8,999 research outputs found
The horizontal and vertical semi-diameters of the Sun observed at the Cape of Good Hope (1834 - 1887) and Paris (1837 - 1906): A report on work in progress
Cape and Paris meridian observations of the solar limbs which permit an estimate to be made of the solar semi-diameter were surveyed, sampled, and compared with Greenwich and U.S. Naval Observatory observations. Significant systematic errors were found in the Paris work and have been correlated with changes of instruments and observers. Results from the Cape series indicate that work should continue on the compilation of data from Cape observations of the Sun
Determination of rotation periods in solar-like stars with irregular sampling: the Gaia case
We present a study on the determination of rotation periods (P) of solar-like
stars from the photometric irregular time-sampling of the ESA Gaia mission,
currently scheduled for launch in 2013, taking into account its dependence on
ecliptic coordinates. We examine the case of solar-twins as well as thousands
of synthetic time-series of solar-like stars rotating faster than the Sun. In
the case of solar twins we assume that the Gaia unfiltered photometric passband
G will mimic the variability of the total solar irradiance (TSI) as measured by
the VIRGO experiment. For stars rotating faster than the Sun, light-curves are
simulated using synthetic spectra for the quiet atmosphere, the spots, and the
faculae combined by applying semi-empirical relationships relating the level of
photospheric magnetic activity to the stellar rotation and the Gaia
instrumental response. The capabilities of the Deeming, Lomb-Scargle, and Phase
Dispersion Minimisation methods in recovering the correct rotation periods are
tested and compared. The false alarm probability (FAP) is computed using Monte
Carlo simulations and compared with analytical formulae. The Gaia scanning law
makes the rate of correct detection of rotation periods strongly dependent on
the ecliptic latitude (beta). We find that for P ~ 1 d, the rate of correct
detection increases with ecliptic latitude from 20-30 per cent at beta ~
0{\deg} to a peak of 70 per cent at beta=45{\deg}, then it abruptly falls below
10 per cent at beta > 45{\deg}. For P > 5 d, the rate of correct detection is
quite low and for solar twins is only 5 per cent on average.Comment: 12 pages, 18 figures, accepted by MNRA
Overcharging: The Crucial Role of Excluded Volume
In this Letter we investigate the mechanism for overcharging of a single
spherical colloid in the presence of aqueous salts within the framework of the
primitive model by molecular dynamics (MD) simulations as well as
integral-equation theory. We find that the occurrence and strength of
overcharging strongly depends on the salt-ion size, and the available volume in
the fluid. To understand the role of the excluded volume of the microions, we
first consider an uncharged system. For a fixed bulk concentration we find that
upon increasing the fluid particle size one strongly increases the local
concentration nearby the colloidal surface and that the particles become
laterally ordered. For a charged system the first surface layer is built up
predominantly by strongly correlated counterions. We argue that this a key
mechanism to produce overcharging with a low electrostatic coupling, and as a
more practical consequence, to account for charge inversion with monovalent
aqueous salt ions.Comment: 7 pages, 3 figs (4 EPS files). To appear in Europhysics Letter
High Efficiency Detection of Argon Scintillation Light of 128nm Using LAAPDs
The possibility of efficient collection and detection of vacuum ultraviolet
light as emitted by argon, krypton, and xenon gas is studied. Absolute quantum
efficiencies of large area avalanche photodiodes (LAAPDs) are derived at these
wavelengths. VUV light of wavelengths down to the 128nm of Ar emission is shown
to be detectable with silicon avalanche photodiodes at quantum efficiencies
above 42%. Flexible Mylar foil overcoated with Al+MgF is measured to have a
specular reflectivity of 91% at argon emission wavelength. Low-pressure
argon gas is shown to emit significant amounts of non-UV radiation. The average
energy expenditure for the creation of non-UV photons in argon gas at this
pressure is measured to be below 378 eV.Comment: 5 pages, 4 figures, Talk given at IEEE 2005 Nuclear Science Symposium
and Medical Imaging Conference, Puerto Ric
Activity cycles in members of young loose stellar associations
Magnetic cycles have been detected in tens of solar-like stars. The
relationship between the cycle properties and global stellar parameters is not
fully understood yet.
