% TEMPLATE FILE memsait.tex TO PREPARE CAMERA-READY PAPERS FOR
% MEMORIE DELLA SOCIETA` ASTRONOMICA ITALIANA.

% NEEDS THE FILE memsait.sty TO RUN.

% MODIFIED BY A. COMASTRI IN DECEMBER 1996 TO ALLOW INCLUSION OF
% FIGURES AND TABLES. NEEDS THE FILE epsf.sty TO RUN.

% USE LATEX COMMANDS WHILE FORMATTING YOUR TEXT. 
% COMPILE USING LATEX.

% PLEASE USE THIS TEMPLATE FILE AND THE ASSOCIATED memsait.sty FILE
% TO PREPARE YOUR CONTRIBUTION TO THE PROCEEDINGS OF CONFERENCES
% TO BE PUBLISHED IN MEMORIE S.A.IT

% Useful abbreviations for the bibliography

\def \SAIT #1 #2 {{\em Mem.\ Soc.\ Astron.\ It.\/} {\bf #1}, #2}
\def \MESS #1 #2 {{\em The Messenger\/} {\bf #1}, #2}
\def \ASTRNACH #1 #2 {{\em Astron. Nach.\/} {\bf #1}, #2}
\def \AAP #1 #2 {{\em Astron. Astrophys.\/} {\bf #1}, #2}
\def \AAL #1 #2 {{\em Astron. Astrophys. Lett.\/} {\bf #1}, L#2}
\def \AAR #1 #2 {{\em Astron. Astrophys. Rev.\/} {\bf #1}, #2}
\def \AAS #1 #2 {{\em Astron. Astrophys. Suppl. Ser.\/} {\bf #1}, #2}
\def \AJ #1 #2 {{\em Astron. J.\/} {\bf #1}, #2}
\def \ANNREV #1 #2 {{\em Ann. Rev. Astron. Astrophys.\/} {\bf #1}, #2}
\def \APJ #1 #2 {{\em Astrophys. J.\/} {\bf #1}, #2}
\def \APJL #1 #2 {{\em Astrophys.. J. Lett.\/} {\bf #1}, L#2}
\def \APJS #1 #2 {{\em Astrophys. J. Suppl.\/} {\bf #1}, #2}
\def \APSS #1 #2 {{\em Astrophys. Space Sci.\/} {\bf #1}, #2}
\def \ASR #1 #2 {{\em Adv. Space Res.\/} {\bf #1}, #2}
\def \BAIC #1 #2 {{\em Bull. Astron. Inst. Czechosl.\/} {\bf #1}, #2}
\def \JSQRT #1 #2 {{\em J. Quant. Spectrosc. Radiat. Transfer\/} {\bf #1}, #2}
\def \MN #1 #2 {{\em Mon. Not. R. Astr. Soc.\/} {\bf #1}, #2}
\def \MEM #1 #2 {{\em Mem. R. Astr. Soc.\/} {\bf #1}, #2}
\def \PLR #1 #2 {{\em Phys. Lett. Rev.\/} {\bf #1}, #2}
\def \PASJ #1 #2 {{\em Publ. Astron. Soc. Japan\/} {\bf #1}, #2}
\def \PASP #1 #2 {{\em Publ. Astr. Soc. Pacific\/} {\bf #1}, #2}
\def \NAT #1 #2 {{\em Nature\/} {\bf #1}, #2}
%
%

\documentstyle{memsait}
\input epsf.sty
\begin{opening}
\title{The Hubble Space Telescope at the beginning of a new millennium}
\author{NINO PANAGIA}
\institute{ESA/Space Telescope Science Institute,
Baltimore, MD 21218, USA; panagia@stsci.edu}
\date{} % DO NOT INSERT ANY DATE HERE !!!
\end{opening}


\begin{document}

%\oddpagefooter{\sf Mem. S.A.It., Vol. ??, ??}{}{\thepage}
%\evenpagefooter{\thepage}{}{\sf Mem. S.A.It., Vol. ??, ??}
\oddpagefooter{}{}{} % LEAVE AS IT IS !
\evenpagefooter{}{}{} % LEAVE AS IT IS !
\ 
\bigskip

