Browsing by Subject "formation"
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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 A Comparative Astrochemical Study Of The High-Mass Protostellar Objects NGC 7538 IRS 9 And IRS 1(2012-10) Barentine, John C.; Lacy, John H.; Barentine, John C.; Lacy, John H.We report the results of a spectroscopic study of the high-mass protostellar object NGC 7538 IRS 9 and compare our observations to published data on the nearby object NGC 7538 IRS 1. Both objects originated in the same molecular cloud and appear to be at different points in their evolutionary histories, offering an unusual opportunity to study the temporal evolution of envelope chemistry in objects sharing a presumably identical starting composition. Observations were made with the Texas Echelon Cross Echelle Spectrograph, a sensitive, high spectral resolution (R = lambda/Delta lambda similar or equal to 100,000) mid-infrared grating spectrometer. Forty-six individual lines in vibrational modes of the molecules C2H2, CH4, HCN, NH3, and CO were detected, including two isotopologues ((CO)-C-13, (CO)-C-12-O-18) and one combination mode (nu(4) + nu(5) C2H2). Fitting synthetic spectra to the data yielded the Doppler shift, excitation temperature, Doppler b parameter, column density, and covering factor for each molecule observed; we also computed column density upper limits for lines and species not detected, such as HNCO and OCS. We find differences among spectra of the two objects likely attributable to their differing radiation and thermal environments. Temperatures and column densities for the two objects are generally consistent, while the larger line widths toward IRS 9 result in less saturated lines than those toward IRS 1. Finally, we compute an upper limit on the size of the continuum-emitting region (similar to 2000 AU) and use this constraint and our spectroscopy results to construct a schematic model of IRS 9.Item Cosmological Impact Of Population III Binaries(2015-03) Chen, Ke-Jung; Bromm, Volker; Heger, Alexander; Jeon, Myoungwon; Woosley, Stan; Chen, Ke-Jung; Woosley, StanWe present the results of the stellar feedback from Population III (Pop III) binaries by employing improved, more realistic Pop III evolutionary stellar models. To facilitate a meaningful comparison, we consider a fixed mass of 60 M-circle dot incorporated in Pop III stars, either contained in a single star, or split up in binary stars of 30 M-circle dot each or an asymmetric case of one 45 and one 15 M-circle dot star. Whereas the sizes of the resulting H II regions are comparable across all cases, the He III regions around binary stars are significantly smaller than that of the single star. Consequently, the He+ 1640 angstrom recombination line is expected to become much weaker. Supernova (SN) feedback exhibits great variety due to the uncertainty in possible explosion pathways. If at least one of the component stars dies as a hypernova about 10 times more energetic than conventional core-collapse SNe, the gas inside the host minihalo is effectively blown out, chemically enriching the intergalactic medium (IGM) to an average metallicity of 10(-4)-10(-3) Z(circle dot), out to similar to 2 kpc. The single star, however, is more likely to collapse into a black hole, accompanied by at most very weak explosions. The effectiveness of early chemical enrichment would thus be significantly reduced, in contrast to. the lower mass binary stars, where at least one component is likely to contribute to heavy element production and dispersal. Important new feedback physics is also introduced if close binaries can form high-mass X-ray binaries, leading to the pre-heating and -ionization of the IGM beyond the extent of the stellar H II regions.Item Dark Matter Scaling Relations And The Assembly Epoch Of Coma Early-Type Galaxies(2009-01) Thomas, Jens; Saglia, R. P.; Bender, Ralf; Thomas, D.; Gebhardt, Karl; Magorrian, John; Corsini, E. M.; Wegner, G.; Gebhardt, KarlAxisymmetric, orbit-based dynamical models are used to derive dark matter scaling relations for Coma early-type galaxies. From faint to bright galaxies, halo core radii and asymptotic circular velocities increase. Compared to spirals of the same brightness, the majority of Coma early-type galaxies-those with old stellar populations-have similar halo core radii but more than two times larger asymptotic halo velocities. The average dark matter density inside 2r(eff) decreases with increasing luminosity and is 6.8 times larger than in disk galaxies of the same B-band luminosity. Compared at the same stellar mass, dark matter densities in ellipticals are 13.5 times higher than in spirals. Different baryon concentrations in ellipticals and spirals cannot explain the higher dark matter density in ellipticals. Instead, the assembly redshift (1 + z) of Coma early-type halos is likely about two times larger than of comparably bright spirals. Assuming that local spirals typically assemble at a redshift of one, the majority of bright Coma early-type galaxy