arXiv subjects

D. Froebrich

Publications and source records attributed to D. Froebrich.

At least 55 records · Page 3Linked to original sources

arXiv2010

Old Star Clusters in the FSR catalogue

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 diagrams 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.

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arXiv2010

The Structure of Molecular Clouds: II - Column Density and Mass Distributions

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 four 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.

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arXiv2009

First detection of acceleration and deceleration in protostellar jets? Time variability in the Cha II outflows

We present a multi-epoch (20 years baseline) kinematical investigation of HH52, 53, and 54 at optical and near-IR wavelengths, along with medium and high- resolution spectroscopic analyses, probing the kinematical and physical time variability conditions of the gas along the flows. By means of multi-epoch and multi-wavelength narrow-band images, we derived proper motions, tangential velocities, velocity and flux variability of the knots. Radial velocities and physical parameters of the gas were derived from spectroscopy. Finally, spatial velocities and inclination of the flows were obtained by combining both imaging and spectroscopy. The P.M. analysis reveals three distinct, partially overlapping outflows. In 20 years, about 60% of the knots show some degree of flux variability. Our set of observations apparently indicates acceleration and deceleration in a variety of knots along the jets. For about 20% of the knots, mostly coincident with working surfaces or interacting knots along the flows, a relevant variability in both flux and velocity is observed. We argue that both variabilities are related and that all or part of the kinetic energy lost by the interacting knots is successively radiated. The analysis indicates the presence of very light, ionised, and hot flows, impacting a denser medium. Several knots are deflected. At least for a couple of them (HH54 G and G0), the deflection originates from the collision of the two. For the more massive parts of the flow, the deflection is likely the result of the flow collision with a dense cloud or with clumps.

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arXiv2009

UKIRT follow-up observations of the old open cluster FSR0358 (Kirkpatrick1)

We aim to characterise the properties of the stellar clusters in the Milky Way. Utilising an expectation-maximisation method we determined that the cluster FSR0358, originally discovered by J.D.Kirkpatrick, is the most likely real cluster amongst the cluster candidates from Froebrich et al.. Here we present new deep high resolution near infrared imaging of this object obtained with UKIRT. The analysis of the data reveals that FSR0358 (Kirkpatrick1) is a 5+-2Gyr old open cluster in the outer Milky Way. Its age, metallicity of Z=0.008 and distance from the Galactic Centre of 11.2kpc are typical for the known old open galactic clusters. So far six of the FSR cluster candidates have been identified as having an age above 5Gyr. This shows the significance of this catalogue in enhancing our knowledge of the oldest open clusters in the Galaxy.

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arXiv2009

The Structure of Molecular Clouds: I - All Sky Near Infrared Extinction Maps

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 from 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.

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arXiv2008

A census of molecular hydrogen outflows and their sources along the Orion A molecular ridge: characteristics and overall distribution

We present wide-field near-IR images of Orion A. K and H2 1-0S(1) images of a contiguous 8 sqr degree region are compared to photometry from Spitzer and dust-continuum maps obtained with MAMBO and SCUBA. We also measure proper motions for H2 features in 33 outflows. We increase the number of known H2 outflows in Orion A to 116. A total of 111 H2 flows were observed with Spitzer; outflow sources are identified for at least 72 of them. The MAMBO 1200 micron maps cover 97 H2 flows; 57 of them are associated with Spitzer sources and dust cores or extended emission. The H2 jets are widely distributed and randomly orientated; the jets do not appear to be orthogonal to large-scale filaments or even to the small-scale cores. Moreover, H2 jet lengths and opening angles are not obviously correlated with indicators of outflow source age - source spectral index or (sub)millimetre core flux. We demonstrate that H2 jet sources are predominantly protostellar with flat or positive spectral indices, rather than disk-excess (or T Tauri) stars. Most protostars in molecular cores drive H2 outflows. However, not all molecular cores are associated with protostars or H2 jets. On statistical grounds, the H2 jet phase may be marginally shorter than the protostellar phase, though must be considerably shorter than the prestellar phase. In terms of their spectral index, H2 jet sources are indistinguishable from protostars. The few true protostars without H2 jets are almost certainly more evolved than their H2-jet-driving counterparts. We also find that protostars that power molecular outflows are no more (nor no less) clustered than protostars that do not. The H2 emission regions in outflows from young stars clearly weaken and fade very quickly, before the source evolves from protostar to pre-main-sequence star.

