We discuss possible secondary scientific results obtainable with the FRESIP satellite in regards to surface features on solar-like stars

Dempsey, Robert C.

English

NASA Technical Reports Server

N95- 14501
l"
Probing Surface Structure on Late-type Stars With
FRESIP
Robert C. Dempsey
Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD
21218
Abstract.
We discuss possible secondary scientific results obtainable with the
FRESIP satellite in regards to surafce features on solar-like stars.
1. Introduction
Currently, for only one star, our Sun, is it possible to resolve surface features such
as sunspots and plages. In the case of the Sun these features are well studied.
For example, it is clear that the dark sunspots never cover more than 1% of the
surface and tend to develop at mid latitudes with formation occurring closer to
the equator as the ll-year solar cycle progresses. Through the remarkable work
of the Mt. Wilson project we now know many stars possess surfaces covered
unevenly with plage-like regions (Wilson 1978) and that stellar activity levels
may undergo cyclic variations similar to the solar cycle (Baliunas & Vaughn
1985). Noyes et al. (1984) demonstrated the connection of the Ca H emission flux
to the magnetic dynamo believed to be operating in rapidly rotating, convective
stars. In general, the Ca II emission is strongest for stars with P, ot "_ 1-3d and
gradually decreases as P, ot increases. Similar relationships have been observed
in other diagnostics (e.g., Simon & Fekel 1987; Strassmeier et al. 1990) and the
rotation-activity relationship is now firmly established, at least for single stars.
Numerous late-type stars show photometric amplitude modulation which is
believed to result from large, sunspot-like regions rotating in and out of view
as the star rotates. The light curves of a few dozen stars have been modeled
successfully using simple spot models (see Dempsey et al. 1992 for references).
Results from modeling the photometric variations have been supported by high
resolution spectroscopic studies, called Doppler imaging, for a very small sub-
set of systems (< 12) and for a very few epochs. However, since good phase
coverage is required and large amplitude photometric variations are easier to
........... ;;_ _:_ de_ect, these studies have been biased towards the short-period "hyper-active"
.......... _'+_: ........... _s:-_t-i_ermore, _ Stars s'tu_li_' ]n this m_er have been relatively
bright which usually means they are giant systems. Results from these studies
are rather puzzeling since the detected spots are almost always at or near the
pole, quite unlike the case of the Sun where spots tend to form closer to the
equator. Does this result from studying-6-nYy the "hyper-active" systems or is
the Sun unusual among late-type stars? Or, perhaps, is the Sun going through
some phase where its spot behavior deviates from the "norm", whatever that
11
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maybe? Moresystemsmustbestudiedto answerthesequestions.
2. FRESIP
A major benefit of the FRESIP project will be the ability to detect and monitor
surface features on thousands of stars. An immediate result of this is that a
statistically significant number of stars of all spectral types can be sampled.
Since the program stars will be dwarfs this will allow us to study the advent of
spots in objects earlier than the Sun and to see how far down the main sequence
such phenomena persist, i.e. do fully convective M stars posses spots. If possible,
it would be desirable to include a sample of 200-300 class IV and HI objects.
This allows us to probe the characteristics of the future Sun.
In particular, surface features on the longer period (P > 10 - 20d), less
active systems will be studied in detail for the first time. This results from the
fact that FRESIP will be able to detect photometric variations with periods
greater than several weeks more easily than can be done from the earth. Earth
based observations are limited by short observing seasons, weather and evolution
of the surface features On timescales less than P_ot. A typically poor light curve
is shown in ]?ig ]. Since it has also been established that the rotation period
generally determines the activity level, the longer period systems will be less
active and more solar-like than those studied to date. Therefore, the miUimag
precision of FRESIP will allow us to probe to lower thresholds_ i.e_
smaller spot or plage areas, available f_om the ground. The short period
systems, and those with large photometric amplitudes, will be studied in greater
detail than available using ground based photometers. This will allow the study
of changes on very short timescales if present.
3. Simulations
To simulate some possible results we used the DOPPEL code discussed in
Dempsey et al (1992) modified to yield the integrated magnitude over the 450
700 am b_S, pas_ _e t_he FRESIP satellitc. The models simulated G and K
dwarfs and a K giant. Only spots cooler than the photosphere were simulated
since warmer areas will have similar results. Also, real life is far more com-
plicated than the models presented here. In general it is likely that several
