15 research outputs found
Signature of a massive rotating metal-poor star imprinted in the Phoenix stellar stream*
The Phoenix stellar stream has a low intrinsic dispersion in velocity and
metallicity that implies the progenitor was probably a low mass globular
cluster. In this work we use Magellan/MIKE high-dispersion spectroscopy of
eight Phoenix stream red giants to confirm this scenario. In particular, we
find negligible intrinsic scatter in metallicity () and a large peak-to-peak range in [Na/Fe] and [Al/Fe]
abundance ratios, consistent with the light element abundance patterns seen in
the most metal-poor globular clusters. However, unlike any other globular
cluster, we also find an intrinsic spread in [Sr II/Fe] spanning 1 dex,
while [Ba II/Fe] shows nearly no intrinsic spread (). This abundance signature is best interpreted as
slow neutron capture element production from a massive fast-rotating metal-poor
star (, ,
). The low inferred cluster mass suggests the system
would have been unable to retain supernovae ejecta, implying that any massive
fast-rotating metal-poor star that enriched the interstellar medium must have
formed and evolved before the globular cluster formed. Neutron capture element
production from asymptotic giant branch stars or magneto-rotational
instabilities in core-collapse supernovae provide poor fits to the
observations. We also report one Phoenix stream star to be a lithium-rich giant
(). At it is among the
most metal-poor lithium-rich giants known.Comment: Accepted to ApJ 2021-07-0
The Southern Stellar Stream Spectroscopic Survey (S5): Chemical Abundances of Seven Stellar Streams
We present high-resolution Magellan/MIKE spectroscopy of 42 red giant stars in seven stellar streams confirmed by the Southern Stellar Stream Spectroscopic Survey (S5): ATLAS, Aliqa Uma, Chenab, Elqui, Indus, Jhelum, and Phoenix. Abundances of 30 elements have been derived from over 10,000 individual line measurements or upper limits using photometric stellar parameters and a standard LTE analysis. This is currently the most extensive set of element abundances for stars in stellar streams. Three streams (ATLAS, Aliqa Uma, and Phoenix) are disrupted metal-poor globular clusters, although only weak evidence is seen for the light-element anticorrelations commonly observed in globular clusters. Four streams (Chenab, Elqui, Indus, and Jhelum) are disrupted dwarf galaxies, and their stars display abundance signatures that suggest progenitors with stellar masses ranging from 106 to 107 M⊙. Extensive description is provided for the analysis methods, including the derivation of a new method for including the effect of stellar parameter correlations on each star's abundance and uncertainty.A.R.C. is supported in part by the
Australian Research Council through a Discovery Early Career
Researcher Award (DE190100656). Parts of this research were
supported by the Australian Research Council Centre of
Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO
3D) through project No. CE170100013. A.B.P. acknowledges
support from NSF grant AST-1813881. S.K. is partially supported
by NSF grants AST-1813881 and AST-1909584 and HeisingSimons foundation grant 2018-1030. S.L.M. and J.D.S. acknowledge support from the Australian Research Council through
Discovery Project grant DP180101791
Diffuse debris entrainment in glacier, lab and model environments
Small quantities of liquid water lining triple junctions in polycrystalline glacier ice form connected vein networks that enable material exchange with underlying basal environments. Diffuse debris concentrations commonly observed in ice marginal regions might be attributed to this mechanism. Following recent cryogenic ring-shear experiments, we observed emplacement along grain boundaries of loess particles several tens of microns in size. Here, we describe an idealized model of vein liquid flow to elucidate conditions favoring such particle transport. Gradients in liquid potential drive flow toward colder temperatures and lower solute concentrations, while deviations of the ice stress state from hydrostatic balance produce additional suction toward anomalously low ice pressures. Our model predicts particle entrainment following both modest warming along the basal interface resulting from anticipated natural changes in effective stress, and the interior relaxation of temperature and solute concentration imposed by our experimental protocols. Comparisons with experimental observations are encouraging, but suggest that liquid flow rates are somewhat higher and/or more effective at dragging larger particles than predicted by our idealized model with nominal parameter choices. Diffuse debris entrainment extending several meters above the glacier bed likely requires a more sophisticated treatment that incorporates effects of ice deformation or other processes
