We use nearly contemporaneus low-resolution IUE observations of Mg II h + k emission and Mount Wilson Observatory Ca II H + K S indices for 33 pairs of observations of lower main sequence stars to formulate a relationship that will permit accurate predictions of S values as a function of (B - V) color and Mg II h + k flux. The resulting relationship is useful because it will extend the set of solar neighborhood stars for which a uniform estimate of chromospheric activity is available to include stars that are not observable from Mount Wilson as well as providing additional estimates of activity levels for stars that are on the Mount Wilson HK Project observing list

Baliunas, Sallie L.

Dobson, A. K.

Donahue, R. A.

Wicklund, B. M.

English

NASA Technical Reports Server

A COMBINED MGII/CAII SURVEY OF STELLAR MAGNETIC
ACTIVITY IN THE SOLAR NEIGHBORHOOD
NASA-CR-204946
NASA Grant NAG5-2444
Final Report
For the period 15 December 1993 through 30 August 1996
1/I - / u
Principal Investigator
Sallie L. Baliunas
June 1997
Prepared for
National Aeronautics and Space Administration
Washington, D.C. 20546
Smithsonian Institution
Astrophysical Observatory
Cambridge, Massachusetts 02138
The Smithsonian Astrophysical Observatory
is a member of the
Harvard-Smithsonian Center for Astrophysics
The NASA Techaical Ott]cer for this Grant is Dr. Donald West, Code 684.1, Laboratory
for Astronomy and Solar Physics, NASA/Goddard Space Flight Center, Greenbelt, MD
20771.
https://ntrs.nasa.gov/search.jsp?R=19970022598 2020-06-16T02:31:07+00:00Z

