Atmospheres Of Red Giants Research Articles

Abundance of barium in the atmospheres of red giants in the Galactic globular cluster NGC 104 (47 Tuc)

Context. While most (if not all) Type I Galactic globular clusters (GGCs) are characterised by spreads in the abundances of light chemical elements (e.g. Li, N, O, Na, Mg, Al), it is not yet well established whether similar spreads may exist in s-process elements as well. Aims. We investigated the possible difference in Ba abundance between the primordial (1P) and polluted (2P) stars in the Galactic globular cluster (GGC) 47 Tuc (NGC 104). For this purpose, we obtained homogeneous abundances of Fe, Na, and Ba in a sample of 261 red giant branch (RGB) stars, which comprises the largest sample used for Na and Ba abundance analysis in any GGC so far. Methods. Abundances of Na and Ba were determined using archival GIRAFFE/VLT spectra and 1D non-local thermodynamic equilibrium (NLTE) abundance analysis methodology. Results. Contrary to the finding of Gratton et al. (2013, A&A, 549, A41), we did not detect any significant Ba–Na correlation or 2P–1P Ba abundance difference in the sample of 261 RGB stars in 47 Tuc. This corroborates the result of D’Orazi et al. (2010, ApJ, 719, L213), who found no statistically significant Ba–Na correlation in 110 RGB stars in this GGC. The average barium-to-iron ratio obtained in the sample of 261 RGB stars, ⟨[Ba/Fe]1D NLTE⟩ = −0.01 ± 0.06, agrees well with those determined in Galactic field stars at this metallicity and may therefore represent the abundance of primordial proto-cluster gas that has not been altered during the subsequent chemical evolution of the cluster.

Depletion of Bright Red Giants in the Galactic Center during Its Active Phases

Abstract Observations in the near-infrared domain showed the presence of the flat core of bright late-type stars inside ∼0.5 pc from the Galactic center supermassive black hole (Sgr A*), while young massive OB/Wolf–Rayet stars form a cusp. Several dynamical processes were proposed to explain this apparent paradox of the distribution of the Galactic center stellar populations. Given the mounting evidence on the significantly increased activity of Sgr A* during the past million years, we propose a scenario based on the interaction between the late-type giants and a nuclear jet, whose past existence and energetics can be inferred from the presence of γ-ray Fermi bubbles and bipolar radio bubbles. Extended, loose envelopes of red giant stars can be ablated by the jet with kinetic luminosity in the range of L j ≈ 1041– within the inner ∼0.04 pc of Sgr A* (S-cluster region), which would lead to their infrared luminosity decrease after several thousand jet–star interactions. The ablation of the atmospheres of red giants is complemented by the process of tidal stripping that operates at distances of ≲1 mpc, and by the direct mechanical interaction of stars with a clumpy disk at ≳0.04 pc, which can explain the flat density profile of bright late-type stars inside the inner half parsec from Sgr A*.

Photospheric nitrogen abundances and carbon 12C/13C ratios of red giant stars

Abstract Nitrogen abundances and carbon isotope ratios (12C$/$13C) in the atmospheres of red giants are known to be influenced by dredge-up of H-burning products, and serve as useful probes to study the nature of evolution-induced envelope mixing. We determined the [N/Fe] and 12C$/$13C ratios for 239 late-G/early-K giant stars by applying the spectrum-fitting technique to the 12CN and 13CN lines in the ∼8002–8005 Å region, with the aim of investigating how these quantities are related to other similar mixing-affected indicators which were already reported in our previous work. It was confirmed that [N/Fe] values are generally supersolar (typically by several tenths of a dex, though widely differing from star to star), anti-correlated with [C/Fe], and correlated with [Na/Fe], as expected from theory. As seen from their dependence upon stellar parameters, it appears that mixing tends to be enhanced with an increase of stellar luminosity (or mass) and rotational velocity, which is also reasonable from the theoretical viewpoint. In contrast, the resulting 12C$/$13C ratios turned out to be considerably diversified in the range of ∼5–50 (with a peak around ∼20), without showing any systematic dependence upon C or N abundance anomalies caused by the mixing of CN-cycled material. It thus appears that our understanding of the photospheric 12C$/$13C ratios in red giants is still incomplete, requiring more observational studies.

Abundance of zirconium in the atmospheres of red giants in Galactic globular cluster 47 Tuc

AbstractWe determined zirconium abundance in the atmospheres of 327 red giant branch (RGB) stars in the globular cluster 47 Tuc. The 1D LTE abundances were obtained from the archival VLT GIRAFFE spectra, using 1D hydrostaticATLAS9 stellar model atmospheres and synthetic Zr I line profiles computed with theSYNTHE package. The average zirconium abundance determined in the sample of RGB stars, 〈[Zr/Fe]〉 = +0.38 ± 0.12, agrees well with zirconium abundances obtained at this metallicity in the Galactic field stars, as well as with those observed in other Galactic globular clusters.

