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Astron. Astrophys. 320, 167-171 (1997)

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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).

[FIGURE] 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.

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.


[TABLE]

Table 1. Object coordinates and photometry


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 [FORMULA] (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 [FORMULA] m to 100 [FORMULA] m. There is clearly excess infrared emission above that of a stellar photosphere (blackbody).

[FIGURE] 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 ([FORMULA] K) fit to the V-band flux and is shown for comparison purposes only. Bottom: Optical spectra of CB31YC1. Notice the presence of the H [FORMULA] emission line as well as the Li I absorption line.

The spectrum of CB31YC1 reveals the presence of Balmer lines (H [FORMULA] and H [FORMULA]) in emission. Fig. 2 (bottom) shows part of the spectrum. The equivalent width of the H [FORMULA] emission is 25 Å. The Li I ([FORMULA] 6707 Å) absorption line is also clearly present. Since the simultaneous presence of H [FORMULA] emission and Li I ([FORMULA] 6707 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 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). H [FORMULA] 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.

[FIGURE] 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 H [FORMULA] emission line is clearly present together with the Li I absorption line.

3.2. A New Candidate FU Ori Star

Fig. 4 (top) presents a [FORMULA] 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.

[FIGURE] Fig. 4. Top: POSS image covering 4 arcmin2 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 (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 [FORMULA], 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.

[FIGURE] Fig. 5. Broad-band spectral energy distribution of the FUor candidate. The dotted line represents a blackbody ([FORMULA] K) fit to the V-band flux and is shown for comparison purposes only.

The low brightness of this object resulted in insufficient signal-to-noise ratio in its spectra (shown in Fig. 6).

[FIGURE] 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 H [FORMULA] 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 2 [FORMULA]. 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 2.3µm CO absorption bands predicted by the FUor disk models.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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