High resolution sub millimetre studies of the chemistry of low mass protostars
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High resolution (sub)millimetre studies of the chemistry of low-mass protostars. ...or “where did all the CO go?”. Jes J ø rgensen (CfA) Fredrik Sch ö ier (Stockholm), Ewine van Dishoeck (Leiden), Michiel Hogerheijde (Leiden), Geoff Blake (Caltech) Tyler Bourke, David Wilner, Phil Myers (CfA).

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

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

...or “where did all the CO go?”

Jes Jørgensen (CfA)Fredrik Schöier (Stockholm), Ewine van Dishoeck (Leiden), Michiel Hogerheijde (Leiden), Geoff Blake (Caltech)Tyler Bourke, David Wilner, Phil Myers (CfA)

ACP

Cardiff, January 6th 2005


Low-mass star formation

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


Protostellar 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.


This study

Establish 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.

  • 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?


SCUBA obs. + Rad. transfer model.

Single-dish obs. + Monte Carlo model.

Approach

Dust continuum emission

  • CO

  • CS, SO

  • HCO+, N2H+

  • HCN, HNC, CN

  • DCN, DCO+

  • H2CO, CH3OH

  • SO2, SiO, H2S, CH3CN(~ 40 transitions)

Physical structure

Molecular excitation

Chemical structure

Detailed chemical model

Interferometry:small scale structure


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


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.


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

CO depletion

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

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


Abundance

  • 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 stages

Caselli et al. (1999), Tafalla et al. (2002), Bergin et al. (2002), Bacmann et al. (2002), Lee et al. (2003)...

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


“Drop” abundance model

nde

Tev


L723:

Constant abundance model

“Drop” abundance model


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

Jørgensen, 2004, A&A, 424, 589


Depletion ~ Time

( 105 yrs)

  • “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?


Chemical effects of CO depletion (HCO+ and N2H+)


Empirical chemical network

CO

CS

HCO+

SO

CN

HNC

HCN

HC3N

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


dust grains

CO

H3+

HCO+

N2

N2H+


HCO+ and N2H+ abundances

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


CO freeze-out X(N2H+)

CO desorption (T> 30 K)

L483:

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

 450 m dust continuum

 N2H+J = 10

 C18O J = 10

1” = 200 AU  3×1015 cm

Jørgensen, 2004, A&A, 424, 589


L483:

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

 450 m dust continuum

 N2H+J = 10

 C18O J = 10

1” = 200 AU  3×1015 cm

Jørgensen, 2004, A&A, 424, 589


Chemistry as a tool...

NGC 1333-IRAS2

SCUBA 850 µm

BIMA: N2H+ 1-0*


2C

2A

2B

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

Chemistry as a tool...

NGC 1333-IRAS2

BIMA: N2H+ 1-0*

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


Depletion ~ Time

( 105 yrs)


2C

2A

2B

Time

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

Chemistry as a tool...

NGC 1333-IRAS2

BIMA: N2H+ 1-0*

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


dust grains

CO

H3+

HCO+

N2

N2H+

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-HCO+-N2H+

BLACK/BLUE: [CO] varying

RED:[CO] & [N2] varying

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


Conclusions

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

  • Continuum emission; dust radiative transferPhysical 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


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