arXiv subjects

Th. Henning

Publications and source records attributed to Th. Henning.

At least 505 records · Page 28Linked to original sources

arXiv2010

Dust, Ice and Gas in Time (DIGIT) Herschel program first results: A full PACS-SED scan of the gas line emission in protostar DK Cha

DK Cha is an intermediate-mass star in transition from an embedded configuration to a star plus disk stage. We aim to study the composition and energetics of the circumstellar material during this pivotal stage. Using the Range Scan mode of PACS on the Herschel Space Observatory, we obtained a spectrum of DK Cha from 55 to 210 micron as part of the DIGIT Key Program. Almost 50 molecular and atomic lines were detected, many more than the 7 lines detected in ISO-LWS. Nearly the entire ladder of CO from J=14-13 to 38-37 (E_u/k = 4080 K), water from levels as excited as E_u/k = 843 K, and OH lines up to E_u/k = 290 K were detected. The continuum emission in our PACS SED scan matches the flux expected from a model consisting of a star, a surrounding disk of 0.03 Solar mass, and an envelope of a similar mass, supporting the suggestion that the object is emerging from its main accretion stage. Molecular, atomic, and ionic emission lines in the far-infrared reveal the outflow's influence on the envelope. The inferred hot gas can be photon-heated, but some emission could be due to C-shocks in the walls of the outflow cavity.

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arXiv2010

From high-mass starless cores to high-mass protostellar objects

Aims: Our aim is to understand the evolutionary sequence of high-mass star formation from the earliest evolutionary stage of high-mass starless cores, via high-mass cores with embedded low- to intermediate-mass objects, to finally high-mass protostellar objects. Methods: Herschel far-infrared PACS and SPIRE observations are combined with existing data at longer and shorter wavelengths to characterize the spectral and physical evolution of massive star-forming regions. Results: The new Herschel images spectacularly show the evolution of the youngest and cold high-mass star-forming regions from mid-infrared shadows on the Wien-side of the spectral energy distribution (SED), via structures almost lost in the background emission around 100mum, to strong emission sources at the Rayleigh-Jeans tail. Fits of the SEDs for four exemplary regions covering evolutionary stages from high-mass starless cores to high-mass protostellar objects reveal that the youngest regions can be fitted by single-component black-bodies with temperatures on the order of 17K. More evolved regions show mid-infrared excess emission from an additional warmer component, which however barely contributes to the total luminosities for the youngest regions. Exceptionally low values of the ratio between bolometric and submm luminosity additionally support the youth of the infrared-dark sources. Conclusions: The Herschel observations reveal the spectral and physical properties of young high-mass star-forming regions in detail. The data clearly outline the evolutionary sequence in the images and SEDs. Future work on larger samples as well as incorporating full radiative transfer calculations will characterize the physical nature at the onset of massive star formation in even more depth.

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arXiv2010

The structured environments of embedded star-forming cores. PACS and SPIRE mapping of the enigmatic outflow source UYSO 1

The intermediate-mass star-forming core UYSO 1 has previously been found to exhibit intriguing features. While deeply embedded and previously only identified by means of its (sub-)millimeter emission, it drives two powerful, dynamically young, molecular outflows. Although the process of star formation has obviously started, the chemical composition is still pristine. We present Herschel PACS and SPIRE continuum data of this presumably very young region. The now complete coverage of the spectral energy peak allows us to precisely constrain the elevated temperature of 26 - 28 K for the main bulge of gas associated with UYSO1, which is located at the interface between the hot HII region Sh 2-297 and the cold dark nebula LDN 1657A. Furthermore, the data identify cooler compact far-infrared sources of just a few solar masses, hidden in this neighbouring dark cloud.

