Diapositiva 1 - Osservatorio Astronomico di Palermo

Transcript

Messaggeri della Conoscenza
Struttura, origine e caratterizzazione
dei pianeti nel Sistema Solare e in
sistemi esterni
STAR FORMING REGIONS AND
YOUNG STELLAR OBJECTS
Antonio Maggio
Istituto Nazionale di Astrofisica
Osservatorio Astronomico di Palermo
Relevance for exoplanets
• Stellar (and planetary) formation
₋ Understanding physical conditions and
mechanisms
⇒ Do stars and planets form at the same time?
⇒ Are planets hosted preferentially by stars with
specific characteristics?
• Planetary growth and dynamical evolution
• What is the role of central stars in early (protoplanetary) phases?
• Planet ageing and life development
• How star-planet interactions influence planetary
environment and habitability conditions?
Some historic milestones
•
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1755, Kant: The Nebular Hypothesis
1796, Laplace: Nebular break-up hypothesis
XVIII – XIX: observations of dark clouds
1904, Hartmann: Discovery of insterstellar gas
1930, Trumpler: Discovery of interstellar dust
1939, Spitzer: Theory of Turbulent Viscosity
1940s, Joy: Identification of T Tauri Stars
1950s: Radio astronomy and the study of ISM
1961, Hayashi: Theory of PMS evolution
1962, Herbig: T Tauri star formation in clouds
1977, Shu: Theory of cloud collapse
1980s – 1990s, IR observations of YSOs from space
 Early-type stars
and Star Forming
Regions are
mainly found
along the spiral
arms of the
Galaxy
 However, detailed
observations of
SFRs are possible
only within few
kpc from the Sun
Mass distribution in our Galaxy
Mass
Scale radius
Bulge
1.7 1010 M⊙
0.5 kpc
Disk
Dark Halo (< 8 kpc)
3.4 1010 M⊙
3.2 kpc
2.7 1010 M⊙
R < Sun distance from
Galactic center
Dark Halo (< 12.5 kpc)
Dark Halo (< 20 kpc)
5.1 1010 M⊙
12.5 kpc
8.9 1010 M⊙
20 kpc
Exercise: estimate the total mass within 8 kpc from
Galactic center, assuming a solar velocity of 220 km/s
along the Galactic orbit; compare your result with the
values reported above (Sofue Y. 2012, PASJ 64, 75).
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Location of Star Forming Regions
 Luminous (massive)
stars mostly in arms
of spiral galaxies
 Spiral arms traced by

