Two new T Tauri stars and a candidate FU Orionis star associated with Bok globules

Astron. Astrophys. 320, 167 171 (1997) ASTRONOMY AND ASTROPHYSICS Two new T Tauri stars and a candidate FU Orionis star associated with Bok globules J.L. Yun 1, M.C. Moreira 1, J.F. Alves 1, and J. Storm 2 1 Departamento de Física, Universidade de Lisboa, Campo Grande, Edif. C1, 1700 Lisboa, Portugal 2 European Southern Observatory, Casilla 19001, Santiago, Chile Received 20 May 1996 / Accepted 25 September 1996 Abstract. We present photometric and spectroscopic evidence of two new T Tauri stars formed in the conditions of isolated small Bok globules. The spectral energy distributions of these objects display excess infrared emission, they are associated with optical reflection nebulae, and their optical spectra reveal Balmer emission lines and the Li I λ6707 Å absorption line. Additionnally, we report the discovery of what may be a new FU Orionis star seen towards Bok globule CB 34. The star is about 4 magnitudes brighter than it appears on the Palomar plates and is associated with the aggregate of young stellar objects forming in Bok globule CB 34. Key words: stars: formation stars: pre-main-sequence ISM: clouds ISM: individual: CB 34 infrared: stars 1. Introduction T Tauri stars were the first group of objects recognized as young pre-main sequence stars (Joy 1945). We now believe that classical T Tauri stars are young low-mass stars ( 2M ) surrounded by circumstellar accretion disks. Since the discovery of the first T Tauri stars, about 800 pre-main sequence stars have been optically identified and listed in the Herbig & Bell (1988) Catalog. T Tauri stars like other young stars tend to form in groups embedded in molecular clouds. Many are typically found in loose clusters known as T associations. Multiple star formation however does not facilitate the study of the star formation process. Observations of the formation of just one or a couple of stars can certainly provide a simpler laboratory for the study of the star formation process. Isolated T Tauri stars exist and are likely to form in the core of the smallest and simplest molecular clouds: the small Bok globules. The physical conditions in Bok globules are similar to those in dark cloud cores where lowmass star formation has been shown to take place (Myers et al. 1987). Moreover, given their small size and isolation (Clemens Send offprint requests to: J.L.Yun & Barvainis 1988), Bok globules are the ideal laboratories for studying the formation of single or a couple of low-mass stars. During the last years, it has been shown that low-mass star formation is a rather common phenomenon in Bok globules (e.g. Yun & Clemens 1990; 1994a), displaying all ordinary signposts of active star formation, such as the presence of high-velocity molecular outflows, (Yun & Clemens 1992, 1994b), optical and/or near-infrared jets (Moreira & Yun 1995), infrared reflection nebulae (Yun et al. 1993 ; Yun & Clemens 1994a), and infrared binary protostars (Yun 1996). Both Class I and Class II objects (Adams, Lada, & Shu 1987) have been found to have formed in Bok globules (Yun & Clemens 1995). Class II objects are thought to be T Tauri stars with their infrared excess emission due to the presence of circumstellar disks. Their T Tauri nature can only be definitely confirmed spectroscopically. In fact, classical T Tauri stars are emission line objects, with prominent Balmer lines and frequently displaying the Li I line (λ6707 Å, in absorption) typical of young stars. Another class of phenomena that is thought to be closely related to the star formation process is named FU Orionis phenomenon. A FU Orionis (FUor) object typically undergoes a rapid increase in its optical brightness that may reach 5 magnitudes in less than a year (Herbig 1966, 1977, 1989; Reipurth 1990). Paczynski (1976) and Hartmann & Kenyon (1985) have attributed these outbursts to large (up to three orders of magnitude) increases in the accretion rate on a central T Tauri star, probably due to instabilities ocurring in a massive circumstellar disk. The FUor stage could be a short-lived but important aspect of the star formation process significantly affecting the evolution of a star. Few FUor objects are known (e.g. Hartmann et al. 1993; Strom & Strom 1993; Evans et al. 1994). All are associated with reflection nebulae and large infrared excesses (e.g. Goodrich 1987) indicating they are pre-main sequence objects. In addition, they seem to display distinctive spectroscopic properties: G-type supergiant optical spectra; broad absorption lines, deep, blue-shifted absorption components on Hα and Na I D; CO absorption bands (e.g. Hartmann et al. 1993). However, it has been noted (Herbig 1989) that the spectra of FUors may not

