arXiv subjects

Thomas Henning

Publications and source records attributed to Thomas Henning.

At least 505 records · Page 28Linked to original sources

arXiv2012

Transient dust in warm debris disks - Detection of Fe-rich olivine grains

(Abridged) Debris disks trace remnant reservoirs of leftover planetesimals in planetary systems. A handful of "warm" debris disks have been discovered in the last years, where emission in excess starts in the mid-infrared. An interesting subset within these warm debris disks are those where emission features are detected in mid-IR spectra, which points towards the presence of warm micron-sized dust grains. Given the ages of the host stars, the presence of these grains is puzzling, and questions their origin and survival in time. This study focuses on determining the mineralogy of the dust around 7 debris disks with evidence for warm dust, based on Spitzer/IRS spectroscopic data, in order to provide new insights into the origin of the dust grains. We present a new radiative transfer code dedicated to SED modeling of optically thin disks. We make use of this code on the SEDs of seven warm debris disks, in combination with recent laboratory experiments on dust optical properties. We find that most, if not all, debris disks in our sample are experiencing a transient phase, suggesting a production of small dust grains on relatively short timescales. From a mineralogical point of view, we find that enstatite grains have small abundances compared to crystalline olivine grains. The main result of our study is that we find evidences for Fe-rich crystalline olivine grains (Fe / [Mg + Fe] ~ 0.2) for several debris disks. This finding contrasts with studies of gas-rich protoplanetary disks. The presence of Fe-rich olivine grains, and the overall differences between the mineralogy of dust in Class II disks compared to debris disks suggest that the transient crystalline dust is of a new generation. We discuss possible crystallization routes to explain our results, and comment on the mechanisms that may be responsible for the production of small dust grains.

astro-ph.SR↗

arXiv2012

The smoothness of the interstellar extinction curve in the UV. Comparison with recent laboratory measurements of PAH mixtures

Context. As revealed by high-resolution spectral investigations in the wavelength range between 300 and 400 nm, the interstellar extinction curve does not display any of the sharp electronic absorption bands that are characteristic for large polyatomic molecules, such as polycyclic aromatic hydrocarbons (PAHs), which belong to the most abundant interstellar molecules. Aims. We aim to verify whether the absorption curves of mixtures of medium-sized PAHs produced in the laboratory can explain the astronomical observations. Methods. The PAH mixtures were synthesized by infrared laser pyrolysis and subsequent chemical extraction and size separation. The matrix isolation technique was used to study the absorption spectra of isolated molecules at low temperature. Results. Our experimental results demonstrate that the UV-visible absorption curves of PAH mixtures can be very smooth, displaying no sharp bands, if the molecular diversity is sufficiently high. Conclusions. In view of the absence of sharp electronic features on the interstellar extinction curve for 300 < λ < 400 nm, we conclude from our experimental findings that the interstellar PAH population must be very diverse. The low fractional abundances of individual species prevent their detection on the basis of spectral fingerprints in the UV.

astro-ph.GA↗

arXiv2012

SMA and Spitzer Observations of Bok Glouble CB17: A Candidate First Hydrostatic Core?

We present high angular resolution SMA and Spitzer observations toward the Bok globule CB17. SMA 1.3mm dust continuum images reveal within CB17 two sources with an angular separation of about 21" (about 5250 AU at a distance of 250 pc). The northwestern continuum source, referred to as CB17 IRS, dominates the infrared emission in the Spitzer images, drives a bipolar outflow extending in the northwest-southeast direction, and is classified as a low luminosity Class0/I transition object (L_bol ~ 0.5 L_sun). The southeastern continuum source, referred to as CB17 MMS, has faint dust continuum emission in the SMA 1.3mm observations (about 6 sigma detection; ~3.8 mJy), but is not detected in the deep Spitzer infrared images at wavelengths from 3.6 to 70 micron. Its bolometric luminosity and temperature, estimated from its spectral energy distribution, are less than 0.04 L_sun and 16 K, respectively. The SMA CO(2-1) observations suggest that CB17 MMS may drive a low-velocity molecular outflow (about 2.5 km/s), extending in the east-west direction. Comparisons with prestellar cores and Class0 protostars suggest that CB17 MMS is more evolved than prestellar cores but less evolved than Class0 protostars. The observed characteristics of CB17 MMS are consistent with the theoretical predictions from radiative/magneto hydrodynamical simulations of a first hydrostatic core, but there is also the possibility that CB17 MMS is an extremely low luminosity protostar deeply embedded in an edge-on circumstellar disk. Further observations are needed to study the properties of CB17 MMS and to address more precisely its evolutionary stage.

