High resolution (sub)millimetre High resolution (sub)millimetre studies of the chemistry of low-mass studies of the chemistry of low-mass

protostarsprotostars

Jes Jørgensen (CfA)

Fredrik Schöier (Stockholm), Ewine van Dishoeck (Leiden), Michiel Hogerheijde (Leiden), Geoff Blake (Caltech)

Tyler Bourke, David Wilner, Phil Myers (CfA)

Cardiff, January 6th 2005

...or “where did all the CO go?”

ACP

Pictures from NASA/Astronomical picture of the day; Myers et al. (1998)

Low-mass star formationLow-mass star formation

Protostellar propertiesProtostellar properties

• Centrally condensed envelope of gas and dust• Ongoing accretion through a circumstellar disk

• Densities ranging from 104 cm-3 to 107-108 cm-3 (H2)

• Temperatures ranging from 10 to a few hundred K.

What is the relation between the physical and chemical properties of low-mass protostars?

What are the useful diagnostics of different protostellar components?

Is it possible to use the chemistry to trace the protostellar evolution?

This studyThis studyEstablish the physical and chemical structure of a sample of ~ 20 low-mass protostars (class 0/I); using single-dish obs. (JCMT), mm interferometry and detailed radiative transfer modeling.

ApproachApproach

Dust continuum emission

Physical structure

Molecular excitationChemical structure

Interferometry:small scale structure

Detailed chemical model

SCUBA obs. + Rad. transfer model.SCUBA obs. + Rad. transfer model.

Single-dish obs. + Monte Carlo model.Single-dish obs. + Monte Carlo model.

• CO• CS, SO

• HCO+, N2H+

• HCN, HNC, CN• DCN, DCO+

• H2CO, CH3OH

• SO2, SiO, H2S, CH3CN

(~ 40 transitions)

Today...Today...

...very little about continuum observations and dust radiative transfer

BUT: Continuum/Physical structures...

...describe star formation/core physical evolution ...are crucial for molecular excitation calculations ...establish reference scale (H2 density) relative to which

abundances are calculated

...include significant simplifying assumptions (e.g., dust properties, dust-gas coupling...)

An example: CO depletion An example: CO depletion

Example: modeling of CO lines toward L723

Adopting n(r) and T(r) from continuum modeling: constrain abundances (and velocity field) from Monte Carlo line radiative transfer by comparison to observed line profiles.

CO freezes out at low temp. ( 35 K) - as seen in pre-stellar cores (e.g., Caselli et al. (1999), Tafalla et al. (2002))

Objects with high envelope masses (younger?) show significantly higher degree of CO depletion

CO depletionCO depletion

Jørgensen, Schöier & van Dishoeck 2002 A&A, 389, 981

“Canonical” CO abundance (Lacy et al. 1994)

Jørgensen, Schöier & van Dishoeck, 2005, A&A submitted

Pre-stellar core:

•Low temperature

•Depletion toward center (high densities ~ time)

•...but not edge

Protostellar core:

•Central heating ~ temperature gradient

•Thermal desorption toward center

•...outside (low T): depletion/no depletion regions as in pre-stellar stagesCaselli et al. (1999), Tafalla et al. (2002), Bergin et al. (2002), Bacmann et

al. (2002), Lee et al. (2003)...

Abundance

“Drop” abundance model

nde

Tev

Constant abundance model

“Drop” abundance model

L723:

C18O 1-0 OVRO observations L483 (class 0 protostar @ 200 pc)

Jørgensen, 2004, A&A, 424, 589

• “Drop abundance structure” needed to account for both single-dish and interferometer observations

• Explains differences in CO abundances between YSOs with envelopes of different masses - but note: no trend between tde and “age”

• Potentially(!) a tracer of the “history” of the core - dense stage (where CO depletes) only 105 years?

Depletion ~ Time

( 105 yrs)

Chemical effects of CO Chemical effects of CO depletion depletion

(HCO(HCO++ and N and N22HH++) )

HCN

HC3N

CNHNC

HCO+

CO CS

SO

Empirical chemical networkEmpirical chemical network

Jørgensen, Schöier & van Dishoeck 2004, A&A, 416, 603

COdust grains

H3+

N2 N2H+

HCO+

HCOHCO++ and N and N22HH++ abundances abundances

Jørgensen, Schöier & van Dishoeck 2004, A&A, 416, 603

L483:L483:

450 m dust continuum

N2H+ J = 10

C18O J = 10

Jørgensen, 2004, A&A, 424, 589

“Typical embedded pro-tostar (quite asymmetric, though) at a distance of approximately 200 pc.”

CO desorption (T> 30 K)

CO freeze-out X(N2H+)

1” = 200 AU 3×1015 cm

L483:L483:

450 m dust continuum

N2H+ J = 10

C18O J = 10

Jørgensen, 2004, A&A, 424, 589

1” = 200 AU 3×1015 cm

“Typical embedded pro-tostar (quite asymmetric, though) at a distance of approximately 200 pc.”

BIMA: N2H+ 1-0*NGC 1333-IRAS2

SCUBA 850 µm

Chemistry as a tool...Chemistry as a tool...

BIMA: N2H+ 1-0*

Chemistry as a tool...Chemistry as a tool...

NGC 1333-IRAS2

Jørgensen, Hogerheijde, van Dishoeck et al., 2004, A&A, 413, 993

2C2A

2B

Dashed line: SCUBA continuum emissionSolid line: Contrast N2H+/SCUBA emission

Depletion ~ Time

( 105 yrs)

BIMA: N2H+ 1-0*

Chemistry as a tool...Chemistry as a tool...

NGC 1333-IRAS2

Jørgensen, Hogerheijde, van Dishoeck et al., 2004, A&A, 413, 993

2C2A

2B

Dashed line: SCUBA continuum emissionSolid line: Contrast N2H+/SCUBA emission

Time

COdust grains

H3+

N2 N2H+

HCO+??

Previously N2 assumed to freeze-out slower than CO (e.g., Bergin & Langer, 1997) – but recent observations show N2H+ depleting towards the centers of pre- and protostellar cores (although slower than CO) (e.g., Bergin et al. (2002), Belloche & André (2004)) and lab. experiments show similar binding energies for CO and N2 (Öberg et al.)

CO-HCOCO-HCO++-N-N22HH++

Chemistry of gas parcel at 106 cm-3 and 20 K after 104 years following model of Doty et al. (2004) with varying CO – and N2 - depletion

BLACK/BLUE: [CO] varying

RED: [CO] & [N2] varying

ConclusionsConclusions

Continuum emission; dust radiative transfer Physical structure of envelopes (down to 500 AU)(The presence or absence of disks)

Molecular line studiesChemical evolution ~ thermal history (e.g., CO)

Important link between high-resolution observations, single-dish surveys and detailed modeling

A A quantitativequantitative framework for the interpretation of framework for the interpretation of the detailed physical and chemical structure of the detailed physical and chemical structure of early protostellar sources has been established.early protostellar sources has been established.