243 research outputs found
The HD5980 multiple system: Masses and evolutionary status
New spectroscopic observations of the LBV/WR multiple system HD5980 in the
Small Magellanic Cloud are used to address the question of the masses and
evolutionary status of the two very luminous stars in the 19.3d eclipsing
binary system. Two distinct components of the N V 4944 A line are detected in
emission and their radial velocity variations are used to derive masses of 61
and 66 Mo, under the assumption that binary interaction effects on this atomic
transition are negligible. We propose that this binary system is the product of
quasi-chemically homogeneous evolution with little or no mass transfer. Thus,
both of these binary stars may be candidates for gamma-ray burst progenitors or
even pair instability supernovae. Analysis of the photospheric absorption lines
belonging to the third-light object in the system confirm that it consists of
an O-type star in a 96.56d eccentric orbit (e=0.82) around an unseen companion.
The 5:1 period ratio and high eccentricities of the two binaries suggest that
they may constitute a hierarchical quadruple system.Comment: 27 pages, 8 tables, 15 figures; accepted A
A Detailed Study of 2S 0114+650 with the RXTE
We present the results of a detailed study of the high mass X-ray binary 2S
0114+650 made with the pointed instruments onboard the Rossi X-ray Timing
Explorer. The spectral and temporal behaviour of this source was examined over
the pulse, orbital, and super-orbital timescales, covering 2 cycles of
the 30.7 d super-orbital modulation. Marginal evidence for variability of the
power law photon index over the pulse period was identified, similar to that
observed from other X-ray pulsars. If this variability is real it can be
attributed to a varying viewing geometry of the accretion region with the spin
of the neutron star. Variability of the neutral hydrogen column density over
the orbital period was observed, which we attribute to the line of sight motion
of the neutron star through the dense circumstellar environment. A reduction in
the power law photon index was observed during the orbital maximum, which we
speculate is due to absorption effects as the neutron star passes behind a
heavily absorbing region near the base of the supergiant companion's wind. No
significant variability of the column density was observed over the
super-orbital period, indicating that variable obscuration by a precessing warp
in an accretion disc is not the mechanism behind the super-orbital modulation.
In contrast, a significant increase in the power law photon index was observed
during the super-orbital minimum. We conclude that the observed super-orbital
modulation is tied to variability in the mass accretion rate due to some as yet
unidentified mechanism.Comment: 22 pages, 27 figures, accepted for publication in MNRAS after
moderate revisio
Emergent properties of electrically coupled smooth muscle cells
Asynchronous and synchronous calcium oscillations occur in a variety of cells. A well-established pathway for intercellular communication is provided by gap junctions which connect adjacent cells and can mediate electrical and chemical coupling. Several experimental studies report that cells presenting only a transient increase when freshly dispersed may oscillate when they are coupled. Such observations suggest that the role of gap junctions is not only to coordinate calcium oscillations of adjacent cells. Gap junctions may also be important to generate oscillations. Here we illustrate the emergent properties of electrically coupled smooth muscle cells using a model that we recently proposed. A bifurcation analysis in the case of two cells reveals that synchronous and asynchronous calcium oscillations can be induced by electrical coupling. In a larger population of smooth muscle cells, electrical coupling may result in the creation of groups of cells presenting synchronous calcium oscillations. The elements of one group may be distant from each other. Moreover, our results highlight a general mechanism by which gap junctional electrical coupling can give rise to out of phase calcium oscillations in smooth muscle cells that are non-oscillating when uncoupled. All these observations remain true in the case of non-identical cells, except that the solution corresponding to synchronous calcium oscillations disappears and that the formation of groups is sensitive to the degree of heterogeneity. © 2005 Society for Mathematical Biology. Published by Elsevier Ltd. All rights reserved
Investigating the Chemically Homogeneous Evolution Channel and its Role in the Formation of the Enigmatic Binary Black Hole Progenitor Candidate HD 5980
Chemically homogeneous evolution (CHE) is a promising channel for forming
massive binary black holes. The enigmatic, massive Wolf-Rayet (WR) binary HD
5980 A&B has been proposed to have formed through this channel. We investigate
this claim by comparing its observed parameters with CHE models. Using MESA, we
simulate grids of close massive binaries then use a Bayesian approach to
compare them with the stars' observed orbital period, masses, luminosities, and
hydrogen surface abundances. The most probable models, given the observational
data, have initial periods ~3 days, widening to the present-day ~20 day orbit
as a result of mass loss -- correspondingly, they have very high initial
stellar masses (150 M). We explore variations in stellar
wind-mass loss and internal mixing efficiency, and find that models assuming
enhanced mass-loss are greatly favored to explain HD 5980, while enhanced
mixing is only slightly favoured over our fiducial assumptions. Our most
probable models slightly underpredict the hydrogen surface abundances.
