arXiv subjects

Rosemary F. G. Wyse

Publications and source records attributed to Rosemary F. G. Wyse.

At least 55 records · Page 3Linked to original sources

arXiv2018

The R-Process Alliance: First Release from the Northern Search for r-Process Enhanced Metal-Poor Stars in the Galactic Halo

This paper presents the detailed abundances and r-process classifications of 126 newly identified metal-poor stars as part of an ongoing collaboration, the R-Process Alliance. The stars were identified as metal-poor candidates from the RAdial Velocity Experiment (RAVE) and were followed-up at high spectral resolution (R~31,500) with the 3.5~m telescope at Apache Point Observatory. The atmospheric parameters were determined spectroscopically from Fe I lines, taking into account <3D> non-LTE corrections and using differential abundances with respect to a set of standards. Of the 126 new stars, 124 have [Fe/H]<-1.5, 105 have [Fe/H]<-2.0, and 4 have [Fe/H]<-3.0. Nine new carbon-enhanced metal-poor stars have been discovered, 3 of which are enhanced in r-process elements. Abundances of neutron-capture elements reveal 60 new r-I stars (with +0.3<=[Eu/Fe]<=+1.0 and [Ba/Eu]<0) and 4 new r-II stars (with [Eu/Fe]>+1.0). Nineteen stars are found to exhibit a `limited-r' signature ([Sr/Ba]>+0.5, [Ba/Eu]<0). For the r-II stars, the second- and third-peak main r-process patterns are consistent with the r-process signature in other metal-poor stars and the Sun. The abundances of the light, alpha, and Fe-peak elements match those of typical Milky Way halo stars, except for one r-I star which has high Na and low Mg, characteristic of globular cluster stars. Parallaxes and proper motions from the second Gaia data release yield UVW space velocities for these stars which are consistent with membership in the Milky Way halo. Intriguingly, all r-II and the majority of r-I stars have retrograde orbits, which may indicate an accretion origin.

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arXiv2018

The escape speed curve of the Galaxy obtained from Gaia DR2 implies a heavy Milky Way

We measure the escape speed curve of the Milky Way based on the analysis of the velocity distribution of $\sim 2850$ counter-rotating halo stars from the Gaia DR2. The distances were estimated through the StarHorse code, and only stars with distance errors smaller than 10 per cent were used in the study. The escape speed curve is measured at Galactocentric radii ranging from $\sim 5$ kpc to $\sim 10.5$ kpc. The local Galactic escape at the Sun's position is estimated to be $v_\mathrm{e}(r_\odot)=580 \pm 63~\mathrm{km~s^{-1}}$, and it rises towards the Galactic center. Defined as the minimum speed required to reach three virial radii, our estimate of the escape speed as a function of radius implies, for a Navarro-Frenk-White profile and local circular velocity of $240~\mathrm{km~s^{-1}}$, a dark matter mass $M_{200}=1.28^{+0.68}_{-0.50} \times 10^{12}~M_\odot$ and a high concentration $c_{200}=11.09^{+2.94}_{-1.79}$. Assuming the mass-concentration relation of $Λ$CDM, we get $M_{200}=1.55_{-0.51}^{+0.64}\times 10^{12}~M_\odot$, $c_{200}=7.93_{-0.27}^{+0.33}$, for a local circular velocity of $228~\mathrm{km~s^{-1}}$.

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arXiv2018

The Fastest Travel Together: Chemical Tagging of the Fastest Stars in Gaia DR2 to the Stellar Halo

The fastest moving stars provide insight into several fundamental properties of the Galaxy, including the escape velocity as a function of Galactocentric radius, the total mass, and the nature and frequency of stellar encounters with the central supermassive black hole. The recent second data release of Gaia has allowed the identification of new samples of stars with extreme velocities. Discrimination among the possible origins of these stars is facilitated by chemical abundance information. We here report the results from our high-resolution spectroscopic followup, using the Apache Point Observatory, of five late-type `hypervelocity' star candidates, characterised by total Galactic rest-frame velocities between 500-600 km/s and estimated to have a probability larger than 50% to be unbound from the Milky Way. Our new results confirm the Gaia DR2 radial velocities to within 1 km/s. We derived stellar atmospheric parameters and chemical abundances for several species including $α$-elements (Mg, Ti, Si, Ca), Fe-peak elements (Fe, Ni, Co, Cr, Mn), neutron-capture elements (Sr, Y, Zr, Ba, La, Nd, Eu) and odd-Z elements (Na, Al, K, V, Cu, Sc). We find that all stars observed are metal-poor giants with -2 $\leq$ [Fe/H] $\leq$ -1 dex and are chemically indistinguishable from typical halo stars. Our results are supported by the chemical properties of four additional stars with extreme space motions which were observed by existing spectroscopic surveys. We conclude that these stars are simply the high-velocity tail of the stellar halo and effectively rule out more exotic origins such as from the Galactic centre or the Large Magellanic Cloud.

