arXiv subjects

Sebastian Wolf

Publications and source records attributed to Sebastian Wolf.

At least 127 records · Page 7Linked to original sources

arXiv2014

Large-scale magnetic fields in Bok globules

Context: The role of magnetic fields in the star formation process is a contentious matter of debate. In particular, no clear observational proof exists of a general influence by magnetic fields during the initial collapse of molecular clouds. Aims: Our aim is to examine magnetic fields and their influence on a wide range of spatial scales in low-mass star-forming regions. Method: We trace the large-scale magnetic field structure on scales of 10^3-10^5 AU in the local environment of Bok globules through optical and near-infrared polarimetry and combine these measurements with existing submillimeter measurements, thereby characterizing the small-scale magnetic field structure on scales of 10^2-10^3 AU. Results: For the first time, we present polarimetric observations in the optical and near-infrared of the three Bok globules B335, CB68, and CB54, combined with archival observations in the submillimeter and the optical. We find a significant polarization signal (P>=2%, P/sigma(P)>3) in the optical and near-infrared for all three globules. Additionally, we detect a connection between the structure on scales of 10^2-10^3 AU to 10^3-10^4 AU for both B335 and CB68. Furthermore, for CB54, we trace ordered polarization vectors on scales of ~10^5 AU. We determine a magnetic field orientation that is aligned with the CO outflow in the case of CB54, but nearly perpendicular to the CO outflow for CB68. For B335 we find a change in the magnetic field oriented toward the outflow direction, from the inner core to the outer regions. Conclusion: We find strongly aligned polarization vectors that indicate dominant magnetic fields on a wide range of spatial scales.

astro-ph.SR↗

arXiv2014

Cooling antihydrogen ions for the free-fall experiment GBAR

We discuss an experimental approach allowing to prepare antihydrogen atoms for the GBAR experiment. We study the feasibility of all necessary experimental steps: The capture of incoming $\bar{\rm H}^+$ ions at keV energies in a deep linear RF trap, sympathetic cooling by laser cooled Be$^+$ ions, transfer to a miniaturized trap and Raman sideband cooling of an ion pair to the motional ground state, and further reducing the momentum of the wavepacket by adiabatic opening of the trap. For each step, we point out the experimental challenges and discuss the efficiency and characteristic times, showing that capture and cooling are possible within a few seconds.

physics.atom-ph↗

arXiv2013

Structures in the protoplanetary disk of HD142527 seen in polarized scattered light

We present H- and Ks-band polarized differential images (PDI) of the Herbig Ae/Be star HD142527, revealing its optically thick outer disk and the nearly empty gap. The very small inner working angle (~0.1") and high resolution achievable with an 8m-class telescope, together with a careful polarimetric calibration strategy, allow us to achieve images that surpass the quality of previous scattered light images. Previously known substructures are resolved more clearly and new structures are seen. Specifically, we are able to resolve 1) half a dozen spiral structures in the disk, including previously known outer-disk spirals as well as new spiral arms and arcs close to the inner rim of the disk; 2) peculiar holes in the polarized surface brightness at position angles of ~0° and ~160°; 3) the inner rim on the eastern side of the disk; 4) the gap between the outer and inner disk, ranging from the inner working angle of 0.1" out to between 0.7 and 1.0", which is nearly devoid of dust. We then use a Markov-chain Monte-Carlo algorithm to determine several structural parameters of the disk, using very simple assumptions, including its inclination, eccentricity, and the scale height of the inner rim. We compare our results to previous work on this object, and try to produce a consistent picture of the system and its transition disk.

