85 research outputs found
Time evolution and rotation of starspots on CoRoT-2 from the modelling of transit photometry
CoRoT-2, the second planet-hosting star discovered by the CoRoT satellite, is
a young and active star. A total of 77 transits were observed for this system
over a period of 135 days. Small modulations detected in the optical light
curve of the planetary transits are used to study the position, size,
intensity, and temporal evolution of the photospheric spots on the surface of
the star that are occulted by the planetary disk. We apply a spot model to
these variations and create a spot map of the stellar surface of CoRoT-2 within
the transit band for every transit. From these maps, we estimate the stellar
rotation period and obtain the longitudes of the spots in a reference frame
rotating with the star. Moreover, the spots temporal evolution is determined.
This model achieves a spatial resolution of 2\circ. Mapping of 392 spots vs.
longitude indicates the presence of a region free of spots, close to the
equator, reminiscent of the coronal holes observed on the Sun during periods of
maximum activity. With this interpretation, the stellar rotation period within
the transit latitudes of -14.\circ 6 \pm 10 \circ is found to be 4.48 days.
This rotation period is shorter than the 4.54 days as derived from the
out-of-transit light modulation. Since the transit data samples a region close
to the stellar equator, while the period determined from out-of-transit data
reflects the average rotation of the star, this is taken as an indication of a
latitudinal differential rotation of about 3% or 0.042 rad/d.Comment: 8 pages, 12 figure
Properties of starspots on CoRoT-2
As a planet eclipses its parent star, a dark spot on the surface of the star
may be occulted, causing a detectable variation in the light curve. A total of
77 consecutive transit light curves of CoRoT-2 were observed with a high
temporal resolution of 32 s, corresponding to an uninterrupted period of 134
days. By analyzing small intensity variations in the transit light curves, it
was possible to detect and characterize spots at fixed positions (latitude and
longitude) on the surface of the star. The model used simulates planetary
transits and enables the inclusion of spots on the stellar surface with
different sizes, intensities (i.e. temperatures), and positions. Fitting the
data by this model, it is possible to infer the spots physical characteristics.
The fits were either in spot longitude and radius, with a fixed intensity, or
in spots longitude and intensity, for spots of constant size. Before the
modeling of the spots were performed, the planetary radius relative to the star
radius was estimated by fitting the deepest transit to minimize the effect of
spots. A slightly larger (3%) radius, 0.172 Rstar, resulted instead of the
previously reported 0.1667 Rstar . The fitting of the transits yield spots, or
spot groups, with sizes of ranging from 0.2 to 0.7 planet radius, Rp, with a
mean of (0.41 +/- 0.13) Rp (~100,000 km), resulting in a stellar area covered
by spots within the transit latitudes of 10-20%. The intensity varied from 0.4
to 0.9 of the disk center intensity, Ic, with a mean of (0.60 +/- 0.19) Ic,
which can be converted to temperature by assuming an effective temperature of
5625 K for the stellar photosphere, the spots temperature ranges mainly from
3600 to 5000 K. The results from the spot modeling are in agreement with those
found for magnetic activity analysis from out of transit data of the same star.Comment: 7 pages, 11 figure
Association of radio polar cap brightening with bright patches and coronal holes
Radio-bright regions near the solar poles are frequently observed in Nobeyama
Radioheliograph (NoRH) maps at 17 GHz, and often in association with coronal
holes. However, the origin of these polar brightening has not been established
yet. We propose that small magnetic loops are the source of these bright
patches, and present modeling results that reproduce the main observational
characteristics of the polar brightening within coronal holes at 17 GHz. The
simulations were carried out by calculating the radio emission of the small
loops, with several temperature and density profiles, within a 2D coronal hole
atmospheric model. If located at high latitudes, the size of the simulated
bright patches are much smaller than the beam size and they present the
instrument beam size when observed. The larger bright patches can be generated
by a great number of small magnetic loops unresolved by the NoRH beam. Loop
models that reproduce bright patches contain denser and hotter plasma near the
upper chromosphere and lower corona. On the other hand, loops with increased
plasma density and temperature only in the corona do not contribute to the
emission at 17 GHz. This could explain the absence of a one-to-one association
between the 17 GHz bright patches and those observed in extreme ultraviolet.
