Laura L. Watkins

Max Planck Institute for Astronomy, Heidelburg, Baden-Württemberg, Germany

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Publications (12)39.47 Total impact

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    Alex Büdenbender, Glenn van de Ven, Laura L. Watkins
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    ABSTRACT: Accurate determination of the local dark matter density is important for understanding the nature and distribution of dark matter in the universe. This requires that the local velocity distribution is characterised correctly. Here, we present a kinematic study of 16,276 SEGUE G-type dwarf stars in the solar neighbourhood, with which we determine the shape of the velocity ellipsoid in the meridional plane. We separate our G-dwarf stars based on their [Fe/H] and [alpha/Fe] abundances and estimate the best-fitting Milky Way model independently for each sub-sample using a maximum-likelihood method that accounts for possible contaminants. We show that the different subpopulations yield consistent results only when we allow the velocity ellipsoid in the disk to be tilted, demonstrating that the common assumption of decoupled radial and vertical motions in the disk is incorrect. Further, we the find that the tilt angle alpha of the velocity ellipsoid increases with height |z| from 5{\deg} at 0.5 kpc to 14{\deg} at 2.0 kpc, consistent with pointing toward the Galactic centre at an angle tan(alpha) ~ |z|/R. We also confirm earlier findings that the subpopulations behave almost isothermally both radially and vertically, about 39 (20) km/s for the chemically-young, metal-rich disk stars to about 60 (48) km/s for the chemically-old, metal-poor disk stars. We conclude that the coupling between radial and vertical motion captured in the velocity ellipsoid tilt cannot be ignored when considering dynamical models of the solar neighbourhood. In a subsequent paper, we will develop a new modelling scheme informed by these results and make an improved determination of the local dark matter density.
    07/2014;
  • L ~L Watkins, G Ven, M Brok, R ~C ~E Bosch
    mnras. 12/2013; 436:2598-2615.
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    ABSTRACT: We analyze line-of-sight velocity and proper motion data of stars in the Galactic globular cluster M15 using a new method to fit dynamical models to discrete kinematic data. Our fitting method maximizes the likelihood for individual stars and, as such, does not suffer the same loss of spatial and velocity information incurred when spatially binning data or measuring velocity moments. In this paper, we show that the radial variation in M15 of the mass-to-light ratio is consistent with previous estimates and theoretical predictions, which verifies our method. Our best-fitting axisymmetric Jeans models do include a central dark mass of $\sim2 \pm 1\cdot 10^3M_\odot$, which can be explained by a high concentration of stellar remnants at the cluster center. This paper shows that, from a technical point of view and with current computing power, spatial binning of data is no longer necessary. This not only leads to more accurate fits, but also avoids biased mass estimates due to the loss of resolution. Furthermore, we find that the mass concentration in M15 is significantly higher than previously measured, and is in close agreement with theoretical predictions for core-collapsed globular clusters without a central intermediate-mass black hole.
    Monthly Notices of the Royal Astronomical Society 11/2013; 438(1). · 5.52 Impact Factor
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    ABSTRACT: We present a new framework for modelling discrete kinematic data. Current techniques typically involve binning. Our approach works directly with the discrete data and uses maximum-likelihood methods to assess the probability of the dataset given model predictions. We avoid making hard cuts on the datasets by allowing for a contaminating population in our models. We apply our models to discrete proper-motion and line-of-sight-velocity data of Galactic globular cluster omega Centauri and find a mildly radial velocity anisotropy beta = 0.10 +/- 0.02, an inclination angle i = 50 +/- 1 deg, a V-band mass-to-light ratio Upsilon = 2.71 +/- 0.05 Msun/Lsun and a distance d = 4.59 +/- 0.08 kpc. All parameters are in agreement with previous studies, demonstrating the feasibility of our methods. We find that the models return lower distances and higher mass-to-light ratios than expected when we include proper motion stars with high errors or for which there is some blending. We believe this not a fault of our models but is instead due to underestimates or missing systematic uncertainties in the provided errors.
