Origin and Early Evolution ofBrown DwarfsLeonardo Testi, Antonella Natta INAF - Osservatorio Astrofisico di Arcetri Fernando Comeron European Southern Observatory Francesca DAntona INAF - Osservatorio Astronomico di RomaAntonio Magazzu`, Ernesto Oliva INAF - Centro Galileo Galilei e TNG

New: Coordinated national project on young BDs(Arcetri, Cagliari, Capodimonte, Pino Torinese, Palermo, Uni Torino)NIR low-res spectroscopy with the Amici deviceThe M-, L-, T-dwarfs Amici spectral libraryThe origin of Brown DwarfsDisks around young Brown Dwarfs

Brown DwarfsBrown Dwarfs are star-like objects with masses below the hydrogen burning limit (~0.07 M)The first bona fide BDs have been discovered in 1995 (Rebolo et al.; Nakajima et al.)In just a few years, from being purely hypothetical, these objects required the definition of two new spectral classes L-type dwarfs Alkali metalsTeff range 1600-2200T-type dwarfs Methane absorptionTeff range 500-1600

Origin of Brown DwarfsDynamical interaction in small (proto-)stellar systems result in the ejection of stellar embiosNo or very short-lived diskSingle or low-mass binariesStandard core collapse and disk accretion scenarioHigh fraction of long lived active disksSimilar properties a more massive TTs systemsFormation in protoplanetary disks and subsequent dynamical ejectionNo disksNo multiples

Young BDs in Star Forming RegionsBDs 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 searchesVery young BDs are still embedded and this makes it very difficult to confirm young candidates by means of optical spectroscopyWe 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 systemsSeveral candidate BDs are known to have infrared excess, an indication of the presence of warm dust and possibly an accretion diskLarger samples are needed to investigate the nature of such systemsWe need an efficient classification/confirmation scheme based on infrared spectroscopy

The Amici Device

TNG/NICS Amici resultsPowerful 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 DwarfsSpectral characteristicsM- and L-dwarfs classification is an extension of the classical optical classification schemeT-dwarfs classification based on NIRWhy NIR?Most of the flux is emitted in the IRClassification based on molecular features (mainly H2O and CH4)Benefits of Low-ResMolecular features are broadHigher sensitivityLarger spectral range covered

L-dwarfT-dwarfKI

CH4

Amici spectra of cool dwarfs

NIR Amici Spectral ClassificationIs it possible to obtain an accurate spectral classification based on very low-resolution NIR spectra?Spectral librarySpectral indicesSuccess with L-dwarfs

NIR Amici Spectral ClassificationExtension to M- and T-dwarfs

Cool Photosphere Models

Model spectra fits

Fits Results: Teff vs Sp.Type

Brown DwarfsBrown Dwarfs are star-like objects with masses below the hydrogen burning limit (~0.7 M)The first bona fide BDs have been discovered in 1995 (Rebolo et al.; Nakajima et al.)In just a few years, from being purely hypothetical, these objects required the definition of two new spectral classes L-type dwarfs 1600-2200T-type dwarfs 500-1600Following the discoveries in the solar neighborhood many BDs and Planetary-Mass Objects have been discovered in SFRs

Origin of Brown DwarfsDynamical interaction in small (proto-)stellar systems result in the ejection of stellar embiosNo or very short-lived diskSingle or low-mass binariesStandard core collapse and disk accretion scenarioHigh fraction of long lived active disksSimilar properties a more massive TTs systemsFormation in protoplanetary disks and subsequent dynamical ejectionNo disksNo multiples

The -Oph SampleThe embedded cluster is known to be very young (age < 1Myr)Previous studies indicate the presence of Young BDs with IR excessThe ISOCAM survey of Bontemps et al (2001) was used to select candidate BDs with infrared excessFinal selection criteria:Class II objectsL

Classification and Photospheric PropertiesFor the 9 objects in the sample we obtained low-resolution near-infrared spectra with the Amici device at the TNGExtinction, Spectral Type and Teff were derived by comparison with field dwarfs and model spectra

Classification and Photospheric PropertiesLuminosities were derived from dereddened J-band magnitudes, using appropriate bolometric corrections

Disk Models Fits to the SEDsWe modeled the SEDs of the 9 objects following the prescriptions of standard disk models that accurately fit more massive systemsMdisk=0.03 MRdisk=65 AUi = 0-60Ri=1-3 RFlared or flat geometry

The Case of GY11/CAM033M~8-12 MJ ; age

ImplicationsMid Infrared excess from young BDs can be interpreted in terms of standard disks scaled down to the appropriate central object parametersThis suggest that the formation mechanism of at least some of the BDs may be similar to those of more massive starsAlternative formation mechanisms predict that a small fraction of young BDs are surrounded by small and short-lived disksWe cannot exclude such mechanismsOur sample is biased towards IR excess objectsMid-IR excess alone cannot constrain the disk size and mass

Future DevelopmentsOur 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 modelsFar 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