Bright H II regions,
associated to earlytype (OB) stars

Dark (dusty) lanes,
associated to Giant
Molecular Clouds
Interstellar Medium in Galaxy Arms
• Hot gas
₋ Ionized hydrogen (H II regions), very bright in Hα
emission (656 nm)
₋ Other ionized low-Z elements, identified by their
forbidden-line emission (⇔ low-density gas)
• Cold gas
₋ Neutral hydrogen (H I regions), mainly observed in radio
(21-cm emission)
₋ Molecular hydrogen (H2) and several other molecules
including the most abundant elements (in particular C, N,
and O). Best observations at mm wavelength from CO.
• Dust
₋ Dark clouds (⇒ high extintion of background stars)
₋ Reflection Nebulas (⇐ Mie scattering)
The Local Bubble and the Galactic
Neighborhood  Supernova
Remnant
(SNR) and H II
region (Gum
Nebula)
 Hot, lowdensity
Interstellar
Medium
 Molecular
Clouds and
Star-Forming
Regions
Linda Huff (American Scientist), Priscilla Frisch (U. Chicago)
A typical stellar nursery
 Massive star forming
regions are complex
environments
 Expanding SNRs
 OB stars and associated wind
shocks
 Stars in different evolutionary
stages (protostars, T Tauri
stars, ecc.)
 Several hydro-dynamical
and radiative effects may
influence the early phases
of stellar formation
 About 40 Star Forming
Regions (SFRs) within 3
kpc already studied with
multi-wavelength
observations
 Best known SFRs, up to date:
Orion, Taurus-Auriga,
Ophiucus
Sequential star formation
• H II regions carved by O/B star radiation inside a giant
molecular cloud in the LMC
Evaporating Gaseous
Globules (EGGs) in
the Eagle Nebula
(M16)
J. Hester, P. Scowen (ASU), HST, NASA
Molecular CO distribution in Orion
CO emission (mm wavelength)
Brunch & Wittke (NAU)
Spectroscopy at millimeter wavelengths provides
diagnostics for temperature, density, and abundance of
cold melecular gas
Stability condition (Virial theorem)
General equation that relates the time-average of the
total kinetic energy, 𝑲 , of a stable system consisting
of N particles, bound by potential forces, with the timeaverage of the total potential energy, 𝑼 :
𝟐 𝑲 + 𝑼 =𝟎
On the other hand, during the evolution of the system,
the total energy is
𝑬= 𝑲 + 𝑼
Any decrease of the total internal energy of the system
goes into radiation
𝑳=−
𝒅𝒅
𝒅𝒅
=−
𝒅𝒅
𝒅𝒅
+
𝒅𝒅
𝒅𝒅
=
𝟏 𝒅𝒅
𝒅𝒅
−
and
𝟐 𝒅𝒅
𝒅𝒅
=
𝟏 𝒅𝒅
−
𝟐 𝒅𝒅
Jeans criterion for collapse
Properties of Molecular Clouds
Mass
Size
Temperature
Density
Molecular
Clouds
Clumps in
Molecular
Clouds
Molecular
Cloud Core
104.5 – 106.5 M⊙
10 – 102 M⊙
1 – 10 M⊙
10 – 100 pc
1 – 10 pc
≤ 1 pc
10 – 30 K
10 – 30 K
1 – 10 K
102 – 103 cm-3
103 – 104 cm-3
104 – 105 cm-3
Exercise: verify if the above astrophysical environments
are gravitationally stable according to the Jean criterion.
But, nature is complex…
• Molecular Clouds cannot be treated as an isolated
system: pressure of the gas in the external ISM
must be taken into account
⇒ Equilibrium solution is a «Bonnor-Ebert’s Sphere»
• Molecular clouds may start with a non-null angular
momentum (Exercise: compute angular momentum
of clumps in a molecular cloud, assuming a
characteristic velocity of 3×104 cm/s, and compare
it with that of the Sun, Earth, and Jupiter)
• Molecular Clouds are permeated with magnetic
fields (10-100 µG measured in Cloud Cores)
⇒ If the medium is ionized, the field is «frozen»,
and the magnetic flux, ∫ 𝐵 ∙ 𝑑𝑑, must be conserved
Pre-collapse dark cloud B68
• Cloud core density structure can be derived from extintion measurements
and compared with physical models of collapse
• Also constrain ISM-to-core dust grain size & composition
Alves et al. (2001), Graham (1998), Foster & Goodman (2006), Steinacker et al. (2010).
Magnetic fields in Orion
Goodman et al.; Goncalves et al. 2005
Hydrodynamical Simulation
Initial Mass Function (IMF)
• The IMF, ξ(M), introduced by Edwin Salpeter in
1955, gives the number of stars per unit mass
range
• The IMF can be deduced from observational
data (stellar counts in SFRs)
• Caveat: Most of the known IMFs are relative to
the local Galactic neighborhood, hence they
may not be valid in very far away, physically
different astropysical environments
According to Salpeter…
• The IMF follows a power law
d NS / d log10 M ~ M-1.35
or, in linear form:
ξ(M) = d NS / d M ~ M-2.35
where ξ(M) is the number of stars with mass
between M and M+dM.
• Low-mass stars dominate the stellar population!
Other IMF determinations
Observed IMFs
Marchi et al. (2010);
Bastian, Covey, & Meyer (2010, ARAA)
 Young SFRs, open
clusters, stellar
associations, and
field stars show very
similar slope at high
masses
 Some difference in
the low-mass branch
 Young SFRs and
associations tend to
have stars with
lower masses than
old open clusters
and globular clusters
(dependence on
metallicity)
Simplified Scheme for the IMF
Possible physical mechanisms behind the IMF
Universality of the IMF
• Open questions
• How can the IMF be independent from
initial conditions, such as rotation of the
protostellar cloud, presence of magnetic
fields, and chemical composition
(metallicity) of the environment?
• Can the physical mechanisms responsible
for the stellar IMF determine also the
planetary population? (see next lecture by
C. Argiroffi)
30
From cloud collapse to main sequence
HR diagram of the Orion Nebula Cluster
In practice, determination of
stellar ages is difficult!
IR images of Protostars
Color code:
Blue 3.6 µm
Green 4.5 µm
Red 8.0 µm
IR images of Protostars
IR images of YSOs with disks
Model of stellar collapse
including magnetic fields
• Problem: how can protostars get rid of excess angular
momentum and magnetic flux?
• Possible solution: Magnetized disks and jets
(see lecture by S. Orlando)
Magnetospheric Accretion Disk Model
Shu, Adams & Lizano 1987, ARAA
Young Stellar Objects classification
Relative lifetimes
Class 0: 0.2 Myr
Class I : 0.5 Myr
Class II:
3 Myr
Class III: 10 Myr
Lada 1987, Andrè et al. 1993
Messaggeri della Conoscenza
Struttura, origine e caratterizzazione
dei pianeti nel Sistema Solare e in
sistemi esterni
STAR FORMING REGIONS AND
YOUNG STELLAR OBJECTS
Part II
Antonio Maggio
Istituto Nazionale di Astrofisica
Osservatorio Astronomico di Palermo
X-ray emission of YSOs
Feigelson & Montmerle, 1999
• X-ray emission
signals the onset of
stellar magnetic
activity (but when?)
• Since then, stellar
activity starts
affecting the
evolution of the
environment
• Feedback on
stellar evolution,
disk longevity,
planetary formation
and primordial “space
climate”
X-ray view of the Orion Nebula Cluster
 About 1600 sources detected in 800 ks Chandra
observattion, mostly Young Stellar Objects with ages
105 −106 yr
Heavily obscured protostellar cores
BN/KL region in the
Orion Molecular
Cloud:
Several new sources
discovered in X-rays,
deeply embedded
22.2 < log NH < 23.6
(the hydrogen column
Grosso et al. (2005)
Red 0.5–1.7 keV Green 1.7–2.8 keV Blue 2.8–8.0 keV
density, 𝑁𝐻 = ∫𝑛𝐻 𝑑𝑑 , is a
measure of the absorbing
material along the line of
sight)
X-ray luminosity of YSOs in Orion
Preibisch et al. (2005)
 Mean X-ray luminosity Lx ≈ 10-3.5 Lbol (saturated level)
for non-accreting (class III) stars
 Depressed X-ray emission and large scatter for
accreting (class I–II) stars
Orion Nebula Cluster: the COUP movie
Continuous flaring activity for essentially all sources




