31 research outputs found
Atomic Diffusion and Mixing in Old Stars V: A deeper look into the Globular Cluster NGC 6752
Abundance trends in heavier elements with evolutionary phase have been shown
to exist in the globular cluster NGC 6752 [Fe/H]=-1.6. These trends are a
result of atomic diffusion and additional (non-convective) mixing. Studying
such trends can provide us with important constraints on the extent to which
diffusion modifies the internal structure and surface abundances of solar-type,
metal-poor stars. Taking advantage of a larger data sample, we investigate the
reality and the size of these abundance trends and address questions and
potential biases associated with the various stellar populations that make up
NGC6752. Based on uvby Str\"omgren photometry, we are able to separate three
stellar populations in NGC 6752 along the evolutionary sequence from the base
of the red giant branch down to the turnoff point. We find weak systematic
abundance trends with evolutionary phase for Ca, Ti, and Fe which are best
explained by stellar-structure models including atomic diffusion with efficient
additional mixing. We derive a new value for the initial lithium abundance of
NGC 6752 after correcting for the effect of atomic diffusion and additional
mixing which falls slightly below the predicted standard BBN value. We find
three stellar populations by combining photometric and spectroscopic data of
194 stars in the globular cluster NGC 6752. Abundance trends for groups of
elements, differently affected by atomic diffusion and additional mixing, are
identified. Although the statistical significance of the individual trends is
weak, they all support the notion that atomic diffusion is operational along
the evolutionary sequence of NGC 6752.Comment: 15 pages, 11 figures, 2 online table
First evidence of multiple populations along the AGB from Str\"omgren photometry
Spectroscopic studies have demonstrated that nearly all Galactic globular
clusters (GCs) harbour multiple stellar populations with different chemical
compositions. Moreover, colour-magnitude diagrams based exclusively on
Str\"omgrem photometry have allowed us to identify and characterise multiple
populations along the RGB of a large number of clusters. In this paper we show
for the first time that Str\"omgren photometry is also very effcient at
identifying multiple populations along the AGB, and demonstrate that the AGB of
M3, M92, NGC362, NGC1851, and NGC6752 are not consistent with a single stellar
population. We also provide a catalogue of RGB and AGB stars photometrically
identified in these clusters for further spectroscopic follow-up studies.We
combined photometry and elemental abundances from the literature for RGB and
AGB stars in NGC6752 where the presence of multiple populations along the AGB
has been widely debated. We find that, while the MS, SGB, and RGB host three
stellar populations with different helium and light element abundances, only
two populations of AGB stars are present in the cluster. These results are
consistent with standard evolutionary theory.Comment: 9 pages, 3 figures, 1 table in the main article, 3 tables in the
appendix of which 2 tables containing coordinates and photometry of
photometrically identified RGB and AGB star
Atomic diffusion and mixing in old stars IV: Weak abundance trends in the globular cluster NGC 6752
Atomic diffusion in stars can create systematic trends of surface abundances
with evolutionary stage. Globular clusters offer useful laboratories to put
observational constraints on this theory as one needs to compare abundances in
unevolved and evolved stars, all drawn from the same stellar population. In
this paper, we show the results of an abundance study of stars in the globular
cluster NGC6752 which shows weak but systematic abundances trends with
evolutionary phase for Fe, Sc, Ti and Ca. The trends are best explained by a
stellar structure model including atomic diffusion with efficient additional
mixing. The model allows to correct for sub-primordial stellar lithium
abundances of the stars on the Spite plateau, and to match it to the
WMAP-calibrated Big-Bang nucleosynthesis predictions to within the mutual
1-sigma errors.Comment: 15 pages, 4 figures and 8 table
Atomic diffusion and mixing in old stars VI: The lithium content of M30
The prediction of the PLANCK-constrained primordial lithium abundance in the
Universe is in discordance with the observed Li abundances in warm Population
II dwarf and subgiant stars. Among the physically best motivated ideas, it has
been suggested that this discrepancy can be alleviated if the stars observed
today had undergone photospheric depletion of lithium. The cause of this
depletion is investigated by accurately tracing the behaviour of the lithium
abundances as a function of effective temperature. Globular clusters are ideal
laboratories for such an abundance analysis as the relative stellar parameters
of their stars can be precisely determined. We performed a homogeneous chemical
abundance analysis of 144 stars in the metal-poor globular cluster M30, ranging
from the cluster turnoff point to the tip of the red giant branch. NLTE
abundances for Li, Ca, and Fe were derived where possible. Stellar parameters
were derived by matching isochrones to the observed V vs V-I colour-magnitude
diagram. Independent effective temperatures were obtained from automated
profile fitting of the Balmer lines and by applying colour-T_eff calibrations
to the broadband photometry. Li abundances of the turnoff and early subgiant
stars form a thin plateau that is broken off abruptly in the middle of the SGB
as a result of the onset of Li dilution caused by the first dredge-up.
