arXiv subjects

Jason Sanders

Publications and source records attributed to Jason Sanders.

9 recordsLinked to original sources

arXiv2023

JASMINE: Near-Infrared Astrometry and Time Series Photometry Science

Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE) is a planned M-class science space mission by the Institute of Space and Astronautical Science, the Japan Aerospace Exploration Agency. JASMINE has two main science goals. One is the Galactic archaeology with Galactic Center Survey, which aims to reveal the Milky Way's central core structure and formation history from Gaia-level (~25 $\mu$as) astrometry in the Near-Infrared (NIR) Hw-band (1.0-1.6 $\mu$m). The other is the Exoplanet Survey, which aims to discover transiting Earth-like exoplanets in the habitable zone from NIR time-series photometry of M dwarfs when the Galactic center is not accessible. We introduce the mission, review many science objectives, and present the instrument concept. JASMINE will be the first dedicated NIR astrometry space mission and provide precise astrometric information of the stars in the Galactic center, taking advantage of the significantly lower extinction in the NIR. The precise astrometry is obtained by taking many short-exposure images. Hence, the JASMINE Galactic center survey data will be valuable for studies of exoplanet transits, asteroseismology, variable stars and microlensing studies, including discovery of (intermediate mass) black holes. We highlight a swath of such potential science, and also describe synergies with other missions.

astro-ph.IM↗

arXiv2022

Simulated Bars May Be Shorter But Are Not Slower Than Observed: TNG50 vs. MaNGA

Galactic bars are prominent dynamical structures within disk galaxies whose size, formation time, strength, and pattern speed influence the dynamical evolution of their hosts galaxies. Yet, their formation and evolution in a cosmological context is not well understood, as cosmological simulation studies have been limited by the classic trade off between simulation volume and resolution. Here we analyze barred disk galaxies in the cosmological magneto-hydrodynamical simulation TNG50 and quantitatively compare the distributions of bar size and pattern speed to those from MaNGA observations at $z=0$. TNG50 galaxies are selected to match the stellar mass and size distributions of observed galaxies, to account for observational selection effects. We find that the high-resolution of TNG50 yields bars with a wide range of pattern speeds (including those with $\geq 40~\mathrm{km}\,\mathrm{s}^{-1}$\,$\mathrm{kpc}^{-1}$) and a mean value of $\sim36~\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{kpc}$ consistent with observations within $6\,\mathrm{km}\,\mathrm{s}^{-1}$\,$\mathrm{kpc}^{-1}$, in contrast with previous lower-resolution cosmological simulations that produced bars that were too slow. We find, however, that bars in TNG50 are on average $\sim 35\%$ shorter than observed, although this discrepancy may partly reflect remaining inconsistencies in the simulation-data comparison. This leads to higher values of $\mathcal{R} = R_\mathrm{corot}/R_\mathrm{bar}$ in TNG50, but points to simulated bars being `too short' rather than `too slow'. After repeating the analysis on the lower-resolution run of the same simulation (with the same physical model), we qualitatively reproduce the results obtained in previous studies: this implies that, along with physical model variations, numerical resolution effects may explain the previously found `slowness' of simulated bars.

