Star Formation, Brown Dwarfs,and Exoplanets

● This is a big topic● Star formation is discussed in your text (a little), in the text by LeBlanc (a

bit more), and in some review articles (linked on the course webpage)

PHY 521: Stars

ISM Summary

PHY 521: Stars

The ISM and Dense Cool Clouds● Interstellar medium: The matter (gas and dust) that exists between the

stars in a galaxy● Mostly H, He, but also 1-2% grains that may be ice, silicates, or metals (in

the physicist sense). Typical density ~10-21 kg m-3.● Density of stars ~ 0.08 pc-3 in the solar neighborhood, or 3 X 10-24 g cm-3

assuming the stars are 0.5M⊙.

– Number density of n ~ 1 cm-3

– Overall, mass of ISM similar to that of stars.● Small fraction of ISM is in molecular clouds, cold regions typically

composed of molecular hydrogen, H2. These clouds are likely places for new stars to form.

PHY 521: Stars

Interstellar Gas● Dust is ~ 1% of the ISM mass. ISM is mostly gas (primarily H)

– This can be neutral (H I), ionized (H II), or molecular (H2)● Most of the H is neutral.

– At low ISM T, electrons are mostly in the ground state.– No emission lines– Absorption lines require the presence of UV photons

PHY 521: Stars

ISM Make-up● Interstellar clouds

– T ~ 100 K, n ~ 1 – 10 cm-3, R ~ 10 pc– About 5% of ISM

● Warm/Hot rarefied gas– T ~ 1000 – 10000 K, n ~ 0.1 cm-3 (pressure equilibrium w/ clouds)– Fills most of the ISM

● Hot bubbles– Million K bubbles evacuated by SNe explosions

● Molecular clouds– Very dense (characterized by large extinction)– Neutron H combines to H2—21 cm emission plateaus

– n ~ 103 – 105 cm-3

All-sky map of neutral hydrogen. J. Dickey (UMn), F. Lockman (NRAO), SkyView

http://antwrp.gsfc.nasa.gov/apod/ap010113.html

T. Dame (CfA, Harvard) et al., Columbia 1.2-m Radio Telescopes

http://antwrp.gsfc.nasa.gov/apod/ap970430.html

Molecular (CO) map

PHY 521: Stars

Local Bubble

A model of the local ISM. The Sun is believed to moving through the local interstellar cloud just on the edge of the local bubble.

Linda Huff (American Scientist), Priscilla Frisch (U. Chicago)http://antwrp.gsfc.nasa.gov/apod/ap020217.html

http://antwrp.gsfc.nasa.gov/apod/ap020210.htmlLinda Huff (American Scientist), Priscilla Frisch (U. Chicago)

PHY 521: Stars

Giant Molecular Clouds● GMC: largest molecular clouds in ISM● M ~ 105 M⊙, but they gather together in

complexes that have M ~ 106 M⊙.

● Dense cores are where star formation happens

CO emission (top) and intensity of ionized gas (bottom) around Orion (B. A. Wilson - T. M. Dame - M. R. W. Masheder - P. Thaddeus, 2005)

PHY 521: Stars

How It Began● Solar nebula: gas between stars● Solar nebula collapsed (see also Bennett et al. Chapter 8)

– Some gravitational PE radiated away, rest heated cloud– Angular momentum conservation → disk forms

● Sun forms at center– Eventually T high enough for fusion– H burning began– Energy produced in core matched energy radiated from surface—

equilibrium● Planets form in disk

– Orbital directions reflect spinning disk

Why does the cloud begin to collapse?

How long does the collapse take?

PHY 521: Stars

Star Forming in Orion

More than 3000 stars are in this image amongst the gas and dust in the nebula(NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team)

(Credit: Mouser Williams)

PHY 521: Stars

Free-fall Timescale● The collapse of a cloud of gas will proceed at the freefall timescale● Consider a spherical cloud. The acceleration of a particle some

distance x from the center is given by:

● The solution to this will be of the form:

● Note: the free-fall time is independent of the size of the cloud.● Initially, the collapse is slow, but accelerates toward the end.