Data for "A power-based abrasion law for use in landscape evolution models"
Experimental data used in the paper: "A power-based abrasion law for use in landscape evolution models," published in Geology.Subglacial abrasion drives erosion for many glaciers, inundating forefields and proglacial marine environments with glaciogenic sediments. Theoretical treatments of this process suggest that bedrock abrasion rates scale linearly with the energy expended through rock-on-rock friction during slip, but this assumption lacks an empirical basis for general implementation. To test this approach, we simulated abrasion by sliding debris-laden ice over rock beds under subglacial conditions in a cryo-ring shear and a direct shear device. Miniscule volumes of erosion that occurred during each run were mapped with a white-light profilometer, and we measured the rock mechanical properties needed to constrain the abrasion energy expenditure. We find that abraded volume per unit area increases linearly with average shear force at the bed and that abrasion rates increase linearly with basal power for plane beds. Lastly, only a small percentage (⪅1%) of the energy partitioned to basal slip is dissipated by abrasion. These results confirm the basal-power abrasion rule is viable to implement in landscape evolution models.This work is supported by NSF 2017185
Kinematics of Antlia 2 and Crater 2 from The Southern Stellar Stream Spectroscopic Survey (S5)
We present new spectroscopic observations of the diffuse Milky Way satellite
galaxies Antlia 2 and Crater 2, taken as part of the Southern Stellar Stream
Spectroscopic Survey (S5). The new observations approximately double the number
of confirmed member stars in each galaxy and more than double the spatial
extent of spectroscopic observations in Antlia 2. A full kinematic analysis,
including Gaia EDR3 proper motions, detects a clear velocity gradient in Antlia
2 and a tentative velocity gradient in Crater 2. The velocity gradient
magnitudes and directions are consistent with particle stream simulations of
tidal disruption. Furthermore, the orbit and kinematics of Antlia 2 require a
model that includes the reflex motion of the Milky Way induced by the Large
Magellanic Cloud. We also find that Antlia 2's metallicity was previously
overestimated, so it lies on the empirical luminosity-metallicity relation and
is likely only now experiencing substantial stellar mass loss. Current
dynamical models of Antlia 2 require it to have lost over 90% of its stars to
tides, in tension with the low stellar mass loss implied by the updated
metallicity. Overall, the new kinematic measurements support a tidal disruption
scenario for the origin of these large and extended dwarf spheroidal galaxies.Comment: 23 pages (+appendix), 11 figures, accepted to ApJ. Updated to
accepted version (1 new figure, minor changes to text) Video of Antlia 2
tidal disruption here: https://www.youtube.com/watch?v=wvt-Q4kRq9
The Southern Stellar Stream Spectroscopic Survey (S5):Chemical Abundances of Seven Stellar Streams
We present high-resolution Magellan/MIKE spectroscopy of 42 red giant stars
in seven stellar streams confirmed by the Southern Stellar Stream Spectroscopic
Survey (S5): ATLAS, Aliqa Uma, Chenab, Elqui, Indus, Jhelum, and Phoenix.
Abundances of 30 elements have been derived from over 10,000 individual line
measurements or upper limits using photometric stellar parameters and a
standard LTE analysis. This is currently the most extensive set of element
abundances for stars in stellar streams. Three streams (ATLAS, Aliqa Uma, and
Phoenix) are disrupted metal-poor globular clusters, although only weak
evidence is seen for the light element anticorrelations commonly observed in
globular clusters. Four streams (Chenab, Elqui, Indus, and Jhelum) are
disrupted dwarf galaxies, and their stars display abundance signatures that
suggest progenitors with stellar masses ranging from .
Extensive description is provided for the analysis methods, including the
derivation of a new method for including the effect of stellar parameter
correlations on each star's abundance and uncertainty.
This paper includes data gathered with the 6.5 meter Magellan Telescopes
located at Las Campanas Observatory, Chile.Comment: Accepted to AJ. 39 pages, 13 figures, 7 table