Formation of Low Mass Stars

Shantanu BasuWestern University, London, Ontario, Canada

Collaborators: Sayantan Auddy (Western), Manuel Gil (McGill), Takahiro Kudoh (Nagasaki), Eduard Vorobyov (Vienna)

SFDE 2016Quy Nhon, Vietnam

Monday July 25, 2016

Key Steps

• Fragmentation of cloud into large scale structures (filaments/ribbons, etc.)

• Formation of dense cores within larger structures

• Core collapse to form hydrostatic protostar• Disk formation, multiplicity, BDs, planets

Cosmological Filaments

Millenium simulation, VIRGO Consortium, Springel et al. (2005)

Galaxy structure from SDSS

Herschel Observations

Arzoumanian et al. (2011)

IC 5146

Avg. spacing between filaments ~ 1 pc.

Avg. observed filament width ~ 0.1 pc over a wide range of column densities.

Magnetic Fields and Filaments

Palmeirim et al. (2012)

Herschel observations of B211 and B213 in Taurus Molecular Cloud

Inferred B directions in green.

Molecular Cloud Scenario

Supercritical high-density regions assembled by large scale flows/turbulence

Subcritical common envelope

cf. Nakamura & Li (2005), Elmegreen (2007), Kudoh & Basu (2008), Nakamura & Li (2008), Basu , Ciolek, Dapp, & Wurster (2009; model shown above).

Magnetic Ribbon Model

Auddy, Basu, & Kudoh (2016)

See poster!

12

00

0

2 1t

A

vL L

v

Observed width depends on turbulent compression scale, Alfvénic Mach number, and viewing angle.

Magnetic Ribbon Model

Auddy, Basu, & Kudoh (2016)

2 JH

LAverage over random viewing angles

Observe from a set of random viewing angles: blue dots.

Dense Cores

Sheets, ribbons, etc. all inevitably fragment into dense cores through gravity-dominated collapse, magnetically–regulated fragmentation, or turbulent fragmentation.

Dense Cores to Stars – Direct Mapping?

Andre et al. (2014). CMF from Herschel data of Aquila – Konyves et al. 2010, Andre et al. 2010

Jeans mass

2/32/1

3-4 K 10cm105.5

Tn

MM sunJ

Can it account for all substellar masses?

Star Formation

Key Questions:

- Star Formation as an accretion process or a fragmentation process?

- Do disks play a role in determining stellar/substellar masses?

New deep image of ONC

IR view of Orion Nebula Cluster. Courtesy: ESO

New deep, wide near near-IR VLT HAWK-1 map

~ 920 low mass stars~ 760 brown dwarfs~ 160 planemos

A multitude of very low mass objects from ejection from multiple systems during the early star-formation process or from circumstellar disks?

New respect for substellar objects?

Low Mass Objects in Orion

Drass et al. (2016)

Chabrier IMF, extrapolated

New ONC IMF, Drass et al.

Binning in D m not D log m

IMF of a dense subregion

Accretion-Ejection Scenario in Cluster-Forming Simulations

Bate (2009) and earlier simulations find a population of ejected BDs. Radiative feedback and other effects (e.g. magnetic fields) play a role in limiting the numbers of BDs.

Bate (2009) - Results of three separate simulations of 50 Msun clouds with radiative feedback in comparison to standard IMFs. Note small number statistics.

High-res Disk Formation and Episodic Accretion

Time (Myr)

0.0 0.1 0.2 0.3 0.4 0.5

Mass a

ccre

tion r

ate

(M

yr-1

)

1e-10

1e-9

1e-8

1e-7

1e-6

1e-5

1e-4

1e-3

smooth mode burst mode

FU Ori eruptions

flickering

residual disk accretion

-200 -100 0 100 200

Radial distance (AU)

-200

-100

0

100

200

Ra

dia

l d

ista

nc

e (

AU

)

678910111213

-250 -150 -50 50 150 250

Radial distance (AU)

-250

-150

-50

50

150

250

Rad

ial

dis

tan

ce (

AU

)

678910111213

Vorobyov & Basu (2006, ApJ, 650, 956 )

Bursts of accretion occur during the early accretion phase, as clumps are formed and driven inward. This is followed by a more quiescent phase that is still characterized by flickering accretion.

