622 research outputs found
Accretion onto the Companion of Eta Carinae During the Spectroscopic Event: III. the He II 4686 Line
We continue to explore the accretion model of the massive binary system eta
Carinae by studying the anomalously high He II 4686 line. The line appears just
before periastron and disappears immediately thereafter. Based on the He II
4686 line emission from O-stars and their modeling in the literature, we
postulate that the He II 4686 line comes from the acceleration zone of the
secondary stellar wind. We attribute the large increase in the line intensity
to a slight increase in the density of the secondary stellar wind in its
acceleration zone. The increase in density could be due to the ionization and
subsequent deceleration of the wind by the enhanced X-ray emission arising from
the shocked secondary wind further downstream or to accretion of the primary
stellar wind. Accretion around the secondary equatorial plane gives rise to
collimation of the secondary wind, which increases its density, hence enhancing
the He II 4686 emission line. In contrast with previous explanations, the
presently proposed model does not require a prohibitively high X-ray flux to
directly photoionize the He.Comment: ApJ, in pres
A Five-year Spectroscopic and Photometric Campaign on the Prototypical alpha Cygni Variable and A-type Supergiant Star Deneb
Deneb is often considered the prototypical A-type supergiant, and is one of
the visually most luminous stars in the Galaxy. A-type supergiants are
potential extragalactic distance indicators, but the variability of these stars
needs to be better characterized before this technique can be considered
reliable. We analyzed 339 high resolution echelle spectra of Deneb obtained
over the five-year span of 1997 through 2001 as well as 370 Stromgren
photometric measurements obtained during the same time frame. Our spectroscopic
analysis included dynamical spectra of the H-alpha profile, H-alpha equivalent
widths, and radial velocities measured from Si II 6347, 6371. Time-series
analysis reveals no obvious cyclic behavior that proceeds through multiple
observing seasons, although we found a suspected 40 day period in two,
non-consecutive observing seasons. Some correlations are found between
photometric and radial velocity data sets, and suggest radial pulsations at two
epochs. No correlation is found between the variability of the H-alpha profiles
and that of the radial velocities or the photometry. Lucy (1976) found evidence
that Deneb was a long period single-lined spectroscopic binary star, but our
data set shows no evidence for radial velocity variations caused by a binary
companion.Comment: 49 pages, 9 figures, 5 tables, accepted for publication in the
Astronomical Journa
The Synthesis and polymerization of oxo-crown ethers
A novel synthetic route to and a polymn. procedure for oxo-crown ethers 2,11-dioxo-18-crown-6 (I), 2-oxo-12-crown-4 (II), and 2-oxo-21-crown-7 (III) are described. Cyclization, of the w-hydroxycarboxylic acid precursors of I-III was achieved by heating these precursors in the presence of a Lewis acid catalyst (CoCl2) and template salt (MCl). Cyclization yields ?82% were reached, using CsCl as template salt. Oxo-crown ethers I-III were polymd. using SnOct2 as catalyst, giving polymers with mol. wts. of approx. 5 kg/mol and dispersities of ca. 1.8 (measured with polyethylene oxide stds.). The polymn. of II was monitored with 1H-NMR and SEC giving information about the influence of monomer/catalyst ratio and temp. on polymn. rate as well as the influence of temp. on mol. wt. (distribution). The possibility of introducing end groups into II homopolymer using SnOct2-catalyzed polymn. was also studie
Galactic Twins of the Ring Nebula Around SN1987A and a Possible LBV-like Phase for Sk-69 202
Some core-collapse supernovae show clear signs of interaction with dense
circumstellar material that often appears to be non-spherical. Circumstellar
nebulae around supernova progenitors provide clues to the origin of that
asymmetry in immediate pre-supernova evolution. Here I discuss outstanding
questions about the formation of the ring nebula around SN1987A and some
implications of similar ring nebulae around Galactic B supergiants. Several
clues hint that SN1987A's nebula may have been ejected in an LBV-like event,
rather than through interacting winds in a transition from a red supergiant to
a blue supergiant.Comment: 2 pages, to appear in procedings of "Massive stars: fundamental
parameters and circumstellar interactions", conference in honor of Virpi
