Download Winds of Main-Sequence Stars - Harvard

Winds of Main-Sequence Stars:
Observational
Limits
& a path to
Theoretical Prediction
Steven R. Cranmer
Harvard-Smithsonian Center for Astrophysics
Winds of Main-Sequence Stars:
Outline:
• Background: the solar wind (M ~ 10–14 M /yr)
• Cool-star winds: observational M methods & results
• How can theory be folded in? Should it be?
Steven R. Cranmer
Harvard-Smithsonian Center for Astrophysics
Brief history: solar wind
• Mariner 2 (1962): first direct confirmation of continuous supersonic solar wind.
• Helios probed in to 0.3 AU, Voyager continues past 100+ AU.
• Ulysses (1990s) left the ecliptic; provided 3D view of the
wind’s magnetic geometry.
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
Brief history: solar wind
• Mariner 2 (1962): first direct confirmation of continuous supersonic solar wind.
• Helios probed in to 0.3 AU, Voyager continues past 100+ AU.
• Ulysses (1990s) left the ecliptic; provided 3D view of the
wind’s magnetic geometry.
• SOHO gave us new views of “source regions” of solar wind.
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
The solar wind mass los rate
• The sphere-averaged “M” isn’t usually considered by solar physicists!
• Wang (1998, CS10) used empirical relationships between B-field, wind speed,
and density to reconstruct M over two solar cycles.
ACE (in ecliptic)
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
Mass loss over the Sun’s lifetime
• T Tauri phase: Does accretion drive wind?
AGB
Pre-MS
HB
(Matt &
Pudritz
2005)
ZAMS
• ZAMS: Was there a “bright young Sun?”
• HB/AGB: Is mass loss the “2nd parameter?”
Do winds clear out “missing ISM” in clusters?
• Close binaries: SN Ia properties!
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
Cool-star winds: “traditional” diagnostics
• Optical/UV spectroscopy: simple blueshifts or full
“P Cygni” profiles
• IR continuum: circumstellar dust causes SED excess
• Molecular lines (mm, sub-mm): CO, OH maser
• Radio: free-free emission from (partially
ionized?) components of the wind
(Bernat 1976)
• Continuum methods need V from
another diagnostic to get mass loss rate.
•
wind
star
• Clumping?
(van den Oord &
Doyle 1997)
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
Cool-star mass loss rates
de Jager et al. (1988)
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
Multi-line spectroscopy
• 1990s: more self-consistent treatments of radiative transfer AND better data
(GHRS, FUSE, high-spectral-res ground-based) led to better stellar wind
diagnostic techniques!
• A nice example: He I 10830 Å for TW Hya (pole-on T Tauri star) . . .
Dupree
et al.
(2006)
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
Cool-star mass loss rates
Hartigan etde
al.Jager et al. (1988)
(1995)
Carpenter,
Harper,
Dupree,
etc.
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
New ideas (1): astrosphere absorption
• Wood et al. (2001, 2002, 2005) distinguished cool ISM H I Lyα absorption from
hotter “piled up” H0 in stellar astrospheres. Derived M depends on models . . .
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
New ideas (2): accretion in pre-CVs
• Some H-rich & He-rich white dwarfs show metal lines in their atmospheres
(classes DAZ, DZ). Accretion from ISM and/or “comets” is problematic.
• Debes (2006) suggested that M-dwarf companions deposit metal-rich gas via
stellar winds onto the WD surfaces.
• Observed abundances (usually from Ca H,
K lines) modeled as steady-state balance
between accretion & downward diffusion;
this provides Macc ;
• Bondi-Hoyle accretion rate provides the
density;
• Mass conservation (spherical geometry)
provides Mwind .
• Largest uncertainty: wind velocity (v4).
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
Cool-star mass loss rates
Wood et al. (2005)
Debes (2006)
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
New ideas (3): charge exchange X-rays
• ISM neutrals flow into an “astrosphere” and CX
with wind ions.
• Ions left in excited state
emit X-rays.
• Wargelin & Drake (2001, 2002) suggest using this
to probe stellar wind properties.
