Origin and Early Evolution of Brown Dwarfs

Leonardo Testi: Formation and Evolution of Brown Dwarfs, Stars in Galaxies, La Palma, Mar 8, 2003

Origin and Early Evolution ofBrown Dwarfs

Leonardo Testi, Antonella Natta – INAF - Osservatorio Astrofisico di Arcetri Fernando Comeron – European Southern Observatory

Francesca D’Antona – INAF - Osservatorio Astronomico di RomaAntonio Magazzu`, Ernesto Oliva – INAF - Centro Galileo Galilei e TNG

New: New: Coordinated national project on young BDs(Arcetri, Cagliari, Capodimonte, Pino Torinese, Palermo, Uni Torino)(Arcetri, Cagliari, Capodimonte, Pino Torinese, Palermo, Uni Torino)

NIR low-res spectroscopy with the Amici device The M-, L-, T-dwarfs Amici spectral library The origin of Brown Dwarfs Disks around young Brown Dwarfs


Brown Dwarfs Brown Dwarfs are star-like objects with masses below the Brown Dwarfs are star-like objects with masses below the

hydrogen burning limit (~0.07 Mhydrogen burning limit (~0.07 M)) The first The first bona fidebona fide BDs have been discovered in 1995 BDs have been discovered in 1995

(Rebolo et al.; Nakajima et al.)(Rebolo et al.; Nakajima et al.) In just a few years, from being purely hypothetical, these In just a few years, from being purely hypothetical, these

objects required the definition of two new spectral classes objects required the definition of two new spectral classes L-type dwarfs L-type dwarfs

Alkali metalsAlkali metals TTeffeff range 1600-2200 range 1600-2200

T-type dwarfs T-type dwarfs Methane absorptionMethane absorption TTeffeff range 500-1600 range 500-1600


Origin of Brown Dwarfs Dynamical interaction in small

(proto-)stellar systems result in the ejection of stellar embios No or very short-lived disk Single or low-mass binaries

Standard core collapse and disk accretion scenario High fraction of long lived

active disks Similar properties a more

massive TTs systems Formation in protoplanetary

disks and subsequent dynamical ejection No disks No multiples


Young BDs in Star Forming Regions BDs are much brighter and hotter when they are young, thus they are

in principle easier to detect and for this reason young clusters and associations have been one of the primary targets of BDs searches

Very young BDs are still embedded and this makes it very difficult to confirm young candidates by means of optical spectroscopy

We showed that the SED of two candidate BDs and one “bona fide” BD in the Chamaeleon I cloud are consistent with disk systems with a similar structure to more massive TTauri and Herbig Ae systems

Several candidate BDs are known to have infrared excess, an indication of the presence of warm dust and possibly an accretion disk

Larger samples are needed to investigate the nature of such systems We need an efficient classification/confirmation scheme based on

infrared spectroscopy


The Amici Device

0.8μm

2.5μmL0A0


TNG/NICS Amici results Powerful wind in a quasar

at z=5.8 (Maiolino et al. 2001)

H2O in Trans-Neptunian objects (Licandro et al. 2001)

Spectral classification of cool dwarfs (Testi et al. 2001; 2003)


Spectral Classification of Late Dwarfs

Spectral characteristicsSpectral characteristics M- and L-dwarfs classification is an M- and L-dwarfs classification is an

extension of the classical optical extension of the classical optical classification schemeclassification scheme

T-dwarfs classification based on T-dwarfs classification based on NIRNIR

Why NIR?Why NIR? Most of the flux is emitted in the IRMost of the flux is emitted in the IR Classification based on molecular Classification based on molecular

features (mainly Hfeatures (mainly H22O and CHO and CH44)) Benefits of Low-ResBenefits of Low-Res

Molecular features are broadMolecular features are broad Higher sensitivityHigher sensitivity Larger spectral range coveredLarger spectral range covered

L-dwarfL-dwarf

T-dwarfT-dwarf

KIKI

CHCH44


Amici spectra of cool dwarfs


NIR Amici Spectral Classification Is it possible to obtain an accurate spectral classification

based on very low-resolution NIR spectra? Spectral library Spectral indices

Success with L-dwarfs


NIR Amici Spectral Classification Extension to M-

and T-dwarfs


Cool Photosphere Models


Model spectra fits

Dusty Mod• …•…

Settle Mod• …•…

Cond Mod• …•…

Smooth

Resample

Least Sq. Fit

λ shift?


Fits Results: Teff vs Sp.Type


Origin of Brown Dwarfs Dynamical interaction in small

(proto-)stellar systems result in the ejection of stellar embios No or very short-lived disk Single or low-mass binaries

Standard core collapse and disk accretion scenario High fraction of long lived

active disks Similar properties a more

massive TTs systems Formation in protoplanetary

disks and subsequent dynamical ejection No disks No multiples


The ρ-Oph Sample The embedded cluster is known to be very young (age <

1Myr) Previous studies indicate the presence of Young BDs with

IR excess The ISOCAM survey of Bontemps et al (2001) was used to select candidate BDs with infrared excess

Final selection criteria: Class II objects L<0.04 L

AV < 8.5 mag

9 Objects detected at both wavelengths, close or below completness


Classification and Photospheric Properties For the 9 objects in the sample we obtained low-

resolution near-infrared spectra with the Amici device at the TNG

Extinction, Spectral Type and Teff were derived by comparison with field dwarfs and model spectra


Classification and Photospheric Properties Luminosities were derived from dereddened J-band

magnitudes, using appropriate bolometric corrections


Disk Models Fits to the SEDs We modeled the SEDs

of the 9 objects following the prescriptions of “standard” disk models that accurately fit more massive systems Mdisk=0.03 M

Rdisk=65 AU i = 0º-60º

Ri=1-3 R

Flared or flat geometry


The Case of GY11/CAM033 M~8-12 MJ ; age<1Myr MIR excess is well fit by a

flared disk with inner hole


Implications

Mid Infrared excess from young BDs can be interpreted in terms of “standard” disks scaled down to the appropriate central object parameters

This suggest that the formation mechanism of at least some of the BDs may be similar to those of more massive stars

Alternative formation mechanisms predict that a small fraction of young BDs are surrounded by small and short-lived disks

We cannot exclude such mechanisms Our sample is biased towards IR excess objects Mid-IR excess alone cannot constrain the disk size and

mass


Future Developments Our models suggest that the infrared excess is small at K-

band, and can be easily detected only at longer wavelengths

L-band surveys of (NIR) spectroscopically classified complete samples are necessary to derive a correct estimate of the fraction of young BDs with disk and to constrain the formation models

Far infrared and millimetric strudies are required to constrain the disk size and mass (SMA, ALMA)

Higher resolution spectroscopy will allow to measure the disk activity and possibly reveal disk accretion features and measure accretion rates

Origin and Early Evolution of Brown Dwarfs

Documents

Transcript of Origin and Early Evolution of Brown Dwarfs