Low-Resolution IUE Observations of Chromospheric Mg II h and k
Emission 1
B. M. Wicklund 2,a
(bml05@columbia.edu)
R. A. Donahue 2'4
(donahue@cfa.harvard.edu)
A. K. Dobson s
(dobson@whitman.edu)
S. L. Baliunas 2'6
Received ; accepted
Draft - Version of 2 December
1Based on observations made at Mount Wilson Observatory, operated by the Mount
Wilson Institute under an agreement with the Carnegie Institution of Washington.
2Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA, 02138,
USA
3Dept. of Applied Physics, Columbia Univ., 500 West 120th St., New York NY, 10027,
USA.
4Mount Wilson Institute, 740 Holladay Road, Pasadena CA, 91106, USA.
SDept. of Astronomy, Whitman College, Walla Walla WA, 99362, USA.
6Center for Automated Space Science and Center of Excellence in Information Systems,
Tennessee State University, 330 10th Ave. N., Nashville TN, 37203, USA.
A Combined MgII/Ca II Survey of Stellar Magnetic Activity
in the Solar Neighborhood
Nearly contemporaneus low-resolution IUE observations of Mg II h+k emission and
Mount Wilson Observatory Ca II H÷K fluxes were used for 33 pairs of observations of
lower main-sequence stars to formulate a relationship that permits accurate predictions
of Ca II H+K flux values as a function of a star's (B-V) color and Mg II h+k flux. The
derived relationship is useful because it expands, by using the IUE observations of Mg
II, the number of stars in the solar neighborhood for which a uniform estimate of chro-
mospheric activity is available, including several solar-like stars that cannot be observed
from the latitude of Mount Wilson Observatory. In addition, additional estimates of ac-
tivity were derived for stars that have only been observed infrequently at Mount Wilson.
This calibration between the Mount Wilson observations and spectra from the IUE
serves as the basis for additional studies utilizing data from the IUE Final Archive, when
it has been constructed. We plan additional proposals in the future to concentrate on
other types of stars using data for the Final Archive made available after the conclusion
of this project.
A paper detailing our calibration and results is in preparation and will be submitted
to the Astrophysical Journal. A poster paper showing interim results was presented at
the AAS meeting in January, 1996 (B.W. Wicklnnd, R.A. Donahue, S. Baliunas, Bulletin
American Astron. Soc., 27, 1430 (1996).
-2-
ABSTRACT
We usenearly contemporaneuslow-resolution IUE observations of Mg II
h + k emission and Mount Wilson Observatory Ca II H + K S indices for 33 pairs
of observations of lower main sequence stars to formulate a relationship that
will permit accurate predictions of S values as a function of (B - V) color and
Mg II h + k flux. The resulting relationship is useful because it will extend the
set of solar neighborhood stars for which a uniform estimate of chromospheric
activity is available to include stars that are not observable from Mount Wilson
as well as providing additional estimates of activity levels for stars that are on
the Mount Wilson HI4 Project observing list.
Subject headings: stars: activity--stars: late-type
31. Introduction
Magnetic activity is ubiquitous among stars on the cool side of the Hertzsprung-Russell
diagram. Such activity may be studied with the help of diagnostics such as the emission in
the line cores of the resonance lines of singly ionized Mg and Ca. In particular, Ca II H + K
emission from lower main sequence stars has been studied systematically for many years.
The HK Project at Mount Wilson Observatory (MWO) began with the establishment
in 1966 by Olin Wilson of a program to measure Ca II H + K fluxes for 91 lower main
sequence stars on a regular basis (Wilson 1968). Near-nightly observations of many stars
have been made at MWO since 1980, revealing, among other things, rotational modulation
and activity cycles, for stars with a range of (B - V) color (i.e., mass) (see, e.g., Noyes et
al. 1984, Baliunas et al. 1995).
Previous studies (e.g., Oranje & Zwaan 1985, Schrijver 1987) have shown that radiative
fluxes from various emission lines formed in the outer atmospheres of lower main sequence
