arXiv subjects

Steven R. Majewski

Publications and source records attributed to Steven R. Majewski.

At least 109 records · Page 6Linked to original sources

arXiv2018

Constraining the Solar Galactic Reflex Velocity Using Gaia Observations of the Sagittarius Stream

Because of its particular orientation around the Galaxy - i.e., in a plane nearly perpendicular to the Galactic plane and containing both the Sun and Galactic center - the Sagittarius (Sgr) stream provides a powerful means by which to measure the solar reflex velocity, and thereby infer the velocity of the Local Standard of Rest (LSR), in a way that is independent of assumptions about the solar Galactocentric distance. Moreover, the solar reflex velocity with respect to the stream is projected almost entirely into the proper motion component of Sgr stream stars perpendicular to the Sgr plane, which makes the inferred velocity relatively immune to most Sgr model assumptions. Using Gaia DR2 proper motions of ~2,000 stars identified to be Sgr stream candidates in concert with the Law and Majewski (2010) Sgr N-body models (which provide a good match to the Gaia observations) we constrain the solar reflex velocity induced by its orbital motion around the Galaxy to be $Θ_{\odot} = 253 \pm 6$ km/s. Assuming a solar peculiar motion in the direction of orbital rotation of 12 km/s, and an LSR velocity of 12 km/s with respect to the local circular speed, the implied circular speed of the Milky Way at the solar circle is $229 \pm 6$ km/s.

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arXiv2018

Orbital Decay in Binaries Containing Post-Main Sequence Stars

The orbital decay of binaries containing a primary sub-giant or red giant star and a stellar or substellar companion is investigated. The tide raised in the primary by the companion leads to an exchange of angular momentum between the orbit and the stellar spin, causing the orbit to contract and the primary to spin up. The rate of orbital decay is computed including both the equilibrium tide, damped by turbulent viscosity in the convective envelope, and the dynamical tide, assumed to be a traveling internal-gravity wave in the radiative core. For close binaries, the tidal forcing period is expected to be much shorter than the eddy turnover timescale in the convective envelope, and the prescription for "reduced" viscosity is an important consideration. The dynamical tide tends to dominate for the closest orbits, while the equilibrium tide dominates for more distant orbits, with the crossover point depending on the stellar mass. The spin up of the primary to synchronous rotation occurs for sufficiently massive secondaries, and this greatly slows the orbital decay until the Darwin instability occurs. A parameter survey is presented for orbital decay as a function of primary and secondary mass, as well as turbulent viscosity prescription. These results are summarized with analytic formulae and numerical results for the age-dependent critical separation, $a_{\rm crit}$, inside of which orbital decay is rapid, and few systems are expected to be observed. The calculations of $a_{\rm crit}$ are compared with APOGEE binaries, as well as solar mass exoplanet host stars.

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arXiv2018

SMHASH: A new mid-infrared RR Lyrae distance determination for the Local Group dwarf spheroidal galaxy Sculptor

We present a new distance estimation for the Milky Way dwarf spheroidal satellite Sculptor obtained from multi-epoch mid-infrared observations of RR Lyrae stars. The 3.6 μm observations have been acquired with the Infrared Array Camera on board the Spitzer Space Telescope as part of the SMHASH Program. Mid-infrared light curves for 42 RRL were obtained, from which we measured Sculptor's distance modulus to be μ = 19.60 $\pm$ 0.02 (statistical) $\pm$ 0.04 (photometric) mag (with $σ_{sys}=$ = 0.09 mag), using the 3.6 μm empirical period-luminosity relations derived from the Galactic globular cluster M4, or μ = 19.57 $\pm$ 0.02 (statistical) $\pm$ 0.04 (photometric) mag (with $σ_{sys}=$ = 0.11 mag) using empirical relations in the same passband recently derived from the Large Magellanic Cloud globular cluster Reticulum. Both these measurements are in good agreement with values presented in previous works with Sculptor RR Lyrae stars in optical bands, and are also consistent with recent near-infrared RR Lyrae results. Best agreement with the literature is found for the latter modulus which is equivalent to a distance of d = 82 $\pm$ 1 (statistical) $\pm$ 2 (photometric) kpc (with $σ_{sys}=$ = 4 kpc). Finally, using a subsample of RR Lyrae stars with spectroscopic metallicities, we demonstrate that these distance estimates are not affected by metallicity effects.

