612 research outputs found
CHARA/MIRC observations of two M supergiants in Perseus OB1: temperature, Bayesian modeling, and compressed sensing imaging
Two red supergiants of the Per OB1 association, RS Per and T Per, have been
observed in H band using the MIRC instrument at the CHARA array. The data show
clear evidence of departure from circular symmetry. We present here new
techniques specially developed to analyze such cases, based on state-of-the-art
statistical frameworks. The stellar surfaces are first modeled as limb-darkened
discs based on SATLAS models that fit both MIRC interferometric data and
publicly available spectrophotometric data. Bayesian model selection is then
used to determine the most probable number of spots. The effective surface
temperatures are also determined and give further support to the recently
derived hotter temperature scales of red su- pergiants. The stellar surfaces
are reconstructed by our model-independent imaging code SQUEEZE, making use of
its novel regularizer based on Compressed Sensing theory. We find excellent
agreement between the model-selection results and the reconstructions. Our
results provide evidence for the presence of near-infrared spots representing
about 3-5% of the stellar flux
Detecting the Companions and Ellipsoidal Variations of RS CVn Primaries: I. sigma Geminorum
To measure the properties of both components of the RS CVn binary sigma
Geminorum (sigma Gem), we directly detect the faint companion, measure the
orbit, obtain model-independent masses and evolutionary histories, detect
ellipsoidal variations of the primary caused by the gravity of the companion,
and measure gravity darkening. We detect the companion with interferometric
observations obtained with the Michigan InfraRed Combiner (MIRC) at Georgia
State University's Center for High Angular Resolution Astronomy (CHARA) Array
with a primary-to-secondary H-band flux ratio of 270+/-70. A radial velocity
curve of the companion was obtained with spectra from the Tillinghast Reflector
Echelle Spectrograph (TRES) on the 1.5-m Tillinghast Reflector at Fred Lawrence
Whipple Observatory (FLWO). We additionally use new observations from the
Tennessee State University Automated Spectroscopic and Photometric Telescopes
(AST and APT, respectively). From our orbit, we determine model-independent
masses of the components (M_1 = 1.28 +/- 0.07 M_Sun, M_2 = 0.73 +/- 0.03
M_Sun), and estimate a system age of 5 -/+ 1 Gyr. An average of the 27-year APT
light curve of sigma Gem folded over the orbital period (P = 19.6027 +/- 0.0005
days) reveals a quasi-sinusoidal signature, which has previously been
attributed to active longitudes 180 deg apart on the surface of sigma Gem. With
the component masses, diameters, and orbit, we find that the predicted light
curve for ellipsoidal variations due to the primary star partially filling its
Roche lobe potential matches well with the observed average light curve,
offering a compelling alternative explanation to the active longitudes
hypothesis. Measuring gravity darkening from the light curve gives beta < 0.1,
a value slightly lower than that expected from recent theory.Comment: Accepted to ApJ, 11 pages, 6 figures, 8 table
X-raying the Beating Heart of a Newborn Star: Rotational Modulation of High-energy Radiation from V1647 Ori
We report a periodicity of ~1 day in the highly elevated X-ray emission from
the protostar V1647 Ori during its two recent multiple-year outbursts of mass
accretion. This periodicity is indicative of protostellar rotation at
near-breakup speed. Modeling of the phased X-ray light curve indicates the
high-temperature (~50 MK), X-ray-emitting plasma, which is most likely heated
by accretion-induced magnetic reconnection, resides in dense (>~5e10 cm-3),
pancake-shaped magnetic footprints where the accretion stream feeds the newborn
star. The sustained X-ray periodicity of V1647 Ori demonstrates that such
protostellar magnetospheric accretion configurations can be stable over
timescales of years.Comment: 26 pages, 10 figure
Magnetospheric Accretion and Ejection of Matter in Resistive Magnetohydrodynamic Simulations
The ejection of matter in the close vicinity of a young stellar object is
investigated, treating the accretion disk as a gravitationally bound reservoir
of matter. By solving the resistive MHD equations in 2D axisymmetry using our
version of the Zeus-3D code with newly implemented resistivity, we study the
effect of magnetic diffusivity in the magnetospheric accretion-ejection
mechanism. Physical resistivity was included in the whole computational domain
so that reconnection is enabled by the physical as well as the numerical
resistivity. We show, for the first time, that quasi-stationary fast ejecta of
matter, which we call {\em micro-ejections}, of small mass and angular momentum
fluxes, can be launched from a purely resistive magnetosphere. They are
produced by a combination of pressure gradient and magnetic forces, in presence
of ongoing magnetic reconnection along the boundary layer between the star and
the disk, where a current sheet is formed. Mass flux of micro-ejection
increases with increasing magnetic field strength and stellar rotation rate,
and is not dependent on the disk to corona density ratio and amount of
resistivity.Comment: 18 pages, many revisions from previous version, accepted in Ap
Solar Flares and Coronal Mass Ejections: A Statistically Determined Flare Flux-CME Mass Correlation
In an effort to examine the relationship between flare flux and corresponding
CME mass, we temporally and spatially correlate all X-ray flares and CMEs in
the LASCO and GOES archives from 1996 to 2006. We cross-reference 6,733 CMEs
having well-measured masses against 12,050 X-ray flares having position
information as determined from their optical counterparts. For a given flare,
we search in time for CMEs which occur 10-80 minutes afterward, and we further
require the flare and CME to occur within +/-45 degrees in position angle on
the solar disk. There are 826 CME/flare pairs which fit these criteria.