astro-ph.SR↗

arXiv2012

WASP-4b Transit Observations With GROND

Ground-based simultaneous multiband transit observations allow an accurate system parameters determination and may lead to the detection and characterization of additional bodies via the transit timing variations (TTVs) method. We aimed to (i) characterize the heavily bloated WASP-4b hot Jupiter and its star by measuring system parameters and the dependence of the planetary radius as a function of four (Sloan g', r', i', z') wavelengths and (ii) search for TTVs. We recorded 987 images during three complete transits with the GROND instrument, mounted on the MPG/ESO-2.2m telescope at La Silla Observatory. Assuming a quadratic law for the stellar limb darkening we derive system parameters by fitting a composite transit light curve over all bandpasses simultaneously. To compute uncertainties of the fitted parameters, we employ the Bootstrap Monte Carlo Method. The three central transit times are measured with precision down to 6 s. We find a planetary radius Rp = 1.413+/-0.020 RJup, an orbital inclination i = 88deg57'+/-0.45deg and calculate a new ephemeris, a period P = 1.33823144+/-0.00000032days and a reference transit epoch T0 = 2454697.798311+/-0.000046 (BJD). Analysis of the new transit mid-times in combination with previous measurements shows no sign of a TTV signal greater than 20s. We perform simplified numerical simulations to place upper-mass limits of a hypothetical perturber in the WASP-4b system.

astro-ph.SR↗

arXiv2012

Planetesimal formation by sweep-up: How the bouncing barrier can be beneficial to growth

The formation of planetesimals is often accredited to collisional sticking of dust grains. The exact process is unknown, as collisions between larger aggregates tend to lead to fragmentation or bouncing rather than sticking. Recent laboratory experiments have however made great progress in the understanding and mapping of the complex physics involved in dust collisions. We want to study the possibility of planetesimal formation using the results from the latest laboratory experiments, particularly by including the fragmentation with mass transfer effect, which might lead to growth even at high impact velocities. We present a new experimentally and physically motivated dust collision model capable of predicting the outcome of a collision between two particles of arbitrary masses and velocities. It is used together with a continuum dust-size evolution code that is both fast in terms of execution time and able to resolve the dust well at all sizes, allowing for all types of interactions to be studied without biases. We find that for the general dust population, bouncing collisions prevent the growth above millimeter-sizes. However, if a small number of cm-sized particles are introduced, for example due to vertical mixing or radial drift, they can act as a catalyst and start to sweep up the smaller particles. At a distance of 3 AU, 100-meter-sized bodies are formed on a timescale of 1 Myr. We conclude that direct growth of planetesimals might be a possibility thanks to a combination of the existence of a bouncing barrier and the fragmentation with mass transfer effect. The bouncing barrier is here even beneficial, as it prevents the growth of too many large particles that would otherwise only fragment among each other, and creates a reservoir of small particles that can be swept up by larger bodies. However, for this process to work, a few seeds of cm in size or larger have to be introduced.

