Diapositiva 1

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Lecture 6 Star Formation

• The formation of stars
• gas clouds collapse
• Initial Mass Functions
• parameterizations
• Observational indicators of Star Formation (SF)
• recombination lines
• UV continuum
• FIR from dust emission
• radio continuum
• CO from molecular clouds
• Lyman Break Galaxies
• Star Burst (SB) galaxies
• The SF along the cosmic time

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• The formation of stars

• star formation process is not well established
  due to non hydrostatic equilibrium state of the
• proto-star entities, the complexity of ISM
  and the unknown internal dynamics of clouds
• (that prevents us of choosing which
  initital condition to use), importance of the
• magnetic fields, interactions of borning
  stars with their neighbours in clusters, etc
• young star clusters (OC) are invariably
  associated with with dense interestellar clouds
  and
• with spiral arms

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• The formation of stars

• Gas clouds collapse
• gas in Giant Molecular Clouds (molecular and
  atomic gas, with some dust, and 106-107 M?)
• may be eventually compressed by shock
  fronts
• fragmentation and gravitational collapse take
  place, against some combination of
• resisting turbulent and magnetic energies
  (as the ionization level of material, governed
• by high energy radiation and cosmic ray
  particles, becomes small, the coupling of
  magnetic
• field weakens and it dissipates)
• as the cloud fragment free-falls, gravitational
  energy is released as radiation (luminosity
• rises rapdly but temperature remains quite
  low, 10-20K, but growing in the center)
• the dense core (proto-star) gradually becomes
  opaque, trapping the radiation and heating up

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• The formation of stars

• at about 2000 K, H2 molecules dissociates into
  HI, sinking energy and accelerating the
• collapse of the core
• temperature continues rising and H becomes
  ionized (HII)
• free e- pressure starts to balance the collapse
  that eventually halts in the core
• convective proto-star reachs the Hayashi limit
  in the CM diagram and enter the hydrostatic
• equilibrium regime (becoming a T-Tauri
  star)
• the core starts to burn D and Li and becomes
  radiative, the envelope continue contracting
• at nearly constant luminosity, the
  effective temperature continues rising, violent
  surface
• activity and strong protostellar winds
  occur (cleaning the remaining envelope)
• ultimately the core temperature rises to the
  point at which the thermonuclear fusion of H
• into He becomes possible, and the star
  settles on to the ZAMS

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• The formation of stars

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• The formation of stars

• Associations
• chain reaction

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• Initial Mass Functions

• distribution of masses of a freshly formed (just
  after a burst of SF) stellar population
• dN N0 ?(M) dM
• where N0 is the normalizing constant
  the number of solar masses contained in the
  burst,
• and ?(M) is the IMF
• we have no a priori reason to suppose that ? is
  a universal function that applies to all SB,
• but observations indicate consistency at
  least for M ? M?
• determinations of the IMF are more difficult and
  unreliable for low mass stars, because these
• stars are slow to settle on the MS and
  their spectra deviate significantly from that of
  a
• black-body

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• Initial Mass Functions

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• Initial Mass Functions

Salpeter 1955, ApJ 121, 161
Miller Scalo 1979, ApJS 41, 513
Scalo 1986, Fundam. Cosmic Physics 11, 1
Kroupa, Tout Gilmore 1993, MNRAS 262, 545
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• Initial Mass Functions

• Parametrizations of the IMF
• Kennicutt, Tamblyn Congdon 1994, ApJ 435, 22
  argued, from the position of their galaxies
• in the B-V, W(Ha) plane, that the IMF
  in these systems has to be about as rich in
  massive
• stars as the Salpeter IMF predicts

Kennicutt 1983, AJ 88, 1094 IMF close to
Salpeter
at high masses
Kennicutt, Tamblyn Congdom 1994, ApJ 435, 22
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• Observational indicators of Star Formation

• Past SF
• colors

• Present SF
• recombination emission lines
• UV continuum from hot stars
• thermal far infra-red (FIR) from dust
• radio continuum
• CO emission from molecular clouds

