70 research outputs found
Steps toward a high precision solar rotation profile: Results from SDO/AIA coronal bright point data
Coronal bright points (CBP) are ubiquitous small brightenings in the solar
corona associated with small magnetic bipoles. We derive the solar differential
rotation profile by tracing the motions of CBPs detected by the Atmospheric
Imaging Assembly (AIA) instrument aboard the Solar Dynamics Observatory (SDO).
We also investigate problems related to detection of coronal bright points
resulting from instrument and detection algorithm limitations. To determine the
positions and identification of coronal bright points we used a segmentation
algorithm. A linear fit of their central meridian distance and latitude versus
time was utilised to derive velocities. We obtained 906 velocity measurements
in a time interval of only 2 days. The differential rotation profile can be
expressed as \degr day. Our result is in agreement with
other work and it comes with reasonable errors in spite of the very short time
interval used. This was made possible by the higher sensitivity and resolution
of the AIA instrument compared to similar equipment as well as high cadence.
The segmentation algorithm also played a crucial role by detecting so many
CBPs, which reduced the errors to a reasonable level. Data and methods
presented in this paper show a great potential to obtain very accurate velocity
profiles, both for rotation and meridional motion and, consequently, Reynolds
stresses. The amount of coronal bright point data that could be obtained from
this instrument should also provide a great opportunity to study changes of
velocity patterns with a temporal resolution of only a few months. Other
possibilities are studies of evolution of CBPs and proper motions of magnetic
elements on the Sun
Searching for Planets in the Hyades. III. The Quest for Short-Period Planets
We have been using the Keck I High Resolution Spectrograph to search for planetary companions in the Hyades cluster. We selected four stars from this sample that showed significant radial velocity variability on short timescales to search for short-period planetary companions. The radial velocities of these four stars were monitored regularly with the Hobby-Eberly Telescope for approximately 2 months, while sparse data were also taken over ∼4 months: we also obtained near-simultaneous photometric observations with one of the automatic photoelectric telescopes at Fairborn Observatory. For three of the stars, we detect photometric variability with the same period present in the radial velocity (vr) measurements, compatible with the expected rotation rates for Hyades members. The fourth star continues to show vr variations and minimal photometric variability but with no significant periodicity. This study shows that for the three stars with periodic behavior, a significant portion of the vr fluctuations are likely due primarily to magnetic activity modulated by stellar rotation rather than planetary companions. Using simple models for the vr perturbations arising from spot and plage, we demonstrate that both are likely to contribute to the observed vr variations. Thus, simultaneous monitoring of photometric (photospheric) and spectroscopic (chromospheric) variations is essential for identifying the cause of Doppler-shifted absorption lines in more active stars
Searching for Planets in the Hyades II: Some Implications of Stellar Magnetic Activity
The Hyades constitute a homogeneous sample of stars ideal for investigating
the dependence of planet formation on the mass of the central star. Due to
their youth, Hyades members are much more chromospherically active than stars
traditionally surveyed for planets using high precision radial velocity (RV)
techniques. Therefore, we have conducted a detailed investigation of whether
magnetic activity of our Hyades target stars will interfere with our ability to
make precise RV searches for substellar companions. We measure chromospheric
activity (which we take as a proxy for magnetic activity) by computing the
equivalent of the R'HK activity index from the Ca II K line. is not
constant in the Hyades: we confirm that it decreases with increasing
temperature in the F stars, and also find it decreases for stars cooler than
mid-K. We examine correlations between simultaneously measured R'HK and RV
using both a classical statistical test and a Bayesian odds ratio test. We find
that there is a significant correlation between R'HK and the RV in only 5 of
the 82 stars in this sample. Thus, simple Rprime HK-RV correlations will
generally not be effective in correcting the measured RV values for the effects
of magnetic activity in the Hyades. We argue that this implies long timescale
activity variations (of order a few years; i.e., magnetic cycles or growth and
decay of plage regions) will not significantly hinder our search for planets in
the Hyades if the stars are closely monitored for chromospheric activity. The
trends in the RV scatter (sigma'_v) with , vsini, and P_rot for our stars
is generally consistent with those found in field stars in the Lick planet
search data, with the notable exception of a shallower dependence of sigma'_v
on for F stars.Comment: 15 pages, 7 figures, 3 tables; To appear in the July 2002 issue of
The Astronomical Journa
X-ray Activity on the Star-Planet Interaction Candidate HD 179949
We carry out detailed spectral and timing analyses of the X-ray
data of HD 179949, a prototypical example of a star with a close-in giant
planet with possible star-planet interaction (SPI) effects. We find a low
coronal abundance Fe/H0.2 relative to the solar photosphere, as well
as lower abundances of high FIP elements O/Fe 1, Ne/Fe
0.1, but with indications of higher abundances of N and Al. This star also has
an anomalous FIP bias of , larger than expected for
stars of this type. We detect significant intensity variability over time
scales ranging from 100~s - 10~ks, and also evidence for spectral variability
over time scales of 1-10~ks. We combine the flux measurements with
and measurements to detect periodicities, and determine
that the dominant signal is tied to the stellar polar rotational period,
consistent with expectations that the corona is rotational-pole dominated. We
also find evidence for periodicity at both the planetary orbital frequency and
at its beat frequency with the stellar polar rotational period, suggesting the
presence of a magnetic connection between the planet and the stellar pole. If
these periodicities represent an SPI signal, the lack of phase dependence in
coronal temperature or flaring suggests that the SPI in this system is driven
by a quasi-continuous form of heating (e.g., magnetic field stretching) rather
than a highly sporadic, hot, impulsive form (e.g., flare-like reconnection).Comment: 23 pages, 12 figures, 9 tables. Submitted to the Astrophysical
Journal. Comments welcome
Stellar Cycles in Fully Convective Stars and a New Interpretation of Dynamo Evolution
An dynamo, combining shear and cyclonic convection in the
tachocline, is believed to generate the solar cycle. However, this model cannot
explain cycles in fast rotators (with minimal shear) or in fully convective
stars (no tachocline); analysis of such stars could therefore provide key
insights into how these cycles work. We reexamine ASAS data for 15 M dwarfs, 11
of which are presumed fully convective; the addition of newer ASAS-SN data
confirms cycles in roughly a dozen of them, while presenting new or revised
rotation periods for five. The amplitudes and periods of these cycles follow
, with (where Ro is the Rossby number), very
similar to that we
find for 40 previously studied FGK stars, although
and are a factor of 20 smaller in the M stars. The very
different -Ro relationship seen here compared to
previous work suggests that two types of dynamo, with opposite Ro dependences,
operate in cool stars. Initially, a (likely or )
dynamo operates throughout the convective zone in mid-late M and fast rotating
FGK stars, but once magnetic breaking decouples the core and convective
envelope, a tachocline dynamo begins and eventually dominates in
older FGK stars. A change in in the tachocline dynamo generates the
fundamentally different -Ro relationship.Comment: 26 pages, 18 figures, submitted to ApJ. Figure sets will be available
in the final prin
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