astro-ph.SR↗

arXiv2013

Evidence of a discontinuous disk structure around the Herbig Ae star HD 139 614

A new class of pre-main sequence objects has been recently identified as pre-transitional disks. They present near-infrared excess coupled to a flux deficit at about 10 microns and a rising mid-infrared and far-infrared spectrum. These features suggest a disk structure with inner and outer dust components, separated by a dust-depleted region (or gap). We here report on the first interferometric observations of the disk around the Herbig Ae star HD 139614. Its infrared spectrum suggests a flared disk, and presents pre-transitional features,namely a substantial near-infrared excess accompanied by a dip around 6 microns and a rising mid-infrared part. In this framework, we performed a study of the spectral energy distribution (SED) and the mid-infrared VLTI/MIDI interferometric data to constrain thespatial structure of the inner dust disk region and assess its possibly multi-component structure. We based our work on a temperature-gradient disk model that includes dust opacity. While we could not reproduce the SED and interferometric visibilities with a one-component disk, a better agreement was obtained with a two-component disk model composed of an optically thin inner disk extending from 0.22 to 2.3 au, a gap, and an outer temperature-gradient disk starting at 5.6 au. Therefore, our modeling favors an extended and optically thin inner dust component and in principle rules out the possibility that the near-infrared excess originates only from a spatially confined region. Moreover, the outer disk is characterized by a very steep temperature profile and a temperature higher than 300 K at its inner edge. This suggests the existence of a warm component corresponding to a scenario where the inner edge of the outer disk is directly illuminated by the central star. This is an expected consequence of the presence of a gap, thus indicative of a pre-transitional structure.

astro-ph.SR↗

arXiv2013

Small vs large dust grains in transitional disks: do different cavity sizes indicate a planet?

Transitional disks represent a short stage of the evolution of circumstellar material. Studies of dust grains in these objects can provide pivotal information on the mechanisms of planet formation. Dissimilarities in the spatial distribution of small (micron-size) and large (millimeter-size) dust grains have recently been pointed out. Constraints on the small dust grains can be obtained by imaging the distribution of scattered light at near-infrared wavelengths. We aim at resolving structures in the surface layer of transitional disks (with particular emphasis on the inner 10 - 50 AU), thus increasing the scarce sample of high resolution images of these objects. We obtained VLT/NACO near-IR high-resolution polarimetric differential imaging observations of SAO 206462 (HD135344B). This technique allows one to image the polarized scattered light from the disk without any occulting mask and to reach an inner working angle of 0.1''. A face-on disk is detected in H and Ks bands between 0.1'' and 0.9''. No significant differences are seen between the H and Ks images. In addition to the spiral arms, these new data allow us to resolve for the first time an inner cavity for small dust grains. The cavity size (about 28 AU) is much smaller than what is inferred for large dust grains from (sub)mm observations (39 to 50 AU). The interaction between the disk and potential orbiting companion(s) can explain both the spiral arm structure and the discrepant cavity sizes for small and large dust grains. One planet may be carving out the gas (and, thus, the small grains) at 28 AU, and generating a pressure bump at larger radii (39 AU), which holds back the large grains. We analytically estimate that, in this scenario, a single giant planet (with a mass between 5 and 15 Jupiter masses) at 17 to 20 AU from the star is consistent with the observed cavity sizes.

astro-ph.EP↗

arXiv2013

A diversity of dusty AGN tori: Data release for the VLTI/MIDI AGN Large Program and first results for 23 galaxies

The AGN-heated dust distribution (the "torus") is increasingly recognized not only as the absorber required in unifying models, but as a tracer for the reservoir that feeds the nuclear Super-Massive Black Hole. Yet, even its most basic structural properties (such as its extent, geometry and elongation) are unknown for all but a few archetypal objects. Since most AGNs are unresolved in the mid-infrared, we utilize the MID-infrared interferometric Instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) that is sensitive to structures as small as a few milli-arcseconds (mas). We present here an extensive amount of new interferometric observations from the MIDI AGN Large Program (2009 - 2011) and add data from the archive to give a complete view of the existing MIDI observations of AGNs. Additionally, we have obtained high-quality mid-infrared spectra from VLT/VISIR. We present correlated and total flux spectra for 23 AGNs and derive flux and size estimates at 12 micron using simple axisymmetric geometrical models. Perhaps the most surprising result is the relatively high level of unresolved flux and its large scatter: The median "point source fraction" is 70 % for type 1 and 47 % for type 2 AGNs meaning that a large part of the flux is concentrated on scales smaller than about 5 mas (0.1 - 10 pc). Among sources observed with similar spatial resolution, it varies from 20 % - 100 %. For 18 of the sources, two nuclear components can be distinguished in the radial fits. While these models provide good fits to all but the brightest sources, significant elongations are detected in eight sources. The half-light radii of the fainter sources are smaller than expected from the size ~ L^0.5 scaling of the bright sources and show a large scatter, especially when compared to the relatively tight size--luminosity relation in the near-infrared.

