Multiwavelength interferometric observations and modeling of circumstellar disks

We investigate the structure of the innermost region of three circumstellar disks around pre-main sequence stars HD 142666, AS 205 N, and AS 205 S. We determine the inner radii of the dust disks and, in particular, search for transition objects where dust has been depleted and inner disk gaps have formed at radii of a few tenths of AU up to several AU. We performed interferometric observations with IOTA, AMBER, and MIDI in the infrared wavelength ranges 1.6-2.5um and 8-13um with projected baseline lengths between 25m and 102m. The data analysis was based on radiative transfer simulations in 3D models of young stellar objects (YSOs) to reproduce the spectral energy distribution and the interferometric visibilities simultaneously. Accretion effects and disk gaps could be considered in the modeling approach. Results from previous studies restricted the parameter space. The objects of this study were spatially resolved in the infrared wavelength range using the interferometers. Based on these observations, a disk gap could be found for the source HD 142666 that classifies it as transition object. There is a disk hole up to a radius of R_in=0.30AU and a (dust-free) ring between 0.35AU and 0.80AU in the disk of HD 142666. The classification of AS 205 as a system of classical T Tauri stars could be confirmed using the canonical model approach, i. e., there are no hints of disk gaps in our observations.Comment: accepted by Astronomy & Astrophysic

Radio structure of the most distant radio-detected quasar at the ten milli- arcsecond scale

We present a high resolution radio image of SDSS 0836+0054 identified recently as the most distant radio-detected quasar at a redshift of z=5.82. The observation was carried out with ten antennas of the European VLBI Network, spread from Europe to China and South Africa, at 1.6 GHz frequency on 2002 June 8. The source is detected with a total flux density of 1.1 mJy, equal to its flux density measured in the VLA FIRST survey. We found no indication of multiple images produced by gravitational lensing. The radio structure of the quasar at ~10-mas angular resolution appears somewhat resolved. It resembles the radio structure typical for lower redshift radio-loud active galactic nuclei. We obtained so far the best astrometric position of the source with an accuracy better than 8 mas, limited mainly by the structural effects in the phase-reference calibrator source.Comment: 5 pages, 2 figures, accepted for publication in MNRAS (Letters

Mid-infrared interferometric variability of DG Tau: implications for the inner-disk structure

Context. DG Tau is a low-mass pre-main sequence star, whose strongly accreting protoplanetary disk exhibits a so-far enigmatic behavior: its mid-infrared thermal emission is strongly time-variable, even turning the 10 $\mu$m silicate feature from emission to absorption temporarily. Aims. We look for the reason for the spectral variability at high spatial resolution and at multiple epochs. Methods. We study the temporal variability of the mid-infrared interferometric signal, observed with the VLTI/MIDI instrument at six epochs between 2011 and 2014. We fit a geometric disk model to the observed interferometric signal to obtain spatial information about the disk. We also model the mid-infrared spectra by template fitting to characterize the profile and time dependence of the silicate emission. We use physically motivated radiative transfer modeling to interpret the mid-infrared interferometric spectra. Results. The inner disk (r<1-3 au) spectra exhibit a 10 $\mu$m absorption feature related to amorphous silicate grains. The outer disk (r>1-3 au) spectra show a crystalline silicate feature in emission, similar to the spectra of comet Hale-Bopp. The striking difference between the inner and outer disk spectral feature is highly unusual among T Tauri stars. The mid-infrared variability is dominated by the outer disk. The strength of the silicate feature changed by more than a factor of two. Between 2011 and 2014 the half-light radius of the mid-infrared-emitting region decreased from 1.15 to 0.7 au. Conclusions. For the origin of the absorption we discuss four possible explanations: a cold obscuring envelope, an accretion heated inner disk, a temperature inversion on the disk surface and a misaligned inner geometry. The silicate emission in the outer disk can be explained by dusty material high above the disk plane, whose mass can change with time, possibly due to turbulence in the disk.Comment: 16 pages, 13 figure

A kozmikus por fejlődése a Tejútrendszerben és a távoli Univerzumban = Evolution of cosmic dust in the Milky Way and in the distant Universe