Note: this assumes that the pressure is small enough that it does not affect the collapse.

PHY 521: Stars

Free-fall Timescale● Assuming constant density during the collapse:

PHY 521: Stars

Jeans Critical Mass● A “gaseous sphere” has two influences acting on it: gravity pulling it

together, and kinetic energy of the constituent particles tending to cause it to fly apart.

● For a relatively static situation, there is a critical mass such that mass lower than this limit will disperse and mass above this limit will collapse. This is the Jeans critical mass.

● For a given density, one can also consider a Jeans radius.

PHY 521: Stars

Jean's Mass● Consider a gas sphere● Total thermal energy of the particles:

● Gravitational potential energy of the spherical configuration:

● Virial theorem applies if stable. Collapse means

PHY 521: Stars

Jean's Mass● Rearranging:

● Introducing mass density:

● Jean's length:

● Jean's mass:

PHY 521: Stars

Jean's Mass● Observations:

– Collapse for: , – Jeans length decreases as density increases

● Typical molecular cloud (GMC core):

PHY 521: Stars

Alternate Derivation● Compare sound speed to free-fall collapse

– Collapse occurs if:

– This gives the same form as the Jean's length

PHY 521: Stars

Collapse● Large, cold cloud of gas (D ~ few ly)

– Collapse begins

– Gravity pulls cloud together● Heating

– Gravitational potential energy released. ½ convert to heat; ½ radiated away (virial theorem)

– Sun forms at hot center● Rotation

– Conservation of angular momentum● Disk formation

– Easier to collapse along rotation axis than against it

– Collisions make orbits circular

– Outer part of disk thin—cools rapidly. Temperature gradient forms.

PHY 521: Stars

Rotation and Collapse● Angular momentum is conserved in rotating cloud

– Disk forms● Smaller fragments of the cloud can collapse more easily

– Total angular momentum split between orbital angular momentum and rotational angular momentum of each cloud

– Fragmentation: can give rise to binary systems

Credit: Hubble Heritage Team (STScI/AURA), N. Walborn (STScI) & R. Barbß (La Plata Obs.), NASA

http://antwrp.gsfc.nasa.gov/apod/ap030630.html

Molecular cloud near Carina nebula. About 2 ly across, It is being eroded by radiation from nearby young stars.

PHY 521: Stars

Star Formation and the HR Diagram● Jeans: free-fall initially● Pressure becomes important

and contraction proceeds on K-H timescale– 1/2 gravitational PE is radiated

—luminosity– This phases is much longer

than free-fall collapse● H- opacity makes the star

convective– Hayashi track

(Paxton et al. 2010)

PHY 521: Stars

Star Formation and the HR Diagram● Hayashi track: boundary

separating hydrostatic stars that can carry energy via convection (left) and those that have no means of effective energy transport (right)

● Move off Hayashi track when reactions kick in

● First part of pp-chain and conversion of 12C to 14N via CNO– Low mass stars never have this

happen● Once hot enough, full H

burning comences

(Paxton et al. 2010)

PHY 521: Stars

Complications● Rotation

– Conservation of angular momentum → cloud spins up– Collapse easier along rotation axis than across it → protoplanetary disk

forms– Protostar can continue to grow via accretion

● Friction between orbits in disk causes inspiral– Strong magnetic field can slow rotation

Credit: NASA, ESA, M. Robberto (STScI/ESA), the HST Orion Treasury Project Team, & L. Ricci (ESO)

http://antwrp.gsfc.nasa.gov/apod/ap091222.html

PHY 521: Stars

Complications● Jets

– Formation uncertain– Aligned with spin axis– Outflow liked channeled by magnetic field

● Binaries– Cloud fragments during collapse– Angular momentum carried in orbits

A disk around a protostar + a jet

PHY 521: Stars

Young Disk● HL Tau: < 1 Myr; 140 pc

ALMA (ESO/NAOJ/NRAO)

PHY 521: Stars

Slightly Older Disk● HD 141569; 5 Myr, 98 pc

(J. C. Augereau and J. C. B. Papaloizou, A&A, 2004)

PHY 521: Stars

An example of a disk: β Pictoris

Beta Pictoris is young—only about 10 million years old.

