Browsing by Subject "astronomy & astrophysics"

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A 12 Minute Orbital Period Detached White Dwarf Eclipsing Binary
(2011-08) Brown, Warren R.; Kilic, Mukremin; Hermes, J. J.; Prieto, Carlos Allende; Kenyon, Scott J.; Winget, D. E.; Hermes, J. J.
We have discovered a detached pair of white dwarfs (WDs) with a 12.75 minute orbital period and a 1315 km s(-1) radial velocity amplitude. We measure the full orbital parameters of the system using its light curve, which shows ellipsoidal variations, Doppler boosting, and primary and secondary eclipses. The primary is a 0.25 M-circle dot tidally distorted helium WD, only the second tidally distorted WD known. The unseen secondary is a 0.55 M-circle dot carbon-oxygen WD. The two WDs will come into contact in 0.9 Myr due to loss of energy and angular momentum via gravitational wave radiation. Upon contact the systems may merge (yielding a rapidly spinning massive WD), form a stable interacting binary, or possibly explode as an underluminous Type Ia supernova. The system currently has a gravitational wave strain of 10(-22), about 10,000 times larger than the Hulse-Taylor pulsar; this system would be detected by the proposed Laser Interferometer Space Antenna gravitational wave mission in the first week of operation. This system's rapid change in orbital period will provide a fundamental test of general relativity.
18 Sco: A Solar Twin Rich In Refractory And Neutron-Capture Elements. Implications For Chemical Tagging
(2014-08) Melendez, Jorge; Ramirez, Ivan; Karakas, Amanda I.; Yong, David; Monroe, TalaWanda R.; Bedell, Megan; Bergemann, Maria; Asplund, Martin; Maia, Marcelo Tucci; Bean, Jacob; do Nascimento, Jose-Dias; Bazot, Michael; Alves-Brito, Alan; Freitas, Fabricio C.; Castro, Matthieu; Ramirez, Ivan
We study with unprecedented detail the chemical composition and stellar parameters of the solar twin 18 Sco in a strictly differential sense relative to the Sun. Our study is mainly based on high-resolution (R similar to 110,000), high signal-to-noise ratio (800-1,000) Very Large Telescope UVES spectra, which allow us to achieve a precision of about 0.005 dex in differential abundances. The effective temperature and surface gravity of 18 Sco are T-eff = 5823 +/- 6 K and log g = 4.45 +/- 0.02 dex, i.e., 18 Sco is 46 +/- 6 K hotter than the Sun and log g is 0.01 +/- 0.02 dex higher. Its metallicity is [Fe/H] = 0.054 +/- 0.005 dex, and its microturbulence velocity is +0.02 +/- 0.01 km s-1 higher than solar. Our precise stellar parameters and differential isochrone analysis show that 18 Sco has a mass of 1.04 +/- 0.02 M-circle dot and that it is similar to 1.6 Gyr younger than the Sun. We use precise High Accuracy Radial velocity Planet Searcher (HARPS) radial velocities to search for planets, but none are detected. The chemical abundance pattern of 18 Sco displays a clear trend with condensation temperature, thus showing higher abundances of refractories in 18 Sco than in the Sun. Intriguingly, there are enhancements in the neutron-capture elements relative to the Sun. Despite the small element-to-element abundance differences among nearby n-capture elements (similar to 0.02 dex), we successfully reproduce the r-process pattern in the Solar System. This is independent evidence for the universality of the r process. Our results have important implications for chemical tagging in our Galaxy and nucleosynthesis in general.
2006 Whole Earth Telescope Observations Of GD358: A New Look At The Prototype DBV
(2009-03) Provencal, J. L.; Montgomery, Michael H.; Kanaan, A.; Shipman, H. L.; Childers, D.; Baran, A.; Kepler, S. O.; Reed, M.; Zhou, A.; Eggen, J.; Watson, T. K.; Winget, D. E.; Thompson, Susan E.; Riaz, B.; Nitta, Atsuko; Kleinman, S. J.; Crowe, R.; Slivkoff, J.; Sherard, P.; Purves, N.; Binder, P.; Knight, R.; Kim, S. L.; Chen, W. P.; Yang, M.; Lin, H. C.; Lin, C. C.; Chen, C. W.; Jiang, X. J.; Sergeev, A. V.; Mkrtichian, D.; Andreev, M.; Janulis, R.; Siwak, M.; Zola, S.; Koziel, D.; Stachowski, G.; Paparo, M.; Bognar, Z.; Handler, G.; Lorenz, D.; Steininger, B.; Beck, P.; Nagel, T.; Kusterer, D.; Hoffman, A.; Reiff, E.; Kowalski, R.; Vauclair, G.; Charpinet, S.; Chevreton, M.; Solheim, J. E.; Pakstiene, E.; Fraga, L.; Dalessio, J.; Montgomery, Michael H.; Winget, D. E.
