1,403 research outputs found
Heritability of daytime cortisol levels in children
INTRODUCTION Cortisol is a steroid hormone secreted by the outer cortex of the adrenal gland. Its secretion is stimulated by ACTH (adrenocorticotrophic hormone), produced in the pituitary in response to corticotropin-releasing hormone (CRH), a product from neurons in the paraventricular nucleus of the hypothalamus.After its release, the major part of cortisol binds to the plasma proteins corticosteroid binding globulin (CBG, or transcortin) and albumin, which prevents the hormone from penetrating the membranes of their target cells. About 3--5% of the total cortisol is the unbound, biologically active fraction. This active fraction has permissive, suppressive, stimulatory, and preparative action effects in the realms of cardiovascular function, fluid volume and hemorrhage, immunity and inflammation, metabolism, neurobiology, and reproductive physiology (Sapolsky, Romero, and Munck, 2000). Although cortisol is mainly known for its pivotal role in generating an adequate response to phy
Sensitive Limits on the Water Abundance in Cold Low Mass Molecular Cores
We present SWAS observations of water vapor in two cold star-less clouds, B68
and Core D in rho Ophiuchus. Sensitive non-detections of the 1(10)-1(01)
transition of o-H2O are reported for each source. Both molecular cores have
been previously examined by detailed observations that have characterized the
physical structure. Using these rather well defined physical properties and a
Monte-Carlo radiation transfer model we have removed one of the largest
uncertainties from the abundance calculation and set the lowest water abundance
limit to date in cold low-mass molecular cores. These limits are < 3 x 10^{-8}
(relative to H2) and < 8 x 10^{-9} in B68 and rho Oph D, respectively. Such low
abundances confirm the general lack of ortho-water vapor in cold (T < 20 K)
cores. Provided that the ortho/para ratio of water is not near zero, these
limits are well below theoretical predictions and appear to support the
suggestion that most of the water in dense low-mass cores is frozen onto the
surfaces of cold dust grains.Comment: 12 pages, 3 figures, accepted by Astrophysical Journal Letter
Predicting deformation-induced polymer-steel interface roughening and failure
A novel integrated framework is presented for the prediction of deformation-induced interface roughening and failure in polymer-coated steels. Crystal plasticity is employed to predict the change in steel surface roughness in-situ. The steel substrate is coated with a thin polymer layer and the polymer-steel interface is modeled using an exponential cohesive zone law. Uniaxial tensile simulations are performed and the results show that the predicted roughness increases with the applied deformation. The local changes in the steel surface profile result in initiation and growth of local interface failure. Furthermore, a compression simulation shows that the roughening rate of the steel is increased compared to tension, with an increase in the predicted interface damage as a result. The presented framework thus allows for a detailed numerical study of the initiation and growth of interface damage in polymer-coated steels during applied deformation. The incorporation of the crystal plasticity model to predict the changes in the steel surface profile complements the cumbersome measurements of detailed experimental displacement fields that accompany deformation-induced roughening and thus enables the analysis of deformation processes where measuring the steel surface profile is difficult if not impossible, e.g. industrial forming processes such as deep-drawing
Current Star Formation in the Ophiuchus and Perseus Molecular Clouds: Constraints and Comparisons from Unbiased Submillimeter and Mid-Infrared Surveys. II
We present a census of the population of deeply embedded young stellar
objects (YSOs) in the Ophiuchus molecular cloud complex based on a combination
of Spitzer Space Telescope mid-infrared data from the "Cores to Disks" (c2d)
legacy team and JCMT/SCUBA submillimeter maps from the COMPLETE team. We have
applied a method developed for identifying embedded protostars in Perseus to
these datasets and in this way construct a relatively unbiased sample of 27
candidate embedded protostars with envelopes more massive than our sensitivity
limit (about 0.1 M_sun). Embedded YSOs are found in 35% of the SCUBA cores -
less than in Perseus (58%). On the other hand the mid-infrared sources in
Ophiuchus have less red mid-infrared colors, possibly indicating that they are
less embedded. We apply a nearest neighbor surface density algorithm to define
the substructure in each of the clouds and calculate characteristic numbers for
each subregion - including masses, star formation efficiencies, fraction of
embedded sources etc. Generally the main clusters in Ophiuchus and Perseus
(L1688, NGC1333 and IC348) are found to have higher star formation efficiencies
than small groups such as B1, L1455 and L1448, which on the other hand are
completely dominated by deeply embedded protostars. We discuss possible
explanations for the differences between the regions in Perseus and Ophiuchus,
such as different evolutionary timescales for the YSOs or differences, e.g., in
the accretion in the two clouds.Comment: Accepted for publication in ApJ (56 pages, 13 figures; abstract
abridged). Version with full-resolution figures available at
http://www.astro.uni-bonn.de/~jes/paper120.pd
Discovery of a Wide Low-mass Binary System in Upper Scorpius
Using the near-infrared spectrometer SpeX and its slit-viewing camera at the
IRTF, I have resolved a low-mass member of the Upper Scorpius OB association
into a double star. From K-band images of the pair, DENIS-P J161833.2-251750.4
A and B, I measure a separation of 0.96" and a magnitude difference of dK=0.42
mag. I present resolved 0.8-2.5 micron spectroscopy of the two objects, both of
which exhibit signatures of youth in the shape of their H- and K-band continua,
demonstrating that both are members of Upper Scorpius rather than field stars.
