440 research outputs found
Calibrating the {\alpha} parameter of convective efficiency using observed stellar properties
Context. Synthetic model atmosphere calculations are still the most commonly
used tool when determining precise stellar parameters and stellar chemical
compositions. Besides three-dimensional models that consistently solve for
hydrodynamic processes, one-dimensional models that use an approximation for
convective energy transport play the major role.
Aims. We use modern Balmer-line formation theory as well as spectral energy
distribution (SED) measurements for the Sun and Procyon to calibrate the model
parameter {\alpha} that describes the efficiency of convection in our 1D
models. Convection was calibrated over a significant range in parameter space,
reaching from F-K along the main sequence and sampling the turnoff and giant
branch over a wide range of metallicities. This calibration was compared to
theoretical evaluations and allowed an accurate modeling of stellar
atmospheres.
Methods. We used Balmer-line fitting and SED fits to determine the convective
efficiency parameter {\alpha}. Both methods are sensitive to the structure and
temperature stratification of the deeper photosphere.
Results. While SED fits do not allow a precise determination of the
convective parameter for the Sun and Procyon, they both favor values
significantly higher than 1.0. Balmer-line fitting, which we find to be more
sensitive, suggests that the convective efficiency parameter {\alpha} is
2.0 for the main sequence and quickly decreases to 1.0 for
evolved stars. These results are highly consistent with predictions from 3D
models. While the values on the main sequence fit predictions very well,
measurements suggest that the decrease of convective efficiency as stars evolve
to the giant branch is more dramatic than predicted by models.Comment: 14 pages, 16 figures, accepted for publication in A&
Star formation in the S233 region
The main objective of this paper is to study the possibility of triggered
star formation on the border of the HII region S233, which is formed by a
B-star. Using high-resolution spectra we determine the spectral class of the
ionizing star as B0.5 V and the radial velocity of the star to be -17.5(1.4)
km/s. This value is consistent with the velocity of gas in a wide field across
the S233 region, suggesting that the ionizing star was formed from a parent
cloud belonging to the S233 region. By studying spatial-kinematic structure of
the molecular cloud in the S233 region, we detected an isolated clump of gas
producing CO emission red-shifted relative to the parent cloud. In the UKIDSS
and WISE images, the clump of gas coincides with the infrared source containing
a compact object and bright-rimmed structure. The bright-rimmed structure is
perpendicular to the direction of the ionizing star. The compact source
coincides in position with IRAS source 05351+3549. All these features indicate
a possibility of triggering formation of a next-generation star in the S233
region. Within the framework of a theoretical one-dimensional model we conclude
that the "collect-and-collapse" process is not likely to take place in the S233
region. The presence of the bright-rimmed structure and the compact infrared
source suggest that the "collapse of the pre-existing clump" process is taking
place.Comment: 12 pages, 10 figure
F.Y.I., 1991-05-24
Newsletter published by Governors State University between 1989-1996
THz parametric gain in semiconductor superlattices in the absence of electric domains
We theoretically show that conditions for THz gain and conditions for
formation of destructive electric domains in semiconductor superlattices are
fairly different in the case of parametric generation and amplification. Action
of an unbiased high-frequency electric field on a superlattice causes a
periodic variation of energy and effective mass of miniband electrons. This
parametric effect can result in a significant gain at some even harmonic of the
pump frequency without formation of electric domains and corruption from pump
harmonics.Comment: 4 pages, 3 figures. Accepted to Appl. Phys. Let
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