8 research outputs found
Unresolved Rossby and gravity modes in 214 A and F stars showing rotational modulation
Here we report an ensemble study of 214 A- and F-type stars observed by
\textit{Kepler}, exhibiting the so-called \textit{hump and spike} periodic
signal, explained by Rossby modes (r~modes) -- the \textit{hump} -- and
magnetic stellar spots or overstable convective (OsC) modes -- the
\textit{spike} -- respectively. We determine the power confined in the
non-resolved hump features and find additional gravity~modes (g~modes) humps
always occurring at higher frequencies than the spike. Furthermore, we derive
projected rotational velocities from FIES, SONG and HERMES spectra for 28 stars
and the stellar inclination angle for 89 stars. We find a strong correlation
between the spike amplitude and the power in the r and g~modes, which suggests
that both types of oscillations are mechanically excited by either stellar
spots or OsC modes. Our analysis suggests that stars with a higher power in
r~modes humps are more likely to also exhibit humps at higher azimuthal
orders ( = 2, 3, or 4). Interestingly, all stars that show g~modes humps are
hotter and more luminous than the observed red edge of the Scuti
instability strip, suggesting that either magnetic fields or convection in the
outer layers could play an important role.Comment: 18 pages, 19 figure
Rotational modulation in A and F stars: Magnetic stellar spots or convective core rotation?
The Kepler mission revealed a plethora of stellar variability in the light curves of many stars, some associated with magnetic activity or stellar oscillations. In this work, we analyse the periodic signal in 162 intermediate-mass stars, interpreted as Rossby modes and rotational modulation - the so-called hump & spike feature. We investigate whether the rotational modulation (spike) is due to stellar spots caused by magnetic fields or due to Overstable Convective (OsC) modes resonantly exciting g modes, with frequencies corresponding to the convective core rotation rate. Assuming that the spikes are created by magnetic spots at the stellar surface, we recover the amplitudes of the magnetic fields, which are in good agreement with theoretical predictions. Our data show a clear anti-correlation between the spike amplitudes and stellar mass and possibly a correlation with stellar age, consistent with the dynamo-generated magnetic fields theory in (sub)-surface convective layers. Investigating the harmonic behaviour, we find that for 125 stars neither of the two possible explanations can be excluded. While our results suggest that the dynamo-generated magnetic field scenario is more likely to explain the spike feature, we assess further work is needed to distinguish between the two scenarios. One method for ruling out one of the two explanations is to directly observe magnetic fields in hump & spike stars. Another would be to impose additional constraints through detailed modelling of our stars, regarding the rotation requirement in the OsC mode scenario or the presence of a convective-core (stellar age)
A Giant Planet Candidate Transiting a White Dwarf
Astronomers have discovered thousands of planets outside the solar system,
most of which orbit stars that will eventually evolve into red giants and then
into white dwarfs. During the red giant phase, any close-orbiting planets will
be engulfed by the star, but more distant planets can survive this phase and
remain in orbit around the white dwarf. Some white dwarfs show evidence for
rocky material floating in their atmospheres, in warm debris disks, or orbiting
very closely, which has been interpreted as the debris of rocky planets that
were scattered inward and tidally disrupted. Recently, the discovery of a
gaseous debris disk with a composition similar to ice giant planets
demonstrated that massive planets might also find their way into tight orbits
around white dwarfs, but it is unclear whether the planets can survive the
journey. So far, the detection of intact planets in close orbits around white
dwarfs has remained elusive. Here, we report the discovery of a giant planet
candidate transiting the white dwarf WD 1856+534 (TIC 267574918) every 1.4
days. The planet candidate is roughly the same size as Jupiter and is no more
than 14 times as massive (with 95% confidence). Other cases of white dwarfs
with close brown dwarf or stellar companions are explained as the consequence
of common-envelope evolution, wherein the original orbit is enveloped during
the red-giant phase and shrinks due to friction. In this case, though, the low
mass and relatively long orbital period of the planet candidate make
common-envelope evolution less likely. Instead, the WD 1856+534 system seems to
demonstrate that giant planets can be scattered into tight orbits without being
tidally disrupted, and motivates searches for smaller transiting planets around
white dwarfs.Comment: 50 pages, 12 figures, 2 tables. Published in Nature on Sept. 17,
2020. The final authenticated version is available online at:
