arXiv subjects

Steven R. Majewski

Publications and source records attributed to Steven R. Majewski.

At least 145 records · Page 8Linked to original sources

arXiv2016

Two Groups of Red Giants with Distinct Chemical Abundances in the Bulge Globular Cluster NGC 6553 Through the Eyes of APOGEE

Multiple populations revealed in globular clusters (GCs) are important windows to the formation and evolution of these stellar systems. The metal-rich GCs in the Galactic bulge are an indispensable part of this picture, but the high optical extinction in this region has prevented extensive research. In this work, we use the high resolution near-infrared (NIR) spectroscopic data from APOGEE to study the chemical abundances of NGC 6553, which is one of the most metal-rich bulge GCs. We identify ten red giants as cluster members using their positions, radial velocities, iron abundances, and NIR photometry. Our sample stars show a mean radial velocity of $-0.14\pm5.47$ km s$^{-1}$, and a mean [Fe/H] of $-0.15\pm 0.05$. We clearly separate two populations of stars in C and N in this GC for the first time. NGC 6553 is the most metal-rich GC where the multiple stellar population phenomenon is found until now. Substantial chemical variations are also found in Na, O, and Al. However, the two populations show similar Si, Ca, and iron-peak element abundances. Therefore, we infer that the CNO, NeNa, and MgAl cycles have been activated, but the MgAl cycle is too weak to show its effect on Mg. Type Ia and Type II supernovae do not seem to have significantly polluted the second generation stars. Comparing with other GC studies, NGC 6553 shows similar chemical variations as other relatively metal-rich GCs. We also confront current GC formation theories with our results, and suggest possible avenues for improvement in the models.

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arXiv2016

The Stellar Density Profile of the Distant Galactic Halo

We use extensive gravity-sensitive DDO 51 photometry over 5100 square degrees, combined with SDSS broadband photometry, to select a catalog of $\sim 4,000$ giant stars covering a large fraction of the high Galactic latitude sky and reaching out to $\sim 80$ kpc in the Galactic halo. This sample of bright and unbiased tracers enables us to measure the radial profile and 3D structure of the stellar halo to large distance which had previously only been measured with sparse tracers or small samples. Using population synthesis models to reproduce the observed giant star luminosity function, we find that the halo maintains a $r^{-3.5}$ profile from $30$ to $80$ kpc with no signs of a truncation or sharp break over this range. The radial profile measurement is largely insensitive to individual halo substructure components, but we find that attempting to measure the shape of the halo is overwhelmed by the Sagittarius stream such that no ellipsoidal shape is a satisfactory description in this region. These measurements allow us to begin placing the Milky Way in context with the growing sample of external galaxies where similar halo profile measurements are available, with the goal of further linking the properties of stellar halos to the accretion histories that formed them.

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arXiv2016

Chemical tagging can work: Identification of stellar phase-space structures purely by chemical-abundance similarity

Chemical tagging promises to use detailed abundance measurements to identify spatially separated stars that were in fact born together (in the same molecular cloud), long ago. This idea has not yielded much practical success, presumably because of the noise and incompleteness in chemical-abundance measurements. We have succeeded in substantially improving spectroscopic measurements with The Cannon, which has now delivered 15 individual abundances for ~100,000 stars observed as part of the APOGEE spectroscopic survey, with precisions around 0.04 dex. We test the chemical-tagging hypothesis by looking at clusters in abundance space and confirming that they are clustered in phase space. We identify (by the k-means algorithm) overdensities of stars in the 15-dimensional chemical-abundance space delivered by The Cannon, and plot the associated stars in phase space. We use only abundance-space information (no positional information) to identify stellar groups. We find that clusters in abundance space are indeed clusters in phase space. We recover some known phase-space clusters and find other interesting structures. This is the first-ever project to identify phase-space structures at survey-scale by blind search purely in abundance space; it verifies the precision of the abundance measurements delivered by The Cannon; the prospects for future data sets appear very good.