halos must have formed around z approximate to 2-3. For about half of our Coma galaxies, the assembly redshifts match with constraints derived from stellar populations. We find dark matter densities and estimated assembly redshifts of our observed Coma galaxies in reasonable agreement with recent semi-analytic galaxy formation models.Item Discovery Of A Low-Mass Bipolar Molecular Outflow From L1014-Irs With The Submillimeter Array(2005-11) Bourke, Tyler L.; Crapsi, Antonio; Myers, Phillip C.; Evans, Neal J.; Wilner, David J.; Huard, Tracy L.; Jorgensen, Jes K.; Young, Chadwick H.; Evans, Neal J.; Young, Chadwick H.Using the Submillimeter Array, we report the discovery of a compact low-mass bipolar molecular outflow from L1014-IRS and confirm its association with the L1014 dense core at 200 pc. Consequently, L1014-IRS is the lowest luminosity (L similar to 0.09 L-.), and perhaps the lowest mass, source known to be driving a bipolar molecular outflow, which is one of the smallest known in size (similar to 500 AU), mass (<10(-4) M-.), and energetics (e.g., with a M, force <10(-7) M-. km s(-1) yr(-1)).Item The First Stars: A Low-Mass Formation Mode(2014-04) Stacy, Athena; Bromm, Volker; Bromm, VolkerWe perform numerical simulations of the growth of a Population III stellar system under photodissociating feedback. We start from cosmological initial conditions at z = 100, self-consistently following the formation of a minihalo at z = 15 and the subsequent collapse of its central gas to high densities. The simulations resolve scales as small as similar to 1 AU, corresponding to gas densities of 1016 cm(-3). Using sink particles to represent the growing protostars, we evolve the stellar system for the next 5000 yr. We find that this emerging stellar group accretes at an unusually low rate compared with minihalos which form at earlier times (z = 20-30), or with lower baryonic angular momentum. The stars in this unusual system will likely reach masses ranging from <1 M-circle dot to similar to 5 M-circle dot by the end of their main-sequence lifetimes, placing them in the mass range for which stars will undergo an asymptotic giant branch (AGB) phase. Based upon the simulation, we predict the rare existence of Population III stars that have survived to the present day and have been enriched by mass overflow from a previous AGB companion.Item Gravitational Fragmentation In Turbulent Primordial Gas And The Initial Mass Function Of Population III Stars(2011-02) Clark, Paul C.; Glover, Simon C. O.; Klessen, Ralf S.; Bromm, Volker; Bromm, VolkerWe report results from numerical simulations of star formation in the early universe that focus on the dynamical behavior of metal-free gas under different initial and environmental conditions. In particular we investigate the role of turbulence, which is thought to ubiquitously accompany the collapse of high-redshift halos. We distinguish between two main cases: the birth of Population III. 1 stars-those which form in the pristine halos unaffected by prior star formation-and the formation of Population III. 2 stars-those forming in halos where the gas has an increased ionization fraction. We find that turbulent primordial gas is highly susceptible to fragmentation in both cases, even for turbulence in the subsonic regime, i.e., for rms velocity dispersions as low as 20% of the sound speed. Fragmentation is more vigorous and more widespread in pristine halos compared to pre-ionized ones. If such levels of turbulent motions were indeed present in star-forming minihalos, Population III. 1 stars would be on average of somewhat lower mass, and form in larger groups, than Population III. 2 stars. We find that fragment masses cover over two orders of magnitude, suggesting that the Population III initial mass function may have been much broader than previously thought. This prompts the need for a large, high-resolution study of the formation of dark matter minihalos that is capable of resolving the turbulent flows in the gas at the moment when the baryons become self-gravitating. This would help to determine the applicability of our results to primordial star formation.Item Ionized Gas Toward Molecular Clumps: Physical Properties Of Massive Star-Forming Regions(2009-12) Johnston, Katharine G.; Shepherd, Debra S.; Aguirre, James E.; Dunham, Miranda K.; Rosolowsky, Erik; Wood, Kenneth; Dunham, Miranda K.We have conducted a search for ionized gas at 3.6 cm, using the Very Large Array, toward 31 Galactic intermediate- and high-mass clumps detected in previous millimeter continuum observations. In the 10 observed fields, 35 H II regions are identified, of which 20 are newly discovered. Many of the H II regions are multiply peaked indicating the presence of a cluster of massive stars. We find that the ionized gas tends to be associated toward the millimeter clumps; of the 31 millimeter clumps observed, nine of these appear to be physically related to ionized gas, and a further six have ionized gas emission within 1'. For clumps with associated ionized gas, the