356 research outputs found
Slow Radiation-Driven Wind Solutions of A-Type Supergiants
The theory of radiation-driven winds succeeded in describing terminal
velocities and mass loss rates of massive stars. However, for A-type
supergiants the standard m-CAK solution predicts values of mass loss and
terminal velocity higher than the observed values. Based on the existence of a
slow wind solution in fast rotating massive stars, we explore numerically the
parameter space of radiation-driven flows to search for new wind solutions in
slowly rotating stars, that could explain the origin of these discrepancies. We
solve the 1-D hydrodynamical equation of rotating radiation-driven winds at
different stellar latitudes and explore the influence of ionization's changes
throughout the wind in the velocity profile. We have found that for particular
sets of stellar and line-force parameters, a new slow solution exists over the
entire star when the rotational speed is slow or even zero. In the case of slow
rotating A-type supergiant stars the presence of this novel slow solution at
all latitudes leads to mass losses and wind terminal velocities which are in
agreement with the observed values. The theoretical Wind Momentum-Luminosity
Relationship derived with these slow solutions shows very good agreement with
the empirical relationship. In addition, the ratio between the terminal and
escape velocities, which provides a simple way to predict stellar wind energy
and momentum input into the interstellar medium, is also properly traced.Comment: 7 Pages, 3 figures, Astrophysical Journal, Accepte
The wind of rotating B supergiants, I : domains of slow and fast solution regimes
In the scenario of rotating radiation-driven wind theory for massive stars, three types of stationary hydrodynamic solutions are currently known: the classical ( fast) m-CAK solution, the Ω-slow solution that arises for fast rotators, and the so-called δ-slow solution if high values of the δ line-force parameter are allowed independently of the rotation speed. Compared to the fast solution, both ?slow solutions? have lower terminal velocities. As the study ofthe parameter domain for the slow solution is still incomplete, we perform a comprehensive analysis of the distinctive flow regimes for B supergiants that emerge from a fine grid of rotation values, Ω, and various ionizationconditions in the wind (δ) parameter. The wind ionization defines two domains: one for fast outflowing winds and the other for slow expanding flows. Both domains are clear-cut by a gap, where a kink/plateau structure of thevelocity law could exist for a finite interval of δ. The location and width of the gap depend on Teff and Ω. There is a smooth and continuous transition between the Ω-slow and δ-slow regimes, a single Ω δ-slow regime. We discussdifferent situations where the slow solutions can be found and the possibility of a switch between fast and slow solutions in B supergiant winds. We compare the theoretical terminal velocity with observations of B and Asupergiants and find that the fast regime prevails mostly for early B supergiants while the slow wind regime matches better for A and B mid- and late-type supergiants.Fil: Venero, Roberto Oscar José. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Curé, M.. Universidad de Valparaíso; ChileFil: Cidale, Lydia Sonia. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Araya, I.. Universidad de Valparaíso; Chil
Improving prediction performance of stellar parameters using functional models
This paper investigates the problem of prediction of stellar parameters, based on the star's electromagnetic spectrum. The knowledge of these parameters permits to infer on the evolutionary state of the star. From a statistical point of view, the spectra of different stars can be represented as functional data. Therefore, a two-step procedure decomposing the spectra in a functional basis combined with a regression method of prediction is proposed. We also use a bootstrap methodology to build prediction intervals for the stellar parameters. A practical application is also provided to illustrate the numerical performance of our approach
Ionization structure in the winds of B[e] supergiants II. Influence of rotation on the formation of equatorial hydrogen neutral zones
Context: B[e] supergiants are known to have non-spherical winds, and the
existence of disks that are neutral in hydrogen close to their stellar surface
has been postulated. A suitable mechanism to produce non-spherical winds seems
to be rapid rotation, and at least for three B[e] supergiants in the Magellanic
Clouds rotation velocities at a substantial fraction of their critical velocity
have been found. Aims: We want to find suitable recombination distances in the
equatorial plane of rapidly rotating stars that explain the observed huge
amounts of neutral material in the vicinity of B[e] supergiants. Methods: We
perform ionization structure calculations in the equatorial plane around
rapidly rotating luminous supergiants. The restriction to the equatorial plane
allows us to treat the ionization balance equations 1-dimensionally, while the
stellar radiation field is calculated 2-dimensionally, taking into account the
latitudinal variation of the stellar surface parameters. The stellar parameters
used correspond to those known for B[e] supergiants. The assumptions made in
the computations all have in common that the total number of available ionizing
photons at any location within the equatorial plane is overestimated, resulting
in upper limits for the recombination distances. Results: We find that despite
the drop in equatorial surface density of rapidly rotating stars (neglecting
effects like bi-stability and/or wind compression), hydrogen and helium
recombine at or close to the stellar surface, for mass loss rates Mdot > 5d-5
M_sun/yr and rotation speeds in excess of v(rot,eq)/v(crit)=0.8.Comment: 10 pages, 5 figures, accepted for publication in A&A (03/05/2006),
Eq. (3) corrected, language improve
Do Hummingbirds See in Ultraviolet?