100x M
• So far, upper limits only (M dwarfs).
• Better spatial & spectral res. needed.
• With good enough spectra, one can
also obtain wind speed, composition,
and ionization state.
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
Theory
As M goes down, reliance on modeling goes up . . .
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
Theory: dimensional analysis . . .
• Stellar wind power:
• Reimers (1975, 1977) proposed a semi-empirical scaling:
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
Theory: dimensional analysis . . .
• Stellar wind power:
• Reimers (1975, 1977) proposed a semi-empirical scaling:
• Schröder & Cuntz (2005) investigated an explanation via convective turbulence
generating atmospheric waves . . .
• Funny things happen during rapid evolutionary stages!
(e.g., Willson 2000, Ann. Rev.)
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
Cool-star mass loss rates
Schröder & Cuntz (2005)
scaling for I, III, V
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
What sets solar mass loss?
• Coronal heating must be
ultimately responsible!
• Hammer (1982) & Withbroe (1988) suggested a steady-state energy balance:
• Only a fraction of total coronal
heat conduction
heat flux conducts down, but in
general, we expect something
close to
. . . along open flux tubes!
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
radiation
losses
5
— ρvkT
2
S. R. Cranmer
Cool Stars 14, November 10, 2006
Stellar coronal heating
• The well-known “rotation-age-activity” relationship shows how coronal heating
weakens as young (solar-type) stars spin down.
• Heating rates also scale with mean magnetic field.
open or closed fields?
K, M stars
Sun
Judge, Güdel, Kürster, Garcia-Alvarez, Preibisch, Feigelson, Jeffries
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
Saar (2001, CS11)
S. R. Cranmer
Cool Stars 14, November 10, 2006
Solar X-rays & magnetic flux
• Empirically, does solar mass loss really scale with FX ~ Φ ? It depends on which
field lines are considered!
Coronal hole (open)
Quiet
regions
Schwadron
et al. (2006)
Active
regions
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
Solar X-rays & magnetic flux
• Empirically, does solar mass loss really scale with FX ~ Φ ? It depends on which
field lines are considered!
M ~ ΦAR0.06
M ~ ΦQR0.41
Active regions
Quiet
regions
Schwadron
et al. (2006)
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
Sun’s mass loss history
• Did liquid water exist on Earth 4 Gyr ago? If “standard” solar models are correct,
a strong greenhouse effect was needed.
• Sackmann & Boothroyd (2003) argued that a more massive (~1.07 M) young Sun
could have been luminous enough to solve this problem, but it would have needed
strong early mass loss . . .
Sackmann & Boothroyd
(2003)
M ~ LX1.3
M ~ LX1.0
M ~ LX0.4
M ~ LX0.1
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
S. R. Cranmer
Cool Stars 14, November 10, 2006
Heating & wind acceleration
• Models of how coronal heating (FX) scales with magnetic flux (Φ) are growing
more sophisticated . . .
• Closed loops:
Magnetic reconnection
e.g., Longcope &
Kankelborg 1999
• Open field lines: MHD turbulence!
Z–
T
(K)
Z+
Z–
Cranmer & van Ballegooijen (2005, 2007)
T. Suzuki (CS14, Poster II)
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
reflection
coefficient
S. R. Cranmer
Cool Stars 14, November 10, 2006
Conclusions
Observations:
• Combined multi-ion spectroscopy & atmosphere
modeling still has unexplored potential.
• Magnetic field measurements are also key to
constraining stellar wind properties. ZDI!
• Coming soon: X-ray charge exchange M’s ?
Theory:
• Understanding mass loss depends on modeling
coronal heating on open & closed field lines.
• Coming soon: 3D convection simulations for
rapid rotators, with implications for how the
photospheric waves affect coronal heating . . .
• Here now: turbulence-driven wind models
with “real” coronal heating!
Winds of Main-Sequence Stars: Observational
Limits and a Path to Theoretical Prediction
B. Brown et al. (2004)
S. R. Cranmer
Cool Stars 14, November 10, 2006