We searched for activity cycles in 90 solar-like stars with ages between 4
and 95 Myr aiming to investigate the properties of activity cycles in this age
range.
We measured the length of a given cycle by analyzing the long-term
time-series of three activity indexes. For each star, we computed also the
global magnetic activity index that is proportional to the amplitude of
the rotational modulation and is a proxy of the mean level of the surface
magnetic activity. We detected activity cycles in 67 stars. Secondary cycles
were also detected in 32 stars. The lack of correlation between and
suggest that these stars belong to the Transitional Branch and that
the dynamo acting in these stars is different from the solar one. This
statement is also supported by the analysis of the butterfly diagrams.
We computed the Spearman correlation coefficient between ,
and different stellar parameters. We found that is
uncorrelated with all the investigated parameters. The index is
positively correlated with the convective turn-over time-scale, the magnetic
diffusivity time-scale , and the dynamo number , whereas
it is anti-correlated with the effective temperature , the
photometric shear and the radius at which
the convective zone is located.
We found that is about constant and that decreases with the
stellare age in the range 4-95 Myr. We investigated the magnetic activity of AB
Dor A by merging ASAS time-series with previous long-term photometric data. We
estimated the length of the AB Dor A primary cycle as .Comment: 19 pages , 15 figures, accepte
The Impact of Demographic Variables on Differentiation of Self During the Course of Counseling
Bowen\u27s natural systems theory has been identified as a foundational theory in learning of family processes (Miller, Anderson, & Keala, 2004). Despite the importance of this theory to the counseling profession, there is a lack of research examining the moderators of Bowen\u27s key construct, differentiation of self. There are few studies that explore the effect of demographic variables on levels of differentiation of self. There is also a lack of literature regarding the impact of these demographic moderators on an individual\u27s ability to increase their level of differentiation of self through counseling. This study investigated the impact of counseling on levels of differentiation of self and the moderating role of various demographic variables on differentiation in an adult clinical population. Participants were adults recruited from a community mental health center in northwestern Pennsylvania at their initial presentation for counseling services.
At the initial counseling session demographic questionnaires and Differentiation of Self Inventory-Revised (DSI-R) were administered with a follow-up administration of the DSI-R was conducted three months later. The results indicated a significant increase in overall differentiation of self, as well as in DSI-R subscale scores of Emotional Reactivity and I Position. The study also found that changes in differentiation of self varied as a function of age, with the most change occurring for younger (18-25 year old) participants; no other demographic variables were related to changes in differentiation of self. Potential implications for future research and potential explanations for these results are discussed
RACE-OC Project: Rotation and variability in the open cluster M11 (NGC6705)
Rotation and magnetic activity are intimately linked in main-sequence stars
of G or later spectral types. The presence and level of magnetic activity
depend on stellar rotation, and rotation itself is strongly influenced by
strength and topology of the magnetic fields. Open clusters represent
especially useful targets to investigate the rotation/activity/age connection.
