14,001 research outputs found
Angular Momentum Evolution of Stars in the Orion Nebula Cluster
(Abridged) We present theoretical models of stellar angular momentum
evolution from the Orion Nebula Cluster (ONC) to the Pleiades and the Hyades.
We demonstrate that observations of the Pleiades and Hyades place tight
constraints on the angular momentum loss rate from stellar winds. The observed
periods, masses and ages of ONC stars in the range 0.2--0.5 M, and the
loss properties inferred from the Pleiades and Hyades stars, are then used to
test the initial conditions for stellar evolution models. We use these models
to estimate the distribution of rotational velocities for the ONC stars at the
age of the Pleiades (120 Myr). The modeled ONC and observed Pleiades
distributions of rotation rates are not consistent if only stellar winds are
included. In order to reconcile the observed loss of angu lar momentum between
these two clusters, an extrinsic loss mechanism such as protostar-accretion
disk interaction is required. Our model, which evolves the ONC stars with a
mass dependent saturation threshold normalized such that at 0.5 \m, and which includes a distribution of disk lifetimes
that is uniform over the range 0--6 Myr, is consistent with the Pleiades. This
model for disk-locking lifetimes is also consistent with inferred disk
lifetimes from the percentage of stars with infrared excesses observed in young
clusters. Different models, using a variety of initial period distributions and
different maximum disk lifetimes, are also compared to the Pleiades. For
disk-locking models that use a uniform distribution of disk lifetimes over the
range 0 to , the acceptable range of the maximum lifetime is Myr.Comment: 21 pages, 7 figures, submitted to Ap
The Clustering of AGN in the Sloan Digital Sky Survey
We present the two--point correlation function (2PCF) of narrow-line active
galactic nuclei (AGN) selected within the First Data Release of the Sloan
Digital Sky Survey. Using a sample of 13605 AGN in the redshift range 0.055 < z
< 0.2, we find that the AGN auto--correlation function is consistent with the
observed galaxy auto--correlation function on scales 0.2h^{-1}Mpc to
>100h^{-1}Mpc. The AGN hosts trace an intermediate population of galaxies and
are not detected in either the bluest (youngest) disk--dominated galaxies or
many of the reddest (oldest) galaxies. We show that the AGN 2PCF is dependent
on the luminosity of the narrow [OIII] emission line (L_{[OIII]}), with low
L_{[OIII]} AGN having a higher clustering amplitude than high L_{[OIII]} AGN.
This is consistent with lower activity AGN residing in more massive galaxies
than higher activity AGN, and L_{[OIII]} providing a good indicator of the
fueling rate. Using a model relating halo mass to black hole mass in
cosmological simulations, we show that AGN hosted by ~ 10^{12} M_{odot} dark
matter halos have a 2PCF that matches that of the observed sample. This mass
scale implies a mean black hole mass for the sample of M_{BH} ~ 10^8 M_{odot}.Comment: 5 pages, 4 figures. Accepted for publication in ApJ
Sympathetic ground state cooling and coherent manipulation with two-ion-crystals
We have cooled a two-ion-crystal to the ground state of its collective modes
of motion. Laser cooling, more specific resolved sideband cooling is performed
sympathetically by illuminating only one of the two Ca ions in the
crystal. The heating rates of the motional modes of the crystal in our linear
trap have been measured, and we found them considerably smaller than those
previously reported by Q. Turchette {\em et. al.} Phys. Rev. A 61, 063418
(2000) in the case of trapped Be ions. After the ground state is
prepared, coherent quantum state manipulation of the atomic population can be
performed. Within the coherence time, up to 12 Rabi oscillations are observed,
showing that many coherent manipulations can be achieved. Coherent excitation
of each ion individually and ground state cooling are important tools for the
realization of quantum information processing in ion traps
Method and apparatus for determining time, direction, and composition of impacting space particles
A space particle collector for recording the time specific particles are captured, and its direction at the time of capture, utilizes an array of targets, each comprised of an MOS capacitor on a chip charged from an external source and discharged upon impact by a particle through a tab on the chip that serves as a fuse. Any impacting particle creates a crater, but only the first will cause a discharge of the capacitor. A substantial part of the metal film around the first crater is burned off by the discharge current. The time of the impulse which burns the tab, and the identification of the target, is recorded together with data from flight instruments. The metal film is partitioned into pie sections to provide a plurality of targets on each of an array of silicon wafers, thus increasing the total number of identified particles that can be collected. It is thus certain which particles were captured at what specific times
Development of magnetostrictive active members for control of space structures
The goal of this Phase 2 Small Business Innovative Research (SBIR) project was to determine the technical feasibility of developing magnetostrictive active members for use as truss elements in space structures. Active members control elastic vibrations of truss-based space structures and integrate the functions of truss structure element, actively controlled actuator, and sensor. The active members must control structural motion to the sub-micron level and, for many proposed space applications, work at cryogenic temperatures. Under this program both room temperature and cryogenic temperature magnetostrictive active members were designed, fabricated, and tested. The results of these performance tests indicated that room temperature magnetostrictive actuators feature higher strain, stiffness, and force capability with lower amplifier requirements than similarly sized piezoelectric or electrostrictive active members, at the cost of higher mass. Two different cryogenic temperature magnetostrictive materials were tested at liquid nitrogen temperatures, both with larger strain capability than the room temperature magnetostrictive materials. The cryogenic active member development included the design and fabrication of a cryostat that allows operation of the cryogenic active member in a space structure testbed
Clustering Analyses of 300,000 Photometrically Classified Quasars--I. Luminosity and Redshift Evolution in Quasar Bias
Using ~300,000 photometrically classified quasars, by far the largest quasar
sample ever used for such analyses, we study the redshift and luminosity
evolution of quasar clustering on scales of ~50 kpc/h to ~20 Mpc/h from
redshifts of z~0.75 to z~2.28. We parameterize our clustering amplitudes using
realistic dark matter models, and find that a LCDM power spectrum provides a
superb fit to our data with a redshift-averaged quasar bias of b_Q =
2.41+/-0.08 () for . This represents a better
fit than the best-fit power-law model (; ). We find b_Q increases with redshift.
