Using HST/GHRS, HST/STIS and FUSE archival data for alpha Tau and the CHIANTI spectroscopic code, we have derived line shifts, volumetric emission measures, and plasma density estimates, and calculated filling factors for a number of UV lines forming between 10,000 K and 300,000 K in the outer atmosphere of this red giant star. The data suggest the presence of low-temperature extended regions and high-temperature compact regions, associated with magnetically open and closed structures in the stellar atmosphere, respectively. The signatures of UV lines from alpha Tau can be consistently understood via a model of upward-traveling Alfven waves in a gravitationally stratified atmosphere. These waves cause non-thermal broadening in UV lines due to unresolved wave motions and downward plasma motions in compact magnetic loops heated by resonant Alfven wave heating

Airapetian, Vladimir

Carpenter, Kenneth G.

NASA Technical Reports Server

The Atmospheric Dynamics of α Tau (K5 III) –  
Clues to Understanding the Magnetic Dynamo 
in Late-Type Giant Stars 
Kenneth G. Carpentera and Vladimir Airapetianb 
aExoplanets and Stellar Astrophysics Laboratory, Code 667, NASA-Goddard Space Flight Center, 
Greenbelt, MD 20771  
bCatholic University of America at NASA-GSFC, Greenbelt, MD 20771 
Abstract. Using HST/GHRS, HST/STIS and FUSE archival data for α Tau and the CHIANTI 
spectroscopic code, we have derived line shifts, volumetric emission measures, and plasma 
density estimates, and calculated filling factors for a number of UV lines forming between 
10,000 K and 300,000 K in the outer atmosphere of this red giant star.  The data suggest the 
presence of low-temperature extended regions and high-temperature compact regions, associated 
with magnetically open and closed structures in the stellar atmosphere, respectively. The 
signatures of UV lines from α Tau can be consistently understood via a model of upward-
traveling Alfvén waves in a gravitationally stratified atmosphere. These waves cause non-
thermal broadening in UV lines due to unresolved wave motions and downward plasma motions 
in compact magnetic loops heated by resonant Alfvén wave heating.  
Keywords: Stars – atmospheres, Stars – chromospheres, Stars – Winds, Alfvén waves, Stars – 
atmospheric heating 
PACS: 97.10.Ex; 97.10.Jb; 97.10Me; 97.20.Li 
OBJECTIVES OF THE PROGRAM 
The primary objective of this program is to determine and use the properties of 
select UV emission lines in the spectrum of α Tau to further our understanding of the 
structure and heating of the outer atmospheres of red giant stars and obtain clues as to 
the nature of their magnetic dynamos.  For reference, the physical properties of α Tau 
are given in Table 1.  
TABLE 1.  The physical properties of the Red Giant α Tau 
Parameter Value 
Spectral Type K5 III 
Distance (pc) 20 
Radius (in solar radii) 44 
Effective Temperature 3898 K 
Mass (in solar masses) 1.3 
Turbulence in the photosphere  5 km/s 
Mean turbulent velocity at the wind base 24 km/s 
Asymptotic Flow Speed 30 km/s 
 
https://ntrs.nasa.gov/search.jsp?R=20080037989 2019-08-30T05:26:38+00:00Z
UV EMISSION LINES IN RED GIANTS 
Almost all late-type evolved giants show signatures of plasma at T of 10,000 - 
300,000 K. We have compiled and analyzed data obtained with HST/GHRS, 
HST/STIS and FUSE on evolved late-type stars [1, 2, and 3] for this study.  These data 
indicate that: 
 UV emission lines are non-thermally broadened, up to as much as V~120 km/s 
 UV emission lines are mostly red- shifted with respect to the photosphere  
 The profiles of CII 1335, Si IV 1393, C III 977, OVI 1032 Å show that their 
non-thermal broadening increases with the temperature of UV emitting plasma  
 
Sample UV emission line profiles from the α Tau spectrum are shown in Figure 1 and 
the properties of a broad sample of lines are listed in Table 2. 
 
