357 research outputs found
The X-ray cycle in the solar-type star HD 81809
(abridged) Our long-term XMM-Newton program of long-term monitoring of a
solar-like star with a well-studied chromospheric cycle, HD 81809 aims to study
whether an X-ray cycle is present, along with studying its characteristics and
its relation to the chromospheric cycle. Regular observations of HD 81809 were
performed with XMM-Newton, spaced by 6 months from 2001 to 2007. We studied the
variations in the resulting coronal luminosity and temperature, and compared
them with the chromospheric CaII variations. We also modeled the observations
in terms of a mixture of active regions, using a methodology originally
developed to study the solar corona. Our observations show a well-defined cycle
with an amplitude exceeding 1 dex and an average luminosity approximately one
order of magnitude higher than in the Sun. The behavior of the corona of HD
81809 can be modeled well in terms of varying coverage of solar-like active
regions, with a larger coverage than for the Sun, showing it to be compatible
with a simple extension of the solar case.Comment: In press, Astronomy & Astrophysic
High-amplitude, long-term X-ray variability in the solar-type star HD 81809: the beginning of an X-ray activity cycle?
We present the initial results from our XMM program aimed at searching for
X-ray activity cycles in solar-type stars. HD 81809 is a G2-type star (somewhat
more evolved than the Sun, and with a less massive companion) with a pronounced
8.2 yr chromospheric cycle, as evident from from the Mt. Wilson program data.
We present here the results from the initial 2.5 years of XMM observations,
showing that large amplitude (a factor of approx. 10) modulation is present in
the X-ray luminosity, with a clearly defined maximum in mid 2002 and a steady
decrease since then. The maximum of the chromospheric cycle took place in 2001;
if the observed X-ray variability is the initial part of an X-ray cycle, this
could imply a phase shift between chromospheric and coronal activity, although
the current descent into chromospheric cycle minimum is well reflected into the
star's X-ray luminosity. The observations presented here provide clear evidence
for the presence of large amplitude X-ray variability coherent with the
activity cycle in the chromosphere in a star other than the Sun.Comment: Accepted for publication in A&
Time series of high resolution spectra of SN 2014J observed with the TIGRE telescope
We present a time series of high resolution spectra of the Type Ia supernova
2014J, which exploded in the nearby galaxy M82. The spectra were obtained with
the HEROS echelle spectrograph installed at the 1.2 m TIGRE telescope. We
present a series of 33 spectra with a resolution of R = 20, 000, which covers
the important bright phases in the evolution of SN 2014J during the period from
January 24 to April 1 of 2014. The spectral evolution of SN 2014J is derived
empirically. The expansion velocities of the Si II P-Cygni features were
measured and show the expected decreasing behaviour, beginning with a high
velocity of 14,000 km/s on January 24. The Ca II infrared triplet feature shows
a high velocity component with expansion velocities of > 20, 000 km/s during
the early evolution apart from the normal component showing similar velocities
as Si II. Further broad P-Cygni profiles are exhibited by the principal lines
of Ca II, Mg II and Fe II. The TIGRE SN 2014J spectra also resolve several very
sharp Na I D doublet absorption components. Our analysis suggests interesting
substructures in the interstellar medium of the host galaxy M82, as well as in
our Milky Way, confirming other work on this SN. We were able to identify the
interstellar absorption of M82 in the lines of Ca II H & K at 3933 and 3968 A
as well as K I at 7664 and 7698 A. Furthermore, we confirm several Diffuse
Interstellar Bands, at wavelengths of 6196, 6283, 6376, 6379 and 6613 A and
give their measured equivalent widths.Comment: 11 pages, 10 figures, accepted for publication in MNRA
Simultaneous X-ray and optical spectroscopy of the Oef supergiant lambda Cep
Probing the structures of stellar winds is of prime importance for the
understanding of massive stars. Based on their optical spectral morphology and
variability, the stars of the Oef class have been suggested to feature
large-scale structures in their wind. High-resolution X-ray spectroscopy and
time-series of X-ray observations of presumably-single O-type stars can help us
understand the physics of their stellar winds. We have collected XMM-Newton
observations and coordinated optical spectroscopy of the O6Ief star lambda Cep
to study its X-ray and optical variability and to analyse its high-resolution
X-ray spectrum. We investigate the line profile variability of the He II 4686
and H-alpha emission lines in our time series of optical spectra, including a
search for periodicities. We further discuss the variability of the broadband
X-ray flux and analyse the high-resolution spectrum of lambda Cep using
line-by-line fits as well as a code designed to fit the full high-resolution
X-ray spectrum consistently. During our observing campaign, the He II 4686 line
varies on a timescale of ~18 hours. On the contrary, the H-alpha line profile
displays a modulation on a timescale of 4.1 days which is likely the rotation
period of the star. The X-ray flux varies on time-scales of days and could in
fact be modulated by the same 4.1 days period as H-alpha, although both
variations are shifted in phase. The high-resolution X-ray spectrum reveals
broad and skewed emission lines as expected for the X-ray emission from a
distribution of wind-embedded shocks. Most of the X-ray emission arises within
less than 2R* above the photosphere.Comment: Accepted for publication in Astronomy & Astrophysic
On the Relation between Solar Activity and Clear-Sky Terrestrial Irradiance
The Mauna Loa Observatory record of direct-beam solar irradiance measurements
for the years 1958-2010 is analysed to investigate the variation of clear-sky
terrestrial insolation with solar activity over more than four solar cycles.
