STAR FORMING REGIONS AND YOUNG STELLAR...

STAR FORMING REGIONS AND YOUNG STELLAR OBJECTS

Antonio Maggio Istituto Nazionale di Astrofisica

Osservatorio Astronomico di Palermo

Messaggeri della Conoscenza Struttura, origine e caratterizzazione

dei pianeti nel Sistema Solare e in sistemi esterni

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 (proto-planetary) phases?

• Planet ageing and life development • How star-planet interactions influence planetary

environment and habitability conditions?

Some historic milestones • 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

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Note di presentazione

Vedi PSPF_Lec7 (Mayer) and amanda.pdf

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

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Observed (normal lines) and extrapolated (dotted lines) structure of the spiral arms. The gray lines radiating from the Sun's position (upper center) list the three-letter abbreviations of the corresponding constellations. H II regions are marked as dots colored in the same color as their spiral arm. They come in three sizes, measured by the excitation parameter U: small - U > 200 pc cm-2 medium - 200 > U > 110 pc cm-2 large - 110 > U > 70 pc cm-2

Mass distribution in our Galaxy

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).

Mass Scale radius Bulge 1.7 1010 M⊙ 0.5 kpc

Disk 3.4 1010 M⊙ 3.2 kpc

Dark Halo (< 8 kpc) 2.7 1010 M⊙ R < Sun distance from Galactic center

Dark Halo (< 12.5 kpc) 5.1 1010 M⊙ 12.5 kpc

Dark Halo (< 20 kpc) 8.9 1010 M⊙ 20 kpc

A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo

Luminous (massive) stars mostly in arms of spiral galaxies

Spiral arms traced by Bright H II regions,

associated to early-type (OB) stars

Dark (dusty) lanes, associated to Giant Molecular Clouds

Location of Star Forming Regions


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)

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Halpha (Balmer series transition n=3->2)

Linda Huff (American Scientist), Priscilla Frisch (U. Chicago)

The Local Bubble and the Galactic Neighborhood

Supernova

Remnant (SNR) and H II region (Gum Nebula)

Hot, low-

density Interstellar Medium

Molecular

Clouds and Star-Forming Regions

http://antwrp.gsfc.nasa.gov/apod/image/0004/galacticneighborhood_frisch_big.gif

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


• H II regions carved by O/B star radiation inside a giant molecular cloud in the LMC

Sequential star formation

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The star-forming region, catalogued as N11B, lies in the Large Magellanic Cloud (LMC), located only 160,000 light-years from Earth. The image illustrates a perfect case of sequential star formation in a nearby galaxy where new star birth is being triggered by previous-generation massive stars. A collection of blue- and white-colored stars near the left of the image are among the most massive stars known anywhere in the universe. The region around the cluster of hot stars in the image is relatively clear of gas, because the stellar winds and radiation from the stars have pushed the gas away. When this gas collides with and compresses surrounding dense clouds, the clouds can collapse under their own gravity and start to form new stars. The cluster of new stars in N11B may have been formed this way, as it is located on the rim of the large, central interstellar bubble of the N11 complex. The stars in N11B are now beginning to clear away their natal cloud, and are carving new bubbles in turn. Yet another new generation of stars is now being born in N11B, inside the dark dust clouds in the center and right-hand side of the Hubble image. This chain of consecutive star birth episodes has been seen in more distant galaxies, but it is shown very clearly in this new Hubble image. Farther to the right of the image, along the top edge, are several smaller dark clouds of interstellar dust with odd and intriguing shapes. They are seen silhouetted against the glowing interstellar gas. Several of these dark clouds are bright-rimmed because they are illuminated and are being evaporated by radiation from neighboring hot stars.

J. Hester, P. Scowen (ASU), HST, NASA

Evaporating Gaseous Globules (EGGs) in

the Eagle Nebula (M16)

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These pictures from NASA's Hubble Space Telescope show newborn stars emerging from rather dense, compact pockets of interstellar gas called evaporating gaseous globules (EGGs). Hubble found the "EGGs" in the Eagle nebula, a nearby star-forming region 6,500 light- years away in the constellation Serpens. Hubble gives a clear look at what happens as a torrent of ultraviolet light from nearby young, hot stars heats the gas along the surface of the pillars, "boiling it away" into interstellar space — a process called "photoevaporation. The Hubble pictures show photoevaporating gas as ghostly streamers flowing away from the columns. But not all of the gas boils off at the same rate. The EGGs, which are denser than their surroundings, are left behind after the gas around them is gone.

http://antwrp.gsfc.nasa.gov/apod/image/0111/pillars4_hst_big.jpg

Molecular CO distribution in Orion


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http://www.ualberta.ca/~pogosyan/teaching/ASTRO_122/lect14/lecture14.html

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 time-average 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 = − 𝒅𝑬𝒅𝒅

= − 𝒅𝑲𝒅𝒅

+ 𝒅𝑼𝒅𝒅

= −𝟏𝟐𝒅𝑼𝒅𝒅

and 𝒅𝑲𝒅𝒅

= −𝟏𝟐𝒅𝑼𝒅𝒅


Jeans criterion for collapse


Properties of Molecular Clouds

Exercise: verify if the above astrophysical environments are gravitationally stable according to the Jean criterion.

