The <Emphasis Type="Italic">r</Emphasis>- and <Emphasis Type="Italic">s</Emphasis>-process contributions to heavy-element abundances in the halo star HD 29907

The abundances of 22 heavy elements from Sr to Pb have been determined for the halo star HD 29907 (T _eff = 5500 K, log g = 4.64) with [Fe/H] = −1.55 using high-quality VLT/UVES spectra (ESO, Chile). The star has a moderate enhancement of r-process elements (Eu-Tm) with [r/Fe] = 0.63. In the range from Ba to Yb, the derived abundance pattern agrees well with those for strongly r-process enhanced stars (r-II stars with [Eu/Fe] > 1 and [Ba/Eu] < 0), such as CS 22892-052 and CS 31082-001, as well as with the scaled solar r-process curve and the r-process model HEW. Thus, Ba-Yb in HD 29907 originate in the r-process. Just as other moderately r-process enhanced stars studied in the literature, HD 29907 exhibits higher Sr, Y, and Zr abundances than those for r-II stars. These results confirm the assumption by other authors about the existence of an additional Sr-Zr synthesis mechanism in the early Galaxy before the onset of nucleosynthesis in asymptotic giant branch (AGB) stars. The same mechanism can be responsible for the enhancement of Mo-Ag in the star being investigated compared to r-II stars. There are no grounds to suggest the presence of s-nuclei of lead in the material of the star being investigated, because its measured abundance ratio log ɛ(Pb/Eu) = 1.20 lies within the range for the comparison stars: from log ɛ(Pb/Eu) = 0.17 (CS 31082-001) to < 1.55 (HE 1219-0312). Thus, even if there was a contribution of AGB stars to the heavy-element enrichment of the interstellar medium at the epoch with [Fe/H] = −1.55, it was small, at the level of the abundance error.     

The Yields of r-Process Elements and Chemical Evolution of the Galaxy

The supernova yields of r-process elements are obtained as a function of the mass of their progenitor stars from the abundance patterns of extremely metal-poor stars on the left-side [{Ba/Mg}]--[{Mg/H}] boundary with a procedure proposed by Tsujimoto and Shigeyama. The ejected masses of r-process elements associated with stars of progenitor mass M _ms ≤ 18 M _⊙ are infertile sources and the SNe II with 20 M _⊙ ≤ M _ms ≤ 40 M _⊙ are the dominant source of r-process nucleosynthesis in the Galaxy. The ratio of these stars 20 M _⊙ ≤ M _ms ≤ 40 M _⊙ with compared to the all massive stars is about∼ 18%. In this paper, we present a simple model that describes a star's [r/Fe] in terms of the nucleosynthesis yields of r-process elements and the number of SN II explosions. Combined the r-process yields obtained by our procedure with the scatter model of the Galactic halo, the observed abundance patterns of the metal-poor stars can be well reproduced.     

Two R-Process Components in Ultra-Metal-Poor Stars: The Neutron-Capture Element Distribution of CS 22892-052

The abundance patterns of neutron-capture elements in very metal-poor halo stars play a crucial role in guiding and constraining theoretical models of nucleosynthesis. Many studies have suggested that the abundance patterns of the heavier (Z≥ 56) stable neutron-capture elements in very metal-poor halo stars are consistent with the solar system r-process abundance distribution, but this concordance breaks down for the lighter neutron-capture elements in the range of 40<Z<56. Some studies argue that there are two separate r-processes respectively responsible for the productions of the heavier and lighter neutron-capture elements. The new observed data of the lighter n-capture elements in the 40<Z<56 domain (Nb, Ru, Rh, Pd, Ag and Cd) in CS 22892-052 makes it available to examine whether or not there are two different r-processes. Based upon these observed abundances of n-capture elements in ultra metal-poor star CS22892-052, we present a phenomenological model to identify the characters of the different nucleosynthesis processes in very metal-poor stars. The results show that the model predictions can well match the observations in CS 22892-052, which truly means that there are different r-processes for the lighter and heavier neutron-capture elements, and the stellarr-process patterns are similar to the solar system r-process abundance distribution. This revised version was published online in July 2006 with corrections to the Cover Date.     