Kutatómunkánk során foglalkoztunk az eredeti munkatervben megjelölt mindhárom témakörrel: a por szerepe és tulajdonságai csillagközi felhőkben, csillagok körüli korongokban, valamint kozmológiai távolságú rendszerekben. Megmutattuk, hogy a sűrű csillagközi felhőkben a porszemcsék optikai tulajdonságai megváltoznak, arra utalva, hogy méretük valamilyen fizikai hatás következtében (összetapadás, jégköpeny képződés) megnő. Az extragalaktikus por tanulmányozására távoli (z <= 1) galaxisokat figyeltünk meg a Spitzer Űrtávcsővel. A legnagyobb hangsúlyt a hirtelen kifényesedést mutató eruptív fiatal csillagok körüli por- és gázkorongok tanulmányozására fektettük. A V1647 Ori kitörése során interferometrikus mérésekkel fel tudtuk bontani a csillagkörüli korong belső részét, és megmutattuk, hogy a kitörés során a belső rész geometriája megváltozik. Az EX Lupi 2008-as kitörését az Európai Déli Obszervatórium és a Spitzer Űrtávcső műszereivel követtük. A kitörés előtt felvett színkép tanúsága szerint az EX Lupit körülvevő korong felszínét kisméretű és amorf szerkezetű por borította, a kitörés csúcsát követően azonban már nagyrészt kristályos porszemcsék spektroszkópiai nyomát láthattuk. Így első ízben sikerült közvetlenül megfigyelni a porszemcsék kristályosodását egy csillagkörüli korongban, lehetséges forgatókönyvet javasolva naprendszerbeli üstökösök anyagának keletkezésére is. A felfedezést a Nature közölte 2009-ben. | We made progress in all three research areas defined in the workplan of the OTKA project: the role and properties of dust in interstellar clouds, in circumstellar disks, and in galaxies at cosmological distances. We demonstrated that the infrared emisivity of the dust particles increases in dense interstellar clouds, suggesting a particle growth via coagulation or the appearance of ice mantels. In order to study the extragalactic dust we performed far-infrared observations of SN Ia host galaxies with the Spitzer Space Telescope. Our most productive research line was, however, the investigation of gas- and dust disks around young eruptive stars. Using interferometric observations, we were able to resolve the inner disk around V1647 Ori, and concluded that the the geometrical structure of the inner part changed during the outburst. The 2008 Jan-Sep eruption of EX Lupi was followed using ESO and Spitzer instruments. Comparing pre-outburst and outburst mid-infrared spectra, we showed that while in quiescence the surface of the disk was dominated by submicron size amorphous particles, during outburst signatures of crystalline dust appeared. It was the first direct detection of on-going crystal formation in a cosmic object, offering also a scenario for the formation of the silicate crystals located in comet nuclei in the Solar System. This result was published in Nature in 2009 May

Deep Extragalactic VLBI-Optical Survey (DEVOS) II. Efficient VLBI detection of SDSS quasars

Context: The Deep Extragalactic VLBI-Optical Survey (DEVOS) aims at constructing a large sample of compact radio sources up to two orders of magnitude fainter than those studied in other Very Long Baseline Interferometry (VLBI) surveys. Optical identification of the objects is ensured by selecting them from the Sloan Digital Sky Survey (SDSS) list. Aims: While continuing to build up the DEVOS data base, we investigated how the VLBI detection rate could be enhanced by refining the initial selection criteria introduced in the first paper of this series. Methods: We observed 26 sources in two adjacent, slightly overlapping 2 deg radius fields with the European VLBI Network (EVN) at 5 GHz frequency on 2 March 2007.The phase-reference calibrator quasars were J1616+3621 and J1623+3909. The objects selected were unresolved both in the Faint Images of the Radio Sky at Twenty-centimeters (FIRST) survey catalogue and the SDSS Data Release 4. Results: We present images of milli-arcsecond (mas) scale radio structures and accurate coordinates of 24 extragalactic sources. Most of them have never been imaged with VLBI. Twenty-two compact radio sources (85% of our initial sample) are considered as VLBI detections of the corresponding optical quasars in SDSS. We found an efficient way to identify quasars as potential VLBI targets with mas-scale compact radio stucture at >1 mJy level, based only on the FIRST and SDSS catalogue data by applying simple selection criteria.Comment: 14 pages, 7 figures (one multi-page); accepted for publication in Astronomy & Astrophysic

Radio interferometric observations of two core-dominated triple radio sources at z>3

