disks in the sub-stellar regime ray jayawardhana university of toronto
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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
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