Until now it has been impossible to observationally measure how star cluster scale height evolves beyond 1Gyr as only small samples have been available. Here we establish a novel method to determine the scale height of a cluster sample using modelled
distributions and Kolmogorov-Smirnov tests. This allows us to determine the scale height with a 25% accuracy for samples of 38 clusters or more. We apply our method to investigate the temporal evolution of cluster scale height, using homogeneously selected sub-samples of Kharchenko et al. (MWSC), Dias et al. (DAML02), WEBDA, and Froebrich et al. (FSR). We identify a linear relationship between scale height and log(age/yr) of clusters, considerably different from field stars. The scale height increases from about 40pc at 1Myr to 75pc at 1Gyr, most likely due to internal evolution and external scattering events. After 1Gyr, there is a marked change of the behaviour, with the scale height linearly increasing with log(age/yr) to about 550pc at 3.5Gyr. The most likely interpretation is that the surviving clusters are only observable because they have been scattered away from the mid-plane in their past. A detailed understanding of this observational evidence can only be achieved with numerical simulations of the evolution of cluster samples in the Galactic Disk. Furthermore, we find a weak trend of an age-independent increase in scale height with galactocentric distance. There are no significant temporal or spatial variations of the cluster distribution zero point. We determine the Suns vertical displacement from the Galactic Plane as $Z_odot=18.5pm1.2$pc.
Determining star cluster distances is essential to analyse their properties and distribution in the Galaxy. In particular it is desirable to have a reliable, purely photometric distance estimation method for large samples of newly discovered cluster
candidates e.g. from 2MASS, UKIDSS-GPS and VISTA-VVV. Here, we establish an automatic method to estimate distances and reddening from NIR photometry alone, without the use of isochrone fitting. We employ a decontamination procedure of JHK photometry to determine the density of stars foreground to clusters and a galactic model to estimate distances. We then calibrate the method using clusters with known properties. This allows us to establish distance estimates with better than 40% accuracy. We apply our method to determine the extinction and distance values to 378 known open clusters and 397 cluster candidates from the list of Froebrich, Scholz and Raftery (2003). We find that the sample is biased towards clusters of a distance of approximately 3kpc, with typical distances between 2 and 6kpc. Using the cluster distances and extinction values, we investigate how the average extinction per kiloparsec distance changes as a function of Galactic longitude. We find a systematic dependence that can be approximated by A_H(l)[mag/kpc]=0.10+0.001*|l-180deg|/deg for regions more than 60deg from the Galactic Centre.
Jets and outflows accompany the mass accretion process in protostars and young stellar objects. Using a large and unbiased sample, they can be used to study statistically the local feedback they provide and the typical mass accretion history. Here we
analyse such a sample of Molecular Hydrogen emission line Objects in the Serpens and Aquila part of the Galactic Plane. Distances are measured by foreground star counts with an accuracy of 25%. The resulting spacial distribution and outflow luminosities indicate that our objects sample the formation of intermediate mass objects. The outflows are unable to provide a sizeable fraction of energy and momentum to support, even locally, the turbulence levels in their surrounding molecular clouds. The fraction of parsec scale flows is one quarter and the typical dynamical jet age of the order of 1E4yrs. Groups of emission knots are ejected every 1E3yrs. This might indicate that low level accretion rate fluctuations and not FU-Ori type events are responsible for the episodic ejection of material. Better observational estimates of the FU-Ori duty cycle are needed.
Jets and outflows from Young Stellar Objects (YSOs) are important signposts of currently ongoing star formation. In order to study these objects we are conducting an unbiased survey along the Galactic Plane in the 1-0S(1) emission line of molecular h
ydrogen at 2.122mu using the UK Infrared Telescope. In this paper we are focusing on a 33 square degree sized region in Serpens and Aquila (18deg < l < 30deg; -1.5deg < b < +1.5deg). We trace 131 jets and outflows from YSOs, which results in a 15 fold increase in the total number of known Molecular Hydrogen Outflows. Compared to this, the total integrated 1-0S(1) flux of all objects just about doubles, since the known objects occupy the bright end of the flux distribution. Our completeness limit is 3*10^-18Wm^-2 with 70% of the objects having fluxes of less than 10^-17Wm^-2. Generally, the flows are associated with Giant Molecular Cloud complexes and have a scale height of 25-30pc with respect to the Galactic Plane. We are able to assign potential source candidates to about half the objects. Typically, the flows are clustered in groups of 3-5 objects, within a radius of 5pc. These groups are separated on average by about half a degree, and 2/3rd of the entire survey area is devoid of outflows. We find a large range of apparent outflow lengths from 4arcsec to 130arcsec. If we assume a distance of 3kpc, only 10% of all outflows are of parsec scale. There is a 2.6sigma over abundance of flow position angles roughly perpendicular to the Galactic Plane.