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arXiv2010

From interstellar abundances to grain composition: the major dust constituents Mg, Si and Fe

We analyse observational correlations for three elements entering into the composition of interstellar silicate and oxide grains. Using current solar abundances (Asplund et al. 2009), we convert the gas-phase abundances into dust-phase abundances for 196 sightlines. We deduce a sharp difference in abundances for sightlines located at low ($|b|<30\degr$) and high ($|b|>30\degr$) galactic latitudes. For high-latitude stars the ratios Mg/Si and Fe/Si in dust are close to 1.5. For disk stars they are reduced to ${\rm Mg/Si} \sim 1.2$ and ${\rm Fe/Si} \sim 1.05$. The derived numbers indicate that 1) the dust grains cannot be the mixture of silicates with olivine and pyroxene composition only and some amount of magnesium or iron (or both) should be in another population and 2) the destruction of Mg-rich grains in the warm medium is more effective than of Fe-rich grains. We reveal a decrease of dust-phase abundances and correspondingly an increase of gas-phase abundances with distance $D$ for stars with $D\ga 400$\,pc. We attribute this fact to an observational selection effect: a systematic trend toward smaller observed hydrogen column density for distant stars. We find differences in abundances for disk stars with low ($E({\rm B-V}) \la 0.2$) and high ($E({\rm B-V}) \ga 0.2$) reddenings which reflect the distinction between the sightlines passing through diffuse and translucent interstellar clouds. For Scorpius-Ophiuchus we detect an uniform increase of dust-phase abundances of Mg and Si with an increase of the ratio of total to selective extinction $R_{\rm V}$ and a decrease of the strength of the far-UV extinction. This is the first evidence for a growth of Mg-Si grains due to accretion in the interstellar medium.

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arXiv2010

New Herbig Ae/Be stars confirmed via high-resolution optical spectroscopy

We present FEROS high-resolution (R~45000) optical spectroscopy of 34 Herbig Ae/Be star candidates with previously unknown or poorly constrained spectral types. Within the sample, 16 sources are positionally coincident with nearby (d<250 pc) star-forming regions (SFRs). All the candidates have IR excess. We determine the spectral type and luminosity class of the sources, derive their radial and rotational velocities, and constrain their distances employing spectroscopic parallaxes. We confirm 13 sources as Herbig Ae/Be stars and find one classical T Tauri star. Three sources are emission line early-type giants and may be Herbig Ae/Be stars. One source is a main-sequence A-type star. Fourteen sources are post-main-sequence giant and supergiant stars. Two sources are extreme emission-line stars. Most of the sources appear to be background stars at distances over 700 pc. We show that high-resolution optical spectroscopy is a crucial tool for distinguishing young stars from post-main sequence stars in samples taken from emission-line star catalogs based on low-resolution spectroscopy. Within the sample, 3 young stars (CD-38 4380, Hen 3-1145, and HD 145718) and one early-type luminosity class III giant with emission lines (Hen 3-416) are at distances closer than 300 pc and are positionally coincident with a nearby SFR. These 4 sources are likely to be nearby young stars and are interesting for follow-up observations at high-angular resolution. Furthermore, seven confirmed Herbig Ae/Be stars at d>700 pc (Hen 2-80, Hen 3-1121 N&S, HD 313571, MWC 953, WRAY 15-1435, and Th 17-35) are inside or close (<5') to regions with extended 8 micron continuum emission and in their 20' vicinity have astronomical sources characteristic of SFRs. These 7 sources are likely to be members of SFRs. These regions are attractive for future studies of their stellar content.

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arXiv2010

Looking into the hearts of Bok globules: MM and submm continuum images of isolated star-forming cores

We present the results of a comprehensive infrared, submillimetre, and millimetre continuum emission study of isolated low-mass star-forming cores in 32 Bok globules, with the aim to investigate the process of star formation in these regions. The submillimetre and millimetre dust continuum emission maps together with the spectral energy distributions are used to model and derive the physical properties of the star-forming cores, such as luminosities, sizes, masses, densities, etc. Comparisons with ground-based near-infrared and space-based mid and far-infrared images from Spitzer are used to reveal the stellar content of the Bok globules, association of embedded young stellar objects with the submm dust cores, and the evolutionary stages of the individual sources. Submm dust continuum emission was detected in 26 out of the 32 globule cores observed. For 18 globules with detected (sub)mm cores we derive evolutionary stages and physical parameters of the embedded sources. We identify nine starless cores, most of which are presumably prestellar, nine Class 0 protostars, and twelve Class I YSOs. Specific source properties like bolometric temperature, core size, and central densities are discussed as function of evolutionary stage. We find that at least two thirds (16 out of 24) of the star-forming globules studied here show evidence of forming multiple stars on scales between 1,000 and 50,000 AU. However, we also find that most of these small prototstar and star groups are comprised of sources with different evolutionary stages, suggesting a picture of slow and sequential star formation in isolated globules

