High-mass star forming regions:An ALMA view

Riccardo CesaroniINAF - Osservatorio Astrofisico di Arcetri

IR-dark (cold) cloudfragmentation

(hot) molecular coreinfall+rotation

(proto)star+disk+outflowaccretion

hypercompact HII regionexpansion

extended HII region

Possible evolutionary sequence for high-mass stars

turbulence?

gravitation?

magnetic field?

IR-dark clouds

• Detected in absorption at 8 µm with ISO, MSX, SPITZER (Perault et al. 1996; Egan et al. 1998, GLIMPSE) cold and dense

• Confirmed in sub-mm cont. emission with SCUBA (Feldman et al. 2000) and H2CO line (Carey et al. 1998) 2-8 kpc, 103-104 MO, 1-5 pc, 105 cm-3, < 20 K

• Mapped in NH3 line with 100-m telescope (Pillai et al. 2006) 10-20 K, 103-104 MO, line FWHM < 3.5 km/s

MSX 8 m SCUBA 850 m

Carey et al. (2000)

MSX 8 m MSX 8 m

SCUBA 850 m SCUBA 850 m

IR-dark clouds

• Detected in absorption at 8 µm with ISO, MSX, SPITZER (Perault et al. 1996; Egan et al. 1998, GLIMPSE) cold and dense

• Confirmed in sub-mm cont. emission with SCUBA (Feldman et al. 2000) and H2CO line (Carey et al. 1998) 2-8 kpc, 103-104 MO, 1-5 pc, 105 cm-3, < 20 K

• Mapped in NH3 line with 100-m telescope (Pillai et al. 2006) 10-20 K, 103-104 MO, line FWHM < 3.5 km/s

NH3 in IR-dark clouds

Pillai et al. (2006)

NH3 line FWHM and temperature in IR-dark clouds

Sridharan et al. (2005)

IR-darkclouds IR-dark

clouds

• Evidence of sub-structure (cores) from PdBI maps of 1mm cont. & CO isotopomers (Rathborn et al. 2005) 10-2000 MO, embedded stars (outflows) in 30% of cores

• Evidence of embedded protostars from Spitzer images at 3.6 & 24 µm (Carey et al. 2002) low- to intermediate-mass stars

IR-dark clouds may be the very first stage of the high-mass star formation process

Cloud structure: core MF = stellar IMF ? hint on star formation process: IMF set before or after fragmentation?

Cloud/core velocity field: turbulence (Mc Kee & Tan 2002) or gravitation (Bonnell et al. 2004)? discriminate between different models

ALMA contribution: will resolve cloud structure & velocity field on

all scales from 500 AU to >1 pc will detect all cold cores up to 20 kpc

Beuther & Schilke (2004)

core MF = stellar (Salpeter) IMF

dN/dM~M-2.5

Cloud structure: core MF = stellar IMF ? hint on star formation process: IMF set before or after fragmentation?

Cloud/core velocity field: turbulence (Mc Kee & Tan 2002) or collapse (Bonnell et al. 2004)? discriminate between different models

ALMA contribution: will resolve cloud structure & velocity field on

all scales from 500 AU to >1 pc will detect all cold cores up to 20 kpc

Proper motions in Orion (Rodriguez et al. 2006)

ALMA can do the same up to 10 kpc!

12 km/s

27 km/s

500 AU

1985 2002

Cloud structure: core MF = stellar IMF ? hint on star formation process: IMF set before or after fragmentation?

Cloud/core velocity field: turbulence (Mc Kee & Tan 2002) or gravitation (Bonnell et al. 2004)? discriminate between different models

ALMA contribution: will resolve cloud structure & velocity field on

all scales from 500 AU to >1 pc

will detect cold cores >0.1 MO up to 10 kpc

Numerical simulationsof 1-pc clump collapse

Bate et al. (2003)

ALMA beam350GHz 10kpc

Continuum spectrum of cold core

(sensitivity estimates for 5 hr ON-source)

Note: MJeans ≈ 0.5 MO

3σ ALMA

3σ SMA

3σ PdBI

3σ VLA

3σ ALMA

3σ SMA

3σ PdBI

3σ VLA

Hot molecular cores

• Typically: <0.1 pc, >100 K, 107 cm-3, >104 LO

• Rich chemistry: evaporation of grain mantles

• Sometimes with embedded UC HII regions

Believed to be the cradles of OB stars Association with outflow, infall, and rotation

(disks) expected

Cesaroni et al. (1998); Hofner (pers. comm.)

