ALMA Observations of "Chemicall-young Starless'' Cores - - PowerPoint PPT Presentation

Shigehisa Takakuwa, N. Ohashi, S. Takahashi, J. Karr (ASIAA) S.-P. Lai (NTHU)

ALMA Observations of "Chemicall-young Starless'' Cores

The 5th ALMA Taiwan - Japan Science Workshop

Table of Contents

• 1. Chemistry as a Diagnostics of Star Formation
• 2. VeLLOs: Candidates of the Youngest Protostars
• 3. ALMA Observations

1. Chemistry as a diagnostics of Star Formation

Hunt for the earliest stage of protostellar formation the initial stage is likely to constrain the properties of the final product such as the stellar mass and the planetary system

Chemical Status of Dense Cores,

• ut of which protostars form, might help, since…

The Molecular Abundances vary as a function of time, mainly because of C+ --> C --> CO CCS (and other Carbon-Chain Molecules)---> produced from C+, then traces early evolutionary stage CO, HCO+ ---> from CO, later evolutionary stage NH3, N2H+ --> neutral-neutral reaction, later evolutionary stage,

• A. the time evolution of the gas-phase status of Carbon
• B. Depletion of molecules onto dust grains

NH3, N2H+ also depleted but only at the latest evolutionary stage CO, HCO+ depletion at the cold-gas center

• C. Desorption of molecules from dust grains

due to the protostellar heating CO, and organic molecules

Molecular Desorption

Once a Protostar forms, heating from the star desorbs molecules which were depleted onto dust grains in the starless phase. Temperature enhancement in the star-forming dense core

Radius (AU)

Molecular desorption due to the heating CO sublimate to gas phase @ TK > 20 K

• -> compact CO emission at the center as a sign of star formation ?

Aikawa et al. 2008

• 2. VeLLOs: Candidates of the Youngest Protostars

Class 0 Like SED ---------------> Candidates of the youngest protostars (But Not easy to dintiguish youngest or lowest-mass) So far, 15 VeLLOs found

• ut of 50 dense cores,

~ 70 % still starless (Dunham et al. 2008) L1014 (Young et al. 2004) Tbol = 50 K Lbol/Lsubmm = 20 Higher central gas density (> a few × 105 cm-3) than that of starless cores (Kauffmann et al. 2008) Very Low-Luminosity Objects (< 0.1 Lsolar), detected by Spitzer in previously-thought ``starless cores’’, cannot be detected with IRAS

L1014: The First VeLLO

Young et al. 2004

Compact (~ 500 AU) CO outflow (Bourke et al. 2005)

Molecular condensation --> (Crapsi et al. 2005) Lint ~ 0.09 Lsolar

L1521F or MC27 as the youngest source in Taurus (Onishi et al. 1999)

Spitzer Image (Bourke et al. 2006) Outflow-like Cavity, though no extended outflow found (Crapsi et al. 2004) CCS Ring and N2H+ Core (Shinnaga et al. 2004)

IRAM 04191+1522: The most-evolved VeLLO ?

Spitzer image (Dunham et al. 2006) N2H+ depletion ---> Chemically most evolved (Belloche & Andre 2004) Extended (~ 15000 AU) Outflow, different from L1014 and L1521F

Lee et al. 2002

Carbon-Chain Molecules in L1521F and IRAM04191

Abundant carbon-chain molecules in L1521F

• --> Chemically-young

Clear Chemical difference between L1521F and IRAM 04191

• --> Different Evolutionary stage ?

i n

L1521F Takakuwa et al. in prep

i n

IRAM 04191

• --> CO desorption ?

900 AU 24 micron vs 13CO (Green), C18O (Red), and 230GHz continuum (Blue) 13CO (Green), C18O (Red), and 230GHz continuum (color), Cross: Spitzer ps position 900 AU 13CO C18O & Cont.

L1521F IRAM 04191

• --> Compact (~ 500 AU) C18O condensation associated with the VeLLO

A few times abundance return from the depleted (factor ~ 20) abundance ?

• --> CO desorption started ??

SMA observations of L1521F and IRAM 04191 in CO 2-1 and cont.

Chemical Evolution of Starless Cores and VeLLOs

IRAM 04191 Crapsi et al. 2005 L1014 1 1 Carbon-Chain Abundance CO outflow Extent, Molecular Desorption (?) VeLLOs are not necessarily chemically the most evolved

• --> Star Formation initiate when chemically still young ?

• 3. ALMA Observations

Band 3 CCS, C2H, HC3N, N2H+ Band 6 1.3 mm cont. + CO 2-1, 13CO 2-1, C18O 2-1 Band 7 0.8 mm cont. + CO 3-2, 13CO 3-2, C18O 3-2 Here, we propose ALMA obs. of ``chemicall-young starless cores’’;;

• 1. Search for the earliest Protostellar Objects in submm continuum
• 2. Chemical status and Evolution in Carbon-Chain and N2H+ lines
• 3. Search for Molecular Desorption, and the earliest outflows in CO Lines

Target Objects

~ 25 nearby ``Starless Cores’’ (10 sources in Taurus, 5 in Oph and isolated Bok Globules, and 10 in Cha) Kauffmann et al. 2008 MAMBO Survey

TMC-1C TMC-1 L1521B TMC-2

Observational Parameters

0.1 arcsec ~ 20 AU for dust continuum, 0.5 arcsec ~ 100 AU, 0.05 km s-1 for molecular lines small (7-field ~ 20000 AU) mosaic for Band 3 molecular-line obs. single-field for Band 6 and 7 CO and continuum observations Typical continuum (Jy): ~ 0.5 mJy @230 GHz (c.f. 4.5 mJy in IRAM 04191+1522, 1.2 mJy in L1521F) rms 0.017 mJy @230 GHz for 1 hour integ. (30 sigma) CCS, C2H, N2H+ ~ 0.5 – 3 K rms 75 mK per one field @5 arcsec C18O 2-1, 3-2 ~ 2 – 5 K CO 2-1, 3-2 ~ 10 – 100 K (warm desorption region) 70 mK (Band 6) and 490 mK (Band 7) per single field @0.5 arcsec 1 hour x 25 sources x 3 band x 1.5 (overhead) = 113 hours

Summary

ALMA Observations of ``Chemically-Young Starless’’ Cores, Where Carbon-Chain Molecules are still rich, To search for the candidates of the earliest phase of protostars (first core ?!) Objects in submm continuum Chemical status in carbon-chain and N2H+ lines Molecular Desorption and CO outflow