Browsing by Subject "infrared-spectroscopy"

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The Bulge Radial Velocity Assay (BRAVA). II. Complete Sample And Data Release
(2012-03) Kunder, Andrea; Koch, Andreas; Rich, R. Michael; de Propris, Roberto; Howard, Christian D.; Stubbs, Scott A.; Johnson, Christian I.; Shen, Juntai T.; Wang, Yougang G.; Robin, Annie C.; Kormendy, John; Soto, Mario; Frinchaboy, Peter; Reitzel, David B.; Zhao, HongSheng; Origlia, Livia; Kormendy, John
We present new radial velocity measurements from the Bulge Radial Velocity Assay, a large-scale spectroscopic survey of M-type giants in the Galactic bulge/bar region. The sample of similar to 4500 new radial velocities, mostly in the region -10 degrees < l < +10 degrees and b approximate to -6 degrees, more than doubles the existent published data set. Our new data extend our rotation curve and velocity dispersion profile to +20 degrees, which is similar to 2.8 kpc from the Galactic center. The new data confirm the cylindrical rotation observed at -6 degrees and -8 degrees and are an excellent fit to the Shen et al. N-body bar model. We measure the strength of the TiO epsilon molecular band as a first step toward a metallicity ranking of the stellar sample, from which we confirm the presence of a vertical abundance gradient. Our survey finds no strong evidence of previously unknown kinematic streams. We also publish our complete catalog of radial velocities, photometry, TiO band strengths, and spectra, which is available at the Infrared Science Archive as well as at UCLA.
The c2d Spitzer Spectroscopic Survey Of Ices Around Low-Mass Young Stellar Objects. II. CO2
(2008-05) Pontoppidan, Klaus M.; Boogert, Adwin C. A.; Fraser, Helen J.; van Dishoeck, Ewine F.; Blake, Geoffrey A.; Lahuis, Fred; Oberg, Karin I.; Evans, Neal J.; Salyk, Colette; Evans, Neal J.
This paper presents Spitzer IRS lambda/Delta lambda similar to 600 spectroscopy of the CO2 15.2 mu m bending mode toward 50 embedded young low-mass stars, taken mostly from the "Cores to Disks'' (c2d) Legacy program. The average abundance of solid CO2 relative to water in low-mass protostellar envelopes is 0.32 +/- 0.02, significantly higher than that found in quiescent molecular clouds and in massive star-forming regions. A decomposition of the observed CO2 bending mode profiles requires a minimum of five unique components. Roughly 2/3 of the CO2 ice is found in a water-rich environment, while most of the remaining 1/3 is found in a CO environment with strongly varying relative concentrations of CO2 to CO along each line of sight. Ground-based observations of solid CO toward a large subset of the c2d sample are used to further constrain the CO2:CO component and suggest a model in which low-density clouds form the CO2:H2O component and higher density clouds form the CO2:CO ice during and after the freezeout of gas-phase CO. The abundance of the CO2:CO component is consistent with cosmic-ray processing of the CO-rich part of the ice mantles, although a more quiescent formation mechanism is not ruled out. It is suggested that the subsequent evolution of the CO2 and CO profiles toward low-mass protostars, in particular the splitting of the CO2 bending mode due to pure, crystalline CO2, is first caused by distillation of the CO2:CO component through evaporation of CO due to thermal processing to similar to 20-30 K. The formation of pure CO2 via segregation from the H2O rich mantle may contribute to the band splitting at higher levels of thermal processing (greater than or similar to 50 K) but is harder to reconcile with the physical structure of protostellar envelopes around low-luminosity objects.
Chemical Similarities Between Galactic Bulge And Local Thick Disk Red Giant Stars
(2008-06) Melendez, J.; Asplund, M.; Alves-Brito, A.; Cunha, K.; Barbuy, B.; Bessell, M. S.; Chiappini, C.; Freeman, K. C.; Ramirez, I.; Smith, V. V.; Yong, D.; Ramirez, I.
