47 research outputs found
A New Stellar Atmosphere Grid and Comparisons with HST/STIS Calspec Flux Distributions
The Space Telescope Imaging Spectrograph (STIS) has measured the spectral
energy distributions (SEDs) for several stars of types O, B, A, F, and G. These
absolute fluxes from the CALSPEC database are fit with a new spectral grid
computed from the ATLAS-APOGEE ATLAS9 model atmosphere database using a
chi-square minimization technique in four parameters. The quality of the fits
are compared for complete LTE grids by Castelli & Kurucz (CK04) and our new
comprehensive LTE grid (BOSZ). For the cooler stars, the fits with the MARCS
LTE grid are also evaluated, while the hottest stars are also fit with the NLTE
Lanz & Hubeny OB star grids. Unfortunately, these NLTE models do not transition
smoothly in the infrared to agree with our new BOSZ LTE grid at the NLTE lower
limit of Teff =15,000K.
The new BOSZ grid is available via the Space Telescope Institute MAST archive
and has a much finer sampled IR wavelength scale than CK04, which will
facilitate the modeling of stars observed by the James Webb Space Telescope
(JWST). Our result for the angular diameter of Sirius agrees with the ground-
based interferometric value.Comment: 11 figure
NGC 7789: An Open Cluster Case Study
We have obtained high-resolution spectra of 32 giants in the open cluster NGC
7789 using the Wisconsin-Indiana-Yale-NOAO Hydra spectrograph. We explore
differences in atmospheric parameters and elemental abundances caused by the
use of the linelist developed for the Gaia-ESO Survey (GES) compared to one
based on Arcturus used in our previous work. [Fe/H] values decrease when using
the GES linelist instead of the Arcturus-based linelist; these differences are
probably driven by systematically lower (~ -0.1 dex) GES surface gravities.
Using the GES linelist we determine abundances for 10 elements - Fe, Mg, Si,
Ca, Ti, Na, Ni, Zr, Ba, and La. We find the cluster's average metallicity
[Fe/H] = 0.03 +/- 0.07 dex, in good agreement with literature values, and a
lower [Mg/Fe] abundance than has been reported before for this cluster (0.11
+/- 0.05 dex). We also find the neutron-capture element barium to be highly
enhanced - [Ba/Fe] = +0.48 +/- 0.08 - and disparate from cluster measurements
of neutron-capture elements La and Zr (-0.08 +/- 0.05 and 0.08 +/- 0.08,
respectively). This is in accordance with recent discoveries of supersolar Ba
enhancement in young clusters along with more modest enhancement of other
neutron-capture elements formed in similar environments.Comment: 15 pages, 9 figures, Table 1 typo fixe
Chemical Abundances in Field Red Giants from High-Resolution H-Band Spectra using the APOGEE Spectral Linelist
High-resolution H-band spectra of five bright field K, M, and MS giants,
obtained from the archives of the Kitt Peak National Observatory (KPNO) Fourier
Transform Spectrometer (FTS), are analyzed to determine chemical abundances of
16 elements. The abundances were derived via spectrum synthesis using the
detailed linelist prepared for the SDSS III Apache Point Galactic Evolution
Experiment (APOGEE), which is a high-resolution near-infrared spectroscopic
survey to derive detailed chemical abundance distributions and precise radial
velocities for 100,000 red giants sampling all Galactic stellar populations.