Aims. We selected two radio quasars (J1036+1326 and J1353+5725) based on their 1.4-GHz radio structure, which is dominated by a bright central core and a pair of weaker and nearly symmetric lobes at ~10" angular separation. They are optically identified in the Sloan Digital Sky Survey (SDSS) at spectroscopic redshifts z>3. We investigate the possibility that their core-dominated triple morphology can be a sign of restarted radio activity in these quasars, involving a significant repositioning of the radio jet axis. Methods. We present the results of high-resolution radio imaging observations of J1036+1326 and J1353+5725, performed with the European Very Long Baseline Interferometry (VLBI) Network (EVN) at 1.6 GHz. These data are supplemented by archive observations from the Very Large Array (VLA).We study the large- and small-scale radio structures and the brightness temperatures, then estimate relativistic beaming parameters. Results. We show that the central emission region of these two high-redshift, core-dominated triple sources is compact but resolved at ~10 milli-arcsecond resolution. We find that it is not necessary to invoke large misalignment between the VLBI jet and the large-scale radio structure to explain the observed properties of the sources.Comment: 5 pages, 4 figures, accepted for publication in A&

Fundamental Vibrational Transition of CO During the Outburst of EX Lupi in 2008

We report monitoring observations of the T Tauri star EX Lupi during its outburst in 2008 in the CO fundamental band at 4.6–5.0 μm. The observations were carried out at the Very Large Telescope and the Subaru Telescope at six epochs from 2008 April to August, covering the plateau of the outburst and the fading phase to a quiescent state. The line flux of CO emission declines with the visual brightness of the star and the continuum flux at 5 μm, but composed of two subcomponents that decay with different rates. The narrow-line emission (50 kms^(−1) in FWHM) is near the systemic velocity of EX Lupi. These emission lines appear exclusively in v =1–0. The line widths translate to a characteristic orbiting radius of 0.4 AU. The broad-line component (FWZI ~ 150 km s^(−1)) is highly excited up to v ≤ 6. The line flux of the component decreases faster than the narrow-line emission. Simple modeling of the line profiles implies that the broad-line emitting gas is orbiting around the star at 0.04–0.4 AU. The excitation state, the decay speed of the line flux, and the line profile indicate that the broad-line emission component is physically distinct from the narrow-line emission component, and more tightly related to the outburst event

High-resolution images of five radio quasars at early cosmological epochs

Context: Until now, there have only been seven quasars at z>4.5 whose the high-resolution radio structure had been studied in detail with Very Long Baseline Interferometry (VLBI) imaging. Aims: We almost double the number of VLBI-imaged quasars at these high redshifts with the aim of studying their redshift-dependent structural and physical properties in a larger sample. Methods: We observed five radio quasars (J0813+3508, J1146+4037, J1242+5422, J1611+0844, and J1659+2101) at 4.5<z<5 with the European VLBI Network (EVN) at 1.6 GHz on 29 October 2008 and at 5 GHz on 22 October 2008. The angular resolution achieved ranges from 1.5 to 25 milli-arcseconds (mas), depending on the observing frequency, the position angle in the sky, and the source's celestial position. Results: The sources are all somewhat extended on mas scales, but compact enough to be detected at both frequencies. With one exception of a flat-spectrum source (J1611+0844), their compact emission is characterised by a steep radio spectrum. We found no evidence of Doppler-boosted radio emission in the quasars in our sample. The radio structure of one of them (J0813+3508) is extended to ~7", which corresponds to 43 kpc projected linear size. Many of the highest redshift compact radio sources are likely to be young, evolving objects, far-away cousins of the powerful gigahertz peaked-spectrum (GPS) and compact steep-spectrum (CSS) sources that populate the Universe at lower redshifts.Comment: 7 pages, 2 figures, accepted for Astronomy & Astrophysic

V1647 Orionis: One Year into Quiescence

We present new optical, near-IR, and mid-IR observations of the young eruptive variable star V1647 Orionis that went into outburst in late 2004 for approximately two years. Our observations were taken one year after the star had faded to its pre-outburst optical brightness and show that V1647Ori is still actively accreting circumstellar material. We compare and contrast these data with existing observations of the source from both pre-outburst and outburst phases. From near-IR spectroscopy we identify photospheric absorption features for the first time that allow us to constrain the classification of the young star itself. Our best fit spectral type is M0+-2 sub-classes with a visual extinction of 19+-2 magnitudes and a K-band veiling of rK~1.5+-0.2. We estimate that V1647Ori has a quiescent bolometric luminosity of ~9.5Lsun and a mass accretion rate of ~1.10^-6Msun yr^-1. Our derived mass and age, from comparison with evolutionary models, are 0.8+-0.2 Msun and ~0.5Myrs, respectively. The presence towards the star of shock excited optical [S II] and [Fe II] emission as well as near-IR H2 and [Fe II] emission perhaps suggests that a new Herbig-Haro flow is becoming visible close to the star.Comment: 22 pages, 19 Figures, accepted AJ 13 October 200