Ultraviolet data obtained with the IUE spacecraft are presented for a dozen solar-type stars in the field. The stars are of spectral type F6 V - G1 V; on the basis of their high Li content, they range in age from 0.1 to 2.8 Gyr. The evolution of transition regions and chromospheric emission with stellar age is studied along with the surface distribution of magnetically active regions as revealed by rotational modulation of UV emission line fluxes

Boesgaard, A. M.

Simon, T.

English

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ORIGINAL PAGE IS
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IUE OBSERVATIONS OF THE CHROMOSPHERIC ACTIVITY-AGE
RELATION IN YOUNG SOLAR-TYPE STARS
Theodore Simon and Ann Merchant Boesgaard
Institute for Astronomy
2680 Woodlawn Drive
Honolulu, Hawaii 96&22 USA
k1l
IMODUCTION
The difficulties of measuring magnetic fields in late-type stars
other than the sun are well known, as one is reminded by other contri-
butions to these Proceedings. This Symposium nevertheless comes at a
very opportune time, as we are now at the point where we can begin to
explore the relationship of stellar magnetism to flare activity and
quiescent cool star chromospheres, transition regions (TRs), and
coronae.
Except for the synoptic observations of the chromospheric Ca II
H K lines by Wilson ( 1978), in which he sought evidence for magnetic
activity cycles, there is still scant data on stellar activity, espe-
cially at UV and X-ray wavelengths where 10 5 K TRs and 106 - 107 K
coronae are expected to radiate. This paper presents new UV data,
obtained with the IUE spacecraft, for a dozen solar-type stars in the
field. The stars are of spectral type F6 V - G1 V; on the basis of
their high Li content, they range in age from 0.1 to 2 .8 Gyr (Duncan
1981). Our purpose is to study the evolution of TR and chromospheric
emission with stellar age, and also the surface distribution of mag-
netically active regions as revealed by rotational modulation of UV
emission line fluxes.
OBSERVATIONS
Low resolution (^-6 A) spectra from 1200-1950 A were obtained
with the SWP camera of IUE. We have measured the strengths of chromo-
spheric and TR ea'_ssicr. features ( .mostly unresolved blends) of 0 I,
C I, C II, C IV, Si IV, N V, and He II. The bright continuum at
11 > 1750 A is generally overexposed in our deep exposures, saturating
the Si II, Si III, and C III features. Echelle spectra of the Mg II
lines at 2800 A (resolution 220 mA) were obtained for all but one star
with the LWR camera. A dozen exposures of X 1 Ori, covering three
rotational cycles, were made with both cameras in order to assess the
effects of surface activity (Boesgaard and Simon 1982).
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CHRUdUSPHERIC ACTIVITY-AGE RELATION IN YOUNG
SOLAR-TYPE STABS ( Hawaii Univ., Honolulu.)
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ORIGINAL PAGE IS
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CORRELATIONS: ACTIVITY AND AGE
We find that the hottest TR lines are more strongly enhanced
above quiet sun fluxes in the young stars than are the l ines formed in
the low chromosphere, and thus the high temperature lines follow a
steeper relation between emission strength and age than do the chromo—
spheric lines. As representative of 10 5 K plasma, the observed C IV
1549 A strengths are shown in Figure 1. The line fluxes are normal -
ised to the apparent stellar bolometric luminosity, and the symbols
are keyed to stellar rotational velocities from Soderblom (1980) or
Kraft (19671. Normalized fluxes are also given for T Tauri variables
(Imhoff and Giampapa 1982), corrected for W extinction, for stars in
the Hyades cluster (Zolcinski et al. 1982), and for the active and
quiet sun. A power law, flux a Age -0.8 , fits the C IV data reasonably
Drell, despite some significant deviations. The range of variation
observed fni X 1 Ori (at 0.6 Gyr), denoted by a shaded box in Figure 1,
presumably gives a fair indication of the scatter in C IV brightness
expected as stellar active regions or bright supergranulation network
rotate-on and off the visible stellar disk.
60	 7D	 8D	 W
by Ape (y►)
6D
	