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arXiv2010

Chemistry in Disks. III. -- Photochemistry and X-ray driven chemistry probed by the ethynyl radical (CCH) in DM Tau, LkCa 15, and MWC 480

We studied several representative circumstellar disks surrounding the Herbig Ae star MWC 480 and the T Tauri stars LkCa 15 and DM Tau at (sub-)millimeter wavelengths in lines of CCH. Our aim is to characterize photochemistry in the heavily UV-irradiated MWC 480 disk and compare the results to the disks around cooler T Tauri stars. We detected and mapped CCH in these disks with the IRAM Plateau de Bure Interferome- ter in the C- and D-configurations in the (1-0) and (2-1) transitions. Using an iterative minimization technique, the CCH column densities and excitation conditions are con- strained. Very low excitation temperatures are derived for the T Tauri stars. These values are compared with the results of advanced chemical modeling, which is based on a steady-state flared disk structure with a vertical temperature gradient, and a gas- grain chemical network with surface reactions. Both model and observations suggest that CCH is a sensitive tracer of the X-ray and UV irradiation. The predicted radial dependency and source to source variations of CCH column densities qualitatively agree with the observed values, but the predicted column densities are too low by a factor of several. The chemical model fails to reproduce high concentrations of CCH in very cold disk midplane as derived from the observed low excitation condition for both the (1-0) and (2-1) transitions.

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arXiv2010

2MASS J03105986+1648155AB - A new binary at the L/T transition

The transition from the L to the T spectral type of brown dwarfs is marked by a very rapid transition phase, remarkable brightening in the J-band and a higher binary frequency. Despite being an active area of inquiry, this transition regime still remains one of the most poorly understood phases of brown dwarf evolution. We resolved the L dwarf 2MASS J03105986+1648155 for the first time into two almost equally bright components straddling the L/T transition. Since such a co-eval system with common age and composition provides crucial information of this special transition phase, we monitored the system over 3 years to derive first orbital parameters and dynamical mass estimates, as well as a spectral type determination. We obtained resolved high angular resolution, near-IR images with HST and the adaptive optics instrument NACO at the VLT including the laser guide star system PARSEC. Based on two epochs of astrometric data we derive a minimum semi-major axis of 5.2 +- 0.8 AU. The assumption of a face-on circular orbit yields an orbital period of 72 +- 4 years and a total system mass of 30-60 Mjup. This places the masses of the individual components of the system at the lower end of the mass regime of brown dwarfs. The achieved photometry allowed a first spectral type determination of L9 +- 1 for each component. In addition, this seems to be only the fifth resolved L/T transition binary with a flux reversal. While ultimate explanations for this effect are still owing, the 2MASS J03105986+1648155 system adds an important benchmark object for improving our understanding of this remarkable evolutionary phase of brown dwarfs. Additionally, the observational results of 2MASS J03105986+1648155 AB derived with the new PARSEC AO system at the VLT show the importance of this technical capability. The updated AO system allows us to significantly extend the sample of brown dwarfs observable with high-resolution from the ground and hence to reveal more of their physical properties.

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arXiv2010

Electronic spectroscopy of medium-sized polycyclic aromatic hydrocarbons: Implications for the carriers of the 2175 Å UV bump

Mixtures of polycylic aromatic hydrocarbons (PAHs) have been produced by means of laser pyrolysis. The main fraction of the extracted PAHs were primarily medium-sized, up to a maximum size of 38 carbon atoms per molecule. The use of different extraction solvents and subsequent chromatographic fractionation provided mixtures of different size distributions. UV-VIS absorption spectra have been measured at low temperature by matrix isolation spectroscopy and at room temperature with PAHs as film-like deposits on transparent substrates. In accordance with semi-empirical calculations, our findings suggest that large PAHs with sizes around 50 to 60 carbon atoms per molecule could be responsible for the interstellar UV bump at 217.5 nm.

