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Astron. Astrophys. 327, 758-770 (1997)

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2. The L 1287 star forming region

Shortly after the discovery of a molecular outflow in the dark cloud L 1287 by Snell et al. (1990), Yang et al. (1991) mapped the outflow and the dense cloud gas in HCN (1-0) and HCO [FORMULA] (1-0), and identified the driving source with the far infrared source IRAS 00338+6312, whose position is centered on the dense cloud core and near the suspected outflow origin. Using three different distance indicators, Yang et al. found fairly good agreement for the distance to L 1287, which was finally adopted to 850 pc.

L 1287 received much attention when Staude & Neckel (1991) found a FU Orionis star (RNO 1B) near the suspected origin of the outflow and an indication for another nearby YSO. Indeed, Kenyon et al. (1993) presented strong evidence in favor of a second FU Orionis star (RNO 1C), only 6" northeast of RNO 1B. A comparison of the optical and infrared spectral energy distribution of the well known FU Orionis star (FUor) V 1735 Cyg with the combined flux of RNO 1B/1C gave very good agreement between 0.5 and about 10 [FORMULA] m. However, Kenyon et al. noticed excess emission in the far infrared using the flux data of IRAS 00338+6312, which they identified with the FUors RNO 1B/C. Although, the formal position of IRAS 00338+6312 is offset by 11" from RNO 1B and 5" from RNO 1C, the suggested identification seemed reasonable, taking into account the positional error ellipse of the IRAS source, which covers both the position of RNO 1B and RNO 1C.

However, the far infrared excess indicates that the spectral energy distribution of IRAS 00338+6312 is not quite typical for a FUor.

Fig. 1 shows a color diagram using the 12 [FORMULA] m, 25 [FORMULA] m, and 60 [FORMULA] m fluxes of IRAS point sources that can be identified with known FUors (Weintraub et al. 1991). The 100 [FORMULA] m IRAS data have not been taken into account since they have the highest probability of being contaminated by extended emission of cold dust. Weintraub (1990) found that almost all of the known FUors surveyed by IRAS were detected.

[FIGURE] Fig. 1. Color diagram of known FUors (according to Weintraub et al. 1991). For comparison, the region typically occupied by T Tau stars is indicated by the dashed box (Emerson 1987). The upper right area separated by solid lines represents the region where ultracompact HII regions are located (Wood & Churchwell 1989). The arrows indicate color temperatures [K] and the solid line represents the loci of black bodies up to 250 K.

Besides V 1057 Cyg, which was known to be a T Tau star before its outburst, Herbig (1977) concluded from the frequency of observed FUor eruptions and an estimate of the population of T Tau stars that could appear as FUors, that the FUor phenomenon is repetitive and occurs several times in the evolution of every T Tau star.

On the other hand, Weintraub et al. (1991) carried out submm observations of seven FUors and found their results to be consistent with the interpretation that FUors are among the youngest T Tau stars.

As suggested by the evolutionary relationship between FUors and T Tau stars (Herbig 1977), FUors might be expected to occupy a region in the color diagram that is shifted to higher F25 /F12 and F60 /F12 flux ratios as compared to T Tau stars, indicating somewhat lower color temperatures (i.e., steeper spectra). This tendency, already noticed by Weintraub et al., is in accordance with the evolution of dust envelopes around YSOs, where later evolutionary stages reveal deeper and hotter dust layers.

The color indices of IRAS 00338+6312 are still larger than those of other FUors implying a steeper spectrum and hence lower color temperatures than what is typical for a FUor. In terms of the evolutionary relationship between T Tau stars and FUors with respect to their color temperatures, it might be tempting to assume that IRAS 00338+6312 represents a still younger and deeper embedded YSO than a typical FUor; hence being not to be identified with RNO 1B/C. The IRAS fluxes indicate a broader than single temperature black body, and a positive spectral index [FORMULA] - d log [FORMULA] / d log [FORMULA] between 12 [FORMULA] m and 60 [FORMULA] m, which classifies IRAS 00338+6312 as a Class I object (Lada 1987).

