377 research outputs found
The Dynamical Evolution of the Pleiades
We present the results of a numerical simulation of the history and future
development of the Pleiades. This study builds on our previous one that
established statistically the present-day structure of this system. Our
simulation begins just after molecular cloud gas has been expelled by the
embedded stars. We then follow, using an N body code, the stellar dynamical
evolution of the cluster to the present and beyond. Our initial state is that
which evolves, over the 125 Myr age of the cluster, to a configuration most
closely matching the current one.
We find that the original cluster, newly stripped of gas, already had a
virial radius of 4 pc. This configuration was larger than most observed,
embedded clusters. Over time, the cluster expanded further and the central
surface density fell by about a factor of two. We attribute both effects to the
liberation of energy from tightening binaries of short period. Indeed, the
original binary fraction was close to unity. The ancient Pleiades also had
significant mass segregation, which persists in the cluster today.
In the future, the central density of the Pleiades will continue to fall. For
the first few hundred Myr, the cluster as a whole will expand because of
dynamical heating by binaries. The expansion process is aided by mass loss
through stellar evolution, which weakens the system's gravitational binding. At
later times, the Galactic tidal field begins to heavily deplete the cluster
mass. It is believed that most open clusters are eventually destroyed by close
passage of a giant molecular cloud. Barring that eventuality, the density
falloff will continue for as long as 1 Gyr, by which time most of the cluster
mass will have been tidally stripped away by the Galactic field.Comment: 45 pages, 13 figures, 2 tables; Accepted for publication in MNRA
The diagnosis of mental disorders: the problem of reification
A pressing need for interrater reliability in the diagnosis of mental disorders
emerged during the mid-twentieth century, prompted in part by
the development of diverse new treatments. The Diagnostic and Statistical
Manual of Mental Disorders (DSM), third edition answered this need
by introducing operationalized diagnostic criteria that were field-tested
for interrater reliability. Unfortunately, the focus on reliability came at a
time when the scientific understanding of mental disorders was embryonic
and could not yield valid disease definitions. Based on accreting
problems with the current DSM-fourth edition (DSM-IV) classification,
it is apparent that validity will not be achieved simply by refining
criteria for existing disorders or by the addition of new disorders. Yet
DSM-IV diagnostic criteria dominate thinking about mental disorders
in clinical practice, research, treatment development, and law. As a result,
the modernDSMsystem, intended to create a shared language, also
creates epistemic blinders that impede progress toward valid diagnoses.
Insights that are beginning to emerge from psychology, neuroscience,
and genetics suggest possible strategies for moving forward
Hubble-COS Observations of Galactic High-Velocity Clouds: Four AGN Sight Lines through Complex C
We report ultraviolet spectra of Galactic high-velocity clouds (HVCs) in
Complex C, taken by the Cosmic Origins Spectrograph (COS) on the Hubble Space
Telescope (HST), together with new 21-cm spectra from the Green Bank Telescope.
The wide spectral coverage and higher S/N, compared to previous HST spectra,
provide better velocity definition of the HVC absorption, additional ionization
species, and improved abundances in this halo gas. Complex C has a metallicity
of 0.1-0.3 solar and a wide range of ions, suggesting dynamical and thermal
interactions with hot gas in the Galactic halo. Spectra in the COS
medium-resolution G130M (1133-1468 A) and G160M (1383-1796 A) gratings detect
ultraviolet absorption lines from 8 elements in low ionization stages (O I, N
I, C II, S II, Si II, Al II, Fe II, P II) and 3 elements in intermediate and
high-ionization states (Si III, Si IV, C IV, N V). Our four AGN sight lines
toward Mrk 817, Mrk 290, Mrk 876, and PG1259+593 have high-velocity H I and O
VI column densities, log N_HI = 19.39-20.05 and log N_OVI = 13.58-14.10, with
substantial amounts of kinematically associated photoionized gas. The high-ion
abundance ratios are consistent with cooling interfaces between photoionized