astro-ph.EP↗

arXiv2011

The Initial Mass Function of the Orion Nebula Cluster across the H-burning limit

We present a new census of the Orion Nebula Cluster (ONC) over a large field of view (>30'x30'), significantly increasing the known population of stellar and substellar cluster members with precisely determined properties. We develop and exploit a technique to determine stellar effective temperatures from optical colors, nearly doubling the previously available number of objects with effective temperature determinations in this benchmark cluster. Our technique utilizes colors from deep photometry in the I-band and in two medium-band filters at lambda~753 and 770nm, which accurately measure the depth of a molecular feature present in the spectra of cool stars. From these colors we can derive effective temperatures with a precision corresponding to better than one-half spectral subtype, and importantly this precision is independent of the extinction to the individual stars. Also, because this technique utilizes only photometry redward of 750nm, the results are only mildly sensitive to optical veiling produced by accretion. Completing our census with previously available data, we place some 1750 sources in the Hertzsprung-Russel diagram and assign masses and ages down to 0.02 solar masses. At faint luminosities, we detect a large population of background sources which is easily separated in our photometry from the bona fide cluster members. The resulting initial mass function of the cluster has good completeness well into the substellar mass range, and we find that it declines steeply with decreasing mass. This suggests a deficiency of newly formed brown dwarfs in the cluster compared to the Galactic disk population.

astro-ph.GA↗

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

On the stability of radiation-pressure-dominated cavities

Context: When massive stars exert a radiation pressure onto their environment that is higher than their gravitational attraction, they launch a radiation-pressure-driven outflow. It has been claimed that a radiative Rayleigh-Taylor instability should lead to the collapse of the outflow cavity and foster the growth of massive stars. Aims: We investigate the stability of idealized radiation-pressure-dominated cavities, focusing on its dependence on the radiation transport approach for the stellar radiation feedback. Methods: We compare two different methods for stellar radiation feedback: gray flux-limited diffusion (FLD) and ray-tracing (RT). We also derive simple analytical models to support our findings. Results: Only the FLD cases lead to prominent instability in the cavity shell. The RT cases do not show such instability. The gray FLD method underestimates the opacity at the location of the cavity shell and leads to extended epochs of marginal Eddington equilibrium in the cavity shell, making them prone to the radiative Rayleigh-Taylor instability. In the RT cases, the radiation pressure exceeds gravity by 1-2 orders of magnitude. The radiative Rayleigh-Taylor instability is then consequently suppressed. Conclusions: Treating the stellar irradiation in the gray FLD approximation underestimates the radiative forces acting on the cavity shell. This can lead artificially to situations that are affected by the radiative Rayleigh-Taylor instability. The proper treatment of direct stellar irradiation by massive stars is crucial for the stability of radiation-pressure-dominated cavities.

astro-ph.SR↗

arXiv2011

The alignment of molecular cloud magnetic fields with the spiral arms in M33

The formation of molecular clouds, which serve as stellar nurseries in galaxies, is poorly understood. A class of cloud formation models suggests that a large-scale galactic magnetic field is irrelevant at the scale of individual clouds, because the turbulence and rotation of a cloud may randomize the orientation of its magnetic field. Alternatively, galactic fields could be strong enough to impose their direction upon individual clouds, thereby regulating cloud accumulation and fragmentation, and affecting the rate and efficiency of star formation. Our location in the disk of the Galaxy makes an assessment of the situation difficult. Here we report observations of the magnetic field orientation of six giant molecular cloud complexes in the nearby, almost face-on, galaxy M33. The fields are aligned with the spiral arms, suggesting that the large-scale field in M33 anchors the clouds.

astro-ph.GA↗

arXiv2011

Observations and modeling of the massive young star AFGL 4176: From large scales to small

We present spatially-resolved mid-infrared interferometric observations of the massive young stellar object AFGL 4176, together with literature and survey data. We model these observations using a simple, spherically-symmetric radiative transfer model, and find that the observational data are consistent with a highly luminous star surrounded by a thick envelope.

astro-ph.SR↗

arXiv2011

MIDI interferometry of massive YSOs: Updates on the MPIA programme

Information about the inner structure of young stellar objects is crucial for understanding how the central forming stars gain their mass. However, especially for observations of (usually further away) high-mass young stellar objects, conventional imaging has limitations in spatial resolution. By means of mid-infrared interferometry, we can peek deeply into the strongly extinguished central 100 AU of such objects. Here, we report on new data we obtained within our programme using MIDI at the VLTI. Shown are preliminary results on the known outflow sources AFGL 2136 IRS 1 and Mon R2 IRS 3A. In particular, we describe how quantities like differential phases give additional geometrical structure information. We demonstrate how the combined interpretation of single-telescope and interferometric data at different wavelength regimes can lead to a more complete picture of the nature of such MYSOs.

astro-ph.SR↗

arXiv2011

Can neutral and ionized PAHs be carriers of the UV extinction bump and the diffuse interstellar bands?

