Spots and granulation as a function of mass and age
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SPOTS AND GRANULATION as a Function of Mass and Age. Mark Giampapa National Solar Observatory Tucson, Arizona USA. Granulation. First and most apparent non-uniformity observed in the Quiet Sun Bright cells of irregular polygonal shape separated by dark lanes

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SPOTS AND GRANULATION as a Function of Mass and Age

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Spots and granulation as a function of mass and age

SPOTS AND GRANULATIONas a Function of Mass and Age

Mark GiampapaNational Solar ObservatoryTucson, Arizona USA

2010 Sagan Summer Workshop

Stars as Homes for Habitable Planetary Systems


Granulation

Granulation

  • First and most apparent non-uniformity observed in the Quiet Sun

  • Bright cells of irregular polygonal shape separated by dark lanes

  • Solar granulation ~ 1000 km (1-2 arcsec) and lifetimes of 8 min – 20 min.

  • High uniformity of brightness and brightness variations across a cell

Mark Giampapa

2010 Sagan Summer Workshop


Irradiance variability in the sun

Irradiance Variability in the Sun

  • Magnetic features appear to give rise to long- and short-term variations in the solar irradiance

  • Random variations in the number and geometry of turbulent convective cells should produce global variations in heat flux

  • But….it is the enormous thermal inertia of the deeper solar layers that is at the root of solar irradiance variations. But this same inertia tends to damp luminosity variations we might otherwise expect from structural changes deeper in the sun.

Mark Giampapa

2010 Sagan Summer Workshop


Line bisectors

Line Bisectors

  • The effect of granulation in stellar spectra can be described by the “C-shape” of the so-called line bisector

Mark Giampapa

2010 Sagan Summer Workshop


C shape of line bisectors

C-shape of Line Bisectors

Mark Giampapa

2010 Sagan Summer Workshop

From Gray (2005)


Spectral line asymmetries due to granulation

Spectral line asymmetries due to granulation

  • Velocity-brightness correlation of granulation leads to natural line asymmetry, i.e., the correlation between rise and fall velocities and temperature of the granulation structure

  • Convective blueshifts can hinder measurement of true radial velocities to accuracies better than a few hundred m s-1

Mark Giampapa

2010 Sagan Summer Workshop


Line bisectors in the sun

Line Bisectors in the Sun

Mark Giampapa

2010 Sagan Summer Workshop

Courtesy W. C. Livingston


Sun as a star bisector variation

Sun-as-a-star bisector variation

Courtesy W. C. Livingston (NSO)


Spots and granulation as a function of mass and age

Solar bisector data courtesy of W. Livingston (National Solar Observatory)


Bisector span and color

Bisector Span and Color

Mark Giampapa

2010 Sagan Summer Workshop

From Povich et al. (2001)


Bisectors and effective temperature

Bisectors and effective temperature

Typical changes in bisector shape with effective temperature; each panel is for a separate luminosity class. The thin lines show the rms uncertainty in the measurements of the mean bisectors (thick lines). Horizontal placement of the bisectors is arbitrary but ordered by spectral type.

From Gray (2005)


Bisectors and luminosity class

Bisectors and Luminosity Class

Height of the blue‐most point of the mean bisectors, plotted against absolute magnitude. Different luminosity classes are denoted by different symbols. The stars included are restricted in temperature class. Along a vertical line of constant absolute magnitude, hotter stars are higher (Gray 2005; PASP, 117, 711)


Granulation and line strengths

Granulation and line strengths

Mark Giampapa

2010 Sagan Summer Workshop

From I. Ramírez, C. Allende Prieto, and D. L. Lambert (2008)


Inverse c shapes

Inverse C-shapes

Gray & Nagel (1989)

Mark Giampapa

2010 Sagan Summer Workshop


Spots and granulation as a function of mass and age

Gray & Nagel (1989)

Mark Giampapa

2010 Sagan Summer Workshop


Spots and granulation as a function of mass and age

Gray, Carney & Yong (2008)


Summary granulation

Summary: Granulation

  • The tops of convection cells produce the patchwork known as granulation on the Sun and late-type stars. Granulation in giant stars appears to be characterized by larger spatial scales than in dwarfs, i.e., larger convective elements.

  • Line bisectors reveal the asymmetries in line profiles as a result of convection and the ensuing granulation pattern. Bisectors in solar-type stars have a characteristic “C-shape” with a blueward amplitude due to the velocity-brightness correlation of granulation.

  • Bisector amplitudes decrease toward later dwarf spectral types.

  • In bright giants the largest upward velocities due to the convective granulation pattern occur near the line cores; the blue-most point moves out into the wings toward subgiants and dwarfs.

Mark Giampapa

2010 Sagan Summer Workshop


Granulation cont d

Granulation (cont’d.)

  • Short and long-term variability in bisector amplitudes have been observed in stars and the Sun-as-a-star. The variations may or may not be periodic.

  • In the Sun, average photospheric bisector amplitudes are anti-correlated with the solar cycle, i.e., lower amplitudes at solar maximum and higher amplitudes at solar minimum.