We report on the analysis of 436.1 hr of nearly continuous high-speed photometry on the pulsating db white dwarf GD358 acquired with the Whole Earth Telescope (WET) during the 2006 international observing run, designated XCOV25. The Fourier transform (FT) of the light curve contains power between 1000 and 4000 mu Hz, with the dominant peak at 1234 mu Hz. We find 27 independent frequencies distributed in 10 modes, as well as numerous combination frequencies. Our discussion focuses on a new asteroseismological analysis of GD358, incorporating the 2006 data set and drawing on 24 years of archival observations. Our results reveal that, while the general frequency locations of the identified modes are consistent throughout the years, the multiplet structure is complex and cannot be interpreted simply as l = 1 modes in the limit of slow rotation. The high-k multiplets exhibit significant variability in structure, amplitude and frequency. Any identification of the m components for the high-k multiplets is highly suspect. The k = 9 and 8 modes typically do show triplet structure more consistent with theoretical expectations. The frequencies and amplitudes exhibit some variability, but much less than the high-k modes. Analysis of the k = 9 and 8 multiplet splittings from 1990 to 2008 reveal a long-term change in multiplet splittings coinciding with the 1996 sforzando event, where GD358 dramatically altered its pulsation characteristics on a timescale of hours. We explore potential implications, including the possible connections between convection and/or magnetic fields and pulsations. We suggest future investigations, including theoretical investigations of the relationship between magnetic fields, pulsation, growth rates, and convection.
The 69 μm forsterite band in spectra of protoplanetary disks. Results from the Herschel DIGIT programme
(2013-05) Sturm, B.; Bouwman, J.; Henning, T.; Evans, N. J.; Waters, Lbfm; van Dishoeck, E. F.; Green, J. D.; Olofsson, J.; Meeus, G.; Maaskant, K.; Dominik, C.; Augereau, J. C.; Mulders, G. D.; Acke, B.; Merin, B.; Herczeg, G. J.; Digit Team; Evans, Neal J.; Green, J. D.
Context. We have analysed far-infrared spectra of 32 circumstellar disks around Herbig Ae/Be and T Tauri stars obtained within the Herschel key programme Dust, Ice and Gas in Time (DIGIT). The spectra were taken with the Photodetector Array Camera and Spectrometer (PACS) on board the Herschel Space Observatory. In this paper we focus on the detection and analysis of the 69 mu m emission band of the crystalline silicate forsterite. Aims. This work aims at providing an overview of the 69 mu m forsterite bands present in the DIGIT sample. We use characteristics of the emission band (peak position and FWHM) to derive the dust temperature and to constrain the iron content of the crystalline silicates. With this information, constraints can be placed on the spatial distribution of the forsterite in the disk and the formation history of the crystalline grains. Methods. The 69 mu m forsterite emission feature is analysed in terms of position and shape to derive the temperature and composition of the dust by comparison to laboratory spectra of that band. The PACS spectra are combined with existing Spitzer IRS spectra and we compare the presence and strength of the 69 mu m band to the forsterite bands at shorter wavelengths. Results. A total of 32 disk sources have been observed. Out of these 32, 8 sources show a 69 mu m emission feature that can be attributed to forsterite. With the exception of the T Tauri star AS 205, all of the detections are for disks associated with Herbig Ae/Be stars. Most of the forsterite grains that give rise to the 69 mu m bands are found to be warm (similar to 100-200 K) and iron-poor (less than similar to 2% iron). AB Aur is the only source where the emission cannot be fitted with iron-free forsterite requiring approximately 3-4% of iron. Conclusions. Our findings support the hypothesis that the forsterite grains form through an equilibrium condensation process at high temperatures. The large width of the emission band in some sources may indicate the presence of forsterite reservoirs at different temperatures. The connection between the strength of the 69 and 33 mu m bands shows that at least part of the emission in these two bands originates fom the same dust grains. We further find that any model that can explain the PACS and the Spitzer IRS observations must take the effects of a wavelength dependent optical depth into account. We find weak indications of a correlation of the detection rate of the 69 mu m band with the spectral type of the host stars in our sample. However, the sample size is too small to obtain a definitive result.
