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Young Stars Near Earth: Progress and Prospects
ASP Conference Series, Vol. 244, 2001
Ray Jayawardhana and Thomas P. Greene, eds.
The Young Stellar Group Associated with HD 199143
Mario E. van den Ancker
Harvard­Smithsonian Center for Astrophysics, 60 Garden Street, MS
42, Cambridge, MA 02138
Mario R. P'erez
Emergent­IT Corp., 9315 Largo Drive West, Suite 250, Largo MD 20774
Dolf de Winter
TNO­TPD, Stieltjesweg 1, 2600 AD Delft, The Netherlands
Abstract. Recently, several groups of young stars in the solar neigh­
borhood have been discovered. Given their proximity, these systems are
ideally suited for detailed studies of star and planet formation. Here we
report on a group of young stars associated with the bright F8V star
HD 199143. At a distance of only 48 pc, this is the closest YSO group
containing a classical T Tauri star (HD 358623; K7--M0e). New ground­
based mid­infrared data shows that both HD 199143 and HD 358623 have
large infrared excesses due to circumstellar disks. A systematic search for
new members of this Capricornius association has yielded four new prob­
able members, which we use to derive an age of 5--10 Myr for the group
as a whole.
1. Introduction
In recent years, a fascinating picture of the recent star formation history of
the solar neighborhood has emerged: 10--40 million years ago an ensemble of
molecular clouds were forming stars at a modest rate near the present position
of the Sun. About 10 Myrs ago, the most massive of these newly formed stars
exploded as a supernova, terminating the star formation episode and generating
the very low density region seen in most directions from the present Sun. This
scenario can not only explain the presence of young stellar groups close to the
earth such as the TW Hydrae and the newly identified Tucanae Association
(Kastner et al. 1997; Zuckerman & Webb 2000 and these proceedings), but
also explains how the fi Pic moving group can be so young (20 Myr; Barrado y
Navascu'es et al. 1999), and yet so close.
In a recent A&A letter (van den Ancker et al. 2000) we have identified two
nearby objects, the bright F­type star HD 199143 and the late­type emission­line
star HD 358623, as young stars. In these proceedings we will extend this work by
presenting new mid­infrared observations of HD 199143 and HD 358623, demon­
strating that both possess circumstellar disks. We will also present the results
69

70 van den Ancker, P'erez & de Winter
Figure 1. Spectral Energy Distributions of HD 199143 (top) and HD
358623 (bottom). Also shown are Kurucz models for the stellar photo­
spheres, fitted to the observed energy distribution. Note the presence
of strong infrared excesses in both sources and the presence of excess
UV emission in HD 199143.

The Young Stellar Group Associated with HD 199143 71
of a systematic search for further candidates, showing that both stars belong
to a larger group of young stars, which we tentatively name the Capricornius
association. In the final section of this contribution we will discuss its relation to
the other young stellar groups discussed in these proceedings and briefly touch
upon its formation history.
2. HD 199143 and HD 356823
HD 199143 is a bright (V = 7.27), nearby (Hipparcos distance of 47.7 \Sigma 2.4 pc)
F8V star, which would be completely inconspicuous if it hadn't been detected
as a bright extreme­ultraviolet source by the ROSAT and EUVE missions. A
recent study of the optical and UV spectrum of HD 199143 by van den Ancker
et al. (2000) revealed the presence of emission lines of Mg ii, C i, C ii, C iii, C iv,
Si iv, He ii and Nv and a large amount variability, both in the continuum and
line fluxes. The fact that these phenomena were only found in the ultraviolet
part of the spectrum suggests that HD 199143 is a binary system, consisting
of a rapidly rotating F­type primary and a low­mass chromospherically active
companion which dominates the ultraviolet and infrared light of the system.
A literature search for sources near HD 199143 revealed that a photomet­
rically variable K7--M0e dwarf, HD 358623 (BD\Gamma17 ffi 6128), is located only a
few arcminutes from HD 199143. This star was previously studied by Math­
ioudakis et al. (1995), who found strong Hff emission and evidence for a high
Li abundance, i.e. the characteristics of a classical T Tauri star. Data from the
Tycho­2 catalog shows that HD 199143 and HD 358623 have identical proper
motions. Both the closeness of the two stars and the similarity of the space
motions strongly suggest that the two stars must form a physical group. The
only explanation for the presence of two active stars of such different masses in
such close proximity is to pose that the two stars are young.
New observations of HD 199143 and HD 356823 were obtained with the
TIMMI2 instrument on the ESO 3.6m telescope. Both sources were detected in
the N (11 ¯m) and Q (19 ¯m) bands. Spectral Energy Distributions (SEDs) of
HD 199143 and HD 358623, which include the newly determined N and Q band
data, are shown in Fig. 1. As can be seen clearly from this figure, both stars
possess large excesses above photospheric levels in the mid­infrared. We explain
these infrared excesses as being due to the presence of circumstellar disks in both
systems. In the case of HD 199143 it is not clear whether this disk is in the form
of a disk around the low­mass companion, or is in the form of a circumbinary
disk.
3. Further Capricornius Association Members
Based on a systematic search for stars near HD 199143 with strong X­ray fluxes
we have identified four probable new members of our newly discovered associa­
tion in Capricornius. All four have ROSAT Point Source Catalog fluxes which
are an order of magnitude higher than those of normal late­type stars at 48 pc
and have a projected distance of less than five degrees (4.2 pc at d = 48 pc)
from HD 199143. Similarly strong X­ray sources are not found in adjacent fields
of identical size, leading us to believe that these stars are indeed members of the

