SPOTS AND GRANULATION as a Function of Mass and Age Mark Giampapa National Solar Observatory Tucson, Arizona USA 7/26/ Sagan Summer Workshop Stars as Homes for Habitable Planetary Systems

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 7/26/2010

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 7/26/2010

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 7/26/2010

C-shape of Line Bisectors From Gray (2005) Mark Giampapa 2010 Sagan Summer Workshop 7/26/2010

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 7/26/2010

Line Bisectors in the Sun Courtesy W. C. Livingston Mark Giampapa 2010 Sagan Summer Workshop 7/26/2010

Sun-as-a-star bisector variation Courtesy W. C. Livingston (NSO)

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

Bisector Span and Color From Povich et al. (2001) Mark Giampapa 2010 Sagan Summer Workshop 7/26/2010

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 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 From I. Ramírez, C. Allende Prieto, and D. L. Lambert (2008) Mark Giampapa 2010 Sagan Summer Workshop 7/26/2010

Inverse C-shapes Gray & Nagel (1989) Mark Giampapa 2010 Sagan Summer Workshop 7/26/2010

Gray & Nagel (1989) Mark Giampapa 2010 Sagan Summer Workshop 7/26/2010

Gray, Carney & Yong (2008)

Povich et al. (2001)

51 Peg (Povich et al. 2001)

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 7/26/2010

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 7/26/2010

Spots Most prominent manifestation of magnetic activity on the visible Sun 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 7/26/2010

Modulates the irradiance of the Sun and the brightness of late-type stars in photometric bands 7/26/2010

Spot Color Signatures Mark Giampapa 2010 Sagan Summer Workshop 7/26/2010

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 7/26/2010

Observational evidence for magnetic fields across the HR diagram, IAUS 259, Nov 5, 2008, Tenerife Berdyugina Zeeman-Doppler Imaging Doppler Imaging Zeeman-Doppler Imaging Atomic linesMolecular lines (P. Petit) Mark Giampapa 2010 Sagan Summer Workshop 7/26/2010

Observational evidence for magnetic fields across the HR diagram, IAUS 259, Nov 5, 2008, Tenerife Berdyugina Mark Giampapa 2010 Sagan Summer Workshop 7/26/2010

Spot modulation of stellar light curves Hyades member VB 73 (G1 V)—from Radick et al. (1995) Mark Giampapa 2010 Sagan Summer Workshop 7/26/2010

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

7/26/2010

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

Mark Giampapa 2010 Sagan Summer Workshop 7/26/2010

Messina et al. (2001)

Fekel & Henry (1998) Mark Giampapa 2010 Sagan Summer Workshop 7/26/2010

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 7/26/2010

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 7/26/2010

END 7/26/2010 Mark Giampapa 2010 Sagan Summer Workshop