261 research outputs found
Bright OB stars in the Galaxy.II. Wind variability in O supergiants as traced by H-alpha
We investigate the line-profile variability (lpv) of H-alpha for a large
sample of O-type supergiants. We used the Temporal Variance Spectrum (TVS)
analysis, developed by Fullerton et al 1996 and modified by us to take into
account the effects of wind emission. By means of a comparative analysis we put
a number of constraints on the properties of the variability as a function of
stellar and wind parameters. The results of our analysis show that all the
stars in the sample show evidence of significant lpv in H-alpha, mostly
dominated by processes in the wind. The variations occur between zero and 0.3
v_inf (i.e., below ~1.5 R_star), in good agreement with the results from
similar studies. A comparison between observations and line-profile simulations
indicates that for stars with intermediate wind densities the H-alpha
variability can be explained by simple models, consisting of coherent or broken
shells (blobs) uniformly distributed over the wind volume, with an intrinsic
scatter in the maximum density contrast of about a factor of two. For stars at
lower and higher wind densities, on the other hand, we found certain
inconsistencies between observations and our predictions, most importantly
concerning the mean amplitude and the symmetry properties of the TVS. This
disagreement might be explained with the presence of coherent large-scale
structures, partly confined in a volume close to the star. Interpreted in terms
of a variable mass-loss rate, the observed variations of H-alpha indicate
changes of 4% with respect to the mean value of M_dot for stars with stronger
winds and of 16% for stars with weaker winds. The effect of these variations on
the corresponding wind momenta is rather insignificant (<0.16 dex), increasing
the local scatter without affecting the Wind Momentum Luminosity Relationship.Comment: 22 pages, 11 figures. Accepted by Astronomy and Astrophysic
Orbital and stochastic far-UV variability in the nova-like system V3885 Sgr
Highly time-resolved time-tagged FUSE satellite spectroscopic data are
analysed to establish the far-ultraviolet (FUV) absorption line characteristics
of the nova-like cataclysmic variable binary, V3885 Sgr. We determine the
temporal behaviour of low (Ly_beta, CIII, NIII) and high (SIV, PV, OVI) ion
species, and highlight corresponding orbital phase modulated changes in these
lines. On average the absorption troughs are blueshifted due to a low velocity
disc wind outflow. Very rapid (~ 5 min) fluctuations in the absorption lines
are isolated, which are indicative of stochastic density changes. Doppler
tomograms of the FUV lines are calculated which provide evidence for structures
where a gas stream interacts with the accretion disc. We conclude that the line
depth and velocity changes as a function of orbital phase are consistent with
an asymmetry that has its origin in a line-emitting, localised disc-stream
interaction region.Comment: Accepted for publication in MNRA
Kinematic model inversions of hot star recurrent DAC data - tests against dynamical CIR models
The Discrete Absorption Components (DACs) commonly observed in the ultraviolet lines of hot stars have previously been modelled by dynamical simulations of Corotating Interaction Regions (CIRs) in their fine-driven stellar winds. Here we apply the kinematic DAC inversion method of Brown et al. to the hydrodynamical CIR models and test the reliability of the results obtained. We conclude that the inversion method is able to recover valuable information on the velocity structure of the mean wind and to trace movement of velocity plateaux in the hydrodynamical data, though the recovered density profile of the stream is correct only very near to the stellar surface
An ALMA 3mm continuum census of Westerlund 1
Context. Massive stars play an important role in both cluster and galactic evolution and the rate at which they lose mass is a key driver of both their own evolution and their interaction with the environment up to and including their terminal SNe explosions. Young massive clusters provide an ideal opportunity to study a co-eval population of massive stars, where both their individual properties and the interaction with their environment can be studied in detail.
Aims. We aim to study the constituent stars of the Galactic cluster Westerlund 1 in order to determine mass-loss rates for the diverse post-main sequence population of massive stars.
Methods. To accomplish this we made 3mm continuum observations with the Atacama Large Millimetre/submillimetre Array.
Results. We detected emission from 50 stars in Westerlund 1, comprising all 21 Wolf-Rayets within the field of view, plus eight cool and 21 OB super-/hypergiants. Emission nebulae were associated with a number of the cool hypergiants while, unexpectedly, a number of hot stars also appear spatially resolved.