gas and collisionally ionized gas: N(C IV)/N(O VI) = 0.3-0.5, N(Si IV)/N(O VI)
= 0.05-0.11, N(N V)/N(O VI) = 0.07-0.13, and N(Si IV)/N(Si III) = 0.2.Comment: 43 pages, 11 figures (appearing in ApJ, Sept 1, 2011
HST-COS Observations of Hydrogen, Helium, Carbon and Nitrogen Emission from the SN 1987A Reverse Shock
We present the most sensitive ultraviolet observations of Supernova 1987A to
date. Imaging spectroscopy from the Hubble Space Telescope-Cosmic Origins
Spectrograph shows many narrow (dv \sim 300 km/s) emission lines from the
circumstellar ring, broad (dv \sim 10 -- 20 x 10^3 km/s) emission lines from
the reverse shock, and ultraviolet continuum emission. The high signal-to-noise
(> 40 per resolution element) broad LyA emission is excited by soft X-ray and
EUV heating of mostly neutral gas in the circumstellar ring and outer supernova
debris. The ultraviolet continuum at \lambda > 1350A can be explained by HI
2-photon emission from the same region. We confirm our earlier, tentative
detection of NV \lambda 1240 emission from the reverse shock and we present the
first detections of broad HeII \lambda1640, CIV \lambda1550, and NIV]
\lambda1486 emission lines from the reverse shock. The helium abundance in the
high-velocity material is He/H = 0.14 +/- 0.06. The NV/H-alpha line ratio
requires partial ion-electron equilibration (T_{e}/T_{p} \approx 0.14 - 0.35).
We find that the N/C abundance ratio in the gas crossing the reverse shock is
significantly higher than that in the circumstellar ring, a result that may be
attributed to chemical stratification in the outer envelope of the supernova
progenitor. The N/C abundance ratio may have been stratified prior to the ring
expulsion, or this result may indicate continued CNO processing in the
progenitor subsequent to the expulsion of the circumstellar ring.Comment: 12 pages, 8 figures. ApJ - accepte
A peculiar HI cloud near the distant globular cluster Pal 4
We present 21-cm observations of four Galactic globular clusters, as part of
the on-going GALFA-HI Survey at Arecibo. We discovered a peculiar HI cloud in
the vicinity of the distant (109 kpc) cluster Pal 4, and discuss its properties
and likelihood of association with the cluster. We conclude that an association
of the HI cloud and Pal 4 is possible, but that a chance coincidence between
Pal 4 and a nearby compact high-velocity cloud cannot be ruled out altogether.
New, more stringent upper limits were derived for the other three clusters: M
3, NGC 5466, and Pal 13. We briefly discuss the fate of globular cluster gas
and the interaction of compact clouds with the Galactic Halo gas.Comment: Accepted for publication in MNRA
Variable Sodium Absorption in a Low-Extinction Type Ia Supernova
Recent observations have revealed that some Type Ia supernovae exhibit
narrow, time-variable Na I D absorption features. The origin of the absorbing
material is controversial, but it may suggest the presence of circumstellar gas
in the progenitor system prior to the explosion, with significant implications
for the nature of the supernova progenitors. We present the third detection of
such variable absorption, based on six epochs of high-resolution spectroscopy
of the Type Ia supernova SN 2007le from Keck and the HET. The data span ~3
months, from 5 days before maximum light to 90 days after maximum. We find that
one component of the Na D absorption lines strengthened significantly with
time, indicating a total column density increase of ~2.5 x 10^12 cm^-2. The
changes are most prominent after maximum light rather than at earlier times
when the UV flux from the SN peaks. As with SN 2006X, we detect no change in
the Ca II H&K lines over the same time period, rendering line-of-sight effects
improbable and suggesting a circumstellar origin for the absorbing material.
Unlike the previous two SNe exhibiting variable absorption, SN 2007le is not
highly reddened (E_B-V = 0.27 mag), also pointing toward circumstellar rather
than interstellar absorption. Photoionization models show that the data are
consistent with a dense (10^7 cm^-3) cloud or clouds of gas located ~0.1 pc
from the explosion. These results broadly support the single-degenerate
scenario previously proposed to explain the variable absorption, with mass loss
from a nondegenerate companion star responsible for providing the circumstellar
gas. We also present tentative evidence for narrow Halpha emission associated
with the SN, which will require followup observations at late times to confirm.
