Abstract
It is unknown whether or not low-mass stars can form at low metallicity. While theoretical simulations of Population III (Pop III) star formation show that protostellar disks can fragment, ...it is impossible for those simulations to discern if those fragments survive as low-mass stars. We report the discovery of a low-mass star on a circular orbit with orbital period
P
= 34.757 ± 0.010 days in the ultra metal-poor (UMP) single-lined spectroscopic binary system 2MASS J18082002–5104378. The secondary star 2MASS J18082002–5104378 B has a mass
M
2
=
0.14
−
0.01
+
0.06
M
⊙
, placing it near the hydrogen-burning limit for its composition. The 2MASS J18082002–5104378 system is on a thin disk orbit as well, making it the most metal-poor thin disk star system by a considerable margin. The discovery of 2MASS J18082002–5104378 B confirms the existence of low-mass UMP stars and its short orbital period shows that fragmentation in metal-poor protostellar disks can lead to the formation and survival of low-mass stars. We use scaling relations for the typical fragment mass and migration time along with published models of protostellar disks around both UMP and primordial composition stars to explore the formation of low-mass Pop III stars via disk fragmentation. We find evidence that the survival of low-mass secondaries around solar-mass UMP primaries implies the survival of solar-mass secondaries around Pop III primaries with masses
10
M
⊙
≲
M
*
≲
100
M
⊙
. If true, this inference suggests that solar-mass Pop III stars formed via disk fragmentation could survive to the present day.
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. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.
Globular clusters are some of the oldest bound stellar structures observed in the Universe
. They are ubiquitous in large galaxies and are believed to trace intense star-formation events and the ...hierarchical build-up of structure
. Observations of globular clusters in the Milky Way, and a wide variety of other galaxies, have found evidence for a 'metallicity floor', whereby no globular clusters are found with chemical (metal) abundances below approximately 0.3 to 0.4 per cent of that of the Sun
. The existence of this metallicity floor may reflect a minimum mass and a maximum redshift for surviving globular clusters to form-both critical components for understanding the build-up of mass in the Universe
. Here we report measurements from the Southern Stellar Streams Spectroscopic Survey of the spatially thin, dynamically cold Phoenix stellar stream in the halo of the Milky Way. The properties of the Phoenix stream are consistent with it being the tidally disrupted remains of a globular cluster. However, its metal abundance (Fe/H = -2.7) is substantially below the empirical metallicity floor. The Phoenix stream thus represents the debris of the most metal-poor globular clusters discovered so far, and its progenitor is distinct from the present-day globular cluster population in the local Universe. Its existence implies that globular clusters below the metallicity floor have probably existed, but were destroyed during Galactic evolution.
We present the first detailed chemical abundance study of the ultra-faint dwarf galaxy Tucana II, based on high-resolution Magellan/MIKE spectra of four red giant stars. The metallicities of these ...stars range from Fe/H = -3.2 to -2.6, and all stars are low in neutron-capture abundances (Sr/Fe and Ba/Fe < -1). However, a number of anomalous chemical signatures are present. One star is relatively metal-rich (Fe/H = -2.6) and shows Na, alpha, Sc/Fe < 0, suggesting an extended star formation history with contributions from AGB stars and SNe Ia. Two stars with Fe/H < -3 are mildly carbon-enhanced (C/Fe ~ 0.7) and may be consistent with enrichment by faint supernovae, if such supernovae can produce neutron-capture elements. A fourth star with Fe/H = -3 is carbon-normal, and exhibits distinct light element abundance ratios from the carbon-enhanced stars. This carbon-normal star implies that at least two distinct nucleosynthesis sources, both possibly associated with Population III stars, contributed to the early chemical enrichment of this galaxy. Despite its very low luminosity, Tucana II shows a diversity of chemical signatures that preclude it from being a simple "one-shot" first galaxy yet still provide a window into star and galaxy formation in the early universe.
