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Origin and Early Evolution of Brown Dwarfs PowerPoint Presentation
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Origin and Early Evolution of Brown Dwarfs

Origin and Early Evolution of Brown Dwarfs

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Origin and Early Evolution of Brown Dwarfs

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  1. 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 Roma Antonio 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 device • The M-, L-, T-dwarfs Amici spectral library • The origin of Brown Dwarfs • Disks around young Brown Dwarfs Leonardo Testi: Formation and Evolution of Brown Dwarfs, Stars in Galaxies, La Palma, Mar 8, 2003

  2. Brown Dwarfs • Brown 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 metals • Teff range 1600-2200 • T-type dwarfs • Methane absorption • Teff range 500-1600 Leonardo Testi: Formation and Evolution of Brown Dwarfs, Stars in Galaxies, La Palma, Mar 8, 2003

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

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

  5. A0 L0 2.5μm 0.8μm The Amici Device Leonardo Testi: Formation and Evolution of Brown Dwarfs, Stars in Galaxies, La Palma, Mar 8, 2003

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

  7. Spectral Classification of Late Dwarfs L-dwarf • Spectral characteristics • M- and L-dwarfs classification is an extension of the classical optical classification scheme • T-dwarfs classification based on NIR • Why NIR? • Most of the flux is emitted in the IR • Classification based on molecular features (mainly H2O and CH4) • Benefits of Low-Res • Molecular features are broad • Higher sensitivity • Larger spectral range covered KI T-dwarf CH4 Leonardo Testi: Formation and Evolution of Brown Dwarfs, Stars in Galaxies, La Palma, Mar 8, 2003

  8. Amici spectra of cool dwarfs Leonardo Testi: Formation and Evolution of Brown Dwarfs, Stars in Galaxies, La Palma, Mar 8, 2003

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

  10. NIR Amici Spectral Classification • Extension to M- and T-dwarfs Leonardo Testi: Formation and Evolution of Brown Dwarfs, Stars in Galaxies, La Palma, Mar 8, 2003

  11. Cool Photosphere Models Leonardo Testi: Formation and Evolution of Brown Dwarfs, Stars in Galaxies, La Palma, Mar 8, 2003

  12. Dusty Mod • … • … • Settle Mod • … • … • Cond Mod • … • … Smooth Resample Least Sq. Fit λ shift? Model spectra fits Leonardo Testi: Formation and Evolution of Brown Dwarfs, Stars in Galaxies, La Palma, Mar 8, 2003

  13. Fits Results: Teff vs Sp.Type Leonardo Testi: Formation and Evolution of Brown Dwarfs, Stars in Galaxies, La Palma, Mar 8, 2003

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

  15. 9 Objects detected at both wavelengths, close or below completness 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 Leonardo Testi: Formation and Evolution of Brown Dwarfs, Stars in Galaxies, La Palma, Mar 8, 2003

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

  17. Classification and Photospheric Properties • Luminosities were derived from dereddened J-band magnitudes, using appropriate bolometric corrections Leonardo Testi: Formation and Evolution of Brown Dwarfs, Stars in Galaxies, La Palma, Mar 8, 2003

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

  19. The Case of GY11/CAM033 • M~8-12 MJ ; age<1Myr • MIR excess is well fit by a flared disk with inner hole Leonardo Testi: Formation and Evolution of Brown Dwarfs, Stars in Galaxies, La Palma, Mar 8, 2003

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

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