  • A “granulation boundary” exists in the H-R diagram where on the “hot” side the late-type stars exhibit an inverse C-shape while on the “cool” side the normal C-shape is observed. However, red giant branch and red horizontal branch giants cooler than 4100 K show an inverse C-shape. The origin of the inverse C-shape is not known.

  • In red giant branch stars, small variations in radial velocity at the 500 – 150 m/sec level are seen in objects that have either regular or inverse C-shapes. This is thought to be a manifestation of upward moving and downward flowing large convection cells.

Mark Giampapa

2010 Sagan Summer Workshop


Spots

Spots

  • Most prominent manifestation of magnetic activity on the visibleSun

  • Modulates the luminous output in the Sun and late-type stars as seen in photometric bands

  • Direct detection in molecular lines

  • Can produce spectral line distortions

  • Can affect interpretation of astrometric and transit measurements

Mark Giampapa

2010 Sagan Summer Workshop


Modulates the irradiance of the sun and the brightness of late type stars in photometric bands

Modulates the irradiance of the Sun and the brightness of late-type stars in photometric bands


Spot color signatures

Spot Color Signatures

Mark Giampapa

2010 Sagan Summer Workshop


Astrometric signature of spots

Astrometric signature of spots

  • Star spots can shift photocenter of the star by 0.1% - 0.2%

  • Earth-Sun astrometric amplitude at 10 pc is 0.3 μas or 0.03% of the stellar diameter

  • Spots can shift photocenter 3 – 6 times the size of the astrometric signature

Mark Giampapa

2010 Sagan Summer Workshop


Zeeman doppler i maging

Doppler Imaging

Zeeman-Doppler Imaging

Zeeman-DopplerImaging

Atomic lines

Molecular lines

(P. Petit)

Observational evidence for magnetic fields across the HR diagram, IAUS 259, Nov 5, 2008, Tenerife Berdyugina

Mark Giampapa

2010 Sagan Summer Workshop


Spots and granulation as a function of mass and age

Observational evidence for magnetic fields across the HR diagram, IAUS 259, Nov 5, 2008, Tenerife Berdyugina

Mark Giampapa

2010 Sagan Summer Workshop


Spot modulation of stellar light curves

Spot modulation of stellar light curves

Hyades member VB 73 (G1 V)—from Radick et al. (1995)

Mark Giampapa

2010 Sagan Summer Workshop


Spots and granulation as a function of mass and age

Data sources: Messina et al. (2001); Radick et al. (1995); Saar, Barnes & Meibom (2010—NGC 3532—private communication)


Spots and granulation as a function of mass and age

Data sources: Messina et al. (2001); Radick et al. (1995); Saar, Barnes & Meibom (2010—NGC 3532—private communication)


Spots and granulation as a function of mass and age

Data sources: Messina et al. (2001); Radick et al. (1995); Saar, Barnes & Meibom (2010—NGC 3532—private communication)


Spots and granulation as a function of mass and age

Data sources: Messina et al. (2001); Radick et al. (1995); Saar, Barnes & Meibom (2010—NGC 3532—private communication)

Mark Giampapa

2010 Sagan Summer Workshop


Spots and granulation as a function of mass and age

Messina et al. (2001)


Spots and granulation as a function of mass and age

Fekel & Henry (1998)

Mark Giampapa

2010 Sagan Summer Workshop


Summary spots

Summary: Spots

  • Sunspots and starspots are characterized by lower temperatures and stronger magnetic fields than their surrounding respective photospheres. Spots can produce distortions in line profiles, unique spectral line features, polarimetric signals, and color signatures. Spots also can contribute “astrometric noise.”

  • Irradiance and brightness variations in the Sun and stars are due to spatial inhomogeneities defined by magnetic structures combined with the large thermal inertia of the outer convection zone.

  • Spots modulate the luminous output of the Sun and late-type stars.

  • The data tentatively suggest that spot amplitudes increase toward cooler spectral types, i.e., stars with larger fractional convection zone depths.

Mark Giampapa

2010 Sagan Summer Workshop


Spots cont d

Spots (cont’d.)

  • Rotation and fractional convection zone depth appear to be the primary factors that affect the filling factor and surface distribution of spots. Spot modulation amplitudes increase with increasing rotation rates, appearing to reach a maximum near a period of about 0.35 days in dwarf stars.

  • Looking at spot coverage as a function of stellar age, the amplitude of modulation of the V-band light curve by spots is ~ 10% (with considerable “scatter”) in young, solar-type dwarfs with ages ~ 30 Myr. This amplitude declines rapidly by the age of the Hyades (~ 600 Myr) to ~ 1% – 3%. The decline appears to be more gradual to the age of the Sun (4.6 Gyr) where spot coverage is ~0.1%.

  • The data for giants are more limited but amplitudes of variation can be ~7% in M0 giants, declining to about 1% in K0 giants.

  • The correlation between photometric light-curve modulation and age arises from the evolutionary relationship between stellar activity and rotation rates via dynamo processes and the coupling of stellar winds to magnetic fields.

Mark Giampapa

2010 Sagan Summer Workshop


Spots and granulation as a function of mass and age

END

Mark Giampapa

2010 Sagan Summer Workshop


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