Download Planets around Stars Beyond the Main Sequence (Evolved Stars)

Planets
around
Stars
Beyond
the
Main
Sequence
(Evolved
Stars)
1.  RV measurements of Giant Stars
2.  Timing Variations
a)  Pulsar Planets
b)  Planets around oscillating stars
c)  Planets in eclipsing binaries
Exoplanets around Giant stars
Mass on main
sequence
Difficult on the main sequence, easier (in principle) for evolved stars
One way to detect planets around more massive stars with the
RV method:
A 1.9 M‫ סּ‬main sequence star
A 1.9 M‫ סּ‬K giant star
Early Evidence for Planets around Giant stars (Hatzes & Cochran
1993)
P = 1.5 yrs
Frink et al. 2002
M = 9 MJ
Planet around the giant star Iota Dra (M ~ 2.2 MSun)
The Planet around Pollux
McDonald 2.1m
CFHT
McDonald 2.7m
TLS
The RV variations of β Gem taken with 4 telescopes over a time span of 26 years. The
solid line represents an orbital solution with Period = 590 days, m sin i = 2.3 MJup.
Mass of star = 1.9 solar masses
The Planet around Aldebaran
CFHT
McDonald 2.1m DAO
McDonald 2.7m
TLS
The RV variations of α Tau taken. The solid line represents an orbital solution with Period
= 633 days, m sin i =5.3 MJup.
Mass of star = 1.06 solar masses
The first Tautenburg Planet: HD 13189
P = 471 d
Msini = 14 MJ
M* = 3.5 Msun
HD 13189 : Short Term Variations
All Giant stars show stellar oscillations with periods of hours to days and
amplitudes of 10-50 m/s
HD 13189
Sp. Type
Mass
V sin i
K2 II–III
3.5 Msun
2.4 km/s
HD 13189 b
Period
471 ± 6 d
RV Amplitude
e
a
m sin i
173 ± 10 m/s
0.27 ± 0.06
1.5 – 2.2 AU
14 MJupiter
From Michaela Döllinger‘s Ph.D thesis
P = 517 d
Msini = 10.6 MJ e = 0.09
M* = 1.84 M‫סּ‬
P = 272 d
Msini = 6.6 MJ e = 0.53
M* = 1.2 M‫סּ‬
P = 657 d
Msini = 10.6 MJ e = 0.60
M* = 1.2 M‫סּ‬
P = 159 d
Msini = 3 MJ e = 0.03
M* = 1.15 M‫סּ‬
P = 1011 d
Msini = 9 MJ e = 0.08
M* = 1.3 M‫סּ‬
P = 477 d
Msini = 3.8 MJ e = 0.37
M* = 1.0 M‫סּ‬
M sin i = 3.5 – 10 MJupiter
N
Stellar Mass Distribution: Tautenburg Sample
Mean = 1.4 M‫סּ‬
Median = 1.3 M‫סּ‬
M (M‫)סּ‬
~20% of the intermediate mass stars have
giant planets
Johnson et al. (2010): Planets around „retired“ A stars
Johnson et al. also estimate that ~25% of stars with mass > 1.5
Msun have giant planets
Eccentricity versus Period
Blue points: Giant stars with planets
Open points: Main sequence stars with planets
Planet Mass Distribution for
Solar-type main sequence
stars with P> 100 d
Planet Mass Distribution
for Giant and Main
Seq
u
ence stars with M > 1.1 M‫סּ‬
N
More massive stars tend to
have more massive planets
and at a higher frequency
M sin i (Mjupiter)
The Planet-Metallicity Connection Revisitied
Valenti & Fischer
There is believed to be a connection
between metallicity and planet formation.
Stars with higher metalicity tend to have a
higher frequency of planets.
Percent
Planet-Metallicity Effect in Giant stars?
[Fe/H]
Giant stars show no metallicity effect
Maybe pollution can explain the metallicity-planet connection
Figure 1: Metal distribution for planet-hosting (P-H)
giants (full line), P-H dwarfs with periods larger than 180
days (dashed line) and all P-H dwarfs (dotted). The
giants show a distribution shifted to lower metallicity by
about 0.2-0.3 dex with respect to the dwarfs
Giant hosting planet stars do not show a metallicity enhancement such as the
planet hosting stars on the main sequence. Pasquini et al. (2007) hypothesize that
the high metal content is due to pollution by planets. When the stars evolve to
giants they have deeper convection zones which mixes the chemicals.
Pollution hypothesis: During planet formation the giant
planets migrate in and collide with the star. They have a
higher content of metals and thus pollute the outer layers of
the stellar atmosphere. Because the convection zone for
main sequence stars is not as deep, the „polluted“ layers
survive for some time. For giant stars that have a deep
convection zone, this polluted layer gets mixed and one does
not see a higher metal content.
Timing Variations: Pulsars
time
Due to the orbital motion the
distance the Earth changes.
This causes differences in
the light travel time
time
Change in arrival time =
apmpsini
M* c
ap, mp = semimajor axis, mass of planet
The Progenitors to Pulsars: Exploding Massive stars
The burning stages of
massive stars
Main sequence lifetime ~ 10 million years
Helium burning ~ 1 million years
Carbon burning ~ 300 years
Oxygen burning ~ 2/3 year
Silicon burning ~ 2 days
After Si burning the core
collapses resulting in a
supernova explosion. What
is left behind is a neutron
star. These Type II
supernova
Type Ia: Exploding White Dwarf that has accreted matter to send it over the
Chandrasekhar Limit of ~ 1.4 Msun
Energy output 1049 – 1051 ergs
Properties of Neutron Stars (Pulsars)
Progenitor Mass: 8-20 Msun
Remnant mass: < 3 Msun (otherwise it becomes a black hole)
Pressure support: Neutron degeneracy pressure
Radius: ~10 km
Density: 200 million tons/cm3
Magnetic field strength: ~ 1013 Gauss
Periods: 1.5 millisecs to 8.5 s
Rotation period of B1937+21:
P = 0.0015578064924327 ±0.0000000000000004 secs
These are very stable clocks!
