Showing papers by "Isabelle Baraffe published in 2004"

The effect of evaporation on the evolution of close-in giant planets

Abstract: We include the effect of evaporation in our evolutionary calculations of close-in giant planets, based on a recent model for thermal evaporation taking into account the XUV flux of the parent star (Lammer et al 2003) Our analysis leads to the existence of a critical mass for a given orbital distance mcrit(a) below which the evaporation timescale becomes shorter than the thermal timescale of the planet For planets with initial masses below mcrit, evaporation leads to a rapid expansion of the outer layers and of the total planetary radius, speeding up the evaporation process Consequently, the planet does not survive as long as estimated by a simple application of mass loss rates without following consistently its evolution We find out that the transit planet HD 209458b might be in such a dramatic phase, although with an extremely small probability As a consequence, we predict that, after a certain time, only planets above a value mcrit(a) should be present at an orbital distance a of a star For planets with initial masses above mcrit, evaporation does not affect the evolution of the radius with time

First determination of the dynamical mass of a binary L dwarf

Abstract: We present here the results of astrometric, photometric and spectroscopic observations leading to the determination of the orbit and dynamical masses of the binary L dwarf 2MASSW J0746425+2000321. High angular resolution observations spread over almost 4 years and obtained with the Hubble Space Telescope (HST), the ESO Very Large Telescope (VLT), and a the W. M. Keck Observatory (Keck) allow us to cover 36% of the period, corresponding to 60% of the orbit, and, for the first time, to derive a precise estimate of the total and individual masses of such a late-type object. We find an orbital period of 3850.9$^{+904}_{-767}$ days. The corresponding total mass is 0.146$^{+0.016}_{-0.006}$ M$_{\sun}$, with uncertainties depending on the distance. Spatially resolved low resolution optical (550--1025 nm) spectra have been obtained with HST/STIS, allowing us to measure the spectral types of the two components (L0$\pm$0.5 for the primary and L1.5$\pm$0.5 for the secondary). We also present precise photometry of the individual components measured on the high angular resolution images obtained with HST/ACS and WFPC2 (visible), VLT/NACO (J, H and Ks bands) and Keck I (Ks) band). These spectral and photometric measurements enable us to estimate their effective temperatures and mass ratio, and to place the object accurately in a H--R diagram. The binary system is most likely formed by a primary with a mass of 0.085$\pm$0.010 M$_{\sun}$ and a secondary with a mass of 0.066$\pm$0.006 M$_{\sun}$, thus clearly substellar, for an age of approximately 300$\pm$150 Myr. H$\alpha$ variability indicates chromospheric and/or magnetic activity.

First determination of the dynamical mass of a binary L dwarf

Abstract: We present here the results of astrometric, photometric and spectroscopic observations leading to the determination of the orbit and dynamical masses of the binary L dwarf 2MASSW J0746425+2000321. High angular resolution observations spread over almost 4 years and obtained with the Hubble Space Telescope (HST), the ESO Very Large Telescope (VLT), and at heW. M. Keck Observatory (Keck) allow us to cover ∼36% of the period, corresponding to 60% of the orbit, and, for the first time, to derive a precise estimate of the total and individual masses of such a late-type object. We find an orbital period of 3850.9 +904 −767 days. The corresponding total mass is 0.146 +0.016 −0.006 Mwith uncertainties depending on the distance. Spatially resolved low resolution optical (550-1025 nm) spectra have been obtained with HST/STIS, allowing us to measure the spectral types of the two components (L0 ± 0.5 for the primary and L1.5 ± 0.5 for the secondary). We also present precise photometry of the individual components measured on the high angular resolution images obtained with HST/ACS and WFPC2 (visible), VLT/NACO (J, H and KS bands) and Keck I (KS band). These spectral and photometric measurements enable us to estimate their effective temperatures and mass ratio, and to place the object accurately in a H-R diagram. The binary system is most likely formed by a primary with a mass of 0.085 ± 0.010 Mand a secondary with a mass of 0.066 ± 0.006 M� , thus clearly substellar, for an age of approximately 300 ± 150 Myr. Hα variability indicates chromospheric and/or magnetic activity.

The Evolution of Irradiated Planets: Application to Transits

Abstract: Extending the theory that we derived recently for HD 209458b to different cases of strongly irradiated gaseous exoplanets, we have calculated the consistent evolution of a new transiting planet, OGLE-TR-56b, for its recently revised mass determination. The theory is shown to successfully reproduce the observed radius for the proper age of the system. We also examine the dissipation of kinetic energy at the planet's internal adiabat due to atmospheric winds and place constraints on the efficiency of this process. We show that a fraction ~0.1%-0.5% of the incident flux transformed into thermal energy deposited at the adiabatic level can accommodate the observed radii of both OGLE-TR-56b and HD 209458b. The present theory yields quantitative predictions on the evolution of the emergent spectrum and fundamental properties of hot jupiters. The predictions for radius, luminosity, temperature as a function of the planet's mass, and orbital distance can be used as benchmarks for future detections of transit planets.

