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\begin{document}

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\title*{Imaging and Spectroscopy of Hot (Young)
``Ultracool'' Companions}
%

\toctitle{Imaging and Spectroscopy of Hot (Young)
``Ultracool'' Companions}

%

\titlerunning{Young ``Ultracool'' Companions}

%

\author{G. Schneider\inst{1}
\and P.J. Lowrance\inst{2}
\and E.E. Becklin\inst{2}
\and J.D. Kirkpatrick\inst{3}
\and P. Plait\inst{4}
\and S.R. Heap\inst{5}
\and E. Malumuth\inst{6}
\and R.J. Terille\inst{7}
\and C. Dumas\inst{7}
\and A.B. Schultz\inst{8}
\and B.A. Smith\inst{9}
\and A. J. Weinberger\inst{2}
\and D.C. Hines\inst{1}}
%

\authorrunning{Glenn Schneider et al.}

% if there are more than two authors,

% please abbreviate author list for running head

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%

\institute{Steward Observatory, University of Arizona, Tucson Arizona 85750
\and Dept. of Physics and Astronomy, UCLA, Los, Angeles, CA 90095
\and IPAC, California Institute of Technology, Pasadena, CA 91125
\and Advanced Computer Concepts, Inc., GSFC, Greenbelt, MD 20771
\and GSFC, Greenbelt, MD 20771
\and Raytheon ITSS, GSFC, Greenbelt, MD 20771
\and JPL, California Institute of Technology, Pasadena, CA 91101
\and Astronomy Programs, Computer Sciences Corp., Lanham-Seabrook, MD 20706
\and Institute of Astronomy, University of Hawaii, Honolulu, HI 96822}


\maketitle              % typesets the title of the contribution



\begin{abstract}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
We report the initial results from a
program of HST/STIS spectroscopy of ``hot'' low-mass (brown dwarf) 
companions to nearby young stars identified in our HST/NICMOS
coronagraphic imaging survey.  We present spectra of the likely 20
and 40 Jupiter mass companions to CD~-33$^{\circ}$~7795 and HR~7329,
respectively, the binary companions to GL~577 (both, possibly, brown
dwarfs), and the low-mass star GL~503.2B.  We discuss the state of 
our recent follow-up observations of the putative companion to TWA~6.  
We add these companions to the very few already known, 
allowing the first steps to be taken toward answering many 
fundamental questions regarding the formation and evolution of 
multiple systems with components near and below the 
stellar/substellar boundary.



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\end{abstract}

\section{Introduction}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
The discovery and subsequent characterization of sub-stellar 
objects in stellar systems continues to be one of the key 
goals of contemporary observational astronomy.  From the low-mass 
end of the mass-luminosity function (which is still very poorly 
constrained for M$_*$ $<$0.2M$_{\odot}$) through the regime 
of brown dwarfs and into the domain of 
giant planets, the near and sub-stellar bestiary remains fertile ground 
for exploration.  Knowledge of stellar and sub-stellar 
masses and luminosities at and below the $\sim$0.08 M$_{\odot}$ 
H-burning limit is of fundamental 
importance in many inter-related areas such as the determination of 
the stellar mass function at the low-mass ``end'' of the main sequence, 
the theory of stellar evolution, the resolution of age/evolution issues 
and the origins of stellar and planetary systems. Yet, for
{\it companions}, the transition region 
between very low-mass stars and giant planets is poorly understood and 
ill-observed (due in large part to
their intrinsic faintness and close proximities to their primaries). 

With the discovery and imaging of the brown dwarf companion to GL 229 
\cite{Nakajima95}, the very existence of such 
objects moved them from the realm of conjecture to observation.  And, 
since the first detection of an extra-solar planet in the Jovian-mass 
regime, the companion to 51 Peg \cite{Mayor95}, continuing 
discoveries of 1--10 Jupiter mass companions to 
solar-like stars by indirect methods have revealed an 
unanticipated diversity in mass ranges, dynamical properties, and 
primary-star characteristics \cite{Marcy00a}.

