198 research outputs found
Solar model with CNO revised abundances
Recent three-dimensional, NLTE analyses of the solar spectrum have shown a
significant reduction in the C, N, O and Ne abundances leading to a Z/X ratio
of the order of 0.0177. We have computed solar models with this new mixture in
the OPAL opacity tables. The present He abundance we find seems rather
consistent with the helioseismic value. However, the convective envelope is too
shallow, and diffusion, even if it reduces the discrepancy, is not able to give
the current value. We present some numerical experiments consisting in changing
the diffusion velocities and/or the value of opacity at the base of the
convective envelope.Comment: 3 pages, Proc. SOHO 14 - GONG 2004 Meeting, Yale, 12-16 July 2004,
added reference to Basu, S., Antia, H.M., 2004, ApJ 606, L8
Higher metal abundances do not solve the solar problem
Context. The Sun acts as a cornerstone of stellar physics. Thanks to
spectroscopic, helioseismic and neutrino flux observations, we can use the Sun
as a laboratory of fundamental physics in extreme conditions. The conclusions
we draw are then used to inform and calibrate evolutionary models of all other
stars in the Universe. However, solar models are in tension with helioseismic
constraints. The debate on the ``solar problem'' has hitherto led to numerous
publications discussing potential issues with solar models and abundances.
Aims. Using the recently suggested high-metallicity abundances for the Sun, we
investigate whether standard solar models, as well as models with macroscopic
transport reproducing the solar surface lithium abundances and analyze their
properties in terms of helioseismic and neutrino flux observations. Methods. We
compute solar evolutionary models and combine spectroscopic and helioseismic
constraints as well as neutrino fluxes to investigate the impact of macroscopic
transport on these measurements. Results. When high-metallicity solar models
are calibrated to reproduce the measured solar lithium depletion, tensions
arise with respect to helioseismology and neutrino fluxes. This is yet another
demonstration that the solar problem is also linked to the physical
prescriptions of solar evolutionary models and not to chemical composition
alone. Conclusions. A revision of the physical ingredients of solar models is
needed in order to improve our understanding of stellar structure and
evolution. The solar problem is not limited to the photospheric abundances if
the depletion of light elements is considered. In addition, tighter constraints
on the solar beryllium abundance will play a key role in the improvement of
solar models.Comment: Accepted for publication in Astronomy and Astrophysic
Grids of Stellar Models and Frequencies with CLES + LOSC
We present a grid of stellar models, obtained with the CLES evolution code,
following the specification of ESTA-Task1, and the corresponfing seismic
properties, computed with the LOSC code. We provide a complete description of
the corresponding files that will be available on the ESTA web-pages.Comment: 8 pages, accepted for publication in Astrophys. Space Sci.
(CoRoT/ESTA Volume
An asteroseismic study of the O9V star HD 46202 from CoRoT space-based photometry
The O9V star HD 46202, which is a member of the young open cluster NGC 2244,
was observed by the CoRoT satellite in October/November 2008 during a short run
of 34 days. From the very high-precision light curve, we clearly detect beta
Cep-like pulsation frequencies with amplitudes of ~0.1 mmag and below. A
comparison with stellar models was performed using a chi^2 as a measure for the
goodness-of-fit between the observed and theoretically computed frequencies.
The physical parameters of our best-fitting models are compatible with the ones
deduced spectroscopically. A core overshooting parameter alpha_ov = 0.10 +-
0.05 pressure scale height is required. None of the observed frequencies are
theoretically excited with the input physics used in our study. More
theoretical work is thus needed to overcome this shortcoming in how we
understand the excitation mechanism of pulsation modes in such a massive star.
A similar excitation problem has also been encountered for certain pulsation
modes in beta Cep stars recently modelled asteroseismically.Comment: Accepted for publication in Astronomy and Astrophysics on 17/12/2010,
9 pages, 7 figures, 4 table
Know Your Heritage: Exploring the Effects of Fit in Cultural Knowledge on Chinese Canadians’ Heritage Identification
In the present research, we introduce the notion of fit in cultural knowledge (FICK) – which we define as a match between the self and others in representing a cultural tradition. For ethnic minorities, FICK can be manifested in different degrees of matching their personal beliefs about their heritage culture with outgroup as well as ingroup beliefs about their heritage culture. We conducted two studies with the objective of exploring the potentially negative effects of FICK on Chinese Canadians’ heritage identification. In both studies, Chinese Canadian university students (N = 102; N = 156) indicated their personal beliefs about what values are normative in Chinese culture. Ingroup beliefs were assessed by beliefs about Chinese values that Chinese Canadians ascribed to their parents (Study 2), whereas outgroup beliefs were assessed by beliefs about Chinese values that were held by Euro-Canadians (Study 1) or that Chinese Canadians ascribed to Euro-Canadians (Study 2). The main findings based on a series of path models are as follows: (1) a stronger FICK generally predicted lower Chinese identification (centrality, ingroup ties, and affect), yet those negative effects were largely manifested in the openness to change versus conservation rather than in the self-transcendence versus self-enhancement value dimension. (2) The negative effects could be explained by Chinese Canadians’ experience of bicultural conflict (Study 1) and the frustration of continuity, meaning, and belonging identity motives (Study 2), suggesting that it matters which specific views of Chinese culture are matched in FICK. 3) Individuals who agreed with the perceived outgroup beliefs, and parental beliefs to a lesser extent, were more likely to apply the model minority stereotype to other Chinese Canadians (Study 2). Taken together, those findings demonstrate the challenges FICK presents to heritage identity maintenance among Chinese Canadian young adults. Implications for enculturation and cultural fit are discussed