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arXiv2018

APOGEE Data Releases 13 and 14: Stellar Parameter and Abundance Comparisons With Independent Analyses

Data from the SDSS-IV / Apache Point Observatory Galactic Evolution Experiment (APOGEE-2) have been released as part of SDSS Data Releases 13 (DR13) and 14 (DR14). These include high resolution H-band spectra, radial velocities, and derived stellar parameters and abundances. DR13, released in August 2016, contained APOGEE data for roughly 150,000 stars, and DR14, released in August 2017, added about 110,000 more. Stellar parameters and abundances have been derived with an automated pipeline, the APOGEE Stellar Parameter and Chemical Abundance Pipeline (ASPCAP). We evaluate the performance of this pipeline by comparing the derived stellar parameters and abundances to those inferred from optical spectra and analysis for several hundred stars. For most elements -- C, Na, Mg, Al, Si, S, Ca, Cr, Mn, Ni -- the DR14 ASPCAP analysis have systematic differences with the comparisons samples of less than 0.05 dex (median), and random differences of less than 0.15 dex (standard deviation). These differences are a combination of the uncertainties in both the comparison samples as well as the ASPCAP-analysis. Compared to the references, magnesium is the most accurate alpha-element derived by ASPCAP, and shows a very clear thin/thick disk separation, while nickel is the most accurate iron-peak element (besides iron).

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arXiv2018

APOGEE Data Releases 13 and 14: Data and Analysis

Data and analysis methodology used for the SDSS/APOGEE Data Releases 13 and 14 are described, highlighting differences from the DR12 analysis presented in Holtzman (2015). Some improvement in the handling of telluric absorption and persistence is demonstrated. The derivation and calibration of stellar parameters, chemical abundances, and respective uncertainties are described, along with the ranges over which calibration was performed. Some known issues with the public data related to the calibration of the effective temperatures (DR13), surface gravity (DR13 and DR14), and C and N abundances for dwarfs (DR13 and DR14) are highlighted. We discuss how results from a data-driven technique, The Cannon (Casey 2016), are included in DR14, and compare those with results from the APOGEE Stellar Parameters and Chemical Abundances Pipeline (ASPCAP). We describe how using The Cannon in a mode that restricts the abundance analysis of each element to regions of the spectrum with known features from that element leads to Cannon abundances can lead to significantly different results for some elements than when all regions of the spectrum are used to derive abundances.

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arXiv2018

44 New & Known M Dwarf Multiples In The SDSS-III/APOGEE M Dwarf Ancillary Science Sample

Binary stars make up a significant portion of all stellar systems. Consequently, an understanding of the bulk properties of binary stars is necessary for a full picture of star formation. Binary surveys indicate that both multiplicity fraction and typical orbital separation increase as functions of primary mass. Correlations with higher order architectural parameters such as mass ratio are less well constrained. We seek to identify and characterize double-lined spectroscopic binaries (SB2s) among the 1350 M dwarf ancillary science targets with APOGEE spectra in the SDSS-III Data Release 13. We measure the degree of asymmetry in the APOGEE pipeline cross-correlation functions (CCFs), and use those metrics to identify a sample of 44 high-likelihood candidate SB2s. At least 11 of these SB2s are known, having been previously identified by Deshapnde et al, and/or El Badry et al. We are able to extract radial velocities (RVs) for the components of 36 of these systems from their CCFs. With these RVs, we measure mass ratios for 29 SB2s and 5 SB3s. We use Bayesian techniques to fit maximum likelihood (but still preliminary) orbits for 4 SB2s with 8 or more distinct APOGEE observations. The observed (but incomplete) mass ratio distribution of this sample rises quickly towards unity. Two-sided Kolmogorov-Smirnov tests and probabilities of 18.3% and 18.7%, demonstrating that the mass ratio distribution of our sample is consistent with those measured by Pourbaix et al. and Fernandez et al., respectively.

