A robust unconstrained orbital solution is obtained for the G2 V star HIP 16853 = HD 22705 at 42 pc, which is a probable member of the 28-30 Myr old Tucana-Horologium stream of post-T Tauri stars. The solution yields an apparent semimajor axis of 5.1 ± 0.7 mas, a period of 201 ± 2 days, and an inclination of 80° ± 7°. Assuming a mass of 1 M_⊙ for the primary, the close companion is only 0.4 M_⊙, which implies a spectral type M0.5 at this age. The expected maximum separation (a) between the companions is 18 mas, which makes this system amenable for high-resolution observations. The wide companion HIP 16853 B at 14" is investigated as a possible tertiary component but rejected on account of the near-infrared photometric data inconsistent with the well-defined H-R diagram of the Tucana-Horologium group

Makarov, Valeri V.

English

Caltech Authors - Main

L81
The Astrophysical Journal, 654: L81–L82, 2007 January 1
 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A.
BINARITY OF THE NEARBY 30 Myr OLD SOLAR ANALOG HIP 16853 IN THE TUCANA-HOROLOGIUM STREAM
Valeri V. Makarov
Michelson Science Center, California Technology Institute, Pasadena, CA; vvm@caltech.edu
Received 2006 October 24; accepted 2006 November 16; published 2006 December 18
ABSTRACT
A robust unconstrained orbital solution is obtained for the G2 V star HIP 16853p HD 22705 at 42 pc, which
is a probable member of the 28–30 Myr old Tucana-Horologium stream of post–T Tauri stars. The solution yields
an apparent semimajor axis of mas, a period of days, and an inclination of . Assuming5.1 0.7 201 2 80  7
a mass of 1 for the primary, the close companion is only 0.4 , which implies a spectral type M0.5 atM M, ,
this age. The expected maximum separation (a) between the companions is 18 mas, which makes this system
amenable for high-resolution observations. The wide companion HIP 16853 B at 14 is investigated as a possible
tertiary component but rejected on account of the near-infrared photometric data inconsistent with the well-defined
H-R diagram of the Tucana-Horologium group.
Subject headings: astrometry — binaries: close —
open clusters and associations: individual (Tucana-Horologium) —
stars: individual (HIP 16853)
The star HIP 16853 (HD 22705) is a very young analog of
the Sun at a distance of 42 pc from the Earth. It is a probable
member of the stream of post–T Tauri stars found in the con-
stellations of Tucana and Horologium in the southern sky (Tor-
res et al. 2000; Zuckerman & Webb 2000). Owing to the prox-
imity of this group and a relatively high proper motion on the
sky (∼80–90 mas yr1), identification of new members is fairly
easy. HIP 16853 is a recent addition to the list of proposed
members (Zuckerman & Song 2004). The youth of this star is
also betrayed by the remarkable chromospheric activity re-
ported in Henry et al. (1996) with a Ca ii H and K lines emission
of . The star passes some of the selection′log R (HK)p 4.33
criteria for TUCHOR membership in Makarov (2007), including
a few close conjunctions with other members in the past; how-
ever, no close encounter with the Upper Scorpius OB association
(or its progenitor cloud) has been found, which is considered to
be the likely origin of TUCHOR, or of its major part. A possible
reason for this miss is that the radial velocity used in Makarov
(2007) is perturbed by the orbital motion of the bright primary
star around the center of mass in this fairly close binary. Indeed,
HIP 16853 was found to be a single-lined spectroscopic binary
in Nordstro¨m et al. (2004). A radial velocity of 14.4 is given
in this extensive spectroscopic survey with a formal error of
0.9 km s1 and a rms scatter of 4 km s1 on 18 measurements.
The mean radial velocity may still be affected by the orbital
variation, since no orbital solution has been published.
HIP 16853 is also an astrometric binary with accelerating
proper motion (ESA 1997; Makarov & Kaplan 2005). An ac-
celeration of (1.54, 5.61)  (1.86, 1.66) mas yr2 is spec-
ified in the Hipparcos catalog. Despite its statistically signifi-
cant value in declination, this determination is spurious,
because the model of accelerated motion is not applicable in
this case. Such spurious acceleration determinations occur
when a relatively short period system ( yr) is sampledP ! 3
nonuniformly along its orbit according to the scanning law of
Hipparcos. Meaningful parameters can only be derived from
complete astrometric orbital solutions.
In the absence of a spectroscopic orbital solution, I compute
an unconstrained 12-parameter fit on the 78 individual data
points available in the Hipparcos Intermediate Astrometry Data
(HIAD; ESA 1997). More accurate fits can be obtained in
constrained astrometric solutions when spectroscopic estimates
of e, P, T, and q are used as constraints (e.g., Torres et al.
2003; Jancart et al. 2005). The technique of unconstrained or-
bital solutions and the difficulties arising in it are discussed in
detail in Goldin & Makarov (2006). For this star, however,
there was no need in running the computationally heavy genetic
optimization program described in that paper, because the or-
bital signature was so clear and prominent that a sufficiently
good initial approximation could be done by eye. After a few
iterations using the regular Powell optimization algorithm, the
fit converged to the set of parameters specified in Table 1. The
formal reduced drops from 1.56 to 0.93 on 78–12 degrees2x
of freedom. It should be mentioned that the formal F-test
(which yields a confidence of 0.9997 for this orbital fit) may
