316 research outputs found
Multi-dimensional models of the interior of stars
No full textThis is a talk entitled "Multi-dimensional models of the interior of stars" given at the 22nd ACME Conference on Computational Mechanics on 4 April, 2014. The talk describes the numerical choices made during the early development of a time-implicit code for stellar hydrodynamics, MUSIC. Authors: J. Pratt, I. Baraffe, T. Goffrey, et al.<br
Nonradial oscillations in classical Cepheids: the problem revisited
No full textContext.We analyse the presence of nonradial oscillations in Cepheids, a problem that has not been theoretically revised since the work of Dziembowski (1977, Acta Astron., 27, 95) and Osaki (1977, PASJ, 29, 235). Our analysis is motivated by a work of Moskalik et al. (2004, ASPC, 310, 498), which reports the detection of low-amplitude periodicities in
a few Cepheids of the large Magellanic cloud. These newly discovered periodicities
were interpreted as nonradial modes.
Aims.Based on linear nonadiabatic stability analysis, our goal is to reanalyse the presence and stability of nonradial modes, taking into account improvement in the main input physics required for the modelling of Cepheids.
Methods.We compare the results obtained from two different numerical methods used to solve
the set of differential equations: a matrix method and the Ricatti method.
Results.We show the limitation of the matrix method for finding low-order p-modes (), because of their dual character in evolved stars such as Cepheids. For higher order
p-modes, we find excellent agreement between the two methods.
Conclusions.No nonradial instability is found below , whereas many unstable nonradial modes
exist for higher orders. We also find that nonradial modes remain unstable,
even at hotter effective temperatures than the blue edge of the Cepheid
instability strip, where no radial pulsations are expected
The importance of non-local effects: numerical studies of compressible convection in the young sun
No full textThis is a short talk "The importance of non-local effects: numerical studies of compressible convection in the young sun" given as an Astrophysics Seminar "Tuesday Talks" at the University of Exeter on 14 June 2016. This talk presents a numerical study of stellar convection that reveals the impact of non-local convection on stellar dynamics and stellar structure. The particular focus is on the changing statistics for convective overshooting at the base of the convection zone, and preliminary results are reported. Authors: J. Pratt, I. Baraffe, T. Goffrey, et al.<br
Understanding the Depths of Brown Dwarfs and Giant Exoplanets: Modelling Substellar Atmospheres
No full textThe absence or lack of steady hydrogen fusion in the cores of brown dwarfs means these objects cool over time by radiating away their internal thermal energy. The rate at which these objects cool is regulated by the atmosphere, which imprints its complex and changing chemical composition of molecules and condensates onto the emitted radiation. A reliable model of the atmosphere and its evolution over time therefore lies at the core of our understanding of brown dwarfs and substellar objects. Over the last decade the WISE mission has uncovered the coolest spectral type known as the Y dwarfs. These objects have effective temperatures a few times greater than Jupiter, and thus provide excellent analogs for Jovian-like worlds outside of our solar system. Accurate and reliable atmosphere and evolution models are important for placing mass and age constraints on these newly discovered objects and understanding the rich chemistry and physics taking place in their atmospheres. In this thesis, I present a new set of solar metallicity atmosphere and evolutionary models for very cool brown dwarfs and self-luminous giant exoplanets, which is termed ATMO 2020. Atmosphere models are generated with the state-of-the-art 1D radiative-convective equilibrium code ATMO, and are used as surface boundary conditions to calculate the interior structure and evolution of 0.0005-0.075 solar mass objects. These models include several key improvements to the input physics used in previous models available in the literature. First, the use of a new H-He equation of state including ab initio quantum molecular dynamics calculations has raised the mass by ~1-2% at the stellar-substellar boundary and has altered the cooling tracks around the hydrogen and deuterium burning minimum masses. A second key improvement concerns updated molecular opacities in our atmosphere model ATMO, which now contains significantly more line transitions required to accurately capture the opacity in these hot atmospheres. This leads to warmer atmospheric temperature structures, further changing the cooling curves and predicted emission spectra of substellar objects. I present significant improvement for the treatment of the collisionally broadened potassium resonance doublet, and highlight the importance of these lines in shaping the red-optical and near-infrared spectrum of brown dwarfs. This is highlighted through improved comparisons to the observed spectra of benchmark objects. I generate three different grids of model simulations, one using equilibrium chemistry and two using non-equilibrium chemistry due to vertical mixing, all three computed self-consistently with the pressure-temperature structure of the atmosphere. I show the impact of vertical mixing on emission spectra and in colour-magnitude diagrams, and highlight wavelength regions which can be used of infer the strength of vertical mixing in cool brown dwarfs.Science and Technology Facilities Counci