Conclusions. We were able to measure the mass-loss rates for a unique population of massive post-main sequence stars at every stage of evolution, confirming a significant increase as stars transitioned from OB supergiant to WR states via LBV and/or cool hypergiant phases. Fortuitously, the range of spectral types exhibited by the OB supergiants provides a critical test of radiatively-driven wind theory and in particular the reality of the bi-stability jump. The extreme mass-loss rate inferred for the interacting binary Wd1-9 in comparison to other cluster members confirmed the key role binarity plays in massive stellar evolution. The presence of compact nebulae around a number of OB and WR stars is unexpected; by analogy to the cool super-/hypergiants we attribute this to confinement and sculpting of the stellar wind via interaction with the intra-cluster medium/wind. Given the morphology of core collapse SNe depend on the nature of the pre-explosion circumstellar environment, if this hypothesis is correct then the properties of the explosion depend not just on the progenitor, but also the environment in which it is located
Large-scale wind structures in OB supergiants: a search for rotationally modulated H\alpha variability
We present the results of a long-term monitoring campaign of the H\alpha line
in a sample of bright OB-supergiants (O7.5-B9) that aims at detecting
rotationally modulated changes potentially related to the existence of
large-scale wind structures. A total of 22 objects were monitor ed during 36
nights spread over 6 months in 2001-2002. Coordinated broad-band photometric
observations were also obtained for some targets. Conspicuous evidence for
variability in H\alpha is found for the stars displaying a feature contaminated
by wind emission. Most changes take place on a daily time-scale, although
hourly variations are also occasionally detected. Convincing evidence for a
cyclical pattern of variability in H\alpha has been found in 2 stars: HD 14134
and HD 42087 (periodic signals are also detected in other stars, but
independent confirmation is required). Rotational modulation is suggested from
the similarity between the observed recurrence time-scales (in the range 13-25
days) and estimated periods of stellar rotation. We call attention to the
atypical case of HD 14134 which exhibits a clear 12.8-d periodicity both in the
photometric and in the spectroscopic data sets. This places this object among a
handful of early-type stars where one may observe a clear link between extended
wind structures and photospheric disturbances. Further modelling may test the
hypothesis that azimuthally-extended wind streams are responsible for the
patterns of spectral variability in our target stars.Comment: 18 pages, accepted for publication in MNRA
Terminal velocities of luminous, early-type SMC stars
Ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS) are
used to determine terminal velocities for 11 O and B-type giants and
supergiants in the Small Magellanic Cloud (SMC) from the Si IV and C IV
resonance lines. Using archival data from observations with the Goddard
High-Resolution Spectrograph and the International Ultraviolet Explorer
telescope, terminal velocities are obtained for a further five B-type
supergiants. We discuss the metallicity dependence of stellar terminal
velocities, finding no evidence for a significant scaling between Galactic and
SMC metallicities for Teff < 30,000 K, consistent with the predictions of
radiation driven wind theory for supergiant stars. A comparison of the
ratio between the SMC and Galactic samples, while
consistent with the above statement, emphasizes that the uncertainties in the
distances to galactic O-stars are a serious obstacle to a detailed comparison
with theory. For the SMC sample there is considerable scatter in this ratio at
a given effective temperature, perhaps indicative of uncertainties in stellar
masses.Comment: 28 pages, 8 figures, accepted by ApJ; minor revisions prior to
acceptanc
Cost of hospitalization for childbirth in India: how equitable it is in the post-NRHM era?
BACKGROUND AND OBJECTIVE: Information on out-of-pocket (OOP) expenditure during childbirth in public and private health facilities in India is needed to make rational decisions for improving affordability to maternal care services. We undertook this study to evaluate the OOP expenditure due to hospitalization from childbirth and its impact on households. METHODS: This is a secondary data analysis of a nationwide household survey by the National Sample Survey Organization in 2014. The survey reported health service utilization and health care related expenditure by income quintiles and type of health facility. The recall period for hospitalization expenditure was 365 days. OOP expenditure amounting to more than 10% of annual consumption expenditure was termed as catastrophic. RESULTS: Median expenditure per episode of hospitalisation due to childbirth was US$54. The expenditure incurred was about six times higher among the richest quintile compared to the poorest quintile. Median private sector OOP hospitalization expenditure was nearly nine times higher than in the public sector. Hospitalization in a private sector facility leads to a significantly higher prevalence of catastrophic expenditure than hospitalization in a public sector (60% vs. 7%). Indirect cost (43%) constituted the largest share in the total expenditure in public sector hospitalizations. Urban residence, poor wealth quintile, residing in eastern and southern regions of India and delivery in private hospital were significantly associated with catastrophic expenditure. CONCLUSIONS: We strongly recommend cash transfer schemes with effective pro-poor targeting to reduce the impact of catastrophic expenditure. Strengthening of public health facilities is required along with private sector regulation
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