stars are highly correlated, although also dependent on mass, which can be represented
by (B - V) color. [do you want to go into why?, i.e., convective zone depth and effective
temperature?]
The International Ultraviolet Explorer (IUE), launched in 1978, has measured the UV
spectra of over 700 G-M-type lower main-sequence stars. Boggess et al. (1978a, b) describe
the spacecraft and its performance. The NASA Data Archive and Distribution Service
(NDADS) has been developing a final archive of IUE data, the result of which has been
improved signal-to-noise ratios and photometric accuracy and increased uniformity of the
data set.
The resonance lines of Mg II, formed at heights similar to Ca II H + K, are also useful
activity diagnostics. In particular, there are IUE observations for many stars that have
are not a part of the HK Project data base. For less active stars, Mg is useful because the
-4-
photospheric line wings are darker at 280 nm and interpretation thus more straightforward
than for Ca H + K. Using high resolution IUE data, Schrijver et al. (1992) established
a relationship between Ca II fluxes and Mg II fluxes for stars with 0.5 _< (B - V) _> 1.0.
High resolution IUE observations, while desirable, are available for fewer stars than
low-resolution observations, however. Given this, Hartmann et al. (1984) examined the
relationship between Mg h + k and rotation, demonstrating that Mg h + k emission is fairly
well correlated with Ca H + K emission.
Low-resolution IUE data are now readily available for a larger sample of stars. Using
this sample, we have determined a relationship between Mg II and Ca II emission that will
allow us to predict values of Ca IIS. This in turn will expand the set of chromospheric
activity indices for stars in the solar neighborhood sample (Vaughan & Preston 1980).
2. Observational Data
2.1. IUE Mg II observations
We have identified all available low-resolution spectra made by IUE's Long Wavelength
Prime (LWP) camera / spectrograph combination between 1978 and 1990 of lower
main-sequence stars which are already in the IUE final archive and for which MWO
Ca II observations have also been made. More recent LWP images, and, in particular,
hihg-resolution spectra, are expected to be available from NDADS in the near future. From
each image we extracted the flux measurements in three bandpasses: a 3-nm-wide passband,
hk, centered at 279.9 nm, which contains both the Mg IIh and k lines (located at 279.55
and 280.27 rim, respectively), and two 2-nm bands, v and r, located on the violet and
red sides, respectively, of the hk band. These are the same bandpasses used successfully
by Fanelli et al. (1990) to measure Mg II chromospheric emission. From these fluxes we
5constructedan index s, dividing the flux in the hk bandpass by the sum of the fluxes in the
two pseudo-continuum bands:
s = hk/(v + r). (1)
The parameter s provides a measure of the Mg II flux relative to the surrounding continuum
and is similar to the construction of the MWO Ca II S index (see below). Because of the
low resolution of the instrument (NNN [?]nm) the s index contains a greater contribution
from the photospheric continuum than does the S index. [sure this is true?]
2.2. MWO CaII S indices
Comparison Ca II H + K observations have been taken from the MWO HK Project
data set and were made either with the coud@ scanner on the 100-inch telescope (Wilson
1968) or with the HKP2 spectrophotometer on the 60-inch telescope (Vaughan et al. 1978,
Baliunas et al. 1995). The present system permits the selection of either 0.1 nm- or 0.2
nm-wide slits for the H &: K lines, centered at 396.8 and 393.4 nm, respectively. A chopper
wheel sequentially measures the flux in the H + K bandpasses and two 2-nm bandpasses,
V and R, centered at 390.1 and 400.1 nm. The exit slits are translated to correct for a
star's velocity relative to the Earth. Several nightly observations of a standard lamp and of
standard stars permit photometric calibration. The activity index S is defined as
(g+g)
s= (v+R ) , (2)
the ratio of the counts in the Ca II passbands to the sum of the counts in the pseudo-