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arXiv2016

Carbon Stars in the Satellites and Halo of M31

We spectroscopically identify a sample of carbon stars in the satellites and halo of M31 using moderate-resolution optical spectroscopy from the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo survey. We present the photometric properties of our sample of 41 stars, including their brightness with respect to the tip of the red giant branch (TRGB) and their distributions in various color-color spaces. This analysis reveals a bluer population of carbon stars fainter than the TRGB and a redder population of carbon stars brighter than the TRGB. We then apply principal component analysis to determine the sample's eigenspectra and eigencoefficients. Correlating the eigencoefficients with various observable properties reveals the spectral features that trace effective temperature and metallicity. Putting the spectroscopic and photometric information together, we find the carbon stars in the satellites and halo of M31 to be minimally impacted by dust and internal dynamics. We also find that while there is evidence to suggest that the sub-TRGB stars are extrinsic in origin, it is also possible that they are are particularly faint members of the asymptotic giant branch.

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arXiv2016

Chemical tagging with APOGEE: Discovery of a large population of N-rich stars in the inner Galaxy

Formation of globular clusters (GCs), the Galactic bulge, or galaxy bulges in general, are important unsolved problems in Galactic astronomy. Homogeneous infrared observations of large samples of stars belonging to GCs and the Galactic bulge field are one of the best ways to study these problems. We report the discovery by APOGEE of a population of field stars in the inner Galaxy with abundances of N, C, and Al that are typically found in GC stars. The newly discovered stars have high [N/Fe], which is correlated with [Al/Fe] and anti-correlated with [C/Fe]. They are homogeneously distributed across, and kinematically indistinguishable from, other field stars in the same volume. Their metallicity distribution is seemingly unimodal, peaking at [Fe/H]~-1, thus being in disagreement with that of the Galactic GC system. Our results can be understood in terms of different scenarios. N-rich stars could be former members of dissolved GCs, in which case the mass in destroyed GCs exceeds that of the surviving GC system by a factor of ~8. In that scenario, the total mass contained in so-called "first-generation" stars cannot be larger than that in "second-generation" stars by more than a factor of ~9 and was certainly smaller. Conversely, our results may imply the absence of a mandatory genetic link between "second generation" stars and GCs. Last, but not least, N-rich stars could be the oldest stars in the Galaxy, the by-products of chemical enrichment by the first stellar generations formed in the heart of the Galaxy.

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arXiv2016

Companions to APOGEE Stars I: A Milky Way-Spanning Catalog of Stellar and Substellar Companion Candidates and their Diverse Hosts

In its three years of operation, the Sloan Digital Sky Survey (SDSS-III) Apache Point Observatory Galactic Evolution Experiment (APOGEE-1) observed $>$14,000 stars with enough epochs over a sufficient temporal baseline for the fitting of Keplerian orbits. We present the custom orbit-fitting pipeline used to create this catalog, which includes novel quality metrics that account for the phase and velocity coverage of a fitted Keplerian orbit. With a typical RV precision of $\sim100-200$ m s$^{-1}$, APOGEE can probe systems with small separation companions down to a few Jupiter masses. Here we present initial results from a catalog of 382 of the most compelling stellar and substellar companion candidates detected by APOGEE, which orbit a variety of host stars in diverse Galactic environments. Of these, 376 have no previously known small separation companion. The distribution of companion candidates in this catalog shows evidence for an extremely truncated brown dwarf (BD) desert with a paucity of BD companions only for systems with $a < 0.1-0.2$ AU, with no indication of a desert at larger orbital separation. We propose a few potential explanations of this result, some which invoke this catalog's many small separation companion candidates found orbiting evolved stars. Furthermore, 16 BD and planet candidates have been identified around metal-poor ([Fe/H] $< -0.5$) stars in this catalog, which may challenge the core accretion model for companions $>10 M_{Jup}$. Finally, we find all types of companions are ubiquitous throughout the Galactic disk with candidate planetary-mass and BD companions to distances of $\sim6$ and $\sim16$ kpc, respectively.

