Figure 1: Spectral energy distribution of the FU Orionis-type star V1057 Cyg. Measurements from 1983 are indicated with blue dots, data taken between 1996 and 2000 are indicated with red dots.

The fading phase after the outburst is well-documented in the optical/near-infrared wavelength regime. Very few data have been available so far at mid- and far-infrared wavelengths where thermal emission of the disk and the circumstellar envelope can be observed. Recently the Infrared Space Observatory (ISO) provided new photometric data on FUors in the 4.8-200 μm range.

We search for systematic brightness variations during the post-outburst phase of seven FUors at mid- and far-infrared wavelengths. We compiled two SEDs for each star: the first one is based on IRAS photometry as well as ground-based observations and is representative of the evolutionary status around 1983. The second SED was compiled from observations taken around 1996-2000 including data from ISOPHOT, the infrared photometer on-board ISO, MSX and 2MASS. The comparison of the two SEDs provides information on the wavelength dependence of the far-infrared flux evolution during a period of 15 years. (See e.g. the SED of V1057 Cyg in Fig. 1.)

Our results indicate that while at shorter wavelengths (< 25 μm) the stars show various behaviour (some brightened, some faded, some did not change), at longer wavelengths (≥ 60 μm) none of the stars changed its brightness within the measurement uncertainties. This result contradicts the predictions of models developed to describe the circumstellar structure of FUors. Via a detailed analysis of the case of V1057 Cyg we suggested three different sources of the far-infrared emission of V1057 Cyg, among which the most likely is an optically thick disk, whose temperature profile is controlled by the ambient radiation field.

The results are published in the following papers:

Title: Long-term evolution of FU Orionis objects at infrared wavelengths

Authors:Ábrahám, P., Kóspál, Á., Csizmadia, Sz., Kun, M., Moór, A., Prusti, T.

Journal: Astronomy & Astrophysics, in press (2004)

Title: The long-term evolution of 7 FU Orionis-type stars at infrared wavelengths

Authors: Kóspál, Á., Ábrahám, P., Csizmadia, Sz., Kun, M., Moór, A., Prusti, T.

Journal: 3rd Workshop of Young Researchers in Astronomy and Astrophysics and British-Hungarian N+N Workshop for Young Researchers, Publications of the Astronomy Department of the Eötvös University, v. 14, p. 119-125 (2004)

Title: Long-term evolution of FU Ori-type stars at infrared wavelengths

Authors: Kóspál, Á., Ábrahám, P., Csizmadia, Sz.

Journal: Baltic Astronomy, v. 13, p. 518-521 (2004)

Title: Long-term evolution of FU Ori-type stars at infrared wavelengths

Authors: Kóspál, Á., Ábrahám, P., Kun, M., Csizmadia, Sz., Verdugo, E.

Journal: Magnetic Fields and Star Formation: Theory Versus Observations, Kluwer Academic Press, Astrophysics and Space Science, 291, vols. 1-4, in press (2004)

On January 23 2004 a new nebula (McNeil's Nebula) was discovered towards the Orion B molecular cloud. The appearence of the nebula is probably connected to the outburst of its illuminating source, V1647 Ori (also known as IRAS 05436-0007). (Fig. 2. shows the Orion Cloud at infrared wavelengths).

Figure 2: High-resolution far-infrared image of the Orion Cloud. (RBG colour composite of 60 μm (R), 25 μm (G) and 12 μm (B) IRAS images.)

Using infrared data from ISO, IRAS, 2MASS and ground-based submm and radio observations we investigated the infared properties of this intriguing new source. We constructed the 1-1300 μm SED of V1647 Ori representative of the quiescent phase. According to this SED V1647 Ori proved to be a flat-spectrum source, which may be the result of az extended envelope around the source. The slope of the SED at the far-infrared also suggests a large amount of cold material. From the location of V1647 Ori in the J-H H-K graph we calculated the extinction towards the star, and integrating over the dereddened SED we determined its infrared-to-submillimetre luminosity. We compared the SED with a representative sample of FUors and EXors, and we found V1647 Ori to be more similar to FUors than to EXors. From its bolometric temperature and luminosity we calculated an approximate age of 400 000 years. In this evolutionary stage an accretion disk is already fully developed, though a circumstellar envelope may also be present.

Related paper:

Title: The infrared properties pf the new outburst star IRAS 05436-0007 in quiescent phase

Authors: Ábrahám, P., Kóspál, Á., Csizmadia, Sz., Moór, A., Kun, M., Stringfellow, G.

Journal: Astronomy & Astrophysics, v. 419, p. L39 (2004)

We are also doing new optical and near-infrared photometry and spectroscopy on V1647 Ori using the telescopes of Konkoly Observatory, Teide Observatory and the Observatorio del Roque de Los Muchachos. We computed J-H and H-K_S colour-maps of McNeil's nebula which revealed a flattened structure whose main axis is perpendicular to the axis of the optical nebula. Examining the I_C-band light-curves of different parts of the nebula and the star itself, we derived a possible geometry of the system. The SED, the light-curve and the spectroscopic properties suggests that V1647 Ori is a FUor.

On going ...

Related paper:

Title: Optical and near-infrared observations of V1647 Ori and McNeil's Nebula in February-April 2004

Authors: Kun, M., Acosta-Pulido, J.A., Moór, A., Ábrahám, P., Charcos-Llorens, M., Kóspál, Á., Csizmadia, Sz., Manchado, A., Vidal-Nú\~nez, M.J., Benkõ, J.M.

Journal: [astro-ph/0408432]

Figure 3: High-resolution far-infrared image if the Serpens Cloud. (RBG colour composite of 60 μm (R), 25 μm (G) and 12 μm (B) IRAS images.)

OO Ser, which had an outburst in 1995, is a poorly studied, deeply embedded star in the Serpens North-West star-forming region. (Fig. 3. shows the Serpens Cloud at infrared wavelengths). We are currently working on the ISOPHOT data on OO Ser. The source was monitored at infrared wavelengths in the fading phase by ISO between February 1996 and September 1997.

On going ...

Figure 4: RGB colour composite image of the FU Orionis-type star Parsamian 21 (infrared (R), red (G) and blue (B) POSS2 images).

Parsamian 21 is the central source of a compact, bright cometary nebula in Aquila. This star shows many spectroscopic characteristics of FUors and is likely surrounded by a circumstellar disk. We are currently working on near-infrared AO polarimetric data on Par 21. (Fig. 4. shows an infrared, red and blue colour composite image of Parsamian 21.

On going ...