 
FIGURE 1.  A single  Gaussian is sufficient to fit the profile of the CII λ1335.70 line (left), as well as 
the CIII λ977, but the OVI λ1032 emission line (right) has extended wings signifying anisotropically 
distributed turbulence.  The latter also provides signatures of plasma downflows in one or a few 
magnetic loops in the atmosphere of α Tau, as indicated by the red-shifted emission lines and 
anisotropically distributed turbulence produced by a non-linear Alfven wave at the Alfven speed. 
 
TABLE 2.  Properties of select UV emission lines in the spectrum of α Tau 
Emission Line Log T Vshift 
(in km/s) 
Dλ1/2 
(in km/s) 
Log 
(VEM) 
Data Source 
CII] 2325.5Å 4.1 12 124±  ±  54.7 [1] 
SiII 2350.9 Å 4.3 23 ±  221±  54.0 [1] 
C III 1908 Å 4.4 N/A ≤ 25 53.6 [3] 
CII 1335 Å 4.6 9.56622 ±  ±  51.5 [3] 
CII 1334Å 4.8 9.56622 ±  ±  51.3 [3] 
CIII 977Å 4.85 998 886±  ±  49.9 [3] 
NIII 991Å 4.9 94.92 876±  ±  50.4 [3] 
OVI  1032Å 5.5 53.5 106.121±  ±  49.5 [3] 
O VI 1037Å 5.5 53.5 106.121±  ±  49.5 [3] 
 
CLUES TO THE STRUCTURE AND HEATING OF THE OUTER 
ATMOSPHERES OF RED GIANTS:  CII, CIII, & O VI LINES 
Analysis of the UV lines from the CII ion indicates that: 
 Opacity effects are not important in formation of the line profile  
 CII 1335Å is well fit with a single Gaussian, while CII 2325Å requires 2  
 The ratio of FWHM’s of CII 1335Å/CII 2325Å ~ 3 
 I(1335Å)/I(2325Å) ~ 1.6 - 2 indicates that 2325Å line forms at T~10,000K 
 Filling factor for CII 1335Å is f ~ 3x10-4 
 
Analysis of the UV lines from the CIII ion indicates that: 
 Opacity effects are not important in the formation of these line profiles 
 CIII 977Å is well fit with a single Gaussian 
 Ratio of the FWHM’s of CIII 977Å/CIII 1908Å ~ 4 
 CIII 977Å and NIII 991 Å line shifts are 100 km/s, while the CIII 1908Å line 
does not show a shift > 10 km/s 
 I(977Å)/I(1908Å) ~ 1.375 indicates that 2325Å line forms at T~12,000K 
 977Å/1176Å line ratio => Ne > 1010cm-3 (technique from [4]) 
 Filling factor of CIII 977Å is  f  < 10-5 
 
Analysis of the UV lines from the OVI ion indicates that 
 Opacity effects are not important in the formation of 1032Å and 1038Å line 
 OVI 1032Å, 1038Å is well fit with 2 Gaussians 
 OVI 1032Å, 1038Å lines are unshifted and show >100 km/s non-thermal 
broadening 
 
   This provides evidence for the existence of both low-T, high-f (open magnetic 
field) and high-T, low-f (magnetic loops) regions in the outer atmosphere of α Tau. 
ALFVEN WAVES IN THE ATMOSPHERES OF COOL GIANTS 
Heating by Alfven Waves 
Our modeling [5] has shown that Alfven Waves (AW) can provide the required 
plasma heating in loops via resonant absorption and that the observed non-thermal 
broadening can be understood in terms of the amplitudes of unresolved wave motion 
at V≤VA. For weakly dissipated AWs, the amplitudes of wave motions increase with 
T, as measured (see Table 1).  Magnetic loops serve as resonant cavities at P=2L/VA~1 
day (consistent with [6]) with the absorption rate of waves [7] of E~BVAP/L3~4x106 
erg/cm2/s. Cool condensations within a magnetic loop are formed due to radiatively 
driven thermal instability at T~100,000K, at scales <0.005Rstar.  These cool 
condensations slide down along the magnetic field line at free-fall velocities V~100 
km/s, consistent with the red-shift observed in CIII 977Å.   This suggests no 
dependence of Vredshift on T, consistent with Table 1. 
 The Source of Alfven Waves in these Atmospheres 
In our models, radial pulsations ignite sound waves, and therefore, a periodic 
variation of density along large-scale magnetic field lines in the atmosphere.  The 
Alfven waves with the half frequency of pulsation can be amplified in the atmosphere 
due to the swing wave-wave interaction [8] and have periods of a few days.  This is 
consistent with the period of waves required by the resonant heating mechanism [7]. 
CONCLUSIONS 
A comparison of our models with observations of α Tau and other red giant stars 
leads to the following results:  
 
