4,657 research outputs found
Long-term radial-velocity variations of the Sun as a star: The HARPS view
Stellar radial velocities play a fundamental role in the discovery of
extrasolar planets and the measurement of their physical parameters as well as
in the study of stellar physical properties. We investigate the impact of the
solar activity on the radial velocity of the Sun using the HARPS spectrograph
to obtain measurements that can be directly compared with those acquired in the
extrasolar planet search programs. We use the Moon, the Galilean satellites,
and several asteroids as reflectors to measure the radial velocity of the Sun
as a star and correlate it with disc-integrated chromospheric and magnetic
indexes of solar activity that are similar to stellar activity indexes. We
discuss in detail the systematic effects that affect our measurements and the
methods to account for them. We find that the radial velocity of the Sun as a
star is positively correlated with the level of its chromospheric activity at
about 95 percent significance level. The amplitude of the long-term variation
measured in the 2006-2014 period is 4.98 \pm 1.44 m/s, in good agreement with
model predictions. The standard deviation of the residuals obtained by
subtracting a linear best fit is 2.82 m/s and is due to the rotation of the
reflecting bodies and the intrinsic variability of the Sun on timescales
shorter than the activity cycle. A correlation with a lower significance is
detected between the radial velocity and the mean absolute value of the
line-of-sight photospheric magnetic field flux density. Our results confirm
similar correlations found in other late-type main-sequence stars and provide
support to the predictions of radial velocity variations induced by stellar
activity based on current models.Comment: 11 pages, 7 figures, 2 tables, 1 Appendix; accepted by Astronomy and
Astrophysic
III.2 Activity modelling and impact on planet’s parameters
This book is dedicated to all the people interested in the CoRoT mission and the beautiful data that were delivered during its six year duration. Either amateurs, professional, young or senior researchers, they will find treasures not only at the time of this publication but also in the future twenty or thirty years. It presents the data in their final version, explains how they have been obtained, how to handle them, describes the tools necessary to understand them, and where to find them. It also highlights the most striking first results obtained up to now. CoRoT has opened several unexpected directions of research and certainly new ones still to be discovered
Space Velocities of L- and T-type Dwarfs
(Abridged) We have obtained radial velocities of a sample of 18 ultracool
dwarfs (M6.5-T8) using high-resolution, near-infrared spectra obtained with
NIRSPEC and the Keck II telescope. We have confirmed that the radial velocity
of Gl 570 D is coincident with that of the K-type primary star Gl 570 A, thus
providing additional support for their true companionship. The presence of
planetary-mass companions around 2MASS J05591914-1404488 (T4.5V) has been
analyzed using five NIRSPEC radial velocity measurements obtained over a period
of 4.37 yr. We have computed UVW space motions for a total of 21 L and T dwarfs
within 20 pc of the Sun. This population shows UVW velocities that nicely
overlap the typical kinematics of solar to M-type stars within the same spatial
volume. However, the mean Galactic (44.2 km/s) and tangential (36.5 km/s)
velocities of the L and T dwarfs appear to be smaller than those of G to M
stars. A significant fraction (~40%) of the L and T dwarfs lies near the Hyades
moving group (0.4-2 Gyr), which contrasts with the 10-12% found for
earlier-type stellar neighbors. Additionally, the distributions of all three
UVW components (sigma_{UVW} = 30.2, 16.5, 15.8 km/s) and the distributions of
the total Galactic (sigma_{v_tot} = 19.1 km/s) and tangential (sigma_{v_t} =
17.6 km/s) velocities derived for the L and T dwarf sample are narrower than
those measured for nearby G, K, and M-type stars, but similar to the
dispersions obtained for F stars. This suggests that, in the solar
neighborhood, the L- and T-type ultracool dwarfs in our sample (including brown
dwarfs) is kinematically younger than solar-type to early M stars with likely
ages in the interval 0.5-4 Gyr.Comment: Accepted for publication in Ap
Planets and Stellar Activity: Hide and Seek in the CoRoT-7 system
Since the discovery of the transiting super-Earth CoRoT-7b, several
investigations have yielded different results for the number and masses of
planets present in the system, mainly owing to the star's high level of
activity. We re-observed CoRoT-7 in January 2012 with both HARPS and CoRoT, so
that we now have the benefit of simultaneous radial-velocity and photometric
data. This allows us to use the off-transit variations in the star's light
curve to estimate the radial-velocity variations induced by the suppression of
convective blueshift and the flux blocked by starspots. To account for
activity-related effects in the radial-velocities which do not have a
photometric signature, we also include an additional activity term in the
radial-velocity model, which we treat as a Gaussian process with the same
covariance properties (and hence the same frequency structure) as the light
curve. Our model was incorporated into a Monte Carlo Markov Chain in order to
make a precise determination of the orbits of CoRoT-7b and CoRoT-7c. We measure
the masses of planets b and c to be 4.73 +/- 0.95 Mearth and 13.56 +/- 1.08
Mearth, respectively. The density of CoRoT-7b is (6.61 +/- 1.72)(Rp/1.58
Rearth)^(-3) g.cm^(-3), which is compatible with a rocky composition. We search
for evidence of an additional planet d, identified by previous authors with a
period close to 9 days. We are not able to confirm the existence of a planet
with this orbital period, which is close to the second harmonic of the stellar
rotation at around 7.9 days. Using Bayesian model selection we find that a
model with two planets plus activity-induced variations is most favoured.Comment: Accepted 2014 July 2. Received 2014 June 30; in original form 2013