PHY 521: Stars

ES

O/A

.-M. L

a grange

For the first time, astronomers have been able to directly follow the motion of an exoplanet as it moves to the other side of its host star. The planet has the smallest orbit so far of all directly imaged exoplanets, lying as close to its host star as Saturn is to the Sun.

The above composite shows the reflected light on the dust disc in the outer part, as observed in 1996 with the ADONIS instrument on ESO's 3.6-metre telescope. In the central part, the observations of the planet obtained in 2003 and autumn 2009 with NACO are shown. The possible orbit of the planet is also indicated, albeit with the inclination angle exaggerated.

http://www.eso.org/public/images/eso1024a/

PHY 521: Stars

Debris Disk● Formalhautl 440 Myr, 7.6 pc

At this stage, a planet is actively shaping the debris

PHY 521: Stars

Protostars● Even though it is

luminous, most of the radiation is in the IR

● Dust obscures our view

PHY 521: Stars

Initial Mass Function● Low mass stars form more

frequently than high mass stars● Salpeter function:

– Salpeter, power is -2.35● Now we typically have different

powers for low and high mass

(Ivan Baldry)

PHY 521: Stars

T Tauri Stars● Young (pre-main sequence) variable

stars found near molecular clouds– High activity, variability ~days– Many have H, Ca II, K emission lines

● P Cygni profile has blue shifted absorption—indicates disk

– Still contracting along vertical path in HR diagram

– < 2 solar masses● Many have circumstellar disks

Outburst from XZ Tauri, showing bubble of hot gas flowing out from the system

John Krist (STScI), Karl Stapelfeldt (NASA Jet Propulsion Laboratory), Jeff Hester (Arizona State University), Chris Burrows (ESA/STScI)

http://hubblesite.org/newscenter/archive/releases/2000/32/

PHY 521: Stars

T Tauri Stars

(Carroll & Ostlie)

PHY 521: Stars

Next Step: Main Sequence● Main-sequence begins when central temperature rises to ~10 million

K → hot enough for fusion– Low mass stars remain on the main-sequence for 100 billion years– High mass stars live for ~ 10 million years

● Planets can form in disk

Artists conception of the β-Pic solar systemhttp://www.nasa.gov/vision/universe/starsgalaxies/betapic.html(Credit: NASA/FUSE/Lynette Cook)

PHY 521: Stars

Outstanding Questions● Do we need a trigger to initiate the collapse?

– Shock wave from nearby supernova?– Stellar wind from young star?

● How much of the mass of the cloud winds up in stars?● What is the initial mass function of stars?● OB associations: unbound stars from a bound cloud—how?● Do high and low mass stars form in the same place?

– High mass stars may form is bursts

PHY 521: Stars

H II Regions● Hot stars (O, early B) have significant flux in UV

– Able to ionize H– H II region: region surrounding star where ionization takes place—

extends out to the point where ionization and recombination balance

PHY 521: Stars

H II RegionsH II regions (red spots) in M51(NASA, ESA, S. Beckwith (STScI), and The Hubble Heritage Team (STScI/AURA))

M33(Chris Schur)http://antwrp.gsfc.nasa.gov/apod/ap061123.html

PHY 521: Stars

Exoplanets● What about the disk?

PHY 521: Stars

Two Types of Planets

(NASA)

PHY 521: Stars

Debris

(Minor Planet Center, Gareth Williams)

• Main asteroids (green)• Near-earth asteroids (red)• Comets (blue squares)• Trojan asteroids (blue dots)

PHY 521: Stars

Debris

PHY 521: Stars

Center of Mass Motion of the Sun

(Wikipedia)

PHY 521: Stars

Planet Detection Overview● Gravitational: observe motion of star around center of mass

– Directly: astrometric– Doppler: radial velocity

● Photometric– Transits – Direct Imaging

● Microlensing● All methods have their own strengths/weaknesses and biases

PHY 521: Stars

Planet Detection Methods

(M. Perryman/Exoplanet.eu)

