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Joergens V.,Institute For Theoretische Astrophysik | Joergens V.,Max Planck Institute for Astronomy | Pohl A.,Max Planck Institute for Astronomy | Sicilia-Aguilar A.,Autonomous University of Madrid | Henning T.,Max Planck Institute for Astronomy
Astronomy and Astrophysics | Year: 2012

We show that the very young brown dwarf candidate ISO 217 (M6.25) is driving an intrinsically asymmetric bipolar outflow with a stronger and slightly faster red-shifted component based on spectro-astrometry of forbidden [S II] emission lines at 6716Å and 6731Å observed in UVES/VLT spectra taken in 2009. ISO 217 is only one of a handful of brown dwarfs and very low-mass stars (M5-M8) for which an outflow has been detected and that show that the T Tauri phase continues at the substellar limit. We measure a spatial extension of the outflow in [S II] of up to ± 190 mas (about ± 30 AU) and velocities of up to ± 40-50 km s -1. We find that the basic outflow properties (spatial extension, velocities, and outflow position angle) are of similar order as those determined in the discovery spectra from May 2007 of Whelan and coworkers. We show that the velocity asymmetry between both lobes is variable on timescales of a few years and that the strong asymmetry of a factor of two found in 2007 might be smaller than originally anticipated when using a more realistic stellar rest-velocity. We also detect forbidden line emission of [Fe II]λ7155Å, for which we propose as a potential origin the hot inner regions of the outflow. To comprehensively understand the ISO 217 system, we determine the properties of its accretion disk based on radiative transfer modeling of the SED from 0.66 to 24 μm. This disk model agrees very well with Herschel/PACS data at 70 μm. We find that the disk is flared and intermediately inclined (i ∼ 45°). The total disk mass of the best-fit model is 4 × 10 -6 M ·, which is low compared to the accretion and outflow rate of ISO 217 from the literature (∼). We propose that this discrepancy can be explained by either a higher disk mass than inferred from the model because of strong undetected grain growth and/or by an on average lower accretion rate and outflow rate than the determined values. We show that a disk inclination significantly exceeding 45°, as suggested from Hα modeling and from both lobes of the outflow being visible, is inconsistent with the SED data. Thus, despite its intermediate inclination angle, the disk of this brown dwarf does not appear to obscure the red outflow component in [S II], which is very rarely seen for T Tauri objects (only one other case). © 2012 ESO. Source

Walch S.,University of Cardiff | Whitworth A.P.,University of Cardiff | Girichidis P.,University of Cardiff | Girichidis P.,Institute For Theoretische Astrophysik
Monthly Notices of the Royal Astronomical Society | Year: 2012

The collapse of weakly turbulent pre-stellar cores is a critical stage in the process of star formation. Being highly non-linear and stochastic, the outcome of collapse can only be explored theoretically by performing large ensembles of numerical simulations. Standard practice is to quantify the initial turbulent velocity field in a core in terms of the amount of turbulent energy (or some equivalent) and the exponent in the power spectrum (n≡-dlogP k/dlogk). In this paper, we present a numerical study of the influence of the details of the turbulent velocity field on the collapse of an isolated, weakly turbulent, low-mass pre-stellar core. We show that, as long asn≳ 3 (as is usually assumed), a more critical parameter thannis the maximum wavelength in the turbulent velocity field, λ MAX. This is because λ MAX carries most of the turbulent energy, and thereby influences both the amountandthe spatial coherence of the angular momentum in the core. We show that the formation of dense filaments during collapse depends critically on λ MAX, and we explain this finding using a force balance analysis. We also show that the core has only a high probability of fragmenting if λ MAX >R CORE/2 (whereR CORE is the core radius), the dominant mode of fragmentation involves the formation and break-up of filaments and although small protostellar discs (with radiusR DISC≲ 20 au) form routinely, more extended discs are rare. In turbulent, low-mass cores of the type we simulate here, the formation of large, fragmenting protostellar discs is suppressed by early fragmentation in the filaments. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS. Source

Andrae R.,Max Planck Institute for Astronomy | Jahnke K.,Max Planck Institute for Astronomy | Melchior P.,Institute For Theoretische Astrophysik
Monthly Notices of the Royal Astronomical Society | Year: 2011

