We present near-infrared photometry and K-band spectra of newly-identified massive stars in the Quintuplet Cluster, one of the three massive clusters projected within 50 pc of the Galactic Center. We find that the cluster contains a variety of massive stars, including more unambiguously identified Wolf-Rayet stars than any cluster in the Galaxy, and over a dozen stars in earlier stages of evolution, i.e., LBV, Ofpe/WN9, and OB supergiants. One newly identified star is the second ``Luminous Blue Variable'' in the cluster, after the ``Pistol Star.'' Given the evolutionary stages of the identified stars, the cluster appears to be about 4 \pm 1 Myr old, assuming coeval formation. The total mass in observed stars is $\sim 10^3 \Msun$, and the implied mass is $\sim 10^4 \Msun$, assuming a lower mass cutoff of 1 \Msun and a Salpeter initial mass function. The implied mass density in stars is at least a few thousand $\Msun pc^{-3}$. The newly-identified stars increase the estimated ionizing flux from this cluster by about an order of magnitude with respect to earlier estimates, to 10^{50.9} photons/s, or roughly what is required to ionize the nearby ``Sickle'' HII region (G0.18 - 0.04). The total luminosity from the massive cluster stars is $\approx 10^{7.5}$ \Lsun, enough to account for the heating of the nearby molecular cloud, M0.20 - 0.033. We propose a picture which integrates most of the major features in this part of the sky, excepting the non-thermal filaments. We compare the cluster to other young massive clusters and globular clusters, finding that it is unique in stellar content and age, except, perhaps, for the young cluster in the central parsec of the Galaxy. In addition, we find that the cluster is comparable to small ``super star clusters.'

We report Hubble Space Telescope (HST) Near-infrared Camera and Multiobject Spectrometer (NICMOS) observations of the Arches and Quintuplet clusters, two extraordinary young clusters near the Galactic Center. For the first time, we have identified main sequence stars in the Galactic Center with initial masses well below 10 M ⊙ . We present the first determination of the initial mass function (IMF) for any population in the Galactic Center, finding an IMF slope which is significantly more positive (Γ ≈ −0.65) than the average for young clusters elsewhere in the Galaxy (Γ ≈ −1.4). The apparent turnoffs in the color-magnitude diagrams suggest cluster ages which are consistent with the ages implied by the mixture of spectral types in the clusters; we find τ age ∼ 2±1 Myr for the Arches cluster, and τ age ∼ 4±1 Myr for the Quintuplet. We estimate total cluster masses by adding the masses of observed stars down to the 50% completeness limit, and then extrapolating down to a lower mass cutoff of 1 M ⊙ . Using this method, we find ∼ >10 4 M ⊙ for the total mass of the Arches cluster. Such a determination for the Quintuplet cluster is complicated by the double-valued mass-magnitude relationship for clusters with ages ∼ > 3 Myr. We find a lower limit of 6300 M ⊙ for the total cluster mass, and suggest a best -2estimate of twice this value which accounts for the outlying members of the cluster. Both clusters have masses which place them as the two most massive clusters in the Galaxy.

We present and use new spectra and narrow-band images, along with previously published broad-band images, of stars in the Arches cluster to extract photometry, astrometry, equivalent width, and velocity information. The data are interpreted with a wind/atmosphere code to determine stellar temperatures, luminosities, mass-loss rates, and abundances. We have doubled the number of known emission-line stars, and we have also made the first spectroscopic identification of the main sequence for any population in the Galactic Center. We conclude that the most massive stars are bona-fide Wolf-Rayet (WR) stars and are some of the most massive stars known, having M_{initial} > 100 Msun, and prodigious winds, Mdot > 10^{-5} Msun yr^{-1}, that are enriched with helium and nitrogen; with these identifications, the Arches cluster contains about 5% of all known WR stars in the Galaxy. We find an upper limit to the velocity dispersion of 22 kms^{-1}, implying an upper limit to the cluster mass of 7(10^4) Msun within a radius of 0.23 pc; we also estimate the bulk heliocentric velocity of the cluster to be v_{cluster,odot} approximately +95 kms^{-1}

There is no accepted upper mass limit for stars. Such a basic quantity eludes both theory and observation, because of an imperfect understanding of the star-formation process and because of incompleteness in surveying the Galaxy. The Arches cluster is ideal for investigating such limits, being large enough to expect stars at least as massive as approximately 500 solar masses (approximately 500 Mo; based on a typical mass function), and young enough for its most massive members to still be visible. It is also old enough to be free of its natal molecular cloud, it is at a well-established distance, and it is close enough for us to discern its individual stars. Here I report an absence of stars with initial masses greater than 130 Mo in the Arches cluster, whereas the typical mass function predicts 18. I conclude that this indicates a firm limit of 150 Mo for stars; the probability that the observations are consistent with there being no upper limit is 10(-8).