72 van den Ancker, P'erez & de Winter
Figure 2. Hertzsprung­Russell diagram of the Capricornius group.
Also shown are the evolutionary tracks (solid lines) and isochrones
(dash­dotted) by Palla & Stahler (1993).
Capricornius association. For two of the newly identified candidate members,
proper motions are available, which are in agreement with those of HD 199143
and HD 358623.
Further indications for the hypothesis that our four new candidate mem­
bers belong to the Capricornius association comes from the observation that if
we assume a common distance of 48 pc, HD 199143 and HD 358623, as well
as the four new candidate members form a smooth curve in the Hertzsprung­
Russell diagram (Fig. 2). According to the models by Palla & Stahler (1993),
all stars are located between the isochrones with ages between 5 \Theta 10 6 and 10 7
years. Although the youth of our newly selected candidate members remains to
be confirmed through spectroscopic means, we conclude that most likely these
four stars are indeed members of a more extended group of young stars associ­
ated with HD 199143. Since the search we performed here for new members is
certainly not complete (it is based on the catalog data in the Simbad database),
we expect to be able to find additional low­mass members of the Capricornius
association through dedicated imaging. Such observations are currently planned.

The Young Stellar Group Associated with HD 199143 73
4. Discussion and Conclusions
In these proceedings we have argued that HD 199143 and HD 358623 are part
of a larger group of young stars, which we tentatively name the Capricornius
association. Several of the properties of this newly discovered group make it
unique: not only is it at a distance of 48 pc the closest association containing
a bona fide classical T Tauri star (the TW Hya group is at slightly more than
50 pc), but its declination of \Gamma17 ffi also clearly separates it from the other newly
discovered YSO groups, which are all located much further to the South.
Yet kinematically our Capricornius association may be related to the other
nearby regions of recent star formation. HD 199143 has a galactic space velocity
(U; V; W ) of (\Gamma10 \Sigma 13; \Gamma13 \Sigma 6; \Gamma13 \Sigma 6) km s \Gamma1 , similar to that found for
the Tucanae and TW Hydra associations (Zuckerman & Webb 2000). This, as
well as the similarity in the ages (5--10 Myr for Capricornius vs. ¸ 10 Myr for
both Tuc and TW Hya), suggests that all three associations may have formed
from the same cloud complex. Star formation in this large cloud may have
progressed linearly, as is also observed commonly in more distant star forming
regions (Elmegreen et al. 2000 and references therein), starting with the most
southern association (Horlogium, Tucanae, TW Hydra) and progressing to our
newly identified Capricornius group. Whether star formation ended here or
whether more northern associations, and possibly even remnants of the parent
molecular cloud, exist and remain to be discovered is a question that awaits
further investigation. In any case the Capricornius group represents a unique
opportunity to not only gain a better understanding of the star formation history
in the solar neighborhood, but to also allow us more insight in the structure of
protoplanetary disks and hence our own origins.
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