 Volume Emission Measures  and line ratios of FUV lines suggests the presence 
of dense and compact regions in the atmosphere of α Tau 
 Red-shifted and non-thermally broadened UV lines can be explained in terms 
of AW induced heating and subsequent formation of radiatively driven  
condensations sliding down from the top of the loop 
 The filling-factor (f) of open structures is least 100 times greater than that of 
loop structures 
 The energy density flux of AWs in the atmosphere derived from UV lines is 
consistent with calculated heating rates and the f of magnetic structures 
 This is consistent with HD simulations of convective cells in giant stars [9]. 
The estimated magnetic flux in loops is (1-2) x 1022 Maxwell 
ACKNOWLEDGMENTS 
This work was supported in part by NASA through STScI grants for HST GO/AR 
grants 9967 and 10956. 
REFERENCES 
1. Robinson, R. D. Carpenter, K. G. & Brown, A. 1998, ApJ, 503, 396. 
2. Dupree, A. K., Lobel, A., Young, P. R., Ake, T. B., Linsky, J. L. & Redfield, S. 2005, ApJ, 622, 629. 
3. Ayres, T. R., Brown, A. & Harper, G. M. 2006, ApJ, 651, 1126. 
4. Guinan, E. F., Ribas, I. & Harper, G. M.. 2003, ApJ, 594, 561. 
5. Airapetian, V. and Carpenter, K. G., 2008, ApJ, submitted. 
6. Koen, C., Laney, D., & Van Wyk, F.  2002, MNRAS, 335, 223. 
7. Hollweg, J. V. 1987, ApJ, 312, 880. 
8. Zaqarashvili, T. V & Belvedere, G. 2005, MNRAS Letters, 362, L35 
9. Freytag, B. et al. 2002, Astron. Nachr., 323, 213. 
 
 
 


The Atmospheric Dynamics of alpha Tau (K5 III) - Clues to Understanding the Magnetic Dynamo in Late-Type Giant Stars

The Atmospheric Dynamics of α Tau (K5 III) –  
Clues to Understanding the Magnetic Dynamo 
in Late-Type Giant Stars 
Kenneth G. Carpentera and Vladimir Airapetianb 
aExoplanets and Stellar Astrophysics Laboratory, Code 667, NASA-Goddard Space Flight Center, 
Greenbelt, MD 20771  
bCatholic University of America at NASA-GSFC, Greenbelt, MD 20771 
Abstract. Using HST/GHRS, HST/STIS and FUSE archival data for α Tau and the CHIANTI 
spectroscopic code, we have derived line shifts, volumetric emission measures, and plasma 
density estimates, and calculated filling factors for a number of UV lines forming between 
10,000 K and 300,000 K in the outer atmosphere of this red giant star.  The data suggest the 
presence of low-temperature extended regions and high-temperature compact regions, associated 
with magnetically open and closed structures in the stellar atmosphere, respectively. The 
signatures of UV lines from α Tau can be consistently understood via a model of upward-
traveling Alfvén waves in a gravitationally stratified atmosphere. These waves cause non-
thermal broadening in UV lines due to unresolved wave motions and downward plasma motions 
in compact magnetic loops heated by resonant Alfvén wave heating.  
Keywords: Stars – atmospheres, Stars – chromospheres, Stars – Winds, Alfvén waves, Stars – 
atmospheric heating 
PACS: 97.10.Ex; 97.10.Jb; 97.10Me; 97.20.Li 
OBJECTIVES OF THE PROGRAM 
The primary objective of this program is to determine and use the properties of 
select UV emission lines in the spectrum of α Tau to further our understanding of the 
structure and heating of the outer atmospheres of red giant stars and obtain clues as to 
the nature of their magnetic dynamos.  For reference, the physical properties of α Tau 
are given in Table 1.  
TABLE 1.  The physical properties of the Red Giant α Tau 
Parameter Value 
Spectral Type K5 III 
Distance (pc) 20 
Radius (in solar radii) 44 
Effective Temperature 3898 K 
Mass (in solar masses) 1.3 
Turbulence in the photosphere  5 km/s 
Mean turbulent velocity at the wind base 24 km/s 
Asymptotic Flow Speed 30 km/s 
 