The open cluster M11 has been studied as a part of the RACE-OC project
(Rotation and ACtivity Evolution in Open Clusters), which is aimed at exploring
the evolution of rotation and magnetic activity in the late-type members of
open clusters with different ages. Photometric observations of the open cluster
M11 were carried out in June 2004 using LOAO 1m telescope. The rotation periods
of the cluster members are determined by Fourier analysis of photometric data
time series. We further investigated the relations between the surface
activity, characterized by the light curve amplitude, and rotation. We have
discovered a total of 75 periodic variables in the M11 FoV, of which 38 are
candidate cluster members. Specifically, among cluster members we discovered 6
early-type, 2 eclipsing binaries and 30 bona-fide single periodic late-type
variables. Considering the rotation periods of 16 G-type members of the almost
coeval 200-Myr M34 cluster, we could determine the rotation period distribution
from a more numerous sample of 46 single G stars at an age of about 200-230 Myr
and determine a median rotation period P=4.8d. A comparison with the younger
M35 cluster (~150 Myr) and with the older M37 cluster (~550 Myr) shows that G
stars rotate slower than younger M35 stars and faster than older M37 stars. The
measured variation of the median rotation period is consistent with the
scenario of rotational braking of main-sequence spotted stars as they age.Comment: Accepted by Astronomy and Astrophysics on Dec 15, 200
Evidence from stellar rotation of enhanced disc dispersal: (I) The case of the triple visual system BD-21 1074 in the Pictoris association
The early stage of stellar evolution is characterized by a star-disc locking
mechanism. The disc-locking prevents the star to spin its rotation up, and its
timescale depends on the disc lifetime. Some mechanisms can significantly
shorten this lifetime, allowing a few stars to start spinning up much earlier
than other stars. In the present study, we aim to investigate how the
properties of the circumstellar environment can shorten the disc lifetime. We
have identified a few multiple stellar systems, composed of stars with similar
masses, which belong to associations with a known age. Since all parameters
that are responsible for the rotational evolution, with the exception of
environment properties and initial stellar rotation, are similar for all
components, we expect that significant differences among the rotation periods
can only arise from differences in the disc lifetimes. A photometric timeseries
allowed us to measure the rotation periods of each component, while
high-resolution spectra provided us with the fundamental parameters,
and chromospheric line fluxes. The rotation periods of the components differ
significantly, and the component B, which has a closer companion C, rotates
faster than the more distant and isolated component A. We can ascribe the
rotation period difference to either different initial rotation periods or
different disc-locking phases arising from the presence of the close companion
C. In the specific case of BD21 1074, the second scenario seems to be more
favored. In our hypothesis of different disc-locking phase, any planet orbiting
this star is likely formed very rapidly owing to a gravitational instability
mechanism, rather than core accretion. Only a large difference of initial
rotation periods alone could account for the observed period difference,
leaving comparable disc lifetimes.Comment: Accepted by Astronomy & Astrophysics on July 31, 2014; Pages 12,
Figs.
Lower limit for differential rotation in members of young loose stellar associations
Surface differential rotation (SDR) plays a key role in dynamo models. SDR
estimates are therefore essential for constraining theoretical models. We
measure a lower limit to SDR in a sample of solar-like stars belonging to young
associations with the aim of investigating how SDR depends on global stellar
parameters in the age range (4-95 Myr). The rotation period of a solar-like
star can be recovered by analyzing the flux modulation caused by dark spots and
stellar rotation. The SDR and the latitude migration of dark-spots induce a
modulation of the detected rotation period. We employ long-term photometry to
measure the amplitude of such a modulation and to compute the quantity
DeltaOmega_phot =2p/P_min -2pi/P_max that is a lower limit to SDR. We find that
DeltaOmega_phot increases with the stellar effective temperature and with the
global convective turn-over time-scale tau_c. We find that DeltaOmega_phot is
proportional to Teff^2.18pm 0.65 in stars recently settled on the ZAMS. This
power law is less steep than those found by previous authors, but closest to
recent theoretical models. We find that DeltaOmega_phot steeply increases
between 4 and 30 Myr and that itis almost constant between 30 and 95 Myr in a 1
M_sun star. We find also that the relative shear increases with the Rossby
number Ro. Although our results are qualitatively in agreement with
hydrodynamical mean-field models, our measurements are systematically higher
than the values predicted by these models. The discrepancy between
DeltaOmega_phot measurements and theoretical models is particularly large in
stars with periods between 0.7 and 2 d. Such a discrepancy, together with the
anomalous SDR measured by other authors for HD 171488 (rotating in 1.31 d),
suggests that the rotation period could influence SDR more than predicted by
the models.Comment: 23 pages, 15 figures, 5 tables,accepted by Astronomy and Astrophysic
- …