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arXiv2018

Improved distances and ages for stars common to TGAS and RAVE

We combine parallaxes from the first Gaia data release with the spectrophotometric distance estimation framework for stars in the fifth RAVE survey data release. The combined distance estimates are more accurate than either determination in isolation - uncertainties are on average two times smaller than for RAVE-only distances (three times smaller for dwarfs), and 1.4 times smaller than TGAS parallax uncertainties (two times smaller for giants). We are also able to compare the estimates from spectrophotometry to those from Gaia, and use this to assess the reliability of both catalogues and improve our distance estimates. We find that the distances to the lowest log g stars are, on average, overestimated and caution that they may not be reliable. We also find that it is likely that the Gaia random uncertainties are smaller than the reported values. As a byproduct we derive ages for the RAVE stars, many with relative uncertainties less than 20 percent. These results for 219566 RAVE sources have been made publicly available, and we encourage their use for studies that combine the radial velocities provided by RAVE with the proper motions provided by Gaia. A sample that we believe to be reliable can be found by taking only the stars with the flag notification flag_any=0

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arXiv2018

Correlations between age, kinematics, and chemistry as seen by the RAVE survey

We explore the connections between stellar age, chemistry, and kinematics across a Galactocentric distance of $7.5 < R\,(\mathrm{kpc}) < 9.0$, using a sample of $\sim 12\,000$ intermediate-mass (FGK) turnoff stars observed with the RAdial Velocity Experiment (RAVE) survey. The kinematics of this sample are determined using radial velocity measurements from RAVE, and parallax and proper motion measurements from the Tycho-Gaia Astrometric Solution (TGAS). In addition, ages for RAVE stars are determined using a Bayesian method, taking TGAS parallaxes as a prior. We divide our sample into young ($0 < τ< 3$ Gyr) and old ($8 < τ< 13$ Gyr) populations, and then consider different metallicity bins for each of these age groups. We find significant differences in kinematic trends of young and old, metal-poor and metal-rich, stellar populations. In particular, we find a strong metallicity dependence in the mean Galactocentric radial velocity as a function of radius ($\partial {V_{\rm R}}/\partial R$) for young stars, with metal-rich stars having a much steeper gradient than metal-poor stars. For $\partial {V_ϕ}/\partial R$, young, metal-rich stars significantly lag the LSR with a slightly positive gradient, while metal-poor stars show a negative gradient above the LSR. We interpret these findings as correlations between metallicity and the relative contributions of the non-axisymmetries in the Galactic gravitational potential (the spiral arms and the bar) to perturb stellar orbits.

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arXiv2018

Constraints on Radial Migration in Spiral Galaxies - II. Angular momentum distribution and preferential migration

The orbital angular momentum of individual stars in galactic discs can be permanently changed through torques from transient spiral patterns. Interactions at the corotation resonance dominate these changes and have the further property of conserving orbital circularity. We derived in an earlier paper an analytic criterion that an unperturbed stellar orbit must satisfy in order for such an interaction to occur i.e. for it to be in a trapped orbit around corotation. We here use this criterion in an investigation of how the efficiency of induced radial migration for a population of disc stars varies with the angular momentum distribution of that population. We frame our results in terms of the velocity dispersion of the population, this being an easier observable than is the angular momentum distribution. Specifically, we investigate how the fraction of stars in trapped orbits at corotation varies with the velocity dispersion of the population, for a system with an assumed flat rotation curve. Our analytic results agree with the finding from simulations that radial migration is less effective in populations with 'hotter' kinematics. We further quantify the dependence of this trapped fraction on the strength of the spiral pattern, finding a higher trapped fraction for higher amplitude perturbations.

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arXiv2017

Galactic Surveys in the Gaia Era

The final astrometric data from the Gaia mission will transform our view of the stellar content of the Galaxy, particularly when complemented with spectroscopic surveys providing stellar parameters, line-of-sight kinematics and elemental abundances. Analyses with Gaia DR1 are already demonstrating the insight gained and the promise of what is to come with future Gaia releases. I present a brief overview of results and puzzles from recent Galactic Archaeology surveys for context, focusing on the Galactic discs.

