87 research outputs found
First Asteroseismic Analysis of the Globular Cluster M80: Multiple Populations and Stellar Mass Loss
Asteroseismology provides a new avenue for accurately measuring the masses of
evolved globular cluster (GC) stars through the detection of their solar-like
oscillations. We present the first detections of solar-like oscillations in 47
red giant branch (RGB) and early asymptotic giant branch (EAGB) stars in the
metal-poor GC M80; only the second ever with measured seismic masses. We
investigate two major areas of stellar evolution and GC science; the multiple
populations and stellar mass-loss. We detected a distinct bimodality in the
EAGB mass distribution. We showed that this is likely due to sub-population
membership. If confirmed, it would be the first direct measurement of a mass
difference between sub-populations. A mass difference was not detected between
the sub-populations in our RGB sample. We instead measured an average RGB mass
of 0.782\pm0.009~\msun, which we interpret as the average between the
sub-populations. Differing mass-loss rates on the RGB has been proposed as the
second parameter that could explain the horizontal branch (HB) morphology
variations between GCs. We calculated an integrated RGB mass-loss separately
for each sub-population: 0.12\pm0.02~\msun (SP1) and 0.25\pm0.02~\msun
(SP2). Thus, SP2 stars have greatly enhanced mass-loss on the RGB. Mass-loss is
thought to scale with metallicity, which we confirm by comparing our results to
a higher metallicity GC, M4. We also find that M80 stars have insignificant
mass-loss on the HB. This is different to M4, suggesting that there is a
metallicity and temperature dependence in the HB mass-loss. Finally, our study
shows the robustness of the -independent mass scaling relation in
the low-metallicity (and low-surface gravity) regime.Comment: 20 pages, 11 figure
Principal Component Analysis on Chemical Abundances Spaces
In preparation for the HERMES chemical tagging survey of about a million
Galactic FGK stars, we estimate the number of independent dimensions of the
space defined by the stellar chemical element abundances [X/Fe]. [...] We
explore abundances in several environments, including solar neighbourhood
thin/thick disk stars, halo metal-poor stars, globular clusters, open clusters,
the Large Magellanic Cloud and the Fornax dwarf spheroidal galaxy. [...] We
find that, especially at low metallicity, the production of r-process elements
is likely to be associated with the production of alpha-elements. This may
support the core-collapse supernovae as the r-process site. We also verify the
over-abundances of light s-process elements at low metallicity, and find that
the relative contribution decreases at higher metallicity, which suggests that
this lighter elements primary process may be associated with massive stars.
[...] Our analysis reveals two types of core-collapse supernovae: one produces
mainly alpha-elements, the other produces both alpha-elements and Fe-peak
elements with a large enhancement of heavy Fe-peak elements which may be the
contribution from hypernovae. [...] The extra contribution from low mass AGB
stars at high metallicity compensates the dimension loss due to the
homogenization of the core-collapse supernovae ejecta. [...] the number of
independent dimensions of the [X/Fe]+[Fe/H] chemical space in the solar
neighbourhood for HERMES is about 8 to 9. Comparing fainter galaxies and the
solar neighbourhood, we find that the chemical space for fainter galaxies such
as Fornax and the Large Magellanic Cloud has a higher dimensionality. This is
consistent with the slower star formation history of fainter galaxies. [...]Comment: 28 pages, 25 figures, 3 tables, MNRAS (Accepted for publication- 2011
December 14
High-resolution elemental abundance analysis of the open cluster IC 4756
We present detailed elemental abundances of 12 subgiants in the open cluster
IC 4756 including Na, Al, Mg, Si, Ca, Ti, Cr, Ni, Fe, Zn and Ba. We measure the
cluster to have [Fe/H] = -0.01 +/- 0.10. Most of the measured star-to-star
[X/H] abundance variation is below sigma < 0.03, as expected from a coeval
stellar population preserving natal abundance patterns, supporting the use of
elemental abundances as a probe to reconstruct dispersed clusters. We find
discrepancies between Cr I and Cr II abundances as well as between Ti I and Ti
II abundances, where the ionized abundances are larger by about 0.2 dex. This
follows other such studies which demonstrate the effects of overionization in
cool stars. IC 4756 are supersolar in Mg, Si, Na and Al, but are solar in the
other elements. The fact that IC 4756 is supersolar in some alpha-elements (Mg,
Si) but solar in the others (Ca, Ti) suggests that the production of
alpha-elements is not simply one dimensional and could be exploited for
chemical tagging.Comment: 13 pages, 13 figures, 10 tables, MNRAS (Accepted for publication-
2012 August 25
The GALAH survey: New diffuse interstellar bands found in residuals of 872,000 stellar spectra
We use more than 872,000 mid-to-high resolution (R 20,000) spectra of
stars from the GALAH survey to discern the spectra of diffuse interstellar
bands (DIBs). We use four windows with the wavelength range from 4718 to 4903,
5649 to 5873, 6481 to 6739, and 7590 to 7890 \AA, giving a total coverage of
967 \AA. We produce 400,000 spectra of interstellar medium (ISM)
absorption features and correct them for radial velocities of the DIB clouds.