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arXiv2016

Chemical abundance gradients from open clusters in the Milky Way disk: results from the APOGEE survey

Metallicity gradients provide strong constraints for understanding the chemical evolution of the Galaxy. We report on radial abundance gradients of Fe, Ni, Ca, Si, and Mg obtained from a sample of 304 red-giant members of 29 disk open clusters, mostly concentrated at galactocentric distances between ~8 - 15 kpc, but including two open clusters in the outer disk. The observations are from the APOGEE survey. The chemical abundances were derived automatically by the ASPCAP pipeline and these are part of the SDSS III Data Release 12. The gradients, obtained from least squares fits to the data, are relatively flat, with slopes ranging from -0.026 to -0.033 dex/kpc for the alpha-elements [O/H], [Ca/H], [Si/H] and [Mg/H] and -0.035 dex/kpc and -0.040 dex/kpc for [Fe/H] and [Ni/H], respectively. Our results are not at odds with the possibility that metallicity ([Fe/H]) gradients are steeper in the inner disk (R_GC ~7 - 12 kpc) and flatter towards the outer disk. The open cluster sample studied spans a significant range in age. When breaking the sample into age bins, there is some indication that the younger open cluster population in our sample (log age < 8.7) has a flatter metallicity gradient when compared with the gradients obtained from older open clusters.

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arXiv2015

Comparison of Hipparcos Trigonometric and Mount Wilson Spectroscopic Parallaxes for 90 Subgiants that Defined the Class in 1935

A history is given of the discovery between 1914 and 1935 of stars of intermediate luminosity between giants and dwarfs with spectral types between G0 to K3. The Mt Wilson spectroscopists identified about 90 such stars in their 1935 summary paper of spectroscopic absolute magnitudes for 4179 stars. Called "subgiants" by Strömberg, these 90 stars defined the group at the time. The position of the Mt Wilson subgiants in the HR diagram caused difficulties in comparisons of high weight trigonometric parallaxes being measured and with Russell's prevailing evolution proposal, and critics questioned the reality of the Mt Wilson subgiants. We compare, star-by-star, the Mt Wilson spectroscopic absolute magnitudes of the 90 stars defining their sample against those absolute magnitudes derived from Hipparcos (HIP) trigonometric parallaxes. We address concerns over biases in the Mt Wilson calibration sample and biases created by the adopted methodology for calibration. Historically, these concerns were sufficient to discredit the discovery of subgiants in the Mt Wilson sample. However, as shown here, the majority of the Mount Wilson stars identified as subgiants that also have reliable HIP trigonometric parallaxes do lie among the subgiant sequence in the HIP HR diagram. Moreover, no significant offset is seen between the M(V) brightnesses derived from the Mt Wilson spectroscopic parallaxes and the M(V) values derived from Hipparcos trigonometric parallaxes with a fractional error of 10%, which confirms in an impressive manner the efficacy of the original Mt Wilson assessments. The existence of subgiants proved that Russell's contraction proposal for stellar evolution from giants to the main sequence was incorrect. Instead, Gamow's 1944 unpublished conjecture that subgiants are post main-sequence stars just having left the main sequence was very nearly correct but was a decade before its time.

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arXiv2015

ASPCAP: The Apogee Stellar Parameter and Chemical Abundances Pipeline

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) has built the largest moderately high-resolution (R=22, 500) spectroscopic map of the stars across the Milky Way, and including dust-obscured areas. The APOGEE Stellar Parameter and Chemical Abundances Pipeline (ASPCAP) is the software developed for the automated analysis of these spectra. ASPCAP determines atmospheric parameters and chemical abundances from observed spectra by comparing observed spectra to libraries of theoretical spectra, using chi-2 minimization in a multidimensional parameter space. The package consists of a fortran90 code that does the actual minimization, and a wrapper IDL code for book-keeping and data handling. This paper explains in detail the ASPCAP components and functionality, and presents results from a number of tests designed to check its performance. ASPCAP provides stellar effective temperatures, surface gravities, and metallicities precise to 2%, 0.1 dex, and 0.05 dex, respectively, for most APOGEE stars, which are predominantly giants. It also provides abundances for up to 15 chemical elements with various levels of precision, typically under 0.1 dex. The final data release (DR12) of the Sloan Digital Sky Survey III contains an APOGEE database of more than 150,000 stars. ASPCAP development continues in the SDSS-IV APOGEE-2 survey.