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arXiv2018

The Remarkable Be+sdOB Binary HD 55606 I: Orbital and Stellar Parameters

Prompted by peculiar spectroscopic variability observed in SDSS/APOGEE $H$-band spectra, we monitored the Be star HD 55606 using optical spectroscopy and found that it is an exotic double-lined spectroscopic binary (SB2) consisting of a Be star and a hot, compact companion that is probably an OB subdwarf (sdOB) star. Motion of the sdOB star is traced by its impact on the strong He~I lines, observed as radial velocity ($V_{r}$) variable, double-peaked emission profiles with narrow central absorption cores. Weak He II 4686 Å absorption associated with the companion star is detected in most spectra. Use of the emission peaks of low-ionization emission lines to trace the Be star $V_{r}$ and the He I lines to trace the companion star $V_{r}$ yields a circular orbital solution with a 93.8-day period and masses of $M_{\rm Be}=6.2$ $M_{\rm \odot}$ and $M_{\rm sdOB}=0.9$ $M_{\rm \odot}$ in the case of $i=80^{\circ}$. HD 55606 exhibits a variety of phase-locked variability, including the development of shell lines twice per orbit. The shell phases coincide with variation in the double emission peak separations, and both forms of variability are likely caused by a two-armed spiral density perturbation in the Be disk. The intensity ratios of the double emission peaks are also phase-locked, possibly indicating heating by the sdOB star of the side of the Be disk facing it. HD 55606 is a new member of the growing sample of Be+sdOB binaries, in which the Be star's rapid rotation and ability to form a disk can be attributed to past mass transfer.

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arXiv2018

Stellar and Planetary Characterization of the Ross 128 Exoplanetary System from APOGEE Spectra

The first detailed chemical abundance analysis of the M dwarf (M4.0) exoplanet-hosting star Ross 128 is presented here, based upon near-infrared (1.5--1.7 \micron) high-resolution ($R$$\sim$22,500) spectra from the SDSS-APOGEE survey. We determined precise atmospheric parameters $T_{\rm eff}$=3231$\pm$100K, log$g$=4.96$\pm$0.11 dex and chemical abundances of eight elements (C, O, Mg, Al, K, Ca, Ti, and Fe), finding Ross 128 to have near solar metallicity ([Fe/H] = +0.03$\pm$0.09 dex). The derived results were obtained via spectral synthesis (1-D LTE) adopting both MARCS and PHOENIX model atmospheres; stellar parameters and chemical abundances derived from the different adopted models do not show significant offsets. Mass-radius modeling of Ross 128b indicate that it lies below the pure rock composition curve, suggesting that it contains a mixture of rock and iron, with the relative amounts of each set by the ratio of Fe/Mg. If Ross 128b formed with a sub-solar Si/Mg ratio, and assuming the planet's composition matches that of the host-star, it likely has a larger core size relative to the Earth. The derived planetary parameters -- insolation flux (S$_{\rm Earth}$=1.79$\pm$0.26) and equilibrium temperature ($T_{\rm eq}$=294$\pm$10K) -- support previous findings that Ross 128b is a temperate exoplanet in the inner edge of the habitable zone.

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arXiv2018

Kepler-503b: An Object at the Hydrogen Burning Mass Limit Orbiting a Subgiant Star

Using spectroscopic radial velocities with the APOGEE instrument and Gaia distance estimates, we demonstrate that Kepler-503b, currently considered a validated Kepler planet, is in fact a brown-dwarf/low-mass star in a nearly circular 7.2-day orbit around a subgiant star. Using a mass estimate for the primary star derived from stellar models, we derive a companion mass and radius of $0.075\pm0.003 \ M_{\odot}$ ($78.6\pm3.1 \ M_{Jup}$) and $0.099^{+0.006}_{-0.004}\ R_{\odot}$ ($0.96^{+0.06}_{-0.04}\ R_{Jup}$), respectively. Assuming the system is coeval, the evolutionary state of the primary indicates the age is $\sim6.7$ Gyr. Kepler-503b sits right at the hydrogen burning mass limit, straddling the boundary between brown dwarfs and very low-mass stars. More precise radial velocities and secondary eclipse spectroscopy with James Webb Space Telescope will provide improved measurements of the physical parameters and age of this important system to better constrain and understand the physics of these objects and their spectra. This system emphasizes the value of radial velocity observations to distinguish a genuine planet from astrophysical false positives, and is the first result from the SDSS-IV monitoring of Kepler planet candidates with the multi-object APOGEE instrument.

