Progresses on our understanding the processes of star formation in the

Milky Way from Herschel observations

Davide EliaINAF-IAPS, Roma

Part IIThe Herschel

photometric surveys

Herschel and star formation

The wavelength range covered by the cameras on board Herschel containsthe emission peak of the cold dust. It is suited for studying the dense cloudsand the early stages of star formation!

Nature of the compact sources

• Warm Cores SED sources are under-luminous with respect to UCHII/HotCores of similar envelope mass

• Concurring indications suggesting that the dominant source in the Warm Core objects is not yet on the ZAMS

ZAMS

ACCRETION

Molinari et al. 2008

Hot Core

Warm Core

In a pre-Herschel SED analysis of sample of 42 intermediate and high-mass star forming region from the sample of Molinari et al. (1996), a Class 0-I-II sequence analogous to the low-mass regime was suggested: Warm Cores to Hot Cores to HII-driving objects

Nature of the compact sources

ZAMS

ACCRETIONKrumholz & McKee (2008)

≈1 g cm-2

A significant fraction of the clumps should be already forming high-mass protostars (M≥10M)

Molinari+ 2008Elia+ 2010

Problem: Sources in Hi-GAL are mostly clumps, while SED models are available for single YSOs (Robitaille et al. 2006)

Herschel photometric surveys of star forming regions

Gould Belt

HOBYS

Hi-GAL

André et al. 2010, A&A, 518, L102

Motte et al. 2010, A&A, 518, L77

460 hrs

Molinari et al. 2010, PASP, 122, 314

125 hrs

900 hrs

Photometric imaging of nearby (d < 0.5 kpc)molecular clouds

Formation of solar-type stars

Reasonably well established evolutionary sequence, but physics of early stages unclear

• What determines the distribution of stellar masses = the IMF?

• What generates prestellar cores & what governs their evolution to protostars?

• Timescale of core/star formation? Quasi-static or dynamic process?

?

Cloud complexResponsible

teams

Responsible

teams

Taurus Cardiff/Saclay Saclay

Ophiuchus Saclay/Cardiff Saclay

Pipe nebula Saclay Saclay

Polaris flare Orsay/SaclaySAG3/SAG4

Lupus Rome/RALRome/

Leuven

Coalsack Saclay Saclay

Cham I-III & Musca HSC/SaclayLeuven/

HSC

Corona Australis RAL/CardiffHeidelb

erg

Serpens/Aquila riftRome/RAL/

SaclayRome/

Arcetri

Perseus Rome/Canada Rome

IC 5146 Marseille Saclay

Cepheus flare CanadaSAG3/

Canada

Orion A/Orion B Rome/CanadaSaclay/Cardiff

RomeSaclay

Aquila rift and Polaris flareAndré et al. 2010, A&A, 518, L102; Könives et al. 2010, A&A, 518, L106

BTW: How to calculate N(H2) and T maps?

Pixel-to-pixel grey-body fit

• Regrid the Herschel maps onto the map at the largest wavelength available (usually λMAX = 500 μm).

• Reconvolve them with the Herschel FWHM at λMAX

• Perform the pixel-to-pixel fit (time consuming: parallel computing is recommended)

2

ref refMF B T

d

Aquila rift and Polaris flareAndré et al. 2010, A&A, 518, L102; Könives et al. 2010, A&A, 518, L106

Prestellar cores are only observed above the threshold AV = 7 because they form out of a filamentary background and only the supercritical, gravitationally unstable filaments are able to fragment into bound cores.

Two First Hydrostatic Cores in PerseusPezzuto et al. 2012, A&A, 547, A54

FHCDifficult to see it, because it is:

• Short-lived (t = 102-103 yr)

• Invisible in the MIR

• Hard to resolve, even at near distances (size = several AU)

Perseus, d 235 pc

The two sources are situated a few 10^3 AU apart, corresponding

to a few Jeans lengths. It is then possible that these two sources

formed at almost the same time from the fragmentation of a larger

structure.