168 J.L. Yun et al.: Two new T Tauri stars and a candidate FU Orionis star associated with Bok globules Fig. 1. V-band CCD image of CB31YC1. The new T Tauri star, CB31YC1, is the bright star at the center of the image. Notice the associated optical nebulosity. necessarily point to the existence of a self-luminous accretion disk. Instead, the spectra may alternatively be interpreted as indicating a peculiar, rapidly-rotating star with a semi-transparent atmosphere plus circumstellar material. During our survey of T Tauri stars in Bok globules, we confirmed the T Tauri nature of a subset of our Class II sources. In addition, we found an object that is likely to be a new FU Ori star. In this paper, we report these findings. In Sect. 2 we describe observations and data reduction. In Sect. 3 we present and discuss our results, and in Sect. 4 we summarize our conclusions. Fig. 2. Top: Broad-band spectral energy distribution of CB31YC1. Filled circles correspond to flux values, open circles represent upper limits (in the IRAS PSC catalog). The dotted line represents a blackbody (T = 3700 K) fit to the V-band flux and is shown for comparison purposes only. Bottom: Optical spectra of CB31YC1. Notice the presence of the Hα emission line as well as the Li I absorption line. 2. Observations and data reduction Optical imaging observations towards Class II YSOs in Bok globules (Yun & Clemens 1995) were carried out during the nights of 1993 December 13 16 using the European Southern Observatory 0.9 m Dutch telescope equipped with a 512 512 pixel CCD detector. Images were obtained using a set of filters which included broad-band B, V, R, I filters. The detector was used at a plate scale of 0.46 arcsec per pixel, resulting in a field of view of 3.9 3.9 arcmin 2 on the sky. In addition, in 1994 December and 1995 December, we obtained medium resolution ( 7Å per pixel) optical spectra and additional R-band images, using the ESO Danish 1.54 m telescope equipped with the Danish Faint Object Spectrograph and Camera (DFOSC) detector. Calibration lamps (He and Ne) were observed immediately before or after each object spectrum. Near-infrared observations were made in 1993 December 20 using the IRAC2 NICMOS 3 infrared array camera mounted on the ESO 2.2 m telescope. We obtained a set of 9 dithered frames in the JHK-bands, with a total integration time per frame of 30 seconds. Standard CCD image reduction procedures (dark and bias subtraction, and flat-fielding) were applied to the optical CCD images, using the NOAO Image Reduction and Analysis Facility (IRAF 1 ). Similarly, standard IRAF packages were used to make the spectroscopic reductions (SPECRED). Reduction of the near-infrared images were performed by subtraction of a sky frame (computed by median filtering of 1 IRAF is distributed by National Optical Astronomy Observatories which are operated by the Association of Universities for Research in Astronomy, Inc., under contract to the National Science Foundation.

J.L. Yun et al.: Two new T Tauri stars and a candidate FU Orionis star associated with Bok globules 169 Table 1. Object coordinates and photometry CB31YC1 CB32 T Tauri CB 34 FU R. A. (1950) 5 h 30 m 45 s 5 h 33 m 57 s 5 h 44 m 04.5 s Dec. (1950) 00 38 15 00 20 40 20 59 31 B 15.6... 17.1 V 14.1... 15.4 R 13.1... 14.3 I 12.1... 12.9 J 10.6 10.2 11.9 H 9.5 9.3 10.9 K 8.8 8.8 10.3 the 9 dithered images) followed by flat-fielding. The final flatfielded images were subsequently corrected for the presence of bad pixels and mosaicked, co-adding the overlapping regions. 3. Results and discussion 3.1. Two new T Tauri stars 3.1.1. CB31YC1 CB31YC1 is a relatively bright and isolated optical visible star located to the northwest of the center of Bok globule CB31. It is associated with tenuous optical nebular emission (Fig 1). Photometry of this object was performed and transformed to the Johnson standard system. Magnitudes in the B, V, R, I, J, H, and K bands are given in Table 1. CB31YC1 was classified as a Class II source by Yun & Clemens (1995). Its broad-band spectral energy distribution is shown in Fig. 2 (top), where the dotted line represents a blackbody fit to the V-band flux and is shown for comparison purposes only. The blackbody temperature was determined from the color index (B V ) (Carney et al. 1987). Given the good spatial coincidence with IRAS 05307 0019, and the absence of other candidates, this source was considered as being the optical