The inner disks of EXor-type eruptive stars

EX Lupi-type young stars (EXors) show sporadic brightenings of several magnitudes, caused by the episodic increase in the accretion rate of the circumstellar matter onto the young star. As the inner disk plays a crucial role during the onset of the outburst, we examined the quiescent properties of the circumstellar environment of EXors, focusing on the inner regions. We found that in case of three EXors (VY Tau, V1143 Ori and EX Lup) the spectral energy distributions show no or weak excess above the stellar photosphere at NIR-MIR wavelengths, indicative of inner disk clearing. A detailed radiative transfer modeling of the sources revealed that the inner regions of these disks had to go through significant evolution, either the inner radius of the dusty disk is beyond the sublimation radius and/or the inner disks are flattene

On the Relationship Between Debris Disks and Planets

Dust in debris disks is generated by collisions among planetesimals. The existence of these planetesimals is a consequence of the planet formation process, but the relationship between debris disks and planets has not been clearly established. Here we analyze Spitzer/MIPS 24 and 70 μm data for 150 planet-bearing stars, and compare the incidence of debris disks around these stars with a sample of 118 stars around which planets have been searched for, but not found. Together they comprise the largest sample ever assembled to deal with this question. The use of survival analysis techniques allows us to account for the large number of nondetections at 70 μm. We discovered 10 new debris disks around stars with planets and one around a star without known planets. We found that the incidence of debris disks is marginally higher among stars with planets, than among those without, and that the brightness of the average debris disk is not significantly different in the two samples. We conclude that the presence of a planet that has been detected via current radial velocity techniques is not a good predictor of the presence of a debris disk detected at infrared wavelengths

Outbursts in Global Protoplanetary Disk Simulations

While accreting through a circumstellar disk, young stellar objects are observed to undergo sudden and powerful accretion events known as FUor or EXor outbursts. Although such episodic accretion is considered to be an integral part of the star formation process, the triggers and mechanisms behind them remain uncertain. We conducted global numerical hydrodynamics simulations of protoplanetary disk formation and evolution in the thin-disk limit, assuming both magnetically layered and fully magnetorotational instability (MRI)-active disk structure. In this paper, we characterize the nature of the outbursts occurring in these simulations. The instability in the dead zone of a typical layered disk results in "MRI outbursts". We explore their progression and their dependence on the layered disk parameters as well as cloud core mass. The simulations of fully MRI-active disks showed an instability analogous to the classical thermal instability. This instability manifested at two temperatures--above approximately 1400 K and 3500 K--due to the steep dependence of Rosseland opacity on the temperature. The origin of these thermally unstable regions is related to the bump in opacity resulting from molecular absorption by water vapor and may be viewed as a novel mechanism behind some of the shorter duration accretion events. Although we demonstrated local thermal instability in the disk, more investigations are needed to confirm that a large-scale global instability will ensue. We conclude that the magnetic structure of a disk, its composition, as well as the stellar mass, can significantly affect the nature of episodic accretion in young stellar objects.Comment: 16 figure

Physical conditions of gas components in debris disks of 49 Ceti and HD 21997

Characterization of gas component in debris disks is of fundamental importance for understanding its origin. Toward this goal, we have conducted non-LTE (local thermodynamic equilibrium) analyses of the rotational spectral lines of CO including those of rare isotopologues ($^{13}$CO and C$^{18}$O) observed toward the gaseous debris disks of 49 Ceti and HD 21997 with the Atacama Large Millimeter/submillimeter Array (ALMA) and Atacama Compact Array (ACA). The analyses have been carried out for a wide range of the H$_{2}$ density, and the observed line intensities are found to be reproduced, as far as the H$_{2}$ density is higher than 10$^{3}$ cm$^{-3}$. The CO column density and the gas temperature are evaluated to be (1.8-5.9)$\times$10$^{17}$ cm$^{-2}$ and 8-11 K for 49 Ceti and (2.6-15)$\times$10$^{17}$ cm$^{-2}$ and 8-12 K for HD 21997, respectively, where the H$_{2}$ collision is assumed for the rotational excitation of CO. The results do not change significantly even if electron collision is considered. Thus, CO molecules can be excited under environments of no H$_{2}$ or a small number of H$_{2}$ molecules, even where the collision with CO, C, O, and C$^{+}$ would make an important contribution for the CO excitation in addition to H$_{2}$. Meanwhile, our result does not rule out the case of abundant H$_{2}$ molecules. The low gas temperature observed in the debris disks is discussed in terms of inefficient heating by interstellar and stellar UV radiation.Comment: 25 pages, 5 figures, 9 tables, accepted for publication in Ap