Abstract
We measure the variation of the escape speed of the Milky Way across a range of ∼40 kpc in Galactocentric radius. The local escape speed is found to be $521^{+46}_{-30}{\,{\rm km\,s}^{-1}}$, ...in good agreement with other studies. We find that this has already fallen to $379^{+34}_{-28}{\,{\rm km\,s}^{-1}}$ at a radius of 50 kpc. Through measuring the escape speed and its variation, we obtain constraints on the Galactic mass profile and rotation curve. The gradient in the escape speed suggests that the total mass contained within 50 kpc is $30^{+7}_{-5}\times 10^{10}{\,\mathrm{M}_{\odot }}$, implying a relatively light dark halo for the Milky Way. The local circular speed is found to be $v_{\rm c}(R_0) = 223^{+40}_{-34}{\,{\rm km\,s}^{-1}}$ and falls with radius as a power law with index −0.19 ± 0.05. Our method represents a novel way of estimating the mass of the Galaxy, and has very different systematics to more commonly used models of tracers, which are more sensitive to the central parts of the halo velocity distributions. Using our inference on the escape speed, we then investigate the orbits of high-speed Milky Way dwarf galaxies. For each considered dwarf, we predict small pericentre radii and large orbital eccentricities. This naturally explains the large observed ellipticities of two of the dwarfs, which are likely to have been heavily disrupted at pericentre.
ABSTRACT There exists an inordinate amount of spectral data in both public and private astronomical archives that remain severely under-utilized. The lack of reliable open-source tools for analyzing ...large volumes of spectra contributes to this situation, which is poised to worsen as large surveys successively release orders of magnitude more spectra. In this article I introduce sick, the spectroscopic inference crank, a flexible and fast Bayesian tool for inferring astrophysical parameters from spectra. sick is agnostic to the wavelength coverage, resolving power, or general data format, allowing any user to easily construct a generative model for their data, regardless of its source. sick can be used to provide a nearest-neighbor estimate of model parameters, a numerically optimized point estimate, or full Markov Chain Monte Carlo sampling of the posterior probability distributions. This generality empowers any astronomer to capitalize on the plethora of published synthetic and observed spectra, and make precise inferences for a host of astrophysical (and nuisance) quantities. Model intensities can be reliably approximated from existing grids of synthetic or observed spectra using linear multi-dimensional interpolation, or a Cannon-based model. Additional phenomena that transform the data (e.g., redshift, rotational broadening, continuum, spectral resolution) are incorporated as free parameters and can be marginalized away. Outlier pixels (e.g., cosmic rays or poorly modeled regimes) can be treated with a Gaussian mixture model, and a noise model is included to account for systematically underestimated variance. Combining these phenomena into a scalar-justified, quantitative model permits precise inferences with credible uncertainties on noisy data. I describe the common model features, the implementation details, and the default behavior, which is balanced to be suitable for most astronomical applications. Using a forward model on low-resolution, high signal-to-noise ratio spectra of M67 stars reveals atomic diffusion processes on the order of 0.05 dex, previously only measurable with differential analysis techniques in high-resolution spectra. sick is easy to use, well-tested, and freely available online through GitHub under the MIT license.