Molecular Clouds

Clumps in Molecular Clouds

Molecular Cloud Core

Mass 104.5 – 106.5 M⊙ 10 – 102 M⊙ 1 – 10 M⊙

Size 10 – 100 pc 1 – 10 pc ≤ 1 pc

Temperature 10 – 30 K 10 – 30 K 1 – 10 K

Density 102 – 103 cm-3 103 – 104 cm-3 104 – 105 cm-3


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


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http://ned.ipac.caltech.edu/level5/Sept10/Krumholz/Krumholz4.html

Alves et al. (2001), Graham (1998), Foster & Goodman (2006), Steinacker et al. (2010).

• 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

Pre-collapse dark cloud B68

Magnetic fields in Orion

Goodman et al.; Goncalves et al. 2005

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http://www.berkeley.edu/news/media/releases/2006/01/12_helical.shtml

Hydrodynamical Simulation

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The largest, most complex hydrodynamical star formation calculation ever performed (2009) (http://www.astro.ex.ac.uk/people/mbate/Animations/)

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


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)

Observed IMFs


Simplified Scheme for the IMF


Possible physical mechanisms behind the IMF


30

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)


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


Relatore


New evidence from NASA's Spitzer Space Telescope is showing that tight-knit twin stars might be triggered to form by asymmetrical envelopes like the ones shown in this image. All stars, even single ones like our sun, are known to form from collapsing clumps of gas and dust, called envelopes, which are seen here around six forming star systems as dark blobs, or shadows, against a dusty background. The greenish color shows jets coming away from the envelopes. The envelopes are all roughly 100 times the size of our solar system. Two of the six star systems are known to have already formed twin, or binary stars (Spitzer can see the envelopes but not the stars themselves). Astronomers believe that the irregular shapes of the envelopes, revealed in detail by Spitzer, might trigger binary stars to form, or might have already triggered them to form. From top left, moving clockwise, the stars are: IRAS 03282+3035, CB230, IRAS 16253-2429, L1152, L483, HH270 VLA1. IRAS 03282+3035 and CB230 are the two known to have already formed binary stars. Infrared light with a wavelength of 3.6 microns has been color-coded blue; 4.5-micron light is green; and 8.0-micron light is red.

IR images of Protostars


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The "Cores to Disks" Spitzer Legacy team is using the two infrared cameras on NASA's Spitzer Space Telescope to search dense regions of interstellar molecular clouds (known as "cores") for evidence of star formation. Part of the study targeted a group of objects with no known stars to study the properties of such regions before any stars have formed. The first of these "starless cores" to be examined held a surprise: a source of infrared light appeared where none was expected. The core is known as L1014, the 1,014th object in a list of dark, dusty "clouds" compiled by astronomer Beverly Lynds over 40 years ago. These have proved to be homes to a rich variety of molecules and are the birthplaces of stars and planets. The Spitzer image is a 3.6 micron (blue), 8.0 micron (green) and 24.0 micron (red) composite image. The light seen in the infrared image originates from very different sources. The bright yellow object at the center of the image is the object detected in the "starless core". The red ring surrounding the object is an artifact of the reduced spatial resolution of the telescope at 24 microns. At 3.6 microns the light comes mainly from the object at the heart of the core. At longer wavelengths, the light from the object becomes stronger, a signature that it is not a background star. Also in the longer wavelengths (8.0 to 24.0 microns), astronomers saw the glow from interstellar dust, glowing green to red in the Spitzer composite image. This dust consists mainly of a variety of carbon-based organic molecules known collectively as polycyclic aromatic hydrocarbons. The red color traces a cooler dust component. No previous observations showed any hint of a source in L1014. For example, the visible light image is from the Digital Sky Survey and is a B-, R-, and I-band composite image (wavelengths ranging from 0.4 to 0.7 microns). The dark cloud in the center of the image is the core, completely opaque in the visible due to obscuration by dust. The L1014 core lies in the direction of Cygnus. It is thought to be about 600 light years away, but the distance is somewhat uncertain. The results from this study are published by C. Young and the "Cores to Disks" team in the Astrophysical Journal.