combined mass of the ionizing massive stars is compared to the clump masses to provide an estimate of the instantaneous star formation efficiency. These values range from a few percent to 25%, and have an average of 7% +/- 8%. We also find a correlation between the clump mass and the mass of the ionizing massive stars within it, which is consistent with a power law. This result is comparable to the prediction of star formation by competitive accretion that a power-law relationship exists between the mass of the most massive star in a cluster and the total mass of the remaining stars.Item Letter to Dan E. Feray from H.B. Stenzel on 1948-03-25(1948-03-25) Stenzel, Henryk B.Item Letter to G. Bevelander from H.B. Stenzel on 1969-05-12(1969-05-12) Stenzel, Henryk B.Item Letter to G. Bevelander from H.B. Stenzel on 1970-02-17(1970-02-17) Stenzel, Henryk B.Item Letter to H.B. Stenzel from D.J. Beets on 1964-04-04(1964-04-04) Beets, D.J.Item Letter to H.B. Stenzel from E.Wallace Moore on Undated(0000-00-00) Moore, E.WallaceItem Letter to H.B. Stenzel from G. Bevelander on 1968-10-21(1968-10-21) Bevelander, G.Item Letter to H.B. Stenzel from O.M. Ball on 1936-04-29(1936-04-29) Ball, O.M.Item Letter to H.B. Stenzel from W.H. Matthews, III on 1948-12-17(1948-12-17) Matthews, W.H., IIIItem Letter to O.H. Eichblott from H.B. Stenzel on 1936-03-05(1936-03-05) Stenzel, Henryk B.Item Letter to Rene Marliere from H.B. Stenzel on 1963-04-02(1963-04-02) Stenzel, H.B.Item The Near-Ultraviolet Luminosity Function Of Young, Early M-Type Dwarf Stars(2015-01) Ansdell, Megan; Gaidos, Eric; Mann, Andrew W.; Lepine, Sebastien; James, David; Buccino, Andrea; Baranec, Christoph; Law, Nicolas M.; Riddle, Reed; Mauas, Pablo; Petrucci, Romina; Mann, Andrew W.Planets orbiting within the close-in habitable zones of M dwarf stars will be exposed to elevated high-energy radiation driven by strong magnetohydrodynamic dynamos during stellar youth. Near-ultraviolet (NUV) irradiation can erode and alter the chemistry of planetary atmospheres, and a quantitative description of the evolution of NUV emission from M dwarfs is needed when modeling these effects. We investigated the NUV luminosity evolution of early M-type dwarfs by cross-correlating the Lepine & Gaidos catalog of bright M dwarfs with the Galaxy Evolution Explorer (GALEX) catalog of NUV (1771-2831 angstrom) sources. Of the 4805 sources with GALEX counterparts, 797 have NUV emission significantly (>2.5 sigma) in excess of an empirical basal level. We inspected these candidate active stars using visible-wavelength spectra, high-resolution adaptive optics imaging, time-series photometry, and literature searches to identify cases where the elevated NUV emission is due to unresolved background sources or stellar companions; we estimated the overall occurrence of these "false positives" (FPs) as similar to 16%. We constructed an NUV luminosity function that accounted for FPs, detection biases of the source catalogs, and GALEX upper limits. We found the NUV luminosity function to be inconsistent with predictions from a constant star-formation rate and simplified age-activity relation defined by a two-parameter power law.Item PHIBSS: Molecular Gas, Extinction, Star Formation, And Kinematics In The Z=1.5 Star-Forming Galaxy Egs13011166(2013-08) Genzel, R.; Tacconi, L. J.; Kurk, Jaron; Wuyts, S.; Combes, Francoise; Freundlich, J.; Bolatto, A.; Cooper, Michael C.; Neri, R.; Nordon, R.; Bournaud, F.; Burkert, A.; Comerford, J.; Cox, P.; Davis, M.; Schreiber, N. M. F.; Garcia-Burillo, S.; Gracia-Carpio, J.; Lutz, D.; Naab, T.; Newman, S.; Saintonge, A.; Griffin, K. S.; Shapley, A.; Sternberg, Assaf; Weiner, B.; Comerford, J.We report matched resolution imaging spectroscopy of the CO 3-2 line (with the IRAM Plateau de Bure millimeter interferometer) and of the Ha line (with LUCI at the Large Binocular Telescope) in the massive z = 1.53 main-sequence galaxy EGS 13011166, as part of the "Plateau de Bure high-z, blue-sequence survey" (PHIBSS: Tacconi et al.). We combine these data with Hubble Space Telescope V-I-J-H-band maps to derive spatially resolved distributions of stellar surface density, star formation rate, molecular gas surface density, optical extinction, and gas kinematics. The spatial distribution and kinematics of the ionized and molecular gas are remarkably similar and are well modeled by a turbulent, globally Toomre unstable, rotating disk. The stellar surface density distribution is smoother than the clumpy rest-frame UV/ optical light distribution and peaks in an obscured, star-forming massive bulge near the dynamical center. The molecular gas surface density and the effective optical screen extinction track each other and are well modeled by a "mixed" extinction model. The inferred slope of the spatially resolved molecular gas to star formation rate relation, N = d(log)Sigma(star form)/dlog Sigma(mol) (gas), depends strongly on the adopted extinction model, and can vary from 0.8 to 1.7. For the preferred mixed dust-gas model, we find N = 1.14 +/- 0.1.