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arXiv2018

Speckle Imaging Through a Coherent Fiber Bundle

Speckle imaging is a well known method to achieve diffraction-limited (DL) imaging from ground-based telescopes. The traditional observing method for speckle has been to observe a single, unresolved, source per telescope pointing over a relatively small field of view (FOV). The need for large DL surveys of targets with high sky density motivates a desire for simultaneous speckle imaging over large FOVs, however it is currently impractical to attain this by covering the entire focal plane with fast readout detectors. An alternative approach is to connect a relatively small number of detector pixels to multiple interesting targets spanning a large FOV through the use of optical fibers, a technique employed in spectroscopy for decades. However, for imaging we require the use of coherent fiber bundles (CFBs). We discuss various design considerations for coherent fiber speckle imaging with an eye toward a multiplexed system using numerous configurable CFBs, and we test its viability with a prototype instrument that uses a single CFB to transport speckle images from the telescope focal plane to a traditionally designed, fast readout speckle camera. Using this device on University of Virginia's Fan Mountain Observatory 40-inch telescope we have for the first time successfully demonstrated speckle imaging through a CFB, using both optical and NIR detectors. Results are presented of DL speckle imaging of well-known close (including subarcsecond) binary stars resolved with this fiber-fed speckle system and compared to both literature results and traditional speckle imaging taken with the same camera directly, with no intervening CFB.

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arXiv2018

Disk-Like Chemistry of the Triangulum-Andromeda Overdensity as Seen by APOGEE

The nature of the Triangulum-Andromeda (TriAnd) system has been debated since the discovery of this distant, low-latitude Milky Way (MW) overdensity more than a decade ago. Explanations for its origin are either as a halo substructure from the disruption of a dwarf galaxy or a distant extension of the Galactic disk. We test these hypotheses using chemical abundances of a dozen TriAnd members from the Sloan Digital Sky Survey's 14th Data Release of Apache Point Observatory Galactic Evolution Experiment (APOGEE) data to compare to APOGEE abundances of stars with similar metallicity from both the Sagittarius (Sgr) dSph, and the outer MW disk. We find that TriAnd stars are chemically distinct from Sgr across a variety of elements, (C+N), Mg, K, Ca, Mn, and Ni, with a separation in [X/Fe] of about 0.1 to 0.4 dex depending on the element. Instead, the TriAnd stars, with a median metallicity of about -0.8, exhibit chemical abundance ratios similar to those of the lowest metallicity ([Fe/H] ~ -0.7) stars in the outer Galactic disk, and are consistent with expectations of extrapolated chemical gradients in the outer disk of the MW. These results suggest that TriAnd is associated with the MW disk, and, therefore, that the disk extends to this overdensity --- i.e., past a Galactocentric radius of 24 kpc --- albeit vertically perturbed about 7 kpc below the nominal disk midplane in this region of the Galaxy.

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arXiv2018

Exploring the Very Extended Low Surface Brightness Stellar Populations of the Large Magellanic Cloud with SMASH

We present the detection of very extended stellar populations around the Large Magellanic Cloud (LMC) out to R~21 degrees, or ~18.5 kpc at the LMC distance of 50 kpc, as detected in the Survey of the MAgellanic Stellar History (SMASH) performed with the Dark Energy Camera on the NOAO Blanco 4m Telescope. The deep (g~24) SMASH color magnitude diagrams (CMDs) clearly reveal old (~9 Gyr), metal-poor ([Fe/H]=-0.8 dex) main-sequence stars at a distance of 50 kpc. The surface brightness of these detections is extremely low with our most distant detection having 34 mag per arcsec squared in g-band. The SMASH radial density profile breaks from the inner LMC exponential decline at ~13-15 degrees and a second component at larger radii has a shallower slope with power-law index of -2.2 that contributes ~0.4% of the LMC's total stellar mass. In addition, the SMASH densities exhibit large scatter around our best-fit model of ~70% indicating that the envelope of stellar material in the LMC periphery is highly disturbed. We also use data from the NOAO Source catalog to map the LMC main-sequence populations at intermediate radii and detect a steep dropoff in density on the eastern side of the LMC (at R~8 deg) as well as an extended structure to the far northeast. These combined results confirm the existence of a very extended, low-density envelope of stellar material with disturbed shape around the LMC. The exact origin of this structure remains unclear but the leading options include a classical accreted halo or tidally stripped outer disk material.

