The transition region spectra of 87 late A and early F dwarfs and subgiants were analyzed. The emission line fluxes are uniformly strong in the early F stars, and drop off rapidly among the late A stars. The basal flux level in the F stars is consistent with an extrapolation of that observed among the G stars, while the magnetic component displays the same flux-flux relations seen among solar-like stars. Despite the steep decrease in transition region emission flux for B-V less than 0.28, C II emission is detected in alpha Aql (B-V = 0.22). The dropoff in emission is inconsistent with models of the mechanically generated acoustic flux available. It is concluded that, although the nonmagnetic basal heating is an increasingly important source of atmospheric heating among the early F stars, magnetic heating occurs in any star which has a sufficiently thick convective zone to generate acoustic heating

Boyd, William

Schrijver, Carolus J.

Walter, Frederick M.

English

NASA Technical Reports Server

-N89- 107 4 7
323
TRANSITION REGION FLUXES IN A-F DWARFS:
BASAL FLUXES AND DYNAMO ACTIVITY
F, ede.riek M Walter Carolus J. Sclnijver William Boyd
CASA J1LA CASA
University of Colorado
Bouhh.'r CO $0309
Univ_-rsity of Colorado
Bouldm CO 80309
University of Colorado
Boulder CO 80309
ABSTRACT
We report analysis of the transition region spectra of a
sample of 87 late A and early F dwarfs and subgiants. The
emissmn line fluxes are uniformly strong m the early F
stars, and drop off rapidly among dm late A stars. The
basal flux level in the F stars is consistent with an extrap-
olation of that observed among the G stats, while the mag-
netic component displays the same flux-flux relations seen
among the solar like stars. Despite the steep decrease in
wansiuon region emission flux for B V<0.28, C II ennsslOn
is detected an a Aql (BV=0.22). The dropoff in emissmn
is inconsistent with models of the mechanically generated
acoustic flux available. We conchtde that, although the
non-magnetic basal heating is an increasingly hnportant
source of atmospheric heating among the early F stars,
magnetic heating occurs in any star which has a sufficiently
thick convective zone to generate acoustic heating
Keywords: Transition Regions, Convection, F stars, A stars
1. INTRODUCTION
Stellar magnetic actiwty is a uniqmtous property of stars
on the cool side of the H-R diagram (Ref. 1). The cool
stars have deep convective envelopes in which the inter-
action of rotatmn and convection are thought to gener-
ate magnetic fiol_ls (\rough the, dynamo mechanism. On
the main sequence' the [x_ansltion betvkeen the hot and the
cool stars - in terms of activity - occurs in the late-A or
early F type stars (Ref. 2). Theory predicts that stars
hotter than B-V,-_ 0.30 do not have well-developed convec-
tive envelopes. These stars should not be able to maintain
convectively-driven dynamos, nor should convectively gen
crated noise be available to heat the atmosphere acousti-
cally. The study of the early F and late A stars on and
near the main sequence, where this transition occurs, may
show us how the atmospheric heating depends on convec
tion zone parameters.
\Valter and Linsky (Ref. 3) reported on the initial results
of this investigation. Wolff, Boesgaard, and Simon (Ih,f.
4) &scussed actiwty in the F dwarfs based on observations
of the C IV M550._. and He I )_5876X lines. Both groups
.bhow that:
• All early F (hv_ffs exhib,t strong transition region
enussion, with C II and C IX; surface fluxes over 10 _
erg cm-2s -1 .
• The obse,'ved transition region flux actually incr('ases
with decreasing B-V, a_ the thlckuess of the convec-
live envelope decreases.
• There is remarkably little scatter m the surface fluxes
among the early F dwarfs (.3<B-V<.42), whih," the
cooler F dwarfs exhibit solar like, rotation-activity cor-
reh_tions.
X-ray observations (Refs. 5 and 6) showed sunilar result._,
with all early F dwarfs being detected at essentially the
same surface flux. F stars with B-V>0 45 clearly exhibit a
