48 research outputs found
Towards a long-term record of solar total and spectral irradiance
The variation of total solar irradiance (TSI) has been measured since 1978
and that of the spectral irradiance for an even shorter amount of time.
Semi-empirical models are now available that reproduce over 80% of the measured
irradiance variations. An extension of these models into the more distant past
is needed in order to serve as input to climate simulations. Here we review our
most recent efforts to model solar total and spectral irradiance on time scales
from days to centuries and even longer. Solar spectral irradiance has been
reconstructed since 1947. Reconstruction of solar total irradiance goes back to
1610 and suggests a value of about 1-1.5 Wm for the increase in the
cycle-averaged TSI since the end of the Maunder minimum, which is significantly
lower than previously assumed but agrees with other modern models. First steps
have also been made towards reconstructions of solar total and spectral
irradiance on time scales of millennia
Solar cycle variations in the growth and decay of sunspot groups
We analysed the combined Greenwich (1874-1976) and Solar Optical
Observatories Network (1977-2011) data on sunspot groups. The daily rate of
change of the area of a spot group is computed using the differences between
the epochs of the spot group observation on any two consecutive days during its
life-time and between the corrected whole spot areas of the spot group at these
epochs. Positive/negative value of the daily rate of change of the area of a
spot group represents the growth/decay rate of the spot group. We found that
the total amounts of growth and decay of spot groups whose life times > or = 2
days in a given time interval (say one-year) well correlate to the amount of
activity in the same interval. We have also found that there exists a
reasonably good correlation and an approximate linear relationship between the
logarithmic values of the decay rate and area of the spot group at the first
day of the corresponding consecutive days, largely suggesting that a
large/small area (magnetic flux) decreases in a faster/slower rate. There
exists a long-term variation (about 90-year) in the slope of the linear
relationship. The solar cycle variation in the decay of spot groups may have a
strong relationship with the corresponding variations in solar energetic
phenomena such as solar flare activity. The decay of spot groups may also
substantially contribute to the coherence relationship between the total solar
irradiance and the solar activity variations.Comment: 12 pages, 7 figures, Accepted for publication in Astrophysics & Space
Science. arXiv admin note: substantial text overlap with arXiv:1105.106
Phi meson production in Au+Au and p+p collisions at sqrt (s)=200 GeV
We report the STAR measurement of Phi meson production in Au+Au and p+p
collisions at sqrt (s)=200 GeV. Using the event mixing technique, the Phi
spectra and yields are obtained at mid-rapidity for five centrality bins in
Au+Au collisions and for non-singly-diffractive p+p collisions. It is found
that the Phi transverse momentum distributions from Au+Au collisions are better
fitted with a single-exponential while the p+p spectrum is better described by
a double-exponential distribution. The measured nuclear modification factors
indicate that Phi production in central Au+Au collisions is suppressed relative
to peripheral collisions when scaled by the number of binary collisions. The
systematics of versus centrality and the constant Phi/K- ratio versus beam
species, centrality, and collision energy rule out kaon coalescence as the
dominant mechanism for Phi production.Comment: 6 pages, 3 figures, submitted to Phys. Rev. Let
Azimuthal anisotropy at RHIC: the first and fourth harmonics
We report the first observations of the first harmonic (directed flow, v_1),
and the fourth harmonic (v_4), in the azimuthal distribution of particles with
respect to the reaction plane in Au+Au collisions at the Relativistic Heavy Ion
Collider (RHIC). Both measurements were done taking advantage of the large
elliptic flow (v_2) generated at RHIC. From the correlation of v_2 with v_1 it
is determined that v_2 is positive, or {\it in-plane}. The integrated v_4 is
about a factor of 10 smaller than v_2. For the sixth (v_6) and eighth (v_8)
harmonics upper limits on the magnitudes are reported.Comment: 6 pages with 3 figures, as accepted for Phys. Rev. Letters The data
tables are at
http://www.star.bnl.gov/central/publications/pubDetail.php?id=3
Pion, kaon, proton and anti-proton transverse momentum distributions from p+p and d+Au collisions at GeV
Identified mid-rapidity particle spectra of , , and
from 200 GeV p+p and d+Au collisions are reported. A
time-of-flight detector based on multi-gap resistive plate chamber technology
is used for particle identification. The particle-species dependence of the
Cronin effect is observed to be significantly smaller than that at lower
energies. The ratio of the nuclear modification factor () between
protons and charged hadrons () in the transverse momentum
range GeV/c is measured to be
(stat)(syst) in minimum-bias collisions and shows little
centrality dependence. The yield ratio of in minimum-bias d+Au
collisions is found to be a factor of 2 lower than that in Au+Au collisions,
indicating that the Cronin effect alone is not enough to account for the
relative baryon enhancement observed in heavy ion collisions at RHIC.Comment: 6 pages, 4 figures, 1 table. We extended the pion spectra from
transverse momentum 1.8 GeV/c to 3. GeV/
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
ρ\u3csup\u3e0\u3c/sup\u3e production and possible modification in Au + Au and p + p collisions at √s\u3csub\u3eNN\u3c/sub\u3e = 200 GeV
We report results on ρ(770)0→π+π− production at midrapidity in p+p and peripheral Au+Au collisions at √sNN=200 GeV. This is the first direct measurement of ρ(770)0→π+π− in heavy-ion collisions. The measured ρ0 peak in the invariant mass distribution is shifted by ∼40 MeV/c2 in minimum bias p+p interactions and ∼70 MeV/c2 in peripheral Au+Au collisions. The ρ0 mass shift is dependent on transverse momentum and multiplicity. The modification of the ρ0 meson mass, width, and shape due to phase space and dynamical effects are discussed
Event-wise ⟨p\u3csub\u3et\u3c/sub\u3e⟩ fluctuations in Au-Au collisions at √s\u3csub\u3eNN\u3c/sub\u3e = 130 GeV
We present the first large-acceptance measurement of event-wise mean transverse momentum ⟨pt⟩ fluctuations for Au-Au collisions at nucleon-nucleon center-of-momentum collision energy √sNN = 130 GeV. The observed nonstatistical ⟨pt⟩ fluctuations substantially exceed in magnitude fluctuations expected from the finite number of particles produced in a typical collision. The r.m.s. fractional width excess of the event-wise ⟨pt⟩ distribution is 13.7±0.1(stat) ±1.3(syst)% relative to a statistical reference, for the 15% most-central collisions and for charged hadrons within pseudorapidity range |η|c. The width excess varies smoothly but nonmonotonically with collision centrality and does not display rapid changes with centrality which might indicate the presence of critical fluctuations. The reported ⟨pt⟩ fluctuation excess is qualitatively larger than those observed at lower energies and differs markedly from theoretical expectations. Contributions to ⟨pt⟩ fluctuations from semihard parton scattering in the initial state and dissipation in the bulk colored medium are discussed