Lecture 26Low-Mass Young Stellar Objects

1. Nearby Star Formation2. General Properties of Young Stars3. T Tauri Stars4. Herbig Ae/Be Stars

ReferencesAdams, Lizano & Shu ARAA 25 231987Lada OSPS 1999Stahler & Palla Chs. 17 & 18

Local Star Forming Regions

Stahler & Palla Fig 1.1

Taurus-Auriga & Perseus

Much of our knowledge of star formation comes from a few nearby regions

– 150 pclow mass (sun-like) stars

Representative for the Galaxy as a whole?

Orion – 450 pchigh & low mass stars

[Grey – Milky WayBlack – Molecular clouds]

TMC-1

B5

L1551

T Tau

NGC 1333

AURIGA

PERSEUS

TAURUS

IC 348NGC 1579

with famous objects

Taurus, Auriga & Perseus

• A cloud complexrich in cores & YSOs

• NGC1333/IC 348• Pleiades• TMC-1,2• T Tauri & other TTSs• L 1551

CO

Wilking et al. 1987 AJ 94 106

Ophiuchus

Andre PP IV

Orion

L1630 star clusters

L1630

L1630 in Orion

NIR star clusters on CS(2-1) mapE Lada, ApJ 393 25 1992

M(L1630) ~ 8x104 Msun

5 massive cores (~ 200 Msun)associated with NIR star clusters

2. General PropertiesYoung 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 youngRevealed stars - seen at optical wavelengths or shorter, usually presumed to be older

What makes young stars particularly interesting isCircumstellar 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 dust continuum and green-scale H2 emission

Gueth & GuilloteauA&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Unpublished observations, probably at sub arc second resolution.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

Original cartoon for integratingoutflows into star formation theory.

CO contours on optical photo Snell et al. ApJ 239 L17 1980

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 FormationShu, Adams, & Lizano ARAA 25 23 1987

3

2

Shu et al. (1987 ARAA 25 23)

embedded protostarmolecular cloud core

classicalT Tauri star

thin disk ...

SED Classification of YSOsDue 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

Lada OSPS 1999

Due to Adams, Lada & ShuApJ 312 788 1987based on the spectral index

υυ υ

log)(log

dFd

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 wereyoung stellar objects (YSOs) (~1957)

T Tauri stars are revealed YSOs

Optical Spectra of T Tauri Stars

Kenyon et al. 1998 AJ 115 2491

⊕

Hα

[SII]

[OI]

Increasing emissionline strengths clock-wise from upper right.

LkCa3 is a “weak-line”T Tauri star (TTS).

BP & DG are “classical”TTSs.

DG Tau has very strong lines.

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 continuumIs difficult to see

– [NII] λλ 6548,6584 tend to be strong

– Emission-line equivalent widths are comparable to those in Herbig-Haroobjects

These observations suggest that these TTSs have warm ionized regions and strong outflows and jets.

Measuring Disk Accretion Rates

LkCa7, V819 Tau & V836 Tau areweak line T Tauri stars

The 3200-5200 Å spectra trace excess hot continuum emission fromaccretion onto the central star. Classical TTSs have appreciableBalmer 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 RatesGullbring et al. ApJ 492 323 1998.

The above rates are in the range 10-9 - 10-7 M yr-1 , with accretion providing no more than 20% of the total luminosity. More recent analyses of the UV veiling of the stellar absorption lines suggest smaller rates,

⎟⎟⎠

⎞⎜⎜⎝

⎛−≈

•

inacc R

RR

MGML *

*

* 1

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 StarsAB AurGroup I

PV CepGroup II

Herbig (1960) searched for these more massive analogs of TTS.Comprehensive studiesof the SEDs by Hillenbrand et al. (ApJ 397 613 1992 etc.).

The IR excess starts already at 1-2 µm.

Hillenbrand’s classification (I - declining & II – rising)is the opposite of the acceptedClasses 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• λFλ ~ λS

s = -3 stars = -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

λ(µm)

• ISO spectra of HAe/Be stars show a rich variety of solid state bands– Silicates (amorphous

and crystalline)– FeO,– PAHs – Crystalline H2O ice

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 CO2 ice• 7.7 µm CH4

d’Hendecourt et al. 1996 AA 315 L365

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.