909 research outputs found
Signature of wide-spread clumping in B supergiant winds
We seek to establish additional observational signatures of the effects of
clumping in OB star winds. The action of clumping on strategic wind-formed
spectral lines is tested to steer the development of models for clumped winds
and thus improve the reliability of mass-loss determinations for massive
stars.The SiIV 1400 resonance line doublets of B0 to B5 supergiants are
analysed using empirical line-synthesis models. The focus is on decoding
information on wind clumping from measurements of ratios of the radial optical
depths (tau_(rad)(w)) of the red and blue components of the SiIV doublet. We
exploit in particular the fact that the two doublet components are decoupled
and formed independently for targets with relatively low wind terminal
velocities. Line-synthesis analyses reveal that the mean ratio of tau_(rad)(w)
of the blue to red SiIV components are rarely close to the canonical value of ~
2 (expected from atomic constants), and spread instead over a range of values
between ~1 and 2. These results are interpreted in terms of a photosphere that
is partially obscured by optically thick structures in the outflowing gas.The
spectroscopic signatures established in this study demonstrate the wide-spread
existence of wind clumping in B supergiants. The additional information in
unsaturated doublet profiles provides a means to quantify the porosity of the
winds.Comment: Accepted for publication in A&A Letter
UV diagnostic of porosity-free mass-loss estimates in B stars
We seek to establish evidence in UV P Cygni line profiles that the signs of
wind clumping and porosity vary with velocity. We aim to demonstrate
empirically that while at most wind velocities optically thick clumps cover
only a fraction of the stellar surface, close to the terminal velocity where
narrow absorption components (NACs) appear in UV lines the covering factor is
approximately unity. SEI line-synthesis models are used to determine the radial
optical depths of blue and red components of the SiIV 1400 resonance line
doublet in a sample of 12 B0 to B4 supergiants. We focus on stars with well
developed NACs and relatively low terminal velocity so that the SiIV doublet
components can be treated as radiatively decoupled and formed independently.
For all 12 stars the mean optical depth ratio of the blue to red components is
closer to ~ 2 (i.e. the ratio of oscillator strengths) in the NACs than at
intermediate and lower velocities. The product of mass-loss rate and Si^3+ ion
fraction calculated from the NAC optical depths is a factor of ~ 2 to 9 higher
compared to mass-loss values sampled at ~ 0.4 to 0.6 of the terminal velocity.
Since the wind effectively becomes `smooth' at the high NAC velocities and the
column density is uniformly distributed over the stellar disk, the optical
depths of the NACs are not seriously affected by porosity and this feature thus
provides the most reliable measurement of mass-loss rate in the UV lines.
Applications of this result to the weak-wind problem of late O-dwarf stars and
the "PV mass loss discordance" in early O supergiants are discussed.Comment: Accepted for publication in Astronomy and Astrophysics; 7 pages; 3
figure
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
The development of a competence framework for engineering analysis and simulation
Engineering analysis and simulation has always played a significant role in the nuclear sector and its use continues to increase across all branches of industry. To remain competitive in an increasingly global environment and to ensure the safety and reliability of products, companies must prepare effectively for the challenges that new engineering simulation technologies will bring. Concerns surrounding the inappropriate use of simulation by staff without the appropriate competences persist, as analyses become more advanced, increasingly embracing more complex physical phenomena and interactions, often in an effort to model reality more faithfully. These trends and the associated competencies required, emphasize the need for life-long learning and continual staff development. Organisations clearly require a sufficient and ongoing supply of well-qualified engineers and the recently funded EASIT2 project is directly aimed at addressing and managing these issues
Transferring simulation skills from other industries to nuclear
