The FU Orionis Binary System and the Formation of Close Binaries 1

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1 Accepted by ApJ Letters The FU Orionis Binary System and the Formation of Close Binaries 1 Bo Reipurth 1 and Colin Aspin 2 1: Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI reipurth@ifa.hawaii.edu 2: Gemini Observatory, 670 North A ohoku Place, University Park, Hilo, HI caa@gemini.edu ABSTRACT The faint star next to FU Orionis recently discovered by Wang et al. has been observed with adaptive optics at the Subaru telescope. Infrared JHK L photometry shows clear infrared excess, indicating that the object is a pre-main sequence star. Its infrared K-band spectrum is very different from that of FU Orionis, and suggests a star of spectral type K. We discuss these observations in light of the hypothesis that FUors may be newborn binaries that have become bound when a small non-hierarchical multiple system breaks up. This scenario predicts that FU Orionis must be a close binary (<10 AU), and if so the newly discovered companion is the outlying member in a triple system. We discuss various implications of this scenario, in particular we note that FUor eruptions should commonly occur during a relatively brief evolutionary phase partly overlapping with and immediately following the formation of Herbig-Haro jets. If this is the dominant mechanism to trigger FUor outbursts, then FUor eruptions should preferentially occur in close binaries, i.e. in about 20% of all stars. Subject headings: stars: formation stars: pre-main sequence binaries (including multiple): close binaries: visual circumstellar matter ISM: jets and outflows 1 Based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan

2 2 1. Introduction FU Orionis eruptions represent a rare, but very important phenomenon in early stellar evolution, during which a young low-mass star brightens by up to 6 magnitudes and displays the optical spectrum of an F or G type supergiant (Herbig 1977). This has been interpreted as the brightening of a circumstellar disk around a T Tauri-like star due to a large increase in accretion (Hartmann & Kenyon 1985). While this accretion disk hypothesis has been very successful in explaining a number of observed characteristics of FU Orionis (FUor) eruptions, it still leaves certain observational details unexplained, suggesting that further refinements are required (Herbig 1989; Petrov & Herbig 1992; Herbig et al. 2003). Few FUor eruptions have been observed (named the classical FUors), but a steadily increasing number of premain sequence (PMS) objects are found which share some of the characteristics of FUors (e.g. Hartmann et al. 1989; Kenyon et al. 1993; Reipurth & Aspin 1997; Reipurth et al. 2002, Aspin & Reipurth 2003). Such FUor-like objects could well represent objects whose eruptions occurred before major sky surveys commenced and thus were not observed. The onset of increased mass transfer through the disk may be triggered by a thermal instability (Bell & Lin 1994). Alternatively, the close passage of a companion star may cause an increase in the accretion rate (Bonnell & Bastien 1992; Clarke & Syer 1996). The prototype of the classical FUors, FU Orionis itself, erupted in 1936 and has since then decayed only imperceptibly (Herbig 1966, 1977; Kenyon et al. 2000). It is the brightest and best studied of the classical FUors, and as such may provide us with crucial information on the nature of FUor outbursts. One significant question is whether FU Ori is a binary. Malbet et al. (1998) obtained interferometric observations suggesting the possible presence of a very close companion (<1 AU in projection), although the data can be equally well interpreted in terms of a circumstellar disk. Recently, Wang et al. (2004) discovered a faint red star 0.5 arcsec south of FU Ori and, based on statistical arguments, reasoned that this is likely to be a true companion. In this Letter, we present infrared photometry and spectroscopy obtained with adaptive optics at the Subaru telescope, and demonstrate that indeed the companion to FU Ori is a pre-main sequence star. 2. Observations FU Oriwas observed onutdecember 15, 2003with the NAOJ 8 meter Subaru telescope on Mauna Kea and the Infrared Camera and Spectrograph (IRCS, Kobayashi et al. 2000), assisted by the facility Adaptive Optics (AO) Unit. A pixel scale of /pixel was used. The natural seeing at K was variable around 0.8 arcsec, but the AO image quality at K was as good as 0.08, very close to the diffraction limit of an 8 meter telescope. The AO