We investigate the old star clusters in the sample of cluster candidates from Froebrich, Scholz & Raftery 2007 -- the FSR list. Based on photometry from the 2-Micron All Sky Survey we generated decontaminated colour-magnitude and colour-colour diagra
ms to select a sample of 269 old stellar clusters. This sample contains 63 known globular clusters, 174 known open clusters and 32 so far unclassified objects. Isochrone fitting has been used to homogeneously calculate the age, distance and reddening to all clusters. The mean age of the open clusters in our sample is 1Gyr. The positions of these clusters in the Galactic Plane show that 80% of open clusters older than 1Gyr have a Galactocentric distance of more than 7kpc. The scale height for the old open clusters above the Plane is 375pc, more than three times as large as the 115pc which we obtain for the younger open clusters in our sample. We find that the mean optical extinction towards the open clusters in the disk of the Galaxy is 0.70mag/kpc. The FSR sample has a strong selection bias towards objects with an apparent core radius of 30 to 50 and there is an unexplained paucity of old open clusters in the Galactic Longitude range of 120deg < l < 180deg.
The formation of stars is inextricably linked to the structure of their parental molecular clouds. Here we take a number of nearby giant molecular clouds (GMCs) and analyse their column density and mass distributions. This investigation is based on f
our new all-sky median colour excess extinction maps determined from 2MASS. The four maps span a range of spatial resolution of a factor of eight. This allows us to determine cloud properties at a common spatial scale of 0.1pc, as well as to study the scale dependence of the cloud properties. We find that the low column density and turbulence dominated part of the clouds can be well fit by a log-normal distribution. However, above a universal extinction threshold of 6.0 pm 1.5mag A_V there is excess material compared to the log-normal distribution in all investigated clouds. This material represents the part of the cloud that is currently involved in star formation, and thus dominated by gravity. Its contribution to the total mass of the clouds ranges over two orders of magnitude from 0.1 to 10%. This implies that our clouds sample various stages in the evolution of GMCs. Furthermore, we find that the column density and mass distributions are extremely similar between clouds if we analyse only the high extinction material. On the other hand, there are significant differences between the distributions if only the low extinction, turbulence dominated regions are considered. This shows that the turbulent properties differ between clouds depending on their environment. However, no significant influence on the predominant mode of star formation (clustered or isolated) could be found. Furthermore, the fraction of the cloud actively involved in star formation is only governed by gravity, with the column density and mass distributions not significantly altered by local feedback processes.
We are studying the column density distribution of all nearby giant molecular clouds. As part of this project we generated several all sky extinction maps. They are calculated using the median near infrared colour excess technique applied to data fro
m the Two Micron All-Sky Survey (2MASS). Our large scale approach allows us to fit spline functions to extinction free regions in order to accurately determine the colour excess values. Two types of maps are presented: i) Maps with a constant noise and variable spatial resolution; ii) Maps with a constant spatial resolution and variable noise. Our standard Av map uses the nearest 49 stars to the centre of each pixel for the determination of the extinction. The one sigma variance is constant at 0.28mag Av in the entire map. The distance to the 49th nearest star varies from below 1arcmin near the Galactic Plane to about 10arcmin at the poles, but is below 5arcmin for all giant molecular clouds (|b|< 30degr). A comparison with existing large scale maps shows that our extinction values are systematically larger by 20% compared to Dobashi et al. and 40% smaller compared to Schlegel et al.. This is most likely caused by the applied star counting technique in Dobashi et al. and systematic uncertainties in the dust temperature and emissivity in Schlegel et al.. Our superior resolution allows us to detect more small scale high extinction cores compared to the other two maps.
We are conducting a large program to classify newly discovered Milky Way star cluster candidates from the list of Froebrich, Scholz & Raftery (2007). Here we present deep NIR follow-up observations from ESO/NTT of 14 star cluster candidates. We show
that the combined analysis of star density maps and colour-colour/magnitude diagrams derived from deep near-infrared imaging is a viable tool to reliably classify new stellar clusters. This allowed us to identify two young clusters with massive stars, three intermediate age open clusters, and two globular cluster candidates among our targets. The remaining seven objects are unlikely to be stellar clusters. Among them is the object FSR1767 which has previously been identified as a globular cluster using 2MASS data by Bonatto et al. (2007). Our new analysis shows that FSR1767 is not a star cluster. We also summarise the currently available follow-up analysis of the FSR candidates and conclude that this catalogue may contain a large number of new stellar clusters, probably dominated by old open clusters.
We present a 127deg x 63deg extinction map of the Anticenter of the Galaxy, based on <J-H> and <H-K> colour excess maps from 2MASS. This 8001 square degree map with a resolution of 4 arcminutes is provided as online material. The colour excess ratio
<J-H> / <H-K> is used to determine the power law index of the reddening law (beta) for individual regions contained in the area (e.g. Orion, Perseus, Taurus, Auriga, Monoceros, Camelopardalis, Cassiopeia). On average we find a dominant value of beta=1.8+-0.2 for the individual clouds, in agreement with the canonical value for the interstellar medium. We also show that there is an internal scatter of beta values in these regions, and that in some areas more than one dominant beta value is present. This indicates large scale variations in the dust properties. The analysis of the A_V values within individual regions shows a change in the slope of the column density distribution with distance. This can either be attributed to a change in the governing physical processes in molecular clouds on spatial scales of about 1pc or an A_V dilution with distance in our map.
