Á. Kóspál

The outburst and nature of two young eruptive stars in the North America/Pelican Nebula Complex

Context. In August 2010, the sudden optical brightening of two young stellar objects, HBC 722 and VSX J205126.1+440523, located in the North America/Pelican Nebula Complex, was announced. Early photometric and spectroscopic observations of these objects indicated that they may belong to the FUor or EXor class of young eruptive stars. The eruptions of FUors and EXors are often ex- plained by enhanced accretion of material from the circumstellar disk to the protostar. Aims. In order to determine the true nature of these two objects, we started an optical and near-infrared monitoring program, a nd complemented our data with archival observations and data from the literature. Methods. We plot and analyze pre-outburst and outburst spectral energy distributions (SEDs), multi-filter light curves, and col or-color diagrams. Results. The quiescent SED of HBC 722 is consistent with that of a slightly reddened normal T Tauri-type star. The source bright- ened monotonically in about two months, and the SED obtained during maximum brightness indicates the appearance of a hot, single-temperature blackbody. The current fading rate implies that the star will return to quiescence in about a year, q uestioning its classification as a bone fide FUor. The quiescent SED of VSX J20 5126.1+440523 looks like that of a highly embedded Class I source. The outburst of this source happened more gradually, but reached an unprecedentedly high amplitude. At 2.5 months after the peak, its light curves show a deep minimum, when the object was close to its pre-outburst optical brightness. Further monitori ng indicates that it is still far from being quiescent. Conclusions. The shape of the light curves, as well as the bolometric luminosities and accretion rates suggest that these objects do no t fit into the classic FUor group. Although HBC 722 exhibit all s pectral characteristics of a bona fide FUor, its luminosity a nd accretion rate is too low, and its timescale is too fast compared to clas sical FUors. VSX J205126.1+440523 seems to be an example where quick extinction changes modulate the light curve.

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.

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 (SED) of EX Lup in the outburst from optical to millimeter wavelengths with a two-dimensional 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 μm increased right after the end of the outburst. Six months later, however, the crystallinity in the 10 μm silicate feature complex decreased. Our modeling of the mid-infrared spectral evolution of EX Lup 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.

EX Lupi in quiescence

Aims. EX Lupi is the prototype of EXors, a subclass of low-mass pre-main sequence stars whose episodic eruptions are attributed to temporarily increased accretion. In quiescence the optical and near-infrared properties of EX Lup cannot be distinguished from those of normal T Tau stars. Here we investigate whether it is the circumstellar disk structure that makes EX Lup an atypical Class II object. During outburst the disk might undergo structural changes. Our characterization of the quiescent disk is intended to serve as a reference for studying the physical changes related to one of EX Lupi’s strongest known eruptions in 2008 Jan–Sep. Methods. We searched the literature for photometric and spectroscopic observations including ground-based, IRAS, ISO, and Spitzer data. After constructing the optical–infrared spectral energy distribution (SED), we compared it with the typical SEDs of other young stellar objects and modeled it using the Monte Carlo radiative transfer code RADMC. We determined the mineralogical composition of the 10 μm silicate emission feature and also gave a description of the optical and near-infrared spectra. Results. The SED is similar to that of a typical T Tauri star in most aspects, though EX Lup emits higher flux above 7 μm. The quiescent phase data suggest low-level variability in the optical–mid-infrared domain. By integrating the optical and infrared fluxes, we derived a bolometric luminosity of 0.7 L� . The 10 μm silicate profile could be fitted by a mixture consisting of amorphous silicates,

The infrared properties of the new outburst star IRAS 05436-0007 in quiescent phase

We compiled and investigated the infrared/sub-mm/mm SED of the new outburst star IRAS 05436−0007 in quiescent phase. The star is a flat-spectrum source, with an estimated total luminosity of Lbol ≈ 5.6 L� , typical of low-mass T Tauri stars. The derived circumstellar mass of 0.5 Mis rather high among low-mass YSOs. The observed SED differs from the SEDs of typical T Tauri stars and of 4 well-known EXors, and resembles more the SEDs of FU Orionis objects indicating the presence of a circumstellar envelope. IRAS 05436−0007 seems to be a class II source with an age of approximately 4 × 10 5 yr. In this evolutionary stage an accretion disk is already fully developed, though a circumstellar envelope may also be present. Observations of the present outburst will provide additional knowledge on the source.