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arXiv2018

The APOGEE-2 Survey of the Orion Star Forming Complex: I. Target Selection and Validation with early observations

The Orion Star Forming Complex (OSFC) is a central target for the APOGEE-2 Young Cluster Survey. Existing membership catalogs span limited portions of the OSFC, reflecting the difficulty of selecting targets homogeneously across this extended, highly structured region. We have used data from wide field photometric surveys to produce a less biased parent sample of young stellar objects (YSOs) with infrared (IR) excesses indicative of warm circumstellar material or photometric variability at optical wavelengths across the full 420 square degrees extent of the OSFC. When restricted to YSO candidates with H < 12.4, to ensure S/N ~100 for a six visit source, this uniformly selected sample includes 1307 IR excess sources selected using criteria vetted by Koenig & Liesawitz and 990 optical variables identified in the Pan-STARRS1 3$π$ survey: 319 sources exhibit both optical variability and evidence of circumstellar disks through IR excess. Objects from this uniformly selected sample received the highest priority for targeting, but required fewer than half of the fibers on each APOGEE-2 plate. We fill the remaining fibers with previously confirmed and new color-magnitude selected candidate OSFC members. Radial velocity measurements from APOGEE-1 and new APOGEE-2 observations taken in the survey's first year indicate that ~90% of the uniformly selected targets have radial velocities consistent with Orion membership.The APOGEE-2 Orion survey will include >1100 bona fide YSOs whose uniform selection function will provide a robust sample for comparative analyses of the stellar populations and properties across all sub-regions of Orion.

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arXiv2018

Binary companions of evolved stars in APOGEE DR14: Search method and catalog of ~5,000 companions

Multi-epoch radial velocity measurements of stars can be used to identify stellar, sub-stellar, and planetary-mass companions. Even a small number of observation epochs can be informative about companions, though there can be multiple qualitatively different orbital solutions that fit the data. We have custom-built a Monte Carlo sampler (The Joker) that delivers reliable (and often highly multi-modal) posterior samplings for companion orbital parameters given sparse radial-velocity data. Here we use The Joker to perform a search for companions to 96,231 red-giant stars observed in the APOGEE survey (DR14) with $\geq 3$ spectroscopic epochs. We select stars with probable companions by making a cut on our posterior belief about the amplitude of the stellar radial-velocity variation induced by the orbit. We provide (1) a catalog of 320 companions for which the stellar companion properties can be confidently determined, (2) a catalog of 4,898 stars that likely have companions, but would require more observations to uniquely determine the orbital properties, and (3) posterior samplings for the full orbital parameters for all stars in the parent sample. We show the characteristics of systems with confidently determined companion properties and highlight interesting systems with candidate compact object companions.

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arXiv2018

Age-resolved chemistry of red giants in the solar neighbourhood

In the age of high-resolution spectroscopic stellar surveys of the Milky Way, the number of stars with detailed abundances of multiple elements is rapidly increasing. These elemental abundances are directly influenced by the evolutionary history of the Galaxy, but this can be difficult to interpret without an absolute timeline of the abundance enrichment. We present age-abundance trends for [M/H], [α/M], and 17 individual elements using a sample of 721 solar neighbourhood Hipparcos red giant stars observed by APOGEE. These age trends are determined through a Bayesian hierarchical modelling method presented by Feuillet et al. (2016). We confirm that the [α/M]- age relation in the solar neighbourhood is steep and relatively narrow (0.20 dex age dispersion), as are the [O/M]- and [Mg/M]-age relations. The age trend of [C/N] is steep and smooth, consistent with stellar evolution. The [M/H]-age relation has a mean age dispersion of 0.28 dex and a complex overall structure. The oldest stars in our sample are those with the lowest and highest metallicities, while the youngest stars are those with solar metallicity. These results provide strong constraints on theoretical models of Galactic chemical evolution (GCE). We compare them to the predictions of one-zone GCE mod- els and multi-zone mixtures, both analytic and numerical. These comparisons support the hypothesis that the solar neighbourhood is composed of stars born at a range of Galactocentric radii, and that the most metal-rich stars likely migrated from a region with earlier and more rapid star formation such as the inner Galaxy.