astro-ph.GA↗

arXiv2020

Keeping it Cool: Much Orbit Migration, yet Little Heating, in the Galactic Disk

A star in the Milky Way's disk can now be at a Galactocentric radius quite distant from its birth radius for two reasons: either its orbit has become eccentric through radial heating, which increases its radial action $J_R$ (`blurring'); or merely its angular momentum $L_z$ has changed and thereby its guiding radius (`churning'). We know that radial orbit migration is strong in the Galactic low-$\alpha$ disk and set out to quantify the relative importance of these two effects, by devising and applying a parameterized model for the distribution $p(L_z, J_R, \tau, \mathrm[Fe/H])$ in the stellar disk. This model describes the orbit evolution for stars of age $\tau$ and metallicity [Fe/H], presuming coeval stars were initially born on (near-)circular orbits, and with a unique [Fe/H] at a given birth angular momentum and age. We fit this model to APOGEE red clump stars, accounting for the complex selection function of the survey. The best fit model implies changes of angular momentum of $\sqrt{\langle \Delta L_z \rangle^2} \approx 619\, \mathrm{kpc~km/s~}(\tau/\mathrm{6~Gyr})^{0.5}$, and changes of radial action as $\sqrt{\langle \Delta J_R \rangle^2} \approx 63\, \mathrm{kpc~km/s~} (\tau/\mathrm{6~Gyr})^{0.6}$ at 8 kpc. This suggests that the secular orbit evolution of the disk is dominated by diffusion in angular momentum, with radial heating being an order of magnitude lower.

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arXiv2019

The Inside-out Growth of the Galactic Disk

We quantify the inside-out growth of the Milky Way's low-alpha stellar disk, modelling the ages, metallicities and Galactocentric radii of APOGEE red clump stars with 6 < R < 13 kpc. The current stellar distribution differs significantly from that expected from the star formation history due to the redistribution of stars through radial orbit mixing. We propose and fit a global model for the Milky Way disk, specified by an inside-out star formation history, radial orbit mixing, and an empirical, parametric model for its chemical evolution. We account for the spatially complex survey selection function, and find that the model fits all data well. We find distinct inside-out growth of the Milky Way disk; the best fit model implies that the half-mass radius of the Milky Way disk has grown by 43\% over the last 7 Gyr. Yet, such inside-out growth still results in present-day age gradient weaker than 0.1 Gyr/kpc. Our model predicts the half-mass and half-light sizes of the Galactic disk at earlier epochs, which can be compared to the observed redshift -size relations of disk galaxies. We show that radial orbit migration can reconcile the distinct disk-size evolution with redshift, also expected from cosmological simulations, with the modest present-day age gradients seen in the Milky Way and other galaxies.

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arXiv2019

Lessons from the curious case of the `fastest' star in Gaia DR2

Gaia DR2 5932173855446728064 was recently proposed to be unbound from the Milky Way based on the $-614.3\pm2.5\;\mathrm{km}\;\mathrm{s}^{-1}$ median radial velocity given in Gaia DR2. We obtained eight epochs of spectroscopic follow-up and find a very different median radial velocity of $-56.5 \pm 5.3\;\mathrm{km}\;\mathrm{s}^{-1}$. If this difference were to be explained by binarity, then the unseen companion would be an intermediate-mass black hole; we therefore argue that the Gaia DR2 radial velocity must be in error. We find it likely that the spectra obtained by Gaia were dominated by the light from a star $4.3\;\mathrm{arcsec}$ away, and that, due to the slitless, time delay integration nature of Gaia spectroscopy, this angular offset corresponded to a spurious $620\;\mathrm{km}\;\mathrm{s}^{-1}$ shift in the calcium triplet of the second star. We argue that such unanticipated alignments between stars may account for 105 of the 202 stars with radial velocities faster than $500\;\mathrm{km}\;\mathrm{s}^{-1}$ in Gaia DR2 and propose a quality cut to exclude stars that are susceptible. We propose further cuts to remove stars where the colour photometry is suspect and stars where the radial velocity measurement is based on fewer than four transits, and thus produce an unprecedentedly clean selection of Gaia RVS stars for use in studies of Galactic dynamics.

astro-ph.SR↗

arXiv2018

MADE: A spectroscopic Mass, Age, and Distance Estimator for red giant stars with Bayesian machine learning