Up to now, no laboratory-based study has investigated polycyclic aromatic hydrocarbon (PAH) species as potential carriers of both the diffuse interstellar bands (DIBs) and the 2175 A UV bump. We examined the proposed correlation between these two features by applying experimental and theoretical techniques on two specific medium-sized/large PAHs (dibenzorubicene C30H14 and hexabenzocoronene C42H18) in their neutral and cationic states. It was already shown that mixtures of sufficiently large, neutral PAHs can partly or even completely account for the UV bump. We investigated how the absorption bands are altered upon ionization of these molecules by interstellar UV photons. The experimental studies presented here were realized by performing matrix isolation spectroscopy with subsequent far-UV irradiation. The main effects were found to be a broadening of the absorption bands in the UV combined with slight red shifts. The position of the complete pi - pi* absorption structure around 217.5 nm, however, remains more or less unchanged which could explain the observed position invariance of the interstellar bump for different lines of sight. This favors the assignment of this feature to the interstellar PAH population. As far as the DIBs are concerned, neither our investigations nor the laboratory studies carried out by other research groups support a possible connection with this class of molecules. Instead, there are reasonable arguments that neutral and singly ionized cationic PAHs cannot be made responsible for the DIBs.

astro-ph.GA↗

arXiv2011

Infrared spectroscopy of intermediate mass young stellar objects

In this paper we present Spitzer Infrared Spectrograph spectroscopy for 14 intermediate-mass young stellar objects. We use Spitzer spectroscopy to investigate the physical properties of these sources and their environments. Our sample can be divided into two types of objects: young isolated, embedded objects with spectra that are dominated by ice and silicate absorption bands, and more evolved objects that are dominated by extended emission from polycyclic aromatic hydrocarbons (PAHs) and pure H2 rotational lines. We are able to constrain the illuminating FUV fields by classifying the PAH bands below 9micron. For most of the sources we are able to detect several atomic fine structure lines. In particular, the [NeII] line appearing in two regions could originate from unresolved photodissociation regions (PDRs) or J-shocks. We relate the identified spectral features to observations obtained from NIR through submillimeter imaging. The spatial extent of several H2 and PAH bands is matched with morphologies identified in previous Spitzer/IRAC observations. This also allows us to distinguish between the different H2 excitation mechanisms. In addition, we calculate the optical extinction from the silicate bands and use this to constrain the spectral energy distribution fit, allowing us to estimate the masses of these YSOs.

astro-ph.SR↗

arXiv2011

A Herschel Search For Cold Dust in Brown Dwarf Disks: First Results

We report initial results from a {\it Herschel} program to search for far-infrared emission from cold dust around a statistically significant sample of young brown dwarfs. The first three objects in our survey are all detected at 70\micron, and we report the first detection of a brown dwarf at 160\micron. The flux densities are consistent with the presence of substantial amounts of cold dust in the outer disks around these objects. We modeled the SED's with two different radiative transfer codes. We find that a broad range of model parameters provides a reasonable fit to the SED's, but that the addition of our 70\micron, and especially the 160\micron\ detection enables strong lower limits to be placed on the disk masses since most of the mass is in the outer disk. We find likely disk masses in the range of a few $\times 10^{-6}$ to $10^{-4}$ \msun. Our models provide a good fit to the SED's and do not require dust settling.

astro-ph.GA↗

arXiv2011

The Ubiquity of Micrometer-Sized Dust Grains in the Dense Interstellar Medium

Cold molecular clouds are the birthplaces of stars and planets, where dense cores of gas collapse to form protostars. The dust mixed in these clouds is thought to be made of grains of an average size of 0.1 micrometer. We report the widespread detection of the coreshine effect as a direct sign of the existence of grown, micrometer-sized dust grains. This effect is seen in half of the cores we have analyzed in our survey, spanning all Galactic longitudes, and is dominated by changes in the internal properties and local environment of the cores, implying that the coreshine effect can be used to constrain fundamental core properties such as the three-dimensional density structure and ages and also the grain characteristics themselves.