astro-ph.CO↗

arXiv2013

Vertical settling and radial segregation of large dust grains in the circumstellar disk of the Butterfly Star

Context: Circumstellar disks are considered to be the environment for the formation of planets. The growth of dust grains in these disks is the first step in the core accretion-gas capture planet formation scenario. Indicators and evidence of disk evolution can be traced in spatially resolved images and the spectral energy distribution (SED) of these objects. Aims: We develop a model for the dust phase of the edge-on oriented circumstellar disk of the Butterfly Star which allows one to fit observed multi-wavelength images and the SED simultaneously. Methods: Our model is based on spatially resolved high angular resolution observations at 1.3 mm, 894 micron, 2.07 micron, 1.87 micron, 1.60 micron, and 1.13 micron and an extensively covered SED ranging from 12 micron to 2.7 mm, including a detailed spectrum obtained with the Spitzer Space Telescope in the range from 12 micron to 38 micron. A parameter study based on a grid search method involving the detailed analysis of every parameter was performed to constrain the disk parameters and find the best-fit model for the independent observations. The individual observations were modeled simultaneously, using our continuum radiative transfer code. Results: We derived a model that is capable of reproducing all of the observations of the disk at the same time. We find quantitative evidence for grain growth up to ~100 micron-sized particles, vertical settling of larger dust grains toward the disk midplane, and radial segregation of the latter toward the central star. Within our best-fit model the large grains have a distribution with a scale height of 3.7 AU at 100 AU and a radial extent of 175 AU compared to a hydrostatic scale height of 6.9 AU at 100 AU and an outer disk radius of 300 AU. Our results are consistent with current theoretical models for the evolution of circumstellar disks and the early stages of planet formation.

astro-ph.SR↗

arXiv2013

Mid-infrared observations of the circumstellar disks around PDS 66 and CRBR 2422.8-3423

Aims: We present mid-infrared observations and photometry of the circumstellar disks around PDS 66 and CRBR 2422.8-3423, obtained with VISIR/VLT in the N band and for the latter also in the Q band. Our aim is to resolve the inner regions of these protoplanetary disks, which carry potential signatures of intermediate or later stages of disk evolution and ongoing planet formation. Methods: We determined the radial brightness profiles of our target objects and the corresponding PSF reference that were observed before and after our target objects. Background standard deviations, the standard errors, and the seeing variations during the observations were considered. Adopting a simple radiative transfer model based on parameters taken from previous studies, we derived constraints on the inner-disk hole radius of the dust disk. Results: Neither of the circumstellar disks around our science targets are spatially resolved in our observations. However, we are able to constrain the inner-disk hole radius to <15.0(+0.5,-0.5) AU and <10.5(+0.5,-1.0) AU for PDS 66 and CRBR 2422.8-3423, respectively. The photometry we performed yields N-band flux densities of 599+-8 mJy for PDS 66 and 130+-14 mJy for CRBR 2422.8-3423, as well as a Q-band flux density of 858+-109 mJy for CRBR 2422.8-3423.

astro-ph.SR↗

arXiv2013

Porous dust grains in debris disks

When modeling the density and grain size distribution in debris disks, the minimum particle size is often significantly larger than the corresponding blowout size. While the dust particles are usually modeled as compact, homogenous spheres, we instead investigate the impact of porosity. The optical properties of porous particles are determined, and the influences of porosity on the blowout size and dust temperatures investigated. Using the method of discrete dipole approximation, we calculate the scattering and absorption cross sections of porous particles and derive the blowout size and the behavior of the dust temperature. We investigate the influence on the beta-ratio. Blowout sizes are calculated for various stellar luminosities and porosities, and an approximation equation is derived to estimate the blowout size as a function of these parameters. Furthermore, we investigate the influence of the porosity on the dust equilibrium temperature. The blowout size increases with the particle porosity and stellar luminosity. The dust temperature of porous particles is lower than the one of the compact spheres, in particular the temperature of blowout grains decreases for porous particles.

astro-ph.SR↗

arXiv2013

Gaps in the HD169142 protoplanetary disk revealed by polarimetric imaging: Signs of ongoing planet formation?

