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The discovery of an extreme jet in Rosette that shedding light on the formation of free-floating brown dwarfs and giant planets

We report on the discovery of an optical jet with a striking morphology in the Rosette Nebula. It could be the most extreme case known of an accretion disk and jet system directly exposed to strong ionization fields that impose strong effects on its disk evolution. Unlike typical optical flows, this jet system is found to have a high excitation nature mainly due to disruptive interaction with the violent environment. As a result, the extension of the highly-collimated jet and possible former episodes of the degenerated counterjet all show bow-shocked structures. Our results provide implications on how incipience of massive stars in giant molecular clouds prevents further generations of low-mass star formation, and offers an evolutionary solution on how isolated substellar/planetary mass objects in regions of massive star formation are formed.


Introduction
Herbig-Haro (HH) jets are the optical manifestations of collimated outflows from young stellar objects (YSOs) in their early stages of evolution and, along with typical HH objects, are apparent tracers of active star formation. YSOs, unfortunately, are usually deeply embedded in dense, extended envelopes and/or opaque molecular cloud cores that strongly impede efforts for investigations in the optical regime and make our view of the early stages of star formation still quite a puzzle. In some cases, however, YSOs are located within the confines of HII regions (Reipurth et al. 1998) and their power sources suffer very low extinction and are therefore optically visible. Similar jets immersed in a photoionized medium were identified in the Orion nebula and near the late B stars in NGC 1333 (Bally et al. 2000;Bally & Reipurth 2001). However, some properties of photoionized jets in HII regions are found to be highly dependent on their environment. Those immersed in the outskirts of HII regions, or near soft-UV radiation sources such as late B stars, tend to emerge as bipolar jets which usually share a "C" shaped appearance (Bally et al. 2000;Bally & Reipurth 2001), whereas jets deeply embedded in HII regions endure a more disruptive environment and seem to favor highly asymmetric or even monopolar jet formation (Reipurth et al. 1998).
The Rosette Nebula is a spectacular HII region excavated by the strong stellar winds from O stars in the center of the young open cluster NGC 2244, which has an age of about 3 x 10 6 yr (Ogura & Ishida 1981). This on-going star forming region (Meaburn & Walsh 1986;Clayton et al. 1998;Phelps & Lada 1997;Li et al. 2002;Li 2003) is located at a distance of ∼1500 pc at the tip of a giant molecular cloud complex with an extent of around 100 pc (Dorland & Montmerle 1987). In this paper, we present the discovery of an optical jet system embedded in the Rosette Nebula with a fascinating morphology that displays exceptional new features not known before.

Narrowband Imaging
Narrow band images of the Rosette Nebula were obtained 3 March 1999 with the Kitt Peak National Observatory 0.9-meter telescope and the 8k×8k MOSAIC I camera (Muller et al. 1998). Images were taken with the Hα, [OIII] and [SII] filters, whose central wavelengths/FWHMs are 6569Å/80Å, 5021Å/55Å, and 6730Å/80Å, respectively. For each filter, five exposures of 600 seconds were taken, each image slightly offset to fill in physical gaps between the MOSAIC CCDs. The pixel scale is 0.423 ′′ pixel −1 , resulting in a 59 ′ x 59 ′ field of view. An astrometric solution for these images was determined with the use of stars in the Guide Star Catalog 1.2 (Roser et al. 1998). The astrometric accuracy in the optical images presented is ∼1 ′′ .

Optical Spectroscopy
Optical spectroscopy of the jet system, with a 200Å mm −1 , 4.8Å pixel −1 dispersion, and a 2.5 ′′ slit, was carried out with the 2.16m telescope of the National Astronomical Observatories of China on 10 and 16 January 2003, with slit positions along and orthogonal to the jet direction. A possible physical companion of the apparent power source was also observed with the orthogonal slit position to avoid contamination from the jet system. An OMR (Optomechanics Research Inc.) spectrograph and a Tektronix 1024×1024 CCD detector were used in this run of observations.
The spectral data were reduced following standard procedures in the NOAO Image Reduction and Analysis Facility (IRAF, version 2.11) software package. The CCD reductions included bias and flat-field correction, successful nebular background subtraction, and cosmic rays removal. Wavelength calibration was performed based on helium-argon lamps exposed at both the beginning and the end of the observations every night. Flux density calibration of each spectrum was conducted based on observations of at least two of the KPNO spectral standards (Massey et al. 1988) per night.

