arXiv subjects

Dirk Froebrich

Publications and source records attributed to Dirk Froebrich.

At least 37 records · Page 2Linked to original sources

arXiv2018

YSO jets in the Galactic Plane from UWISH2: V - Jets and Outflows in M17

Jets and outflows are the first signposts of stellar birth. Emission in the H2 1-0S(1) line at 2.122 micron is a powerful tracer of shock excitation in these objects. Here we present the analysis of 2.0 x 0.8 square degrees data from the UK Widefield Infrared Survey for H2 (UWISH2) in the 1-0S(1) line to identify and characterize the outflows of the M17 complex. We uncover 48 probable outflows, of which, 93 per cent are new discoveries. We identified driving source candidates for 60 per cent of the outflows. Among the driving source candidate YSOs: 90 per cent are protostars and the reminder 10 per cent are Class II YSOs. Comparing with results from other surveys, we suggest that H2 emission fades very quickly as the objects evolve from protostars to pre-main-sequence stars. We fit SED models to 14 candidate outflow driving sources and conclude that the outflows of our sample are mostly driven by moderate-mass YSOs that are still actively accreting from their protoplanetary disc. We examined the spatial distribution of the outflows with the gas and dust distribution of the complex, and observed that the filamentary dark-cloud "M17SWex" located at the south-western side of the complex, is associated with a greater number of outflows. We find our results corroborate previous suggestions, that in the M17 complex, M17SWex is the most active site of star formation. Several of our newly identified outflow candidates are excellent targets for follow up studies to better understand very early phase of protostellar evolution.

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arXiv2018

Discovery of a magnetic white dwarf with unusual short-period variability

We report the discovery of a magnetic white dwarf which shows periodic variability with P=110 min, color-dependent amplitudes and a transient phase shift in the blue compared to the red lightcurve - a previously unknown type of variability for this type of object. We attribute the variations either to a close ultracool (thus far undetected) companion or, more likely, to magnetic spots with unusual temperature structure.

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arXiv2017

Extreme infrared variables from UKIDSS - II. an end-of-survey catalogue of eruptive YSOs and unusual stars

We present a catalogue of 618 high amplitude infrared variable stars (1 < DeltaK < 5 mag) detected by the two widely separated epochs of 2.2 um data in the UKIDSS Galactic plane survey, from searches covering 1470 deg^2. Most were discovered by a search of all fields at 30 < l < 230 deg. Sources include new dusty Mira variables, three new CV candidates, a blazar and a peculiar source that may be an interacting binary system. However, c. 60 per cent are YSOs, based on spatial association with star forming regions at distances ranging from 300 pc to over 10 kpc. This confirms our initial result in Contreras Pena et al.(Paper I) that YSOs dominate the high amplitude infrared variable sky in the Galactic disc. It is also supported by recently published VVV results at 295 < l < 350 deg. The spectral energy distributions of the YSOs indicate class I or flat spectrum systems in most cases, as in the VVV sample. A large number of variable YSOs are associated with the Cygnus X complex and other groups are associated with the North America/Pelican nebula, the Gemini OB1 molecular cloud, the Rosette complex, the Cone nebula, the W51 star forming region and the S86 and S236 HII regions. Most of the YSO variability is likely due to variable/episodic accretion on timescales of years, albeit usually less extreme than classical FUors and EXors. Luminosities at the 2010 WISE epoch range from c. 0.1 Lsun to 10^3 Lsun but only rarely exceed 10^2.5 Lsun.

