Browsing by Subject "ism: abundances"
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Item 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.Item The c2d Spitzer Spectroscopic Survey Of Ices Around Low-Mass Young Stellar Objects. IV. NH3 And CH3OH(2010-08) Bottinelli, Sandrine; Boogert, Adwin C. A.; Bouwman, Jordy; Beckwith, Martha; van Dishoeck, Ewine F.; Oberg, Karin I.; Pontoppidan, Klaus M.; Linnartz, H.; Blake, Geoffrey A.; Evans, Neal J.; Lahuis, Fred; Evans, Neal J.NH3 and CH3OH are key molecules in astrochemical networks leading to the formation of more complex N- and O-bearing molecules, such as CH3CN and CH3OCH3. Despite a number of recent studies, little is known about their abundances in the solid state. This is particularly the case for low-mass protostars, for which only the launch of the Spitzer Space Telescope has permitted high-sensitivity observations of the ices around these objects. In this work, we investigate the similar to 8-10 mu m region in the Spitzer IRS (InfraRed Spectrograph) spectra of 41 low-mass young stellar objects (YSOs). These data are part of a survey of interstellar ices in a sample of low-mass YSOs studied in earlier papers in this series. We used both an empirical and a local continuum method to correct for the contribution from the 10 mu m silicate absorption in the recorded spectra. In addition, we conducted a systematic laboratory study of NH3- and CH3OH- containing ices to help interpret the astronomical spectra. We clearly detect a feature at similar to 9 mu m in 24 low-mass YSOs. Within the uncertainty in continuum determination, we identify this feature with the NH3 nu(2) umbrella mode and derive abundances with respect to water between similar to 2% and 15%. Simultaneously, we also revisited the case of CH3OH ice by studying the nu(4) C-O stretch mode of this molecule at similar to 9.7 mu m in 16 objects, yielding abundances consistent with those derived by Boogert et al. based on a simultaneous 9.75 and 3.53 mu m data analysis. Our study indicates that NH3 is present primarily in H2O-rich ices, but that in some cases, such ices are insufficient to explain the observed narrow FWHM. The laboratory data point to CH3OH being in an almost pure methanol ice, or mixed mainly with CO or CO2, consistent with its formation through hydrogenation on grains. Finally, we use our derived NH3 abundances in combination with previously published abundances of other solid N-bearing species to find that up to 10%-20% of nitrogen is locked up in known ices.Item The Chemical Composition Of Cernis 52 (BD+31 Degrees 640)(2009-11) Hernandez, J. I. G.; Iglesias-Groth, S.; Rebolo, R.; Garcia-Hernandez, D. Anibal; Manchado, A.; Lambert, David L.; Lambert, David L.We present an abundance analysis of the star Cernis 52 in whose spectrum we recently reported the naphthalene cation in absorption at 6707.4 angstrom. This star is on a line of sight to the Perseus molecular complex. The analysis of high-resolution spectra using a chi(2)-minimization procedure and a grid of synthetic spectra provides the stellar parameters and the abundances of O, Mg, Si, S, Ca, and Fe. The stellar parameters of this star are found to be T(eff) = 8350 +/- 200 K, log(g/cm s(2))= 4.2 +/- 0.4 dex. We derived a metallicity of [Fe/H] = -0.01 +/- 0.15. These stellar parameters are consistent with a star of similar to 2 M(circle dot) in a pre-main-sequence evolutionary stage. The stellar spectrum is significantly veiled in the spectral range lambda lambda 5150-6730 angstrom up to almost 55% of the total flux at 5150 angstrom and decreasing toward longer wavelengths. Using Johnson-Cousins and Two Micron All Sky Survey photometric data, we determine a distance to Cernis 52 of 231(-85)(+135) pc considering the error bars of the stellar parameters. This determination places the star at a similar distance to the young cluster IC 348. This together with its radial velocity, v(r) = 13.7 +/- 1 kms(-1), its proper motion and probable young age support Cernis 52 as a likely member of IC 348. We determine a rotational velocity of v sin i = 65 +/- 5 kms(-1) for this star. We confirm that the stellar resonance line of Li I at 6707.8 angstrom is unable to fit the broad feature at 6707.4 angstrom. This feature should have a interstellar origin and could possibly form in the dark cloud L1470 surrounding all the cluster IC 348 at about the same distance.Item Evidence for the Naphthalene Cation in a Region of the Interstellar Medium with Anomalous Microwave Emission(2008-09) Iglesias-Groth, S.; Manchado, A.; Garcia-Hernandez, D. A.; Hernandez, J. I. G.; Lambert, David L.; Lambert, D. L.We report high-resolution spectroscopy of the moderately reddened (A(v) = 3) early-type star Cernis 52 located in a region of the Perseus molecular cloud complex with anomalous microwave emission. In addition to the presence of the most common diffuse interstellar bands (DIBs) we detect two new interstellar or circumstellar bands