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Probing the Structure of Low-mass Star-forming Cores with Dust Continuum Emission

This study investigates the structure of low-mass star-forming cores using dust continuum emission. It explores the evolutionary sequence of protostellar formation and compares theoretical models with observed data. The study also examines the density, temperature, and velocity structure of the cores.

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Probing the Structure of Low-mass Star-forming Cores with Dust Continuum Emission

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  1. Probing the Structure of Low-mass Star-forming Cores with Dust Continuum Emission Yancy L. Shirley Collaborators: Claire Chandler, Neal J. Evans II, Jonathan M. C. Rawlings, Chad H. Young NRAO Colloquium January 2003

  2. Where Do Stars Form ? NASA Hubble Heritage

  3. Molecular Cloud Complexes Rho Ophiuchus Lupus L. Cambresy 1999

  4. Dense Cores Motte, Andre, & Neri 1998

  5. Star Formation within Cores

  6. Isolated Molecular Clouds BHR-71 VLT

  7. Low-mass Star Formation • Star formation occurs within dense molecular cores • Low-mass star formation may occur in isolation or in clustered environments • There exists a putative evolutionary sequence for low-mass star formation • Theories of protostar formation predict the evolution of the core’s density, temperature, and velocity structure • In particular the density structure is a strong discriminator of theoretical models

  8. Low-mass Evolutionary Scheme • Protostar evolves from deeply embedded phase (Class 0) to optically visible T-Tauri star (Class II & III). • Object classified based on shape of the SED: • a(NIR) slope • Tbol • Lbol / Lsmm Figure from P.Andre 2002

  9. SCUBA Survey Shirley et al. 2000, Young et al. 2002 • 16 nights at the JCMT 15-m telescope • 39 nearby (D < 450 pc), low-mass (Lbol < 13Lsun) cores • 6 Pre-Protostellar Cores (PPCs) • 15 Class 0 Cores • 18 Class I Cores • SCUBA 850 & 450 mm jiggle maps (3.5 x 3.5 arcmin) • Sensitivity 20 mJy beam-1 or roughly Av = 3 mag at edge • qmb ~ 15 arcsec at 850 mm • qmb ~ 8 arcsec at 450 mm • 120 arcsec chop throw

  10. Goals of Continuum Survey • Use optically thin submm emission to characterize the envelope structure of star forming cores • Low-mass sample: 500 to 104 AU • Compare evolutionary indicators • Test putative evolutionary scheme for low-mass cores • More realistic density and temperature profiles for radiative transfer models of molecular line observations

  11. Preprotostellar Cores L1544 L1512 L1689B 10,000 AU L1498 B133 L1689A

  12. Class 0 Cores B335 L1527 B228 10,000 AU 10,000 AU IRAS03282 L1448C/N L483

  13. Class I Cores IRAS04166 L43 IRAS04302 10,000 AU IRAS04169 IRAS04248 IRAS04264

  14. Low Mass Evolution ? • Dust continuum emission qualitatively consistent with putative evolutionary sequence • Class I Bias: All but 3 Class I sources located in Taurus Class 0 Class I PPC

  15. Dust Opacity

  16. Evolutiony Indicators • Tbol and Lbol/Lsmminconsistent Class 0 = Lbol/Lsmm < 200 & Tbol < 70 K

  17. Submm Continuum Emission • Submillimeter continuum emission is optically thin. The specific intensity along a line-of-sight is given by:

  18. Why must we model ? • Rayleigh-Jeans approximation fails in outer envelope of low-mass cores • hn/k = 16.9 K at 850 mm • Heating from ISRF is very important in outer envelopes of low-mass cores • Radiative transfer is optically thick at short l • Observed brightness distribution is convolved with complicated beam pattern, scanning, and chopping

  19. SCUBA Beam Profiles 15.2” 7.9”

  20. Radiative Transfer Procedure nd(r) L kn Sn(l) I(b) Radiative Transfer Simulate Obs. Td(r) Nearly orthogonal constraints: SEDMass x Opacity I(b)n(r) Gas to Dust Physical Model n(r) Iterate Observations

  21. Initial Density Structure • Solutions to the equation of hydrostatic equilibrium: • Bonnor-Ebert spheres: (Bonnor 1956, Ebert 1955) • Limit is Singular Isothermal Sphere (n ~ r-2)

  22. BE Models of submm emission

  23. L1544 N2H+ • Pre-protostellar core • Taurus Molecular Cloud • 140 pc • Evidence for extended infall ? Caselli et al. 2002 SCUBA 850 mm SCUBA 450 mm ISO 200 mm 10,000 AU Ward-Thompson et al. 2002 3.5’ x 3.5’ 12’ x 12’

  24. Power Law Model

  25. B335 N2H+ • Class 0 protostar in Bok globule • Isolated: 1 deg from Aquila Rift • D = 250 pc • Best collapse candidate Caselli et al. 2002 SCUBA 850 mm DSS - optical SCUBA 450 mm 10,000 AU 3.5’ x 3.5’ 6’ x 6’

  26. Power Law Models: n(r) ~ r -p

  27. Testing Model Parameters • Uncertainty on power law model is • Neglects possible contribution from a disk, outflow, etc.