astro-ph.GA↗

arXiv2011

The 2008 outburst of EX Lup - silicate crystals in motion

EX Lup is the prototype of the EXor class of eruptive young stars. These objects show optical outbursts which are thought to be related to runaway accretion onto the star. In a previous study we observed in-situ crystal formation in the disk of EX Lup during its latest outburst in 2008, making the object an ideal laboratory to investigate circumstellar crystal formation and transport. This outburst was monitored by a campaign of ground-based and Spitzer Space Telescope observations. Here we modeled the spectral energy distribution of EX Lup in the outburst from optical to millimeter wavelengths with a 2D radiative transfer code. Our results showed that the shape of the SED at optical wavelengths was more consistent with a single temperature blackbody than a temperature distribution. We also found that this single temperature component emitted 80-100 % of the total accretion luminosity. We concluded that a thermal instability, the most widely accepted model of EXor outbursts, was likely not the triggering mechanism of the 2008 outburst of EX Lup. Our mid-infrared Spitzer spectra revealed that the strength of all crystalline bands between 8 and 30 um increased right after the end of the outburst. Six months later, however, the crystallinity in the 10 um silicate feature complex decreased. Our modeling of the mid-infrared spectral evolution of EXLup showed that, although vertical mixing should be stronger during the outburst than in the quiescent phase, fast radial transport of crystals (e.g., by stellar/disk wind) was required to reproduce the observed mid-infrared spectra.

astro-ph.SR↗

arXiv2011

Collapse of massive magnetized dense cores using radiation-magneto-hydrodynamics: early fragmentation inhibition

We report the results of radiation-magneto-hydrodynamics calculations in the context of high mass star formation, using for the first time a self-consistent model for photon emission (i.e. via thermal emission and in radiative shocks) and with the high resolution necessary to resolve properly magnetic braking effects and radiative shocks on scales <100 AU. We investigate the combined effects of magnetic field, turbulence, and radiative transfer on the early phases of the collapse and the fragmentation of massive dense cores. We identify a new mechanism that inhibits initial fragmentation of massive dense cores, where magnetic field and radiative transfer interplay. We show that this interplay becomes stronger as the magnetic field strength increases. Magnetic braking is transporting angular momentum outwards and is lowering the rotational support and is thus increasing the infall velocity. This enhances the radiative feedback owing to the accretion shock on the first core. We speculate that highly magnetized massive dense cores are good candidates for isolated massive star formation, while moderately magnetized massive dense cores are more appropriate to form OB associations or small star clusters.

astro-ph.SR↗

arXiv2011

CID: Chemistry in disks VI.sulfur-bearing molecules in the protoplanetary disks surrounding LkCa15, MWC480, DM Tau, and GO Tau

We study the content in S-bearing molecules of protoplanetary disks around low-mass stars. We used the new IRAM 30-m receiver EMIR to perform simultaneous observations of the $1_{10}-1_{01}$ line of H$_2$S at 168.8 GHz and $2_{23}-1_{12}$ line of SO at 99.3 GHz. We compared the observational results with predictions coming from the astrochemical code NAUTILUS, which has been adapted to protoplanetary disks. The data were analyzed together with existing CS J=3-2 observations. We fail to detect the SO and H$_2$S lines, although CS is detected in LkCa15, DM\,Tau, and GO\,Tau but not in MWC\,480. However, our new upper limits are significantly better than previous ones and allow us to put some interesting constraints on the sulfur chemistry. Our best modeling of disks is obtained for a C/O ratio of 1.2, starting from initial cloud conditions of H density of $2\times 10^5$ cm$^{-3}$ and age of $10^6$ yr. The results agree with the CS data and are compatible with the SO upper limits, but fail to reproduce the H$_2$S upper limits. The predicted H$_2$S column densities are too high by at least one order of magnitude. H$_2$S may remain locked onto grain surfaces and react with other species, thereby preventing the desorption of H$_2$S.

astro-ph.SR↗