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arXiv2010

Sequential Star Formation in RCW 34: A Spectroscopic Census of the Stellar Content of High-mass Star-forming Regions

We present VLT/SINFONI integral field spectroscopy of RCW 34 along with Spitzer/IRAC photometry of the surroundings. RCW 34 consists of three different regions. A large bubble has been detected on the IRAC images in which a cluster of intermediate- and low-mass class II objects is found. At the northern edge of this bubble, an HII region is located, ionized by 3 OB stars. Intermediate mass stars (2 - 3 Msun) are detected of G- and K- spectral type. These stars are still in the pre-main sequence (PMS) phase. North of the HII region, a photon-dominated region is present, marking the edge of a dense molecular cloud traced by H2 emission. Several class 0/I objects are associated with this cloud, indicating that star formation is still taking place. The distance to RCW 34 is revised to 2.5 +- 0.2 kpc and an age estimate of 2 - 1 Myrs is derived from the properties of the PMS stars inside the HII region. The most likely scenario for the formation of the three regions is that star formation propagates from South to North. First the bubble is formed, produced by intermediate- and low-mass stars only, after that, the HII region is formed from a dense core at the edge of the molecular cloud, resulting in the expansion as a champagne flow. More recently, star formation occurred in the rest of the molecular cloud. Two different formation scenarios are possible: (a) The bubble with the cluster of low- and intermediate mass stars triggered the formation of the O star at the edge of the molecular cloud which in turn induces the current star-formation in the molecular cloud. (b) An external triggering is responsible for the star-formation propagating from South to North. [abridged]

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arXiv2010

Disk and outflow signatures in Orion-KL: The power of high-resolution thermal infrared spectroscopy

We used the CRIRES spectrograph on the VLT to study the ro-vibrational 12CO/13CO, the Pfund beta and H2 emission between 4.59 and 4.72mu wavelengths toward the BN object, the disk candidate source n, and a proposed dust density enhancement IRC3. We detected CO absorption and emission features toward all three targets. Toward the BN object, the data partly confirm the results obtained more than 25 years ago by Scoville et al., however, we also identify several new features. While the blue-shifted absorption is likely due to outflowing gas, toward the BN object we detect CO in emission extending in diameter to ~3300AU. Although at the observational spectral resolution limit, the 13CO line width of that feature increases with energy levels, consistent with a disk origin. If one attributes the extended CO emission also to a disk origin, its extent is consistent with other massive disk candidates in the literature. For source n, we also find the blue-shifted CO absorption likely from an outflow. However, it also exhibits a narrower range of redshifted CO absorption and adjacent weak CO emission, consistent with infalling motions. We do not spatially resolve the emission for source n. For both sources we conduct a Boltzmann analysis of the 13CO absorption features and find temperatures between 100 and 160K, and H2 column densities of the order a few times 10^23cm^-2. The observational signatures from IRC3 are very different with only weak absorption against a much weaker continuum source. However, the CO emission is extended and shows wedge-like position velocity signatures consistent with jet-entrainment of molecular gas, potentially associated with the Orion-KL outflow system. We also present and discuss the Pfund beta and H2 emission in the region.

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arXiv2009

Timescale of Mass Accretion in Pre-Main-Sequence Stars

We present initial result of a large spectroscopic survey aimed at measuring the timescale of mass accretion in young, pre-main-sequence stars in the spectral type range K0 - M5. Using multi-object spectroscopy with VIMOS at the VLT we identified the fraction of accreting stars in a number of young stellar clusters and associations of ages between 1 - 50 Myr. The fraction of accreting stars decreases from ~60% at 1.5 - 2 Myr to ~2% at 10 Myr. No accreting stars are found after 10 Myr at a sensitivity limit of $10^{-11}$ Msun yr-1. We compared the fraction of stars showing ongoing accretion (f_acc) to the fraction of stars with near-to-mid infrared excess (f_IRAC). In most cases we find f_acc < f_IRAC, i.e., mass accretion appears to cease (or drop below detectable level) earlier than the dust is dissipated in the inner disk. At 5 Myr, 95% of the stellar population has stopped accreting material at a rate of > 10^{-11} Msun yr-1, while ~20% of the stars show near-infrared excess emission. Assuming an exponential decay, we measure a mass accretion timescale (t_acc) of 2.3 Myr, compared to a near-to-mid infrared excess timescale (t_IRAC) of 2.9 Myr. Planet formation, and/or migration, in the inner disk might be a viable mechanism to halt further accretion onto the central star on such a short timescale.