    08/2013;
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    Laura L. Watkins, N. Wyn Evans, Glenn van de Ven
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    ABSTRACT: We present an analysis of the dynamics of the M31 satellite system. Proper motion data exist for only two of the M31 satellites. We account for this incompleteness in velocity data by a statistical analysis using a combination of the timing argument and phase-space distribution functions. The bulk of the M31 satellites are well fit by these models and we offer a table of orbital properties, including period, eccentricity and semi-major axis. This enables us to search for evidence of group infall based on orbital similarity rather than propinquity on the sky. Our results favour an association between Cass II and NGC 185, as the orbital parameters are in close agreement, but not for NGC 185 and NGC 147, which have often been associated in the past. Other possible satellite groupings include the pair And I and And XVII; the pair And IX and And X; and the triple And V, And XXV and NGC 147. And XXII has been claimed as a satellite of M33; we find that they are not moving independently along the same orbit, but cannot determine whether they are orbiting each other or are unrelated. Two satellites, And XII and And XIV, have high line-of-sight velocities, consistent with very recent infall from the edge of the Local Group. They are not well described by our underlying smooth phase space distribution function, and are reanalysed without priors on their orbital parameters. For And XIV, multiple pericentric passages are possible and improved distance information is needed to draw further conclusions. For And XII, orbits which assume at least one pericentric passage can be ruled out and it must be on its first infall into the M31 system.
    Monthly Notices of the Royal Astronomical Society 11/2012; 430(2). · 5.52 Impact Factor
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    ABSTRACT: We present VLT/FORS2 spectroscopy of candidate blue horizontal branch (BHB) stars in the vicinity of the Hercules ultrafaint dwarf galaxy. We identify eight convincing Hercules BHB members, and a further five stars with similar systemic velocities to that of Hercules, but ~ 0.5 kpc from the centre of the galaxy along its major axis. It is likely that these stars once belonged to Hercules, but have been tidally stripped and are now unbound. We emphasise the usefulness of looking for any gradient in the systemic velocity of this stretched system, which would further support our interpretation of the origin of its elongated and distended morphology.
    Monthly Notices of the Royal Astronomical Society 07/2012; 425(1). · 5.52 Impact Factor
  • 04/2012;
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    Laura L. Watkins
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    ABSTRACT: Using a sample of 407 RR Lyrae stars extracted from SDSS Stripe 82, we study the degree of substructure in the Galactic halo. We identify overdensities associated with the known substructures of the Hercules-Aquila Cloud and the Sagittarius Stream, and find a further previously-unknown substructure, the Pisces Overdensity, at ~100 kpc from the Sun. Together, the three substructures account for ~80% of our RR Lyrae sample. We also study the density distribution of RR Lyraes in the halo and find that the profile is best fit by a broken power law with an inner slope of -2.4 and a break radius of ~25 kpc, consistent with previous studies. We conclude that the halo is predominantly made up of the debris from in-falling satellites, with little or no underlying smooth component.
    11/2011;
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    Laura L. Watkins, N. Wyn Evans, Jin H. An
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    ABSTRACT: We present a family of robust tracer mass estimators to compute the enclosed mass of galaxy haloes from samples of discrete positional and kinematical data of tracers, such as halo stars, globular clusters and dwarf satellites. The data may be projected positions, distances, line-of-sight velocities or proper motions. The estimators all assume that the tracer population has a scale-free density and moves in a scale-free potential in the region of interest. The circumstances under which the boundary terms can be discarded and the estimator converges are derived. Forms of the estimator tailored for the Milky Way Galaxy and for M31 are given. Monte Carlo simulations are used to quantify the uncertainty as a function of sample size.For the Milky Way Galaxy, the satellite sample consists of 26 galaxies with line-of-sight velocities. We find that the mass of the Milky Way within 300 kpc is M300= 0.9 ± 0.3 × 1012 M⊙ assuming velocity isotropy. However, the mass estimate is sensitive to the assumed anisotropy and could plausibly lie between 0.7 × 1012 and 3.4 × 1012 M⊙, if anisotropies implied by simulations or by the observations are used. Incorporating the proper motions of six Milky Way satellites into the data set, we find M300= 1.4 ± 0.3 × 1012 M⊙. The range here if plausible anisotropies are used is still broader, from 1.2 × 1012 to 2.7 × 1012 M⊙. Note that our error bars only incorporate the statistical uncertainty. There are much greater uncertainties induced by velocity anisotropy and by selection of satellite members.For M31, there are 23 satellite galaxies with measured line-of-sight velocities, but only M33 and IC 10 have proper motions. We use the line-of-sight velocities and distances of the satellite galaxies to estimate the mass of M31 within 300 kpc as M300= 1.4 ± 0.4 × 1012 M⊙ assuming isotropy. There is only a modest dependence on anisotropy, with the mass varying between 1.3 × 1012 and 1.6 × 1012 M⊙. Incorporating the proper motion data set does not change the results significantly. Given the uncertainties, we conclude that the satellite data by themselves yield no reliable insights into which of the two galaxies is actually the more massive.Leo I has long been known to dominate mass estimates for the Milky Way due to its substantial distance and line-of-sight velocity. We find that And XII and And XIV similarly dominate the estimated mass of M31. As such, we repeat the calculations without these galaxies, in case they are not bound – although on the balance of the evidence, we favour their inclusion in mass calculations.