We report the discovery of an extraordinarily massive young cluster of stars in the Galaxy, having an inferred total initial cluster mass comparable to the most massive young clusters in the Galaxy. Using {\it IRMOS}, {\it 2MASS}, and {\it Spitzer} observations, we conclude that there are 14 red supergiants in the cluster, compared with five, in what was previously thought to be the richest Galactic cluster of such stars. We infer spectral types from near-infrared spectra that reveal deep CO bandhead absorption that can only be fit by red supergiants. We identify a gap of $\Delta${\it K}$_s$$\sim$4 magnitudes between the stars and the bulk of the other stars in the region that can only be fit by models if the brightest stars in the cluster are red supergiants. We estimate a distance of 5.8~\kpc to the cluster by associating an OH maser with the envelope of one of the stars. We also identify a ``yellow'' supergiant of G6~I type in the cluster. Assuming a Salpeter IMF, we infer an initial cluster mass of 20,000 to 40,000~\Msun for cluster ages of 7-12~\Myr. Continuing with these assumptions, we find 80% of the intial mass and 99% of the number of stars remain at the present time. We associate the cluster with an x-ray source (detected by {\it ASCA} and {\it Einstein}), a recently discovered very high energy $\gamma$-ray source (detected by {\it INTEGRAL} and {\it HESS}), and several non-thermal radio sources, finding that these objects are likely related to recent supernovae in the cluster. In particular, we claim that the cluster has produced at least one recent supernova remnant with properties similar to the Crab nebula. It is not unlikely to find such a source in this cluster, given our estimated supernova rate of one per 40,000 to 80,000~{\it yr}.Comment: ApJ, accepte

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We present new near-infrared data and analysis, which indicate that the Pistol Star is one of the most luminous stars known, adding another test point for massive star formation and stellar evolution theories. We estimate an extinction of using the near-infrared colors of the star and of sur-A K \ 3.2^0.5 rounding stars in the young Quintuplet cluster. Using our wind/atmosphere code, we Ðnd two families of models that Ðt the spectral energy distribution and detailed line proÐles. The lower luminosity models give L \ 106.6B0.2 and K, while the higher luminosity models give L \ 107.2B0.2 L _ T eff \ 104.15B0.01 and K ; the error in luminosity assumes an uncertainty of^0.5 in while theis constrained by detailed line modeling. The models also reveal a helium enriched surface. T eff As previously existing stellar evolution models do not extend to such high luminosities, we employ new evolutionary tracks for very massive stars to determine the initial mass and age of the Pistol Star, and estimate and an age of 1.7È2.1 Myr. The inferred luminosity and temperature M initial \ 200È250 M _ place the star in a sparsely populated zone in the H-R diagram where luminous blue variables (LBVs) are often found. This is consistent with our evolutionary models, which predict that the star is in an unstable evolutionary stage. We interpret the star and its surrounding nebula as an LBV that has recently ejected large amounts of material. Our K-band speckle-imaging data reveal the star to be single down to a projected separation of 110 AU.

Abstract.We have analysed high resolution adaptive optics (AO) science demonstration data of the young, massive stellar cluster Arches near the Galactic Center, obtained with the Gemini North telescope in combination with the University of Hawai'i AO system Hokupa'a. The AO H and K photometry is calibrated using HST/NICMOS observations in the equivalent filters F160W and F205W obtained by Figer et al. (1999). The calibration procedure allows a detailed comparison of the ground-based adaptive optics observations against diffraction limited space-based photometry. The spatial resolution as well as the overall signal-to-noise ratio of the Gemini/Hokupa'a data is comparable to the HST/NICMOS data. The low Strehl ratio of only a few percent is the dominant limiting factor in the Gemini AO science demonstration data as opposed to space-based observations. After a thorough technical comparison, the Gemini and HST data are used in combination to study the spatial distribution of stellar masses in the Arches cluster. Arches is one of the densest young clusters known in the Milky Way, with a central density of ∼3 × 10 5 M pc −3 and a total mass of about 10 4 M . A strong colour gradient is observed over the cluster field. The visual extinction increases by ∆A V ∼ 10 mag over a distance of 15 from the cluster core. Extinction maps reveal a low-extinction cavity in the densest parts of Arches (R ≤ 5 ), indicating the depletion of dust due to stellar winds or photo-evaporation. We correct for the change in extinction over the field and show that the slope of the mass function is strongly influenced by the effects of differential extinction. We obtain present-day mass function slopes of Γ ∼ −0.8 ± 0.2 in the mass range 6 < M < 65 M from both data sets. The spatial analysis reveals a steepening of the mass function slope from close to zero in the cluster center to Γ ∼ −1.7 ± 0.7 at R > 10 , in accordance with a Salpeter slope (Γ = −1.35). The bias in the mass function towards high-mass stars in the Arches center is a strong indication for mass segregation. The dynamical and relaxation timescales for Arches are estimated, and possible mass segregation effects are discussed with respect to cluster formation models.