202 research outputs found
The X-ray binary 2S0114+650=LSI+65 010:A slow pulsar or tidally-induced pulsations?
The X-ray source 2S0114+650=LSI+65 010 is a binary system containing a B-type
primary and a low mass companion believed to be a neutron star. The system has
three reported periodicities: the orbital period, P{orb}~11.6 d, X-ray flaring
with P{flare}~2.7 hr, and a "superorbital" X-ray periodicity P{super}~30.7 d.
The objective of this paper is to show that the puzzling periodicities in the
system may be explained in the context of scenarios in which tidal interactions
drive oscillations in the B-supergiant star. We calculate the solution of the
equations of motion for one layer of small surface elements distributed along
the equator of the star, as they respond to the forces due to gas pressure,
centrifugal, coriolis, viscous forces, and the gravitational forces of both
stars. This calculation provides variability timescales that can be compared
with the observations. In addition, we use observational data obtained at the
Observatorio Astron\'omico Nacional en San Pedro M\'artir (OAN/SPM) between
1993-2004 to determine which of the periodicities may be present in the optical
region. We suggest that the tidal oscillations lead to a structured stellar
wind which, when fed to the neutron star, produces the X-ray modulations. The
connection between the stellar oscillations and the modulation of the mass
ejection may lie in the shear energy dissipation generated by the tangential
motions that are produced by the tidal interaction, particularly in the tidal
bulge region. The tidal oscillation scenario weakens the case for 2S0114+650
containing a magnetar descendent.Comment: 12 pages, 14 figure
A Changing Wind Collision
We report on the first detection of a global change in the X-ray emitting properties of a wind–wind collision, thanks to XMM-Newton observations of the massive Small Magellenic Cloud (SMC) system HD 5980. While its light curve had remained unchanged between 2000 and 2005, the X-ray flux has now increased by a factor of ~2.5, and slightly hardened. The new observations also extend the observational coverage over the entire orbit, pinpointing the light-curve shape. It has not varied much despite the large overall brightening, and a tight correlation of fluxes with orbital separation is found without any hysteresis effect. Moreover, the absence of eclipses and of absorption effects related to orientation suggests a large size for the X-ray emitting region. Simple analytical models of the wind–wind collision, considering the varying wind properties of the eruptive component in HD 5980, are able to reproduce the recent hardening and the flux-separation relationship, at least qualitatively, but they predict a hardening at apastron and little change in mean flux, contrary to observations. The brightness change could then possibly be related to a recently theorized phenomenon linked to the varying strength of thin-shell instabilities in shocked wind regions
Massive stars exploding in a He-rich circumstellar medium. I. Type Ibn (SN 2006jc-like) events
We present new spectroscopic and photometric data of the type Ibn supernovae
2006jc, 2000er and 2002ao. We discuss the general properties of this recently
proposed supernova family, which also includes SN 1999cq. The early-time
monitoring of SN 2000er traces the evolution of this class of objects during
the first few days after the shock breakout. An overall similarity in the
photometric and spectroscopic evolution is found among the members of this
group, which would be unexpected if the energy in these core-collapse events
was dominated by the interaction between supernova ejecta and circumstellar
medium. Type Ibn supernovae appear to be rather normal type Ib/c supernova
explosions which occur within a He-rich circumstellar environment. SNe Ibn are
therefore likely produced by the explosion of Wolf-Rayet progenitors still
embedded in the He-rich material lost by the star in recent mass-loss episodes,
which resemble known luminous blue variable eruptions. The evolved Wolf-Rayet
star could either result from the evolution of a very massive star or be the
more evolved member of a massive binary system. We also suggest that there are
a number of arguments in favour of a type Ibn classification for the historical
SN 1885A (S-Andromedae), previously considered as an anomalous type Ia event
with some resemblance to SN 1991bg.Comment: 17 pages including 12 figures and 4 tables. Slightly revised version,
conclusions unchanged, 1 figure added. Accepted for publication in MNRA
Reduced Wolf-Rayet Line Luminosities at Low Metallicity
New NTT/EMMI spectrophotometry of single WN2-5 stars in the Magellanic Clouds
are presented, from which HeII 4686 line luminosities have been derived, and
compared with observations of other Magellanic Cloud WR stars. SMC WN3-4 stars
possess line luminosities which are a factor of 4 times lower than LMC
counterparts, incorporating several binary SMC WN3-4 stars. Similar results are
found for WN5-6 stars, despite reduced statistics, incorporating observations
of single LMC WN5-9 stars. CIV 5808 line luminosities of carbon sequence WR
stars in the SMC and IC1613 (both WO subtypes) are a factor of 3 lower than LMC
WC stars from Mt Stromlo/DBS spectrophotometry, although similar results are
also obtained for the sole LMC WO star. We demonstrate how reduced line
luminosities at low metallicity follow naturally if WR winds are Z-dependent,
as recent results suggest. We apply mass loss-Z scalings to atmospheric non-LTE
models of Milky Way and LMC WR stars to predict the wind signatures of WR stars
in the metal-poor star forming WR galaxy IZw18. WN HeII 4686 line luminosities
are 7-20 times lower than in Z-rich counterparts of identical bolometric
luminosity, whilst WC CIV 5808 line luminosities are 3-6 times lower.