cool spots will be present along with hot regions thus making the light curve
rather complicated. However, we can use experience gained from modeling KS
CVn systems (e.g., Strassmeier 1988; Strassmeier et al 1988; Jetsu et al. 1990;
Dempsey et al. 1992, 1993) and concentrate on what new is detectable. The
models assume a single, square spot with a temperature several hundred degrees
lower than the surrounding photosphere. Note, we can only detect assymmetries
in the spot/plage distribution since a unform spot, e.g., an equatorial belt, will
affect the light curve equally at all phases. Furthermore, the detection threshold
for our work can be increased easily by binning the data. For example, consider
a star with a rotation period of 33 days. We could bin 8 hours of data into
steps of 0.01 in phase with an accuracy of 0.001 mag/sqrt(8) or 0.0004. Even
with such large bins we will have better phase coverage than achieved from the
ground.
12
For a G dwarf it is possibleto easily detectspotswith a surfacearea
covering _ 0.04% of the total surface without binning the data (Fig 2). To date
the smallest coverage measured on a star other than the Sun has been about
5_. Therefore it is clear that we will, for the first time, be able to detect solar-
like spots which have never covered more than 1% of the Sun! Similar limits are
found for the K dwarf and giant models differing due to contrast effects and spot
brightness relative to total brightness. Binning the data as well as using readily
available period finding algorithms well allow detection of features covering <
0.1%.
4. Discussion
Question we hope to address with this project for the first time:
Do the long period stars have spots like the Sun does, i.e. as we go to
longer and longer period systems will we observe a continuous spectrum
of smaller and smaller spot coverage?
What is the statistical properties of surface features on G and K dwarfs.
Since the convection zone depth is a critical parameter in dynamo theory
we would expect to see differences, e.g., size, between spectral types.
Do all stars have the same magnitude and direction of differential rotation
as does the Sun? As indicated in Fig 3, we can also measure differential
rotation on a number of stars by observing the change in period as the spots
evolve to different latitudes. Preliminary results from Doppler imaging
has shown conflicting results but there are only a couple of systems with
adequate data.
Is there any relation between the area spot coverage and the rotation
period or cycle period? For example, do stars with small spot coverage
tend to have longer cycle periods than those with large spots? It will
not be possible to to measure cycles longer than 5-6 years but some of
the "hyper-active" stars might have periods measurable in the lifetime in
FRESIP.
How do spots evolve on rotation and cycle times scales? Limited evidence
exists that spot may stay stable for many rotation periods or change daily
(Dempsey et al. 1993).
What is important to realize is none of these questions can be an-
swered with ground based instruments.
References
Baliunas, S. L. & Vaughn, A. H. 1985, Ann. Rev. Astron. Astrophys. 23, 379
Dempsey, R, Bopp, B, Strassmeier, K., Granados, A., Henry, G. & Hall, D.
1992, ApJ, 392, 187
13
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>
<1
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1992 Sept-Oct
I I I
-0.90 -
-0.80 -
-0.70
N
[ I I
0.00 0.25 0.50 0.75 1.00
Phose
FIGURE I The measured light curve for IM Peg (Dempsey et al.
1993). Individual observations are indicated by plus signs. A model is
shown by the solid line. Note the poor sampling, especially in the later
portion of the light curve.
Dempsey, R. C., Bopp, B. W., Henry, G. & Hall, D. 1993, in proc of the 8th
Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, in
press
Herbst, A. 1987, AJ 94, 150
Jetsu, L. et al. 1990, A&A 236,423
Noyes, R. W., Hartmann, L. W., Baliunas, S. L., Duncan, D. K., & Vaughn, A.
N. 1984, ApJ, 279, 763
Simon, T. & Fekel, F. C. 1987, ApJ, 316, 434
Strassmeier, K. G. 1988, Ap&SS, 140, 223
Strassmeier et al. 1988, A&A 192, 135
Strassmeier, K. G., Fekel, F. C., Bopp, B. W., Dempsey, R. C:, & Henry, G. W.
1990, ApJS, 72, 191
Wilson, O. C. 1978, ApJ, 226, 379
14
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-0.998 -
i --0.996
i , , I , , I I , , , I , , , I , , ,
0.0 0.2 0.4 0.6 0.8 1.0
Phase
FIGURE II The predicted light curve of a G dwarf. Parameters used
are: Tphotosphe,.e = 5800K, T, pot= 3800, radius = 1 Ro, inclination =
45 degrees. A single spot with an area of 0.04% was placed arbitrarily
on the surface. The y-axis is in magnitudes. The amplitude is approx-
imately 0.002 mag and would be easily detectable with the FRESIP
satellite.
15
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5000 6500 8000 9500
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FIGURE III Longitude of spots (top) and mean brightness (bottom)
on IM Peg over 15 years. This illustrates that while we know spots
evolve on long timesc_es the information is rather poor.
16


Probing surface structure on late-type stars with FRESIP

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