Context. The evolution of the Milky Way bulge and its relationship with the other Galactic populations is still poorly understood. The bulge has been suggested to be either a merger-driven classical bulge or the product of a dynamical instability of the inner disk. Aims. To probe the star formation history, the initial mass function and stellar nucleosynthesis of the bulge, we performed an elemental abundance analysis of bulge red giant stars. We also completed an identical study of local thin disk, thick disk and halo giants to establish the chemical differences and similarities between the various populations. Methods. High-resolution infrared spectra of 19 bulge giants and 49 comparison giants in the solar neighborhood were acquired with Gemini/Phoenix. All stars have similar stellar parameters but cover a broad range in metallicity. A standard 1D local thermodynamic equilibrium analysis yielded the abundances of C, N, O and Fe. A homogeneous and differential analysis of the bulge, halo, thin disk and thick disk stars ensured that systematic errors were minimized. Results. We confirm the well-established differences for [O/Fe] (at a given metallicity) between the local thin and thick disks. For the elements investigated, we find no chemical distinction between the bulge and the local thick disk, which is in contrast to previous studies relying on literature values for disk dwarf stars in the solar neighborhood. Conclusions. Our findings suggest that the bulge and local thick disk experienced similar, but not necessarily shared, chemical evolution histories. We argue that their formation timescales, star formation rates and initial mass functions were similar.
CO2 Ice Toward Low-Luminosity Embedded Protostars: Evidence For Episodic Mass Accretion Via Chemical History
(2012-10) Kim, Hyo Jeeong; Evans, Neal J.; Dunham, Michael M.; Lee, Jeong-Eun; Pontoppidan, Klaus M.; Kim, Hyo Jeeong; Evans, Neal J.
We present Spitzer IRS spectroscopy of CO2 ice bending mode spectra at 15.2 mu m toward 19 young stellar objects (YSOs) with luminosity lower than 1L(circle dot) (3 with luminosity lower than 0.1 L-circle dot). Ice on dust grain surfaces can encode the history of heating because pure CO2 ice forms only at elevated temperature, T > 20 K, and thus around protostars of higher luminosity. Current internal luminosities of YSOs with L < 1L(circle dot) do not provide the conditions needed to produce pure CO2 ice at radii where typical envelopes begin. The presence of detectable amounts of pure CO2 ice would signify a higher past luminosity. Many of the spectra require a contribution from a pure, crystalline CO2 component, traced by the presence of a characteristic band splitting in the 15.2 mu m bending mode. About half of the sources (9 out of 19) in the low-luminosity sample have evidence for pure CO2 ice, and 6 of these have significant double-peaked features, which are very strong evidence of pure CO2 ice. The presence of the pure CO2 ice component indicates that the dust temperature, and hence luminosity of the central star/accretion disk system, must have been higher in the past. An episodic accretion scenario, in which mixed CO-CO2 ice is converted to pure CO2 ice during each high-luminosity phase, explains the presence of pure CO2 ice, the total amount of CO2 ice, and the observed residual (CO)-O-18 gas.
High-Velocity Stars In The Cores Of Globular Clusters: The Illustrative Case Of NGC 2808
(2012-07) Lutzgendorf, N.; Gualandris, A.; Kissler-Patig, M.; Gebhardt, K.; Baumgardt, H.; Noyola, E.; Kruijssen, J. M. D.; Jalali, B.; de Zeeuw, P. T.; Neumayer, N.; Gebhardt, K.
Context. We report the detection of five high-velocity stars in the core of the globular cluster NGC 2808. The stars lie on the red giant branch and show total velocities between 40 and 45 kms(-1). For a core velocity dispersion sigma(c) = 13.4 kms(-1), this corresponds to up to 3.4 sigma(c). These velocities are close to the estimated escape velocity (similar to 50 km s(-1)) and suggest an ejection from the core. Two of these stars have been confirmed in our recent integral field spectroscopy data and we will discuss them in more detail here. These two red giants are located at a projected distance of similar to 0.3 pc from the center. According to their positions on the color magnitude diagram, both stars are cluster members. Aims. We investigate several possible origins for the high velocities of the stars and conceivable ejection mechanisms. Since the velocities are close to the escape velocity, it is not obvious whether the stars are bound or unbound to the cluster. We therefore consider both cases in our analysis. Methods. We perform numerical simulations of three-body dynamical encounters between binaries and single stars and compare the resulting velocity distributions of escapers with the velocities of our stars. If the stars are bound, the encounters must have taken place when the stars were still on the main sequence. We compare the predictions for a single dynamical encounter with a compact object with those of a sequence of two-body encounters due to relaxation. If the stars are unbound, the encounter must have taken place recently, when the stars were already in the giant phase. Results. After including binary fractions and black-hole retention fractions, projection effects, and detection probabilities from Monte-Carlo simulations, we estimate the expected numbers of detections for all the different scenarios. Based on these numbers, we conclude that the most likely scenario is that the stars are bound and were accelerated by a single encounter between a binary of main-sequence stars and a similar to 10 M-circle dot black hole. Finally, we discuss the origin of previously discovered fast stars in globular clusters, and conclude that the case of NGC2808 is most likely a representative case for most other detections of fast stars in globular clusters. We show that with the present analysis we are able to explain high-velocity stars in the clusters M3 and 47 Tucanae with simple dynamical encounters.