Significant He^+ Lyman continuum fluxes are predicted for Z-poor early-type WR
stars. Consequently, our results suggest the need for larger population of WR
stars in IZw18 than is presently assumed, particularly for WN stars,
potentially posing a severe challenge to evolutionary models at very low Z.
Finally, reduced wind strengths from WR stars at low Z impacts upon the
immediate circumstellar environment of long duration GRB afterglows,
particularly since the host galaxies of high-redshift GRBs tend to be Z-poor.Comment: 14 pages, 12 figures, accepted for A&A, revision fixes error with Eqn
Two New LBV Candidates in the M33 Galaxy
We present two new luminous blue variable (LBV) candidate stars discovered in
the M33 galaxy. We identified these stars (Valeev et al. 2010) as massive star
candidates at the final stages of evolution, presumably with a notable
interstellar extinction. The candidates were selected from the Massey et al.
(2006) catalog based on the following criteria: emission in Halpha, V<18.5 and
0.35<(B-V)<1.2. The spectra of both stars reveal a broad and strong Halpha
emission with extended wings (770 and 1000 km/s). Based on the spectra we
estimated the main parameters of the stars. Object N45901 has a bolometric
luminosity log(L/Lsun)=6.0-6.2 with the value of interstellar extinction
Av=2.3+-0.1. The temperature of the star's photosphere is estimated as
Tstar~13000-15000K its probable mass on the Zero Age Main Sequence is
M~60-80Msun. The infrared excess in N45901 corresponds to the emission of warm
dust with the temperature Twarm~1000K, and amounts to 0.1% of the bolometric
luminosity. A comparison of stellar magnitude estimates from different catalogs
points to the probable variability of the object N45901. Bolometric luminosity
of the second object, N125093, is log(L/Lsun)=6.3-6.6, the value of
interstellar extinction is Av=2.75+-0.15. We estimate its photosphere's
temperature as Tstar~13000-16000K, the initial mass as M~90-120Msun. The
infrared excess in N125093 amounts to 5-6% of the bolometric luminosity. Its
spectral energy distribution reveals two thermal components with the
temperatures Twarm~1000K and Tcold~480K. The [CaII] lines (7291A and 7323A),
observed in LBV-like stars VarA and N93351 in M33, are also present in the
spectrum of N125093. These lines indicate relatively recent gas eruptions and
dust activity linked with them. High bolometric luminosity of these stars and
broad Halpha emissions allow classifying the studied objects as LBV candidates.Comment: 14 pages, 4 figure
An X-ray investigation of the NGC346 field in the SMC (1) : the LBV HD5980 and the NGC346 cluster
We present results from a Chandra observation of the NGC346 cluster. This
cluster contains numerous massive stars and is responsible for the ionization
of N66, the most luminous HII region and the largest star formation region in
the SMC. In this first paper, we will focus on the characteristics of the main
objects of the field. The NGC346 cluster itself shows only relatively faint
X-ray emission (with L_X^{unabs} ~ 1.5 times 10^{34} erg s^{-1}), tightly
correlated with the core of the cluster. In the field also lies HD5980, a LBV
star in a binary (or possibly a triple system) that is detected for the first
time at X-ray energies. The star is X-ray bright, with an unabsorbed luminosity
of L_X^{unabs} ~ 1.7 times 10^{34} erg s^{-1}, but needs to be monitored
further to investigate its X-ray variability over a complete 19d orbital cycle.
The high X-ray luminosity may be associated either with colliding winds in the
binary system or with the 1994 eruption. HD5980 is surrounded by a region of
diffuse X-ray emission, which is a supernova remnant. While it may be only a
chance alignment with HD5980, such a spatial coincidence may indicate that the
remnant is indeed related to this peculiar massive star.Comment: 12 pages, 9 figures (3 in jpg and 6 in ps), to be published in Ap
New insights into the nature of the SMC WR/LBV binary HD 5980
We present the results of optical wavelength observations of the unusual SMC
eclipsing binary system HD 5980 obtained in 1999 and 2004--2005. Radial
velocity curves for the erupting LBV/WR object (star A) and its close WR-like
companion (star B) are obtained by deblending the variable emission-line
profiles of N IV and N V lines under the simplistic assumption that these lines
originate primarily in the winds of star A and star B. The derived masses
M_A=58--79 Mo and M_B=51--67 Mo, are more consistent with the stars' location
near the top of the HRD than previous estimates. The presence of a wind-wind
interaction region is inferred from the orbital phase-dependent behavior of He
I P Cygni absorption components. The emission-line intensities continued with
the declining trend previously seen in UV spectra. The behavior of the
photospheric absorption lines is consistent with the results of Schweickhardt
(2002) who concludes that the third object in the combined spectrum, star C, is
also a binary system with P(starC)~96.5 days, e=0.83. The data used in this
paper will be made publicly available for further analysis.Comment: 48 pages, 26 figure
A 2.3-Day Periodic Variability in the Apparently Single Wolf-Rayet Star WR 134: Collapsed Companion or Rotational Modulation?