Moreover, the emission arising from small magnetic loops located close to the
limb may merge with the usual limb brightening profile, increasing its
brightness temperature and width.Comment: 8 pages, 6 figures, 1 table. Accepted for publication in The
Astrophysical Journa
The 6 September 2017 X9 super flare observed from submillimeter to mid-IR
Active Region 12673 is the most productive active region of solar cycle 24: in a few days of early September 2017, four Xâclass and 27 Mâclass flares occurred. SOL2017â09â06T12:00, an X9.3 flare also produced a twoâribbon white light emission across the sunspot detected by Solar Dynamics Orbiter/Helioseismic and Magnetic Imager. The flare was observed at 212 and 405 GHz with the arcminuteâsized beams of the Solar Submillimeter Telescope focal array while making a solar map and at 10 ÎŒm, with a 17 arcsec diffractionâlimited infrared camera. Images at 10 ÎŒm revealed that the sunspot gradually increased in brightness while the event proceeded, reaching a temperature similar to quiet Sun values. From the images we derive a lower bound limit of 180âK flare peak excess brightness temperature or 7,000 sfu if we consider a similar size as the white light source. The rising phase of midâIR and white light is similar, although the latter decays faster, and the maximum of the midâIR and white light emission is âŒ200 s delayed from the 15.4âGHz peak occurrence. The submillimeter spectrum has a different origin than that of microwaves from 1 to 15 GHz, although it is not possible to draw a definitive conclusion about its emitting mechanism
Photospheric activity, rotation, and star-planet interaction of the planet-hosting star CoRoT-6
The CoRoT satellite has recently discovered a hot Jupiter that transits
across the disc of a F9V star called CoRoT-6 with a period of 8.886 days. We
model the photospheric activity of the star and use the maps of the active
regions to study stellar differential rotation and the star-planet interaction.
We apply a maximum entropy spot model to fit the optical modulation as observed
by CoRoT during a uninterrupted interval of about 140 days. Photospheric active
regions are assumed to consist of spots and faculae in a fixed proportion with
solar-like contrasts. Individual active regions have lifetimes up to 30-40
days. Most of them form and decay within five active longitudes whose different
migration rates are attributed to the stellar differential rotation for which a
lower limit of \Delta \Omega / \Omega = 0.12 \pm 0.02 is obtained. Several
active regions show a maximum of activity at a longitude lagging the
subplanetary point by about 200 degrees with the probability of a chance
occurrence being smaller than 1 percent. Our spot modelling indicates that the
photospheric activity of CoRoT-6 could be partially modulated by some kind of
star-planet magnetic interaction, while an interaction related to tides is
highly unlikely because of the weakness of the tidal force.Comment: 9 pages, 7 figures, accepted to Astronomy & Astrophysic
A Tentative Detection of a Starspot During Consecutive Transits of an Extrasolar Planet from the Ground: No Evidence of a Double Transiting Planet System Around TrES-1
There have been numerous reports of anomalies during transits of the planet
TrES-1b. Recently, Rabus and coworkers' analysis of HST observations lead them
to claim brightening anomalies during transit might be caused by either a
second transiting planet or a cool starspot. Observations of two consecutive
transits are presented here from the University of Arizona's 61-inch Kuiper
Telescope on May 12 and May 15, 2008 UT. A 5.4 +/- 1.7 mmag (0.54 +/- 0.17%)
brightening anomaly was detected during the first half of the transit on May 12
and again in the second half of the transit on May 15th. We conclude that this
is a tentative detection of a r greater than or equal to 6 earth radii starspot
rotating on the surface of the star. We suggest that all evidence to date
suggest TrES-1 has a spotty surface and there is no need to introduce a second
transiting planet in this system to explain these anomalies. We are only able
to constrain the rotational period of the star to 40.2 +22.9 -14.6 days, due to
previous errors in measuring the alignment of the stellar spin axis with the
planetary orbital axis. This is consistent with the previously observed P_obs =
33.2 +22.3 -14.3 day period. We note that this technique could be applied to
other transiting systems for which starspots exist on the star in the transit
path of the planet in order to constrain the rotation rate of the star.
(abridged)Comment: 21 pages, 3 tables, 6 figures, Accepted to Ap
Submillimeter and X-ray observations of an X Class flare
The GOES X1.5 class flare that occurred on August 30,2002 at 1327:30 UT is
one of the few events detected so far at submillimeter wavelengths. We present
a detailed analysis of this flare combining radio observations from 1.5 to 212
GHz (an upper limit of the flux is also provided at 405 GHz) and X-ray.