Comparing the flare fluxes with CME masses of these paired events, we find CME
mass increases with flare flux, following an approximately log-linear, broken
relationship: in the limit of lower flare fluxes, log(CME mass)~0.68*log(flare
flux), and in the limit of higher flare fluxes, log(CME mass)~0.33*log(flare
flux). We show that this broken power-law, and in particular the flatter slope
at higher flare fluxes, may be due to an observational bias against CMEs
associated with the most energetic flares: halo CMEs. Correcting for this bias
yields a single power-law relationship of the form log(CME mass)~0.70*log(flare
flux). This function describes the relationship between CME mass and flare flux
over at least 3 dex in flare flux, from ~10^-7 to 10^-4 W m^-2.Comment: 28 pages, 16 figures, accepted to Solar Physic
Review of the Properties of the W Boson at LEP, and the Precision Determination of its Mass
We review the precision measurement of the mass and couplings of the W Boson
at LEP. The total and differential W+W- cross section is used to extract the
WWZ and WWgamma couplings. We discuss the techniques used by the four LEP
experiments to determine the W mass in different decay channels, and present
the details of methods used to evaluate the sources of systematic uncertainty.Comment: 97 page
Surprising dissimilarities in a newly formed pair of 'identical twin' stars
The mass and chemical composition of a star are the primary determinants of
its basic physical properties--radius, temperature, luminosity--and how those
properties evolve with time. Thus, two stars born at the same time, from the
same natal material, and with the same mass are 'identical twins,' and as such
might be expected to possess identical physical attributes. We have discovered
in the Orion Nebula a pair of stellar twins in a newborn binary star system.
Each star in the binary has a mass of 0.41 +/- 0.01 solar masses, identical to
within 2 percent. Here we report that these twin stars have surface
temperatures that differ by ~300K (~10%), and luminosities that differ by ~50%,
both at high confidence level. Preliminary results indicate that the stars'
radii also differ, by 5-10%. These surprising dissimilarities suggest that one
of the twins may have been delayed by several hundred thousand years in its
formation relative to its sibling. Such a delay could only have been detected
in a very young, definitively equal-mass binary system3 such as that reported
here. Our findings reveal cosmic limits on the age synchronisation of young
binary stars, often used as tests for the age calibrations of star-formation
models.Comment: Published in Nature, 19 June 200
Penetrance of HNPCC-related cancers in a retrolective cohort of 12 large Newfoundland families carrying a MSH2 founder mutation: an evaluation using modified segregation models
<p>Abstract</p> <p>Background</p> <p>Accurate risk (penetrance) estimates for associated phenotypes in carriers of a major disease gene are important for genetic counselling of at-risk individuals. Population-specific estimates of penetrance are often needed as well. Families ascertained from high-risk disease clinics provide substantial data to estimate penetrance of a disease gene, but these estimates must be adjusted for possible specific sources of bias.</p> <p>Methods</p> <p>A cohort of 12 independently ascertained HNPCC families harbouring a founder MSH2 mutation was identified from a cancer genetics clinic in St. John's, Newfoundland, Canada. Carrier status was known for 247 family members but phenotype information on up to 85 additional relatives with unknown carrier status was available; using modified segregation models these additional individuals could be included in the analyses. Three HNPCC-related phenotypes were evaluated as age at diagnosis of: any HNPCC cancer (first cancer), colorectal cancer (CRC), and endometrial cancer (EC) for females.</p> <p>Results</p> <p>Lifetime (age 70) risk estimates for male and female carriers were similar for developing any HNPCC cancer (Males = 98.2%, 95% Confidence Interval (CI) = (93.8%, 99.9%); Females = 92.8%, 95% CI = (82.4%, 99.1%)) but female carriers experienced substantially reduced lifetime risk for developing CRC compared to male carriers (Females = 38.9%, 95% CI = (24.2%, 62.1%); Males = 84.5%, 95% CI = (67.3%, 91.3%)). Female non-carriers had very low lifetime risk for these two outcomes while male non-carriers had lifetime risks intermediate to the female carriers and non-carriers. Female carriers had a lifetime risk of developing EC of 82.4%. Relative risks for developing any HNPCC cancer (carriers relative to non-carriers) were substantially greater for females compared to their male counterparts (Females = 54.8, 95%CI = (4.4, 379.8); Males = 9.7, 95% CI = (0.3, 23.8)). Relative risks for developing CRC at age 70 were substantially greater for females compared to their male counterparts (Females = 23.7, 95%CI = (5.6, 137.9); Males = 6.8%, 95% CI = (2.3, 66.2)). However, the risk of developing CRC decreased with age among both genders.</p> <p>Conclusion</p> <p>The proposed modified segregation-based models used to estimate age-specific risks for HNPCC phenotypes can reduce bias due to ascertainment and missing genotype information as well as provide estimates of absolute and relative risks.</p
Charged Scalar Particles and Leptonic Decay
Charged scalar particles introduced in some extensions of the standard model
can induce leptonic decay at tree level. We find that with some charged
SU(2)-singlet scalar particles, like ones introduced in Zee-type models,
leptonic decay width is always smaller than what is predicted by the standard
model, therefore they may offer a natural solution to decay puzzle. To
be more specific, we examine some Zee-type models in detail to see if at the
same time they are acceptable in particle physics, cosmology and astrophysics.
It is shown that decay data do put some constrains on these models.Comment: ICTP Report No. IC/93/31, 12 pages, Latex, one figure is not
included, it is available upon deman
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