counterpart of this IRAS source. Thus, the broad-band spectral energy distribution shown includes data points from 0.4µm to 100 µm. There is clearly excess infrared emission above that of a stellar photosphere (blackbody). The spectrum of CB31YC1 reveals the presence of Balmer lines (Hα and Hβ) in emission. Fig. 2 (bottom) shows part of the spectrum. The equivalent width of the Hα emission is 25 Å. The Li I (λ6707 Å) absorption line is also clearly present. Since the simultaneous presence of Hα emission and Li I (λ6707 Å) is a good spectroscopic criterium to decide on the T Tauri nature of a star, we conclude that CB31YC1 is a bona fide classical T Tauri star. 3.1.2. CB32 T Tauri The YSO candidate CB32YC1 whose spectral energy distribution was presented in Yun & Clemens (1995) was not included in this study. Instead, a nearby star associated with optical nebulosity was observed and its spectra obtained. We refer to this star Fig. 3. Top: V-band CCD image of CB32 T Tauri. This new T Tauri star is the bright star at the lower center of the image. Notice the associated optical nebulosity. Bottom: Optical spectrum of CB32 T Tauri. The Hα emission line is clearly present together with the Li I absorption line. as CB32 T Tauri. Fig. 3 (top) shows this star in a V-band CCD image of Bok globule CB32. CB32 T Tauri is the bright star at the lower center of the image. Coordinates and infrared magnitudes are given in Table 1. This star has no IRAS PSC counterpart and could not be discovered in IRAS-based searches. Its spectrum is presented in Fig 3 (bottom). Hα emission is seen (equivalent width = 3.6 Å) as well as the Li I absorption line. We conclude that this star is a weak-line T Tauri star. Being relatively bright (we estimate its Johnson V-band magnitude to be about 12), CB32 T Tauri could be an interesting object in which to study the wealth of photospheric phenomena usually associated with T Tauri stars.

170 J.L. Yun et al.: Two new T Tauri stars and a candidate FU Orionis star associated with Bok globules Fig. 5. Broad-band spectral energy distribution of the FUor candidate. The dotted line represents a blackbody (T = 3600 K) fit to the V-band flux and is shown for comparison purposes only. 3.2. A New Candidate FU Ori Star Fig. 4. Top: POSS image covering 4 arcmin 2 of Bok globule CB 34 including the new FUor candidate. Note carefully the triplet of three stars very close to one another. Middle: R-band CCD image of the same region obtained in 1993; Bottom: R-band CCD image of the same region obtained in 1994. Notice that the southwestern component of the stellar triplet has strongly increased its brightness. It also displays a small fan-shaped nebula. Fig. 4 (top) presents a 2 2 image obtained from the Palomar Observatory Sky Survey (POSS E) via the Digital Sky Survey. Note carefully, at the center of the image, the three stars very close to one another. Fig. 4 (middle), obtained with a CCD and an R-band filter in December 1993, shows the same region. Comparison of the triplet of stars located at the centers of these two images reveals that the southwest star (FU Orionis candidate, here tentatively named CB 34 FU) has brightened considerably between the times when the two images were taken (1950 and 1994). Fig. 4 (bottom) obtained in December 1994 with a CCD and an R-band filter shows the same region and reveals that the FUor candidate has kept its higher brightness for at least a year. Photometry of the stars present in the R-band CCD image together with a careful comparison of the brightness of CB 34 FU and the brightness of a field star in the Palomar image, lead us to an estimate of about 4.0 magnitudes for the change in the R-band magnitude of CB 34 FU. In addition, differential photometry between CB 34 FU and a field star indicates a slight increase in brightness, of about 0.1 mag, from the 1993 R-band image to the corresponding 1994 image. Magnitudes of CB 34 FU corresponding to December 1993, in the B, V, R, I, J, H, and K band are given in Table 1. Other evidence in favour of this object being a young star is provided by the near infrared excess emission revealed in its spectral energy distribution shown in Fig. 5. In this figure, the blackbody curve is shown for comparison purposes only, revealing the infrared excess emission above that of a photosphere. In addition, the presence of nebulosity surrounding the Fu Ori candidate (Fig. 4), looks similar to the one around the star FU Ori itself (fan-shaped), though less extended. The low brightness of this object resulted in insufficient signal-to-noise ratio in its spectra (shown in Fig. 6).

