Transcript Rotation and Activity in Cool Fusors

Brown Dwarfs : Up Close and Physical
In the mass range intermediate between
stars and planets are the substellar objects
known as brown dwarfs. The first BDs
were discovered in 1995. The first
confirmations were based in one case on
interior properties (the lithium test), and in
the other case on external temperature (the
methane test).
I will concentrate on what we have learned
about their physical properties. We are only
beginning to directly test masses and
evolutionary models, but are learning about
temperatures and atmospheric properties. I
also touch on rotation, magnetic and
accretion activity, in young and old BDs
over their entire mass range.
I will NOT cover many topics, including
search techniques and results, the mass
function or space density, binarity, or
formation mechanisms.
Luminosity History of Low Mass Objects
Burrows et al
Minimum Stellar Temperature
You could view brown dwarfs as stars
which only have a deuterium main
sequence (which is short). Regular
stars also have hydrogen and helium
main sequences, and massive stars
have additional burning phases for
heavier elements.
History of Substellar Sizes
Burrows et al.
Core Temperature depends on Age and Mass
Lithium & Hydrogen burning limit
Deuterium burning limit
The Lithium Test
Basri 1997
The New Cool Spectral Types : L Dwarfs
“L” and “T” have been added to cover the changes in spectra at very cool temperatures.
The L dwarfs are marked by a change from domination by oxide to hydride molecular
species. Refractory metals condense out. This has big ramifications in the optical.
2200K – 1400K
Kirkpatrick et al. 1999
Spectra of L Dwarfs
Geballe et al. 2002
The New Cool Spectral Types
: T Dwarfs
The infrared spectrum shows methane
in preference to carbon monoxide; the
optical spectrum is dominated by
resonance line wings of alkali metals.
1300K – 700K
Alkalai Resonance Lines in the Optical
In very cool objects, the lines of sodium and potassium dominate the
optical opacity. This yields a “magenta” color for brown dwarfs. The
first measurement on an extrasolar planet shows sodium
Burrows
Spectra of T Dwarfs
Geballe et al. 2002
Spectral Typing by IR Molecular Indices
Burgasser et al. 2002
Allard 1999
Dust formation
The opacities and atmospheric chemistry
in brown dwarfs becomes increasingly
tied to the physics of condensates.
Tsuji 2002
Basri 1997
Atmospheric Structure Changes
Tsuji 1300K
condensation
No condensation
Marley et al. 2002
Cloud Formation in Brown Dwarfs
The formation of clouds is poorly understood (not that great even here
on Earth). Particle size distributions, saturation regimes, horizontal
inhomogeneities, global and turbulent currents are all crucial to how
optically thick the clouds will be, what their height of formation,
thickness, and covering fraction is, and knowing when precipitation
will occur. Observed spectra may reflect a blend of different heights
and compositions.
Photometric Evidence of Rotation and Weather
The vsini of BDs implies that the rotation
periods should be hours. Direct evidence
for this has been found. Some vary on
longer timescales; this could be due to
condensation features (clouds).
4.5 hr
Several days – not periodic:
weather (dust clouds)?
Gelino et al. 2002
7.5 hr
The Weather Report for Brown Dwarfs
Burgasser
et al. 2002
Dusty
Clear
Partly Cloudy
Cloudy
Effect of Clouds on Spectra
Models for T=500K, 1000K, 1500K. Flat
spectra result if dust clouds are optically thick;
spectral features are for clear atmospheres.
Marley et al. 2002
There is evidence for cloud formation and then
clearing in the behavior of FeH near the L/T
transition. The FeH should disappear as liquid
iron droplets form, but it reappears even as the
temperature cools further, likely due to breaks in
the clouds that expose hotter interior regions.
Burgasser et al. 2002
L
Atmospheric Changes with
Spectral Type
M
L/T
T
Y?
“Fine Analysis” vs Structural Effective Temperatures
“Structural” temperatures
are defined by measuring
the luminosity (from
photometry adjusted with a
bolometric correction),
combined with the parallax,
then using theory to define a
relation between bolometric
luminosity and radius.
High resolution
spectroscopy yields results
that don’t quite seem to
agree with theoretical
models (problems may be
bolometric corrections,
atmospheric models). The
cooler objects are inferred to
be smaller by spectroscopy
than in the models.
Smith et al. 2003 ApJ
Rotation and Magnetic Activity on Brown Dwarfs
Solar-type stars form with a variety of rotations, perhaps due to disklocking. They initially show signs of accretion and outflow. They are
active in their youth because of relatively rapid spin. The fields carry off
angular momentum, and the stars spin down and become less active.
Does this story apply all the way down the main sequence into brown
dwarfs? Does this story even extend below the brown dwarf mass limit?
Ha falls at the bottom of the
Main Sequence
There is a dramatic fall-off in activity
at the cool end of the M spectral type.
Is this due to rotation, or something
else?
