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Lecture 26 Pre-Main Sequence Evolution

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Lecture 26 Pre-Main Sequence Evolution Lecture 26 Low-Mass Young Stellar Objects 1. Nearby Star Formation 2. General Properties of Young Stars 3. T Tauri Stars 4. Herbig Ae/Be Stars References Adams, Lizano & Shu ARAA 25 231987 Lada OSPS 1999 Stahler & Palla Chs. 17 & 18 Local Star Forming Regions Much of our knowledge of star formation comes from a few nearby regions Taurus-Auriga & Perseus – 150 pc low mass (sun-like) stars Orion – 450 pc high & low mass stars [Grey – Milky Way Black – Molecular clouds] Representative for the Galaxy as a whole? Stahler & Palla Fig 1.1 PERSEUS with famous objects AURIGA NGC 1579 B5 IC 348 NGC 1333 TMC-1 T Tau L1551 TAURUS Taurus, Auriga & Perseus • A cloud complex rich in cores & YSOs • NGC1333/IC 348 • Pleiades •TMC-1,2 • T Tauri & other TTSs • L 1551 Ophiuchus Wilking et al. 1987 AJ 94 106 CO Andre PP IV Orion L1630 L1630 star clusters L1630 in Orion NIR star clusters on CS(2-1) map E Lada, ApJ 393 25 1992 4 M(L1630) ~ 8x10 Msun 5 massive cores (~ 200 Msun) associated with NIR star clusters 2. General Properties Young stars are associated with molecular clouds. Observations are affected by extinction, which decreases with increasing wavelength. Loosely speaking, we can distinguish two types: Embedded stars - seen only at NIR or longer wavelengths, usually presumed to be very young Revealed stars - seen at optical wavelengths or shorter, usually presumed to be older What makes young stars particularly interesting is Circumstellar gas and dust – both flowing in as well as out, e.g., jets, winds, & disks. Examples follow ... The HH 211 Outflow in IC348 • 1.3 mm continuum: circumstellar disk • 12CO(2 – 1) at 1.5”: molecular outflow • Top: |v| < 10 km/s • Bottom: |v| > 10 km/s • H2 2.12 µm: shocks • embedded protostar Molecular line contours overlain on 1.3 mm Gueth & Guilloteau dust continuum and green-scale H2 emission A&A 343 571 1999 SMA Observations of HH 211 Unpublished observations, probably at sub arc second resolution. Grey scale is H2 2.12 µm HH 111 Jet in Orion B NICMOS WFPC2 Bo Reipurth Star/Disk Systems (3 with Jets) Infrared Images Dusty Wide-Angle Winds? NASA/Padgett, Brandner, & Stapelfeldt (1999) Paradigm For Low-mass Star Formation L 1551-IRS5 CO contours on optical photo Original cartoon for integrating Snell et al. ApJ 239 L17 1980 outflows into star formation theory. The Basic Scenario • Star formation can be described in terms of four stages – Formation of dense cores in molecular clouds • Initially supported by turbulent & magnetic pressure, which gradually decrease due to ambipolar diffusion. • Rapid collapse at center, less so in outer layers (inside-out collapse) – Matter originating far from the rotation axis has too much angular momentum to fall onto the deeply embedded protostar and settles into a circumstellar disk • Luminosity is accretion powered – Bipolar outflows develop perpendicular to the disk • Deuterium burning ignites in central regions – Both infall & outflow decrease, and a newly formed star emerges with a circumstellar disk that may make planets Important questions about timing and other details need to be addressed. Scenario for Single Star Formation Four Stages of Low-Mass Star Formation Shu, Adams, & Lizano ARAA 25 23 1987 molecular cloud core embedded2 protostar Shu et al. thin disk ... classical (1987 ARAA 3 25 23) T Tauri star SED Classification of YSOs Due to Lada & Wilking (ApJ 287 610 1984), Adams, Lada & Shu (ApJ 312 788 1987) etc. Based on the fact that, when YSOs emerge as optically visible stars, they remain partially obscured. • Class I YSOs – (stage 2) Completely embedded objects have SEDs with positive spectral index in the far-IR: the youngest sources emit the bulk of their energy in the sub-mm & mm • Class II YSOs (stage 2 &3) Older YSOs with SEDs from a reddened stellar component and an IR excess • Class III YSOs (stages 3 & 4) The IR excess has disappeared; circumstellar gas still observable in atomic lines • Class 0 (added by André et al. ApJ 406 122 1993) Class I sources just after collapse has started, Classification by SED Due to Adams, Lada & Shu ApJ 312 788 1987 based on the spectral index d(logυFυ ) d logυ Lada OSPS 1999 3. T Tauri Stars • Optical detection of young stars or pre-main-sequence stars (YSOs) by AH Joy (ApJ 102 168 1945) – The class of T Tauri stars was defined after the prototype T Tau • Irregular variability by as much as 3 magnitudes • Spectral types F5 