The Absence of Radio Emission from the Globular Cluster G1
(2012-08) Miller-Jones, J. C. A.; Wrobel, J. M.; Sivakoff, G. R.; Heinke, C. O.; Miller, R. E.; Plotkin, R. M.; Di Stefano, R.; Greene, J. E.; Ho, L. C.; Joseph, T. D.; Kong, A. K. H.; Maccarone, T. J.; Greene, J. E.
The detections of both X-ray and radio emission from the cluster G1 in M31 have provided strong support for existing dynamical evidence for an intermediate-mass black hole (IMBH) of mass (1.8 +/- 0.5) x 10(4) M-circle dot at the cluster center. However, given the relatively low significance and astrometric accuracy of the radio detection, and the non-simultaneity of the X-ray and radio measurements, this identification required further confirmation. Here we present deep, high angular resolution, strictly simultaneous X-ray and radio observations of G1. While the X-ray emission (L-X = 1.74(-0.44)(+0.53) x 10(36) (d/750 kpc)(2) erg s(-1) in the 0.5-10 keV band) remained fully consistent with previous observations, we detected no radio emission from the cluster center down to a 3 sigma upper limit of 4.7 mu Jy beam(-1). Our favored explanation for the previous radio detection is flaring activity from a black hole low-mass X-ray binary (LMXB). We performed a new regression of the "Fundamental Plane" of black hole activity, valid for determining black hole mass from radio and X-ray observations of sub-Eddington black holes, finding log M-BH = (1.638 +/- 0.070) log L-R - (1.136 +/- 0.077) log L-X - (6.863 +/- 0.790), with an empirically determined uncertainty of 0.44 dex. This constrains the mass of the X-ray source in G1, if a black hole, to be <9.7 x 10(3) M-circle dot at 95% confidence, suggesting that it is a persistent LMXB. This annuls what was previously the most convincing evidence from radiation for an IMBH in the Local Group, though the evidence for an IMBH in G1 from velocity dispersion measurements remains unaffected by these results.
The Absolute Magnitude Of RRc Variables From Statistical Parallax
(2013-09) Kollmeier, Juna A.; Szczygiel, Dorota M.; Burns, Christopher R.; Gould, Andrew; Thompson, Ian B.; Preston, George W.; Sneden, Christopher; Crane, Jeffrey D.; Dong, Subo; Madore, Barry F.; Morrell, Nidia; Prieto, Jose L.; Shectman, Stephen; Simon, Joshua D.; Villanueva, Edward; Sneden, Christopher
We present the first definitive measurement of the absolute magnitude of RR Lyrae c-type variable stars (RRc) determined purely from statistical parallax. We use a sample of 242 RRc variables selected from the All Sky Automated Survey for which high-quality light curves, photometry, and proper motions are available. We obtain high-resolution echelle spectra for these objects to determine radial velocities and abundances as part of the Carnegie RR Lyrae Survey. We find that M-V,M-RRc = 0.59 +/- 0.10 at a mean metallicity of [Fe/H] = -1.59. This is to be compared with previous estimates for RRab stars (M-V,M-RRab = 0.76 +/- 0.12) and the only direct measurement of an RRc absolutemagnitude (RZ Cephei, M-V,M-RRc = 0.27 +/- 0.17). We find the bulk velocity of the halo relative to the Sun to be (W-pi, W-theta, W-z) = (12.0,-209.9, 3.0) km s(-1) in the radial, rotational, and vertical directions with dispersions (sigma(W pi), sigma(W theta), sigma(Wz)) = (150.4, 106.1, 96.0) km s(-1). For the disk, we find (W-pi, W-theta, W-z) = (13.0,-42.0,-27.3) km s(-1) relative to the Sun with dispersions (sigma(W pi), sigma(W theta), sigma(Wz)) = (67.7, 59.2, 54.9) km s(-1). Finally, as a byproduct of our statistical framework, we are able to demonstrate that UCAC2 proper-motion errors are significantly overestimated as verified by UCAC4.