UC HII

HMC

B0.5

B0.5

B0

B1

Hot molecular cores

• Typically: <0.1 pc, >100 K, 107 cm-3, >104 LO

• Rich chemistry: evaporation of grain mantles

• Sometimes with embedded UC HII regions

Believed to be the cradles of OB stars Associated with outflow, infall, and rotation

Hot molecular cores: outflows

High angular resolution needed to resolve multiple outflows, not to image single outflow

Requirements: • star separation in cluster ≈ 0.05 pc = 0.5”-10” • line wings >> 1 km/s• line intensity = few K very easy for ALMA! E.g. 1” resol., 1 hr ON-

source, 1 km/s resol. 1σ = 0.1 K can image any outflow in the Galaxy

Beuther et al. (2002, 2003)

IRAM 30m2 outflows

IRAM PdBI:6 outflows!

Hot molecular cores: outflows

High angular resolution needed to resolve multiple outflows, not to image single outflow

Requirements: • star separation in cluster ≈ 0.05 pc = 0.5”-10” • line wings >> 1 km/s• line intensity = few K very easy for ALMA! E.g. 1” resol., 1 hr ON-

source, 1 km/s resol. 1σ = 0.1 K can image any outflow in the Galaxy

Other advantages of ALMA for outflow studies:

• Measurement of proper motions: 100 km/s at 1 kpc imply 20 mas/yr (at 90 GHz, 1/3 beam ≈ 15 mas) outflow inclination wrt l.o.s. from Vl.o.s./Vp.m. derivation of deprojected outflow parameters

• Imaging from 0.01 pc to 1 pc (in different tracers) possible outflow precession

0.7 pc

200 AU

Lebròn et al.(2006)

Moscadelli et al. (2005)

IRAS20126+4104

ALMA

Hot molecular cores: infall

Important to test models for OB star formation, but difficult to detect/recognize: e.g. line broadening towards star may be due to optical depth and/or turbulence

Methods & requirements:

• Red-shifted self-absorption temperature gradient and thick line(s) [for any star]

• Red-shifted absorption optically thick, embedded HII region [only for OB stars]

Absorption line tracing

infall ina core with

embedded HII region

HMC

100 K

104 K

Infall velocity field

from NH3 absorption

towards HII region

Sollins et al. (2005)

beam=0.24”=1400 AU

maximum redshifttowards star

G10.6-0.4

Red-shifted absorption is a very powerful method to measure infall, but can be used only if:

1. instrumental beam matches HII region diameter 2 RHII = HPBW(ν) = 0.012” [350/ν(GHz)]

2. free-free emission is optically thick ff(ν) > 1 TB=104 K

3. Core opacity is low dust(ν) < 1

relationships between distance & NLyman and between frequency & NLyman

RHII = 50-1000 AU for B0.5-O4 star

Absorption experiment: HII regions usable to trace infall in absorption:

– all HIIs in B0.5 stars (or earlier) up to 1 kpc– all HIIs in O stars up to galactic center (and beyond)

frequencies < 100 GHz preferred: plenty of lines of many molecules!

typical target: hypercompact HII region with = 1 and RHII = 50-1000 AU

Note that HII regions like these are observed!

Hypercompact HII regions from De Pree et al. (1998)

RHII = 160-900 AUfree-free= 0.1-0.8

B0-O8.5

Hot molecular cores: rotation

Conservation of angular momentum rotation speed up during infall disk formationDisks in OB stars may solve radiation pressure problem:• photon escape along axis reduces radiation pressure• accretion focused through disk boosts ram pressurePresent situation: • a handful of disks (Mdisk< Mstar) seen in early B stars• a few rotating toroids (Mtoroid>>Mstar) seen in O stars Lack of disks in O stars may be observational bias!? ALMA sensitivity and resolution needed

IRAS 20126+4104Cesaroni et al.Hofner et al.

Moscadelli et al.Keplerian rotation:M*=7 MO

Hot molecular cores: rotation

Conservation of angular momentum rotation speed up during infall disk formationDisks in OB stars may solve radiation pressure problem:• photon escape along axis reduces radiation pressure• accretion focused through disk boosts ram pressurePresent situation: • a handful of disks (Mdisk< Mstar) seen in early B stars• a few rotating toroids (Mtoroid>>Mstar) seen in O stars Lack of disks in O stars may be observational bias!? ALMA sensitivity and resolution needed

Beltran et al. (2004)Beltran et al. (2005)Furuya et al. (2002)

hypercompact HII + dust

O9.5 (20 MO) + 130 MO

Beltran et al.