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arXiv2009

Mid-infrared interferometry of massive young stellar objects. I. VLTI and Subaru observations of the enigmatic object M8E-IR

[abridged] Our knowledge of the inner structure of embedded massive young stellar objects is still quite limited. We attempt here to overcome the spatial resolution limitations of conventional thermal infrared imaging. We employed mid-infrared interferometry using the MIDI instrument on the ESO/VLTI facility to investigate M8E-IR, a well-known massive young stellar object suspected of containing a circumstellar disk. Spectrally dispersed visibilities in the 8-13 micron range were obtained at seven interferometric baselines. We resolve the mid-infrared emission of M8E-IR and find typical sizes of the emission regions of the order of 30 milli-arcseconds (~45 AU). Radiative transfer simulations have been performed to interpret the data. The fitting of the spectral energy distribution, in combination with the measured visibilities, does not provide evidence for an extended circumstellar disk with sizes > 100 AU but requires the presence of an extended envelope. The data are not able to constrain the presence of a small-scale disk in addition to an envelope. In either case, the interferometry measurements indicate the existence of a strongly bloated, relatively cool central object, possibly tracing the recent accretion history of M8E-IR. In addition, we present 24.5 micron images that clearly distinguish between M8E-IR and the neighbouring ultracompact HII region and which show the cometary-shaped infrared morphology of the latter source. Our results show that IR interferometry, combined with radiative transfer modelling, can be a viable tool to reveal crucial structure information on embedded massive young stellar objects and to resolve ambiguities arising from fitting the SED.

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arXiv2009

Star and protoplanetary disk properties in Orion's suburbs

(Note: this is a shortened version of the original "structured" A&A format abstract.) We performed a large optical spectroscopic and photometric survey of the Lynds~1630N and 1641 clouds. We provide a catalog of 132 confirmed young stars in L1630N and 267 such objects in L1641. We identify 28 transition disk systems, 20 of which were previously unknown, as well as 42 new transition disk candidates for which we have broad-band photometry but no optical spectroscopy. We estimate mass accretion rates M_acc from the equivalent widths of the H_alpha, H_beta, and HeI 5876Åemission lines, and find a dependence on stellar mass of M_acc propto Mstar^alpha, with alpha~3.1 in the subsolar mass range that we probe. An investigation of a large literature sample of mass accretion rate estimates yields a similar slope of alpha~2.8 in the subsolar regime, but a shallower slope of alpha~2.0 if the whole mass range of 0.04 M_sun-5 Msun is included. Among the transition disk objects, the fraction of stars that show significant accretion activity is relatively low compared to stars with still optically thick disks (26\pm11% vs. 57\pm6%, respectively). However, those transition disks that do show significant accretion have the same median accretion rate as normal optically thick disks of 3-4*10^{-9} M_sun/yr. We find that the ages of the transition disks and the WTTSs without disks are statistically indistinguishable, and both groups are significantly older than the CTTSs. These results argue against disk-binary interaction or gravitational instability as mechanisms causing a transition disk appearance. Our observations indicate that disk lifetimes in the clustered population are shorter than in the distributed population. We propose refined Halpha equivalent width criteria to distinguish WTTSs from CTTSs.

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arXiv2009

Spatially resolved mid-infrared observations of the triple system T Tauri

To enhance our knowledge of the characteristics and distribution of the circumstellar dust associated with the individual components of the young hierarchical triple system T Tau, observations in the N-band with MIDI at the VLTI were performed. Our study is based on both the interferometric and the spectrophotometric measurements and is supplemented by new visual and infrared photometry. Also, the phases were investigated to determine the dominating mid-infrared source in the close southern binary. The data were fit with the help of a sophisticated physical disc model. This model utilises the radiative transfer code MC3D that is based on the Monte-Carlo method. Extended mid-infrared emission is found around all three components of the system. Simultaneous fits to the photometric and interferometric data confirm the picture of an almost face-on circumstellar disc around T Tau N. Towards this star, the silicate band is seen in emission. This emission feature is used to model the dust content of the circumstellar disc. Clear signs of dust processing are found. Towards T Tau S, the silicate band is seen in absorption. This absorption is strongly pronounced towards the infrared companion T Tau Sa as can be seen from the first individual N-band spectra for the two southern components. Our fits support the previous suggestion that an almost edge-on disc is present around T Tau Sa. This disc is thus misaligned with respect to the circumstellar disc around T Tau N. The interferometric data indicate that the disc around T Tau Sa is oriented in the north-south direction, which favours this source as launching site for the east-western jet. We further determine from the interferometric data the relative positions of the components of the southern binary.