The „Lighthouse“ Beacons of Pulsars
PSR B0329+54 P = 0.7s
Vela P = 0.089 s
Crab P = 0.033 s
Why do they think it is a planet?
•  Checked the barycentric correction of the Earth: Ok
•  300 other pulsars observed and no 6 month periodicity was
found. If it is due to the wrong barycentric why was it not
seen in the other pulsars?
Possible problems
•  Pulsars have a rotational instability. Unlikely, especially
since it is periodic
•  The barycentric correction is in fact
wrong….hmmmmmm…
• 
ini9al
posi9on
of
the
pulsar
used
in
the
barycentric
mo9on
of
the
Earth
was
off
by
7
arcmin
• 
They
detected
the
ellip9city
of
the
Earth
98 d orbit removed, 66 d
orbit remains
66 d orbit removed, 98 d
orbit remains
The “radiation from the star” is due to the rotational
energy loss from the star:
dE/dt = I ω dω/dt
ω = 2π/P
dω/dt = rate of rotational change
dE/dt ~ 4 x 107 ergs cm–2 s–1
Solar radiation at Earth = 1.42 x 106 ergs cm–2 s–1
This is about 30 times the flux at the Earth, so the
temperature of the planets should be ~670 K,
comparable to Mercury
• 
Pulsar
with
a
0.3
Msun
mass
companion
in
a
191
d
orbit
• 
AHer
removing
the
9ming
varia9ons
of
the
stellar
companion
there
are
addi9onal
varia9ons
in
the
residuals
• 
Phinney
1993:
Period
varia9ons
due
to
planet
14‐400
Mearth
with
P
>
15
yrs.
• 
ThorseO
et
al.
1993
:
Varia9ons
are
consistent
either
with
a
planet
at
~10
AU,
or
a
star
at
~50
AU
orbit
This planet is uncertain. Currently there is only one pulsar with planetary
companions
Origin of the Pulsar Planets
1. 
First Generation Planets: These „rocks“ are remnants of
planets (maybe giant planets) that survived the supernova
explosion
2. 
Second Generation Planets: Planets that formed in the debris
disk left behind after the supernova explosion
Unfortunately, we only have one example of a pulsar with planets, until
we find more such systems the nature of pulsar planets will be
unknown.
One can also use stellar oscillations as „clocks“ for timing variations
Searching for Planets Around Oscillating White Dwarfs
Optical Light Curves of ZZ Ceti Stars
Mullally et al. (2008, ApJ, 676, 573) looked at a sample of 15
pulsating white dwarfs
One, GD 66 looks promising:
But the amplitude of the
mode shows variations
Arrival time variations consistent with a ~2 MJup companion in a 4.5
year orbit…but one has to be careful:
•  Evolutionary changes can cause period changes
•  Unstable modes can cause period changes
•  Beating of modes can cause period changes (WD stars tend to be
multiperiodic pulsators).
Subdwarf B Stars (sdB)
•  sdB stars are believed to be core He-burning stars of 0.5 M on
the extended horizontal branch that have lost their envelope
•  Teff ~ 22.000 – 40.000 K
•  Periods 100 – 250 secs
V391 Peg
O-C for two pulsation frequencies look the
same
Prototype sdB
pulsating star
Sub-stellar Objects in Strange Places: The Sub-stellar companion to the sdB
star HD 149382 found with traditional radial velocity variations
A 8-20 MJup mass object in a 2.9 d orbital period…so why is this
interesting?
sdB
stars
P = 2.9 days → a = 0.05 AU (assuming a 2 solar mass star) = 10
solar radii.
On giant branch: Stellar radius 10-50 Rsun
At one point this companion was in the envelope of the star!
Planets around the cataclysmic eclipsing binary NN Ser
Orbital Period 3.12 hours
White Dwarf:
Mass: 0.535 Solar masses
Temperature = 37000 K
Mass transfer
M4 Dwarf companion:
Mass: 0.11 Solar masses
Temperature ~ 3000 K
NN Ser is an eclipsing system. If there are additional companions
around one or both stars this will change the expected time of the
eclipse.
One planet fit to the variations in
the eclipse timing:
P1 = 15.5 years
M1 = 6.9 MJup
2:1 resonance
Two planet fit:
P2 = 7.7 years
M2 ~ 2 MJup
These planets have to be circumbinary planets:
X
Formation scenarios: First or Second Generation planets
First Generation: Planets formed with stars, but these would have to
have survived the supernova explosion
Second Generation: Planets formed after the common envelope
phase
Planets around the cataclysmic eclipsing binary DP Leo
Orbital Period = 89 minutes
White Dwarf companion with
Mass > 1 Solar mass
Companion is a cool star
that is transfering mass to
the W.D.
DP Leo is a post Common
Envelope star (CE). During
the evolution of the W.D. the
secondary was in the
envelope of the companion
star.
Timing variations can also be used on transiting planets
Kepler-9b