The Abundance Of Boron In Diffuse Interstellar Clouds
(2011-02) Ritchey, A. M.; Federman, S. R.; Sheffer, Y.; Lambert, David L.; Sheffer, Y.
We present a comprehensive survey of boron abundances in diffuse interstellar clouds from observations made with the Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope. Our sample of 56 Galactic sight lines is the result of a complete search of archival STIS data for the B II lambda 1362 resonance line, with each detection confirmed by the presence of absorption from O I lambda 1355, Cu II lambda 1358, and Ga II lambda 1414 (when available) at the same velocity. Five previous measurements of interstellar B II from Goddard High Resolution Spectrograph observations are incorporated in our analysis, yielding a combined sample that more than quadruples the number of sight lines with significant boron detections. Our survey also constitutes the first extensive analysis of interstellar gallium from STIS spectra and expands on previously published results for oxygen and copper. The observations probe both high-and low-density diffuse environments, allowing the density-dependent effects of interstellar depletion to be clearly identified in the gas-phase abundance data for each element. In the case of boron, the increase in relative depletion with line-of-sight density amounts to an abundance difference of 0.8 dex between the warm and cold phases of the diffuse interstellar medium. The abundance of boron in warm, low-density gas is found to be B/H = (2.4 +/- 0.6) x 10(-10), which represents a depletion of 60% relative to the meteoritic boron abundance. Beyond the effects of depletion, our survey reveals sight lines with enhanced boron abundances that potentially trace the recent production of B-11, resulting from spallation reactions involving either cosmic rays or neutrinos. Future observations will help to disentangle the relative contributions from the two spallation channels for B-11 synthesis.
Abundances Of C, N, Sr, And Ba On The Red Giant Branch Of Omega Centauri
(2010-05) Stanford, Laura M.; Da Costa, G. S.; Norris, John E.; Stanford, Laura M.
Abundances relative to iron for carbon, nitrogen, strontium, and barium are presented for 33 stars on the red giant branch (RGB) of the globular cluster omega Centauri. They are based on intermediate-resolution spectroscopic data covering the blue spectral region analyzed using spectrum synthesis techniques. The data reveal the existence of a broad range in the abundances of these elements, and a comparison with similar data for main-sequence stars enables insight into the evolutionary history of the cluster. The majority of the RGB stars were found to be depleted in carbon, i.e., [C/Fe] < 0, while [N/Fe] for the same stars shows a range of similar to 1 dex, from [N/Fe] approximate to 0.7 to 1.7 dex. The strontium-to-iron abundance ratios varied from solar to mildly enhanced (0.0 <= [Sr/Fe] <= 0.8), with [Ba/Fe] generally equal to or greater than [Sr/Fe]. The carbon and nitrogen abundance ratios for the one known CH star in the sample, ROA 279, are [C/Fe] = 0.6 and [N/Fe] = 0.5 dex. Evidence for evolutionary mixing on the RGB is found from the fact that the relative carbon abundances on the main sequence are generally higher than those on the RGB. However, comparison of the RGB and main-sequence samples shows that the upper level of nitrogen enhancement is similar in both sets at [N/Fe] approximate to 2.0 dex. This is most likely the result of primordial rather than evolutionary mixing processes. One RGB star, ROA 276, was found to have Sr and Ba abundance ratios similar to the anomalous Sr-rich main-sequence star S2015448. High-resolution spectra of ROA 276 were obtained with the Magellan Telescope/mike spectrograph combination to confirm this result, revealing that ROA 276 is indeed an unusual star. For this star, calculations of the depletion effect, the potential change in surface abundance that results from the increased depth of the convective envelope as a star moves from the main sequence to the RGB, strongly suggest that the observed Sr enhancement in ROA 276 is of primordial origin, rather than originating from a surface accretion event.
