Browsing by Subject "ism : clouds"

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Distances To Galactic High-Velocity Clouds. I. Cohen Stream, Complex Gcp, Cloud G1
(2008-01) Wakker, B. P.; York, D. G.; Wilhelm, R.; Barentine, John C.; Richter, P.; Beers, Timothy C.; Ivezic, Z.; Howk, J. C.; Barentine, John C.
The high- and intermediate-velocity interstellar clouds (HVCs/IVCs) are tracers of energetic processes in and around the Milky Way. Clouds with near-solar metallicity about 1 kpc above the disk trace the circulation of material between disk and halo (the Galactic fountain). The Magellanic Stream consists of gas tidally extracted from the SMC, tracing the dark matter potential of the Milky Way. Several other HVCs have low metallicity and appear to trace the continuing accretion of infalling intergalactic gas. These assertions are supported by the metallicities (0.1 to 1 solar) measured for about 10 clouds in the past decade. Direct measurements of distances to HVCs have remained elusive, however. In this paper we present four new distance brackets, using VLT observations of interstellar Ca II H and K absorption toward distant Galactic halo stars. We derive distance brackets of 5.0 to 11.7 kpc for the Cohen Stream (likely to be an infalling low-metallicity cloud), 9.8 to 15.1 kpc for Complex GCP (also known as the Smith Cloud or HVC 40-15+100 and with still unknown origin), 1.0 to 2.7 kpc for an IVC that appears associated with the return flow of the fountain in the Perseus arm, and 1.8 to 3.8 kpc for cloud g1, which appears to be in the outflow phase of the fountain. Our measurements further demonstrate that the Milky Way is accreting substantial amounts of gaseous material, which influences the Galaxy's current and future dynamical and chemical evolution.
Distances To Galactic High-Velocity Clouds: Complex C
(2007-12) Wakker, B. P.; York, D. G.; Howk, J. C.; Barentine, John C.; Wilhelm, R.; Peletier, R. F.; van Woerden, H.; Beers, Timothy C.; Ivezic, Z.; Richter, P.; Schwarz, U. J.; Barentine, John C.
We report the first determination of a distance bracket for the high- velocity cloud (HVC) complex C. Combined with previous measurements showing that this cloud has a metallicity of 0.15 times solar, these results provide ample evidence that complex C traces the continuing accretion of intergalactic gas falling onto the Milky Way. Accounting for both neutral and ionized hydrogen as well as He, the distance bracket implies a mass of (3-14) x 10(6) M-circle dot, and the complex represents a mass inflow of 0.1-0.25 M-circle dot yr(-1). We base our distance bracket on the detection of Ca II absorption in the spectrum of the blue horizontal branch (BHB) star SDSS J120404.78 + 623345.6, in combination with a significant nondetection toward the BHB star BS 16034-0114. These results set a strong distance bracket of 3.7-11.2 kpc on the distance to complex C. A more weakly supported lower limit of 6.7 kpc may be derived from the spectrum of the BHB star BS 16079-0017.
MAMBO Mapping Of Spitzer c2d Small Clouds And Cores
(2008-09) Kauffmann, J.; Bertoldi, F.; Bourke, T. L.; Evans, Neal J.; Lee, C. W.; Evans, Neal J.
Aims. To study the structure of nearby (<500 pc) dense starless and star-forming cores with the particular goal to identify and understand evolutionary trends in core properties, and to explore the nature of Very Low Luminosity Objects (<= 0.1 L(circle dot); VeLLOs). Methods. Using the MAMBO bolometer array, we create maps unusually sensitive to faint (few mJy per beam) extended (approximate to 5 ') thermal dust continuum emission at 1.2 mm wavelength. Complementary information on embedded stars is obtained from Spitzer, IRAS, and 2MASS. Results. Our maps are very rich in structure, and we characterize extended emission features (>subcores>) and compact intensity peaks in our data separately to pay attention to this complexity. We derive, e. g., sizes, masses, and aspect ratios for the subcores, as well as column densities and related properties for the peaks. Combination with archival infrared data then enables the derivation of bolometric luminosities and temperatures, as well as envelope masses, for the young embedded stars. Conclusions. Starless and star-forming cores occupy the same parameter space in many core properties; a picture of dense core evolution in which any dense core begins to actively form stars once it exceeds some fixed limit in, e. g., mass, density, or both, is inconsistent with our data. A concept of necessary conditions for star formation appears to provide a better description: dense cores fulfilling certain conditions can form stars, but they do not need to, respectively have not done so yet. Comparison of various evolutionary indicators for young stellar objects in our sample (e. g., bolometric temperatures) reveals inconsistencies between some of them, possibly suggesting a revision of some of these indicators. Finally, we challenge the notion that VeLLOs form in cores not expected to actively form stars, and we present a first systematic study revealing evidence for structural differences between starless and candidate VeLLO cores.
The Mass Distribution And Lifetime Of Prestellar Cores In Perseus, Serpens, And Ophiuchus
(2008-09) Enoch, Melissa L.; Evans, Neal J.; Sargent, Anneila I.; Glenn, Jason; Rosolowsky, Erik; Myers, Phillip; Evans, Neal J.
