158 research outputs found
First Evidence of a Retrograde Orbit of Transiting Exoplanet HAT-P-7b
We present the first evidence of a retrograde orbit of the transiting
exoplanet HAT-P-7b. The discovery is based on a measurement of the
Rossiter-McLaughlin effect with the Subaru HDS during a transit of HAT-P-7b,
which occurred on UT 2008 May 30. Our best-fit model shows that the spin-orbit
alignment angle of this planet is \lambda = -132.6 (+10.5, -16.3) degrees. The
existence of such a retrograde planet have been predicted by recent planetary
migration models considering planet-planet scattering processes or the Kozai
migration. Our finding provides an important milestone that supports such
dynamic migration theories.Comment: PASJ Letters, in press [13 pages
Spin-Orbit Alignment of the TrES-4 Transiting Planetary System and Possible Additional Radial Velocity Variation
We report new radial velocities of the TrES-4 transiting planetary system,
including observations of a full transit, with the High Dispersion Spectrograph
of the Subaru 8.2m telescope. Modeling of the Rossiter-McLaughlin effect
indicates that TrES-4b has closely aligned orbital and stellar spin axes, with
. The close spin-orbit alignment angle
of TrES-4b seems to argue against a migration history involving planet-planet
scattering or Kozai cycles, although there are two nearby faint stars that
could be binary companion candidates. Comparison of our out-of-transit data
from 4 different runs suggest that the star exhibits radial velocity
variability of 20 ms^-1 in excess of a single Keplerian orbit. Although
the cause of the excess radial velocity variability is unknown, we discuss
various possibilities including systematic measurement errors, starspots or
other intrinsic motions, and additional companions besides the transiting
planet.Comment: 10 pages, 3 figures, 3 tables, PASJ in pres
Wavelength Dependence of Activity-Induced Photometric Variations for Young Cool Stars in Hyades
We investigate photometric variations due to stellar activity which induce
systematic radial-velocity errors (so-called "jitter") for the four targets in
the Hyades open cluster observed by the K2 mission (EPIC 210721261, EPIC
210923016, EPIC 247122957, and EPIC 247783757). Applying Gaussian process
regressions to the K2 light curves and the near-infrared (NIR) light curves
observed with the IRSF 1.4-m telescope, we derive the wavelength dependences of
the photometric signals due to stellar activity. To estimate the temporal
variations in the photometric variability amplitudes between the two
observation periods of K2 and IRSF, separated by more than 2 years, we analyze
a number of K2 targets in Hyades that have also been observed in Campaigns 4
and 13 and find a representative variation rate over 2 years of 38%pm71%.
Taking this temporal variation into account, we constrain projected sizes and
temperature contrast properties of the starspots in the stellar photosphere to
be approximately 10% and 0.95, respectively. These starspot properties can
induce relatively large differences in the variability amplitude over different
observational passbands, and we find that radial-velocity jitter may be more
suppressed in the NIR than previously expected. Our result supports profits of
on-going exoplanet search projects that are attempting to detect or confirm
young planets in open clusters via radial-velocity measurements in the NIR.Comment: 19 pages, 14 figures, 5 tables; Accepted for publication to A
Analytic Description of the Rossiter-McLaughlin Effect for Transiting Exoplanets: Cross-Correlation Method and Comparison with Simulated Data
We obtain analytical expressions for the velocity anomaly due to the
Rossiter- McLaughlin effect, for the case when the anomalous radial velocity is
obtained by cross-correlation with a stellar template spectrum. In the limit of
vanishing width of the stellar absorption lines, our result reduces to the
formula derived by Ohta et al. (2005), which is based on the first moment of
distorted stellar lines. Our new formula contains a term dependent on the
stellar linewidth, which becomes important when rotational line broadening is
appreciable. We generate mock transit spectra for four existing exoplanetary
systems (HD17156, TrES-2, TrES- 4, and HD209458) following the procedure of
Winn et al. (2005), and find that the new formula is in better agreement with
the velocity anomaly extracted from the mock data. Thus, our result provides a
more reliable analytical description of the velocity anomaly due to the
Rossiter-McLaughlin effect, and explains the previously observed dependence of
the velocity anomaly on the stellar rotation velocity.Comment: 31 pages, 9 figures, Astrophysical Journal in pres
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