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arXiv2017

FSR 1716: A new Milky Way Globular Cluster confirmed using VVV RR Lyrae stars

We use deep multi-epoch near-IR images of the VISTA Variables in the Vía Láctea (VVV) Survey to search for RR Lyrae stars towards the Southern Galactic plane. Here we report the discovery of a group of RR Lyrae stars close together in VVV tile d025. Inspection of the VVV images and PSF photometry reveals that most of these stars are likely to belong to a globular cluster, that matches the position of the previously known star cluster FSR\,1716. The stellar density map of the field yields a $>100$ sigma detection for this candidate globular cluster, that is centered at equatorial coordinates $RA_{J2000}=$16:10:30.0, $DEC_{J2000}=-$53:44:56; and galactic coordinates $l=$329.77812, $b=-$1.59227. The color-magnitude diagram of this object reveals a well populated red giant branch, with a prominent red clump at $K_s=13.35 \pm 0.05$, and $J-K_s=1.30 \pm 0.05$. We present the cluster RR Lyrae positions, magnitudes, colors, periods and amplitudes. The presence of RR Lyrae indicates an old globular cluster, with age $>10$ Gyr. We classify this object as an Oosterhoff type I globular cluster, based on the mean period of its RR Lyrae type ab, $ =0.540$ days, and argue that this is a relatively metal-poor cluster with $[Fe/H] = -1.5 \pm 0.4$ dex. The mean extinction and reddening for this cluster are $A_{K_s}=0.38 \pm 0.02$, and $E(J-K_s)=0.72 \pm 0.02$ mag, respectively, as measured from the RR Lyrae colors and the near-IR color-magnitude diagram. We also measure the cluster distance using the RR Lyrae type ab stars. The cluster mean distance modulus is $(m-M)_0 = 14.38 \pm 0.03$ mag, implying a distance $D = 7.5 \pm 0.2$ kpc, and a Galactocentric distance $R_G=4.3$ kpc.

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arXiv2014

Properties of Star Clusters - II: Scale Height Evolution of Clusters

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 Sun's vertical displacement from the Galactic Plane as $Z_\odot=18.5\pm1.2$pc.

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arXiv2014

UKIRT Widefield Infrared Survey for Fe$^+$

The United Kingdom Infrared Telescope (UKIRT) Widefield Infrared Survey for Fe$^+$ (UWIFE) is a 180 deg$^2$ imaging survey of the first Galactic quadrant (7$^{\circ}$ < l < 62$^{\circ}$; |b| < 1.5$^{\circ}$) using a narrow-band filter centered on the [Fe II] 1.644 μm emission line. The [Fe II] 1.644 μm emission is a good tracer of dense, shock-excited gas, and the survey will probe violent environments around stars: star-forming regions, evolved stars, and supernova remnants, among others. The UWIFE survey is designed to complement the existing UKIRT Widefield Infrared Survey for H2 (UWISH2; Froebrich et al. 2011). The survey will also complement existing broad-band surveys. The observed images have a nominal 5σ detection limit of 18.7 mag for point sources, with the median seeing of 0.83". For extended sources, we estimate surface brightness limit of 8.1 x 10$^{-20}$ W m$^{-2}$ arcsec$^{-2}$ . In this paper, we present the overview and preliminary results of this survey.

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arXiv2013

Properties of Star Clusters - I: Automatic Distance and Extinction Estimates

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.

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arXiv2013

The Main Sequence of three Red Supergiant Clusters

Massive clusters in our Galaxy are an ideal testbed to investigate the properties and evolution of high mass stars. They provide statistically significant samples of massive stars of uniform ages. To accurately determine the intrinsic physical properties of these stars we need to establish the distances, ages and reddening of the clusters. One avenue to achieve this is the identification and characterisation of the main sequence members of red supergiant rich clusters. Here we utilise publicly available data from the UKIDSS galactic plane survey. We show that point spread function photometry in conjunction with standard photometric decontamination techniques allows us to identify the most likely main sequence members in the 10-20Myr old clusters RSGC1, 2, and 3. We confirm the previous detection of the main sequence in RSGC2 and provide the first main sequence detection in RSGC1 and RSGC3. There are in excess of 100 stars with more than 8Msun identified in each cluster. These main sequence members are concentrated towards the spectroscopically confirmed red supergiant stars. We utilise the J-K colours of the bright main sequence stars to determine the K-band extinction towards the clusters. The differential reddening is three times as large in the youngest cluster RSGC1 compared to the two older clusters RSGC2 and RSGC3. Spectroscopic follow up of the cluster main sequence stars should lead to more precise distance and age estimates for these clusters as well as the determination of the stellar mass function in these high mass environments.