3 3 corrections allowed us to extract accurate photometry of the companion. In these images, FU Ori was saturated in all filters, and we therefore repeated the images using a 1% neutral density filter. The photometry and total on-source integration times are listed in Table 1. The Mauna Kea photometric system is employed at IRCS/Subaru, and we list the central wavelengths in Table 1. Due to differences in longward cut-off in K and L among the various infrared systems, observers may find that FU Ori has rather different colors depending on the photometric system employed. We also observed FU Ori and its companion with AO-assisted long-slit spectroscopy covering the µm range with 16 min integration time at a resolving power of 800 using a 0.15 arcsec wide slit. Even with AO, the spectra of the two components are partially blended, and we are indebted to Rob Hynes for applying his special technique to extract the companion spectrum (Hynes 2002). 3. Results The companion to FU Orionis is well separated in our images. We derive a separation of 0.493±.003 arcsec at a position angle of 162.6±.4 degrees. The uncertainties are standard deviations based on gaussian fitting of 9 independent images. This is in excellent agreement with the somewhat more uncertain values derived by Wang et al. (2004). At the assumed distance of 460 pc, this corresponds to a projected separation of 227 AU. Figure 1 shows a K-band image which has been rotated around FU Orionis and subtracted. All structure within the 0.5 diameter circle is spurious due to saturation. In Table 1 we list photometry of the two components. Figure 2 shows that both FU Ori and its companion have substantial infrared excesses. The K-band spectra of FU Ori and its companion are very different (Fig. 3), giving confidence that the extraction has been successful. While FU Ori shows the deep CO bands characteristic of FUors, virtually no CO is seen in the companion. However, weak heavily veiled lines of Na I and Ca I appear to be present in the companion spectrum. Combined with the absence of CO bands this suggests a spectral type between late G and late K, which roughly corresponds to temperatures in the K range. Although a best fit to the continuum slope of the companion spectrum is found for a temperature of 2500 K, Fig. 3 shows that a higher temperature could account for the considerable infrared excess detected in the photometry. We conclude that the FU Ori companion is probably a star of spectral type K.

4 4 4. FUors as Binary Systems The detection of a companion to FU Orionis adds one more object to a growing number of FUors and FUor-like objects which are binaries, including Z CMa (Koresko et al. 1991), L1551 IRS5 (Rodríguez et al. 1998), RNO 1B/C (Kenyon et al. 1993), and AR 6A/B (Aspin & Reipurth 2003). In RNO 1B/C and AR 6A/B both components of the binary show FUor characteristics. Given the rarity of FUors, finding two FUor-like objects within a few arcseconds of each other in any star forming region is exceedingly improbable. In other words, whatever has triggered the FUor outburst in one component of these binaries seems likely to be somehow connected to whatever triggered the FUor outburst in the other. In principle, the components could form a highly eccentric binary, and a recent periastron passage could have disturbed their disks, as suggested by Bonnell & Bastien (1992), leading to the present simultaneous FUor characteristics. However, both RNO 1B/C and AR 6A/B have such large separations that it would take many thousands of years to reach their present configuration, and it is currently unclear whether the FUor states could have persisted that long. A way around this time-scale problem is to assume that each component of a FUor-like pair is a close binary, i.e. that they altogether form a hierarchical quadruple system which, through dynamical interactions, was recently transformed from an unstable non-hierarchical system, following a close encounter among the components. This transformation implies that two close stellar pairs are formed that may either remain bound to each other or may eventually disperse as two independent close binaries. On statistical grounds, the transformation of a non-hierarchical system is most likely to occur at such an early stage that the individual components are still surrounded by considerable amounts of circumstellar material (Reipurth 2000). The binaries will therefore evolve with significant viscous interactions, leading to angular momentum transfer and thus to rapid orbital shrinkage of the components (e.g. Artymowicz & Lubow 1996; Bate, Bonnell, & Bromm 2002). As the components spiral in, they may trigger disk instabilities leading to (one or more) FUor eruptions (Bonnell & Bastien 1992; Clarke & Syer 1996). This may explain the existence of pairs of FUor-like objects. In an extension of this scenario, we here explore the possibility that at least some FUor events may represent a stage in the formation of a close binary. It is of interest to consider how FUors in this view relate to other well defined stages of early stellar evolution. Reipurth (2000) suggested that giant Herbig-Haro (HH) flows are the result of dynamical interactions in small multiple systems. Massive disk truncation results from close triple approaches, accompanied by large-scale accretion, with a consequent burst of outflow activity that produces the observed giant HH bow shocks. Some of the material culled from the individual