Evolutionary models for low-mass stars and brown dwarfs: uncertainties and limits at very young ages
No full text12 pages, Latex file, uses aa.clsInternational audienceWe analyse pre-Main Sequence evolutionary tracks for low mass stars with masses based on the Baraffe et al. (1998) input physics. We also extend the recent Chabrier et al. (2000) evolutionary models based on dusty atmosphere to young brown dwarfs down to one mass of Jupiter. We analyse current theoretical uncertainties due to molecular line lists, convection and initial conditions. Simple tests on initial conditions show the high uncertainties of models at ages 1 Myr. We find a significant sensitivity of atmosphere profiles to the treatment of convection at low gravity and K, whereas it vanishes as gravity increases. This effect adds another source of uncertainty on evolutionary tracks at very early phases. We show that at low surface gravity (,) the common picture of vertical Hayashi lines with constant is oversimplified. The effect of a variation of initial deuterium abundance is studied. We compare our models with evolutionary tracks available in the literature and discuss the main differences. We finally analyse to which extent current observations of young systems provide a good test for pre-Main Sequence tracks
Brown dwarfs and very low mass stars in the Hyades cluster: a dynamically evolved mass function
No full textAims: We conducted a search for brown dwarfs (BDs) and very low mass (VLM) stars in the 625 Myr-old Hyades cluster in order to derive the cluster's mass function across the stellar-substellar boundary.
Methods: We performed a deep (I=23, z=22.5) photometric survey over 16 deg2 around the cluster center and followed up with K-band photometry to measure the proper motion of candidate members and with optical and near-IR spectroscopy of probable BD and VLM members.
Results: We report the discovery of the first 2 BDs in the Hyades cluster. The 2 objects have a spectral type early-T and their optical and near-IR photometry as well as their proper motion are consistent with them being cluster members. According to models, their mass is 50 Jupiter masses at an age of 625 Myr. We also report the discovery of 3 new very low mass stellar members of the cluster and confirm the membership of 16 others. We combine these results with a list of previously known cluster members to build the present-day mass function (PDMF) of the Hyades cluster from 50 Jupiter masses to 3 M_⊙. We find the Hyades PDMF to be strongly deficient in very low mass objects and BDs compared to the IMF of younger open clusters such as the Pleiades. We interpret this deficiency as the result of dynamical evolution over the past few 100 Myr, i.e., the preferential evaporation of low mass cluster members due to weak gravitational encounters.
Conclusions: We thus estimate that the Hyades cluster currently hosts about 10-15 BDs, while its initial substellar population may have amounted to up to 150-200 members
Formation and structure of the three Neptune-mass planets system around HD 69830
No full textAccepted in AA LettersSince the discovery of the first giant planet outside the solar system in 1995 (Mayor & Queloz 1995), more than 180 extrasolar planets have been discovered. With improving detection capabilities, a new class of planets with masses 5-20 times larger than the Earth, at close distance from their parent star is rapidly emerging. Recently, the first system of three Neptune-mass planets has been discovered around the solar type star HD69830 (Lovis et al. 2006). Here, we present and discuss a possible formation scenario for this planetary system based on a consistent coupling between the extended core accretion model and evolutionary models (Alibert et al. 2005a, Baraffe et al. 2004,2006). We show that the innermost planet formed from an embryo having started inside the iceline is composed essentially of a rocky core surrounded by a tiny gaseous envelope. The two outermost planets started their formation beyond the iceline and, as a consequence, accrete a substantial amount of water ice during their formation. We calculate the present day thermodynamical conditions inside these two latter planets and show that they are made of a rocky core surrounded by a shell of fluid water and a gaseous envelope
Birth and fate of hot-Neptune planets
No full textThis paper presents a consistent description of the formation and the subsequent evolution of gaseous planets, with special attention to short-period, low-mass hot-Neptune planets characteristic of μ Ara-like systems. We show that core accretion, including migration and disk evolution, and subsequent evolution, taking irradiation and evaporation into account, provides a viable formation mechanism for this type of strongly irradiated light planets. At an orbital distance AU,
this revised core accretion model leads to the formation of planets with total masses ranging from ~14 (0.044 MJ) to ~400 (1.25 MJ). The newly born planets have a dense core of ~6 , independent of the total mass, and heavy element enrichments in the envelope, , varying
from 10% to 80% from the largest to the smallest planets. We examine the dependence of the evolution of the born planet on the evaporation rate due to the incident XUV stellar flux. In order to reach a μ Ara-like mass (~14 ) after ∼1 Gyr, the initial planet mass must range from 166 (~0.52 MJ) to about
20 , for evaporation rates varying by 2 orders of magnitude, which corresponds
to 90% to 20% mass loss during evolution.
