4,413 research outputs found
The Sun in Time: Age, Rotation, and Magnetic Activity of the Sun and Solar-type Stars and Effects on Hosted Planets
Multi-wavelength studies of solar analogs (G0-5 V stars) with ages from ~50
Myr to 9 Gyr have been carried out as part of the "Sun in Time" program for
nearly 20 yrs. From these studies it is inferred that the young (ZAMS) Sun was
rotating more than 10x faster than today. As a consequence, young solar-type
stars and the early Sun have vigorous magnetohydrodynamic (MHD) dynamos and
correspondingly strong coronal X-ray and transition region / chromospheric
FUV-UV emissions. To ensure continuity and homogeneity for this program, we use
a restricted sample of G0-5 V stars with masses, radii, T(eff), and internal
structure (i.e. outer convective zones) closely matching those of the Sun. From
these analogs we have determined reliable rotation-age-activity relations and
X-ray - UV (XUV) spectral irradiances for the Sun (or any solar-type star) over
time. These XUV irradiance measures serve as input data for investigating the
photo-ionization and photo-chemical effects of the young, active Sun on the
paleo-planetary atmospheres and environments of solar system planets. These
measures are also important to study the effects of these high energy emissions
on the numerous exoplanets hosted by solar-type stars of different ages.
Recently we have extended the study to include lower mass, main-sequence
(dwarf) dK and dM stars to determine relationships among their rotation
spin-down rates and coronal and chromospheric emissions as a function of mass
and age. From rotation-age-activity relations we can determine reliable ages
for main-sequence G, K, M field stars and, subsequently, their hosted planets.
Also inferred are the present and the past XUV irradiance and plasma flux
exposures that these planets have endured and the suitability of the hosted
planets to develop and sustain life.Comment: 12 pages, 6 figures; to appear in the proceedings of IAU 258: The
Ages of Star
Investigating ICAPM with Dynamic Conditional Correlations
This paper examines the intertemporal relation between expected return and risk for 30 stocks in the Dow Jones Industrial Average. The mean-reverting dynamic conditional correlation model of Engle (2002) is used to estimate a stock’s conditional covariance with the market and test whether
the conditional covariance predicts time-variation in the stock’s expected return. The risk-aversion coefficient, restricted to be the same across stocks in panel regression, is estimated to be between
two and four and highly significant. This result is robust across different market portfolios, different sample periods, alternative specifications of the conditional mean and covariance processes, and including a wide variety of state variables that proxy for the intertemporal hedging demand component of the ICAPM. Risk premium induced by the conditional covariation of individual stocks with the market portfolio remains economically and statistically significant after controlling for risk premiums induced by conditional covariation with macroeconomic variables (federal funds rate, default spread, and term spread), financial factors (size, book-to-market, and momentum), and volatility measures (implied, GARCH, and range volatility)
Living With a Red Dwarf: The Rotation-Age Relationship of M Dwarfs
Age is a fundamental stellar property, yet for many stars it is difficult to
reliably determine. For M dwarfs it has been notoriously so. Due to their lower
masses, core hydrogen fusion proceeds at a much slower rate in M dwarfs than it
does in more massive stars like the Sun. As a consequence, more customary age
determination methods (e.g. isochrones and asteroseismology) are unreliable for
M dwarfs. As these methods are unavailable, many have searched for reliable
alternatives. M dwarfs comprise the overwhelming majority of the nearby stellar
inventory, which makes the determination of their fundamental parameters even
more important. Further, an ever-increasing number of exoplanets are being
found to orbit M dwarfs and recent studies have suggested they may relatively
higher number of low-mass planets than other spectral types. Determining the
ages of M dwarfs then allows us to better study any hosted exoplanets, as well.
Fortunately, M dwarfs possess magnetic activity and stellar winds like other
cool dwarf stars. This causes them to undergo the spindown effect (rotate with
longer periods) as they age. For this reason, stellar rotation rate has been
considered a potentially powerful age determination parameter for over 50
years. Calibrating reliable age-rotation relationships for M dwarfs has been a
lengthy process, but here we present the age-rotation relationships for ~M0-6.5
dwarfs, determined as part of the Living with a Red Dwarf program. These
relationships should prove invaluable for a wide range of stellar astrophysics
and exoplanetary science applications.Comment: Accepted for publication in ApJ Letter
X-Ray, UV and Optical Observations of Classical Cepheids: New Insights into Cepheid Evolution, and the Heating and Dynamics of Their Atmospheres
To broaden the understanding of classical Cepheid structure, evolution and
atmospheres, we have extended our continuing secret lives of Cepheids program
by obtaining XMM/Chandra X-ray observations, and Hubble space telescope (HST) /
cosmic origins spectrograph (COS) FUV-UV spectra of the bright, nearby Cepheids
Polaris, {\delta} Cep and {\beta} Dor. Previous studies made with the
international ultraviolet explorer (IUE) showed a limited number of UV emission
lines in Cepheids. The well-known problem presented by scattered light
contamination in IUE spectra for bright stars, along with the excellent
sensitivity & resolution combination offered by HST/COS, motivated this study,
and the spectra obtained were much more rich and complex than we had ever
anticipated. Numerous emission lines, indicating 10^4 K up to ~3 x 10^5 K
plasmas, have been observed, showing Cepheids to have complex, dynamic outer
atmospheres that also vary with the photospheric pulsation period. The FUV line
emissions peak in the phase range {\phi} ~ 0.8-1.0 and vary by factors as large
as 10x. A more complete picture of Cepheid outer atmospheres is accomplished
when the HST/COS results are combined with X-ray observations that we have
obtained of the same stars with XMM-Newton & Chandra. The Cepheids detected to
date have X-ray luminosities of log Lx ~ 28.5-29.1 ergs/sec, and plasma
temperatures in the 2-8 x 10^6 K range. Given the phase-timing of the enhanced
emissions, the most plausible explanation is the formation of a
pulsation-induced shocks that excite (and heat) the atmospheric plasmas
surrounding the photosphere. A pulsation-driven {\alpha}^2 equivalent dynamo
mechanism is also a viable and interesting alternative. However, the tight
phase-space of enhanced emission (peaking near 0.8-1.0 {\phi}) favor the shock
heating mechanism hypothesis.Comment: 11 pages, 6 figures, Published in Journal of Astronomy and Space
Sciences (JASS), vol. 29, no. 2, pp 181-189, June, 201
Low-energy electron transport with the method of discrete ordinates
The one-dimensional discrete ordinates code ANISN was adapted to transport low energy (a few MeV) electrons. Calculated results obtained with ANISN were compared with experimental data for transmitted electron energy and angular distribution data for electrons normally incident on aluminum slabs of various thicknesses. The calculated and experimental results are in good agreement for a thin slab (0.2 of the electron range), but not for the thicker slabs (0.6 of the electron range). Calculated results obtained with ANISN were also compared with results obtained using Monte Carlo methods
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