• Future SF
• amount of gas available

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• Observational indicators of Star Formation

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• Observational indicators of current Star
  Formation

• Recombination emission lines
• line emission is characteristic of HII regions
  (zones of ionezed gas, around young star
  clusters)
• Mechanism ? H is ionized by absorption of
  Lyman continuum photons (? lt 912 Å, energy
• above 13.6 eV)
  produced by the hot OB stars
• ? the line radiation we
  detect arises from the recombination of the e so
  released with
• another p, and a
  cascade toward the ground state

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• Observational indicators of current Star
  Formation

• Recombination emission lines
• predominantly Balmer lines, especially Ha (the
• strongest and easiest to deal with)
• narrow band imaging at Ha is usually used to
  find
• HII regions
• SF rates may be calculated from the intensity of
  Ha
• lines (and by measuring equivalent
  widths)
• SFR M?/year 8.93 ? 10?42 L(Ha) erg/s
• SFR M?/year 1.4 ? 10?41 L(OII?3727) erg/s

Kennicutt 1983, ApJ 272, 54
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• Observational indicators of current Star
  Formation

• Recombination emission lines
• not surprinsingly, the SFR per unit starlight
  climbs to later Hubble types

Kennicutt Kent 1983, AJ 88, 1094 ?
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• Observational indicators of current Star
  Formation

• Ultraviolet continuum
• stars not massive or hot enough to produce HII
  regions, but also young (less than 109 years
• for early A stars, p.e.) can be traced by
  their brightness in UV
• Mechanism ? they produce a UV continuum in SF
  galaxies at wavelengths longer than the
• Lyman limit (? 912Å)
• this continuum is remarkbly flat, as can be
  modelled by spectral synthesis codes (first noted
• by Lilly Cowie 1987, Infrared
  Astronomy w/ Arrays and Cowie 1988, ApJL 332,
  L29) this
• is due to the fact that, although these
  luminous stars have short lifetimes, they are
  constantly
• being replaced by new stars
• the intensity of the flat part of the
  UVcontinuum is directly proportional to the rate
  of
• formation of heavy elements, since these
  stars are the ones that produce supernovaes
• reddening (extinction) is a strong effect and
  correspondly serious uncertainty

SFR M?/year 1.7 ? 10?28 L(UV1250-2500) erg
s?1 Hz?1
White 1989, The Epoch of Galaxy Formation
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• Observational indicators of current Star
  Formation

• Thermal far infra-red from dust emission
• SF galaxies are also strong emitters in the FIR
  waveband because of the presence of dust
• in the SF regions (on average, SF
  galaxies are stronger emitters in FIR than in the
  UV), as
• was first shown by IRAS survey
• Mechanism ? dust grains are heated by
  absorption of starlight, which operates most
  efficiently
• in the blue and UV (as
  the waveband comes closer to the characteristic
  grain size)
• ? dust cools again by
  (approximately) black-body emission, with peak
  about 20-40 K
• although the mechanism is not well understood,
  the total UV-optical energy from young stars,
• removed by dust absorption, must emerge
  in FIR (the luminosity from 10-300 µm may
• equals that emitted originally from
  912-3000 Å)m
• since there is an empirical tight relation
  between total FIR emission and Ha, this radiation
  
• may be strongly coupled to current SFR

SFR M?/year 1.3 ? 10?29 L(FIR60?m) erg s?1
Hz?1
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• Observational indicators of current Star
  Formation

M82

• Radio continuum
• SF galaxies also emit in cm radio waveband, much
  of which must be connected, directly or
• no, with SF
• Mechanism ? emission is nonthermal, from
  synchrotron process (accelerated particles,
  perhaps
• in SNe remnants,
  radiating while spiralling through large-scale
  magnetic fields)
• radio emission is a valuable tracer in heavily
  obscured regions VLA mapping proved to be
• a useful tool in identifying IR-loud
  galaxies at faint optical magnitudes since its
  positions
• are much more accurate than IRAS
  centroids.