rotation-activity correlauon, while the hotter stars do not.
Stats hotter that B-V_0 30 are not strong X-ray sources
\\'alter and Schrijver (Ref 7) show that upon subtraction
of api,_opriate basal fluxes from the total observed enns
slon ttux, the remaining emiss,on fluxes (the excess flux)
exlnblt flux flux colrelations smfilar to those seen in cooler
stars (of. Ref 8.). The basal flux component may well
be due to heating of the atmosphere by acoustic waves,
while the rotation-activity and flux-flux correlations of the
non-basal component suggest that it is due to magnetic
processes similar to those observed on the Sun.
In this paper we concentrate on the behavior of the transi-
tion region fluxes in the late A stars, where the convecnon
zone thickness becomes small.
2. OBSERVATIONS AND ANALYSIS
The observational data consist of deep SWP-LO observa-
tions of 87 late A and early F stars (0.21_<B-V_<0.53). The
stars are, in general, close to the ZAMS (Fig. 1), based
on Stromgren uvby/3 photometry (Ref. 9). None exhibit
significant reddening.
A Decade of UV A_tronomy with IUE, Pro,. Celebratory Sympovtum, GSFC, Greenbelt, USA, 12 15 Aprd 1988, ESA SP-281, Vol. 1 (June 1988)
https://ntrs.nasa.gov/search.jsp?R=19890001376 2020-03-20T06:08:22+00:00Z
324 F M WALTER, C J SCHRI.IVER & W BOYD
2.5
I i I I I
2 • % 0 ° 0-
1.5 • _
• O0 • • 0 0-
' & •°°o°°° I
o.5 • • • °eme°J 4
d
0 D • •
7500 7000 6500 6000
Weff
Figure 1: Distances in magnitudes above the ZAMS
(_Mv-0) for the 87 stars in our sample, based on the
Stromgren dCl I)arameter.
The observations were obtained during programs CCDJL,
AFFJL, AFGJL, and ADIFW, with a minority of obser-
vaUons flora the IUE archives. The spectra were well ex-
posed in order to bring up tile relatively weak transit\on
regaon lines, at the expense of greatly overexposing the
i)hotospheric continuum longward of about 1600. x. (Rcf.
3). Enlission line fluxes are nlcasured above a quadratic
backgromtd fit thlough the neighboring pseudo-continuum
(_trongly affected by scattered light m these stars) after
smoothing the data with a Fourier filter. Upper limits are
d('duced from the magnitude of the ]argeat noise feature
near the location of the line of interest. Surface areas are
computed using the Barnes-Evans relation.
The basic data are presented in Figure 2. F_. H {=0.5x(Fc H
+ 0.63xFc zv)} is a mean C II surface flux, where the
factor of 0.63 accounts for the observed proportionality of
C IV to C II. This averagmg of the C II and C IV fluxes
should reduce the random measurement errors.
1
E+6 l i _ _ _
o
o ".
o • • °°°
vv 8 0+
v v o"
V V •
10000 I I I , OI V//Pi}1%
0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55
B-V
Figure 2: C II surface fluxes versus B-V. Filled circles are
tile mean value F'; open circles are C II alone (C IV is
overexposed); + indicates a C IV detection only (underex-
posed spectrum), while the inverted carets are C II upper
limits.
3 (_ AQUILAE AND THE HOTTEST COOL STARS
We have obtained deep SWP-LO observations of 8 stars
with 0.21<B-V_< 0.27. Three of these yielded detections
of C II A1335_, while the rest yielded significant upper
limits. There have been few reports of solar-like activity
(including transxtiou region and X-ray enusslon and He Da
al)sotption) in stars hotter than B-V=0.29. One exception
is ct Aql (B V=0.22), which has been detected as an X-ray
source (Ref. 10), and in H I Lyman a. (Ref. 11). We ob-
tained 3 deep trailed SWP-LO exposures over an interval
of 10 months. The coadded spectrum, shown in Fig. 3,
shows likely emzsszon above the local continuum at wave-
lengths consistent with identification with C II A1335_ and
Si IV A1400._. These emission features are visible in the
individual spectra. The C II emission feature is enhanced
upon subtraction of a template A7 dwarf star composed
of 2 stats from our sample with B-V=0.21 and 0.22. The
ratio of C II to X-ray surface flux is comparable to that
observed among the early F dwarfs. For these reasons we
[_11 I ] L Cf Aql
I
[-I 2
 iii
-0 r i
