AN FU ORIONIS OUTBURST OBJECT in the CYGNUS OB7 MOLECULAR CLOUD. T.A. Movsessian, T.Yu. Magakian, E.H. Nikogossian, Byurakan
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Protostars and Planets V 2005 8135.pdf AN FU ORIONIS OUTBURST OBJECT IN THE CYGNUS OB7 MOLECULAR CLOUD. T.A. Movsessian, T.Yu. Magakian, E.H. Nikogossian, Byurakan Astrophysical Observatory, 378433 Aragatsotn reg., Armenia, ([email protected]), T. Khanzadyan, Max-Planck Institut fur¨ Astronomie, Konigstuhl¨ 17, D-69117 Heidelberg, Germany ([email protected]), C. Aspin, T. Beck, Gemini Observatory, 670 N. Aohoku Place, Hilo, HI 96720, USA, A. Moiseev, Special Astrophysical Observatory,N.Arkhyz, Karachaevo-Cherkesia, 369167 Russia, M.D. Smith, Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK. 1,6 1,4 1,2 1,0 0,8 F l u x 0,6 0,4 0,2 0,0 6450 6500 6550 6600 Wavelength (A) Figure 1: The appearance of the reflection nebula in various spectral ranges and epochs. Introduction:We present an optical and near-infrared in- vestigation of a new FUor-like outburst recently discovered in an active star formation region surrounding RNO 127, located in the Cygnus OB7 dark cloud complex [1]. In this region, Figure 2: Optical and NIR spectrum of the source. several new cometary nebulae, Herbig-Haro objects, and out- flows/jets were found [1]. The detection of a new near-IR reflection nebula in this region immediately brought our at- Near infrared imaging and spectroscopy: For the near- tention to the possibility of its creation by a FUor outburst. IR images we first used the Omega Prime camera at the Calar This nebula, designated ‘IR-Neb’, has already been described Alto 3.5m telescope, Spain. A Ks broad-band filter with a in detail in [1]. Here, we bring together all available evidence passband centered at λ2.196µm was used and images were on the nature of the optical and near-IR nebulosity and the un- acquired on December 2000. derlying young star with the aim of determining whether the Additional near-IR K band images of the IR Nebula were associated young star has undergone a FUor-like outburst. obtained at the "Fredrick C. Gillett" Gemini North Telescope Optical imaging and spectroscopy: The first direct op- on Mauna Kea, Hawaii in 22 August 2003. The facility near- tical images of the region containing RNO 127 were obtained IR imager NIRI (REF) was used for the observations with a with the 2.6 m telescope of the Byurakan Observatory, Ar- K-band filter. The seeing during the acquisition sequence was menia, in August 2000 with the ByuFOSC-2 camera at prime 0.600. focus and CCD detector. Additional images were acquired in Finally, we have utilized the 2MASS Sky Atlas to study 2001 and 2002 using the multi-mode ’SCORPIO’ camera. the region at an earlier epoch. Long-slit optical spectroscopy of the brightest section of We have acquired K-band near-IR spectroscopy of the the IR nebula was obtained using the SCORPIO camera with stellar source associated with the IR Nebula. These data were 10A˚ resolution on the 2.6m telescope on 2003. The spectral taken at the NASA InfraRed Telescope Facility (IRTF) on range was 4000A˚ to 7000A.˚ Manua Kea, Hawaii on UT 31 May 2004 using the spectrom- A spectrum with higher spectral resolution was obtained eter SpeX. on the 6m telescope of Special Astrophysical Observatory Results: The nebulous object is totally absent on the plate (Russia) in September 2004 using a similar SCORPIO camera. from the Digital Sky Survey POSS-II, which was taken on A dispersing holographic grism gave a resolution about 2.5A˚ UT 24 August 1990 and does not show any optical nebulosity, in the red spectral range. The same brightest region in the IR though it is well seen on all our images starting from 2000 [1]. nebula was observed. In the Fig. 1 we present the appearance of the nebula in Scanning Fabry-Perot interferometry: These observa- the various wavelengths. In optics it resembles a curving or tions were carried out at the prime focus of the 6m SAO tele- possibly helical filament and since it is seen in all passbands scope on May 2003. We used a scanning Fabry-Perot interfer- it is likely at least partly of a reflection origin. The upper ometer with the SCORPIO focal reducer. A field-of-view of two images on Fig.1 show the light propagation in the nebula. 