The Jet System
An optical jet system with a striking morphology was identified when scrutinizing the narrow band images of the Rosette Nebula ( Fig. 1). A prominent jet with a position angle of 312 • is clearly seen traced back to a faint visible star, indicating rather low extinction along the line of sight. The circumstellar envelope of the YSO might have been stripped away, leaving a photoablating disk exposed to the strong UV radiation field of the exciting sources in the HII region. The optical jet remains highly collimated for a projected distance of >8000 AU from the energy source if a mean distance of 1500 pc to Rosette is adopted. At the end of the collimated jet is a side-impacted shock structure, probably due to severe radiation pressure from the strong UV field present and strong stellar winds encountered. A prominent knot or compact mass bullet resembling a point source is immediately noticed in the highly collimated part of the jet with a projected separation of 7 ′′ from the source. Another one or two knots can be marginally noticed in the bow shaped extension of the jet, indicating episodic eruption events from the driving source.
We infer the possible existence of a counterjet from the presence of a promising bow shock structure on the opposite side of the jet. Successive episodes of counterjet ejection may have ceased or may be too faint. Alternatively, this structure may simply be shocked interstellar material that was swept up and photoionized by the strong radiation field, which happen to be projected onto the vicinity of the jet exciting source. Another possible explanation is that it is disrupted jet material from a possible T Tauri companion (please refer to Fig. 1) or from nearby young stars. If it is indeed a degenerated counterjet, however, it may be the only existing observational evidence for how bipolar jets evolve into monopolar or highly asymmetric jets. We further argue that the long-puzzling high-velocity components from the photoionized gas fronts aggregated in the Rosette (Meaburn & Walsh 1986;Clayton & Meaburn 1995;Clayton, Meaburn & Lopez et al. 1998) could actually be from rapidly disrupted young disk-jet systems in the inner part of the HII region. Diffuse [OIII] emission is prominent from all parts of the jet system, which is likely due to its photo-dissipating nature.
[SII] emission from the highly-collimated part of the jet decreases rapidly from the base of the jet and is undetected in the shocked structures, indicating a steep decline of internal shock effects or rather overwhelming external ionization.
Spectroscopic observation along the jet direction reveals prominent Hα (W λ ∼ 35Å) as well as [SII] emission. The presence of significant [OIII] λλ4959, 5007 line emission confirms our suspicion that the jet is in a high-excitation state. Fig. 2 clearly shows the existence of a weak counterjet as discussed above. Note that enhanced [NII] emission is detected along the jet, which is not shown here.

The exciting source
Spectroscopic observations of the exciting source reveal primarily the spectrum of a normal F8Ve star with weak Hα (W λ ∼ 6.7Å) and prominent [OIII] emission. However, the spectrum also shows a significant red-displaced absorption component in Hα, with a receding velocity of 500±50 km s −1 (Fig. 3). Two weak but significant emission features with unclear nature are also present immediately to the red of the rest Hα emission and its red-displaced absorption. One is centered at 6582.5Å with an accuracy of ± 1Å , and can be the pronounced [NII] λ 6583.6 emission detected also along the jet. This gives a high [NII] to Hα flux ratio of about 1:3. Albeit the other with a central wavelength of 6613.3Å cannot be identified with any known emission lines with rest velocities. A possible interpretation is a red-displaced emission component of Hα. This however would suggest an extraordinarily high receding velocity of 2300±50 km s −1 and should be treated with great caution. The possible ejection of high velocity, compact mass bullets from the energy source, if true, can be partially supported by the clear existence of a compact knot in the highly collimated part of the jet as mentioned in the previous subsection, and may offer a big challenge to currently available disk-jet models. This scenario, however, seems to match well with Meaburn's (2003) recent speculations on the formation of ablated jets in HII regions.
Spectral energy distribution (Kurucz 1991) fitting of the energy source based on spectroscopic observations and data extracted from 2MASS indicated a relic disk mass of only 0.006 M ⊙ , consistent with a photoablating origin of the system. It is noteworthy that Inverse P Cygni (IPC) profiles seldom appear in low Balmer emission lines such as Hα. The most well known case is the YY Orionis type, weak-lined T Tauri star T Cha (Alcala, Covino & Franchini et al. 1993), from which, at most, a relic circumstellar disk can be expected. An IPC profile in Hα is usually attributed to its optically thin nature, which normally leads to the conclusion of a high inclination angle of the relic disk. Furthermore, this must happen under tight conditions such as low-density in the residual circumstellar material and weak Hα emission (Alcala, Covino & Franchini et al. 1993). Both are consistent with properties of this jet system.

