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Publications of the Astronomical Society of the Pacific 99:116-125, February 1987

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Publications of the Astronomical Society of the Pacific 99:116-125, February 1987 Publications of the Astronomical Society of the Pacific 99:116-125, February 1987 THE RING-SHAPED NEBULAE AROUND FU ORIONIS STARS* ROBERT W. GOODRICH Lick Observatory, Board of Studies in Astronomy and Astrophysics University of California, Santa Cruz, California 95064 Received 1986 October 27 ABSTRACT All five of the known FU Orionis stars are associated with ring-shaped nebulae. This type of nebula is not unique to the FU Orionis class, but it is rather unusual among other young stars. Deep CCD images and spectra are presented of three of the nebulae, those associated with V1057 Cyg, V1515 Cyg, and V1735 Cyg (= Elias 1-12). In the latter case the star is more highly reddened than the nebula, possibly indicating a localized concentration of dust near the star. The spectra of all three nebulae are clearly from reflection. A crude model for the dust shells is briefly discussed, and it is argued that the shells may indicate a relatively advanced evolutionary state for the FU Orionis stars. The shells may represent one lobe of a bipolar structure, with the opposite lobe hidden from view by dust in the equatorial "waist" of the structure. Alternatively, the elongated shape of the shells may be the result of the relative motion of the star through the parent molecular cloud, in which case there is only a single cavity. The FU Orionis outburst itself may then occur when the star reaches the edge of the cavity and fresh material is accreted. Some of the more recent models for the outburst itself are discussed and some observational tests of these models suggested. Key words: circumstellar matter-nebulae-stars: young I. Introduction changes in FU Orionis stars have been put forth. Herbig The existence of the FU Orionis stars as a distinct class (1977) discusses several of these, and the reader is re- of variable stars has been known for some time now. ferred to that paper for further details. Subsequent to that Herbig (1977) gave a thorough review of both the observa- summary, however, some other theories have been pro- tional and the theoretical status of this phenomenon as it posed. On the basis of infrared spectra and excesses, both was known at that time. In 1977 there were three known Elias (1978) and Mould et al. (1978) proposed that the FU members: FU Orionis itself, V1057 Cygni, and V1515 Orionis stars were binaries, in which a large, late-type M Cygni. Herbigs paper helped to define the observational star was orbiting an F or G supergiant. Larson (1980) characteristics of the class—a large and usually rapid rise suggested that a single, rapidly rotating star could explain in brightness, an optical spectrum at maximum light re- the phenomenon and also provide a natural explanation sembling that of an F or G supergiant, a spectral class for the estimated frequency of occurrence of these types which is dependent on the wavelength region in which of outbursts. More recently, Hartmann and Kenyon the classification is made, association with a molecular (1985) have suggested that what we are seeing at maxi- cloud and a peculiar reflection nebula, a characteristic mum light is not the photosphere of a star at all, but rather infrared excess, etc. Subsequent to his work Elias (1978) an optically thick accretion disk undergoing an outburst. found what is regarded as the fourth member of the class, Little work has been presented on the reflection nebu- Elias 1-12 = V1735 Cygni, a star in the IC 5146 dark cloud lae associated with the FU Orionis stars. Herbig (1977) complex that had brightened by five or more magnitudes. noted that the ring-shaped nebulae associated with FU Recently Graham (1983; see also Graham and Frogel Orí, V1057 Cyg, and V1515 Cyg are rather unusual struc- 1985) has discovered yet another example, a star associ- tures, although not unique to these three stars. Similar ated with the Herbig-Haro (HH) object H H 57 in the nebulae have been found to be associated with the other Norma I association. This find was significant in being the two FU Orionis stars V1735 Cyg and HH 57 1RS 1. Her- first documented association of an FU Orionis star with an big also noted that while spectra of the nebulae had not HH object. been obtained, photographic plates taken with interfer- On the theoretical side, many explanations for the ence filters isolating the Ha emission line and the nearby sudden increase in brightness and the apparent structural