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arXiv2008

NTT follow-up observations of star cluster candidates from the FSR catalogue

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.

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arXiv2008

Molecular jets driven by high-mass protostars: a detailed study of the IRAS 20126+4104 jet

We present here an extensive analysis of the protostellar jet driven by IRAS 20126+4104, deriving the kinematical, dynamical, and physical conditions of the H2 gas along the flow. The jet has been investigated by means of near-IR H2 and [FeII] narrow-band imaging, high resolution spectroscopy of the 1-0S(1) line (2.12 um), NIR (0.9-2.5 um) low resolution spectroscopy, along with ISO-SWS and LWS spectra (from 2.4 to 200 um). The flow shows a complex morphology. In addition to the large-scale jet precession presented in previous studies, we detect a small-scale wiggling close to the source, that may indicate the presence of a multiple system. The peak radial velocities of the H2 knots range from -42 to -14 km s^-1 in the blue lobe, and from -8 to 47 km s^-1 in the red lobe. The low resolution spectra are rich in H_2 emission, and relatively faint [FeII] (NIR), [OI] and [CII] (FIR) emission is observed in the region close to the source. A warm H2 gas component has an average excitation temperature that ranges between 2000 K and 2500 K. Additionally, the ISO-SWS spectrum reveals the presence of a cold component (520 K), that strongly contributes to the radiative cooling of the flow and plays a major role in the dynamics of the flow. The estimated L(H2) of the jet is 8.2+/-0.7 L_sun, suggesting that IRAS20126+4104 has an accretion rate significantly increased compared to low-mass YSOs. This is also supported by the derived mass flux rate from the H2 lines (Mflux(H2)~7.5x10^-4 M_sun yr^-1). The comparison between the H2 and the outflow parameters strongly indicates that the jet is driving, at least partially, the outflow. As already found for low-mass protostellar jets, the measured H2 outflow luminosity is tightly related to the source bolometric luminosity.

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arXiv2007

FSR0190 - Another old distant galactic cluster

We are conducting a large program to classify newly discovered Milky Way star cluster candidates from Froebrich et al. (2007). Here we present NIR follow-up observations of FSR0190 (RA=20h05m31.3s, DEC=33deg34'09" J2000). The cluster is situated close to the Galactic Plane (l=70.7302deg, b=+0.9498deg). It shows a circular shape, a relatively large number of core helium burning stars -- which clearly distinguishes the cluster from the rich field -- but no centrally condensed star density profile. We derive an age of more than 7Gyr, a Galactocentric distance of 10.5kpc, a distance of 10kpc from the Sun, and an extinction of A_K=0.8mag. The estimated mass is at least of the order of 1E5M_sun, and the absolute brightness is M_V<=-4.7mag; both are rather typical properties for Palomar-type globular clusters.

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arXiv2007

A large scale extinction map of the Galactic Anticenter from 2MASS

We present a 127deg x 63deg extinction map of the Anticenter of the Galaxy, based on and 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 / is used to determine the power law index of the reddening law (β) for individual regions contained in the area (e.g. Orion, Perseus, Taurus, Auriga, Monoceros, Camelopardalis, Cassiopeia). On average we find a dominant value of β=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 βvalues in these regions, and that in some areas more than one dominant βvalue 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.