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)


Shu, Adams & Lizano 1987, ARAA

Magnetospheric Accretion Disk Model


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

Young Stellar Objects classification

STAR FORMING REGIONS AND YOUNG STELLAR OBJECTS

Part II Antonio Maggio

Istituto Nazionale di Astrofisica Osservatorio Astronomico di Palermo

Messaggeri della Conoscenza Struttura, origine e caratterizzazione

dei pianeti nel Sistema Solare e in sistemi esterni

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


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 density, 𝑁𝐻 = ∫𝑛𝐻 𝑑𝑑 , is a measure of the absorbing material along the line of sight)

Heavily obscured protostellar cores

Red 0.5–1.7 keV Green 1.7–2.8 keV Blue 2.8–8.0 keV


Grosso et al. (2005)

Relatore


43 sources found in 40''×50'' region around the Becklin-Neugebauer object and Kleinmann-Low nebula (collectively BN-KL), 18 newly discovered. Colors indicate photon energies.

X-ray luminosity of YSOs in Orion

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

Preibisch et al. (2005)


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

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

Favata et al. (2005)

Sizes of flaring regions and peak plasma temperatures

• 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)

SYNCHROTRON

NON-THERMAL Bremsstrahlung

Active stars

Orion bright flares

Super-hot

flares

Getman et al. (2008a)

Magnetospheres of accreting YSOs

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...

Getman et al. (2008b)


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

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)

Getman et al. (2008b) Sun 4x


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? A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo

X-ray Diagnostics of X-rayed Disks

Reflection of X-rays off the

disk ⇒ Fluorescent 6.4 keV iron emission line following photo-ionization of a K-shell electron

Absorption of X-ray emission from central stars by gas in disks with edge-on orientation (called proplyds)

Imanishi et al. (2001) Kastner et al. (2005)


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

Carr et al. (2004)

H2O

CO


XUV Irradiation of Circumstellar Disks 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)

Temperature

Ionization Fraction

Verti

cal H

eigh

t [A

U]

Radial distance [AU]

Verti

cal H

eigh

t [A

U]


Magneto-Rotational Instability (MRI) (simple mechanical model)


Disk ionization models Ingredients: X-ray photo-

ionization, 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

Ilgner & Nelson (2006)

Col

umn

Den

sity

Radial distance

dead zone

X-ray active zone

Thermal active zone


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


Relatore


Johansen et al. 2007, Nature During the initial stages of planet formation in circumstellar gas disks, dust grains collide and build up larger and larger bodies1. How this process continues from metre-sized boulders to kilometre-scale planetesimals is a major unsolved problem2: boulders are expected to stick together poorly3, and to spiral into the protostar in a few hundred orbits owing to a 'headwind' from the slower rotating gas4. Gravitational collapse of the solid component has been suggested to overcome this barrier1, 5, 6. But even low levels of turbulence will inhibit sedimentation of solids to a sufficiently dense midplane layer2, 7, and turbulence must be present to explain observed gas accretion in protostellar disks8. Here we report that boulders can undergo efficient gravitational collapse in locally overdense regions in the midplane of the disk. The boulders concentrate initially in transient high pressure regions in the turbulent gas9, and these concentrations are augmented a further order of magnitude by a streaming instability10, 11, 12 driven by the relative flow of gas and solids. We find that gravitationally bound clusters form with masses comparable to dwarf planets and containing a distribution of boulder sizes. Gravitational collapse happens much faster than radial drift, offering a possible path to planetesimal formation in accreting circumstellar disks.

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


Sun

Ribas et al. (2005)

X-ray emission at later epochs X-ray luminosity

decreases with age Wind-driven loss of

stellar angular momentum causes less effective dynamo action

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

Oce

ans

and

atm

osph

ere

on E

arth

Firs

t bac

teria

Euc

ario

tes

Now

Preibisch & Feigelson (2005)

Med

ian

log(

L x) [

erg/

s]

log(age) [yr]

1.0 M

0.7 M

0.3 M


Sun

Ribas et al. (2005)

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

Oce

ans

and

atm

osph

ere

on E

arth

Firs

t bac

teria

Euc

ario

tes

Now

1.0 M

0.7 M

0.3 M


Sun compared with Active Stars Sun Young Active Stars

X-ray luminosities

Lx/Lbol ~ 10-6 (quiescent)

Lx/Lbol ~ 10-5 (large flares)

Lx/Lbol ~ 10-3 (quiescent)

Lx/Lbol ~ 10-1 (large flares)

Occurrence of large flares

1 every 10 days (at max of solar cycle)

A few per day (no magnetic cycle?)

Flare time scales up to a few hours up to a few days

Coronal plasma temperatures

≈ 106 K (quiescent)

≈ 107 K (flaring)

≈ 107 K (quiescent)

≈ 108 K (flaring) !!!


Flare energy distributions


Relatore


Normalized cumulative distributions of flare total energies observed in X-rays in the Orion Nebula Cluster (ONC) with the Chandra satellite (COUP project) and in the Taurus-Auriga Molecular Cloud (TMC) with the XMM-Newton satellite (XEST project).

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)

http://www.astro.ethz.ch/education/courses/Physics_of_Star_and_Planet_Formation/pspf_program

http://www.astro.ethz.ch/education/courses/Physics_of_Star_and_Planet_Formation/pspf_program

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