Li production in the big bang

Li abundance is determined for 23 halo subdwarfs. About half of the stars show [Fe/H] < −1.4 and a space velocityV > 160 km s⁻¹ Li appears to be present in all our halo stars, with an abundance within about ± 0.2 dex of the value logn (Li) = 2.0 found by Spite & Spite (1982). Thus our results provide confirmation of the main conclusion of Spite & Spite.     

The site for r-process nucleosynthesis

Following our hypothesis that each supernova (SN) event triggers star formation in the swept-up gas, so that newly formed stars inherit the elemental abundance pattern of individual SNe, we deduce the production sites and yields for r-process elements. We further show that a strong evidence for the origin of r-process nucleosynthesis products was just there in our backyard - supernova SN1987A -, and conclude that 20 M _⊙ SNe are the predominant production sites for r-process elements. This revised version was published online in September 2006 with corrections to the Cover Date.     

Interstellar Grains in Primitive Meteorites: Diamond,Silicon Carbide,and Graphite

Abstract— Primitive meteorites contain a few parts per million (ppm) of pristine interstellar grains that provide information on nuclear and chemical processes in stars. Their interstellar origin is proven by highly anomalous isotopic ratios, varying more than 1000-fold for elements such as C and N. Most grains isolated thus far are stable only under highly reducing conditions (C/O > 1), and apparently are “stardust” formed in stellar atmospheres. Microdiamonds, of median size ～ 10 Å, are most abundant (～ 400–1800 ppm) but least understood. They contain anomalous noble gases including Xe-HL, which shows the signature of the r- and p-processes and thus apparently is derived from supernovae. Silicon carbide, of grain size 0.2–10 μm and abundance ～ 6 ppm, shows the signature of the s-process and apparently comes mainly from red giant carbon (AGB) stars of 1–3 solar masses. Some grains appear to be ≥10⁹ a older than the Solar System. Graphite spherules, of grain size 0.8–7 μm and abundance <2 ppm, contain highly anomalous C and noble gases, as well as large amounts of fossil ²⁶Mg from the decay of extinct ²⁶Al. They seem to come from at least three sources, probably AGB stars, novae, and Wolf-Rayet stars.     

Chemical evolution of the galaxy: Abundances of the light elements (sodium to calcium)

The abundances of the light (Na to Ca) elements in disc and halo stars are reviewed. New analyses are emphasized. Elements considered are the α-nuclei (Mg, Si, and Ca), and the odd-even nuclei (Na and Al, also²⁵Mg and²⁶Mg). The α-nuclei are overabundant (relative to Fe) in the old disc and halo stars. Halo stars ([Fe/H] < —1.2) have [α/Fe] ∼0.3 with extreme halo ([Fe/H] ≲ −2.0) stars showing possibly higher overabundances. The scatter in [α/Fe] at a given [Fe/H] is small. To within the observational errors, the abundance patterns for Mg, Si, and Ca are identical. For disc stars, the Na and Al abundances relative to Mg are almost independent of the [Fe/H]. Halo stars ([Fe/H] < −1) show [Na/Mg] < 0 and [AI/Mg] < 0, but the form of the mean relation and the scatter about the relation between [odd-even/Mg] and [Fe/H] remains uncertain.     