We present a numerical model to fit the electroretinogram (ERG), a gross evoked eye visual potential, that originate in the retina through photons absorption by photoreceptors and then involve the contribution form others retinal neurons. We use the ERG measured in a hummingbird, to evaluate the most likely retinal mechanism - cones visual pigments and oil-droplets - that participate in their high dimensional tetra or pentachromatic color hyperspace. The model - a nonlinear fit - appears to be a very useful tool to predict the underlying contribution visual mechanism for a variety of retinal preparation
Resolving the kinematics of the discs around Galactic B[e] supergiants
B[e] supergiants are luminous evolved massive stars. The mass-loss during this phase creates a complex circumstellar environment with atomic, molecular, and dusty regions usually found in rings or disc-like structures. For a better comprehension of the mechanisms behind the formation of these rings, detailed knowledge about their structure and dynamics is essential. To address that, we obtained high-resolution optical and near-infrared (near-IR) spectra for eight selected Galactic B[e] supergiants, for which CO emission has been detected. Assuming Keplerian rotation for the disc, we combine the kinematics obtained from the CO bands in the near-IR with those obtained by fitting the forbidden emission [OI] λ5577, [O I] λλ6300,6363, and [Ca II] λλ7291,7323 lines in the optical to probe the disc structure. We find that the emission originates from multiple ring structures around all B[e] supergiants, with each one of them displaying a unique combination of rings regardless of whether the object is part of a binary system. The confirmed binaries display spectroscopic variations of their line intensities and profiles as well as photometric variability, whereas the ring structures around the single stars are stable.Fil: Maravelias, G.. Universidad de Valparaíso; Chile. Academia de la República de Checa; República ChecaFil: Kraus, Michaela. Academia de la República de Checa; República Checa. Universidad de Tartu; EstoniaFil: Cidale, Lydia Sonia. Universidad de Valparaíso; Chile. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Borges Fernades, M.. Ministério de Ciencia, Tecnologia e Innovacao. Observatorio Nacional; BrasilFil: Arias, María Laura. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Curé, Michel. Universidad de Valparaíso; ChileFil: Vasilopoulos, G.. Max-planck-institut Extraterrestrische Physik; Alemani
Outflowing disk formation in B[e] supergiants due to rotation and bi--stability in radiation driven winds
The effects of rapid rotation and bi-stability upon the density contrast
between the equatorial and polar directions of a B[e] supergiant are
re-investigated. Based upon a new slow solution for different high rotational
radiation driven winds (Cur\'e 2004) and the fact that bi--stability allows a
change in the line--force parameters (, , and ), the
equatorial densities are about -- times higher than the polar ones.
These values are in qualitative agreement with the observations.Comment: 5 pages, 5 figures, Accepted for publication in Astronomy &
Astrophysic
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