High resolution (sub)millimetre studies of the chemistry of low-mass protostars
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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
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Pictures from NASA/Astronomical picture of the day; Myers et al. (1998)
Low-mass star formationLow-mass star formation
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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.
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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.
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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)
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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...)
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An example: CO depletion An example: CO depletion
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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.
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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)
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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
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“Drop” abundance model
nde
Tev
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Constant abundance model
“Drop” abundance model
L723:
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C18O 1-0 OVRO observations L483 (class 0 protostar @ 200 pc)
Jørgensen, 2004, A&A, 424, 589
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• “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)
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Chemical effects of CO Chemical effects of CO depletion depletion
(HCO(HCO++ and N and N22HH++) )
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HCN
HC3N
CNHNC
HCO+
CO CS
SO
Empirical chemical networkEmpirical chemical network
Jørgensen, Schöier & van Dishoeck 2004, A&A, 416, 603
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COdust grains
H3+
N2 N2H+
HCO+
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HCOHCO++ and N and N22HH++ abundances abundances
Jørgensen, Schöier & van Dishoeck 2004, A&A, 416, 603
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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
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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.”
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BIMA: N2H+ 1-0*NGC 1333-IRAS2
SCUBA 850 µm
Chemistry as a tool...Chemistry as a tool...
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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
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Depletion ~ Time
( 105 yrs)
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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
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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.)
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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
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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.