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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
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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
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Cosmological Filaments
Millenium simulation, VIRGO Consortium, Springel et al. (2005)
Galaxy structure from SDSS
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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.
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Magnetic Fields and Filaments
Palmeirim et al. (2012)
Herschel observations of B211 and B213 in Taurus Molecular Cloud
Inferred B directions in green.
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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).
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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.
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Magnetic Ribbon Model
Auddy, Basu, & Kudoh (2016)
2 JH
LAverage over random viewing angles
Observe from a set of random viewing angles: blue dots.
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Dense Cores
Sheets, ribbons, etc. all inevitably fragment into dense cores through gravity-dominated collapse, magnetically–regulated fragmentation, or turbulent fragmentation.
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Dense Cores to Stars – Direct Mapping?
Andre et al. (2014). CMF from Herschel data of Aquila – Konyves et al. 2010, Andre et al. 2010
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Jeans mass
2/32/1
3-4 K 10cm105.5
Tn
MM sunJ
Can it account for all substellar masses?
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Star Formation
Key Questions:
- Star Formation as an accretion process or a fragmentation process?
- Do disks play a role in determining stellar/substellar masses?
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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?
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New respect for substellar objects?
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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
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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.
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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
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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!
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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.
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Luminosity distribution in embedded phase
A combination of declining accretion rate and episodic bursts can resolve the luminosity problem.
Dunham and Vorobyov (2012)
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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.
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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
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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.
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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.
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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