Ultimately, we combine the 33,115 best ISM spectra into six reddening bins with
a range of . A total
of 183 absorption features in these spectra qualify as DIBs, their fitted model
parameters are summarized in a detailed catalogue. From these, 64 are not
reported in the literature, among these 17 are certain, 14 are probable and 33
are possible. We find that the broad DIBs can be fitted with a multitude of
narrower DIBs. Finally, we create a synthetic DIB spectrum at unit reddening
which should allow us to narrow down the possible carriers of DIBs and explore
the composition of the ISM and ultimately better model dust and star formation
as well as to correct Galactic and extragalactic observations. The majority of
certain DIBs show a significant excess of equivalent width when compared to
reddening. We explain this with observed lines of sight penetrating more
uniform DIB clouds compared to clumpy dust clouds.Comment: 28 pages, 15 figures, 11 tables, accepted for publication in MNRA
Abundances in the Milky Way across Five Nucleosynthetic Channels from 4 Million LAMOST Stars
Large stellar surveys are revealing the chemodynamical structure of the Galaxy across a vast spatial extent. However, the many millions of low-resolution spectra observed to date are yet to be fully exploited. We employ The Cannon, a data-driven approach for estimating chemical abundances, to obtain detailed abundances from low-resolution (R = 1800) LAMOST spectra, using the GALAH survey as our reference. We deliver five (for dwarfs) or six (for giants) estimated abundances representing five different nucleosynthetic channels, for 3.9 million stars, to a precision of 0.05–0.23 dex. Using wide binary pairs, we demonstrate that our abundance estimates provide chemical discriminating power beyond metallicity alone. We show the coverage of our catalog with radial, azimuthal and dynamical abundance maps and examine the neutron capture abundances across the disk and halo, which indicate different origins for the in situ and accreted halo populations. LAMOST has near-complete Gaia coverage and provides an unprecedented perspective on chemistry across the Milky Way
The GALAH survey: A new constraint on cosmological lithium and Galactic lithium evolution from warm dwarf stars
Lithium depletion and enrichment in the cosmos is not yet well understood. To help tighten
constraints on stellar and Galactic evolution models, we present the largest high-resolution
analysis of Li abundances A(Li) to date, with results for over 100 000 GALAH (Galactic Archeology with HERMES) field stars spanning effective temperatures 5900 K Teff 7000 K and
metallicities −3 [Fe/H] +0.5. We separated these stars into two groups, on the warm
and cool sides of the so-called Li dip, a localized region of the Kiel diagram wherein lithium
is severely depleted. We discovered that stars in these two groups show similar trends in the
A(Li)–[Fe/H] plane, but with a roughly constant offset in A(Li) of 0.4 dex, the warm group
having higher Li abundances. At [Fe/H] −0.5, a significant increase in Li abundance with
increasing metallicity is evident in both groups, signalling the onset of significant Galactic
production. At lower metallicity, stars in the cool group sit on the Spite plateau, showing a
reduced lithium of around 0.4 dex relative to the primordial value predicted from big bang
nucleosynthesis (BBN). However, stars in the warm group between [Fe/H] = −1.0 and −0.5
form an elevated plateau that is largely consistent with the BBN prediction. This may indicate
that these stars in fact preserve the primordial Li produced in the early Universe.XDG, KL, AMA, and SB acknowledge funds from the Alexander
von Humboldt Foundation in the framework of the Sofja Kovalevskaja Award endowed by the Federal Ministry of Education and Research. KL also acknowledges funds from the Swedish
Research Council (VR 2015-004153) and Marie Skłodowska
Curie Actions (cofund project INCA 600398), and AMA also
acknowledges support from the Swedish Research Council (VR
2016-03765), and the project grant ‘The New Milky Way’ (KAW
2013.0052) from the Knut and Alice Wallenberg Foundation. TZ
acknowledges financial support of the Slovenian Research Agency
(research core funding no. P1-0188). SLM and JDS acknowledge
the support of the Australian Research Council through Discovery
Project grant DP180101791. Parts of this research were conducted
by the Australian Research Council Centre of Excellence for
All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through