Abstract
The Transiting Exoplanet Survey Satellite (TESS) will provide high-precision time series photometry for millions of stars with at least a half-hour cadence. Of particular interest are the ...circular regions of 12° radius centred around the ecliptic poles that will be observed continuously for a full year. Spectroscopic stellar parameters are desirable to characterize and select suitable targets for TESS, whether they are focused on exploring exoplanets, stellar astrophysics or Galactic archaeology. Here, we present spectroscopic stellar parameters (Teff, log g, Fe/H, v sin i, vmicro) for about 16 000 dwarf and subgiant stars in TESS’ southern continuous viewing zone. For almost all the stars, we also present Bayesian estimates of stellar properties including distance, extinction, mass, radius and age using theoretical isochrones. Stellar surface gravity and radius are made available for an additional set of roughly 8500 red giants. All our target stars are in the range 10 < V < 13.1. Among them, we identify and list 227 stars belonging to the Large Magellanic Cloud. The data were taken using the High Efficiency and Resolution Multi-Element Spectrograph (HERMES; R ∼ 28 000) at the Anglo–Australian Telescope as part of the TESS–HERMES survey. Comparing our results with the TESS Input Catalogue (TIC) shows that the TIC is generally efficient in separating dwarfs and giants, but it has flagged more than 100 cool dwarfs (Teff < 4800 K) as giants, which ought to be high-priority targets for the exoplanet search. The catalogue can be accessed via http://www.physics.usyd.edu.au/tess-hermes/, or at Mikulski Archive for Space Telescopes (MAST).
Abstract
The chemical abundances of a galaxy’s metal-poor stellar population can be used to investigate the earliest stages of its formation and chemical evolution. The Magellanic Clouds are the most ...massive of the Milky Way’s satellite galaxies and are thought to have evolved in isolation until their recent accretion by the Milky Way. Unlike the Milky Way’s less massive satellites, little is known about the Magellanic Clouds’ metal-poor stars. We have used the mid-infrared metal-poor star selection of Schlaufman & Casey and archival data to target nine LMC and four SMC giants for high-resolution Magellan/MIKE spectroscopy. These nine LMC giants with −2.4 ≲ Fe/H ≲ −1.5 and four SMC giants with −2.6 ≲ Fe/H ≲ −2.0 are the most metal-poor stars in the Magellanic Clouds yet subject to a comprehensive abundance analysis. While we find that at constant metallicity these stars are similar to Milky Way stars in their
α
, light, and iron-peak elemental abundances, both the LMC and SMC are enhanced relative to the Milky Way in the
r
-process element europium. These abundance offsets are highly significant, equivalent to 3.9
σ
for the LMC, 2.7
σ
for the SMC, and 5.0
σ
for the complete Magellanic Cloud sample. We propose that the
r
-process enhancement of the Magellanic Clouds’ metal-poor stellar population is a result of the Magellanic Clouds’ isolated chemical evolution and long history of accretion from the cosmic web combined with
r
-process nucleosynthesis on a timescale longer than the core-collapse supernova timescale but shorter than or comparable to the thermonuclear (i.e., Type Ia) supernova timescale.
The chemical abundances of large samples of extremely metal-poor (EMP) stars can be used to investigate metal-free stellar populations, supernovae, and nucleosynthesis as well as the formation and ...galactic chemical evolution of the Milky Way and its progenitor halos. However, current progress on the study of EMP stars is being limited by their faint apparent magnitudes. The acquisition of high signal-to-noise spectra for faint EMP stars requires a major telescope time commitment, making the construction of large samples of EMP star abundances prohibitively expensive. We have developed a new, efficient selection that uses only public, all-sky APASS optical, 2MASS near-infrared, and WISE mid-infrared photometry to identify bright metal-poor star candidates through their lack of molecular absorption near 4.6 microns. We have used our selection to identify 11,916 metal-poor star candidates with V < 14, increasing the number of publicly available candidates by more than a factor of five in this magnitude range. Their bright apparent magnitudes have greatly eased high-resolution follow-up observations that have identified seven previously unknown stars with Fe/H lap -3.0. Our follow-up campaign has revealed that 3.8 super(+1.3) sub(-1.1)% of our candidates have Fe/H lap -3.0 and 32.5 super(+3.0) sub(-2.9)% have -3.0 lap Fe/H lap -2.0. The bulge is the most likely location of any existing Galactic Population III stars, and an infrared-only variant of our selection is well suited to the identification of metal-poor stars in the bulge. Indeed, two of our confirmed metal-poor stars with Fe/H lap -2.7 are within about 2 kpc of the Galactic center. They are among the most metal-poor stars known in the bulge.
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