7.0
	 8.0	 W
bgAgo (y
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Figure 1 (left). Normalized flux of C IV 1549 A. The range in line
flux observed for RW Aur, X1 Ori, and the sun are shown.
Figure 2 (right). Ratio of C IV 1549 A flux to the Mg II 2800 A
emission line flux.
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ORIGINAL PAGE t3
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UV emission lines of lower excitation (e.g., C II 1335 A) follow
similar power-law relations with shallower slopes. The chromospheric
Mg II lines decline in strength with the square root of stellar age, a
result given previously by Skumanich (1972) for the Ca II H-K rever-
sals and axial rotation. The oldest stars in our sample tend to have
both the weakest C IV and weakest Mg II emission. The fC IV /fMg II
ratio, which measures the relative amounts of 10 5 K and chromospheric
material, is correlated with age as sho n in Figure 2: the younger
the star, the larger the differential emission measure at Nigh temper-
ature, although the observational scatter is considerable. The scat-
ter may be related to the fact that, in contrast to the large varia-
tion in the TR emission of X 1 Ori, we found little variation in simul-
taneous observations of the chromospheric Mg II lines, with no more
than a 13% range in average fig flux over a year.
DISCUSSION
Our WE observations indicate that emission fluxes of high tem-
perature lines in-the UV, and consequently the outer atmosphere heat-
ing and cooling rates, are correlated with the ages of solar-type
stars. Stars one--tenth the age of the sun have emission measures in
105 K plasma an order of magnitude greater than the solar TR. This
trend extends to the very young T Tau stars, although Hartmann et al.
(1982) believe that their UV emission may arise from strong stellar
hinds. Among the young stars we find a greater enhancement of the
emission strengths of high temperature lines (C IV, Si IV, N V) than
those of low temperature lines (Si II, C I, Mg II).
Because a similar trend is present in UV spectra of solar active
regions, and because high dispersion WE spectra of active dwarf stars
(including X 1 Ori) show no opacity broadening by comparison with old
solar-type stars like a Cen A, Ayres et al. (1982) conclude that mag-
netic structures on quiet and active dwarfs must be similar. They
attribute differences in activity levels among dwarf stars to changes
in the fraction of surface occupied by magnetic regions, e.g., plages.
There are several difficulties with this model. First, in UV observa-
tions of the sun there is no difference in TR line widths between
supergranulation cell interiors and boundaries (Feldman et al. 1976),
coronal holes and the quiet sun (Doschek et al. 1981), or active
regions and the quiet sun (Feldman and Doschek 1978). Furthermore,
the Mg emission reversal, although more intense in solar active
regions, is not perceptibly broader than quiet sun profiles from the
chromospheric network or supergranulation cell interiors (Lemaire and
Skumanich 1973). Second, the C IV surface flux of young field stars
Is -20 times brighter than the quiet sun C IV emission. Since the
contrast between active and quiet regions of the sun is a factor of
5-10 (Cook et al. 1980), the greater emission of young stars is diffi-
cult to explain by a larger coverage of solar plage, as suggested by
Ayres et al. (1982), except possibly with filling factors very near to
unity. But high values are precluded by the large .rotational varia-
tions seen among the youngest stars. The surface fluxes of the T Tau
ORIGINAL D
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stars, three orders of magnitude larger than the sun's, are impossible
to understand with this kind of model. A more compact sup=rgranula-
tion network can't account for the enhanced C IV among young stars,
especially the T Tauris, Since the solar TR lines are only 3-5 times
brighter in the network than in cell interiors and the area coverage
of the quiet sun network is -40% (Reeves 1976). Therefore, a decided
change in the temperature-pressure structure of young star atmospheres
seems inescapable, and a change in the dimensions of their supergranu-
lation patterns seems likely as well.
Although an enhanced network, varying in cell size across the
stellar disk, is our preferred model for the UV activity-age correla-
tions found here, intense active regions with lifetimes of a year or
more may exist. We note however that brightness variations at the
M1 II kl feature and the nearby continuum were not observed for
X Ori, nor was any variation observed in the continuum below 2000 A,
as is found in the brightest solar plages (Cook et al. 1980).
REFERENCES
Ayres, T.R., Linsky, J.L., Brown, A., Jordan, C., and Simon, T. 1982,
in Advances in Ultraviolet Astron.: Four Years of IUE Research,
in press.
Boesgaard, A.M., and Simon, T. 1982, these Proceedings.
Cook, J.W., Brueckner, G.E., and VanHoosier, M.E. 1980, J. Geophys.
Res. 85, 2257.
Doschek, G.A., Mariska, J.T., and Feldman, U. 1981, Mon.Not.Roy.
Astron.Soc. 195, 107.
Duncan, D. 1981, Astrophys. J. 248, 651.
Feldman, U., and Doschek, G.A. 1978, Astrophys. J. Supp. 37, 443.
Feldman, U., Doschek, G.A., and Patterson, N.P. 1976, Astrophys. J.
209, 270.
Hartmann, L., Edwards, S., and Avrett, E. 1982, Astrophys. J., in
press.
Imhoff, C.L., and Giampapa, M.S. 1982, in Advances in Ultraviolet
Astron.: Four Years of IUE Research, in press.
Kraft, R.P. 1967, Astrophys. J. 150, 551.
Lemaire, P., and Skumanich, A. 1973, Astron. Astrophys. 22, 61.
Reeves, E.M. 1976, Solar Phys. 46, 53.
Skumanich, A. 1972, Astrophys. J. 171, 565.
Soderblom, D. 1980, Ph.D. thesis, Univ. of Calif. at Santa Cruz.
Wilson, O.C. 1978, Astrophys. J. 226, 379.
Zolcinski, M.-C., Antiochos, S.K., Stern, R.A., and Walker, A.B.C.
1982, Astrophys. J. 258, 177.


IUE observations of the chromospheric activity-age relation in young solar-type stars

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