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Exors and the Stellar Birthline Mem. S.A.It. Vol. 88, 616 c SAIt 2017 Memorie della EXors and the stellar birthline S. Stahler Deptartment of Astronomy University of California, Berkeley, CA 94720, USA e-mail: [email protected] Abstract. EXors are young stars whose luminosities periodically increase by up to a factor of 100 within a year. Unlike the better-known FUors, their decay time is also relatively brief, and single objects have been observed to flare multiple times. A popular notion is that EXors rep- resent an especially early phase of pre-main-sequence evolution. Mackenzie Moody and I have shown that classical EXors, those which are visible prior to outburst, have the same age range as classical T Tauri stars. However, embedded EXors, those seen in quiescence at infrared and submillimeter wavelengths, are indeed very young. They lie close to the birthline in the HR di- agram. Photometrically, these objects are typical Class I or flat-spectrum sources. Thus, both categories include stars that are in the pre-main-sequence, and not the true protostellar, phase. An “embedded pre-main-sequence” phase should be included in a more complete description of low-mass stellar evolution. Key words. Stars: pre-main sequence – Stars: variables: general 1. Introduction have been seen to flare multiple times. Thus, the amount of data available to study the phe- One ubiquitous property of young stars is their nomenon is quite extensive. Nevertheless, the variability. The stars which vary in the most current attempts at modeling are even more in- dramatic fashion are the FUors. These objects, conclusive than for FUors, simply because few named for the prototype FU Ori, increase their theorists have tackled the subject. luminosity by a factor of up to 100 in the Even before attempting any direct assault course of a year or so, and then take many on the physical cause of flaring, it is worth- decades to fade (Vittone & Errico 2005). The while asking how common the phenomenon recurrence time of any flareup is so long that actually is. EXors that are optically visible we have never witnessed more than one such prior to an outburst are known to be clas- eruption in any star. As a consequence, our sical T Tauri stars. Herbig (2008) observed records of the actual flaring are generally rather that they are indistinguishable, both in spec- sparse. troscopy and photometry, from other stars of The observational situation is vastly im- this type (see his Fig. 10). EXor eruptions oc- proved for the less well-studied EXors. Like cur so frequently in any object that their total FUors, these low-mass, young stars brighten in number would be vastly larger if most classi- a year by as much as a factor of 100. However, cal T Tauri stars had this behavior. Clearly this the decay lasts only a decade or less, compa- cannot be the case, i.e. only a small fraction rable to the recurrence period of flares (Herbig of low-mass, pre-main sequence stars undergo 1989). Most of the several dozen known EXors this kind of outburst. Stahler: EXors and the stellar birthline 617 Classical T Tauri stars have disks, and these may be responsible, either directly or indi- 1.5 rectly, for the flaring activity. Might this ac- tivity occur only in the very youngest subset 1.0 ) ⊙ of classical T Tauri stars? Such is the view of /L ∗ (Hartmann 2008). He and others have specu- L 0.5 lated that both FUor and EXor-type outbursts 0.0 occur in all low-mass, young stars, the former when the object is still undergoing infall from ( Luminosity log −0.5 its parent dense core. In this picture, the EXors are direct descendents of the FUors (see also −1.0 Reipurth & Aspin 2010). The more frequent 3.75 3.70 3.65 3.60 3.55 3.50 Temperature log (Teff) (K) outbursts occur once the star is no longer em- bedded, but still very early in the optically visi- Fig. 1. Exors in the HR diagram. Open and filled ble pre-main-sequence phase (Hartmann 2008, circles represent classical and embedded EXors, re- Fig. 1.12). spectively. The upper, dashed curve is the stellar The hypothesis of extreme youth is easy to birthline of ref, and the lower, heavy solid curve is the zero-age main sequence. The lighter, dashed test, at least in principle. One simply places all curves are the pre-main-sequence tracks of ref. known, visible EXors in the HR diagram, and From right to left, the corresponding masses, in so- reads off stellar ages (and masses) from a set lar units, are: 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.5, 2.0, of underlying, pre-main-sequence tracks. One 2.5, 3.0, and 3.5. should, of course, put stars in the diagram ac- cording to their quiescent, pre-outburst effec- tive temperatures