Transcript radius M

Planet or Star ?
Physics mainly determined by the object's mass
- Gravitation
- Nuclear processes
10 M
13 Mjup
(0.013 M )
80 MJup
(0.08 M)
1.5-2 M
p-p cycle
Gas
Accretion
Planets
Terrestrials
Giants
Planet : no deuterium fusion
Deuterium fusion
Brown dwarfs:
radius  M-1/3
Mass
CNO cycle
Hydrogen fusion
Stars :
radius  M
Brief historical: observers era
• 1931 : Berman : m= 45Mj
• 1960 : Van de Kamp : Barnard star
• 1981 :  pictoris
• 1985 : Mc Carthy et al. : Van Biesbroeck 8 B
• 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
 Pictoris
Retour
PSR 1257+12
3 planets
M (m)
0.015
3.4
2.8
a (au) period (j)
0.19
0.36
0.47
25.34
66.54
98.22
Retour
51 Pégase
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
2 hc
B 
 e
5
2


 hc  1


  k T



h : Planck’s Cte
= 6.62620 10-34 J.s
c : speed of light
= 2.9979 108 ms-1
k : Boltzmann’s Cte
= 1.38 10-23 J.K-1
Stefan’s Law and effective temperature
Total Flux emitted by a blackbody at T
F T
4
Avec  = Cte 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 Teff according to
Stefan’s law.
Planet in radiative equilibrium
S
D
A, Rpl, Teff
Flux received from star = flux emitted by planet
F (R/D)2 (1 - A) f = Teff4
Avec F : stellar flux ( T4)
A : planet’s albedo
D : orbital distance
Teff : planet’s effective temperature (Teq)
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
Teff:
150010,000K
Logg:
3.5 - 6.0
[M/H]:
0.3 - -4.0
Very Low Mass Stars to Brown Dwarfs Spectral
Sequence according to NextGen (Allard et al.1990,
1995, 1997) and Hauschildt et al. (1999).
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.
Brown Dwarfs Spectral Properties
Mol: 3500-2500K
Dust: 2500-1700K
CH4 : ≤ 1700K
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.
The spectral energy distribution of Gl229B
(S.K. Leggett, UKIRT) is compared to our
model AMES-Cond with Teff=1000K (blue curve).
When Stars meet Planets
Allard et al. (ARA&A 35, 137, 1997)
Allard 1996
2000K
1000K
1000K
500K
Marley 1996
Cooler Brown Dwarfs/Planemos
As Teff decreases
further:
H2O sediments out
CH4 keeps growing
The lack of CH4
high energy
opacities prevents
an accurate
modeling of
Jupiter (Teff=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 Teff=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,
Teff=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
• H2-alkali absorption line profiles