5 5 circumstellar disks may settle into a circumbinary disk around the newly bound stellar pair. The small remaining and truncated circumstellar disks are fed from the circumbinary disk through gas streams, and this as well as other dynamical effects cause the binary orbit to shrink (Artymowicz & Lubow 1996). Gas streams together with disk interactions at periastron drive cyclic accretion modulated on an orbital time scale. As the stellar components gradually spiral towards each other, the increasingly frequent mass loss events form chains of HH objects until eventually the binary has a semi-major axis of only 9-12 AU, at which point the closely spaced shocked ejecta are produced on the yr time scale observed in finely collimated jets. Thus, such HH flows can be read as a fossil record of the evolution of orbital motions of a binary, newly formed in a triple disintegration event, as it shrinks from a typical separation of 100 AU or more to 10 AU or less. At this point the components are becoming so close that they, in combination with their eccentric orbits, begin to have difficulties maintaining stable circumstellar disks. This implies that the jet collimation mechanism, which is likely to involve magnetic fields partly anchored in the disk (e.g. Shu et al. 1995), becomes increasingly ineffective. Although mass loss continues, and indeed sporadically increases greatly, it soon no longer can take the form of collimated jets. At the same time the proximity of the two stars near periastron causes significant perturbations of the remaining disk material, leading to enhanced accretion (Bonnell & Bastien 1992; Clarke & Syers 1996), which we observe as FUor outbursts. In this view, at least some FUor outbursts will preferentially occur during a brief evolutionary phase in the formation of a close binary that is partly overlapping with and immediately following the formation of HH jets. It should be emphasized that any perturbation of a disk that will lead to a major increase in accretion is likely to appear as an outburst with FUor characteristics. If the final phase of the formation of a close binary will lead to FUor outbursts, this does not, for example, exclude that the thermal instability of Bell and Lin or the throttling mechanism of Hartmann and Kenyon (in which the accretion rate through the disk sometimes differs from the infall rate from the envelope) may play roles at earlier times. 5. Open Issues In the following we discuss several questions that arise from the discovery by Wang et al. (2004) of a companion to FU Orionis and of the scenario outlined above. Is the FU Ori companion related to the outburst of FU Ori? This could be the case if the companion had been ejected in 1936 when FU Ori erupted. If so, the eruption of FU Ori