We present H-band VLT/NACO polarized light images of the Herbig Ae/Be star HD169142 probing its protoplanetary disk as close as ~0.1" to the star. Our images trace the face-on disk out to ~1.7" (~250 AU) and reveal distinct sub-structures for the first time: 1) the inner disk (<20 AU) appears to be depleted in scattering dust grains; 2) an unresolved disk rim is imaged at ~25 AU; 3) an annular gap extends from ~40 - 70 AU; 4) local brightness asymmetries are found on opposite sides of the annular gap. We discuss different explanations for the observed morphology among which ongoing planet formation is a tempting - but yet to be proven - one. Outside of ~85 AU the surface brightness drops off roughly r^{-3.3}, but describing the disk regions between 85-120 AU / 120-250 AU separately with power-laws r^{-2.6} / r^{-3.9} provides a better fit hinting towards another discontinuity in the disk surface. The flux ratio between the disk integrated polarized light and the central star is ~4.1 * 10^{-3}. Finally, combining our results with those from the literature, ~40% of the scattered light in the H-band appears to be polarized. Our results emphasize that HD169142 is an interesting system for future planet formation or disk evolution studies.

astro-ph.GA↗

arXiv2012

Tracing large-scale structures in circumstellar disks with ALMA

Planets are supposed to form in circumstellar disks. The gravitational potential of a planet perturbs the disk and leads to characteristic structures, i.e. spiral waves and gaps, in the disk's density profile. We perform a large-scale parameter study of the observability of these planet-induced structures in circumstellar disks with ALMA. On the basis of HD and MHD simulations, we calculated the disk temperature structure and (sub)mm images of these systems. These were used to derive simulated ALMA images. Because appropriate objects are frequent in Taurus, we focused on a distance of 140pc and a declination of 20°. The explored range of star-disk-planet configurations consists of 6 HD simulations (including magnetic fields and different planet masses), 9 disk sizes, 15 total disk masses, 6 different central stars, and two different grain size distributions. On almost all scales and in particular down to a scale of a few AU, ALMA is able to trace disk structures induced by planet-disk interaction or by the influence of magnetic fields on the wavelength range between 0.4 and 2.0mm. In most cases, the optimum angular resolution is limited by the sensitivity. However, within the range of typical masses of protoplanetary disks (0.1-0.001Msun) the disk mass has a minor impact on the observability. It is possible to resolve disks down to 2.67e-6Msun and trace gaps induced by a planet with M_p/M_s = 0.001 in disks with 2.67e-4Msun with a signal-to-noise ratio greater than three. The central star has a major impact on the observability of gaps, as well as the considered maximum grainsize of the dust in the disk. In general, it is more likely to trace planet-induced gaps in our MHD models, because gaps are wider in the presence of magnetic fields. We also find that zonal flows resulting from MRI create gap-like structures in the disk's re-emission radiation, which are observable with ALMA.

astro-ph.SR↗

arXiv2012

A Comparison of Approaches in Fitting Continuum SEDs

We present a detailed comparison of two approaches, the use of a pre-calculated database and simulated annealing (SA), for fitting the continuum spectral energy distribution (SED) of astrophysical objects whose appearance is dominated by surrounding dust. While pre-calculated databases are commonly used to model SED data, only few studies to date employed SA due to its unclear accuracy and convergence time for this specific problem. From a methodological point of view, different approaches lead to different fitting quality, demand on computational resources and calculation time. We compare the fitting quality and computational costs of these two approaches for the task of SED fitting to provide a guide to the practitioner to find a compromise between desired accuracy and available resources. To reduce uncertainties inherent to real datasets, we introduce a reference model resembling a typical circumstellar system with 10 free parameters. We derive the SED of the reference model with our code MC3D at 78 logarithmically distributed wavelengths in the range [0.3um, 1.3mm] and use this setup to simulate SEDs for the database and SA. Our result shows directly the applicability of SA in the field of SED modeling, since the algorithm regularly finds better solutions to the optimization problem than a pre-calculated database. As both methods have advantages and shortcomings, a hybrid approach is preferable. While the database provides an approximate fit and overall probability distributions for all parameters deduced using Bayesian analysis, SA can be used to improve upon the results returned by the model grid.

