1,031 research outputs found
Fundamental properties of solar-like oscillating stars from frequencies of minimum : II. Model computations for different chemical compositions and mass
The large separations between the oscillation frequencies of solar-like stars
are measures of stellar mean density. The separations have been thought to be
mostly constant in the observed range of frequencies. However, detailed
investigation shows that they are not constant, and their variations are not
random but have very strong diagnostic potential for our understanding of
stellar structure and evolution. In this regard, frequencies of the minimum
large separation are very useful tools. From these frequencies, in addition to
the large separation and frequency of maximum amplitude, Y\i ld\i z et al.
recently have developed new methods to find almost all the fundamental stellar
properties. In the present study, we aim to find metallicity and helium
abundances from the frequencies, and generalize the relations given by Y\i ld\i
z et al. for a wider stellar mass range and arbitrary metallicity () and
helium abundance (). We show that the effect of metallicity is {
significant} for most of the fundamental parameters. For stellar mass, for
example, the expression must be multiplied by (Z/Z_{\sun})^{0.12}. For
arbitrary helium abundance, M \propto (Y/Y_{\sun})^{0.25} . Methods for
determination of and from pure asteroseismic quantities are based on
amplitudes (differences between maximum and minimum values of \Dnu) in the
oscillatory component in the spacing of oscillation frequencies. Additionally,
we demonstrate that the difference between the first maximum and the second
minimum is very sensitive to . It also depends on and small separation between the frequencies. Such a dependence leads us
to develop a method to find (and ) from oscillation frequencies. The
maximum difference between the estimated and model values is about 14 per
cent. It is 10 per cent for .Comment: 8 pages, 13 figures; published in MNRAS (2015
Comparison of Gaia and asteroseismic distances
Asteroseismology provides fundamental properties (mass, radius and effective
temperature) of solar-like oscillating stars using so-called scaling relations.
These properties allow the computation of the asteroseismic distance of stars.
We compare the asteroseismic distances with the recently released Gaia
distances for 74 stars studied in Y{\i}ld{\i}z et al. There is a very good
agreement between these two distances; for 64 of these stars, the difference is
less than 10 per cent. However, a systematic difference is seen if we use the
effective temperature obtained by spectroscopic methods; the Gaia distances are
about 5 per cent greater than the asteroseismic distances.Comment: 4 pages, 4 figures, accepted by MNRA
On the structure and evolution of planets and their host stars effects of various heating mechanisms on the size of giant gas planets
It is already stated in the previous studies that the radius of the giant
planets is affected by stellar irradiation. The confirmed relation between
radius and incident flux depends on planetary mass intervals. In this study, we
show that there is a single relation between radius and irradiated energy per
gram per second (), for all mass intervals. There is an extra increase in
radius of planets if is higher than 1100 times energy received by the
Earth (). This is likely due to dissociation of molecules. The tidal
interaction as a heating mechanism is also considered and found that its
maximum effect on the inflation of planets is about 15 per cent. We also
compute age and heavy element abundances from the properties of host stars,
given in the TEPCat catalogue (Southworth 2011). The metallicity given in the
literature is as [Fe/H]. However, the most abundant element is oxygen, and
there is a reverse relation between the observed abundances [Fe/H] and [O/Fe].
Therefore, we first compute [O/H] from [Fe/H] by using observed abundances, and
then find heavy element abundance from [O/H]. We also develop a new method for
age determination. Using the ages we find, we analyse variation of both radius
and mass of the planets with respect to time, and estimate the initial mass of
the planets from the relation we derive for the first time. According to our
results, the highly irradiated gas giants lose 5 per cent of their mass in
every 1 Gyr.Comment: 15 pages, 13 figures, 3 tables. Accepted by MNRA
On the structure of the Sun and alpha Centauri A and B in the light of seismic and non-seismic constraints
The small separation (delta nu 01, delta nu 02 and delta nu 13) between the
oscillations with low degree l is dependent primarily on the sound speed
profile within the stellar core, where nuclear evolution occurs. The detection
of such oscillations for a star offers a very good opportunity to determine the
stage of its nuclear evolution, and hence its age. In this context, we
investigate the Sun and alpha Cen A and B. For alpha Cen A and B, each of the
small separations delta nu 01, delta nu 02 and delta nu 13 gives a different
age. Therefore, in our fitting process, we also employ the second difference,
defined as nu n2 - 2 nu n1 + nu n0, which is 2 delta nu 01- delta nu 02. In
addition to this, we also use frequency ratio (nu n0/ nu n2). For the Sun,
these expressions areequivalent and give an age of about 4.9-5.0 Gyr. For alpha
Cen A and B, however, the small separation and the second difference give very
different ages. This conflict may be solved by the detection of oscillation
frequencies that can be measured much more precisely than the current