These radial velocity surveys suggest that $\sim$ 5\% of main-sequence
stars possess 0.5-8 Jupiter-mass companions at distances
typically $<$ 3AU from their primaries, but $<$1\% of these stars
have brown dwarf companions. (e.g., \cite{Marcy00b}).
Speculations as to the cause of this ``brown dwarf 
desert" (e.g., \cite{Halbwachs00}) may be observationally 
biased as these surveys are currently insensitive to higher 
mass objects at distances from tens to hundreds of AUs 
because temporal baselines of 
decades to centuries are required to detect them.

Recently, however, many isolated ``field" brown dwarfs have been 
discovered from large ground-based surveys such as the 2-Micron 
All-Sky and Sloan Digital Sky surveys and subsequently confirmed 
spectroscopically (\cite{Kirkpatrick00}; \cite{Fan00}). And, brown 
dwarfs (or ``free floating planets") have been turning up in 
proliferation in very young clusters associated with 
star-forming regions (e.g., \cite{Luhman00}; \cite{Mundt00}). 
Indeed, population statistics in both the field and young
clusters suggest that single brown dwarfs, 
once considered somewhat exotic objects, 
may be as common as stars (e.g., \cite{Tsvetanov00};
\cite{Gizis00}). Yet, the {\it companion}
mass and number fractions of substellar objects, which occupy a 
niche in the mass function between stars and planets, remain 
essentially unknown.  How common are they? 
Only a few brown dwarf companions to stars have thus far been 
found and confirmed (\cite{Oppenheimer95}; \cite{Burgasser00};
\cite{Lowrance00}). The disparity may simply be a contemporary
observational selection effect, or the formation processes for field
vs. companion brown dwarfs may be intrinsically very different.
Does the star formation process end abruptly for masses below the
H-burning limit in multiple systems? Do companion brown dwarfs
form in processes more like planets than 
stars (i.e., fragmentation vs. core collapse; e.g., \cite{Boss00})?
And, how might the presence of a brown dwarf 
companion effect the evolution of a newly-forming solar system?

The unequivocal identification and characterization of 
companion substellar objects in young stellar systems not only helps to 
enumerate the population statistics, but will ultimately lead to 
follow-up dynamical astrometric studies to ascertain their masses
directly and unambiguously.  Though many such objects 
will have to be discovered 
and studied before questions such as those posited above
are fully answered, with those now in hand we have a logical starting 
point to begin investigating the intriguing possibilities.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Observations}
Long before the adoption of NASA's {\it Origins} theme, which contains 
in its foundation the establishment of scientific activities to 
investigate the birth and early evolution of stars and planets 
\cite{Dressler96}, the NICMOS Instrument Definition Team embraced 
these goals in formulating our Environments of Nearby Stars (EONS)
program.  A cornerstone of our EONS investigations was a systematic
search for previously unknown low-mass candidate companions
to young and nearby stars. Carried out during HST Cycle 7,
this survey was designed to find companion candidates
spanning two decades in the mass-spectrum down into
the regime of Jovian mass planets (for objects as young as $\sim$ 10 Myr).
Using the Near-Infrared Camera and Multi-Object 
Spectrometer (NICMOS) on the Hubble Space Telescope (HST) we conducted
a coronagraphic imaging survey of young and 
nearby stars searching for, and finding, 
candidate brown dwarf and giant planet companions.  We have 
now confirmed the physical association of a subset 
of those objects through the establishment of common proper 
motions of the candidates with their putative primaries.  
Further, we have determined their spectral types and 
photospheric temperatures using near-IR/optical spectra obtained 
with the Space Telescope Imaging Spectrograph (STIS).

\subsection{NICMOS Imaging - Detection}

Approximately $\sim$ 18\% of the NICMOS IDT's observing time 
on HST was dedicated to NICMOS/EONS survey, as
previously discussed in detail in the literature
(\cite{Schneider98a}, \cite{Lowrance98}, \cite{Schneider00}). 
In summary, we coronagraphically imaged the circumstellar 
regions of eighty-three young and nearby stars, primarily
in H-band.  In particular we observed:

1) Thirty-eight very young main-sequence stars with mean 
distances of $\sim$30pc. Young sub-stellar companions 
will still be in a higher luminosity phase and thus more readily detectable. The median age for our candidates with 
well-established ages is $\sim$ 90Myrs with 8 candidates 
as young, or younger than, 10Myrs. This sample including 
several members of the TW Hydrae association (d $\sim$ 55pc),
the nearest site of recent star-formation to the 
Earth \cite{Kastner97}.

