Physics and chemistry of irradiated protostars Johan E. Lindberg 1,2 - - PowerPoint PPT Presentation

Physics and chemistry of irradiated protostars

Johan E. Lindberg1,2

Jes K. Jørgensen2,1,

• J. D. Green3, G. J. Herczeg4,5, O. Dionatos6, N. J. Evans II3, A. Karska5, S. F. Wampfler1,2,
• C. Brinch2,1, T. Haugbølle1, E. A. Bergin7, D. Harsono8,9, M. V. Persson8, R. Visser7, S. Yamamoto10,
• Y. Watanabe10, S. E. Bisschop1,2, N. Sakai10

1Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen 2Niels Bohr Institute, University of Copenhagen,

3University of Texas at Austin, 4Kavli Institute for Astronomy and Astrophysics, Beijing, 5Max Planck Institute for Extraterrestrial Physics, 6University of Vienna, 7University of Michigan, 8Leiden Observatory, 9SRON Netherlands, 10Department of Physics, University of Tokyo

Lund, February 11th, 2014

Evolution of low-mass Young Stellar Objects

Persson (2013), after Shu et al. (1987) Adapted from Lada (1987), André et al. (2000), and Smith (2004).

Tbol increases with time. Theory: Observations:

Stars form in clusters!

• Most stars form in clusters.
• Environmental effects on star

formation must be considered.

• Irradiation

implications on → physics (temperature).

• Also profound implications on

the chemistry?

Porras et al. (2003): A Catalog of Young Stellar Groups and Clusters within 1 Kiloparsec of the Sun

Stars with relatively high masses

• The Initial Mass Function (IMF; Salpeter 1955) describes

the distribution of initial stellar masses.

• ~ 6% of all stars: M★ > 2 M☉ (L★ > 16L☉)
• ~ 1% of all stars: M★ > 6 M☉ (L★ > 800L☉)
• A large proportion of all stars will be formed close to a

fairly massive star. Implications?

Modication of figure from Herbst & van Dishoeck 2009, ARA&A, 47, 1, 427

Chemistry in protostellar envelopes

Complex organics form in ice mantles of dust grains, but require CO, H2O (and other molecules) in these mantles. The presence of CO in the ice mantles requires low T at large scales in the envelope.

Jørgensen et al. 2002, A&A, 389, 908

Hot corino

?

What is the physical impact from external irradiation

• n the envelope?

What are the effects on the chemistry?

• One of the most nearby star-forming regions (d ≈ 130 pc)
• Situated in the southern sky (δ ≈ -37°)
• ~100 young stellar objects within the region.
• Most of the youngest sources (Class 0/I) situated nearby

the luminous Herbig Be star R CrA (Peterson et al 2011).

Our lab: Corona Australis

SMA and APEX observations of H2CO

• SMA observations of molecular gas around R CrA

showed very extended emission.

• APEX was used for zero-spacing observations.
• The two datasets were combined: High-resolved spectral

images of all emission on 400 – 8 000 AU scales.

+ = H2CO 303-202 at 218.2 GHz

Lindberg & Jørgensen, 2012, A&A, 548, A24

2000 AU

H2CO 303-202 H2CO 322-221

Strong H2CO emission not associated with YSOs

Northern ridge Southern ridge Greyscale: JCMT/SCUBA 850 µm dust continuum

Lindberg & Jørgensen, 2012, A&A, 548, A24

Temperature map assuming Local Thermodynamic Equilibrium

The rotational temperatures were estimated using 3 H2CO lines at 218-219 GHz [K]

Lindberg & Jørgensen, 2012, A&A, 548, A24

Herschel/PACS 120 µm dust continuum

Colour contours: PACS 120 µm continuum Greyscale: H2CO 303 → 202 (SMA+APEX) All emission Point source emission removed using POMAC deconvolution algorithm

Lindberg et al., accepted (A&A)

Lc = 4 L☉, χISRF = 1 Lc = 4 L☉, χISRF = 750

Transphere and RATRAN models of IRS7B envelope

Dust radiative transfer code for spherically symmetric envelopes; Radiative transfer and molecular excitation