%% MY own definitions
\def\hst{{\it HST}}
\def\ngst{{\it NGST}}
\newcommand{\kms}{$km~s^{-1}$ }
\newcommand{\eg}{{\it e.g., }} 
\newcommand{\ie}{{\it i.e.}} 
\newcommand{\etal}{{\it et al.}} 
\newcommand{\msun}{$M_\odot$ } 
\newcommand{\apj}{\APJ } 
\newcommand{\aap}{A\&A, }
\newcommand{\apjl}{\APJ } 
\newcommand{\aj}{AJ, } 
\newcommand{\mnras}{MNRAS, }


\begin{abstract}
I present and illustrate some of the most exciting and spectacular
\hst~ results of the last year,  such as(1) the study of solar system
planets, (2) the discovery and complete characterization of an
extra-solar planet, (3) the expansion of a hot bubble ejected by a
young star, (4) the history of formation and spatial distribution of a
bright cluster in the Large Magellanic Cloud,  (5) the repeated 
measurements of the collision of SN 1987A ejecta with its inner
circumstellar ring, that show a marked increase of high energy
interactions, (6) the possible detection of a dwarf galaxy at the edge
of the Universe, and (7) the study of type Ia SNe at redshifts close
to  unity and beyond, which provide clear evidence for an accelerated
Universe.
\end{abstract}

\section{Introduction }



The dawn of the 21st century finds the ``middle-aged" Hubble Space 
Telescope  (\hst) still playing a major role in astronomical research
as one of the most important and most  prolific  projects in progress.
Suffices  to say that the \hst~archive contains over 100,000
observations of more than 20,000 objects.

The Hubble Space Telescope (\hst) is a cooperative program of  the
National Aeronautics and Space Administration (NASA) and the European
Space Agency (ESA) to operate a long-lived space-based observatory for
the benefit of the international astronomical community. \hst~is an
observatory first conceived in the 1940s, designed and built in the
1970s and 80s, and become operational only in the 1990s. \hst~was
designed to be a different type of mission for NASA -- a long term
space-based observatory. To accomplish this goal and protect the
spacecraft against instrument and equipment failures, NASA had planned
on regular servicing missions.  A three year cycle of on-orbit
servicing was adopted. 

\hst~ was deployed in low-Earth orbit ($\sim 600 ~km$) by the crew of
the space shuttle Discovery (STS-31) on 25 April 1990. Everybody can
remember that at the beginning the mission looked severely faulted
because the \hst~ primary mirror was discovered to be spherically
aberrated. In addition, problems with the solar panels flexing as the
spacecraft passed from the Earth's shadow into sunlight caused problems
with the pointing stability. However, the first maintenance mission
(STS-61 Endeavour, December 1993), successfully fixed all of these
problems and fully restored the functionality of \hst. In fact, all
\hst~servicing missions so far, \ie~  December 1993,  February 1997 and
December 1999, were enormous successes. In particular, the last
refurbishing mission, 3A, which begun on 19 December 1999,  (see Fig.
1) and lasted 9 days, fully restored the operations of HST that had
come to a halt in mid-November 1999, when the failure of a gyroscope
left the spacecraft unable to maneuver with full capabilities. 

\hst's current science instruments include one camera, the Wide Field -
Planetary Camera 2 ({\it WFPC2}), one  spectrograph, the Space
Telescope Imaging Spectrograph ({\it STIS} ), and the Fine Guidance
Sensors ({\it FGS}, primarily used for astrometric observations).
Future servicing missions are planned for February 2002 (3B), which
includes  the installation of the Advanced Camera for Surveys ({\it
ACS}), a mechanical cooling system for {\it NICMOS}, and new solar
arrays, and  mid-2004 (4), to install the Cosmic Origins Spectrograph
({\it COS}, a high-resolution UV spectrograph) and the Wide Field
Camera Three ({\it WFC3}, with imaging capabilities extending from the
near-UV to the near-IR). Contingency flights could still be added to
the shuttle manifest to perform specific tasks that cannot wait for the
next regularly scheduled servicing mission (and/or required tasks that
were not completed on a given servicing mission). Because of the
extraordinary scientific results obtained so far and expected in the
years to come, the length of the {\it HST} mission has been extended
five more years with respect to the original 15-year design life, so
that {\it HST} will be operated until the year 2010. 