Gizis et al. 2001
Activity in L dwarfs is very
minimal; almost none have
detected Ha or X-rays.
All objects cooler than about
L3 are brown dwarfs (and
significant fraction above).
Rotation in very cool objects
Basri
Mohanty
2000,2003
Very
Low Mass
Stars
Stellar and
Substellar
Objects
Brown
Dwarfs
The decrease in activity is clearly NOT due to slow rotation! Rather, the increase in
spindown times is due to temperature. The atmospheres are becoming very neutral,
and cannot couple to the magnetic field to remove angular momentum.
Accretion and Activity in Young BDs:
Ha Strength vs Width
Going to very early ages, activity is generally stronger (the objects are warmer), and
some of them show accretion from disks. The width of Ha can be used as a direct
accretion diagnostic in high-resolution spectra.
Jayawardhana, Mohanty, Basri 2003
These are late M types (5.5-9.5)
Rotation vs Ha strength in Young BDs
Evidence for disk-locking?
Accretion line
Deriving Fundamental Physical Parameters
For objects in a star-forming region, one might hope to get their
fundamental stellar parameters (testing the untested
evolutionary calculations for low masses and young ages).
The procedure is:
1) Find an effective temperature from a spectroscopic
diagnostic that is largely temperature-dependent
2) Find a surface gravity from a pressure-sensitive line
3) Get the radius from the luminosity (which obtains from the
observed brightness, coupled with a known distance to the
region) and derived temperature
4) Find the mass from the radius and surface gravity
5) Assume all objects are coeval and check with isochrones
Note: there have been no fundamental mass determinations for
visible substellar objects, nor has there been confirmation of the
claims that some of these are below the fusion boundary.
Sensitivities to Teff and log(g)
TiO is sensitive
primarily to
temperature.
NaI is sensitive
to both
temperature and
gravity.
Mohanty et al. 2004
Breaking the Degeneracy
One can get good fits for different combinations of T and g in both TiO
and NaI. For NaI an increase of log(g)=0.5 can be offset by an increase
of T=200K. There is only one set of parameters that works for both. This
is further confirmed by checking the TiO region surrounding NaI.
Deriving Fundamental Physical Parameters
For objects in a star-forming region, one might hope to get their
fundamental stellar parameters (testing the untested
evolutionary calculations for low masses and young ages).
The procedure is:
1) Find an effective temperature from a spectroscopic
diagnostic that is largely temperature-dependent
2) Find a surface gravity from a pressure-sensitive line
3) Get the radius from the luminosity (which obtains from
the observed brightness, coupled with a known distance
to the region) and derived temperature
4) Find the mass from the radius and surface gravity
5) Assume all objects are coeval and check with isochrones
Note: there have been no fundamental mass determinations for
visible substellar objects, nor has there been confirmation of the
claims that some of these are below the fusion boundary.
Mass-Luminosity Relation
GG Tau Ba
(!)
GG Tau Bb
We confirm that the lowest free-floating objects being found may be below the D-burning limit!
Radius and
Temperature
vs Mass
GG Ba
GG Bb
GG Bb
Once again we find a problem
between temperatures found by
high resolution spectroscopy
and models. The slope of the
M-T relation is wrong, and the
radii of very low-mass objects
are too small in the
evolutionary models.
It is amazing that the model
spectra can fit so well in detail
if the model atmospheres are
wrong (and clouds don’t form).
Model Gravities and Ages
GG Tau Ba
GG Tau Bb
Mohanty et al. 2004
Evolutionary Model Uncertainties
A somewhat arbitrary starting point is used (without accounting
for accretion effects): >30 jupiter start at D ignition;
<30 jupiter start at log(g)=3.5 (higher than what we measure).
While D burning is occurring, the collapse of the object is slowed,
so this can cause objects to remain at lower gravity and larger
radius. These initial conditions will cause very low mass objects
to appear too young for the first 1.5 Myr. This problem should be
gone, however, by the age of Upper Sco
If D burning really starts at lower gravity (3.25) for the lowest
mass objects, they take a very long time to complete it (>20 Myr),
so they could be hung up in the state we find them (while 30
jupiter objects would be done by 5 Myr).
The gravity at which D-burning starts has decreased by 40% in
the last 10 years in the D’Antona/Mazzitelli models.
Conclusions
•We have learned a lot about “substellar” objects in 8 years!
•We have seen a large range of masses, temperatures, and ages for
substellar objects, down to the substellar mass limit.
•Model atmospheres do amazingly well at reproducing spectra, but
there is still cause for refinements (especially in the infrared).
•Dust and cloud formation, along with precipitation and meteorology,
are key to understanding the appearance of some objects, but are very
complicated and much work remains in this area.
•The magnetic and angular momentum history of these objects is very
different from all but the lowest mass stars.
•Evolutionary models have many good features, but we cannot
consider them well-tested yet.
Thank You!