The raw irradiance data exhibit a marked seasonal cycle, extended periods of
lower irradiance due to emissions of volcanic aerosols, and a long-term
decrease in atmospheric transmission independent of solar activity. After
correcting for these effects, it is found that clear-sky terrestrial irradiance
typically varies by about 0.2 +/- 0.1% over the course of the solar cycle, a
change of the same order of magnitude as the variations of the total solar
irradiance above the atmosphere. An investigation of changes in the clear-sky
atmospheric transmission fails to find a significant trend with sunspot number.
Hence there is no evidence for a yet unknown effect amplifying variations of
clear-sky irradiance with solar activity.Comment: 16 pages, 7 figures, in press at Solar Physics; minor changes to the
text to match final published versio
The Active Corona of HD 35850 (F8 V)
We present Extreme Ultraviolet Explorer spectroscopy and photometry of the
nearby F8 V star HD 35850 (HR 1817). The EUVE spectra reveal 28 emission lines
from Fe IX and Fe XV to Fe XXIV. The Fe XXI 102, 129 A ratio yields an upper
limit for the coronal electron density, log n < 11.6 per cc. The EUVE SW
spectrum shows a small but clearly detectable continuum. The line-to-continuum
ratio indicates approximately solar Fe abundances, 0.8 < Z < 1.6. The resulting
emission-measure distribution is characterized by two temperature components at
log T of 6.8 and 7.4. The EUVE spectra have been compared with non-simultaneous
ASCA SIS spectra of HD 35850. The SIS spectrum shows the same temperature
distribution as the EUVE DEM analysis. However, the SIS spectral firs suggest
sub-solar abundances, 0.34 < Z < 0.81. Although some of the discrepancy may be
the result of incomplete X-ray line lists, we cannot explain the disagreement
between the EUVE line-to-continuum ratio and the ASCA-derived Fe abundance.
Given its youth (t ~ 100 Myr), its rapid rotation (v sin i ~ 50 km/s), and its
high X-ray activity (Lx ~ 1.5E+30 ergs/s), HD 35850 may represent an activity
extremum for single, main-sequence F-type stars. The variability and EM
distribution can be reconstructed using the continuous flaring model of Guedel
provided that the flare distribution has a power-law index of 1.8. Similar
results obtained for other young solar analogs suggest that continuous flaring
is a viable coronal heating mechanism on rapidly rotating, late-type,
main-sequence stars.Comment: 32 pages incl. 14 figures and 3 tables. To appear in the 1999 April
10 issue of The Astrophysical Journa
High-Temperature Quantum Tunneling and Hydrogen Bonding Rearrangements Characterize the Solid-Solid Phase Transitions in a Phosphonium-Based Protic Ionic Liquid
We report the complex phase behavior of the glass
forming protic ionic liquid (PIL) d3-octylphosphonium
bis(trifluoromethylsulfonyl)imide [C8H17PD3][NTf2] by means of
solid-state NMR spectroscopy. Combined line shape and spin
relaxation studies of the deuterons in the PD3 group of the
octylphosphonium cation allow to map and correlate the
phase behavior for a broad temperature range from 71 K to
343 K. In the solid PIL at 71 K, we observed a static state,
characterized by the first deuteron quadrupole coupling
constant reported for PD3 deuterons. A transition enthalpy of
about 12 kJmol 1 from the static to the mobile state with
increasing temperature suggests the breaking of a weak,
charge-enhanced hydrogen bond between cation and anion.