PHY 521: Stars

Astrometric Technique● Look for wobble of parent star as it moves in its orbit around the

galaxy– Works very well for binary stars– Wobble is very small for planets– More pronounced for:

● Massive planet● Larger orbits (but this requires longer observations)● Closer stars

PHY 521: Stars

Radial Velocity Technique● Up until recently (Kepler) most exoplanets were discovered this way● Typical orbital velocities are 10s m/s● Mass of the star is estimated via spectroscopy● Observing a whole period tells use:

– Orbital period, and therefore semi-major axis– Eccentricity of the orbit– Minimum mass of the planet (M sin i)– Number of planets

● Best for massive planets close to the star– Higher v– Shorter P

PHY 521: Stars

Radial Velocity Technique

PHY 521: Stars

Radial Velocity Technique● 51 Pegasi b discovered this way

Radial velocity curve for 51 Pegasi (from http://www.extrasolar.net/planettour.asp?PlanetID=1)

Planet properties:

M = 0.47 MJ

P = 4.23 daysdistance = 0.052 AU

PHY 521: Stars

Radial Velocity● Multiple planets

Upsilon Andromeda system (Butler et al. 1999)

PHY 521: Stars

Transits● No dynamical information

● Period: from time between transits

● Planet radius: from amount of dimming and knowledge of parent star

● Composition: spectra during and outside of transit

● Surface T: IR observations—look for drop in flux

PHY 521: Stars

Transits● Weaknesses:

– Needs inclination of 90 degrees– Biased toward planets with short periods– Doesn't give mass

● Strengths:– Works for smaller (Earth-like planets)– Get's radius– Can be done with small telescopes

● Best of both worlds: transits + radial velocities– Density can then be determined

PHY 521: Stars

Direct Detection● High contrast● IR helps (for young

planets)● Only a handful

discovered this way

PHY 521: Stars

Direct Detection● Planet around β Pictoris● Note the debris disk

http://antwrp.gsfc.nasa.gov/apod/ap081128.html

Credit: ESO, A.-M. Lagrange (LAOG), et al.

PHY 521: Stars

Direct Detection● HR 8799

– 4 known planets– Large distances: challenge for

formation theory

(Marois et al. 2010, Nature)

(Wikipedia)

PHY 521: Stars

Direct Detection● Kuiper belt analogs (debris disks)

Credit: NASA, ESA, P. Kalas, J. G

ra ham, E. Chian g, E. Kite (U

niv. California , Berkeley), M. Clam

pin (N

ASA/Goddard), M

. Fitzgerald (Lawrence Liver m

ore NL), K. Stapelfeld t, J. Krist (N

ASA/JPL)

PHY 521: Stars

ImagingFirst image of a planet being formed around its star.

http://keckobservatory.org/news/first_close-up_view_of_a_planet_being_formed/

Figure 1 Left: The transitional disk around the star LkCa 15. All of the light at this wavelength is emitted by cold dust in the disk. the hole in the center indicates an inner gap with radius of about 55 times the distance from the Earth to the Sun. Right: An expanded view of the central part of the cleared region, showing a composite of two reconstructed images (blue: 2.1 microns, from November 2010; red: 3.7 microns) for LkCa 15. The location of the central star is also marked. Credit: Kraus & Ireland 2011

Artist’s conception of the view near the planet LkCa 15 b. Credit: Karen L. Teramura, UH IfA

PHY 521: Stars

Kepler Mission● Transit mission

– Launched 2009– Monitor 100,000 stars over 4 years– Look for transits lasting hours, recurring on month-year timescales– ~ 1 in 200 systems have proper orientation

● Planned to find ~ 50 earth's over the 4 year mission– Start are more active than they thought originally

● Will find many more Jovian planets● ~3000 candidates (orbits not confirmed)

– 134 confirmed exoplanets– Extrapolation: 40 billion Earth-sized planets orbiting habitable zones around

Sun-like a lower mass stars in our Galaxy● Gyroscope failure resulted in original mission termination.

