# Presentation: 670826

## The formation of stars at very low metallicities

-           (16 July 2007)

- The formation of stars at very low metallicities

- Kazu Omukai  (NAOJ)

- Collaborators: Toru Tsuribe (Osaka U.)
-                        Raffaella Schneider (Arcetri)
-                       Andrea Ferrara (SISSA)

## CONTENTS

- What determines the mass of stars
- Physical Processes in low-Z clouds
- The critical Z for low-mass star formation
- Lack of Ultra metal-poor stars

## Why thermal evolution ?

**M****frag****~ M****Jeans****@T min**

- Yoshida, KO, Hernquist, Abel 2006

## Thermal evolution of clouds with different Z

- K.O., Tsuribe, Schneider & Ferrara (2005)

## Important processes in low-Z gas

- Primordial gas process
- H&D chemistry
- H2 (line, collision-induced emission) & HD line cooling
- +
- Cooling by metals in gas phase
- fine-structure lines ([CII], [CI], [OI])
- H2O, CO lines
- Dust processes
- H2 formation on dust
- cooling by dust thermal emission

## Cooling in low-metallicity gas

- dominant cooling process

- 10-6Zsun

- 10-5Zsun

- Z=0
- H2 lines (<1013cm-3)
- H2 CIE (1013-1016cm-3)

- H2O

- H2

- HD

- [C, O]

- CIE

- CO

- dust

- 0

- 4

- 8

- 12

- 16

- log n (cm-3)

- Z=10-4Zsun
- H2 lines (<104cm-3)
- HD    (104-106cm-3)
- H2O  (106-1010cm-3)
- dust (1010-1013cm-3)

- Z=10-2Zsun
- [C, O]    (<103cm-3)
- CO  (103-106cm-3)
- dust (106-1012cm-3)

## Thermal Evolution of clouds with different Z

- K.O., Tsuribe, Schneider & Ferrara (2005)

## Two characteristic fragmentation epochs

## Fragmentation epoch and mass

- Two temperature minima except for Z=0, Zsun
- Fragmentation due to line-cooling (104cm-3, 100-1000Msun) and that due to dust-cooling (higher density,0.1-1Msun)
- Mass scales weakly dependent on Z

- (Schneider, K.O., Inoue, Ferrara 2006)

- Fragmentation density

- Fragmentation mass

## How much is the critical metallicity (gas-dust ratio) by dust cooling ?

## Dust-induced fragmentation

- With gas-dust ratio Z>~10-5Zsun (standard dust)low-mass fragments by dust cooling

- Tsuribe & K.O. (2006)

- Zcr~10-6-10-5 Zsun

- Standard dust

- First dust

- Z=10-5Zsun

## More realistic simulation

- Clark, Glover, & Klessen (2007) confirmed fragmentation
- during the dust-cooling phase.

- See Glover’s poster

- [Z/H]=-5

- dust
- cooling

## Low-mass fragments by fine-structure-line cooling?

- Some studies claim that low-mass fragments are formed by fine-structure line cooling. (Bromm & Leob 2003, Santoro&Shull2006 )
- Those arguments compareH2 vs fine-str. @ T min of Z=0 Zcr ~10-3.5Zsun
- This only assures  Mfrag < Mfrag(Z=0) ~1000Msun

- Here

## If there were no dust...

- With only metal-line cooling, T increases for n>~108cm-3
- Fragmentation mass  ~10-100Msun
- Dust is indispensable for low-mass star formation

- No Dust  Cases

## Is low-mass SF always possible for Z > Zcr~10-6-10-5Zsun ?

## Lack of UMP stars

- EMP stars~200, HMP stars ~2 found
- None of UMP stars has been found
- (2 HMPstars are C,O enhanced. Here we suppose they gained after the formation of the stars for example by binary accretion.)
**We need mechanism which allows low-mass HMP stars but does not low-mass UMP stars.**

- desert

- EMP

- UMP

- HMP

- Salvadori et al. 2006

## Case of [Z/H]~-4.5

- Sudden H2 formation heating  very spherical hydrostatic core
- No fragmentation in the dust-cooling phase

- heating

- No
- fragmentation

- spherical

- Tsuribe & K.O. (2007)

## Cases of [Z/H]=-5.5, -3.5

- Long filament formation during dust-cooling phase fragmentation

- [Z/H]=-5.5

## Why sudden heating only at [Z/H]~-4.5 ?

- 3-body reaction n>108cm-3
-  At [Z/H]~-5.5, temperature is already high and heating is not so remarkable.
- At [Z/H]~-3.5, H2 formation is almost completed by dust reaction before 3 body reaction starts.
- At [Z/H]~-4.5, 3-body reaction starts at low-temperature. This results in sudden heating.
-  fragment mass    ~10-100Msun

- Temperature evolution

- 3-body
- reaction

- fragment

## SUMMARY

- Dust is indispensable for low-mass star formation
- [Z/H]>~-5.5  low-mass star formation
- Except at [Z/H]~-4.5, where H2 formation heating prohibits low-M SF. (>~10Msun) ‘metallicity desert’ of UMP stars.