Nonlinear instability clumps efficient angular momentum transport

Quiescent period

Just before a burst

Spitzer Telescope Survey Episodic Accretion Paradigm Required

Lyman Spitzer Jr. (1914-1997)

Spitzer Space Telescope, infrared wavelengths Enoch et al. (2009), Evans et al. (2009)

Source counts lead to estimated lifetime of main mass accumulation phase (Class 0 and Class I) of ~ 0.5 Myr. For mean stellar mass ~ 0.5 Msun, mean accretion rate is ~ 10-6 Msun/yr (Blue horizontal line).

But most luminosities of sources fall far below this line, with a small fraction lying above the line episodic accretion is required!

Luminosity distribution in embedded phase

3

10ssun

McGMML L

R R

for 0.5 , 3 , 10K.sun sunM M R R T

Dunham et al. (2010)

Dashed line is predicted luminosity distribution of embedded protostarsusing smooth accretion of inside-out collapse of a singular isothermal sphere.

Luminosity distribution in embedded phase

A combination of declining accretion rate and episodic bursts can resolve the luminosity problem.

Dunham and Vorobyov (2012)

Ejection during Disk AccretionR

adia

l dis

tan

ce (

AU

)

Basu & Vorobyov (2012, ApJ, 750, 30)Ejection correlated with higher mass and angular momentum in initial state.

2

0.95

1.3 10

sunM M

Ejection of gaseous clump during multiple object interaction.

Ejections occur in many models

Basu & Vorobyov (2012, ApJ, 750, 30)

Ejected clumps span the substellar to low mass star regime, and have moderate ejection speeds 0.8 +/- 0.35 km/s.

Some models exhibit multiple ejections

Lowest mass objects more likely to be sheared by tidal effects arising from ejection

SF as a killed process

Power-law index ad/g is the ratio of characteristic growth time of stars to the characteristic time of accretion termination.

Model developed in order to understand intermediate and high mass power-law tail of IMF (Basu & Jones 2004; Basu, Gil, & Auddy 2015). Also Myers (2000, 2009, 2010, 2014).

1. An initial lognormal2. Lognormal plus

exponential growth for fixed time

3. MLP: lognormal plus exponential growth for an exponential distribution of times

12

3

Modified Lognormal Power-Law (MLP) Distribution

. where

,ln

2

1 erfc

2/exp2

)(

0

00

12

0

2

0

gda

a

aaa a

m

mmf

( ) tf t e dd

0 ,tm m e g

.

2

ln -exp

2

1)(

2

2

m

mmf

3 parameters: 0, 0, a.

MLP Distribution for IMF

Basu, Gil, & Auddy (2015)

Best fit parameters :

This means the underlying lognormal distribution had a mode (peak ) at

Close to mass of first hydrostatic core.

0 = -2.404, 0 = 1.044, a = 1.396

0

00

)1(2

0

2

0

ln

2

1erfc 2exp

2)(

aaa

a a mmmf

exp (0 – 02) = 0.03 Msun.

Summary

• Observations: first stage of molecular cloud fragmentation occurs with ~ pc scale spacing and into ~ 0.1 pc wide filaments

• Dynamically oscillating quasi-equilibrium magnetic ribbons can provide an explanation for relatively flat ~ 0.1 pc observed widths (Auddy et al. poster)

• New detection of numerous substellar objects in ONC down to planet scale may imply that killed accretion is primary determinant of low mass IMF

• Episodic accretion now the standard paradigm for early protostellarevolution, when most of the stellar mass is actually assembled

• Ejections are a part of the episodic accretion paradigm and may explain the large number of free-floating substellar objects

• Accretion history driven IMF models may explain low mass IMF as well as provide a fit to high mass power law tail of IMF