In the following section I shall present some of the telescope's most
compelling results obtained in the last year. 

\begin{figure} 
\centerline{\epsfxsize=10.cm\epsfbox{vulcano2001_fig1.ps}}
\caption{ Mars' images taken before (left) and during (right) the
recent global dust storm. [Credit: NASA]}
\end{figure}


\section{Solar System Planets }



\subsection{A global dust storm in Mars}

In our solar system, observations made with the  HST-WFPC2 (see Figure
1)  and Mars Global Surveyor (MGS) have revealed the biggest global
dust storm occurred on Mars in decades.  While \hst~ did not provide
continuous Mars coverage, its contribution was of crucial importance
thanks to  its capability of capturing the whole  planet in a single
snapshot, that allows one to study the full range of dust activity
from sunrise to sunset.


The combined  observations have shown that the storm is the result of a
planet-wide series of events that were triggered in the Hellas Basin
and propagated rapidly across the planet equator.  These extreme storm
events provide invaluable clues to how climate changes operate on Mars.


\subsection{A change of season in Saturn}

HST-WFPC2 images (Figure 2), obtained from 1996 to 2000 (Cuzzi \etal~
2000), show Saturn's rings open up from just past edge-on
to nearly fully open.   The first image in this sequence, on the lower
left, was taken soon after the autumnal equinox in Saturn's Northern
Hemisphere (the spring equinox in its Southern Hemisphere). By the
final image in the sequence, on the upper right, the tilt is nearing
its extreme, or winter solstice in the Northern Hemisphere (summer
solstice in the Southern Hemisphere).  The analysis of this set of
images is in progress (Poulet \etal~ 2000, and in preparation). The
study of the   detailed variations in the color and brightness of the
rings will help clarifying such problems as  what is the rings'
composition, how were they formed, and how long they might last.

\begin{figure} 
\centerline{\epsfxsize=8.cm\epsfbox{vulcano2001_fig2.ps}}
\caption{ Images of Saturn taken between 1996 (lower left) and 2000
(upper right). [Credit: NASA]}
\end{figure}


\section{Extrasolar Planets: A Giant Planet around HD209458}


The low mass companion to HD~209458 is the first extrasolar planet
found to transit the disk of its parent star (Charboneau  \etal~2000,  
Mazeh \etal~2000).

\begin{figure} 
\centerline{\epsfxsize=10.cm\epsfbox{vulcano2001_fig3.ps}}
\caption{ HD~209458 HST-STIS light curve provides evidence for a planet
transit (Brown \etal~2001)}
\end{figure}

The importance of studying transits of planets in front of their parent
star resides in the fact that one can directly determine the size of
the planet relative to the star.  Since the star radius is known at
least from its spectral type, one can obtain an accurate measure of the
planet absolute size. Moreover, from measurements of radial velocities
of the parent star, which trace the wobble of the star due to the
presence of the planet, one can unambiguously determine the planet
mass. Having the mass and the size of the orbiting planet, one can
derive the average density of the planet, which can put stringent
constraints to the theoretical models of planet formation.

HST-STIS observations of the HD 209458 system (see Figure 3)
indicated the presence of a companion planet with orbital period of
3.5247 days, a lower limit to its mass of $M_p=0.70\pm0.05~M_J$, a
planet radius $R_p=1.35\pm0.06~R_J$ and a separation of 0.00468~AU (
Brown \etal~2001. A detailed analysis of the transit light curve led to
the conclusion that the planet had no rings (which would have
appreciably altered the shape of the light curve) nor an orbiting
satellite with mass higher than 3$M_{Earth}$ (whose presence would have
affected the symmetry of the light curve).

On the other hand, spectrophotometric observations made at the time of
four transits revealed that the NaI line at 5889\AA~ absorption line
was becoming detectably stronger, \ie~$(2.32\pm0.57)\times 10^{-4}$,
providing clear evidence for the presence of a Na-rich atmosphere in
the planet (Charbonneau \etal~2001).  This is the first detection of an
atmosphere in an extrasolar planet that marks the beginning of a new
and exciting era in which extrasolar planets begin being fully
characterized instead of being merely detected gravitationally as point
sources with mysterious properties.

\section{Tracing an Expanding ``Bubble" around XZ Tauri}

\begin{figure} 
\centerline{\epsfxsize=10.cm\epsfbox{vulcano2001_fig4.ps}}
\caption{ The XZ Tau ``bubble" expands from 1995 (left) to 2000 (right). 
[Credit: NASA]}
\end{figure}

XZ Tauri is a young binary system in the Taurus-Auriga Complex, which
consists of a M3V T Tauri star and a cooler ($T_{eff}\simeq1900~K$)
companion.  HST-WFPC2 observations made in January 1995 had discovered
the presence of a bubble-like structure of nebular emission, which was
extending 4" (\ie~approximately 600~AU at the distance of about 150~pc
of the Taurus-Auriga complex) in the direction of a previously known
stellar jet (Krist \etal~ 1997).