Niemela's 70th birthda
Instability of LBV-stars against radial oscillations
In this study we consider the nonlinear radial oscillations exciting in
LBV--stars with effective temperatures 1.5e4 K <= Teff <= 3e4 K, bolometric
luminosities 1.2e6 L_odot <= L <= 1.9e6 L_odot and masses 35.7 M_odot <= M <=
49.1 M_odot. Hydrodynamic computations were carried out with initial conditions
obtained from evolutionary sequences of population I stars (X=0.7, Z=0.02) with
initial masses from 70M_odot to 90 M_odot. All hydrodynamical models show
instability against radial oscillations with amplitude growth time comparable
with dynamical time scale of the star. Radial oscillations exist in the form of
nonlinear running waves propagating from the boundary of the compact core to
the upper boundary of the hydrodynamical model. The velocity amplitude of outer
layers is of several hundreds of km/s while the bolometric light amplitude does
not exceed 0.2 mag. Stellar oscillations are not driven by the kappa-mechanism
and are due to the instability of the gas with adiabatic exponent close to the
critical value Gamma_1 = 4/3 due to the large contribution of radiation in the
total pressure. The range of the light variation periods (6 day <= P <= 31 day)
of hydrodynamical models agrees with periods of microvariability observed in
LBV--stars.Comment: 14 pages, 5 figures, submitted to Astronomy Letter
Travel-Associated Zika Virus Disease Acquired in the Americas Through February 2016
BACKGROUND: Zika virus has spread rapidly in the Americas and has been imported into many nonendemic countries by travelers.
OBJECTIVE: To describe clinical manifestations and epidemiology of Zika virus disease in travelers exposed in the Americas.
DESIGN: Descriptive, using GeoSentinel records. SETTING: 63 travel and tropical medicine clinics in 30 countries.
PATIENTS: Ill returned travelers with a confirmed, probable, or clinically suspected diagnosis of Zika virus disease seen between January 2013 and 29 February 2016.
MEASUREMENTS: Frequencies of demographic, trip, and clinical characteristics and complications.
RESULTS: Starting in May 2015, 93 cases of Zika virus disease were reported. Common symptoms included exanthema (88%), fever (76%), and arthralgia (72%). Fifty-nine percent of patients were exposed in South America; 71% were diagnosed in Europe. Case status was established most commonly by polymerase chain reaction (PCR) testing of blood and less often by PCR testing of other body fluids or serology and plaque-reduction neutralization testing. Two patients developed Guillain–Barre syndrome, and 3 of 4 pregnancies had adverse outcomes (microcephaly, major fetal neurologic abnormalities, and intrauterine fetal death).
LIMITATION: Surveillance data collected by specialized clinics may not be representative of all ill returned travelers, and denominator data are unavailable.
CONCLUSION: These surveillance data help characterize the clinical manifestations and adverse outcomes of Zika virus disease among travelers infected in the Americas and show a need for global standardization of diagnostic testing. The serious fetal complications observed in this study highlight the importance of travel advisories and prevention measures for pregnant women and their partners. Travelers are sentinels for global Zika virus circulation and may facilitate further transmission
HD148937: a multiwavelength study of the third Galactic member of the Of?p class
Three Galactic O-type stars belong to the rare class of Of?p objects: HD108,
HD191612, and HD148937. The first two stars show a wealth of phenomena,
including magnetic fields and strong X-ray emission, light variability, and
dramatic periodic spectral variability. We present here the first detailed
optical and X-ray study of the third Galactic Of?p star, HD148937.
Spectroscopic monitoring has revealed low-level variability in the Balmer and
HeII4686 lines, but constancy at HeI and CIII4650. The Ha line exhibits profile
variations at a possible periodicity of ~7d. Model atmosphere fits yield
T_{eff}=41000+-2000K, log(g)=4.0+-0.1, Mdot_{sph}<~ 10^{-7}Msol/yr and a
surabondance of nitrogen by a factor of four. At X-ray wavelengths, HD148937
resembles HD108 and HD191612 in having a thermal spectrum dominated by a
relatively cool component (kT=0.2keV), broad lines (>1700km/s), and an
order-of-magnitude overluminosity compared to normal O stars (log
[L_X^unabs/L_BOL] ~ -6).Comment: accepted by AJ; 15p, 15fig available in jp
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