Although the flux ratios and the derived luminosity ([FORMULA] 680 [FORMULA], Kenyon et al. 1993 ) are compatible with a high mass object, an active low mass YSO cannot be excluded. Provided that this embedded object would be another low mass YSO ([FORMULA] 1 [FORMULA], [FORMULA] 3 R [FORMULA]) as the neighboring FUors, the derived luminosity could be understood in terms of accretion luminosity, [FORMULA] [FORMULA], if the mass accretion rate is presently rather high (≳ 7 [FORMULA] 10-5 [FORMULA] yr-1); [FORMULA] 1) is the accretion efficiency. Since this mass accretion rate compares well with the derived value for FUor outbursts ([FORMULA] [FORMULA] yr-1, Hartmann & Kenyon 1985), it might be speculated that the deeply embedded protostar in L 1287 could be another FUor that currently encounters a phase of elevated activity.

From infrared polarimetric imaging, Weintraub & Kastner (1993) obtained images of the reflection nebula GN 00.33.9 (Neckel & Vehrenberg 1985), from which they derived patterns of the polarization vectors. These patterns indicate illumination of the reflection nebula by a deeply embedded single source surrounded by a dusty disk (Bastien & Ménard 1990), whose plane of symmetry is oriented perpendicular to the projected direction of the molecular outflow. The embedded YSO lies about 5" northeast of RNO 1C, at a position that is within 1&bsec;5 of the formal position of IRAS 00338+6312.

The suggestion of a third object, besides RNO 1B/C, is supported by VLA observations of the radio continuum at 8 GHz by Anglada et al. (1994) who detected a compact source (VLA 3) within 1" of the formal IRAS position.

At the same position, typically a dozen of strongly varying 22 GHz [FORMULA] masers were found, confined to an elongated region with a projected largest diameter of about 70 AU (Fiebig et al. 1996). The position-velocity distribution of the masers clearly indicates an underlying systematic velocity field (Fig. 2).

[FIGURE] Fig. 2. Position-velocity distribution of identified 22 GHz [FORMULA] masers in L 1287 (H2 O) (adopted from Fiebig et al. 1996). The LSR-velocities were binned into six equally spaced, consecutive velocity intervals, covering a velocity range of 26 [FORMULA]. It is important to notice that the positional error bars indicate one-channel-errors (as defined by Fiebig et al.) and not standard deviations. Some data point representations exceed the corresponding error bars. The offset coordinates are centered on R.A. (1950) = 00h 33m 53[FORMULA] 15, Dec (1950) = +63 [FORMULA] 12 [FORMULA] 32 [FORMULA] 1.

If the molecular outflow, that was detected on a rather large scale, originates at the center of VLA 3, Fiebig et al. argued that a simple outflow motion is hardly sufficient to explain the observed position-velocity distribution.

However, jet-driven outflows seem to be able to account for a variety of problems associated with molecular outflows (Masson & Chernin 1993), so that the maser emission might eventually be due to the interaction between the jet and the interstellar medium. Although the S-shaped position-velocity distribution of the masers displayed in Fig. 2 is reminiscent of a precessing jet structure, the most positive and negative local standard of rest (LSR) velocities would be expected to appear along the projected direction of the symmetry axis of the precession cones, i.e., toward the northeast and southwest; along the outflow direction. Contrary to that orientation, the masers' position-velocity distribution clearly indicates the extreme LSR-velocities toward the east/southeast and west/northwest.

An alternative interpretation considered by Fiebig et al. was maser emission from an accretion disk around an embedded YSO in VLA 3. Although the elongated position-velocity distribution of the masers agrees with an expected disk orientation that is perpendicular to the direction of the molecular outflow, the measured LSR-velocities could only be accounted for if a velocity component toward the disk center was assumed that is comparable with the rotational velocity component of disk rotation. This requirement, which had to be adopted for a reasonable agreement with the measured data, finds a simple explanation in terms of a clumpy structure of the infalling matter that impacts on an accretion disk (accretion shock), as will be shown below.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998