Additional observations made in March 1998, February 1999, and February
2000 have shown that the ``bubble" has dramatically expanded (see
Figure 4), with a longitudinal velocity of $145~km~s^{-1}$, a
transverse velocity of  $30~km~s^{-1}$, which imply a dynamical age of
about 23 years (Krist \etal~1999, 2001).

Still it is not clear if this event represents a discrete eruption of
hot gas or rather it is the result of a back-boosting provided to
pre-existing material by the collimated jet.  Future observations of
proper motions of a number of  bubble sub-structure will be able to
discern between the two possibilities. 


\section{Young Stellar Populations in the Large Magellanic Cloud: NGC~1850}


\begin{figure} 
\centerline{\epsfxsize=7.cm\epsfbox{vulcano2001_fig5.ps}}
\caption{ The double cluster NGC~1850: component A displays a
globular-like structure (center) whereas component B is centered on a
compact group of O-type stars (lower center) (Romaniello \etal~2002a). }
\end{figure}

\begin{figure} 
\centerline{\epsfxsize=9.cm\epsfbox{vulcano2001_fig6.ps}}
\caption{ The spatial distribution of O-type stars (crosses), Be-Ae
stars (diamonds) and T Tauri stars (open circles)(Romaniello \etal~2002a).}
\end{figure}


NGC~1850 is the second brightest stellar cluster in the Large
Magellanic Cloud after the 30 Doradus complex.  It consists of two
distinct sub-entities (see Figure 5), \ie~$(a)$ NGC~1850A which has a
globular-like appearance, has an age of about 50~Myrs and a rather flat
Initial Mass Function (IMF, with $\Gamma\simeq0.4$), and $(b)$
NGC~1850B which a compact  core of O-type stars plus a spatially more 
diffuse stellar population, is much younger
($\sim4~$Myrs) and has a more ``normal" IMF with $\Gamma\simeq1.6$
(Gilmozzi \etal~1994, ApJ 435, L43).  

Their study provides invaluable clues to  star formation and evolution.
For example, supernova explosions of NGC~1850A massive stars appear to
have triggered star formation in the younger NGC~1850B. HST-WFPC2
observations obtained with a narrow-band H$\alpha$ filter have allowed
the detection of a large number of stars with considerable H$\alpha$
emission (W$_{eq}>$10\AA), \ie~about 100 Be-Ae stars and about 300 T
Tauri stars, out of a total of about 14,000 stars measured in the field
down to $M_V\simeq5$ (Romaniello \etal~2002a). While the
Be-Ae stars are strongly concentrated in and around NGC~1850A, the 
Tauri stars are more dispersed over the field.  

It is important to note that T Tau stars display an considerably higher
density toward the strong nebulosity (see Fig. 5 and 6) and their
spatial distribution appears to be anti-correlated with the
distribution of O-type stars.  Since T Tau stars are essentially coeval
with O-type stars, this result provides evidence for different
formation processes operating for high-mass and low-mass stars.  A
corollary of this is that any IMF  measured in star forming regions can
only be meaningful in a statistical sense, \ie~when averaged over a
suitably large volume and a suitably large number of stars.  Similar
results are found in other LMC and SMC regions (\eg SN~1987A field:
Panagia \etal~2000; selected LMC and SMC  clusters: Romaniello
\etal~2001, 2002b), which indicate that the spatial separation of birth
of low mass stars relative  to more massive ones is a general feature
of star formation.  

A simple explanation would be that the strong radiation field from
massive stars raises the average temperature  and, therefore, the Jeans
mass in the vicinities of their birth places.  As a consequence the
formation rate of low mass stars is strongly reduced within some
distance from massive stars, while it is highest in those portions of a
parent cloud where both the density is high and the radiation from
massive stars is efficiently shielded off.