Accretion process, magnetic fields, and apsidal motion in the pre-main sequence binary DQ Tau

Classical T Tauri stars (CTTSs) are young stellar objects that accrete materials from their accretion disc influenced by their strong magnetic field. The magnetic pressure truncates the disc at a few stellar radii and forces the material to leave the disc plane and fall onto the stellar surface by following the magnetic field lines. However, this global scheme may be disturbed by the presence of a companion interacting gravitationally with the accreting component. This work is aiming to study the accretion and the magnetic field of the tight eccentric binary DQ Tau, composed of two equal-mass ($\sim$ 0.6 \msun ) CTTSs interacting at different orbital phases. We investigated the variability of the system using a high-resolution spectroscopic and spectropolarimetric monitoring performed with ESPaDOnS at the CFHT. We provide the first ever magnetic field analysis of this system, the Zeeman-Doppler imaging revealed a stronger magnetic field for the secondary than the primary (1.2 kG and 0.5 kG, respectively), but the small-scale fields analysed through Zeeman intensification yielded similar strengths (about 2.5 kG). The magnetic field topology and strengths are compatible with the accretion processes on CTTSs. Both components of this system are accreting, with a change of the main accretor during the orbital motion. In addition, the system displays a strong enhancement of the mass accretion rate at periastron and apastron. We also discovered, for the first time in this system, the apsidal motion of the orbital ellipse.Comment: 18 pages, 20 figures. Accepted for publication in MNRA

Eruptive Behavior of Magnetically Layered Protoplanetary Disks in Low-metallicity Environments

A protoplanetary disk typically forms a dead zone near its midplane at the distance of a few au from the central protostar. Accretion through such a magnetically layered disk can be intrinsically unstable and has been associated with episodic outbursts in young stellar objects. We present the first investigation into the effects of low metallicity environment on the structure of the dead zone as well as the resulting outbursting behavior of the protoplanetary disk. We conducted global numerical hydrodynamic simulations of protoplanetary disk formation and evolution in the thin-disk limit. The consequences of metallicity were considered via its effects on the gas and dust opacity of the disk, the thickness of the magnetically active surface layer, and the temperature of the prestellar cloud core. We show that the metal poor disks accumulate much more mass in the innermost regions, as compared to the solar metallicity counterparts. The duration of the outbursting phase also varies with metallicity - the low metallicity disks showed more powerful luminosity eruptions with a shorter burst phase, which was confined mostly to the early, embedded stages of the disk evolution. The lowest metallicity disks with the higher cloud core temperature showed the most significant differences. The occurrence of outbursts was relatively rare in the disks around low mass stars and this was especially true at lowest metallicities. We conclude that the metal content of the disk environment can have profound effects on both the disk structure and evolution in terms of episodic accretion.Comment: 24 pages, 11 figure

A mid-infrared interferometric survey of the planet-forming region around young Sun-like stars

We present our results from a mid-infrared interferometric survey targeted at the planet-forming region in the circumstellar disks around low- and intermediate-mass young stars. Our sample consist of 82 objects, including T Tauri stars, Herbig Ae stars, and young eruptive stars. Our main results are: 1) Disks around T Tauri stars are similar to those around Herbig Ae stars, but are relatively more extended once we account for stellar luminosity. 2) From the distribution of the sizes of the mid-infrared emitting region we find that inner dusty disk holes may be present in roughly half of the sample. 3) Our analysis of the silicate spectral feature reveals that the dust in the inner $\sim$1 au region of disks is generally more processed than that in the outer regions. 4) The dust in the disks of T Tauri stars typically show weaker silicate emission in the N band spectrum, compared to Herbig Ae stars, which may indicate a general difference in the disk structure. Our data products are available at VizieR, and at the following web page: http://konkoly.hu/MIDI_atlasComment: 4 pages, 2 figures, published in "Origins: From the Protosun to the First Steps of Life. Proceedings of the International Astronomical Union, Volume 345