[abridged]Comment: 16 pages, 10 figures (8 in color), 5 tables. Accepted for publication
in Ap
The ionization fraction in alpha-models of protoplanetary disks
We calculate the ionization fraction of protostellar alpha disks, taking into
account vertical temperature structure, and the possible presence of trace
metal atoms. Both thermal and X-ray ionization are considered. Previous
investigations of layered disks used radial power-law models with isothermal
vertical structure. But alpha models are used to model accretion, and the
present work is a step towards a self-consistent treatment. The extent of the
magnetically uncoupled (``dead'') zone depends sensitively on alpha, on the
assumed accretion rate, and on the critical magnetic Reynolds number, below
which MHD turbulence cannot be self-sustained. Its extent is extremely
model-dependent. It is also shown that a tiny fraction of the cosmic abundance
of metal atoms can dramatically affect the ionization balance. Gravitational
instabilities are an unpromising source of transport, except in the early
stages of disk formation.Comment: 25 pages including 8 figures, Latex in the MN style - Accepted by
MNRA
Galaxy Zoo: dust and molecular gas in early-type galaxies with prominent dust lanes
We study dust and associated molecular gas in 352 nearby early-type galaxies
(ETGs) with prominent dust lanes. 65% of these `dusty ETGs' (D-ETGs) are
morphologically disturbed, suggesting a merger origin. This is consistent with
the D-ETGs residing in lower density environments compared to the controls
drawn from the general ETG population. 80% of D-ETGs inhabit the field
(compared to 60% of the controls) and <2% inhabit clusters (compared to 10% of
the controls). Compared to the controls, D-ETGs exhibit bluer UV-optical
colours (indicating enhanced star formation) and an AGN fraction that is more
than an order of magnitude greater (indicating higher incidence of nuclear
activity). The clumpy dust mass residing in large-scale features is estimated,
using the SDSS r-band images, to be 10^{4.5}-10^{6.5} MSun. A comparison to the
total (clumpy + diffuse) dust masses- calculated using the far-IR fluxes of 15%
of the D-ETGs that are detected by the IRAS- indicates that only ~20% of the
dust resides in these large-scale features. The dust masses are several times
larger than the maximum value expected from stellar mass loss, ruling out an
internal origin. The dust content shows no correlation with the blue
luminosity, indicating that it is not related to a galactic scale cooling flow.
No correlation is found with the age of the recent starburst, suggesting that
the dust is accreted directly in the merger rather than being produced in situ
by the triggered star formation. Using molecular gas-to-dust ratios of ETGs in
the literature we estimate that the median current and initial molecular gas
fraction are ~1.3% and ~4%, respectively. Recent work suggests that the merger
activity in nearby ETGs largely involves minor mergers (mass ratios between
1:10 and 1:4). If the IRAS-detected D-ETGs form via this channel, then the
original gas fractions of the accreted satellites are 20%-44%. [Abridged]Comment: 11 pages, 18 figures, 1 table, MNRAS (Accepted for publication- 2012
March 19
Predictors of Peritraumatic Reactions and PTSD Following the September 11th Terrorist Attacks
In this study the authors characterize peritraumatic reactions of residents of New
York City during and immediately following the September 11th terrorist attacks,
identify predictors of those reactions, and identify predictors of PTSD 4 months
later. A cross–sectional sample of New York residents (n = 2,001) responded to
questions about sociodemographics, historical factors, event–related exposure;
acute cognitive, emotional, and physiological reactions to the September 11th terrorist
attacks; and current (past month) PTSD symptoms. Factor analyses of
peritraumatic reactions yielded three related, but distinct, peritraumatic response
patterns—dissociation, emotional reactions, and panic/physiological arousal. Several
demographic, historical, and exposure–related variables predicted one or
more peritraumatic reaction patterns. After controlling for demographic, historical,
and exposure factors, each of the peritraumatic reactions factors, one historical
factor and one event–related exposure factor remained as significant predictors
of PTSD. These results support a growing literature concerning the predictive value
of peritraumatic reactions in relation to PTSD. Implications for preventive efforts
and suggestions for future research are discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/40315/2/Lawyer_Predictors of peritraumatic reactions and PTSD_2006.pd
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