\section{Fireworks in SN 1987A equatorial ring}

\begin{figure} 
\centerline{\epsfxsize=9.cm\epsfbox{vulcano2001_fig7.ps}}
\caption{ Series of images of SN 1987A and its inner circumstellar ring
obtained with HST-WFPC2 between October 1994 and May 2001. It appears
that the quiescent ring has developed at least nine hot spots in the
last five years. [Courtesy of Peter Challis (CfA), on behalf of the
SINS project (Supernova INtensive Study, PI: R.P Kirshner)]}
\end{figure}

 
Supernova 1987A exploded on 1987 February 23 in the Large Magellanic
Cloud. Although \hst~ was not yet in orbit when this rare chance to
observe a nearby supernova appeared, it took advantage of the
opportunity as soon as it became operational. The European Space
Agency's Faint Object Camera took the first images of SN 1987A on 1990
August 23-24, which revealed the supernova crowned by a glorious
circumstellar ring. Later, two more rings were discovered.

\hst~ has kept an attentive eye on SN 1987A, and its observations have
produced many fundamental results, including a direct measure of the
supernova expansion, the detailed properties of its surrounding rings,
the distance to the supernova, and the origin of the stars associated
with the supernova.

Recently, Hubble has observed the high-velocity material from the
supernova explosion starting to overtake and crash into the
slower-moving inner ring. Figure 7 shows the dramatic evidence of these
collisions. The circumstellar ring, which until 1996 was relatively
quiescent, started to develop bright spots in 1997, and  in May 2001
one can identify almost a dozen bright spots. These bright spots are
the result of the fast moving component of the ejecta colliding with
the stationary equatorial ring. The fastest debris, moving at 15,000
\kms or 1/20 the speed of light, are now colliding with the slower
moving gas which was ejected when the progenitor was a red supergiant
(\eg Sonneborn \etal~1998).  

Over the next decades, as ``slower" ejecta layers (keeping in mind that
the ``slow" moving layers have velocities of several thousand kilometers
per second!) reach the ring, more and more spots will light up and the
whole ring will shine as it did in the first several months after
explosion.  Eventually, the bulk of the ejecta will completely sweep
the ring up, clearing the circumstellar space of that remnant of the
pre-supernova wind activity. 


\section{A Distant Dwarf Galaxy: A building block at z$\sim6$?}




Combined HST-WFPC2 and Keck observations have revealed the most distant
dwarf galaxy ever found. It may represent a galaxy building block, i.e.
one of those dwarf galaxies which are expected to form first at the
infancy of the Universe and whose merging may lead to the formation of
more massive galaxies.

Gravitationally lensed by the Abell 2218 galaxy cluster, with an
estimated magnification of a factor of $\sim33$, the small red galaxy
(see Figure 8) was discovered in  archival images of the galaxy cluster
and recognized by the double image that is a characteristic of strong
gravitational lensing (Figure 8, upper right panel).  Spectroscopy
obtained with the Keck telescope has shown a spectrum characterized by
a very weak continuum and one strong, clearly asymmetrical line, which
has been identified as the Ly-$\alpha$ line.  On this basis the galaxy
redshift has been measured to be $z\simeq5.6$.  Also, the strength of
the Ly-$\alpha$ line is indicative of the presence a very young stellar
population, with an age less than few Myrs and a stellar mass of about
$10^6~M_\odot$ (Ellis \etal~ 2001). 

\begin{figure} 
\centerline{\epsfxsize=10.cm\epsfbox{vulcano2001_fig8.ps}}
\caption{ A distant dwarf galaxy gravitationally lensed by the 
Abell 2218 galaxy cluster [Credit: NASA}
\end{figure}

\section{Type Ia Supernovae (SNIa) and Cosmology}

\subsection{SNIa Standard Candles and the Expansion of the Universe}

Since supernovae are very bright objects and are relatively common (in
an ``average cluster of galaxies" with, say, $10^{13}$~M$_\odot$ in
stars, one may expect several SN explosions to occur per year), they
constitute prime candidates to probe distances to galaxies. 