Blind data detection in the presence of PLL phase noise by sequential Monte Carlo method
In this paper, based on a sequential Monte Carlo method, a computationally efficient algorithm is presented for blind data detection in the presence of residual phase noise generated at the output the phase tracking loop employed in a digital receiver. The basic idea is to treat the transmitted symbols as" missing data" and draw samples sequentially of them based on the observed signal samples up to time t. This way, the Bayesian estimates of the phase noise and the incoming data are obtained through these samples, sequentially drawn,together with their importance weights. The proposed receiver structure is seen to be ideally suited for high-speed parallel implementation using VLSI technology. © 2006 IEEE
Blind phase noise estimation in OFDM systems by sequential Monte Carlo method
In this paper, based on a sequential Monte Carlo method, a computationally efficient algorithm is presented for estimating the residual phase noise, blindly, generated at the output the phase tracking loop employed in OFDM systems. The basic idea is to treat the transmitted symbols as "missing data" and draw samples sequentially of them based on the observed signal samples up to time t. This way, the Bayesian estimates of the phase noise is obtained through these samples, sequentially drawn, together with their importance weights. The proposed receiver structure is seen to be ideally suited for high-speed parallel implementation using VLSI technology. © 2006 Springer
Helioseismic analysis of the hydrogen partition function in the solar interior
The difference in the adiabatic gradient gamma_1 between inverted solar data
and solar models is analyzed. To obtain deeper insight into the issues of
plasma physics, the so-called ``intrinsic'' difference in gamma_1 is extracted,
that is, the difference due to the change in the equation of state alone. Our
method uses reference models based on two equations of state currently used in
solar modeling, the Mihalas-Hummer-Dappen (MHD) equation of state, and the OPAL
equation of state (developed at Livermore). Solar oscillation frequencies from
the SOI/MDI instrument on board the SOHO spacecraft during its first 144 days
in operation are used. Our results confirm the existence of a subtle effect of
the excited states in hydrogen that was previously studied only theoretically
(Nayfonov & Dappen 1998). The effect stems from internal partition function of
hydrogen, as used in the MHD equation of state. Although it is a pure-hydrogen
effect, it takes place in somewhat deeper layers of the Sun, where more than
90% of hydrogen is ionized, and where the second ionization zone of helium is
located. Therefore, the effect will have to be taken into account in reliable
helioseismic determinations of the astrophysically relevant helium-abundance of
the solar convection zone.Comment: 30 pages, 4 figures, 1 table. Revised version submitted to Ap
New Constraints on the Origin of the Short-Term Cyclical Variability of the Wolf-Rayet Star WR 46
The Wolf-Rayet star WR 46 is known to exhibit a very complex variability
pattern on relatively short time scales of a few hours. Periodic but
intermittent radial velocity shifts of optical lines as well as multiple
photometric periods have been found in the past. Non-radial pulsations, rapid
rotational modulation or the presence of a putative low-mass companion have
been proposed to explain the short-term behaviour. In an effort to unveil its
true nature, we observed WR 46 with FUSE (Far Ultraviolet Spectroscopic
Explorer) over several short-term variability cycles. We found significant
variations on a time scale of ~8 hours in the far-ultraviolet (FUV) continuum,
in the blue edge of the absorption trough of the OVI {\lambda}{\lambda}1032,
1038 doublet P Cygni profile and in the SVI {\lambda}{\lambda}933, 944 P Cygni
absorption profile. We complemented these observations with X-ray and UV
light-curves and an X-ray spectrum from archival XMM-Newton (X-ray Multi-Mirror
Mission - Newton Space Telescope) data. The X-ray and UV light-curves show
variations on a time scale similar to the variability found in the FUV. We
discuss our results in the context of the different scenarios suggested to
explain the short-term variability of this object and reiterate that non-radial
pulsations is the most likely to occur.Comment: 36 pages, 11 figures. Accepted for publication in Ap
CoRoT observations of O stars: diverse origins of variability
Six O-type stars were observed continuously by the CoRoT satellite during a
34.3-day run. The unprecedented quality of the data allows us to detect even
low-amplitude stellar pulsations in some of these stars (HD 46202 and the
binaries HD 46149 and Plaskett's star). These cover both opacity-driven modes
and solar-like stochastic oscillations, both of importance to the
asteroseismological modelling of O stars. Additional effects can be seen in the
CoRoT light curves, such as binarity and rotational modulation. Some of the
hottest O-type stars (HD 46223, HD 46150 and HD 46966) are dominated by the
presence of red-noise: we speculate that this is related to a sub-surface
convection zone.Comment: 5 pages, 3 figures, conference paper. To be published in "Four
decades of Research on Massive Stars", Astronomical Society of the Pacific.
Eds. C. Robert, N. St-Louis and L. Drisse
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