Star Formationin the Small Magellanic Cloud

E. Sabbi (STScI)

Why Star Formation?

Galaxy evolution scheme

Collapse ofprotogalaxy

and/or mergers

Star formation

Stellar evolution,nucleosynthesis

Stellar mass lossand death

Gasin/out-flows

Mass and composition of the ISM

To understand galaxy evolution

theoretical models observational constraints

galaxy formationchemical evolution

dynamical evolution

chemical abundancesgas/star/dark masses

kinematicsstar formation history

IMF

The Color-Magnitude Diagram:Stellar Evolutionary Phases

The Sun

The color of a star is a measure of its temperature

The magnitude of a star is a measure of its luminosity

The position of a star in the CMD is uniquely related to its mass, age, and chemical composition

In a Simple Stellar population stars are characterized by:

the same agethe same chemical

compositionthey are all at the same

distance

Simple Stellar populations

old globular clusters

young stellar clusters

Simple Stellar populations

TO

Simple Stellar populations

chemical composition similar to those of high redshift galaxies but no high mass stars

high and low mass stars but high metallicity

The Small Magellanic Cloud

MW

LMC

SMC

Star Formation TracersUV OPTICAL IR RADIO

hot & massive stars

dust supernovaestars & ionized gas

The Small Magellanic Cloud (II)

It has been greatly distorted by tidal interaction with the MW;

It differs from the MW:1) for it gas content (gas-

richer)2) metal abundance (metal-

poorer)

The SMC in Space and in Time: HST

4 young clusters

7 old clusters

7 fields

The 4 Young Clusters

Deep HST/ACS V,I,Ha, & UV imagingHST/NICMOS Near-IR imaging

NGC 346 NGC 602 NGC 299 NGC 376

NGC 346

NGC 346 is a complex region shaped by the powerful winds of itsmost massive stars, some of which have

already exploded as SN

SNR0056-7233

SNR0057-7226HD5980

~5’ x 5’ 88 x 88 pc

BS90

NGC 346: CMD~80000 stars with σ <0.1in both bands, down to F555W = 26.2 (0.65 Mo)

Old SMC population

?Young Population

(NGC 346)

The inner region

Pre-MS spatial distribution

Sub-clusters

NGC 346: ongoing SF

IR: Spitzer data

A model

BS90

Mass ~ 7 x 104 Mo

Age ~ 4 Gyr

NGC 602NGC 602 is a small bright star forming region located in the SMC “Bridge”

(Westerlund 1964, Massey et al. 2000)

IRAC 3.0μmIRAC 4.5μmIRAC 8.0μm

YSO

NGC 602 in the IR

Two twin clusters :NGC 376 & NGC 299

NGC 299NGC 376

ConclusionsNGC 346 NGC 602 NGC 299 NGC 376

3 Myr oldThe parent cloud of molecular gas fragmentized in many coeval sud-clustersSecond generation of stars in the periphery

5 Myr old

Broken shell structure

Second generation of stars in the periphery

25 Myr oldEven if located in environments which differ for stellar density and gas content, these two clusters appear almost identical.

2 Myr 30 Myr