That SNIa could be used as standard candles has been proposed for many
years. However, most of the progress in this field has occurred over
the last decade. Extensive ground-based surveys have identified a large
number of new supernovae and characterized their global properties in a
statistically meaningful way.  At the same time, using the Hubble Space
Telescope, a team led by Sandage  has carried out an extensive program
to determine the absolute brightness of a selected sample of supernovae
(\eg Saha \etal~2001 and references therein). This has allowed to place
the Hubble diagram for SNIa on an absolute scale(\eg Hamuy \etal~ 1996,
Riess \etal~ 1999,  Parodi \etal~2000)  and the Hubble constant itself
to be determined with a precision of 10\% or better. In particular,
they show that the best values are $M_B=-19.50\pm0.06$  and
$M_V=-19.49\pm0.06$. The derived value for the Hubble constant is $H_0
=59\pm6~km~s^{-1}~Mpc^{-1}$. This implies possible ages of the Universe
in the range 11-17 $Gyrs$, depending of the acceleration/deceleration
history of the Universe itself. 

\subsection{Clues to the acceleration of the Universe} 

Once the expansion rate is determined, the next step is to determine
its variation in the course of the evolution of the Universe, \ie~ the
derivative of its expansion rate. It is interesting to note that, since
one wants to measure the derivative of the expansion rate, it is only
necessary to verify that SNe Ia are indeed standard candles, \ie~they
have the same properties at all redshifts (or, equivalently, at all
look-back times), with no need to determine their absolute 
luminosities.  In other words, one could measure the deceleration
parameters without measuring $H_0$, and viceversa. 


\begin{figure} 
\centerline{\epsfxsize=10.cm\epsfbox{vulcano2001_fig9.ps}}
\caption{ The Hubble diagram for SNIa up to z$\sim0.8$ (Perlmutter \etal
1999).}
\end{figure}


The acceleration of the Universe can be determined using SNIa as
standard candles, if these are observed at suitably large distances in
order to reveal a measurable deviation from a constant expansion.
Moreover, Goobar and Perlmutter (1995) showed the possibility of
separating the relative contributions of the mass density, $\Omega_M$,
and the cosmological constant, $\Lambda$ (or, equivalently, its
dimensionless parameter $\Omega_\Lambda=\Lambda/3H_0^2$), to changes
in the expansion rate by studying supernovae at a range of redshifts. 

Pioneering work by Danish astronomers (N{\o}rgaard-Nielsen \etal~ 1989)
led to the discovery of one SNIa at $z=0.31$ as the result of a
multi-year observational effort. It was only when Perlmutter's
Supernova Cosmology Project (SCP) took off that SNIa searches at high
redshifts became an efficient reality. Thanks to their use of large
format CCDs, a ``clever" observational strategy, and sophisticated
image analysis techniques, in 1992 they discovered their first high
redshift SNIa, SN~1992bi at $z=0.46$ (Perlmutter \etal~ 1995), followed
in 1994 by 6 more at $z>0.35$ (Perlmutter \etal~ 1997a). Currently they
discover about a dozen SNIa during each observing run, typically
twice a year (\eg Perlmutter \etal~ 1997b). Friendly competition promptly
followed suit, when the High-Z Supernova Search Team (Schmidt \etal~
1998) started their systematic searches in 1995, essentially adopting
Perlmutter's strategy and, consistently, also discovering a dozen
supernova candidates per run. By mid-1998 the SCP team has discovered
and studied about 78 SNIa, most of which in the redshift range
0.2--0.8, and the HZSS team has discovered about 32 SNIa. 

In the last few years, both groups have augmented their SNIa samples
and, for selected SNIa, have complemented their ground-based
observations with higher quality photometry obtained with the {\it
HST--WFPC2\/}.  The obvious improvement provided by the superior
angular resolution of \hst~ is that contamination by the host galaxy
light is significantly reduced and becomes an easily treatable problem,
thus providing higher photometric accuracy. 

\begin{figure} 
\centerline{\epsfxsize=8.cm\epsfbox{vulcano2001_fig10.ps}}
\caption{ Constraints to the matter density, $\Omega_M$, and the vacuum
energy density, $\Omega_\Lambda$, as derived from 42 SNIa observations
up to z$\sim$0.8. Also shown are the constraints derived from CMB
fluctuations (De Bernardis \etal~ 2000) and  galaxy cluster dynamics
(Bahcall \& Fan 1998) .}
\end{figure}