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arXiv2013

Near-infrared H2 and Continuum Survey of Extended Green Objects. II. Complete Census for the Northern Galactic Plane

We discuss 94 Extended Green Objects (EGOs) in the northern Galactic plane cataloged by Cyganowski et al, based on near-infrared narrowband H2 (2.122 μm and continuum observations from the United Kingdom Infrared Telescope. This data set is three times larger than our previous study, and is unbiased by preselection. As discussed in the previous paper, the morphologies of the 4.5 μm emission generally resemble those of the near-infrared continuum, but are different from those of the H2 emission. Of our sample, only 28% of EGOs with H2 emission show similar morphologies between 4.5 μm and H2 emission. These results suggest that the 4.5 μm emission mainly comes from scattered continuum from the embedded young stellar object (YSO), and partially from H2 emission. About half of EGOs are associated with H2 outflows, if the H2 outflow incompleteness is considered. The H2 outflow detection rate for EGOs with K-band detections (61%) is significantly higher than for those without K-band detections (36%). This difference may be due to the fact that both H2 and K-band emissions are associated with outflows, i.e., H2 emission and K-band continuum are associated with shocks and outflow cavities, respectively. We also compared the correlation between the H2 outflows and Class I 44 GHz methanol masers from literature. The methanol masers can be located upstream or downstream of the H2 outflows and some bright H2 spots or outflows are not associated with methanol masers, suggesting that methanol masers and H2 emission trace different excitation conditions.

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arXiv2013

V2494 Cyg: A unique FU Ori type object in the Cygnus OB7 complex

A photometric and spectral study of the variable star V2494 Cyg in the L 1003 dark cloud is presented. The brightness of the star, formerly known as HH 381 IRS, increased by 2.5 mag in R (probably in the 1980s) and since then has remained nearly constant. Since the brightness increase, V2494 Cyg has illuminated a bipolar cometary nebula. The stellar spectrum has several features typical of the FU Ori type, plus it exhibits very strong Halpha and forbidden emission lines with high-velocity components. These emission lines originate in the HH jet near the star. The kinematic age of the jet is consistent with it forming at the time of the outburst leading to the luminosity increase. V2494 Cyg also produces a rather extended outflow; it is the first known FUor with both an observed outburst and a parsec-sized HH flow. The nebula, illuminated by V2494 Cyg, possesses similar morphological and spectral characteristics to Hubble's Variable Nebula (R Monocerotis/NGC 2261).

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arXiv2013

A systematic survey for eruptive young stellar objects using mid-infrared photometry

Accretion in young stellar objects (YSOs) is at least partially episodic, i.e. periods with high accretion rates ('bursts') are interspersed by quiescent phases. These bursts manifest themselves as eruptive variability. Here we present a systematic survey for eruptive YSOs aiming to constrain the frequency of accretion bursts. We compare mid-infrared photometry from Spitzer and WISE separated by ~5 yr for two samples of YSOs, in nearby star forming regions and in the Galactic plane, each comprising about 4000 young sources. All objects for which the brightness at 3.6 and 4.5 mu is increased by at least 1 mag between the two epochs may be eruptive variables and burst candidates. For these objects, we carry out follow-up observations in the near-infrared. We discover two new eruptive variables in the Galactic plane which could be FU Ori-type objects, with K-band amplitudes of more than 1.5 mag. One object known to undergo an accretion burst, V2492 Cyg, is recovered by our search as well. In addition, the young star ISO-Oph-50, previously suspected to be an eruptive object, is found to be better explained by a disk with varying circumstellar obscuration. In total, the number of burst events in a sample of 4000 YSOs is 1-4. Assuming that all YSOs undergo episodic accretion, this constraint can be used to show that phases of strong accretion (>10^-6 Msol/yr) occur in intervals of about 10^4 yr, most likely between 5000 and 50000 yr. This is consistent with the dynamical timescales for outflows, but not with the separations of emission knots in outflows, indicating that episodic accretion could either trigger or stop collimated large-scale outflows.