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arXiv2007

Proper Motion Studies of Outflows from Classical T Tauri Stars

In a previous paper (McGroarty & Ray 2004, 420, 975) we examined the environment of a number of evolved low-mass young stars, i.e. Classical T Tauri Stars, to see if they are capable of driving parsec-scale outflows. These stars - CW Tau, DG Tau, DO Tau, HV Tau C and RW Aur - were previously known to drive only "micro-jets" or small-scale outflows of < 1' or 0.04 pc at the distance of the Taurus-Auriga Cloud. We found that they drive outflows of 0.5pc - 1pc, based on the morphology and alignment of newly discovered and previously known HH objects with these sources and their "micro-jets". Here, we use a cross-correlation method to determine the proper motions of the HH objects in these five outflows (HH 220, HH 229, HH 702, HH 705 and HH 826 - HH 835) which in turn allows us to confirm their driving sources. Moreover, the tangential velocities of HH objects at large distances from their origin are currently poorly known so these proper motions will allow us to determine how velocities evolve with distance from their source. We find tangential velocities of typically 200 kms^-1 for the more distant objects in these outflows. Surprisingly, we find similar tangential velocities for the "micro-jets" that are currently being ejected from these sources. This leads us to suggest that either the outflow velocity was much higher 10^3 years ago when the more distant objects were ejected and that these objects have decelerated to their current velocity or that the outflow velocity at the source has remained approximately constant and the more distant objects have not undergone significant deceleration due to interactions with the ambient medium. Numerical simulations are needed before we can decide between these scenarios.

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arXiv2007

FSR1735 -- A new globular cluster candidate in the inner Galaxy

We carry out a large program to classify newly discovered star clusters from Froebrich et al. (2006) in the inner Galaxy. Here, we present a first analysis of a new high-priority globular cluster candidate, FSR1735 at l=339.1879; b=-1.8534, based on new deep near infrared observations from Sofi at the NTT. A significant peak in the K-band luminosity function is found, which is interpreted as the clump of post-He-flash stars. The distance and the reddening of the cluster are determined to 9.1kpc and A_K=0.5mag, respectively, the metallicity is estimated to [M/H]=-0.8. Radial star density profiles are used to measure the core radius and the tidal radius of the cluster. The lack of signs for on-going star formation and the position in the Galaxy pose strong arguments against the interpretation of this object as a young or old open cluster. All the observational evidence is in agreement with the interpretation that FSR1735 is a so far unknown globular cluster in the inner Galaxy.

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arXiv2006

WFCAM, Spitzer-IRAC and SCUBA observations of the massive star forming region DR21/W75: I. The collimated molecular jets

We present wide-field near-infrared images of the DR21/W75 high-mass star forming region, obtained with the Wide Field Camera, WFCAM, on the United Kingdom Infrared Telescope. Broad-band JHK and narrow-band H2 1-0S(1) images are compared to archival mid-IR images from the Spitzer Space Telescope, and 850 micron dust-continuum maps obtained with the Submillimeter Common User Bolometer Array (SCUBA). Together these data give a complete picture of dynamic star formation across this extensive region, which includes at least four separate star forming sites in various stages of evolution. The H2 data reveal knots and bow shocks associated with more than 50 individual flows. Most are well collimated, and at least five qualify as parsec-scale flows. Most appear to be driven by embedded, low-mass protostars. The orientations of the outflows, particularly from the few higher-mass sources in the region (DR21, DR21(OH), W75N and ERO~1), show some degree of order, being preferentially orientated roughly orthogonal to the chain of dusty cores that runs north-south through DR21. Clustering may inhibit disk accretion and therefore the production of outflows; we certainly do not see enhanced outflow activity from clusters of protostars. Finally, although the low-mass protostellar outflows are abundant and widely distributed, the current generation does not provide sufficient momentum and kinetic energy to account for the observed turbulent motions in the DR21/W75 giant molecular clouds. Rather, multiple epochs of outflow activity are required over the million-year timescale for turbulent decay.

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arXiv2006

A systematic survey for infrared star clusters with |b|<20deg using 2MASS

We used star density maps obtained from 2MASS to obtain a sample of star clusters in the entire Galactic Plane with |b|<20deg. A total of 1788 star cluster candidates are identified in this survey. Among those are 681 previously known open clusters and 86 globular clusters. A statistical analysis indicates that our sample of 1021 new cluster candidates has a contamination of about 50%. Star cluster parameters are obtained by fitting a King profile to the star density. These parameters are used to statistically identify probable new globular cluster candidates in our sample. A detailed investigation of the projected distribution of star clusters in the Galaxy demonstrates that they show a clear tendency to cluster on spatial scales in the order of 12-25pc, a typical size for molecular clouds.