coincident to within 0.01% in wavelength with the two strongest bands of the naphthalene cation (C(10)H(8)(+)) as measured in gas-phase laboratory spectroscopy at low temperatures and find marginal evidence for the third strongest band. Assuming these features are caused by the naphthalene cation, from the measured intensity and available oscillator strengths we find that 0.008% of the carbon in the cloud could be in the form of this molecule. We expect hydrogen additions to cause hydronaphthalene cations to be abundant in the cloud and to contribute via electric dipole radiation to the anomalous microwave emission. The identification of new interstellar features consistent with transitions of the simplest polycyclic aromatic hydrocarbon adds support to the hypothesis that this type of molecules are the carriers of both diffuse interstellar bands and anomalous microwave emission.Item Interstellar CN And CH+ In Diffuse Molecular Clouds: C-12/C-13 Ratios And Cn Excitation(2011-02) Ritchey, A. M.; Federman, S. R.; Lambert, David L.; Ritchey, A. M.; Federman, S. R.; Lambert, David L.We present very high signal-to-noise ratio absorption-line observations of CN and CH+ along 13 lines of sight through diffuse molecular clouds. The data are examined to extract precise isotopologic ratios of (CN)-C-12/(CN)-C-13 and (CH+)-C-12/(CH+)-C-13 in order to assess predictions of diffuse cloud chemistry. Our results on (CH+)-C-12/(CH+)-C-13 confirm that this ratio does not deviate from the ambient C-12/C-13 ratio in local interstellar clouds, as expected if the formation of CH+ involves nonthermal processes. We find that (CN)-C-12/(CN)-C-13, however, can be significantly fractionated away from the ambient value. The dispersion in our sample of (CN)-C-12/(CN)-C-13 ratios is similar to that found in recent surveys of (CO)-C-12/(CO)-C-13. For sight lines where both ratios have been determined, the (CN)-C-12/(CN)-C-13 ratios are generally fractionated in the opposite sense compared to (CO)-C-12/(CO)-C-13. Chemical fractionation in CO results from competition between selective photodissociation and isotopic charge exchange (ICE). An inverse relationship between (CN)-C-12/(CN)-C-13 and (CO)-C-12/(CO)-C-13 follows from the coexistence of CN and CO in diffuse cloud cores. However, an ICE reaction with CN may mitigate the enhancements in (CN)-C-12/(CN)-C-13 for lines of sight with low (CO)-C-12/(CO)-C-13 ratios. For two sight lines with high values of (CO)-C-12/(CO)-C-13, our results indicate that about 50% of the carbon is locked up in CO, which is consistent with the notion that these sight lines probe molecular cloud envelopes where the transition from C+ to CO is expected to occur. An analysis of CN rotational excitation yields a weighted mean value for T-01((CN)-C-12) of 2.754 +/- 0.002 K, which implies an excess over the temperature of the cosmic microwave background (CMB) of only 29 +/- 3 mK. This modest excess eliminates the need for a local excitation mechanism beyond electron and neutral collisions. The rotational excitation temperatures in (CN)-C-13 show no excess over the temperature of the CMB.Item the Li-7/Li-6 Isotope Ratio Near the Supernova Remnant IC 443(2012-05) Taylor, C. J.; Ritchey, A. M.; Federman, S. R.; Lambert, David L.; Lambert, David L.We present an analysis of Li-7/Li-6 isotope ratios along four sight lines that probe diffuse molecular gas near the supernova remnant IC 443. Recent gamma-ray observations have revealed the presence of shock-accelerated cosmic rays interacting with the molecular cloud surrounding the remnant. Our results indicate that the Li-7/Li-6 ratio is lower in regions more strongly affected by these interactions, a sign of recent Li production by cosmic rays. We find that Li-7/Li-6 approximate to 7 toward HD 254755, which is located just outside the visible edge of IC 443, while Li-7/Li-6 approximate to 3 along the line of sight to HD 43582, which probes the interior region of the supernova remnant. No evidence of Li-7 synthesis by neutrino-induced spallation is found in material presumably contaminated by the ejecta of a core-collapse supernova. The lack of a neutrino signature in the Li-7/Li-6 ratios near IC 443 is consistent with recent models of Galactic chemical evolution, which suggest that the nu-process plays only a minor role in Li production.Item Modeling The Pollution Of Pristine Gas In The Early Universe(2013-10) Pan, Lubin B.; Scannapieco, Evan; Scalo, John; Scalo, JohnWe conduct a comprehensive theoretical and numerical investigation of the pollution of pristine gas in turbulent flows, designed to provide useful new tools for modeling the evolution of the first generation of stars. The properties of such Population III (Pop III) stars are thought to be very different than those of later stellar generations, because cooling is dramatically different in gas with a metallicity below a critical value Z(c), which lies between similar to 10(-6) and similar to 10(-3) Z(circle