Abundances Of Light Elements In Metal-Poor Stars IV. Fe/O And Fe/Mg Ratios And The History Of Star Formation In The Solar Neighborhood
(2000-06) Gratton, Raffaele Gratton; Carretta, Eugenio; Matteucci, Francesca; Sneden, Christopher; Sneden, Christopher
The accurate O, Mg and Fe abundances derived in previous papers of this series from a homogeneous reanalysis of high quality data for a large sample of stars are combined with stellar kinematics in order to discuss the history of star formation in the solar neighborhood. We found that the Fe/O and Fe/Mg abundance ratios are roughly constant in the (inner) halo and the thick disk; this means that the timescale of halo collapse was shorter than or of the same order of typical lifetime of progenitors of type Ia SNe (similar to 1 Gyr), this conclusion being somewhat relaxed (referring to star formation in the individual fragments) in an accretion model for the Galaxy formation. Both Fe/O and Fe/Mg ratios raised by similar to 0.2 dex while the O/H and Mg/H ratios hold constant during the transition from the thick to thin disk phases, indicating a sudden decrease in star formation in the solar neighbourhood at that epo;ch. These results are discussed in the framework of current views of Galaxy formation; they fit in a scenario where both dissipational collapse and accretions were active on a quite similar timescale.
Abundances Of Light Elements In Metal-Poor Stars IV. Fe/O And Fe/Mg Ratios And The History Of Star Formation In The Solar Neighborhood
(2000-06) Gratton, Raffaele Gratton; Carretta, Eugenio; Matteucci, Francesca; Sneden, Christopher; Sneden, Christopher
The accurate O, Mg and Fe abundances derived in previous papers of this series from a homogeneous reanalysis of high quality data for a large sample of stars are combined with stellar kinematics in order to discuss the history of star formation in the solar neighborhood. We found that the Fe/O and Fe/Mg abundance ratios are roughly constant in the (inner) halo and the thick disk; this means that the timescale of halo collapse was shorter than or of the same order of typical lifetime of progenitors of type Ia SNe (similar to 1 Gyr), this conclusion being somewhat relaxed (referring to star formation in the individual fragments) in an accretion model for the Galaxy formation. Both Fe/O and Fe/Mg ratios raised by similar to 0.2 dex while the O/H and Mg/H ratios hold constant during the transition from the thick to thin disk phases, indicating a sudden decrease in star formation in the solar neighbourhood at that epo;ch. These results are discussed in the framework of current views of Galaxy formation; they fit in a scenario where both dissipational collapse and accretions were active on a quite similar timescale.
The Abundances of Light Neutron-Capture Elements in Planetary Nebulae. II. S-Process Enrichments and Interpretation
(2008-01) Sterling, N. C.; Dinerstein, Harriet L.; Sterling, N. C.; Dinerstein, Harriet L.
We present the results of a large-scale survey of neutron(n)-capture elements in Galactic planetary nebulae (PNe), undertaken to study enrichments from s-process nucleosynthesis in their progenitor stars. From new K-band observations of over 100 PNe supplemented by data from the literature, we have detected the emission lines [Kr III] 2.199 mu m and/or [Se IV] 2.287 mu m in 81 of 120 objects. We determine Se and Kr elemental abundances, employing ionization correction formulae derived in the first paper of this series. We find a significant range in Se and Kr abundances, from near solar (no enrichment) to enhanced by > 1.0 dex relative to solar, which we interpret as self-enrichment due to in situ s-process nucleosynthesis. Kr tends to be more strongly enriched than Se; in 18 objects exhibiting both Se and Kr emission, we find that [Kr/Se] = 0.5 +/- 0.2. Our survey has increased the number of PNe with n-capture element abundance determinations by a factor of 10, enabling us for the first time to search for correlations with other nebular properties. As expected, we find a positive correlation between s-process enrichments and the C/O ratio. Type I and bipolar PNe, which arise from intermediate-mass progenitors (> 3-4 M-circle dot), exhibit little to no s-process enrichments. Finally, PNe with H-deficient Wolf-Rayet central stars do not exhibit systematically larger s-process enrichments than objects with H-rich nuclei. Overall, 44% of the PNe in our sample display significant s-process enrichments (> 0.3 dex). Using an empirical PN luminosity function to correct for incompleteness, we estimate that the true fraction of s-process enriched Galactic PNe is at least 20%.