continuum bands; a is the nightly calibration factor. The integration times are determined
such that the uncertainty due to the photon statistics is on the order of 1.0 - 1.5%.
-6-
3. Analysis
We have identified 33 pairs of observationsin which IUE Mg II observations were
made within 6 days of a corresponding MWO Ca II observation. While a larger data
sample would be helpful, observations also need to be nearly contemporaneous to reduce
the scatter in the relationship between Ca II S and Mg II s due to changes in a star's level
of activity over time. (Six days is arbitrary: we compared each IUE observation with the
MWO observation closest to it in time; the distribution of time differences (At's) has a
distinct break between 6 and 11 days.) Table 2 lists the values of Ca II S, Mg II s, At, and
(B - V) for these 33 pairs of observations.
EDITOR: PLACE TABLE ?? HERE.
Figure 1 shows the distribution of Ca S values vs. a) Mg s and b) (B- V). Correlation
coefficients between the variables are given in Table 1.
EDITOR: PLACE FIGURE ?? HERE.
We have fit MWO Ca II S as a function of IUE Mg s and (B - V) using a linear
least squares routine, including terms (and cross-terms) to third order in the independent
variables. As each independent variable is considered an F-test is performed, and only
those variables which contribute significantly to improving the percentage of the variance
in the data which can be accounted for by the fit (i.e., F _> 3) are retained. Due to the
broader range of activity levels for later-type stars, we found it impractical to fit the entire
(B - V) range (0.42 - 1.37) with one fit and so split the data set into two parts, with some
overlap, constructing separate fits for stars with (B - V) _< 0.9 and (B - V) _> 0.8.
-7-
For (B - V) < 0.85,
Spred_ct_d = --0.310 + 1.362S 2 + 3.270(B - V) 2 - 3.570(B - V) 3,
and for (B - V) _> 0.85,
Spr_d_¢_d = --0.684 + 7.769S -- 9.986s(B - V) + 3.394s(B - V) 2.
(3)
(4)
The former fit accounts for 87.0% of the variance for 28 points, with a multiple
correlation coefficient, r, = 0.933; the latter fit accounts for 90.9% of the variance, with 18
points and r = 0.953. Figure 2 shows the residuals as functions of Mg II s and B - V.
[insert another figure here somehow]
The data set includes also 92 other observational pairs for which At > 6 days. We
applied the relationship, determined above, to the IUE Mg II s values for these stars. In
some cases the time difference between the IUE and MWO observations is several years,
and therefore we would not expect to reproduce the observed Ca II S value precisely, but
rather would expect to obtain a predicted S value that would fall within the range of values
that might be expected for that star. Because we are investigating the possibilities of using
low-resolution observations we have not attempted to model and subtract the photospheric
contribution between the h and k lines. We find that for several very inactive stars in the
high (B - V) group, stars with Mg II s values well below any in the group from which
the relationship was constructed, this results in predictions of negative Ca II S values. In
other words, there is an empirical lower limit Mg II s envelope, as a function of (B - V),
below which we are not able to produce a useful relationship between Ca II S and (Mg II s,
(B- V)).
For (B - V) > 0.85, the lower limit for application of the above relationship is
s = -1.967 + 3.306(B- V)- 1.829(B- V) 3 + 1.252(B- V) 4. (5)
-8-
With the exceptionof thosehalf-dozenstars, however,the predictedvaluesof Ca II S
for thesenon-contemporaneousobservationsarequite reasonable.Figure 3 w/ Mg II s vs
(B - V) showing lower limit and the non-contemp and non-MWO Mg II ,, and the residuals
for the non-contemp obs vs (B - V).
4. Discussion
Equation 3 provides a method of converting Mg s values, derived from low-resolution
spectra, into reasonably reliable Ca S values. From the non-contemporaneous observations
we estimate that precision with which Ca II S is estimated from the low-resolution IUE
observations is about 5% [CHECK THIS], i.e., roughly the uncertainty that would be
expected taking any single instantaneous measurement without any knowledge of the
