631 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
On the photometric variability of blue supergiants in NGC 300 and its impact on the Flux-weighted Gravity-Luminosity Relationship
We present a study of the photometric variability of spectroscopically
confirmed supergiants in NGC 300, comprising 28 epochs extending over a period
of five months. We find 15 clearly photometrically variable blue supergiants in
a sample of nearly 70 such stars, showing maximum light amplitudes ranging from
0.08 to 0.23 magnitudes in the V band, and one variable red supergiant. We show
their light curves, and determine semi-periods for two A2 Ia stars. Assuming
that the observed changes correspond to similar variations in the bolometric
luminosity, we test for the influence of this variability on the Flux-weighted
Gravity--Luminosity Relationship and find a negligible effect, showing that the
calibration of this relationship, which has the potential to measure
extragalactic distances at the Cepheid accuracy level, is not affected by the
stellar photometric variability in any significant way.Comment: 9 pages, 3 figures, 3 tables. Accepted for publication in the
Astrophysical Journa
Accretion onto the Companion of Eta Carinae During the Spectroscopic Event. IV. the Disappearance of Highly Ionized Lines
We show that the rapid and large decrease in the intensity of high-ionization
emission lines from the Eta Carinae massive binary system can be explained by
the accretion model. These emission lines are emitted by material in the nebula
around the binary system that is being ionized by radiation from the hot
secondary star. The emission lines suffer three months long deep fading every
5.54 year, assumed to be the orbital period of the binary system. In the
accretion model, for ~70 day the less massive secondary star is accreting mass
from the primary wind instead of blowing its fast wind. The accretion event has
two effects that substantially reduce the high-energy ionizing radiation flux
from the secondary star. (1) The accreted mass absorbs a larger fraction of the
ionizing flux. (2) The accreted mass forms a temporarily blanked around the
secondary star that increases its effective radius, hence lowering its
effective temperature and the flux of high energy photons. This explanation is
compatible with the fading of the emission lines at the same time the X-ray is
declining to its minimum, and with the fading being less pronounced in the
polar directions.Comment: ApJ, in pres
The Purple Haze of Eta Carinae: Binary-Induced Variability?
Asymmetric variability in ultraviolet images of the Homunculus obtained with
the Advanced Camera for Surveys/High Resolution Camera on the Hubble Space
Telescope suggests that Eta Carinae is indeed a binary system. Images obtained
before, during, and after the recent ``spectroscopic event'' in 2003.5 show
alternating patterns of bright spots and shadows on opposite sides of the star
before and after the event, providing a strong geometric argument for an
azimuthally-evolving, asymmetric UV radiation field as one might predict in
some binary models. The simplest interpretation of these UV images, where
excess UV escapes from the secondary star in the direction away from the
primary, places the major axis of the eccentric orbit roughly perpendicular to
our line of sight, sharing the same equatorial plane as the Homunculus, and
with apastron for the hot secondary star oriented toward the southwest of the
primary. However, other orbital orientations may be allowed with more
complicated geometries. Selective UV illumination of the wind and ejecta may be
partly responsible for line profile variations seen in spectra. The brightness
asymmetries cannot be explained plausibly with delays due to light travel time
alone, so a single-star model would require a seriously asymmetric shell
ejection.Comment: 8 pages, fig 1 in color, accepted by ApJ Letter
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
I Zw 18 revisited with HST/ACS and Cepheids: New Distance and Age
We present new V and I-band HST/ACS photometry of I Zw 18, the most
metal-poor blue compact dwarf (BCD) galaxy in the nearby universe. It has been
argued in the past that I Zw 18 is a very young system that started forming
stars only 1 Gyr) red
giant branch (RGB) stars may also exist. Our new data, once combined with
archival HST/ACS data, provide a deep and uncontaminated optical
color-magnitude diagram (CMD) that now strongly indicates an RGB. The RGB tip
(TRGB) magnitude yields a distance modulus (m-M)_0 = 31.30 +/- 0.17, i.e., D =
18.2 +/- 1.5 Mpc. The time-series nature of our observations allows us to also
detect and characterize for the first time three classical Cepheids in I~Zw~18.
The time-averaged Cepheid and magnitudes are compared to the VI
reddening-free Wesenheit relation predicted from new non-linear pulsation
models specifically calculated at the metallicity of I Zw 18. For the one
bona-fide classical Cepheid with a period of 8.63 days this implies a distance
modulus (m-M)_0 = 31.42 +/- 0.26. The other two Cepheids have unusually long
periods (125.0 and 129.8 d) but are consistent with this distance. The coherent
picture that emerges is that I Zw 18 is older and farther away than previously
believed. This rules out the possibility that I Zw 18 is a truly primordial
galaxy formed recently (z < 0.1) in the local universe.Comment: 12 pages, 3 figures, submitted to ApJ
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
RXTE All-Sky Monitor Detection of the Orbital Period of Scorpius X-1
The orbital period of Scorpius X-1 has been accepted as 0.787313 d since its
discovery in archival optical photometric data by Gottlieb, Wright, & Liller
(1975). This period has been confirmed in both photometric and spectroscopic
optical observations, though to date only marginal evidence has been reported
for modulation of the X-ray intensity at that period. We have used data taken
with the RXTE All Sky Monitor to search for such a modulation. A major
difficulty in detecting the orbit in X-ray data is presented by the flaring
behavior of Sco X-1, which contributes white noise to Fourier transforms of the
intensity time series, and tends to obscure weak modulations. We present a new
technique for substantially reducing the effects of the flaring behavior while
retaining much of any periodic orbital modulation, provided only that the two
temporal behaviors exhibit different spectral signatures. Through such a
search, we have found evidence for orbital modulation at about the 1% level
with a period of 0.78893 d, equal within our accuracy to a period which differs
by 1 cycle per year from the accepted value. If we compare our results with the
period of the 1 year sideband cited by Gottlieb et al. we conclude that the
actual period may be 0.78901 d.Comment: AASTeX, 20 pages, 5 Postscript figure
The highly polarized open cluster Trumpler 27
We have carried out multicolor linear polarimetry (UBVRI) of the brightest
stars in the area of the open cluster Trumpler 27. Our data show a high level
of polarization in the stellar light with a considerable dispersion, from to . The polarization vectors of the cluster members appear to be
aligned. Foreground polarization was estimated from the data of some non-member
objects, for which two different components were resolved: the first one
associated with a dust cloud close to the Sun producing
and degrees, and a second component, the main source of
polarization for the cluster members, originated in another dust cloud, which
polarizes the light in the direction of degrees. From a detailed
analysis, we found that the two components have associated values for the first one, and for the other. Due the
difference in the orientation of both polarization vectors, almost 90 degrees
(180 degrees at the Stokes representation), the first cloud (
degrees) depolarize the light strongly polarized by the second one ( degrees).Comment: 12 Pages, 6 Figures, 2 tables (9 Pages), accepted for publication in
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