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arXiv2015

The Data Reduction Pipeline for the Apache Point Observatory Galactic Evolution Experiment

The Apache Point Observatory Galactic Evolution Experiment (APOGEE), part of the Sloan Digital Sky Survey III, explores the stellar populations of the Milky Way using the Sloan 2.5-m telescope linked to a high resolution (R~22,500), near-infrared (1.51-1.70 microns) spectrograph with 300 optical fibers. For over 150,000 predominantly red giant branch stars that APOGEE targeted across the Galactic bulge, disks and halo, the collected high S/N (>100 per half-resolution element) spectra provide accurate (~0.1 km/s) radial velocities, stellar atmospheric parameters, and precise (~0.1 dex) chemical abundances for about 15 chemical species. Here we describe the basic APOGEE data reduction software that reduces multiple 3D raw data cubes into calibrated, well-sampled, combined 1D spectra, as implemented for the SDSS-III/APOGEE data releases (DR10, DR11 and DR12). The processing of the near-IR spectral data of APOGEE presents some challenges for reduction, including automated sky subtraction and telluric correction over a 3 degree diameter field and the combination of spectrally dithered spectra. We also discuss areas for future improvement.

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arXiv2015

The Apache Point Observatory Galactic Evolution Experiment (APOGEE)

The Apache Point Observatory Galactic Evolution Experiment (APOGEE), one of the programs in the Sloan Digital Sky Survey III (SDSS-III), has now completed its systematic, homogeneous spectroscopic survey sampling all major populations of the Milky Way. After a three year observing campaign on the Sloan 2.5-m Telescope, APOGEE has collected a half million high resolution (R~22,500), high S/N (>100), infrared (1.51-1.70 microns) spectra for 146,000 stars, with time series information via repeat visits to most of these stars. This paper describes the motivations for the survey and its overall design---hardware, field placement, target selection, operations---and gives an overview of these aspects as well as the data reduction, analysis and products. An index is also given to the complement of technical papers that describe various critical survey components in detail. Finally, we discuss the achieved survey performance and illustrate the variety of potential uses of the data products by way of a number of science demonstrations, which span from time series analysis of stellar spectral variations and radial velocity variations from stellar companions, to spatial maps of kinematics, metallicity and abundance patterns across the Galaxy and as a function of age, to new views of the interstellar medium, the chemistry of star clusters, and the discovery of rare stellar species. As part of SDSS-III Data Release 12, all of the APOGEE data products are now publicly available.

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arXiv2015

Young alpha-enriched giant stars in the solar neighbourhood

We derive age constraints for 1639 red giants in the APOKASC sample for which seismic parameters from Kepler, as well as effective temperatures, metallicities and [alpha/Fe] values from APOGEE DR12 are available. We investigate the relation between age and chemical abundances for these stars, using a simple and robust approach to obtain ages. We first derive stellar masses using standard seismic scaling relations, then determine the maximum possible age for each star as function of its mass and metallicity, independently of its evolutionary stage. While the overall trend between maximum age and chemical abundances is a declining fraction of young stars with increasing [alpha/Fe], at least 14 out of 241 stars with [alpha/Fe]>0.13 are younger than 6 Gyr. Five stars with [alpha/Fe]>0.2 have ages below 4 Gyr. We examine the effect of modifications in the standard seismic scaling relations, as well as the effect of very low helium fractions, but these changes are not enough to make these stars as old as usually expected for alpha-rich stars (i.e., ages greater than 8-9 Gyr). Such unusual alpha-rich young stars have also been detected by other surveys, but defy simple explanations in a galaxy evolution context.