Engineering analysis and simulation has always played a significant role in the nuclear sector and its use continues to increase across all branches of industry. To remain competitive in an increasingly global environment and to ensure the safety and reliability of designs, the nuclear industry must take advantage of the new engineering simulation technologies. Concerns surrounding the inappropriate use of simulation by staff without the appropriate competency persist, as analyses become more advanced, increasingly embracing more complex physical phenomena, often in an effort to model reality more faithfully. Furthermore, the age profile of the skilled staff in the nuclear sector in the UK is such that the skills shortage is likely to increase in future. These trends emphasize the need for life-long learning and continual staff development along with transfer of skills from other industry sectors to the nuclear sector. The nuclear industry has taken some initiatives to address skill shortages through the National Skills Academy for Nuclear and Nuclear Energy Skills Alliance (NESA) but these are mostly focused on manufacturing and R&D skills. The recently completed EU funded EASIT2 project is directly aimed at addressing the engineering analysis and simulation skills. This paper gives a brief overview of the EASIT2 project and its deliverables and points out how it can help the skills issues being faced by the nuclear industry. INTRODUCTIO
Wind variability of B supergiants
We present the most suitable data sets available in the International Ultraviolet Explorer (IUE) archive for the study of time-dependent stellar winds in early B supergiants. The UV line profile variability in 11 B0 to B3 stars is analysed, compared and discussed, based on 16 separate data sets comprising over 600 homogeneously reduced high-resolution spectrograms. The targets include ``normal'' stars with moderate rotation rates and examples of rapid rotators. A gallery of grey-scale images (dynamic spectra) is presented, which demonstrates the richness and range of wind variability and highlights different structures in the winds of these stars. This work emphasises the suitability of B supergiants for wind studies, under-pinned by the fact that they exhibit unsaturated wind lines for a wide range of ionization. The wind activity of B supergiants is substantial and has highly varied characteristics. The variability evident in individual stars is classified and described in terms of discrete absorption components, spontaneous absorption, bowed structures, recurrence, and ionization variability and stratification. Similar structures can occur in stars of different fundamental parameters, but also different structures may occur in the same star at a given epoch. We discuss the physical phenomena that may be associated with the spectral signatures. The diversity of wind patterns evident likely reflects the role of stellar rotation and viewing angle in determining the observational characteristics of azimuthally extended structure rooted at the stellar surface. In addition, SEI line-synthesis modelling of the UV wind lines is used to provide further information about the state of the winds in our program stars. Typically the range, implied by the line profile variability, in the product of mass-loss rate and ion fraction (mdot q_i) is a factor of ~ 1.5, when integrated between 0.2 and 0.9 v_infty ; it can however be several times larger over localised velocity regions. At a given effective temperature the mean relative ion ratios can differ by a factor of 5. The general excess in predicted (forward-scattered) emission in the low velocity regime is discussed in terms of structured outflows. Mean ion fractions are estimated over the B0 to B1 spectral classes, and trends in the ionic ratios as a function of wind velocity are described. The low values obtained for the ion fractions of UV resonance lines may reflect the role of clumping in the wind
PN fast winds: Temporal structure and stellar rotation
To diagnose the time-variable structure in the fast winds of central stars of
planetary nebulae (CSPN), we present an analysis of P Cygni line profiles in
FUSE satellite far-UV spectroscopic data. Archival spectra are retrieved to
form time-series datasets for the H-rich CSPN NGC 6826, IC 418, IC 2149, IC
4593 and NGC 6543. Despite limitations due to the fragmented sampling of the
time-series, we demonstrate that in all 5 CSPN the UV resonance lines are
variable primarily due to the occurrence of blueward migrating discrete
absorption components (DACs). Empirical (SEI) line-synthesis modelling is used
to determine the range of fluctuations in radial optical depth, which are
assigned to the temporal changes in large-scale wind structures. We argue that
DACs are common in CSPN winds, and their empirical properties are akin to those
of similar structures seen in the absorption troughs of massive OB stars.
Constraints on PN central star rotation velocities are derived from
Fast-Fourier Transform analysis of photospheric lines for our target stars.
Favouring the causal role of co-rotating interaction regions, we explore
connections between normalised DAC accelerations and rotation rates of PN
central stars and O stars. The comparative properties suggest that the same
physical mechanism is acting to generate large-scale structure in the
line-driven winds in the two different settings.Comment: Accepted for publication in MNRAS; 10 pages, 5 figure
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