https://www.nature.com/articles/s41586-020-2713-
A Possible Alignment Between the Orbits of Planetary Systems and their Visual Binary Companions
Astronomers do not have a complete picture of the effects of wide-binary companions (semimajor axes greater than 100 au) on the formation and evolution of exoplanets. We investigate these effects using new data from Gaia Early Data Release 3 and the Transiting Exoplanet Survey Satellite mission to characterize wide-binary systems with transiting exoplanets. We identify a sample of 67 systems of transiting exoplanet candidates (with well-determined, edge-on orbital inclinations) that reside in wide visual binary systems. We derive limits on orbital parameters for the wide-binary systems and measure the minimum difference in orbital inclination between the binary and planet orbits. We determine that there is statistically significant difference in the inclination distribution of wide-binary systems with transiting planets compared to a control sample, with the probability that the two distributions are the same being 0.0037. This implies that there is an overabundance of planets in binary systems whose orbits are aligned with those of the binary. The overabundance of aligned systems appears to primarily have semimajor axes less than 700 au. We investigate some effects that could cause the alignment and conclude that a torque caused by a misaligned binary companion on the protoplanetary disk is the most promising explanation
A giant planet candidate transiting a white dwarf
Astronomers have discovered thousands of planets outside the Solar System, most of which orbit stars that will eventually evolve into red giants and then into white dwarfs. During the red giant phase, any close-orbiting planets will be engulfed by the star, but more distant planets can survive this phase and remain in orbit around the white dwarf. Some white dwarfs show evidence for rocky material floating in their atmospheres, in warm debris disks or orbiting very closely, which has been interpreted as the debris of rocky planets that were scattered inwards and tidally disrupted. Recently, the discovery of a gaseous debris disk with a composition similar to that of ice giant planets demonstrated that massive planets might also find their way into tight orbits around white dwarfs, but it is unclear whether these planets can survive the journey. So far, no intact planets have been detected in close orbits around white dwarfs. Here we report the observation of a giant planet candidate transiting the white dwarf WD 1856+534 (TIC 267574918) every 1.4 days. We observed and modelled the periodic dimming of the white dwarf caused by the planet candidate passing in front of the star in its orbit. The planet candidate is roughly the same size as Jupiter and is no more than 14 times as massive (with 95 per cent confidence). Other cases of white dwarfs with close brown dwarf or stellar companions are explained as the consequence of common-envelope evolution, wherein the original orbit is enveloped during the red giant phase and shrinks owing to friction. In this case, however, the long orbital period (compared with other white dwarfs with close brown dwarf or stellar companions) and low mass of the planet candidate make common-envelope evolution less likely. Instead, our findings for the WD 1856+534 system indicate that giant planets can be scattered into tight orbits without being tidally disrupted, motivating the search for smaller transiting planets around white dwarfs
A Possible Alignment Between the Orbits of Planetary Systems and their Visual Binary Companions
Astronomers do not have a complete picture of the effects of wide-binary
companions (semimajor axes greater than 100 AU) on the formation and evolution
of exoplanets. We investigate these effects using new data from Gaia EDR3 and
the TESS mission to characterize wide-binary systems with transiting
exoplanets. We identify a sample of 67 systems of transiting exoplanet
candidates (with well-determined, edge-on orbital inclinations) that reside in
wide visual binary systems. We derive limits on orbital parameters for the
wide-binary systems and measure the minimum difference in orbital inclination
between the binary and planet orbits. We determine that there is statistically
significant difference in the inclination distribution of wide-binary systems
with transiting planets compared to a control sample, with the probability that
the two distributions are the same being 0.0037. This implies that there is an
overabundance of planets in binary systems whose orbits are aligned with those
of the binary. The overabundance of aligned systems appears to primarily have
semimajor axes less than 700 AU. We investigate some effects that could cause
the alignment and conclude that a torque caused by a misaligned binary
companion on the protoplanetary disk is the most promising explanation.Comment: 30 pages, 19 figures, 2 csv files included in Arxiv source; accepted
for publication in A