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arXiv2015

Hubble Space Telescope Proper Motions along the Sagittarius Stream: I. Observations and Results for Stars in Four Fields

We present a multi-epoch Hubble Space Telescope (HST) study of stellar proper motions (PMs) for four fields spanning 200 degrees along the Sagittarius (Sgr) stream: one trailing arm field, one field near the Sgr dwarf spheroidal tidal radius, and two leading arm fields. We determine absolute PMs of dozens of individual stars per field, using established techniques that use distant background galaxies as stationary reference frame. Stream stars are identified based on combined color-magnitude diagram and PM information. The results are broadly consistent with the few existing PM measurements for the Sgr galaxy and the trailing arm. However, our new results provide the highest PM accuracy for the stream to date, the first PM measurements for the leading arm, and the first PM measurements for individual stream stars; we also serendipitously determine the PM of the globular cluster NGC~6652. In the trailing-arm field, the individual PMs allow us to kinematically separate trailing-arm stars from leading-arm stars that are 360 degrees further ahead in their orbit. Also, in three of our fields we find indications that two distinct kinematical components may exist within the same arm and wrap of the stream. Qualitative comparison of the HST data to the predictions of the Law & Majewski and Penarrubia et al. N-body models show that the PM measurements closely follow the predicted trend with Sgr longitude. This provides a successful consistency check on the PM measurements, as well as on these N-body approaches (which were not tailored to fit any PM data).

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arXiv2015

Exploring Anticorrelations and Light Element Variations in Northern Globular Clusters Observed by the APOGEE Survey

We investigate the light-element behavior of red giant stars in Northern globular clusters (GCs) observed by the SDSS-III Apache Point Observatory Galactic Evolution Experiment (APOGEE). We derive abundances of nine elements (Fe, C, N, O, Mg, Al, Si, Ca, and Ti) for 428 red giant stars in 10 globular clusters. The intrinsic abundance range relative to measurement errors is examined, and the well-known C-N and Mg-Al anticorrelations are explored using an extreme-deconvolution code for the first time in a consistent way. We find that Mg and Al drive the population membership in most clusters, except in M107 and M71, the two most metal-rich clusters in our study, where the grouping is most sensitive to N. We also find a diversity in the abundance distributions, with some clusters exhibiting clear abundance bimodalities (for example M3 and M53) while others show extended distributions. The spread of Al abundances increases significantly as cluster average metallicity decreases as previously found by other works, which we take as evidence that low metallicity, intermediate mass AGB polluters were more common in the more metal poor clusters. The statistically significant correlation of [Al/Fe] with [Si/Fe] in M15 suggests that $^{28}$Si leakage has occurred in this cluster. We also present C, N and O abundances for stars cooler than 4500 K and examine the behavior of A(C+N+O) in each cluster as a function of temperature and [Al/Fe]. The scatter of A(C+N+O) is close to its estimated uncertainty in all clusters and independent on stellar temperature. A(C+N+O) exhibits small correlations and anticorrelations with [Al/Fe] in M3 and M13, but we cannot be certain about these relations given the size of our abundance uncertainties. Star-to-star variations of $α-$elements (Si, Ca, Ti) abundances are comparable to our estimated errors in all clusters.

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arXiv2015

Hydra II: a faint and compact Milky Way dwarf galaxy found in the Survey of the Magellanic Stellar History

We present the discovery of a new dwarf galaxy, Hydra II, found serendipitously within the data from the ongoing Survey of the MAgellanic Stellar History (SMASH) conducted with the Dark Energy Camera on the Blanco 4m Telescope. The new satellite is compact (r_h = 68 +/- 11 pc) and faint (M_V = -4.8 +/- 0.3), but well within the realm of dwarf galaxies. The stellar distribution of HydraII in the color-magnitude diagram is well-described by a metal-poor ([Fe/H] = -2.2) and old (13 Gyr) isochrone and shows a distinct blue horizontal branch, some possible red clump stars, and faint stars that are suggestive of blue stragglers. At a heliocentric distance of 134 +/- 10 kpc, Hydra II is located in a region of the Galactic halo that models have suggested may host material from the leading arm of the Magellanic Stream. A comparison with N-body simulations hints that the new dwarf galaxy could be or could have been a satellite of the Magellanic Clouds.

