Disks in the Sub-Stellar Regime

Ray JayawardhanaUniversity of Toronto

Context and Motivation

* Brown dwarfs occupy the mass range between stars and planets: ~75 -- ~12 MJupiter (and below)

* Found both as companions to stars and as free-floating objects (vast majority so far)

* At ages of a few Myrs, their Teff ~ 3000-1500K (~M6-mid L)

Key Observational Question:

Do Young Sub-Stellar Objects undergo a T Tauri-like Phase?

Jets/Outflows

from Hartmann (1998)

Investigating Young Brown Dwarfs: A

Multi-Faceted Approach* Measurements of Disk Excess

* Disk Mass and Radius Estimates from mm Fluxes

* Search for Spectroscopic Signatures of Accretion

* Looking for Evidence of Jets/Outflows---------------------------------

* Binary frequency and properties

* Determination of Sub-Stellar Physical Properties (Mohanty et al. 2004a; Mohanty, Jayawardhana & Basri 2004b)

* Studies of Angular Momentum Evolution (v sin i, photometric periods) and Activity (H, X-rays, radio)

(e.g., Stelzer et al. 2006; Osten & Jayawardhana 2006)

Disk Excess Measurements

Sample

50+ objects with known spectral types later than M5

in nearby star-forming regions

Observations

JHKL’ photometry at VLT, Keck and IRTF, plus 2MASS

K-L’ is a more reliable measure of disk fractions: less susceptible

to geometric effects, smaller extinction corrections, easily measurable above photosphere

Jayawardhana, Ardila, Stelzer & Haisch (2003)

also see Comeron et al. (1998); Natta & Testi (2001); Natta et al. (2002); Liu et al. (2003)

Mohanty, Jayawardhana, Natta, et al.

(2004)

Flared disks in GY 310 and GY 11

Inner disk holes of a few (sub-)stellar radii in all three

Possible silicate feature in GY 310

Constraining the Disk Geometry: Ground-based Mid-IR Observations

Scholz, Jayawardhana, Wood & Meeus

Constraining Disk Geometry and Evolution:

Spitzer Mid-IR Observations

-disk flaring

-inner hole size

-grain growth and/or processing

~20 BDs in Taurus @ ~2 Myr

~30 BDs in Upper Sco @ ~5 Myr

Apai et al. (2005)

Grain Processing in Brown Dwarf Disks

Scholz, Jayawardhana, Wood & Meeus

Evidence for dust settling and inner holes?

Taurus

Taurus

Upper Sco

Upper Sco

Zooming in on H

Disk Fraction among Upper Sco BDs (~5 Myr)

Scholz, Jayawardhana, Wood & Meeus

Masses of Brown Dwarf Disks

IRAM 1.3mm survey of 20 BDs in Taurus

6 detected

with disk masses ~ 0.7 - 2.5 MJupiter

14 not detected

implying disk masses < 0.6 MJupiter(3

Disk masses range from < ~0.7% -- ~5% of BD mass

Scholz, Jayawardhana & Wood (2006)

Two BDs detected at mm by Pascucci et al. (2003) & Klein et al. (2003)

Ratio of Disk/Object Mass vs. Object Mass

Scholz, Jayawardhana & Wood (2006)

Minimum Disk Radii

at least 25% with r > 10 AU

(in contrast to Bate et al. 2002 prediction of ~5% with r > 10 AU)

Scholz, Jayawardhana &

Wood (2006)

Spectroscopic Signatures of Accretion

Sample

Objects with known spectral types later than M5 in Upper Scorpius, Ophiuchus, IC 348, Taurus, Chamaeleon I, and TW Hydrae

Total ~ 82 (!), including 55 brown dwarfs (M6 and later)

Observations

High-resolution optical spectra at Keck and Magellan

R~33,000 (HIRES), R~20,000 (MIKE)

H line profiles in high-resolution spectra is a good diagnostic of accretion (also OI, HeI, CaII in some cases)

H Criterion for Accretion

* H EW > 10A are usually categorized as CTTS

* But this threshold value varies with spectral type

•H line width at 10% of the peak may be a better indicator

* Nearly free-falling flow between disk inner edge and star

* For brown dwarfs at a few Myr,

adopted accretion threshold: ~200 km/s (+ other diagnostics)