Evolution of the distribution of neutron exposures in the Galaxy disc: An analytical model

In this work, based on the analytical model with delayed production approximation developed by Pagel & Tautvaišienė (1995) for the Galaxy, the analytic solutions of the distribution of neutron exposures of the Galaxy (hereafter NEG) are obtained. The present results appear to reasonably reproduce the distribution of neutron exposures of the solar system (hereafter NES). The strong component and the main component of the NES are built up in different epochs. Firstly, the strong component is produced by the s-process nucleosynthesis in the metal-poor AGB stars, starting from [Fe/H] ≈ −1.16 to [Fe/H] ≈ −0.66, corresponding to the time interval 1.06 < t < 2.6 Gyr. Secondly, the main component is produced by the s-process in the galactic disk AGB stars, starting from [Fe/H] ≈ −0.66 to [Fe/H] ≈ 0, corresponding to the time interval t > 2.6 Gyr. The analytic solutions have the advantage of an understanding of the structure and the properties of the NEG. The NEG is believed to be an effective tool to study the s-process element abundance distributions in the Galaxy at different epochs and the galactic chemical evolution of the neutron-capture elements.     

The R Coronae Borealis stars — A few mere facts

This review presents a selection of recent highlights of observations of R Coronae Borealis variables. Emphasis is placed on an abundance analysis of a complete sample (18 stars) of the warm galactic RCBs. It is shown that 14 of the 18 have very similar compositions: the iron mass fraction ranges about a factor of 3 around the solar value (assuming C/He = 3%) but abundance ratios X/Fe for elements from Na to Ba show little variation. By contrast, the other 4 stars are deficient in iron but not in Na, Si, S and some other elements. With for example, [Si/Fe] ≃ 2, the quartet is indeed ‘peculiar’. One of the quartet, V854 Cen shows depletions of elements (other than CNO) similar to the depletions seen in interstellar medium corresponding to average logn(H _tot) = − 1.5. Scenarios for creating RCB from normal single and double stars are summarised. Invited review talk presented at the Asia-Pacific regional IAU meeting held at Pune from 16–20 August 1993.     

Oxygen Abundances in Solar-Type Stars

The absolute solar oxygen abundance,ε ⊙ = 8.80± 0.06, has been determined from various oxygen abundance indicators in different solar atlases, and a new method is proposed to test collision rate coefficientsfor the NLTE model of OI.Using effective temperatures derived from Balmer lines, oxygen abundances from O triplets in 83 solar-type stars within the solar neighborhood spanning a metallicity range of [Fe/H] = −2.3 ... +0.4 have been determined.NLTE effects are not negligible, especially in warm stars(T_eff ≥ 5800) with [Fe/H] ≥ −0.5. This revised version was published online in July 2006 with corrections to the Cover Date.     

Model atmospheres of carbon giants with high carbon abundance

We calculated 240 model atmospheres of carbon giants with high carbon abundance (2800 K ≤ T _eff ≤ 3400 K, 0.06 ≤ log(C/O) ≤ 2.7). This set of models was used for modelling of the energy distribution in the spectrum of an evolved carbon star DY Per demonstrating photometric features of the R CrB type stars. Most of the models are metal-poor (−3.5 ≤ [Fe/H] < 0) and a portion of them was calculated with the hydrogen deficiency (1/9 ≤ H/He < 9/1). The calculations of the models were carried out taking into account specific for carbon stars sources of opacity in the frame of the classic approaches. The opacity sampling method was used to calculate the opacity due to the atomic and molecular line absorption.     

The Os-Pt-Hg abundance peak in Ap stars and the problem of very heavy cosmic rays

Relative abundances in the region 74Z83 (W to Bi) are determined for 73 Dra, HR 4072, and some other Ap stars. Abundance peaks occur at atomic massesA=191±2 on 73 Dra, atA=201±3 on HR 4072, atA=199±5 on other main group Ap stars, and atA=201±2 on Mn stars. Pb has a relatively low abundance on Ap stars and also in cosmic rays which have an abundance peak atA=193±3. The abundance peaks on main group Ap stars are due to the cyclicr-process which occurred in explosions of former companion stars. Fission products of transuranic elements are recycled by further rapid neutron captures. At the end of ther-process, the high neutron flux decreases gradually so that the final -decays take place in a neutron-rich environment; superheavy elements (Z110) formed in ther-process may be partly destroyed by neutron-induced fission. The pulsar remnants of the explosions accelerater-process elements to cosmic-ray energies. The peak atA 201 on Mn stars is discussed briefly.     