2) Twenty-seven M-dwarfs which are a) very nearby (d $<$ 6pc), 
with spectral types later than $\sim$M3.5; b) young 
(age $<$ 10 Myr) and (d $<$ 25 pc); and c) spectrally 
the latest known (i.e., ``ultra-cool" dwarfs later than 
$\sim$M8.5).

3) Eighteen primarily main-sequence stars with large IRAS 
excesses and other indicators of dust, to image circumstellar material at 1.1 and/or 1.6$\mu$m.

Each target was observed in a single orbit (providing a total
of $\sim$ 25 minutes of integration time), but at two
different spacecraft orientation angles to permit the
discrimination of point spread function artifacts from
intrinsic circumstellar sources.  By subtracting two
coronagraphically occulted images of a star, rolled about
the target axis, the NICMOS/HST system can achieve a
per-pixel reduction in background intensity approaching
10$^{-6}$ of the peak stellar intensity at a distance of
1". In H-band this allowed us to achieve statistically 
significant detections of companions as faint as H $\sim$ 24, 
10--15 magnitudes fainter than their primaries at separations
of 0.5"--3.0" \cite{Schneider98b}.  For the youngest objects in our sample
these levels of performance enable the imaging of
``hot" Jupiters at distances of tens of AUs from their
primaries.  Typically, the stability of the instrumental system
allows us to determine the positions of coronagraphically
imaged putative companions with a precision of 
about 10mas with respect to their suspected
primaries in unocculted target acquisition images,
establishing an astrometric baseline for follow-up
observations.

\subsection{Ground-Based (AO) Imaging - Confirmation}
The unequivocal determination of companionship for a candidate
rests in the confirmation of physical association with its
putative primary through the establishment of common proper motions.
A subset of our targets, those with less severe contrast
ratios and/or larger separations, are observable with ground-based 
adaptive optics systems. With our first-epoch NICMOS observations,
and later ground-based AO imaging which we are obtaining on the 
Palomar, Keck, and CFHT AO systems (for those observationally 
possible), we are able to discriminate against background objects 
for many of these targets on relatively short time-scales. 
For several high proper motion objects such astrometric confirmation 
of companionship has already been 
secured.  For four astrometrically confirmed objects, of six thus far 
also studied spectroscopically, we have found CD~-33$^{\circ}$ 7795B,
HR~7329B, and GL~577B/C to be brown dwarfs (the latter a brown-dwarf 
binary, the first such hierarchical companion found), and GL~503.2B
to be a very-low-mass star near the stellar/brown dwarf boundary. 
The astrometric results for 
TWA 6``B'', an extrasolar giant planet (EJP) candidate, 
are indeterminate at this time, as a longer temporal baseline 
is required to confirm or reject its companionship.

%%%%%%%%%%%%%%%%%%%FIGURE%%%%%%%%%%%%%%%
%\newpage
\begin{figure}[h]
\special{psfile=PROCEED_2.2.eps voffset=-363 hoffset=35 vscale=50 hscale=50 angle=0}
\vspace{7.25truecm}
\caption[]{Companionship is confirmed (or rejected) by differential
proper motions. As an example we show angular separation 
measures for CD~-33$^{\circ}$~7795A/B over an 18 month 
baseline which, statistically, remained constant (within the 
measurement error).  
We compare this with the anticipated change in separation 
from the proper motion of CD~-33$^{\circ}$~7795A if the 
companion was actually background star, and conclude they 
are indeed a common proper motion pair.}
\end{figure}
%%%%%%%%%%%%%%%%%END FIGURE%%%%%%%%%%%%%