be significantly hampered by the strong nonlinearity and in-
trinsic correlations of the parameters e, P, and T, and the com-
puted confidence is likely to be strongly overestimated.
The orbit of the primary component has a semimajor axis
of about 5 mas and is inclined at a large angle (almost edge-
on) to the plane of the sky. With a period of roughly 200 days,
assuming a mass of 1 for the primary component (spectralM,
type G2), the total semimajor axis is AU, or 18 masap 0.75
on the sky, and the secondary mass is 0.4 . At the maximumM,
separation, the companion can be resolved with high-resolution
observations in the near-infrared, for example, by the new tech-
nique of spectroastrometry in combination with adaptive optics.
Note that the parallax has not changed in the orbital solution
with respect to the value specified in the Hipparcos catalog.
The fit is consistent with (circular orbit). For this reason,ep 0
the parameters T (periastron time) and q (longitude of perias-
tron) become degenerate in the multiparameter least-squares
adjustment and in the covariance calculus described in Goldin
& Makarov (2006).
A Hertzsprung-Russell (H-R) diagram based on the J and
magnitudes from the Two Micron All Sky Survey (2MASS)Ks
survey and the Hipparcos parallaxes is shown in Figure 1 for
the following members of the Tucana-Horologium group: HIP
1113, 1481, 9141, 9892, 9902, 16853, 30034, 32435, 107947,
and 108195. The star under consideration, HIP 16853, is
marked with a cross on the graph. For comparison, two model
isochrones for age 27 Myr and are shown from SiessZp 0.01
L82 MAKAROV Vol. 654
TABLE 1
Orbital Parameters of HIP 16853
Parameter Value
(mas)…a0 5.1 0.7
P (days)… 201 2
i (deg)… 80 7
e … 0.0 0.3
Q (deg)… 36  7
Parallax (mas)… 24.0  0.5
q (deg)… 65
…T0 JD 2,448,400
Fig. 1.—Near-infrared H-R diagram for bona fide members of the Tucana-
Horologium stream of young stars. Model isochrones from (Siess et al. 2000;
solid line) and (Pietrinferni et al. 2004; dashed line) are drawn for age
27 Myr, , and no reddening. The position of the star HIP 16853 isZp 0.01
indicated with a cross and a legend; the alleged wide companion HIP 16853
B is shown with an asterisk and a legend.
et al. (2000, solid line) and Pietrinferni et al. (2004, dashed
line). An age of 27 Myr was inferred from kinematical con-
siderations in Makarov (2007), where a past encounter of
TUCHOR with the Upper Scorpius OB association was de-
tected. Generally, the stars seem to follow the Siess et al. (2000)
isochrone very well, except for the star HIP 108195, which
has a K-band excess of over 1 mag. In particular, the star HIP
16853 lies very close to the Siess et al. isochrone. It may even
be shifted by the expected 0.1 mag upward (to brighter ) byKs
the contribution of light from the astrometric companion. How-
ever, this agreement cannot be considered a significant proof
that the group as a whole and our object are indeed 27 Myr
old. Inconsistencies between the theoretical models are obvi-
ous; moreover, solar-like stars do not provide accurate age de-
termination because the isochrones are already fairly com-
pressed at this age. For example, a 1 star at 27 Myr willM,
be brighter than a similar star on the zero-age main sequence
(ZAMS) by only 0.2 mag in J and in , an amount within theKs
model uncertainty. This difficulty motivates many researches to
look for late-type members of young T associations, where the
upturn of the main sequence becomes large. A 0.4 star, byM,
contrast, will have a magnitude difference of 0.66 between
27 Myr and ZAMS in both J and .Ks
We therefore should examine the faint visual companion HIP
16853 B, listed in the CCDM (Components of Double and
Multiple Stars) catalog of double stars (Dommanget & Nys
1994). It is located at 14.6 from the primary star at position
angle 259, according to CCDM. Hipparcos apparently did not
resolve the alleged B companion, probably because of its op-
tical faintness (11.8 mag). This star is, however, listed in the
2MASS catalog of pointlike sources with near-infrared mag-
nitudes , , and . Assuming thatJp 10.47 Hp 9.97 K p 9.83s
this star is a physical binary companion, I can place it on the
H-R diagram in Figure 1, where its locus is indicated with an
asterisk. Obviously, this result is inconsistent with the as-
sumption of physical association with HIP 16853 and with the
TUCHOR group. Furthermore, a proper motion for the B com-
panion specified in the CCDM (of unknown origin) is com-
pletely inconsistent with the proper motion of the primary.
Therefore, this companion is probably optical, and the closer
astrometric companion HIP 16853 C remains the only physical
companion in this binary system.
The research described in this Letter was carried out at the
Jet Propulsion Laboratory, California Institute of Technology,
under a contract with the National Aeronautics and Space Ad-
ministration. This research has made use of the SIMBAD da-
tabase, operated at CDS, Strasbourg, France; and data products
from the 2MASS, which is a joint project of the University of
Massachusetts and the Infrared Processing and Analysis Center,
California Technology Institute, funded by NASA and the NSF.
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Binarity of the Nearby 30 Myr Old Solar Analog HIP 16853 in the Tucana-Horologium Stream

The Hipparcos and Tycho Catalogs, Vol. 1–17 (ESA SP-1200; Noordwijk: ESA)

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Binarity of the Nearby 30 Myr Old Solar Analog HIP 16853 in the Tucana-Horologium Stream

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