(White & Basri 2003;

Jayawardhana, Mohanty, & Basri 2003)

Natta et al. (2004)

Accretion Rate vs. H 10% Width

Jayawardhana, Mohanty & Basri (2002, 2003);

Mohanty, Jayawardhana & Basri (2005)

also see White & Basri (2003);

Muzerolle et al. (2003)

H line profiles of

some accretors

Jayawardhana, Mohanty & Basri (2002)

H line profiles for Upper Sco (~5 Myr)

Mohanty, Jayawardhana & Barrado y Navascués (2003)

Mohanty, Jayawardhana & Basri (2005)

The Interesting Case of the 8-Myr-old Brown Dwarf 2MASS 1207-3932 in the TW Hydrae Association

Accretion and outflow in BDs can persist for up to ~8 Myrs

inner disk hole

Jayawardhana et al.(2003); Sterzik et al. (2004)

on-going accretion and possible wind?

Mohanty, Jayawardhana & Basri (2005)

also see Natta et al. (2004); Muzerolle et al. (2005)

Accretion Rate vs. (Sub-)Stellar Mass

Fraction of Accretors

Region Fraction Est. Age

Oph 6/9 < 1 Myr

Taurus 12/30 ~1 Myr

IC 348 & Cha I 10/32 ~2 Myr

Upper Sco 1/20 ~5 Myr

TW Hya 1/3 ~8 Myr

In Taurus, Cha I and Upper Sco, several objects with K-L’ excess do not show accretion-like H profiles

Disks persist after accretion rates have dropped?

2MASS1207-3932 in TWA: accretion and mid-IR excess, no L’ excess

Fraction of Accretors as a Function of Age: Comparison with higher mass

stars

Mohanty, Jayawardhana & Basri (2005)

Blue squares: K0-M4 stars using BM03

Red circles: VLM objects using BM03

Black triangles: VLM objects with our high-res. analysis

Fernandez & Comeron (2001); Barrado y Navascués, Mohanty & Jayawardhana

(2004)

Outflows in the VLM Domain?

LS RCrA 1 Cha II-C41

Barrado y Navascués, & Jayawardhana (2004)

Whelan et al. (2005)

Outflows in the VLM Domain?

Summary* A large fraction of young sub-stellar objects harbor K-L’ excess consistent with optically thick dusty disks

* Mid-IR and mm observations suggest a range of disk geometries and dust properties similar to those of TTS

-Possible evidence of dust settling and inner holes by ~5 Myr

-Disk masses <~0.7% - ~5% of object mass, some have disk radii > 10 AU

* Many brown dwarfs show signs of accretion at very young ages

* Disks appear to persist after accretion rates have dropped below measurable levels (e.g., Upper Sco)

* Inner disk lifetimes of brown dwarfs do not appear to be vastly different from those of T Tauri stars

* Tantalizing evidence of outflows/winds in a number of objects

Compelling Evidence for a T Tauri Phase in Young Sub-Stellar Objects

Planetary Mass Companion to a Nearby Young Brown Dwarf

Chauvin et al. (2004, 2005)

Planetary Mass Companion to a Nearby Young Brown Dwarf

Mohanty, Jayawardhana,Huelamo & Mamajek (2006)

Companion mass ~ 8x Jupiter

Companion Teff ~ 1600 K

Mohanty, Jayawardhana,Huelamo & Mamajek (2006)

A disk around the secondary as well?

Other Planetary Mass Objects with Disks?

Using Spitzer, Allers et al. (2006) identified 6 candidates in nearby SFRs with inferred masses of ~6-15 MJupiter and mid-IR excesses possibly from dusty disks

also see Luhman et al. (2006)

Optical spectra from VLT and NTT

Two ~5-10 MJ

One ~10-20 MJ

Two ~30-40 MJ

One likely background source!

Jayawardhana & Ivanov (2006)

Jayawardhana & Ivanov (2006)

Zooming in on H

© Don Dixon / Scientific American