The results for a first subset (42 out of 78) of the SCP sample of SNIa
was presented and discussed by Perlmutter \etal~ (1999). The Hubble
diagram for these supernovae as well as for the sample of lower $z$
SNIa from the Cal\'an/Tololo survey (Hamuy \etal~ 1996) is shown in
Figure~9. The deviation of the high $z$ points from a constant
expansion rate, which had already been noticed with  a more  limited
sample (Perlmutter \etal~1998), is now clearly apparent. The best fit
confidence regions in the $\Omega_M$--$\Omega_\Lambda$ plane (Fig.~10)
show that neither an empty Universe ($\Omega_M=0$,
~$\Omega_\Lambda=0$), nor a non-inflationary Universe
($\Omega_\Lambda=0$) are consistent with the data, but rather the
best-fit solution lies along the line $1.34\Omega_M -
\Omega_\Lambda=-0.45$.  For a flat, $\Omega_M + \Omega_\Lambda=1$,
cosmology this corresponds to $\Omega_M^{flat} = 0.24\pm0.08
(statistical)^{+0.06}_{-0.03} (systematic)$. An open, $\Lambda=0$
cosmology also does not fit the data well: the data strongly suggest
that the cosmological constant is non-zero and positive, with a
confidence level of $P(\Lambda>0)=99.1$\%, including the systematic
uncertainty. Similar results, but still with a somewhat larger
uncertainty,  have also been found by the HZSS team from the analysis
of 10 of their high $z$ SNIa (Riess \etal~ 1998). 


\begin{figure} 
\centerline{\epsfxsize=9.cm\epsfbox{vulcano2001_fig11.ps}}
\caption{SN~1997ff discovery: a section of the HDF-N (upper), the
supernova host galaxy (lower left) and the supernova itself (lower
right)}.
\end{figure}

These results are rather unexpected and puzzling: for example, the
fact that the cosmological constant value is comparable to the current
mass-energy density (which varies as $[1+z]^3$) would place us at a
``special" time in the evolution of the Universe. It is clear that the
problem is far from solved, but {\it can\/} be solved: on the one
hand, one needs to study more SNIa, over a wider range of redshifts
to reduce the uncertainty region and to test for the presence of
possibly ``unseen" systematic effects, e.g., evolution of the SN~Ia
properties with redshift and/or metallicity. On the other hand, one
needs to find constraints on the cosmological parameters which are
``orthogonal" to the ones derived from SNIa observations: a promising
possibility is provided by the forthcoming balloon and satellite
studies of the Cosmic Background Radiation which should provide an
excellent measurement of the sum ($\Omega_M+\Omega_\Lambda$).



Meanwhile the SNIa constraints on  $\Omega_M$ and $\Omega_\Lambda$ have
been tighten up  by the discovery and the study of SNIa at redshifts
higher than 1.  The most distant SNIa found so far is SN~1997ff. The
supernova was discovered by Gilliland and Phillips (1998, IAUC~6810;
see also Gilliland \etal~1999) in a repeat HST-WFPC2 observation of the
Hubble Deep Field North (HDF-N) and was serendipitously monitored with
HST-NICMOS  throughout the Thompson \etal~ Guaranteed-Time Observer 
campaign (see Figure 11).


The photometric record of SN~1997ff includes a dozen flux measurements
in the I, J, and H bands spanning 35 days. Riess \etal~ (2001) 
determined the supernova type and redshift from the observed colors
and their temporal behavior, both of which match a typical SN Ia at
redshift z$\sim$1.7. This makes SN 1997ff the farthest supernova
observed to date.  Fits to observations of the SN provide constraints
for the redshift-distance relation of SNe Ia that lend strong support
to the currently preferred cosmological model, $\Omega_M\sim0.3$,
$\Omega_\Lambda\sim0.7$. 

The results obtained for four additional high redshift SNIa, namely
SN~1998eq (z$\simeq1.2$; Perlmutter \etal~2002), SNe 1999fm, 1999fk
and  1999fv (z$\simeq0.94, 1.05, 1.17$, respectively; Tonry \etal~2002)
provide strong support to the cosmological interpretation (see Figure
12)  while excluding conclusively  grey dust absorption and/or
evolutionary effects, which could explain the dimming (relative to the
expections for an empty Universe) of SNIa  up to z$\sim0.5$, but
definitely not the  brightening as found for higher z SNIa. 


\begin{figure} 
\centerline{\epsfxsize=12.cm\epsfbox{vulcano2001_fig12.ps}}
\caption{ Differences in magnitude between SNIa observations and
expections for an  empty Universe. High redshift supernovae exclude
grey dust and/or evolutionary effects to explain the SNIa data and
strongly favor the cosmological interpretation with $\Omega_M\sim0.3$,
$\Omega_\Lambda\sim0.7$. [Courtesy Adam Riess]}
\end{figure}


% \acknowledgements
% Type here the acknowledgements.