astro-ph.EP↗

arXiv2012

Observing planet-disk interaction in debris disks

Context. Structures in debris disks induced by planetdisk interaction are promising to provide valuable constraints on the existence and properties of embedded planets. Aims. We investigate the observability of structures in debris disks induced by planet-disk interaction. Methods. The observability of debris disks with the Atacama Large Millimeter/submillimeter Array (ALMA) is studied on the basis of a simple analytical disk model. Furthermore, N-body simulations are used to model the spatial dust distribution in debris disks under the influence of planet-disk interaction. Images at optical scattered light to millimeter thermal re-emission are computed. Available information about the expected capabilities of ALMA and the James Webb Space Telescope (JWST) are used to investigate the observability of characteristic disk structures through spatially resolved imaging. Results. Planet-disk interaction can result in prominent structures. This provides the opportunity of detecting and characterizing extrasolar planets in a range of masses and radial distances from the star that is not accessible to other techniques. Facilities that will be available in the near future are shown to provide the capabilities to spatially resolve and characterize structures in debris disks. Limitations are revealed and suggestions for possible instrument setups and observing strategies are given. In particular, ALMA is limited by its sensitivity to surface brightness, which requires a trade-off between sensitivity and spatial resolution. Space-based midinfrared observations will be able to detect and spatially resolve regions in debris disks even at a distance of several tens of AU from the star, where the emission from debris disks in this wavelength range is expected to be low. [Abridged]

astro-ph.IM↗

arXiv2012

A Herschel Survey of Cold Dust in Disks Around Brown Dwarfs and Low-Mass Stars

We report the complete photometric results from our Herschel study which is the first comprehensive program to search for far-infrared emission from cold dust around young brown dwarfs. We surveyed 50 fields containing 51 known or suspected brown dwarfs and very low mass stars that have evidence of circumstellar disks based on Spitzer photometry and/or spectroscopy. The objects with known spectral types range from M3 to M9.5. Four of the candidates were subsequently identified as extragalactic objects. Of the remaining 47 we have successfully detected 36 at 70micron and 14 at 160micron with S/N greater than 3, as well as several additional possible detections with low S/N. The objects exhibit a range of [24]--[70] micron colors suggesting a range in mass and/or structure of the outer disk. We present modeling of the spectral energy distributions of the sample and discuss trends visible in the data. Using two Monte Carlo radiative transfer codes we investigate disk masses and geometry. We find a very wide range in modeled total disk masses from less than 1e-6 solar masses up to 1e-3 solar masses with a median disk mass of order 3e-5 solar masses, suggesting that the median ratio of disk mass to central object mass may be lower than for T Tauri stars. The disk scale heights and flaring angles, however, cover a range consistent with those seen around T Tauri stars. The host clouds in which the young brown dwarfs and low-mass stars are located span a range in estimated age from ~1-3 Myr to ~10 Myr and represent a variety of star-forming environments. No obvious dependence on cloud location or age is seen in the disk properties, though the statistical significance of this conclusion is not strong.

astro-ph.SR↗

arXiv2012

Circumstellar disks and planets. Science cases for next-generation optical/infrared long-baseline interferometers

We present a review of the interplay between the evolution of circumstellar disks and the formation of planets, both from the perspective of theoretical models and dedicated observations. Based on this, we identify and discuss fundamental questions concerning the formation and evolution of circumstellar disks and planets which can be addressed in the near future with optical and infrared long-baseline interferometers. Furthermore, the importance of complementary observations with long-baseline (sub)millimeter interferometers and high-sensitivity infrared observatories is outlined.