The Abundances of Light Neutron-Capture Elements in Planetary Nebulae. III. The Impact of New Atomic Data on Nebular Selenium and Krypton Abundance Determinations
(2015-06) Sterling, N. C.; Porter, R. L.; Dinerstein, Harriet L.; Dinerstein, Harriet L.
The detection of neutron(n)-capture elements in several planetary nebulae (PNe) has provided a new means of investigating s-process nucleosynthesis in low-mass stars. However, a lack of atomic data has inhibited accurate trans-iron element abundance determinations in astrophysical nebulae. Recently, photoionization (PI) and recombination data were determined for Se and Kr, the two most widely detected n-capture elements in nebular spectra. We have incorporated these new data into the photoionization code Cloudy. To test the atomic data, numerical models were computed for 15 PNe that exhibit emission lines from multiple Kr ions. We found systematic discrepancies between the predicted and observed emission lines that are most likely caused by inaccurate PI and recombination data. These discrepancies were removed by adjusting the Kr+-Kr3+ PI cross sections within their cited uncertainties and the dielectronic recombination rate coefficients by slightly larger amounts. From grids of models spanning the physical conditions encountered in PNe, we derive new, broadly applicable ionization correction factor (ICF) formulae for calculating Se and Kr elemental abundances. The ICFs were applied to our previous survey of near-infrared [Kr III] and [Se IV] emission lines in 120 PNe. The revised Se and Kr abundances are 0.1-0.3 dex lower than former estimates, with average values of [Se/(O, Ar)] = 0.12 +/- 0.27 and [Kr/(O, Ar)] = 0.82 +/- 0.29, but correlations previously found between their abundances and other nebular and stellar properties are unaffected. We also find a tendency for high-velocity PNe that can be associated with the Galactic thick disk to exhibit larger s-process enrichments than low-velocity PNe belonging to the thin-disk population.
Abundances Of Neutron-Capture Elements In Metal-Poor Stars
(1994-06) Gratton, Raffaele Gratton; Sneden, Christopher; Sneden, Christopher
We use a large set of high S/N, high resolution spectra of 19 stars with -2.8 <[Fe/H]< 0 to study the abundances of neutron-rich elements in metal-poor stars. Basic data (atmospheric parameters, iron abundances, abundance indices, atomic and line parameters) are carefully examined both for the Sun and for the program stars, and extensive use is made of comparisons with synthetic spectra. New analyses of solar abundances of Sr, La, and Ce are presented; deduced abundances agree well with meteoritic results. Our stellar abundances are briefly compared with nucleosynthesis predictions. The main results are: 1. Those elements whose solar abundances are mainly attributed to the s-process (e.g. Ba and La) are overdeficient in extremely metal-poor stars ([Fe/H]< -2) with respect to those elements whose solar abundances are mainly attributed to the r-process (e.g. Eu). We did not find any clear evidence for a plateau in abundance ratios like [Ba/Eu] at these low values of [Fe/H]. 2. Eu itself begins to decline (with respect either to Fe or Mg) in the most metal-poor stars, with perhaps a sharp drop in stars with [Fe/H]< -2.5. If the r-process mainly occurs in SN explosions of massive stars, then the abundances of its products in the ejecta are a function of initial stellar mass and/or metallicity. 3. The abundance pattern of neutron-capture elements in metal-poor stars show clear differences with respect to scaled solar-system r-process nucleosynthesis predictions; e.g. there is a relative excess of Ba (attributed mainly to the s-process). This pattern may be explained if the contribution of the s-process to the solar abundances of heavy elements is overestimated, or the production of heavy elements through the r - process was different when [Fe/H] was much lower than the present value, or there was an early onset of the contribution by the main component of the s-process in the Galactic chemical evolution.