A young binary Brown Dwarf in the R-CrA star formation region

Abstract: We present imaging and spectroscopic observations with HST (WFPC2, ACS/HRC and STIS), VLT (FORS2) and Keck (HIRES) of the dM8 ultra-cool dwarf DENIS-P J185950.9-370632, located in the R-CrA region. The presence of lithium absorption at 670.8 nm and the strong Halpha emission indicate a young age and a sub-stellar mass. Our diffraction-limited images resolve a companion at the separation limit of HST/ACS (~0.06"). The 2.1 mJy flux in the LW2 filter (5.0-8.5 microns) of the Infrared Space Observatory (see 1999A&A...350..883) likely corresponds to an infrared excess, suggesting the presence of circumstellar material. Proper motion and photometric measurements, as well as the Halpha activity, confirm membership in the R-CrA star forming region. If confirmed by further observations, DENIS-P J185950.9-370632 would be the first accreting sub-stellar multiple system observed to date.

Stability of Supernova Ia Progenitors against Radial Oscillations

Abstract: We analyze the possible existence of a pulsational instability excited by the -mechanism during the last few centuries of evolution of a Chandrasekhar-mass white dwarf prior to its explosion as a Type Ia supernova. Our analysis is motivated by the temperature sensitivity of the nuclear energy generation rate (~T23) in a white dwarf whose structural adiabatic index is near 4/3. Based on a linear stability analysis, we find that the fundamental mode and higher order radial modes are indeed unstable and that the fundamental mode has the shortest growth timescale. However, the growth timescale for such instability never becomes shorter than the evolutionary timescale. Therefore, even though the star is pulsationally unstable, we do not expect these radial modes to have time to grow and to affect the structure and explosion properties of Type Ia supernovae.

Stability of SN Ia progenitors against radial oscillations

Abstract: We analyze the possible existence of a pulsational instability excited by the $\epsilon$-mechanism during the last few centuries of evolution of a Chandrasekhar mass white dwarf prior to its explosion as a Type Ia supernova. Our analysis is motivated by the temperature sensitivity of the nuclear energy generation rate ($\sim T^{23}$) in a white dwarf whose structural adiabatic index is near 4/3. Based upon a linear stability analysis, we find that the fundamental mode and higher order radial modes are indeed unstable and that the fundamental mode has the shortest growth time scale. However, the growth time scale for such instability never becomes shorter than the evolutionary timescale. Therefore, even though the star \emph{is} pulsationally unstable, we do not expect these radial modes to have time to grow and to affect the structure and explosion properties of Type Ia supernovae.

A young binary Brown Dwarf in the R-CrA star formation region

Abstract: We present imaging and spectroscopic observations with HST (WFPC2, ACS/HRC and STIS), VLT (FORS2) and Keck (HIRES) of the dM 8 ultra-cool dwarf DENIS-P J185950.9-370632, located in the R-CrA region. The presence of lithium absorption at 670.8 nm and the strong Hα emission indicate a young age and a sub-stellar mass. Our diffraction-limited images resolve a companion at the separation limit of HST/ACS (∼0. 06). The 2.1 mJy flux in the LW2 filter (5.0−8.5 µm) of the Infrared Space Observatory (Olofsson et al. 1999) likely corresponds to an infrared excess, suggesting the presence of circumstellar material. Proper motion and photometric measurements, as well as the Hα activity, confirm membership in the R-CrA star forming region. If confirmed by further observations, DENIS-P J185950.9-370632 would be the first accreting sub-stellar multiple system observed to date.

The effect of evaporation on the evolution of close-in giant planets

Abstract: We include the effect of evaporation in our evolutionary calculations of close-in giant planets, based on a recent model for thermal evaporation taking into account the XUV flux of the parent star (Lammer et al. 2003). Our analysis leads to the existence of a critical mass for a given orbital distance $m_{\rm crit}(a)$ below which the evaporation timescale becomes shorter than the thermal timescale of the planet. For planets with initial masses below $m_{\rm crit}$, evaporation leads to a rapid expansion of the outer layers and of the total planetary radius, speeding up the evaporation process. Consequently, the planet does not survive as long as estimated by a simple application of mass loss rates without following consistently its evolution. We find out that the transit planet HD 209458b might be in such a dramatic phase, although with an extremely small probability. As a consequence, we predict that, after a certain time, only planets above a value $m_{\rm crit}(a)$ should be present at an orbital distance $a$ of a star. For planets with initial masses above $m_{\rm crit}$, evaporation does not affect the evolution of the radius with time.