Implications
A similar red-displaced absorption profile is also extracted around the power source from the spectra taken along the jet direction, which helps to elucidate the reliability of the spectral reduction and time elapsed mass accretion of the central YSO. The simultaneous existence of signatures of mass inflow and outflow, along with the absence of apparent veiling or blue excess, the absence of signatures of chromospheric activity, and its weak or absent Balmer emission in combination is strong evidence that the forming star is highly affected by its location in the strong UV radiation fields. Indeed, Reipurth et al. (1998) state that external ionization may help on the feeding and/or launching of the jet, which makes jet formation in such systems survive. We further speculate that the aforementioned effects eventually change the configuration of the stellar-disk magnetosphere such that photodissipated material in the relic disk is easily loaded onto the magnetic channels, especially on the side facing the strong radiation fields. It is conceivable that most or all of this material could have been ejected in the form of a jet on the opposite side of the mass loading, instead of eventually ramming into the contracting infant star embarrassed with starvation. It's therefore hard for the energy source to further grow in mass, its accretion disk could be dissipated on a comparatively short time scale and its evolution toward the main sequence is either highly accelerated or in some cases even led to the formation of a failed star that would have evolved into a higher mass star under normal conditions. Many young, very low mass stars and brown dwarfs are found to show evidence of mass accretion from its circumstellar environment (Fernandez & Comeron 2001;Luhman, Briceno & Stauffer 2003;Jayawardhana, Mohanty & Basri 2003;White & Basri, 2003). This study presents further implications on how incipience of massive stars in giant molecular clouds inhibits further generations of low-mass star formation. It could also well serve as an inspiring case of how isolated substellar/planetary mass objects in regions of massive star formation, especially those found in HII regions (Zapatero Osorio et al. 2000), were prohibited from growing in mass, ceased their accretion process and came into being. Fig. 1.-The Hα imaging of the jet system (3 ′ .2 x 2 ′ .7). North is up and east is to the left. The prominent jet, at the lower center of the image, and its extended bow shock structure as a whole extended to ∼0.13 pc away from the driving source. Assuming that the jet has an inclination of 45 • off the plane of the sky and a typical flow velocity of 150 km s −1 , a lower limit of the kinematic age of the jet of ∼1200 yr is derived. The apparent power source is located at R.A. = 06h32m20.76s, Dec = 04d53m02.9s (J2000). A possible companion also exists, for which the coordinates from 2MASS are R.A. = 06h32m20.79s, Dec = 04d52m58.1s (J2000). Optical spectroscopy of this star shows prominent Balmer emission lines (W λ of Hα and Hβ are 72Å & 51Å, respectively) are superimposed on spectrum of a low-reddening K dwarf. Both components of the possible binary system are therefore T Tauri stars previously unknown in this region. However, no strong intervention between the two is apparent. Fig. 2.-Contour map of the Hα emission along the jet direction. Note that low-resolution spectroscopy was employed in this run of observations and no prominent velocity difference between the opposite jets can be noticed. However, there is clear indication of the existence of a poorly developed or degenerated counterjet. Compact Hα emission at the upper part of the map is from the shocked structure located on the opposite side of the prominent jet.