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arXiv2015

Chemical Cartography with APOGEE: Metallicity Distribution Functions and the Chemical Structure of the Milky Way Disk

Using a sample of 69,919 red giants from the SDSS-III/APOGEE Data Release 12, we measure the distribution of stars in the [$α$/Fe] vs. [Fe/H] plane and the metallicity distribution functions (MDF) across an unprecedented volume of the Milky Way disk, with radius $3 11$ kpc. The peak of the midplane MDF shifts to lower metallicity at larger $R$, reflecting the Galactic metallicity gradient. Most strikingly, the shape of the midplane MDF changes systematically with radius, with a negatively skewed distribution at $3 1$ kpc or [$α$/Fe]$>0.18$, the MDF shows little dependence on $R$. The positive skewness of the outer disk MDF may be a signature of radial migration; we show that blurring of stellar populations by orbital eccentricities is not enough to explain the reversal of MDF shape but a simple model of radial migration can do so.

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arXiv2015

The power spectrum of the Milky Way: Velocity fluctuations in the Galactic disk

We investigate the kinematics of stars in the mid-plane of the Milky Way on scales between 25 pc and 10 kpc with data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), the Radial Velocity Experiment (RAVE), and the Geneva-Copenhagen Survey (GCS). Using red-clump stars in APOGEE, we determine the large-scale line-of-sight velocity field out to 5 kpc from the Sun in (0.75 kpc)^2 bins. The solar motion V_{sun-c} with respect to the circular velocity V_c is the largest contribution to the power on large scales after subtracting an axisymmetric rotation field; we determine the solar motion by minimizing the large-scale power to be V_{sun-c} = 24+/-1 (ran.)+/-2 (syst [V_c])+/-5 (syst. [large-scale]) km/s, where the systematic uncertainty is due to (a) a conservative 20 km/s uncertainty in V_c and (b) the estimated power on unobserved larger scales. Combining the APOGEE peculiar-velocity field with red-clump stars in RAVE out to 2 kpc from the Sun and with local GCS stars, we determine the power spectrum of residual velocity fluctuations in the Milky Way's disk on scales between 0.2/kpc < k < 40/kpc. Most of the power is contained in a broad peak between 0.2/kpc < k < 0.9/kpc. We investigate the expected power spectrum for various non-axisymmetric perturbations and demonstrate that the central bar with commonly used parameters but of relatively high mass can explain the bulk of velocity fluctuations in the plane of the Galactic disk near the Sun. Streaming motions ~10 km/s on >~3 kpc scales in the Milky Way are in good agreement with observations of external galaxies and directly explain why local determinations of the solar motion are inconsistent with global measurements.

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arXiv2015

The Puzzling Li-rich Red Giant Associated with NGC 6819

A Li-rich red giant star (2M19411367+4003382) recently discovered in the direction of NGC 6819 belongs to the rare subset of Li-rich stars that have not yet evolved to the luminosity bump, an evolutionary stage where models predict Li can be replenished. The currently favored model to explain Li enhancement in first-ascent red giants like 2M19411367+4003382 requires deep mixing into the stellar interior. Testing this model requires a measurement of 12C/13C, which is possible to obtain from APOGEE spectra. However, the Li-rich star also has abnormal asteroseismic properties that call into question its membership in the cluster, even though its radial velocity and location on color-magnitude diagrams are consistent with membership. To address these puzzles, we have measured a wide array of abundances in the Li-rich star and three comparison stars using spectra taken as part of the APOGEE survey to determine the degree of stellar mixing, address the question of membership, and measure the surface gravity. We confirm that the Li-rich star is a red giant with the same overall chemistry as the other cluster giants. However, its log g is significantly lower, consistent with the asteroseismology results and suggestive of a very low mass if the star is indeed a cluster member. Regardless of the cluster membership, the 12C/13C and C/N ratios of the Li-rich star are consistent with standard first dredge-up, indicating that Li dilution has already occurred, and inconsistent with internal Li enrichment scenarios that require deep mixing.

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