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
M(L1630) ~ 8x104 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
dust continuum and green-scale H2 emission
Gueth & Guilloteau
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Unpublished observations, probably at sub arc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
Snell et al. ApJ 239 L17 1980
Original cartoon for integrating
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
•
•
–
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
•
–
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)
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
embedded
2 protostar
classical
3
T Tauri star
thin disk ...
Shu et al.
(1987 ARAA
25 23)
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 log υ
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 clockwise from upper right.
LkCa3 is a “weak-line”
T Tauri star (TTS).
BP & DG are “classical”
TTSs.
⊕
[OI] Hα
[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
accretion providing no more than 20% of the total
Lacc ≈
R*
luminosity. More recent analyses of the UV veiling of the
stellar absorption lines suggest smaller rates,
⎛
R ⎞
⎜⎜1 − * ⎟⎟
⎝ Rin ⎠
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
for these more massive
analogs of TTS.
Comprehensive studies
of the SEDs by Hillenbrand
et al. (ApJ 397 613 1992 etc.).
AB Aur
Group I
The IR excess starts already
at 1-2 µm.
PV Cep
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
• λFλ ~ λS
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
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.