The connection between processes that produce optical chromospheric activity indicators and those that produce x-rays in RS CVn binary systems by taking advantage of the ROSAT All-Sky Survey (RASS) results and our unique ground-based data set was investigated. In RS CVn systems, excess emission in the Ca 2 resonance (K & H) and infrared triplet (IRT) lines and in the Balmer lines of hydrogen is generally cited as evidence for chromospheric activity, which is usually modeled as scaled up solar-type activity. X-ray emission in RS CVn systems is believed to arise from coronal loop structures. Results from spectra data obtained from RASS observations are discussed and presented

Ramsey, Lawrence W.

English

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NASA-CR-Z9Z933
f,
;J
Final Technical Report
For NAG 5-1435
ROSAT X-Ray Survey Observations of Active
Choromspheric Binary Systems and Other Selected
Sources
Principle Investigator:
Lawrence W. Ramsey
Professor of Astronomy & Astrophysics
The Pennsylvania State University
(NASA-CR-192933) ROSAT X RAY
SURVEY OBSERVATIONS OF ACTIVE
CHROMOSPHERIC BINARY SYSTEMS AND
OTHFR SELECTED SOURCES Final
Technical Rpport (Pennsylvania
State Univ.) 7 p
G]/89
N93-26426
Uncl as
0158837
https://ntrs.nasa.gov/search.jsp?R=19930017237 2020-03-17T05:41:22+00:00Z
Personnel on this grant were Ph.D. candidate Robert J. Patterer and post-doctoral
research associate Alan D. Welty, with the PI providing oversight.
Our purpose in this grant was to investigate the connection between processes that
produce optical chromospheric activity indicators and those that produce x-rays in RS CVn
binary systems by taking advantage of the ROSAT All-Sky Survey (RASS) results and our
unique ground-based data set. In RS CVn systems, excess emission in the Ca II resonance
(K & H) and infrared triplet (IRT) lines and in the Balmer lines of Hydrogen is generally
cited as evidence for chromospheric activity, which is usually modeled as scaled up solar-type
activity. X-ray emission in RS CVn systems is believed to arise from coronal loop structures
(see Rosner, Tucker, & Vaiana 1978).
Results for RASS observations of a large sample of RS CVn binaries have been presented
by Dempsey et al. (1993). Their results may be summarized as follows: X-ray surface flux,
F_, decreases with increasing rotational period, but is level for P < 3 days. For dwarf
systems F, is independent of P, different from the behavior of single dwarfs. No correlation
is seen between Fx or Lx and v,ot or v sin i. There is no dependance of F_ on spectral type.
Neither did they find any dependence of F_ on Roche lobe filling factor, which would indicate
that the secondary does not influence activity. They quote a ROSAT PSPC counts-to-x-ray
luminosity conversion factor for use when spectral information is lacking, and present x-ray
luminosities for all 112 of their detected systems.
We obtained spectra of 39 RS CVn binaries with the 1.6m telescope at Penn State's
Black Moshannon Observatory, and with the Coud_ Feed telescope of the Kitt Peak National
Observatory. The former has R = 5000, and the latter R = 12000. Wavelength coverage
spanned the optical region, but there were gaps between echelle orders. This incomplete
coverage, and poor signal-to-noise ratio for most systems limited us to the Ha line and the
8542/_ IRT line for BMO targets; for KPNO targets those two lines and H/_ were useful.
The acquisition and reduction of the optical spectra was funded by a grant from the National
Science Foundation.
Our optical spectra were reduced in the usual manner for fiber/echelle/CCD data. To
PRE_--DING P/_GE BLANK NOT FILMED
isolate the portions of the spectra arising from stellar active regions, we used our technique
spectral subtraction to determine the non-photospheric contributions to activity sensitive
lines. Spectra of two non-active stars are shifted in velocity, artificially rotationally broad-
ened, and weighted to fit the "quiet" regions of an object spectrum. The photospheric com-
ponent to activity sensitive lines is calculated with the best fit parameters and subtracted
from the object spectrum, yielding the spectrum of the active region spectrum.
All spectra were obtained during the ROSAT All-Sky Survey, although few are simul-
taneous with RASS observations. Because results from the RASS were not immediately
available, we initially compared our optical results with Einstein IPC luminosities. Those
results were presented at the Seventh Cambridge Workshop on Cool Star, Stellar Systems,
and the Sun, and published in the conference proceedings. Recently, RASS luminosities for
most of our targets have become available, and this report contains these updated results
accordingly.
In Figure 1 we show excess Ha luminosity vs. Einstein IPC x-ray luminosity. We use
x-ray fluxes from Schmitt et al. (1991) when available, x-ray luminosities from Strassmeier
et al. (1988) otherwise. Distances, V magnitudes, and colors (used to convert flux to
luminosity) are those reported in Strassmeier et al. (1988). Each point represents a single
observation of a program object. We estimate the uncertainties in the excess line luminosities
to be 20%. Triangles are for mass transfer systems known to us; pentagons are for AR Mon.
The most striking result from Figure 1 is that known mass transfer systems all populate
the high range of the x-ray luminosity distribution. Excluding the high x-ray luminosity
points (L, < 2 x 1031 erg/s; mostly mass transfer systems), points for AR Mon (mass
transfer and highly variable Ha), and points with LH_ < 0, a correlation (slope 0.63 + 0.06)