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arXiv2017

The selection function of the RAVE survey

We characterize the selection function of RAVE using 2MASS as our underlying population, which we assume represents all stars which could have potentially been observed. We evaluate the completeness fraction as a function of position, magnitude, and color in two ways: first, on a field-by-field basis, and second, in equal-size areas on the sky. Then, we consider the effect of the RAVE stellar parameter pipeline on the final resulting catalogue, which in principle limits the parameter space over which our selection function is valid. Our final selection function is the product of the completeness fraction and the selection function of the pipeline. We then test if the application of the selection function introduces biases in the derived parameters. To do this, we compare a parent mock catalogue generated using Galaxia with a mock-RAVE catalogue where the selection function of RAVE has been applied. We conclude that for stars brighter than I = 12, between $4000 \rm K < T_{\rm eff} < 8000 \rm K$ and $0.5 < \rm{log}\,g < 5.0$, RAVE is kinematically and chemically unbiased with respect to expectations from Galaxia.

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arXiv2017

Cardinal kinematics: I. Rotation fields of the APOGEE Survey

Correlations between stellar chemistry and kinematics have long been used to gain insight into the evolution of the Milky Way Galaxy. Orbital angular momentum is a key physical parameter and it is often estimated from three-dimensional space motions. We here demonstrate the lower uncertainties that can be achieved in the estimation of one component of velocity through selection of stars in key directions and use of line-of-sight velocity alone (i.e. without incorporation of proper motion data). In this first paper we apply our technique to stars observed in the direction of Galactic rotation in the APOGEE survey. We first derive the distribution of azimuthal velocities, $v_ϕ$, then from these and observed radial coordinates, estimate the stellar guiding centre radii, $R_g$, within $6.9\leq R \leq 10$ kpc with uncertainties smaller than (or of the order of) 1kpc. We show that there is no simple way to select a clean stellar sample based on low errors on proper motions and distances to obtain high-quality 3D velocities and hence one should pay particular attention when trying to identify kinematically peculiar stars based on velocities derived using the proper motions. Using our azimuthal velocity estimations, we investigate the joint distribution of elemental abundances and rotational kinematics free from the blurring effects of epicyclic motions, and we derive the $\partial v_ϕ/ \partial R_g$ trends for the thin and thick discs as a function of radius. Our analysis provides further evidence for radial migration within the thin disc and hints against radial migration playing a significant role in the evolution of the thick disc.

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arXiv2016

The Radial Velocity Experiment (RAVE): Fifth Data Release

Data Release 5 (DR5) of the Radial Velocity Experiment (RAVE) is the fifth data release from a magnitude-limited (9< I < 12) survey of stars randomly selected in the southern hemisphere. The RAVE medium-resolution spectra ($R\sim7500$) covering the Ca-triplet region (8410-8795Å) span the complete time frame from the start of RAVE observations in 2003 to their completion in 2013. Radial velocities from 520,781 spectra of 457,588 unique stars are presented, of which 255,922 stellar observations have parallaxes and proper motions from the Tycho-Gaia astrometric solution (TGAS) in Gaia DR1. For our main DR5 catalog, stellar parameters (effective temperature, surface gravity, and overall metallicity) are computed using the RAVE DR4 stellar pipeline, but calibrated using recent K2 Campaign 1 seismic gravities and Gaia benchmark stars, as well as results obtained from high-resolution studies. Also included are temperatures from the Infrared Flux Method, and we provide a catalogue of red giant stars in the dereddened color $(J-Ks)_0$ interval (0.50,0.85) for which the gravities were calibrated based only on seismology. Further data products for sub-samples of the RAVE stars include individual abundances for Mg, Al, Si, Ca, Ti, Fe, and Ni, and distances found using isochrones. Each RAVE spectrum is complemented by an error spectrum, which has been used to determine uncertainties on the parameters. The data can be accessed via the RAVE Web site or the Vizier database.

