41 research outputs found
Fingerprints of giant planets in the photospheres of Herbig stars
Around 2% of all A stars have photospheres depleted in refractory elements.
This is hypothesized to arise from a preferential accretion of gas rather than
dust, but the specific processes and the origin of the material -- circum- or
interstellar -- are not known. The same depletion is seen in 30% of young,
disk-hosting Herbig Ae/Be stars. We investigate whether the chemical
peculiarity originates in a circumstellar disk. Using a sample of systems for
which both the stellar abundances and the protoplanetary disk structure are
known, we find that stars hosting warm, flaring group I disks typically have
Fe, Mg and Si depletions of 0.5 dex compared to the solar-like abundances of
stars hosting cold, flat group II disks. The volatile, C and O, abundances in
both sets are identical. Group I disks are generally transitional, having
radial cavities depleted in millimetre-sized dust grains, while those of group
II are usually not. Thus we propose that the depletion of heavy elements
emerges as Jupiter-like planets block the accretion of part of the dust, while
gas continues to flow towards the central star. We calculate gas to dust ratios
for the accreted material and find values consistent with models of disk
clearing by planets. Our results suggest that giant planets of ~0.1 to 10 M_Jup
are hiding in at least 30% of Herbig Ae/Be disks.Comment: 5 pages, 3 figures, accepted for publication in A&A Letter
From solar to stellar corona: the role of wind, rotation and magnetism
Observations of surface magnetic fields are now within reach for many stellar
types thanks to the development of Zeeman-Doppler Imaging. These observations
are extremely useful for constraining rotational evolution models of stars, as
well as for characterizing the generation of magnetic field. We recently
demonstrated that the impact of coronal magnetic field topology on the
rotational braking of a star can be parametrized with a scalar parameter: the
open magnetic flux. However, without running costly numerical simulations of
the stellar wind, reconstructing the coronal structure of the large scale
magnetic field is not trivial. An alternative -broadly used in solar physics-
is to extrapolate the surface magnetic field assuming a potential field in the
corona, to describe the opening of the field lines by the magnetized wind. This
technique relies on the definition of a so-called source surface radius, which
is often fixed to the canonical value of 2.5Rsun. However this value likely
varies from star to star. To resolve this issue, we use our extended set of
2.5D wind simulations published in 2015, to provide a criteria for the opening
of field lines as well as a simple tool to assess the source surface radius and
the open magnetic flux. This allows us to derive the magnetic torque applied to
the star by the wind from any spectropolarimetric observation. We conclude by
discussing some estimations of spin-down time scales made using our technique,
and compare them to observational requirements.Comment: Accepted for publication in the Astrophysical Journa
KELT-9 and its ultra-hot Jupiter: stellar parameters, composition, and planetary pollution
KELT-9b is an ultra-hot Jupiter observed to be undergoing extreme mass loss.
Its A0-type host star has a radiative envelope, which makes its surface layers
prone to retaining recently accreted material. To search for potential signs of
planetary material polluting the stellar surface, we carry out the most
comprehensive chemical characterisation of KELT-9 to-date. New element
detections include Na and Y, which had previously been detected in the
ultra-hot Jupiter but not studied in the star; these detections complete the
set of nine elements measured in both star and planet. In comparing KELT-9 with
similar open cluster stars we find no strong anomalies. This finding is
consistent with calculations of photospheric pollution accounting for stellar
mixing and using observationally estimated KELT-9b mass loss rates. We also
rule out recent, short-lived intensive mass transfer such as the stellar
ingestion of an Earth-mass exomoon.Comment: 7 pages, 7 figures, accepted for publication in MNRA
First results of a magnetic survey of classical Cepheids
We report recent ESPaDOnS and HARPSpol spectropolarimetric observations from
our ongoing magnetic survey of the brightest twenty-five classical Cepheids.
Stokes magnetic signatures are detected in eight of fifteen targets
observed to date. The Stokes profiles show a diversity of morphologies with
weak associated longitudinal field measurements of order 1 G. Many of the
Stokes profiles are difficult to interpret in the context of the normal
Zeeman effect. They consist of approximately unipolar single or double lobe(s)
of positive or negative circular polarization. We hypothesize that these
unusual signatures are due to the Zeeman effect modified by atmospheric
velocity or magnetic field gradients. In contrast, the Stokes profiles of
Polaris and MY Pup appear qualitatively similar to the complex magnetic
signatures of non-pulsating cool supergiants, possibly due to the low pulsation
amplitudes of these two stars.Comment: 2 pages, 1 figure, to appear in Proceedings of the IAU, Symposium No.
361: Massive Stars Near and Far, 2022, N. St-Louis, J. S. Vink and J. Mackey,
ed
Magnetic field and wind of Kappa Ceti: towards the planetary habitability of the young Sun when life arose on Earth
We report magnetic field measurements for Kappa1~Cet, a proxy of the young
Sun when life arose on Earth. We carry out an analysis of the magnetic
properties determined from spectropolarimetric observations and reconstruct its
large-scale surface magnetic field to derive the magnetic environment, stellar
winds and particle flux permeating the interplanetary medium around Kappa1~Cet.