Although the observations of radio emission up to 212 GHz indicates that
relativistic electrons with energies of a few MeV were accelerated, no
significant hard X-ray emission was detected by RHESSI above ~ 250 keV. Images
at 12--20 and 50--100 keV reveal a very compact, but resolved, source of about
~ 10" x 10". EUV TRACE images show a multi-kernel structure suggesting a
complex (multipolar) magnetic topology. During the peak time the radio spectrum
shows an extended flatness from ~ 7 to 35 GHz. Modeling the optically thin part
of the radio spectrum as gyrosynchrotron emission we obtained the electron
spectrum (spectral index delta, instantaneous number of emitting electrons). It
is shown that in order to keep the expected X-ray emission from the same
emitting electrons below the RHESSI background at 250 keV, a magnetic field
above 500 G is necessary. On the other hand, the electron spectrum deduced from
radio observations >= 50 GHz is harder than that deduced from ~ 70 - 250 keV
X-ray data, meaning that there must exist a breaking energy around a few
hundred keV. During the decay of the impulsive phase, a hardening of the X-ray
spectrum is observed which is interpreted as a hardening of the electron
distribution spectrum produced by the diffusion due to Coulomb collisions of
the trapped electrons in a medium with an electron density of n_e ~ 3E10 - 5E10
cm-3.Comment: Accpeted in Astronomy & Astrophysics. 9 Pages, 6 Figures ADDED
REFERENCE
Statistical analysis of the onset temperature of solar flares in 2010-2011
Understanding the physical processes that trigger solar flares is paramount
to help with forecasting space weather and mitigating the effects on our
technological infrastructure. A previously unknown phenomenon was recently
identified in solar flares: the plasma temperature, derived from soft X-ray
(SXR) data, at the onset of four flares, was revealed to be in the range 10-15
MK, without evidence of gradual heating. To investigate how common the
hot-onset phenomenon may be, we extend this investigation to solar flares of
B1.2- X6.9 classes recorded by the X-ray Sensor (XRS) on-board the GOES-14 and
GOES-15 satellites between 2010 and 2011. For this statistical study, we
employed the same methodology as in recent work, where the pre-flare SXR flux
of each flare is obtained manually, and the temperature and emission measure
values are obtained by the flux ratio of the two GOES/XRS channels using the
standard software. From 3224 events listed in the GOES flare catalog for
2010-2011, we have selected and analyzed 745 events for which the flare
heliographic location was provided in the list, to investigate center-to-limb
effects of the hot-onset phenomenon. Our results show that 559 out of 745
flares (75%) exhibit an onset temperature above 8.6 MK (the first quartile),
with respective log10 of the emission measure values between 46.0 - 47.25 cm-3,
indicating that small amounts of plasma are quickly heated to high
temperatures. These results suggest that the hot-onset phenomenon is very
common in solar flares.Comment: 6 pages,7 figure
Signals of exomoons in averaged light curves of exoplanets
The increasing number of transiting exoplanets sparked a significant interest
in discovering their moons. Most of the methods in the literature utilize
timing analysis of the raw light curves. Here we propose a new approach for the
direct detection of a moon in the transit light curves via the so called
Scatter Peak. The essence of the method is the valuation of the local scatter
in the folded light curves of many transits. We test the ability of this method
with different simulations: Kepler "short cadence", Kepler "long cadence",
ground-based millimagnitude photometry with 3-min cadence, and the expected
data quality of the planned ESA mission of PLATO. The method requires ~100
transit observations, therefore applicable for moons of 10-20 day period
planets, assuming 3-4-5 year long observing campaigns with space observatories.
The success rate for finding a 1 R_Earth moon around a 1 R_Jupiter exoplanet
turned out to be quite promising even for the simulated ground-based
observations, while the detection limit of the expected PLATO data is around
0.4 R_Earth. We give practical suggestions for observations and data reduction
to improve the chance of such a detection: (i) transit observations must
include out-of-transit phases before and after a transit, spanning at least the
same duration as the transit itself; (ii) any trend filtering must be done in
such a way that the preceding and following out-of-transit phases remain
unaffected.Comment: 9 pages, 5 figures, accepted for publication in MNRAS. Typos
correcte
The barycentric motion of exoplanet host stars: tests of solar spin-orbit coupling
Empirical evidence suggests a tantalising but unproven link between various
indicators of solar activity and the barycentric motion of the Sun. The latter
is exemplified by transitions between regular and more disordered motion
modulated by the motions of the giant planets, and rare periods of retrograde
motion with negative orbital angular momentum. An examination of the
barycentric motion of exoplanet host stars, and their stellar activity cycles,
has the potential of proving or disproving the Sun's motion as an underlying
factor in the complex patterns of short- and long-term solar variability
indices, by establishing whether such correlations exist in other planetary
systems. A variety of complex patterns of barycentric motions of exoplanet host
stars is demonstrated, depending on the number, masses and orbits of the
planets. Each of the behavioural types proposed to correlate with solar
activity are also evident in exoplanet host stars: repetitive patterns
influenced by massive multiple planets, epochs of rapid change in orbital
angular momentum, and intervals of negative orbital angular momentum. The study
provides the basis for independent investigations of the widely-studied but
unproven suggestion that the Sun's motion is somehow linked to various
indicators of solar activity. We show that, because of the nature of their
barycentric motions, the host stars HD168443 and HD74156 offer particularly
powerful tests of this hypothesis.Comment: 7 pages, 3 figures. Accepted for publication in A&
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