61 research outputs found
The born again (VLTP) scenario revisited: The mass of the remnants and implications for V4334 Sgr
We present 1-D numerical simulations of the very late thermal pulse
(VLTP) scenario for a wide range of remnant masses. We show that by taking
into account the different possible remnant masses, the observed evolution of
V4334 Sgr (a.k.a. Sakurai's Object) can be reproduced within the standard
1D-MLT stellar evolutionary models without the inclusion of any
reduced mixing efficiency. Our simulations hint at a consistent picture with
present observations of V4334 Sgr. From energetics, and within the standard MLT
approach, we show that low mass remnants \hbox{(\msun)} are
expected to behave markedly different than higher mass remnants
\hbox{(\msun)} in the sense that the latter are not expected to
expand significantly as a result of the violent H-burning that takes place
during the VLTP. We also assess the discrepancy in the born again times
obtained by different authors by comparing the energy that can be liberated by
H-burning during the VLTP event.Comment: Submitted to MNRAS. In includes an appendix regarding the treatment
of reduced convective motions within the Mixing Length Theor
Nightside Pollution of Exoplanet Transit Depths
Out of the known transiting extrasolar planets, the majority are gas giants
orbiting their host star at close proximity. Both theoretical and observational
studies support the hypothesis that such bodies emit significant amounts of
flux relative to the host star, increasing towards infrared wavelengths. For
the dayside of the exoplanet, this phenomenon typically permits detectable
secondary eclipses at such wavelengths, which may be used to infer atmospheric
composition. In this paper, we explore the effects of emission from the
nightside of the exoplanet on the primary transit lightcurve, which is
essentially a self-blend. Allowing for nightside emission, an exoplanet's
transit depth is no longer exclusively a function of the ratio-of-radii. The
nightside of an exoplanet is emitting flux and the contrast to the star's
emission is of the order of ~10^(-3) for hot-Jupiters. Consequently, we show
that the transit depth in the mid-infrared will be attenuated due to flux
contribution from the nightside emission by ~10^(-4). We show how this effect
can be compensated for in the case where exoplanet phase curves have been
measured, in particular for HD 189733b. For other systems, it may be possible
to make a first-order correction by using temperature estimates of the planet.
Unless the effect is accounted for, transmission spectra will also be polluted
by nightside emission and we estimate that a Spitzer broadband spectrum on a
bright target is altered at the 1-sigma level. Using archived Spitzer
measurements, we show that the effect respectively increases the 8.0um and
24.0um transit depths by 1-sigma and 0.5-sigma per transit for HD 189733b.
Consequently, we estimate that this would be 5-10 sigma effect for near-future
JWST observations.Comment: Accepted in MNRA
New evolutionary calculations for the Born Again scenario
We present evolutionary calculations aimed at describing the born-again
scenario for post-AGB remnant stars of 0.5842 and 0.5885 \msun. Results are
based on a detailed treatment of the physical processes responsible for the
chemical abundance changes. We considered two theories of convection: the
standard mixing length theory (MLT) and the double-diffusive GNA convection
developed by Grossman et al. The latter accounts for the effect of the chemical
gradient () in the mixing processes and in the transport of energy.
We also explore the dependence of the born-again evolution on some physical
hypothesis, such as the effect of the existence of non-zero chemical gradients,
the prescription for the velocity of the convective elements and the size of
the overshooting zones. Attention is given to the behavior of the born-again
times and to the chemical evolution during the ingestion of protons. We find
that in our calculations born again times are dependent on time resolution. In
particular when the minimum allowed time step is below yr we
obtain, with the standard mixing length theory, born again times of 5-10 yr.