Duration from minutes
to days
Peak X-ray luminosities
Lx ≈ 1032 erg/s
Total radiated energy
up to E ≈ 1036.5 erg
[0.5-8 keV]
Frequency of largest
flares: every few days
per object
X-ray Flare Characteristics
Favata et al. (2005)
 Fast rise and slower exponential decay
 Explained by hydrodynamical models of
magnetically-confined plasma undergoing
impulsive heating
 Analysis of broad-band X-ray spectroscopy data
allows to estimate the size of magnetic
structures where the flaring plasma is confined
Sizes of flaring regions and peak
plasma temperatures
Orion bright flares
SYNCHROTRON
Superhot
flares
Active stars
NON-THERMAL
Bremsstrahlung
Getman et al. (2008a)
• Scale lengths from 5 x 1011 to 5 x 1012 cm,
i.e. up to 10 − 20 R*
• Plasma peak temperatures up to 7 x 108 K
(but extreme values are poorly constrained)
Magnetospheres of accreting YSOs
Getman et al. (2008b)
 In accreting T Tauri stars, coronal extent limited by the disk
(truncated at the co-rotation radius)
 Closed field ⇒ hot plasma ⇒ X-ray emission, flares
 Open field ⇒ stellar wind ⇒ mass & angular momentum losses
 Star-disk field ⇒ accretion ⇒ shocks ⇒ soft X-ray excess
⇒ giant flares? Open issue...
Role of Magnetic Fields
 In accreting T Tauri stars,
magnetic fields may affect
⇒ hot plasma confinement
and heating
⇒ steady X-ray emission and
flares
⇒ mass inflow and outflow
⇒ angular momentum balance
⇒ star-disk locking and
magnetic torques
⇒ disk disruption
⇒inner edge for planet
migration?
Matt & Pudritz (2005)
Magnetospheres in non-accreting stars
Getman et al. (2008b)
Sun 4x
 Coronal extent limited by gas/B-field pressure ratio
 Fast rotation ⇒ efficient magnetic dynamo ⇒ enhanced
activity and X-ray emission
 Intense high-energy irradiation of young planetary bodies
 Coronal Mass Ejections and Stellar Energetic Particle flows
 Possible star-planet magnetospheric interaction (see next
lecture by A. Maggio)
Stellar activity effects on
Circumstellar/Protoplanetary Disks
 Circumstellar disks are subject to high-energy radiation,
winds, and energetic particles originating from the central
star ⇒ heating, ionization, evaporation
 How is planetary formation affected?
X-ray Diagnostics of X-rayed Disks
Kastner et al. (2005)

Absorption of X-ray
emission from central
stars by gas in disks
with edge-on
orientation (called
proplyds)
Imanishi et al. (2001)
 Reflection of X-rays off the
disk ⇒ Fluorescent 6.4 keV
iron emission line following
photo-ionization of a K-shell
electron
IR Diagnostics of X-rayed Disks
 Detection of
[Ne II] 12.81 µm
line emission
Pascucci et al. (2007)