is present in Figure 1, suggesting physical connection between x-ray and excess Ha emission
in the non-transfer systems.
Complicating interpretation of the data is the apparent presence of circumstellar mate-
rial that produces absorption in some subtracted spectra (e.g., Hall _ Ramsey (1992); other
scenarios are possible). Also, the excess Ha luminosity can be highly variable in a single
3
system,asevident in AR Mon.
Figure 2 is the sameasFigure 1 except that the RASSx-ray luminosities (Dempseyet
al. 1993)replace Einstein x-ray luminosities. We are surprised that neither the correlation
described above nor the segregation by mass transfer status is as apparent. We expected
differences for individual sources, but also expected that the trends for the ensemble of
sources would remain the same.
Three explanations for this discrepancy come to mind. First, it is possible, though
highly unlikely, that all of the mass transfer systems happened to be emitting less x-ray
radiation during the RASS observations. Second, the energy sensitivities of Einstein and
ROSAT are different. So our data may provide evidence that mass transfer systems are
intrinsically harder sources than are the non-transfer systems (given that the counts to lumi-
nosity conversion factor does not take the x-ray energy distribution into account). The third
possibility is that high neutral column density toward the mass transfer systems produces
an apparently harder spectrum.
Modeling of the Einstein spectra (Schmitt et al. 1990) of our targets has enabled us
tentatively to distinguish between the latter two options. Five of the six mass transfer
systems have hydrogen column densities log NH :> 20, while only one other target in our
sample has a similarly high log NH. Those mass transfer systems are, not surprisingly then,
among the most distant of our targets. Also, three of the five other systems which are
underluminous in the RASS with respect to Einstein, but at lower L_, also have relatively
high log NH.
This raises more questions, which we are investigating in part with a current ROSAT
program targeting mass-transfer systems. For example: Are the mass transfer systems simply
behind clouds? If so, why are they preferentially behind clouds? Do the mass transfer
systems expel enough material to account for the high NH? On the other hand, we may ask:
does mass transfer induce flare-like activity, in the sense that the induced activity produces
copious hard x-ray emission but Ha emission small compared to the stellar total.
Figure 3 shows excess Ha vs. excess H/3 luminosity. The Balmer line measuremeuts
include the entire non-photosphericcontribution to the lines, and may consist of multiple
componentsarising from different physical mechanisms. Solid circles are for objects with
L_ > 2.0 x 1031 erg/s (RASS), asterisks for objects with L_ less than that value, and stars
represent AR Mon. The presence of extended or distributed material causes some "excess
absorption." The slope, 3.49 4- 0.48 from a chi-square minimization straight line fit (Press
et al. 1986) to non-transfer system, first quadrant points (for which the "excess" in the
Balmer lines is positive, i.e., not dominated by absorption components), gives a clue to
the origin of excess Balmer emission. Buzasi (1989) found that excess Ha/H/3 luminosity
ratios greater than about 3 can be obtained only when viewing material similar to solar
quiescent prominences off-limb. The observed Ha/Hfl ratio would be larger if no absorption
components were present.
In Figure 3, first quadrant points for mass transfer systems share the slope defined
by non-mass transfer systems, but with larger uncertainty. Thus, the Balmer lines do not
appear to be greatly affected in strength by mass transfer.
This result and the segregation seen in Figure 1 suggest that the mechanism for mass
transfer produces x-rays, in addition to the coronal x-rays that correlate with excess Hc_
emission.
Our excess Ho_ vs. excess 8542 ._ luminosity data is consistent with that seen in a recent
study of solar active regions and a small sample of the most active RS CVn systems (Chester
1991).
In summary, our conclusions based on ROSAT data are: 1) The correlation we find
between excess LH,_ and L, for the non-mass transfer systems (using L= (Einstein)) suggests
that the Ha excess and coronal x-ray emission are closely related. That is, the bulk of x-ray
emission appears to arise when prominence-like structures are present. 2) From comparison
of Einstein and RASS x-ray luminosities and modeling of Einstein spectra, the apparently
relatively hard spectra of mass transfer systems arise largely as a result of high neutral
hydrogen column density, whose spatial location has yet to be determined. 3) Other types
of activity likely are present. In particular, we can not rule out the possibility that mass
5
transfer in semi-detatchedsystemsmay induceproduction of somex-ray radiation without
a significant Ha signature.
REFERENCES
Busazi,D. L. 1989,Ph.D. Thesis,PennsylvaniaState University
Chester,M. M. 1991,Ph.D. Thesis,PennsylvaniaState University
Dempsey,R., Linsky, J., Fleming, T. A., & Schmitt, J.H.M.M. 1993,ApJS, in press
Hall, J. C. & Ramsey,L.W. 1992,PASP Conf. Ser. #26
Schmitt, J.H.M.M., Collura, A., Sciortino, S., Vaiana, G. S., Harnden, F. R., &: Rosner, R.
1990, ApJ, 365, 704
Strassmeier, K. G., Hall, D. S., Zeilik, M., Nelson, E., Eker, Z., _: Fekel, F. 1988, A&:ASS,
72, 291
Press, W. H., Flannery, B. P., Teukolsky, S. A., _: Vetterling, W. T. 1986, "Numerical
Recipes," (Cambridge Univ. Press: New York)
Rosner, R., Tucker, W. H., & Vaiana, G. S. 1978, ApJ, 220, 643
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