Laboratory evidence of slow-cooling for carbon droplets in red-giant atmospheres

Laboratory evidence of slow-cooling for carbon droplets in red-giant atmospheres

Planetary Motion in the Atmospheres of Red Giants

When a star reaches the red giant stage in the course of its evolution, its closest planets are in its atmosphere. Numerical gas dynamic models are constructed for hypersonic flow around planets by matter in the atmosphere of a red giant. The results are compared with analytic models for the motion of planets in the atmospheres of stars.

The prospective search for highly ionized technetium in hot (pre-) white dwarfs

The discovery of technetium (Tc) in the atmospheres of red giants by Merrill (1952, ApJ, 116, 21) constituted convincing proof that s-process nucleosynthesis is indeed occurring in evolved stars. In principle, Tc should still be present in the atmospheres of hot post-AGB stars and (pre-) white dwarfs although, due to radioactive decay, it should be present in decreasing quantities along post-AGB evolution. The recent discovery of a large number of trans-iron group elements in hot white dwarfs with atomic numbers in the range A = 30–56 (Zn to Ba) raises the prospect that Tc ( A = 43) may also be detected. However, this is currently not feasible because no atomic data exist for ionization stages beyond Tc ii. As an initial step, we calculated atomic energy levels and oscillator strengths of Tc iv–vi and used these data to compute non-local thermodynamic equilibrium (NLTE) model atmospheres to estimate at which minimum abundance level Tc could be detected. We show that Tc lines can be found in ultraviolet spectra of hot white dwarfs provided Tc is as abundant as other detected trans-Fe elements. We find that radiative levitation can keep Tc in large, easily detectable quantities in the atmosphere. A direct identification of Tc lines is still not feasible because wavelength positions cannot be computed with necessary precision. Laboratory measurements are necessary to overcome this problem. Our results suggest that such efforts are beneficial to the astrophysical community.

Systematic trend of water vapour absorption in red giant atmospheres revealed by high resolution TEXES 12 μm spectra

The structures of the outer atmospheres of red giants are very complex. The notion of large optically thick molecular spheres around the stars (MOLspheres) has been invoked in order to explain e.g. spectro-interferometric observations. However, high-resolution spectra in the mid-IR do not easily fit into this picture. They rule out any large sphere of water vapour in LTE surrounding red giants. Our aim here is to investigate high-resolution, mid-infrared spectra for a range of red giants, from early-K to mid M. We have recorded 12 microns spectra of 10 well-studied bright red giants, with TEXES on the IRTF. We find that all giants in our study cooler than 4300 K, spanning a range of effective temperatures, show water absorption lines stronger than expected. The strengths of the lines vary smoothly with spectral type. We identify several spectral features in the wavelength region that undoubtedly are formed in the photosphere. From a study of water-line ratios of the stars, we find that the excitation temperatures, in the line-forming regions, are several hundred Kelvin lower than expected from a classical photospheric model. This could either be due to an actually lower temperature structure in the outer regions of the photospheres caused by, for example, extra cooling, or due to non-LTE level populations, affecting the source function and line opacities. We have demonstrated that these diagnostically interesting water lines are a general feature of red giants across spectral types, and we argue for a general explanation of their formation rather than explanations requiring specific properties. Since the water lines are neither weak (filled in by emission) nor appear in emission, as predicted by LTE MOLsphere models in their simplest forms, the evidence for the existence of such large optically-thick, molecular spheres enshrouding the stars is weakened. (abbreviated)

Nine new metal-poor stars on the subgiant and red horizontal branches with high levels of r-process enhancement★

We report the discovery of nine metal-poor stars with high levels of r-process enhancement (+0.81<[Eu/Fe]<+1.13), including six subgiants and three stars on the red horizontal branch. We also analyze four previously-known r-process-enhanced metal-poor red giants. From this sample of 13 stars, we draw the following conclusions. (1) High levels of r-process enhancement are found in a broad range of stellar evolutionary states, reaffirming that this phenomenon is not associated with a chemical peculiarity of red giant atmospheres. (2) Only 1 of 10 stars observed at multiple epochs shows radial velocity variations, reaffirming that stars with high levels of r-process enhancement are not preferentially found among binaries. (3) Only 2 of the 13 stars are highly-enhanced in C and N, indicating that there is no connection between high levels of r-process enhancement and high levels of C and N. (4) The dispersions in [Sr/Ba] and [Sr/Eu] are larger than the dispersions in [Ba/Eu] and [Yb/Eu], suggesting that the elements below the second r-process peak do not always scale with those in the rare earth domain, even within the class of highly-r-process-enhanced stars. (5) The light-element (12<Z<30) abundances of highly-r-process-enhanced stars are indistinguishable from those with normal levels of r-process material at the limit of our data, 3.5 per cent (0.015 dex) on average. The nucleosynthetic sites responsible for the large r-process enhancements did not produce any detectable light-element abundance signatures distinct from normal core-collapse supernovae.