Our results show a closer magnetosphere and mass-loss rate of Mdot = 9.7 x
10^{-13} Msol/yr, i.e., a factor 50 times larger than the current solar wind
mass-loss rate, resulting in a larger interaction via space weather
disturbances between the stellar wind and a hypothetical young-Earth analogue,
potentially affecting the planet's habitability. Interaction of the wind from
the young Sun with the planetary ancient magnetic field may have affected the
young Earth and its life conditionsComment: 6 pages, 5 figures, Published at the Astrophysical Journal Letters
(ApJL): Manuscript #LET3358
The winds of young Solar-type stars in the Pleiades, AB Doradus, Columba and Pictoris
Solar-type stars, which shed angular momentum via magnetised stellar winds,
enter the main sequence with a wide range of rotational periods .
This initially wide range of rotational periods contracts and has mostly
vanished by a stellar age Gyr, after which Solar-type stars spin
according to the Skumanich relation .
Magnetohydrodynamic stellar wind models can improve our understanding of this
convergence of rotation periods. We present wind models of fifteen young
Solar-type stars aged from 24 Myr to 0.13 Gyr. With our previous wind models of
stars aged 0.26 Gyr and 0.6 Gyr we obtain thirty consistent three-dimensional
wind models of stars mapped with Zeeman-Doppler imaging - the largest such set
to date. The models provide good cover of the pre-Skumanich phase of stellar
spin-down in terms of rotation, magnetic field, and age. We find that the mass
loss rate with a residual spread of 150% and
that the wind angular momentum loss rate with a residual spread of 500% where is the unsigned
surface magnetic flux. When comparing different magnetic field scalings for
each single star we find a gradual reduction in the power-law exponent with
increasing magnetic field strength.Comment: 19 pages, 7 figure
Estimating magnetic filling factors from Zeeman–Doppler magnetograms
V.S., S.P.M., and A.J.F.acknowledge funding from the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation programme (grant agreement No. 682393 AWESoMeStars). S.B.S. acknowledges funding via the Austrian Space Application Programme (ASAP) of the Austrian Research Promotion Agency (FFG) within ASAP11, the FWF NFN project S11601-N16 and the sub-project S11604-N16. A. A.V. acknowledges funding received from the Irish Research Council Laureate Awards 2017/2018.Low-mass stars are known to have magnetic fields that are believed to be of dynamo origin. Two complementary techniques are principally used to characterize them. Zeeman–Doppler imaging (ZDI) can determine the geometry of the large-scale magnetic field while Zeeman broadening can assess the total unsigned flux including that associated with small-scale structures such as spots. In this work, we study a sample of stars that have been previously mapped with ZDI. We show that the average unsigned magnetic flux follows an activity-rotation relation separating into saturated and unsaturated regimes. We also compare the average photospheric magnetic flux recovered by ZDI, BV, with that recovered by Zeeman broadening studies, BI. In line with previous studies, BV ranges from a few % to ~20% of BI. We show that a power-law relationship between BV and BI exists and that ZDI recovers a larger fraction of the magnetic flux in more active stars. Using this relation, we improve on previous attempts to estimate filling factors, i.e., the fraction of the stellar surface covered with magnetic field, for stars mapped only with ZDI. Our estimated filling factors follow the well-known activity-rotation relation, which is in agreement with filling factors obtained directly from Zeeman broadening studies. We discuss the possible implications of these results for flux tube expansion above the stellar surface and stellar wind models.Publisher PDFPeer reviewe
Genome-Wide Association Study of Coronary Heart Disease and Its Risk Factors in 8,090 African Americans: The NHLBI CARe Project
Coronary heart disease (CHD) is the leading cause of mortality in African Americans. To identify common genetic polymorphisms associated with CHD and its risk factors (LDL- and HDL-cholesterol (LDL-C and HDL-C), hypertension, smoking, and type-2 diabetes) in individuals of African ancestry, we performed a genome-wide association study (GWAS) in 8,090 African Americans from five population-based cohorts. We replicated 17 loci previously associated with CHD or its risk factors in Caucasians. For five of these regions (CHD: CDKN2A/CDKN2B; HDL-C: FADS1-3, PLTP, LPL, and ABCA1), we could leverage the distinct linkage disequilibrium (LD) patterns in African Americans to identify DNA polymorphisms more strongly associated with the phenotypes than the previously reported index SNPs found in Caucasian populations. We also developed a new approach for association testing in admixed populations that uses allelic and local ancestry variation. Using this method, we discovered several loci that would have been missed using the basic allelic and global ancestry information only. Our conclusions suggest that no major loci uniquely explain the high prevalence of CHD in African Americans. Our project has developed resources and methods that address both admixture- and SNP-association to maximize power for genetic discovery in even larger African-American consortia