https://ntrs.nasa.gov/search.jsp?R=20080040681 2019-08-30T05:32:09+00:00Z
UV EMISSION LINES IN RED GIANTS 
Almost all late-type evolved giants show signatures of plasma at T of 10,000 - 
300,000 K. We have compiled and analyzed data obtained with HST/GHRS, 
HST/STIS and FUSE on evolved late-type stars [1, 2, and 3] for this study.  These data 
indicate that: 
 UV emission lines are non-thermally broadened, up to as much as V~120 km/s 
 UV emission lines are mostly red- shifted with respect to the photosphere  
 The profiles of CII 1335, Si IV 1393, C III 977, OVI 1032 Å show that their 
non-thermal broadening increases with the temperature of UV emitting plasma  
 
Sample UV emission line profiles from the α Tau spectrum are shown in Figure 1 and 
the properties of a broad sample of lines are listed in Table 2. 
 
 
FIGURE 1.  A single  Gaussian is sufficient to fit the profile of the CII λ1335.70 line (left), as well as 
the CIII λ977, but the OVI λ1032 emission line (right) has extended wings signifying anisotropically 
distributed turbulence.  The latter also provides signatures of plasma downflows in one or a few 
magnetic loops in the atmosphere of α Tau, as indicated by the red-shifted emission lines and 
anisotropically distributed turbulence produced by a non-linear Alfven wave at the Alfven speed. 
 
TABLE 2.  Properties of select UV emission lines in the spectrum of α Tau 
Emission Line Log T Vshift 
(in km/s) 
Dλ1/2 
(in km/s) 
Log 
(VEM) 
Data Source 
CII] 2325.5Å 4.1 12 124±  ±  54.7 [1] 
SiII 2350.9 Å 4.3 23 ±  221±  54.0 [1] 
C III 1908 Å 4.4 N/A ≤ 25 53.6 [3] 
CII 1335 Å 4.6 9.56622 ±  ±  51.5 [3] 
CII 1334Å 4.8 9.56622 ±  ±  51.3 [3] 
CIII 977Å 4.85 998 886±  ±  49.9 [3] 
NIII 991Å 4.9 94.92 876±  ±  50.4 [3] 
OVI  1032Å 5.5 53.5 106.121±  ±  49.5 [3] 
O VI 1037Å 5.5 53.5 106.121±  ±  49.5 [3] 
 
CLUES TO THE STRUCTURE AND HEATING OF THE OUTER 
ATMOSPHERES OF RED GIANTS:  CII, CIII, & O VI LINES 
Analysis of the UV lines from the CII ion indicates that: 
 Opacity effects are not important in formation of the line profile  
 CII 1335Å is well fit with a single Gaussian, while CII 2325Å requires 2  
 The ratio of FWHM’s of CII 1335Å/CII 2325Å ~ 3 
 I(1335Å)/I(2325Å) ~ 1.6 - 2 indicates that 2325Å line forms at T~10,000K 
 Filling factor for CII 1335Å is f ~ 3x10-4 
 
Analysis of the UV lines from the CIII ion indicates that: 
 Opacity effects are not important in the formation of these line profiles 
 CIII 977Å is well fit with a single Gaussian 
 Ratio of the FWHM’s of CIII 977Å/CIII 1908Å ~ 4 
 CIII 977Å and NIII 991 Å line shifts are 100 km/s, while the CIII 1908Å line 
does not show a shift > 10 km/s 
 I(977Å)/I(1908Å) ~ 1.375 indicates that 2325Å line forms at T~12,000K 
 977Å/1176Å line ratio => Ne > 1010cm-3 (technique from [4]) 
 Filling factor of CIII 977Å is  f  < 10-5 
 