J.L. Yun et al.: Two new T Tauri stars and a candidate FU Orionis star associated with Bok globules 171 CB 34 FU is a candidate FU Ori star that increased its visual brightness by about 4 mag from 1951 to the present time. Further work is needed to confirm its FUor nature. Acknowledgements. This work has been partially supported by a JNICT grant to J.L.Y. Support from JNICT to M.C.M. and J.F.A. in the form of scholarships is gratefully acknowledged. References Fig. 6. Optical spectra of the FUor candidate. No special features are evident possibly due to insuffcient signal-to-noise ratio. The sodium doublet is marked with an arrow. The expected positions of the Hα and Li I lines are also indicated. The sodium doublet is present in absorption (with an equivalent width of 2.9 Å) which is one of the characteristics of the FU Ori stars. However, the doublet is not broad or blue-shifted as is common with FUors. The Li I line appears to be present but at a S/N of only about 2σ. Also, a higher spectral resolution and a more extended spectral coverage into the blue are needed in order to spectral type this object. Thus, the spectra are not conclusive and spectroscopic confirmation will require obtaining new optical spectra of this object with both higher resolution and higher signal-to-noise ratio. Near-infrared spectra would also be useful to search for the 2.3 µm CO absorption bands predicted by the FUor disk models. Adams, F. C., Lada, C. J., & Shu, F. H. 1987, ApJ, 312, 788 Carney, B. W., Laird, J. B., Latham, D. W., & Kurucz, R. L. 1987, AJ, 94, 1066 Clemens, D. P., & Barvainis, R. 1988, ApJS, 68, 257 Evans II, N. J., Balkum, S., Levreault, R. M., Hartmann, L., & Kenyon, S. 1994 ApJ, 424, 793 Goodrich, R. 1987, PASP, 99, 116 Hartmann, L, & Kenyon, S. J., 1985, ApJ, 299, 462 Hartmann, L, Kenyon, S. J., & Hartigan, P. 1993, in Protostars & Planets III, edited by E. H. Levy and J. Lunine (University of Arizona Press, Tucson), p. 497 Herbig, G. H. 1966, Vistas in Astronomy, 8, 10 Herbig, G. H. 1977, ApJ, 217, 693 Herbig, G. H., & Bell, K. R. 1988, Lick Obs. Bull. No. 1111 Herbig, G. H. 1989, in ESO Workshop on Low-Mass Star Formation and Pre-Main Sequence Objects, ed. B. Reipurth (Garching:ESO), pp. 233 Joy, A. H. 1945, ApJ, 102, 168 Moreira, M. C., & Yun, J. L. 1995, ApJ, 454, 850 Myers, P. C., Fuller, G. A., Mathieu, R. D., Beichman, C. A., Benson, P. J., Schild, R. E., and Emerson, J. P. 1987, ApJ, 319, 340. Paczynski, B. 1976, QJRAS, 17, 31 Reipurth, B. 1990, in IAU Symp. 136, Flare Stars in Star Clusters, Associations, and the Solar Vicinity, ed. L. V. Mirzoyan, B. R. Peterson, & M. K. Tsvetkov (Dordrecht:Kluwer), 285 Strom, K. M., & Strom, S. E. 1993, ApJ, 412, L63 Yun, J. L., & Clemens, D. P. 1990, ApJ, 365, L73 Yun, J. L., & Clemens, D. P. 1992, ApJ, 385, L21 Yun, J. L., Clemens, D. P., McCaughrean, M., & Rieke, M. 1993, ApJ, 408, L101 Yun, J. L., & Clemens, D. P. 1994a, AJ, 108, 612 Yun, J. L., & Clemens, D. P. 1994b, ApJS, 92, 145 Yun, J. L., & Clemens, D. P. 1995, AJ, 109, 742 Yun, J. L. 1996, AJ, 111, 930 4. Conclusions We confirm the T Tauri nature of two bright Class II young stellar objects in Bok globules CB31 and CB32. This article was processed by the author using Springer-Verlag LaT E X A&A style file L-AA version 3.