244 research outputs found
The Secret Lives of Cepheids: A Multi-Wavelength Study of the Atmospheres and Real-Time Evolution of Classical Cepheids
The primary goal of this study is to observe how complex the behaviors of
Cepheids can be, and to show how the continued monitoring of Cepheids at
multiple wavelengths can begin to reveal their "secret lives."
We aim to achieve this through optical photometry, UV spectroscopy and X-ray
imaging. Through Villanova's guaranteed access to ground-based telescopes, we
have secured well-covered light curves as regularly as possible. Amplitudes and
times of max brightness were obtained and compared to previous literature
results. At UV wavelengths, we have secured hi-res spectra of 2 nearby Cepheids
- delta Cep and beta Dor - with HST-COS. Also, we have obtained X-ray images of
5 Cepheids with XMM-Newton and the Chandra X-ray Observatory, and further
observations with both satellites have been proposed for (XMM) and approved
(Chandra).
Optical photometry has shown that 8 of the 10 observed Cepheids have
amplitude variability, or hints thereof, and all 10 show period variability
(recent, long-term or possibly periodic). The UV spectra reveal emission lines
from heated atmospheric plasmas of 10^4 - 10^5 K that vary in phase with the
Cepheid pulsations. The X-ray images have detected the three nearest Cepheids
observed (Polaris, delta Cep and beta Dor), while the distances of the other
two place their fluxes likely at or below detector background levels. The X-ray
fluxes for delta Cep show possible phased variability, but anti-correlated with
the UV emission lines (i.e. high X-ray flux during low UV flux, and vice
versa).
Further data are required to ultimately confirm Blazhko-like cycles in
Cepheids, X-ray variability with phase and the particulars of the high-energy
variability such as phase-lags between atmospheric plasma emissions of
different temperature and the exact contributions of the possible heating
mechanism.Comment: Thesis (Ph.D.) -- James Cook University, 201
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
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
Classical Cepheids Require Enhanced Mass Loss
Measurements of rates of period change of Classical Cepheids probe stellar
physics and evolution. Additionally, better understanding of Cepheid structure
and evolution provides greater insight into their use as standard candles and
tools for measuring the Hubble constant. Our recent study of the period change
of the nearest Cepheid, Polaris, suggested that it is undergoing enhanced mass
loss when compared to canonical stellar evolution model predictions. In this
work, we expand the analysis to rates of period change measured for about 200
Galactic Cepheids and compare them to population synthesis models of Cepheids
including convective core overshooting and enhanced mass loss. Rates of period
change predicted from stellar evolution models without mass loss do not agree
with observed rates whereas including enhanced mass loss yields predicted rates
in better agreement with observations. This is the first evidence that enhanced
mass loss as suggested previously for Polaris and delta Cephei must be a
ubiquitous property of Classical Cepheids.Comment: 6 pages, 4 figures, Accepted for publication in ApJ Letter
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