PHY 521: Stars

Kepler Mission

http://kepler.nasa.gov/media/art.html#lomberg

http://www.nasa.gov/mission_pages/kepler/multimedia/images/fullFFIHot300.html

PHY 521: Stars

Kepler Candidates

PHY 521: Stars

Discovery History

PHY 521: Stars

Exoplanet Properties● Very few known exoplanets have orbits beyond 5 AU

● Different databases give different #s of known (and well-known) exoplanets

PHY 521: Stars

Exoplanet Properties

Mass of Earth ~ 0.003 MJ

Mass of Neptune ~ 0.05 MJ

PHY 521: Stars

Exoplanet Properties

PHY 521: Stars

Exoplanet Properties

PHY 521: Stars

Exoplanet Properties(from

Udry a nd Santos, ARAA 2007)

PHY 521: Stars

Exoplanet Properties

PHY 521: Stars

Exoplanet Properties(from

Udry a nd Santos, ARAA 2007)

PHY 521: Stars

Exoplanet Properties● Metals are likely important in making planets

PHY 521: Stars

Competing Ideas on Planet Formation

● For Jovian planets, there is some uncertainty

(NASA/ESA and A. Feild (STScI)

http://www.spacetelescope.org/images/html/opo0319f.html

PHY 521: Stars

Core Accretion vs. Gravitational Instability(Baraffe et al. 2010)

● Core accretion difficulties

– Growth of cores takes longer than typical disk lifetimes (few Myr)● Larger disks or lower opacity can help

– Turbulence in the disk may help● Gravitational instability difficulties

– Requires disk to cool– Only operates in massive disks and at large orbital distances (> 100 AU)– May not be able to explain the higher metallicities in giant planets

● Gravitational instability works quickly—young (1 Myr old) stars should already have gas planets

PHY 521: Stars

Core Accretion vs. Gravitational Instability

● HD 149026b is significantly denser than Saturn– May have 70 M⊙ of

heavy material– Supports core-accretion

model

(Baraffe et al. 2010)

PHY 521: Stars

Brown Dwarfs(Baraffe et al. 2010)

● At the high end of the mass scale of exoplanets, with brown dwarfs● Brown dwarfs are thought to form like stars, fragmentation of a giant

molecular cloud

– Doesn't have to be in the plane of the planetary disk● Planets form in the protoplanetary disk after star formation

– Either through core accretion or gravitational instability in the disk● Planets should have higher amounts of heavy metals than brown dwarfs

PHY 521: Stars

Large Sizes(Baraffe et al. 2010)

● Some hot Jupiters have very large radii– Close to parent star—stellar irradiation

● Doesn't explain the entire effect– Tidal heating?– Atmospheric circulation (slows cooling, larger radius at given age)?– Other physics (Ohmic heating?)

PHY 521: Stars

Comparing to Our Solar System● Few terrestrial planets in these systems (selection effect)● Much closer to star + higher eccentricities than our SS● Hot Jupiters: composition probably same as our Jovian planets—H/He

– Larger radii/lower density is due to closer orbit● Moving Jupiter 0.05 AU would make it 50% bigger in radius

– Higher T (1000 K) → rock-dust clouds!– Cloud bands, strong winds (extreme heating + synchronous rotation)

PHY 521: Stars

Solar System Formation Problems● How do you make a hot Jupiter?

– Nebular theory suggests that planet formation is a natural consequence of star formation

– Based on our solar system, Jupiters should be at large distances from their parent star

● Revisions based on exoplanet discoveries– Jovian planets migrate inward

PHY 521: Stars

Planetary Migration● Planet moving through disk creates density waves● Type I migration

– Difference in torque from inner and outer waves transports angular momentum

– Operates for planets < 0.1 Jupiter masses● Type II migration

– Planet clears a gap in the disk– Gas flow in the disk moves the gap, planet moves with it

PHY 521: Stars

Planetary Migration

PHY 521: Stars

Encounters and Resonances● Gravitational encounters → eccentric orbits

– Two Jovian planets get close: 1 ejected, one spirals inward, elliptical orbit– Small planetesimals ejected (to Oort cloud), Jovian planet loses orbital energy

● Happened in our SS● Resonances

– Lead to eccentric orbits– Can yield migration or ejection