The outburst of the eruptive young star OO Serpentis between 1995 and 2006

Aims. OO Serpentis is a deeply embedded pre-main sequence star in the Serpens NW star-forming region. The star went into outburst in 1995 and gradually faded afterwards. In many respects its eruption resembled the well-known FU Orionis-type (FUor) or EX Lupi-type (EXor) outbursts. Since very few such events have ever been documented at infrared wavelengths, our aim is to study the temporal evolution of 00 Ser in the infrared. Methods. OOSer was monitored with the Infrared Space Observatory in the 3.6-100μm wavelength range, starting 4 months after peak brightness and covering a period of 20 months. Eight years later, in 2004-2006 we again observed OO Ser at 2.2 and 12 μm from the ground and complemented this dataset with archival Spitzer observations also from 2004. We analysed these data with special attention to source confusion and constructed light curves at 10 different wavelengths as well as spectral energy distributions. Results. The outburst caused brightening in the whole infrared regime. According to the infrared light curves, OOSer started a wavelength-independent fading after the peak brightness. Later the flux decay became slower but stayed practically wavelength-independent. The fading is still ongoing, and current fading rates indicate that 00 Ser will not return to quiescent state before 2011. The outburst timescale of OO Ser seems to be shorter than that of FUors, but longer than that of EXors. Conclusions. The outburst timescale and the moderate luminosity suggest that OO Ser is different from both FUors and EXors, and shows some similarities to the recently erupted young star V 1647 Ori. Based on its SED and bolometric temperature, OO Ser seems to be an early class I object, with an age of <10 5 yr. As proposed by outburst models, the object is probably surrounded by an accretion disc and a dense envelope. This picture is also supported by the wavelength-independence of the fading. Due to the shorter outburst timescales, models developed for FUors can only work for OO Ser if the viscosity parameter in the circumstellar disc, α, is set to an order of magnitude higher value than usual for FUors.

Long-term evolution of FU Orionis objects at infrared wavelengths

We investigate the brightness evolution of seven FU Orionis systems in the 1−100 µm wavelength range using data from the Infrared Space Observatory (ISO). The ISO measurements were supplemented by 2MASS and MSX observations performed in the same years as the ISO mission (1995−98). The spectral energy distributions (SEDs) based on these data points were compared with earlier ones derived from the IRAS photometry as well as from ground-based observations carried out around the epoch 1983. In three cases (Z CMa, Parsamian 21, V1331 Cyg) no difference between the two epochs was seen within the measurement uncertainties. V1057 Cyg, V1515 Cyg and V1735 Cyg have become fainter at near-infrared wavelengths while V346 Nor has become slightly brighter. V1057 Cyg exhibits a similar flux change in the mid-infrared. At λ ≥ 60 µ mm ost of the sources remained constant; only V346 Nor seems to fade. Our data on the long-term evolution of V1057 Cyg agree with the model predictions of Kenyon & Hartmann (1991) and Turner et al. (1997) at near- and mid-infrared wavelengths, but disagree at λ> 25 µm. We discuss if this observational result at far-infrared wavelengths could be understood in the framework of the existing models.

The discovery of new warm debris disks around F-type stars

We report the discovery of four rare debris disks with warm excesses around F stars, significantly increasing the number of such systems known in the solar neighborhood. Three of the disks are consistent with the predictions of steady-state planetesimal disk evolution models. The oldest source, HD 169666, displays a dust fractional luminosity too high to be in a steady state and we suggest that this system recently underwent a transient event of dust production. In addition, two spectra of this star separated by approximately three years show silicate emission features, indicative of submicron- to micron-sized grains. We argue that such small grains would be rapidly depleted and their presence in both spectra suggests that the production of small dust is continuous over a timescale of at least a few years. We predict that systems showing variable mid-infrared spectra, if they exist, will provide valuable help in distinguishing the possible scenarios proposed for dust replenishment.

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.

Episodic Accretion in Young Stars

In the last twenty years, the topic of episodic accretion has gained significant interest in the star formation community. It is now viewed as a common, though still poorly understood, phenomenon in low-mass star formation. The FU Orionis objects (FUors) are long-studied examples of this phenomenon. FUors are believed to undergo accretion outbursts during which the accretion rate rapidly increases from typically