473 research outputs found
On the Method to Infer an Atmosphere on a Tidally-Locked Super Earth Exoplanet and Upper limits to GJ 876d
We develop a method to infer or rule out the presence of an atmosphere on a
tidally-locked hot super Earth. The question of atmosphere retention is a
fundamental one, especially for planets orbiting M stars due to the star's
long-duration active phase and corresponding potential for stellar-induced
planetary atmospheric escape and erosion. Tidally-locked planets with no
atmosphere are expected to show a Lambertian-like thermal phase curve, causing
the combined light of the planet-star system to vary with planet orbital phase.
We report Spitzer 8 micron IRAC observations of GJ 876 taken over 32
continuous hours and reaching a relative photometric precision of 3.9e-04 per
point for 25.6 s time sampling. This translates to a 3 sigma limit of 5.13e-05
on a planet thermal phase curve amplitude. Despite the almost photon-noise
limited data, we are unable to conclusively infer the presence of an atmosphere
or rule one out on the non-transiting short-period super Earth GJ 876d. The
limiting factor in our observations was the miniscule, monotonic photometric
variation of the slightly active host M star, because the partial sine wave due
to the planet has a component in common with the stellar linear trend. The
proposed method is nevertheless very promising for transiting hot super Earths
with the James Webb Space Telescope and is critical for establishing
observational constraints for atmospheric escape.Comment: Published in Ap
Searching for Star-Planet interactions within the magnetosphere of HD 189733
HD 189733 is a K2 dwarf, orbited by a giant planet at 8.8 stellar radii. In
order to study magnetospheric interactions between the star and the planet, we
explore the large-scale magnetic field and activity of the host star.
We collected spectra using the ESPaDOnS and the NARVAL spectropolarimeters,
installed at the 3.6-m Canada-France-Hawaii telescope and the 2-m Telescope
Bernard Lyot at Pic du Midi, during two monitoring campaigns (June 2007 and
July 2008).
HD 189733 has a mainly toroidal surface magnetic field, having a strength
that reaches up to 40 G. The star is differentially rotating, with latitudinal
angular velocity shear of domega = 0.146 +- 0.049 rad/d, corresponding to
equatorial and polar periods of 11.94 +- 0.16 d and 16.53 +- 2.43 d
respectively. The study of the stellar activity shows that it is modulated
mainly by the stellar rotation (rather than by the orbital period or the beat
period between the stellar rotation and the orbital periods). We report no
clear evidence of magnetospheric interactions between the star and the planet.
We also extrapolated the field in the stellar corona and calculated the
planetary radio emission expected for HD 189733b given the reconstructed field
topology. The radio flux we predict in the framework of this model is time
variable and potentially detectable with LOFAR
Magnetic cycles of the planet-hosting star Tau Bootis: II. a second magnetic polarity reversal
In this paper, we present new spectropolarimetric observations of the
planet-hosting star Tau Bootis, using ESPaDOnS and Narval spectropolarimeters
at Canada-France-Hawaii Telescope (CFHT) and Telescope Bernard Lyot (TBL),
respectively. We detected the magnetic field of the star at three epochs in
2008. It is a weak magnetic field of only a few Gauss, oscillating between a
predominant toroidal component in January and a dominant poloidal component in
June and July. A magnetic polarity reversal was observed relative to the
magnetic topology in June 2007. This is the second such reversal observed in
two years on this star, suggesting that Tau Boo has a magnetic cycle of about 2
years. This is the first detection of a magnetic cycle for a star other than
the Sun. The role of the close-in massive planet in the short activity cycle of
the star is questioned.
Tau Boo has strong differential rotation, a common trend for stars with
shallow convective envelope. At latitude 40 deg., the surface layer of the star
rotates in 3.31 d, equal to the orbital period. Synchronization suggests that
the tidal effects induced by the planet may be strong enough to force at least
the thin convective envelope into corotation. Tau Boo shows variability in the
Ca H & K and Halpha throughout the night and on a night to night time scale. We
do not detect enhancement in the activity of the star that may be related to
the conjunction of the planet. Further data is needed to conclude about the
activity enhancement due to the planet.Comment: 9 pages, 5 figures, 3 tables Accepted to MNRA
MOST detects variability on tau Bootis possibly induced by its planetary companion
(abridged) There is considerable interest in the possible interaction between
parent stars and giant planetary companions in 51 Peg-type systems. We
demonstrate from MOST satellite photometry and Ca II K line emission that there
has been a persistent, variable region on the surface of tau Boo A which
tracked its giant planetary companion for some 440 planetary revolutions and
lies ~68deg (phi=0.8) in advance of the sub-planetary point. The light curves
are folded on a range of periods centered on the planetary orbital period and
phase dependent variability is quantified by Fourier methods and by the mean
absolute deviation (MAD) of the folded data for both the photometry and the Ca
II K line reversals. The region varies in brightness on the time scale of a
rotation by ~1 mmag. In 2004 it resembled a dark spot of variable depth, while
in 2005 it varied between bright and dark. Over the 123 planetary orbits
spanned by the photometry the variable region detected in 2004 and in 2005 are
synchronised to the planetary orbital period within 0.0015 d. The Ca II K line
in 2001, 2002 and 2003 also shows enhanced K-line variability centered on
phi=0.8, extending coverage to some 440 planetary revolutions. The apparently
constant rotation period of the variable region and its rapid variation make an
explanation in terms of conventional star spots unlikely. The lack of
complementary variability at phi=0.3 and the detection of the variable region
so far in advance of the sub-planetary point excludes tidal excitation, but the
combined photometric and Ca II K line reversal results make a good case for an
active region induced magnetically on the surface of tau Boo A by its planetary
companion.Comment: 7 pages, 7 figures; accepted for publication in A&
Magnetic field, differential rotation and activity of the hot-Jupiter hosting star HD 179949
HD 179949 is an F8V star, orbited by a giant planet at ~8 R* every 3.092514
days. The system was reported to undergo episodes of stellar activity
enhancement modulated by the orbital period, interpreted as caused by
Star-Planet Interactions (SPIs). One possible cause of SPIs is the large-scale
magnetic field of the host star in which the close-in giant planet orbits.