astro-ph.IM↗

arXiv2012

The Debris Disk Candidates: Eleven 24$μ$m excess stars in Spitzer SWIRE Fields

We present the optical to mid-infrared SEDs of 11 debris disk candidates from $Spitzer$ SWIRE fields. All these candidates are selected from SWIRE 24$μ$m sources matched with both the SDSS star catalog and the 2MASS point source catalog. They show an excess in the mid-infrared at 24$μ$m ($K_S$-[24]$_{Vega}$ $\ge$ 0.44) indicating the presence of a circumstellar dust disk. The observed optical spectra show that they are all late type main-sequence stars covering the spectral types of FGKM. Their fractional luminosities are well above 5$\times10^{-5}$, even up to the high fractional luminosity of 1$\times10^{-3}$. The high galactic latitudes of SWIRE fields indicate that most of these candidates could belong to the oldest stars in the thick disk. Our results indicate that the high fractional luminosity debris disks could exist in the old solar-like star systems, though they are now still quite rare. Their discoveries at high-galactic latitudes will also provide us an excellent opportunity to the further studies of properties and evolution of the debris disk in the ISM poor environments.

astro-ph.SR↗

arXiv2011

Resolving the inner regions of the HD97048 circumstellar disk with VLT/NACO polarimetric differential imaging

Circumstellar disks are the cradles of planetary systems and their physical and chemical properties directly influence the planet formation process. As most planets supposedly form in the inner disk regions, i.e., within a few tens of AU, it is crucial to study circumstellar disk on these scales to constrain the conditions for planet formation. Our aims are to characterize the inner regions of the circumstellar disk around the young Herbig Ae/Be star HD97048 in polarized light. We use VLT/NACO to observe HD97048 in polarimetric differential imaging (PDI) mode in the H and Ks band. We spatially resolve the disk around HD97048 in polarized flux in both filters on scales between ~0.1"-1.0" corresponding to the inner ~16-160 AU. Fitting isophots to the flux calibrated H-band image between 13 - 14 mag/arcsec^2 and 14 - 15 mag/arcsec^2 we derive a apparent disk inclination angle of 34+-5 deg and 47+-2 deg, respectively. The disk position angle in both brightness regimes is almost identical and roughly 80 deg. Along the disk major axis the surface brightness of the polarized flux drops from ~11 mag/arcsec^2 at ~0.1" (~16 AU) to ~15.3 mag/arcsec^2 at ~1.0" (~160 AU). The brightness profiles along the major axis are fitted with power-laws falling off as ~r^(-1.78+-0.02) in H and ~r^(-2.34+-0.04) in Ks. As the surface brightness drops off more rapidly in Ks compared to H the disks becomes relatively bluer at larger separations possibly indicating changing dust grain properties as a function of radius. For the first time the inner ~0.1"-1.0" (~16-160 AU) of the surface layer of the HD97048 circumstellar disk have been imaged in scattered light demonstrating the power of ground-based imaging polarimetry. Our data fill an important gap in a large collection of existing data including resolved thermal dust and PAH emission images as well as resolved gas emission lines.

astro-ph.GA↗

arXiv2011

Transition disk chemistry and future prospects with ALMA

We explore the chemical structure of a disk that contains a large central gap of R ~ 45 AU, as is commonly seen in transitional disk systems. In our chemical model of a disk with a cleared inner void, the midplane becomes revealed to the central star so that it is directly irradiated. The midplane material at the truncation radius is permissive to reprocessed optical heating radiation, but opaque to the photo-dissociating ultraviolet, creating an environment abundant in gas-phase molecules. Thus the disk midplane, which would otherwise for a full disk be dominated by near complete heavy element freeze-out, should become observable in molecular emission. If this prediction is correct this has exciting prospects for observations with the Atacama Large Millimeter/Submillimeter Array (ALMA), as the inner transition region should thus be readily detected and resolved, especially using high-J rotational transitions excited in the high density midplane gas. Therefore such observations will potentially provide us with a direct probe of the physics and chemistry at this actively evolving interface.

astro-ph.SR↗