\subsection{STIS Spectroscopy - Characterization}
During HST Cycle 8 we obtained STIS spectra for a 
subset of our young (less than $\sim$ 0.3 Gyr) NICMOS candidate companions
to obtain their spectral types and effective temperatures. The 
instrumental characteristics of STIS+HST 
are uniquely suited to the task of characterizing the 
substellar nature of these targets. 
The ability of STIS to obtain spectra in the regions of key atomic 
lines and molecular bands to differentiate between very low mass stars, 
brown dwarfs, and extra-solar giant planets, for very faint companions 
in close angular proximity to their bright primaries, is unmatched 
by any current ground-based capabilities.
%%%%%%%%%%%%%%%%%%%FIGURE%%%%%%%%%%%%%%%
\begin{figure}[h]
\special{psfile=PROCEED_2.3.eps voffset=-370 hoffset=40 vscale=50 hscale=50 angle=0}
\vspace{6.6truecm}
\caption[]{Young, and therefore hot, brown dwarfs have
spectra resembling M dwarf stars of the same photospheric
temperatures, rather than exhibiting spectral characteristics
of older L and T dwarfs.  Here we compare our STIS spectra
of TWA~5B (CD~-33$^{\circ}$~7795B)and HR~7329B, both 
presumed to be one to a few x 10$^{7}$ yrs of age, 
with several M stars.  We ascertain their spectral 
types on the basis of the continuum, the strength of the 
NaII doublet, and the depths of the TiO band-heads.}
\end{figure}
Representative spectra of two young objects 
CD~-33$^{\circ}$ 7795B and HR~7329B,
in the 8000--9000\AA\ range, are compared to several late
M-dwarf standards of similar effective temperatures in Figure 2.
In the case of CD -33$^{\circ}$ 7795B we find a very 
good fit for M9V, and an effective temperature of 2600K. 
For HR~7329B we deduce a spectral 
class of M7.5V, and effective temperature of 2700K. 
%%%%%%%%%%%%%%%%%END FIGURE%%%%%%%%%%%%%
STIS spectra in $\sim$ 7500--9500\AA\ range allow us 
to compare the strengths and morphologies of diagnostic spectral 
tracers in late M, L, and T dwarfs (and giant planets).  Young, and 
therefore hot, brown dwarfs have spectra resembling 
M dwarf stars of the same photospheric temperatures.
In this spectral region, M dwarfs are dominated by bands of TiO and, 
for the latest M dwarfs, VO.  For cooler L-types, the oxide molecules 
disappear onto grains, leaving cleaner spectra showing only bands of 
the hydride molecules CrH 8611\AA\ and FeH 8692\AA\ along with sharp 
atomic lines of the alkali metals Na 8183 \& 8195\AA, 
Rb 7800 \& 7948 \AA, and Cs 8521 \& 8943\AA. In the spectra of the 
T class, which abut the L dwarfs in temperature, the hydrides have 
disappeared onto grains and some of the alkali metals have formed 
into chloride compounds, leaving Rb and Cs lines (as in GL~229B) and,
for cooler spectra, CH$_4$ 8890\AA.  H$_2$O vapor bands 
($\sim$~9300--9600\AA), which gain strength throughout the sequence from 
late-M through the methane types, are significantly
compromised in ground-based spectra due to atmospheric absorption
(e.g., see figure 4 of \cite{Schultz98} ).