This is true without altering the prescription for the efficiency of convective
mixing during the proton ingestion. On the other hand we find that the
inclusion of the chemical gradients in the calculation of the mixing velocity
tend to increase the born again times by about a factor of two. In addition we
find that proton ingestion can be seriously altered if the occurrence of
overshooting is modified by the -barrier at the H-He interface,
strongly altering born again times.Comment: 15 pages including 13 figures and 2 table
Discovery and characterization of WASP-6b, an inflated sub-Jupiter mass planet transiting a solar-type star
We report the discovery of WASP-6b, an inflated sub-Jupiter mass planet transiting every 3.3610060^{\rm + 0.0000022 }_ days a mildly metal-poor solar-type star of magnitude V = 11.9. A combined analysis of the WASP photometry, high-precision followup transit photometry and radial velocities yield a planetary mass M_{\rm p} = 0.503^_ and radius R_{\rm p} = 1.224^_ , resulting in a density . The mass and radius for the host star are M_\ast = 0.88^_ and R_\ast = 0.870^_ . The non-zero orbital eccentricity e = 0.054^{\rm +0.018}_ that we measure suggests that the planet underwent a massive tidal heating ~1 Gyr ago that could have contributed to its inflated radius. High-precision radial velocities obtained during a transit allow us to measure a sky-projected angle between the stellar spin and orbital axis \beta = 11^_ deg. In addition to similar published measurements, this result favors a dominant migration mechanism based on tidal interactions with a protoplanetary disk
Improved parameters for the transiting planet HD 17156b: a high-density giant planet with a very eccentric orbit
We report high-precision transit photometry for the recently detected planet
HD 17156b. Using these new data with previously published transit photometry
and radial velocity measurements, we perform a combined analysis based on a
Markov Chain Monte Carlo approach. The resulting mass M_p = 3.09 (+0.22-0.17)
M_Jup and radius R_p = 1.23 (+0.17-0.20) R_Jup for the planet places it at the
outer edge of the density distribution of known transiting planets with rho_p =
1.66 (+1.37-0.60) rho_Jup. The obtained transit ephemeris is T_tr =
2454438.48271 (+0.00077-0.00057) + N x 21.21747 (+0.00070-0.00067) BJD. The
derived plausible tidal circularization time scales for HD 17156b are larger
than the age of the host star. The measured high orbital eccentricity e =
0.6719 (+0.0052-0.0063) can thus not be interpreted as the clear sign of the
presence of another body in the system.Comment: 6 pages, 3 figures, 1 table. Accepted for publication in A&
SUPERSHARP - Segmented Unfolding Primary for Exoplanet Research via Spectroscopic High Angular Resolution Photography
We propose to search for biosignatures in the spectra of reflected light from
about 100 Earth-sized planets that are already known to be orbiting in their
habitable zones (HZ). For a sample of G and K type hosts, most of these planets
will be between 25 and 50 milli-arcsec (mas) from their host star and 1 billion
to 10 billion times fainter. To separate the planet's image from that of its
host star at the wavelength (763nm) of the oxygen biosignature we need a
telescope with an aperture of 16 metres. Furthermore, the intensity of the
light from the host star at the position in the image of the exoplanet must be
suppressed otherwise the exoplanet will be lost in the glare.
This presents huge technical challenges. The Earth's atmosphere is turbulent
which makes it impossible to achieve the required contrast from the ground at
763nm. The telescope therefore needs to be in space and to fit the telescope in
the rocket fairing it must be a factor of 4 or more times smaller when folded
than when operational. To obtain spectroscopy of the planet's biosignature at
763nm we need to use an integral field spectrometer (IFS) with a field of view
(FOV) of 1000 x 1000 milli-arcsec (mas) and a spectral resolution of 100. This
is a device that simultaneously takes many pictures of the exoplanet each at a
slightly different wavelength which are then recorded as a data cube with two
spatial dimensions and one wavelength dimension. In every data cube wavelength
slice, the background light from the host star at the location of the planet
image must be minimised. This is achieved via a coronagraph which blocks the
light from the host star and active/adaptive optics techniques which
continuously maintain very high accuracy optical alignment to make the images
as sharp as possible. These are the technical challenges to be addressed in a
design study.Comment: A proposal in response to the ESA New Science Ideas call. Sept 2016.
