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%astro-ph/0711.0657

\begin{document}

   \title{The most massive stars in the Arches cluster \thanks{Based on 
observations collected at the ESO Very Large Telescope (program 
075.D-0736(A))}}

   \subtitle{}

   \author{F. Martins\inst{1}
          \and
          D.J. Hillier\inst{2}
          \and
          T. Paumard\inst{3}
          \and
          F. Eisenhauer\inst{1}
          \and
          T. Ott\inst{1}
          \and
          R. Genzel\inst{1,4}
          }

   \offprints{F. Martins}

   \institute{Max-Planck Instit$\ddot{\rm u}$t f$\ddot{\rm u}$r 
extraterrestrische Physik, Postfach-
1312, D-85741, Garching bei M$\ddot{\rm u}$nchen, Germany \\
              \email{martins@mpe.mpg.de}
         \and
             Department of Physics and Astronomy, University of 
Pittsburgh, 3941 O'Hara St., Pittsburgh, PA 15260, USA\\
         \and
             LESIA, Observatoire de Paris, CNRS, UPMC, Universit\'e 
Paris Direrot, 5 Place Jules Janssen, F-92195, Meudon CEDEX, France\\
         \and
             Department of Physics, University of California, CA 94720, 
Berkeley, USA\\
             }

   \date{Received 13 August 2007; accepted 5 November 2007}

  \abstract
   {}
   {We study a sample composed of 28 of the brightest stars in the 
Arches cluster. Our aim is to constrain their stellar and wind 
properties and to establish th
eir nature and evolutionary status.}
   {We analyze K-band spectra obtained with the integral field 
spectrograph SINFONI on the VLT. Atmosphere models computed with the 
code CMFGEN are used to deri
ve the effective temperatures, luminosities, stellar abundances, mass 
loss rates and wind terminal velocities.}
   {We find that the stars in our sample are either H-rich WN7--9 stars 
(WN7--9h) or supergiants, where two are classified as OIf$^{+}$. All 
stars are 2--4 Myr
old. There is marginal evidence for a younger age among the most massive 
stars. The WN7--9h stars reach luminosities as high as $2 \times 10^{6} 
L_{\odot}$, con
sistent with initial masses of $\sim$ 120 \msun. They are still quite 
H-rich, but show both N enhancement and C depletion. They are thus 
identified as core H-bu
rning objects showing products of the CNO equilibrium on their surface. 
Their progenitors are most likely supergiants of spectral types earlier 
than O4--6 and i
nitial masses $>$ 60 \msun. Their winds follow a well-defined modified 
wind momentum -- luminosity relation (WLR): this is a strong indication 
that they are rad
iatively driven. Stellar abundances tend to favor a slightly super-solar 
metallicity, at least for the lightest metals. We note, however, that 
the evolutionary
models seem to under-predict the degree of N enrichment. }