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arXiv2016

A Synoptic Map of Halo Substructures from the Pan-STARRS1 3π Survey

We present a panoramic map of the entire Milky Way halo north of dec~-30 degrees (~30,000 deg^2), constructed by applying the matched-filter technique to the Pan-STARRS1 3Pi Survey dataset. Using single-epoch photometry reaching to g~22, we are sensitive to stellar substructures with heliocentric distances between 3.5 and ~35 kpc. We recover almost all previously-reported streams in this volume and demonstrate that several of these are significantly more extended than earlier datasets have indicated. In addition, we also report five new candidate stellar streams. One of these features appears significantly broader and more luminous than the others and is likely the remnant of a dwarf galaxy. The other four streams are consistent with a globular cluster origin, and three of these are rather short in projection (<10 degrees), suggesting that streams like Ophiuchus may not be that rare. Finally, a significant number of more marginal substructures are also revealed by our analysis; many of these features can also be discerned in matched-filter maps produced by other authors from SDSS data, and hence they are very likely to be genuine. However, the extant 3Pi data is currently too shallow to determine their properties or produce convincing CMDs. The global view of the Milky Way provided by Pan-STARRS1 provides further evidence for the important role of both globular cluster disruption and dwarf galaxy accretion in building the Milky Way's stellar halo.

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arXiv2016

Chemical separation of disc components using RAVE

We present evidence from the RAdial Velocity Experiment (RAVE) survey of chemically separated, kinematically distinct disc components in the solar neighbourhood. We apply probabilistic chemical selection criteria to separate our sample into $α$-low (`thin disc') and $α$-high (`thick disc') sequences. Using newly derived distances, which will be utilized in the upcoming RAVE DR5, we explore the kinematic trends as a function of metallicity for each of the disc components. For our $α$-low disc, we find a negative trend in the mean rotational velocity ($V_{\mathrmϕ}$) as a function of iron abundance ([Fe/H]). We measure a positive gradient $\partial V_{\mathrmϕ}$/$\partial$[Fe/H] for the $α$-high disc, consistent with results from high-resolution surveys. We also find differences between the $α$-low and $α$-high discs in all three components of velocity dispersion. We discuss the implications of an $α$-low, metal-rich population originating from the inner Galaxy, where the orbits of these stars have been significantly altered by radial mixing mechanisms in order to bring them into the solar neighbourhood. The probabilistic separation we propose can be extended to other data sets for which the accuracy in [$α$/Fe] is not sufficient to disentangle the chemical disc components a priori. For such datasets which will also have significant overlap with Gaia DR1, we can therefore make full use of the improved parallax and proper motion data as it becomes available to investigate kinematic trends in these chemical disc components.

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arXiv2016

The chemical evolution of the Bootes I ultra-faint dwarf galaxy

We present chemical abundance measurements of two metal-poor red giant stars in the ultra-faint dwarf galaxy Bootes I, based on Magellan/MIKE high-resolution spectra. For Boo I-980, with [Fe/H]=-3.1, we present the first elemental abundance measurements while Boo I-127, with [Fe/H]=-2.0, shows abundances in good agreement with previous measurements. Light and iron-peak element abundance ratios in the two Bootes I stars, as well as those of most other Boootes I members, collected from the literature, closely resemble those of regular metal-poor halo stars. Neutron-capture element abundances Sr and Ba are systematically lower than the main halo trend, and also show a significant abundance spread. Overall, this is similar to what has been found for other ultra-faint dwarf galaxies. We apply corrections to the carbon abundances (commensurate with stellar evolutionary status) of the entire sample and find 21% of stars to be carbon-enhanced metal-poor (CEMP) stars, compared to 13% without using the carbon correction. We reassess the metallicity distribution functions (MDF) for the CEMP stars and non-CEMP stars, and confirm earlier claims that CEMP stars might belong to a different, earlier population. Applying a set of abundance criteria to test to what extent Bootes I could be a surviving first galaxy suggests that it is one of the earliest assembled systems that perhaps received gas from accretion from other clouds in the system, or from swallowing a first galaxy or building block type object. This resulted in the two stellar populations observable today.

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arXiv2016

Galactic Archaeology: Current Surveys

I present an overview of the science goals and achievements of ongoing spectroscopic surveys of individual stars in the nearby Universe. I include a brief discussion of the development of the field of Galactic Archaeology - using the fossil record in old stars nearby to infer how our Galaxy evolved and place the Milky Way in cosmological context.

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arXiv2016

Dispersal of tidal debris in a Milky-Way-sized dark matter halo

We simulate the tidal disruption of a collisionless N-body globular star cluster in a total of 300 different orbits selected to have galactocentric radii between 10 and 30 kpc in four dark matter halos: (a) a spherical halo with no subhalos, (b) a spherical halo with subhalos, (c) a realistic halo with no subhalos, and (d) a realistic halo with subhalos. This allows us to isolate and study how the halo's (lack of) dynamical symmetry and substructures affect the dispersal of tidal debris. The realistic halos are constructed from the snapshot of the Via Lactea II simulation at redshift zero. We find that the overall halo's lack of dynamical symmetry disperses tidal debris to make the streams fluffier, consistent with previous studies of tidal debris of dwarf galaxies in larger orbits than ours in this study. On the other hand, subhalos in realistic potentials can locally enhance the densities along streams, making streams denser than their counterparts in smooth potentials. We show that many long and thin streams can survive in a realistic and lumpy halo for a Hubble time. This suggests that upcoming stellar surveys will likely uncover more thin streams which may contain density gaps that have been shown to be promising probes for dark matter substructures.

