The FRESIP project offers an unprecedented opportunity to study pulsations in hot stars (which vary on time scales of a day) over a several year period. The photometric data will determine what frequencies are present, how or if the amplitudes change with time, and whether there is a connection between pulsation and mass loss episodes. It would initiate a new field of asteroseismology studies of hot star interiors. A search should be made for selected hot stars for inclusion in the list of project targets. Many of the primary solar mass targets will be eclipsing binaries, and I present estimates of their frequency and typical light curves. The photometric data combined with follow up spectroscopy and interferometric observations will provide fundamental data on these stars. The data will provide definitive information on the mass ratio distribution of solar-mass binaries (including the incidence of brown dwarf companions) and on the incidence of planets in binary systems

Gies, Douglas R.

English

NASA Technical Reports Server

OBSERVATIONS OF HOT STARS
AND ECLIPSING BINARIES WITH FRESIP
Douglas R. Gies
Department of Physics and Astronomy
Georgia State University, Atlanta, GA 30303-3083
I: gies@chara.gsu.edu
ABSTRACT
N95-14508
The FRESIP project offers an unprecedented opportunity to study pulsations in hot
stars (which vary on time scales of a day) over a several year period. The photometric
data will determine what frequencies are present, how or if the amplitudes change with
time, and whether there is connection between pulsation and mass loss episodes. It
would initiate a new field of asteroseismology studies of hot star interiors. A search
should be made for selected hot stars for inclusion in the list of project targets. Many
of the primary solar mass targets will be eclipsing binaries, and I present estimates of
their frequency and typical light curves. The photometric data combined with follow
up spectroscopy and interferometric observations will provide fundamental data on
these stars. The data will provide definitive information on the mass ratio distribution
of solar-mass binaries (including the incidence of brown dwarf companions) and on the
incidence of planets in binary systems.
1. Hot Star Variability
There is growing evidence that many O- and B-type stars are nonradial pulsators (NRP) with
periods on the order of a day (Baade 1987; Fullerton 1991; Cuypers 1991; Walker 1991). This time
scale is problematical for ground-based observers, and we have only a preliminary notion of the
pulsation characteristics of massive stars. Continous photometric monitoring of these stars over
several years would lead to a vast improvement in our understanding. Many objects are known to
be multi-periodic (Smith 1985; Gies & Kullavanijaya 1988; Waelkens 1991), and I suspect that
most will turn out to be multi-periodic. However, to approach the kind of results found for white
dwarf pulsations, where observers can obtain continuous data over 1000 cycles (cf. Winger et al.
1992), would require 2 years of continous monitoring for a Be star pulsator like ,_ Eri (Balona et
al. 1992). The great attraction of studying NRP in hot stars is that in most cases the stars are
rapid rotators, and consequently the rotational Doppler line broadening reflects the longitudinal
distribution of the pulsations across the visible hemisphere. Thus combined spectroscopic and
photometric studies can lead to the detection of relatively high-order pulsations.
There are many potential scientific gains from continuous monitoring. The following are some
representative cases involving time scales on the order of a day:
63 PIllglIX_4 pA_IIt.ANK NO T FILMED
https://ntrs.nasa.gov/search.jsp?R=19950008094 2020-06-16T10:24:27+00:00Z
Be stars: Most Be stars display line profile variability consistent with low order NRP (Baade
1987), and the same periods are found in the photometry (Am _ 0.05 mag; Percy 1987). Both
the photometric amplitude and Be emission activity vary on time scales of months to years. Since
NRP is a potential source of energy to promote mass loss into a circumsteUar disk, it is important
to search for a correlation between pulsation activity and Be emission episodes.
0 stars: A few O stars are known multi-periodic line profile variables (cf. _" Oph, Reid
et al. 1993). The stars are also photometric variables but the variations are complex (termed
"microvariability"; cf. Balona 1992).
Slowly pulsating B stars: A growing number of B stars are found to be multi-periodic,
photometric variables (cf. 53 Per, Buta & Smith 1979; Waelkens 1991). Beating between the signal
frequencies can only be sorted out over very extended time scales (often longer than a typical
observing season).