The spectral investigation of seven HII regions in Kazarian galaxies

Spectra from SDSS DR5 are used for a spectral study of seven HII regions in Kazarian galaxies. The abundances of helium and heavy elements, and also the quantity of ionizing stars and the star formation rate in these galaxies, are determined. The oxygen abundance, 12+log(O/H) lies between 7.94 and 8.35. The mean abundance ratios log(S/O), log(Ar/O), and log(Ne/O) are equal to −1.63, −2.37, and −0.78, respectively. For these HII regions, log(N/O) lies between −0.63 and −1.37. On an N/O-O/H diagram they occupy the same area as high excitation HII regions. In all likelihood the ages of the HII regions studied here exceed the 100–300 million years, required for the enrichment in nitrogen by intermediate-mass stars.. The star formation rate is one order as in the HII regions of spiral and irregular galaxies and ranges from 0.05 ÷ 0.81 M_⊙. year⁻¹. __________ Translated from Astrofizika, Vol. 51, No. 1, pp. 75–87 (February 2008).     

Distribution of stars perpendicular to the plane of the galaxy

We present here rigorous analytical solutions for the Boltzmann-Poisson equation concerning the distribution of stars above the galactic plane. The number density of stars is considered to follow a behaviour n(m,0) ∼H(m - m₀)m^−x, wherem is the mass of a star andx an arbitrary exponent greater than 2 and also the velocity dispersion of the stars is assumed to behave as < v²(m)> ∼ m^−θ the exponent θ being arbitrary and positive. It is shown that an analytic expression can be found for the gravitational field K_z, in terms of confluent hypergeometric functions, the limiting trends being K_z∼z for z →0, while K_z → constant for z → infinity. We also study the behaviour of < |z(m)|²>,i.e. the dispersion of the distance from the galactic disc for the stars of massm. It is seen that the quantity < |z(m)|²>∼ m^t-θ, for m→ t, while it departs significantly from this harmonic oscillator behaviour for stars of lighter masses. It is suggested that observation of < |z(m)|²> can be used as a probe to findx and hence obtain information about the mass spectrum.     

Cataclysmic variables from the USNO-B1.0 catalog: Stars with outbursts on infrared palomar plates

Cataclysmic variables with outlying infrared magnitudes have been searched for in the USNO-B1.0 catalog. The sample was limited to objects in the northern hemisphere with (B1−R1) < 1 and (R2−I) > 3.5. The search method is described, and individual stars are considered in detail. A total of 27 variable objects have been found; 20 of them are previously known ones and 7 are new discoveries. Four of the newly found stars are cataclysmic variables, most likely dwarf novae, while the remaining three objects are pulsating red Mira or semiregular variables, including the heavily reddened star in the Pelican Nebula. The pulsation periods have been determined for two stars discovered previously by other researchers. It has been confirmed that one suspected dwarf nova is actually such a star. Prospects for the proposed search technique within the framework of the Virtual Observatory are discussed.     

Deficiencies in the classical model of s-process nucleosynthesis

Abstract— The classical model of s-process nucleosynthesis, based on the concept of a steady neutron flux under astrophysical conditions pertaining to the He-burning phase of red giant stars, has successfully described observed isotopic abundances and provided information on the physical conditions of the s-process environment. Because most of the isotopes on the s-process path are stable, their relevant nuclear parameters can be measured in the laboratory so that as more accurate elemental abundance and neutron capture cross-section data have become available, the classical model has been tested under increasingly stringent conditions. Accurate determinations of the neutron capture cross sections at appropriate astrophysical conditions for the Ba isotopes have shown that the abundance of the s-only isotope ¹³⁶Ba is under-produced by ～20% according to the classical model. This paper describes the accurate assessment of the meteoritic abundance of Ba by the stable isotope dilution mass spectrometric technique, based on the Cl carbonaceous chondrites Orgueil and Ivuna. Repeated analyses of these two Cl chondrites give an abundance that is identical to the presently accepted solar system value for Ba within experimental errors, which indicates a deficiency in the classical model. When combined with similar data for the s-only nuclides ¹¹⁶Sn and ¹⁴²Nd, it is apparent that the classical model, having served a valuable function for many years, must be replaced by stellar models that more accurately reflect the dynamic nature of the He-burning phase in red giant stars, in particular, during the thermal pulses of low-mass asymptotic giant branch (AGB) stars.     