Two First Hydrostatic Cores in PerseusPezzuto et al. 2012, A&A, 547, A54

envelope: T = 9 K , M = 7.3 Mʘ envelope: T = 9.4 K , M = 8 Mʘ

Photometric imaging of all the high-mass star forming regions at d < 3 kpc

Molecular complexes

Distance(kpc)

Area(deg2)

Vela 0.7 2.75

Mon OB1/NGC2264 Mon R2

0.8 1.65

Rosette 1.5 1.15

Cygnus X 1.7 5.90

M 16/M17/Sh40 1.7 2.15

NGC 6334/NGC 6357 1.7 3.10

W3/KR 140 2.2 1.55

NGC 7538 2.8 0.55

W48 3.0 2.75

Sh 104 4

RCW 79 4

RCW 82 2.9

RCW 120 1.3

These data can allow us to determine the importance of external triggers for high-mass star formation in the nearest massive molecular cloud complexes.

O-stars from NGC 2244

Filaments in the Rosette molecular cloud

“Confidence map” highligthing the filament junctions

Existing infrared clusters and the most massive dense cores (potential sites of future massive starformation) identified in the same data set are overlaid on the image. All sources lie in the proximity of junctions

Schneider et al. 2012, A&A 540, L11

O-stars from NGC 2244

Schneider et al. 2012, A&A 540, L11

PDFs of the Rosette molecular cloud

The Vela–C cloud

b = 0º

It is the cloud “C” of the Vela Molecular Ridge (Murphy & May, 1991)

distance = 700 ± 200 pc (Liseau et al. 1992)

Site of star formation on a wide range of masses (Massi et al. 2003; Baba et al. 2006)

BLAST 250 μm BLAST 250 μm

3 deg2

HOBYS

Giannini et al. 2012, A&A 539, A156

Vela–C - Compact source extraction

Sources are searched separately on each map

• CuTEx: sources detected as local maxima in the curvature map (2nd derivative)

• An elliptical Gaussian is fitted on them, and geometric parameters estimated

• A list of sources with S/N>5 is obtained at each λ

Vela–C - SED fitting

The SEDs eligible for the grey-body fit have been selected applying few constraints: i) fluxes at least 3 adjacent bands between 160 and 500 μm; ii) without concavities; iii) no peak at 500 μm; iv) spatially resolved at 160 μm; v) not presenting multiple associations at λ ≥ 160 μm; vi) not belonging to the RCW34 region

268 objects selected for fit

20/ /ref refM d k

1 ( )F e B T

0/

Giannini et al. 2012, A&A 539, A156

Vela–C - Pre-stellar sources

206 out of 218 starless sources shave been recognized as pre-stellar (~94%, probably affected by selection).

In the mass vs size plot, all the unbound starless sources lie below the Bonnor-Ebert mass curve at T = 8 K.

1 1(2.4 )

2 2BE s

pre BE

R cM M

G

To determine if a starless source is gravitationally bound (then pre-stellar), a comparison of its mass with the corresponding Bonnor-Ebert mass has been performed:

Giannini et al. 2012, A&A 539, A156

Vela–C - An evolutionary framework

Class 0

Although not

completely

separated,

the pre- and

proto-stellar core

samples show a

global trend to

populate different

regions of the

diagram.

For proto-stellar cores, Lbol is probably underestimated, resulting in an underestimate

of their actual age.

Giannini et al. 2012, A&A 539, A156

Vela–C - The Source Mass Distribution

(log )N M M

Vela-C : γ=1.1±0.2

Aquila RiftKönives et al. (2010): γ=1.45±0.2 (M > 0.3 Mʘ)

Orion A Polychroni et al. γ=1.5±0.5, Ikeda et al. γ=1.3±0.1 (M > 9.3 Mʘ)

Orion B Johnstone et al. 2006 γ=1.5±0.42

Perseus+Serpens+OphiuchusEnoch et al. 2008: γ=1.3±0.2 (M > 0.8 Mʘ)

Kroupa 2001

Kramer et al. 1998

This work

Chabrier 2005

D< 0.08 pc

Giannini et al. 2012, A&A 539, A156