The long periods indicate the presence of g-mode pulsations which should represent an
important probe of hot star interiors. Saio & Lee (1991 and references therein) show that NRP
in Be stars could result from coupling with convective motions in the core; thus the phase motion
of NRP in the photosphere reflects the core rotation rate. Data accumulated over several years
would provide the means to begin significant asteroseismology studies of hot star interiors.
2. Hot Star Content in the Selected Region
Are there interesting hot star targets in the regions near (l,b) = (90 °, +15 °) or
(l,b) = (89 °, +11°)? Unfortunately, the massive star population dwindles rapidly at high
Galactic latitudes, so the sample will be limited. There are no O stars with V < 9.0 in these
regions (Cruz-Gonz£1ez et al. 1974). Since V = 9.0 corresponds to a height of z = 1 kpc for an
08 V star at b = +15 °, and since the the scale height for O stars is 73 pc, there will be few if any
O stars found in the proposed fields of view (FOV). There is one B5 V star, HD 188665 (HR 7608
= 23 Cyg), near the center of the b = 15 ° FOV. The Bright Star Catalogue (Hoffieit 1982) lists the
star as a possible velocity variable, so this may be a pulsator. There are several known B stars in
the b = 11 ° FOV (HD 195554, HD 196421, and the Be star HD 194883). However, all these stars
are bright (V < 8), so a neutral density filter would be required.
I expect that there are many fainter B stars that remain undetected in these fields. The
number of B stars, N, fainter than some limiting Vb can be estimated from
where /k_z is the angular area of the field, the local space density of B stars is po = 1 x 10 -4 pc -3
(Mihalas &: Binney 1981), a is the ratio of sin b to the scale height fl = 69 pc (Abt 1987), and r
is the distance corresponding to I_. The extinction is Av = 0.64 (1.66) mag in the b = 15 ° (11 °)
64
FOV(Burstein& Heiles1982);thus,for _ = 10, the distance of a B3 V star (with My = -2.0) is
r = 1870 (1170) pc. Therefore, the number of B stars in the FOV fainter than V = 10 is _ 37 (92);
18% of these will be type Be (Abt & Cardona 1984). Thus even after deletion of the bright stars
(and the need for neutral density filters), there will be a significant number of targets for study.
The great potential of continous monitoring with the FRESIP mission could easily be
extended to this sample of B and Be stars in the FOV. Such targets should be selected by
spectroscopic observations of blue objects in the FOV.
3. Eclipsing Binaries among the Main Targets
Many of the solar mass targets of the FRESIP program win be eclipsing binaries with stellar
companions. The light curve data on these systems will provide fundamental data (masses and
radii) when combined with spectroscopic data.
Mayor et al. (1992) report on the binary properties of a volume limited sample of G- and
K-type main sequence stars (from CORAVEL radial velocity measurements of some 900 stars over
a time span of > 10 years combined with visual binary and common proper motion data). They
find that the period distribution has a Gaussian shape in log P (mean period of 170 y). The mass
ratio distribution is approximately flat but they argue that the numbers could rise at the low mass
end.
The period distribution can be transformed to a semi-major axis distribution for an assumed
mass ratio. This in turn can be used to obtain a probability distribution for a stellar eclipse.
This probability distribution is illustrated as a function of semi-major axis a in Figure 1. The
integrated probability of finding an eclipsing system at any a is 0.4% (or 0.9% according to the
binary frequency found by Abt & Willmarth 1992). This result is essentially independent of the
assumed mass ratio. The lower limit for the integration, a = 2 Re, probably represents a contact
binary (Mochnacki 1981). The mean observed semi-major axis for these ecfipsing systems is
0.13 AU (P = 14.6 d).
I have calculated eclipse fight curves (V-band) for several types of companions. These models
take P = 14.6 d and i = 90 °, and assume that the primary is a solar mass star with the Sun's
radius. The assumed flux (Kurucz 1970) and limb darkening law (Wade & Rucinksi 1985) for the
solar mass primary were taken from a Ten = 5500 K and log g = 4.5 model atmosphere. The input
parameters and eclipse depths (Amp for primary superior conjunction) are given in Table 1. The
full duration of the eclipse is given by At.
The K star model light curve illustrated in Figure 2 was made with the synthesis program
of Mochnacki & Doughty (1972). The model flux and limb darkening for Te_ = 4000 K and
log g = 4.0 were taken from Carbon _: Gingerich (1969). Both primary and secondary eclipse are
easily visible.