(2006)

Hot molecular cores: rotation

Conservation of angular momentum rotation speed up during infall disk formationDisks in OB stars may solve radiation pressure problem:• photon escape along axis reduces radiation pressure• accretion focused through disk boosts ram pressurePresent situation: • handful of disks (Mdisk< Mstar) seen in early B stars• a few rotating toroids (Mtoroid>>Mstar) seen in O stars Lack of disks in O stars may be observational bias!? ALMA sensitivity and resolution needed

ALMA

PdBI

Assumptions:HPBW = Rdisk/4

FWHMline = Vrot(Rdisk)

Mdisk Mstar

same <Ncol> in all disks

TB > 20 K

obs. freq. = 230 GHz

5 hours ON-source

spec. res. = 0.2 km/s

S/N = 20

edge

-on

i = 35

°

Assumptions:HPBW = Rdisk/4

FWHMline = Vrot(Rdisk)

Mdisk Mstar

same <Ncol> in all disks

TB > 20 K

obs. freq. = 230 GHz

5 hours ON-source

spec. res. = 0.2 km/s

S/N = 20

ALMA

PdBI

no st

ars

edge

-on

i = 35

°

Hot molecular cores: rotation

Conservation of angular momentum rotation speed up during infall disk formationDisks in OB stars may solve radiation pressure problem:• photon escape along axis reduces radiation pressure• accretion focused through disk boosts ram pressurePresent situation: • handful of disks (Mdisk< Mstar) seen in early B stars• a few rotating toroids (Mtoroid>>Mstar) seen in O stars Lack of disks in O stars may be observational bias!? ALMA sensitivity and resolution needed!

Summary: ALMA and OB star formation

• Assess structure of IR-dark clouds in the Galaxy mass function and 3D velocity of cores prior to star formation

• Resolve multiple outflows from cluster and measure their (3D) velocity accurate estimate of outflow parameters

• Reveal infall in O stars up to galactic center estimate accretion rates

• Image circumstellar disks in OB stars up to Galactic center discriminate between high-mass star formation theories

What ALMA cannot do…

Spectrum of deeply embedded OB stars peaks in the far-IR, hence:

precise luminosity estimate impossible with ALMA! High resolution imaging in the sub-mm and mid-IR insufficient (see Orion)

(sub)arcsec resolution at 50-100 µm!!! Herschel and FIRI (Far-InfraRed

Interferometer) needed

Orion KL

105 LO: where from?

sub-mmBeuther et al.

(2005)

NIR-MIRShuping et al.

(2004)

FIR

?

ALMA

What ALMA cannot do…

Spectrum of deeply embedded OB stars peaks in the far-IR, hence:

precise luminosity estimate impossible with ALMA! High resolution imaging in the sub-mm and mid-IR insufficient (see Orion)

(sub)arcsec resolution at 50-100 µm!!! Herschel and FIRI (Far-InfraRed

Interferometer) needed

HPBW=0.3” obs.freq.=230GHz int.time=5h spec.res.=0.2km/s

ALMA

HPBW=0.3” obs.freq.=230GHz int.time=5h spec.res.=0.2km/s

PdBI

ALMA can detect all disks (if any…) in O stars up to galactic center!

Also important: 8 GHz bandwidth with high spectral resolution simultaneous imaging of many lines from different species, with different optical depths and different excitation energies

ALMA will make it possible to discriminate between theories of massive star formation (e.g. disk accretion, competitive accretion, etc.)

compact ALMA

extended ALMA extended ALMA

compact ALMA

Core angular diameter

Note: RJeans ≈ 0.03 pc

ACAACA

HII region

molecular core

Orion I

Beuther et al. (2005)

SMA

HII

opaque c

ore

maximum ALMA resolution: HPBW = 0.012” (350 GHz/ν)

Example:All HIIs in O9

stars usable up

to 10 kpc with

HII radius of

200 AU matching

ALMA beam of

0.05” at 90 GHz

A primer for high-mass star formation

• IMF problem: OB stars born in clusters clump fragmentation core MF = stellar IMF?

• Radiation pressure problem: for Mstar > 8 MO tKH < tacc reach ZAMS deeply embedded radiation pressure halts accretion!?

• Lifetime problem: typical accretion rates in low-mass stars 10-5 MO/yr embedded phase of high-mass stars >106 yr MS lifetime!?