fraction have suitably high orbital inclinations for direct photometric and spectroscopic radial-velocity analysis. Even in the small number of cases in which a high-inclination binary near the upper mass limit can be identified, rotational broadening and contamination of spectral line features from thick circumstellar material (either natal clouds or produced by strong stellar wind driven mass loss from one or both of he stellar components) biases the analysis. In the wilds of the upper HR diagram, we're often left with indirect and circumstantial means of determining mass, a rather unsatisfactory state of affairs
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