We present a new approach (MADE) that generates mass, age, and distance estimates of red giant stars from a combination of astrometric, photometric, and spectroscopic data. The core of the approach is a Bayesian artificial neural network (ANN) that learns from and completely replaces stellar isochrones. The ANN is trained using a sample of red giant stars with mass estimates from asteroseismology. A Bayesian isochrone pipeline uses the astrometric, photometric, spectroscopic, and asteroseismology data to determine posterior distributions for the training outputs: mass, age, and distance. Given new inputs, posterior predictive distributions for the outputs are computed, taking into account both input uncertainties, and uncertainties in the ANN parameters. We apply MADE to $\sim10\,000$ red giants in the overlap between the 14$^{\mathrm{th}}$ data release from the APO Galactic Evolution Experiment (APOGEE, Abolfathi et al. 2018) and the Tycho-Gaia astrometric solution (TGAS, Michalik et al. 2015). The ANN is able to reduce the uncertainty on mass, age, and distance estimates for training-set stars with high output uncertainties allocated through the Bayesian isochrone pipeline. The fractional uncertainties on mass are $<10\%$ and on age are between $10$ to $25\%$. Moreover, the time taken for our ANN to predict masses, ages, and distances for the entire catalogue of APOGEE-TGAS stars is of a similar order of the time taken by the Bayesian isochrone pipeline to run on a handful of stars. Our resulting catalogue clearly demonstrates the expected thick and thin disc components in the [M/H]-[$\alpha$/M] plane, when examined by age.

astro-ph.GA↗

arXiv2016

The RAVE-on catalog of stellar atmospheric parameters and chemical abundances for chemo-dynamic studies in the Gaia era

The orbits, atmospheric parameters, chemical abundances, and ages of individual stars in the Milky Way provide the most comprehensive illustration of galaxy formation available. The Tycho-Gaia Astrometric Solution (TGAS) will deliver astrometric parameters for the largest ever sample of Milky Way stars, though its full potential cannot be realized without the addition of complementary spectroscopy. Among existing spectroscopic surveys, the RAdial Velocity Experiment (RAVE) has the largest overlap with TGAS ($\gtrsim$200,000 stars). We present a data-driven re-analysis of 520,781 RAVE spectra using The Cannon. For red giants, we build our model using high-fidelity APOGEE stellar parameters and abundances for stars that overlap with RAVE. For main-sequence and sub-giant stars, our model uses stellar parameters from the K2/EPIC. We derive and validate effective temperature $T_{\rm eff}$, surface gravity $\log{g}$, and chemical abundances of up to seven elements (O, Mg, Al, Si, Ca, Fe, Ni). We report a total of 1,685,851 elemental abundances with a typical precision of 0.07 dex, a substantial improvement over previous RAVE data releases. The synthesis of RAVE-on and TGAS is the most powerful data set for chemo-dynamic analyses of the Milky Way ever produced.

astro-ph.SR↗

arXiv2012

Angle-action estimation in a general axisymmetric potential

The usefulness of angle-action variables in galaxy dynamics is well known, but their use is limited due to the difficulty of their calculation in realistic galaxy potentials. Here we present a method for estimating angle-action variables in a realistic Milky Way axisymmetric potential by locally fitting a St\"ackel potential over the region an orbit probes. The quality of the method is assessed by comparison with other known methods for estimating angle-action variables of a range of disc and halo-type orbits. We conclude by projecting the Geneva-Copenhagen survey into angle-action space.

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arXiv2012

Determining the velocity dispersion of the thick disc

We attempt to recover the mean vertical velocity and vertical velocity dispersion as a function of the Galactic height for a sample drawn from a realistic Galaxy distribution function by following the method presented in Moni Bidin et al. (2012). We find that, for the sample size used, the observational error in the velocities is much smaller than the Poisson noise which has not been accounted for by Moni Bidin et al. We repeat the analysis on a large number of samples to estimate the contribution of the Poisson noise and to uncover any systematics. We find that the dispersion is systematically overestimated at low Galactic heights and slightly underestimated at high Galactic heights leading to an underestimate of the gradient of the dispersion with Galactic height. The causes of the systematics are revealed by repeating the calculation using a method inspired by Girard et al. (2006). This method recovers the expected dispersion much more successfully and in particular yields a gradient of the dispersion with Galactic height which is approximately three times that found using the method presented by Moni Bidin et al.

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