continuum indicated that they are predominantly reflec- tion. Duncan, Harlan, and Herbig (1981) performed area *Lick Observatory Bulletin No. 1055. photometry on the nebula associated with V1057 Cyg, 116 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System FU ORIONIS STARS 117 concluding that the nebula was fading in proportion to the TABLE I fading of the star, thus establishing conclusively the con- nection between nebula and star. Images This paper will begin by presenting some observational data on the morphology (Section III) and spectra (Section IV) of the nebulae surrounding three FU Orionis stars. Object Date Exposure The implications of these and other data for the evolution- (m) ary status of the stars are discussed in Section V, while Section VI reviews some of the outburst models currently being discussed in the literature. ■V1057 Cyg 24 Oct. 1984 60 25 Oct. 1984 60 25 Oct. 1984 60 II. Observations Images of the reflection nebulae around the stars V1057 V1515 Cyg 5 May 1984 10 5 May 1984 10 Cyg, V1515 Cyg, and V1735 Cyg were obtained with the 29 Sept. 1984 20 1-m Nickel telescope on Mount Hamilton, using a G.E.C. charge-coupled device (CCD) at the //17 V1735 Cyg 27 Sept. 1984 30 Cassegrain focus of the telescope. This detector has a 24 Oct. 1984 60 scale of 0''26 pixel-1 and a field of view of 1.'7 by 2.'5. A red 25 Oct. 1984 60 interference filter with a broad passband and high throughput (described by Djorgovski 1985) was used to Haro 2-249 24 Oct. 1984 30 avoid most of the strong emission lines from nearby San a Jose. To obtain lower noise and a wider areal coverage, RNO 54 3 Jan. 1986 1 3 Jan. 1986 3 several images of the fields were taken, often with differ- ent centers. These individual fields were then combined into the mosaics presented in Figure 1. Also shown in a 3-m images; all others from 1-m. Figure 1 are an image of the FU Ori star near H H 57 (from Schwartz, Jones, and Sirk 1984) and images of the two young stars RNO 54 and Haro 2-249. The H H 57 image used, with each pixel perpendicular to the slit represent- was taken on a photographic plate with the CTIO 4-m ing 0''71 on the sky. In the case of V1057 Cyg and V1515 reflector, while the RNO 54 image is a CCD montage Cyg, the slit was placed only across the nebulae to avoid from the 3-m Shane telescope on Mount Hamilton. The saturation of the CCD by the stars themselves. In the case Haro 2-249 was taken in the same manner and with the of V1735 Cyg, however, the star is heavily reddened and same equipment as the first three images. The relevant hence could be placed on the slit without worry. This has data for the individual exposures are given in Table I. the added advantage of obtaining a simultaneous long-ex- Another set of data was taken through a narrow-pas s- posure spectrum of the star itself. Even so, the exposure band interference filter centered on the red [S π] emis- of VI735 Cyg and its nebula was split in two to avoid sion lines at 6717 A and 6731 A. These observations were potential saturation of the CCD at the red end of the taken with the imaging mode of the CCD spectrograph on spectrum. Table II contains pertinent exposure informa- the 1-m telescope. A Hasselblad imaging lens of 110-mm tion and Figure 2 presents the spectra. focal length was used to give an effective focal ratio of about 2.4 and a scale of roughly Γ/Ι pixel-1. These images, III. Nebular Morphology covering almost 9' on the sky, were used to search for One rather striking characteristic of the nebulae of the [S ll] excesses in the vicinities of the stars. Light scattered FU Orionis stars is their ring-like morphology. This ex- into the optical path by infrared-emitting LEDs in the tends even to the cases of Η Η 57 1RS 1 and FU Ori spectrograph limit somewhat the usefulness of these data, (although the latter is also associated with a bright fan- but there is no evidence for any [S ll]-emitting nebulosity shaped nebula). As Herbig (1977) noted, there is a bright around these three stars. Hence, these images are not arc of nebulosity extending from FU Ori, similar to a part presented in this paper. of the rings seen around V1057 Cyg and V1515 Cyg. Of Spectra were obtained of these three nebulae using the course, one can find other examples of elliptical ring- Cassègrain spectrograph of the 3-m Shane telescope at shaped reflection nebulae among young stars. RNO 54 Mount Hamilton. This spectrograph employs a virtual- and Haro 2-249 (Fig. 1) are two well-defined examples, phase, 800 X 800 pixel T.I. CCD at an effective focal ratio and V1331 Cygni is seen almost at the center of a ring of of 1.2.
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