Probing the Structure of Low-Probing the Structure of Low-mass Star-forming Cores with mass Star-forming Cores with

Dust Continuum EmissionDust Continuum Emission

Yancy L. Shirley

NRAO Colloquium

January 2003

Collaborators: Claire Chandler, Neal J. Evans II, Jonathan M. C. Rawlings, Chad H. Young

Where Do Stars Form ?Where Do Stars Form ?

NASA Hubble Heritage

Molecular Cloud ComplexesMolecular Cloud ComplexesRho Ophiuchus

L. Cambresy 1999

Lupus

Dense CoresDense Cores

Motte, Andre, & Neri 1998

Star Formation within CoresStar Formation within Cores

Isolated Molecular CloudsIsolated Molecular Clouds

VLT

BHR-71

Low-mass Star FormationLow-mass Star Formation

Star formation occurs within dense molecular Star formation occurs within dense molecular corescores

Low-mass star formation may occur in Low-mass star formation may occur in isolation or in isolation or in clustered environmentsclustered environments

There exists a putative evolutionary sequence for There exists a putative evolutionary sequence for low-mass star formationlow-mass star formation

Theories of protostar formation predict the Theories of protostar formation predict the evolution of the core’s density, temperature, and evolution of the core’s density, temperature, and velocity structurevelocity structure

In particular In particular the density structure is a strong the density structure is a strong discriminator of theoretical modelsdiscriminator of theoretical models

Low-mass Low-mass Evolutionary Evolutionary

SchemeScheme

Protostar evolves from Protostar evolves from deeply embedded phase deeply embedded phase (Class 0) to optically visible (Class 0) to optically visible T-Tauri star (Class II & III).T-Tauri star (Class II & III).

Object classified based on Object classified based on shape of the SED:shape of the SED:

(NIR) slope(NIR) slope

TTbolbol

LLbolbol / L / Lsmmsmm

Figure from P.Andre 2002

SCUBA SurveySCUBA Survey

16 nights at the JCMT 15-m telescope16 nights at the JCMT 15-m telescope

39 nearby (D < 450 pc), low-mass (L39 nearby (D < 450 pc), low-mass (Lbolbol < 13L < 13Lsunsun) cores) cores

6 Pre-Protostellar Cores (PPCs)6 Pre-Protostellar Cores (PPCs)

15 Class 0 Cores15 Class 0 Cores

18 Class I Cores18 Class I Cores

SCUBA 850 & 450 SCUBA 850 & 450 m jiggle maps (3.5 x 3.5 arcmin)m jiggle maps (3.5 x 3.5 arcmin)Sensitivity 20 mJy beam-1 or roughly Av = 3 mag at edgeSensitivity 20 mJy beam-1 or roughly Av = 3 mag at edge

mbmb ~ 15 arcsec at 850 ~ 15 arcsec at 850 mm

mbmb ~ 8 arcsec at 450 ~ 8 arcsec at 450 mm

120 arcsec chop throw120 arcsec chop throw

Shirley et al. 2000, Young et al. 2002

Goals of Continuum SurveyGoals of Continuum Survey

Use Use optically thinoptically thin submm emission to submm emission to characterize the envelope structure of star characterize the envelope structure of star forming coresforming cores

Low-mass sample: 500 to 10Low-mass sample: 500 to 1044 AU AU

Compare evolutionary indicatorsCompare evolutionary indicatorsTest putative evolutionary scheme for low-mass coresTest putative evolutionary scheme for low-mass cores

More realistic More realistic densitydensity and and temperaturetemperature profiles profiles for radiative transfer models of molecular line for radiative transfer models of molecular line observationsobservations

L1544 L1689B L1512

L1498 L1689A B133

Preprotostellar CoresPreprotostellar Cores

10,000 AU

B335 B228 L1527

IRAS03282 L483 L1448C/N

Class 0 CoresClass 0 Cores

10,000 AU10,000 AU

IRAS04166 L43 IRAS04302

IRAS04264 IRAS04169 IRAS04248

Class I CoresClass I Cores

10,000 AU

PPC

Low Mass Evolution ?Low Mass Evolution ?

Class 0 Class I

Dust continuum emission qualitatively consistent with putative evolutionary sequence

Class I Bias: All but 3 Class I sources located in Taurus

Dust OpacityDust Opacity

Evolutiony IndicatorsEvolutiony IndicatorsTTbolbol and L and Lbolbol/L/Lsmmsmm inconsistentinconsistent

Class 0 = Lbol/Lsmm < 200 & Tbol < 70 K

Submm Continuum EmissionSubmm Continuum Emission

Submillimeter continuum emission is Submillimeter continuum emission is optically thinoptically thin. The . The specific intensity along a line-of-sight is given by:specific intensity along a line-of-sight is given by:

Why must we model ?Why must we model ?