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arXiv2015

Sagittarius II, Draco II and Laevens 3: three new Milky Way satellites discovered in the Pan-STARRS 1 3pi Survey

We present the discovery of three new Milky Way satellites from our search for compact stellar overdensities in the photometric catalog of the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS 1, or PS1) 3pi survey. The first satellite, Laevens 3, is located at a heliocentric distance of d=67+/-3 kpc. With a total magnitude of Mv=-4.4+/-0.3 and a half-light radius rh=7+/-2 pc, its properties resemble those of outer halo globular clusters. The second system, Draco II/Laevens 4 (Dra II), is a closer and fainter satellite (d~20 kpc, Mv =-2.9+/-0.8), whose uncertain size (rh = 19 +8/-6 pc) renders its classification difficult without kinematic information; it could either be a faint and extended globular cluster or a faint and compact dwarf galaxy. The third satellite, Sagittarius II/Laevens 5 (Sgr II), has an ambiguous nature as it is either the most compact dwarf galaxy or the most extended globular cluster in its luminosity range (rh = 37 +9/-8 pc and Mv=-5.2+/-0.4). At a heliocentric distance of 67+/-5 kpc, this satellite lies intriguingly close to the expected location of the trailing arm of the Sagittarius stellar stream behind the Sagittarius dwarf spheroidal galaxy (Sgr dSph). If confirmed through spectroscopic follow up, this connection would locate this part of the trailing arm of the Sagittarius stellar stream that has so far gone undetected. It would further suggest that Sgr II was brought into the Milky Way halo as a satellite of the Sgr dSph.

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arXiv2015

Galaxy UV-luminosity function and reionization constraints on axion dark matter

If the dark matter (DM) were composed of axions, then structure formation in the Universe would be suppressed below the axion Jeans scale. Using an analytic model for the halo mass function of a mixed DM model with axions and cold dark matter, combined with the abundance-matching technique, we construct the UV-luminosity function. Axions suppress high-$z$ galaxy formation and the UV-luminosity function is truncated at a faintest limiting magnitude. From the UV-luminosity function, we predict the reionization history of the universe and find that axion DM causes reionization to occur at lower redshift. We search for evidence of axions using the Hubble Ultra Deep Field UV-luminosity function in the redshift range $z=6$-$10$, and the optical depth to reionization, $τ$, as measured from cosmic microwave background polarization. All probes we consider consistently exclude $m_a\lesssim 10^{-23}\text{ eV}$ from contributing more than half of the DM, with our strongest constraint ruling this model out at more than $8σ$ significance. In conservative models of reionization a dominant component of DM with $m_a=10^{-22}\text{ eV}$ is in $3σ$ tension with the measured value of $τ$, putting pressure on an axion solution to the cusp-core problem. Tension is reduced to $2σ$ for the axion contributing only half of the DM. A future measurement of the UV-luminosity function in the range $z=10$-$13$ by JWST would provide further evidence for or against $m_a=10^{-22}\text{ eV}$. Probing still higher masses of $m_a=10^{-21}\text{ eV}$ will be possible using future measurements of the kinetic Sunyaev-Zel'dovich effect by Advanced ACTPol to constrain the time and duration of reionization.

astro-ph.CO↗

arXiv2015

Simulating tidal streams in a high resolution dark matter halo

We simulate tidal streams in the presence and absence of substructures inside the zero redshift snapshot of the Via Lactea II (VL-2) simulation. A halo finder is used to remove and isolate the subhalos found inside the high resolution dark matter halo of VL-2, and the potentials for both the main halo and all the subhalos are constructed individually using the self-consistent field (SCF) method. This allows us to make direct comparison of tidal streams between a smooth halo and a lumpy halo without assuming idealized profiles or triaxial fits. We simulate the kinematics of a star cluster starting with the same orbital position but two different velocities. Although these two orbits are only moderately eccentric and have similar apo- and pericentric distances, we find that the two streams have very different morphologies. We conclude that our model of the potential of VL-2 can provide insights about tidal streams that have not been explored by previous studies using idealized or axisymmetric models.

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