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arXiv2018

Chemical Abundances of Main-Sequence, Turn-off, Subgiant and red giant Stars from APOGEE spectra I: Signatures of Diffusion in the Open Cluster M67

Detailed chemical abundance distributions for fourteen elements are derived for eight high-probability stellar members of the solar metallicity old open cluster M67 with an age of $\sim$4 Gyr. The eight stars consist of four pairs, with each pair occupying a distinct phase of stellar evolution: two G-dwarfs, two turnoff stars, two G-subgiants, and two red clump K-giants. The abundance analysis uses near-IR high-resolution spectra ($λ$1.5 -- 1.7$μ$m) from the APOGEE survey and derives abundances for C, N, O, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, and Fe. Our derived stellar parameters and metallicity for 2M08510076+113115 suggest that this star is a solar-twin, exhibiting abundance differences relative to the Sun of $\leq$ 0.04 dex for all elements. Chemical homogeneity is found within each class of stars ($\sim$0.02 dex), while significant abundance variations ($\sim$0.05 -- 0.20 dex) are found across the different evolutionary phases; the turnoff stars typically have the lowest abundances, while the red clump tend to have the largest. Non-LTE corrections to the LTE-derived abundances are unlikely to explain the differences. A detailed comparison of the derived Fe, Mg, Si, and Ca abundances with recently published surface abundances from stellar models that include chemical diffusion, provides a good match between the observed and predicted abundances as a function of stellar mass. Such agreement would indicate the detection of chemical diffusion processes in the stellar members of M67.

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arXiv2018

The Metal-poor non-Sagittarius (?) Globular Cluster NGC 5053: Orbit and Mg, Al and Si Abundances

Metal-poor globular clusters (GCs) exhibit intriguing Al-Mg anti-correlations and possible Si-Al correlations, which are important clues to decipher the multiple-population phenomenon. NGC 5053 is one of the most metal-poor GCs in the nearby Universe, and has been suggested to be associated with the Sagittarius (Sgr) dwarf galaxy, due to its similarity in location and radial velocity with one of the Sgr arms. In this work, we simulate the orbit of NGC 5053, and argue against a physical connection between Sgr and NGC 5053. On the other hand, the Mg, Al, and Si spectral lines, which are difficult to detect in the optical spectra of NGC 5053 stars, have been detected in the near-infrared APOGEE spectra. We use three different sets of stellar parameters and codes to derive the Mg, Al, and Si abundances. Regardless of which method is adopted, we see a large Al variation, and a substantial Si spread. Along with NGC 5053, metal-poor GCs exhibit different Mg, Al, and Si variations. Moreover, NGC 5053 has the lowest cluster mass among the GCs that have been identified to exhibit an observable Si spread until now.

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arXiv2018

Stellar Multiplicity Meets Stellar Evolution And Metallicity: The APOGEE View

We use the multi-epoch radial velocities acquired by the APOGEE survey to perform a large scale statistical study of stellar multiplicity for field stars in the Milky Way, spanning the evolutionary phases between the main sequence and the red clump. We show that the distribution of maximum radial velocity shifts (\drvm) for APOGEE targets is a strong function of \logg, with main sequence stars showing \drvm\ as high as $\sim$300 \kms, and steadily dropping down to $\sim$30 \kms\ for \logg$\sim$0, as stars climb up the Red Giant Branch (RGB). Red clump stars show a distribution of \drvm\ values comparable to that of stars at the tip of the RGB, implying they have similar multiplicity characteristics. The observed attrition of high \drvm\ systems in the RGB is consistent with a lognormal period distribution in the main sequence and a multiplicity fraction of 0.35, which is truncated at an increasing period as stars become physically larger and undergo mass transfer after Roche Lobe Overflow during H shell burning. The \drvm\ distributions also show that the multiplicity characteristics of field stars are metallicity dependent, with metal-poor ([Fe/H]$\lesssim-0.5$) stars having a multiplicity fraction a factor 2-3 higher than metal-rich ([Fe/H]$\gtrsim0.0$) stars. This has profound implications for the formation rates of interacting binaries observed by astronomical transient surveys and gravitational wave detectors, as well as the habitability of circumbinary planets.

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