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arXiv2012

Near-Infrared H2 and Continuum Survey of Extended Green Objects

The Spitzer GLIMPSE survey has revealed a number of "Extended Green Objects" (EGOs) which display extended emission at 4.5 micron. These EGOs are potential candidates for high mass protostellar outflows. We have used high resolution (< 1") H2 1-0 S(1) line, K, and H-band images from the United Kingdom Infrared Telescope to study 34 EGOs to investigate their nature. We found that 12 EGOs exhibit H2 outflows (two with chains of H2 knotty structures; five with extended H2 bipolar structures; three with extended H2 lobes; two with pairs of H2 knots). In the 12 EGOs with H2 outflows, three of them exhibit similar morphologies between the 4.5 micron and H2 emission. However, the remaining 9 EGOs show that the H2 features are more extended than the continuum features, and the H2 emission is seldom associated with continuum emission. Furthermore, the morphologies of the near-infrared continuum and 4.5 micron emission are similar to each other for those EGOs with K-band emission, implying that at least a part of the IRAC-band continuum emission of EGOs comes from scattered light from the embedded YSOs.

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arXiv2011

New Galactic Star Clusters Discovered in the VVV Survey

VISTA Variables in the V\'ıa Láctea (VVV) is one of the six ESO Public Surveys operating on the new 4-meter Visible and Infrared Survey Telescope for Astronomy (VISTA). VVV is scanning the Milky Way bulge and an adjacent section of the disk, where star formation activity is high. One of the principal goals of the VVV Survey is to find new star clusters of different ages. In order to trace the early epochs of star cluster formation we concentrated our search in the directions to those of known star formation regions, masers, radio, and infrared sources. The disk area covered by VVV was visually inspected using the pipeline processed and calibrated $K_{\rm S}$-band tile images for stellar overdensities. Subsequently, we examined the composite $JHK_{\rm S}$ and $ZJK_{\rm S}$ color images of each candidate. PSF photometry of $15\times15$ arcmin fields centered on the candidates was then performed on the Cambridge Astronomy Survey Unit reduced images. After statistical field-star decontamination, color-magnitude and color-color diagrams were constructed and analyzed. We report the discovery of 96 new infrared open clusters and stellar groups. Most of the new cluster candidates are faint and compact (with small angular sizes), highly reddened, and younger than 5\,Myr. For relatively well populated cluster candidates we derived their fundamental parameters such as reddening, distance, and age by fitting the solar-metallicity Padova isochrones to the color-magnitude diagrams.

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arXiv2010

A near-infrared variability study in the cloud IC1396W: low star-forming efficiency and two new eclipsing binaries

Identifying the population of young stellar objects (YSOs) in high extinction regions is a prerequisite for studies of star formation. This task is not trivial, as reddened background objects can be indistinguishable from YSOs in near-infrared colour-colour diagrams. Here we combine deep JHK photometry with J- and K-band lightcurves, obtained with UKIRT/WFCAM, to explore the YSO population in the dark cloud IC1396W. We demonstrate that a colour-variability criterion can provide useful constraints on the star forming activity in embedded regions. For IC1396W we find that a near-infrared colour analysis alone vastly overestimates the number of YSOs. In total, the globule probably harbours not more than ten YSOs, among them a system of two young stars embedded in a small (~10000 AU) reflection nebula. This translates into a star forming efficiency SFE of ~1%, which is low compared with nearby more massive star forming regions, but similar to less massive globules. We confirm that IC1396W is likely associated with the IC1396 HII region. One possible explanation for the low SFE is the relatively large distance to the ionizing O-star in the central part of IC1396. Serendipitously, our variability campaign yields two new eclipsing binaries, and eight periodic variables, most of them with the characteristics of contact binaries.

