279 research outputs found
Some Statistical Picture of Magnetic CP Stars Evolution
We discuss some statistical results on the evolution of magnetic CP stars in
the framework of the supposition about their binary nature.Comment: 3 pages, 3 figure
Some evolutionary aspects of the binary stellar systems containing neutron star
The obvious lack of the binary stellar systems that contain neutron stars
(NS) is observed at present. Partly it is caused by the fact that it is very
difficult to detect neutron star in a binary system if this relativistic
component does not manifest itself as a radio pulsar. Among 1879 pulsars that
are listed in the ATNF pulsar catalogue, only 141 pulsars are known to be the
companions in binary systems. Only 81 objects having median mass estimation of
more than 0.2 constitute the binary systems with pulsars.
Nevertheless, such systems should be much more numerous and their investigation
is of the great interest because thier structure and evolution can certainly
help in our understanding of many unique properties that are seen in some
stars.Comment: Accepted to published in the Odessa Astronomical Publications, 2012,
vol. 25/1, p.35-3
Neutron Star Mergers Are the Dominant Source of the r-process in the Early Evolution of Dwarf Galaxies
There are many candidate sites of the r-process: core-collapse supernovae
(including rare magnetorotational core-collapse supernovae), neutron star
mergers, and neutron star/black hole mergers. The chemical enrichment of
galaxies---specifically dwarf galaxies---helps distinguish between these
sources based on the continual build-up of r-process elements. This technique
can distinguish between the r-process candidate sites by the clearest
observational difference---how quickly these events occur after the stars are
created. The existence of several nearby dwarf galaxies allows us to measure
robust chemical abundances for galaxies with different star formation
histories. Dwarf galaxies are especially useful because simple chemical
evolution models can be used to determine the sources of r-process material. We
have measured the r-process element barium with Keck/DEIMOS medium-resolution
spectroscopy. We present the largest sample of barium abundances (almost 250
stars) in dwarf galaxies ever assembled. We measure [Ba/Fe] as a function of
[Fe/H] in this sample and compare with existing [alpha/Fe] measurements. We
have found that a large contribution of barium needs to occur at more delayed
timescales than core-collapse supernovae in order to explain our observed
abundances, namely the significantly more positive trend of the r-process
component of [Ba/Fe] vs. [Fe/H] seen for [Fe/H] <~ -1.6 when compared to the
[Mg/Fe] vs. [Fe/H] trend. We conclude that neutron star mergers are the most
likely source of r-process enrichment in dwarf galaxies at early times.Comment: Accepted to ApJ on 2018 October 2
High-resolution abundance analysis of HD 140283
HD 140283 is a reference subgiant that is metal poor and confirmed to be a
very old star. The abundances of this type of old star can constrain the nature
and nucleosynthesis processes that occurred in its (even older) progenitors.
The present study may shed light on nucleosynthesis processes yielding heavy
elements early in the Galaxy. A detailed abundance analysis of a high-quality
spectrum is carried out, with the intent of providing a reference on stellar
lines and abundances of a very old, metal-poor subgiant. We aim to derive
abundances from most available and measurable spectral lines. The analysis is
carried out using high-resolution (R = 81 000) and high signal-to-noise ratio
(800 < S/N/pixel < 3400) spectrum, in the wavelength range 3700 - 10475,
obtained with a seven-hour exposure time, using the ESPaDOnS at the CFHT. The
calculations in LTE were performed with the OSMARCS 1D atmospheric model and
the spectrum synthesis code Turbospectrum, while the analysis in NLTE is based
on the MULTI code. We present LTE abundances for 26 elements, and NLTE
calculations for the species C I, O I, Na I, Mg I, Al I, K I, Ca I, Sr II, and
Ba II lines. The abundance analysis provided an extensive line list suitable
for metal-poor subgiant stars. The results for Li, CNO, alpha-, and iron peak
elements are in good agreement with literature. The newly NLTE Ba abundance,
along with a NLTE Eu correction and a 3D Ba correction from literature, leads
to [Eu/Ba] = +0.59 +/- 0.18. This result confirms a dominant r-process
contribution, possibly together with a very small contribution from the main
s-process, to the neutron-capture elements in HD 140283. Overabundances of the
lighter heavy elements and the high abundances derived for Ba, La, and Ce
favour the operation of the weak r-process in HD 140283.Comment: 34 pages, 27 figure
Reddenings of FGK supergiants and classical Cepheids from spectroscopic data
Accurate and homogeneous atmospheric parameters (Teff, log (g), Vt, [Fe/H])
are derived for 74 FGK non-variable supergiants from high-resolution, high
signal-to-noise ratio, echelle spectra. Extremely high precision for the