1200 1250 1300 1350 1400 1450
,_ngs_rom_
Figure 3, High S/N SWP-LO spectrum of o_ Aql. Tins
_s tlie sum of 3 1S0s trailed exposures. Note the emission
featme near tile expected location of C II.
me confident that the identification of C II emission in a
Aql is correct.
The stats detected m C II at B-V=0.25 are HD28052 and
HD90132. Tile former is a strong X-ray source, a Hyad,
and a bim_ry with a G dwarf. The latter is an apparently
single AS dwarf. Simon and Landsman (Ref. 12) show that
$3 Tau (B-V=0.26) also exhibits strong C II emission.
4. FLUX-COLOR DIAGRAMS
The C II flux (Fig. 2) increases with increasing T_// to
B-V_,0.3, but drops lapidly thereafter, although significant
enlissmn is xuslble as far as B-V=0.22. The logarithmic
range m observed CII fluxes is over an order of magnitude
for B-V>0 4, but only a factor of 3 for 0.3_<B-V<0.4. The
mmimunl in tile range is actually an artifact of the sharply
increasing minimum flux level. The scatter again increases
into tile late A dwarfs. It is not clear how this is to be
interpIeted, as the limits imply a very fa_t, almost zn_tan-
tar_eou_ drop in the minimum flux level while the 3 detec-
tions are consistent with a much more gradual decline. In
any event, the decline in transition region heating in the
ORIGINAL
OF POOR
PAGE' IS
QUALITY
I'RANSIFION REGION FI,U'XI:S IN A 1; DWARFS 325
A .q.m s i_ much fa,_tei than pred:cted by models of acous-
tic wave heating of the lower atmosphele (Re'f. 13). The
COmlmted acoustic fluxes are overl)lotted on the obse'rved
transitio:l ,egion fluxes in Fig. 4.
E+6 ' ' _ ' ' ' _ 1
v v
v v
10000 , J *,
0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55
B-V
F:gm'e 4: As Figure 2, lint with the acou,_t,w flux (Ref. 13)
available overplotted. The two curves are for log g=4 0
a:M 4 5. The scaling is arbitrary The shape of the com-
puted craves matches the observed atpper envelope of the
t:ansition :egion fluxes, but neither the lower hlmt (which
is 1)resumably acoustic in ha:me) nor the turnover in the
late-A stats. Also ovmplotted is om itrehnnnary estimate
of Lit:, basal flux level.
5 FLUX-FLUX RELATIONS AND BASAL FLUXES
The powe:-law relations })eB\'een flux densiues in chromo-
sl)he:ic,transition region, and coronal ennssions have Been
shown to hold for late-F through emly-M type dwarfs and
grants, for both single .,,tars and members of muh, iple sys
terns, provided an emp:rical lower lira:t, or basal, flux den-
_:t)"is sulolractcd (Ref. ]-4) Basal fluxes arc large,st fol
the chlomosl)heric emissions, and likely to be negligable
for the co:onal X-rays. The fluxes corrected for the basal
fluxes (the c'xce',b fluxes) ate tdcntlticd w:th tee (mag:iet-
1tally) active part of tit(" stellar atmosphe:e. Tit{" basal
flllXO.% ill}iy 1)e {l:le t.6) :toll magnetic acou.,,tic hc'atmg
Since thc basal component in latc-A and eazly-F type stm s
is vc:v silent, the C II and C IV exce>s fluxes a,e very ,,c'n
s:li\'e lo e:rors in the observed fluxes a:M colors. Walter
and Schruver{Eef 7) demonstrated that if anempi, wally
det.e: mined basal flux :s subti acted from the (>l>berved stel-
lm flux then tit,: early F stars follow the iehltlon between
the X-ray and C lI excess flux densities defined by coole:
stars (the:: Fig. 2). whe:e the basal flux co::ection is ten
erally :wgl:gable We have used he:e ,_ dtffc':ent basal flux
an ext:apolatton of Rutten's C II power-law (Ref. 8) flon
the G _ta:'s Altho::gl: a good dcscril>tion of the lowe: cn-
velolm of the F _tais, this basal flux gives po{>:el flux-flux
CoilelatlOll5 than did tit{: steeper power law used in Rcf 7.
These d:ff<enccs point out the iml_(>_tam ('of being alde
u_ define the txuc' basal flux accmatelv, as it is _,t, h a l;,rgc
fr;tct:(m of the total flux in t.h{'se sims
6. CONCLUSIONS
TEc chromospheres and transition regions of the early F
dwa: fs app(.ar to Be strongly affected by the Basal heating,
winch _s uncorrelated with the X-ray emission or stellar
tot.at:on r_tte. The color dependence of tit(" basal flux hevel