Given the enormous galaxy data bases of modern sky surveys, parametrizing galaxy morphologies is a very challenging task due to the huge number and variety of objects. We assess the different problems faced by existing parametrization schemes (CAS, Gini, M20, Sérsic profile, shapelets) in an attempt to understand why parametrization is so difficult and in order to suggest improvements for future parametrization schemes. We demonstrate that morphological observables (e.g. steepness of the radial light profile, ellipticity, asymmetry) are intertwined and cannot be measured independently of each other. We present strong arguments in favour of model-based parametrization schemes, namely reliability assessment, disentanglement of morphological observables and point spread function modelling. Furthermore, we demonstrate that estimates of the concentration and Sérsic index obtained from the Zurich Structure & Morphology catalogue are in excellent agreement with theoretical predictions. We also demonstrate that the incautious use of the concentration index for classification purposes can cause a severe loss of the discriminative information contained in a given data sample. Moreover, we show that, for poorly resolved galaxies, concentration index and M20 suffer from strong discontinuities, i.e. similar morphologies are not necessarily mapped to neighbouring points in the parameter space. This limits the reliability of these parameters for classification purposes. Two-dimensional Sérsic profiles accounting for centroid and ellipticity are identified as the currently most reliable parametrization scheme in the regime of intermediate signal-to-noise ratios and resolutions, where asymmetries and substructures do not play an important role. We argue that basis functions provide good parametrization schemes in the regimes of high signal-to-noise ratios and resolutions. Concerning Sérsic profiles, we show that scale radii cannot be compared directly for profiles of different Sérsic indices. Furthermore, we show that parameter spaces are typically highly non-linear. This implies that significant caution is required when distance-based classification methods are used. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS. Source

Andrae R.,Max Planck Institute for Astronomy | Melchior P.,Institute For Theoretische Astrophysik | Melchior P.,Ohio State University | Jahnke K.,Max Planck Institute for Astronomy
Monthly Notices of the Royal Astronomical Society | Year: 2011

Parametrization of galaxy morphologies is a challenging task, for instance in shear measurements of weak gravitational lensing or investigations of formation and evolution of galaxies. The huge variety of different morphologies requires a parametrization scheme that is highly flexible and that accounts for certain morphological observables, such as ellipticity, steepness of the radial light profile and azimuthal structure. In this article, we revisit the method of sérsiclets, where galaxy morphologies are decomposed into a set of polar basis functions that are based on the Sérsic profile. This approach is justified by the fact that the Sérsic profile is the first-order Taylor expansion of any real light profile. We show that sérsiclets indeed overcome the modelling failures of shapelets in the case of early-type galaxies. However, sérsiclets implicate an unphysical relation between the steepness of the light profile and the spatial scale of the polynomial oscillations, which is not necessarily obeyed by real galaxy morphologies and can therefore give rise to modelling failures. Moreover, we demonstrate that sérsiclets are prone to undersampling, which restricts sérsiclet modelling to highly resolved galaxy images. Analysing data from the weak-lensing great08 challenge, we demonstrate that sérsiclets should not be used in weak-lensing studies. We conclude that although the sérsiclet approach appears very promising at first glance, it suffers from conceptual and practical problems that severely limit its usefulness. In particular, sérsiclets do not provide high-precision results in weak-lensing studies. Finally, we show that the Sérsic profile can be enhanced by higher order terms in the Taylor expansion, which can drastically improve model reconstructions of galaxy images. When orthonormalized, these higher order profiles can overcome the problems of sérsiclets, while preserving their mathematical justification. However, this method is computationally expensive. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS. Source

Joergens V.,Max Planck Institute for Astronomy | Joergens V.,Institute For Theoretische Astrophysik | Muller A.,Max Planck Institute for Astronomy | Muller A.,European Southern Observatory | Reffert S.,Zentrum fur Astronomie Heidelberg
Astronomy and Astrophysics | Year: 2010

The very young brown dwarf candidate ChaHα 8 was recently discovered to have a close (∼1 AU) companion by means of radial velocity monitoring.We present additional radial velocity data obtained with UVES/VLT between 2007 and 2010, which significantly improve the orbit determination of the system. The combined data set spans ten years of radial velocity monitoring for ChaHα 8. A Kepler fit to the data yields an orbital period of 5.2 years, an eccentricity of e = 0.59, and a radial velocity semi-amplitude of 2.4 kms-1. A companion mass M2 sin i (which is a lower limit due to the unknown orbital inclination i) of 25 ± 7 MJup and of 31 ± 8 MJup is derived when using model-dependent mass estimates for the primary of 0.07 M⊙ and 0.10 M⊙, respectively. Assuming random orientation of orbits in space, we find a very high probability that the companion of ChaHa 8 is of substellar nature: With a greater than 50% probability (i = 60.), the companion mass is between 30 and 35 MJup and the mass ratio M2/M1 smaller than 0.4; with a greater than 87% probability (i = 30.) the companion mass is between 30 and 69 MJup and the mass ratio smaller than 0.7. The absence of any evidence of the companion in the cross-correlation function together with the size of the radial velocity amplitude also indicate a mass ratio of at most 0.7, and likely smaller. Furthermore, the new data exclude the possibility that the companion has a mass in the planetary regime (≤13 MJup). We show that the companion contributes significantly to the total luminosity of the system: model-dependent estimates provide a minimum for the luminosity ratio L2/L1 of 0.2. ChaHα 8 is the fourth known spectroscopic brown dwarf or very low-mass stellar binary with determined orbital parameters, and the second known very young one. With an age of only about 3 Myr, it is of particular interest to very low-mass formation and evolution theories. In contrast to most other spectroscopic binaries, it has a relatively long orbital period and it might be possible to determine the astrometric orbit of the primary and, thus, the orbital inclination. © ESO, 2010. Source

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