SFR M?/year 5.9 ? 10?29 L(HI1.42GHz) erg s?1
Hz?1
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• Observational indicators of current Star
  Formation

• CO emission lines from molecular clouds
• since Giant Molecular Clouds are the immediate
  precursors of SF, their mapping is also an
• indicator of SF
• CO molecules, the most abundant after H2,
  detected
• at 1.3 and 2.6 mm (230 and 115 GHz) are
  usually
• the tracers (H2 cannot be observed in
  radio domain
• because it is symmetric and does not
  possess an
• eletric dipole)

Antenae
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• Lyman Break Galaxies

• Photometric search of high z
• SF galaxies
• a color-selection technique for identifying
• high redshift galaxies (with flat rest UV
• continuum, like SF galaxies, and small
• extinction) was first used by Steidel
• Hamilton 1992, AJ 104, 941, by Lilly et
• al. 1995, ApJ 455, 108 for CFRS data
• and Madau et al. 1996, MNRAS 283,
• 1388 for HDF data, the last ones to
• measure SF at earlier times.
• since Lyman break is observed at
• 912(1z) Å, for a z 2.5 it is found
• in the optical galaxies with z 3
• will be seen in B, V and R filters,
• with similar magnitudes, but will not
• be seen in U filter. These are called
• Lyman break galaxies

Steidel 1999, PNAS USA 96, 4232
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Mark 357

• Star Burst (SB) galaxies

• some galaxies show evidence of a
• recent and transient increase in SFR
• by as much as a factor of 50
• (hundreds of M?/year)
• since the galaxy gas is rapdly
• consumed in the SF, exhaution
• timescales are of order 107-108
• years
• the burst is often confined to a few hundred pc
  near the nucleus, although disc-wide bursts are
• also common
• SB are usually found in interacting galaxies,
  merging systems
• and bursting dwarves
• global (or super) winds are also found, powered
  by energy of
• starlight, stellar winds and supernovae

LMC (IRAS)
30 Dor
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• Star Burst (SB) galaxies

• Observational characteristics of SB
• large Balmer lines luminosity and equivalent
  width
• high ratio LFIR/LB
• unusual strong radio continuum emission
• optical spectra resembles those of HII regions

• Possible SB mechanisms
• cloud collisions in a perturbed disc
• collisions between clouds originally belonging
• to different galaxies
• channelling of gas through bars towards the
  center
• tidally induced density waves
• disk instabilities produced by perturbations in
  the
• gravitational potential
• physical transfer of gas during encounter (over
  a
• critical limit)
• direct impact of gas rich dwarf satellites into
  disks

Poggianti et al. 1999, ApJ 518, 576
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• Star Burst (SB) galaxies

M82 Ha SII, WIYN
M82 X-rays, Chandra
M82 UV
M82 IR, SAO
M82 radio, VLA
M82 opt, HST
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• SF along the cosmic time

• The Madau-Lilly plot
• the greatest rates of global SF occurred at z
  1-2!

Steidel 1999, PNAS USA 96, 4232
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• Star Burst (SB) galaxies

• SB ? AGN
• for very luminous galaxies, which are dusty
  enough that most of their power emerges in the
• FIR (once known as IRAS galaxies, now
  sharing such acronyms as LIRGs, ULIRGs, PIGs
• or ELFs) it is difficult to determine
  whether the dominant energy source is a SB or a
  AGN
• many galaxies exhibit both nuclear activity and
  considerable SF activity, specially
• interacting and merging systems

• SB
• energy supplied by OB stars
• more diffuse radio emission
• lack of high ionization species
• strong PAH features (6.2 µm, p.e.)
• flat UV continuum

• AGN
• energy comes from continuum produced by
• central accretion disk
• compact, flat radio spectrum
• high ionization species
• PAH features are destroyed by the intense
• hard radiation
• UV continuum inclined