STIS target acquisition and dispersed spectral images 
also allow us to further constrain the relative proper 
motions of the presumed systemic components.  For example,
a spectrum was secured for HD~102982B \cite{Lowrance98} 
in the 0.2" wide slit, which would have failed for most possible geometries if it was a background object.  Conversely, in 
the case of HD~177996 companionship for the candidate was 
rejected as the object did not appear in the 
slit (because of its differential proper motion as
a background star) after the "primary" was successfully acquired.
\section{Hot Brown Dwarf Companions}
While the spectra of objects like CD~-33$^{\circ}$ 7795B 
and HR~7329B clearly resemble late M dwarfs, their youth 
belies their true nature - ``hot" brown dwarfs.  A fundamental, 
and reasonable, assumption is that the low-mass companions 
are coeval with their primaries. With effective temperatures
determined from our spectra, and bolometric magnitudes derived from 
our H-band imaging, we can then place the companions on
H-R diagrams demanding coevality with their primaries, and
estimate their masses through evolutionary cooling models.
\subsection{CD~-33$^{\circ}$~7795 (TWA~5)}
%%%%%%%%%%%%%%%%%%FIGURE%%%%%%%%%%%%%%%
\begin{figure}[h]
\special{psfile=PROCEED_3.1a.eps voffset=-457 hoffset=10 vscale=60 hscale=60 angle=0}
\vspace{6.81truecm}
\caption[]{Direct imaging of CD~-33$^{\circ}$ 7795A.
Top Left - NICMOS 1.65$\mu$m target acquisition (TA) images at two spacecraft
orientations (three stretches to show PSFs of A and B components).
Top Right - Keck 1.6$\mu$m AO image, 22 months later, revealing
the primary as two equal magnitude components separated by 60mas.
Bottom Left - Combined NICMOS TA image and radial profile of
A component shows no evidence of duplicity.
Bottom right - PSF core of A component fell at different
sub-pixel locations in the two rolled NICMOS TA images.
Difference of the two TA images (left), stretched to show
residuals from imperfect subtraction, bears no resemblance
to the difference image of two equal magnitude point sources
separated by 60mas (right).}
\end{figure}
%%%%%%%%%%%%%%%%%END FIGURE%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%FIGURE%%%%%%%%%%%%%%%
\begin{figure}[h]
\special{psfile=PROCEED_3.1b.eps voffset=-364 hoffset=40 vscale=49 hscale=49 angle=0}
\vspace{4.12truecm}
\caption[]{Left - Keck/LRIS I-band image of CD~-33$^{\circ}$ 7795,
self-subtracted after 180$^{\circ}$ image rotation shows
(positive and negative) images of B component.
Right - NICMOS H-band coronagraphic images 
CD~-33$^{\circ}$ 7795B, spacecraft target point rolled
around component A and subtracted. Inset - positive
and negative image conjugates from subtraction combined.}
\end{figure}
%%%%%%%%%%%%%%%%%END FIGURE%%%%%%%%%%%%%

CD -33$^{\circ}$ 7795A, an M1.5 dwarf (H=7.2) is a 
member of the member of TW Hydrae Association 
\cite{Kastner97}.  With an estimated age of 10 Myrs 
(\cite{Kastner97}; \cite{Soderblom98})
the primary exhibits X-ray and H-$\alpha$ emission, and strong Li 
absorption. CD~-33$^{\circ}$~7795 was a suggested spectroscopic binary, 
so chromospheric activity may be enhanced \cite{Barrado96}, 
but its space motions and Li absorption are consistent with 
other TW Hydrae association members.  Keck AO imaging resolved
the primary as a close ($\sim$ 0.06") binary with nearly-equal 
magnitude components (see Fig. 3).  The close binary orbit 
must be in a nearly edge-on, however, as it was unresolved at 
the time of the NICMOS observations.

CD -33$^{\circ}$ 7795B was the first brown dwarf companion 
candidate identified in the NICMOS survey \cite{Lowrance99}, 
and co-discovered by \cite{Webb99} in speckle observations taken
at the NASA IRTF. An H=12.14 magnitude companion to TWA 5A
($\Delta$H=4.92) was readily seen in the NICMOS F160W (1.6$\mu$m) 
coronagraphic difference image with a separation of 1.96"
($\sim$ 110 AU projected distance) at a PA of 358.9$^{\circ}$. 
Near-IR colors were measured from KeckII/LRIS acquisition
imaging, possible given the modest primary/secondary
contrast ratio and sufficiently large separation.
Differential astrometric measures from HST/NICMOS,
CFHT AO and Keck AO images over a baseline of 18 months
confirmed companionship for the secondary through common
proper motions (see Fig. 1). From the NICMOS and Keck 
photometry, and a 
bolometric correction of BC(H) = 2.8, we ascertained its 
luminosity as 0.0021 L$_{\odot}$ and its 
effective temperature as $\sim$ 2600K. 
From these measures we determined a photometric spectral 
classification of M8.5V ($\pm$~0.5 spectral class). These results
were confirmed by the STIS spectra (see Fig. 2), though a better
fit to the template spectra is obtained at M9V (still within the
estimated photometric uncertainty). Recently \cite{Neuhauser00}
obtained lower resolution spectra using FORS2 on the VLT
which are in agreement with these results.