The highly mobile phase above 100 K exhibits an almost
disappearing activation barrier, strongly indicating quantum
tunneling. Thus, we provide first evidence of tunneling driven
mobility of the hydrogen bonded P D moieties in the glassy
state of PILs, already at surprisingly high temperatures up to
200 K. Above 250 K, the mobile phase turns from anisotropic
to isotropic motion, and indicates strong internal rotation of
the PD3 group. The analyzed line shapes and spin relaxation
times allow us to link the structural and dynamical behavior
at molecular level with the phase behavior beyond the DSC
traces
Stellar Lyman-alpha Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres
We search the Hubble Space Telescope (HST) archive for previously unanalyzed
observations of stellar H I Lyman-alpha emission lines, our primary purpose
being to look for new detections of Lyman-alpha absorption from the outer
heliosphere, and to also search for analogous absorption from the astrospheres
surrounding the observed stars. The astrospheric absorption is of particular
interest because it can be used to study solar-like stellar winds that are
otherwise undetectable. We find and analyze 33 HST Lyman-alpha spectra in the
archive. All the spectra were taken with the E140M grating of the Space
Telescope Imaging Spectrograph (STIS) instrument on board HST. The HST/STIS
spectra yield 4 new detections of heliospheric absorption (70 Oph, Xi Boo, 61
Vir, and HD 165185) and 7 new detections of astrospheric absorption (EV Lac, 70
Oph, Xi Boo, 61 Vir, Delta Eri, HD 128987, and DK UMa), doubling the previous
number of heliospheric and astrospheric detections. When combined with previous
results, 10 of 17 lines of sight within 10 pc yield detections of astrospheric
absorption. This high detection fraction implies that most of the ISM within 10
pc must be at least partially neutral, since the presence of H I within the ISM
surrounding the observed star is necessary for an astrospheric detection. In
contrast, the detection percentage is only 9.7% (3 out of 31) for stars beyond
10 pc. Our Lyman-alpha analyses provide measurements of ISM H I and D I column
densities for all 33 lines of sight, and we discuss some implications of these
results. Finally, we measure chromospheric Lyman-alpha fluxes from the observed
stars. We use these fluxes to determine how Lyman-alpha flux correlates with
coronal X-ray and chromospheric Mg II emission, and we also study how
Lyman-alpha emission depends on stellar rotation.Comment: 56 pages, 15 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty;
accepted by ApJ
Spectroscopic variability of two Oe stars
The Oe stars HD45314 and HD60848 have recently been found to exhibit very
different X-ray properties: whilst HD60848 has an X-ray spectrum and emission
level typical of most OB stars, HD45314 features a much harder and brighter
X-ray emission, making it a so-called gamma Cas analogue. Monitoring the
optical spectra could provide hints towards the origin of these very different
behaviours. We analyse a large set of spectroscopic observations of HD45314 and
HD60848, extending over 20 years. We further attempt to fit the H-alpha line
profiles of both stars with a simple model of emission line formation in a
Keplerian disk. Strong variations in the strengths of the H-alpha, H-beta, and
He I 5876 emission lines are observed for both stars. In the case of HD60848,
we find a time lag between the variations in the equivalent widths of these
lines. The emission lines are double peaked with nearly identical strengths of
the violet and red peaks. The H-alpha profile of this star can be successfully
reproduced by our model of a disk seen under an inclination of 30 degrees. In
the case of HD45314, the emission lines are highly asymmetric and display
strong line profile variations. We find a major change in behaviour between the
2002 outburst and the one observed in 2013. This concerns both the relationship
between the equivalent widths of the various lines and their morphologies at
maximum strength (double-peaked in 2002 versus single-peaked in 2013). Our
simple disk model fails to reproduce the observed H-alpha line profiles of
HD45314. Our results further support the interpretation that Oe stars do have
decretion disks similar to those of Be stars. Whilst the emission lines of
HD60848 are explained by a disk with a Keplerian velocity field, the disk of
HD45314 seems to have a significantly more complex velocity field that could be
related to the phenomenon that produces its peculiar X-ray emission.Comment: Accepted for Publication in A&
New rotation periods in the Pleiades: Interpreting activity indicators
We present results of photometric monitoring campaigns of G, K and M dwarfs
in the Pleiades carried out in 1994, 1995 and 1996. We have determined rotation
periods for 18 stars in this cluster. In this paper, we examine the validity of
using observables such as X-ray activity and amplitude of photometric
variations as indicators of angular momentum loss. We report the discovery of
cool, slow rotators with high amplitudes of variation. This contradicts
previous conclusions about the use of amplitudes as an alternate diagnostic of
the saturation of angular momentum loss. We show that the X-ray data can be
used as observational indicators of mass-dependent saturation in the angular
momentum loss proposed on theoretical grounds.Comment: 24 pages, LaTex (AASTeX); includes 8 postscript figures and 4 Latex
tables. To appear in ApJ, Feb. 1, 1998. Postscript version of preprint can be
obtained from http://casa.colorado.edu/~anitak/pubs.htm
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