Regardless of its prior history, this system is a likely binary black hole
progenitor. We model its further evolution under our fiducial and enhanced wind
assumptions, finding that both stars produce black holes with masses ~19-37
M. The projected final orbit is too wide to merge within a Hubble time
through gravitational waves alone. However, the system is thought to be part of
a 2+2 hierarchical multiple. We speculate that secular effects with the
(possible) third and fourth companions may drive the system to promptly become
a gravitational-wave source.Comment: 23 pages, 9 figures, 2 tables, 2 appendices. Accepted for publication
in Ap
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
BRITE-Constellation reveals evidence for pulsations in the enigmatic binary Carinae
Car is a massive, eccentric binary with a rich observational history.
We obtained the first high-cadence, high-precision light curves with the
BRITE-Constellation nanosatellites over 6 months in 2016 and 6 months in 2017.
The light curve is contaminated by several sources including the Homunculus
nebula and neighboring stars, including the eclipsing binary
CPD592628. However, we found two coherent oscillations in the light
curve. These may represent pulsations that are not yet understood but we
postulate that they are related to tidally excited oscillations of Car's
primary star, and would be similar to those detected in lower-mass eccentric
binaries. In particular, one frequency was previously detected by van Genderen
et al. and Sterken et al. through the time period of 1974 to 1995 through
timing measurements of photometric maxima. Thus, this frequency seems to have
been detected for nearly four decades, indicating that it has been stable in
frequency over this time span. These pulsations could help provide the first
direct constraints on the fundamental parameters of the primary star if
confirmed and refined with future observations.Comment: 8 pages, 4 figures, accepted to MNRA
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
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
Eta Carinae across the 2003.5 minimum: Spectroscopic Evidence for Massive Binary Interactions
We have analyzed high spatial, moderate spectral resolution observations of
Eta Carinae obtained with the STIS from 1998.0 to 2004.3. The spectra show
prominent P-Cygni lines in H I, Fe II and He I which are complicated by blends
and contamination by nebular emission and absorption along the line-of-sight
toward the observer. All lines show phase and species dependent variations in
emission and absorption. For most of the cycle the He I emission is blueshifted
relative to the H I and Fe II P-Cygni emission lines, which are approximately
centered at system velocity. The blueshifted He I absorption varies in
intensity and velocity throughout the 2024 day period. We construct radial
velocity curves for the absorption component of the He I and H I lines. The He
I absorption shows significant radial velocity variations throughout the cycle,
with a rapid change of over 200 km/s near the 2003.5 event. The H I velocity
curve is similar to that of the He I absorption, though offset in phase and
reduced in amplitude. We interpret the complex line profile variations in He I,
H I and Fe II to be a consequence of the dynamic interaction of the dense wind
of Eta Car A with the less dense, faster wind plus the radiation field of a hot
companion star, Eta Car B. During most of the orbit, Eta Car B and the He+
recombination zone are on the near side of Eta Car A, producing blueshifted He
I emission. He I absorption is formed in the part of the He+ zone that
intersects the line-of-sight toward Eta Car. We use the variations seen in He I
and the other P-Cygni lines to constrain the geometry of the orbit and the
character of Eta Car B.Comment: 16 pages, 18 figure
- …