\subsection{HR 7329}
HR 7329A, a member of the recently discovered Tucanae association
\cite{Zuckerman00} is of comparable distance 
(d = 47.61 $\pm$ 1.6pc as determined by the Hipparcos mission)
and age as CD~-33$^{\circ}$ 7795.  Though somewhat uncertain,
it is likely to be 10--30 Myr, based upon its location on a HR diagram, 
very large Vsini (330 km/sec; \cite{Abt95}), and
Tucanae association membership.  A brown dwarf companion 
candidate was identified with NICMOS, in close proximity to 
the H=5.05 A0V primary  at an angular 
distance of 4.17" \cite{Lowrance00}.  Again, assuming physical association 
for the unresolved object (196 AU projected distance), 
the H=11.90 companion candidate at PA of  166.8$^{\circ}$ was estimated 
to have a luminosity of 0.0026 L$_{\odot}$ 
(Habs = 8.54 with BC(H)=2.67).
%%%%%%%%%%%%%%%%%%%FIGURE%%%%%%%%%%%%%%%
\begin{figure}[h]
\special{psfile=PROCEED_3.2.eps voffset=-455 hoffset=20 vscale=60 hscale=60 angle=0}
\vspace{8.0truecm}
\caption[]{Top (a,b) - NICMOS coronagraphic images
of HR~7329A \& B.  In panel (b) the companion PSF is
partially off of the detector.  (c) - Difference image
(rolled 29.9$^{\circ}$ about primary component). (d) - Combined 
difference image conjugates. Inset (e) - Detailed 
recovery of roll combined PSF.}
\end{figure}
%%%%%%%%%%%%%%%%%END FIGURE%%%%%%%%%%%%%
\subsection{GL 577}
While outside of the NICMOS coronagraphic aperture, a faint 
``extended'' object was seen in two short NICMOS 
target acquisition images of the young star GL~577 \cite{Messina99}
(H=6.88, G5V) at a distance of 5.35" ($\sim$240 AU projected 
separation). Using the $\sim$�50x brighter image of GL~577A as a 
reference PSF, the "extended" object was nulled by two point sources 
differing by $<$15\% in H-band brightness separated by $\sim$0.1". 
The binary nature of this suspected companion was recently confirmed
with follow-up imagery obtained with the PHARO 
adaptive optics camera on the Palomar 200" telescope \cite{Lowrance00b}.
Companionship (thus coevality) for the 
GL~577B/C pair was established by their common proper motions with 
GL~577A over the $\sim$ 2 year baseline since the NICMOS observation.
%%%%%%%%%%%%%%%%%%%FIGURE%%%%%%%%%%%%%%%
\begin{figure}[h]
\special{psfile=PROCEED_3.3a.eps voffset=-435 hoffset=20 vscale=57 hscale=57 angle=0}
\vspace{8.9truecm}
\caption[]{Top  - NICMOS TA image of GL~577A~\&~B/C
(flux renormalized x50 for B/C companions on right
side of frame).  Bottom - BOTTOM: Palomar 200" AO image 
(bright spot below-right of primary is a filter ghost).}
\end{figure}
%%%%%%%%%%%%%%%%%END FIGURE%%%%%%%%%%%%%