Rayleigh-Jeans approximation Rayleigh-Jeans approximation failsfails in outer envelope of in outer envelope of low-mass coreslow-mass cores

hh/k = 16.9 K at 850 /k = 16.9 K at 850 mm

Heating from ISRF is very importantHeating from ISRF is very important in outer envelopes in outer envelopes of low-mass coresof low-mass cores

Radiative transfer is Radiative transfer is optically thick at short optically thick at short

Observed brightness distribution is convolved with Observed brightness distribution is convolved with complicated beam pattern, scanning, and choppingcomplicated beam pattern, scanning, and chopping

SCUBA Beam ProfilesSCUBA Beam Profiles

15.2”

7.9”

Radiative Transfer ProcedureRadiative Transfer Procedure

nd(r)

L Td(r)

SI(b)

Nearly orthogonal constraints:Nearly orthogonal constraints:

SEDSED Mass x OpacityMass x OpacityI(b)I(b) n(r)n(r)

IterateIteratePhysical Model

n(r) Observations

Gas to

Dust

Radiative

Transfer

Simulate

Obs.

Initial Density StructureInitial Density Structure

Solutions to the equation of hydrostatic equilibrium:Solutions to the equation of hydrostatic equilibrium:Bonnor-Ebert spheres: Bonnor-Ebert spheres: (Bonnor 1956, Ebert 1955)(Bonnor 1956, Ebert 1955)

Limit is Singular Isothermal Sphere Limit is Singular Isothermal Sphere (n ~ r(n ~ r-2-2))

BE Models of submm emissionBE Models of submm emission

L1544L1544

10,000 AU

N2H+

Caselli et al. 2002

Pre-protostellar core

Taurus Molecular Cloud

140 pc

Evidence for extended infall ?

Ward-Thompson et al. 2002

SCUBA 850 m SCUBA 450 m ISO 200 m

3.5’ x 3.5’ 12’ x 12’

Power Law ModelPower Law Model

B335B335Class 0 protostar in Bok globuleClass 0 protostar in Bok globule

Isolated: 1 deg from Aquila RiftIsolated: 1 deg from Aquila Rift

D = 250 pcD = 250 pc

Best collapse candidateBest collapse candidate

10,000 AU

DSS - opticalSCUBA 850 m

6’ x 6’3.5’ x 3.5’

SCUBA 450 m

N2H+

Caselli et al. 2002

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

Testing Model ParametersTesting Model Parameters

Uncertainty on power law Uncertainty on power law model is model is

Neglects possible Neglects possible contribution from a disk, contribution from a disk, outflow, etc.outflow, etc.

2.0p

Class 0 and I ModelsClass 0 and I ModelsSingle power law models are good fitsSingle power law models are good fits

No variation in <p>No variation in <p> observed between Class 0 and Class I observed between Class 0 and Class I

3.07.1 p 4.06.1 p

Density vs. pDensity vs. p

Density of best fit model Density of best fit model at 1000 AU vs. pat 1000 AU vs. p

Class I cores order of Class I cores order of magnitude lower densitymagnitude lower density

<n> = 1.2 x 10<n> = 1.2 x 1066 Class 0Class 0

<n> = 1.5 x 10<n> = 1.5 x 1055 Class IClass I

Deconvolved Size vs. pDeconvolved Size vs. p Convolution of a Gaussian beam pattern with a power

law intensity profile yields a deconvolved source size that varies with p

Dust-determined MassDust-determined Mass Can use best fit model to determine envelope mass

<M> = 3.00 ± 1.00 Msun Class 0

<M> = 0.19 ± 0.11 Msun Class 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

Virial MassVirial 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

Gas around Class IGas around Class IVirial mass does Virial mass does NOTNOT agree with dust-determined mass towards agree with dust-determined mass towards Class I sourcesClass I sources

MMvv/M/Mdd up to 20! up to 20!

Problem: Gas tracer, such as NProblem: Gas tracer, such as N22HH++ or C or C1818O appears to be probing O appears to be probing

nearby PPC or background cloudnearby PPC or background cloudLooking for a dense gas tracer of Class I envelopes!Looking for a dense gas tracer of Class I envelopes!

Caselli et al. 2002

N2H+

Variation of Dust Opacity ?Variation of Dust Opacity ?

No evidenceNo evidence for for variation in opacity in variation in opacity in outer envelope of low-outer envelope of low-mass coresmass cores

Beam convolution Beam convolution cannot be ignoredcannot be ignored!!

Comparison limited Comparison limited between 450 and 850 between 450 and 850 microns.microns.

Standard Star Formation ModelsStandard Star Formation Models

MODEL

INITIAL CONFIG.

Larson & Penston (69)

Shu & Hunter (77)

Foster & Chevalier (93)

Uniform n(r) SIS Critical BE- Sphere

higher const higher initially

Perturbations to Shu77:

ROTATION: TSC (84)

B FIELDS: Galli & Shu (93a,b)

ACCRET.RATE

Shu Inside-Out Collapse ModelShu Inside-Out Collapse ModelInitial configuration is SIS Initial configuration is SIS (Shu 1977)(Shu 1977)

Infall radius, rInfall radius, rinfinf, propagates outward at sound speed, propagates outward at sound speed

Animation provided by C. H. Young

B335 Collapse ModelB335 Collapse Model

Shu77 collapse model of molecular lines: HShu77 collapse model of molecular lines: H22CO and CS CO and CS

by by Choi et al. 1995Choi et al. 1995 Best fit: rBest fit: rinfinf = 6200 AU = 6200 AU

Testing Shu Collapse ModelsTesting Shu Collapse ModelsBest fit model of Best fit model of Choi et al (1995)Choi et al (1995) does not fit! does not fit!