4.80 square was observed with a scale of 0.5600 per pixel. After 2001 there were no major changes in nebula illumina- Protostars and Planets V 2005 8135.pdf disk and HH635A north-east of the IR Nebula. Our FP scanning interferometry of this region confirms the suggestion in [1] LVF that these HH objects represent a bipolar outflow from the source stellar source associated with the IR-nebula. HH629 has a 1 negative radial velocity of 81 km s− while HH635A has a −1 positive value of +57 km s− (and also a significant velocity gradient). The FP data also confirm that the optical nebula is HVF purely reflective in nature with no trace of any emission lines such as Hα emission along its extent. Figure 3: Image of the nebula in K band (contours) and in I The 2 µm spectrum of the near-IR star-like source is shown band (grayscale) (left panel) and the scheme of formation of in the lower part of Fig. 2. This spectrum shows a rising double helical structure in high and low velocity flows (right red continuum with no strong features other than deep CO panel). absorption from 2.3–2.4 µm. Yet again, this type of spectrum is typical of FUor objects where the continuum emission is dominated by the circumstellar disk and the absorption by the tion/excitation, which is confirmed also by the photometry of low surface gravity atmosphere of the disk itself (rather than its brightest curving region. the stellar atmosphere). We conclude from this spectrum that In near-IR Ks band image (see Fig. 1 lower left) the source the circumstellar disk around this young star is dominant at appears dramatically different from the optical one. At 2 µm near-IR wavelengths. we observe a cometary reflection nebula, with very bright core. Conclusion: To summarize, without doubts, we see that The nebula is 1’ in total extent and again exhibits a curving, ∼ the variable optical nebula is illuminated by the bright IR possibly helical morphology. The same region imaged in the object which increased in NIR in several magnitudes between 2MASS survey does not show any nebular or stellar source in pre- and post-outburst. We did not observe directly the process this point. Photometry of the IR Nebula indicates a near-IR of the brightening of the IR object; nevertheless, it took place brightening of at least 4 magnitudes. shortly after 21.06.1999 (set by 2MASS observation date) If we look at the core star-like object at slightly higher This object represents a still rare case of FUor with op- spatial resolution from our Gemini NIRI image (Fig. 1 lower tically detected outburst, which also is a source of bipolar right), we see that at low contrast, the star is resolved in a point- outflow. As in other similar cases: e.g., V346 Nor [4] and like source with linear extensions protruding to the south-west probable FUors Z CMa [5] and Re 50 [6], the age of outflow and west. These are most likely the walls of the inner region (about 2000 years in our case) is much greater than the period of the outflow cavity from the young star. The full-width of outburst. This brings two possibilities: either each FUor half-maximum (FWHM) of the stellar profile is 0.800 in this ∼ outburst adds new HH condensations in the flow (thus such image while other stellar sources im the region have a FWHM outbursts much be recurrent), either these two phenomena are of 0.5500. This suggests that even at 2 µm, we have still ∼ not directly related. not detected the stellar photosphere itself probably due to the As was already mentioned in [1], the appearance of the presence of a dense, overlying circumstellar envelope around reflection nebula is quite different in optics and NIR. How- it. We also note that at high contrast we detect the existence ever, morphologically both images have indications to helical of a faint counter-flow, extending to the north-east from the structure, but with different opening angle and step. This can brightest point of the nebula. be schematically repersented as two helices enclosing one an- Our optical spectra of the brightest knot of the optical other (Fig.3). Such structure can be formed if two components nebula show strong and wide Hα absorption with a small with significantly different velocities exist in the outflow; each redshifted emission component. The BaII absorption blend one of them will be responsible for the formation of separate is also present (see Fig. 2). The existence of the young star helix. indicator absorption line λ6707A˚ Li I is yet to be confirmed. All these results strongly support the idea that the source These features make this spectrum similar to that of FU Orionis star, still invisible in optics, can be the genuine FU Ori object. itself and another known FUor, BBW 76 [2]. This work was mainly supported by INTAS grant 03-51- One noteworthy point is the strong asymmetric Hα ab- 4838 and by grant of CRDF/NFSAT AS 062-02/CRDF 12009. sorption profile. The central radial velocity of Hα absorption T.Yu.M. and T.A.M. thank the administration of Gemini Ob- 1 (deepest absorption) corresponds to 220 km s− , and its blue servatory for the support of their visit.