% References. We avoided using the \bibitem commmand since we found it is
% somewhat platform-dependent. We also avoided using the \cite{keyword}
% command since we found it cumbersome. However, if you are an expert 
% LateX user you may use the various LateX tools for the references 
% provided they give the same printout formats of the examples given here.

\begin{thebibliography}{} 
% 
\item  Bahcall, N.A., \& Fan, X., 1998, \APJ 504 1
%
\item  Brown, T.M., \etal, 2001, \APJ 552 699 
%
\item  Charboneau, D.,  \etal, 2000, \APJ 529 L45
%
\item  Charbonneau, D., \etal~2001, in preparation. 
%
\item  Cuzzi, J.N., \etal,~2000, DPS, 32, 4805
%
\item  De Bernardis, P. \etal, 2000, Nature, 404, 955.
%
\item  Ellis, R. \etal,~ 2002, {\em Astrophys. J.\/}, in press [astro-ph/0109249].
%
\item  Gilliland, R.L., \&  Phillips, M.M., 1998, IAUC~6810.
%
\item  Gilliland, R.L., Nugent, P., \&  Phillips, M.M., 1999, \APJ 521 30
%
\item  Gilmozzi, R.,  \etal,~1994, \APJ 435 L43
%
\item  Goobar, A., \& Perlmutter, S., 1995, \APJ 450 14
%
\item  Hamuy, M., Phillips, M.M, Suntzeff, N.B., Schommer, R.A., Maza, 
J., \& Aviles, R., 1996, \AJ 112 2391
%
\item  Krist, J. E., \etal, 1997, \APJ 481 447 
%
\item  Krist, J.E., \etal,~1999, \APJ 515 L35
%
\item  Krist, J.E., \etal,~2001, in preparation.
%
\item  Mazeh, T., \etal,~2000, \APJ 532 L55
%
\item  N{\o}rgaard-Nielsen, H.U., \etal, 1989, {\em Nature\/}, 339, 523.
%
\item  Panagia, N., \etal,~2000, \APJ 539 197
%
\item  Parodi, B.R., Saha, A., Sandage, A., Tammann, G.A., 2000, \APJ
540, 634
%
\item  Perlmutter, S., \etal, 1995, \APJ 440 41 
%
\item  Perlmutter, S., \etal, 1997a, \APJ 483 565
%
\item  Perlmutter, S., \etal, 1997b, in {\it Thermonuclear 
Supernovae}, eds. P. Ruiz-Lapuente, R. Canal \& J. Isern, Dordrecht: 
Kluver, p. 749.
%
\item  Perlmutter, S., \etal, 1998,  {\em Nature\/}, 391, 51.
%
\item  Perlmutter, S., \etal, 1999,  \APJ 517 565
%
\item  Perlmutter, S., \etal, 2002,  in preparation.
%
\item  Poulet, F.,  \etal,~2000, DPS, 32, 4901.
%
\item  Riess, A.G., \etal, 1998, \AJ 116 1009 
%
\item  Riess, A.G., \etal, 1999, \AJ 117 707 
%
\item  Riess, A.G., \etal, 2001, \APJ 560 49
%
\item  Romaniello, M., Panagia, N., Scuderi, S., Gilmozzi, R.,
Tolstoy, E., Favata, F., \& Kirshner, R.P., : in Proceedings ESO
Workshop ``The Origin of Stars and Planets: The VLT View", 2001, 
eds. Joao Alves and Mark McCaughrean, ESO, in press. 
%
\item  Romaniello \etal~2002a, {\em Astrophys. J.\/}, submitted.
%
\item  Romaniello \etal~2002b, in preparation
%
\item  Saha, A., Sandage, A., Tammann, G.A., Dolphin, A.E.,
  Christensen, J., Panagia, N., \& Macchetto, F.D., 2001, \APJ 562 314
%
\item  Schmidt, B.P., \etal, 1998, \APJ 507 46
%
\item Sonneborn, G.,  \etal,~1998, \APJ 492 139

\item Tonry, J.L., \etal, 2002, in preparation.

% IT IS VERY IMPORTANT TO LEAVE THE EMPTY LINE ABOVE AT THE END OF YOUR
% REFERENCE LIST !!!
\end{thebibliography}



\end{document}
