Abstract
We present the first spectroscopic observations of the very metal-poor Milky Way globular cluster ESO280-SC06. Using spectra acquired with the 2dF/AAOmega spectrograph on the ...Anglo-Australian Telescope, we have identified 13 members of the cluster, and estimate from their infrared calcium triplet lines that the cluster has a metallicity of $\textrm{Fe}/\textrm{H}=-2.48^{+0.06 }_{ -0.11}$. This would make it the most metal-poor globular cluster known in the Milky Way. This result was verified with comparisons to three other metal-poor globular clusters that had been observed and analysed in the same manner. We also present new photometry of the cluster from EFOSC2 and SkyMapper and confirm that the cluster is located 22.9 ± 2.1 kpc from the Sun and 15.2 ± 2.1 kpc from the Galactic Centre, and has a radial velocity of $92.5^{+2.4 }_{ -1.6}$ km s−1. These new data finds the cluster to have a radius about half that previously estimated, and we find that the cluster has a dynamical mass of the cluster of (12 ± 2) × 103 M⊙. Unfortunately, we lack reliable proper motions to fully characterize its orbit about the Galaxy. Intriguingly, the photometry suggests that the cluster lacks a well-populated horizontal branch, something that has not been observed in a cluster so ancient or metal poor.
ABSTRACT
We report the first radial velocity measurements of the recently identified globular cluster FSR1758. From the two member stars with radial velocities from the Gaia Radial Velocity ...Spectrograph reported in Gaia DR2, we find FSR1758 has a radial velocity of 227 ± 1$\, \textrm{km}\, \textrm{s}^{-1}$. We also find potential extra-tidal star lost from the cluster in the surrounding 1 deg. Combined with Gaia proper motions and photometric distance estimates, this shows that FSR1758 is on a relatively retrograde, radial orbit with a pericentre of $3.8_{-0.9}^{+0.9}$ kpc, an apocentre of $16_{-5}^{+8}$ kpc, and eccentricity of $0.62_{-0.04}^{+0.05}$. Although it is currently at a Galactocentric distance of $3.8_{-0.9}^{+0.9}$ kpc – at the edge of the bulge – it is an intruder from the halo. We investigate whether a reported ‘halo’ of stars around FSR1758 is related to the cluster, and find that most of these stars are likely foreground dwarf stars. We conclude that FSR1758 is not a dwarf galaxy, but rather a globular cluster.
ABSTRACT
Metal-poor stars are important tools for tracing the early history of the Milky Way, and for learning about the first generations of stars. Simulations suggest that the oldest metal-poor ...stars are to be found in the inner Galaxy. Typical bulge surveys, however, lack low metallicity ($\rm {Fe/H} \lt -1.0$) stars because the inner Galaxy is predominantly metal-rich. The aim of the Pristine Inner Galaxy Survey (PIGS) is to study the metal-poor and very metal-poor (VMP, $\rm {Fe/H} \lt -2.0$) stars in this region. In PIGS, metal-poor targets for spectroscopic follow-up are selected from metallicity-sensitive CaHK photometry from the CFHT. This work presents the ∼250 deg2 photometric survey as well as intermediate-resolution spectroscopic follow-up observations for ∼8000 stars using AAOmega on the AAT. The spectra are analysed using two independent tools: ULySS with an empirical spectral library, and FERRE with a library of synthetic spectra. The comparison between the two methods enables a robust determination of the stellar parameters and their uncertainties. We present a sample of 1300 VMP stars – the largest sample of VMP stars in the inner Galaxy to date. Additionally, our spectroscopic data set includes ∼1700 horizontal branch stars, which are useful metal-poor standard candles. We furthermore show that PIGS photometry selects VMP stars with unprecedented efficiency: 86 per cent/80 per cent (lower/higher extinction) of the best candidates satisfy $\rm {Fe/H} \lt -2.0$, as do 80 per cent/63 per cent of a larger, less strictly selected sample. We discuss future applications of this unique data set that will further our understanding of the chemical and dynamical evolution of the innermost regions of our Galaxy.
ABSTRACT
Streams of stars from captured dwarf galaxies and dissolved globular clusters are identifiable through the similarity of their orbital parameters, a fact that remains true long after the ...streams have dispersed spatially. We calculate the integrals of motion for 31 234 stars, to a distance of 4 kpc from the Sun, which have full and accurate 6D phase space positions in the Gaia DR2 catalogue. We then apply a novel combination of data mining, numerical, and statistical techniques to search for stellar streams. This process returns five high confidence streams (including one which was previously undiscovered), all of which display tight clustering in the integral of motion space. Colour–magnitude diagrams indicate that these streams are relatively simple, old, metal-poor populations. One of these resolved streams shares very similar kinematics and metallicity characteristics with the Gaia-Enceladus dwarf galaxy remnant, but with a slightly younger age. The success of this project demonstrates the usefulness of data mining techniques in exploring large data sets.
ABSTRACT
In order to accurately determine stellar properties, knowledge of the effective temperature of stars is vital. We implement Gaia and 2MASS photometry in the InfraRed Flux Method and apply it ...to over 360 000 stars across different evolutionary stages in the GALAH DR3 survey. We derive colour-effective temperature relations that take into account the effect of metallicity and surface gravity over the range $4000\, \rm {K}\lesssim T_{\rm {eff}}\lesssim 8000\, \rm {K}$, from very metal-poor stars to supersolar metallicities. The internal uncertainty of these calibrations is of order 40–80 K depending on the colour combination used. Comparison against solar-twins, Gaia benchmark stars, and the latest interferometric measurements validates the precision and accuracy of these calibrations from F to early M spectral types. We assess the impact of various sources of uncertainties, including the assumed extinction law, and provide guidelines to use our relations. Robust solar colours are also derived.
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).
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.
Abstract
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 (
S
5
). 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.
ABSTRACT
Gaia DR2 has revealed new small-scale and large-scale patterns in the phase-space distribution of stars in the Milky Way. In cylindrical Galactic coordinates $(R,\phi ,z)$, ridge-like ...structures can be seen in the $(R,V_\phi)$ plane and asymmetric arch-like structures in the $(V_R,V_\phi)$ plane. We show that the ridges are also clearly present when the third dimension of the $(R,V_\phi)$ plane is represented by $\langle z \rangle$, $\langle V_z \rangle$, $\langle V_R \rangle$, $\langle$Fe/H$\rangle$, and $\langle \alpha /{\rm Fe}\rangle$. The maps suggest that stars along the ridges lie preferentially close to the Galactic mid-plane ($|z|\lt 0.2$ kpc), and have metallicity and $\alpha$ elemental abundance similar to that of the Sun. We show that phase mixing of disrupting spiral arms can generate both the ridges and the arches. It also generates discrete groupings in orbital energy – the ridges and arches are simply surfaces of constant energy. We identify eight distinct ridges in the Gaia DR2 data: six of them have constant energy while two have constant angular momentum. Given that the signature is strongest for stars close to the plane, the presence of ridges in $\langle z \rangle$ and $\langle V_z \rangle$ suggests a coupling between planar and vertical directions. We demonstrate, using N-body simulations that such coupling can be generated both in isolated discs and in discs perturbed by an orbiting satellite like the Sagittarius dwarf galaxy.
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.
PDF