Hot H2O and CO
molecular layer
observed is some
YSOs
CO
H2O
Carr et al. (2004)
Vertical Height [AU]
XUV Irradiation of Circumstellar Disks
Temperature
Vertical Height [AU]
Radial distance [AU]
Ionization
Fraction
 Heating and ionization of
gas in disk outer layers out
to several AU
 B-field freezing
⇒ disk truncation
 MRI turbulence
⇒ angular momentum
transport
⇒ mass accretion
⇒ planetary migration?
 Out of equilibrium
molecular chemistry in the
disk interior
Ilgner & Nelson (2006)
Magneto-Rotational Instability (MRI)
(simple mechanical model)
Column Density
Disk ionization models
Thermal
active
zone
dead zone
X-ray active zone
Radial distance
Ilgner & Nelson (2006)
 Ingredients: X-ray photoionization, viscous heating,
radiative cooling, and
turbulent mixing of gas,
dust and grain phases, with
given chemical composition
 Boundary between active
(= turbulent) and dead
(= laminar) zone occurs at
very low ionization fraction,
log Xe ~ -12
 The size of the dead zone
likely depends on the
frequency, energetics, and
hardness of stellar flares
Effects on planets
 Traditional theory of growth
from interstellar grains to
larger bodies requires calm
dynamics and gravitational
settling towards the disk
midplane




X-ray induced MRI turbulence produces inhomogeneities and
gravitational torques ⇒ inhibited sedimentation of solids
⇒ planetary formation possibly occurring in dead zones only
When formed, planetesimals undergo random walks, rather than
simple migration
Close-in gaseous planets have magnetosphere which interact
with the stellar one ⇒ enhanced activity
Planet atmospheres are subject to high-energy irradiation and
stellar winds ⇒ heating, evaporation
Stellar X-ray source variability
• Stellar coronal sources are known to vary
on several time scales
₋ Short-term (from minutes to a few days)
variability due to flares
₋ Medium-term variability (from a few hours to
tens of days): rotational modulation
₋ Long-term variability (years) due to magnetic
cycles
• X-ray emission from YSOs may vary, at
least in principle, also due to
• Variable accretion rate
• Absorption by the circumstellar disk
Preibisch & Feigelson (2005)
Ribas et al. (2005)
log(age) [yr]
Sun
Now
0.3 M
 Wind-driven loss of
stellar angular
momentum causes
less effective dynamo
action
Eucariotes
0.7 M
 X-ray luminosity
decreases with age
First bacteria
Median log(Lx) [erg/s]
1.0 M
Oceans and atmosphere on Earth
X-ray emission at later epochs
 Stellar coronae
become cooler
 Softening of the
spectrum, i.e. less
high-energy photons
 Decreasing flaring
activity
 Smaller increase of
X-ray emission, less
frequent high-energy
events
Sun
 Decreasing flaring
activity
Now
0.3 M
 Softening of the
spectrum, i.e. less
high-energy photons
Eucariotes
0.7 M
 Stellar coronae
become cooler
First bacteria
1.0 M
Oceans and atmosphere on Earth
X-ray emission at later epochs
 Smaller increase of
X-ray emission, less
frequent high-energy
events
Ribas et al. (2005)
Sun compared with Active Stars
Sun
X-ray
luminosities
Lx/Lbol ~ 10-6 (quiescent)
Young Active Stars
Lx/Lbol ~ 10-3 (quiescent)
Lx/Lbol ~ 10-5 (large flares) Lx/Lbol ~ 10-1 (large flares)
Occurrence of 1 every 10 days
large flares
(at max of solar cycle)
A few per day
(no magnetic cycle?)
Flare time
scales
up to a few days
up to a few hours
Coronal
≈ 106 K (quiescent)
plasma
7 K (flaring)
≈
10
temperatures
≈ 107 K (quiescent)
≈ 108 K (flaring)
!!!
Flare energy distributions
BIBLIOGRAPHY
• M. Zeilik, Astronomy (9th edition)
Chapter 11: The Origin and Evolution of the Solar System
Chapter 14: Starbirth and Interstellar Matter
(Elementary level)
• M. Harwit, Astrophysical Concepts (2nd edition)
Chapter 1 and Chapter 9 (Cosmic Gas and Dust)
(Intermediate level)
• M. Meyer (ETH/IfA on-line course)
Physics of Star and Planet Formation
http://www.astro.ethz.ch/education/courses/Physics_of_Star_a
nd_Planet_Formation/pspf_program
(Advanced level)
• E.D. Feigelson, T. Montmerle
High-Energy Processes in Young Stellar Objects
1999, ARAA, Vol. 37, p. 363
(Advanced level)

Diapositiva 1 - Osservatorio Astronomico di Palermo

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