dot). The critical value is much smaller than the typical overall average metallicity, < Z >, and therefore the mixing efficiency of the pristine gas in the interstellar medium plays a crucial role in determining the transition from Pop III to normal star formation. The small critical value, Z(c), corresponds to the far left tail of the probability distribution function (PDF) of the metal abundance. Based on closure models for the PDF formulation of turbulent mixing, we derive evolution equations for the fraction of gas, P, lying below Z(c), in statistically homogeneous compressible turbulence. Our simulation data show that the evolution of the pristine fraction P can be well approximated by a generalized "self-convolution" model, which predicts that (P) over dot = -(n/tau(con))P(1 - P-1/n), where n is a measure of the locality of the mixing or PDF convolution events and the convolution timescale tau(con) is determined by the rate at which turbulence stretches the pollutants. Carrying out a suite of numerical simulations with turbulent Mach numbers ranging from M = 0.9 to 6.2, we are able to provide accurate fits to n and tau(con) as a function of M, Z(c)/< Z >, and the length scale, L-p, at which pollutants are added to the flow. For pristine fractions above P = 0.9, mixing occurs only in the regions surrounding blobs of pollutants, such that n = 1. For smaller values of P, n is larger as the mixing process becomes more global. We show how these results can be used to construct one-zone models for the evolution of Pop III stars in a single high-redshift galaxy, as well as subgrid models for tracking the evolution of the first stars in large cosmological numerical simulations.Item Rubidium In The Interstellar Medium(2009-11) Walker, Kyle M.; Federman, S. R.; Knauth, David C.; Lambert, David L.; Lambert, David L.We present observations of interstellar rubidium toward o Per, zeta Per, AE Aur, HD 147889, chi Oph, zeta Oph, and 20 Aql. Theory suggests that stable (85)Rb and long-lived (87)Rb are produced predominantly by high-mass stars, through a combination of the weak s- and r-processes. The (85)Rb/(87)Rb ratio was determined from measurements of the Rb I line at 7800 angstrom and was compared to the solar system meteoritic ratio of 2.59. Within 1 sigma uncertainties, all directions except HD 147889 have Rb isotope ratios consistent with the solar system value. The ratio toward HD 147889 is much lower than themeteoritic value and similar to that toward rho Oph A; both lines of sight probe the Rho Ophiuchus Molecular Cloud. The earlier result was attributed to a deficit of r-processed (85)Rb. Our larger sample suggests instead that (87)Rb is enhanced in these two lines of sight. When the total elemental abundance of Rb is compared to the K elemental abundance, the interstellar Rb/K ratio is significantly lower than the meteoritic ratio for all the sight lines in this study. Available interstellar samples for other s- and r-process elements are used to help interpret these results.Item Ultraviolet Survey Of CO And H-2 In Diffuse Molecular Clouds: The Reflection Of Two Photochemistry Regimes In Abundance Relationships(2008-11) Sheffer, Y.; Rogers, M.; Federman, S. R.; Abel, N. P.; Gredel, R.; Lambert, David L.; Shaw, G.; Federman, S. R.We carried out a comprehensive far-UV survey of (CO)-C-12 and H-2 column densities along diffuse molecular Galactic sight lines. This sample includes new measurements of CO from HST spectra along 62 sight lines and new measurements of H-2 from FUSE data along 58 sight lines. In addition, high-resolution optical data were obtained at the McDonald and European Southern Observatories, yielding new abundances for CH, CH+, and CN along 42 sight lines to aid in interpreting the CO results. These new sight lines were selected according to detectable amounts of CO in their spectra and provide information on both lower density (<= 100 cm(-3)) and higher density diffuse clouds. A plot of log N(CO) versus log N(H-2) shows that two power-law relationships are needed for a good fit of the entire sample, with a break located at log N(CO, cm(-2)) = 14.1 and log N(H-2) = 20.4, corresponding to a change in production route for CO in higher density gas. Similar logarithmic plots among all five diatomic molecules reveal additional examples of dual slopes in the cases of CO versus CH (break at log N 14: 1, 13.0), CH+ versus H-2 (13.1, 20.3), and CH+ versus CO (13.2, 14.1). We employ both analytical and numerical chemical schemes in order to derive details of the molecular environments. In the denser gas, where C-2 and CN molecules also reside, reactions involving C+ and OH are the dominant factor leading to CO formation via equilibrium chemistry. In the low-density gas, where equilibrium chemistry studies have failed to reproduce the abundance of CH+, our numerical analysis shows that nonequilibrium chemistry must be employed for correctly predicting the abundances of both CH+ and CO.