  28. Class 0 and I Models • Single power law models are good fits • No variation in <p> observed between Class 0 and Class I

  29. Density vs. p • Density of best fit model at 1000 AU vs. p • Class I cores order of magnitude lower density • <n> = 1.2 x 106Class 0 • <n> = 1.5 x 105Class I

  30. Deconvolved Size vs. p • Convolution of a Gaussian beam pattern with a power law intensity profile yields a deconvolved source size that varies with p

  31. Dust-determined Mass • Can use best fit model to determine envelope mass • <M> = 3.00 ± 1.00 MsunClass 0 • <M> = 0.19 ± 0.11 MsunClass I • Are masses indicative of evolution of envelope or is there a bias towards low-mass objects in Taurus? • Inclusion of Visser et al. 2002 data set will elucidate • Determine model “isothermal” temperature • <Tiso> = 11.1 ± 1.2 K PPC • <Tiso> = 12.4 ± 2.0 K Class 0 • <Tiso> = 16.9 ± 3.5 K Class I

  32. Virial Mass • Calculate virial mass from narrow linewidth spectra • N2H+ or H13CO+ • Correct virial mass for: • density power law • thermal component • Virial mass agrees with dust-determined mass within a factor of 2 for Class 0 sources

  33. Gas around Class I • Virial mass does NOT agree with dust-determined mass towards Class I sources • Mv/Md up to 20! • Problem: Gas tracer, such as N2H+ or C18O appears to be probing nearby PPC or background cloud • Looking for a dense gas tracer of Class I envelopes! N2H+ Caselli et al. 2002

  34. Variation of Dust Opacity ? • No evidence for variation in opacity in outer envelope of low-mass cores • Beam convolution cannot be ignored! • Comparison limited between 450 and 850 microns.

  35. Standard Star Formation Models Larson & Penston (69) Shu & Hunter (77) Foster & Chevalier (93) MODEL Critical BE- Sphere INITIAL CONFIG. Uniform n(r) SIS ACCRET.RATE higher const higher initially • Perturbations to Shu77: • ROTATION: TSC (84) • B FIELDS: Galli & Shu (93a,b)

  36. Shu Inside-Out Collapse Model • Initial configuration is SIS (Shu 1977) • Infall radius, rinf, propagates outward at sound speed Animation provided by C. H. Young

  37. B335 Collapse Model • Shu77 collapse model of molecular lines: H2CO and CS by Choi et al. 1995 • Best fit: rinf = 6200 AU

  38. Testing Shu Collapse Models • Best fit model of Choi et al (1995) does not fit! • The infall radius is within the central beam • Density too low by a factor of 5 to match 850 mm flux

  39. B335 NIR Extinction Profile Harvey et al. 2001 • NICMOS H-K color vs. Radius • Outer envelope density profile consistent with dust model • Shu model from Choi et al. (1995) may fit, but NIR extinction map looses sensitivity at R < 5000 AU • Density also too low for Shu model by factor of 3-5

  40. Interferometric Continuum Observations Best fit Model • Probe structure of inner envelope / disk • PdBI 1mm & 3mm images (Harvey et al. 2003) • Power law fit to inner envelope is flatter (n ~ r-1.6) r < 5000 AU • Shu model does NOT fit

  41. Continuum Modeling Summary • PPCs well fitted by Bonnor-Ebert spheres with central densities of 105 to 106 cm-3 • Power law viable because T(r) NOT ISOTHERMAL • Cores with protostars are well fitted by power law density profiles • Low-mass and high-mass distribution similar with p ~ 1.7 • No difference in p observed between Class 0 and Class I cores • Elongated cores appear to be fit by flatter power laws (p ~ 1) • Modeling is consistent with NIR extinction map of B335 • No evidence for infall radius in Shu models of Class 0 sources • Evolutionary indicators, Tbol and Lbol/Lsmm, inconsistent for Class I definition • No evidence for evolution of p with “standard” indicators (Tbol, Lbol/Lsmm, etc.) • Class I dust-determined mass smaller but biased towards Taurus

  42. Modeling Caveats L1544 L1157 • Asymmetric density structure • Observed aspect ratios up to 2 • 3D radiative transfer needed • Effects of outflows • Effects of heating in outflow cavity observed towards a few sources • However, no effect seen at submm l towards B335 • Crowded regions resulting in truncated Router

  43. Caveat : Disk Contribution • Disk may contribute a significant fraction of the emission within the central beam at submm l. • CSO-JCMT interferometer few observed disk fluxes at submm l • Worst-case model indicates flatter p by up to -0.6 • BIMA 2.7mm continuum observations towards Class 0 cores • Shirley et al. 2003, in prep. • L1527, B335, L483, L723 • Strong constraints await submm interferometers

  44. Future Work • Include Visser et al. 2002 Lynds dark cloud SCUBA survey • (Shirley, Chandler, et al. 2003) • BIMA study of low-mass Class 0 cores • (Shirley et al. 2003) • Contribution from a disk • Combination of submm interferometer + single dish bolometer cameras powerful probe of disk and entire envelope • Chemical survey of low-mass regions using density and temperature structure determined by continuum modeling • (Lee et al. 2003) • (Shirley & Wooten 2004) • Includes VLA and GBT projects • SIRTF Legacy: From Molecular Cores to Planet-forming Disks (c2d) (Evans et al.)

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