Evolution of the titanium and oxygen abundances from observations of FGK dwarfs in a wide metallicity range

For a sample of dwarf stars close to the Sun with well-known atmospheric parameters and an iron abundance in the range ?2.6 < [Fe/H] < 0.2, we have determined the titanium and oxygen abundances by taking into account the departures from LTE. The dependence of the [O/Fe] and [Ti/Fe] abundance ratios on [Fe/H] has been refined in comparison with the published data. We have established that [O/Fe] increases from ?0.2 to 0.6 as the metallicity [Fe/H] decreases from 0.2 to ?0.8 and remains constant at a lower metallicity. A similar behavior has been found for [Ti/Fe], but the plateau is formed by stars with [Fe/H] > ?0.7, and the titanium overabundance relative to iron is 0.3. The results confirm that not only oxygen but also titanium are synthesized in the α-process. Our data can be used to test the Galactic chemical evolution models.     

Two new LBV candidates in the M 33 galaxy

We present two new luminous blue variable (LBV) candidate stars discovered in the M33 galaxy. We identified these stars as massive star candidates at the final stages of evolution, presumably with a notable interstellar extinction. The candidates were selected from the Massey et al. catalog based on the following criteria: emission in H _α , V<18^./m 5 and 0_.^m 35 < (B - V) < 1_.^m 2. The spectra of both stars reveal a broad and strong H _α emission with extended wings (770 and 1000 kms⁻¹). Based on the spectra we estimated the main parameters of the stars. Object N45901 has a bolometric luminosity log(L/L_⊙) = 6.0–6.2 with the value of interstellar extinction A _V = 2.3 ± 0.1. The temperature of the star’s photosphere is estimated as T⋆ ∼ 13000–15000 K, its probable mass on the Zero Age Main Sequence is M∼ 60–80 M_⊙. The infrared excess in N 45901 corresponds to the emission of warm dust with the temperature Twarm ∼ 1000 K, and amounts to 0.1%of the bolometric luminosity. A comparison of stellar magnitude estimates from different catalogs points to the probable variability of the object N45901. Bolometric luminosity of the second object, N125093, is log(L/L_⊙) = 6.3 − 6.6, the value of interstellar extinction is A _V = 2.75 ± 0.15. We estimate its photosphere’s temperature as T⋆∼ 13000–16000K, the initial mass as M ∼ 90–120 M_⊙. The infrared excess in N125093 amounts to 5–6% of the bolometric luminosity. Its spectral energy distribution reveals two thermal components with the temperatures T_warm ∼ 1000K and T_cold ∼ 480 K. The [Ca II] λλ7291, 7323 lines, observed in LBV-like stars Var A and N93351 in M33 are also present in the spectrum of N 125093. These lines indicate relatively recent gas eruptions and dust activity linked with them. High bolometric luminosity of these stars and broad H α emissions allow classifying the studied objects as LBV candidates.     

Trigonometric parallaxes of 29 stars with large proper motions

While implementing the first stage of the Pulkovo program of research on stars with large proper motions, we determined the trigonometric parallaxes of 29 stars (12 ^m < V < 16 ^m ) based on CCD observations with a 26-inch refractor. The mean standard error was 3.7 mas. Comparison of the Pulkovo parallaxes with those obtained at the Observatory of the Yale University and the US Naval Observatory (USNO) has shown that the parallax differences (Pulkovo-Yale/USNO) lie within the limits of their measurement errors in an overwhelming majority of cases. On average, they are −0.6 ± 1.0 mas. No systematic dependences on stellar distance, magnitude, and color in this set of differences have been found. Our comparisons show that the observing and data reduction techniques used in the Pulkovo program of research on fast stars allow highly accurate trigonometric parallaxes of these objects to be obtained. All program stars are within 50 pc of the Sun; most of them belong to the immediate solar neighborhood (D < 25 pc). For two stars (J0522+3814 and J1202+3636), the trigonometric parallaxes have been determined for the first time.     