55
1.0000
0.1000
_ O.OLOO
Q.
"o
0.0010
0.0001
0.00
TRANSIT PROBABILITY
.... ' .... I ......... i I
ii ...... I ......... I ......... i ......... I .........
0.10 0.20 0.30 0.40 0.50
SEMI-MAJOR AXIS (AU)
Fig. 1.-- The probability of a transit of the center of a star across a solar mass dwarf as a function
of a (based on the period distribution of Mayor et al. 1992). The solid line shows the distribution
for a mass ratio q = M2/M1 = 1 while the plus signs show the nearly identical distribution for
q=O.
E
-200
2OO
400
600
8O0
K6 V COMPANION
1000 , , , I I 1 , , ,
-4 -2 0 2 4
TIME (HOURS)
Fig.
star.
2.-- The primary (solid) and secondary (dashed) eclipses of a K-dwarf around a solar type
66
2_1
E 4
-2
BROWN DWARFCOMPANION
I I I I I I i i t
-4 -2 0 2 4
TIME (HOURS)
Fig. 3.-- The primary (solid) and secondary (dashed) eclipses of a late M-dwarf (or brown dwarf)
around a solar type star.
-0.5
0.0
0.5
E 1.0
1.5
WHITE DWARF COMPANION
I I
2.0 _ i q I 1 I
-4 -2 0 2 4
TIME (HOURS)
Fig. 4.-- The primary (solid) and secondary (dashed) eclipses of a DA white dwarf around a solar
type star.
6?
An eclipseby a late M-dwarfor browndwarfis shownin Figure3. Parametersfor a possible
browndwarf companionweretakenfrom Kirkpatrick et al. (1993)for GD 165B.For this case
(and the next), a simpleanalyticallight curvewasused.A star this faint actsonly asanopaque
disk;nosecondaryeclipseis seen.Thevariationduringprimaryeclipsereflectsthe limb darkening
of the solarmassprimary (thusthe shapeof the eclipseshouldhelpdeterminethe inclination).
Finally I calculatedan eclipsefor a white dwarf star (Fig. 4). Parametersfor a DA white
dwarf companionweretakenfrom Bergeronet al. (1992)for Teg = 11000 K and log g = 8.0.
Because of the larger surface flux of the hotter white dwarf, the secondary eclipse (white dwarf
behind) is the deeper one in this case.
The final column in Table 1 lists the amplitude of the reflex motion that would be observed
in the solar mass star. Each of these cases would be detected with high resolution spectroscopy,
and the combined light curve and spectroscopic data would yield the inclination and limits on the
masses (masses follow if the system is double-lined or a mass estimate is made for the primary).
Precise secondary radii could also be determined with a time resolution better than At = 1 hour.
The FRESIP data will yield a definitive mass ratio distribution for solar-mass binaries. This
will be of crucial importance in determing the frequency of low mass stellar or brown dwarf
companions. If any of these eclipsing binaries also host planetary systems, then it is probable that
planetary transits will also be observed since the stellar and planetary orbital planes are likely
to be nearly coincidental. Thus the FRESIP program will provide key data on the occurrence of
planets in binary systems.
Table 1. MODEL ECLIPSE DEPTHS
M R Flux Ratio At Amp Ams K1
Star (M_) (Re) (F,/Fp) (h)(mmag) (mmag)(kms -1)
K6 V 0.64 0.718 7.5 x 10 .2 6.5 885 79 40.1
GD 165B 0.08 0.069 2.6 x 10 .4 4.6 6.6 2.8 x 10 -s 6.6
DA 0.52 0.012 1.4 x 10 .3 3.9 0.20 1.56 34.2
68
4. Complementary High Angular Resolution Studies
Overthe next decademanyclosebinarieswill be resolvedwith optical interferometers.The
Centerfor High AngularResolutionAstronomyat GeorgiaStateUniversity(H. A. McAlister,
Director)is completingplansfor the CHARA Array, a 7-element optical/IR interferometer with
a 400-m baseline. This telescope will resolve binaries down to separations of 0.2 milliarcsec and
magnitude differences of Am = 3 (for objects with V < 12). For solar-type targets of V = 10 at
a distance of 100 pc, all binaries with P > 0.8 d will be resolved. Such binaries will probably be
double-lined systems, and the combination of interferometric, spectroscopic, and light curve data
will yield accurate inclinations, masses, radii, and distances to the systems. The CHARA Array
will also be used to search for Jupiter-sized planets in wide binaries by measuring reflex motion in
the residuals from the binary orbit.
