Stars, dwarf

Dwarf stars are relatively small stars. By this definition, our Sun is a dwarf star. Dwarfs can be up to 20 times larger than our Sun and up to 20,000 times brighter. 

The evolution of a. star after it leaves the red-giant phase depends to some extent on its mass. If it is not more than about 1.4 M it may contract appreciably again and then enter an oscillatory phase of its life before becoming a white dwarf (p. 7). When core contraction following helium and carbon depletion raises the temperature above I0 K the y-ray.s in the stellar assembly become sufficiently energetic to promote the (endothermic) reaction Ne(y,a) 0. The a-paiticle released can penetrate the coulomb barrier of other neon nuclei to form " Mg in a strongly exothermic reaction ... 

Soon after the discovery of the first extra-solar planets, it has been noticed that planet-host stars these were particularly metal-rich when compared with single field dwarfs [12], i.e., on average they present a metal-content that is above the one found in stars now known to have any planetary-mass companion. This result, clearly confirmed by an uniform spectroscopic analysis of large samples of stars with and without detected giant planets [22] is obtained by using different... 

Abstract. [La/Eu] and [Ba/Eu] for a sample of Barium stars were determined in order to evaluate the ratio of abundances of s- and r-elements. The results have been compared to previous work dealing with normal red giants and dwarfs with metallicities in the range -3 < [Fe/H] < +0.3. 

Barium stars were recognized as a distinct group of peculiar stars by [1], The objects initially included in this group were red giants of spectral type G and K, which showed strong lines of s-process elements, particularly Ba II and Sr II, as well as enhanced CH, CN and C2 bands. The discovery that HR 107, a dwarf star, shows a composition similar to that of a mild Barium giant by [6] has pushed the search for new Barium dwarfs. 

Fig. 1. Oxygen abundances as a function of the activity index, Rx, derived from X-ray data (left-hand panels) and the excitation temperature Texc (right-hand panels). The bottom panels show the difference between [O/Fe] yielded by the OI triplet at about 7774 A and the [OI] A6300 line. Filled circles RS CVn binaries ([2] and [3]), filled squares field subgiants [3], filled triangles Pleiades stars, open triangles Hyades stars, open circles, squares and hexagons disk dwarfs. The source of the literature data for the open cluster and Galactic disk stars can be found in [4].

The results are summarized in the Table. Our metallicity value for NGC 3680, though still subsolar, is slightly higher than the values given in [1] and [13], [Fe/H]=-0.14 and -0.17 respectively. We need to be observe more stars in this cluster before claiming firm conclusions. The scantiness of the sample is due to the fact that most of the G dwarfs escaped the cluster during its lifetime. 

Abstract. In an effort to determine accurate stellar parameters and abundances for a large sample of nearby stars, we have performed the detailed analysis of 350 high-resolution spectra of FGK dwarfs and giants. This sample will be used to investigate behavior of chemical elements and kinematics in the thick and thin disks, in order to better constrain models of chemical and dynamical evolution of the Galaxy. 

Abstract. The most recently discovered Galactic component - thick disk - still needs high-resolution spectral investigations since its origin and evolution is not understood enough. Elemental abundance ratios in the metallicity range —0.68 < [Fe/H] < —0.10 were determined in a sample of 10 thick-disk dwarfs and compared with results of other stars investigated as well as with models of thin disk chemical evolution. 

Fig. 2. Left, middle and right panel run of the [C/Fe], [N/Fe] and carbon isotopic ratios with absolute magnitude. Filled triangles, circles and squares are dwarfs/subgiants in NGC 6397, NGC 6752 and 47 Tuc, respectively ([7]). Open symbols are RGB stars in the same clusters, from a number of literature studies (see [7] for references). Crosses are the field stars from [4].