    Monthly Notices of the Royal Astronomical Society 07/2010; 406(1):264 - 278. · 5.52 Impact Factor
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    ABSTRACT: We report the discovery of two new Milky Way satellites in the neighboring constellations of Pisces and Pegasus identified in data from the Sloan Digital Sky Survey. Pisces II, an ultra-faint dwarf galaxy lies at the distance of ~180 kpc, some 15° away from the recently detected Pisces I. Segue 3, an ultra-faint star cluster lies at the distance of 16 kpc. We use deep follow-up imaging obtained with the 4-m Mayall Telescope at Kitt Peak National Observatory to derive their structural parameters. Pisces II has a half-light radius of ~60 pc, while Segue 3 is 20 times smaller at only 3 pc.
    The Astrophysical Journal Letters 01/2010; 712(1). · 6.35 Impact Factor
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    ABSTRACT: We present an analysis of the substructure revealed by RRLyraes in Sloan Digital Sky Survey Stripe 82, which covers in declination on the celestial equator over the right ascension range to . We use the new public archive of light-motion curves in Stripe 82, published by Bramich et al. in 2008, to identify a set of high-quality RRLyrae candidates. Period estimates are determined to high accuracy using a string-length method. A subset of 178 RRLyraes with spectrally derived metallicities are employed to derive metallicity-period-amplitude relations, which are then used, together with archive magnitude data and light-curve Fourier decomposition, to estimate metallicities and hence distances for the entire sample. The RRLyraes lie 5-115kpc from the Galactic Centre, with distance estimates accurate to ~8 per cent. The RRLyraes are further divided into subsets of 316 RRab types and 91 RRc types based on their period, colour and metallicity. We fit a smooth density law to the distribution as a simple representation of the data. For Galactocentric radii 5-25kpc the number density of RRLyraes falls as r-2.4, but beyond 25kpc, the number density falls much more steeply, as r-4.5. However, we stress that in practice the density distribution is not smooth, but dominated by clumps and substructure. Samples of 55 and 237 RRLyraes associated with the Sagittarius Stream and the Hercules-Aquila Cloud, respectively, are identified. Hence, ~70 per cent of the RRLyraes in Stripe 82 belong to known substructure, and the sharp break in the density law reflects the fact that the dominant substructure in Stripe 82 - the Hercules-Aquila Cloud and the Sagittarius Stream - lie within 40kpc. In fact, almost 60 per cent of all the RRLyraes in Stripe 82 are associated with the Hercules-Aquila Cloud alone, which emphasizes the cloud's pre-eminence. Additionally, evidence of a new and distant substructure - the Pisces Overdensity - is found, consisting of 28 faint RRLyraes centred on Galactic coordinates (l ~ 80°, b ~ -55°), with distances of ~80kpc. The total stellar mass in the Pisces Overdensity is ~104Msolar and its metallicity is [Fe/H] ~ -1.5.
    Monthly Notices of the Royal Astronomical Society 01/2009; 398(4):1757-1770. · 5.52 Impact Factor
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    ABSTRACT: We present a new public archive of light-motion curves in Sloan Digital Sky Survey (SDSS) Stripe 82, covering 99° in right ascension from to and spanning in declination from to , for a total sky area of ∼249 deg2. Stripe 82 has been repeatedly monitored in the u, g, r, i and z bands over a seven-year baseline. Objects are cross-matched between runs, taking into account the effects of any proper motion. The resulting catalogue contains almost 4 million light-motion curves of stellar objects and galaxies. The photometry are recalibrated to correct for varying photometric zero-points, achieving ∼20 and 30 mmag rms accuracy down to 18 mag in the g, r, i and z bands for point sources and extended sources, respectively. The astrometry are recalibrated to correct for inherent systematic errors in the SDSS astrometric solutions, achieving ∼32 and 35 mas rms accuracy down to 18 mag for point sources and extended sources, respectively.For each light-motion curve, 229 photometric and astrometric quantities are derived and stored in a higher level catalogue. On the photometric side, these include mean exponential and point spread function (PSF) magnitudes along with uncertainties, rms scatter, χ2 per degree of freedom, various magnitude distribution percentiles, object type (stellar or galaxy), and eclipse, Stetson and Vidrih variability indices. On the astrometric side, these quantities include mean positions, proper motions as well as their uncertainties and χ2 per degree of freedom. The light-motion curve catalogue presented here is complete down to r∼ 21.5 and is at present the deepest large-area photometric and astrometric variability catalogue available.
    Monthly Notices of the Royal Astronomical Society 05/2008; 386(2):887 - 902. · 5.52 Impact Factor