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arXiv2006

WFCAM, Spitzer-IRAC and SCUBA observations of the massive star forming region DR21/W75: II. Stellar content and star formation

Wide field near-infrared observations and Spitzer Space Telescope IRAC observations of the DR21/W75 star formation regions are presented. The photometric data are used to analyse the extinction, stellar content and clustering in the entire region by using standard methods. A young stellar population is identified all over the observed field, which is found to be distributed in embedded clusters that are surrounded by a distributed halo population extending over a larger projected area. The Spitzer/IRAC data are used to compute a spectral index value, "alpha", for each YSO in the field. We use these data to separate pure photospheres from disk excess sources. We find a small fraction of sources with "alpha" in excess of 2 to 3 (plus a handful with "alpha"~4), which is much higher than the values found in the low mass star forming region IC348 ("alpha" < 2). The sources with high values of "alpha" spatially coincide with the densest regions of the filaments and also with the sites of massive star formation. Star formation is found to be occuring in long filaments stretching to few parsecs that are fragmented over a scale of ~ 1 pc. The spatial distribution of young stars are found to be correlated with the filamentary nebulae that are prominently revealed by 8micron and 850micron observations. Five filaments are identified that appear to converge on a center that includes the DR21/DR21(OH) regions. The morphological pattern of filaments and clustering compare well with numerical simulations of star cluster formation by Bate et al. 2003.

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arXiv2006

Evolution of Class0 protostars: Models vs. Observations

The rates at which mass accumulates into protostellar cores can now be predicted in numerical simulations. Our purpose here is to develop methods to compare the statistical properties of the predicted protostars with the observable parameters. This requires (1) an evolutionary scheme to convert numerically-derived mass accretion rates into evolutionary tracks and (2) a technique to compare the tracks to the observed statistics of protostars. Here, we use a 3D-Kolmogorov-Smirnov test to quantitatively compare model evolutionary tracks and observations of Class0 protostars. We find that the wide range of accretion functions and timescales associated with gravoturbulent simulations naturally overcome difficulties associated with schemes that use a fixed accretion pattern. This implies that the location of a protostar on an evolutionary track does not precisely determine the present age or final accrued mass. Rather, we find that predictions of the final mass for protostars from observed Tbol-Lbol values are uncertain by a factor of two and that the bolometric temperature is not always a reliable measure of the evolutionary stage. Furthermore, we constrain several parameters of the evolutionary scheme and estimate a lifetime of Class0 sources of 2-6*10^4yrs, which is related to the local free-fall time and thus to the local density at the onset of the collapse. Models with Mach numbers smaller than six are found to best explain the observational data. Generally, only a probability of 70% was found that our models explain the current observations. This is caused by not well understood selection effects in the observational sample and the simplified assumptions in the models.

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arXiv2005

Number ratios of young stellar objects in embedded clusters

Embedded clusters usually contain young stellar objects in different evolutionary stages. We investigate number ratios of objects in these classes in the star-forming regions rho Ophiuchi, Serpens, Taurus, Chamaeleon I, NGC 7129, IC 1396A, and IC 348. They are compared to the temporal evolution of young stars in numerical simulations of gravoturbulent fragmentation in order to constrain the models and to possibly determine the evolutionary stage of the clusters. It turns out that Serpens is the youngest, and IC 348 the most evolved cluster, although the time when the observations are best represented varies strongly depending on the model. Furthermore, we find an inverse correlation of the star formation efficiency (SFE) of the models with the Mach number. However, the observational SFE values cannot be reproduced by the current isothermal models. This argues for models that take into account protostellar feedback processes and/or the effects of magnetic fields.

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arXiv2005

VPMS J1342+2840 - an unusual quasar from the variability and proper motion survey

We report the discovery of the highly peculiar, radio-loud quasar VPMS J1342+2840 (z ~ 1.3) from the variability and proper motion survey. We present spectroscopic, imaging and photometric observations. The unusual spectrum shows a strong depression of the continuum over a wide wavelength range in the blue part without the typical structures of broad absorption line (BAL) troughs. The image of the quasar is unresolved and there is no evidence for a foreground object on the line of sight. The broad-band spectral energy distribution is not consistent with obvious dust reddening with the standard SMC extinction curve. The downturn of the continuum flux of VPMS J1342+2840 at short wavelengths can be caused by dust reddening only if the reddening curve is steeper then the SMC curve in the ultraviolet and is very flat at longer wavelengths. Alternatively, the dominant spectral features can be explained by low-ionization BALs forming unusually wide, overlapping absorption troughs.

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