6 6 would represent a close triple encounter, thus implying that FU Ori itself must be a close binary. However, to reach the current tangential separation of about 230 AU, the mean transverse velocity of the companion must be 16 km s 1. Most ejections occur with much, much lower velocities (e.g. Delgado-Donate et al. 2004), so this is highly unlikely. But there could be an indirect relation if the companion was ejected (into a bound or unbound orbit) maybe one or a few thousand years ago, leading to a spiral-in phase of the close binary and one or more subsequent FUor outbursts. In the scenario outlined in the previous section, FU Orionis itself must be a close binary, with a semi-major axis of 10 AU or (probably) less. Numerical models of the formation of binaries from disintegrating multiple systems show that the configurations that tend to survive are those in which the two most massive objects constitute the central binary and the remaining low mass members are hierarchically distributed at larger distances (e.g. Delgado-Donate et al. 2004). The orbital velocity of a close binary consisting of, say, two 0.5 M stars will be of the order of 5 km s 1 if the semi-major axis is around 10 AU, but with a large variation if the eccentricity is high. In principle, radial velocity measurements should reveal such a spectroscopic binary without too much difficulty. However, in FUors only a small fraction of the light we observe comes from the central star(s), since the hot accretion disk dominates the luminosity at all wavelengths. The resulting broad lines make high-precision radial velocity measurements more difficult. The best chance to observe radial velocity changes would be near periastron. Should we expect FUor eruptions in an object to be periodic? A central issue in the binary scenario is how many FUor outbursts one can expect during the spiral-in phase. Taken at face value, the binary scenario suggests that FUor outbursts should be at least quasi-periodic, occurring at or near periastron passage of the close binary. However, many other factors come into play, principally how long it takes to replenish and reconfigure the disturbed disks after an eruption. If a disk requires much longer to reconstitute than the time interval to the next periastron passage, then there will be little or no eruption. Replenishment of a disk must occur through infall from an envelope or from a circumbinary disk. Therefore, a triple disintegration event occurring during, say, the Class I phase would lead to a longer series of FUor outbursts than one that happened during the Class II phase, which might even lead to only one outburst. The interval between two periastron passages is also sensitively dependent on the eccentricity for a given periastron distance. Why do FU Orionis and many other FUors or FUor-like objects not display HH jets? This is another key question in the study of FUors. There is ample evidence that FUors have massive winds, but somehow most of the known FUors have lost their ability to form jets. In the binary scenario, this is probably because the binaries are now so close in most of these objects that the organization of magnetic fields required for jet launch and collimation have been destroyed. However, there are exceptions, including Z CMa, V346 Nor, and L1551 IRS5.

7 7 In L1551 IRS5, radio jets are seen to still emanate from each of the components in the radio binary (Rodríguez et al. 2003), suggesting that IRS 5 is a quadruple system and that the eruption may be one of the first of various outbursts still to come (this could explain the surprisingly low luminosity of L1551 IRS5 for a FUor). Do all stars go through a FUor phase? If FUor events can be triggered by a variety of perturbations, then all stars may experience such eruptions. This would, however, not be the case if only newly formed binaries would trigger such outbursts. We may get an estimate of the frequency of stars formed as close binaries by examining more evolved stars, and for metal-poor field stars (for which the statistics are particularly good) 18±4% are spectroscopic binaries (Carney et al. 2003). We note that in a recent paper, Herbig et al. (2003) have considered the possibility that FUor events may not be a property of ordinary T Tauri stars, but of a particular subset of them. In other words, not all young low mass stars may go through FUor phases, in contrast to widely accepted current views. However, while fewer stars are involved in FUor outbursts in the close binary interpretation, this may be compensated by them going through several FUor eruptions, depending on the particular orbital characteristics of each binary. 6. Conclusions We have demonstrated that the companion to FU Orionis recently found by Wang et al. (2004) is a pre-main sequence star with considerable infrared excess and a probable spectral type of K. We re-examine the hypothesis by Bonnell & Bastien (1992) that FUor outbursts may be due to a perturbation induced by a companion at periastron passage. We conclude that such events may be a natural consequence of interactions between two components that spiral in towards each other following the break-up of an unstable triple system. We recognize, however, that it may be simplistic to assume there is only one way to trigger FUor outbursts, and note that they may also occur during the actual disintegration of a triple system, as well as by diverse instabilities in disks around single stars. The role of binaries in the formation of FUors can be further explored by searching for close binary components in the known classical FUors employing imaging and/or spectroscopic techniques. We are much indebted to Rob Hynes for extracting the spectrum of the FU Ori companion, and to Hiroshi Terada and the observing team at Subaru for excellent support during the observations. Supported by the Gemini Observatory. We are indebted to George Herbig and Lee Hartmann and an anonymous referee for valuable comments.

