Cambridge, June 13-16, 2005 A Study of Massive Proto- and Pre-stellar Candidates with the SEST...

Cambridge, June 13-16, 2005

A Study of Massive Proto- and Pre-stellar Candidates with the SEST Antenna

Maite Beltrán

Universitat de Barcelona

J. Brand – IRA, BolognaR. Cesaroni – Arcetri, FirenzeF. Fontani – Arcetri, FirenzeL. Testi – Arcetri, FirenzeS. Pezzuto, S. Molinari – IFSI, Roma


Introduction

• Massive stars (> 8 MO) play a crucial role in the appeareance and evolution of galaxies: heavy elements + energetic winds, SN, …

• Massive star formation understanding behind that of lower mass counterparts• Study of clouds associated with UC HII embedded luminous IRAS with no radio

continuum emission• Palla et al. (1991) selected plausible candidate of massive protostars with > -30O:

Criteria Selection: 1. IRAS colours according to criteria of Richards et al (1987) to identify Compact Molecular Clouds2. not associated with HII regions (no cm emission)

Sub-samples: Low and High: according to the IRAS colour criteria ofWood & Churchwell (1989) for identifying UC HII regions


[25-12] > 0.57

[60-

12] >

1.3

0

IRAS colour-colour diagram


Introduction

• H2O masers (Palla et al. 1991); different continuum and molecular line tracers, from cm no NIR wavelengths (Molinari et al. 1996, 1998, 2000, 2003; Brand et al. 2001; Zhang et al. 2001)

• Main findings: 1. High and Low have L > 103 LO

2. Low sources lower association with H2O masers

3. High higher association with UC HII regions

4. Low have Tdust ~ 30 K (Molinari et al. 2000) much lower

than that of “hot cores” (> 100 K)

Low massive protostar candidates are younger than High


Southern Survey

• Extend the study to the southern hemisphere, we selected sources with < -30O from the IRAS-PSC following the Palla et al. (1991) criteria:

• 115 Low observed in C17O and/or CS by Fontani et al. (2005)

• Low + 93 High previously observed in CS by Bronfman et al. (1996)

• Additional Low with > - 30O observed in NH3 by Molinari et al. (1996)

• 235 sources with Luminosities > 103 LO

• OBSERVATIONS:1.2 mm continuum observations with 37-channel bolometer

array SIMBA at the 15-m SEST antenna

maps 900” x 400” + 3 mosaics

HPBW ~ 24”

<LLow> ~ 3 x 104 LO

<LHigh> ~ 1x 105 LO


Morphology of the sources

• Clumps identified with Clumpfind 2D (Williams et al. 1994).

• Massive dust clumps detected in all but 7 regions, usually more than one clump per region: > 650 clumps in 235 regions

• No milimeter source or faint emission mostly towards Low sources with no C17O or CS detected

• Dust emission presents a variety of complex morphologies:

- sometimes a single millimeter source

- sometimes with massive clumps clustered towards the IRAS position

- sometimes with multiple clumps linked forming chains or filaments


Morphology of the sources: Low

1.2 mm + MSX @ 8 m


Morphology of the sources: High

1.2 mm + MSX @ 8 m


IRAS 17225-3426 and IRAS 18014-2428

1.2 mm + MSX @ 8 m

1.2 mm + MSX @ 21 m


Low versus High

Low High

<Mclumps> 163 MO 561 MO

<Rclumps> 0.23 pc 0.33 pc

<nH2 clumps> 1.4 x 106 cm-3 2.6 x 105 cm-3

<L> 2.6 x 104 LO 1.3 x 105 LO

<L/M> 98 LO/MO 101 LO/MO

M calculated for Tdust = 30 K 4-5 times lower than L/M of UC HII regions with same M

High and Low could be in a pre-UC HII phase


MSX versus non MSX• We have searched for prestellar or precluster cores in the surroundings of the

candidate massive protostars, as one may expect that these cores are located close to massive luminous cores hiding already formed high-mass YSOs.

• Such cores are expected to have similar density and size than those with embedded protostars but lower L and T.

• The bulk of its L is expected to be emitted at mm and sub-mm wavelengths with faint mid- and far-IR emission.

• To search for such cores we have cross-correlated our sample with mid-IR MSX PSC + compared the mm continuum maps with MSX images.

• 130 massive mm clumps not associated with mid-IR emission, which are potential prestellar/precluster cores 94 Low (25% of Low clumps)

36 High (15% of High clumps)


MSX versus non MSX

1.2 mm + MSX @ 8 m

1.2 mm + MSX @ 21 m

1.2 mm + MSX @ 8 m


MSX versus non MSX

<M >nonMSX ~ 101 MO

<M >MSX ~ 370 MO

T lower??

Other surveys suggest T < 20 K; e.g. Garay et al. (2004) have found T < 17

K for clumps not associated with MSX

for T = 30 K


The Mass Spectrum of the clumps

Completeness limit Normalized cumulative mass distribution

d < 2 kpc 10 < M < 130 MO

d < 6 kpc M > 92 MO dN/dM ~ M-

= 2.35

= 1.70

Salpeter (1955)

Molecular clouds

from gas obs.



Williams et al. (2004)

Reid & Wilson (astro-ph/053190)

Sample of 68 high-mass protostellar candidates

NGC 7538 @ 2.8 kpc

15 MO

97 MO

25 MO

120 MO

~3000 MO

100 MO

10 MO 800 MO

SCUBA observations

=1.14

=2.32 =2.0

=0.9



Mass Range (Mo)

Dust emission 1.7-25 10-35 30-100 100-103 103 -104

Tothill et al. (2002) 1.7

Beuther & Schilke (2004) 2.5

Williams et al. (2004) ----- 1.14 ----------- 2.32

This Work ----- 1.7 ---------- --------- 2.35 ------------

Reid & Wilson (2005) ------ 0.9 ---------- ---------- 2.0 ------------

Gas emission

Kramer et al. (1998) 1.6-1.7 1.6-1.8 1.7-1.8 1.7-1.8

•Power-law at the high-mass end (M > 100 Mo) consistent with stellar IMF (e.g. Salpeter 1955) -> IMF of high-mass (proto)clusters is determined at the very early stages of evolution?

•Power-law for M <100 Mo flatter: is this break real? or an artifact, due to incompleteness, or to low spatial resolution of the single-dish observations?

Need of higher angular resolution and sensitivity observationsBeuther & Schilke (2004)

IRAS 19410+2336 @ ~ 2 kpc

PdBI


Summary

• We have carried out a 1.2mm dust continuum emission of 235 massive protostar candidates with L > 103 LO, mostly with < -30O.

• Massive dust clumps have been detected in all but 7 out of 235 regions, usually more than one clump per region: > 650 clumps in 235 regions. The dust emission presents a variety of complex morphologies: single sources, clusters of clumps, chains or filaments of clumps.

• Low and High samples have similar physical properties, and a similar mean value of the L/M ratio, much lower than that of known UC HII regions. This suggests that the sources in our survey could be in a younger pre-UC HII phase.

• There are 130 massive mm clumps in the surroundings of candidate massive protostars not associated with mid-IR MSX emission that are potential prestellar/precluster cores. Their properties are similar to those of embedded YSOs, although their mean mass is smaller.

• The mass spectrum of clumps with M > 92 MO resembles the Salpeter stellar IMF, while for 10 MO < M < 130 MO is better fit with a power-law with = 1.7.

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