to G5, with strong Ca II H & K emission lines as well as H Balmer lines • Low luminosity • Association with either bright or dark nebulosity • Later extend to allow any type later than F5, and requiring strong Li 6707 A absorption (as an age indicator) – It was recognized from the start that the emission line patterns resemble those of the solar chromosphere, but much stronger compared to the stellar photospheric emission • Ambartsumian proposed that T Tauri stars were young stellar objects (YSOs) (~1957) T Tauri stars are revealed YSOs Optical Spectra of T Tauri Stars Increasing emission line strengths clock- wise from upper right. LkCa3 is a “weak-line” ⊕ T Tauri star (TTS). BP & DG are “classical” [OI] Hα TTSs. [SII] DG Tau has very strong lines. Kenyon et al. 1998 AJ 115 2491 T Tauri Stars in Taurus-Auriga Class I YSOs in Taurus-Auriga – Some discovered by IRAS – All show strong emission lines – HH30 IRS has strong [OI] λ6300 & [SII] λλ 6717, 6731 – Three show the TiO band at 7200-7500 Å typical of M stars – Spectra decline sharply between 7000 - 5800 Å T Tauri stars in Taurus-Auriga • In some extreme Class I objects, emission lines dominate the spectrum – The stellar continuum Is difficult to see –[NII] λλ 6548,6584 tend to be strong – Emission-line equivalent widths are comparable to those in Herbig-Haro objects These observations suggest that these TTSs have warm ionized regions and strong outflows and jets. Measuring Disk Accretion Rates LkCa7, V819 Tau & V836 Tau are weak line T Tauri stars The 3200-5200 Å spectra trace excess hot continuum emission from accretion onto the central star. Classical TTSs have appreciable Balmer jumps (not dips) as well as emission lines. The spectral types are K3-M4, as judged from the red part of the spectrum Disk Accretion Rates Gullbring et al. ApJ 492 323 1998. The above rates are in the range 10-9 -10-7 M yr-1 , with • ~ GM M ⎛ R ⎞ accretion providing no more than 20% of the total * ⎜ * ⎟ Lacc ≈ ⎜1− ⎟ luminosity. More recent analyses of the UV veiling of the R* ⎝ Rin ⎠ stellar absorption lines suggest smaller rates, T Tauri Stars without Accretion • By definition, weak-lined T Tauri stars (WTTS) have little or no line emission and are not accreting • Prior to IR observations, line emission was thought to be vigorous chromospheric activity – YSOs must (and do) have very strong magnetic fields – YSO magnetic fields are probably not strong enough to directly produce the line emission without a disk or outflows • Even though WTTS are not now accreting, it does not mean that always was the case – Accretion is episodic, as evidenced by the structure of jets – WTTS probably still have disks around them 4. Herbig Ae/Be Stars Herbig (1960) searched AB Aur Group I for these more massive analogs of TTS. Comprehensive studies of the SEDs by Hillenbrand et al. (ApJ 397 613 1992 etc.). The IR excess starts already PV Cep at 1-2 µm. Group II Hillenbrand’s classification (I - declining & II – rising) is the opposite of the accepted Classes I,II,III for TTSs. Squares: observed fluxes Circles: extinction corrected IR Properties of YSOs • IRAS/ISO data for of HAe/Be stars illustrate how IR observations provide fundamental (and at times the only) information about embedded YSOs – IR spectral energy distribution classification S • λFλ ~ λ s = -3 star s = -4/3 accretion disk s < -4/3 Class III -4/3 < s < 0 Class II s > 0 Class I The R properties of YSO cannot be explained with spherical dust distributions. IR Spectra of HAe/Be Stars • ISO spectra of HAe/Be stars show a rich variety of solid state bands – Silicates (amorphous and crystalline) –FeO, – PAHs – Crystalline H2O ice λ(µm) HD 100546 & Comet Hale-Bopp ISO-SWS spectra Herbig Ae star HD 100546 Comet Hale-Bopp Interplanetary & Interstellar Dust IR Spectrum of a Class I Object Deeply embedded source with most of the stellar energy radiated is re-radiated in the IR. Cool dust continuum (with T ≈ 35 K) with many absorption features: • Deep & broad 9.7 & 18 µm silicate absorption •3 & 6 µm H2O ice • 4.27 & 15.2 µm d’Hendecourt et al. 1996 AA 315 L365 CO2 ice • 7.7 µm CH4 IR Spectrum of a Class I Object The 2.5 -18 µm spectrum of RAFGL 7009S compared to the lab spectrum of a UV photolysed ice mixture (H2O:CO:CH4:NH3:O2). HAe/Be Stars and Debris Disk Stars QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Debris disks have their dust replenished by erosion of planetesimals. The prototypes former Herbig Ae stars: β Pic, Vega, & Fomalhaut. Planets can affect the distribution of dust..
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