Mid-Infrared Variability and Mass Accretion Toward NGC2264 Protostars

Susan Terebey

CSULA Dept of Physics & Astronomy

Ann Marie Cody, Luisa M. Rebull, John R. Stauffer, YSOVAR Science Team

Workshop on Dense Cores; Monterey July 2014

Premise: in protostar regime

Mass accretion -> accretion luminosity

Motivation

Protostar mass and mass accretion rates have been difficult to determine. We are in better shape with older class II YSOs.

Variability provides new view of mass accretion

L = L* + Lacc

ORIONSpitzer data

~250 hours of our observing time ~ 1 square degree region of the Orion Nebula cluster Cadence: 40 days, with 2 ∼epochs each day.~1400 Class I and II Orion YSOs with good quality time series (1-2% accuracy)more than half of them show significant variability. period Sep 2009 -- June 2011

• Distance ~ 760 pc• Age ~ 2-4 Myr• Known members: ~2000 + >3000 additional candidates• Large photometric & spectroscopic database• Many objects with disks

Coordinated Synoptic Investigation of NGC 2264

IRAC/MIPSP. S. Teixeira

Variable young stars and their disks:

A dynamic environment at many different wavelengths

Spitzer0.75°

5.2’

0.1

0.1

0.1

0.1

30d

Light curves display distinct patterns…

“spotted multi-periodic”

Spots + binary?

Spots?

“spotted”

“spotted irregular”

Rapidly evolving spots?

40d

0.02

0.1 mag

0.04

“AA Tau”

“accretor”

Dust obscuration?

Accretion bursts?

0.5 mag

0.2 mag

0.1

40d

30d

40d

Light curves display distinct patterns…

Structure function analysis

• 176 candidate protostars in NGC 2264

• Analysis of light curves for subset of69 candidates with good light curves (I2<14)

• Similar statistics for Orion

Light curve

What is the structure function?

0.1 mag

0.1 mag

28 days

0.1 mag

Orion shows same structure function

Magnetic support near 0.1 AU

V J

3.6

60o

0.8 AU

Turner etal 2014

Accretion LuminosityL = L* + Lacc

Lacc = ε GM⁄R *dM/dt

Characteristic 0.1 mag variability Suggests 10% or more accretion luminosity

Characteristic 30 day time scale for accretion events. If 30day = Porbital

Then R = 0.15 AU for M=0.5Msun

ConclusionsThe CSI2264 and YSOVAR Spitzer campaigns produced high quality multi-wavelength light curves for thousands of young stars. Emission at 4.5um is sensitive to the inner regions of young star disks.

Results for class I protostars in NGC2264 and Orion. • Light curve morphology of Class I protostars suggests accretion (variable

but not periodic).

Structure Function (SF) analysis shows characteristic• magnitude difference of 0.1 mag at 4.5 micron• timescale of T ~ 30 days• Power-law behavior for <30 day• Same structure function for protostars in Orion and NGC 2264

turbulent mass-accretion can lead to power-laws.