The infall radius is within the central beamThe infall radius is within the central beam

Density too low by a factor of 5 to match 850 Density too low by a factor of 5 to match 850 m fluxm flux

B335 NIR Extinction ProfileB335 NIR Extinction Profile

NICMOS H-K color vs. RadiusNICMOS H-K color vs. Radius

Outer envelope density profile Outer envelope density profile consistent with dust modelconsistent with dust model

Shu model from Shu model from Choi et al. Choi et al. (1995)(1995) may fit, but NIR may fit, but NIR extinction map extinction map looses looses sensitivity at R < 5000 AUsensitivity at R < 5000 AU

Density also too low for Shu Density also too low for Shu model by factor of 3-5model by factor of 3-5

Harvey et al. 2001

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

Interferometric ContinuumInterferometric ContinuumObservations Best fit Model

Continuum Modeling SummaryContinuum Modeling Summary

PPCs well fitted by Bonnor-Ebert spheres with central PPCs well fitted by Bonnor-Ebert spheres with central densities of 10densities of 1055 to 10 to 1066 cm cm-3-3

Power law viable because Power law viable because T(r) NOT ISOTHERMALT(r) NOT ISOTHERMAL

Cores with protostars are Cores with protostars are well fitted by power law density well fitted by power law density profilesprofiles

Low-mass and high-mass Low-mass and high-mass distribution similar with p ~ 1.7distribution similar with p ~ 1.7No difference in pNo difference in p observed between Class 0 and Class I cores observed between Class 0 and Class I coresElongated cores appear to be fit by flatter power laws (p ~ 1)Elongated cores appear to be fit by flatter power laws (p ~ 1)Modeling is consistentModeling is consistent with NIR extinction map of B335 with NIR extinction map of B335No evidence for infall radius in Shu models of Class 0 sourcesNo evidence for infall radius in Shu models of Class 0 sources

Evolutionary indicators, TEvolutionary indicators, Tbolbol and L and Lbolbol/L/Lsmmsmm, , inconsistent for inconsistent for Class I definitionClass I definition

No evidence for evolution of p with “standard” indicators (TNo evidence for evolution of p with “standard” indicators (Tbolbol, , LLbolbol/L/Lsmmsmm, etc.), etc.)Class I dust-determined mass smaller but biased towards TaurusClass I dust-determined mass smaller but biased towards Taurus

Modeling CaveatsModeling Caveats

AsymmetricAsymmetric density structure density structureObserved aspect ratios up to 2Observed aspect ratios up to 2

3D radiative transfer needed3D radiative transfer needed

Effects of outflowsEffects of outflowsEffects of heating in outflow Effects of heating in outflow cavity observed towards a few cavity observed towards a few sourcessources

However, no effect seen at However, no effect seen at submm submm towards B335 towards B335

Crowded regions resulting in Crowded regions resulting in truncated Rtruncated Routerouter

L1544 L1157

Caveat : Disk ContributionCaveat : Disk Contribution

Disk Disk may contribute a significant fractionmay contribute a significant fraction of the emission of the emission within the central beam at submm within the central beam at submm ..

CSO-JCMT interferometer few observed disk fluxes at submm CSO-JCMT interferometer few observed disk fluxes at submm Worst-case model Worst-case model indicates indicates flatter p by up to -0.6flatter 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

Future WorkFuture Work

Include Visser et al. 2002 Lynds dark cloud SCUBA Include Visser et al. 2002 Lynds dark cloud SCUBA surveysurvey

(Shirley, Chandler, et al. 2003)(Shirley, Chandler, et al. 2003)

BIMA study of low-mass Class 0 coresBIMA study of low-mass Class 0 cores (Shirley et al. 2003)(Shirley et al. 2003)Contribution from a diskContribution from a diskCombination of submm interferometer + single dish Combination of submm interferometer + single dish bolometer cameras powerful probe of disk and entire bolometer cameras powerful probe of disk and entire envelopeenvelope

Chemical survey of low-mass regions using density Chemical survey of low-mass regions using density and temperature structure determined by and temperature structure determined by continuum modeling continuum modeling

(Lee et al. 2003)(Lee et al. 2003)(Shirley & Wooten 2004)(Shirley & Wooten 2004)Includes VLA and GBT projectsIncludes VLA and GBT projects

SIRTF Legacy: From Molecular Cores to Planet-SIRTF Legacy: From Molecular Cores to Planet-forming Disks (c2d) forming Disks (c2d) (Evans et al.)(Evans et al.)