In this paper we present spectropolarimetric observations of HD 179949 during
two observing campaigns (2009 September and 2007 June). We detect a weak
large-scale magnetic field of a few Gauss at the surface of the star. The field
configuration is mainly poloidal at both observing epochs. The star is found to
rotate differentially, with a surface rotation shear of dOmega=0.216\pm0.061
rad/d, corresponding to equatorial and polar rotation periods of 7.62\pm0.07
and 10.3\pm0.8 d respectively. The coronal field estimated by extrapolating the
surface maps resembles a dipole tilted at ~70 degrees. We also find that the
chromospheric activity of HD 179949 is mainly modulated by the rotation of the
star, with two clear maxima per rotation period as expected from a highly
tilted magnetosphere. In September 2009, we find that the activity of HD 179949
shows hints of low amplitude fluctuations with a period close to the beat
period of the system.Comment: Accepted for publication in Monthly Notices of The Royal Astronomical
Societ
Magnetic cycles of the planet-hosting star tauBootis
We have obtained new spectropolarimetric observations of the planet-hosting
star tauBootis, using the ESPaDOnS and NARVAL spectropolarimeters at the
Canada-France-Hawaii Telescope and Telescope Bernard-Lyot. With this data set,
we are able to confirm the presence of a magnetic field at the surface of
tauBoo and map its large-scale structure over the whole star. The overall
polarity of the magnetic field has reversed with respect to our previous
observation (obtained a year before), strongly suggesting that tauBoo is
undergoing magnetic cycles similar to those of the Sun. This is the first time
that a global magnetic polarity switch is observed in a star other than the
Sun; we speculate that the magnetic cycle period of tauBoo is much shorter than
that of the Sun.
Our new data also allow us to confirm the presence of differential rotation
from the latitudinal shearing that the magnetic structure is undergoing. The
differential rotation surface shear that tauBoo experiences is found to be 6 to
10 times larger than that of the Sun. We propose that the short magnetic cycle
period is due to the strong level of differential rotation. With a rotation
period of 3.0 and 3.9 d at the equator and pole respectively, tauBoo appears as
the first planet-hosting star whose rotation (at intermediate latitudes) is
synchronised with the orbital motion of its giant planet (period 3.3 d).
Assuming that this synchronisation is not coincidental, it suggests that the
tidal effects induced by the giant planet can be strong enough to force the
thin convective enveloppe (though not the whole star) into corotation and thus
to play a role in the activity cycle of tauBoo.Comment: MNRAS, in pres
A detailed spectropolarimetric analysis of the planet hosting star WASP-12
The knowledge of accurate stellar parameters is paramount in several fields
of stellar astrophysics, particularly in the study of extrasolar planets, where
often the star is the only visible component and therefore used to infer the
planet's fundamental parameters. Another important aspect of the analysis of
planetary systems is the stellar activity and the possible star-planet
interaction. Here we present a self-consistent abundance analysis of the planet
hosting star WASP-12 and a high-precision search for a structured stellar
magnetic field on the basis of spectropolarimetric observations obtained with
the ESPaDOnS spectropolarimeter. Our results show that the star does not have a
structured magnetic field, and that the obtained fundamental parameters are in
good agreement with what was previously published. In addition we derive
improved constraints on the stellar age (1.0-2.65 Gyr), mass (1.23-1.49 M/M0),
and distance (295-465 pc). WASP-12 is an ideal object to look for pollution
signatures in the stellar atmosphere. We analyse the WASP-12 abundances as a
function of the condensation temperature and compare them with those published
by several other authors on planet hosting and non-planet hosting stars. We
find hints of atmospheric pollution in WASP-12's photosphere, but are unable to
reach firm conclusions with our present data. We conclude that a differential
analysis based on WASP-12 twins will probably clarify if an atmospheric
pollution is present, the nature of this pollution and its implications in the
planet formation and evolution. We attempt also the direct detection of the
circumstellar disk through infrared excess, but without success.Comment: 49 pages, 11 figures, Accepted for publication on Ap
The magnetic field of the planet-hosting star Bootis
We have obtained high resolution spectropolarimetric data for the
planet-hosting star Bootis, using the ESPaDOnS spectropolarimeter at
CFHT. A weak but clear Stokes signature is detected on three of the four
nights of June 2006 during which we have recorded data. This polarimetric
signature indicates with no ambiguity the presence of a magnetic field at the
star's surface, with intensity of just a few Gauss.