8 8 REFERENCES Artymowicz, P., & Lubow, S.H. 1996, ApJ, 467, L77 Aspin, C., & Reipurth, 2003, AJ, 126, 2936 Bate, M.R., Bonnell, I.A., & Bromm, V. 2002, MNRAS, 336, 705 Bell, K.R., & Lin, D.N.C. 1994, ApJ, 427, 987 Bonnell, I., & Bastien, P. 1992, ApJ, 401, L31 Carney, B.W., Latham, D.W., Stefanik, R.P., Laird, J.B., & Morse, J.A. 2003, AJ, 125, 293 Clarke, C.J., & Syer, D. 1996, MNRAS, 278, L23 Delgado-Donate, E.J., Clarke, C.J., Bate, M.R., & Hodgkin, S.T. 2004, MNRAS, in press Hartmann, L., & Kenyon, S.J. 1985, ApJ, 299, 462 Hartmann, L., Kenyon, S.J., Hewett, R., Edwards, S., Strom, K.M., Strom, S.E., & Stauffer, J.R. 1989, ApJ, 338, 1001 Herbig, G.H. 1966, Vistas in Astronomy, 8, 109 Herbig, G.H. 1977, ApJ, 217, 693 Herbig, G.H. 1989, in ESO Workshop on Low Mass Star Formation and Pre-Main Sequence Objects, ed. B. Reipurth, p. 233 Herbig, G.H., Petrov, P.P., & Duemmler, R. 2003, ApJ, 595, 384 Hynes, R.I. 2002, A&A, 382, 752 Kenyon, S.J., Hartmann, L., Gomez, M., Carr, J.S., & Tokunaga, A. 1993, AJ, 105, 1505 Kenyon, S.J., Kolotilov, E.A., Ibragimov, M.A., & Mattei, J.A. 2000, ApJ, 531, 1028 Kobayashi, N. et al. 2000, in Proc. SPIE 4008: Optical and IR Telescope Instrumentation and Detectors, eds M. Iye & A. F. Moorwood, 1056 Koresko, C.D., Beckwith, S., Ghez, A., Matthews, K., & Neugebauer, G. 1991, AJ, 102, 2073 Malbet, F. et al. 1998, ApJ, 507, L149

9 9 Petrov, P.P., & Herbig, G.H. 1992, ApJ, 392, 209 Reipurth, B. 2000, AJ, 120, 3177 Reipurth, B., & Aspin, C. 1997, AJ, 114, 2700 Reipurth, B., Hartmann, L., Kenyon, S.J., Smette, A., & Bouchet, P. 2002, AJ, 124, 2194 Rodríguez, L.F. et al. 1998, Nature, 395, 355 Rodríguez, L.F., Porras, A., Claussen, M.J., Curiel, S., Wilner, D.J., & Ho, P.T.P. 2003, ApJ, 586, L137 Shu, F.H., Najita, J., Ostriker, E.C., & Shang, H. 1995, ApJ, 455, L155 Wang, H., Apai, D., Henning, Th., & Pascucci, I. 2004, ApJ, 601, L83 This preprint was prepared with the AAS L A TEX macros v5.2.

10 Declination (J2000) 9:04: :45: Right Ascension (J2000) Fig. 1. The companion to FU Orionis as seen in an AO assisted K -band image obtained at Subaru. FU Orionis itself is partly subtracted by rotating the image and subtracting it from itself. The circle has a diameter of 0.5, and structure within it has no meaning. Fig. 2. A J H vs.k L diagram showing FU Ori (1) and its companion (2). Error bars are shown to the right.

11 11 Fig. 3. K-band spectra of FU Orionis and its companion. Both spectra are normalized to 1 at their shortward end, but the companion spectrum has been offset for clarity. Three blackbody curves are shown.

12 12 Table 1. Photometry of the FU Ori Binary Filter λ c FU Ori a FU Ori-S On-source b J ± ± H ± ± K ± ± L ± ± a We list the combined photometry of the two components for comparison with previous photometry. The effect of the companion on the combined photometry is close to negligible. b Total exposure time in seconds.