A blended spectrum of GL~577B/C (see Fig. 7), recently obtained with 
STIS, resembles that of an M5.5V star. Differences in detail arising 
from the superposition of the individual spectra are noted and 
careful deblending of the spectra will be required to properly 
ascertain the spectral types and effective temperatures of the 
components. However, as both components are roughly equal in 
luminosity (derived from the spectral type, H magnitude and distance), 
and they each have effective spectral temperatures of $\sim$ 2900K.
%%%%%%%%%%%%%%%%%%%FIGURE%%%%%%%%%%%%%%%
\begin{figure}[h]
\special{psfile=PROCEED_3.3b.eps voffset=-420 hoffset=15 vscale=55 hscale=55 angle=0}
\vspace{9.1truecm}
\caption[]{Spectrum of GL~503.2B (top) and blended spectra
of the B/C companions to GL~577 (bottom) compared to those
of late M dwarfs (dotted lines).  Effective photospheric 
temperatures of $\sim$~3150K for GL~503.2 and $\sim$~2900K
for each component of $\sim$~2900K is found.}
\end{figure}
%%%%%%%%%%%%%%%%%END FIGURE%%%%%%%%%%%%%
\subsection{Mass Estimates}
Brown dwarfs cool as their luminosities decline with age, approximately 
as L~$\sim$~M$^{-2.24}$T$^{-1.3}$.  Hence, mass estimates depend
upon the efficacy of the age determinations, and of the details of 
cooling models themselves.  For older stars determining the ages of the 
more massive primaries becomes more difficult and uncertain.
Obtaining independent age estimates for the newly determined companions 
via, for example, Lithium abundance, will be a very important check and 
constraint for these mass estimates.  In all cases considered here we 
currently rely on the ages determinations of the primaries.  

Assuming coevality with their respective primaries, we 
place the companions on evolutionary tracks (e.g., Fig. 8) from 
\cite {Baraffe98}, \cite{Burrows98}, and \cite{Dantona97} to
estimate their masses.  For CD~-33$^{\circ}$~7795B we find a mass of 
$\sim$ 20 Jupiters, and for HR~7329B $\sim$ 40 Jupiters.  In the case of 
GL~577B and C, we assume a B/C bolometric luminosity ratio of at most 
1.15:1, implicating both as transitional objects, 
with one, and likely both brown dwarfs.
%%%%%%%%%%%%%%%%%%%FIGURE%%%%%%%%%%%%%%%
\begin{figure}[h]
\special{psfile=PROCEED_3.5.eps voffset=-460 hoffset=20 vscale=60 hscale=60 angle=0}
\vspace{8.1truecm}
\caption[]{Placement of companions on an H-R diagram
and in the context of evolutionary isochrones of low-mass stars
and substellar objects \cite{Baraffe98} based upon luminosities
derived from NICMOS imaging and effective
temperatures inferred from STIS spectra.}
\end{figure}
%%%%%%%%%%%%%%%%%END FIGURE%%%%%%%%%%%%%

\subsection{Low Mass, but Not Low Enough}

With an estimated age of 300 Myrs for the H=5.45 solar-like 
(G2V) UMa group primary GL~503.2A (n(Li)=2.56, EW=77\AA, 
vsini=25km/s; \cite{Soderblom93}), 
the $\sim$ M4.5V spectrum of its companion is indicative of a 
low mass star rather than a hot brown dwarf (Fig 7, top).  
Thus, we reject 
the possibility of the sub-stellar nature of this 1.56" distant 
companion (projected separation of 40 AU at d = 25.7 pc.) 
With an estimated effective temperature of 3150 $\pm$ 100K, we 
find the companion mass to be in the range of $\sim$ 
0.1 to 0.15 solar masses.