Abundance trends with effective temperature for Fe and Ca are observed and
compared to predictions from stellar structure models including atomic
diffusion and ad hoc additional mixing below the surface convection zone. The
comparison shows that the stars in M30 are affected by atomic diffusion and
additional mixing. After applying a conservative correction for atomic
diffusion, we find an initial Li abundance of A(Li) = for the
globular cluster M30.Comment: 13 pages, 7 tables (Tab. 1 and 5 can be obtained at ADS) and 8
figure
The Lyman alpha reference sample. VII. Spatially resolved H kinematics
We present integral field spectroscopic observations with the Potsdam Multi
Aperture Spectrophotometer of all 14 galaxies in the Lyman Alpha
Reference Sample (LARS). We produce 2D line of sight velocity maps and velocity
dispersion maps from the Balmer (H) emission in our data
cubes. These maps trace the spectral and spatial properties of the LARS
galaxies' intrinsic Ly radiation field. We show our kinematic maps
spatially registered onto the Hubble Space Telescope H and Lyman
(Ly) images. Only for individual galaxies a causal connection
between spatially resolved H kinematics and Ly photometry can
be conjectured. However, no general trend can be established for the whole
sample. Furthermore, we compute non-parametric global kinematical statistics --
intrinsic velocity dispersion , shearing velocity ,
and the ratio -- from our kinematic maps. In
general LARS galaxies are characterised by high intrinsic velocity dispersions
(54\,km\,s median) and low shearing velocities (65\,km\,s
median). values range from 0.5 to 3.2 with an
average of 1.5. Noteworthy, five galaxies of the sample are dispersion
dominated systems with and are thus
kinematically similar to turbulent star forming galaxies seen at high redshift.
When linking our kinematical statistics to the global LARS Ly
properties, we find that dispersion dominated systems show higher Ly
equivalent widths and higher Ly escape fractions than systems with
. Our result indicates that turbulence in
actively star-forming systems is causally connected to interstellar medium
conditions that favour an escape of Ly radiation.Comment: 26 pages, 15 figures, accepted for publication in A&
Exploring the Chemical Evolution of Globular Clusters and their Stars : Observational Constraints on Atomic Diffusion and Cluster Pollution in NGC 6752 and M4
Through the cosmic matter cycle, the chemical evolution of the Milky Way is imprinted in the elemental abundance patterns of late-type stars (spectral types F to K). Due to their long lifetimes ( 1 Hubble time), these stars are of particular importance when it comes to studying the build-up of elements during the early times of our Galaxy. The chemical composition of the atmospheric layers of such stars is believed to resemble the gas from which they were formed. However, recent observations in globular clusters seem to contradict this assumption. The observations indicate that processes are at work that alter the surface compositions in these stars. The combined effect of processes responsible for an exchange of material between the stellar interior and atmosphere during the main sequence lifetime of the star, is referred to as atomic diffusion. Yet, the extent to which these processes alter surface abundances is still debated. By comparing abundances in unevolved and evolved stars all drawn from the same stellar population, any surface abundance anomalies can be traced. The anomalies, if found, can be compared to theoretical predictions from stellar structure models including atomic diffusion. Globular clusters provide stellar populations suitable to conduct such a comparison. In this thesis, the results of three independent analyses of two globular clusters, NGC 6752 and M4, at different metallicities are presented. The comparison between observations and models yields constraints on the models and finally a better understanding of the physical processes at work inside stars
Exploring the Chemical Evolution of Globular Clusters and their Stars : Observational Constraints on Atomic Diffusion and Cluster Pollution in NGC 6752 and M4
Through the cosmic matter cycle, the chemical evolution of the Milky Way is imprinted in the elemental abundance patterns of late-type stars (spectral types F to K). Due to their long lifetimes ( 1 Hubble time), these stars are of particular importance when it comes to studying the build-up of elements during the early times of our Galaxy. The chemical composition of the atmospheric layers of such stars is believed to resemble the gas from which they were formed. However, recent observations in globular clusters seem to contradict this assumption. The observations indicate that processes are at work that alter the surface compositions in these stars. The combined effect of processes responsible for an exchange of material between the stellar interior and atmosphere during the main sequence lifetime of the star, is referred to as atomic diffusion. Yet, the extent to which these processes alter surface abundances is still debated. By comparing abundances in unevolved and evolved stars all drawn from the same stellar population, any surface abundance anomalies can be traced. The anomalies, if found, can be compared to theoretical predictions from stellar structure models including atomic diffusion. Globular clusters provide stellar populations suitable to conduct such a comparison. In this thesis, the results of three independent analyses of two globular clusters, NGC 6752 and M4, at different metallicities are presented. The comparison between observations and models yields constraints on the models and finally a better understanding of the physical processes at work inside stars