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Astron. Astrophys. 346, 441-452 (1999)

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6. IRAS 05327+3404 (Holoea) in an evolutionary context

The evolutionary status of Holoea is something of a puzzle. It is clearly a young stellar object of some type, given the spectral energy distribution and the existence of molecular and ionized outflows. However, several of the observed features appear to lead towards contradictory conclusions. The central star is observed directly with only a minimal amount of extinction ([FORMULA] mag), as would be expected in a fairly evolved pre-main sequence star. But, there is the strong far-IR emission and CO outflows typically seen in more embedded systems. As we discussed above, there is the possibility that the optically visible star is either a reflection or a binary system. While either of the two possibilities would make it easier to understand the SED, we have presented arguments why they are unlikely for other reasons.

As mentioned in Paper I, the central star of Holoea has become brighter in the last 40 years. On red POSS plate of this field taken Nov 11, 1954, the central star is barely visible (Fig. 7). A comparison with other stars in the field shows that the central star at the time of our first observations on October 2, 1993, the central star has brightened by [FORMULA]1.5 mag (see Fig. 7). We have ruled out a color effect, as stars with colors similar to that of Holoea do not appear substantially fainter on the POSS plate image. The POSS II survey also observed this field Oct 4, 1989. In the red POSS II image, the central star appears to already have reached the level of the Oct 1993 observations. Observations since Oct 1993 have shown no further evidence of variability.

[FIGURE] Fig. 7. This figure shows the 1954 red POSS plate and 1993 R CCD images of Holoea (circled). Holoea has brightened substantially between these two images.

Several types of young stellar objects show variability. Many T Tauri stars exhibit small levels of stochastic variability which may be due to changing accretion rates or changing amounts of obscuration. FU Orionis (FUor) systems show outbursts characterized by sudden, large increases and slow decreases in their brightness, thought to be the result of large, short-lived accretion events. EXor systems usually show smaller, shorter outbursts, which may be due to a similar mechanism to the one responsible for FUor outbursts (Lehmann et al. 1995). This class is named for the star EX Lupi, and invokes the similarity with the FUors. However, the observed brightening of Holoea does not seem to be consistent with either of these types of variability. The increase seems to have been a single change, as there is now no evidence of variability or the slow decay seen in FUors. In the case of the FUor and EXor systems, the increase in brightness is associated with an increase in the flux from the accretion disk. For EXors, which have a strong UV excess and strong high-Balmer lines, the increase in the veiling continuum accounts for most of the increase in flux (Lehmann et al. 1995). Since the veiling in Holoea is now only [FORMULA]25%, this could not be responsible for a 1.5 mag increase in brightness. In the case of FUors, the accretion rate is so high that the boundary expands and takes on the spectral characteristics of a late F or G type star. The brightness increase is attributed to this increased emission (Hartmann & Kenyon 1996). However, our spectra show that the flux at R from Holoea is dominated by a stellar photosphere of spectral type early K. If this is the emission from an extended boundary layer, it is quite cool compared to those of FUors.

An alternative mechanism by which the central star may have gotten brighter is if the amount of extinction has decreased. In Paper I, we suggested that Holoea represents a transitory phase in the evolution of low-mass YSOs from the embedded to the exposed phases, also called Class I and Class II T Tauri stars (Lada & Wilking 1984). This suggestion was based principally on the combination of a visible central source and a flat spectral energy distribution. Given the presence of strong outflows, seen in both ionized and molecular gas, the observed increase in brightness could be the result of clearing of the polar holes.

It would have been interesting to trace the process by which a YSO clears out the central regions if we could have followed the increase in the brightness of the central source since 1954, when the POSS plate was taken. We have searched the Harvard plate stack library for any observations in the intervening 40 years, but without success. Given the V magnitude of 18.6 and the crowded field, only very deep and large-scale photographic plates could have detected Holoea. None of the existing Harvard plate stack images of the region are both deep enough and have high enough spatial resolution. We have also searched the literature for observations, particularly of M 36, since Holoea is within the field of the cluster. Several old photographs of M 36 exist, and these were used for measuring the proper motion (Hagen 1970). However, the only one taken after the POSS plate image was from fall 1954 (Bronnikova 1958), at roughly the same time as the POSS plate image. Unless unpublished observations of M 36 exist in other plate archives, it is unlikely that we will obtain additional, detailed information on the long-term lightcurve of Holoea.

Even if the brightening of Holoea is due to the clearing of the central hole, the evolutionary status of the object is not certain. Our attempts to compare the SED to models suggest that the unusually wide SED is not the result of evolution but rather to the large angular momentum of the circumstellar material. In Sect. 5, we showed that Holoea is part of a very small cloud with only a small number of other possible nearby stars being formed. This small cloud is on or somewhat beyond the periphery of the Taurus-Auriga complex. Lada et al. (1993) discuss the Taurus-Auriga complex and the Rho Ophiuchus complex, and point out that in the first, star formation is taking place in low concentration regions, while in the second the star formation occurs in dense clusters. They also show that the dense, clustered mode of star formation is apparently more common, at least in our part of the Galaxy. Holoea is isolated even compared with the typical star in the Taurus-Auriga complex. We hypothesize that the relative isolation of Holoea compared to the bulk of young stars may be related to the unusually wide SED, perhaps by allowing the formation of large circumstellar disks with high angular momentum which would otherwise be disrupted. It is conceivable that such a high mass, high angular momentum disk may survive much longer than a low angular momentum disk, as the rotation inhibits accretion of the circumstellar material onto the star. This would give the central star time to evolve while the cloud is retained. These last points are speculative, but provide a starting point to consider the differences between stars with different SEDs and apparently similar evolutionary states.

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© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999