and luminosities. Mackenzie ages that span just the range one would expect. Moody and I recently did this exercise (Moody This finding is fully consistent with the earlier & Stahler 2017). results of Herbig (2008). We first tabulated essential data on all the However, there is an interesting twist to known EXors, including a record of which this story. Starting about a decade ago, a new stars are have binary companions. It turns out type of EXor was identified (Lorenzetti et al. that most EXors are, in fact, binaries, either 2012). These are optically faint in quiescence, visual or spectroscopic. The binary periods and their outbursts were picked up in large, in- are vastly discrepant from the recurrence time frared surveys. The spectral energy distribu- of outbursts –much longer for visual binaries, tion of such an “embedded EXor” falls into and much shorter for spectroscopic ones. This the Class I, or occasionally, the flat-spectrum discrepancy suggests that binaries are not re- category (Fig. 2). Thus far, the strength and ponsible for triggering the flareups. However, frequency of their outbursts appear similar to finding additional companions with periods of those of classical EXors, but manifested in the about a decade would be very challenging in infrared. In some cases, the extinction dimin- practice. Until such observations are under- ishes substantially during outburst, but the flar- taken, the question of whether companions ing is intrinsic, and not simply a fluctuation in trigger the outbursts must remain open. coverage by obscuring matter (see, e.g., Aspin Figure 1 is an HR diagram, which includes & Reipurth 2009). both a standard, zero-age main sequence and Placing embedded EXors in the HR dia- the pre-main-sequence evolutionary tracks of gram is more problematic than for their clas- Palla & Stahler (1999). The open circles rep- sical counterparts. For one thing, we rarely resent all of our tabulated EXors that are opti- have a direct measure of the stellar luminosity, cally visible prior to outburst. Again, these are only the bolometric one. But these two quanti- objects that are already known to be classical ties cannot differ greatly. If the bolometric lu- T Tauri stars. It is apparent that these “classi- minosity were much higher, then the excess, cal EXors” are not especially young, but have accretion-powered component would also be 618 Stahler: EXors and the stellar birthline is still enshrouded in a remnant envelope of gas and dust. It is only after the residual material disappears that the star emerges as an optically visible, pre-main-sequence object, a T Tauri star observationally. A small minority of pre-main-sequence stars undergo repeated flaring, both in their embedded and revealed stages. It is not at all clear physically how the remanant envelope is suspended during the “embedded pre-main-sequence” phase, nor how the envelope later vanishes. Nevertheless, the existence of embedded EXors demonstrates two general points. First, Fig. 2. Spectral energy distribution of V2492 Cyg, not all Class I sources are true protostars, and an embedded EXor. From Aspin (2011). the terms should not be conflated. Second, the embedded pre-main-sequence phase needs to relatively large. Assuming this accretion stems be included in a more complete account of from a disk, the lifetime of the latter would be low-mass stellar evolution. Future observa- unreasonably brief, much less than the star’s tional studies should probe more carefully the own contraction (Kelvin-Helmholtz) time. In configuration of the remnant, dusty envelope, summary, it is probably safe to use the rela- so that theorists can discern the balance of tively accesible Lbol as a substitute for L∗, and forces that allows it to persist. we have done so in practice. With the help of this device, we may now position all known embedded EXors in the References HR diagram. In practice, we labeled EXors as Aspin, C. 2011, AJ, 141, 196 embedded according to their observed flux at Aspin, C. & Reipurth, R. 2009, AJ, 138, 1137 12 µm, as compared to that in the visual band. Hartmann, L. 2008, Accretion Processes in Defining R ≡ (λ Fλ)12 = (λ Fλ)V ; we take em- Star Formation (Cambridge Univ. Press, bedded EXors to be those with R > 10. Cambridge) As seen by the filled circles in Figure 1, Herbig, G. H. 1989, in ESO Workshop on these objects mostly lie close to the stellar Low-Mass Star Formation and Pre-Main- birthline. In other words, the embedded pop- Sequence Objects, ed. B. Reipurth (ESO, ulation is an especially young subgroup of pre- Garching bei Munchen), 33 main-sequence stars. And we can be certain Kun, M., Szegedi-Elek, E., Moor,´ A., et al.
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