Analysis of the UV lines from the OVI ion indicates that 
 Opacity effects are not important in the formation of 1032Å and 1038Å line 
 OVI 1032Å, 1038Å is well fit with 2 Gaussians 
 OVI 1032Å, 1038Å lines are unshifted and show >100 km/s non-thermal 
broadening 
 
   This provides evidence for the existence of both low-T, high-f (open magnetic 
field) and high-T, low-f (magnetic loops) regions in the outer atmosphere of α Tau. 
ALFVEN WAVES IN THE ATMOSPHERES OF COOL GIANTS 
Heating by Alfven Waves 
Our modeling [5] has shown that Alfven Waves (AW) can provide the required 
plasma heating in loops via resonant absorption and that the observed non-thermal 
broadening can be understood in terms of the amplitudes of unresolved wave motion 
at V≤VA. For weakly dissipated AWs, the amplitudes of wave motions increase with 
T, as measured (see Table 1).  Magnetic loops serve as resonant cavities at P=2L/VA~1 
day (consistent with [6]) with the absorption rate of waves [7] of E~BVAP/L3~4x106 
erg/cm2/s. Cool condensations within a magnetic loop are formed due to radiatively 
driven thermal instability at T~100,000K, at scales <0.005Rstar.  These cool 
condensations slide down along the magnetic field line at free-fall velocities V~100 
km/s, consistent with the red-shift observed in CIII 977Å.   This suggests no 
dependence of Vredshift on T, consistent with Table 1. 
 The Source of Alfven Waves in these Atmospheres 
In our models, radial pulsations ignite sound waves, and therefore, a periodic 
variation of density along large-scale magnetic field lines in the atmosphere.  The 
Alfven waves with the half frequency of pulsation can be amplified in the atmosphere 
due to the swing wave-wave interaction [8] and have periods of a few days.  This is 
consistent with the period of waves required by the resonant heating mechanism [7]. 
CONCLUSIONS 
A comparison of our models with observations of α Tau and other red giant stars 
leads to the following results:  
 
 Volume Emission Measures  and line ratios of FUV lines suggests the presence 
of dense and compact regions in the atmosphere of α Tau 
 Red-shifted and non-thermally broadened UV lines can be explained in terms 
of AW induced heating and subsequent formation of radiatively driven  
condensations sliding down from the top of the loop 
 The filling-factor (f) of open structures is least 100 times greater than that of 
loop structures 
 The energy density flux of AWs in the atmosphere derived from UV lines is 
consistent with calculated heating rates and the f of magnetic structures 
 This is consistent with HD simulations of convective cells in giant stars [9]. 
The estimated magnetic flux in loops is (1-2) x 1022 Maxwell 
ACKNOWLEDGMENTS 
This work was supported in part by NASA through STScI grants for HST GO/AR 
grants 9967 and 10956. 
REFERENCES 
1. Robinson, R. D. Carpenter, K. G. & Brown, A. 1998, ApJ, 503, 396. 
2. Dupree, A. K., Lobel, A., Young, P. R., Ake, T. B., Linsky, J. L. & Redfield, S. 2005, ApJ, 622, 629. 
3. Ayres, T. R., Brown, A. & Harper, G. M. 2006, ApJ, 651, 1126. 
4. Guinan, E. F., Ribas, I. & Harper, G. M.. 2003, ApJ, 594, 561. 
5. Airapetian, V. and Carpenter, K. G., 2008, ApJ, submitted. 
6. Koen, C., Laney, D., & Van Wyk, F.  2002, MNRAS, 335, 223. 
7. Hollweg, J. V. 1987, ApJ, 312, 880. 
8. Zaqarashvili, T. V & Belvedere, G. 2005, MNRAS Letters, 362, L35 
9. Freytag, B. et al. 2002, Astron. Nachr., 323, 213. 
 
 
 


The Atmospheric Dynamics of alpha Tau (K5 III) - Clues to Understanding the Magnetic Dynamo in Late-Type Giant Stars

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