We present the first data release of the Radial Velocity Experiment ( RAVE), an ambitious spectroscopic survey to measure radial velocities and stellar atmosphere parameters (temperature, metallicity, and surface gravity) of up to one million stars using the Six Degree Field multiobject spectrograph on the 1.2 m UK Schmidt Telescope of the Anglo-Australian Observatory. The RAVE program started in 2003, obtaining medium-resolution spectra (median R ¼ 7500) in the Ca-triplet region (8410-8795 8) for southern hemisphere stars drawn from the Tycho-2 and SuperCOSMOS catalogs, in the magnitude range 9 < I < 12. The first data release is described in this paper and contains radial velocities for 24,748 individual stars (25,274 measurements when including reobservations). Those data were obtained on 67 nights between 2003 April 11 and 2004 April 3. The total sky coverage within this data release is $4760 deg 2 . The average signal-to-noise ratio of the observed spectra is 29.5, and 80% of the radial velocities have uncertainties better than 3.4 km s À1 . Combining internal errors and zero-point errors, the mode is found to be 2 km s À1 . Repeat observations are used to assess the stability of our radial velocity solution, resulting in a variance of 2.8 km s À1 . We demonstrate that the radial velocities derived for the first data set do not show any systematic trend with color or signal-to-noise ratio. The RAVE radial velocities are complemented in the data release with proper motions from Starnet 2.0, Tycho-2, and SuperCOSMOS, in addition to photometric data from the major optical and infrared catalogs (Tycho-2, USNO-B, DENIS, and the Two Micron All Sky Survey). The data release can be accessed via the RAVE Web site.
We present new results on the evolution of the mass function of the globular cluster system of the Milky Way, taking the effect of residual gas expulsion into account. We assume that gas embedded star clusters start with a power‐law mass function with slope β= 2, similar to what is observed for the Galactic open clusters and young, massive star clusters in interacting galaxies. The dissolution of the clusters is then studied under the combined influence of residual gas expulsion driven by energy feedback from massive stars, stellar mass loss, two‐body relaxation and an external tidal field. The influence of residual gas expulsion is studied by applying results from a large grid of N‐body simulations computed by Baumgardt & Kroupa. In our model, star clusters with masses less than 105 M⊙ lose their residual gas on time‐scales much shorter than their crossing time and residual gas expulsion is the main dissolution mechanism for star clusters, destroying about 95 per cent of all clusters within a few tens of Myr. We find that in this case the final mass function of globular clusters is established mainly by the gas expulsion and therefore nearly independent of the strength of the external tidal field, and that a power‐law mass function for the gas embedded star clusters is turned into a present‐day lognormal one, verifying the theory proposed by Kroupa & Boily. Our model provides a natural explanation for the observed (near‐)universality of the peak of the globular cluster mass function within a galaxy and among different galaxies. Our simulations also show that globular clusters must have started a factor of a few more concentrated than as we see them today. Another consequence of residual gas expulsion and the associated strong infant mortality of star clusters is that the Galactic halo stars come from dissolved star clusters. Since field halo stars would come mainly from low‐mass, short‐lived clusters, our model would provide an explanation for the observed abundance variations of light elements among globular cluster stars and the absence of such variations among the halo field stars. Furthermore, our modelling suggests a natural tendency of >107 M⊙ gas clouds to retain their residual gas despite multiple supernova events, possibly explaining the complex stellar populations observed in the most massive globular clusters.
Context. A positive power-law trend between the local surface densities of molecular gas, Σ gas , and young stellar objects, Σ , in molecular clouds of the solar neighbourhood has recently been identified. How it relates to the properties of embedded clusters, in particular to the recently established radius-density relation, has so far not been investigated. Aims. We model the development of the stellar component of molecular clumps as a function of time and initial local volume density. Our study provides a coherent framework able to explain both the molecular-cloud and embedded-cluster relations quoted above. Methods. We associate the observed volume density gradient of molecular clumps to a density-dependent free-fall time. The molecular clump star formation history is obtained by applying a constant star formation efficiency per free-fall time, ff . Results. For the volume density profiles typical of observed molecular clumps (i.e. power-law slope −1.7), our model gives a stargas surface-density relation of the form Σ ∝ Σ 2 gas , which agrees very well with the observations. Taking the case of a molecular clump of mass M 0 10 4 M and radius R 6 pc experiencing star formation during 2 Myr, we derive what star formation efficiency per free-fall time matches the normalizations of the observed and predicted (Σ , Σ gas ) relations best. We find ff 0.1. We show that the observed growth of embedded clusters, embodied by their radius-density relation, corresponds to a surface density threshold being applied to developing star-forming regions. The consequences of our model in terms of cluster survivability after residual starforming gas expulsion are that, owing to the locally high star formation efficiency in the inner part of star-forming regions, global star formation efficiency as low as 10% can lead to the formation of bound gas-free star clusters.
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