25 page
SUPERSHARP - Segmented Unfolding Primary for Exoplanet Research via Spectroscopic High Angular Resolution Photography
A proposal in response to the ESA New Science Ideas call. Sept 2016. 25 pagesWe propose to search for biosignatures in the spectra of reflected light from about 100 Earth-sized planets that are already known to be orbiting in their habitable zones (HZ). For a sample of G and K type hosts, most of these planets will be between 25 and 50 milli-arcsec (mas) from their host star and 1 billion to 10 billion times fainter. To separate the planet's image from that of its host star at the wavelength (763nm) of the oxygen biosignature we need a telescope with an aperture of 16 metres. Furthermore, the intensity of the light from the host star at the position in the image of the exoplanet must be suppressed otherwise the exoplanet will be lost in the glare. This presents huge technical challenges. The Earth's atmosphere is turbulent which makes it impossible to achieve the required contrast from the ground at 763nm. The telescope therefore needs to be in space and to fit the telescope in the rocket fairing it must be a factor of 4 or more times smaller when folded than when operational. To obtain spectroscopy of the planet's biosignature at 763nm we need to use an integral field spectrometer (IFS) with a field of view (FOV) of 1000 x 1000 milli-arcsec (mas) and a spectral resolution of 100. This is a device that simultaneously takes many pictures of the exoplanet each at a slightly different wavelength which are then recorded as a data cube with two spatial dimensions and one wavelength dimension. In every data cube wavelength slice, the background light from the host star at the location of the planet image must be minimised. This is achieved via a coronagraph which blocks the light from the host star and active/adaptive optics techniques which continuously maintain very high accuracy optical alignment to make the images as sharp as possible. These are the technical challenges to be addressed in a design study
MAYA: An active-target detector for binary reactions with exotic beams
International audienceWith recent improvements in the production of radioactive beams in facilities such as SPIRAL at GANIL, a larger area of the nuclear chart is now accessible for experimentation. For these usually low-intensity and low-energy secondary beams, we have developed the new MAYA detector based on the active-target concept. This device allows to use a relatively thick target without loss of resolution by using the detection gas as target material. Dedicated 3D tracking, particle identification, energy loss and range measurements allow complete kinematic reconstruction of reactions taking place inside MAYA
The EBLM project: VI. Mass and radius of five low-mass stars in F+M binaries discovered by the WASP survey
Some M-dwarfs around F-/G-type stars have been measured to be hotter and larger than predicted by stellar evolution models. Inconsistencies between observations and models need to be addressed with more mass, radius, and luminosity measurements of low-mass stars to test and refine evolutionary models. Our aim is to measure the masses, radii and ages of the stars in five low-mass eclipsing binary systems discovered by the WASP survey. We used WASP photometry to establish eclipse-time ephemerides and to obtain initial estimates for the transit depth and width. Radial velocity measurements were simultaneously fitted with follow-up photometry to find the best-fitting orbital solution. This solution was combined with measurements of atmospheric parameters to interpolate evolutionary models and estimate the mass of the primary star, and the mass and radius of the M-dwarf companion. We assess how the best fitting orbital solution changes if an alternative limb-darkening law is used and quantify the systematic effects of unresolved companions. We also gauge how the best-fitting evolutionary model changes if different values are used for the mixing length parameter and helium enhancement. We report the mass and radius of five M-dwarfs and find little evidence of inflation with respect to evolutionary models. The primary stars in two systems are near the “blue hook” stage of their post sequence evolution, resulting in two possible solutions for mass and age. We find that choices in helium enhancement and mixing-length parameter can introduce an additional 3−5% uncertainty in measured M-dwarf mass. Unresolved companions can introduce an additional 3−8% uncertainty in the radius of an M-dwarf, while the choice of limb-darkening law can introduce up to an additional 2% uncertainty. The choices in orbital fitting and evolutionary models can introduce significant uncertainties in measurements of physical properties of such systems
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