We obtained 28 independent radial velocity measurements on the star HD114762, and are now able to derive orbit solution. We confirm the existence of a companion object in an orbit with a semimajor axis about 0.4 au. The new orbit solution is discussed. The result show that the companion is not a planet, but instead is a brown dwarf or a low mass star in a system viewed nearly pole-on

Cochran, William D.

English

NASA Technical Reports Server

N92-12865
The Companion Object to HDl14762 .................. W. D. Cochran
The most interesting results obtained so far in the McDonald Observatory Planetary search
concern the star HD114762. Lower precision radial velocity variation results by David
Latham of Harvard indicated that this star might have a companion object, with an orbital
period of about 84 days. We have obtained 28 independent radial velocity measurements on
this star, and we are now able to derive our own orbit solution. We confirm the existence of
a companion object in an orbit with a semimajor axis about 0.4 au. Our new orbit solution is
shown in the figure below. The solid line is the radial velocity curve from our orbit solution.
The large dots are our radial velocity observations, and the small markers are the discovery
observations of Latham, phased to our orbit solution. The mass function indicates that the
companion has M sin i = 0.011 M,,. If we are viewing the system nearly equator-on, then the
companion object could easily be a planet. A year ago we tested for the case of an exactly
equator-on orientation by searching unsuccessfully for transits of the secondary object across
the disk of the star (Robinson et al. 1990, A. J. 99 672-674). We have since taken our
observed spectra which were used to calculate the radial velocity variations, and we have
analyzed the stellar spectral line profile shapes. In the profile of a stellar photospheric
absorption line, there are two dominant line broadening processes. These are
macroturbulence, which results from Doppler shifts due to photospheric convection patterns,
and rotational broadening due to the component of the stellar rotation along the line of sight.
For a given star, the macroturbulence will be independent of the viewing geometry, while the
rotational broadening will depend on the sine of the inclination angle. When the stellar
photospheric line profiles are analyzed in the Fourier domain, the two types of broadening can
be separated. We have modeled the photospheric line profiles in HD114762, and have
concluded that the best fit is with a macroturbulent broadening of 4.7 km s 1 (which is well
within the range of what is to be expected fl)r this type of star), and a rotational broadening
of 0.0 km s_. Our upper limit on the projected rotational velocity V sin i is 1.0 km s l. This
very low value of the projected rotati_mal velocity is quite significant. There is a very tight
observed relationship between stellar mass and true equatorial rotational velocity. An F9V
star such as HD114762 should have a true rotational velocity of about 13 km s_. Therefore,
we are able to place an upper limit on sin i, the sine of the stellar inclination angle, of 0.08.
However, in our solar system the solar equator is inclined by 7.25 degrees to the ecliptic. If
we allow a similar misalignment between the stellar rotational angular momentum vector and
the orbital angular momentum of the companion object in HD114762, then our constraint
becomes sin i < 0.20 for the companion object. Since we had determined from our high
precision radial velocity measurements that the companion object had M sin i = 11 Jupiter
masses, we can now detern_ine that the companion object mass is at least 55 Jupiter masses,
or 0.055 M,_. This would mean that the compani_)n is n_t a planet, but instead is a brown
dwarf or a low mass star in a system viewed nearly pC)It-on. We have determined that this
system is a "false alarm" for planet detccti()n?
153
https://ntrs.nasa.gov/search.jsp?R=19920003647 2020-03-17T14:05:47+00:00Z
HD114762 (McDonald Solution)
• i
1 o* •
,.-., , ". ;.
" .5 .. "" .; .:. . . ....... "" .- -...-" . _
v : ; • '=, • .I
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_ -,5 " •
0 ,
t'Y •= • o •
ID
--=_ - _ -o -I
..an
-1.5 " = McDonald Data
- Latharn Data (CfA and CORAVEL)
-2
0 .2
,4 ,6 .8 t 1.2 1.4 1.6 1.8 2
Orbital Phase
Radial velocity curve for HD114762. The McDonald Data are shown as the large dots, and
the original discovery data of Latham et al. (1989) are show as the small markers. The solid
line is the radial velocity curve derived from the McDonald data orbital solution.
154


The companion object to HD114762

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