\section{Extrasolar Giant Planets?}

\subsection{Imaging}

The feasibility of imaging young Jovian-mass planets by 
NICMOS coronagraphy was demonstrated with the acquisition of 
a S/N=50 1.6$\mu$m image of a candidate extra-solar giant planet 
companion to ROSAT 116 (H=6.9, K7V), also a member of the TW 
Hydrae association (TWA~6). The H=20.1 putative companion seen in
Fig. 9 is 2.5"  ($\sim$ 140 AU projected distance) from
and 13.2 magnitudes fainter than TWA6. 
In a 0.9$\mu$m NICMOS camera 1 
observation the object was not seen to a limiting magnitude 
of $>$ 22, indicative of a very red source (if not a highly 
reddened background object). {\it If} the object is physically 
associated and coeval with ROSAT 116, its absolute H 
magnitude suggests an effective surface temperature of 
$\sim$ 800K (for a surface gravity of 
7.5x10$^{4}$ cm$^{2}$ s$^{-1}$) and a mass of 
$\sim$ 1 Jupiter based upon evolutionary models 
updated from \cite{Burrows98}.
%%%%%%%%%%%%%%%%%%%FIGURE%%%%%%%%%%%%%%%
\begin{figure}[h]
\special{psfile=PROCEED_4.1.eps voffset=-470 hoffset=60 vscale=60 hscale=60 angle=0}
\vspace{8.05truecm}
\caption[]{An EJP companion {\it candidate} to ROSAT 116 (TWA 6)
is readily seen in this NICMOS F160W coronagraphic image.  Though
the nature of the object is still to be determined, the viability
of direct detection of young EJPs via NICMOS differential
coronagraphy is demonstrated by this image.}
\end{figure}
%%%%%%%%%%%%%%%%%END FIGURE%%%%%%%%%%%%%

We are attempting to use the Keck AO system to establish (or 
reject) the physical association of this candidate with TWA 
6. Currently, with less than a two year baseline, the 
measurement errors in the relative proper motions are of the 
same order of magnitude as the proper motion of TWA 6 itself. 
Further astrometric calibration (and a longer temporal 
baseline) is required to improve the relative astrometry 
which, at the moment, is inconclusive.

\subsection{Spectroscopy}

Because of the extreme contrast ratios and close angular 
proximities of EJP companions, spectroscopy is much more of
a challenge than imaging.  We recently obtained a low 
resolution STIS spectrum of TWA 6`B' in six HST orbits. Not 
unexpectedly its faint spectrum is highly contaminated by the 
dispersed light of the much brighter primary, which is in 
close proximity to the slit. A ``spray" of scattered light in 
between the companion spectrum from those arising from the 
diffraction spikes of the primary is due to instrumental 
scattering and diffraction effects.  To  date  attempts at 
modeling  and removing the  background contamination have 
been only partially successful. Additional work is required 
to better understand the instrumental scattering and the 
limits imposed by the STIS optical system before a 
photometrically calibrated spectrum of the candidate object 
can be properly extracted.  Scattered light calibration 
observations are scheduled for late 2000.  Unfortunately, 
at this juncture, any interpretation would be premature.  

\section{Summary}
%%%%%%%%%%%%%%%%%%FIGURE%%%%%%%%%%%%%%%
\begin{figure}[h]
\special{psfile=PROCEED_4.0.eps voffset=-505 hoffset=-22 vscale=69 hscale=69 angle=0}
\vspace{6.9truecm}
\caption[]{With ages assumed from the characteristics of 
their primaries we estimate masses for the well-established 
companions observed spectroscopically with STIS (circles), 
based upon evolutionary cooling models for very low mass 
stars and substellar objects from \cite{Burrows98}.  The 
classification of ``transitional'' objects (such as GL~577B/C
and possibly GL~503.2B), and the characterization of all 
objects during their cooling phases depends critically upon 
their presumed ages.  Intrinsically fainter objects, such as the possible
companion to TWA 6 (diamond) may be observed into the planetary-mass
domain for objects of sufficient youth.}
\end{figure}
%%%%%%%%%%%%%%%%%END FIGURE%%%%%%%%%%%%%
The NICMOS coronagraphic survey, follow-up
ground-based AO imaging and STIS spectroscopy have given
rise to the discovery, confirmation, and preliminary
characterization of several very low-mass and sub-stellar 
companions.  Here we have reported on the first results
from this survey. Additional candidates identified by
NICMOS are being followed up in a similar manner.  By
assuming coevality with their primaries we can estimate
the masses of the companions through evolutionary cooling
models (e.g., see Fig. 10, adapted from \cite{Burrows98}).
Age determinations of the primaries, however, are somewhat
uncertain (particularly for the older stars in our sample).
Hence, independent determinations of the ages of the
companions (e.g., via Lithium abundance and H-$\alpha$
equivalent widths) is essential to better constrain the
nature and properties of the newly discovered companions.
%

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