The analysis of the photospheric lines of Boo at ultra-high
signal-to-noise reveals the presence of an 18% relative differential rotation.
Tentative Zeeman-Doppler imaging, using our spectropolarimetric observations
covering only a fraction of the star's rotational phase, indicates a magnetic
field with a dominant potential field component. The data are best fitted when
a 3.1d period of modulation and an intermediate inclination are assumed.
Considering the level of differential rotation of Boo, this implies a
rotation period of 3.0d at the equator and of 3.7d at the pole, and a topology
of the magnetic field where its main non-axisymmetric part is located at low
latitudes.
The planet is probably synchronised with the star's rotation at intermediate
latitudes, while the non-axisymmetric part of the magnetic field seems located
at lower latitudes. Our limited data do not provide sufficient constraints on
the magnetic field to study a possible interaction of the planet with the
star's magnetosphere. Investigating this issue will require data with much
better phase coverage. Similar studies should also be performed for other stars
hosting close-in giant planets.Comment: 6 pages, 4 figures, accepted by MNRA
The On/Off Nature of Star-Planet Interactions
Evidence suggesting an observable magnetic interaction between a star and its
hot Jupiter appears as a cyclic variation of stellar activity synchronized to
the planet's orbit. In this study, we monitored the chromospheric activity of 7
stars with hot Jupiters using new high-resolution echelle spectra collected
with ESPaDOnS over a few nights in 2005 and 2006 from the CFHT. We searched for
variability in several stellar activity indicators (Ca II H, K, the Ca II
infrared triplet, Halpha, and He I). HD 179949 has been observed almost every
year since 2001. Synchronicity of the Ca II H & K emission with the orbit is
clearly seen in four out of six epochs, while rotational modulation with
P_rot=7 days is apparent in the other two seasons. We observe a similar
phenomenon on upsilon And, which displays rotational modulation (P_rot=12 days)
in September 2005, in 2002 and 2003 variations appear to correlate with the
planet's orbital period. This on/off nature of star-planet interaction (SPI) in
the two systems is likely a function of the changing stellar magnetic field
structure throughout its activity cycle. Variability in the transiting system
HD 189733 is likely associated with an active region rotating with the star,
however, the flaring in excess of the rotational modulation may be associated
with its hot Jupiter. As for HD 179949, the peak variability as measured by the
mean absolute deviation for both HD 189733 and tau Boo leads the sub-planetary
longitude by 70 degrees. The tentative correlation between this activity and
the ratio of Mpsini to the planet's rotation period, a quantity proportional to
the hot Jupiter's magnetic moment, first presented in Shkolnik et al. 2005
remains viable. This work furthers the characterization of SPI, improving its
potential as a probe of extrasolar planetary magnetic fields.Comment: Accepted for publication in the Astrophysical Journa
Low-Mass Eclipsing Binaries in the Initial Kepler Data Release
We identify 231 objects in the newly released Cycle 0 dataset from the Kepler
Mission as double-eclipse, detached eclipsing binary systems with Teff < 5500 K
and orbital periods shorter than ~32 days. We model each light curve using the
JKTEBOP code with a genetic algorithm to obtain precise values for each system.
We identify 95 new systems with both components below 1.0 M_sun and eclipses of
at least 0.1 magnitudes, suitable for ground-based follow-up. Of these, 14 have
periods less than 1.0 day, 52 have periods between 1.0 and 10.0 days, and 29
have periods greater than 10.0 days. This new sample of main-sequence,
low-mass, double-eclipse, detached eclipsing binary candidates more than
doubles the number of previously known systems, and extends the sample into the
completely heretofore unexplored P > 10.0 day period regime. We find
preliminary evidence from these systems that the radii of low-mass stars in
binary systems decrease with period. This supports the theory that binary
spin-up is the primary cause of inflated radii in low-mass binary systems,
although a full analysis of each system with radial-velocity and multi-color
light curves is needed to fully explore this hypothesis. As well, we present 7
new transiting planet candidates that do not appear among the recently released
list of 706 candidates by the Kepler team, nor in the Kepler False Positive
Catalog, along with several other new and interesting systems. We also present
novel techniques for the identification, period analysis, and modeling of
eclipsing binaries.Comment: 22 pages in emulateapj format. 9 figures, 4 tables, 2 appendices.
Accepted to AJ. Includes a significant addition of new material since last
arXiv submission and an updated method for estimating masses and radi
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