particular circumstances of a star's activity or variability. Thus it is possible for researchers
to use low-resolution IUE data to predict S-index values for stars for which direct MWO
observations are not possible. This significantly expands the number of stars in the solar
neighborhood for which estimates of chromospheric magnetic activity are available.
5. Acknowledgements
This research was supported by an REU grant from the National Science Foundation
to the I-Iarvard-Smithsonian Center for Astrophysics. Use made...of SIMBAD database,
operated at CDS... Colleagues on the MWO HK Project... NDADS .... for IUE dater.
-9-
REFERENCES
Baliunas,S.L., et al. 1995,ApJ, 438,269
Boggess,A., et al. 1978a,Nature, 275,372.
• et al. 1978a,Nature, 275,377.
Fanelli, M.N., O'Connell, R.W., Burstein, D., &:Wu, C. 1990,ApJ, 364,272
Hartmann, L., Baliunas, S.L., Duncan, D.K., _: Noyes,R.W. 1984,ApJ, 279,778
Noyes,R.W., Hartmann, L., Baliunas, S.L., Duncan, D.K., &:Vaughan,A.H. 1984,ApJ,
279,763
Oranje, B.J. _: Zwaan,C. 1985,A&A, 147,265
Schrijver, C.J. 1987,A&A, 172,111
Schrijver, C.J., Dobson,A.K., & Radick, R.R. 1992,A&A, 258,432
Vaughan,A.H. _: Preston, G.W. 1980PASP,92, 385.
Vaughan,A.H., Preston, G.W., & Wilson, O.C. 1978,PASP,90, 267
Wilson, O.C. 1968,ApJ, 153,221
This manuscript waspreparedwith the AAS LATF_ macros v4.0.
- 10-
Table 1. Correlation Matrix
Mg s (B - V) Ca S
Mg II s
(B - V)
Ca II S
1.00 0.86 0.81
1.00 0.76
1.00
-11-
Table 2. Data
HD Ca II S Mg II s At (days) (B - V)
10780 0.245 0.482 1.93 0.81
10780 0.245 0.456 1.98 0.81
17925 0.621 0.682 0.63 0.87
17925 0.591 0.725 0.86 0.87
22049 0.520 0.741 0.04 0.88
22049 0.525 0.768 0.16 0.88
22049 0.542 0.778 0.19 0.88
22049 0.567 0.756 0.69 0.88
22049 0.442 0.718 1.06 0.88
22049 0.442 0.729 1.10 0.88
22049 0.490 0.709 1.85 0.88
22049 0.426 0.701 2.80 0.88
26965 0.229 0.541 1.12 0.82
26965 0.229 0.513 1.21 0.82
26965 0.228 0.533 1.92 0.82
26965 0.228 0.529 1.95 0.82
35171 1.183 1.924 0.26 1.09
72905 0.364 0.416 3.99 0.62
103095 0.180 0.294 0.42 0.75
114710 0.191 0.321 2.44 0.57
115043 0.319 0.363 1.49 0.60
141004 0.154 0.299 0.47 0.60
142373 0.143 0.244 0.50 0.56
166620 0.175 0.527 2.87 0.87
- 12-
Table2--Continued
HD CaII S Mg II s At (days) (B - V)
185144 0.184 0.393 2.85 0.80
187691 0.153 0.297 0.08 0.55
166620 0.175 0.532 2.60 0.87
201091 0.660 1.683 0.77 1.18
201091 0.660 2.100 0.81 1.18
201091 0.597 1.910 4.86 1.18
201092 0.990 3.608 4.83 1.37
206860 0.334 0.354 4.85 0.59
207978 0.148 0.314 5.73 0.42
- 13-
Near-contemporaneous Data
1.4
1.2
1.0
--08
0.6
0.4
0.2
0.0
0.0
+
Near-contemporaneous Data
+
+
1.4
1.2
1.0
r_
08
0.6
0.4
0.2
0.0
+++ _ _t
-¢_-_ + -¢+ +_+
0.5 1.0 1.5 2.0 2.5 0.4 0.6 0.8 1.0 1.2 1.4
Mg II s (B-V)
Fig. 1.-- Near-contemporaneous Data; a) MWO Ca II S vs. IUE Mg II s for 33 observation
pairs; b) the distribution of MWO Ca II S vs. (B - V) color for these observations.
- 14-
O
(.9
0.4
0.2
0.0
-0.2
-0.4
0.0
Near-contemporaneous Data
<>
O
O
°_
0.2
> 0.0
_ -0.2
-0.4
Near-contemporaneous Data
0.4 ..................
(>
+ _+ o
0
0.5 1.0 1.5 2.0 2.5 0.4 0.6 0.8 1.0 1.2 1.4
Mg II s (B-V)
Fig. 2.-- The residuals for the 33 nearly contemporaneous observations used in the linear
least squares fits vs. a) IUE Mg II s and b) (B - V) color. The + symbols are for the
low-(B - V) fit, the diamonds for the high-(B - V) fit.
- 15-
_o
5
4
3
2
1
0
other IUE data
#
+ :
+!Co>
i _+. ,¢.o<>
0.4 0.6 0.8 1.0 1.2 1.4
(B-V)
1
0
o
-1
-2
non-contemporaneous residuals
. . . , . . , . . , . , . . . , .
+
2
+
+
+
_+ + +
+
+
+
0.4 0.6 0.8 1.0 1.2 1.4
(B-V)
Fig. 3.-- Predicted Ca II S values for other IUE observations. In a) the + symbols are
the IUE data for which non-contemporaneous MWO observations exist, the small dots are
IUE observations for which there are no corresponding MWO, and the diamonds indicate
the lower limit of fit applicability. The residuals obtained by applying the predictive fit to
the IUE data for which non-contemporaneous MWO observations exist are shown in b).


A Combined MG II/CA II Survey of Stellar Magnetic Activity in the Solar Neighborhood

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