Nap típusú csillagok korai fejlődési szakaszainak vizsgálata = Study of early evolutionary stages of Solar type stars

1. A V1647 Ori 2004. januártól 2005. októberig tartó kitörését nemzetközi együttműködéssel, több műszerrel követtük. Analizáltuk a csillag fényességének és színképvonalainak változását a kitörés alatt, és infravörösben az elhalványodás után is, egészen 2006. szeptemberéig. 2. Nap típusú fiatal csillagokat és protocsillagokat azonosítottunk a Lynds 1333, Lynds 1622, Lynds 1340 közeli molekulafelhőkben és a Cepheus flare óriás molekulafelhő-komplexum mintegy 200 négyzetfoknyi területén. Az objektumok nagy része korábban nem ismert fiatal csillag. Saját spektroszkópiai és fotometriai méréseink alapján meghatároztuk a fiatal csillagok spektráltípusát, luminozitását, tömegét és korát. Középinfravörös (Spitzer Space Telescope) adatbázis alapján vizsgáltuk az akkréciós korongjaik tulajdonságait, ezek környezetfüggését. 3. A Spitzer Space Telescope középinfravörös adatainak feldolgozásával vizsgáltuk az NGC 2244 fiatal nyílthalmaz szerkezetét és a halmaztagok akkréciós korongjainak tulajdonságait. 337 fősorozat előtti csillagot és 25 I osztályú protocsillagot azonosítottunk. Spitzer és HST mérések alapján elemeztük a por elpárolgásának folyamatát a forró csillagok közelében. A h és chi Persei Spitzer-adatainak elemzésével megállapítottuk, hogy a 24 mikronon megfigyelhető törmelékkorongok tipikus kora 5 és 15 millió év között van. 4. A pályázat támogatásával elkészült egy kis felbontású spektrográf a piszkés-tetői RCC teleszkópra. | 1. We followed the outburst of V1647 Ori between 2004 January and 2005 October using several instruments. We analysed the variations of the brightness and spectral line intensities during the outburst, and even after the dimming of the star in the infrared, up to 2006 September. 2. We identified Solar type young stars and protostars in the nearby molecular clouds Lynds 1333, Lynds 1622, as well as over the whole surface of some 200 square degrees of the Cepheus flare molecular complex. Most of the objects are newly identified young stars. Based on our own spectroscopic and photometric observations we determined the spectral types, luminosities, masses, and ages of the stars. Based on the mid-infrared Spitzer Space Telescope archive data we studied their accretion disks, and the dependence of disk properties on the environment. 3. We studied the structure of the young open cluster NGC 2244 based on the mid-infrared data obtained by the Spitzer Space Telescope. We identified 337 pre-main sequence stars and 25 Class I protostars. Based on Spitzer and HST measurements we analysed the evaporation of the dust in the vicinity of hot stars. Based on Spitzer observations of the open cluster h and chi Persei we established that the typical age of the debris disks observed at 24 micron is 5-15 million years. 4. A low-resolution spectrograph has been devised and built for the RCC telescope with the support of this project

The Accretion Process in the DQ Tau Binary System

Mass accretion from the circumstellar disk onto the protostar is a fundamental process during star formation. Measuring the mass accretion rate is particularly challenging for stars belonging to binary systems, because it is often difficult to discriminate which component is accreting. DQ Tau is an almost equal-mass spectroscopic binary system where the components orbit each other every 15.8 days. The system is known to display pulsed accretion, i.e., the periodic modulation of the accretion by the components on eccentric orbit. We present multi-epoch ESO/Very Large Telescope X-Shooter observations of DQ Tau, with the aim of determining which component of this system is the main accreting source. We use the absorption lines in the spectra to determine the radial velocity of the two components, and measure the continuum veiling as a function of wavelength and time. We fit the observed spectra with nonaccreting templates to correct for the photospheric and chromospheric contribution. In the corrected spectra, we study in detail the profiles of the emission lines and calculate mass accretion rates for the system as a function of orbital phase. In accordance with previous findings, we detect elevated accretion close to periastron. We measure the accretion rate as varying between 10-8.5 and 10-7.3 M ⊙ yr-1. The emission line profiles suggest that both stars are actively accreting, and the dominant accretor is not always the same component, varying in a few orbits