This evolution is significant at >99.6% using our data set alone, increasing to
>99.9999% if stellar contamination is not explicitly parameterized. We measure
the quasar classification efficiency across our full sample as a = 95.6 +/-
^{4.4}_{1.9}%, a star-quasar separation comparable with the star-galaxy
separation in many photometric studies of galaxy clustering. We derive the mean
mass of the dark matter halos hosting quasars as MDMH=(5.2+/-0.6)x10^{12}
M_solar/h. At z~1.9 we find a deviation from luminosity-independent
quasar clustering; this suggests that increasing our sample size by a factor of
1.8 could begin to constrain any luminosity dependence in quasar bias at z~2.
Our results agree with recent studies of quasar environments at z < 0.4, which
detected little luminosity dependence to quasar clustering on proper scales >50
kpc/h. At z < 1.6, our analysis suggests that b_Q is constant with luminosity
to within ~0.6, and that, for g < 21, angular quasar autocorrelation
measurements are unlikely to have sufficient statistical power at z < 1.6 to
detect any luminosity dependence in quasars' clustering.Comment: 13 pages, 9 figures, 2 tables; uses amulateapj; accepted to Ap
Hot Horizontal Branch Stars in the Galactic Bulge. I
We present the first results of a survey of blue horizontal branch (BHB)
stars in the Galactic bulge. 164 candidates with 15 < V < 17.5 in a field
7.5deg from the Galactic Center were observed in the blue at 2.4A FWHM
resolution with the AAT 2dF spectrograph. Radial velocities were measured for
all stars. For stars with strong Balmer lines, their profiles were matched to
theoretical spectrum calculations to determine stellar temperature Teff and
gravity log g; matches to metal lines yielded abundances. CTIO UBV photometry
then gave the reddening and distance to each hot star. Reddening was found to
be highly variable, with E(B-V) from 0.0 to 0.55 around a mean of 0.28.
Forty-seven BHB candidates were identified with Teff >= 7250K, of which seven
have the gravities of young stars, three are ambiguous, and 37 are HB stars.
They span a wide metallicity range, from solar to 1/300 solar. The warmer BHB's
are more metal-poor and loosely concentrated towards the Galactic center, while
the cooler ones are of somewhat higher metallicity and closer to the center.
Their red B-V colors overlap main-sequence stars, but the U-B vs. B-V diagram
separates them until E(B-V) > 0.5. We detect two cool solar-metallicity HB
stars in the bulge of our own Galaxy, the first such stars known. Still elusive
are their hot counterparts, the metal-rich sdB/O stars causing excess UV light
in metal-rich galaxies; they have V ~ 20.5 in the Bulge.Comment: 29 pages, 4 figures (the third with 4 panels, the fourth with 2
panels). To appear in the Astrophysical Journal v571n1, Jan. 20, 2000.
Abstract is shortened here, and figures compresse
In Memoriam: Daniel J. Meltzer
Sedan år 2006 har den syntetiskt framställda drogen spice existerat i Sverige, men först år 2008 blev drogen populär och tidningar började rikta uppmärksamhet mot den. Genom att drogen kan byta skepnad i uppbyggnaden kan den förbli laglig, därför kan egentligen namnet “spice” inte betraktas som ett enhetligt begrepp. Vårt syfte med denna studie blev att granska hur drogen spice beskrivs i tidningar och forskning. Även hur drogen betraktas gällande psykosociala och medicinska avseenden. Detta i förhållande till socialkonstruktivistiskt perspektiv. Den metod som vi valde att använda oss av i studien var kvantitativ innehållsanalys. Genom analys av 80 stycken svenska tidningsartiklar hämtade ur databasen mediearkivet, samt vetenskapliga artiklar och litteratur har vi besvarat våra frågeställningar. De resultat vi fick från analyser visade på att tidningar kan vara av betydande roll för ungdomars attityder. Tidningar kan utföra bland annat skrämselpropaganda för att upplysa och förhindra brukandet av spice, dessvärre visar resultatet en motsatt effekt. Forskning visar att bruk av spice kan påverka relationer och arbete negativt. Dessutom är några bieffekter av spice bröstsmärtor, vanföreställningar, självmordstankar och hjärtstopp
A Catalog of Spectroscopically Confirmed White Dwarfs from the Sloan Digital Sky Survey Data Release 4
We present a catalog of 9316 spectroscopically confirmed white dwarfs from
the Sloan Digital Sky Survey Data Release 4. We have selected the stars through
photometric cuts and spectroscopic modeling, backed up by a set of visual
inspections. Roughly 6000 of the stars are new discoveries, roughly doubling
the number of spectroscopically confirmed white dwarfs. We analyze the stars by
performing temperature and surface gravity fits to grids of pure hydrogen and
helium atmospheres. Among the rare outliers are a set of presumed helium-core
DA white dwarfs with estimated masses below 0.3 Msun, including two candidates
that may be the lowest masses yet found. We also present a list of 928 hot
subdwarfs.Comment: Accepted by the Astrophysical Journal Supplements, 25 pages, 24
figures, LaTeX. The electronic catalog, as well as diagnostic figures and
links to the spectra, is available at http://das.sdss.org/wdcat/dr4
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