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arXiv2009

Imaging search for the unseen companion to Eps Ind A -- Improving the detection limits with 4 micron observations

Eps Ind A is one of the nearest sun-like stars, located only 3.6 pc away. It is known to host a binary brown dwarf companion, Eps Ind Ba/Bb, at a large projected separation of 6.7", but radial velocity measurements imply that an additional, yet unseen component is present in the system, much closer to Eps Ind A. Previous direct imaging has excluded the presence of any stellar or high-mass brown dwarf companion at small separations, indicating that the unseen companion may be a low-mass brown dwarf or high-mass planet. We present the results of a deep high-contrast imaging search for the companion, using active angular differential imaging (aADI) at 4 micron, a particularly powerful technique for planet searches around nearby and relatively old stars. We also develop an additional PSF reference subtraction scheme based on locally optimized combination of images (LOCI) to further enhance the detection limits. No companion is seen in the images, although we are sensitive to significantly lower masses than previously achieved. Combining the imaging data with the known radial velocity trend, we constrain the properties of the companion to within approximately 5-20 Mjup, 10-20 AU, and i > 20 deg, unless it is an exotic stellar remnant. The results also imply that the system is probably older than the frequently assumed age of ~1 Gyr.

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arXiv2009

The Long-Lived Disks in the Eta Chamaeleontis Cluster

We present IRS spectra and revised MIPS photometry for the 18 members of the Eta Chamaeleontis cluster. Aged 8 Myr, the Eta Cha cluster is one of the few nearby regions within the 5-10 Myr age range, during which the disk fraction decreases dramatically and giant planet formation must come to an end. For the 15 low-mass members, we measure a disk fraction ~50%, high for their 8 Myr age, and 4 of the 8 disks lack near-IR excesses, consistent with the empirical definition of "transition'' disks. Most of the disks are comparable to geometrically flat disks. The comparison with regions of different ages suggests that at least some of the "transition" disks may represent the normal type of disk around low-mass stars. Therefore, their flattened structure and inner holes may be related to other factors (initial masses of the disk and the star, environment, binarity), rather than to pure time evolution. We analyze the silicate dust in the disk atmosphere, finding moderate crystalline fractions (~10-30%) and typical grain sizes ~1-3 micron, without any characteristic trend in the composition. These results are common to other regions of different ages, suggesting that the initial grain processing occurs very early in the disk lifetime (<1 Myr). Large grain sizes in the disk atmosphere cannot be used as a proxy for age, but are likely related to higher disk turbulence. The dust mineralogy varies between the 8-12micron and the 20-30 micron features, suggesting high temperature dust processing and little radial mixing. Finally, the analysis of IR and optical data on the B9 star Eta Cha reveals that it is probably surrounded by a young debris disk with a large inner hole, instead of being a classical Be star.

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arXiv2009

Episodic formation of cometary material in the outburst of a solar-like young star

Our Solar System originated in interstellar gas and dust; the latter is in the form of amorphous silicate particles and carbonaceous dust. The composition of cometary material shows that a significant fraction of the amorphous silicates was transformed into crystalline form during the early evolution of the protosolar nebula. How and when this transformation happened has been controversial, with the main options being heating by the young Sun or shock heating. Here we report mid-infrared features in the outburst spectrum of the young solar-like star EX Lupi that were not present in quiescence. We attribute them to crystalline forsterite; the crystals were produced via thermal annealing in the surface layer of the inner disk by heat from the outburst, a process that has hitherto not been considered. The observed lack of cold crystals excludes shock heating at larger radii.

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