In addition, through a comparison to optically-classified pre-main-sequence
objects, I derive a spectral type near M5 for each component, corresponding to
a mass of ~0.15 Msun with the evolutionary models of Chabrier and Baraffe. The
probability that this pair is composed of unrelated M-type members of Upper
Scorpius is ~10^-5. When added to the recent discoveries of other wide, easily
disrupted low-mass binaries, this new system further establishes that the
formation of low-mass stars and brown dwarfs does not require ejection from
multiple systems. These observations also indicate that wide low-mass binaries
can form in OB associations as well as in smaller clusters where the previously
known wide pairs have been found. Thus, the available data show no perceptible
effect of star-forming environment on the prevalence of loosely bound low-mass
systems.Comment: 5 page
Evolution of brown dwarf disks: A Spitzer survey in Upper Scorpius
We have carried out a Spitzer survey for brown dwarf (BD) disks in the ~5 Myr
old Upper Scorpius (UpSco) star forming region, using IRS spectroscopy from 8
to 12\mu m and MIPS photometry at 24\mu m. Our sample consists of 35 confirmed
very low mass members of UpSco. Thirteen objects in this sample show clear
excess flux at 24\mu m, explained by dust emission from a circum-sub-stellar
disk. Objects without excess emission either have no disks at all or disks with
inner opacity holes of at least ~5 AU radii. Our disk frequency of 37\pm 9% is
higher than what has been derived previously for K0-M5 stars in the same region
(on a 1.8 sigma confidence level), suggesting a mass-dependent disk lifetime in
UpSco. The clear distinction between objects with and without disks as well as
the lack of transition objects shows that disk dissipation inside 5 AU occurs
rapidly, probably on timescales of <~10^5 years. For the objects with disks,
most SEDs are uniformly flat with flux levels of a few mJy, well modeled as
emission from dusty disks affected by dust settling to the midplane, which also
provides indirect evidence for grain growth. The silicate feature around 10\mu
m is either absent or weak in our SEDs, arguing for a lack of hot, small dust
grains. Compared with younger objects in Taurus, BD disks in UpSco show less
flaring. Taken together, these results clearly demonstrate that we see disks in
an advanced evolutionary state: Dust settling and grain growth are ubiquituous
in circum-sub-stellar disks at ages of 5 Myr, arguing for planet forming
processes in BD disks. For almost all our targets, results from high-resolution
spectroscopy and high-spatial resolution imaging have been published before,
thus providing a large sample of BDs for which information about disks,
accretion, and binarity is available. (abridged)Comment: 39 pages, 7 figures, accepted for publication in Ap
A Survey for Low-mass Stars and Brown Dwarfs in the Upper-Scorpius OB Association
The Upper-Scorpius association is the OB association nearest to the Sun (145
pc). Its young age (5 Myr) makes it an ideal place to search for low-mass stars
and brown dwarfs, as these objects should be relatively bright. We have
performed a photometric search for the low-mass members of the association,
using the R, I, and Z filters. The completeness limit is I=18.5 and the
saturation limit is I=13. We obtain 138 candidate members, covering nearly the
entire M spectral type range. We find an excess of brown dwarf candidates over
the number predicted by a Miller-Scalo Initial Mass Function. In addition, we
have performed infrared imaging and low resolution optical spectroscopy of
selected candidates. We find that the infrared observations confirm the
spectral types obtained with the optical photometry. Furthermore, we find
H_alpha in emission in 20 of the 22 objects observed spectroscopically. As
H_alpha is an indicator of youth, we believe that these 20 objects may belong
to the association. One of them, UScoCTIO 128 has a very strong and constant
H_alpha line (equivalent width: -130 A), and its position in the color
magnitude diagram suggests that it is a brown dwarf with mass equal to 0.02
msun. Confirmation of this and the other candidates will have to wait for
higher resolution observations that can reveal spectroscopic mass indicators
like Li I and gravity indicators, such as K I and the subordinate lines of Na
I.Comment: 20 pages, 7 figures, 3 tables, accepted in the Astronomical Journa
Hot Organic Molecules Toward a Young Low-Mass Star: A Look at Inner Disk Chemistry
Spitzer Space Telescope spectra of the low mass young stellar object (YSO)
IRS 46 (L_bol ~ 0.6 L_sun) in Ophiuchus reveal strong vibration-rotation
absorption bands of gaseous C2H2, HCN, and CO2. This is the only source out of
a sample of ~100 YSO's that shows these features and the first time they are
seen in the spectrum of a solar-mass YSO. Analysis of the Spitzer data combined
with Keck L- and M-band spectra gives excitation temperatures of > 350 K and
abundances of 10(-6)-10(-5) with respect to H2, orders of magnitude higher than
those found in cold clouds. In spite of this high abundance, the HCN J=4-3 line
is barely detected with the James Clerk Maxwell Telescope, indicating a source
diameter less than 13 AU. The (sub)millimeter continuum emission and the
absence of scattered light in near-infrared images limits the mass and
temperature of any remnant collapse envelope to less than 0.01 M_sun and 100 K,
respectively. This excludes a hot-core type region as found in high-mass YSO's.