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arXiv2007

Multiplicity Among Young Brown Dwarfs and Very Low Mass Stars

Characterizing multiplicity in the very low mass (VLM) domain is a topic of much current interest and fundamental importance. Here we report on a near-IR AO imaging survey of 31 young brown dwarfs and VLM stars, 28 of which are in Chamaeleon I, using the ESO VLT. Our survey is sensitive enough to detect equal mass binaries down to separations of 0.04-0.07" (6-10 AU at 160 pc) and, typically, companions with mass ratios as low as 0.2 outside of 0.2" (~30 AU). We resolve the suspected 0.16" (~26 AU) binary Cha_Halpha 2 and present two new binaries, Hn 13 and CHXR 15, with separations of 0.13" (~20 AU) and 0.30" (~50 AU); the latter is one of the widest VLM systems discovered to date. We do not find companions around the majority of our targets, giving a binary frequency of 11% [+9,-6], thus confirming the trend for a lower binary frequency with decreasing mass. By combining our work with previous surveys, we arrive at the largest sample of young VLMOs (72) with high angular resolution imaging to date. Its multiplicity fraction is in statistical agreement with that for VLMOs in the field. In addition we note that many field stellar binaries with lower binding energies and/or wider cross sections have survived dynamical evolution and that statistical models suggest tidal disruption by passing stars is unlikely to affect the binary properties of our systems. Thus, we argue that there is no significant evolution of multiplicity with age among brown dwarfs and VLM stars in OB and T associations between a few Myr to several Gyr. Instead, the observations to date suggest that VLM objects are either less likely to be born in fragile multiple systems than solar mass stars or such systems are disrupted very early.

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arXiv2007

Herbig-Haro Objects - Tracers of the Formation of Low-mass Stars and Sub-stellar Objects

Herbig-Haro objects (HHOs) are caused by outflows from young objects. Since the outflow relies on mass accretion from a circumstellar disk, it indicates ongoing growth. Recent results of infrared observations yielded evidence for disks around brown dwarfs. This suggests that at least a certain fraction of brown dwarfs forms like stars. Thus, young sub-stellar objects might cause HHOs as well. We present selected results of a general survey for HHOs based on DSS-II plates and CCD images taken with the Tautenburg Schmidt telescope. Numerous young objects could be identified due to their association with newly detected HHOs. In some cases the luminosity is consistent with very low-mass stars or close to sub-stellar values. This holds for L1415-IRS and a few infrared sources embedded in other dark clouds (e.g., GF9, BHR111). The question on the minimum mass for outflow activity is addressed.

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arXiv2006

Extinction techniques and impact on dust property determination

The near infrared extinction powerlaw index ($β$) and its uncertainty is derived from three different techniques based on star counts, colour excess and a combination of them. We have applied these methods to 2MASS data to determine maps of $β$ and near infrared extinction of the small cloud IC 1396 W. The combination of star counts and colour excess results in the most reliable method to determine $β$. It is found that the use of the correct $β$-map to transform colour excess values into extinction is fundamental for column density profile analysis of clouds. We describe how artificial photometric data, based on the model of stellar population synthesis of the Galaxy (Robin et al. 2003), can be used to estimate uncertainties and derive systematic effects of the extinction methods presented here. We find that all colour excess based extinction determination methods are subject to small but systematic offsets, which do not affect the star counting technique. These offsets occur since stars seen through a cloud do not represent the same population as stars in an extinction free control field.

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arXiv2005

Rotation and variability of young very low mass objects

Variability studies are an important tool to investigate key properties of stars and brown dwarfs. From photometric monitoring we are able to obtain information about rotation and magnetic activity, which are expected to change in the mass range below 0.3 solar masses, since these fully convective objects cannot host a solar-type dynamo. On the other hand, spectroscopic variability information can be used to obtain a detailed view on the accretion process in very young objects. In this paper, we report about our observational efforts to analyse the variability and rotational evolution of young brown dwarfs and very low mass stars.

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