inferred effective temperatures (10-40 K) is achieved by using the line-depth
ratio method. The new data are combined with atmospheric values for 164
classical Cepheids, observed at 675 different pulsation phases, taken from our
previously published studies. The derived values are correlated with unreddened
B-V colours compiled from the literature for the investigated stars in order to
obtain an empirical relationship of the form: (B-V)o = 57.984 - 10.3587(log
Teff)^2 + 1.67572(log Teff)^3 - 3.356(log (g)) + 0.0321(Vt) + 0.2615[Fe/H] +
0.8833((log (g))(log Teff)). The expression is used to estimate colour excesses
E(B-V) for individual supergiants and classical Cepheids, with a precision of
+-0.05 mag. for supergiants and Cepheids with n=1-2 spectra, reaching +-0.025
mag. for Cepheids with n>2 spectra, matching uncertainties for the most
sophisticated photometric techniques. The reddening scale is also a close match
to the system of space reddenings for Cepheids. The application range is for
spectral types F0--K0 and luminosity classes I and II.Comment: accepted for publication (MNRAS
Grid of theoretical NLTE equivalent widths of four Ba II lines and barium abundance in cool stars
We present a grid of computed non-local thermodynamic equilibrium (NLTE)
equivalent widths (EW) and NLTE abundance corrections for four Ba II lines:
4554, 5853, 6141, and 6496 A. The grid can be useful in deriving the NLTE
barium abundance in stars having parameters in the following ranges: effective
temperature from 4000 K to 6500 K, surface gravity log g from 0 to 5,
microturbulent velocity 0 km s^-1 to 3 km s^-1, metallicity [Fe/H] from -2 to
+0.5, and [Ba/Fe] from -0.4 to +0.6. The NLTE abundance can be either derived
by EW interpolation (using the observed Ba II line EW) or by using the NLTE
correction applied to a previously determined LTE abundance. Ba II line
equivalent widths and the NLTE corrections were calculated using the updated
MULTI code and the Ba II atomic model that was previously applied to determine
the NLTE barium abundance in different types of stars. The grid is available
on-line through the web, and we find that the grid Ba NLTE corrections are
almost as accurate as direct NLTE profile fitting (to within 0.05-0.08 dex).
For the weakest Ba II line (5853 A) the LTE abundances almost agree with the
NLTE abundances, whereas the other three Ba II lines, 4554, 6141, and 6496 A,
need NLTE corrections even at the highest metallicities tested here. The 4554 A
line is extremely strong and should not be used for abundance analysis above
[Fe/H]= -1. Furthermore, we tested the impact of different model atmospheres
and spectrum synthesis codes and found average differences of 0.06 dex and 0.09
dex, respectively, for all four lines. At these metallicities we find an
average Delta NLTE of +/-0.1 dex for the three useful Ba lines for subsolar
cool dwarfs.Comment: 9 pages 8 figures submitted to A&
Oxygen, -element and iron abundance distributions in the inner part of the Galactic thin disc. II
We have derived the abundances of 36 chemical elements in one Cepheid star,
ASAS 181024--2049.6, located R kpc from the Galactic center.
This star falls within a region of the inner thin disc poorly sampled in
Cepheids. Our spectral analysis shows that iron, magnesium, silicon, calcium
and titanium LTE abundances in that star support the presence of a plateau-like
abundance distribution in the thin disc within 5 kpc of the Galactic center, as
previously suggested by \cite{Maret15}. If confirmed, the flattening of the
abundance gradient within that region could be the result of a decrease in the
star formation rate due to dynamic effects, possibly from the central Galactic
bar.Comment: 5 pages, 3 figure
Sulphur in the Sculptor dwarf spheroidal galaxy - Including NLTE corrections
In Galactic halo stars, sulphur has been shown to behave like other
-elements, but until now, no comprehensive studies have been done on
this element in stars of other galaxies. Here, we use high-resolution ESO
VLT/FLAMES/GIRAFFE spectra to determine sulphur abundances for 85 stars in the
Sculptor dwarf spheroidal galaxy, covering the metallicity range . The abundances are derived from the S~I triplet at
9213, 9228, and 9238~\AA. These lines have been shown to be sensitive to
departure from local thermodynamic equilibrium, i.e. NLTE effects. Therefore,
we present new NLTE corrections for a grid of stellar parameters covering those
of the target stars. The NLTE-corrected sulphur abundances in Sculptor show the
same behaviour as other -elements in that galaxy (such as Mg, Si, and
Ca). At lower metallicities () the abundances are
consistent with a plateau at , similar to what is
observed in the Galactic halo, . With increasing
[Fe/H], the [S/Fe] ratio declines, reaching negative values at
. The sample also shows an increase in [S/Mg] with
[Fe/H], most probably because of enrichment from Type Ia supernovae.Comment: 9 pages, 11 figures, 2 tables + 3 online tables, accepted in A&
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