The Abundances Of Neutron-Capture Species In The Very Metal-Poor Globular Cluster M15: A Uniform Analysis Of Red Giant Branch And Red Horizontal Branch Stars
(2011-06) Sobeck, Jennifer S.; Kraft, Robert P.; Sneden, Christopher; Preston, George W.; Cowan, John J.; Smith, Graeme H.; Thompson, Ian B.; Shectman, Stephen A.; Burley, Gregory S.; Sneden, Christopher
The globular cluster M15 is unique in its display of star-to-star variations in the neutron-capture elements. Comprehensive abundance surveys have been previously conducted for handfuls of M15 red giant branch (RGB) and red horizontal branch (RHB) stars. No attempt has been made to perform a single, self-consistent analysis of these stars, which exhibit a wide range in atmospheric parameters. In the current effort, a new comparative abundance derivation is presented for three RGB and six RHB members of the cluster. The analysis employs an updated version of the line transfer code MOOG, which now appropriately treats coherent, isotropic scattering. The apparent discrepancy in the previously reported values for the metallicity of M15 RGB and RHB stars is addressed and a resolute disparity of Delta(RHB-RGB) approximate to 0.1 dex in the iron abundance was found. The anti-correlative behavior of the light neutron-capture elements (Sr, Y, Zr) is clearly demonstrated with both Ba and Eu, standard markers of the s- and r-process, respectively. No conclusive detection of Pb was made in the RGB targets. Consequently for the M15 cluster, this suggests that the main component of the s-process has made a negligible contribution to those elements normally dominated by this process in solar system material. Additionally for the M15 sample, a large Eu abundance spread is confirmed, which is comparable to that of the halo field at the same metallicity. These abundance results are considered in the discussion of the chemical inhomogeneity and nucleosynthetic history of M15.
The Abundances Of Polyacetylenes Toward CRL618
(2011-02) Fonfria, J. P.; Cernicharo, J.; Richter, Matthew J.; Lacy, John H.; Lacy, John H.
We present a mid-infrared high spectral resolution spectrum of CRL618 in the frequency ranges 778-784 and 1227-1249 cm(-1) (8.01-8.15 and 12.75-12.85 mu m) taken with the Texas Echelon-cross-Echelle Spectrograph (TEXES) and the Infrared Telescope Facility (IRTF). We have identified more than 170 rovibrational lines arising from C2H2, HCN, C4H2, and C6H2. We have found no unmistakable trace of C8H2. The line profiles display a complex structure suggesting the presence of polyacetylenes in several components of the circumstellar envelope (CSE). We derive total column densities of 2.5x10(17), 3.1x10(17), 2.1x10(17), 9.3x10(16) cm(-2), and less than or similar to 5x10(16) cm(-2) for HCN, C2H2, C4H2, C6H2, and C8H2, respectively. The observations indicate that both the rotational and vibrational temperatures in the innermost CSE depend on the molecule, varying from 100 to 350 K for the rotational temperatures and 100 to 500 K for the vibrational temperatures. Our results support a chemistry in the innermost CSE based on radical-neutral reactions triggered by the intense UV radiation field.
Accretion Disk Temperatures Of QSOs: Constraints From The Emission Lines
(2013-06) Bonning, E. W.; Shields, Gregory A.; Stevens, A. C.; Salviander, Sarah; Shields, Gregory A.; Stevens, A. C.; Salviander, Sarah
We compare QSO emission-line spectra to predictions based on theoretical ionizing continua of accretion disks. The observed line intensities do not show the expected trend of higher ionization with theoretical accretion disk temperature as predicted from the black hole mass and accretion rate. Consistent with earlier studies, this suggests that the inner disk does not reach temperatures as high as expected from standard disk theory. Modified radial temperature profiles, taking account of winds or advection in the inner disk, achieve better agreement with observation. The emission lines of radio-detected and radio-undetected sources show different trends as a function of the theoretically predicted disk temperature.
Accretion Onto "Seed" Black Holes In The First Galaxies
(2009-06) Milosavljevic, Milos; Bromm, Volker; Couch, Sean M.; Oh, S. Peng; Milosavljevic, Milos; Bromm, Volker; Couch, Sean M.