Measured chemical abundances include the cosmochemically important isotopes
12C, 13C, 14N, and 16O, along with Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Co,
Ni, and Cu. A comparison of the abundances derived here with published values
for these stars reveals consistent results to ~0.1 dex. The APOGEE spectral
region and linelist is, thus, well-suited for probing both Galactic chemical
evolution, as well as internal nucleosynthesis and mixing in populations of red
giants using high-resolution spectroscopy.Comment: Accepted for publication in The Astrophysical Journal. 42 pages, 12
figure
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.Comment: accepted for publication in the Ap
The PuZZling Li-Rich Red Giant Associated With NGC 6819
A Li-rich red giant (RG) 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 RGs like 2M19411367+4003382 requires deep mixing into the stellar interior. Testing this model requires a measurement of C-12/C-13, which is possible to obtain from Apache Point Observatory Galactic Evolution Experiment (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 RG 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 C-12/C-13 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.National Science Foundation AST1109888NSF AST-1358862, AST 1109718, AST 1312863Alfred P. Sloan FoundationNational Science FoundationU.S. Department of Energy Office of ScienceUniversity of ArizonaBrazilian Participation GroupBrookhaven National LaboratoryCarnegie Mellon UniversityUniversity of FloridaFrench Participation GroupGerman Participation GroupHarvard UniversityInstituto de Astrofisica de CanariasMichigan State/NotreDame/JINA Participation GroupJohns Hopkins UniversityLawrence Berkeley National LaboratoryMax Planck Institute for AstrophysicsMax Planck Institute for Extraterrestrial PhysicsNew Mexico State UniversityNew York UniversityOhio State UniversityPennsylvania State UniversityUniversity of PortsmouthPrinceton UniversitySpanish Participation GroupUniversity of TokyoUniversity of UtahVanderbilt UniversityUniversity of VirginiaUniversity of WashingtonYale UniversityNational Aeronautics and Space AdministrationTwo Micron All Sky SurveyUniversity of MassachusettsInfrared Processing and Analysis Center/California Institute of TechnologyU.S. Government NAG W-2166Astronom
High-resolution, H band Spectroscopy of Be Stars with SDSS-III/APOGEE: I. New Be Stars, Line Identifications, and Line Profiles
APOGEE has amassed the largest ever collection of multi-epoch,
high-resolution (R~22,500), H-band spectra for B-type emission line (Be) stars.
The 128/238 APOGEE Be stars for which emission had never previously been
reported serve to increase the total number of known Be stars by ~6%. We focus
on identification of the H-band lines and analysis of the emission peak
velocity separations (v_p) and emission peak intensity ratios (V/R) of the
usually double-peaked H I and non-hydrogen emission lines. H I Br11 emission is
found to preferentially form in the circumstellar disks at an average distance
of ~2.2 stellar radii. Increasing v_p toward the weaker Br12--Br20 lines
suggests these lines are formed interior to Br11. By contrast, the observed IR
Fe II emission lines present evidence of having significantly larger formation
radii; distinctive phase lags between IR Fe II and H I Brackett emission lines
further supports that these species arise from different radii in Be disks.
Several emission lines have been identified for the first time including
~16895, a prominent feature in the spectra for almost a fifth of the sample
and, as inferred from relatively large v_p compared to the Br11-Br20, a tracer
of the inner regions of Be disks. Unlike the typical metallic lines observed
for Be stars in the optical, the H-band metallic lines, such as Fe II 16878,
never exhibit any evidence of shell absorption, even when the H I lines are
clearly shell-dominated. The first known example of a quasi-triple-peaked Br11
line profile is reported for HD 253659, one of several stars exhibiting intra-
and/or extra-species V/R and radial velocity variation within individual
spectra. Br11 profiles are presented for all discussed stars, as are full
APOGEE spectra for a portion of the sample.Comment: accepted in A
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.Comment: 4 pages, 3 figures, To appear in Astronomische Nachrichten, special
issue "Reconstruction the Milky Way's History: Spectroscopic surveys,
Asteroseismology and Chemo-dynamical models", Guest Editors C. Chiappini, J.
Montalb\'an, and M. Steffen, AN 2016 (in press)
Sodium and Oxygen Abundances in the Open Cluster NGC 6791 from APOGEE H-Band Spectroscopy
The open cluster NGC 6791 is among the oldest, most massive and metal-rich
open clusters in the Galaxy. High-resolution -band spectra from the Apache
Point Observatory Galactic Evolution Experiment (APOGEE) of 11 red giants in
NGC 6791 are analyzed for their chemical abundances of iron, oxygen, and
sodium. The abundances of these three elements are found to be homogeneous
(with abundance dispersions at the level of 0.05 - 0.07 dex) in these
cluster red giants, which span much of the red-giant branch (T
3500K - 4600K), and include two red-clump giants. From the infrared
spectra, this cluster is confirmed to be among the most metal-rich clusters in
the Galaxy ( = 0.34 0.06), and is found to have a roughly
solar value of [O/Fe] and slightly enhanced [Na/Fe]. Non-LTE calculations for
the studied Na I lines in the APOGEE spectral region (16373.86\AA\ and
16388.85\AA) indicate only small departures from LTE ( 0.04 dex)
for the parameter range and metallicity of the studied stars. The previously
reported double population of cluster members with different Na abundances is
not found among the studied sample.Comment: Accepted for publication at ApJ Letter