Lc = 1 0-4 L☉ , χISRF = 7 5

CO evaporation T = 20 K

Transphere model fitted to Herschel and SCUBA continuum data points. Images (ray-tracing) made using RATRAN. χISRF = 750, Lc = 4 L☉:

Transphere: Dullemond et al. (2002), A&A, 389, 464 RATRAN: Hogerheijde & van der Tak (2000), A&A, 362, 697

n ~ R -1.5 n = 1.3×106 cm-3 at 1000 AU Rin = 200 AU Rout = 10 000 AU

APEX line surveys of IRS7B and other sources in CrA

Lindberg et al., in prep.

• Motivation: Is the chemistry

affected by the irradiation?

• APEX was used to survey IRS7B

at 218-245 GHz

• CH3CCH was the most complex

species detected – no typical complex organic molecules.

• 17 additional Class 0/I sources

in CrA were observed targeting H2CO, CH3OH, and c-C3H2

H2CO CH3OH c-C3H2

Results of line survey

Bisschop et al. (2013), A&A, 552, A122 Caux et al. (2011), A&A, 532, A23 Lindberg et al. (in prep.)

Hot core: High-mass star- forming region with lots of complex organics Hot corino: Low-mass young stellar object with lots of complex organics IRS7B: Low-mass young stellar object with strong CN emission and no complex organics except CH3OH

Rotational temperatures as function of distance to R CrA

H2CO c-C3H2 Transphere model of R CrA heating

VV CrA IRS7B

Lindberg et al., in prep.

Bright CH3OH – from where?

• The APEX and ASTE line surveys showed bright CH3OH lines

(Schöier et al., 2006 – hot corino?; Watanabe et al, 2012)?

• The line survey provided strong upper limits on complex
• rganic molecules with significantly lower abundances vs.

CH3OH than seen in hot corinos.

• ALMA map shows extended CH3OH.
• Study the 30 AU scales!

CH3OH 70→60 Continuum

C17O line emission traces kinematics

Solid lines: Keplerian rotation Dashed lines: Infall with conservation of angular momentum

[km s–1]

Menv ≈ 2.2 M☉ Mstar ≈ 2.0 M☉ Mdisc ≈ 0.024 M☉

Lindberg et al., submitted to A&A

Position-inverted-velocity diagram Lower abs. velocities v ~ r-1/2 v ~ r-1

RATRAN CH3OH models

Data Model

Inner density: Power law Xinner(CH3OH) = 1e-10 Xouter(CH3OH) = 1e-10 Inner density: Power law Xinner(CH3OH) = 1e-8 Xouter(CH3OH) = 1e-10 Inner density: Flat Xinner(CH3OH) = 1e-8 Xouter(CH3OH) = 1e-10 Inner density: Flat Xinner(CH3OH) = 1e-10 Xouter(CH3OH) = 1e-12

Radiative transfer models show that CH3OH either has a low abundance or that the density profile is flat in the central envelope. A flattened envelope profile at R < 100 AU could be related to the presence of a circumstellar disc at those scales. Currently difficult to separate Model 1 from Models 3-4.

Spectra from IRS7B 1.2''*1.0'' box: Lindberg et al., submitted to A&A

Conclusions

• Intermediate-mass stars have a significant influence on the

temperature in star-forming regions. The temperature is affected by R CrA on scales as large as 30 000 AU (0.15 pc).

• This seems to affect the large-scale chemistry in the IRS7 cloud.
• The absence of complex organics could either be caused by this, or

by the flattening of the inner envelope by a newly-formed protoplanetary disc around IRS7B.

Outlook

• The effect should be studied in other, more massive, star-forming
• regions. How common is this effect?
• ALMA will provide higher-sensitivity images of discs and complex
• rganic molecules.
• Stars are often formed in clusters, which seem to strongly affect

the physics and chemistry of the star-forming envelope. What can this tell us about the formation of our own Solar System?