The validity of the hypothesis that the massive black holes in high redshift quasars grew from stellar-sized "seeds" is contingent on a seed's ability to double its mass every few 10 million years. This requires that the seed accrete at approximately the Eddington-limited rate. In the specific case of radiatively efficient quasi-radial accretion in a metal-poor protogalactic medium, for which the Bondi accretion rate is often prescribed in cosmological simulations of massive black hole formation, we examine the effects of the radiation emitted near the black hole's event horizon on the structure of the surrounding gas flow. We find that photoheating and radiation pressure from photoionization significantly reduce the steady-Stateaccretion rate and potentially render the quasi-radial accretion flow unsteady and inefficient. The time-averaged accretion rate is always a small fraction of the "Bondi" accretion rate calculated ignoring radiative feedback. The pressure of Ly alpha photons trapped near the H II region surrounding the black hole may further attenuate the inflow. These results suggest that an alternative to quasi-radial, radiatively efficient Bondi-like accretion should be sought to explain the rapid growth of quasar-progenitor seed black holes.
Accretion onto Intermediate-Mass Black Holes in Dense Protogalactic Clouds
(2009-05) Milosavljevi?, Miloš; Couch, Sean M.; Bromm, Volker; Milosavljevi?, Miloš; Couch, Sean M.; Bromm, Volker
We present the first results from two-dimensional simulations of radiatively efficient accretion of metal-free gas onto intermediate-mass black holes. We fix the shape of the spectral energy distribution of the radiation produced near the event horizon and study the structure of the irradiated low-angular-momentum accretion flow over 3 orders of magnitude in radius from the black hole, 10(14)-10(17) cm for a 100 M(circle dot) black hole. The luminosity of the central source is made to be proportional to the rate at which gas accretes across the inner boundary, which we set just inside the sonic radius. We find that photoionization heating and radiation pressure modify the structure of the flow. When the ambient gas density is 10(7) cm(-3), accretion is intermittent and on average reduced to 32% of the Eddington-limited rate, over 2 orders of magnitude below the "Bondi" rate evaluated ignoring radiation, in agreement with theoretical models. Even if the vicinity of the black hole is supplied with high-density gas, accretion is rendered inefficient through heating and radiation pressure.
Accretion Onto Planetary Mass Companions of Low-Mass Young Stars
(2014-03) Zhou, Yifan; Herczeg, Gregory J.; Kraus, Adam L.; Metchev, Stanimir; Cruz, Kelle L.; Kraus, Adam L.
Measurements of accretion rates onto planetary mass objects may distinguish between different planet Formation mechanisms, which predict different accretion histories. In this Letter, we use Hubble Space Telescope (HST)/WFC3 UVIS optical photometry to measure accretion rates onto three accreting objects, GSC 06214-00210 b, GQ Lup b, and DH Tau b, that are at the planet/brown dwarf boundary and are companions to solar mass stars. The excess optical emission in the excess accretion continuum yields mass accretion rates of 10(-9)-10(-11) M-circle dot yr(-1) for these three objects. Their accretion rates are an order of magnitude higher than expected from the correlation between mass and accretion rates measured from the UV excess, which is applicable if these wide planetary mass companions formed by protostellar core fragmentation. The high accretion rates and large separation from the central star demonstrate the presence of massive disks around these objects. Models for the Formation and evolution of wide planetary mass companions should account for their large accretion rates. High ratios of Ha luminosity over accretion luminosity for objects with low accretion rates suggest that searches for Ha emission may be an efficient way to find accreting planets.
The ACS Nearby Galaxy Survey Treasury. III. Cepheids In The Outer Disk Of M81
(2009-06) McCommas, Les P.; Yoachim, Peter; Williams, Benjamin F.; Dalcanton, Julianne J.; Davis, Matthew R.; Dolphin, Andrew E.; Yoachim, Peter
The ACS Nearby Galaxy Survey Treasury (ANGST) has acquired deep ACS imaging of a field in the outer disk of the large spiral galaxy M81. These data were obtained over a total of 20 Hubble Space Telescope orbits, providing a baseline long enough to reliably identify Cepheid variable stars in the field. Fundamental mode and first overtone types have been distinguished through comparative fits with corresponding Cepheid light curve templates derived from principal component analysis of confirmed Cepheids in the Large Magellanic Cloud (LMC), Small Magellanic Cloud, and Milky Way. A distance modulus of 27.78 +/- 0.05(r) +/- 0.14(s) with a corresponding distance of 3.60 +/- 0.23 Mpc has been calculated from a sample of 11 fundamental mode and two first overtone Cepheids (assuming an LMC distance modulus of mu(LMC) = 18.41 +/- 0.10(r) +/- 0.13(s)).