▪ Abstract We summarize the properties of FU Orionis variables, and show how accretion disk models simply explain many peculiarities of these objects. FU Ori systems demonstrate that disk accretion in early stellar evolution is highly episodic, varying from ∼ 10−7 yr−1 in the low (T Tauri) state to 10−4 yr−1 in the high (FU Ori) state. This variability in mass accretion is matched by a corresponding variability in mass ejection, with mass loss rates reaching ∼ 10−1 of the mass accretion rates in outburst. It appears that the FU Ori phenomenon is restricted to early phases of stellar evolution, probably with infall still occuring to the disk, which may help drive repetitive outbursts. Thermal instabilities are a promising way to produce FU Ori disk outbursts, although many uncertainties remain in the theory; triggering by interactions with companion stars on eccentric orbits may also play a role.

The FU Orionis Phenomenon

Dust opacities in dense regions

The Structure of Jupiter's Ring System as Revealed by the Galileo Imaging Experiment

?????We have obtained deep [S II] ??6716, 6731 and continuum images of the five classical T Tauri stars (CTTSs) DP Tau, RW Aur, V536 Aql, LkH? 321, and V1331 Cyg. These CTTSs were selected for our imaging study because they either showed strong P Cygni absorptions in their spectra (LkH? 321, V1331 Cyg) or were known from long-slit spectroscopic studies to have bipolar jetlike outflows (DP Tau, RW Aur, V536 Aql). For the latter three objects, the spatial extent and structure of the outflows are studied here for the first time. In addition, LkH? 321 and V1331 Cyg were found to be associated with Herbig-Haro (HH) objects and/or HH jets. In four cases, the jets or outflows are bipolar, with the largest spatial extent seen in RW Aur (145''; 0.1 pc for d = 150 pc) and V1331 Cyg (360''; 1 pc for d = 550 pc). About 170'' west of LkH? 321, we have serendipitously discovered a faint HH source, which we designate LkH? 321B.

T Tauri Stars Associated with Herbig-Haro Objects and Jets

FU Orionis stars (FUors) are eruptive pre-main sequence objects thought to represent quasi-periodic or recurring stages of enhanced accretion during the low-mass star-forming process. We characterize the sample of known and candidate FUors in a homogeneous and consistent way, deriving stellar and circumstellar parameters for each object. We emphasize the analysis in those parameters that are supposed to vary during the FUor stage. We modeled the spectral energy distributions of 24 of the 26 currently known FUors, using the radiative transfer code of Whitney et al. We compare our models with those obtained by Robitaille et al. for Taurus class II and I sources in quiescence periods by calculating the cumulative distribution of the different parameters. FUors have more massive disks: we find that ~80% of the disks in FUors are more massive than any Taurus class II and I sources in the sample. Median values for the disk mass accretion rates are ~10–7 M ☉ yr–1 versus ~10–5 M ☉ yr–1 for standard young stellar objects (YSOs) and FUors, respectively. While the distributions of envelope mass accretion rates for class I FUors and standard class I objects are similar, FUors, on average, have higher envelope mass accretion rates than standard class II and class I sources. Most FUors (~70%) have envelope mass accretion rates above 10–7 M ☉ yr–1. In contrast, 60% of the classical YSO sample has an accretion rate below this value. Our results support the current scenario in which changes experimented by the circumstellar disk explain the observed properties of these stars. However, the increase in the disk mass accretion rate is smaller than theoretically predicted, although in good agreement with previous determinations.

/pdf/spectral-energy-distribution-analysis-of-class-i-and-class-52e22ks019.pdf

Spectral energy distribution analysis of class i and class ii fu orionis stars

Galileo NIMS Near-Infrared Observations of Jupiter's Ring System

High resolution (approximately 4 sec) aperture synthesis maps of the CO (1 yields 0), (13)CO (1 yields 0), (13)CO (2 yields 1), and asociated continuum emission from the FU Orions candidate V1331 Cygni reveal a massive, 0.5 +/- 0.15 solar mass, circumstellar disk surrounded by a flattened gaseous envelope, 6000 x 4400 AU in size, mass greater than or equal to 0.32 solar mass. These images and lower resolution measurements also trace a bipolar outflow and gaseous ring, 4.1 by 2.8 x 10(exp 4) AU, mass greater than or equal to 0.07 solar mass, radially expanding at 22 +/- 4 km/s. We suggest this ring is a swept-up gaseous torus from an energetic mass ejection stage, possibly an FU Orionis outburst or outburts, approximately 4 x 10(exp 3) yr ago that imparted greater than or equal to 10(exp 45) ergs into the ambient cloud.

The circumstellar environment of the FU Orionis pre-outburst candidate V1221 Cygni

High resolution (~4") aperture synthesis maps of the CO (1→ 0), ^(13)CO (1→0), ^(13)CO (2→1), and asociated continuum emission from the FU Orions candidate V1331 Cygni reveal a massive, 0.5 ± 0.15 M_☉, circumstellar disk surrounded by a flattened gaseous envelope, 6000 x 4400 AU in size, mass >0.32 M_☉. These images and lower resolution measurements also trace a bipolar outflow and gaseous ring, 4.1 by 2.8 x 10^4 AU, mass greater than or equal to 0.07 M_☉, radially expanding at 22 ± 4 kms^(-1). We suggest this ring is a swept-up gaseous torus from an energetic mass ejection stage, possibly an FU Orionis outburst or outburts, ~4 x 10^3 yr ago that imparted >10^(45) ergs into the ambient cloud.

/pdf/the-circumstellar-environment-of-the-fu-orionis-pre-outburst-2gkm4xgtje.pdf

The Circumstellar Environment of the FU Orionis Pre-Outburst Candidate V1331 Cygni

Observations of CS (7→6) emission reveal a ≥3M_⊙ core, 1.8×10^4 AU in size, surrounding the FU Orionis binary system RNO 1B/1C. Fractional chemical abundances, calculated from LVG and LTE codes, are mostly similar to those in the cold core TMC 1. However, values for Si0/H_2 and CH_(3)0H/H_2 are enhanced, possibly by sputtering reactions or grain-grain collisions in tile outflow associated with the young stars. Aperture syntllesis maps of tile 2.6 and 3.1 mm continuum emission at ~5" and ~9" resolution, respectively, reveal that RNO 1C is surrounded by a flattened, dusty envelope, ~5000 AU in size, with mass
≥1.1 M_⊙. High spatial resolution (~3") interferometer observations of CS (2→1) emission may trace the dense walls of ail outflow cavity comprised of two concentric arcs with dynamical ages of 4×10^3 and 1×10^4 yr. The velocity structure of lower density gas imaged in the CO (1→0) transition is consistent with the arcs being formed by two energetic FU Orionis outbursts. Each event may have imparted more than 4 M_⊙km s^(-1) to the outflow, implying outburst mass loss rates of ~10^(-4) M_⊙ yr^(-1). It appears that RNO 1C is probably the driving source for the outflow and tllat, while pre-main sequence stars are in tile FU Orionis stage, outbursts may dominate both outflow morphology and energetics.

/pdf/the-fu-orionis-binary-system-rno-1b-1c-2nds2b9y7d.pdf

The FU Orionis binary system RNO 1B/1C

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Galileo NIMS Near-Infrared Observations of Jupiter's Ring System

The circumstellar environment of the FU Orionis pre-outburst candidate V1221 Cygni

The Circumstellar Environment of the FU Orionis Pre-Outburst Candidate V1331 Cygni

The FU Orionis binary system RNO 1B/1C