To test if dilution of the products of CNO burning may explain the difference in abundance pattern with evolved giants and a possible excess in 12 C visible in N-rich stars (see left panel of Fig. 4), we use simple models in the plane [C/N] vs [O/N] (right panel of Fig. 4). Starting from the approximate composition of N-poor stars, the trend for different fractions of gas processed in the complete CNO-cycle (solid line) reproduces fairly well the data, albeit it predict too low C abundances for N-rich dwarfs. Pollution from RGB stars with composition N-rich from very deep mixing (complete CNO and Na enrichment involved, dotted line) reproduces also rather well the data, apart for N-rich dwarfs. On the other hand, the N-poor case, typical of the chemical composition of field RGB stars, is a very poor match (dashed line). Moreover, in this case, the model would predict C-poor, Na-poor stars, whereas no one is observed among over 40 dwarfs/subgiants in the 3 clusters. 

A weak but useful carbon line [Cl] 8727.13 A disappears in halo dwarfs with metallicities below —1. To measure carbon abundance in halo stars one can use four Cl high excitation lines near 9100 A and the CH band at 4300 A. The Cl lines at 9100 A together with the OI triplet at 7771 A have been used by Tomkin et al. (1992) and Akerman et al. (2004) to study the behaviour of C/O versus metallicity. However, Cl and OI lines employed in these papers are sensitive to a non-LTE effects and one has to bare in mind that this sensitivity is different for C and O. The CH band at 3145 A used by Israelian et al. (1999) is almost saturated in disk stars and several blends makes the abundance analysis less accurate. To ensure a homogeneous analysis of the C/O and N/O ratio from NH,CH and OH lines in the near-UV, we used the same model atmospheres and tools as in our previous studies. The oxygen abundances were compiled from Israelian et al. (1998, 2001) and Boesgaard et al. (1999). 

Various spectral features can be used to derive the nitrogen abundance in dwarfs. Unfortunately weak high excitation (x=10.34 eV) near-infrared NI lines at 7468.31, 8216.34, 8683.4, 8703.25 and 8718.83 A disappear at metallicities [Fe/H] < -1 and for the analysis of N in metal-poor stars we are left with the CN and NH molecular bands at 3883 and 3360 A, respectively. It must be mentioned... 

Since most (if not all) low-metallicity objects that are currently observed in the halo are not in the AGB phase, material enriched in carbon and the s-process elements is assumed to have accreted from the companion AGB stars, which have already evolved to faint white dwarfs, to the surface of the surviving companion. This scenario is the same as that applied to classical CH stars [4], Unfortunately, long-term radial velocity monitoring has been obtained for only a limited number of objects a clear binarity signature has been established for six objects in our sample to date. However, there exists additional support for the mass-accretion scenario for the Ba-rich CEMP stars. Fig. lb shows [C/H] as a function of luminosity roughly estimated from the effective temperature... 

Abstract. We examine outstanding issues in the analysis and interpretation of the halo Li plateau. We show that the majority of very Li-poor halo Li-plateau stars (5 out of 8) have high projected rotation velocities usim between 4.7 and 10.4 km s 1. Such stars have very different evolutionary histories to Li-normal plateau stars, and hence cannot be included in studies of Li depletion by normal halo dwarfs. Uncertainties in the effective temperature scale for metal-poor stars continue to challenge the analysis of Li. 

Two completely different scenarios attempt to explain the presence of large Li abundances among the RGB stars. One is the result of an external contamination (pollution) produced by the engulfing of near giant planets or brown dwarfs companions. The second one is the result of an internal action known as the Cameron-Fowler 7Be mechanism. Here, we will make a brief discussion of both. 

Abstract. This aims to be an overview of what detailed observations of individual stars in nearby dwarf galaxies may teach us about galaxy evolution. This includes some early results from the DART (Dwarf Abundances and Radial velocity Team) Large Programme at ESO. This project has used 2.2m/WFI and VLT/FLAMES to obtain spectra of large samples of individual stars in nearby dwarf spheroidal galaxies and determine accurate abundances and kinematics. These results can be used to trace the formation and evolution of nearby galaxies from the earliest times to the present. 

The DART large programme at ESO made v ei and [Fe/H] measurements from FLAMES spectroscopy of 401 red giant branch (RGB) stars in the Sculptor (Scl) dSph [6]. The relatively high signal/noise, S/N ( 10-20 per pixel) resulted in both accurate metallicities ( 0.1 dex from internal errors) and radial velocities ( 2 km/s). This is the first time that a large sample of accurate velocities and metallicities have been measured in a dwarf galaxy. 

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