189 research outputs found
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&
Combined ultraviolet studies of astronomical sources
Topics addressed include: Cygnus Loop; P Cygni profiles in dwarf novae; YY Gem; nova shells; HZ Herculis; activity cycles in cluster giants; Alpha Ori; metal deficient giant stars; ultraviolet spectra of symbiotic stars detected by the Very Large Array; time variability in symbiotic stars; blue galaxies; and quasistellar objects with X-ray spectra
No planet for HD 166435
The G0V star HD166435 has been observed by the fiber-fed spectrograph ELODIE
as one of the targets in the large extra-solar planet survey that we are
conducting at the Observatory of Haute-Provence. We detected coherent,
low-amplitude, radial-velocity variations with a period of 3.7987days,
suggesting a possible close-in planetary companion. Subsequently, we initiated
a series of high-precision photometric observations to search for possible
planetary transits and an additional series of CaII H and K observations to
measure the level of surface magnetic activity and to look for possible
rotational modulation. Surprisingly, we found the star to be photometrically
variable and magnetically active. A detailed study of the phase stability of
the radial-velocity signal revealed that the radial-velocity variability
remains coherent only for durations of about 30days. Analysis of the time
variation of the spectroscopic line profiles using line bisectors revealed a
correlation between radial velocity and line-bisector orientation. All of these
observations, along with a one-quarter cycle phase shift between the
photometric and the radial-velocity variationss, are well explained by the
presence of dark photospheric spots on HD166435. We conclude that the
radial-velocity variations are not due to gravitational interaction with an
orbiting planet but, instead, originate from line-profile changes stemming from
star spots on the surface of the star. The quasi-coherence of the
radial-velocity signal over more than two years, which allowed a fair fit with
a binary model, makes the stability of this star unusual among other active
stars. It suggests a stable magnetic field orientation where spots are always
generated at about the same location on the surface of the star.Comment: 9 pages, 8 figures, Accepted for publication in A&
A Survey of Chromospheric Activity in the Solar-Type Stars in the Open Cluster M67
We present the results of a spectroscopic survey of the Ca II H & K core
strengths in a sample of 60 solar-type stars that are members of the solar-age
and solar-metallicity open cluster M67. We adopt the HK index, defined as the
summed H+K core strengths in 0.1 nm bandpasses centered on the H and K lines,
respectively, as a measure of the chromospheric activity that is present. We
compare the distribution of mean HK index values for the M67 solar-type stars
with the variation of this index as measured for the Sun during the
contemporary solar cycle. We find that the stellar distribution in our HK index
is broader than that for the solar cycle. Approximately 17% of the M67 sun-like
stars exhibit average HK indices that are less than solar minimum. About 7%-12%
are characterized by relatively high activity in excess of solar maximum values
while 72%-80% of the solar analogs exhibit Ca II H+K strengths within the range
of the modern solar cycle. The ranges given reflect uncertainties in the most
representative value of the maximum in the HK index to adopt for the solar
cycle variations observed during the period A.D. 1976--2004. Thus, ~ 20% - 30%
of our homogeneous sample of sun-like stars have mean chromospheric H+K
strengths that are outside the range of the contemporary solar cycle. Any
cycle-like variability that is present in the M67 solar-type stars appears to
be characterized by periods greater than ~ 6 years. Finally, we estimate a mean
chromospheric age for M67 in the range of 3.8--4.3 Gyr.Comment: Accepted in The Astrophysical Journa
Multiple and changing cycles of active stars II. Results
We study the time variations of the cycles of 20 active stars based on
decades-long photometric or spectroscopic observations. A method of
time-frequency analysis, as discussed in a companion paper, is applied to the
data. Fifteen stars definitely show multiple cycles; the records of the rest
are too short to verify a timescale for a second cycle. The cycles typically
show systematic changes. For three stars, we found two cycles in each of them
that are not harmonics, and which vary in parallel, indicating that a common
physical mechanism arising from a dynamo construct. The positive relation
between the rotational and cycle periods is confirmed for the inhomogeneous set
of active stars. Stellar activity cycles are generally multiple and variable.Comment: Accepted to Astronomy and Astrophysic
Rotation periods of exoplanet host stars
The stellar rotation periods of ten exoplanet host stars have been determined
using newly analysed Ca II H & K flux records from Mount Wilson Observatory and
Stromgren b, y photometric measurements from Tennessee State University's
automatic photometric telescopes (APTs) at Fairborn Observatory. Five of the
rotation periods have not previously been reported, with that of HD 130322 very
strongly detected at Prot = 26.1 \pm 3.5 d. The rotation periods of five other
stars have been updated using new data. We use the rotation periods to derive
the line-of-sight inclinations of the stellar rotation axes, which may be used
to probe theories of planet formation and evolution when combined with the
planetary orbital inclination found from other methods. Finally, we estimate
the masses of fourteen exoplanets under the assumption that the stellar
rotation axis is aligned with the orbital axis. We calculate the mass of HD
92788 b (28 MJ) to be within the low-mass brown dwarf regime and suggest that
this object warrants further investigation to confirm its true nature.Comment: Accepted for publication in MNRAS. 15 pages, 11 figure
Combined ultraviolet studies of astronomical source
As part of its Ultraviolet Studies of Astronomical Sources the Smithsonian Astrophysical Observatory for the period 1 Feb. 1985 to 31 July 1985 observed the following: the Cygnus Loop; oxygen-rich supernova remnants in 1E0102-72; the Large Magellanic Cloud supernova remnants; P Cygni profiles in dwarf novae; soft X-ray photoionization of interstellar gas; spectral variations in AM Her stars; the mass of Feige 24; atmospheric inhomogeneities in Lambda Andromedae and FF Aquarii; photometric and spectroscopic observation of Capella; Alpha Orionis; metal deficient giant stars; M 67 giants; high-velocity winds from giant stars; accretion disk parameters in cataclysmic variables; chromospheric emission of late-type dwarfs in visual binaries; chromospheres and transient regions of stars in the Ursa Major group; and low-metallicity blue galaxies
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