ABSTRACT We present the line-of-sight (LOS) velocities for 13 distant main sequence Milky Way halo stars with published proper motions (PMs). The PMs were measured using long baseline (5-7 years) ...multi-epoch Hubble Space Telescope/Advanced Camera for Surveys photometry, and the LOS velocities were extracted from deep (5-6 hr integrations) Keck II/DEIMOS spectra. We estimate the parameters of the velocity ellipsoid of the stellar halo using a Markov chain Monte Carlo ensembler sampler method. The velocity second moments in the directions of the Galactic (l, b, LOS) coordinate system are km s−1, , and . We use these ellipsoid parameters to constrain the velocity anisotropy of the stellar halo. Ours is the first measurement of the anisotropy parameter β using 3D kinematics outside of the solar neighborhood. We find , consistent with isotropy and lower than solar neighborhood β measurements by 2 (βSN ∼ 0.5-0.7). We identify two stars in our sample that are likely members of the known TriAnd substructure, and excluding these objects from our sample increases our estimate of the anisotropy to , which is still lower than solar neighborhood measurements by 1 . The potential decrease in β with Galactocentric radius is inconsistent with theoretical predictions, though consistent with recent observational studies, and may indicate the presence of large, shell-type structure (or structures) at r ∼ 25 kpc. The methods described in this paper will be applied to a much larger sample of stars with 3D kinematics observed through the ongoing HALO7D program.
We present the design and performance of the multi-object fiber spectrographs for the Sloan Digital Sky Survey (SDSS) and their upgrade for the Baryon Oscillation Spectroscopic Survey (BOSS). ...Originally commissioned in Fall 1999 on the 2.5 m aperture Sloan Telescope at Apache Point Observatory, the spectrographs produced more than 1.5 million spectra for the SDSS and SDSS-II surveys, enabling a wide variety of Galactic and extra-galactic science including the first observation of baryon acoustic oscillations in 2005. The spectrographs were upgraded in 2009 and are currently in use for BOSS, the flagship survey of the third-generation SDSS-III project. BOSS will measure redshifts of 1.35 million massive galaxies to redshift 0.7 and Ly alpha absorption of 160,000 high redshift quasars over 10,000 deg super(2) of sky, making percent level measurements of the absolute cosmic distance scale of the universe and placing tight constraints on the equation of state of dark energy. The twin multi-object fiber spectrographs utilize a simple optical layout with reflective collimators, gratings, all-refractive cameras, and state-of-the-art CCD detectors to produce hundreds of spectra simultaneously in two channels over a bandpass covering the near-ultraviolet to the near-infrared, with a resolving power R = lambda/FWHM ~ 2000. Building on proven heritage, the spectrographs were upgraded for BOSS with volume-phase holographic gratings and modern CCD detectors, improving the peak throughput by nearly a factor of two, extending the bandpass to cover 360 nm < lambda < 1000 nm, and increasing the number of fibers from 640 to 1000 per exposure. In this paper we describe the original SDSS spectrograph design and the upgrades implemented for BOSS, and document the predicted and measured performances.
We present the metallicity distribution function (MDF) for 24,270 G and 16,847 K dwarfs at distances from 0.2 to 2.3 kpc from the Galactic plane, based on spectroscopy from the Sloan Extension for ...Galactic Understanding and Exploration (SEGUE) survey. This stellar sample is significantly larger in both number and volume than previous spectroscopic analyses, which were limited to the solar vicinity, making it ideal for comparison with local volume-limited samples and Galactic models. For the first time, we have corrected the MDF for the various observational biases introduced by the SEGUE target-selection strategy. SEGUE is particularly notable for its sample of K dwarfs, which are too faint to examine spectroscopically far from the solar neighborhood. The MDF of both spectral types becomes more metal-poor with increasing |Z|, which reflects the transition from a sample with small alpha/Fe values at small heights to one with enhanced alpha/Fe above 1 kpc. Comparison of our SEGUE distributions to those of two different Milky Way models reveals that both are more metal-rich than our observed distributions at all heights above the plane. Our unbiased observations of G and K dwarfs provide valuable constraints over the |Z|-height range of the Milky Way disk for chemical and dynamical Galaxy evolution models, previously only calibrated to the solar neighborhood, with particular utility for thin- and thick-disk formation models.
Using G dwarfs from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey, we have determined the vertical metallicity gradient in the Milky Way's disk and examined how this ...gradient varies for different alpha /Fe subsamples. Our sample contains over 40,000 stars with low-resolution spectroscopy over 144 lines of sight. It also covers a significant disk volume, between ~0.3 and 1.6 kpc from the Galactic plane, and allows us to examine the disk in situ, whereas previous analyses were more limited in scope. Furthermore, this work does not presuppose a disk structure, whether composed of a single complex population or distinct thin and thick disk components. We employ the SEGUE Stellar Parameter Pipeline to obtain estimates of stellar parameters, Fe/H, and alpha /Fe and extract multiple volume-complete subsamples of approximately 1000 stars each. Based on SEGUE's target-selection algorithm, we adjust each subsample to determine an unbiased picture of disk chemistry; consequently, each individual star represents the properties of many. The metallicity gradient is -0.243 super(+0.039) sub(-0.053) dex kpc super(-1) for the entire sample, which we compare to various literature results. This gradient stems from the different alpha /Fe populations inhabiting different ranges of height above the Galactic plane. Each alpha /Fe subsample shows little change in median Fe/H with height. If we associate alpha /Fe with age, the negligible gradients of our alpha /Fe subsamples suggest that stars formed in different epochs exhibit comparable vertical structure, implying similar star formation processes and evolution.
This paper describes the discovery of seven dwarf objects of spectral type L (objects cooler than the latest M dwarfs) in commissioning imaging data taken by the Sloan Digital Sky Survey (SDSS). ...Low-resolution spectroscopy shows that they have spectral types from L0 to L8. Comparison of the SDSS and the Two Micron All Sky Survey (2MASS) photometry for several of these objects indicates the presence of significant opacity at optical wavelengths. This comparison also demonstrates the high astrometric accuracy (better than 1{sup ''} for these faint sources) of both surveys. The L dwarfs are shown to occupy a distinctive region of color-color space as measured in the SDSS filters, which should enables their identification in a straightforward way. This should lead eventually to a complete sample of many hundreds of these low-mass objects, or about 1 per 15 deg2 to i{sup '} {approx_equal}20, in the complete SDSS data set. (c) (c) 2000. The American Astronomical Society.
ABSTRACT There is an overwhelming evidence that white dwarfs host planetary systems; revealed by the presence, disruption, and accretion of planetary bodies. A lower limit on the frequency of white ...dwarfs that host planetary material has been estimated to be ≃ 25–50 per cent; inferred from the ongoing or recent accretion of metals on to both hydrogen-atmosphere and warm helium-atmosphere white dwarfs. Now with the unbiased sample of white dwarfs observed by the Dark Energy Spectroscopic Instrument (DESI) survey in their Early Data Release (EDR), we have determined the frequency of metal enrichment around cool-helium atmosphere white dwarfs as 21 ± 3 per cent using a sample of 234 systems. This value is in good agreement with values determined from previous studies. With the current samples we cannot distinguish whether the frequency of planetary accretion varies with system age or host-star mass, but the DESI data release 1 will contain roughly an order of magnitude more white dwarfs than DESI EDR and will allow these parameters to be investigated.
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