frequencies. When we fit the models to the observations, we find (i) Z 0=0.020,
t=3.50 Gyr and M B=1.006 Msun from the small separations delta nu 01, delta nu
02 and delta nu 13 of alpha Cen B; and (ii) a variety of solutions from the
non-seismic constraints and delta nu 02 of alpha Cen A and B, in which the
masses of alpha Cen A and B are slightly modified and the age of the system is
about 5.2-5.3 Gyr. For Z=0.025, the closest masses we find to the observed
masses are M B=0.922 Msun and M A=1.115 Msun.The differences between these
masses and the corresponding observed masses are about 0.01 Msun.Comment: 9 Pages and 9 Figure
Effect of freezing and thawing on strength and permeability of lime-stabilized clays
AbstractIn this study, the effect of freezing and thawing on the strength and permeability of two clayey soils (high and low plasticity), which had been stabilized with lime, were investigated. Before and after stabilization, the permeability and strength of the specimens were determined with various freeze-thaw cycles. Results of this study indicated that for both clays, 6% lime addition increased the hydraulic conductivity of the specimens 1000 times. However, the hydraulic conductivity of clay with 6% lime increased 10–20 times after only 3 freeze-thaw cycles. The results of strength tests exhibited different trends. The strength of stabilized high plasticity clay increased approximately 15 times at the end of 28 day curing, whereas the strength of stabilized low plasticity clay increased about 3 times only. The strength of both stabilized clays decreased 10–15% at the end of the freeze-thaw cycles
The Solar and Centauri A and B models improved by opacity enhancement - a possible explanation for the oversize cool stars
The Sun and Cen A and B are the nearest stars to us. Despite the
general agreement between their models and seismic and non-seismic constraints,
there are serious problems pertaining to their interior. The good agreement
between the sound speed and base radius of the convective zone of the Sun and
the solar models is broken apart by a recent revision in solar chemical
composition. For Cen A and B, however, it is not possible to fit
models with the same age and chemical composition to all seismic and
non-seismic observational constraints. At the age deduced from seismic
constraints, the luminosity ratio () of the models is
significantly lower than the ratio taken from the observed luminosities.
Enhancement of opacity as a function of temperature is one way to restore the
agreement between solar models and the Sun, but such an enhancement does not
alter the situation for Cen A and B. The reason is that models of both
components are influenced in a similar manner and consequently the luminosity
ratio doesn't change much. In the present study, problems pertaining to the
interior of these three stars with a single expression for opacity enhancement
are modelled. The opacity enhancement is expressed as a function of density,
ionization degree of heavy elements (oxygen), and temperature. According to
this expression, for improvement of the models the required opacity enhancement
for Cen A and B at = 6.5, for example, is about 7 and 22 per
cent, respectively. The enhancement tak es place in the region in which
pressure ionization is effective, and is higher for low-mass stars than for
high-mass stars. This result seems to be a possible explanation for the serious
differences between models and observational results of cool stars.Comment: 9 pages, 8 figures, accepted by MNRA
Classifying the embedded young stellar population in Perseus and Taurus & the LOMASS database
Context. The classification of young stellar objects (YSOs) is typically done
using the infrared spectral slope or bolometric temperature, but either can
result in contamination of samples. More accurate methods to determine the
evolutionary stage of YSOs will improve the reliability of statistics for the
embedded YSO population and provide more robust stage lifetimes. Aims. We aim
to separate the truly embedded YSOs from more evolved sources. Methods. Maps of
HCO+ J=4-3 and C18O J=3-2 were observed with HARP on the James Clerk Maxwell
Telescope (JCMT) for a sample of 56 candidate YSOs in Perseus and Taurus in
order to characterize emission from high (column) density gas. These are
supplemented with archival dust continuum maps observed with SCUBA on the JCMT
and Herschel PACS to compare the morphology of the gas and dust in the
protostellar envelopes. The spatial concentration of HCO+ J=4-3 and 850 micron
dust emission are used to classify the embedded nature of YSOs. Results.
Approximately 30% of Class 0+I sources in Perseus and Taurus are not Stage I,
but are likely to be more evolved Stage II pre-main sequence (PMS) stars with
disks. An additional 16% are confused sources with an uncertain evolutionary
stage. Conclusions. Separating classifications by cloud reveals that a high
percentage of the Class 0+I sources in the Perseus star forming region are
truly embedded Stage I sources (71%), while the Taurus cloud hosts a majority
of evolved PMS stars with disks (68%). The concentration factor method is
useful to correct misidentified embedded YSOs, yielding higher accuracy for YSO
population statistics and Stage timescales. Current estimates (0.54 Myr) may
overpredict the Stage I lifetime on the order of 30%, resulting in timescales
of 0.38 Myr for the embedded phase.Comment: 33 pages, 21 figures, 6 tables, Accepted to be published in A&
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