We present the results of an intensive campaign of spectroscopic and
photometric monitoring of the peculiar Wolf-Rayet star WR 134 from 1989 to
1997.
This unprecedentedly large data set allows us to confirm unambiguously the
existence of a coherent 2.25 +/- 0.05 day periodicity in the line-profile
changes of He II 4686, although the global pattern of variability is different
from one epoch to another. This period is only marginally detected in the
photometric data set. Assuming the 2.25 day periodic variability to be induced
by orbital motion of a collapsed companion, we develop a simple model aiming at
investigating (i) the effect of this strongly ionizing, accreting companion on
the Wolf-Rayet wind structure, and (ii) the expected emergent X-ray luminosity.
We argue that the predicted and observed X-ray fluxes can only be matched if
the accretion on the collapsed star is significantly inhibited. Additionally,
we performed simulations of line-profile variations caused by the orbital
revolution of a localized, strongly ionized wind cavity surrounding the X-ray
source. A reasonable fit is achieved between the observed and modeled
phase-dependent line profiles of He II 4686. However, the derived size of the
photoionized zone substantially exceeds our expectations, given the observed
low-level X-ray flux. Alternatively, we explore rotational modulation of a
persistent, largely anisotropic outflow as the origin of the observed cyclical
variability. Although qualitative, this hypothesis leads to greater consistency
with the observations.Comment: 34 pages, 16 figures. Accepted by the Astrophysical Journa
The evolution of rotating stars
First, we review the main physical effects to be considered in the building
of evolutionary models of rotating stars on the Upper Main-Sequence (MS). The
internal rotation law evolves as a result of contraction and expansion,
meridional circulation, diffusion processes and mass loss. In turn,
differential rotation and mixing exert a feedback on circulation and diffusion,
so that a consistent treatment is necessary.
We review recent results on the evolution of internal rotation and the
surface rotational velocities for stars on the Upper MS, for red giants,
supergiants and W-R stars. A fast rotation is enhancing the mass loss by
stellar winds and reciprocally high mass loss is removing a lot of angular
momentum. The problem of the ``break-up'' or -limit is critically
examined in connection with the origin of Be and LBV stars. The effects of
rotation on the tracks in the HR diagram, the lifetimes, the isochrones, the
blue to red supergiant ratios, the formation of W-R stars, the chemical
abundances in massive stars as well as in red giants and AGB stars, are
reviewed in relation to recent observations for stars in the Galaxy and
Magellanic Clouds. The effects of rotation on the final stages and on the
chemical yields are examined, as well as the constraints placed by the periods
of pulsars. On the whole, this review points out that stellar evolution is not
only a function of mass M and metallicity Z, but of angular velocity
as well.Comment: 78 pages, 7 figures, review for Annual Review of Astronomy and
Astrophysics, vol. 38 (2000
Synergy Between Intercellular Communication and Intracellular Ca2+ Handling in Arrhythmogenesis
Calcium is the primary signalling component of excitation-contraction coupling, the process linking electrical excitability of cardiac muscle cells to coordinated contraction of the heart. Understanding Ca2þ handling processes at the cellular level and the role of intercellular communication in the emergence of multicellular synchronization are key aspects in the study of arrhythmias. To probe these mechanisms, we have simulated cellular interactions on large scale arrays that mimic cardiac tissue, and where individual cells are represented by a mathematical model of intracellular Ca2þ dynamics. Theoretical predictions successfully reproduced experimental findings and provide novel insights on the action of two pharmacological agents (ionomycin and verapamil) that modulate Ca2þ signalling pathways via distinct mechanisms. Computational results have demonstrated how transitions between local synchronisation events and large scale wave formation are affected by these agents. Entrainment phenomena are shown to be linked to both ntracellular Ca2þ and coupling-specific dynamics in a synergistic manner. The intrinsic variability of the cellular matrix is also shown to affect emergent patterns of rhythmicity, providing insights into the origins of arrhythmogenic Ca2þ perturbations in cardiac tissue in situ
- …