This work was supported in part by NSF grant AST-9115121.
REFERENCES
Abt, H. A. 1987, in Physics of Be Stars, ed. A. Slettebak & T. P. Snow (Cambridge: Cambridge
University Press), 470
Abt, H. A., & Cardona, O. 1984, ApJ, 285,190
Abt, H. A., _: Willmarth, D. W. 1992, in Complementary Approaches to Double and Multiple
Star Research, A. S. P. Conf. Series Vol. 32, ed. H. A. McAlister & W. I. Hartkopf (San
Francisco: ASP), 82
Baade, D. 1987, in Physics of Be Stars, ed. A. Slettebak &: T. P. Snow (Cambridge: Cambridge
University Press), 361
Balona, L. A. 1992, MNRAS, 254,404
Balona, L. A., Cuypers, J., & Marang, F. 1992, A&AS, 92, 533
Bergeron, P., Wesemael, F., & Fontaine, G. 1992, ApJ, 387, 288
Burstein, D., & Heiles, C. 1982, AJ, 87, 1165
Buta, R. J., & Smith, M. A. 1979, ApJ, 232,213
Carbon, D. F., & Gingerich, O. 1969, in Theory and Observation of Normal Stellar Atmospheres,
ed. O. Gingerich (Cambridge: MIT Press), 377
Cruz-Gonz£1ez, C., Recillas-Cruz, E., Costero, R., Peimbert, M., & Torres-Piembert, S. 1974, Rev.
Mex. Astr. Astrofis., 1,211
Cuypers, J. 1991, in Rapid Variability of OB-stars: Nature and Diagnostic Value, ed. D. Baade
(Garching-bei-Mfinchen: ESO), 83
59
FuUerton,A. W. 1991,in RapidVariability of OB-stars:Natureand DiagnosticValue,ed. D.
Baade(Garching-bei-M/inchen:ESO),3
Gies,D. R., & Kullavanijaya,A. 1988,ApJ, 326,813
Hoffieit,D. 1982,The Bright StarCatalogue (4th ed.) (New Haven: Yale University Obs.)
Kirkpatrick, J. D., Henry, T. J., & Liebert, J. 1993, ApJ, 406, 701
Kurucz, R. L., 1970, ApJS, 40, 1
Mayor, M., Duquennoy, A., Halbwachs, J.-L., & Mermilliod, J.-C. 1992, in Complementary
Approaches to Double and Multiple Star Research, A. S. P. Conf. Series Vol. 32, ed. H. A.
McAlister &: W. I. Hartkopf (San Francisco: ASP), 73
Mihalas, D., _z Binney, J. 1981, Galactic Astronomy, Structure and Kinematics (2nd ed.) (New
York: Freeman)
Mochnacki, S. W. 1981, ApJ, 245, 650
Mochnacki, S. W., & Doughty, N. A. 1972, MNRAS, 156, 51
Percy, J. R. 1987, in Physics of Be Stars, ed. A. Slettebak & T. P. Snow (Cambridge: Cambridge
University Press), 49
Reid, A. H. N., et al. 1993, ApJ, 417, 320
SaJo, H., & Lee, U. 1991, in Rapid Variability of OB-stars: Nature and Diagnostic Value, ed. D.
Baade (Garching-bei-Miinchen: ESO), 293
Smith, M. A. 1985, ApJ, 297, 206
Waelkens, C. 1991, A&A, 246,453
Wade, R. A., & Rucinksi, S. M. 1985, A&AS, 60,471
Walker, G. A. H. 1991, in Rapid Variability of OB-stars: Nature and Diagnostic Value, ed. D.
Baade (Garching-bei-Miinchen: ESO), 27
Winget, D. E., et al. 1992, ApJ, 378, 326
This preprint was prepared with the AAS _Tff/X macros v3.0.
7O
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