The most plausible origin of this hot gas rich in organic molecules is in the
inner (<6 AU radius) region of the disk around IRS 46, either the disk itself
or a disk wind. A nearly edge-on 2-D disk model fits the spectral energy
distribution (SED) and gives a column of dense warm gas along the line of sight
that is consistent with the absorption data. These data illustrate the unique
potential of high-resolution infrared spectroscopy to probe organic chemistry,
gas temperatures and kinematics in the planet-forming zones close to a young
star.Comment: 4 pages, 4 figures; To appear in Astrophysical Journal Letter
Optical Spectroscopy of the Surface Population of the rho Ophiuchi Molecular Cloud: The First Wave of Star Formation
We present the results of optical spectroscopy of 139 stars obtained with the
Hydra multi-object spectrograph. The objects extend over a 1.3 square degree
area surrounding the main cloud of the rho Oph complex. The objects were
selected from narrowband images to have H alpha in emission. Using the presence
of strong H alpha emission, lithium absorption, location in the
Hertzsprung-Russell diagram, or previously reported x-ray emission, we were
able to identify 88 objects as young stars associated with the cloud. Strong H
alpha emission was confirmed in 39 objects with line widths consistent with
their origin in magnetospheric accretion columns. Two of the strongest
emission-line objects are young, x-ray emitting brown dwarf candidates with M8
spectral types. Comparisons of the bolometric luminosities and effective
temperatures with theoretical models suggest a medianage for this population of
2.1 Myr which is signifcantly older than the ages derived for objects in the
cloud core. It appears that these stars formed contemporaneously with low mass
stars in the Upper Scorpius subgroup, likely triggered by massive stars in the
Upper-Centaurus subgroup.Comment: 35 pages of postscript which includes seven figures (some of which
are multi-panel) and four postscript tables. Astronomical Journal (in press
Quiescent Dense Gas in Protostellar Clusters: the Ophiuchus A Core
We present combined BIMA interferometer and IRAM 30 m Telescope data of N2H+
1-0 line emission across the nearby dense, star forming core Ophiuchus A (Oph
A) at high linear resolution (e.g., ~1000 AU). Six maxima of integrated line
intensity are detected which we designate Oph A-N1 through N6. The N4 and N5
maxima are coincident with the starless continuum objects SM1 and SM2
respectively but the other maxima are not coincident with previously-identified
objects. In contrast, relatively little N2H+ 1-0 emission is coincident with
the starless object SM2 and the Class 0 protostar VLA 1623. The FWHM of the
N2H+ 1-0 line, Delta V, varies by a factor of ~5 across Oph A. Values of Delta
V < 0.3 km/s are found in 14 locations in Oph A, but only that associated with
N6 is both well-defined spatially and larger than the beam size. Centroid
velocities of the line, V_LSR, vary relatively little, having an rms of only
\~0.17 km/s. Small-scale V_LSR gradients of <0.5 km/s over ~0.01 pc are found
near SM1, SM1N, and SM2, but not N6. The low N2H+ abundances of SM2 or VLA 1623
relative to SM1, SM1N, or N6 may reflect relatively greater amounts of N2
adsorption onto dust grains in their colder and probably denser interiors. The
low Delta V of N6, i.e., 0.193 km/s FWHM, is only marginally larger than the
FWHM expected from thermal motions alone, suggesting turbulent motions in the
Oph A core have been reduced dramatically at this location. The non-detection
of N6 in previous thermal continuum maps suggests that interesting sites
possibly related to star formation may be overlooked in such data.Comment: LaTex with 7 figures, produces 36 pages. Accepted for publication in
ApJ. Typo related to Equation 3 fixed, caused derived values of N(N2H+) and
X(N2H+) to be low by factors of ~40%. Conclusions of paper are unchange
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