project no. CE170100013. YST is grateful to be supported by
the NASA Hubble Fellowship grant HST-HF2-51425 awarded by
the Space Telescope Science Institute. SWC acknowledges federal
funding from the Australian Research Council through the Future
Fellowship grant entitled ‘Where are the Convective Boundaries in
Stars?’ (FT160100046). GT acknowledges support by the project
grant ‘The New Milky Way’ from the Knut and Alice Wallenberg
Foundation and by the grant 2016-03412 from the Swedish Research
Council
The K2-HERMES Survey: Age and Metallicity of the Thick Disc
Asteroseismology is a promising tool to study Galactic structure and
evolution because it can probe the ages of stars. Earlier attempts comparing
seismic data from the {\it Kepler} satellite with predictions from Galaxy
models found that the models predicted more low-mass stars compared to the
observed distribution of masses. It was unclear if the mismatch was due to
inaccuracies in the Galactic models, or the unknown aspects of the selection
function of the stars. Using new data from the K2 mission, which has a
well-defined selection function, we find that an old metal-poor thick disc, as
used in previous Galactic models, is incompatible with the asteroseismic
information. We show that spectroscopic measurements of [Fe/H] and
[/Fe] elemental abundances from the GALAH survey indicate a mean
metallicity of for the thick disc. Here is the
effective solar-scaled metallicity, which is a function of [Fe/H] and
[/Fe]. With the revised disc metallicities, for the first time, the
theoretically predicted distribution of seismic masses show excellent agreement
with the observed distribution of masses. This provides an indirect
verification of the asteroseismic mass scaling relation is good to within five
percent. Using an importance-sampling framework that takes the selection
function into account, we fit a population synthesis model of the Galaxy to the
observed seismic and spectroscopic data. Assuming the asteroseismic scaling
relations are correct, we estimate the mean age of the thick disc to be about
10 Gyr, in agreement with the traditional idea of an old -enhanced
thick disc.Comment: 21 pages, submitted to MNRA
The K2-HERMES Survey: age and metallicity of the thick disc
Asteroseismology is a promising tool to study Galactic structure and evolution because it can probe the ages of stars. Earlier attempts comparing seismic data from the Kepler satellite with predictions from Galaxy models found that the models predicted more low-mass stars compared to the observed distribution of masses. It was unclear if the mismatch was due to inaccuracies in the Galactic models, or the unknown aspects of the selection function of the stars. Using new data from the K2 mission, which has a well-defined selection function, we find that an old metal-poor thick disc, as used in previous Galactic models, is incompatible with the asteroseismic information. We use an importance-sampling framework, which takes the selection function into account, to fit for the metallicities of a population synthesis model using spectroscopic data. We show that spectroscopic measurements of [Fe/H] and [α/Fe] elemental abundances from the GALAH survey indicate a mean metallicity of log (Z/Z⊙) = −0.16 for the thick disc. Here Z is the effective solar-scaled metallicity, which is a function of [Fe/H] and [α/Fe]. With the revised disc metallicities, for the first time, the theoretically predicted distribution of seismic masses show excellent agreement with the observed distribution of masses. This indirectly verifies that the asteroseismic mass scaling relation is good to within five per cent. Assuming the asteroseismic scaling relations are correct, we estimate the mean age of the thick disc to be about 10 Gyr, in agreement with the traditional idea of an old α-enhanced thick disc.MJH is supported by an ASTRO-3D Fellowship.
SB and KL acknowledge funds from the Alexander von Humboldt
Foundation in the framework of the Sofja Kovalevskaja Award
endowed by the Federal Ministry of Education and Research.
KL acknowledges funds from the Swedish Research Council
(Grant nr. 2015-00415 3) and Marie Sklodowska Curie Actions
(Cofund Project INCA 600398). JK, KC, and TZ acknowledge
financial support from the Slovenian Research Agency (research
core funding No. P1-0188). DMN was supported by the Allan
C. and Dorothy H. Davis Fellowship. JZ acknowledges support
from NASA grants 80NSSC18K0391 and NNX17AJ40
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