Non-LTE Abundances of Sodium in the Atmospheres of Red Giants of the Thick and Thin Galactic Disks

AbstractWe determined NLTE abundances of Na in the atmospheres of 80 red giants including the 33 stars of the thin disk, 35 stars of the thick disk and 12 stars of Hercules stream.

Barium abundance in red giants of NGC 6752

(Abridged) Aims: We study the effects related to departures from non-local thermodynamic equilibrium (NLTE) and homogeneity in the atmospheres of red giant stars in Galactic globular cluster NGC 6752, to assess their influence on the formation of Ba II lines. Methods: One-dimensional (1D) local thermodynamic equilibrium (LTE) and 1D NLTE barium abundances were derived using classical 1D ATLAS stellar model atmospheres. The three-dimensional (3D) LTE abundances were obtained for 8 red giants on the lower RGB, by adjusting their 1D LTE abundances using 3D-1D abundance corrections, i.e., the differences between the abundances obtained from the same spectral line using the 3D hydrodynamical (CO5BOLD) and classical 1D (LHD) stellar model atmospheres. Results: The mean 1D barium-to-iron abundance ratios derived for 20 giants are <[Ba/Fe]>_{1D NLTE} = 0.05 \pm0.06 (stat.) \pm0.08 (sys.). The 3D-1D abundance correction obtained for 8 giants is small (~+0.05 dex), thus leads to only minor adjustment when applied to the mean 1D NLTE barium-to-iron abundance ratio for the 20 giants, <[Ba/Fe]>_{3D+NLTE} = 0.10 \pm0.06(stat.) \pm0.10(sys.). The intrinsic abundance spread between the individual cluster stars is small and can be explained in terms of uncertainties in the abundance determinations. Conclusions: Deviations from LTE play an important role in the formation of barium lines in the atmospheres of red giants studied here. The role of 3D hydrodynamical effects should not be dismissed either, even if the obtained 3D-1D abundance corrections are small. This result is a consequence of subtle fine-tuning of individual contributions from horizontal temperature fluctuations and differences between the average temperature profiles in the 3D and 1D model atmospheres: owing to the comparable size and opposite sign, their contributions nearly cancel each other.

Chemical composition of the atmospheres of red giants with high space velocities

The results of a comparative analysis of the elemental abundances in the atmospheres of 14 red giants with high Galactic space velocities are presented. For almost all of the chemical elements considered, the their abundance trends with metallicity correspond to those constructed for thick-disk dwarfs. In the case of sodium, the main factor affecting the [Na/Fe] abundance in the stellar atmosphere for red giants is the surface gravity that characterizes the degree of development of the convective envelope. The difference between the [Na/Fe] abundances in the atmospheres of thin-and thick-disk red giants has been confirmed.

Chemical abundance analysis for the atmospheres of red giants in the Hercules moving group

The results of a comparative analysis of the kinematics, ages, and elemental abundances for 17 red giants in the Hercules moving group are presented. Model atmospheres are used to determine the parameters of the stellar atmospheres and the abundances of about 20 elements. The masses and ages of the stars are estimated, and the components of their Galactic velocities and the elements of their Galactic orbits are calculated. Our analysis demonstrates that the Hercules stream is a heterogeneous group of objects from the thin and thick disks.

SpS1-Infrared and submillimetre-wave spectroscopy as probes of stellar evolution

For over a hundred years, optical spectroscopy has been the main tool to study stellar structure and evolution. Photospheric spectra of the electronic transitions of atoms and ions are used to determine the temperature and elemental abundance. Beyond atomic and ionic lines, only the electronic transitions of a few simple molecules (C2, CN, H2O, TiO, CH, etc.) appear in the optical photospheric spectra. With the recent development of infrared and submm spectroscopy, a wide range of molecules have been observed, specially in cool atmospheres of red giants and brown dwarfs. We also realize that beyond the photosphere, a stellar system consists of chromosphere, corona, and stellar wind. Both young and evolved stars possess extensive circumstellar regions and the atoms, molecules, and solid particles in this environment radiate a wide range of lines and bands observable at infrared and submm wavelengths.