(F% 4) is much more pronounced than the calculated
vat:ariel: with color of the mechanical energy generated in
tit{' upper pint of the convective zone. Schtijver (Ref. 15)
qhowed that :,he basal flu× for the Mg II h and k lines does
exhilnt the color dependence expected of the acoustic flux.
If the Mg lI h and k and C II lower bomMs both inca-
sm c purely 0.coustlc he.ating, then the upt>er chromosphere
and trans:t,um iegion respond non-linearly to the goner-
atcd mechanical flux, and hence appeai to be very s('nsi_l\'e
to changes in the l}hotospherm and lowel-chron:osl)hertc
%_ I 1 l('tlll'e.
\\;ah,e: and Sch, ijver (Ref. 7) suggested that the dynamo
cflic:en%" ,s sup:essed at a given rotational veloctty or :o
t.,,uon;,l l,e, tod for stars hotter than B-V_0 4, although
the (lyltamo :cmains operational down to B V_0.3. Be-
{a:>e of the magI:_tu(le of the l>asal flux co, rectio,_ in the F
< a: s. and the lack of knowledge of its true funct:onM for m,
tins :e.bttlt is questionable. Using the power-law basal flux
(h,_wn m F_gure 4, we find no such decrease in dynamo
cIli_ wncy flora t,he mid- to early F stars (Fig. 5)
I
E+6
= I
E+5
<1
10000
I I I I I III I i ' i 1 i1111[ I i i i i ii
0
0
, , ,ey,,I
10
v sin
1 I I ,,1''1 , i | till
I00 I000
i (km s-')
17:,_,t:: ' .B ThP mean C II flux F', corrected for lhe basal
IilIX ;IN ,I fllllC{IOll ()f V Slit i. Solid ci:clcs a:c stars
wath 028_<13 V_(140, open ('itcles :eptesent star_ with
0 t0<I3-\7<_0 50 The iml_hed :otat:on-activ:ty :elat:on has
F' c',_ _m i (u_i'°us) but an equally acceptable alternat.ive
h,ls a steepel :clarion winch satu:at, es for v sm i>40km s -_
The: e ,._ m, dClmItdence of the excess flux on distance above
{lie ltI{llll ,s(?qtlelli.e, ah bhowIl 111 Figure 13. or o2t color (as:de
f:(m: the gene:al co_relation of v sm i xxuth B-V
T).' &op m the tza:tsitton z(.'gum anti X-:ay fluxes below
B-\;_0 3 (Refs 1 2,6) suggests that witE the dtsapi)ear-
ant,' of the v{}nv(2('tive envelope, both the basal and active
]U';ll liiE, Ill('{'}ldlll%lll% C{'dSO tO fllll('l'l()II C'Oll\'O: selv, if a ('o1/
\_,_t:\_' ,'Hvcl, q : > ._uflic:entty thick to l)ro(htce acoustw
hc,:l::tg. :h{':_ _{, _('{'u> a]s(} Cal)ablc, of SUpl}ort]:lg dynamo
,1{ I 1\ 1I\
326 F M WALTER, C J SCHRIJVER & W BOYD
Im
<1
1
E+6
10000
.J [ I I
I, OoO°
1000 _ I I I
-0.05 0 0.05 0.1 0.15 0.2 0.25
d_c t
Figure 6: The scatter of AF_ 1I with the Stromgren lumi-
nosity parameter _cl. No correlation of the excess flux with
luminosity (or stellar evolutionary state, age, and mass) is
evident.
7. ACKNOWLEDGEMENTS
These data were reduced and analyzed at tile Colorado
RDAF, which is supported by NASA contract number NAS5-
28731. Tins work was supported in part by NASA contract
NAG5-429 to the University of Colorado.
S. REFERENCES
1 Lmsky J L 1980, AT_n Rev A,_tron A,_trophy., 18, 439.
2. Bohm-Vitense E & Dcttmmm T 1980, Ap. J 236, 561}
3 Walter F M & Linsky .3 b 1986, in New ln,_wht,_ zv,
Astrophyszc,s, ESA SP-263, pg 1(13.
4. Wolff S C & Boesgaard A M & Silnon T 1986, Ap. ,I.
310,360.
5. Walter F M 1983, Ap. J 274, 294.
6. Schmitt J H MMet al 1985, Ap. J. 290, 307.
7. Walter F M and Schrijver C J 1987, in Cools Stars,
Stellar Sy3tems, and the Sun, eds. J L Linsky and R
E Stencel, (Berlin:Springer-Verlag), p. 262.
S. Rutten R G M, Lemmens, A F P, and Zwaan, C 1988,
Astron.Astrophys., in press.
9 Crawford D L 1975, A_tron.J. 80,955.
10. Golub L et al 1983, Ap. J. 271,264.
11. BIanco C, Catalano S and Marilli E 1980, ESA SP-
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]2 Simon T and Landsman W 1987 in Cools Stars, Stel-
lar Systems, and the Sun, eds. J L Linsky and R E
Stducel, (Berlin:Springer-Verlag), p. 265
13. Bohn U H 1984, Astron.Astrophys. 136,338.
14. Schrijver C a 1987 in Cools Stara, Stellar Systems, and
tl_e Sun, ads. J L Lmsky and R E Stencel, (Berlin:Sprmger-
\;ctlag), p. 135.
15 Sdn'ijvet C J 1987 A,_tron A._t,rophy._ 172,111.


Transition region fluxes in A-F Dwarfs: Basal fluxes and dynamo activity

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