Kinematics At the Edge of the Galactic Bulge: Evidence for Cylindrical Rotation
(2009-09) Howard, Christian D.; Rich, R. Michael; Clarkson, Will; Mallery, Ryan; Kormendy, John; De Propris, Roberto; Robin, Anne C.; Fux, Roger; Reitzel, David B.; Zhao, HongSheng; Kuijken, Konrad; Koch, Andreas; Kormendy, John
We present new results from BRAVA, a large-scale radial velocity survey of the Galactic bulge, using M giant stars selected from the Two Micron All Sky Survey catalog as targets for the Cerro Tololo Inter-American Observatory 4 m Hydra multi-object spectrograph. The purpose of this survey is to construct a new generation of self-consistent bar models that conform to these observations. We report the dynamics for fields at the edge of the Galactic bulge at latitudes b = -8 degrees and compare to the dynamics at b = -4 degrees. We find that the rotation curve V (r) is the same at b = -8 degrees as at b = -4 degrees. That is, the Galactic boxy bulge rotates cylindrically, as do boxy bulges of other galaxies. The summed line-of-sight velocity distribution at b = -8 degrees is Gaussian, and the binned longitude-velocity plot shows no evidence for either a (disk) population with cold dynamics or for a (classical bulge) population with hot dynamics. The observed kinematics are well modeled by an edge-on N-body bar, in agreement with published structural evidence. Our kinematic observations indicate that the Galactic bulge is a prototypical product of secular evolution in galaxy disks, in contrast with stellar population results that are most easily understood if major mergers were the dominant Formation process.
Very Metal-Poor Stars in the Outer Galactic Bulge Found By the APOGEE Survey
(2013-04) Garcia Perez, Ana E.; Cunha, Katia; Shetrone, Matthew; Majewski, Steven R.; Johnson, Jennifer A.; Smith, Verne V.; Schiavon, Ricardo P.; Holtzman, Jon; Nidever, David; Zasowski, Gail; Allende Prieto, Carlos; Beers, Timothy C.; Bizyaev, Dmitry; Ebelke, Garrett; Eisenstein, Daniel J.; Frinchaboy, Peter M.; Girardi, Leo; Hearty, Fred R.; Malanushenko, Elena; Malanushenko, Viktor; Meszaros, Szabolcs; O'Connell, Robert W.; Oravetz, Daniel; Pan, Kaike; Robin, Annie C.; Schneider, Donald P.; Schultheis, Mathias; Skrutskie, Michael F.; Simmonsand, Audrey; Wilson, John C.; Shetrone, Matthew
Despite its importance for understanding the nature of early stellar generations and for constraining Galactic bulge Formation models, at present little is known about the metal-poor stellar content of the central Milky Way. This is a consequence of the great distances involved and intervening dust obscuration, which challenge optical studies. However, the Apache Point Observatory Galactic Evolution Experiment (APOGEE), a wide-area, multifiber, high-resolution spectroscopic survey within Sloan Digital Sky Survey III, is exploring the chemistry of all Galactic stellar populations at infrared wavelengths, with particular emphasis on the disk and the bulge. An automated spectral analysis of data on 2403 giant stars in 12 fields in the bulge obtained during APOGEE commissioning yielded five stars with low metallicity ([Fe/H] <= -1.7), including two that are very metal-poor [Fe/H] similar to -2.1 by bulge standards. Luminosity-based distance estimates place the 5 stars within the outer bulge, where 1246 of the other analyzed stars may reside. A manual reanalysis of the spectra verifies the low metallicities, and finds these stars to be enhanced in the alpha-elements O, Mg, and Si without significant alpha-pattern differences with other local halo or metal-weak thick-disk stars of similar metallicity, or even with other more metal-rich bulge stars. While neither the kinematics nor chemistry of these stars can yet definitively determine which, if any, are truly bulge members, rather than denizens of other populations co-located with the bulge, the newly identified stars reveal that the chemistry of metal-poor stars in the central Galaxy resembles that of metal-weak thick-disk stars at similar metallicity.