8,453 research outputs found
Chemical abundances of damped Lyman alpha systems in the XQ-100 survey
The XQ-100 survey has provided high signal-noise spectra of 100 redshift
3-4.5 quasars with the X-Shooter spectrograph. The metal abundances for 13
elements in the 41 damped Lyman alpha systems (DLAs) identified in the XQ-100
sample are presented, and an investigation into abundances of a variety of DLA
classes is conducted. The XQ-100 DLA sample contains five DLAs within 5000 km/s
of their host quasar (proximate DLAs; PDLAs) as well as three sightlines which
contain two DLAs within 10,000 km/s of each other along the same line-of-sight
(multiple DLAs; MDLAs). Combined with previous observations in the literature,
we demonstrate that PDLAs with logN(HI)<21.0 show lower [S/H] and [Fe/H]
(relative to intervening systems with similar redshift and N(HI)), whilst
higher [S/H] and [Si/H] are seen in PDLAs with logN(HI)>21.0. These abundance
discrepancies are independent of their line-of-sight velocity separation from
the host quasar, and the velocity width of the metal lines (v90). Contrary to
previous studies, MDLAs show no difference in [alpha/Fe] relative to single
DLAs matched in metallicity and redshift. In addition, we present follow-up
UVES data of J0034+1639, a sightline containing three DLAs, including a
metal-poor DLA with [Fe/H]=-2.82 (the third lowest [Fe/H] in DLAs identified to
date) at z=4.25. Lastly we study the dust-corrected [Zn/Fe], emphasizing that
near-IR coverage of X-Shooter provides unprecedented access to MgII, CaII and
TiII lines (at redshifts 3-4) to provide additional evidence for subsolar
[Zn/Fe] ratio in DLAs.Comment: Accepted to MNRAS. 19 pages plus Appendix material (102 pages total
The Investigation of Space Charge Dominated Beams in a Synchrotron
This research was sponsored by the National Science Foundation Grant NSF PHY-931478
Clues to the nature of high-redshift OVI absorption systems from their (lack of) small-scale structure
We present results of the first survey of high-redshift ( ~ 2.3) OVI
absorption systems along parallel lines of sight toward two lensed QSOs. After
a careful and well-defined search, we find ten intervening OVI systems. Within
the errors, all OVI systems appear at the same redshift and have similar line
strengths in front of both QSO images, whereas in most cases CIV or SiIV show
more differences across the lines of sight, either in radial velocity or line
strength. We conclude that (1) the coherence length of OVI must be much larger
than ~ 1 kpc, and (2) an important fraction of the CIV absorbers may not reside
in the same volume as OVI. Since Doppler parameters are consistent with
photoionization, we propose a model in which CIV occurs in two different
photoionized phases, one large, with characteristic sizes of a few hundred kpc
and bearing OVI, and another one a factor of ten smaller and containing CIII.
This model is able to explain the various transverse differences observed in
column density and kinematics. We apply the model successfully to 2 kinds of
absorbers, with low and high metallicity. In the low-metallicity regime, [C/H]
\~ -2, we find that [C/O] ~ -0.7 is required to explain the observations, which
hints at late (z < 6) rather than early metal enrichment. In the
high-metallicity regime, the observed dissociation between OVI and CIV gas
might be produced by galactic outflows. Altogether, the relative abundances,
inhomogeneous CIV and featureless OVI are consistent with gas that has been
processed recently before the absorption occurred (thus close to star-forming
regions). Finally, we discuss briefly three associated systems (z_abs ~ z_em)
that also show OVI. (abridged)Comment: Accepted by A&A, 22 page
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A factor analysis approach to examining relationships among ovarian steroid concentrations, gonadotrophin concentrations and menstrual cycle length characteristics in healthy, cycling women
STUDY QUESTION:
How are ovarian steroid concentrations, gonadotrophins and menstrual cycle characteristics inter-related within normal menstrual cycles?
SUMMARY ANSWER:
Within cycles, measures of estradiol production are highly related to one another, as are measures of progesterone production; however, the two hormones also show some independence from one another, and measures of cycle length and gonadotrophin concentrations show even greater independence, indicating minimal integration within cycles.
WHAT IS KNOWN ALREADY:
The menstrual cycle is typically conceptualized as a cohesive unit, with hormone levels, follicular development and ovulation all closely inter-related within a single cycle. Empirical support for this idea is limited, however, and to our knowledge, no analysis has examined the relationships among all of these components simultaneously.
STUDY DESIGN, SIZE, DURATION:
A total of 206 healthy, cycling Norwegian women participated in a prospective cohort study (EBBA-I) over the duration of a single menstrual cycle. Of these, 192 contributed hormonal and cycle data to the current analysis.
PARTICIPANTS/MATERIALS, SETTING, METHODS:
Subjects provided daily saliva samples throughout the menstrual cycle from which estradiol and progesterone concentrations were measured. FSH and LH concentrations were measured in serum samples from three points in the same menstrual cycle and cycle length characteristics were calculated based on hormonal data and menstrual records. A factor analysis was conducted to examine the underlying relationships among 22 variables derived from the hormonal data and menstrual cycle characteristics.
MAIN RESULTS AND THE ROLE OF CHANCE:
Six rotated factors emerged, explaining 80% of the variance in the data. Of these, factors representing estradiol and progesterone concentrations accounted for 37 and 13% of the variance, respectively. There was some association between measures of estradiol and progesterone production within cycles; however, cycle length characteristics and gonadotrophin concentrations showed little association with any measure of ovarian hormone concentrations.
LIMITATIONS, REASONS FOR CAUTION:
Our summary measures of ovarian hormones may be imprecise in women with extremely long or short cycles, which could affect the patterns emerging in the factor analysis. Given that we only had data from one cycle on each woman, we cannot address how cycle characteristics may covary within individual women across multiple cycles.
WIDER IMPLICATIONS OF THE FINDINGS:
Our findings are generalizable to other healthy populations with typical cycles, however, may not be applicable to cycles that are anovulatory, extreme in length or otherwise atypical. The results support previous findings that measures of estradiol production are highly correlated across the cycle, as are measures of progesterone production. Estradiol and progesterone concentrations are associated with one another, furthermore. However factor analysis also revealed more complex underlying patterns in the menstrual cycle, highlighting the fact that gonadotrophin concentrations and cycle length characteristics are virtually independent of ovarian hormones. These results suggest that despite integration of follicular and luteal ovarian steroid production across the cycle, cycle quality is a multi-faceted construct, rather than a single dimension.
STUDY FUNDING/COMPETING INTEREST(S):
The EBBA-I study was supported by a grant from the Norwegian Cancer Society (49 258, 05087); Foundation for the Norwegian Health and Rehabilitation Organizations (59010-2000/2001/2002); Aakre Foundation (5695-2000, 5754-2002) and Health Region East. The current analyses were completed under funding from the National Institutes of Health (K12 ES019852). No competing interests declared.Anthropolog
Clustering of vacancy defects in high-purity semi-insulating SiC
Positron lifetime spectroscopy was used to study native vacancy defects in
semi-insulating silicon carbide. The material is shown to contain (i) vacancy
clusters consisting of 4--5 missing atoms and (ii) Si vacancy related
negatively charged defects. The total open volume bound to the clusters
anticorrelates with the electrical resistivity both in as-grown and annealed
material. Our results suggest that Si vacancy related complexes compensate
electrically the as-grown material, but migrate to increase the size of the
clusters during annealing, leading to loss of resistivity.Comment: 8 pages, 5 figure
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