Presentation on theme: "Brown Dwarfs and Extrasolar Giant Planets Model Atmospheres France Allard Centre de Recherche Astronomique de Lyon."— Presentation transcript:
Brown Dwarfs and Extrasolar Giant Planets Model Atmospheres France Allard Centre de Recherche Astronomique de Lyon
Planet or Star ? Physics mainly determined by the object's mass - Gravitation - Nuclear processes Mass p-p cycleCNO cycle 1.5-2 M 13 M jup (0.013 M ) Deuterium fusion Brown dwarfs: radius M -1/3 Planets 10 M Gas Accretion GiantsTerrestrials Planet : no deuterium fusion Hydrogen fusion Stars : radius M 80 M Jup (0.08 M )
Brief historical: observers era 1931 : Berman : m= 45Mj 1960 : Van de Kamp : Barnard star 1985 : Mc Carthy et al. : Van Biesbroeck 8 B 1981 : pictoris 1992 : Wolszczan : PSR 1257 +12 1995 : Walker : nothing above 3 Mj around 21 stars 1995 : Mayor et Queloz : 51 Pegasus
Barnard Star Observations 1916-1963 2413 shots m = 1.6 Mj e = 0.77 a = 4.42 au retour
In parallel, the detection of young brown dwarfs in the Pleiades was confirmed by the lithium test (Rebolo, Martín and Magazzú, ApJ 389, L83, 1992) using Keck spectroscopy (Basri et al. 1996). Li I 6707.8,8126Å
Blackbody Emission Planck’s Law h : Planck’s C te = 6.62620 10 -34 J.s c : speed of light = 2.9979 10 8 ms -1 k : Boltzmann’s C te = 1.38 10 -23 J.K -1
Stefan’s Law and effective temperature Total Flux emitted by a blackbody at T Avec = C te de Stefan-Boltzmann = 5.66956 10 -8 Wm -2 K -4 To any source that emit F (as measured by a bolometer for example), can be associated an effective temperature T eff according to Stefan’s law.
Planet in radiative equilibrium Flux received from star = flux emitted by planet Avec F : stellar flux ( T 4 ) A : planet’s albedo D : orbital distance T eff : planet’s effective temperature (T eq ) A, R pl, T eff S D F (R /D) 2 (1 - A) f = T eff 4
The First Confirmed Brown Dwarf Nakajima et al. (Nature 378, 463, 1995) The discovery of this first evolved brown dwarf (black curve, Oppenheimer et al.,Science 270, 1478, 1995), confirms model predictions (blue curve, Allard & Hauschildt, ApJ 445, 433, 1995) that brown dwarfs emit more flux between molecular bands at 1.1, 1.3, 1.6 et 2.2 µm than a blackbody would do.
The Models 1D, Static, Chemical Equilibrium Detailed opacity spectra Departures from LTE Spherical symmetry Detailed impinging spectrum one point of the surface one moment in time L redistribution, phase
First Grid Of Brown Dwarfs models:NextGen Very Low Mass Stars to Brown Dwarfs Spectral NextGen Sequence according to NextGen (Allard et al.1990, 1995, 1997) and Hauschildt et al. (1999). T eff : 1500- 10,000K Logg: 3.5 - 6.0 [M/H]: 0.3 - -4.0
Theory vs Angular diameters (VLTI) Segransan et al. (2003) Press release (Photo 27c/02) comparing radii and masses of four red dwarfs observed with the VLTI, GJ 205, GJ 887, GJ 191 ("Kapteyn star") et Proxima Centauri (red points), to the NextGen (1997) models with 400 Myr (dashed red line) and 5 Gyr (full black curve). Jupiter’s position is indicated by a black triangle.
Mol: 3500-2500K Dust: 2500-1700K CH 4 : ≤ 1700K Brown Dwarfs Spectral Properties 2500 18001000K Brown dwarf, cooling off through spectral types M, L and T according to models by Allard et al. (2001), Chabrier et al. (2000). The surface temperatures are from top to bottom: 2500, 1800 et 1000K. Iron and silicate dust grain formation produce a greenhouse effect that heats up and reddens dwarfs of type L, while the cooler T-type dwarfs remain dust free.
When Stars meet Planets Allard et al. (ARA&A 35, 137, 1997) Allard 1996 2000K1000K1000K500K Marley 1996
Cooler Brown Dwarfs/Planemos As T eff decreases further: H 2 O sediments out CH 4 keeps growing The lack of CH 4 high energy opacities prevents an accurate modeling of Jupiter (T eff =128K).
AMES-Cond/Dusty 2001 Allard et al. (ApJ 556, 357) M dwarfs L dwarfs T dwarfs A transition between Dusty and Cond regimes begins to be observed, suggesting the existence of a physical process removing dust from the photosphere.
Line formation in brown dwarfs The observed spectrum of Gl229b (thick black line) is compared to Allard et al. 2001’s Cond model with T eff =1000 K, log g=5.5 (blue dotted line). The model accounts for the opacity contribution of the line wings out to 5000Å from the line centre. Two other are shown where the line opacity is accounted for out to 15000Å from the line centre (green and red lines), one where molecular opacities have been suppressed (red line), to underline the atomic line opacity contributions. One can see that the KI doublet at 7665,7648Å determines the entire emerging spectrum bluewards of about 1 m. The Lorentz profile used in our models is however no longer valid so far from the line centre, and overestimates the far-wing opacity contributions. Hence, when using 15000Å search window around the line centre, our models under-predict the flux peak at 1.25 m by as much as 25%. This stresses the importance of an accurate modelling of the far-wings of alkali lines in brown dwarf atmospheres.
Allard, N. F., Allard, F., et al. A&A 2003 Models obtained with the van der Waals approximation (in blue); and with unified profiles (in magenta) are compared to the observed brown dwarf SDSS1624 spectrum (in black, T eff =1000K, logg=5.5, solar composition).
Conclusions Very low mass stars are relatively well understood To distinguish brown dwarfs from VLM stars and free-floating planemos we must rely upon models that need to be improved on the front of: cloud modeling cloud modeling H2-alkali absorption line profiles H2-alkali absorption line profiles