Oxygen abundance in local disk and bulge: chemical evolution with a strictly universal IMF

This paper has two parts: one about observational constraints related to the empirical differential oxygen abundance distribution (EDOD), and the other about inhomogeneous models of chemical evolution, in particular the theoretical differential oxygen abundance distribution (TDOD). In the first part, the EDOD is deduced from subsamples related to two different samples involving (i) N=532 solar neighbourhood (SN) stars within the range, −1.5<[Fe/H]<0.5, for which the oxygen abundance has been determined both in presence and in absence of the local thermodynamical equilibrium (LTE) approximation (Ramirez et al. in Astron. Astrophys. 465:271, 2007); and (ii) N=64 SN thick disk, SN thin disk, and bulge K-giant stars within the range, −1.7<[Fe/H]<0.5, for which the oxygen abundance has been determined (Melendez et al. in Astron. Astrophys. 484:L21, 2008). A comparison is made with previous results implying use of [O/H]–[Fe/H] empirical relations (Caimmi in Astron. Nachr. 322:241, 2001b; New Astron. 12:289, 2007) related to (iii) 372 SN halo subdwarfs (Ryan and Norris in Astron. J. 101:1865, 1991); and (iv) 268 K-giant bulge stars (Sadler et al. in Astron. J. 112:171, 1996). The EDOD of the SN thick + thin disk is determined by weighting the mass, for assumed SN thick to thin disk mass ratio within the range, 0.1–0.9. In the second part, inhomogeneous models of chemical evolution for the SN thick disk, the SN thin disk, the SN thick + thin disk, the SN halo, and the bulge, are computed assuming the instantaneous recycling approximation. The EDOD data are fitted, to an acceptable extent, by their TDOD counterparts with the exception of the thin or thick + thin disk, where two additional restrictions are needed: (i) still undetected, low-oxygen abundance thin disk stars exist, and (ii) a single oxygen overabundant star is removed from a thin disk subsample. In any case, the (assumed power-law) stellar initial mass function (IMF) is universal but gas can be inhibited from, or enhanced in, forming stars at different rates with respect to a selected reference case. Models involving a strictly universal IMF (i.e. gas neither inhibited from, nor enhanced in, forming stars with respect to a selected reference case) can also reproduce the data to an acceptable extent. Our main conclusions are (1) different models are necessary to fit the (incomplete) halo sample, which is consistent with the idea of two distinct halo components: an inner, flattened halo in slow prograde rotation, and an outer, spherical halo in net retrograde rotation (Carollo et al. in Nature 450:1020, 2007); (2) the oxygen enrichment within the inner SN halo, the SN thick disk, and the bulge, was similar and coeval within the same metallicity range, as inferred from observations (Prochaska et al. in Astron. J. 120:2513, 2000); (3) the fit to thin disk data implies an oxygen abundance range similar to its thick disk counterpart, with the extension of conclusion (2) to the thin disk, and the evolution of the thick + thin disk as a whole (Haywood in Mon. Not. R. Astron. Soc. 388:1175, 2008) cannot be excluded; (4) leaving outside the outer halo, a fit to the data related to different environments is provided by models with a strictly universal IMF but different probabilities of a region being active, which implies different global efficiencies of the star formation rate; (5) a special case of stellar migration across the disk can be described by models with enhanced star formation, where a fraction of currently observed SN stars were born in situ and a comparable fraction is due to the net effect of stellar migration, according to recent results based on high-resolution N-body + smooth particle hydrodynamics simulations (Roškar et al. in Astrophys. J. Lett. 684:L79, 2008).