A&A 385, 900-903 (2002)
C. B. Pereira - C. S. Franco - S. J. C. Landaberry
Observatório Nacional-MCT, Rua José Cristino, 77, CEP 20921-400, São Cristóvão, Rio de Janeiro-RJ, Brazil
Received 3 December 2001 / Accepted 17 January 2002
We report spectroscopic observations of a new symbiotic star. This star is located in our Galaxy in the direction of the bulge and was discovered during a southern spectroscopic survey of Hemission line objects. The star, SS7371, after Sanduleak & Stephenson (1973), shows characteristics of a symbiotic star: the presence of TiO bands and emission lines of ions of higher ionization such as He II4686 Å and [O III]5007 Å. Here, we present some of the main spectroscopic features, line intensities and, whenever possible, some physical parameters.
Key words: binaries: symbiotic - stars: emission line, Be
In this work we present spectroscopic observations of the
emission-line star SS7371, first recognized as an H
line object after a objective prism survey of Sanduleak & Stephenson
(1973), hereafter SS73. SS73 classified this object as a
"Very-Steep-Balmer-Decrement'', X symbol in their Col. 7, which
might be an indication of symbiotic object or a highly reddened Be
star. Allen (1978) classified it as a "Be star with weak Balmer
emission and He I emission''. We find that SS7371 displays
characteristics of a symbiotic object, e.g., TiO absorption bands and
emission lines of ions of higher ionization such as He
II4686 Å and [O III]5007 Å. This symbiotic is not given
in the recent catalogue of Belczynski et al. (2000).
A finding chart of the object is present in Fig. 1. The
equatorial coordinates of SS7371 are
The V magnitude
was derived convolving the calibrated spectrum with a V filter profile.
The result is shown in Table 2.
|Figure 1: Finding chart of the region around SS7371. SS7371 is the faint star at the center of the field. The field has dimensions of . North is at the top and east is to the left.|
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|Figure 2: Blue spectrum of SS7371. Notice the presence of the He II4686 Å line and the [O III] forbidden line at 5007 Å.|
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|Figure 3: Optical spectrum of SS7371.|
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Spectroscopic observations were performed using a Boller & Chivens spectrograph at the Cassegrain focus of the ESO 1.52 m telescope in La Silla (Chile) at March 24, 2000, June 15, 2001 and September 25, 2001. A UV-flooded thinned Loral Lesser CCD #39 ( , 15 m/pixel) was used as the detector; it gives a high quantum efficiency in the blue and in the UV range. The setup employed is the same as that used to investigate suspected symbiotic objects with the 1.52 m telescope (Pereira et al. 1998). Two different instrumental setups were employed. The first one made use of the grating #23 with 600 l/mm providing a resolution of 4.6 Å in the range 3500 Å-8000 Å and the other was the grating #33 with 1200 l/mm, resulting in a resolution of 1.9 Å in the 3200 Å-5300 Å range.
The spectra were reduced using standard IRAF tasks, from bias subtraction and flat-field correction, through spectral extraction and wavelength and flux calibration. Spectrophotometric standards from Oke (1974) and Hamuy et al. (1994) were also observed. The slit orientation in the range (3200 Å-5300 Å) was aligned with the parallactic angle in order to minimize the light loss due to atmospheric refraction.
The flux of emission lines have been
measured by the conventional method adjusting a Gaussian function to
the line profile thereby obtaining the intensity, the central
wavelength and the line width at half power level. Uncertainties in
the line intensities come mainly from the position of the underlying
continuum. We estimate the flux error to be about 20% for the
weak lines (line fluxes 10 on the scale of H)
and about 10% for stronger lines. Figures 2 and 3 show the reduced
Table 1 shows the observed line flux of the identified lines. The spectrum of SS7371 shows typical features of a symbiotic star with late K-early M component when compared with spectral atlas at same resolution (Jacoby et al. 1984; Turnschek et al. 1985). The spectrum of SS7371 looks similar to the spectrum of Hen 1342 (Medina-Tanco & Steiner 1995) and AS 289 (Gutiérrez-Moreno et al. 1999), which have spectral types, respectively, of K7-M1 and M 3.5 according to Mürset & Schmid (1999). We identify the TiO absorption bands in the spectrum of SS7371 at 6200 Å, 6852 Å and at 7160 Å. The absorption at 6494 Å it is a blend of the absorption features of Ba II, Ca II and Fe I. Around 5890 Å the Na I is present in absorption. In the range 3100 Å-5100 Å some absorption features were identified such as Fe I at 3920 Å, 4045 Å and at 4064 Å; Ca II at 3933 Å and the diffuse absorption band at 4430 Å. The emission spectrum shows the Balmer lines, the major lines of He I, the He II line at 4686 Å and the oxygen forbidden lines at 4363 Å, 4959 Å and 5007 Å the latter of which is partly blended with the He I5015 line in our spectra.
In this section we will provide some physical parameters
of SS7371 such as reddening, electron
density, temperature of the hot component and the infra-red type
(S-type or D-type). Based on infrared color, symbiotics can be divided
into two groups : S- and D-types (Webster & Allen 1975;
Allen 1982). Those that show dust continuum emission between wavelengths
1.0 and 5.0 m are classified as D-type, and these systems contain
a mass-losing Mira variable as a cool component. Those with a normal
stellar spectrum in the infrared are classified as S-types.
Table 2 contains the results.
|Log cm-3||7.0 -0.05+0.22|
|He I 6678/5876|
a: From [O III] lines; For K.
b: From O+2 ion.
c: From He+2 ion.
Reddening estimates were based on Netzer's (1975) curves for H/H versus H/H. It was assumed that the reddening law can be represented by the standard interstellar extinction curve (Seaton 1979).
It was not possible to make good plama diagnostics with the emission lines available. In fact we do not have lines for electron temperature determination for densities as high as those involved in these kinds of symbiotic systems. The [O III] lines at 5007 Å, 4959 Å and 4363 Å are not good indicators of temperature but are approriate for electron densities. Using the [O III] line ratios corrected for reddening and assuming K we obtained the electron density.
The temperature of the hot component was determined using the method proposed by Mürset & Nussbaumer (1994) ( ) (hereafter MN94) and Ijima (1981) ( ). The MN94 method has the advantage that it takes into considaration the observed ion with the highest ionization potential, but may fail for the S-type systems, because some forbidden lines may be absent because of the high density. In the method of Ijima (1981) the temperature derived is an upper limit for the hot component since optical depths may alter the intensity of Balmer lines.
Proga et al. (1994) demonstrated that He I emission line ratio can be used to distinguish between S-type (stellar continuum) and D-type (dust) symbiotic stars, besides their IR-colors. Since the S-type systems have higher electron densities ( cm3) than the D-type systems ( cm3), the emission-line ratio, as defined by the ratio I(6678/5876), distinguishes the two types. Those that have are D type and those that have are S-types.
Based on a spectroscopic survey between 3200 Å-5200 Å and 3500 Å-8000 Å of H emission-line stars from the SS73 sample we discovered a new symbiotic star (SS7371). Its continuum spectrum seems to indicate the spectral type of late K-early M with Balmer lines, helium lines and forbidden lines of oxygen in emission. We also provide a table with the identified features and some physical parameters.