CN Abundance Inhomogeneities among Bright Red Giants in the Globular Clusters NGC 288 and NGC 362

ABSTRACTElement inhomogeneities are studied in the second-parameter pair of globular clusters NGC 288 and NGC 362. Spectra obtained with the CTIO 1.5 m telescope of relatively high luminosity red giants (MV < -1.0) in each cluster have been used to measure a λ3883 CN band index denoted S(3839), and two indices centered on CH absorption in the λ4300 G band. Giants in both clusters exhibit a comparable spread in CN band strength, with only a small difference in mean G band absorption between CN-strong and CN-weak giants. By contrast, the S(3839) index anticorrelates with the [O/Fe] abundance measured by Shetrone & Keane. Part of this anticorrelation can be explained by the effect of reduced oxygen on the equilibria of CO and CN molecule formation in red giant atmospheres. However, the difference in S(3839) between CN-weak and CN-strong giants is sufficiently large that a factor of ∼10 enhancement in nitrogen in CN-strong stars also appears to be necessary. Additionally, the 3883 Å CN band strength correlates with [Na/Fe] abundance. These CN-O-Na relationships among the red giants of NGC 362 and NGC 288 are also a characteristic of other globular clusters of the Milky Way. At a given S(3839) and [Na/Fe] abundance, the CN-strong red giants of NGC 288 have a higher oxygen abundance than red giants in NGC 362. As such, NGC 288 and NGC 362 do not appear to follow a correlation between minimum [O/Fe] abundance and the blueward extent of the horizontal branch in the color-magnitude diagram.

Cobalt in stars and the galaxy

An analysis of the abundance of cobalt in atmospheres of red giants, indicates they can be divided into two groups: stars with the normal [Co/Fe] abundance and those with a small [Co/Fe] excess. A comparative analysis of the spectrograms taking into account the effect of superfine splitting provides evidence for a [Co/Fe] excess in some stars. We have also detected physical and kinematical differences between these groups. Stars with a [Co/Fe] excess are related to the thick-disk population of the Galaxy. These stars are older and less massive, display lower metallicities, and have Galactic velocities corresponding to those of thick-disk objects. It is suggested that the observed pattern of a [Co/Fe] excess in the halo and thick disk reflects the chemical composition of the Galaxy at a very early stage of its evolution, when Population III objects existed. The lower abundance excess in the thick disk compared to the halo and the absence of an excess in the thin disk are due to the contributiuon from Type I supernovae at later stages of the Galaxy’s evolution. We have found that the thick disk of the Galaxy displays gradients of its cobalt and iron abundances, possibly providing evidence that the thick disk formed as a result of the collapse of a protogalactic cloud.

Analysis of the Na, Mg, Al, and Si abundances in the atmospheres of red giants of different spectral subgroups

We analyze the Na, Mg, Al, and Si abundances in the atmospheres of more than 40 stars, includingred giants of different spectral subgroups (normal red giants, mild and classical barium stars) and several supergiants. All these elements exhibit abundance excesses, with the overabundance increasing with the star’s luminosity. The dependence of the overabundances for each of these elements on the luminosity (or log g) is the same for all the spectral subgroups, testifying to a common origin: they are all products of hydrogen burning in the NeNa and MgAl cycles that have been dredged up from the stellar interiors to the outer atmospheric layers by convection that gradually develops during the star’s evolution from the main sequence to the red-giant stage. The sodium abundances derived for several stars are lower than for other stars with similar atmospheric parameters. The ages and kinematic characteristics of these two groups of stars suggest that they probably belong to different stellar generations.

3D hydrodynamical model stellar atmospheres of metal-poor red giants

We investigate the main differences between static 1D and 3D time-dependent model stellar atmospheres of red giants at very low metallicities. We focus in particular on the impact of 3D LTE spectral line formation on the derivation of elemental abundances for the extremely metal-poor ([Fe/H] 5.3) red giant HE 0107-5240.

Model atmospheres of red giants

Techniques and results of computations of model atmospheres are discussed for M and C giants and for giants with abundance anomalies. The SAM12 code, a modification of the ATLAS12 code developed by Kurucz, is used. Blanketing effects due to atomic and molecular line absorption are taken into account using the opacity-sampling approach. The computed model atmosphere for the Sun (G2V) is used as a test of the SAM12 code. The model red-giant atmospheres are compared with models reported in other studies. Comparisons between the computed and observed spectral energy distributions are given for the C giant WX Cyg (C-J6) and Sakurai's object (V4334 Sgr).

A comparative analysis of chemical abundances in the atmospheres of red giants of different age groups

We analyze previously published chemical abundances in the atmospheres of red giants. Excess abundances are observed not only for Na, but also for Al and Si, with the overabundances increasing with the stars’ luminosity. The observed anomalies provide evidence that, in addition to the CNO hydrogen-burning cycle, the Mg-Al and Ne-Na cycles operate in the interiors of main-sequence stars; their products are brought to the stellar atmospheres by convection after the transition to the red-giant phase. The abundance anomalies for s-process elements, also observed in the atmospheres of field stars, testify to the presence of a substantial number of neutrons. The s-process abundance anomalies are absent from giants of the young Hyades cluster.