We present an unbiased census of starless cores in Perseus, Serpens, and Ophiuchus, assembled by comparing large-scale Bolocam 1.1 mm continuum emission maps with Spitzer c2d surveys. We use the c2d catalogs to separate 108 starless from 92 protostellar cores in the 1.1 mm core samples from Enoch and Young and their coworkers. A comparison of these populations reveals the initial conditions of the starless cores. Starless cores in Perseus have similar masses but larger sizes and lower densities on average than protostellar cores, with sizes that suggest density profiles substantially flatter than rho alpha r(-2). By contrast, starless cores in Serpens are compact and have lower masses than protostellar cores; future star formation will likely result in lower mass objects than the currently forming protostars. Comparison to dynamical masses estimated from the NH3 survey of Perseus cores by Rosolowsky and coworkers suggests that most of the starless cores are likely to be gravitationally bound, and thus prestellar. The combined prestellar core mass distribution includes 108 cores and has a slope of alpha = -2.3 +/- 0.4 for M > 0.8 M-circle dot. This slope is consistent with recent measurements of the stellar initial mass function, providing further evidence that stellar masses are directly linked to the core formation process. We place a lower limit on the core-to-star efficiency of 25%. There are approximately equal numbers of prestellar and protostellar cores in each cloud; thus the dense prestellar core lifetime must be similar to the lifetime of embedded protostars, or 4.5 x 10(5) yr, with a total uncertainty of a factor of 2. Such a short lifetime suggests a dynamic, rather than quasi-static, core evolution scenario, at least at the relatively high mean densities (n > 2 x 10(4) cm(-3)) to which we are sensitive.
The Origin Of Radio Scintillation In The Local Interstellar Medium
(2008-03) Linsky, Jeffrey L.; Rickett, Barney J.; Redfield, Seth; Redfield, Seth
We study three quasar radio sources (B1257-326, B1519-273, and J1819+385) that show large-amplitude intraday and annual scintillation variability produced by the Earth's motion relative to turbulent-scattering screens located within a few parsecs of the Sun. We find that the lines of sight to these sources pass through the edges of partially ionized warm interstellar clouds where two or more clouds may interact. From the gas flow vectors of these clouds, we find that the relative radial and transverse velocities of these clouds are large and could generate the turbulence that is responsible for the observed scintillation. For all three sight lines the flow velocities of nearby warm local interstellar clouds are consistent with the fits to the transverse flows of the radio scintillation signals.
The Relationship Between The Optical Depth Of The 9.7 ?m Silicate Absorption Feature And Infrared Differential Extinction In Dense Clouds
(2007-09) Chiar, J. E.; Ennico, K.; Pendleton, Y. J.; Boogert, Adwin C. A.; Greene, T.; Knez, Claudia; Lada, C.; Roellig, T.; Tielens, Aggm; Werner, M.; Whittet, D. C. B.; Knez, Claudia
We have examined the relationship between the optical depth of the 9.7 mm silicate absorption feature (tau(9.7)) and the near-infrared color excess E(J - K-s), in the Serpens, Taurus, IC 5146, Chameleon I, Barnard 59, and Barnard 68 dense clouds/cores. Our data set, based largely on Spitzer IRS spectra, spans E(J - K-s) p = 0.3-10 mag (corresponding to visual extinction between about 2 and 60 mag). All lines of sight show the 9.7 mu m silicate feature. Unlike in the diffuse ISM where a tight linear correlation between the 9.7 mu m silicate feature optical depth and the extinction (A(V)) is observed, we find that the silicate feature in dense clouds does not show a monotonic increase with extinction. Thus, in dense clouds, tau(9.7) is not a good measure of total dust column density. With few exceptions, the measured values fall well below the diffuse ISM correlation line for E(J - K-s) > 2 mag (AV > 12 mag). Grain growth via coagulation is a likely cause of this effect.
The Spitzer c2d Survey Of Large, Nearby, Interstellar Clouds. VII. Ophiuchus Observed With MIPS
(2008-01) Padgett, Deborah L.; Rebull, L. M.; Stapelfeldt, Karl R.; Chapman, Nicholas L.; Lai, Shih-Ping; Mundy, Lee G.; Evans, Neal J.; Brooke, Tyler Y.; Cieza, Lucas A.; Spiesman, William J.; Noriega-Crespo, Alberto; McCabe, Caer-Eve; Allen, Lori E.; Blake, Geoffrey A.; Harvey, Paul M.; Huard, Tracy L.; Jorgensen, Jes K.; Koerner, David W.; Myers, Phillip C.; Sargent, Anneila I.; Teuben, Peter; van Dishoeck, Ewine F.; Wahhaj, Zahed; Young, Kaisa E.; Spiesman, William J.; Evans, Neal J.; Harvey, Paul M.; Young, Kaisa E.
We present maps of 14.4 deg(2) of the Ophiuchus dark clouds observed by the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer ( MIPS). These high-quality maps depict both numerous point sources and extended dust emission within the star-forming and non-star-forming portions of these clouds. Using PSF-fitting photometry, we detect 5779 sources at 24 mu m and 81 sources at 70 mu m at the 10 sigma level of significance. Three hundred twenty-three candidate young stellar objects ( YSOs) were identified according to their positions on the MIPS/2MASS K versus K - [24] color-magnitude diagrams, as compared to 24 mu m detections in the SWIRE extragalactic survey. We find that more than half of the YSO candidates, and almost all those with protostellar Class I spectral energy distributions, are confined to the known cluster and aggregates.