May 30 (17 pages, 9 figures
Predicting convective blueshift and radial-velocity dispersion due to granulation for FGK stars
To detect Earth-mass planets using the Doppler method, a major obstacle is to
differentiate the planetary signal from intrinsic stellar variability (e.g.,
pulsations, granulation, spots and plages). Convective blueshift, which results
from small-scale convection at the surface of Sun-like stars, is relevant for
Earth-twin detections as it exhibits Doppler noise on the order of 1 m/s. Here,
we present a simple model for convective blueshift based on fundamental
equations of stellar structure. Our model successfully matches observations of
convective blueshift for FGK stars. Based on our model, we also compute the
intrinsic noise floor for stellar granulation in the radial velocity
observations. We find that for a given mass range, stars with higher
metallicities display lower radial-velocity dispersion due to granulation, in
agreement with MHD simulations. We also provide a set of formulae to predict
the amplitude of radial-velocity dispersion due to granulation as a function of
stellar parameters. Our work is vital in identifying the most amenable stellar
targets for EPRV surveys and radial velocity follow-up programmes for TESS,
CHEOPS, and the upcoming PLATO mission.Comment: 11 pages, 5 figures, 3 tables. Submitted, under revie
The Upper Limit for CH4 in the Protostellar Disk toward HL Tauri
We used high-resolution infrared spectra of the heavily embedded T Tauri star HL Tau to search for evidence of absorption due to the R0, R1, and R2 gas-phase CH4 ν3 lines near 3.3 μm. From this, we report a 3 σ upper limit of 1.3 × 1015 cm-2 for the CH4 gas column density toward HL Tau. Our results are compared to those found for CO gas toward this source and to the recent model for chemistry in the inner (10 AU) disks around T Tauri stars by Markwick et al. We find that the upper limit of methane ice+gas column density toward HL Tau, when compared to CO, is somewhat lower than but consistent with that measured toward other interstellar sources (~1%) but that it is much lower than that predicted in the Markwick et al. model and much less than the CH4/CO ratio (10%-80%) found in cometary volatiles. This has important implications for the processing of interstellar material and its incorporation into planetary bodies
An Accurate Mass Determination for Kepler-1655b, a Moderately Irradiated World with a Significant Volatile Envelope
We present the confirmation of a small, moderately irradiated (F = 155 ± 7 F⊕) Neptune with a substantial gas envelope in a P = 11.8728787 ± 0.0000085 day orbit about a quiet, Sun-like G0V star Kepler-1655. Based on our analysis of the Kepler light curve, we determined Kepler-1655b's radius to be 2.213 ± 0.082 R⊕. We acquired 95 high-resolution spectra with Telescopio Nazionale Galileo/HARPS-N, enabling us to characterize the host star and determine an accurate mass for Kepler-1655b of 5.0±^(3.1)_(2.8) M⊕ via Gaussian-process regression. Our mass determination excludes an Earth-like composition with 98% confidence. Kepler-1655b falls on the upper edge of the evaporation valley, in the relatively sparsely occupied transition region between rocky and gas-rich planets. It is therefore part of a population of planets that we should actively seek to characterize further
The incidence and risk factors for new onset atrial fibrillation in the PROSPER study
Aims Atrial fibrillation/flutter (AF) is the most common arrhythmia in older people. It associates with reduced exercise capacity, increased risk of stroke, and mortality. We aimed to determine retrospectively whether pravastatin reduces the incidence of AF and whether any electrocardiographic measures or clinical conditions might be risk factors for its development. Methods and results The PROspective Study of Pravastatin in the Elderly at Risk (PROSPER) was a randomized, double-blind controlled trial that recruited 5804 individuals aged 70-82 years with a history of, or risk factors for, vascular disease. A total of 2891 were allocated to pravastatin and 2913 to placebo; mean follow-up was 3.2 years. Electrocardiograms (ECGs), which were recorded at baseline, annually thereafter, and at run-out, were processed by computer and reviewed manually. In all, 264 of 2912 (9.1%) of the placebo group and 283 of 2888 (9.8%) of the pravastatin-treated group developed AF [hazard ratio 1.08 (0.92,1.28), P = 0.35)]. Multivariate analysis showed that PR and QTc intervals, age, left ventricular hypertrophy, and ST-T abnormalities were related to development of AF after adjustment for many variables including alcohol consumption, which itself was univariately predictive of developing AF. Previous myocardial infarction on the ECG was not a risk factor. A history of vascular disease was strongly linked with developing AF but not diabetes and hypertension. Conclusion Pravastatin does not reduce the incidence of AF in older people at risk of vascular disease, at least in the short-medium term. Risk factors for AF include older age, prolongation of PR or QTc intervals, left ventricular hypertrophy, and ST-T abnormalities on the EC
Evaluation of children's centres in England: strand 2 - baseline survey of families using children's centres in the most disadvantaged areas
This is the final version of the report. Available from the Department for Education via the link in this recorThe evaluation of children’s centres in England is a large scale, six year study that looks at Sure Start children’s centres (SSCCs) in the most disadvantaged areas of England. These are centres that were set up in the first two phases of the programme.
The evaluation, from a survey of families who were using children’s centres when their child was 9- to 18-months-old, will provide a very detailed picture of children’s centre services. This includes how effective they are when they use different approaches in their management and when delivering services and activities for parents and children. It also looks at the cost of delivering different types of services
- …