The presence of a core and heavy elements in the envelope affects the structure and the evolution of the planet appreciably and yields difference in radius compared to coreless objects of solar composition for Saturn-mass planets.
These combinations of evaporation rates and internal compositions
translate into different detection probabilities and thus into different statistical distributions for hot-Neptunes and hot-Jupiters.
These calculations provide an observable diagnostic, namely a mass-radius-age relationship to distinguish between the present core-accretion-evaporation model
and the alternative colliding core scenario for the formation of hot-Neptunes.
Magnetic activity and hot Jupiters of young Suns : the weak-line T Tauri stars V819 Tau and V830 Tau
No full textSGG acknowledges support from the Science & Technology Facilities Council (STFC) via an Ernest Rutherford Fellowship [ST/J003255/1]. SHPA acknowledges financial support from CNPq, CAPES and Fapemig. AAV acknowledges support from the Swiss National Science Foundation (SNSF) via the allocation of an Ambizione Followship. Date of Acceptance: 06/08/2015We report results of a spectropolarimetric and photometric monitoring of the weak-line T Tauri stars (wTTSs) V819 Tau and V830 Tau within the MaTYSSE (Magnetic Topologies of Young Stars and the Survival of close-ingiant Exoplanets) programme, involving the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope. At ≃3 Myr, both stars dissipated their discs recently and are interesting objects for probing star and planet formation. Profile distortions and Zeeman signatures are detected in the unpolarized and circularly polarized lines, whose rotational modulation we modelled using tomographic imaging, yielding brightness and magnetic maps for both stars. We find that the large-scale magnetic fields of V819 Tau and V830 Tau are mostly poloidal and can be approximated at large radii by 350-400 G dipoles tilted at≃30° to the rotation axis. They are significantly weaker than the field of GQ Lup, an accreting classical T Tauri star (cTTS) with similar mass and age which can be used to compare the magnetic properties of wTTSs and cTTSs. The reconstructed brightness maps of both stars include cool spots and warm plages. Surface differential rotation is small, typically ≃4.4 times smaller than on the Sun, in agreement with previous results on wTTSs. Using our Doppler images to model the activity jitter and filter it out from the radial velocity(RV) curves, we obtain RV residuals with dispersions of 0.033 and 0.104km s-1 for V819 Tau and V830 Tau, respectively. RV residuals suggest that a hot Jupiter may be orbiting V830 Tau, though additional data are needed to confirm this preliminary result. We find no evidence for close-in giant planet around V819 Tau.Peer reviewe
Reaction rates for O-18(alpha, gamma)Ne-22, Ne-22(alpha, gamma)Mg-26, and Ne-22(alpha, n)Mg-25 in stellar helium burning and s-process nucleosynthesis in massive stars
No full textNew experimental results in O-18(Li-6, d) and Ne-22(Li-6, d) alpha
transfer reaction studies have been utilized to calculate the resonance
strength of low-energy resonances in the alpha-capture on O-18 and
Ne-22. The present results imply a significant increase of the reaction
rates for O-18(alpha, gamma)Ne-22 and Ne-22(alpha, gamma)Mg-26. The
Ne-22(alpha, n)Mg-25 rate might be strongly enhanced by a possible
resonance at 633 keV. The respective uncertainties are presented as a
function of temperature. The influence of the proposed reaction rates on
the s-process nucleosynthesis during core helium burning has been
investigated in the range 15 less than M/solar mass less than 30. The
resulting uncertainties are compared to those related to the modeling of
core helium burning in massive stars. This comparison indicates that the
s-process may be a useful constraint for the evolution and convection in
massive stars, provided the persistent problem of the 633 keV resonance
in the Ne-22(alpha, n)Mg-25 channel can be solved
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